Effect of prolonged water soaking on structure and chemistry of thermally modified Scots pine and Norway spruce wood

Transcription

1 ECWM8, October 2015, Helsinki, Finland Effect of prolonged water soaking on structure and chemistry of thermally modified Scots pine and Norway spruce wood Henrik Heräjärvi 1), Chenyang Cai 1), Jukka Antikainen 1), Katri Luostarinen 2), Veikko Möttönen 1) 1) Natural Resources Institute Finland 2) University of Eastern Finland Study jointly sponsored by: 1

2 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

3 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 3 Henrik Heräjärvi

4 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 subsample using Valtti furniture and decking oil Soaking in tap water 4 Henrik Heräjärvi

5 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. 5 Henrik Heräjärvi

6 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

7 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. 7 Henrik Heräjärvi

8 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! 8 Henrik Heräjärvi

9 Pilodyn indentation values 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. Pilodyn value for different treatments as a function of soaking time. Scots pine Norway spruce 9 Henrik Heräjärvi

10 Water absorption, shown as relative mass change Water absorption of Scots pine specimens as a function of soaking time Mass change rate in pine: Thermo D < Thermo S < Control Water absorption of Norway spruce specimens as a function of soaking time Mass change rate in spruce: Small differences, but Thermo S > Thermo D and Control 10 Henrik Heräjärvi

11 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. 11 Henrik Heräjärvi

12 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 Henrik Heräjärvi

13 FTIR spectra of 50 mm-thick Thermo D spruce Soaking time: 13 Henrik Heräjärvi

14 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 14 Henrik Heräjärvi

15 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 15 Henrik Heräjärvi

16 The work will be continued Thank you! 16 Henrik Heräjärvi