Chemistry Reference
In-Depth Information
the reaction proceeds, the temperature is increased. Initially, the butanediol
reacts, methanol is removed, and oligomeric esters are formed. Next, vacuum
is applied and butanediol is removed by distillation. The molecular weight
increases as oligomers combine in transesterification reactions. The reaction
is allowed to proceed until the desired molecular weight is achieved, usually
determined by monitoring the viscosity of the molten polyester. The molten
polyester is then extruded through a die and pelletized.
8.6 POLYCARBONATE
Polycarbonate was invented in 1953 in the United States at GE Plastics and
at about the same time in Germany at Bayer. GE marketed the polymer as
Lexan
®
and Bayer as Makrolon
®
. In 2007, GE sold its plastics business to
SABIC (Saudi Basic Industries Corporation), but the Lexan
®
name is still
used. About 30 years after the original invention, Dow Chemical Co. began
offering polycarbonate under the name Calibre
®
. Polycarbonate is an amor-
phous polymer and has excellent clarity, dimensional stability, and toughness.
It is used for bullet-resistant windows and automotive headlamps.
Polycarbonate is made by the reaction of bisphenol-A (BPA) with
phosgene or an equivalent. The reaction can be done in an interfacial
process by adding base and phosgene to a stirring mixture of bisphenol-A
in a water/organic solvent combination. The organic solvent dissolves the
polycarbonate as it forms and is typically a chlorinated solvent such as
methylene chloride or trichloroethane. After the reaction, the aqueous layer
and the solvent are removed to give solid polycarbonate as powder or pellets,
depending on the method of solvent removal. If sodium hydroxide is used as
the base, then the byproduct is sodium chloride which can be converted back
to sodium hydroxide and chlorine. The sodium hydroxide is used as the base
to make more polycarbonate and the chlorine is used to make more phosgene.
H
2
O/NaOH
solvent
catalyst
O
O
+
+ 2x NaCl
Cl
Cl
HO
OH
O
O
Phosgene
BPA
x
Polycarbonate
2 NaCl
2 NaOH + Cl
2
O
Cl
2
+ CO
Cl
Cl
Phosgene
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