microwave drying and conditioning of pinus radiata d. don

Microwave Drying and Conditioning of Pinus radiata D. Don Sawn Timber

Nur Hannani binti Abdul Latif

Submitted in total fulfilment of the requirements of the degree of Master of Wood Science

March 2014

School of Land and Environment

The University of Melbourne

Australia

i

ABSTRACT

Microwave (MW) processing technology is used for the conditioning of wood to specific

moisture contents (MC), generally 12%. MW drying differs from conventional drying in the

way MW energy interacts with wood moisture and its superior penetration. Wood moisture

content can be quite variable towards the end of drying. MW processing reduces the within

charge moisture variation. The objective of this study is to model energy requirements as a

function of starting wood moisture content and wood species. The methodology involves

investigating the influence of microwave conditioning technology in reducing wood drying

time and also wood drying degrade, due to moisture leveling, assisting stress relaxation and

avoiding case hardening of Pinus radiata by using laboratory scale and pilot scale

microwave technology.

For laboratory scale experiments, research methodology involved oven-drying boards for a

range of hours (to get a range of moisture contents) and then conditioning boards with MW

energy until the final weight of each board achieved the equivalent of 12% MC. It was

found that there is a strong correlation between moisture content after oven drying and the

number of microwave passes needed to achieving a final board of 12% moisture content.

Drying quality assessment after microwave processing revealed that checking had occurred

and there were some limitations in evaluating warping on samples due to the limitation of

specimen size.

Pilot scale microwave drying was then conducted to scale up and validate laboratory scale

microwave research and further evaluate the effectiveness of dynamic microwave

processing in optimizing drying with minimal defects. A comparison of sapwood only,

mixed sapwood & heartwood and heartwood only was determined. From this study, it was

shown that microwave drying time was fastest for heartwood samples and that microwave

energy consumption was about 206 kW/h. The moisture content distribution in boards was

also uniform and residual stress tests found that almost 90% of boards were free of case

hardening.

ii

It can be concluded that the application of microwave conditioning is an efficient method of

drying timber. Drying times are fast and there are minimal drying defects.

iii

STATEMENT OF ORIGINALITY

This is to declare that this thesis comprises my own work, except acknowledgement has

been indicated in the text and other materials used.

Nur Hannani Abdul Latif

March 2014

iv

ACKNOWLEDGEMENTS

I owe a special dedicated thanks to the following persons:

My supervisors, Professor Peter Vinden, Dr. Simon Przewloka and Dr. Ian Graham Brodie,

for their constant supports, valuable inputs, guidance and encouragement over the duration

of my research.

Universiti Teknologi MARA (UiTM), Malaysia, by funding me with Young Lecturer’s

Scheme scholarship to make my research possible.

Professor Grigory Torgovnikov, for helping me with the new technology, microwave

conditioning of wood for drying treatment. Mr. Gerry Harris, for his assistance and ideas

with high temperature drying treatment at Burnley Campus, The University of Melbourne.

All academic and administrative members at Department of Forest and Ecosystem Science,

School of Land and Environment, The University of Melbourne (UoM), Creswick for their

help, Mr. Peter Plews for helping me cut my samples, and Mr. Gerry Harris, who assisted

me with the reference finding in the library and UoM Web Portal.

My colleagues, Norashikin Kamarudin, Muliyana Arifudin, Krisdiyanto Sugiyanto and Anil

Kumar Shetty, for their friendship and help in the field.

Last and most importantly, my parents, Abdul Latif bin Muda and Napisah binti Muhd.

Said, my husband, Mohd Redzuan hamzah and my siblings. They supported me and loved

me.

If I have failed to mention someone, I sincerely apologise.

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TABLE OF CONTENTS

ABSTRACT………………………………………………………………………………....i

STATEMENT OF ORIGINALITY……………...………………………………………iii

ACKNOWLEDGEMENTS………………………………………………………………iv

TABLE OF CONTENTS………………..…………………………………………...…....v

LIST OF FIGURES………….…………………………………………………………..viii

LIST OF TABLES…...…………………………………………………...……………….xi

LIST OF PLATES……………………………………………………………...………...xii

CHAPTER 1 GENERAL INTRODUCTION

1.1 Introduction ................................. .............................................................................1

1.2 Sapwood and heartwood .. ........................................................................................2

1.3 Colour ........... ............................................................................................................4

1.4 Wood quality ....... .....................................................................................................5

1.5 General objectives ....... .............................................................................................6

CHAPTER 2 LITERATURE REVIEW

2.1 Introduction .......... ....................................................................................................7

2.2 Wood drying ............ .................................................................................................7

2.3 Drying elements ....... ..............................................................................................10

2.4 Wood drying quality ........ .......................................................................................11

2.4.1 Rupture of wood tissue .......................... ...................................................12

2.4.2 Warp ........................................................................................ .................14

2.4.3 Discoloration.............................................................................................16

2.4.4 Case hardening..........................................................................................18

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2.5 Wood drying methods.............................................................................................23

2.5.1 Air drying ........ .........................................................................................23

2.5.2 Kiln drying................................................................................................24

2.6 Microwave drying...................................................................................................28

2.6.1 Introduction ....... .......................................................................................28

2.6.2 Microwave heating and drying..................................................................28

CHAPTER 3 MICROWAVE WOOD DRYING

3.1 Introduction ..... .......................................................................................................30

3.2 Experiment One: Laboratory scale microwave wood drying.................................31

3.2.1 Research objectives...................................................................................31

3.2.2 Research hypotheses . ................................................................................31

3.2.3 Materials and methods .... ..........................................................................31

3.2.3.1 Sample boards ... .....................................................................31

3.2.3.2 Moisture content profile ... ......................................................32

3.2.3.3 Oven drying ............................................................................35

3.2.3.4 Microwave drying ..................................................................35

3.2.3.5 Moisture content distribution . ................................................37

3.2.3.6 Microwave drying quality.......................................................38

3.2.4 Statistical analysis......................................................................................38

3.2.5 Results.......................................................................................................38

3.2.5.1 Sample variations....................................................................38

3.2.5.2 Relationship between green density and green

moisture content.....................................................................39

3.2.5.3 Relationship between green moisture content,

oven-drying time and moisture content after

oven-dry.................................................................................40

3.2.5.4 Microwave drying treatment . .................................................42



vii

3.2.6 Discussion and conclusions.......................................................................50

3.3 Experiment Two: Pilot scale microwave wood drying...........................................51

3.3.1 Research objectives...................................................................................51

3.3.2 Research hypotheses ...... ...........................................................................51

3.3.3 Materials and methods..............................................................................51

3.3.3.1 Sample boards . .......................................................................51

3.3.3.2 Moisture content profile..........................................................52

3.3.3.3 Microwave drying ... ...............................................................53


3.3.3.5 Prong test/ Drying stress determination..................................60


3.3.4 Statistical analysis......................................................................................62

3.3.5 Results.......................................................................................................63

3.3.5.1 Sample variations . ..................................................................63

3.3.5.2 Relationship between green density, initial

moisture content and basic density.........................................64

3.3.5.3 Microwave drying treatment . .................................................65

3.3.5.4 Drying rate..............................................................................68

3.3.5.5 Moisture content distribution .. ...............................................70

3.3.5.6 Prong test/ Stress test..............................................................72


3.3.6 Discussion and conclusions.......................................................................79

CHAPTER 4 GENERAL DISCUSSION AND CONCLUSION

General discussion and conclusion.............................................................82

REFERENCES...................................................................................................................85

APPENDICES.....................................................................................................................89

viii

LIST OF FIGURES

CHAPTER 1

Figure 1.1 - Log section containing heartwood and sapwood…………………….....……....3

Figure 1.2 – Structure of open bordered pit in wood……….……………………………….4

Figure 1.3 - Incidence of tension (T) and compression wood sites (C)…...……………..…..6

CHAPTER 2

Figure 2.1 - Moisture gradients of wood drying processes at varying

times and thickness…………….…………………………………………...….9

Figure 2.2 - Surface checks in wood.....................................................................................13

Figure 2.3 - End checks in wood...........................................................................................13

Figure 2.4 - Collapse in wood...............................................................................................14

Figure 2.5 - Sapwood with (left) and without discoloration (right)……..………………....16

Figure 2.6 - Colour profiles for samples kiln dried in air (left) and gas………………..….18

Figure 2.7 - Diagram of how casehardening occurred after various

stages of wood drying process…………..…………………………………….20

Figure 2.8 - Summary of Prong test result (prongs curvature)………………………..…....22

Figure 2.9 - Timber is stacked in air drying practice……………………….………….…...23

CHAPTER 3

Figure 3.1 - Sample measurement……………………………………………………….....32

Figure 3.2 - Preparations of sample board…………………………………………..……..34

Figure 3.3 - Relationship between green density and green moisture content………….…40

Figure 3.4 - Relationship between oven-dry moisture content and drying time…………...41

Figure 3.5 - Relationship between percentages of oven-dry moisture

content and moisture content after one pass through the

microwave ………………………………………………………...………….43

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content and moisture content after two passes

through the microwave …………………………………………………….....43


content and moisture content after three passes through the

microwave….……………………………………………………………….....44


content and moisture content after four passes through the

microwave …………………………………………………...………...……..44

Figure 3.9 - Drying rate patterns for various microwave treatments………………………45

Figure 3.10 - Moisture content of samples after each pass of microwave drying…………48

Figure 3.11 - Defects after microwave treatment: checking (left)

and staining (right).………………………………..……………………...….49

Figure 3.12 – Sample measurement (planks) for microwave treatment……………..….…52

Figure 3.13 - Microwave generator and conveyor applicator……………………………...55

Figure 3.14 - Placement of dummy plank to ensure full microwaving

of experimental plank.s……………………………………………………….57

Figure 3.15 - Reaction of wafer samples to the prong test....................................................60

Figure 3.16 - Measurement of bow……………………………………………………...….61

Figure 3.17 - Measurement of spring…………………………………………………….…61

Figure 3.18 - Measurement of twisting……………………………………………………..62

Figure 3.19 - Measurement of cupping……………………………………………………..62

Figure 3.20 - Relationship between green moisture content and green density……….…...65

Figure 3.21 - Drying rate (cumulative weight loss) of microwaved

heartwood planks……………… …..………………………………………...68


mixed planks….……………………………………………………………….69


sapwood planks…………………………………………………………...…..69

x

Figure 3.24 - Average moisture content distribution of microwaved

planks at different layers………………………………………………...…..71

Figure 3.25 - Comparison of prong curvature before and after

24 hours air drying for all planks………………………...….……………….74

Figure 3.26 - Checks degrade that occurring for all groups after microwave processing……………………………………………………….77

CHAPTER 4

Figure 4.1 – Sampling methodology of samples divided into slices with equal thickness for measuring final moisture content of planks……………….......................................................................83

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LIST OF TABLES

CHAPTER 2

Table 2.1- Schedule for the accelerated drying and equalizing of

Pinus radiata framing timber…………………………………………………....25

CHAPTER 3

Table 3.1 - Sample sets using systematic randomized design………………………….…..32

Table 3.2 - Oven drying overview for all set of samples…………………………………...35

Table 3.3 - Statistical description of green density and green moisture

content .……………………………………………………...…………………39

Table 3.4 - Predicted moisture content after microwave treatment………………………..47

Table 3.5 - Summary of the sample prepared for microwave treatment…………………...52

Table 3.6 - Technical data for the 60 kW microwave……………………………………...54

Table 3.7 - Average initial moisture content and green density for

each group……………………………………………………………………....63

Table 3.8 - Summary of wood property variability……………………………………...…64

Table 3.9- Summary of microwave drying schedules used for

each group…………………………………………………………………...…66

Table 3.10 - Average value of moisture content distribution for

microwaved planks……………………………………………………………..70

Table 3.11 - Statistically analysis of F-Value on the final moisture content

of microwaved plank at different groups and layers........................................71

Table 3.12 - Summary of prong curvature during prong test………………………………73

Table 3.13 - Comparisons of warp defects of microwaved planks………………….……..76

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LIST OF PLATES

CHAPTER 3

Plate 3.1 - Microwave equipment used in wood treatment………………………………..36

Plate 3.2 - Sample arrangement in microwaving chamber…………………………………36

Plate 3.3 - Sample cross-section……………………………………………………………37

Plate 3.4 – Cross-section sample prior to division (left) and the

inner sample (right)…………………………………………….………….…….37

Plate 3.5 - Identification of the heartwood (H) and sapwood (S) boundary………………53

Plate 3.6 - 60 kW microwave generator installations………………………………………56

Plate 3.7 - Plank moved in through microwave applicator using conveyor belt…….……..58

Plate 3.8 - Planks exiting from the microwave applicator

(after one microwave treatment pass) ………………………………..………….58

Plate 3.9 -Wood sap (free water) comes out from plank during

microwave processing.………………………………………………..….……...67

Plate 3.10 - Comparisons of prongs curvature with case hardening (left),

remain straight (middle) and reverse case hardening (right)…………………...72

Plate 3.11 – Warp in plank was measured using thread……………………………...…….75

Plate 3.12 - Internal checks that occurred after microwave process………………………..77

CHAPTER 1: GENERAL INTRODUCTION

1.1 Introduction

Timber drying is the most time and energy consuming step in the processing of wood

products. Variability in drying processes and sensitivities to drying defects impose limitations

on the development of standard drying procedures (Simpson, 1999). Different drying

properties occur both between and within species. Most hardwood timbers, for example

eucalypts and oaks, even balsa (which is around 160 kg/m3 is the lightest and softest

commercial timber) are flowering plants and have broad-leaved trees, and have complex

anatomical structure. The textures of the wood also range from fine to coarse (Brandon, 2005).

Due to these properties, and their less complex anatomical structure and better permeability,

certain softwoods are easier to dry than hardwood species. Softwoods are coniferous species

such as pines, firs and spruce, which are normally fine textured but not particularly light

(Brandon, 2005). In green or freshly-cut timber, the moisture content can vary from 30%-

200%. This variation in moisture content is not only dependent on species, but also occurs

within the same species and even the same tree.

Timber commences drying once it is cut down and this continues until the moisture content is

in equilibrium with the surrounding air (Walker, 1993). According to Simpson (1999),

sapwood layers next to the bark contain living cells which have higher moisture contents than

the heartwood. From a commercial prospective, it is desirable to dry timber to a final moisture

content that is suitable for service, as fast as possible, without excessive degrade. Usually, the

moisture content of timber in service is 8%-15% (Harris, 2008) and the difficulty of drying

depends upon use. Moreover, Simpson (1999) found that softwood timber for use as framing

in construction is normally dried to 15%-19% moisture content while, for many other uses,

timber is dried to lower moisture content. Timber for numerous appearance grade applications

is dried to moisture contents of 10%-12% and to as low as 7%-9% moisture content for the

furniture industry. Hardwood timber is dried to an average of 15% moisture content and

around 6%-8% moisture content for the furniture industry (Simpson, 1999).

Numerous techniques exist for drying timber including air drying, shed air drying,

conventional kiln drying, low temperature kiln drying, high temperature drying and vacuum

drying (Denig et al., 2000). During the 1970,s, air drying was the most popular method for

timber drying followed by kiln drying. Timber drying research ultimately aims to produce

better quality dried timbers. The objectives, when drying timber, are to produce a useful

product and minimize any quality losses, thereby conserving natural resources while

delivering profit (Denig et al., 2000). There are a number of reasons why drying timber is

important:

i. To increase the stability of timber once equilibrium moisture content is reached.

ii. To reduce weight, thus decrease shipping and handling costs (for ease of

transportation).

iii. To reduce susceptibility to attack by borers or decay fungi.

iv. To increase strength (dry timber is more than twice as strong as wet timber).

v. Machining and gluing are much more easily accomplished.

1.2 Sapwood and heartwood

The structure of wood and its location within the tree can affect drying processes (Simpson,

1999). Besides bark and pith, wood also consists of heartwood and sapwood. Sapwood is the

layer adjacent to the bark and contains living cells that actively transport water and other

nutrients necessary for the tree. According to Denig et. al. (2000) sapwood is pale in colour.

In contrast, heartwood is located at the centre of the stem and is usually darker and harder than

wood near the pith with physiologically inactive cells. Heartwood starts forming in Pinus

radiata at an age of 12-15 years (Walker, 1993). Heartwood occurs when sapwood cells age

and become less permeable after extractives permeate from the cell wall and cell lumen. The

walls of the parenchyma cells become lignified. The darker region (older cells) that they form

is the heartwood. Heartwood formation includes deposition of resins in softwoods.

Heartwood and sapwood locations are illustrated in Figure 1.1. The sapwood of softwoods

such as Pinus radiata is normally higher in moisture content than the heartwood. The moisture

content of sapwood in hardwood species is usually higher than, or equal to the heartwood

(Simpson, 1999). Some species of timber contain an abnormal type of heartwood known as

wetwood. Wet-wood has higher moisture content than the normal wood of the species

(Simpson, 1999).

Figure 1.1 - Log section containing heartwood and sapwood (Wikipedia, 2008).

Sapwood is more permeable than heartwood, thus sapwood dries faster than heartwood.

Permeability of wood is a measure of the ease with which fluids flow through it (Comstock,

et. al., 1968). Bordered pits control the longitudinal permeability of wood. Bordered pits

consists a centralized thickened disk, torus, and margo (supporting membrane) as illustrated in

Figure 1.2. These pits are quite permeable and allow easy passage of fluids in green sapwood.

However, the permeability of wood decrease during drying processes due to the aspiration of

pits. During the drying process, surface tension forces tend to displace the torus. The torus

comes into contact and adheres with one of the pit borders (Comstock, et. al., 1968). In effect

sapwood

heartwood

the torus acts as a valve. The valves close during drying. This phenomenon is termed pit

aspiration. Drying sapwood may cause a gradual increase of aspirated pits with loss of

moisture down to the vicinity of the fibre saturation point. The lower permeability of

heartwood also makes it more susceptible to certain types of drying defects.

Figure 1.2 - Structure of open bordered pit in wood (Martin, H, 2007).

The sapwood of all wood species is perishable and susceptible to sap-stain. Colonization with

decay fungi can occur within 2-3 months. The heartwood of Pinus radiata is classified as

Class 4 (non-durable) according to Australian Standard AS 1604 and has a life expectancy in

ground contact of approximately five years.

1.3 Colour

The light colour of sapwood gradually changes during tree growth due to the presence of

extractives. Some softwood species such as spruce and fir undergo little or no change in

colour (Simpson, 1999). Unwanted discolouration can develop in trees during log storage or

during drying. During high temperature drying, darkening can occur due to the temperatures

employed. Simpson (1999) stated that colour change in sapwood is common during drying,

but infrequently encountered in heartwood.

1.4 Wood quality

Wood quality comprises inherent physical and technological properties of timber dependent

upon knots, bark, tension and compression wood and spiral grain content (Welling, 1994).

These naturally occurring components directly establish the grade and value of each

individual board. Some of these natural features can be removed by timber processing such as

planning, or trimming (Ward and Simpson, 1985). Sometimes, these structural variations or

attributes can be removed by docking and the timber finger jointed. In some instances the

attributes can give aesthetic value and may influence wood drying quality.

Knots are parts of tree branches that appear on the board. During drying, different types of

knots occur due to differences in shrinkage. Checked knots are aggravated by using low

humidity during drying and are almost impossible to avoid. Knots held in the wood by bark

and pitch invariably loosen during drying and may drop out during handling or machining

(Ward and Simpson, 1985).

Tension wood predominantly occurs at the upper side of leaning trees and is common in

hardwood timber. The fibres are shorter in tension wood compared to the normal fibres

(Waterson, 1997). Shrinkage is greatest in the longitudinal direction. In contrast, compression

wood usually occurs at the lowest part of timber in softwoods. It can be found on the cross-

section part as darker coloured with quarter moon shaped region and heavier than normal

wood (Waterson, 1997). Timber containing compression wood will shrink most along the

length of the board (Simpson, 1999). These two types of wood can cause warp defects during

drying. Both tension and compression wood incidence is shown in Figure 1.3.

Figure 1.3 – Incidence of tension (T) and compression wood sites (C) (Waterson, 1997).

1.5 General objectives

The objectives of this project are:

i. To model energy requirements during microwave processing of Pinus radiata as a

function of the starting moisture content.

ii. To determine microwave drying rate in reducing wood moisture content.

iii. To identify any drying defects that might arise as a result of microwave conditioning.

CHAPTER 2: LITERATURE REVIEW

2.1 Introduction

Drying is the process of seasoning timber to a moisture range depending on the conditions and

purposes for which it is to be used (building, furniture, fencing and flooring). During drying

water is removed from the timber. Depending on species, trees can contain water in quantities

from 40% to more than 200% of the dry mass (Harris, 2008). The location and amount of

water present combined with the wood structure influences drying characteristics. According

to Walker (1993), minimizing degradation is the primary reason for drying timber.

Simpson (1999) stated that control of the drying process is paramount to valuable and efficient

timber drying. Drying degrades or defect can occur due to imperfections in drying practice

and thus affect the quality and serviceability of the product. Shrinkage and checking may

occur during uncontrolled drying. Slow drying could be applied to reduce some problems but

is an uneconomical option.

Discoloration may occur during drying activities. It may be influenced by the combined effect

of temperature and moisture on the wood, also due to oxidation of some constituents.

Previous studies by Pang and Li (2006) reported that kiln drying of Pinus radiata can cause a

discolouration problem that affects the wood processing industry. Pinus radiata softwood is

widely planted in the world’s South Temperate Zone especially in Australia and New Zealand

(Bootle, 2004). It has a rapid drying rate and is usually kiln-dried directly from the green

condition. However, it is very prone to blue stain during the warmer months (Bootle, 2004). It

usually occurs under favorable temperature and moisture conditions during kiln-drying.

2.2 Wood drying

Moisture in wood normally moves from higher to lower zones of moisture content. The

surface of the wood must be drier than the interior if moisture is to be removed (Simpson,

1999). In softwood species, sapwood is usually higher in moisture content than heartwood. A

two phase process is used to dry timber. Initially moisture moves from the interior of the

board to the surface followed by evaporation of the moisture from the surface. In green

timber, wood cells and their interior are saturated with water. Free water inside the cell lumen

is initially lost during drying, followed by evaporation of the water from the cell wall (Harris,

2008).

To reach equilibrium, as outer drying increases, interior moisture is drawn towards the outside

of the timber. During drying the surface fibres of heartwood reach moisture equilibrium with

the surrounding air. The typical moisture gradient is the difference in moisture between the

inner and outer portions of a board (Simpson, 1999). Timber is normally dried to a level

known as the equilibrium moisture content. Relative humidity and air movement affect the

equilibrium moisture content, as do species, wood density and the extractive content of the

wood. Wood dried below 30% moisture content, the fibre saturation point, begins to lose

water from within the cell wall and thus they begin to shrink (Rene, 2007). Simpson (1999)

defined fibre saturation as the moisture content at which the cell walls are saturated but no

free water remains in the cell cavities.

Board thickness can influence drying rate. Drying time increases with board thickness and at a

rate that is more than proportional to thickness. If board thickness is doubled, drying time is

more than doubled (Simpson, 1999). Moisture content variation across the board thickness can

affect the internal drying stress (Waterson, 1997). Poor sawing of timber causes thickness

variations and thus influences kiln drying times. Drying also varies across a board. The

distribution of moisture at the core layer is higher than that of the outer layer. According to

Waterson (1997), initially moisture in a board is constant from core to the outer layer because

in the beginning the board is uniformly wet. The moisture content difference between the core

and the outer layer occurs when water starts to evaporate from the outer layer and the moisture

content decreases below saturation (Waterson, 1997). During the early stages of drying, the

moisture content is distributed with large variation between layers but after many days, the

moisture variations decrease. The distribution of moisture at different times is illustrated in

Figure 2.1. Quality drying is achieved by reducing the variability of factors affecting the

drying process (Culpepper, 1990).

Figure 2.1 - Moisture gradients of wood drying processes at varying times and thickness

(Harris, 2009).

Culpepper (1990) concluded that poor manufacturing can also affect moisture distribution and

grade recovery in wood drying by introducing variability. Efficient manufacture can assist in

reducing variability in drying. Implementation of quality control programs in the sawmill

helps to reduce and maintain target size for thickness, width and length. There also sawmills

use scanners to ensure accurate cutting of timber to reduce size variation.

2.3 Drying elements

In the drying process, there are three major elements that need to be controlled. These are

temperature, relative humidity and air flow. Temperatures can influence the drying process in

many ways. According to Walker (2006), raising the temperature dramatically enhances

diffusion of water molecules across cell walls, the basis for kiln drying at high temperatures.

High temperatures will increase the rate of moisture transfer to the wood surface. However,

each wood species has a critical temperature that must not be exceeded during the initial or

final drying process. Wood is weaker at high temperature and more prone to failure. Drying

defects such as surface checking probably occur at a relatively high temperature during the

early stage of drying (McMillen, 1958). Temperature also affects shrinkage and interior

compressive stresses during the intermediate stages of drying.

The relative humidity of air is a measure of how much of the moisture carrying capacity of the

air, at any particular temperature is used up (Harris, 2009). When the air is totally dry, the

relative humidity will be zero. Whereas it may be 100% once it is fully saturated with water

(Waterson, 1997). Relative humidity is difficult to measure reliably and is determined

indirectly from the wet-bulb and dry-bulb temperatures (Walker, 2006). The wet-bulb

temperature is largely determined by both actual air temperature (dry-bulb temperature) and

humidity, the amount of moisture in the air. Meanwhile, the dry-bulb measures the air

temperature (no cooling effect on the dry-bulb thermometer). Wet-bulb depression and dry-

bulb temperature are the parameters used to control the relative humidity of a kiln (Walker,

2006). Waterson (1997) also states that relative humidity will decrease when a sample or air is

heated without any additional vapour and may increase if it is cooled below its original

temperature and no moisture extracted from it.

Air flow also needs to be controlled because drying rate is dependent on the flow of air past

the wet drying surfaces (Harris, 2009). Airflow performs two roles:

a carrier of heat and

a medium to absorb evaporating moisture.

Higher air flow reduces the boundary layer, increases the evaporation rate and removes larger

volumes of water. Harris (2009) also states that once the surface dries, air flow is less

important during the drying process. In an air drying yard, good access to the surrounding air

provides more efficient circulation for drying. It is different in a kiln, where the air flow is

uniform and adequate through all part of stacks. The reason is because the rate of moisture

removal varies linearly with the air velocity, so that, the higher the temperature, the more

critical the evenness of the air flow (Waterson, 1997).

2.4 Wood drying quality

Wood drying quality is one of the most important aspects of wood drying. Drying rate of

timber is one potential factor that can affect wood drying quality. Drying rates that are too fast

will result in cracks and splits, while drying rates that are too slow will result in stain and warp

(Wengert, 1994). Wengert (1994) also states that an assessment of quality is needed as soon as

possible after drying is completed, e.g. measurement of average final moisture content, the

amount of defects (checks, warp and colour change), shrinkage and the spread of moisture

content. All these variables can be used to determine whether the drying process is under good

control or not.

Drying quality differs from wood quality. Welling (1994) states that drying quality depends

on control and regulation of the drying process but wood quality is influenced by the physical

properties of the unprocessed timber (knots, bark, compression and tension wood). Drying

defects that occur due to imperfections of the drying process can be easily identified. Drying

defects that occur due to wood imperfections such as knots are more difficult to detect

(Welling, 1994).

According to Walker (1993), drying defects occur due to shrinkage or differential shrinkage

within the timber. Moisture variation in timber after drying causes differential shrinkage.

Moisture gradients in boards cause problems such as low dimensional stability. Drying defects

are any characteristic or blemish in a wood product that occurs during the drying process that

degrades the intended value of a product (Ward and Simpson, 1985). Drying degrades and

other drying defects cost the softwood and hardwood timber industries millions of dollars

annually in both lost of value and volume due to poor product performance. Most defects that

develop after drying can be classified under one of the following categories (Ward and

Simpson, 1985).

i) Rupture of wood tissue

ii) Warp

iii) Discolouration

iv) Case-hardening

2.4.1 Rupture of wood tissue

Rupture of wood tissue is a drying defect related to shrinkage (Ward and Simpson, 1985).

Kiln drying is frequently blamed for defects that have occurred during air drying, but most

defects can occur during either process. Some of the defects in this category are:

i) Surface checks

According to Walker (1993), a check is a split parallel to the grain normally a few centimetre

long. It results from the separation of the thinner walled early-wood cells; they also follow the

rays and therefore are usually confined tangentially. They generally occur early in drying but

in some softwood the danger persists beyond the initial stages. Checks occur because the

timber surface dries too quickly as a result of low relative humidity. Surface check defects are

illustrated in Figure 2.2.

Based on work by Ward and Simpson (1985), timber with surface checks formed during air

drying or open surface checks after kiln drying should not be exposed to high relative

humidity before or during kiln drying otherwise surface checks are lengthened, widened and

deepened.

Figure 2.2 - Surface checks in wood (Ward and Simpson, 1985).

ii) End checks and splits

End checks occur at the ends of boards and are visible on the cross-section. They occur when

moisture moves much faster in the longitudinal direction than the transverse direction (Ward

and Simpson, 1985). This problem can also be caused by rapid end drying. According to

McMillen (1958), checks and splits may decrease wood strength. End coating can be used to

reduce end checks. Higher relative humidity also assists in reducing both defects. End check

defects are illustrated in Figure 2.3.

Figure 2.3 - End checks in wood (Ward and Simpson, 1985).

iii) Collapse

Collapse (Figure 2.4) can be defined as abnormal shrinkage accompanied by distortion or

crushing of wood cells (McMillen, 1958). It can also cause internal checking in wood

(Waterson, 1997). Although it occurs in early drying, collapse is not usually visible on the

wood surface until later in the process but remains a serious problem requiring prevention.

Compressive drying stresses in the interior parts of a board and liquid tension in cell cavities

that are full of water result in collapse (Ward and Simpson, 1985).

Species that are susceptible to collapse are generally air-dried before being kiln dried. In many

cases, reconditioning of the timber to remove collapse from the timber is possible. Timber is

steamed at 100°C and 100% relative humidity for four to eight hours, depending on the degree

of collapse (Walker, 1993).

Figure 2.4 - Collapse in wood (Ward and Simpson, 1985).

2.4.2 Warp

Warp can be defined as anisotropic shrinkage of timber that can cause significant volume and

grade loss. According to Ward and Simpson (1985), the irregular or distorted grain and the

presence of abnormal types of wood tissue such as juvenile and reaction wood can influence

this defect. There are five major types of warping:

i) Cupping

Greater shrinkage across the growth rings will result in cupping (Ward and Simpson, 1985).

There is a bigger difference between tangential and radial shrinkage, resulting in a greater

degree of cup. Boards at the top of timber stacks can only cup this way because the other

boards are held flat by the weight of the timber above. Cupping must be avoided because it

can cause excessive losses of timber during machining. Good stacking and avoiding over

drying are the best ways to minimize cup.

ii) Diamonding

Diamonding occurs when the tangential shrinkage is greater than the radial shrinkage during

drying. A square cross-section that is sawn with the growth rings running diagonally becomes

diamond shaped (Walker, 1985). Diamonding can be controlled by air drying or controlling

initial kiln conditions and good stacking (Wengert and Toennisson, 1998).

iii) Bowing

Ward and Simpson (1985) report that bowing is related to longitudinal shrinkage in juvenile

wood near the pith of a tree, and by compression or tension timber that occurs in a leaning tree

and cross grain. The differences in longitudinal shrinkage on opposite faces of a board

influences bowing. If the longitudinal shrinkage is the same on opposite faces, bow can be

avoided.

iv) Crook or spring

This is similar to cupping and bowing except that it occurs on the edge. Good stacking can be

used to minimize crook but it is not thoroughly effective.

v) Twist

Twist occurs when four corners of any face of a board turn on any face of a board, so that they

are no longer in the same plane. Twisting is commonly found in species with spiral grain and

is also related to cross grain. Pre-steaming can be applied after drying to reduce twisting

(Haslett et al., 2001).

2.4.3 Discolouration

Unwanted discolouration may occur during storage of green timber and during drying. These

colour changes occur when water, light or chemicals react with exposed surfaces of wood

(Ward and Simpson, 1985). Discolouration defects reduce the value of wood products that

normally require a natural finish for end use resulting in significant losses of low quality

timber. It also leads to the product being downgraded or being reclassified as waste (McCurdy

et al., 2005). Colour changes during drying have been associated with fungal colonization

(blue stain) or darkening of the wood due to bacteria (brown stain). The formation of

unwanted colour varies with complex interactions of tree species, type of wood tissue

harvesting and transport delays and drying conditions (Ward and Simpson, 1985).

Furthermore, Simpson (1999) states that colour changes in the sapwood region commonly

occur but are rare in the heartwood region. Figure 2.5 shows kiln-dried and planed boards with

and without discolourations.

Figure 2.5 - Sapwood with (left) and without discolouration (right) (Ward and Simpson,

1985).

Ward and Simpson (1985) also define types of wood discolourations occurring in sapwood

and heartwood. They state that in sapwood, there are three categories of discolourations,

consisting of chemical, fungal and bacterial. A reaction between oxidative and enzymatic

chemical constituents in the sapwood may cause chemical discolouration. The colour

produced varies, from pink, blue, yellow and reddish brown to dark brown shades. Hardwoods

are more subject to this degrade than softwood. A chemical brown stain may occur in

sapwood during kiln drying but can be eliminated using steaming at 100°C (Ward and

Simpson, 1985). The application of higher temperatures will encourage the discolouration to

penetrate deeper into the board. For example, above 60°C, brown discolouration will become

more pronounced in the sapwood board of yellow pine. Previous studies show that higher

drying temperatures (100°C) produce dark colouration of wood (Murray et al., 2005)

Blue stain is a fungal stain that is also present in sapwood boards due to fungi that feed on cell

contents in the sapwood. It will not cause any decay of the sapwood and has little effect on the

strength of the wood. This unwanted stain can be avoided by providing unfavorable conditions

for the fungi. It survives but cannot grow in boards at 20% moisture content or lower (Ward

and Simpson, 1985). Temperatures above 66°C will kill fungi.

Discolouration also occurs in sapwood due to bacteria. These bacteria grow on certain

chemical components in the sapwood extractives that discolor during kiln drying (Ward and

Simpson, 1985). An aqueous solution of weak organic acids can be sprayed on to the wood

before drying in controlling these discolourations.

Discolouration in heartwood is normally chemical in nature but not as frequent as found in

sapwood. Heartwood will darken uniformly during drying and the percentage of the

discolouration depends on the chemical nature and drying temperature (Ward and Simpson,

1985). For example, the unwanted colour is often oily-looking blotches. Fungal and bacteria

discolourations will not develop in heartwood in normal situations but bacteria do develop in

discolourations of wetwood, which is an abnormal type of heartwood occurring when logs are

left in the forest for extended periods.

Trial and error is required to find the most suitable agent for a particular stain. Sometimes it is

economical to remove discolourations but it depends on the condition of the wood (Ward and

Simpson, 1985). Pang and Li (2006) used a modified drying medium to prevent discolouration

of Pinus radiata sapwood during kiln drying. In their study, samples were dried using three

gases in a tailored cylinder at different temperatures. Brightness was also examined in this

study. Figure 2.6 illustrates that at 70°C or above, there is an increase in kiln brown stain and

surface darkening. These discolourations decreased at lower temperature of 50°C. Kiln brown

stains were eliminated for all three gases tested except one at 70°C. It was concluded that a

modified medium can be an effective way of reducing surface discolouration.

Figure 2.6 - Colour profiles for samples kiln dried in air (left) and gas dried (Pang and Li, 2006)

2.4.4 Case-hardening

In kiln drying timber, it is important to understand case-hardening. Very rapid drying at a high

temperature can result this degrade (Nolan et al., 2003). During timber drying, a system of

stress, strain and set will occur. According to McMillen (1958), case-hardening occurs in dry

timber with uniform moisture content and is characterized by tension in the core and

compression in the outer layer of the cell. Surface zones of the timber remain under tension or

are stressed while the core zone is under compression.

When timber is dried, the outer layer dries below fibre saturation point but is restrained from

fully shrinking. Given the core is still above the fibre saturation point, no shrinkage occurs in

this zone. This situation are common during the initial stages of drying and as drying

continues; the surface zones dry further but do not shrink extensively because they are

restrained (McMillen 1958). Later, most moisture is lost from the core zones which begin to

dry below fibre saturation point but given this zones connection to the outer regions, shrinkage

of the core is prevented. At this stage, the stresses are reversed, where the surface is in

compression and the core is in tension. Figure 2.7 shows a diagram of how case-hardening

occurs after various stages of wood drying.

Figure 2.7 – Diagram of how case-hardening occurred after various stages of wood drying

process (Harris, 2009).

McMillen (1958) also points out that conditioning treatment can be used to overcome case-

hardening problems. The temperature and relative humidity are increased to make the timber

more plastic. Surface zones reabsorb moisture and compressive stress increases until the

elastic limit is reached, then soft surface zones and induced compression set (Walker, 1993).

These conditions need to be maintained until stresses are relieved but if overdone, reverse

case-hardening can occur. Furthermore, reduction of case-hardening can also be achieved

using higher relative humidity and slower drying rates at early stages of the drying process

(Nolan et al., 2003).

Whether case-hardening is considered as a defect or not depends on the final use of the timber.

If there is no further machining, case-hardening is acceptable but if further fabrication is

necessary, end-checking, splitting and cupping problems may occur. Case-hardened stress

levels can be measured using a prong test. Beutel (1997) stated that the prong test will indicate

the amount of elastic stress in the wood at the time of cutting. The concept of the Prong test is

that, prongs should ideally remain straight or curve out slightly if there has been stress-free

drying but if stress is present, the prongs will pinch in. A summary of prong test results is

presented in Figure 2.8 (Waterson, 1997).

Figure 2.8 – Summary of prong test result (prongs curvature) (Waterson, 1997).

2.5 Wood drying methods

There are two options available for drying timber - exterior stacking with slow air drying or

kiln drying, in which humidity and air speed are controlled using only temperature control

(Walker, 1993).

2.5.1 Air drying

Air drying is generally known as uncontrolled drying. Drying times vary because the drying

rate is dependent upon climatic conditions and air movement. Timber is stacked (Figure 2.8)

with each layer separated by fillets. This drying is suitable for exterior use timber which does

not require low final moisture contents. Timber dries rapidly as temperature and air movement

increases and as the relative humidity decreases (Tomford, 1960). When the reverse of these

conditions occurs, the drying rate becomes slow and encourages the growth of stains.

Extended yard time will raise drying cost and is uneconomical (Rietz and Page, 1971). In

small mills, all production may be air dried but only part of the production (generally lower

grade timber) may be air dried in large operations (Walker, 1993).

Figure 2.9 - Timber is stacked in air drying practice (Harris, 2009).

2.5.2 Kiln drying

Air temperature, relative humidity and air flow are controlled in kiln drying (Harris, 2009).

Higher temperatures and faster air circulation are used to significantly increase drying rates

without increasing defects. Timber is stacked in chambers called drying kilns. Stacks are made

in the chambers to allow forced air flow through the material (Walker, 1993). The main

elements of kiln drying are construction materials, heating, humidification and air circulation.

Unsaturated air is used as the drying medium in this practice.

A conventional kiln removes water from wood by heat provided by steam, hot water coils or a

furnace. Water is extracted by evaporation and exhausted from the kiln with the heated air.

Almost all commercial timber is dried in industrial kilns (Haque, 2006). Kilns provide many

benefits over air drying:

i) Drying time is much faster in kiln drying compared to air drying. Softwoods

can be dried in less than a week and if high temperature schedules are applied, only

hours are required (Walker, 1993).

ii) Drying degrade like checking can be reduced with controlled drying.

iii) Timber can be dried to lower than 18% moisture content.

iv) Kiln drying allows more flexible practices as the operation does not depend

upon uncontrolled climatic conditions.

In order to produce economic and efficient operations suitable kiln schedules have been

developed. Simpson (1999) states that kiln schedules should be different between species due

to variations in physical and mechanical properties, thickness, sawing pattern, grade and end

use of the timber. Pinus radiata for example, can be dried using various kiln schedules (Table

2.1) depending on the intended use.

Table 2.1- Schedule for the accelerated drying and equalizing of Pinus radiata framing timber

(Walker, 1993).

Moisture content of the

wettest timber in stack

Dry bulb

(°C)

Wet bulb

(°C)

RH

(%)

EMC

(%)

Green 71 60 58 9.0

50 75 60 49 6.3

20 80 60 39 4.9

Equalizing 80 73 73 9.9

Conditioning 85 84 96 18.5 Notes: EMC = equilibrium moisture content; R.H = relative humidity

(i) High temperature drying

Temperatures greater than 100°C are advantageous when applied in timber drying (Knight,

1968). Williams and Kininmonth (1984) stated that high temperature drying is more

economic, reduces drying time and requires slightly less energy than conventional kilns. Koch

(1971) also agreed that high temperature drying is quick, simple and efficient. High

temperature drying suits untreated Pinus radiata, but is not recommended for cedar or

redwood because of the collapse problems in these species.

High temperature drying kilns must have adequate fan capacity, walkway width and features

incorporated in their design such as sticker thickness, drying schedule and kiln length to

ensure uniform air movement through the stack. Modern high temperature drying is carried

out in an atmosphere of superheated steam and air with the dry bulb and wet bulb

temperatures commonly set at 120°C and 70°C respectively (Williams and Kininmonth,

1984). The drying process is continued until timber reaches approximately 10% moisture

content. Reconditioning takes place once drying is complete, about four hours with steam

conditioning at 100°C. Timber is then ready for cooling and storage.

Collapse, end-checking and honeycomb are defects likely to occur after high temperature

drying (Wengert, 1972). Furthermore, discolouration is also prevalent in high temperature

drying compared to conventional kiln drying. The method also has some impact on the

mechanical properties of wood. Studies by Gerhard and McMillen (1976) show that increasing

temperature has a corresponding decrease in all strength properties such as modulus of rupture

(MOR), compression perpendicular to the grain, side hardness, toughness and work to

maximum load, but not modulus of elasticity (MOE). Three principal factors that have been

investigated are the length of exposure time to high temperature, and the temperature and

moisture content of the wood (Gerhard and McMillen, 1976). Furthermore, Kozlik (1969) also

studied the effect of temperature, conditions of equilibrium moisture content (EMC) and time

during drying. In these studies, six temperatures were applied (90º, 150º, 180º, 195º, 215º and

230ºF), with the conditions for EMC set at 6% or 12%. Drying times were doubled to

investigate the effect of drying time at each condition, excluding 90ºF (control). Exposure of

wood to high temperature drying can also cause reduction of wood hygroscopicity.

(ii) Vacuum drying

Walker (1993) suggests that vacuum drying provides all the benefits offered by high

temperature drying without the danger of defects that may occur in some species at 100°C.

The concept of vacuum drying is to quickly remove the moisture from the wood by drawing a

vacuum, thus decreasing the boiling point of water (Rice, 1984). Wood can therefore be dried

relatively quickly at lower temperatures and avoid check formation.

(iii) Dehumidification drying

Dehumidification is used globally but only a small percentage of overall timber volume uses

the technology (Brown, 1988). Humidity is removed by condensation on the cold coils of a

heat pump dehumidifier (Burnett, 1991). Two procedures are employed for dehumidifier

drying:

low temperature units operating up to 60°C. These suit small scale operations that

require to dry small parcels of timber only.

higher temperature units operating at 80°C. These are suitable for general commercial

use. The advantages of dehumidifier drying compared to conventional drying are ease

of operation, lower costs, lower energy requirements and less drying defects but the

disadvantage is a slower drying rate.

(iv) Solar drying

A solar-heated drying kiln is suitable for any operation where drying time and rate is not

critical. These kilns are normally suitable for drying small amounts of appearance grade

timber of hardwood and refractory species that need slow drying (Waterson, 1997). It is easier

to dry softwood using these kilns, where the wood can be dried to a lower than equilibrium

moisture content (Waterson, 1997). Drying time is dependent upon sun intensity, timber

moisture content and relative humidity. Solar drying can be combined with other sources of

power such as electricity, oil and gas.

The solar drying kiln converts solar energy into heated air which is passed through the timber

stack by fans (Winturri, 1969). Humidity controls with minimum heat loss and energy storage

are two important factors that need to be considered during unit design. Energy storage by use

of heat-absorbing materials such as stones ensures the consistency of airflow during periods of

little or no sunlight while a slanted roof has been used during winter months to obtain

maximum utilization of energy (Winturri, 1969). The advantage of solar drying is its low

energy cost. However, it has not been widely used due to the uncertainty of potential users. It

may be more widely used in the future if energy storage problem are resolved. Waterson

(1997) identifies three main types of solar kiln. These are:

i. Greenhouse kiln with walls and roof covered with a transparent skin and solar

collectors within the structure.

ii. Semi-greenhouse kiln with roof and some walls glazed.

iii. Solar dehumidifier kilns with a basic solar kiln. The solar kiln is fitted with a

dehumidifier for reducing humidity and returning latent heat to the kiln

2.6 Microwave drying

2.6.1 Introduction

Microwave energy is the portion of the electromagnetic spectrum (300 MHz-300 GHz)

between radio waves and the far infrared. Heating timber by microwaves applies frequencies

of 900 MHz or 2450 MHz with very short wavelengths (0.333 μm and 0.122 μm respectively)

(Harris, 2008; Meredith, 1998). Microwaves are a source of energy and not a heat source. This

heating method differs from conventional heating, in that it does not rely on a transfer of heat

through the material but results from a volumetric interaction between the material and the

electromagnetic fields. Moreover, it can provide more uniform heating of large objects in a

very short time period (Rene, 2007). The penetration and dispersion of drying energy avoids

the formation of steep moisture gradients associated with conventional drying. The

temperature is higher within the wood than on the surface and is highest where there is most

moisture (Walker, 2006).

The use of microwaves (electromagnetic) energy to heat materials in an industrial process

offers many benefits such as developing quality, efficiency and control (Meredith, 1998;

Harris, 2008). Using microwave heating, up to 80% of the total power consumed can be

transferred electromagnetically. Conventional heating can only consume less than 25% power

(Petrie, 2009). The power supplied can be regulated accurately which allows safe and precise

control of an applicator even when applying large power or rapid heating rates. A further

advantage of industrial microwave heating is the inclusion of rapid volumetric heating (Petrie,

2009). Heat travels from below the surface, inwards and outwards, enhancing heating

uniformity and the ability to dissipate extremely high power densities within the material

results in rapid heating times.

2.6.2 Microwave heating and drying

Microwave energy comprises electrical and magnetic components. In practice, the associated

magnetic field constantly switches between polarities with a relaxation or zero point between

the negative and positive magnetism (Mujumdar, 2007). Heating water by microwave energy

involves both dipolar and conduction effects and many industrial microwave systems are

dependent upon the presence of water in the material being heated (Harris, 2008).

Walker (2006) states that water molecules are dipolar by nature and are normally randomly

oriented. These dipoles attempt to remain aligned with the rapidly changing polarity of an

alternating field. As the field changes polarity, the dipoles return to a random orientation

before being pulled the other way (Walker, 2006). Once wave strength decays (resulting in

stress on the molecules), they move back to their former positions and then become aligned in

the opposite directions as the wave strengthens once more (Rene, 2007).

CHAPTER 3: MICROWAVE WOOD DRYING

3.1 Introduction

Drying enhances the properties of timber and increases its value. The chief objective when

drying timber is to produce a quality product without any loss of quality. Research on

microwave drying of wood has been carried out for over 50 years and today it is possible to

carry out continuous measurement of important factors such as pressure, temperature and the

moisture content of wood during drying (Antti and Perre, 1999). Better understanding of

microwave drying processes will encourage a broader application of this technique.

Microwave technology can be used for the conditioning of wood to specific moisture contents

below 12%.

The benefit of microwave drying over conventional kiln drying is a faster drying rate, thus a

decrease in drying time. This occurs due to the interaction of microwave energy with the

timber and its superior penetration. Pinus radiata is one softwood species known to have a

fast growth rate and good timber properties (Bootle, 2004). When drying, this species requires

short periods of time using conventional kiln drying, however, the moisture content after

drying is not always uniformly distributed. Uneven drying conditions encourage defects such

as checking and collapse.

During kiln drying, colour changes are a significant problem that develops in Pinus radiata

boards that can reduce the value of the final wood products. Warping of the timber also may

occur due to internal stresses building up during drying. Little research has been conducted to

evaluate the quality of microwave drying for this species. The use of microwaves in timber

drying processes can have an effect on its final quality as they can affect the timber micro-

structure. Some studies state that microwave drying can provide better quality of the final

wood product compared to kiln drying, where it has been found that drying by microwaves

does not cause case-hardening (Barnes et al., 1976).

3.2 Experiment One: Laboratory scale microwave wood drying

3.2.1 Research objectives To test laboratory scale microwave heating for assessing microwave drying times for Pinus radiata.

3.2.2 Research hypotheses

The hypotheses of this study are:

i) That microwave drying will shorten drying times to reduce wood moisture

content to 12%.

ii) That control of final wood moisture content can be achieved simply and

accurately by “in-line” weighing of each sample prior to microwaving.

3.2.3 Materials and methods

3.2.3.1 Sample boards

Twelve freshly sawn Pinus Radiata D. Don planks were obtained from Central Highland

Timber Sawmill (Ballarat, Victoria, Australia). The planks were cut to size 200 mm

(longitudinal) x 100 mm (tangential) x 50 mm (radial) as outlined in Figure 3.1 and labeled.

The moisture content of all samples ranged from 120-130%. Ten replicate samples were cut

from each board. A total of five sets of samples were examined. Samples for each set were

chosen using a systematic randomized design (Table 3.1). Both ends of each sample were

painted with polyvinyl acetate (PVA) resin to avoid evaporation (Waterson, 1997).

50 mm

50mm

200 mm 100 mm

Figure 3.1- Sample measurement.

Table 3.1 – Sample sets using systematic randomized design.

Set Samples

1 A9 B11 C11 D3 E20 F18 G19 H7 J19 K15 L1 M5

2 A11 B7 C15 D6 E21 F7 G7 H19 J2 K19 L13 M6

3.2.3.2 Moisture content profile

Biscuit samples 25 mm thick were cut from the twelve planks to determine the initial moisture

content and basic density. Moisture content is the weight of water expressed as a percentage

of oven-dry weight of wood. Basic density is defined as the absolutely dry weight to the

maximum volume of wood (kg/m3) as indicated by Equations 1 and 2 respectively. Samples

were weighed and measured (using a micro-caliper) to determine volume. Samples were then

oven-dried for 24 hours at 105°C. Samples were removed, stored in a sealed chamber and

reweighed and measured again.

Moisture content (%) = Green weight (kg) – Dry weight (kg) x 100% Equation 1

Dry weight (kg) 1

Basic density (kg/m3) = Dry weight (kg) Equation 2

Volume (m3)

After calculating the initial moisture content, green density and basic density, the theoretical

weight of each sample at 12% moisture content was calculated as the target weight after

microwave treatment. Waterson (1997) has established protocols for the preparation of a

sample board, as shown in Figure 3.2. Green density is defined as the green weight of wood

per unit of green volume (kg/m3) as outlined by Equations 3 and 4 as below:

Green density (Di) = Green weight (kg) / Green volume (m3) Equation 3

Wf = (Wi x Df ) / Di Equation 4

Where:

Wf = Weight at 12% moisture content

Wi = Green weight

Df = Density at 12% moisture content

Di = Green density

Figure 3.2 – Preparations of sample board (illustrated from Waterson, 1997).

Preparation of sample boards for moisture content and basic density determinations

Biscuit sections Sample board

Biscuit section Sample board Sample B

Measured volume

End sealed both ends and measured volume

Weight (W) samples using electronic weight

balance

Weight

Oven-dry biscuit section for 24 hours at

105°C

Average moisture content = moisture content

for sample board as defined from biscuit

section.

Reweight and remeasure

Calculation of oven-dry weight (ODW)=

ODW = Initial weight x 100

100 + AMC

Moisture content (%) =

W – ODW1 x 100%

ODW1

Calculate moisture at any moisture content

percent (X) =

Weight ODW x Density at X (MC %)

Green density

3.2.3.3 Oven-drying

Table 3.2 – Oven-drying overview for all sets of samples.

Set Oven-drying time

(hours)

Replicates

1 11 10

2 13 10

3 15 10

4 17 10

5 19 10

Samples were weighed before and after oven-drying to allow calculation of oven-dried

moisture content. All samples were oven-dried at specific oven-drying times as schedules

summarized by Table 3.2. The temperature applied was 105ºC. Microwave treatment was then

applied after oven-drying. Before proceeding with microwave treatment, the temperature of

samples was measured. Six holes were drilled in the centre of each sample. Holes were drilled

on each surface of the sample, at two different depths, 8.5 mm and 17.5 mm. The temperature

in each hole was measured to give an indication of the sample temperature (within the board)

once drying commenced using microwave. The average initial temperature of samples ranged

from between 18º C to 22º C. Once the temperature of the samples reached 100ºC, during

microwave drying temperature measurements were discontinued.

3.2.3.4 Microwave drying

The aim of this study is to apply microwave drying to samples that had a range of starting

moisture contents. This was achieved by applying preliminary oven-drying of samples to

achieve a suitable range of wood moisture contents. Australian Standards for Timber Drying

Quality (AS/NZS 4787, 2001) recommend an equilibrium moisture content (EMC) of 10% -

12% for the majority of Australian States, although in extreme cases moisture contents up to

15 - 18% are recommended. The final moisture content is selected to minimize dimensional

changes (or movement) of the final product in service. In these experiments 12% moisture

content was selected as the final moisture content. The microwave (Plate 3.1) used in this

study is capable of a 1 kW power output. Microwave treatment was applied while the samples

were still hot. Each sample was weighed prior to microwave drying and after each successive

pass (Rene, 2007). Sample positioning during treatment is illustrated in Plate 3.2. The duration

of each microwave treatment pass was three minutes. Microwave treatment was discontinued

when the sample reached 12% moisture content. After removal from the microwave, the

samples were allowed to cool.

Plate 3.1 - Microwave equipment used in wood treatment.

Plate 3.2 - Sample arrangement in the microwaving chamber.

3.2.3.5 Moisture content distribution

The pattern of moisture content distribution was determined on completion of microwave

drying and subsequent cooling. Two cross-section samples were cut 25 mm thick from each

board (Plate 3.3). One sample was used to determine the cross-section moisture content the

second sample was divided into nine segments for analysis of the inner part. Plate 3.4

illustrates the cutting patterns used to isolate the inner section. Segments were oven-dried for

24 hours at 105°C.

Plate 3.3 - Sample cross-section.

Plate 3.4 - Cross-section sample prior to division (left) and the inner sample (right).

3.2.3.6 Microwave drying quality

Upon completion of the microwave drying treatments, the evaluation of microwave drying

quality was applied to each plank. Drying degrade such as checking as described in Section

2.4 was recorded and photographed. Warping was not evaluated due to the limitation of plank

size.

3.2.4 Statistical analysis

All of the experimental variables (green density, basic density and moisture content) were

assessed using regression analysis. Further analysis of the coefficient of correlation was also

undertaken to examine the suitability of these variables as predictors of sample weight at 12%

moisture content.

3.2.5 Results

3.2.5.1 Sample variations

Average green density and green moisture content (MC) are summarized in Table 3.3. The

average green density of samples ranged from 952-983 kg/m3. Samples from Sets 2 and 5

were the two highest for both green density and green moisture content (130% to 132%).

Samples from Set 3 had the lowest green density and green moisture content.

Table 3.3 - Statistical description of green density and green moisture content.

VARIABLES SET

Green density (kg/m3) 1 2 3 4 5 Mean 952 980 947 968 983

Std. dev 93 66 61 84 94

Green MC (%) 1 2 3 4 5 Mean 124 130 123 128 132

Std. dev 33 27 28 33 33

3.2.5.2 Relationship between green density and green moisture content

The relationship between green density and green moisture content is shown in Figure 3.3. A

strong relationship exists between both variables, with high correlation (r2=0.9975).

Therefore, with increases in green density, an increase in the green moisture content is also

expected to the extent that green moisture content can be predicted based upon the green

density.

Figure 3.3 - Relationship between green density and green moisture content.

3.2.5.3 Relationship between green moisture content, oven-drying time and moisture content

after oven-drying

Samples oven-dried for 13 hours (Set 2) had higher moisture contents than samples dried for

11 hours (Set 1). This is because the average initial moisture content of Set 2 samples was

higher than Set 1. Furthermore, total moisture content loss for Set 1 and 2 were 69% and 73%

respectively. The result was different for samples from Set 3. After oven-drying for 15 hours,

their average moisture content was 29% (average total moisture loss was 94%). This may have

arisen because of the percentage of sapwood and heartwood in samples from Set 3. The mean

green density of his set of samples was the lowest with a green density of 947kg/m3, which

potentially made them easier to be dried during the oven-drying process. Overall, this situation

affected the correlation between oven-drying time and subsequent wood moisture content

(r2=0.3095) as illustrated by Figure 3.4. Apart from the samples from Set 3, it can be

concluded that increases in oven-drying time systematically reduced the moisture content of

samples.

R² = 0.3095

0

10

20

30

40

50

60

0 5 10 15 20

MC

afte

r ove

ndry

(%)

Ovendry time (hours)

Figure 3.4 - Relationship between oven-dry moisture content and drying time.

Clearly this observation cannot be applied to all sets. There is no control over relative

humidity and there will be variation in the amount of heartwood in samples between sets that

will influence starting moisture content as well as relative humidity during drying. These

factors will influence the amount of moisture lost during oven-drying.

3.2.5.4 Microwave drying treatment

(i) Drying rate

A summary of the results for microwave treatment of Pinus radiata is illustrated in Figures

3.5-3.8. Details of the data generated are presented in Appendix 1. Figures 3.5-3.8 show that a

relationship between oven-dry moisture content and microwave moisture content exists. Four

passes of microwave treatment were carried out and the time for each pass was three minutes.

After one pass, there is strong evidence that moisture content after microwaving could be

predicted from the starting moisture content after oven-drying (r2 = 0.97). The r2 value

decreased after two three and four microwave passes (r2=0.93, 0.89 and 0.78 respectively).

The correlation decreased with the increased of microwave passes.

The amount of moisture loss after one microwave pass ranged from 4% - 15%. Once the

samples had been through the microwave for a second pass, the amount of weight loss

increased gradually (ranging from 14% - 29%). The amount of moisture loss kept increased

until third and fourth passes. After third passes, the total moisture loss were between 19% till

39%.

R² = 0.9671

0

10

20

30

40

50

60

70

80

90

100

0 20 40 60 80

Perc

enta

ge o

ven-

dry

MC

(%)

MC decrease by MW treatment(%)

Figure 3.5 - Relationship between percentage of oven-dry moisture content and moisture

content after one pass through the microwave.


content after two passes through the microwave.


content after three passes through the microwave.

.


content after four passes through the microwave.

Comparison of drying rate patterns between oven-dry moisture content and microwaved

moisture content after each microwave pass is outlined in Figure 3.9. Thus at oven-dry

moisture content of 40%, microwave moisture contents of 35%, 24%, 15% and 10% are

achieved for one pass, two passes, three passes and four passes of microwave treatment

respectively. It can be concluded that with increasing microwave passes, sample drying is

achieved in shorter times. MC was reduced to 10% in 12 minutes (each MW pass required 3

minutes to complete and four MW passes were needed to achieve the desired final moisture

content). Figure 3.9 also shows that between samples, moisture content becomes less variable

after microwave treatment.

1 pass:R² = 0.9671

2 passes: R² = 0.9285

3 passes: R² = 0.8914

4 passes: R² = 0.7821

0

10

20

30

40

50

60

70

80

90

0 20 40 60 80 100

Perc

enta

ge o

ven-

dry

MC

(%)

MC decrease with MW treatment (%)

1 pass

2 Passes

3 passes

4 passes

Figure 3.9 - Drying rate patterns for various microwave treatments.

(ii) Relationship between oven-dry moisture content, microwave moisture content and number

of passes

Multiple regression analysis was used to model the relationship between three variables

(oven-dry moisture content, microwave moisture content and number of microwave treatment

passes). The statistical analysis demonstrates the existence of a strong correlation (r2=0.966).

Any single variable can affect the value of the other two variables. Each variable was

dependent of each other. Detailed data for the multiple regression analysis is summarized in

Appendix 2. The equation outlined below can be used to predict wood moisture content when

all three variables are known. The final moisture content of wood after microwave treatment

can be predicted using the starting moisture content (oven-dry moisture content) and number

of microwave passes.

Percentage moisture content after microwave treatment =

K + starting moisture content (oven-dry moisture content) (x) – Number of passes (y)

Where:

K = 9.245

Oven-dry moisture content = 0.870

Number of passes = 10

Therefore, percentage moisture content after microwave treatment:

= 9.245 + 0.870(x) - 10(y) Equation 5

Given this information, formulation of a drying schedule can be achieved. Over the range of

20-80% moisture content, number of passes and the K-value are applied with the aim of

predicting the moisture content after microwave treatment. Prediction of moisture content

after microwave processing is important so that the number of microwave passes needed to

achieve the desired moisture content can be calculated. This way is a much simpler

methodology for controlling the microwave process. Table 3.5 summarizes the number of

passes required and sample moisture contents following microwave treatment based on

Equation 5. Further details are summarized in Appendix 3.

Table 3.4 - Predicted moisture content after microwave treatment.

Percentage oven-dry moisture content

Number of passes

Predicted percentage moisture content after microwave treatment

20 2 6.6 40 3 14.0 60 5 11.4 80 7 8.8

Table 3.4 shows that for each number of microwave passes, there is an approximate reduction

in moisture content of 10%. For example, samples with 80% moisture content after oven-

drying required seven microwave passes to reduce the moisture content to 8.8%. For a sample

with 20% oven-dried moisture content, two microwave passes are necessary to reduce the

moisture content to 6.6%. Using this information, prediction of the times needed to reach an

identified moisture gradient is possible. Statistical analysis of raw data (Appendix 3a) is

summarized in Figure 3.10. Figure 3.10 shows that for a sample with starting moisture content

of 80% (after oven-drying), one and two microwave passes reduced the moisture content of

samples to 69% and 59% respectively. This pattern of moisture content reduction continued

with further microwave passes. Microwave treatment ceased when the moisture content

reached 12%. Oven-drying samples before continuing with microwave treatment had assisted

in predicting the drying times needed to reduce samples to such a level.

Relationship between oven and microwaved dried moisture content and number of passes

y1 = 0.87x - 0.755R2 = 1

y2 = 0.87x - 10.755R2 = 1

y 3= 0.87x - 20.755R2 = 1

y4 = 0.87x - 30.755R2 = 1

y5 = 0.87x - 40.755R2 = 1

0

10

20

30

40

50

60

70

80

0 20 40 60 80 100Percentage moisture content with oven drying (%)

Perc

enta

ge m

oist

ure

cont

ent w

ith m

icro

wav

e dr

ying

(%)

1 Pass2 Pass3 Pass4 Pass5 Pass6 Pass7 Pass

Figure 3.10 - Moisture content of samples after each pass of microwave drying.


The outer layer of samples was found to be drier than inner layers (Appendix 3b). The

moisture content of outer layers ranged from 15% to 17% MC while inner layers ranged from

14% to 22% MC. Surprisingly, the moisture content of samples from Sets 1, 2 and 3 were

similar even though the sets had been oven-dried for different times. The variation in moisture

distribution occurring for samples in Sets 4 and 5 may have been due to longer oven-drying

time.


After oven-drying and microwaving, small drying defects (internal checks, core checks,

staining and some burning) were observed in samples. Figure 3.11 illustrates core checks,

internal checks and burning defects. Samples having more than three passes of microwave

treatment were prone to checks and burning. Rapid end drying and faster moisture transfer

probably caused internal checking.

Figure 3.11 – Defects after microwave treatment: checking (left) and staining (right).

3.2.6 Discussion and conclusions

All samples of green Pinus radiata showed high initial moisture content (120% to 130%) and

also differing degrees of variability of green density, basic density and oven-dry moisture

content. There is a strong correlation between green density, measured from the weight and

dimensions of samples and wood moisture content determined by taking cross-section samples

and oven-drying. Green density determination is potentially a convenient method for

determining the total microwave energy needed to dry boards to final predetermined moisture

content

Partial oven-drying of samples prior to undertaking microwave treatment was a convenient

method of reducing sample moisture content, so that the effect of a range of wood moisture

contents on subsequent microwave processing could be evaluated. The method of using oven-

drying has disadvantages in that there is no control over the rate of drying because there is no

control over relative humidity.

In some instances, the harsh drying of samples using an oven also probably resulted in the

formation of defects for example internal checking. Nevertheless, the data derived is useful for

determining the range of schedules needed for microwave processing. The use of small

samples also limits the opportunity for evaluating stress buildup or relief during microwave

processing since any stress buildup will be dissipated longitudinally in small samples.

The value of equations derived using small samples at different moisture contents will be

tested in scaled-up microwave processing. Similarly, the wood quality attributes identified in

small samples for example internal checking and charring will be more accurately determined

by scaling up the samples used in microwave processing.

3.3 Experiment Two: Pilot scale microwave wood drying

3.3.1 Research objective

To scale up and validate microwave research completed in Section 3.2 and further evaluate the

effectiveness of dynamic microwave processing in optimizing drying with minimal defects.

3.3.2 Research hypotheses

The hypotheses of this study are:

i) The drying rate of green Pinus radiata (sapwood, heartwood and mixed

boards) is faster using dynamic microwave drying.

ii) A moisture meter can accurately determine the drying end point (by

comparison of moisture meter readings with the standard weighing method).

iii) Weighing boards during microwaving provides an accurate determination of

board moisture content.

iv) That microwave technology will reduce wood drying defects due to moisture

leveling, assisting stress relaxation and eliminating case-hardening compared to drying

process in Experiment One (Section 3.1).

3.3.3 Materials and methods

3.3.3.1 Sample boards

Twenty green sawn Pinus radiata D. Don boards were obtained from Central Highlands

Timber Sawmill (Ballarat, Victoria, Australia) for microwave drying. Sawn timber contained

either sapwood, heartwood or a mixture of both sapwood and heartwood. Details are

summarized in Table 3.5. Twelve boards were chosen for each group depending upon

moisture content and the surplus boards discarded. For heartwood boards, they must have

contained heartwood only or at least must be more than 80% heartwood. This method also

applied in choosing sapwood boards. Mixed sapwood / heartwood boards, were selected to

contain approximately 50% of sapwood and heartwood.

Table 3.5 - Summary of the samples prepared for microwave treatment.

Group Log content Replicates

1 Heartwood only 6

2 Mixed 6

3 Sapwood only 6

Notes: Number 1, 2 and 3 represent heartwood, mixed heartwood & sapwood and sapwood

board groups respectively. These were treated using a 60 kW microwave.

Planks measuring 2000 mm (length) x 90 mm (width) x 50 mm (thickness) as shown in Figure

3.12 and two biscuit samples (25 x 90 x 45 mm) were cut from each board. In total, eighteen

planks and thirty-two biscuit samples were cut for the microwaving study and the moisture

content of the biscuits determined. After resizing, the planks were marked and numbered.

Planks were stored under polythene to prevent drying until microwave drying.

50 mm

2000 mm 90 mm

Figure 3.12- Sample measurement (planks) for microwave treatment.

3.3.3.2 Moisture content profile

Two biscuit samples per board (that prepared in Section 3.3.3.1) were used for moisture

content and basic density determinations. The edges of these samples were scraped free of

splinters using a sharp knife to avoid weight variations from handling (Wallis, 1970). Steps

from Chapter 3 (Section 3.2.3.2) were repeated to determine the initial moisture content and

basic density of each board. The percentages of heartwood and sapwood were also marked as

shown in Plate 3.5. Boundaries were easier to differentiate in boards with green moisture

content in comparison to dried wood. The colour of the sapwood region was lighter than the

heartwood which was darker due to resin deposition during the transition from sapwood to

heartwood (and lower moisture content) (Chapter 1.2).

Plate 3.5 – Identification of the heartwood (H) and sapwood (S) boundary.

Notes: The sapwood (S) was approximately 60% of the board whereas heartwood (H) was

approximately 40%.

The aim of this experiment was to achieve 12% MC or less for each plank. The predicted

weight of each plank at 12% MC was calculated using Equations 3 and 4, as described in

Chapter 3 (Section 3.2.2.2).

3.3.3.3 Microwave drying

All 18 planks prepared in Chapter 3 (section 3.3.3.1) were used in this experiment. Microwave

processing was carried out using a 60 kW microwave unit. Technical data for the microwave

installation is presented in Table 3.6.

Table 3.6 – Technical data for the 60 kW microwave (Torgovnikov and Hann, 2006).

No. Specification Settings

1 Microwave power output 10-60 kW

2 Microwave frequency 0.922 GHz

3 Maximum dimensions of timber 90 x 90 x 4500 mm

4 Speed range 6-60 mm/sec

5 Air heating power 13 kW

6 Air temperature 20-150°C

The microwave installation consisted of a microwave power supply, in-feed and out-feed

systems, wave guides, microwave applicators, water load, gauze and a dynamic air system for

the removal of vapours from the applicator and the prevention of water condensation on the

walls of the applicator (Torgovnikov and Hann, 2006). The microwave plant used is illustrated

in Figures 3.13 and Plate 3.6.

Figure 3.13 – Microwave generator and conveyor applicator (Torgovnikov and Hann, 2006).

Notes: 1-conveyor belt, 2-tunnel, 3-inlet/outlet for heated air, 4-waveguide, 5 drive, 6-timber,

7-belt, 8-teflon, 9-microwave generator and 10-water load.

Plate 3.6 – 60 kW microwave generator installations.

In this microwave drying treatment, a nominal microwave power of 10 kW, with a vector E

orientation perpendicular to the wood grain was applied (Torgovnikov and Hann, 2006). Each

plank was weighed, placed on the conveyor belt as illustrated in Figure 3.13 with the timber

cross-section in the vertical direction, and fed through the microwave field as described by

Torgovnikov and Hann (2006). Planks were moved through the microwave in a continuous

loop. The interaction between the plank and the microwave was about 2000 mm in length. The

feed speed used was 35 mm/sec. Hot air was applied into the microwave applicator at rate of

0.5-1.0 m/sec to avoid condensation forming on the applicator surfaces (Torgovnikov and

Hann, 2006).

The initial power used for heartwood for the first three passes was 15 kW. The objective was

to increase the board’s temperature to the steady state drying temperature of 100°C. This took

three passes. The temperature of each plank was measured immediately after microwave

processing. A preliminary scoping trial was carried out before this experiment to investigate

optimum power and number of passes required to achieve the steady state drying temperature.

After three passes (pre-heating passes), power was reduced to 10 kW for all subsequent passes

to reduce the potential for drying defects. Dummy planks (non-experimental boards) were

used to ensure microwave continuity of exposed boards to ensure that experimental planks

were fully treated (Vinden pers.comm.)

Dummy boards were placed before the first experimental board and after the last board

exposed to microwave treatment. When the first board moving on the conveyor belt came

within the vicinity of the microwave applicator, power was applied as outlined in Figure 3.14.

The gauze provided a “window” to check the location of experimental boards.

Figure 3.14 – Placement of dummy plank to ensure full microwaving of experimental planks.

Notes: Care was taken to ensure there was no gap between the dummy plank and the first

experimental plank and all subsequent planks to avoid arcing.

After the first pass, boards were removed and cooled until all other planks had undergone an

initial pass, treatment then continued with a second pass for each planks. Planks were placed

on trolley (resting) after each successive pass, while waiting for the next pass. Plates 3.7 and

3.8 showed the condition of planks during microwave drying process.

Plate 3.7 – Plank moves through the microwave applicator using a conveyor belt.

Plate 3.8 – Planks exiting from the microwave applicator (after one microwave treatment

pass). Wood sap (free water) is visible on the surfaces of the heated plank.

These steps were repeating depending on the number of passes required to decrease the

moisture content of the plank to 25%, and then to 12%. The number of passes applied to each

plank depended on the weight of each board. When the plank reached the predicted weight at

25% MC, a resistance moisture meter was used to determine the current moisture readings. It

was anticipated that moisture meter readings would be inaccurate above fibre saturation point

but once planks had achieved readings below 25% MC and given suitable correction for

temperature, that the moisture meter would provide an accurate prediction of wood moisture

content (Vinden, pers.comm.)

Microwave drying continued until the plank’s weight reached approximately 100-200 g of the

predicted weight at 12% MC. Treatment time (time of exposure to microwave energy) and

resting times for each pass were recorded. Upon completion of treatment, planks were stacked

cooled and weighed.


Two biscuit samples were cut from the whole board for assessment of moisture content

distribution. The moisture content of the core and cross-section regions were compared by

application of the steps outlined in Chapter 3 (Section 3.2.3.5). Final moisture content was

recorded from the biscuit samples and compared with the final moisture content readings that

were measured with the moisture meter.

3.3.3.5 Prong test/ Drying stress determination

After cooling, drying stress was determined using the prong test (Beutel, 1997). Similarly

sized sample strips from the same direction were compared from a point not less than 200 mm

from the end of each sample board. These sections were immediately hand sawn with one or

more slots across the width of the sample. Cuts did not reach 15 mm from the end of the

section as described in Figure 3.15. After the small section was slotted, the reaction of the

prongs was recorded as turning in, turning out or remaining straight (McMillen, 1958). The

test sections were dried for 12-14 hours and an evaluation of the stresses made by recording

the reaction (turning in, out or remaining straight). The slotted section allowed each layer to

move freely and each could adjust accordingly to any stress present (Beutel, 1997).

Uncut small section Prongs turned in Prongs straight Prongs turned out

Figure 3.15 – Reaction of wafer samples to the prong test (McMillen, 1958).


Following completion of the drying treatments, observations were made to determine the

drying quality of all samples. Any drying degrade such as checking, bowing, twisting, spring

and cupping, (discussed briefly in Chapter 2.4) was recorded and photographed. Each board

was placed on the floor as shown by Figure 3.16, 3.17, 3.18 and 3.19 to measure bowing,

spring, twisting and cupping respectively, using Australian Standard AS2796.1-1999. Then,

thread and ruler were used to measure the size of the defects.

Figure 3.16 – Measurement of bow (AS 2796.1, 1999).

Figure 3.17 – Measurement of spring (AS 2796.1, 1999).

Figure 3.18 – Measurement of twisting (AS 2796.1, 1999).

Figure 3.19 – Measurement of cupping (AS 2796.1, 1999).

3.3.4 Statistical analysis

The variability of properties between boards was analyzed using statistical analysis.

Comparisons of drying time, drying quality and microwave energy consumption between

planks and treatments were made to determine the required drying schedules.

3.3.5 Results

3.3.5.1 Sample variations

The initial moisture content (MC), green density and basic density for each board and groups

were determined. All board measurements included in this analysis are given in Appendix 4.

Sapwood boards had the highest initial moisture content, ranging from 131% to 155% MC,

followed by mixed boards group, with initial moisture contents ranging between 81%-103%,

(depending on the ratio of sapwood and heartwood content in boards). Heartwood had the

lowest initial moisture contents, ranging from 36% to 75%.

Green density had the same pattern of weight distribution.. Sapwood had the highest green

density, ranging from 999 kg/m3 to 1107 kg/m3. In contrast, heartwood had the lowest green

density ranging from 539 kg/m3 to 693 kg/m3. For mixed boards, the green density ranged

from 713 – 877 kg/m3. Table 3.7 summarizes the average initial moisture content and green

density for each groups (sapwood, heartwood and mixed). The average initial moisture

content for heartwood, mixed and sapwood boards were 59%, 90% and 145% respectively.

Table 3.7 – Average initial moisture content and green density for each group.

Group Log content Green MC (%) Green density (kg/m3) Total boards

1 Heartwood 36-75 539 - 693 6 2 Mixed 81-103 713 - 877 6 3 Sapwood 131-155 999 - 1107 6

Table 3.8 provides the mean and standard deviations for the green moisture content, green

density and basic density of boards for each group. Mixed sapwood /heartwood had the most

variable readings, with a standard deviation of 60.4 kg/m3. Sapwood had the least variation

with a standard deviation of 41.5 kg/m3. Details of green moisture content, green density and

basic density for each board are presented in Appendix 4.

.

Table 3.8 – Summary of wood property variability.

Variables Group

Green density (kg/m3) 1 2 3 Mean 622 788 1059

Std. dev 50.4 60.4 41.5

Green moisture content (%) 1 2 3 Mean 59 90 145

Std. dev 17.7 9.7 9.3

Basic density (kg/m3) 1 2 3 Mean 638 805 1087

Std.dev 74.4 59.9 52.7

Notes: Groups 1, 2 and 3 represent heartwood, mixed heartwood & sapwood and sapwood

plank groups respectively that were treated using a 60 kW microwave.

3.3.5.2 Relationship between green density, green moisture content and basic density.

Figure 3.20 shows the relationship between the green density and green moisture content. It

shows that, there is a strong correlation between the two variables with r2 = 0.9992. When the

green moisture content increases, the green density also will be increases.

Figure 3.20 – Relationship between initial (green) moisture content and green density.

3.3.5.3 Microwave drying treatment

The microwave drying schedules used for each board and group are summarized in Table 3.9.

Detailed results have been tabulated in Appendix 5. Microwave energy consumption and

electrical energy consumption were calculated using the following equation:

Microwave energy consumption (kWh/m3)

1) Microwave power = P

2) Time of microwave was applied to each board = T

3) Microwave energy absorbed by wood of 1 board (E1) = P x T

4) Volume of each board = V

5) Microwave energy for 1m3 required (E) = E1 / V

Electric energy consumption (kWh/m3)

1) Microwave generator efficiency = 85% or 0.85

2) Electric energy consumption (EL) = E / 0.85

Table 3.9 – Summary of microwave drying schedules used for each group.

Group 1 2 3 No of boards 6 6 6 Initial moisture content (%) 59 90 145 Final moisture content 13 17 13 Moisture content loss (%) 46 73 132 MW power (kW) 10 10 10 Belt speed (mm/s) 35 35 35 MW pass exposure time (s) 86 86 86 Time between passes (s) 516 516 516 Total pass time (s) 602 1204 2236 Average no. of passes 7 14 26 MW time (min) 10 20 37 Total drying time (min) 70 140 261 MW energy consumption (kW/h) 206 413 767 Electrical consumption (kW/h) 242 484 902 Note: Groups 1, 2 and 3 represent heartwood, mixed heartwood &

sapwood and sapwood planks respectively.

Group 1 (Heartwood)

A single pass took about 602 seconds, comprising 86 seconds of microwave heating and 516

seconds of resting between passes. Due to lower initial moisture content compared to sapwood

and mixed planks, heartwood only need about seven passes to achieve 12% MC. The average

final moisture content was 13%. Wood sap can be seen comes out from planks during the

treatment as shown by Plate 3.9.

Total drying time was 70 minutes, with 10 minutes plank been exposed to microwave and

another 60 minutes for standing time. The amount of microwave energy needed can be

calculated based on the number of passes required to dry the timber, the applied microwave

power and the time of microwave exposure (Torgovnikov and Hann, 2006). In seven passes,

the planks absorbed about 206 KWh.

Plate 3.9 – Wood sap (free water) comes out from planks during microwave processing.

Group 2 Mixed heartwood /sapwood

Mixed planks required a microwave residence time of 86 seconds and a total drying time 140

minutes. This involved 14 passes through the microwave to reduce the planks to 12% MC.

Group 3 Sapwood

Total drying time to reduce each plank to 12% MC took about 261 minutes, involving 26

passes through the microwave. There was some delay in microwave drying due to conveyor

belt breakage during Pass number 22. Repairs to the conveyor took 166 minutes. The average

final moisture content for this group was 13%. Thus, reducing the plank moisture content

from 155% to 13% required 767 KWh/m3 and 902 KWh/m3 of microwave and electrical

energy consumption respectively.

3.3.5.4 Drying rate

The relationship between average cumulative weight loss during each successive pass for

heartwood, mixed and sapwood planks is tabulated in Appendix 5. It shows that there is strong

correlation between weight loss and number of passes through the microwave. The average

cumulative weight loss was fastest for heartwood planks followed by mixed, then sapwood

planks. The pattern of weight loss reduction during each pass for all planks is plotted in

Figures 3.21, 3.22 and 3.23. For the first three passes, the cumulative weight loss for

heartwood was 580 g, while for mixed and sapwood planks was 243.6 g and 344.6 g

respectively. The cumulative weight loss for heartwood increase rapidly after Pass number 4,

ranging from about 250 to 350 g weight loss for each pass. Faster drying rates for mixed

sapwood & heartwood and sapwood planks started from Pass number 6.

Figure 3.21 – Drying rate (cumulative weight loss) of microwaved heartwood planks.

Figure 3.22 – Drying rate (cumulative weight loss) of microwaved mixed planks.

Figure 3.23 – Drying rate (cumulative weight loss) of microwaved sapwood planks.


Table 3.10 – Average value of moisture content distribution for microwaved planks.

Group 1 2 3 Layer Outer Inner Outer Inner Outer Inner Mean 22 19 27 19 14 15 Std. dev 8 8 11 5 3 4

Note: Groups 1, 2 and 3 represent heartwood, mixed heartwood &

sapwood and sapwood planks respectively.

The moisture content distributions for all planks were analyzed and tabulated in Table 3.10.

The results show that the inner layers were drier than outer layers for mixed and heartwood

planks. Normally outer layers will be drier than inner layers in conventional kiln drying. Outer

layers were slightly wetter than inner layers for sapwood plank, see Appendix 6. The standard

deviation for sapwood was the lowest, with 3% and 4% for outer and inner layers respectively.

The standard deviation for mixed planks was the highest and the range of moisture content

distribution between outer and inner layer was also higher, with 11% (outer) and 5% (inner).

The result was most probably due to the different percentage of sapwood and heartwood of the

planks.

Table 3.11 - Statistical analysis of F-Value on the final moisture content of microwaved planks of

different groups and layers.

VARIABLE df MC

Group 2 4.300*

Layer 1 1.639 ns

Group*Layer 2 0845 ns Notes: ns- not significant p>0.05, x - significant at p<0.05, xx - highly significant at p<0.01

The results of statistical analysis found that there is no significant difference in final moisture

content distribution between different layers of the planks (p>0.05) as tabulated in Table 3.11.

The mean moisture content for the outer layer was 20%, compared to 18% for the inner layer,

as shown in Figure 3.24. The inner layer became drier than outer layer probably because of

the concept of moisture removal using microwave. In microwaving processing, energy is

supplied to the inner layer of planks, thus allowing it to heat and achieve mass flow of wood

sap to the outer layer.

Figure 3.24 – Average moisture content distribution of microwaved planks at different layers.

3.3.5.6 Prong test/ stress test

The effect of stress in boards after kiln drying was determined using a prong test. Prong

curvature for all boards was determined before and after 24 hours air drying, the prongs either

remained straight (free of stresses), turned in (case-hardening) or turned out (reverse case-

hardening) as illustrated in Plate 3.10 and Appendix 7.

Plate 3.10 – Comparisons of prongs curvature with case-hardening (left), remain straight

(middle) and reverse case-hardening (right).

The change of prong curvature was divided into eight types as illustrated in Table 3.12. Each

prong was evaluated based upon these groups. Prongs turned out if the surface was in tension

and the centre part is in compression. Prongs turned out when the surface was in compression

and tension was present in the center part. Prongs remained straight when they were totally

free from stresses.

Table 3.12 - Summary of prong curvature during prong test.

Type Before After 1 Turned out Turned in 2 Turned out Turned out 3 Turned in Pinch tighter 4 Turned in Straight 5 Turned in Turned out 6 Remain straight Remain straight 7 Remain straight Turned in 8 Remain straight Turned out

Data recorded and evaluated during the prong test are outlined in Appendix 7. Test samples

were air dried for 24 hours during the prong test. Fourteen out of fifteen planks were free from

stress as their prong curvatures remained straight. One plank from the mixed sapwood &

heartwood group had case-hardening (prongs turn in). After air drying at room temperature for

24 hours, thirteen planks (five sapwood, three mixed and five heartwood planks) demonstrated

Type 6 prong curvature, i.e. the prongs remained straight. One plank from the sapwood group

illustrated Type 7 prong curvature. The prongs were free of stress before air drying but had

turned inwards upon completion of air drying. One mixed plank that had case-hardening at the

start of the trial returned back to its original shape and was free of stress following 24 hours of

air drying. Types 1, 2, 3, 5 and 8 were not observed in any planks.

The pattern of prong curvature before and after 24 hours air drying for sapwood, mixed and

heartwood planks is summarized in Figure 3.25. In summary, more than 90% planks were free

of stress following microwave processing, which should reduce the occurrence of drying

defects during the drying process.

Figure 3.25 – Comparison of prong curvature before and after 24 hours air drying for all

planks.


a) Warping

Warping of planks can be divided into bowing, cupping, twisting and spring. Thread was used

to measure the size of the warps, as illustrated in Plate 3.11.

Plate 3.11 – Warp in planks was measured using thread.

From this study, it was found that some planks are susceptible to warping defects after

microwaving process, whereas for sapwood planks, four boards out of six were susceptible to

bowing. The bow measured at the plank centres ranged from between 5 to 14 mm. Bowing

defects also occurred in all mixed planks, ranging from between 9 to 30 mm. Twisting did not

occur for either of these two groups. In heartwood planks, two boards were susceptible to

bowing and two planks recorded twisting. All data is tabulated in Appendix 9. This finding

may not accurately represent drying quality of microwave processing since no restraints were

applied during the drying process. A previous study by Haslett et.al (1999) stated that this

condition seems an unusual result as warp during drying of timber can cause up to 40%

rejection and twist is the major cause of warp rejection for Pinus radiata, as it has a tendency

to twist during the drying process.

Table 3.13 – Comparisons of warp defects of microwaved planks.

Mean warp (mm) Bowing Twisting Spring

Group 1- Heartwood 4.2a 11.33a 0

Group 2- Mixed 13.0a 0b 0

Group 3- Sapwood 6.5a 0b 0 Notes: - Groups 1, 2 and 3 represent heartwood, mixed and sapwood planks respectively.

- Maximum acceptable warp: spring -5mm; bowing-20mm; twist-5mm

- Letters are for comparison. Different letters are significantly different at 0.05 probability

level.

Table 3.13 shows a statistical comparison of warp measurements for three different groups of

microwaved planks. There is a significant difference in twisting defects between the groups

(p=0.038), while there is no significant difference for bowing (p=0.168) and spring. Haslett et.

al., (1999) stated that the comparisons of warp rejection between groups is applied to

determine if there was any difference in rejection levels of the three groups. Spring defect was

not evaluated in this study as it did not occur in this study. Bowing defects in sapwood and

mixed boards was less than 20 mm and within an acceptable range. An opposite trend is

apparent in heartwood boards. The boards had unacceptable values for twisting (11.33mm).

The maximum acceptable level is 5 mm, although bowing defects were still less than the

maximum acceptable level. It was expected that twist would be the major factor resulting in

warp rejection.

b) Checking

In microwave drying quality determinations, over half (50%) of the mixed planks (heartwood

& sapwood) had surface and internal checks (as shown in Figure 3.26 and Plate 3.12.). They

had the greatest level of surface checks, followed by sapwood and then heartwood. Surface

checking was the most frequent degrade. Too rapid drying and variations in plank moisture

content were probably the major cause of checking. Overall, about 33% from all microwaved

planks were effected with checking degrade.

Figure 3.26 – Checks degrade that occurring for all groups after microwave processing.

Plate 3.12 – Internal checks occurring after microwave processing.

c) Discolourations

Discolouration is identified as degrade associated with fungal attack or chemicals in the wood

and normally occurs after high temperature drying as outlined in Section 2.4.3. Ward and

Simpson (1985) observed that this degrade is normally confined to within 1/8 of the outer

layer of boards dried at temperatures in excess of 100ºC and varies with complex interactions

between tree species, type of wood tissue and drying conditions. From this experiment, it was

found that there is no discolouration associated with microwave drying. This result is

supported by Rene (2007), that the darkening of the exterior timber associated with high

temperature drying is a feature that can be avoided by using microwave drying, where boards

are able to retain their light colour. However, discolouration may occur if heated resins and

sap have extruded from the boards during microwave drying.

3.3.6 Discussion and conclusion

Natural variation in board characteristics arising from growth conditions and position in the

original log influenced the quality attributes such as green density and wood moisture content.

These variations were expected and differing trends may thus result. As expected, heartwood

planks had the lowest initial moisture content, followed by mixed sapwood and heartwood.

Heartwood also had less variable moisture contents than planks from other two groups. The

larger variation in mixed boards was due to different percentages of heartwood and sapwood

in the planks. Furthermore, in heartwood, some variation may also result from the presence of

transition heartwood, where the wood will have moisture contents similar to sapwood but

permeability comparable to heartwood. For more accurate results, it should be ensured that

logs are from similarly aged and tree sized to reduce natural variations.

Total drying time was fastest for heartwood, requiring 70 minutes only. Drying times for

mixed sapwood & heartwood and sapwood groups were 140 minutes and 261 minutes

respectively. These drying times are very fast in comparison to conventional kiln drying. The

longer drying times associated with kiln-drying may cause drying stress in planks and

influence the occurrence of drying defects. . Higher temperatures may improve drying

processes. Waterson (1997), for example, claims that increases in drying temperature increase

the rate of evaporation. Under these conditions, the rate of heat transfer provides latent heat

for vaporization and a reduction in relative humidity. Moisture removal from inner to the outer

sections of the timber then becomes faster. However, total drying time for sapwood planks

should be lower than 261 minutes because that total drying time included the time delay

caused by conveyor belt break-down. Some uncontrolled drying may have affected the planks

during this period.

Energy consumption for microwaving all three groups of timber ranged from between 206

KWh to 767 KWh. However, the energy consumption in the Table 3.9 does not include heat

losses from the plant (5-10% of energy consumption) or energy required for heating fresh air

(8% of electric energy consumption). In addition, a previous study by Torgovnikov and Hann

(2006) stated that the total energy consumption may be 13 to 18% higher than the amount

identified from a microwave experiment.

The trend in moisture content distribution for the three groups of board was similar, where the

inner layer was drier than outer layer. The difference in inner and outer layer moisture content

was lowest in sapwood (about 1%). Mixed heartwood & sapwood was the largest

(approximately 7%). However, this variation in moisture content distribution is substantially

lower compared to the planks dried using conventional kiln drying or high temperature drying.

A study by Haslett and Lenth (2003) stated that high temperature drying generates extreme

moisture content gradients through the cross-section of the timber leading to significant

shrinkage and stresses, which when retained within the dried plank can be responsible for

significant losses due to deformation in reprocessing.

The final moisture content of planks calculated using predicted oven-dried weight and the

moisture meter was compared to the actual final moisture content. It can be concluded that

final moisture content calculated using predicted oven-dry weight provides an approximation

and cannot be totally depended upon for inaccuracy. Inaccuracy arises due to natural sample

variation that arises in initial moisture content and basic density. Measurements using the

moisture meter were more accurate and the results had minor variation compared to the final

moisture content determined after the drying process. In future research, moisture meters can

be used as an alternative method to identify the moisture content of planks.

The prong test was used to evaluate drying stresses in boards after microwave drying. From

the observed results, it can be concluded that fewer than 10% of planks had case-hardening or

unequal moisture content distribution, where the centre was in tension and the surface under

compression. Case-hardened planks can be treated using steam treatment but application

depends upon time needed to avoid reverse case-hardening. One prong that was free of initial

stress turned inwards after air drying but it is possible for this to occur even when there is no

difference in final moisture content between the surface and centre regions. This is an

indication that drying stresses did develop during drying. No reverse case-hardening was

observed in this study.

In drying quality determinations, most drying degrade such as checking was observed on

mixed heartwood & sapwood planks. Other degrades included warping such as bowing and

twisting. Warping may be present due to pressure in the planks such as core tension after

removal from the microwave during the drying process. This caused the wood cells to shrink

and potentially affect the dimensions of the dried planks. Wood cell shrinkage will not be

uniform. Ray cells in the wood structure reduce the shrinkage in the radial direction, while the

tangential direction is more severe. Examples of warping include bowing, spring and twisting.

The degree of warping depends on that variation and its freedom of movement (Wallis, 1970).

In this study, surface checking and bowing occurred after microwave drying but only to a

small extent in all groups. Too rapid drying and variations of moisture content in planks

caused checking. Twisting only occurred on sapwood planks. The prolonged time for

sapwood planks dried due to conveyor belt breakage may have increased surface checking and

twisting. When drying recommenced, the planks needed to be reheated, thus taking a longer

time to achieve a reduction in wood moisture content.

CHAPTER 4: GENERAL DISCUSSION AND CONCLUSION

Microwave conditioning or drying of Pinus radiata was investigated by applying two

different experiments:

1) Laboratory scale microwave wood drying

2) Pilot scale microwave wood drying

The objective was to study the effectiveness of microwave processing on drying rate,

moisture content distribution and drying quality. This chapter summarizes the discussions and

conclusions developed from these studies.

Control of drying by measuring the weight of the boards after each microwave pass provided a

useful indication of drying rate. Cumulative weight loss after each pass through the

microwave provided a methodology for predicting the number of microwave passes needed to

dry Pinus radiata to specific moisture contents.

Moisture content distribution between outer and inner layers of microwaved dried timber is

quite uniform. The differences between inner and outer layers ranged from 1-7% moisture

content. Thus the moisture gradient (the moisture content difference between outer and inner

layers) is low. The decrease in moisture gradient indicates that the potential residual stress

within the plank may be low. This was supported by using the prong test that showed that

ninety percent of planks had no residual stress. Prong tests indicated that ten percent of planks

may have some case-hardening. In getting a more accurate definition of moisture content

distribution in planks, it is recommended that moisture content samples are sliced into six

layers as illustrated below in Figure 4.1.

Slice 1 Slice 2

Slice 3 Slice 4

Slice 5 Slice 6

Figure 4.1 – Sampling methodology of samples divided into slices with equal thickness for measuring final moisture content of planks. The moisture content of Slice 1 and Slice 6 will indicate the moisture content for outer layer

of wood. Slices 2 and 5 will represent the moisture content for middle part. The moisture

content of inner layer (core) will be obtained from Slices 3 and 4. This sampling methodology

will give a more accurate moisture content value for wood and will be easier to evaluate the

drying phases in wood. The information may be used for evaluating the presence of certain

drying defects such as case-hardening and warping.

Observations in relation to drying quality shows that sample size should be at least 2000 mm

in length since warping and case-hardening cannot be evaluated on the smaller samples used

in Experiment 1.

Trials with longer length samples (experiment two) found that drying defects such as warping

(bowing) may occur after microwave drying but in relatively small amount and there was no

significant different between heartwood only, mixed heartwood & sapwood and sapwood

only. Twisting occurred on heartwood planks only. However, the presence of large knots in

heartwood planks contributed to warping because of extensive grain deviation. There was no

restraint applied to the planks during microwave processing to reduce the potential for bowing

and twisting. The much lower bowing and twisting observed using microwave processing than

one might anticipate in high temperature or conventional kiln drying indicated that microwave

processing may have the potential for reducing the volume loss due to drying. It is

recommended that further research into microwave drying of boards is undertaken with

defects removed by cross cutting (as may for example be undertaken for finger jointing) to

remove potential drying degrade arising from knots and grain deviations. Drying processes

without defects also increase the yield and may cut the costs of wood drying. Engineering

design of applicators could also be a possible focus of further research.

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APPENDIX 1:

a) Drying rate of samples after each microwave passes (Experiment 3.2).

Set Sample OD MC (%) Final MC (%) MW passes MC loss (%) Set Sample OD MC (%) Final MC (%) MW Passes MC loss (%)1 62.0 58.1 1 4 1 59.7 45.2 1 152 30.9 26.6 1 4 2 48.1 41.6 1 63 52.7 49.5 1 3 3 42.1 32.5 1 104 63.0 58.5 1 4 4 46.0 39.8 1 65 72.6 62.3 1 10 5 71.4 63.1 1 8

SET 1 6 39.3 34.6 1 5 SET 4 6 37.5 27.3 1 107 73.8 70.4 1 3 7 46.4 32.4 1 148 33.2 31.2 1 2 8 26.7 20.4 1 69 43.5 39.8 1 4 9 42.3 33.2 1 910 74.6 70.4 1 4 10 70.2 66.4 1 411 59.8 55.4 1 4 11 45.9 40.3 1 612 51.7 47.2 1 5 12 35.7 29.6 1 61 69.4 63.3 1 6 1 30.7 21.4 1 92 52.5 46.2 1 6 2 23.1 20.1 1 33 53.9 50.2 1 4 3 44.7 43.3 1 14 44.0 36.7 1 7 4 41.1 35.8 1 55 86.3 73.2 1 13 5 65.4 52.1 1 13

SET 2 6 55.5 51.7 1 4 SET 5 6 16.1 10.7 1 57 54.8 48.4 1 6 7 32.0 23.5 1 88 29.3 23.7 1 6 8 22.3 15.7 1 79 56.4 50.5 1 6 9 39.2 37.1 1 210 77.0 73.0 1 4 10 81.8 67.2 1 1511 60.2 53.4 1 7 11 40.8 36.0 1 512 41.6 32.4 1 9 12 36.6 21.8 1 151 20.4 13.8 1 72 23.7 18.5 1 5

3 30.6 25.7 1 5 MC = Moisture content4 34.9 29.6 1 5 MW = Microwave5 27.1 20.9 1 6

SET 3 6 18.0 12.8 1 57 39.8 33.9 1 68 8.9 4.7 1 49 31.7 26.5 1 510 44.5 37.3 1 711 30.6 25.9 1 512 39.5 33.4 1 6

Set Sample OD MC (%) Final MC (%) MW passes Cum. MC loss (%) Set Sample OD MC (%) Final MC (%) MW Passes Cum. MC loss (%)1 62.0 44.9 2 17 1 59.7 31.2 2 292 30.9 16.4 2 14 2 48.1 30.9 2 173 52.7 36.4 2 16 3 42.1 23.2 2 194 63.0 43.6 2 19 4 46.0 27.4 2 195 72.6 45.8 2 27 5 71.4 48.2 2 23

SET 1 6 39.3 25.2 2 14 SET 4 6 37.5 18.8 2 197 73.8 56.0 2 18 7 46.4 20.0 2 268 33.2 23.2 2 10 8 26.7 12.2 2 159 43.5 30.4 2 13 9 42.3 24.8 2 1710 74.6 57.0 2 18 10 70.2 54.0 2 1611 59.8 44.9 2 15 11 45.9 28.8 2 1712 51.7 34.2 2 17 12 35.7 19.3 2 161 69.4 48.2 2 21 1 30.7 10.7 2 202 52.5 35.7 2 17 2 23.1 12.8 2 103 53.9 40.4 2 14 3 44.7 33.7 2 114 44.0 24.9 2 19 4 41.1 24.2 2 175 86.3 66.3 2 20 5 65.4 39.2 2 26

SET 2 6 55.5 40.2 2 15 SET 5 7 32.0 13.7 2 187 54.8 46.1 2 9 8 22.3 7.8 2 158 29.3 12.8 2 16 9 39.2 28.3 2 119 56.4 40.4 2 16 10 81.8 54.4 2 2710 77.0 60.7 2 16 11 40.8 27.2 2 1411 60.2 42.9 2 17 12 36.6 11.2 2 2512 41.6 20.1 2 222 23.7 12.1 2 12

3 30.6 15.9 2 15 MC = Moisture content4 34.9 17.9 2 17 MW = Microwave5 27.1 8.1 2 196 18.0 5.9 2 12

SET 3 7 39.8 16.5 2 239 31.7 15.5 2 16

10 44.5 23.6 2 2111 30.6 18.0 2 1312 39.5 22.7 2 17

Set Sample OD MC (%) Final MC (%) MW passes Cum. MC loss (%) Set Sample OD MC (%) Final MC (%) MW Passes Cum. MC loss (%)1 62.0 30.2 3 32 1 59.7 20.4 3 392 30.9 9.9 3 21 2 48.1 21.6 3 273 52.7 27.4 3 25 3 42.1 16.3 3 264 63.0 30.8 3 32 4 46.0 16.2 3 305 72.6 33.2 3 39 5 71.4 34.0 3 37

SET 1 6 39.3 17.0 3 22 SET 4 6 37.5 11.1 3 267 73.8 42.9 3 31 7 46.4 10.5 3 368 33.2 13.4 3 20 8 42.3 16.7 3 269 43.5 20.0 3 23 9 70.2 42.1 3 2810 74.6 45.1 3 29 11 45.9 18.7 3 2711 59.8 33.7 3 26 12 35.7 9.4 3 2612 51.7 24.8 3 27 1 30.7 3.5 3 271 69.4 35.5 3 34 3 44.7 24.9 3 202 52.5 25.1 3 27 4 41.1 14.6 3 273 53.9 31.1 3 23 SET 5 5 65.4 28.2 3 374 44.0 13.0 3 31 9 39.2 19.8 3 195 86.3 50.5 3 36 10 81.8 47.2 3 35

SET 2 6 55.5 29.9 3 26 11 40.8 19.0 3 227 54.8 27.5 3 278 29.3 5.3 3 24

9 56.4 31.0 3 25 MC = Moisture content10 77.0 48.1 3 29 MW = Microwave11 60.2 31.9 3 2812 41.6 9.2 3 32

Set Sample OD MC (%) Final MC (%) MW passes Cum. MC loss (%)1 69.4 21.7 4 482 52.5 16.1 4 363 53.9 21.5 4 325 86.3 38.1 4 486 55.5 19.8 4 36

SET 2 7 54.8 18.0 4 379 56.4 21.8 4 3510 77.0 35.5 4 4211 60.2 19.7 4 401 59.7 11.4 4 482 48.1 14.0 4 343 42.1 10.4 4 32

SET 4 5 71.4 23.4 4 48

9 42.3 11.1 4 3110 70.2 31.7 4 3911 45.9 11.2 4 35

SET 5 3 44.7 17.6 4 275 65.4 18.7 4 479 39.2 13.3 4 2610 81.8 42.7 4 39

b) Average moisture content of each set after been microwaved

VARIABLES SET

Oven-drying time 1 2 3 4 5 Hours 11 13 15 17 19

MC after ovendry (%) 1 2 3 4 5 Mean 55 57 29 48 39

Std. dev 15 15 10 13 18

MC AFTER MW (%) 1 2 3 4 5 Mean 29 19 13 15 15

Std. dev 12 10 6 5 11

Notes: MC = moisture content; MW= microwave

APPENDIX 2:

Regression analysis for microwave treatment (Experiment 3.2).

Variables Entered/Removeda

Model Variables Entered Variables Removed Method

1

OD.MC .

Stepwise

(Criteria:

Probability-

of-F-to-enter

<= .050,

Probability-

of-F-to-

remove >=

.100).

2

No.Passes .

Stepwise

(Criteria:

Probability-

of-F-to-enter

<= .050,

Probability-

of-F-to-

remove >=

.100).

a. Dependent Variable: Final.MC

Model Summaryc

Model R R Square

Adjusted R

Square

Std. Error of the

Estimate

1 .748a .560 .558 10.66167

2 .966b .933 .933 4.15875

a. Predictors: (Constant), OD.MC

b. Predictors: (Constant), OD.MC, No.Passes

c. Dependent Variable: Final.MC

ANOVAc

Model Sum of Squares df Mean Square F Sig.

1 Regression 25620.845 1 25620.845 225.394 .000a

Residual 20119.816 177 113.671

Total 45740.661 178

2 Regression 42696.712 2 21348.356 1.234E3 .000b

Residual 3043.949 176 17.295

Total 45740.661 178

a. Predictors: (Constant), OD.MC

b. Predictors: (Constant), OD.MC, No.Passes

c. Dependent Variable: Final.MC

Coefficientsa

Model

Unstandardized Coefficients

Standardized

Coefficients

t Sig. B Std. Error Beta

1 (Constant) -4.586 2.483 -1.847 .066

OD.MC .716 .048 .748 15.013 .000

2 (Constant) 9.245 1.064 8.689 .000

OD.MC .870 .019 .909 45.224 .000

No.Passes -10.092 .321 -.632 -31.422 .000

a. Dependent Variable: Final.MC

APPENDIX 3 (a):

Relationship between oven-dry moisture content, number of microwave passes and moisture

content after microwave treatment (regression) (Experiment 3.2).

Equation:

MC (%) after MW = K + ODMC(x) - No of Passes(y)

K = 9.245 ODMC = 0.870 No of passes = 10.092 = 10

So that,

MC (%) after MW = 9.245 + 0.870x - 10y

Constant (K) ODMC (%) No. of Passes MC (%) after MW MC (%) after MW 9.245 80 1 69 68.8 9.245 60 1 51 51.4 9.245 40 1 34 34.0 9.245 20 1 17 16.6 9.245 80 2 59 58.8 9.245 60 2 41 41.4 9.245 40 2 24 24.0 9.245 20 2 7 6.6 9.245 80 3 49 48.8 9.245 60 3 31 31.4 9.245 40 3 14 14.0 9.245 80 4 39 38.8 9.245 60 4 21 21.4 9.245 40 4 4 4.0 9.245 80 5 29 28.8 9.245 60 5 11 11.4 9.245 80 6 19 18.8 9.245 80 7 9 8.8

APPENDIX 3 (b)

Raw data for moisture content distribution between outer and inner layer (Experiment 3.2).

SET Layer final wt od wt Final mc SET Layer final wt od wt Final mc

1 outer 57.20 44.78 28 3 outer 49.67 44.06 131 outer 59.50 54.41 9 3 outer 50.53 48.10 51 outer 71.00 62.82 13 3 outer 64.27 56.60 141 outer 54.00 46.79 15 3 outer 44.26 37.27 191 outer 53.60 46.07 16 3 outer 45.64 42.46 71 outer 63.80 56.51 13 3 outer 50.35 45.91 101 outer 56.30 48.93 15 3 outer 51.05 43.54 171 outer 68.90 59.53 16 3 outer 70.01 60.20 161 outer 66.70 58.74 14 3 outer 57.20 50.75 131 outer 54.70 47.44 15 3 outer 47.98 40.55 181 outer 54.70 47.76 15 3 outer 52.43 46.30 131 outer 53.80 47.61 13 3 outer 50.51 41.95 201 inner 5.83 5.05 15 3 inner 4.80 4.35 101 inner 7.25 6.57 10 3 inner 6.60 6.15 71 inner 9.40 8.12 16 3 inner 6.80 5.99 141 inner 6.27 5.35 17 3 inner 5.80 4.00 451 inner 6.84 5.92 16 3 inner 4.60 4.37 51 inner 8.17 7.26 13 3 inner 6.80 6.38 71 inner 7.62 6.50 17 3 inner 6.30 5.22 211 inner 9.04 7.65 18 3 inner 6.10 5.06 211 inner 6.85 5.86 17 3 inner 7.60 6.69 141 inner 8.00 6.80 18 3 inner 4.50 3.75 201 inner 7.44 6.37 17 3 inner 5.20 4.79 91 inner 6.98 6.06 15 3 inner 6.60 5.17 282 outer 49.67 44.06 13 4 outer 55.73 49.73 122 outer 50.53 48.10 5 4 outer 62.09 57.11 92 outer 64.27 56.60 14 4 outer 65.37 60.98 72 outer 44.26 37.27 19 4 outer 52.52 41.37 272 outer 45.64 42.46 7 4 outer 50.51 42.77 182 outer 50.35 45.91 10 4 outer 65.53 58.12 132 outer 51.05 43.54 17 4 outer 55.05 47.07 172 outer 70.01 60.29 16 4 outer 74.45 57.83 292 outer 57.20 50.75 13 4 outer 62.07 57.23 82 outer 47.98 40.55 18 4 outer 63.64 46.85 362 outer 52.43 46.30 13 4 outer 50.48 45.24 122 outer 50.51 41.95 20 4 outer 50.97 45.22 132 inner 4.80 4.35 10 4 inner 5.90 5.24 132 inner 6.60 6.15 7 4 inner 8.61 7.94 82 inner 6.80 5.99 14 4 inner 8.54 8.24 42 inner 5.80 4.69 24 4 inner 8.81 6.20 422 inner 4.60 4.37 5 4 inner 8.07 6.33 272 inner 6.80 6.38 7 4 inner 9.30 8.20 132 inner 6.30 5.22 21 4 inner 8.08 6.79 192 inner 6.10 5.06 21 4 inner 11.72 7.99 472 inner 7.60 6.69 14 4 inner 9.59 8.75 102 inner 4.50 3.75 20 4 inner 10.27 6.60 562 inner 5.20 4.79 9 4 inner 7.79 7.12 92 inner 6.60 5.17 28 4 inner 8.24 7.19 15

SET Layer final wt od wt Final mc5 outer 55.73 49.73 125 outer 62.09 57.11 95 outer 65.37 60.98 75 outer 52.52 41.37 275 outer 50.51 42.77 185 outer 65.53 58.12 135 outer 55.05 47.07 175 outer 74.45 57.83 295 outer 62.07 57.23 85 outer 63.64 46.85 365 outer 50.48 45.24 125 outer 50.97 45.22 135 inner 5.90 5.24 135 inner 8.61 7.94 85 inner 8.54 8.24 45 inner 8.81 6.20 425 inner 8.07 6.33 275 inner 9.30 8.20 135 inner 8.08 6.79 195 inner 11.72 7.99 475 inner 9.59 8.75 105 inner 10.27 6.60 565 inner 7.79 7.12 95 inner 8.24 7.19 15

3c) Mean and standard deviation of MC distribution between outer and inner layer

SETLayer Outer Inner Outer Inner Outer Inner Outer Inner Outer Inner

1 28 15 13 10 13 10 12 13 12 132 9 10 5 7 5 7 9 8 9 83 13 16 14 14 14 14 7 4 7 44 15 17 19 24 19 45 27 42 27 425 16 16 7 5 7 5 18 27 18 276 13 13 10 7 10 7 13 13 13 137 15 17 17 21 17 21 17 19 17 198 16 18 16 21 16 21 29 47 29 479 14 17 13 14 13 14 8 10 8 1010 15 18 18 20 18 20 36 56 36 5611 15 17 13 9 13 9 12 9 12 912 13 15 20 28 20 28 13 15 13 15

Mean 15 16 14 15 14 17 17 22 17 22sd 4 2 5 7 5 11 9 17 9 17

1 2 3 4 5

APPENDIX 4:

Samples variability for Experiment 3.3 (heartwood, mixed of sapwood & heartwood and

sapwood boards).

Boards Initial wt (g) Initial MC (%) Green density (kg/m3) Basic density (kg/m3)

H1 5751.5 52 602.9 632.7H2 6858.2 63 693.1 750.0H3 5100 47 539.5 517.3H4 5893.7 36 629 652.0H5 6118.9 83 641.4 649.9H6 5995.8 75 628.2 624.1Mean 5953.0 59.3 622.4 637.7Std. dev 569.3 17.7 50.4 74.4


M1 7853.3 82.0 791.3 771.0M2 7821.3 81.0 775.9 808.9M3 7205.5 97.0 739.5 760.2M4 8535.7 103.0 877.3 907.2M5 8172.0 95.0 833.9 833.9M6 6934.3 81.0 712.7 746.2Mean 7753.7 89.8 788.4 804.6

Std. dev 595.5 9.7 60.4 59.9


S1 9066.5 153.0 999.4 1025.6S2 10906.6 131.0 1055.8 1048.2S3 10146.5 155.0 1043.9 1087.2S4 10707.7 143.0 1107.3 1155.1S5 10783.2 151.0 1106.6 1144.1S6 9777.3 139.0 1043.5 1059.7

Mean 10231.3 145.3 1059.4 1086.6Std. dev 715.2 9.3 41.5 52.7

APP

END

IX 5

:

Mic

row

ave

dryi

ng tr

eatm

ent (

Expe

rimen

t 3.3

)

Micr

owav

e dry

ing f

or he

artw

ood b

oard

sFir

st 3 p

asse

s with

MW

powe

r 15k

w, th

en 10

kw. F

eed s

peed

35m

m/s

ec

Boar

dW

i (g)

AMC

ODW

Wt a

t 25%

Wt a

t 12%

1 pas

s2 p

ass

3 pas

s4 p

ass

5 pas

s6 p

ass

7 pas

s8 p

ass

9 pas

s

H157

51.5

5237

83.9

4729

.942

37.9

5718

.555

51.1

5196

.248

75.5

4634

.3H2

6858

.263

4207

.552

59.4

4712

.468

1566

76.2

6452

.661

08.3

5797

.255

10.9

5238

.450

00.2

H351

0047

3469

.443

36.7

3885

.750

82.4

4861

.245

72.1

4250

.240

26.5

3844

.6H4

5893

.736

4333

.654

17.0

4853

.655

71.1

5333

.250

32.5

4678

.2H5

6118

.959

3848

.448

10.5

4310

.260

60.7

5896

.256

27.3

5208

.249

65.5

4750

.845

10.3

4290

.341

21H6

5995

.875

3426

.242

82.7

3837

.359

41.4

5689

.953

56.2

5016

.546

94.7

4463

.542

73.6

4091

.2

Cum

ulat

ive w

eigh

t los

s afte

r eac

h pas

ses (

g)

Boar

dW

i(g)

AMC(

%)OD

WW

t at 2

5%W

t at 1

2%1 p

ass

2 pas

s3 p

ass

4 pas

s5 p

ass

6 pas

s7 p

ass

8 pas

s9 p

ass

H157

51.5

5237

83.9

4729

.942

37.9

33.0

200.4

555.3

876.0

1117

.2H2

6858

.263

4207

.552

59.4

4712

.443

.218

2.040

5.674

9.910

61.0

1347

.316

19.8

1858

.0H3

5100

.047

3469

.443

36.7

3885

.717

.623

8.852

7.984

9.810

73.5

1255

.4H4

5893

.736

4333

.654

17.0

4853

.632

2.656

0.586

1.212

15.5

H561

18.9

5938

48.4

4810

.543

10.2

58.2

222.7

491.6

910.7

1153

.413

68.1

1608

.618

28.6

1997

.9H6

5995

.875

3426

.242

82.7

3837

.354

.430

5.963

9.697

9.313

01.1

1532

.317

22.2

1904

.6M

ean

88.2

285.1

580.2

930.2

1141

.213

75.8

1650

.218

63.7

1997

.9

Microw

ave dry

ing for

mixed

board

sFirs

t 3 pass

es with

MW po

wer 15

kw, the

n 10 kw

. Feed

speed

35mm/s

ec

Boards

Wi (g)

AMC (%

)OD

WWt

at 25%

Wt at 1

2%1 pa

ss2 pa

ss3 pa

ss4 pa

ss5 pa

ss6 pa

ss7 pa

ss8 pa

ss9 pa

ss10 p

ass11 p

ass12 p

ass13 p

ass14 p

ass15 p

ass16 p

ass17 p

ass18 p

ass

M1785

3.382

4315.0

5393.8

4832.8

7817.5

7757.7

7646.5

7438.2

7184.2

6898

6627.2

6342.5

6062.5

5815.3

5577.6

5360.7

M2782

1.381

4321.2

5401.5

4839.7

7758.6

7695.2

7582.6

7405.2

7178.6

6889.3

6592.5

6318.4

6053

5784.3

5526.4

5163

4922.1

4922.1

M3720

5.597

3657.6

4572.0

4096.5

7125.4

7052.6

6941.2

6732.4

6495

6170.2

5868.2

5564.7

5301.2

5016.6

4788.5

M4853

5.7103

4204.8

5256.0

4709.4

8494.3

8423.2

8313.5

8167.5

7916.3

7640.3

7331.4

7018

6739

6474

6224.1

5990.5

5733.3

5606.6

5479.2

5289.9

5117.8

4941.6

M5817

295

4190.8

5238.5

4693.7

8086.7

8023.7

7916.5

7741.5

7554

7251.4

6946.1

6646.2

6359.2

6110.4

5867.2

5618.6

5367.1

M6693

4.381

3831.1

4788.9

4290.8

6849

6785.2

6660.1

6491.2

6287.5

6016.3

5740.3

5465.8

5217.8

4981.5

4757.5

4584.5

4383.1

Cumula

tive we

ight lo

ss after

each p

asses (

g)

Boards

1 pass

2 pass

3 pass

4 pass

5 pass

6 pass

7 pass

8 pass

9 pass

10 pass

11 pass

12 pass

13 pass

14 pass

15 pass

16 pass

17 pass

18 pass

M135.8

95.6206

.8415

.1669

.1955

.3122

6.1151

0.8179

0.8203

8.0227

5.7249

2.6M2

62.7126

.1238

.7416

.1642

.7932

.0122

8.8150

2.9176

8.3203

7.0229

4.9265

8.3289

9.2289

9.2M3

80.1152

.9264

.3473

.1710

.5103

5.3133

7.3164

0.8190

4.3218

8.9241

7.0M4

41.4112

.5222

.2368

.2619

.4895

.4120

4.3151

7.7179

6.7206

1.7231

1.6254

5.2280

2.4292

9.1305

6.5324

5.8341

7.9359

4.1M5

85.3148

.3255

.5430

.5618

.0920

.6122

5.9152

5.8181

2.8206

1.6230

4.8255

3.4280

4.9M6

85.3149

.1274

.2443

.1646

.8918

1194

1468.5

1716.5

1952.8

2176.8

2349.8

2551.2

Mean

65.1130

.8243

.6424

.4651

.1942

.8123

6.1152

7.8179

8.2205

6.7229

6.8251

9.9276

4.4291

4.2305

6.5324

5.8341

7.9359

4.1

Micro

wave

dryin

g for

sapwo

od bo

ards

First 3

passe

s with

MW

powe

r 15kw

, then

10 kw

. Feed

spee

d 35m

m/sec

Proble

m: M

W sto

pped

at 17

passe

s(*) a

nd 22

passe

s (**)

due t

o con

veyo

r belt

brok

en.

Board

sWi

(g)

AMC (

%)OD

WWt

at 25

%Wt a

t 12%

1 pass

2 pass

3 pass

4 pass

5 pass

6 pass

7 pass

8 pass

9 pass

10 pa

ss11

pass

12 pa

ss

S1906

6.5153

3583.6

4479.5

4013.6

8997.2

8926.7

8832.0

8713.3

8551.0

8340.6

8061.4

7717.8

7506.4

7218.1

6955.4

6681.3

S2107

07.7

143440

6.5550

8.1493

5.2994

1.4989

4.5979

7.4969

6.3955

9.2940

6.3916

5.2899

9.3860

8.2832

4.3801

3.2774

0.7S3

10146.

5155

3979.0

4973.8

4456.5

10078.

2100

07.2

9902.4

9791.4

9658.8

9506.2

9317.2

9052.9

8724.6

8442.0

8138.0

7869.3

S4977

7.3139

4090.9

5113.7

4581.8

9718.5

9668.3

9560.3

9466.7

9329.0

9145.4

8910.4

8639.8

8367.2

8091.5

7787.7

7508.2

S5109

06.6

131472

1.5590

1.8528

8.0108

45.6

10792.

7106

93.4

10573.

4104

16.5

10237.

7100

26.4

9757.4

9454.2

9160.0

8868.9

8581.7

S6107

83.2

150431

3.3539

1.6483

0.9107

21.3

10636.

9105

35.0

10403.

2102

98.2

10142.

7995

0.6967

4.1928

4.7898

9.3868

2.4845

6.8

Board

s13

pass

14 pa

ss15

pass

16 pa

ss17

pass

18 pa

ss19

pass

20 pa

ss21

pass

22 pa

ss23

pass

24 pa

ss25

pass

26 pa

ss27

pass

28 pa

ss29

pass

S1643

5.6617

5.8592

3.4567

7.7542

0.4526

7.4514

1.7492

4.9471

5.4452

4.4443

5.3433

9.7419

3.4S2

7482.6

7234.6

6943.1

6666.3

6413.2

6223.2

6150.5

5924.1

5649.8

5516.2

5406.1

5170.2

4936.7

S3760

0.4732

3.1705

2.8678

5.0652

7.0632

3.4626

1.2607

0.5583

0.6558

2.6545

0.9529

2.3506

1.6482

7.6466

8.6441

5.7S4

7254.2

6978.6

6697.1

6409.0

6145.2

6045.3

5973.3

5741.2

5482.6

4940.4

4856.1

4658

S5832

1.0805

6.4779

6.4753

2.5722

0.0716

1696

1.2672

3646

6.3619

0.4604

5583

0.9571

1.7551

9.5530

2.4S6

8161.8

7881.7

7623.8

7347.7

7115.5

7057.6

6860.1

6617.5

6357.5

6148.5

5960.1

5713.7

5462.2

5246.2

5140.9

4946.4

4769.5

Cum

ulat

ive w

eigh

t los

s afte

r eac

h pas

ses (

g)

Boar

ds1 p

ass

2 pas

s3 p

ass

4 pas

s5 p

ass

6 pas

s7 p

ass

8 pas

s9 p

ass

10 pa

ss11

pass

12 pa

ss13

pass

14 pa

ss15

pass

S169

.313

9.823

4.535

3.251

5.572

5.910

05.1

1348

.715

60.1

1848

.421

11.1

2385

.226

30.9

2890

.731

43.1

S276

6.381

3.291

0.310

11.4

1148

.513

01.4

1542

.517

08.4

2099

.523

83.4

2694

.529

67.0

3225

.134

73.1

3764

.6S3

68.3

139.3

244.1

355.1

487.7

640.3

829.3

1093

.614

21.9

1704

.520

08.5

2277

.225

46.1

2823

.430

93.7

S458

.810

9.021

7.031

0.644

8.363

1.986

6.911

37.5

1410

.116

85.8

1989

.622

69.1

2523

.127

98.7

3080

.2S5

61.0

113.9

213.2

333.2

490.1

668.9

880.2

1149

.214

52.4

1746

.620

37.7

2324

.925

85.6

2850

.231

10.2

S661

.914

6.324

8.238

0.048

5.064

0.583

2.611

09.1

1498

.517

93.9

2100

.823

26.4

2621

.429

01.5

3159

.4M

ean

180.9

243.6

344.6

457.3

595.9

768.2

992.8

1257

.815

73.8

1860

.421

57.0

2425

.026

88.7

2956

.332

25.2

Boar

ds16

pass

17 pa

ss18

pass

19 pa

ss20

pass

21 pa

ss22

pass

23 pa

ss24

pass

25 pa

ss26

pass

27 pa

ss28

pass

29 pa

ss

S133

88.8

3646

.137

99.1

3924

.841

41.6

4351

.145

42.1

4631

.247

26.8

4873

.1S2

4041

.442

94.5

4484

.545

57.2

4783

.650

57.9

5191

.553

01.6

5537

.557

71.0

S333

61.5

3619

.538

23.1

3885

.340

76.0

4315

.945

63.9

4695

.648

54.2

5084

.953

18.9

5477

.957

30.8

S433

68.3

3632

.137

32.0

3804

.040

36.1

4294

.748

36.9

4921

.251

19.3

S533

74.1

3686

.637

45.6

3945

.441

83.6

4440

.347

16.2

4861

.650

75.7

5194

.953

87.1

5604

.2S6

3435

.536

67.7

3725

.639

23.1

4165

.744

25.7

4634

.748

23.1

5069

.553

21.0

5537

.056

42.3

5836

.860

13.7

Mea

n34

94.9

3757

.838

85.0

4006

.642

31.1

4480

.947

47.6

4872

.450

63.8

5249

.054

14.3

5574

.858

36.8

6013

.7

APPENDIX 6

Moisture content distribution for microwaved boards in Experiment 3.3.

a) Raw data

Boards Layer final wt od wt Final mc 1 outer 14.112 12.831 10.0 1 outer 11.754 10.519 11.7 1 outer 10.567 9.29 13.7 1 outer 11.847 10.133 16.9 1 outer 11.508 10.015 14.9 1 outer 14.487 12.236 18.4 1 inner 8.768 7.509 16.8 1 inner 9.463 8.647 9.4 1 inner 7.87 7.067 11.4 1 inner 9.328 8.018 16.3 1 inner 8.317 6.975 19.2 1 inner 9.009 7.708 16.9 2 outer 12.78 10.412 22.7 2 outer 10.235 8.646 18.4 2 outer 8.753 7.16 22.2 2 outer 11.063 7.736 43.0 2 inner 9.546 8.372 14.0 2 inner 7.859 6.364 23.5 2 inner 9.438 8.154 15.7 2 inner 7.806 6.294 24.0 3 outer 10.214 7.619 34.1 3 outer 7.843 6.86 14.3 3 outer 10.275 8.604 19.4 3 outer 10.561 8.852 19.3 3 outer 10.979 8.531 28.7 3 outer 9.36 7.965 17.5 3 inner 7.626 6.776 12.5 3 inner 6.8 6.125 11.0 3 inner 9.756 7.894 23.6 3 inner 7.507 6.532 14.9 3 inner 5.467 4.133 32.3 3 inner 7.525 6.16 22.2

b) Mean value and standard deviation

Group 1 2 3 Layer Outer Inner Outer Inner Outer Inner

1 34.1 12.5 22.7 14.0 10.0 16.8 2 14.3 11.0 18.4 23.5 11.7 9.4 3 19.4 23.6 22.2 15.7 13.7 11.4 4 19.3 14.9 43.0 24.0 16.9 16.3 5 28.7 32.3 14.9 19.2 6 17.5 22.2 18.4 16.9

Mean 22 19 27 19 14 15 sd 8 8 11 5 3 4

c) General Linear Model (GLM)

Between-Subjects Factors

N

group 1 12

2 8

3 12

layer inner 16

outer 16

Tests of Between-Subjects Effects

Dependent Variable:mc

Source

Type III Sum of

Squares df Mean Square F Sig.

Corrected Model 523.407a 5 104.681 2.294 .075

Intercept 11688.343 1 11688.343 256.129 .000

group 392.423 2 196.211 4.300 .024

layer 74.800 1 74.800 1.639 .212

group * layer 77.164 2 38.582 .845 .441

Error 1186.497 26 45.635

Total 13296.130 32

Corrected Total 1709.905 31

a. R Squared = .306 (Adjusted R Squared = .173)

d) Post Hoc Tests

Mc

Duncan

group N

Subset

1 2

3 12 14.6333

1 12 20.8167

2 8 22.9375

Sig. 1.000 .483

Means for groups in homogeneous subsets

are displayed.

Based on observed means.

The error term is Mean Square(Error) =

45.635.

e) T-Test

APPENDIX 7

Raw data for Prong test (Experiment 3.3).

Boards Before After 1 Remain straight Remain straight 1 Remain straight Remain straight 1 Remain straight Remain straight 1 Remain straight Remain straight 1 Remain straight Remain straight 2 Remain straight Remain straight 2 Remain straight Remain straight 2 Remain straight Remain straight 2 Turn in Remain straight 3 Remain straight Remain straight 3 Remain straight Remain straight 3 Remain straight Remain straight 3 Remain straight Remain straight 3 Remain straight Remain straight 3 Remain straight Turn in

BEFORE air drying AFTER 24 hours of air drying Group Turn in Turn out Remain straight Turn in Turn out Remain straight Sapwood 0 0 6 1 0 5 Mixed 1 0 3 0 0 4 Heartwood 0 0 5 0 0 5

APPENDIX 8

Microwave drying defects (Experiment 3.3).

a) Raw data for warping defects

Boards Bowing (mm) Twisting (mm) Spring(mm) 1 17 0 0 1 0 20 0 1 8 33 0 1 0 0 0 1 0 15 0 1 0 0 0 2 14 0 0 2 15 0 0 2 30 0 0 2 10 0 0 2 9 0 0 2 0 0 0 3 6 0 0 3 5 0 0 3 14 0 0 3 0 0 0 3 0 0 0 3 14 0 0

b) Statistical analysis for warping defects

Between-Subjects Factors

N

Group 1 6

2 6

3 6

Tests of Between-Subjects Effects

Source

Dependent

Variable

Type III Sum of

Squares df Mean Square F Sig.

Corrected Model Bowing 251.444a 2 125.722 2.014 .168

Twisting 513.778b 2 256.889 4.085 .038

Spring .000c 2 .000 . .

Intercept Bowing 1120.222 1 1120.222 17.946 .001

Twisting 256.889 1 256.889 4.085 .061

Spring .000 1 .000 . .

Group Bowing 251.444 2 125.722 2.014 .168

Twisting 513.778 2 256.889 4.085 .038

Spring .000 2 .000 . .

Error Bowing 936.333 15 62.422

Twisting 943.333 15 62.889

Spring .000 15 .000

Total Bowing 2308.000 18

Twisting 1714.000 18

Spring .000 18

Corrected Total Bowing 1187.778 17

Twisting 1457.111 17

Spring .000 17

a. R Squared = .212 (Adjusted R Squared = .107)

b. R Squared = .353 (Adjusted R Squared = .266)

c. R Squared = . (Adjusted R Squared = .)

c) Post Hoc Tests

Bowing

Duncan

Group N

Subset

1

1 6 4.1667

3 6 6.5000

2 6 13.0000

Sig. .085

Means for groups in homogeneous subsets are

displayed.


The error term is Mean Square (Error) = 62.422.

Twisting

Duncan

Group N

Subset

1 2

2 6 .0000

3 6 .0000

1 6 11.3333

Sig. 1.000 1.000

Means for groups in homogeneous subsets are displayed.


The error term is Mean Square(Error) = 62.889.

111

d) Checking degrade

Sample Heartwood Mixed Sapwood 1 Free of checks Free of checks Free of checks 2 Free of checks Checks Checks 3 Checks Checks Free of checks 4 Free of checks Checks Free of checks 5 Free of checks Free of checks Checks 6 Free of checks Free of checks Free of checks

Minerva Access is the Institutional Repository of The University of Melbourne

Author/s:

Abdul Latif, Nur Hannani

Title:

Microwave drying and conditioning of Pinus radiata D. Don sawn timber

Date:

2014

Citation:

Abdul Latif, N. H. (2014). Microwave drying and conditioning of Pinus radiata D. Don sawn

timber. Masters Research thesis, Melbourne School of Land and Environment, The

University of Melbourne.

Persistent Link:

http://hdl.handle.net/11343/39760

File Description:

Microwave drying and conditioning of Pinus radiata D. Don sawn timber

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microwave drying and conditioning of pinus radiata d. don

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