Effect of wood extractives on cure of phenolic resin Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/b2773z463

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  • Difficulties have been encountered in gluing some Southeast Asian woods using phenolic resin. Extractives in veneers from the trade groups yellow, white, and red meranti (Shorea, Parashorea, and Pentacme spp. ), keruing (Dipterocarpus spp. ), and kapur (Dryobalanops spp. ) were suspected to be able to modify resin cure and to cause inferior gluelines. The average extractive content for veneers of each trade group was determined by sequential extraction with benzene, ether, ethanol, and water. Yellow meranti had the greatest quantity of extractives (15.6%), followed closely by kapur (10.1%). In general, ethanol and water soluble extractives exceeded other extractives. Extractive migration to the veneer surface seemed to occur with kapur veneers but seemed insignificant for the other groups. Knowing the wood extractive content and assuming various depths of resin penetration, the extractive content of the dry glue film was estimated. Extractives might affect the resin similarly to inert filler, or the extractives might modify the resin by adjusting the solids content or the pH of the resin. Decreasing resin pH from 11 to 10.5 or increasing the resin solids from 42.5% to 49.5% caused a 50% decrease in the resin gel time. The effect of an inert filler (kaolin powder) was negligible on any property other than resin viscosity. These experimental results were then used to evaluate the effects of extractives on a phenolic resin. Ethanol soluble extractives from all of the veneers were found to be most acidic and therefore capable of depressing the resin alkalinity significantly. Kapur and yellow meranti were found to be the most acidic woods because of their high extractive content. Adding these alcohol soluble extractives to a phenolic resin in concentrations similar to those expected to occur in gluelines verified that the acidity of kapur and yellow meranti wood was capable of shortening the resin gel time by 50%. The degree of resin cure was detected by a spectrophotometric method. In general, ethanol and water solubles of kapur and yellow meranti prevented complete resin cure at much lower concentrations than red meranti and keruing extractives. It was concluded that extractives in white meranti, red meranti and keruing veneers were unlikely to interfere with the resin curing process, except by retarding moisture loss from the glueline. However, kapur and yellow meranti extractives were found capable of modifying the curing process in two ways: resin gelation was speeded by the decrease in pH and complete cure was prevented by the chemical reaction between the resin and extractives.
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