18-08-2012, 02:37 PM
Industrial Polymers, Specialty Polymers, and Their Applications
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Introduction
The first completely synthetic plastic, phenol-formaldehyde, was introduced by L. H. Baekeland in 1909,
nearly four decades after J.W. Hyatt had developed a semisynthetic plastic—cellulose nitrate. Both Hyatt
and Baekeland invented their plastics by trial and error. Thus the step from the idea of macromolecules to
the reality of producing them at will was still not made. It had to wait till the pioneering work of
Hermann Staudinger, who, in 1924, proposed linear molecular structures for polystyrene and natural
rubber. His work brought recognition to the fact that the macromolecules really are linear polymers.
After this it did not take long for other materials to arrive. In 1927 poly(vinyl chloride) (PVC) and
cellulose acetate were developed, and 1929 saw the introduction of urea-formaldehyde (UF) resins.
The production of nylon-6,6 (first synthesized by W. H. Carothers in 1935) was started by Du Pont in
1938, and the production of nylon-6 (perlon) by I. G. Farben began in 1938, using the caprolactam
route to nylon developed by P. Schlock. The latter was the first example of ring-opening polymerization.
The years prior toWorldWar II saw the rapid commercial development of many important plastics, such
as acrylics and poly(vinyl acetate) in 1936, polystyrene in 1938, melamine–formaldehyde (formica) in
1939, and polyethylene and polyester in 1941. The amazing scope of wartime applications accelerated
the development and growth of polymers to meet the diverse needs of special materials in different fields
of activity.
The development of new polymeric materials proceeded at an even faster pace after the war. Epoxies
were developed in 1947, and acrylonitrile–butadiene–styrene (ABS) terpolymer in 1948. The polyurethanes,
introduced in Germany in 1937, saw rapid development in the United States as the technology
became available after the war. The discovery of Ziegler–Natta catalysts in the 1950s brought about the
development of linear polyethylene and stereoregular polypropylene. These years also saw the emergence
of acetal, polyethylene terephthalate, polycarbonate, and a host of new copolymers. The next two decades
saw the commercial development of a number of highly temperature-resistant materials, which included
poly(phenylene oxide) (PPO), polysulfones, polyimides, polyamide-imides, and polybenzimidazoles.
Addition Polymers
Addition polymers are produced in largest tonnages among industrial polymers. The most important
monomers are ethylene, propylene, and butadiene. They are based on low-cost petrochemicals or natural
gas and are produced by cracking or refining of crude oil. Polyethylene, polypropylene, poly(vinyl
chloride), and polystyrene are the four major addition polymers and are by far the least-expensive
industrial polymers on the market. In addition to these four products, a wide variety of other addition
polymers are commercially available.
For addition polymers four types of polymerization processes are known: free-radical-initiated chain
polymerization, anionic polymerization, cationic polymerization, and coordination polymerization
(with Ziegler–Natta catalysts). By far the most extensively used process is the free-radical-initiated
chain polymerization. However, the more recent development of stereo regular polymers using certain
1-2 Industrial Polymers, Specialty Polymers, and Their Applications
organometallic coordination compounds called Ziegler–Natta catalysts, which has added a new
dimension to polymerization processes, is expected to play a more important role in coming years.
The production of linear low-density polyethylene (LLDPE) is a good example. Ionic polymerization is
used to a lesser extent. Thus, anionic polymerization is used mainly in the copolymerization of olefins,
such as the production of styrene–butadiene elastomers, and cationic polymerization is used exclusively
in the production of butyl rubber.
High-Molecular-Weight High-Density Polyethylene
High-molecular-weight high-density polyethylene (HMW-HDPE) is defined as a linear homopolymers
or copolymer with a weight-average molecular weight ð M w
Þ in the range of approximately
200,000–500,000. HMW-HDPE resins are manufactured using predominantly two basic catalyst
systems: Ziegler-type catalysts and chromium oxide-based catalysts. These catalysts produce linear
polymers which can be either homopolymers when higher-density products are required or copolymers
with lower density. Typical comonomers used in the latter type of products are butene, hexane, and
octenes.
HMW-HDPE resins have high viscosity because of their high molecular weight. This presents
problems in processing and, consequently, these resins are normally produced with broad MWD.
The combination of high molecular weight and high density imparts the HMW-HDPE good stiffness
characteristics together with above-average abrasion resistance and chemical resistance. Because of the
relatively high melting temperature, it is imperative that HMW-HDPE resins be specially stabilized with
antioxidant and processing stabilizers. HMW-HDPE products are normally manufactured by the
extrusion process; injection molding is seldom used.
Catalyst Technology
The origin of sophisticated catalyst used today are to be found in the early work of Karl Ziegler (1950)
and Guilio Natta (1954). In the last five decades, several distinct “generations” of catalyst technologies
have emerged. The earliest commercial catalysts (first generation) were essentially titanium trichloride,
simply prepared by reducing TiCl4 with alkylaluminums to yield brown (b) TiCl3
, which was
subsequently heated to convert it to the stereospecific purple (g) form.
In the 1970s, improved or second-generation catalysts were developed. The essence of the improvement
was that catalyst poisons AlCl3 or AlEtCl2, which are cocrystallized with or absorbed onto the TiCl3
catalyst, were removed by using dialkyl ethers (especially di-n-butyl ether and di-isoamyl ether).
The 1980s heralded the widespread commercial implementation of supported catalysts. These thirdgeneration
catalysts comprise of TiCl4 on a specially prepared MgCl2 support. Commercially available
MgCl2 is converted to “active MgCl2” by treating with “activating agents,” which are electron donors
(Lewis bases) such as ethyl benzoate, diisobutylphthalate, and phenyl triethoxy silane. These are also used
in conjunction with the cocatalyst (trialkylaluminum) as a “selectivity control agent.”