Page 206 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Ionic Chain-Reaction and Complex Coordination Polymerization 169

High-density polyethylene, produced using organometallic catalysts such as the Ziegler-Natta or
Phillips catalysts, have less than 15 (normally within the range of 1–6) short-alkyl branches (essen-
tially no long branches) for 1,000 ethylene units. Because of the regular structure of the ethylene
units themselves and the low extent of branching, HDPE chains can pack more effi ciently, resulting
in a material with greater crystallinity (generally up to 90%), higher density (0.96), with increased
o
chemical resistance, hardness, stiffness, barrier properties, melting point (about 130 C), and tensile
strength. Low molecular weight (chain lengths in the hundreds) HDPE is a “wax” while “typical”
HDPE is a tough plastic.
Linear low-density polyethylene (LLDPE) can be produced with less than 300 psi and at about
o
100 C. It has a density between 0.915 and 0.925 g/mL. It is actually a copolymer of ethylene with

about 8%–10% of an alpha olefin such as 1-butene, 1-pentene, 1-hexene, or 1-octene. Through con-

trol of the nature and amount of alpha olefin, we are able to produce materials with densities and
properties between those of LDPE and HDPE. LLDPE does not contain the long branches found in
LDPE. Because of its toughness, transparency, and flexibility it is used in film applications as pack-

aging for cables, toys, pipes, and containers.
Very low-density polyethylene, (VLDPE) has a density range between 0.88 and 0.915 g/mL. It is
largely linear chains with a high amount of short-chain branching generally made by copolymeriza-
tion of the ethylene with short-chain alpha olefins such as 1-butene, 1-hexene, and 1-octene so that

it is structurally similar to LLDPE except the alpha olefins are generally longer resulting is a lower

density. VLDPE is generally made employing metallocene catalysts.
Ultrahigh molecular weight polyethylene, UHMWPE, is a high-density PE with chain lengths
over 100,000 ethylene units. Because of the great length of the chains, they “inter-tangle” causing
physical cross-links, increasing the tensile strength and related properties of these materials. (By
comparison, HDPE rarely is longer than 2,000 ethylene units.) UHMWPE is about 45% crystalline
and offers outstanding resistance to corrosion and environmental stress cracking, outstanding abra-
sion resistance and impact toughness, and good resistance to cyclical fatigue and radiation failure,
and with a low-surface friction. It is produced utilizing catalysts systems similar to those employed
for the production of HDPE (i.e., Ziegler-Natta and Phillips catalysts). It has a density of about
0.93.
Ultralinear polyethylene (ULPE) has recently become available through the use of soluble ste-
reoregulating catalysts. Along with a decreased amount of short-chained alkyl branching, ULPE
has a narrower molecular weight spread.
At extremely high chain lengths, fibers can be made of largely linear PE producing a material

that is extremely strong and resistant to punctures and cutting even by scissors. One such product

is “Spectra,” which is about 70,000 units long, with the PE fibers stretched about 100% to align the
chains. Spectra is used in the construction of bullet-resistant vests and surgical gloves where nick-
ing and cutting of surgeon’s hands by the sharp instruments is unwelcomed. Spectra is said to be
10 times stronger than steel on a weight basis and 35% stronger than aramid fi bers such as Kevlar.
Like other woven materials, these ultrahigh chain length PE gloves are vulnerable to punctures by
sharp objects such as needles.
Cross-linked polyethylene, PEX or XLPE, is a medium to high-density PE that contains cross-

linkes resulting in a thermoset material. The cross-linking causes a reduced flow for the material and
permeability and increased chemical resistance. Because of the good retention and stability toward
water, some water-pumping systems use PEX tubing since its cross-linking causes the tubing, once
expanded over a metal nipple, to return to its original shape resulting in a good connection.
Medium-density polyethylene (MDPE) has a density between about 9.3 and 9.4 g/mL. It is mainly
produced employing ion-coordination polymerization. It has good shock and drop resistance.
Polymethylene can be produced through several routes, including the use of diazomethane or a
mixture of carbon monoxide and hydrogen. This polymer has only a little branching.
It is well accepted that the history of a polymer, including polymer processing, infl uences poly-
mer behavior. Some of these influences are just becoming known. Interestingly, as in much of

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