Editing Polyethylene (section)

==Classification==
Polyethylene is classified by its [[density]] and [[Branching (polymer chemistry)|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)===
{{main|Ultra-high-molecular-weight polyethylene}}
[[File:Stainless steel and ultra high molecular weight polythene hip replacement (9672239334).jpg|thumb|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 [[Atomic mass unit|amu]].<ref>{{cite book|author=Kurtz, Steven M. |title=UHMWPE Biomaterials Handbook. Ultra-High Molecular Weight Polyethylene in Total Joint Replacement and Medical Devices|edition= 3rd| page=3|isbn=9780323354356 |year=2015|publisher=Elsevier|doi=10.1016/C2013-0-16083-7}}</ref> The high molecular weight makes it a very [[toughness|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&nbsp;g/cm<sup>3</sup>). 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 [[implant (medicine)|implants]] used for [[hip replacement|hip]] and [[knee replacement]]s. As [[Ultra-high-molecular-weight polyethylene#UHMWfiber|fiber]], it competes with [[aramid]] in [[bulletproof vests]].

===High-density (HDPE)===
{{main|High-density polyethylene}}
[[File:Site Photo of HDPE Pipe.jpg|thumb|HDPE pipe on site during installation in outback Western Australia. The white outer layer, Acu-Therm, is co-extruded to provide a reduction of thermal heating.]]
HDPE is defined by a density of greater or equal to 0.941&nbsp;g/cm<sup>3</sup>. HDPE has a low degree of branching. The mostly linear molecules pack together well, so intermolecular forces are stronger than in highly branched polymers. HDPE can be produced by [[chromium]]/silica catalysts, [[Ziegler–Natta catalyst]]s or [[metallocene]] catalysts; by choosing catalysts and reaction conditions, the small amount of branching that does occur can be controlled. These catalysts prefer the formation of [[free radicals]] at the ends of the growing polyethylene molecules. They cause new ethylene monomers to add to the ends of the molecules, rather than along the middle, causing the growth of a linear chain.

HDPE has high tensile strength. It is used in products and packaging such as milk jugs, detergent bottles, butter tubs, garbage containers, and [[HDPE pipe|water pipes]].

===Cross-linked (PEX or XLPE)===
{{Main|Cross-linked polyethylene}}
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.

===Medium-density (MDPE)===
{{Main|Medium-density polyethylene}}
MDPE is defined by a density range of 0.926–0.940&nbsp;g/cm<sup>3</sup>. 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.

===Linear low-density (LLDPE)===
{{Main|Linear low-density polyethylene}}
LLDPE is defined by a density range of 0.915–0.925&nbsp;g/cm<sup>3</sup>. LLDPE is a substantially linear polymer with significant numbers of short branches, commonly made by [[copolymerization]] of ethylene with short-chain [[alpha-olefin]]s (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 they are 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 films.
===Low-density (LDPE)===
{{Main|Low-density polyethylene}}
LDPE is defined by a density range of 0.910–0.940&nbsp;g/cm<sup>3</sup>. 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 [[Radical polymerization|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.

The radical polymerization process used to make LDPE does not include a catalyst that "supervises" the radical sites on the growing PE chains. (In HDPE synthesis, the radical sites are at the ends of the PE chains, because the catalyst stabilizes their formation at the ends.) Secondary [[free radical|radicals]] (in the middle of a chain) are more stable than primary radicals (at the end of the chain), and tertiary radicals (at a branch point) are more stable yet. Each time an ethylene monomer is added, it creates a primary radical, but often these will rearrange to form more stable secondary or tertiary radicals. Addition of ethylene monomers to the secondary or tertiary sites creates branching.

===Very-low-density (VLDPE)===
VLDPE is defined by a density range of 0.880–0.915&nbsp;g/cm<sup>3</sup>. 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.

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 one in 100 or 1,000 branches per backbone carbon, can significantly affect the [[rheology|rheological]] properties of the polymer.

===Copolymers===
In addition to [[copolymerization]] with alpha-olefins, ethylene can 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 [[acrylate]]s. Applications of [[Acrylic resin|acrylic]] copolymer include packaging and sporting goods, and [[superplasticizer]], used in cement production.

=== Types of polyethylenes ===
The particular material properties of "polyethylene" depend on its molecular structure. Molecular weight and crystallinity are the most significant factors; crystallinity in turn depends on molecular weight and degree of branching. The less the polymer chains are branched, and the lower the molecular weight, the higher the crystallinity of polyethylene. Crystallinity ranges from 35% (PE-LD/PE-LLD) to 80% (PE-HD). Polyethylene has a density of 1.0&nbsp;g/cm<sup>3</sup> in crystalline regions and 0.86&nbsp;g/cm<sup>3</sup> in amorphous regions. An almost linear relationship exists between density and crystallinity.<ref name="Kaiser" />

The degree of branching of the different types of polyethylene can be schematically represented as follows:<ref name="Kaiser" />
{| class="wikitable skin-invert-image"
|-
| PE-HD || [[File:PE-HD schematic.svg|Schematic representation of PE-HD (high-density polyethylene)]]
|-
| PE-LLD ||
[[File:PE-LLD schematic.svg|Schematic representation of PE-LLD (linear low-density polyethylene)]]
|-
| PE-LD || [[File:PE-LD schematic.svg|Schematic representation of PE-LD (low-density polyethylene)]]
|}
The figure shows polyethylene backbones, short-chain branches and side-chain branches. The polymer chains are represented linearly.

=== Chain branches ===
The properties of polyethylene are highly dependent on type and number of chain branches. The chain branches in turn depend on the process used: either the high-pressure process (only PE-LD) or the low-pressure process (all other PE grades). Low-density polyethylene is produced by the high-pressure process by radical polymerization, thereby numerous short chain branches as well as long chain branches are formed. Short chain branches are formed by [[Intramolecular reaction|intramolecular]] [[chain transfer]] reactions, they are always [[butyl group|butyl]] or [[ethyl group|ethyl]] chain branches because the reaction proceeds after the following mechanism:
:[[File:PE-LD mechanism en.svg|class=skin-invert-image|Mechanism for the emergence of side chains during synthesis of polyethylene (PE-LD).]]