Chemistry Reference
In-Depth Information
Alternatively polycarbonate can be made in a melt process using diphenyl
carbonate in lieu of phosgene. Mainly due to some issues with inferior prop-
erties such as color and thermal aging, the melt process was not used commer-
cially to any significant extent until the early 1990s when these problems were
solved. A 1:1 molar ratio of BPA and diphenyl carbonate is combined and
heated in the presence of a catalyst. The BPA dissolves in the molten diphenyl
carbonate and phenol is generated by a transesterification reaction. Vacuum
is applied and removal of phenol shifts the equilibrium to the polycarbonate.
The molten polycarbonate is extruded through a die and pelletized.
H
O
O
O
O
Cataly
s
t
+ 2x
+
O
O
O
Diphenyl carbonate
HO
OH
x
Phenol
Polycarbonate
BPA
Diphenyl carbonate can be made from phenol and phosgene, but if it
is made by a non-phosgene route, it can avoid the use of the toxic gases
phosgene and chlorine. Diphenyl carbonate can be made from dimethyl
carbonate. Dimethyl carbonate is available from the reaction of methanol
with carbon monoxide [14] or with carbon dioxide [15].
O
O
O
O
CO or CO
2
H
3
C
CH
3
CH
3
OH
O
Diphenyl carbonate
O
Dimethyl carbonate
Another advantage of the melt process is that it avoids the use of chlori-
nated solvents. Trace levels of residual chlorinated solvents can be a concern
for polycarbonate used in applications such as water bottles [16]. The sol-
vent levels are very low in manufactured polycarbonate and become even
lower during subsequent melt processing into articles; however use of the
melt process avoids the issue entirely. Both processes have the potential for
residual BPA and BPA is a degradation product of polycarbonate, especially
degradation by hydrolysis. There is currently some concern about the health
effects of BPA and the topic is the subject of much scientific debate and many
studies [17].
8.7 NYLON
The reaction between an amine and a carboxylic acid can, with removal of
water, yield an amide. It is this reaction that is the basis for nylon produc-
tion. Nylon is a polyamide. Commonly, nylons are prepared from aliphatic
Search WWH ::
Custom Search