olyethylene is a good electrical insulator.
It offers good electrical treeing resistance; however, it
becomes easily electrostatically charged (which can be reduced by additions of graphite, carbon
black or antistatic agents). When pure, the dielectric constant is in the range 2.2 to 2.4 depending on
the density[21] and the loss tangent is very low, making it a good dielectric for building capacitors. For
the same reason it is commonly used as the insulation material for high-frequency coaxial and twisted
pair cables.
Optical
[edit]
Depending on thermal history and film thickness, PE can vary between almost clear (transparent),
milky-opaque (translucent) and opaque. LDPE has the greatest, LLDPE slightly less, and HDPE the least
transparency. Transparency is reduced by crystallites if they are larger than the wavelength of visible
light.[22]
Manufacturing process
[edit]
Monomer
[edit]
Ethylene (ethene)
The ingredient or monomer is ethylene (IUPAC name ethene), a gaseous hydrocarbon with the
formula C2H4, which can be viewed as a pair of methylene groups (−CH
2−) connected to each other. Typical specifications for PE purity are <5 ppm for water, oxygen, and
other alkenes contents. Acceptable contaminants include N2, ethane (common precursor to ethylene),
and methane. Ethylene is usually produced from petrochemical sources, but is also generated
by dehydration of ethanol.[7]
Polymerization
[edit]
Polymerization of ethylene to polyethylene is described by the following chemical equation:
n CH
2=CH
2 (gas) → [−CH
2−CH
2−]
[23]
n (solid) ΔH/n = −25.71 ± 0.59 kcal/mol (−107.6 ± 2.5 kJ/mol)
Ethylene is a stable molecule that polymerizes only upon contact with catalysts. The conversion is
highly exothermic. Coordination polymerization is the most pervasive technology, which means that
metal chlorides or metal oxides are used. The most common catalysts consist of titanium(III) chloride,
the so-called Ziegler–Natta catalysts. Another common catalyst is the Phillips catalyst, prepared by
depositing chromium(VI) oxide on silica.[7] Polyethylene can be produced through radical
polymerization, but this route has only limited utility and typically requires high-pressure apparatus.
Joining
[edit]
See also: Solvent bonding
Commonly used methods for joining polyethylene parts together include:[24]
Welding
Hot gas welding
Infrared welding
Laser welding
Ultrasonic welding
Heat sealing
Heat fusion
Fastening
Adhesives[24]
Pressure-sensitive adhesive (PSAs)
Dispersion of solvent-type PSAs
Polyurethane contact adhesives
Two-part polyurethane
Epoxy adhesives
Hot-melt adhesives
Solvent bonding – Adhesives and solvents are rarely used as solvent
bonding because polyethylene is nonpolar and has a high resistance to solvents.
Pressure-sensitive adhesives (PSA) are feasible if the surface chemistry or charge is modified
with plasma activation, flame treatment, or corona treatment.
Classification
[edit]
Polyethylene is classified by its density and branching. Its mechanical properties depend significantly
on variables such as the extent and type of branching, the crystal structure, and the molecular weight.
There are several types of polyethylene:
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)
Chlorinated polyethylene (CPE)
With regard to sold volumes, the most important polyethylene grades are HDPE, LLDPE, and LDPE.
Ultra-high-molecular-weight (UHMWPE)
[edit]
Main article: Ultra-high-molecular-weight polyethylene
Stainless steel and ultra high molecular weight polyethylene hip
replacement
UHMWPE is polyethylene with a molecular weight numbering in the millions, usually between 3.5 and
7.5 million amu.[25] 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, steel cable replacements on ships, and butchers'
chopping boards. It is commonly used for the construction of articular portions of implants used
for hip and knee replacements. As fiber, it competes with aramid in bulletproof vests.
