Chemistry Reference
In-Depth Information
The novolak can then be treated with crosslinkers such as hexamethylenete-
tramine in a mold to give further condensations and a thermoset polymer.
With both the resole and the novolak the structures can be affected by the
reaction conditions and the choice of catalyst. In particular, the ortho:para
ratios can vary. Resoles are characterized by a large number of methylol
groups which can condense with the phenolic rings to evolve water and give
additional methylene bridges in the cure step.
H
H
OH
O
OH
O
H
+
H
H
H
+
−
CH
2
OH
Acid
O
+
OH
H
Rearomatization
H
H
HO
OH
HO
OH
CH
2
OH
Acid
H
O
H
OH
Novolak
N
N
N
N
Hexamethylene tetramine
Cured thermoset
Novolak
Phenolic resins are useful as adhesives, especially for wood composites
such as plywood or particle board. They have high heat capabilities and excel-
lent flame resistance; this makes them useful for brake linings and clutch
facings and more mundane applications such as oven knobs and pot handles.
There are many other applications including printed circuit boards and as an
abrasive such as in waterproof sandpaper.
Epoxy resins are thermoset polymers prepared by the reaction of an
epoxy compound, typically a low molecular weight diepoxide, with a curing
agent. These two parts are combined and a chemical reaction causes the
cure. They are often called two-part epoxies. They come in two tubes for
home use. There are many variants of epoxy compounds and curing agents
but diglycidyl ethers of bisphenols cured with diamines are common. The
diglycidyl ethers are readily prepared from epichlorohydrin. The reaction is
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