POLYETHYLENE (PE)
Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most common plastic. Its primary use is in packaging (plastic bags, plastic films, geomembranes, containers including bottles, etc.). Many kinds of polyethylene are known, with most having the chemical formula (C2H4)n. PE is usually a mixture of similar polymers of ethylene with various values of n. Density is from 0.88 to 0.96 gm/cc.
(Polyethylene)
Properties
The properties of polyethylene can be divided into mechanical, chemical, electrical, optical, and thermal properties etc.
Mechanical properties
Polyethylene is of low strength, hardness and rigidity, but has a high ductility and impact strength as well as low friction. It shows strong creep under persistent force, which can be reduced by addition of short fibers. It feels waxy when touched.
Thermal properties
The usefulness of polyethylene is limited by its melting point of 80 °C (176 °F) (HDPE, types of low crystalline softens earlier). For common commercial grades of medium- and high-density polyethylene the melting point is typically in the range 120 to 180 °C (248 to 356 °F). The melting point for average, commercial, low-density polyethylene is typically 105 to 115 °C (221 to 239 °F). These temperatures vary strongly with the type of polyethylene.
Chemical properties
Polyethylene consists of nonpolar, saturated, high molecular weight hydrocarbons. Therefore, its chemical behaviour is similar to paraffin. The individual macromolecules are not covalently linked. Because of their symmetric molecular structure, they tend to crystallize; overall polyethylene is partially crystalline. Higher crystallinity increases density and mechanical and chemical stability.
Most LDPE, MDPE, and HDPE grades have excellent chemical resistance, meaning they are not attacked by strong acids or strong bases, and are resistant to gentle oxidants and reducing agents. Crystalline samples do not dissolve at room temperature. Polyethylene (other than cross-linked polyethylene) usually can be dissolved at elevated temperatures in aromatic hydrocarbons such as toluene or xylene, or in chlorinated solvents such as trichloroethane or trichlorobenzene.
Polyethylene absorbs almost no water. The gas and water vapor permeability (only polar gases) is lower than for most plastics; oxygen, carbon dioxide and flavourings on the other hand can pass it easily.
PE can become brittle when exposed to sunlight, carbon black is usually used as a UV stabilizer.
Polyethylene burns slowly with a blue flame having a yellow tip and gives off an odour of paraffin (similar to candle flame).
The material continues burning on removal of the flame source and produces a drip.
Polyethylene cannot be imprinted or stuck together without pre-treatment.
Electrical properties
Polyethylene is a good electrical insulator. It offers good tracking resistance; however, it becomes easily electrostatically charged (which can be reduced by additions of graphite, carbon black or antistatic agents).
Optical properties
Depending on thermal history and film thickness PE can vary between almost clear (transparent), milky-opaque (translucent) or opaque. LDPE thereby owns 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.
Joining of PE
Commonly used methods for joining polyethylene parts together include:
- Hot gas welding
- Fastening
- Infrared welding
- Laser welding
- Ultrasonic welding
- Heat sealing
- Heat fusion
Classification
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.
Cross-linked polyethylene (PEX or XLPE)
PEX is a medium- to high-density polyethylene containing cross-link bonds introduced into the polymer structure, changing the thermoplastic into a thermoset. 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.
Linear low-density polyethylene (LLDPE)
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, and 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. It is used for cable coverings, 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 film.
Copolymers
In addition to copolymerization with alpha-olefins, ethylene can also be copolymerized with a wide range of other monomers and ionic composition that creates ionized free radicals. Common examples include vinyl acetate (the resulting product is ethylene-vinyl acetate copolymer, or EVA, widely used in athletic-shoe sole foams) and a variety of acrylates. Applications of acrylic copolymer include packaging and sporting goods, and upper plasticizer, used for cement production.
Environmental issues
Although ethylene can be produced from renewables, polyethylene is mainly made from petroleum or natural gas.
Crosslinking
Various methods can used to prepare cross-linked polyethylene (PE-X) from thermoplastic polyethylene (PE-LD, PE-LLD or PE-HD). By crosslinking low-temperature impact strength, abrasion resistance and environmental stress cracking resistance can be increased significantly, whereas hardness and rigidity are somewhat reduced. PE-X does not melt anymore (analogous to elastomers) and is thermally resistant (over longer periods of up to 120 °C, for short periods without mechanical load up to 250 °C). With increasing crosslinking density also the maximum shear modulus increases (even at higher temperatures). PE-X has significantly enhanced properties compared with ordinary PE. As PE-X is infusible, always the final preproducts or the mold part are cross-linked.
Applications
PE-X is used as insulating material for medium and high voltage cable insulation, for hot water pipes and molded parts in electrical engineering, plant engineering and in automotive industry.
