Effect of wood extractives on cure of phenolic resin / Dong Nguyen.

Tese (M.S.) - Oregon StateUniversity, 1975

This research has explored the influence of extractives from keruing, kapur, and meranti on the cure of phenolic plywood adhesive resins. The first study estimated the average extractive content of the veneers in question through fractional extraction. Yellow meranti had the greatest quantities of extractives (15.6%), followed by kapur (10.1%), red meranti (7.2%), white meranti (5.0%), and keruing (4.8%). In general, ethanol soluble extractives were also recovered; however, the quantity was usually less than the amount of ethanol solubles. Least significant was the amount of benzene and ether extractives. Extractive migration to veneer surfaces seemed to occur with kapur, but seemed insignificant for the other groups. Extractive migration is expected to vary greatly, even within a species. The possibility of extractive migration is a factor that could effect resin cure greatly but these experiments were not sufficient to allow an absolute conclusion. With two major assumptions - (a) all extractives dissolve into the glue as it penetrates the wood, and (b) all the extractives disperse uniformly through the glue - a theoretical approach was used to estimate the percent extractive likely to occur in the dry glue film. The second study determined the effect of inert filler, resin solids content, and pH on the cure of a phenolic resin. PH and resin solids content were confirmed to have major effects on phenolic resin gel time. However, the effect of inert filler (kaolin powder) was considered to be negligible on any property other than viscosity. As a rule of thumb for the resin used in this study, a reduction of pH from 11.0 to 10.5 cuts the resin gel time in half. This slightly less alkaline resin would have much greater viscosity affecting the flow of the glue into the surface of a veneer. Gel time was reduced by 40% for a 5% increase in resin solids content from the undiluted value of 42.5%. Thus any effect extractives might have on rate of water loss from the glueline will affect rate of resin cure. The final study combined the results of the previous studies to estimate the effects of extractives on the adhesive. The water and alcohol soluble extractives from any of the four Southeast Asian hardwoods caused low wood pH and were capable of depressing the alkalinity of the resin. Ethanol solubles were the most acidic, followed by water soluble extractives. The constribution of benzene and ether extractives were negligible because of their small amount. Yellow meranti was the most acidic wood, followed closely by kapur. Gel time measurements of extractive-resin mixtures proved that the gel time was a function of the acidity and quantity of extractives dissolved in resin, and not of the species group. Ether, ethanol, and water extractives accelerated the gelling significantly, but the major effect would be expected from the alcohol solubles because of their greater qualtity. The degree of resin cure was strongly affected by extractives. Ether extractives of kapur prevented complete cure of the resin to greater extent than either ethanol or water extractives, but were considered less of a problem than the water/alcohol solubles beacuse of their small quantity. Benzene extractives had no effect on the polymerization process. In general, kapur and yellow meranti extractives prevented complete cure of the resin more than red and white meranti and keruing extractives.