|Abstract or Summary
- 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.