Study On The Relationship Between Moisture And Pore Size In Wood With Proton Probe

The moisture content in wood is closely related to the wood cell wall pore size,and thereafter the moisture migration and pore changes in wood have an important effect on wood drying.Therefore,the relationship between moisture and pore size changes is an important issue in wood research.In this study,water states and pore size changes in three species（Pinus sylvestris,Populus beijingensis and Ulmus pumila）were explored with different environmental conditions based on hydrogen proton sensor of the time-domain NMR technique.The main conclusions are as follows:（1）The proton sensor’s spin lattice relaxation time T₁ can effectively differentiate the bound water on the both wood cell wall and cell lumen inner surface（120ms-160ms）from the confined free water in the cell wall pores（within 10ms）.（2）The bound water on wood cell wall surface has similar T₂ value with the confined free water in the wood cell wall small pores（within 1ms）.The T₂can distinguish cell wall water（with 10 ms）and cell lumen water（tens to hundreds ms）.（3）The wood cell wall microfibrillar void size is in the range of 2.64nm to 0.99nm with NMR-C method.（4）The surface relaxation rate is linearly related to pore size in the similar size order of magnitude.The average surface relaxation rate for three green wood species cell wall（Ulmus pumila,Populus beijingensis,Pinus Sylvestris）areρ_{2 Ulmus}=0.330 nm/ms,ρ_2Poplar=0.333 nm/ms andρ_{2 Pine}=0.562 nm/ms respectively.（5）Optical microscopy,scanning electron microscopy and other visual detection methods are suitable to obtain the morphological characteristics and size informations for wood large cell lumen.The T₂ relaxation distribution is suitable to characterize the pore size distribution for small cell lumen.The combination of the two methods can obtain more comprehensive informations on cell lulmen pore sizes and characteristics.（6）During the desorption process,the confined free water content and pore size in the wood cell wall gradually decreased,and the microfibril void was partially closed;during the adsorption process,the bound water on the surface of cell wall and cell lumen is mainly dominated when relative humidity is below 31.6%;while the confine free water in cell wall appears in cell wall when relative humidity is above 31.6%.（7）At the beginning stage of vacuum drying,the free water was mainly drained from the cell lumen,accompanied by the loss of a small amount of the cell wall confined free water.In the middle and late drying process,the cell wall confined free water migration mainly occurred in cell wall accompanied by bound water migration.With the same vacuum drying pressure,higher drying temperature can achieve rapid drying.In addition,the changes of cell wall pore sizes are small with higher temperature,so it is easier to maintain the wood dimensional stability.