Abstract:

[Objective]To decrease the wood drying time, the microstructure characteristics of wood was changed. The moisture permeability and flowability were improved and new paths for moisture movement were established. The relationship of the process conditions of rolling-press pretreatment, the variation of the characteristics of cell walls and the drying rate, was constructed. [Method]The present work was based on the theories of viscoelasticity and moisture movement of wood. The lumber was compressed and horizontally passed through the intervals of two vertical arrangement rollers. The diameter of the rollers was 500 mm. The rotation speed of the rollers was 30 rmin^-1. Quarter-sawn and flat-sawn oak boards of 900 mm in length, 100 mm in width and 30 mm in thickness with moisture contents of 47%-55% were compressed by rollers in both radial and tangential directions, with three compression rates of 10%, 20% and 30%, and three compression times of 1, 4 and 9, respectively. The variation characteristics of the vessel element cell wall microstructure of the rolling-compressed samples and the drying rate of samples during conventional drying were studied. [Result]FE-SEM observations showed that the pit membrane of vessel elements broke and the wall of vessel elements cracked because of the pressure from rollers, which formed new paths for moisture moving. The drying period was shortened due to improvement of moisture permeability and flowability when increasing the amount and the broken degree of pit membrane, and the scale and size of cracked cell wall, with the increasing of compression rate and compression times. The results indicated that the drying rate of the treated samples was higher than that of the untreated samples in six drying stages and overall the drying process. The drying rate was increased with increasing the compression times when the compression rate and direction were the same. It was also increased with increasing the compression rate when the compression times and direction were the same. The drying rate of the quarter-sawn samples compressed in the radial direction was higher than that of the flat-sawn samples compressed in the tangential direction.[Conclusion]The drying cycles of treated samples were less than that of untreated samples when the initial and final moisture contents of the samples were 50% and 15%, respectively. The rate of shortened time of the lumber compressed by tangential direction was ranged from 6.67% to 23.64%, while that of compressed by radial direction was ranged from 4.55% to 13.02%. The results showed that the drying period of wood decreased because of the improving of moisture permeability and flowability, which were contributed by the rolling-press pretreatment.

Key words: oak lumber, pit membrane, pre-compressed by rollers, lumber drying, permeability of wood, drying rate