effect of prolonged water soaking on structure and ... · © luonnonvarakeskus objectives •...

© Luonnonvarakeskus © Luonnonvarakeskus

Henrik Heräjärvi1), Chenyang Cai1), Jukka Antikainen1),

Katri Luostarinen2), Veikko Möttönen1)

1) Natural Resources Institute Finland

2) University of Eastern Finland

Effect of prolonged water soaking on

structure and chemistry of thermally

modified Scots pine and Norway

spruce wood

1

Study jointly sponsored by:

ECWM8, 26-27 October 2015, Helsinki, Finland

© Luonnonvarakeskus

Background

• Moisture facilitates thermal degradation: mass loss is higher in case

of heating wet wood than in case of heating dry wood (e.g., Seborg

et al. 1953, Stamm 1956)

– Practical application of this finding: ThermoWood® process, where wood

is dried to near zero MC before the actual thermal treatment

• What does moisture do to thermally modified wood during the

service life?

2 Henrik Heräjärvi


Objectives

• Changes in thermally modified pine and spruce (Thermo S and D)

wood during prolonged water exposure with regards to:

– Anatomical structure: reverse light microscopy

• 13 specimens

– Physical hardness: Pilodyn needle indentation tester

• 62 specimens

– Water absorption: mass change

• 62 specimens

– pH: standard laboratory pH measurement device – two different methods

• 40 specimens

– Surface chemistry changes from the pith side: FTIR spectrometer (ATR)

• 103 specimens

Henrik Heräjärvi 3


Material preparations

Material

• Thermally modified in a lab kiln (Mikkeli University of Applied Sciences)

• 2 species (Scots pine, Norway spruce), 2 treatments (Thermo S & Thermo D) + untreated references, 25 and 50 mm thicknesses

Preparations

• Sawing into dimensions, coding, weighing, surface treatment of a sub-sample using Valtti furniture and decking oil

Soaking in tap water



Changes in anatomy of untreated Scots pine


Degradation of cell structure of untreated Scots pine (50 x magnification). a. after 2 weeks

soaking; b. after 4 weeks soaking; c. after 4 weeks soaking and surface microtomed; d. after 6

weeks soaking; e. after 8 weeks soaking; f. after 12 weeks soaking, g. after 12 weeks soaking

and surface microtomed; h. after 20 weeks soaking.


Changes in heartwood anatomy (Scots pine, Thermo® D)

Degradation of cell structure of Thermo-D Scots pine heartwood (50 x magnification). a. dry

specimen; b. after 2 weeks soaking; c. after 4 weeks soaking; d. after 4 weeks soaking and

surface microtomed; e. after 6 weeks soaking; f. after 8 weeks soaking, g. after 12 weeks

soaking; h. after 12 weeks soaking and surface microtomed; i. after 20 weeks soaking



Changes in sapwood anatomy (Scots pine, Thermo® D)

Degradation of cell structure of Therno D Scots pine sapwood (50 x magnification). a. after 2

weeks soaking; b. after 4 weeks soaking; c. after 4 weeks soaking and surface microtomed; d.

after 6 weeks soaking; e. after 8 weeks soaking; f. after 12 weeks soaking, g. after 12 weeks

soaking and surface microtomed; h. after 20 weeks soaking.



Results – anatomical changes


Increased porosity of cell walls, esp. outer parts. Changes occurred

earlier and more severe in thermally modified wood than in untreated

specimens.

Pores were observed near to the middle lamella both in heartwood

and sapwood. The initial cracks caused by thermal treatment became

wider and deeper during the soaking period.

Sapwood degraded faster and more severely than heartwood.

Less hyphae growth was observed (no real measurements) on

untreated specimens compared to the thermally modified ones…?

NOTE: only 13 anatomy specimens in total => just indicative results!


Pilodyn indentation values


Pilodyn value for different treatments as a function of soaking time.

Scots pine Norway spruce

For dry wood,

Thermo S did not

differ from untreated

control, whereas

Thermo D was

slightly softer.

Wood softened as a function of soaking time, but

the change was very small.


Water absorption,

shown as relative mass

change


Water absorption of Scots pine specimens as a

function of soaking time

Water absorption of Norway spruce specimens as a

function of soaking time

Mass change rate in pine:

Thermo D < Thermo S < Control

Mass change rate in spruce:

Small differences, but

Thermo S > Thermo D and Control


pH


pH of specimens as a function of soaking time

Scots pine Norway spruce

Thermally modified specimens were more acidic than the controls.

Thermo S pine was surprisingly slightly more acidic than Thermo D pine.


Chemistry


FTIR spectra of thermally modified Scots pine and Norway spruce before soaking

Spectral intensities were converted to ATR units and baseline was manually corrected at 13 different

points. All relative intensity ratios were normalized relative to the peak of the band at 1369 cm-1.


FTIR spectra of 50 mm-thick Thermo D spruce


Soaking time:


Changes in chemistry as a result of water soaking

• Decreased intensity of carbonyl stretching (at 1729 cm-1) in

thermally modified specimens indicates further degradation of

remaining hemicelluloses (deacetylation of acetyl groups)

• Increased intensity of aromatic carbonyl stretching (1652 cm-1)

and decreased number of guaiacyl-ring features in lignin (1456

cm-1) represents the degradation of lignin and possible formation

of new lignin aromatic structures

• Hydrogen bond intensity decreases and hygroscopicity increases

in both thermally modified and untreated specimens



Summary

• Some (but not dramatic) strutural changes as a result of almost

half-year soaking

– Slight decrease in hardness

– Increased cracking and porosity of the cell walls of thermally

treated wood, and visible degradation of middle lamellae

• Some chemical changes

– Relative content of lignin may change as a result of acidic

hydrolysis

– Increase in hygroscopicity

– Further degradation of hemicelluloses




Thank you!

The work will be continued…

effect of prolonged water soaking on structure and ... · © luonnonvarakeskus objectives •...

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