Chemistry Reference
In-Depth Information
Acid anhydrides are more effective curing agents for cycloaliphatic epoxy resins than are the
amines. In addition, the amines might also react with ester groups that are present in some of these
materials and form undesirable by-products.
7.12 Phenol-Formaldehyde Resins
The phenolic resins are condensation products of phenol and formaldehyde [
144
-
146
,
148
]. These
materials were among the earliest commercial synthetic plastics. Two different methods [
144
-
146
]
are used to prepare them. In the first one, the condensations are base catalyzed, while in the second
one, they are acid-catalyzed. The products formed with basic catalysts are called
resols
and with
acidic ones
. Phenolic resins are used widely in coatings and laminates. The pure resins are
too friable for use as structural materials by themselves. They become useful plastics, however, when
filled with various fillers.
novolacs
7.12.1 Resols
These thermosetting resins form in reactions of phenols with formaldehyde in water in the presence of
catalytic amounts of bases. Under these conditions, phenol exists as a resonance-stabilized anion:
O
O
O
O
The addition of the phenol anion to formaldehyde is a typical nucleophilic reaction:
O
O
O
OH
protein
transfer
O
+
O
ortho
para
methylolphenols form in the above reaction. Phenol is very reactive and
monosubstituted phenols are hard to isolate from the reaction mixture, because di- and trisubstitution
occurs rapidly. No substitutions were ever shown to take place in the
Both
and
meta
position. The overall
reaction is as follows:
