A. Pizzi et La. Panamgama, DIFFUSION HINDRANCE VS WOOD-INDUCED CATALYTIC ACTIVATION OF MUF ADHESIVE POLYCONDENSATION, Journal of applied polymer science, 58(1), 1995, pp. 109-115
The reaction of hardening of melamine-urea-formaldehyde (MUF) adhesive
resins in the presence of wood and cellulose was confirmed to have a
lower energy of activation than the MUF adhesive alone, both in the pr
esence or absence of ammonium chloride hardener, thus both in mildly a
cid and mildly alkaline environments. DSC exotherms showed that during
hardening of melamine to melamine, melamine to urea, and urea to urea
crosslinks through methylene bridges occur. Only the earliest reactio
n, mainly melamine to melamine crosslinking, presents a decrease in en
ergy of activation which can be assigned to catalytic activation by th
e cellulosic substrate. The other types of crosslinking reactions (i)
appear not to occur due to the more favorable and rapid melamine to me
lamine reaction which precedes them at lower temperature or (ii) do no
t present catalytic activation by the substrate but rather hindrance b
y it or (iii) variation of their energy of activation appears to be du
e to increased diffusion hindrance by the substrate caused by the incr
easing molecular weight of the resin while hardening. This because the
Kissinger equation plots of the resin alone are in the main linear, f
or all the exotherms, indicating that in hardening of the resin alone
diffusion problems appear to be limited. (C) 1995 John Wiley & Sons, I
nc.