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. With regard to sold volumes, the most important polyethylene grades are
HDPE, LLDPE and
LDPE.
- Ultra high molecular weight polyethylene (UHMWPE)
- Ultra low molecular weight polyethylene (ULMWPE or PE-WAX)
- High molecular weight polyethylene (HMWPE)
- High density polyethylene (HDPE)
- High density cross-linked polyethylene (HDXLPE)
- Cross-linked polyethylene (PEX or XLPE)
- Medium density polyethylene (MDPE)
- Linear low density polyethylene (LLDPE)
- Low density polyethylene (LDPE)
- Very low density polyethylene (VLDPE)
UHMWPE
is polyethylene with a molecular weight numbering in the millions,
usually between 3.1 and 5.67 million. The high molecular weight makes
it a very
tough material, but results in less efficient packing of the chains into the
crystal structure
as evidenced by densities of less than high density polyethylene (for
example, 0.930–0.935 g/cm3). UHMWPE can be made through any catalyst
technology, although Ziegler catalysts are most common. Because of its
outstanding toughness and its cut, wear and excellent chemical
resistance, UHMWPE is used in a diverse range of applications. These
include can and
bottle
handling machine parts, moving parts on weaving machines, bearings,
gears, artificial joints, edge protection on ice rinks and butchers'
chopping boards. It competes with
Aramid in
bulletproof vests, under the tradenames Spectra and Dyneema, and is commonly used for the construction of articular portions of
implants used for
hip and
knee replacements.
HDPE is defined by a density of greater or
equal to 0.941 g/cm3. 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 (for example,
chromium catalysts or Ziegler-Natta catalysts) and reaction conditions. HDPE is
used in products and packaging such as milk jugs, detergent bottles,
margarine tubs, garbage containers and water pipes. One third of all
toys are manufactured from HDPE. In 2007 the global HDPE consumption
reached a volume of more than 30 million tons.
PEX 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 because 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.
MDPE is defined by a density range of
0.926–0.940 g/cm3. 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 and screw closures.
LLDPE is defined by a density range of
0.915–0.925 g/cm3. LLDPE is a substantially linear polymer with
significant numbers of short branches, commonly made by
copolymerization of ethylene with short-chain
alpha-olefins (for example,
1-butene,
1-hexene and
1-octene). LLDPE has higher tensile strength than LDPE, it exhibits higher impact and
puncture resistance than LDPE. Lower thickness (gauge) films can be blown, compared with
LDPE, with better environmental stress cracking resistance but is not
as easy to process. LLDPE is used in packaging, particularly film for
bags and sheets. Lower thickness may be used compared to LDPE. Cable
covering, toys, lids, buckets, containers and pipe. While other
applications are available, LLDPE is used predominantly in film
applications due to its toughness, flexibility and relative
transparency. Product examples range from agricultural films, saran
wrap, and bubble wrap, to multilayer and composite films. In 2009 the
world LLDPE market reached a volume of almost 24 billion US-dollars (17
billion Euro).
LDPE is defined by a density range of
0.910–0.940 g/cm3. 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 branching 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. In
2009 the global LDPE market had a volume of circa 22.2 billion
US-dollars (15.9 billion Euro).
VLDPE is defined by a density range of
0.880–0.915 g/cm3. VLDPE is a substantially linear polymer with high
levels of short-chain branches, commonly made by copolymerization of
ethylene with short-chain alpha-olefins (for example, 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. VLDPEs 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.
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 1,000
branches per backbone carbon, can significantly affect the
rheological properties of the polymer.
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