Chemistry Reference
In-Depth Information
Two methylol-substituted phenols react with each other by the same mechanism:
O
O
O
OH
OH
OH
O
+
HO
O
O
O
O
+
H
2
O
HO
HO
The same can be shown for
para
substituted methylolphenols. As the reaction continues, it leads to
formation of trinuclear and tetra nuclear phenolic resins.
A typical liquid resole is quite low in molecular weight. It may contain no more than two or three
benzene rings. Carried a little further, the condensation yields a solid resole. The pH is usually
adjusted to neutral before the resoles are heated further for cross-linking. Under neutral or slightly
acidic conditions, the methylol groups tend to form dibenzyl ethers:
OH
OH
O
OH
OH
+
HO
OH
These dibenzyl ethers are unstable at higher temperatures, such as 150
C, and decompose to yield
methylene bridges and formaldehyde [
147
]:
OH
OH
Δ
OH
+
O
O
O
The structure of a typical resole contains both dimethylene ether and methylene bridges as well as
methylol groups. Fusible and soluble resols are called A-stage resins. Further reactions cause these
resins to pass through a rubbery stage where they can still be swollen by solvents. This is called
B-stage. The finally cross-linked material is called C-stage resin. The cross-linking process involves
complex and competing reactions. Each may be influenced by reaction conditions. When cross-linking
