Classification of polyethylenes
Polyethylene is classified into several
different categories based mostly on its density and branching. The
mechanical properties of PE depend significantly on variables such as
the extent and type of branching, the crystal structure, and the
molecular weight.
UHMWPE
is polyethylene with a molecular weight
numbering in the millions, usually between 3.1 and 5.67 million. The
high molecular weight results in less efficient packing of the chains
into the crystal structure as evidenced by densities less than high
density polyethylene (e.g. 0.935 - 0.930). The high molecular weight
results in a very tough material. UHMWPE can be made through any
catalyst technology, although Ziegler catalysts are most common. Because
of its outstanding toughness, cut, wear and excellent chemical
resistance,
is used in a wide diversity of
applications. These include can and bottle handling machine parts,
moving parts on weaving machines, bearings, gears, artificial joints,
edge protection on ice rinks, butchers' chopping boards. It has even
replaced Kevlar in new bulletproof vests.
is defined by a density of greater or
equal to 0.941 g/cc. HDPE has a low degree of branching and thus
stronger intermolecular forces and tensile strength. HDPE can be
produced by chromium/silica catalysts, Ziegler-Natta catalysts or
metallocene catalysts. The lack of branching is ensured by an
appropriate choice of catalyst (e.g. Chromium catalysts or Ziegler-Natta
catalysts) and reaction conditions. HDPE used in products and packaging
such as milk jugs, detergent bottles, margarine tubs, and garbage
containers.
HDPE is also widely used in the
fireworks community. In tubes of varying length (depending on the size
of the ordnance), HDPE is used as a replacement for the supplied
cardboard mortar tubes for two primary reasons. One, it is much safer
than the supplied cardboard tubes because if a shell were to malfunction
and explode inside (flower pot) an HDPE tube, the tube will not shatter.
The second reason is that they are reusable allowing designers to create
multiple shot mortar racks. Pyrotechnicians discourage the use of PVC
tubing in mortar tubes because it will shatter, sending shards of
plastic at possible spectators, and will not show up in x-rays.
Recently, much research activity has focused on the nature and
distribution of Long Chain Branches in polyethylene. In HDPE, a
relatively small number of these branches (perhaps 1 in 100 or 1000
branches per backbone carbon) can significantly affect the rheological
properties of the polymer.
is a medium- to high-density
polyethylene containing cross-link bonds introduced into the polymer
structure, changing the thermoplast into an elastomer. The
high-temperature properties of the polymer are improved, its flow is
reduced and its chemical resistance is enhanced. PEX is used in some
potable water plumbing systems, as tubes made of the material can be
expanded to fit over a metal nipple, and it will slowly return to its
original shape, forming a permanent, water-tight connection.
is defined by a density range of 0.926 -
0.940 g/cc. MDPE can be produced by chromium/silica catalysts,
Ziegler-Natta catalysts or metallocene catalysts.MDPE has good shock and
drop resistance properties. It also is less notch sensitive than HDPE,
stress cracking resistance is better than HDPE. MDPE is typically used
in gas pipes and fittings, sacks, shrink film, packaging film, carrier
bags, screw closures.
is defined by a density range of 0.915 -
0.925 g/cc. is a substantially linear polymer, with significant numbers
of short branches, commonly made by copolymerization of ethylene with
short-chain alpha-olefins (e.g. 1-butene, 1-hexene, and 1-octene). LLDPE
has higher tensile strength than LDPE. Exhibits higher impact and
puncture resistance than LDPE. Lower thickness (gauge) films can be
blown compared to LDPE, with better environmental stress cracking
resistance compared to LDPE but is not as easy to process LLDPE is used
in packaging, particularly film for bags and sheets. Lower thickness
(gauge) may be used compared to LDPE. Cable covering, toys, lids,
buckets and containers, pipe.While other applications are available,
LLDPE is used predominantly in film applications due to its toughness,
flexibility, and relative transparency.
is defined by a density range of 0.910 -
0.940 g/cc. LDPE has a high degree of short and long chain branching,
which means that the chains do not pack into the crystal structure as
well. It has therefore less strong intermolecular forces as the
instantaneous-dipole induced-dipole attraction is less. This results in
a lower tensile strength and increased ductility. LDPE is created by
free radical polymerization. The high degree of branches with long
chains gives molten LDPE unique and desirable flow properties. LDPE is
used for both rigid containers and plastic film applications such as
plastic bags and film wrap.
is defined by a density range of 0.880 -
0.915 g/cc. is a substantially linear polymer, with high levels of short
chain branches, commonly made by copolymerization of ethylene with
short-chain alpha-olefins (e.g. 1-butene, 1-hexene, and 1-octene). VLDPE
is most commonly produced using metallocene catalysts due to the greater
co-monomer incorporation exhibited by these catalysts. VLDPE’s are used
for hose and tubing, ice and frozen food bags, food packaging and
stretch wrap, as well as impact modifiers when blended with other
polymers.
Ethylene copolymers
In addition to copolymerization with
alpha-olefins, ethylene can also be copolymerized with a wide range of
other monomers and ionic compositon that creates ionized free radicals.
Common examples include vinyl acetate (resulting product is
ethylene-vinyl acetate copolymer, or EVA, widely used in athletic shoe
sole foams), and a variety of acrylates (applications include packaging
and sporting goods).
History
Polyethylene was first synthesized by
the German chemist Hans von Pechmann, who prepared it by accident in
1898 while heating diazomethane. When his colleagues Eugen Bamberger and
Friedrich Tschirner characterized the white, waxy substance he had
created, they recognized that it contained long -CH2- chains and termed
it polymethylene.
The first industrially practical polyethylene synthesis was discovered
(again by accident) in 1933 by Eric Fawcett and Reginald Gibson at the
ICI works in Northwich, England.Upon applying extremely high pressure
(several hundred atmospheres) to a mixture of ethylene and benzaldehyde,
they again produced a white waxy material. Since the reaction had been
initiated by trace oxygen contamination in their apparatus, the
experiment was at first difficult to reproduce. It was not until 1935
that another ICI chemist, Michael Perrin, developed this accident into a
reproducible high-pressure synthesis for polyethylene that became the
basis for industrial LDPE production beginning in 1939.
Subsequent landmarks in polyethylene synthesis have centered around the
development of several types of catalyst that promote ethylene
polymerization at more mild temperatures and pressures. The first of
these was a chromium trioxide based catalyst discovered in 1951 by
Robert Banks and John Hogan at Phillips Petroleum. In 1953, the German
chemist Karl Ziegler developed a catalytic system based on titanium
halides and organoaluminum compounds that worked at even milder
conditions than the Phillips catalyst. The Phillips catalyst is less
expensive and easier to work with, however, and both methods are used in
industrial practice.
By the end of the 1950s both the Phillips and Ziegler type catalysts
were being used for HDPE production. Phillips' initially had
difficulties producing a HDPE product of uniform quality, and filled
warehouses with off-specification plastic. However, financial ruin was
unexpectedly averted in 1957, when the hula hoop, a toy consisting of a
circular polyethylene tube, became a fad among teenagers throughout the
United States.
A third type of catalytic system, one based on metallocenes, was
discovered in 1976 in Germany by Walter Kaminsky and Hansjörg Sinn. The
Ziegler and metallocene catalyst families have since proven to be very
flexible at copolymerizing ethylene with other olefins and have become
the basis for the wide range of polyethylene resins available today,
including VLDPE, and LLDPE. Such resins, in the form of fibers like
Dyneema, have (as of 2005) begun to replace aramids in many
high-strength applications.
Until recently, the metallocenes were the most active single-site
catalysts for ethylene polymerisation known - new catalysts are
typically compared to zirconocene dichloride. Much effort is currently
being exerted on developing new single-site (so-called post-metallocene)
catalysts, that may allow greater tuning of the polymer structure than
is possible with metallocenes. Recently, work by Fujita at the Mitsui
corporation (amongst others) has demonstrated that certain
salicylaldimine complexes of Group 4 metals show substantially higher
activity than the metallocenes.
Physical properties
Depending on the crystallinity and
molecular weight, a melting point and glass transition may or may not be
observable. The temperature at which these occur varies strongly with
the type of polyethylene. For common commercial grades of medium-density
and high-density polyethylene, the melting point is typically in the
range 120-130 °C. The melt point for average commercial low-density
polyethylene is typically 105-115 °C. Most LDPE, MDPE, and HDPE grades
have excellent chemical resistance and do not dissolve at room
temperature because of the crystallinity. Polyethylene (other than
cross-linked polyethylene) usually can be dissolved at elevated
temperatures in aromatic hydrocarbons (i.e. toluene, xylene) or
chlorinated solvents (i.e. trichloroethane, trichlorobenzene).
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