Page 245 - Carrahers_Polymer_Chemistry,_Eighth_Edition

208 Carraher’s Polymer Chemistry

F F
R
R (6.57)
F
F

ETFE has high-corrosion resistance and good strength over a wide range of temperatures of

o
approximately –150 C to 150 C. Compared to glass, ETFE film is 1% of its weight, transmits more
o
light, and is less expensive to install. It is highly resilient, able to bear 400 times its own weight. It
is recyclable and self-cleaning because of the slickness brought about because of the presence of the

tetrafluoroethylene units. On the negative side, as with most fluorine-containing polymers, combus-

tion results in the release of highly corrosive HF.
6.6 POLYSTYRENE
Styrene monomer was discovered by Newman in 1786. The initial formation of PS was by Simon
in 1839. While PS was formed almost 175 years ago, the mechanism of formation, described in
Sections 6.1–6.3, was not discovered until the early twentieth century. Staudinger, using styrene
as the principle model, identified the general free radical polymerization process in 1920. Initially

commercialization of PS, as in many cases, awaited the ready availability of the monomer. While
there was available ethyl benzene, it underwent thermal cracking rather than dehydrogenation until
the appropriate conditions and catalysts were discovered. Dow fi rst successfully commercialized
PS formation in 1938. While most commercial PS has only a low degree of stereoregularity, it is
rigid and brittle because of the resistance of the more bulky phenyl-containing units to move in
comparison, for example, to the methyl-containing units of polypropylene. This is reflected in a

relatively high T of about 100 C for PS. It is transparent because of the low degree of crystalline
o
g
formation.
While PS is largely commercially produced using free radical polymerization, it can be pro-
duced by all four of the major techniques—anionic, cationic, free radical, and coordination-type
systems. All of the tactic forms can be formed employing these systems. The most important of
the tactic forms is syndiotactic PS (sPS). Metallocene-produced sPS is a semicrystalline material
with a T of 270 C. It was initially produced by Dow in 1997 under the tradename of Questra. It
o
m
has good chemical and solvent resistance in contrast to “regular” PS, which has generally poor
chemical and solvent resistance because of the presence of voids that are exploited by the solvents
and chemicals.
Physical properties of PS are dependent on the molecular weight and the presence of additives.
General properties of PS are given in Table 6.8. While higher molecular weight PS offers better strength
and toughness, it also offers poorer processability. Low molecular weight PS allows good processability
but poorer strength and toughness. Generally, a balance is sought where intermediate chain lengths are
used. Typically employed chain lengths are on the order of 1,500–3,500 with standard molecular weight
distributions of about 2.2–3.5. Small amounts of plasticizers are often used to improve processability.
Styrene is employed in the formation of a number of co- and terpolymers. The best known is the
terpolymer ABS.
Major uses of PS are in packaging and containers, toys and recreational equipment, insulation,
disposable food containers, electrical and electronics, housewares, and appliance parts. Expandable
PS is used to package electronic equipment such as TVs, computers, and stereo equipments.
PS is produced in three forms—extruded PS, expanded PS foam, and extruded PS foam (XPS).
Expanded PS (EPS) was developed by the Koppers Company in Pittsburgh, PA in 1959. XPS insula-
tion was developed by Dow Chemical and sold under the trade name Styrofoam. This term is often
used for many other expanded PS materials. EPS and XPS are similar and both generally contain a

9/14/2010 3:39:40 PM
K10478.indb 208 9/14/2010 3:39:40 PM
K10478.indb 208