| Accompanying the declining natural forestry in recent years, the supply of south sea timber is also decreasing. Therefore, the production of plywood made from imported veneer was decreased in demand, and that brings the era of plywood produced by domestically-grown Basswood, Birch and Fraxinus mandshurica to an end as well. The rapid development of China's poplar industry brings new life to plywood production.In order to figure out new methods to enhancement the fast-growing poplar wood, and in light of the manufacturing difficulties which can be attributed to fast-growing poplar wood's characteristics itself; such as low quality texture, inadequate density and hardness, poor in dimensional stability, contraction and deformation also inclination to fluff when applying rotary-cutting veneer, the smoke heat treatment technique was applied.Smoke heat treatment technique is an effective technique before wood-drying process which has purpose to improve the physical-mechanical properties, dimensional properties and cutting property of wood. This technique can accommodate the internal stress and make it distribute uniformly, reduce the moisture content and make it evenly distributed as well. Depend on the species of trees, the required internal temperatures was various; even within the same species, the fact shown different results according to different internal temperatures. The materials which used in this research are 10-year Zhonglin-46 Poplar. Two different smoke-heating treatments were applied: 1) inside of logs were maintained at the temperature of 80℃～90℃about 66h; 2) inside of logs were maintained at the temperature of 90℃～95℃about 20h. Then, smoke heat treated poplar logs were spirally cut into 2.0mm veneers, which were hot pressed to become 3-plyer and 5-plyer plywood. The physical-mechanical properties and dimensional properties were tested according to Japanese Industrial Standards (JIS). The results are summarized as follows:(1) The moduli of rupture (MOR) and elasticity (MOE) of 3-plyer and 5-plyer poplar plywood which manufactured from veneer logs treated by smoke heating were improved about 11.3%～35%. Either dry or wet condition, plywood which treated by smoke heating were higher than plywood which manufactured from veneer logs which no treated by smoke heating.(2) The contact angle of poplar veneers rotary-cut from smoke heat treated logs reduced significantly and the dynamic surface wettability of the same veneers was improved greatly. This condition shows beneficial to glue spreading and glue penetration. Therefore, the shearing strength and internal bond strength (IB) of smoke heat treated poplar plywood were enhanced substantially. Either dry or wet condition, the shearing strength of smoke heat treated poplar plywood was higher than control group. It is about 8.2%～23.3%. The wood failure ratio also increased largely. The IB of 3-plyer and 5-plyer treated poplar plywood improved significantly by about 56.6%～95% and more than 20%, respectively.(3) The MOE and shearing strength of poplar plywood manufactured from veneer logs which inside which maintained at temperature of 90℃were higher than maintained at temperature of 80℃by about 10% and 12%, respectively. It showed that 90℃smoke heat treatment condition was better than 80℃.(4) In the dry/wet cycle accelerated weathering test, the thickness swelling (TS) values of treated poplar plywood were reduced largely than non-treat group. Specially, the TS values of 3-plyer treated poplar plywood under accelerated weathering conditions were below 10% and the residual TS were just only 3%. In addition, during the moisture absorption and desorption processes with high humidity (RH), the equilibrium moisture contents (EMC) of treated plywood was smaller than non-treat group. After moisture absorption and desorption test, the residual TS values of treated plywood were reduce below 2% averagely which was half of value of non-treat group.(5) After the moisture absorption-dry test, the warping values of treated veneer and plywood were much smaller than non-treat group. The maximum warping value among four corners of treated veneer and plywood was below 1/4 time of non-treat group. The summery warping value of four corners of treated veneer and plywood did not reach 1/3 time of non-treat group. Specially, the surface warping value of 5-plyer smoke heat treated poplar plywood was very small. 5-plyer treated plywood had better moisture resistant and dimensional properties.(6) The Coriolus Versicolor Quel (COV) resistance property of treated plywood was reduced, whereas the Tyromyces Versicolor Quel (TPY) resistance property of it was not affected. In addition, either selecting termite resistance test or forced termite resistance test, the weight losses of smoke heat treated plywood was smaller by about 5% than non-treat group. The dead termite ratio of treated plywood was higher by about 10% than non-treat group. Consequently, smoke heat treatment technology can greatly improve the fungal and termite resistance properties of poplar plywood.
|
PDF Full Text Request