What is Polyethylene

Polyethylene is a type of polymer that is classified as a thermoplastic, meaning that it can be melted to a liquid and remolded as it returns to a solid state. As the name implies, polyethylene is chemically synthesized from molecules that contain long chains of ethylene, a monomer that provides the ability to double bond with other carbon-based monomers to form polymers. Polyethylene is known by other, non-official names, such as polythene in the United Kingdom. In addition, it is sometimes spelled as polyethylyne, or abbreviated to simply PE.

The first laboratory creation of polyethylene occurred in 1898 by accident at the hands of Hans von Pechmann while applying heat to another compound the German chemist previously discovered — diazomethane. Ironically, the synthesis of polyethylene via extreme heat and pressure in an industrial setting was again made by accident, but 35 years later. A few years later, another chemist employed by the same England-based chemical company devised a method to consistently produce polyethylene under the same conditions. As a result, polyethylene became the primary source of low-density polyethylene (LDPE) production in 1939.

While polyethylene is essential to the economic health of the plastics industry, most consumers readily recognize the role it plays in everyday life. In fact, this substance is found in many ordinary household items, such as food wrap, shampoo bottles, milk containers, toys, and the common plastic bag used to tote groceries home from the store. However, polyethylene is also present in numerous other products that contain plastic components. For instance, it is used to manufacture artificial knee and hip replacement parts, bulletproof vests, and even glassy flooring for ice skating rinks.

Polyethylene may fall under one of several types. The distinction between them is determined by its molecular weight and branching, which is affected by its crystallization. LDPE is an example of branched polyethylene since its carbon molecules are attached to long chains of polyethylene instead of hydrogen. Otherwise, a linear structure of carbon to hydrogen occurs, which is known as high-density polyethylene (HDPE). However, further variances in structure and molecular weight produce other forms, such as ultra high molecular weight polyethylene (UHMWPE), medium-density polyethylene (MDPE), or very low-density polyethylene (VLDPE).

While polyethylene may help to make numerous useful and durable products possible, its environmental impact is cause for concern. For one thing, it does not readily biodegrade and can reside in a landfill for hundreds of years. However, diligent recycling may significantly reduce this problem. In addition, scientists are exploring the possibility of employing Sphingomonas, an aerobic bacteria shown to shorten biodegrading of some forms of polyethylene to just a few months. Environmental preservation efforts have also led to the development of bioplastics, with the aim of synthesizing polyethylene from ethanol obtained from sugarcane.

By far the most popular thermoplastic commodity used in consumer products (especially products created by rotational moulding), polyethylene is created through the polymerization of ethylene (i.e., ethene).

Chemical Composition

The ethylene molecule is C2H4   (CH2=CH2)

A.K.A.

Polyethene, Polythene, PE, LDPE, HDPE, MDPE, LLDPE

  • LDPE (Low Density Polyethylene) is defined by a density range of 0.910 – 0.940 g/cm3. It 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 branches with long chains gives molten LDPE unique and desirable flow properties.
  • HDPE (High Density Polyethylene) 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.
  • MDPE (Medium Density Polyethylene) 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.
  • LLDPE (Linear-Low Density Polyethylene) is defined by a density range of 0.915 – 0.925 g/cm3. is a substantially linear polymer, with significant numbers of short branches, commonly made by copolymerization of ethylene with short-chain alpha-olefins (e.g. 1-butene, 1-hexene, and 1-octene).

Properties

LDPE Properties:
Semi-rigid, translucent, very tough, weatherproof, good chemical resistance, low water absorption, easily processed by most methods, low cost.

LDPE Physical Properties: Value:
Tensile Strength: 0.20 – 0.40 N/mm2
Notched Impact Strength: no break
Thermal Coefficient of Expansion: 100 – 220 x 10-6
Max. Continued Use Temperature: 65 oC (149 oF)
Melting Point: 110 oC (230 oF)
Glass Transition Temperature: -125 oC (-193 oF)
Density: 0.910 – 0.940 g/cm3

HDPE Properties:
Flexible, translucent/waxy, weatherproof, good low temperature toughness (to -60’C), easy to process by most methods, low cost, good chemical resistance.

HDPE Physical Properties: Value:
Tensile Strength: 0.20 – 0.40 N/mm2
Notched Impact Strength: no break
Thermal Coefficient of Expansion: 100 – 220 x 10-6
Max. Continued Use Temperature: 65 oC (149 oF)
Melting Point: 126 oC (259 oF)
Density: 0.941 – 0.965 g/cm3
From Website
Edited by Leafly Mould Provides Injection Mold, Die Casting Mold, Stamping Mold

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