Page 277 - Carrahers_Polymer_Chemistry,_Eighth_Edition

240 Carraher’s Polymer Chemistry

Sometimes special attractions allow polymers to mix. This is true for the mixture of PS and
poly(phenylene oxide), PPO, where the interaction between the phenyl groups allow the miscibility
of the two polymers. For many combinations, such preferential associations are not present.
Another approach is the use of copolymers. There are a number of variations to this. In some
situations, polymer–copolymer combinations are used where the adage “the enemy of my enemy
is my friend” comes into play. Thus, the random copolymer of styrene and acrylonitrile forms a
miscible blend with PMMA. The copolymer is composed of nonpolar styrene units and polar acry-
lonitrile units that are incompatible with one another. These units will blend with PMMA to avoid
one another.
Another approach is to use copolymers where the structures of the copolymer are similar to that
of the other phase. This is, what occurs for PE and copolymers of ethylene and propylene.
Generally, miscible blends will have properties somewhere between those of the unblended poly-
mers. These properties will be dependent on the ratio of the two polymers and this ratio is often used
to obtain a particular property. These properties include mechanical, chemical, thermal, weather-
o
ing, and so on. For instance, PPO is thermally stable with a high T , about 210 C. While this is good
g
for some applications, it is considered too high for easy processing. Thus, PS, with a T of about
g
o
100 C, is added to allow a lower processing temperature. Noryl is PPO blended with a second poly-
mer which is generally PS or HIPS. Noryl is used in the construction of internal appliance compo-
nents, brackets and structural components in offi ce products, large computer and printer housings,
automotive wheel covers, and high-tolerance electrical switch boxes and connectors.
Industrial companies have long-term strategies. For example, Exxon (now ExxonMobile) is the
third largest chemical company in the United States. Some time ago, they made the decision to
emphasize the ethylene and propylene monomers that are obtained from the petrochemical interests
of ExxonMobile. Thus, ExxonMobile has a research emphasis on the commercialization of prod-
ucts from these monomers. The major materials made from ethylene and propylene are polymeric,
either homopolymers or copolymers. Efforts include developing catalysts that allow the formation
of polymeric materials from the ethylene and propylene monomers and the use of these catalysts to
synthesize polymeric materials that have varying properties, allowing their application in different
marketplaces in society.
A driving force for conversion of gasoline to polymeric materials is increased value in the prod-
ucts made from the polymers. The general trail is gasoline → ethylene, propylene monomers → raw
polymers and copolymers → fi nished products.
As noted above, one major use of HIPS was in automotive bumpers. These have been largely
replaced by another blend, but here it is a miscible blend of PE and PP. HIPS bumpers have a more
rubbery feel to them while the PE–PP materials are more plastic in their behavior and feel. These
PE and PP intense plastics are not only made into automotive bumpers but also as side and bottom
panels. There are several processes employed to produce the raw materials used in the production
of these automotive parts. One process developed by Exxon begins with the production of PP
using a catalyst developed by Exxon. This catalyst system produces isotactic stereoregular poly-
propylene (i-PP) that is stronger and denser than atactic nonstereoregular polypropylene (a-PP).
At some time during this polymerization, some of the liquid polypropylene monomer is removed
and ethylene monomer is added to the mix. Because of the continued presence of the catalyst
that can also polymerize the ethylene monomer, copolymer containing ethylene and i-PP units
is produced. This product can be roughly pictured as being formed from i-PP particles that have
unreacted propylene monomer removed creating open-celled sponge-like particles. These i-PP
particles then become impregnated with ethylene monomer eventually resulting in the formation
of a copolymer that contains both i-PP and PE units with a i-PP outer shell. This product is even-
tually mixed with an ethylene(60%)–i-PP(40%) copolymer giving a blended material that has two

continuous phases with a final i-PP content of 70%. It is this material that is used in molding the
automotive bumpers and panels. The i-PP units contribute stiffness and the PE units contribute

flexibility to the overall product. The inner PE units in the impregnated particles allow stress to

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