| Laminated veneer boards processed higher insulation resistance in dry state,which are widely used in electrical industry.However,due to the existence of polar groups on the chain of the cellulose,hemi-cellulose and other components in the wood,it is easy to absorb water during high temperature,more rain and other water immersion environment and lead to the decrease of the electrical insulation properties and mechanical properties.The phenolformaldehyde(PF)resin is commonly used to improve electrical insulation performance and other physical and mechanical properties after absorbing water.However,there still exists unreacted polar phenolic hydroxyl groups on the molecular chain of PF resin after curing are prone to absorb water and reduce the electrical insulation performance.In present research,environmentally friendly and biodegradable silane coupling agent KH550(?-aminopropyltriethoxysilane)was used to modified low molecular weight PF resin so that the phenolic hydroxyl can be blocked and the electrical insulation was improved;Then insulating laminated veneer boards were prepared by vacuum impregnation and hot-pressed gluing methods.The effects of KH550 on the electrical performance,physical and mechanical properties of laminated veneer boards were studied.At last,the hot pressing process was optimized.The results are as follows:(1)Different adding content of KH550(0,3%,5%,8% based on the mass of phenol)was added in the preparation of PF resin directly to modified the thermosetting low molecular weight PF resin by copolymerization,the electrical insulation performance were tested before and after experiencing hot water soaking.The results showed that the modification by KH550 reduced the water absorption and increased the volume resistance.This preparation process was simple.The modified resin was environmentally friendly and water-soluble with lower free formaldehyde and free phenol content.The 5% usage provided better physical and mechanical properties.The FT-IR spectrum showed that KH550 was grafted onto PF resin,which effectively blocked the polar phenolic hydroxyl group.DSC and DMA analysis showed that curing temperature and curing time of the resins were similar.Curing temperature was kept at around 130?.The curing speed of KH550 modified low molecular weight PF resin was faster and the energy consumption during curing process was not increased.(2)The low molecular weight PF resin modified with KH550 obtained the better penatratability into veneer,the water absorption and thickness swelling of the laminated veneer board was lower.The laminated veneer board exhibited better stability,smaller formaldehyde emission.The electrical insulation properties included volume resistivity were improved;and the mechanical performance included bonging strength were higher;the DMA analysis exhibited that the laminated veneer board processed superior heat resistance and maintained good mechanical properties at higher temperatures.When KH550 was added at 5% based on the mass of phenol,all the electrical properties,physical and mechanical performance before and after boiled were best,enlarged using scope and prolonged effective using time.(3)The low molecular weight PF resin modified by KH550 can also significantly improve the water resistance of glued laminated boards and enhance the volume resistivity before and after boiling in hot water.Comparing to the glued laminated veneer board,the impregnated laminated veneer boards owned the lowest water absorption,highest volume resistivity before and after boiled and exhibited excellent electrical insulation performance.The impregnation method is a more effective way in maintaining the permanent electrical insulation of the laminated veneer boards under hot and humid environment.(4)The hot pressing process was optimized by the orthogonal test.The results showed that the best hot pressing temperature was 130?,unit hot pressing pressure of 3.6 MPa,and hot pressing time was 1.2 min/mm. |
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