Chemistry Reference
In-Depth Information
can depolymerize back to formaldehyde. This was an early problem, essen-
tially making the polymer useless. The thermal stability problem was solved
by a post treatment where the hydroxyl ends were capped, for example by
treatment with acetic anhydride [37].
O
O
O
or
C
H H
O
H
H
O
x
O
O
Ac
2
O
O
C
H
O
C
HO
C
O
H
H
3
C
CH
3
H
H
H
x
x
Acetal resins do have some weaknesses. The acetal linkage can be
hydrolyzed, especially under acidic conditions, back to formaldehyde. The
methylene is prone to oxidation especially if chlorine is present. In the
1970s and 1980s, acetal resins were used for plumbing fittings. Due to
hydrolysis, and attack by the chlorinated water, these parts began to fail
resulting in severe damage and class action law suits. Their use as fittings
was discontinued, but more recently there have been acetal failures in toilet
valve assemblies.
8.12 THERMOSETS
The polymers discussed to this point are reasonably linear. They can be
remelted or redissolved and are members of the general class of thermoplastic
polymers. For example, a used PET water bottle can be chopped into flakes
and then remelted and fabricated into another article such as a flower pot,
golf tees, or many other items. These thermoplastics are predominantly
linear. Even in the case of polyethylene, where there is often some branching,
they soften when heated and can be reshaped or molded into new articles.
However, if the polymer is not linear but three dimensional it cannot be
reshaped. These polymers are formed in the mold and take a set. After the
polymerization is complete, they cannot be remelted and reshaped. These
are called thermoset polymers or thermoset resins.
One type of thermoset polymer is polyurethane. Polyurethane can be linear
or three dimensional, depending on the starting materials for the polymeriza-
tion. The urethane linkage is also known as a carbamate. The polymers are
always called polyurethanes and not polycarbamates. A carbamate can be pre-
pared by the reaction of an alcohol with an isocyanate. Isocyanates are readily
prepared from an amine and phosgene. The reaction sequence is illustrated
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