Effect of Thermal Modification on Wood Cell Structures Observed by Pulsed-Field-Gradient Stimulated-Echo NMR

Abstract

Thermal modification is an environment friendly method for increasing the lifetime and usability of timber products. In our previous work (J. Phys. Chem. B 2009, 113, 1080, we introduced a pulsed-field-gradient stimulated-echo (PGSTE) NMR based method that enables determining the highly anisotropic size distribution of voids (pores) inside wood cell structures in three orthogonal directions. Here, we demonstrate that the method can be used to quantify the effect of thermal modification on the pore dimensions in Pinus sylVestris pine wood. The results show that the modification decreases the dimensions of lumens inside tracheid cells both in the longitudinal and two transverse directions. However, the relative decrease becomes smaller at the highest modification temperature, implying partial destruction of the cell wall structure. The decrease is larger in the radial direction than in the tangential direction at all the modification temperatures.

Magnetic resonance imaging study of water absorption in thermally modified pine wood

“…T 2 values in EW sections were slightly longer in the TM samples (50 -60 ms) than in the reference samples (45 -55 ms). The temptation to interpret this as an increased lumen size in TM wood is not correct because NMR diffusion and field emission scanning electron microscopy (FESEM) measurements showed that, in fact, the lumen size decreases during TM (Kekkonen 2010). Therefore, most likely, the increased T 2 in TM wood is a consequence of removal of extractives and resin from the wood during modification.…”

“…Although the NMR spectrum in this context typically shows only a single and broad peak, bound water and free water can be distinguished by means of T 2 or T 1 relaxation time distributions ( Riggin et al 1979 ;Araujo et al 1992 ;Hartley et al 1994 ;Xu et al 1996 ;Labb é et al 2002 ;Fantazzini et al 2006 ;Almeida et al 2007 ;Thygesen and Elder 2008 ;Cox et al 2010 ;Hoffmeyer et al 2011 ;Elder and Houtman 2013 ;Telkki et al 2013 ;Zhang et al 2013 ;Kekkonen et al 2014 ). NMR self-diffusion measurements enable the observation of moisture transport in wood ( Wycoff et al 2000 ;Hietala et al 2002 ;Topgaard and S ö derman 2002 ;Meder et al 2003 ;Johannessen et al 2006 ;Kekkonen et al 2009 ;Telkki and Jokisaari 2009 ;Kekkonen et al 2010 ). Magnetic resonance imaging (MRI) in medicine is perhaps the best-known application of NMR, but MRI, and specifically high-resolution MR microscopy ( Callaghan 1991 ), is also very important in materials science ( Bl ü mich 2000 ).…”

“…It was demonstrated by remote detection MRI that TM closes the pits connecting wood cells in pine wood . NMR diffusion measurements indicated that the dimensions of lumens decrease in all three orthogonal directions due to TM, and the cell structure begins to decompose when the temperature is increased from 200 ° C to 240 ° C ( Kekkonen et al 2009( Kekkonen et al , 2010. Furthermore, combined NMR cryoporometry and relaxometry experiments showed that TM decreases significantly the amount of both free and bound water absorbed to samples immersed in water, and the technique enabled to determine pore size distribution of micropores in cell walls ( Kekkonen et al 2014 ).…”

Time-domain NMR study of the drying of hemicellulose extracted aspen (Populus tremuloides Michx.)

“…The impact of modification on relaxation time has been reported for polymer-impregnated wood Elder 2008, 2009 ;Thygesen et al 2010 ), thermally degraded or treated material (Hietala et al 2002 ;Elder et al 2006 ;Kekkonen et al 2010 ), biologically degraded wood (Todoruk et al 2011 ), and enzymatically hydrolyzed cellulose (Felby et al 2008 ).…”

Wood Characterization by NMR & MRI of Fluids

“…17 In the hardwoods, the fibers comprising about two-thirds of the wood volume are tubular, 1 and in analysis of results of PGSE experiments, we recommend the use of an echo amplitude equation derived for fluid molecules confined in a cylindrical cavity instead of equation (4). Such an equation can be found in Callaghan's book.…”