Alterations in wood ultrastructure induced by drying Miro Suchy,* Eero Kontturi, Tapani Vuorinen Department of Forest Products Technology School of Chemical Technology Aalto University Finland *) Current address: Metsä Tissue, Raubach, Germany
Outline Background motivation fiber cell wall alterations upon drying (chemical pulp / hornification) deuteration of cellulose / detection with IR spectroscopy Experimental design sampling / deuteration / controlled drying design Results method validation deuteration reversibility, repeatability, reproducibility drying of wood: impact of temperature and relative humidity drying comparison: wood and pulps water retention of pulps WRV and D 2 O / FT-IR correlation
Background Motivation and significance Drying-induced changes in native wood cell wall structure Fundamental study initial changes on ultrastructural level trees grow in water-swollen conditions drying of wood after felling unavoidable logistics or process requirement changes in physical properties of wood after drying are well-known Affects the novel utilization of cellulose from wood Biofuels production accessibility of cellulose acid and enzymatic treatments Cellulose nano-objects fabrication microfibril separation accessibility/aggregation of individual microfibrils
Background Approach Drying-induced changes in native wood cell wall structure Fundamental study initial changes on ultrastructural level Analytical techniques vs. sample preparation water removal or replacement with solvent alteration of native wood cell wall ultrastructure Deuteration of wood no sample preparation FT-IR analysis of deuterated samples minimal sample preparation
Background Pulp drying hornification Irreversible changes in wood pulp upon water removal Reduction of swelling ability feature of low yield (chemical) pulp structural changes internal fiber volume decrease collapse of pores between walls and closure of capillary voids hydrogen bonding / increased degree of cross-linking within cell wall tighter packing of cellulose chains aggregation of fibrils dry drying WATER CELLULOSE re-wetting wet wet Scallan & Tigerström; JPPS 1992 virgin fiber hornified fiber
PAS Response Background Experimental concept Deuterium exchange coupled with FT-IR spectroscopy Exposure of cellulose to D 2 O accessible OH groups readily exchanged: R-OH R-OD Deuterated Control 3600 3200 2800 2400 2000 1600 1200 800 Wavenumber cm -1
Background Experimental concept Deuterium exchange coupled with FT-IR spectroscopy Exposure of cellulose to D 2 O accessible OH groups readily exchanged: R-OH R-OD Photoacoustic (PAS) detection special sample cell absorbed IR radiation >> heat in sample >> partial release to gas at interface resulting pressure changes detected with sensitive microphone no sample preparation (requires dry sample*) measurement depth (10-100 µm)
Background Overall objective and approach Changes in wood ultrastructure during initial dehydration Deuteration of accessible OH groups originally present in fresh wood / fiber Monitoring their conversion to inaccessible OD during drying: Detection of inaccessible OD groups by FT-IR D 2 O H 2 O H / D ex. OH >> OD Controlled drying D / H ex. OD >> OH 22 20 18 16 14 12 10 8 FT- IR PAS 6 4 2 0 Analysis and correlation of pulp and wood testing results
Experimental Wood sampling and pulps freshly felled pine and spruce wood
Experimental Deuteration and controlled drying wood immersed in liquid D 2 O, pulps deuteration in plastic bags controlled D 2 O RH drying: saturated salt solutions for RH control desiccators placed in oven; time of drying: 7 days To vacuum / seal 25 º C 80 º C D 2 O vapor Sample NaCl Estimated RH, % 75 77 Estimated RH, % % 74 76 NaOH 6 7 ~7 Saturated solution
Experimental Design of experiments Fresh wood / pulp D 2 O treatment (60 min / 2 20 min) Drying: D 2 O RH 7% (75%) 25 C 60 C 80 C H 2 O flush (60 min / 2 20 min) 0% RH Drying: 40 C FT- IR PAS measurement
PAS Response Deuteration reversibility Extent of OD exchange and reprotonation fresh wood: pine deuteration of fresh wood fully reversible Fresh wood Deuteration OH > OD Reprotonation OD > OH 3600 3200 2800 2400 2000 1600 1200 800 Wavenumber cm -1 2700 2500 2300
PAS Response Deuteration reversibility Extent of OD exchange and reprotonation fresh wood: pine and spruce deuteration of fresh wood fully reversible Fresh wood Deuteration OH > OD Reprotonation OD > OH 3600 3200 2800 2400 2000 1600 1200 800 2700 2500 2300 Wavenumber cm -1
PAS Response Controlled drying Impact of temperature and relative humidity fresh wood: pine impact of RH at higher temperature 7% RH 7% RH 25 C 80 C 75% RH 75% RH Fresh wood (pine) 3600 3200 2800 2400 2000 1600 1200 800 2700 2500 2300 Wavenumber cm -1
PAS Response Species comparison Impact of temperature and relative humidity fresh wood: pine and spruce similar trend impact of RH at higher temperature 7% RH 75% RH 25 C 7% RH 75% RH 80 C 3600 3200 2800 2400 2000 1600 1200 800 2700 2500 2300 Wavenumber cm -1
PAS Response Reproducibility August / October measurement comparison samples collected in August and October from different locations August October 80 C 25 C 3600 3200 2800 2400 2000 1600 1200 800 Wavenumber cm -1
PAS Response Comparing drying: wood and pulp Wood and pulps impact of temperature greater extent of alterations at higher temperature; more pronounced in pulps Wood 7% D 2 O RH Ground wood pulp Unbleached Pulp Bleached Pulp 80 C 60 C 25 C 2600 2400 2200 2600 2400 2200 2600 2400 2200 2600 2400 2200 Wavenumber cm -1
OD Band Area WRV vs. Deuteration / FT-IR Comparison of techniques for pulp samples good correlation in measurement range deuteration / FT-IR can indicate hornification in pulps similar trend in wood 7% H 2 O / D 2 O RH 800 700 600 500 400 300 200 100 0 20 40 60 80 WRV Reduction Δ% Ground Wood Pulp Unbleached Pulp Bleached Pulp
Summary Main observations Deuteration reversibility and accessibility for wood fully reversible Controlled wood and pulp drying temperature inaccessible OD in dried sample irreversible alterations extent affected by temperature Water retention of pulps and correlation with deuteration studies WRV reduction in dried pulps correlates with amount of OD in dried pulps Changes in wood ultrastructure occur during initial drying and these alterations appear to be similar to the changes occurring in chemical pulp fibers upon drying. Suchy et al. Biomacromolecules 2010, 11, 515-520. Suchy et al. Biomacromolecules 2010, 11, 2161-2168.
Acknowledgement Prof. Mark Hughes Lauri Rautkari Rita Hatakka Petri Huhta Jenni Virtanen Emilia Kauppi UPM-Kymmene Corp. Multi-disciplinary Institute for Digitalization and Energy within Aalto University
Moisture content after drying Impact of temperature pulps groundwood unbleached bleached kraft pulp kraft pulp MC (%) WRV (%) MC (%) WRV (%) MC (%) WRV (%) 65.4 146 67.0 146 66.1 143 5.5 124 5.1 117 4.8 110 0.2 105 1.0 92 1.3 82 0.2 96 1.6 89 0.6 81