Chemistry Reference
In-Depth Information
polystyrene lowers the Tg. When two polymers are miscible, they have a
single Tg which is intermediate between the Tg of each of the component
polymers. The greater the amount of polystyrene added, the lower the Tg of
the PPO blend. Rather than using polystyrene itself, Noryl uses high impact
polystyrene (HIPS). HIPS is prepared by polymerizing styrene in the presence
of polybutadiene, thereby forming a graft copolymer in with polybutadiene
and polystyrene homopolymer. There are different grades of Noryl, but a Tg
of about 150
∘
C is typical for this miscible blend.
Another blend, also invented at General Electric, is Xenoy
®
.Thisisan
immiscible blend of polybutylene terephthalate (PBT) and polycarbonate
(PC) [5, 6]. This blend is immiscible; it has separate phases of PBT and of
PC. An immiscible blend is characterized by two distinct Tgs and thermal
analysis of this blend shows the Tg of PBT and of PC as well as a sharp
melting point for the crystalline PBT. The blend also includes an impact
modifier. It exhibits the good impact associated with PC and the solvent
resistance associated with PBT. Because of these properties, it is suitable for
applications such as automotive bumpers requiring high impact and gasoline
resistance.
The carbonate linkage can react with the ester linkage in a transesterifica-
tion reaction. This reaction is illustrated by the reaction of diphenyl carbonate
with butylene dibenzoate to form phenyl benzoate.
O
O
O
catalyst
O
O
O
O
O
+
O
O
+
O
O
O
O
Diphenyl carbonate
Butylene dibenzoate
Phenyl benzoate
If this reaction proceeds in a PBT/PC blend, the two homopolymers react
and we get a blend that is miscible. This blend now no longer has two distinct
glass transition temperatures and has a broadened and depressed melting
point. The blend no longer has the desirable characteristics. It has poor
solvent resistance and is slow to crystallize, which causes poor injection
molding performance. For commercial blends, the transesterification reaction
must be controlled [7, 8].
There are many other commercial examples of polymer blends. Polycar-
bonate can be blended with an acrylonitrile-butadiene-styrene terpolymer
to give a PC-ABS blend. Polypropylene impact can be improved by
the addition of ethylene-propylene copolymers, which are sometimes
called ethylene-propylene-rubber (EPR). Ethylene, propylene, and a diene
monomer (EPDM), such as ethylidene norbornene, is also used to impart
impact and flexibility to polypropylene.
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