The Pore Distribution And Water Movement In Wood Studied By Time Domain Nuclear Magnetic Resonance Technique

This study investigated the nuclear magnetic relaxation properties of woood by using fixed field nuclear magnetic resonance spectrometer and fast field cycling nuclear magnetic resonance relaxometry based on the time-domain nuclear magnetic resonance technique,and 1Hproton of water in wood was used as the probe.Three hardwood species and two softwood species were employed for the investigations.In present study,we studied the pore distributions of water-saturated wood samples,the water movement of multi-scale pores under different temperatures,and the water migration and dynamic changes of pore size during drying and moisture absorption in different relitive humidity.1.The pore distribution of five wood species was studied by fixed field nuclear magnatic resonence spectrometer and the results showed that:(1)The porosity of five wood samples was about 55%～70%,in which the porosity of cell walls was about 4%～12%,and the porosity of cell lumens was about 46%～65%.(2)The mean diameter of the cell walls among five samples was close to 23～50 nm,and there was a big difference for cell lumens,which ranged from 5.5 μm to 156.7 μm.In addition,the average pore size of cell walls in softwood was smaller than that in hardwood.2.The water movement in wood at low-temperature and high-temperature studied by FFC-NMR relaxometry and the results indicated that:(1)The nuclear magnetic resonance R1(spin-lattice relaxation rate)dispersion differentiated the dynimics characteristics of water in different pores.The high-frequency regions characterized the dipole interactions between water molecules in the macroporous,while the low-frequency regions corresponding to the relatively strong intermolecular dipole interactions between water molecules in the smaller pores.(2)The observed R1 at different temperatures dirived from different relaxation mechanisum.Below zero,the lower temperature lead to an increment in R1 as the bound water migrated into cell lumens.While above the room temperature,the higher temperature caused the water molecules collide rapidly and accelerated the relaxation and hence lead to a R1 increment.In addition,the peak of the relaxation rate dispersion at higher temperature(90 and 100 ℃)reflected the rapid transformation between volume water and surface water of pores.(3)The higher temperature accelerated the water movements and hence reduced the average molecular motional correlation time.The activation energy of water in wood were different among the investigated five wood species.3.The relaxation properties research on the drying process/moisture absorption process of wood under different humidity indicated that:(1)There was a linear relation between FID signal and average moisture content no matter whether it was in drying process or moisture absorption process;In addition,the equilibrium moisture content increased exponentially with the increment of relative humidity.(2)The T2(Spin-spin relaxation time)revealed that the fractional free water was still exist even the moisture content was lower than FSP(fiber saturation point)during wood drying.At the end of drying,the fraction water which adsorbed on the cell lumens surface performed the similar relaxation characteristics of bound water.In comparison,the high-density woods were more likely occuring the moisture transformation between deferent water states in the drying process.Furthermore,the water accumulation during the whole absorption process was bound water,and it did not convert to free water even reached to moisture absorption equilibrium.(3)Free water reduced in a linear function in the effective drying period and bound water reduced in a Logistic function under the different relative humidity.Moreover,the free water drying rate was about 4 times of bound water’s for low-density woods,but it was about 20 to 30 times of high-density woods’.Besides,the average moisture absorption rate inceased exponatially with the incresed relitive humidity during the moisture absorption process.(4)The invastagation of average pore size of cell wall during drying/moisture absorption indicated that the incresed relitive humidity delayed the decrease rate of cell walls during drying.Moveover,the average cell wall pore size of water-saturated wood was about 2.4 times to the average cell wall pore size as wood has been oven-dried firstly and then reached to the moisture absorption equilibrium.