KENAF FIBRES FOR PULPING AND PAPERMAKING

Harshad Pande

A thesis submitted in confonnity with the reqiiirements for the degcee of Dortor of Philosophy

Gnduate Department of Forestry University of Toronto

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Kenaf Fibres for Pulping and Papemaking

Doctor of Philosophy, 1999

Harshad Pan&

Department of Forestry, University of Toronto

ABSTRACT

Kenaf, a non-wood fibrous species used for papermaking has two morphologidly

different portions in its stem: the bast or long fibre fiaction and the core or short fibre M o n . The

objective of this research is to provide an explanation for the observed merences in properties of

paper made fiom the two kenaffractions, in both chernical and morphologicai terms.

Micrepulping experiments were carried out to study the delimiification potential of bast,

core, and whole kenaf fibres. The activation energies for the different fractions were calculated

using the Arrhenius quafion and were 68 Wmole, 91 kJ/mole, and 75 kJ/mole for bast, are, and

whoIe fractions, respectively. M.K.Systems digester followed by PFi beating was used to obtain

pulps with a range of residual Lignin contents and freeness values for making handsheets. Multiple

regression analysis was then used to analyse papa strength properties in terms of the

morphological and chernical characteristics of the pulps. Separate models for bast and core were

developed, as they have a wide variation in activation energies and delignification potentid.

Elasticity or the relative rate of change in each strength property, with respect to a change in

morphological or chernical property was also calculated. This hiselped to elucidate the relative

sensitivity of the independent variables affecthg the models.

The micro-pulping experiments led to the conclusion that the bast fibres in kenaf are

relatively easy to d e l i e during pulping, followed by the whole and the core kenaf M o n s .

Also, for optimal utilisation of kenaf as a raw material for papennaking, the bast and core hctions

should be processeci separately and then biendeci, as per requirement. The regression analysis for

the strength properties suggested that a major portion of the variafion in the strength properties (as

hi& as 96% for burst in a r e nbres) is arplained by the modeis. The models prcsented Li this

research cau serve as initial exploratory tools for fiirther nsearch in n o n - w h in general and

kenaf in parti&. It is concluded that in general, morpho1ogicd factors are more important than

chernical fctors during the development of paper stm@ properties in kenaf fibres.

Table of Contents

Abstracts

Table of Contents

List of Figures

List of Tables

Acknowledgements

GIossary of Symbols

Chapter 1 htroduction

1.1 Global Fibre Sc&o

1.2 Non-Wood Fibres

1.3 Ken& - A Non-Wood Fibre Source 1.4 Litembue Review

1.4.1 Chernical pulping of kenaf fibres

1.4.3 Influence of fibre morphology and chernical composition on the handsheet strength properties

1 -4.3.1 Breaking length and bunt

1.4.3.2 Tear strength

1.4.3.3 Teasile energy absorption

1 -4.3 -4 Stretch

1 -4.3.5 Stifbess

1 .S Purpose of the Present study

1.6 Objectives

1.7 S ignificance

Chapter 2 Materiais and Erperimental Methods

2.1 Raw Material

2.2 ~ ~ 4 3

2.2.1 Micro-pulping

2.2.2 Pulping for handsheet making

2.3 M g

2 -4 Chernical Anaiysis

2.5

2.5.1

2.5 -2

2.5.3

2.5.4

2.6

2.6.1

2.6.2

2.6.3

2-6.4

2.6.5

2.6.6

2.6.7

2.7

2.7.1

2.7.2

2.7.3

2.7.4

Chapter 3

3.1

3.1.1

3.1.2

3 -2

3.2.1

3.2.2

3.3

3.4

3 -5

3.6

3 6.1

Beating and Morphological Characteristics

Freeness

Fibre length, coarseness, fines, and fibre curl

Fibre width and ceIl wall tbickness

Fibre density

Paper Properties

Basis weight, thickness and apparent density

Breakhg length

Tear strength

-g -& Stretch and tensile energy absorption (TEA)

Zero-span breaking length

Stiffuess

Statistical Analysis

Descriptive statistics

Multiple regression mode1 building

Robustness of the mode1

Elasticity

Results and Discussions

Chernical Composition of Raw Material

Organic composition

Inorganic composition

Delignification Study of Different Kenaf Fractions

Delignification selectivlty

Delignification kinetics

The Morphology of Kenaf Fibres

Effect of Beating on Freeness of Kenaf Fibres

Chernical Composition of Kenaf Fibres at Various Pulping Conditions

Influence of Fibre Morphology and Chernical Composition on 61 the Streogth Properties of Handsheets

Breakng length 63

3.6.1.1 Bastfibres 63

3.6.1.2 Core fibres 67

Chapter 4

Chapter 5

5.1

5 -2

5.2.1

5.2.2

5.3

References

Appendices

Tear fcmr

3.6.2.1 Bast fibres

3.6.2.2 Core fibres

Strength index

3.6.3.1 Tear-tensile plots

3 -6.3.2 Strength index of bast fibres

3 -6.3.3 Strength index of core fibres

Sn.etch

3.6.4.1 Bast fibres

3 -6.4.2 Core Fibres

Tensile energy absorption

3.6.5.1 Bastfibres

3.6.5.2 Core fibres

Burst factor

3.6.6.1 Bast fibre

3.6.6.2 Core fibre

Tensile snffriess

3.6.7.1 Bas fibres

3.6.7.2 Core fibres

Synthesis

Conclusions

Ddignification Kinetics

Influence of Fibre Morphology and Chernicd Composition on the Strength Properties of Handsheets

Bast fibres

Core fibres

Recommendations for Future Work

LIST OF F'IGURES

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Figure 14

Figure 15

Figure 16

Figure 17

Figure 18

Figure 19

Figure 20

Figure 2 1

Figure 22

Figure 23

Figure 24

Figure 25

Figure 26

Figure 27

Figure 28

Figure 29

Figure 30

Kenafplant as gram in Mississippi State 5

Lignin rernoval in kraft pulping 10

Experiment.1 set up for micro pdping 29

M/K Systexns Inc. computerised laboratory digester 31

Pulping seldvity curve for Merent kenaf fiactions 47

Delignificatioa kinetics of k e d bast fibres 48

Delignification kinetics of core fibres 48

DeliPnification kinetics of whole fibres 49

Amount of 1ign.b removed during delignrfication

Arrhenius plot of soda pulping of kenaf bast

Arrhenius plot of soda pulping of kenaf core

Arrhenius plot of soda pdping of kenaf whole

Activation energy for different kenaf M o n s

Bast fibres d e r light microscope

Core fibres under light microscope

Transverse section of bast fibres 56

Transverse section of core fibres 56

Unbeaten bast fibres 57

Unbeaten core fibres 57

Beating curve for bast fibres 58

Beating cuve for core fibres 58

Chemicai composition of kenaf bast fibres at difFerent 59 cooking conditions

Chernical composition of kenaf core fibres at different 60 cooking conditions

A typical histograrn of curl index for bast fibres

A typical histogram of curl index for core fibres

Increase in breaking length with increase in fibre density 65

Increase in breaking length with increase in hemicelluIose 65 content

Decrease in breaking length with increase in fkness 66

Decrease in breaking length with increase in fibre width 66

Decrease in breakig length with increase in fibre coarseness 66

Figure 3 1

Figure 32

Figure 33

Figure 34

Figure 35

Figure 36

Figure 37

Figure 38

Figure 39

Figure 40

Figure 4 1

Figure 42

Figure 43

Figure 44

Figure 45

Figure 46

Figure 47

Figure 48

Figure 49

Figure 50

Figure 5 1

Figure 52

Figure 53

Figure 54

Figure 55

Figure 56

Figure 57

Figure 58

Figure 59

Figure 60

Figure 6 1

Figure 62

Figure 63

Figure 64

Increase in breaking length with increase in fines 69

Increase in breaking length with increase in zer0spa.n 69

hcrease in breaking Iengtb with decrease in CSF 69

lncrease in breaking length with decrease in cell wall thickness 69

Increase in tear b r with hcrease in k n e s s 72

Increase in tear h r wih increase in fibre coarseness 72

Inmase in tear W r with increase in curl index 73

Increase in tear fkctor with increase in zero-pan 73

Increase in tear fhctor with increase in fines content 73

Decrease in tear &or with increase in fibre width 73

hcrease in tear with increase in shed bulk 76

Increase in tear with increase in zerespan 76

Increase in tear with increase in cellulose 76

increase in tear with decrease in marsmess 76

Increase in tear with decrease in fibre wicith 77

Increase in tear with decrease in freeness 77

Tear-tensile plots 79

increase in strength index with increase in zero-span 81

Increase in straigth index with increase in fines content 81

increase in strength index with decrease in fibre width 81

Increase in strength index with increase in hemicellulose content 81

lncrease in strength index with increase in zero-span

hcrease in strength index with increase in cellulose content

hcrease in strength index with decrease in ce11 wall thickness

Increase in strength index with decrease in fieeness

Increase in strength index with decrease in fibre Iength

Increase in stretch with uicrease in zero-span

Increase in stretch with decrease in bulk

Increase in stretch with decrease in fibre length

Increase in stretch with increase in h e s content

Increase in stretch with increase in fibre density

Increase in stretch with increase in zero-span

hcrease in stretch with decrease in fieeness

Increase in stretch with decrease in coarseness

Figure 65

Figure 66

Figure 67

Figure 68

Figure 69

Figure 70

Figure 71

Figure 72

Figure 73

Figure 74

Figure 75

Figure 76

Figure 77

Figure 78

Figure 79

Figure 80

Figure 8 1

Figure 82

Figure 83

Figure 84

Figure 85

Figure 86

Figure 87

Figure 88

Figure 89

Figure 90

Figure 9 1

Figure 92

Figure 93

Figure 94

Figure 93

In- in TEA with decrease in sheet bulk

Increase in TEA with increase in zero-spilll

Increase in TEA with increase in fines content

Increase in TEA with increase in fibre density

Increase in TEA with increase in zero-span

Increase in TEA with increase in h e s content

Increase in TEA with decrease in fieeness

Increase in TEA with decrease in coarseness

Increase in burst W r with decrease in sheet bulk

Increase in burst factor with i n m e in zero-span

Increase in burst fctor with increase in fines content

Increase in burst fctor with decrease in fibre coarseness

Increase in burst hctor with increase in fibre density

Increase in burst fctor with increase in hemicellulose

Increase in burst fctor with increase in zero-span

Increase in burst factor with decrease in fieeness

Increase in sti5ess with decrease in sheet bulk

Increase in stifkess with decrease in fibre curl

increase in stifltness with increase in fines content

hcrease in stiffiiess with increase in libre density

Increase in stiflhess with increase in Iignin content

Increase in stiffUess with increase in hemicellulose

Increase in s t i f k s s with decrease in fibre width

Increase in Stifkess with decrease in sheet bulk

Increase in &ess with decrease in fibre density

Increase in stifi%ess with increase in fines content

Increase in stifbess with decrease in fkeness

Effect of morphological and chemical properties on the breaking length, Stpength index, and tear factor of kenaf bast fibres

Effect of morphological and chemical properties on the 112 stretch, T E . burst, and stifhess of kenaf bast fibres

Effect of morphological and chemical properties on the 113 breakhg length, strength i n d q and tear h r of kenaf a r e fibres

Effect of morphological and chernical properties on the 114 strdch, TEA, burst, and stifiiess of kenaf core fibres

Table 1

Table 2

Table 3

Table 4

Table 5

Table 6

Table 7

Table 8

Table 9

Table 10

Table 11

Table 12

Table 13

Table 14

Table 15

Table 16

Table 17

Table 18

Table 19

Table 20

Table 2 1

Table 22

Cooking conditions for the bast and core fibres

Pulps selected for m e r ~~~sing of bast fibres

Pulps select& for M e r pmcessing of core fibres

Organic chernical analysis of different kenaf fiactions

Trace element anaiysis of different kenaffractions

The effective lignin content &,) for different fiactions of kenaf

Activation energy and reacion rates of kenaf fractions

Fibre morphology of kenaf fibres

Elasticity of breaking length for bast fibres

Elasticity of breaking length for core fibres

Elasticity of tear fctor for bast fibres

Elasticity of tear factor for core fibres

Elasticity of strength index for bast fibres

Elasticity of strength index for core fibres

Elasticity of stretch for bast fibres

Elasticity of stretch for core fibres

Elasticity of TEA for bast fibres

Elasticity of TEA for core fibres

Elasticity of burst W o r for bast fibres

Elasticity of burst fctor for core fibres

Elasticity of stiffiiess index for bast fibres

Elasticity of sliffriess index for core fibres

ACKNOWLEDGEMENTS

The author would like to sincerely thank ail the persons who gave their encouragement and

assisbnce to this projed, in particular the following people vuho made the thesis possible:

Prof. D.N.Roy, my supervisor, for his invaluable guidance, encouragement, and support throughout

the project. Prof. D.A.I.Goring (Department of Chemical Engineering), Prof. J. J.Balatinecz, Prof

S.Kant (Faadty of Foresy), Dr. K.Goel (Domtar), Dr. P.Whiting (T.rojan Technologies), and Dr.

A-Chatte j ee (Proctor and Gamble), m y committee members fbr their suggestions, guidance, and

critical review throughout this work. Prof. C.T.J.Dodsoa (Department of Chemical Engineering)

for the initial research help and training.

Abitibi P r i e Research Centre, Mississauga, for allowing me to use their research bilities. 1 am

gratefl to rnany people at the research centre for helping me during the course of the experiments,

especially Ms. K-Lindstol Ms. C-Powell, and Ms. J.Pitcher.

Prof. L.2- (Facufty of Forestry), for initiating this research by intrducing us to kenaf, the raw

material used in this research. Dr. M.Fuller, and Dr. I-Black, Mississippi State University for

providing separated kenaf bast, core fibres alongwith the whole fibres for this reseatch work. Dr.

S-Maheshwari, and Mr. S-ICMittal, Phoenix Pulp and Paper, Thailand for providing insight into the

research with kenaf.

Mr. DCCharles, Mr. J.McCarron, Mr. I.Kennedy for their assistance in various stages of this

work. Ms. M-Wells, Ms. C.Lee, and Ms. AYeneziano for their heIp in the office m e r s pertaining

to this work.

Al1 my colleagues in the w d chernistry laboratory, especially Mr. W-Mabee for his help in

Merent areas of the experimental work and Ms. F-Correia for her help during the chemical

anaiysis of raw material.

My fellow graduate students especially Mr. A-Quoreshi, Mr. V.M- Mr. J.Das, Mr. P-Bhojvaid,

Mr. A.Saxena, Mr. D.Misra, Mt. D.Mohta, and MrAJoon for useful and stimulating discussions in

the graduate lounge.

The Canadian Commonwealth Schoiarship Cornmittee for the award of a post-graduate

schoIarship.

My parents, wife Archana, daughter Trpti, and son Hitesh whose support and understanding are

appreciated more than they wiU ever know.

GLOSSARY OF SYMBOLS AND ACRONYMS

G

0 s

ANOVA

CI

cm

CMR

CSF

CWT

d

E

Ea

ES

F m

g

K

kg

kT

km

L

m

Beta

Eiasticity

Variance

Strain value in stress-sbain curve

Stress value in stress-straln curve

anaIysis of variance

Curl index

Centimetre

C 13 nuclear magnetic resonance

Canadian Standard Freeness

Ceii wall thickness

Fibre dianteter

Expected value

Activation energy

EIastic modulus

Fourier transform infked

Gram

Degree Kelvin

Kilogram

Kilo Joule

Kilometre

Fibre length

Meter

Minute

ml

m

N

NatO

NAA

NaOH

NMR

OC

r

R

R~

SPSS

T

t

TEA

2-span BL

Millilitre

Millimetre

Newton

Sdurn oxide

Neutron activation adysis

Sodium hydroxide

Nuckar magnetic resonance

Degree Celsius

Correlation coefficient

Gas constant (8.3 14kJPK . mole)

Coefficient of detennination

Statistical package for social scientist

Absolute temperame ("Kelvin)

Thickness of the shed

Tensile energy absorption

Zero-span breaking length

CHAPTER 1

INTRODUCTION

1 Giobal Fibre Scenario

Wood is one of the most ubiquitous rnaterials, and has been used by humans ever since the

Stone Age. ?be total demand for roundwood in the world is estimated at apprownately 3.65 billion

cubic meters, and about 54% of this is consumed as fuel for coolong and heating; 26% is usai in the

production of 1umber and other sawn wood products, and about 9% is used as raw materiai for

plywood, panek and other industrial products. The balance, about 1 1% is used in the production of

pulp, paper, and paperboard - ~ p p l e m n t e d by an additional 6% of mi~~lufcuring residues [l, 21.

Unlike other wood based products, papa has a special significance as its consumption is taken as an

important indicator of economic and educational development of a society or nation. As we move

into the twenty-first century, the global fibre supply will change extemively. The intense cornpetition

for resources between corporations and countries around the world means that allteniative fibre

sources, such as recovered and non-wood fibres will becorne more prevalent and desirable for

papemaking. At the same time most indushialised countries are becoming uicreasingiy wncemed

about deforestation and the environmental cost of harvesting the forest. This has resulted in

widespread support for the practice of more sustainable forms of forestry, which d e s the pursuit

of alternative fibre sources even more important.

The closed (> 40% crown axa) forest area of the world is approximately 3.8 billion

hectares. Hagler [2] points out that 2.1 billion hectares or 55% of the total forest area on a global

basis could be considerd available to meet either the fbel andor fibre requirement of the world.

Russia (over 90% of Eastern European conifer production forests), and Brazil (nearly 70% of Latin

America's h a r d w d production forests), hold the key to a balanced global fibre supply. The global

pulp and paper iadustry consumes approxunately 25% of the industrial roundwood harvested each

year, and supplements this harvest with additionai man- residues, making this industry,

with a net co~l~llmption of 38%, the largest industrial fibre consuming industry in the world. On a

global basis fIagler 121 gives the supply to demand d o for wood fibres as 1.27. He concludes that

the world, while not yet in fibre cnsis, is k i n g a period of relative w d fibre scarcity which wiIl

fiindarnentally alter the structure of the global foresi produci industry, particularly for traditionai

suppliers such as North Amenca. in summary we can say that we are faced with a nnite resowce

b t must supply increasing demand for fuel and fibre.

The increased demand for fibre will be met by one or more of several potential supply

sources: (a) increased harvest of the world timber supply, (b) improved production of timber, (c)

increased yield by better control of pests and catastrophes, (d) increased utilisation of forest waste,

(e) increased utilisation of waste paper, andor (f) increased utilisation of n o n - w d fibrous plants. If

increased wood production is to be realised, improved lines will need to be evolved and systerns

deveioped for intensive forest management. Shorter rotations will probably be also necessary.

Greater use of forest waste will require extensive study. The use of waste wi1I have an impact on

properties of raw material available from the forests. Perhaps the greatest potentid for increasing

world fibre supply rests with the utilisation of non-wood fibre [28].

1.2 Non-Wood Fibres

The texm non-wood fibre encompasses a range of plants with widely ciBering

characteristics. Non-woud fibres, commonly referred to as 'alternate fibres', are non-wdy

cellulosic plant materials fiom which papermaking fibres cari be extracted. The most widely used

non-woods for papermaking are straws, bagasse, bamboo, k d , hemp, jute, sisal, abaca, cotton

LinterS. and reeds. Most non-wd plants are a n n d plants tbat develop fidl fibre potential in one

growing season.

Wood is a relaively uew raw maferia1 in pape&. Histoncally, paper was made

exclusively fiom non-wood plant fibres. The first production of paper is credited to T.S'ai Lun in 05

A.D in China. This was made apparently fiom textile waste, old rags and used fish nets, i-e., the

fibres of tme hemp and China gras (rarnie) [3]. C d y wood is the major raw maferid for pulp

and paper. Non-wods are used for paper rnainiy in those countries and regions that are short an

wood resources. The leadhg corntries in non-wuod utilisation for papermaking are China and India,

accounting for about 80% of the world capacity for non-wood papemiakuig pulp capacity. The

U S A r a d s eleventh with 179,000 tons of non-wood capacity. This is a mere 0.3% of its total

papermakhg pulp capacity of 57 million tons [4]. The most cornmon non-wd fibre globally used

for papermaking is straw, which acmunts for 46% of total production. This is followed by bagasse

(14%), and bamboo (6%) [5] .

Non-woods are a critical fibre resource in regions with inadequate forests, and will continue

to play an increasingly important role in these regions. Environmental pressures, restrictions on

forest uses and a dramatic increase in w d and recycled fibre costs are forcing paper companies in

the traditionally forest nch countries to take a renewed look at n o n - w d . Even forest rich North

Amencan countres are experiencing fibre supply problems as the goveniments are restricting

harvesting, and recyclable waste paper prices have increased ten fold in the past few years.

Meanwhile h e r s in the U.S., Canada, and Europe are being told by their governments to move to

low tillage agriculture and not to bum or landfill their agricultural residues. McCloskey [4]

concludes that non-woods hold great potenial, and their future is bright. There is a wealth of

information on non-wods already available, and new tecbnology is king developed wnstantly.

PreServation of forests and increasing environmental awareness bave led to a focus on exploration of

new renewabie fibrous resources. Mall and Upadhyay [6] point out tbat there d l be an increased

use of unconventional raw materials like grass, waste paper, agricultural residua, etc. for

papermaking to supplement he wood supply. Non-wood plants already acaunt for 9.15% of the

total world papermaking capacity as of 1990, and the percentage annual inaease in the non-wood

plant fibre pulp capacity is more than double the average annuai increase in the w d pulp capacity

i-e. 4.7% vs. 2.0% [A.

In 1995, the Food and Agriculture Organisation (FAO) initiateci the "Global Fibre Supply

Study7'(GFSS). This study was aimed to respond to questions concerning the mpply of raw mterial

to meet the forest products need and the productive forest needed to supply the arpeaed future

demand sustainably. Recent publications [ I l 1, 1 12, 1 141 fiom the study have presented cumnt fibre

supply statistics by region, reviewing some of the major factors which have an impact on supply.

The GFSS shidy also shows that non-woods wiii play an increasing role in the future as a fibre

source globaily. A part of the review on non-woob discussed here has been published in "ClnosyLva"

[llZ].

1.3 Kenaf - A Non-Wood Fibre Source

The kenaf plant (Hibiscus cannabinus) is considered one of the most promising alternatives

to virgin softwoods and hardwods for papa production. A herbaceous annuai plant of the

Malvaceae W y , (related to Cotton and okra) kenaf is a member of the mallow M l y indigenous to

West &ca. The United States Department of Agriculture (USDA) began researching kenaf in

194OYs, when World War II put a stop to jute imports fiom Asia. The work started with a scheme

for evaluation of potential plants as new sources of pulp and paper. The evaluation procedure

involved consideration of botanid characteristics, chernical composition, yield, and dimensional

measurements of fibrous constituents, and a qualitative and visual appraisai of the plants [8, 9, 10,

1 Il. A total of 58 plant species, representing 1 1 M i e s , were reported and laboratory scale pulping

studies were Camed out on them [IO]. This work was extendd by dysis and evaluation of an

additional 208 species (32 fhdies) [I l ] . The characteristics of puIpwoods and other accepte.

pulping niateriais served as a guideline in determuiing the properties of the non-woody species to be

measured. Among the speces that were subjected to the screening evaluation, kenaf and hemp were

most prornising [ 12 1.

Fig.1. Kenaf plant as grown in field in Mississippi State

Kenaf is an muai non-wood fibre plant ha- low density (0.256&m3). It is grown in

rows using standard farm equipment. It reaches heighs of 3 to 5 meters, and yields 4 to 8 metric

tons of riry fibre per acre, dependuig on s i lv id tud conditions. Kenaf c m be cultivated under a

wide range of conditions, and requires relatively litde care. It has &ght slender stems, and is

largely unbranched in dense sands [13, 141. Fig.1 shows a kenaf plant, as grown in fields. Kenaf

consists of two morphologicaiiy, and chemidy distinctive regions in its stem. The outer portion of

the kenaf stem is hown as bark, and it contains long bast or cordage fibres, whiie the inner or

woody portion is termed core, which contains short fibres. The whole stalk of kenaf co nstituteS about

60 - 65% core fibres, and 35 - 40% bast fibres [15]. Pnor to 1989 the f m on kenaf as a fibre

source was to utilise the entire stalk of the plant, consisting of the two fibre f k t o r ~ ~ together as one

and not on the utilisation of the fibres in a separated form. In 1990 severai kenaf companies in

United States began workiag on systems to mechanically separate the two fibre fractions. in 1992

the only commercial kenaf fibre processing hcility of its kind came into operation in Mississippi.

The fcility uses the forage chopper concept for harvesting. The fibre separation process uses a

combination of biomass separation, and modified cotton cleanuig equipment to separate the two fibre

M o n s . Another facility for the separation of the two fractions exists in Kenaf International Ltd.,

Raymondville, Texas. It uses the sugarcane concept for harvesting. The fibre sqaration process uses

the Ankal method developed by Ankal Ltd. utilising a series of large perforated drums to separate the

fibres 113, 321. At present kenaf is king use- on a lirnited sale ais a substitute for w d in the

production of pulp and paper in Thaland, and the Peoples Republic of China [ 1 61.

1.4 Literature Review

1.4.1 Chernical pulpiag of kenaf fibres

The objective of pulping is to liberate the fibres from the raw material. Generally, for non-

wood fibres the soda-pulping process is used [40]. Chenical [l 7-20, 24-27, 29, 3 1-33], mechanid

[2 1, 221, and recently biomfhanicai [q pulps have been prepared from kenaf fibres. There is a

significant range in the chernical composition values reportai in the literature [12]. This is partly

atributed to location, because plants develop much more rapidly in southem thau in northem

locations. Larger plants grown in Florida have higher contents of cellulose, pentosans, and Li*

than shorter plants grown in &ois, but the reverse is tme for ash content [23, 121. The Iignin

content in kenaf is signifcantly lowcr than that in woods. The peitosaits contait, which meaaires

about 20% in kenaf, is double that in softwoods and nearly equal to that in hardwoods. There is no

information on the trace elernent composition of the bast and the core fractions of kenaf fibres. One

study [34] reports a few trace elernents for the whole stem k d . Fibre dimensional characteristics

of kenaf stalks are essentially intermediate to those of softwoods and hardwoods. In the case of kenaf

whole fibres it is cuncluded that effective length of the bast and core fibres together is reasonably

good and the ~lanow width of the bast fibres provides a srnooth printing surolce.

hilping studies with whole stalle kenaf [IO, 17, 20, 241 were aii done at fixeci times and

temperatures. In some, the liquor concentration was vared [17,20]. A few shidies [24, 301 Iooked at

mntinuous pulping with a Pandia type digester. Clark et al. [17] conclude that pulps prepared by

cooking with caustic soda are quite sirnilar in properties to those prepared by sulphate cooking. An

exception is the greater breakhg Iength for bleached soda pulps from cooks at 15% liquor charge.

Bagby [25,28] points out that kenafsoda pulps have strength properties equivalent to those of kenaf

sulphate pulps, and a better initial drainage characteristics than &ose of the kenaf sulphate pulps.

Another important observation was tha ail the pulps fiom whote kenaf stalles seerned to exhibit some

peculiar behaviour with respect to tensile, burst, and fold characteristics. This behaviour was

parbcularly noticeable in the shape of the curves for folding endurance. Bagby et al. concluded that

this shape might be caused by the m e r in which the long and the short fibres in the pulp respond

to beater tmatments. A contempfated work on kenaf pdps prepafed fiom isolated bast and core

fibres might provide sorne explanation for this behaviour. In general, strengthwise the kenaf pulps

are superior to hardwcmd pulps, and, with the exception of resisbnce to tear, the k& pulps are

comparable in seength propexties with softwood and superior to soffwood sulphite. Tear for

kena.f pulps was higher than for hardwood pulps but lower than for softwood pulps.

Chernical pulping of manuaily s e p a i e x i bast and core 119, 25-27, 29, 3 1, 321 shows that

the two stem cornponents behave Merentiy during puIping and papermaZang operatiom. The

possible reasom for better cooking of the bast fibre needs more research. Mostly the cooks were

done at just one set of specified amking conditions, wMe in a few studies [19, 3 11 fibres were

cooked at 1 70C, vaqing the time and alkali charge. In these studies with soda pulping, it was found

that 15% active alkali at 170C for 1.5 hr gave a Kappa below 22 for bast fibres, and 18% active

alkali at 170C for 2 hr, or 22% active alkali at 170C for 1 hr gave a simiIar degree of cookuig for

core fibres.

In studies with chernicd pulping, the delignification has been carried out at predetermined

t h e and temperature althouph, as pointed out earlier, at times alkali concentration is varied. The two

hctions in the kenaf stem are quite ciifferent chemidy, morpbologically, and in their strength

development characteristics. A more cornprehensive understanding fkom the delignification kinetics,

and paper strength properties would help in characterishg the two puIps separately. This will put

more emphasis on the doctrine that the bast and the core M o n s wahin kenaf fibre need to be

eeated separately for the optimal utilisation of the keaaf stexn. Thus there is a need to study the

delignification potmtial of the two fibres separately, with t h e and temperature of -king as the

major variables.

Beatllig for the chemid pulps from the bast and the core fibres has been done in most of the

studies discussed above, but in most of the cases it is limited to a single heness value. A few studies

119, 331 have camed out beating in valley beater, PFI mill at di.t%rent beating energies, but a fbil

five point beating curve with tear-mile plats will provide a better explanation of the beating and

strength development potential of the pulps fkom the two fhdons of kenaf.

4 Delignification kinetics

There are three principal ways of weakening the interfibre bonds between fibres in wood: by

increasing temperature, by swelling agents, and by delignification [39]. The two Ccdriving forces" for

d a and kraft pulping reactions are allcali concentration and temperature. Within the normal cooking

temperature range (1 55- 1 7jC), the delignification rate more than doubles for every 1 OC increase

1381. Delignification during the chernid pulping process is commonly divided into three phases:

1. Fast "initial delignification phase".

2. Slower "'bulk deligdcation stage7'.

3. Very slow "residual delignifiaiion stage7'.

In the initial phase, the amount of lignin dissolved in the pulphg liquor is small, about 60%

of the alkali is cotlsumed, and the carbohydrate yield decreases rapidly. The initial very rapid lignin

removal is characterised as an extraction process. In the bulk delipfication stage, most of the lignin

is dissolved. The carbohydrate yield and the alkali concentration of the cooking Liquor decreases only

slightly in this stage. Kraft cooks are typically mnpleted at a lignin content of 4 - 5% for softwoods

and about 3% for hardwods, well within the bulk delignification phase. En the residual delignification

stage, the dissolution of lignin is again retarded. The carbohydmte yield and alMi concentration

starts to decrease rapidly. The transition between the initial and buk delignification phases is

generally detennined by two criteria - the rate of change of alkali consumption with Lignh removal,

and the ratio of carbhydrate to lignin rernoval [371. The three distinct phases are shown in the Fig.2

[35]. H W r expresses the cooking time and temperature as one single variable.

Fig.2. Lignin removal in kraft pulping

The kinetics of soda, kraft, and sulphite pulping have been studied by severai workers who

bave determined the rate of delignification at various stages of the cook at various maximum

temperatures and cooking liquor compositions. Reaction order, activation energy, and collision

fiequency constants have been caldated fiom the experimental data. The kinetics of alkaline

delignification have been much saidied, [35] and found to fit a ht order process with respect to

iignin and hydroxide concentration. Larocque and Maass [36] have show that the soda delignification

rate appears to be proportional to the product of the unremoved lignia, L, and the concentration of

sodium hydroxide in the liquor [OH']. Quantitatively this leads to the rate equation

- - *' - K I I O H ' J . L d t

(1.1

dL Where - = the rate at which Lignin is rernoved

dt

KI = the r a d o n rate mmtmt, independent of liquor concentration.

t = the t h e at temperature of cookuig

L = the residuai l i e content based on the original weight of wood

If pulping is performed under a constant liquor concentration, then the [OH] in Equation (1) is

constant and may be combined with Kt to give a constant, K,

where & = Kl [OH] (2)

Integration of Equaion (1) yields:

h L 0 - l n L = K o t (3)

According to Equation 3, a plot of In (L) versus digestion t he , t, shouid result in a straight

line of dope -Ko, and intercept In &). L is derived fiom the value obtained by extrapolation of thk

linear portion to zero digestion the , and is not e q d to the lignin content of the wood. is referred

to as Cceffective initial lignin content" of wood. The difference between the acnid Ligriin content and Lo

may be thought of as rapidly removed lignin, and this occurs in the initiai extraction stage. Therefore,

a plot of residual lignin versus time gives a straight line with the dope representing the value of Ko

(sec"). The activation energy of the pulping reaction can be detennined by using the Arrhenius

equation

Where A = the Arrhenius constant

&= the activation energy (kJ/moIe)

R = the gas constant (8 -3 14 klPK.mole)

T = the absolute temperature e()

Equation 4 a n be rewritkn as

Thus if the change in the rate of delignincation with increase in temperature durhg pulping takes

place in the mariner predicted by the activation theory of Arrhenius, a straight line relationship shodd

result on plonig In &) against lm with a slope = EJR

Laroque and Maass [36] have found a value for activation energy of 134 U/g-mole for the

soda delimiificaion of spruce. Kieinert [41] found the value of 135 Wg-mole for the laaft bulk

deIignification of spnice, and about two-thirds of this value for the residual delisnifidon. He also

showed that the lignin remaining at the transition point fiom bulk to residual delignification increased

with lower cookig temperatures Wilder and Daleski [42] found an activation energy of 143 W g -

mole for the soda delignification of loblolly pine. WiIson and Proctor [43] reported an activation

energy of 130 Wg-mole for soda delignification of western hemlock. For organosolv pulping of

EucaZyptus globulus the activation energy of 109 H/g-mole for deIi@cation has been reportai by

Curvelo et al. [44]. For bagasse, Curvelo et al. [45] have reported an activation energy of 102 W/g-

mole during -01-water delignification. A more complete review of kinetics of delignifidon can

be found in the literature [35].

1.4.3 Influence of fibre morphology, and chernical composition on the handsheet strength properties

Softwood puIp fibres are long and cylindric.. They vary in length and diameter, and also in

ce11 wall thickness. Hardwood fibres are shorter than soffwood fibres with tapering ends, and are

accompanied by parenchyma cells and vessel elernents. Similarly, non-wood fibres Vary in Imgth and

diameter depending upon the stem pmon they are derived fiom i.e. the outer bark or the inner core.

Generally they contain a lot of parenchyma cells, epiderrnal cells, and vessel elements, which account

for their higher percentage of fines. The non-woods generally have lower lignin, comparable cellulose

and hemiceUulose, higher ash, higher extractives, and higher silica than the woods. Thus any

relationship between fibre dimensions, chernical wmpostion, and paper properties that holds good for

sofhvoods may not hold so for hardw& or for non-woods.

Since it has been recognised for many years that some wood fibres are better than others for

paperniaking, it is not surprishg that there have been many atternpts to determine which fibre

characteristics influence various paper properties. The usual experimental approach in earlier studies

was to hoId al l fibre chacteristics constant, except one, which was allowed to vary in order to

detennine its influence on the quality of the paper [46]. Another more recent approach has been to

d o w several characteristics to vary sirnuitanmusly, and employ multiple regression d y s i s to

detennine the quantitative contribution to paper properties of each characteristic [4q. At the same

tirne it is not correct to compare results obtained h m Mirent species, like hardwciods b softwoods,

which has often been done. In other words the papermaking potmial of pulp fibres, theu behaviour in

beating, web formation and dry;ng, are to a great extent dderznined by the raw material used [48].

For non-woods in general such comprehensive studies relating the paper strength propedes to the

morphological and chernical pulp properties are lacking. For kenaf fibre TouPnsky et al. [19] have

applied statistical techniques to develop a general quadfatic mode1 relating the strength properties to

active alkali charged and the amking tirne. Another study on kenaf fibres by Clark et al. [17] uses

analysis of variance (ANOVA) to relate the effects of type of pulping chemicals, and the levei of

appled chexnicals on the paper strength properties. There is no comprehensive study on kenaf fibres

relating the strength properties of paper to the morphological characteristics and the chemical pulp

properties. Thus the iiterature reviewed here would be based on the work done for the w d fibres.

Important paper strengtb properties would be reviewed in the context of the chemical and

morphological M o r s affecting them.

1.4.3.1 Breaking length and Burst

Two of the principal properties of paper are the tensile (breakulg length) and burst strengths.

Although these parameters are not identical, they are fkquently considered together, rnainly for

convenience [49]. In wmmon with most of the other strength properties of paper it was generaily

assumed that the tende strength and burst strength of paper were deterrnined primaniy by the fibre

length (+)'. A minimum length is required for intefibre boncihg and when two pulps have very

different fibre lengths, such as a hardwood and a softwood, paper made nom the longer fibre pulps

d l be stronger in aii respectr. When the fibre leagth diffrence is d e r , such as one might h d

betwee~l a beaten ami an unbeaten pulp, then the tende straigth will not be greatly different [46].

Whm wood fibres replaced rags and cotton it became ewident that it was no longer possible to predict

papa strength with fibre length alone. In 1916, Cross and Bevan [68], M e stahg the prime

importance of fibre length in determinhg the breakhg length and bu% were amoag the first workers

to appreciate that d e r factors were involveci. They recornmended that the fctors relaing to the

number and nature of contacts behveen ttZp fibres should a h be consider&. Flzrthemmrs, t5e effect

ofdensity of wood, ceil wall thickness, fibre diameter, lumen diameter, cell wall area, cross-sectional

area, and their different ratios were also considered. Dinwoodie 1491 used aunual rings fiom a 26

year-old and a 32 yeardd Sitka spruce tree in the Grst phase, and in the second phase 16 trees

coverhg 14 sofhvood species. Identicai sulphate cooking conditions were used throughout the

investigation, and al1 pulps were prepared at constant cooking conditions. He found thai the principal

fctor deterniining breaking Iength was fibre density (-). This, of course, detennines the flexibility and

degree of collapse of the fibre, both of which control the degree of codormability within the paper

shed and, as such, the size and nwnber of inter-fibre bonds. The Imgth of fibre is aIso important

though at lower level of simiificance (+). In loblolly pine, Barefoot et al. [50] have indicated that fibre

length is important below 4 mm, thereby confirniing the views of Dadswell and Watson [51j for

radiata pine. The eEect of fibre length on breaking length has been ascribed to stress dissipation; the

longer the fibre (up to some critical level) the greater the area over which the stress is dissipated [49,

521. Einsphar f53], studying the correlations of fibre dimensions with fibre and handsheet strength

properties for pine, concluded that fibre length was significantly correlated with bursting strength and

tensile strena. Paavilainen [48] studied the effect of morphoiogical, fine structural, and chernical

' Sign indicates whether the relationship is direct (+) or inverse (-).

properties of wood fibres on pulp fibre properties, and cm fibre network properties. Unbleached, and

bleached industrial pine and spruce sulphae pulps were cooked to Kappa 30, then fractionated with a

h u e r 3 inch hydroclone, using three stage treanent into four different ceU d thichess Iewels. She

concluded t h . the ?ensile strength incmed with the increase in c d wall thickness, and the beang

energy requlled to reach a certain m i l e strength level increased linearly with ceIl wall thickness.

Paavilainen woricing with the same type of fibres in another study [54] concluded that tensile strength

of an unbeaten s o A w d paper sheet was primarily determined by wet fibre flexibility (+). Tensile

strength increased linearly with increasing wet fibre f l e x i b w Le. with increasing bonded area.

However, for the beaten fibres, thicker walled fibres gave the highest tende strength, and the thin

walled the lowest. Dinwoodie [63] in his literature review on the felationship betwee~l fibre

morphology and paper properties states that at least 40% of the fibres are broken in the tende Mure

of unbeaen sheets, and this percentage increased considerably with beating. Thus fibre strength is

dso an important -or, and the influence of fibre strmgth was observeci by various authors [47,64 -

663. Some studies 163,641 found fibre strength to be simiificant ody in beaten pulp.

In practice the papermaking gotential of fibres is not utilised in full; part of the potential is

Iost in pulping. The carbohydrate fraction of wood fibres consists mainly of cellulose and

hemicelluloses . Increasing hemiceIluloses percentage, especially in hardwood pdps [63, 671, results

in an increase in breaking length. Lignin, however, has an adverse e f f i (-) 1631. In chernical pulping

of tropical hardwoods the chernid factors are unimportant [63], while [67, 701 there s e e ~ to be no

evidence fiom eucalyptus and pinus radiata pulps of an optimum lignin and hemicellulose content for

the development of maximum strength. The major influence of pulping processes and process

conditions on cellulose is to decrease the average polymer chain length, which does not seem to affect

the strength properties much until a particular, critical value is reached, after which the strength ofthe

fibres decreases rapidly. Much interest has been focussed on hemiceilulose, the carbohydrate W o n

that is most readily attacked by pdping agents, and at the same t h e is Wrely to play an important

d e in interfibre bonduig and fibre sweIling. It is beIieved that the more accessible hemiceUulose~

&bute greatly to the beathg response of a pulp. The haniaiidoses content n pulps varies greatly

among pdping processes, and also withm pulping processes when oonditions are varied. In general it

can be said tbat hemicelluloses improve interfibre bondhg and those strength properties tbat rnainly

depend on t, such as tensiie and burst strength [4q. A decrease in effective alkali charge has been

found to increase the tensile strength and yield, but to reduce the delignification rate [SI. Paavilainen

[56], in her study of nilphate cooking parameters on the papamakhg potentid of pulp fibres, points

out that the beating energy needed to reach a certain tensile strength level depends on the effective

alkali charge. In this study pine chips were cooked in laboraory at two different sulphidity levels

(30% and 40%), and at three Werent & i v e alkaii charges (18% 21%* and 24% NaOH on wood).

She showed that if the effective aikali charge is increased fiom 18% to 24% NaOH on wood, the

beating t h e required to achieve a tensile index of 80 N.dg will almost double because high

hemicellulose content improves swelling. Leopold and McIntosh [57l measufed the breakuig stress of

loblolly pine summemood and spruigwood holoceiiulose fibres afler caustic extractions of increasing

severity. The yield range based on wood was h m 49% to 72%. Fibre strength feli with incL-ing

alpha cellulose content. They concludeci that hemicelluloses are important for the interna1 cohesion of

the cell wall, and thus their removai weakem the fibre by reducing the interfibrillar adhesion.

Spiegelberg [58] measured breaking stress of long leaf pine holocellulose fibres, caustic extracted to

various extentts. The strength fell with increasing alpha cellulose content. Spiegelberg deduced that

since the cellulosic fibrils are not degraded by d o n , the Ioss in strength is caused by the poor

stress distribution in tbe fibre as inflexible fibril-fibril bonds replace the flexible fibril-hemicellulose-

fibril bonds. Leopold and Thorpe [59] measured the breaking load per fibre in the yield range of 48 - 84% fiom Nonvay spmce pulped by the kraft, bisulphite, and acid sulphite processes. They found

that in ail processes, the fibre strength dropped unifonnly with yield. There is not a perfect agreement

between these studies because of the Mssing fibril angle data, since for higher fibril angles, fidure is

initiated by shear in the matrBr rather than the Mure of the fibriis as pointed by Page et ai. [W.

Page, whiie shidying the stnmgth and chernical composition of wood puip fibres [6 11, concludes that

for low fibril aagle pulps produced by a pulping process that das not degrade the cellulose, fibre

seaigth is directly proportionai (+) to cellulose content up to a value of 80% cellulose. B e y d this

value, fibre strength is reduced, apparently because of the loss of the stress equalising matrix.

There is a lot of berahire about the proposed models for breaking length and burst. Page [62]

gave an equation for the teasile strength of paper in ternis of a few basic fibre and paper properties.

The equation expresses the tende strength as a hc t ion of zero-span strength, average fibre length,

fibre crosssectional are* fibre perimeter, shear strength per unit area, and relative bonded area. in

order to fiditate the use of the formula and avoid the tedious measurernent it entails, especially those

of the perirneter of fibres, the Page equation was maiifid by Clark [71] to include all the five

fimdamental properties, i-e. zero-span strength, fibre cohesiveness, weighted average fibre length,

coarseness, and wet compactability. A similar mode1 for burst factor is given by Clark [71J. The

equation given by Clark contains a b-g load constant for individuai paper grades. This rnay be

acceptable for a few grades, but when a large range of paper grades are de& with, obvious problems

arise. Dinwoodie [49] developed statisticd equations b a d on multiple regression analysis. He has

divided his analysis for unbeaten pulp, and beaten pulp to 400 ml CSF at a =a Kappa. It is

expected that developing just one set of equations over the entire range of cooking and beating

conditions would help in explahhg b e r the overall variations in the strength property (adjusted R*).

Furttier, the introduction of some other important fibre properties such as zero-span strength would

irnprove the models. RecentIy Lee [47l has built regression models using a wide range of woods, and

a spectnim of pulping processes as stone ground wax!, therrno-mechanicai, ultra-high-yield sulphite,

kraft, Alcell, and Cellulon. In multipIe regression analysis he has considerd CSF as an independent

variable; however, this parameter is a more usehl vanable when cornparison is made within the same

type of pulping process and species.

1.4.3-2 Tear strength

The tearing resistance of the pulp sheet has beai related in the past to most of the anatnmical

and chernical properbes of the fibres. As with breaking lengh and burst, the early belief was that

tear was controlled by the length of the fibre (+), and r e d t s to support this earIy view have been

recorded in many investigations [46, 65, 72 - 741. The variations in the dependence of te- resistance on fibre length with bonding can be explaineci in ternis of the tearing mechankm [75, 761.

In a weakly bonded sheet, since more fibres pull out than break in the tear zone, the tearing

resisbnce is controlled more by the nurnber of bonds that break dong the length of the fibres. Thus,

the tearing resistance depends strongly on the fibre length. On the other hand, in a weil bonded sheet,

more fibres break than pull out in the tear zone. Consequently the tearing resistance is controlled

more by fibre breakage than by bond breakage. Hence, provided tbat the fibres are weIl anchored,

length itself will have litle effect on their breakage, and the tearing resistance becornes less

dependent on fibre length. Anather important fibre characteristic is the thickness of the cell d. It is

difficult to obtain accurate measurement of ceil wall tbickness, and s other approaches have been

taken. Recefltly T ' i j a and Kauppinen [80] have reported cell d l thichess data measured with a

new automatic fibre analyser developed by Kajaani, which is less curnbersome tban the traditional

microscopic approach. They have obtained excellent repeatability in their cell wa thichess

measurements, with an average coefficient of variation (CV%) of 1.06%. In many species of wood,

there is a cl- differentiation between earlywood and Iatewood in each annual growth ring. Since

ea r lywd and latewood fibres difEer d y in ce11 wall thickness, they provide a means for studying

the influence of that characteristic on paper properties. Generally earlywood with thin ceIl wall

fibres gives lower tear than l a t e w d with thick ceIl wall fibres. Watson and Dadswell [78] have

related the variation in tear to ce11 wall thickness or the percentage of summerwood content (+).

Dinwoodie 1491, in his study of softwood puIps, found that the principal vanable affecting tear was

the ratio of wall t f c b s to fibre diameter (+), closely foilowed in signifcance by fibre Iength (+).

faavilainen [48] designed experiments to create four cWErent ceU wail thicbess leveis

correspondhg to summerwood content of 5%, 20-30% 40%, 70%. Her results show that the tear

strength of Merent unkaten sofhvood fibres decreases lineariy with higher ceIl wall thichess. With

beaten fibres the trend was opposite; the thick walled fibres gave better strength. Paavilaiaen in

a n d e r article [77l points out hat the fibre Iength is a more important factor for tear than tensiIe

strength. Dadswell and Watson [SI], reviewing their results for both soAwOOdS and h d w &

conclude that fibre Iength is the most important singie variable. They concluded that ce11 wall

thickness is of secondary importance in long fibre pulps, while in short fibre pulps, it is reMve1y

unimportant. Amther fibre characteristics that contributes towards explainhg the variations in tear

strength is intrinsic fibre strength, and a number of researchers have presented results indicating a

direct relationship between the fibre strength and paper strength of both unbeatetl and beaten pulps

[do, 791, Seth and Page [76] have show that sheets made fiom fibres that are Iong, straight, and

adequately bonded, no matter how bonding is changed, the tear index is approWnateIy proportional

to the square of the zero-span tende strength. however, the fibres in the sheet are not straight, but

are curled, kiked, crimped, and microcompressed, the dependence of tear index on zero-span tensile

strength is stronger. In sheets made fiom shorter fibres the dependerice of tear on zero-span tensile

strength is weaker.

The members of the Institute of Paper Chemistry [75], under the leadership of Van den

Akker, presented a theory for the mechanism of tear. Van den Akker explaineci that, the initial rise in

the tearing strengthheating-tirne curve is due to the fact that in the initial stages of beating, the

fictional cira. work increases by v h e of tighter enmeshment caused by slightly increased bonding.

During this time ody a negligible number of fibres fail in tensile rupture. As the beating continues,

however more fibres fil in tensile rupture and thedore, fbver fibres are pulled intact fiom the

mesh. Since the fictional drag force per fibre is much greater tban the rupture work, this decrease in

the number of fibres pulled intact fiom the mesh causes the tear strength t decrease- Dadswell and

Watson [51, 811 have illmtrated dinrences in the inter-rekionship of fibre length and bondiug

between hardwood and softwood puIp. The investigators have i n c i i d the prime importance of

fibre length, and have confirmed the existence of a critical level of bonding. In short fibre pulp this

critical level of bonding is never atained in unbeaten pulp, and only rarely in beaten pulp, so that the

tear fctor increases with beating. The disnliution of stress is Iess widespread than with long nbre

pulp, and the energy required to pull out the unbroken fibres is relatively low, thereby giving rise to a

low tear value. In long fibre pdp the work done in pulling out long fibres is high whic results in

hi& tear strength. In long fibre pulp the degree of bonding before any beating is generally above tbis

critical level, and any increase in bonding wili result in a reduction of the area of stress

concentration, thereby reducing the amount of energy required to rupture the paper. Gailay [52] has

explained the decrease in tear with beating in t e m of stress concentration or localisation of the

applied I d .

Tear is appmt ly also influenced by the chernical nature of the fibres. Tearing strength

generally decreases with an increase in hemicelluloses content and, in some cases, it is desirable to

remove some of the hemicelluloses to obtain paper types where high tear, sheet softness, and opacity

are essential [46& Jayme 1691 has indicated an optimum level of hemicelluloses content. Paavilainen

1561 in her study of the effct of sulphate cooking parameters on the papennaking potential of pulp

fibres shows that an increase in the e f f d v e alkali charge improves tear strength more than a

sulphidity increase. The beating time needed to reach a given tende strength increases with

increasing e f f d v e alkali charge in mking, while the tear strength decreases accordingiy as

bonding increases. For maximal tear strength, the fibres need to have certain optimal bonding

pote&d, wbich can be achievd by uicreasing bonding untif the fibres are so strongly bonded tbat

they break instead of being pulied out. if the unbeaten fibres possess a high bondhg ability, as do

fibres d e d with low &&ive alkali charge, an increase in beating merely reduces tear strength.

Many mathematical models have beai developed for the tear stnqth. Page and MacLeod

[4q related work of tear to zero-pan breaking length and tende s t r e ~ g t h Ih this mudel one paper

property (tear) ih; predicted by another papa property (tensile strength). Paper properties are

generalIy infTumced by the interaction of rnany fibre properties. Therefore, it becornes difEcult to

detennine which of the fibre properties are significady related to the predicted paper property.

Fundamental fibre properties are more meaningfbl in estunatuig papa properties. Clark [7 11 relaies

tear fctor to zero-span breaking leugth, cohesiveness, average fibre length, fibre coarsetless, and the

bulk of the sheet. In this mode1 there is the probfem of measuring cohesiveness or bond strength. Lee

[4q developed multiple regression equation for tear and showed that the tear was related to

coarseness, zero-span breaking length, fibre length, and fines turbidiy. The first two describe fibre

strength, while the last two iIlustrate the bonding strength of the fibre network. The tear was also

related to CSF. This work was carried for a very wide range of fibres using pulping processes fiom

mechanid to chernical, so the inclusion of CSF as an independent variable is not expected to give

much inorrnation.

1.4.3.3 Tende energy absorption (TEA)

TEA is a measure of paper toughness. It is the work required to break a test sarnple under

tension, and is equai to the area under the stress-strain (loadelongation) curve. It is roughly

proportional to the product of the tensile and stretch values at rupture. Only one paper has been

found [471, which deals with the relationship between TEA and pulp fibre properties. Since TEA is

meanired during the same tende test as breaking length, it is r e a s o ~ b i e to assume tbat sirnilar

reiatiomhips between TEA and pulp fibre properties would ex&. This would mean that TEA is

positiveiy relatpA to bonding poteniid and fibre strength. The regression equation developed by Lee

1471 showed that TEA is a fkction of zrospan strengtb, fibre density' CSF, turbidity, and number

of fibres per gram. Lee's study was conducted over a wide range of wood fibres and umsidered oniy

morphologid h r s .

1.4.3.4 Stretch

Strerch is defined as the ability of paper to eloagate. It is measured in cmjuoction with

teasile streagth. In the literature very little information is avaiiable regardmg the properties that rnay

influence the eiongation of the paper prior to rupture. Watson and Dadsweil[82] in their study of the

influence of fibre morphology on papa properties conchdeci that the micellar spiral angle of the

fibres has a positive correlation with the stretch of paper. The stretch of paper bas also k e n relaed

to the straightening of curled fibres during the tende test 1471. The higher the fibre curl, the bigher is

the saetch of the wet web [83, 841. Although fibre curl tends to decrease the temile strength of the

web, the increase in stretch o h more than compensates and thus the toughness of the web is

increased. The presence of curl controls wet-web stretch at low solids, around 20%' because fibres

can slide upon one another and straighten when the web is strauied. At hi& solids, above 40%' since

the fibre-fibre contacts are strong, the relative movernent of fibres is prohibited and gross curl cannot

be removed. The extensibility of the stieet hen depends on the extensibiliy of the fibre betwm the

fibre-fibre contacts, which in turn depends on the presence of crimps and micro-cumpressions 1851.

The extent to which the potentid stretch in fibres due to curl and micro-compressions can be utilised

depends on the de- of fibre-fibre interaction in the web, and this in tum depends on &ors such

as fibre flexibility, fibrillation, and fines. Both curl and microcompression influence the stress-strain

curve of dry paper. Fibres without microcompressions have a steep, almot linear elastic response,

whereas microcompressed fibres are more extensible, and show a yield point foliowed by

appreciable plastic defomtion [85, 861. It follows that highly microcompressed fibres produce

sheets of high extensibility, and the modem methods of manufacturing extensible papers exploit this

effct. Dinwoodie while studying the influence of anatomid and chemicai characteristics of

softwood fibres on the properties of sulphate pulps points out that the important variables

d-nnining the amount of elongaion at rupture are length (l) or the ratio of length to diameter (yd)

(+). An increase in 1 at nxed d will give a more open network of fibres, which c m be distortai more

readily in one diredon- For a given value of 1, an increase in d d l increase the area of bonding,

thereby reducing the amount of bond shearing or fibre rotation during extension and giving rise to a

lower stretch value. The direct relationship between stretch and fibre length is cuntrary to the

findings of Watson and Dadsweli [82], who in their work with pine reported hi& stretch values with

large micellar angle, and miceilar angle is iaversely related to fibre length. L e [4n in the regression

equation for stntch bas related stretch to length, wet fibre flexibility, fibre density, and fines. He has

point& out that fibre length has a log fiindon relationship with stretch. Length is especially

important for stretch in short fibres, but once a critical length is reached it no longer affects -ch

to the same degree. Fines content improves stretch sharply d l approximately 5% but thereafter

increase in fines content bas no effect on stretch.

1.4.3.5 Stiffness

StBhess is an extremely important property for many uses of paper like file folder, index

cards, playing cards, posters, etc. It also deals with the ability of paper or paper board to resist

deformation under stress. Tende stifiess is one of the several meth& use- for the measurernent of

sMhess. The fctors affecting the stifiess of paper and paperboard are thicbess, Young's modulus

of pulp, restraint during dxying, moisture, surface treatments, density, grammage, fibre bonding, and

fibre orientation [46]. There is not much published information on the morphological and chexnical

h t o r s affecting !33Eless.

1.5 Purpose of the Prescnt Study

The literature for the kenaf fibres has no information on the trace elements present in the two

M o n s bark, and core. When present in chemical pulp, transition metal ions, such as iron,

rnanganese and copper, increase the decomposition rate of hydrogen peroxide in peroxide bleachmg

operation. They prornote the rate of oxidation of cellulose in oxidative bleaching yie1diag lower pdp

viscosity. Chelating agents are therefore required to conml the metal ion codent in modern

bleaching proasses. The envimumental &ects of these chelatuig &ects are unclear, and it is

desirable to minimise the use of such substances. Although in this research we are not concentraing

on the bleaching of pulps, but a basic trace eIement data wouId help in a b r characterisation of

kenaf fibres, and in M e r research towards the improvemeent of bleaching practices for these pulps.

From the pulping trials discussed in the literature it is clear that the respoIlSe to pulping

conditions is different for the bast, and the core fibrs. A complete study of the detimiification

kinetics including the cdculation of activation energy would ftlly quantifi. the delignifcation

pattern, and help in creating a better understanding of the Lignin rernoval d u ~ g the pulping.

The global fibre scenario discussed earlier, alongwith the ftwe projections point out that

the use of non-woods as fibre source for papermaking wodd increase globally. A comprehensive

study inkgrathg the knowledge of fibre morphology, fibre chemistry, and mechanical strength of

papa blended with modem statistical concepts wiil provide an initial exploratory tool airned towards

creating a better understanding of the fibre characteristics in k e d . The n o n - w d fibre utilised in

this study is kenaf The work in the literature relates fibre morphology and paper properties on one

hand and the infiuence of cooking conditions on paper properties on the other. AIso the effect of raw

materiai or process conditions on single fibre properties is evaluated. In hese studies the

morphologid variations were achieved by using either diffrent w d species or by amking

different parts of the trunk separatety. The evaluations are based on the regression anaiysis of the

experimental data or on the generally accepted hypohesis about the &ct of morphologid

properties on papa properties. However, the interaction between wood fibre and paper properties

have spanngly been studied sys&r~&cally through pdp fibre and network properties except in the

latest studies 148, 87, 881. Most of the studies do not explain the order and extent of the inauence of

wood fibre properties, and chernical properties on the paper strength properties. The intefactions

between chemical and physical wood fibre properties and paper properties or the &ect of process

conditions on the papemaking potential have not been studied systematically on the basis of pulp

fibre and network properties. In the case of kenaf fibres the paper strength properties of the bast and

the a r e fibres are significantly different. Thus in this research the influence of morphological, and

chemical factors on the papermaking potential of scxia pulp fibres of the bast, and the core fktions

in the kenaf stem is studied by characterising the paperrnaking potential of pulps in terms of pulp

fibre, and nenivork properties. The morphological and chemicai changes are brought in the fibres by

cooking, and beating operatiom.

1.6 Objectives

The goal of this research was to provide an explmation for the observed ciifferences in paper

properties of dinerent kenaffk~ctions in both chemicai, and morphological t e m . To achieve this the

foilowing objectives were proposed:

1. To chemiedy (organic and inorganic) and morphologicaiiy characterise the bast, and the

core fiactions in k d .

2. To study the delignification kinetics of soda pulpiog of kenaf bast and the core fibres in

order to understand the pulpng characteristics of the fibres.

3. To cook the two fractions of kenaf within a wide range of residual Lignin contents, and to

germate beaijng m e s for the bast and the corn fibres by beaing the puip in PFI miil.

4. To meanire the hdamental morphological, chwical properties on the pdp fibres, and

Merent stmqh properties of handsheets including those thai have not been studied

extensively, with the most accurate methods available.

5. To use the correlation analysis and the mulhiple linear regression analysis to deconvolute the

large data set to produce meanin@ rnodels, and to interpret the resuits in the Light of the

conventional scientSc wisdom, and knowledge

1.7 Significance

The present study is expected to be the most comprehensive study on kenaf fibre to-date.

The multiple regression statistical analysis would provide an initial exploratory tool for the

understanding of strength properties behaviour d u ~ g the process of pulping and papermaking. It

would also indicate the possibihty of manipulation of pulp fibres by varying the cooking and the

beating conditions to obtain the desired paper strength properties. The concept of elasticity used in

this thesis, to explain the relative rate of change in dependent variable with respect to a change in the

independent variable, serves as a usefbl tml for understandhg the relative contribution of different

independent variables to explain the strength properties of paper. The delignincation study would

help in understanding the basic pulping botdaries for this raw material. Finally the

conceptualisation of the integrated knowledge gaineci by the extensive data generated on the

chernical, and morphoIogical properties will help towards explainhg the variations in the strength

properties.

CHAPTER 2

MATERIALS AND EXPERIMENTAL METHODS

2.1 Raw Materid

The raw materiai used in ths study was separated kenaf bast and core fibres. The

separaim technique developed at Mississippi State University was u&. The seprator consistai

of a conveyor bel& cnisher roilers, blade rollers, a sloping vibrahg screen, and a series of vibrating

combs. A whole stalk k e d decorticator was also used. 'Zhe decorticaing machine consists of a

series of cnisher and beater rollers tbat crush the stock while separating the core fiom the bast fibre

111 O]. Separated kenaf bast and core fibres were procured fkom Mississippi State University,

U.S.A. The whole supply of raw material was taken fiom one harvest, and it was ensureci within

practical Limits to minimise the variations in the raw material supply. The raw material was air

dried and then stored in bags for fiiture use.

2.2 Pulping

2.2.1 Micro-pulping

Kenaf meal was prepared fiom the bast and the core fibres by milling air dried samples in a

Wiley mil1 until they passed through a 40-mesh screen giving particle size of approximately 420

pn. The samples for the chemicai analysis and pulping were prepared as per Tappi standards (T

257 cm - 85). Kenaf meal was used instead of chips to ensure that the ratedetermining step in

pulping was not the m a s transfer of chernicals into the raw material or the reaction products out of

the raw material. This will reduce the effect of differences in the delignification rates caused by

such hctors as ciifFerences in the rates of accessibility of liquor to various morphological regions

[89]. It is reported [go] thar generally, raw materials having higher specific gravity take longer time

to reach a ceriain yield level as opposed to material with a lower specific gravity. The bast and the

core M o n in kenaf have different specific gravities; it would be expected that grinding the

sample to powder reduces the ciifference in the delignincation rates caused by the specific gravity

differences,

Extractive fiee (EF) samples for bark, and wre were prepared by e>bractig with ethanol:

benzene (1 :2 by volume) following Tappi standard (T64 om - 82). Extraction eliminates the &kt of extractives on pulping. Soda cooking liquor of 32 gpl NaOH with active alkali charge of 15% as

NaD was us&. The concentration of NaOH was checked and adjusted accordingly by Mrating as

follows:

1 0 4 sol-ution of NaOH was pipetted into a conical flask and diluted with distilfed water. Two

drops of phenolphthalein were added and the solution titrateci agaimt standard HCl (IN).

Consurnption of HCl was noted (a). A few drops of methyl orange were added and titraiion

continuai till the red colour appears. The h a l burette reading was also noted (b), and the

concentration of NaOH as Nafl (g/l) = (2a - b) x 3 1/10.

The EF ground meal samples were placed in a staidess steel reactor (25 ml) with screw

caps, and soda cooking liquor was added to maintain the raw material to liquor ratio of 1:6. The

charges of bast were higher compareci to core (2.50 g vs. 1.77 g on oven dry basis) because of the

ciifference in buk densities. A pre-soaking period of 30 min. at rmm temperature was provideci

before irnmersing the reactors in a silicone oil bah, thermostatically controlled at desired

temperature. The experimental set up is shown in Fig.3. The range of temperatures studied was

140C to 170C. The time to reach the reaction temperature is the time required to regain the

temperatures after the reactors are immersed in oil bath, and it varied with temperature (25 min- at

170C, 20 min. at 15SC, and 15 min. at 140C). The reaction was stopped by quenching the reactor

in a bath of m i n g cold water. Before dipping the reactors in cold water, they were cleaned in a

cooled kerosene bath that removed the silicone oil fiom the surfce of the reactors, thereby

elirninating the possibility of contamination by these substances. The reaction time reported is the

time a. temperature, and it does not take into account the time taken to reacb the temperature. The

resulting pulp with liquor was transferred to a Buchner &el, and washed with enough distilled

water to tramfer the pulp with liquor fiom the reacton. The pu$ was washed with water, and yield

and residual iignin content of the pulp were determined. The reported residual lignin content is the

sum of the Klason lignin and the acid soluble lignin content determined as per Tappi standard T

222 om 83 and Tappi UM 250. Holocellulose and alphacellulose were determined as per procedure

described by Zobel et al. [go]. Tappi T 2 1 1 om-85 was followed for ash content determination.

2.2.2 Puiping for handsheet making

The micro pulping expenments are unsuitable for handsheet making. For making pulps

suitable for handsheets an MIK. System digester was used. The experimental set up is shown in

Fig.4. The MIK Systems, Inc. computensed laboratory digester consists of a pulping system, which

is made up of the following components :

1. The pressure vesse1 which is the digester proper.

2. The liquor circulating pump which takes liquor f?om the bottom of the digester and discharges it

back at the top.

3. The heat exchanger, consisting of two 1300-watt elements.

4. The pressure gauge

The rate of heating the liquor can be reguiated fiam 4O0C/hr to 30O0C/hr. The rate of

heahg during ail the experments was kept constant at 150C/hr. The objective of the cooking

experiments was to make cooks over a wide range of residual lignin content values. The couking

conditions maintaineci for the bas& and the core fibres are aven in Table 1.

1 1. 1 Soaking time for the raw rnatenaf at room 1 30 min. S.NO.

I

2. Raw material to liquor ratio 1:6

Process Conditions

3.

4.

5.

Table 1. Cooking conditions for the bast, and core fibres

Rate of heating (or time to reach cooking temperature) T h e at temperature'

6.

COOks at times 60 min and less gave pulps that were dinicult to process M e r , due to too much rejects. 140C temperature gave uncooked puip.

1 50C/hr.

90 min., 120 min., 150 min., 180 min.,

Temperature of cooking3 and 210 min.

155C and 170C 1

Cooking Iiquor charge (as NazO) Bark - 15% Core - 15% 18%

Fig.4. M/K Systems, Inc. computerised laboratory digester

After cooking the pulps were washed thoroughly, and then screened in slotted screens. The

screened pulp was kept in the cold room for frther analysis and processing. In al1 twenty difTerent

pulps for bast fibres and core fibres were prepared. The residual lignin content values for al1 the

cooks are given in Appendix 1. Out of the twenty pulps five each were selected for bast, and cores

fibres to give a range of residual lignin content values. The selected pulps are marked in Appendix

1, and are given in Table 2 and Table 3 with denotations that would be used Iater on in this thesis.

Kappa nurnber was caIculated using the foliowing relationship (Tappi Standard T 236 cm - 85).

Klason Lignin = 0.15 x Kappa Nurnber

TabIe 2. Pulps selected for further processing of bast fibres

1 hilping conditions

KappaNo. Denotations

155C 150 min. 15%

31 Ab

155C 180 min. 15%

28 Bb

170C 90 min. 15%

18 Cb

170C 150 min. 15%

15 Db

170C 180 min. 15%

21 Eb

Table 3. Pulps seIected for further processing of core fibres

Pubhg conditions

KappaNo. Denatations

In the case of bast fibres the pulps with residuai lignin greaer than 6.2% had too much

rejects and it was not possible to make good handsheets for physical testing. In the case of core

fibres the 15SC cooks had a very high lignin content (> 15.8 %), and had too much ~ c o o k e d

material and it was not possible to make handsheets fiom them.

2.3 Beating

Beating of the screened pulp samples was done in a PFI mill, and Tappi standard T 248

cm-85 was followed. In the PFI, a measured arnount of pulp at specified concentration is beaten

between a roll with bars and a smooth wdled beater housing, both rotating in the sarne direction

but at Merent peripheral speeds. Beating action is achieved through the differential rotational

action and the application of a specified load between the beater roll and housing for a specified

nwnber of revolutions. The amount of pulp sample taken for one run was 30 g (0.D basis) at 10%

consistency. From the beaten sample 6 g was kept aside for fieeness determinaion, while the rest

was utilised for handsheet making, and for chemical and morphological analysis.

170C 90min,15%

52 Ac

2.4 Chernical Analysis

The chemical analysis carried out included Klason Iignin content, acid soluble lignin,

holocellulose and the alpha ce1Iulose. The reportai hemicelluloses content is obtained fiom the

holocellulose, and alpha cellulose content. Extractives, a&., and trace element adysis was done

for the raw material samples only. The procedures foliowed for the determination of extractives,

170C 150min.18%

21 Ec

170C 90min.18%

35 Dc

170C 150min.15%

44 Bc

170C 180rnin.15%

47 Cc

lignin, holoceliulose, alpha cellulose, and a& are the same as discussed earlier in the micro pulping

experiments. The trace element d y s i s was done using the instrumental neutron activation

analysis (INAA), a nonilestnicive technique, at the University of Toronto. Sarnples were put in a

srnaii polythene v i a and radiated with neutrons at LOkw for five minutes in the slowpoke reactor.

The atorns of each element in the sample absorb neutrons forrning new isotopes. These isotopes are

radioactive, and each has a characteristic energy. The radioactive emissions (gamma rays) of each

element were measured for five minutes a b a delay of 2 minutes. The intensity of the gamma

rays emitted by each sample for different isotopes were detected and recorded. Chernical standards

were aiso prepared for each elements. These standards were irradiated in the same way as the pulp

samples. The elemental concentration of each sample was detennined by cornparhg the counts of

each sample to the standard sample. Further details with the formulae used for the calculation of

the concentration of dflerent eIements can be found in the literaure [9 11.

2.5 Beating and Morphological Characteristics

2.5.1 Freeness

Freeness was measured using the Canadian Standard Freeness (CSF), following CPPA

Standard C. 1. The CSF test is designed to give a measure of the rate at which a dilute suspension of

pulp may be dewatered. The drainage rate, or fieeness, has b e n shown to be related to the surface

conditions and the swelling of the fibres, and is a usefiil index for the amount of mechanical

treatment given to the pulp. Standard testing conditions for the stock are consistency 0.3%, and

temperature 20C. Corrections for temperature and consistency were applied on the measured value

using the tables given in the CPPA Standard. The reportai CSF value is the correcteci CSF, and is a

very useful tool to compare pulps fiom the same raw material subjected to different process

conditions, using the same pulping technique. The fieeness test gives Iittle or no definite

information about fibre properties as the freeness test is based upon an empirical procedure, the

result of which depend upon the degree of fibrillation, the fines content, as well as the particular

conditions of pressure head, temperature, construction of tester, and d e r %rs. Freeness values

indicate how the stock will drain on