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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
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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?
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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
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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
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Changes in anatomy of untreated Scots pine
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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.
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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
6 Henrik Heräjärvi
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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.
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Results – anatomical changes
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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!
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Pilodyn indentation values
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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.
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Water absorption,
shown as relative mass
change
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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
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pH
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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.
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Chemistry
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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.
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FTIR spectra of 50 mm-thick Thermo D spruce
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Soaking time:
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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
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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
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Thank you!
The work will be continued…