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PE

PE (Polyethylene)
Polyethylene

A universal plastic used in every corner of life from industrial materials to daily sundries

Overview

Polyethylene is a resin discovered by accident in 1933 during a high-pressure experiment conducted in the lab of a British company, ICI. It began to be produced commercially in the United States in the early 1940s and came under the spotlight as a popular petrochemical product and as a representative of the plastics of universal purposes. There were researches on the method of making polyethylene with ethyl alcohol as its raw material, but it currently is made by way of ethylene polymerization, which uses naphtha (gasoline preparation) obtained from the distillation of crude oil as a raw material. Material properties change in a variety of manners depending on the molecular structure generated into the different forms, and the material properties of polyethylene are largely determined by crystallinity of the molecular structure (density), average molecular weight (melting resin), and distribution of molecular weight. Polyethylene has not only the common properties of the plastics - light, rustproof, and corrosion proof - but also a wide range of excellent properties including chemical stability, weather resistance, water resistance, low temperature impact resistance, insulative property, moldability, etc. Moreover, it belongs to the lightest group of plastics, which makes it affordable. As mentioned above, it is used in a variety of applications in everyday life ranging from industrial materials to daily sundries.

Features

① By density, it is divided into
● HD-PE [high density polyethylene] ⇒ (high-density polyethylene = rigid polyethylene) and
● LD-PE [low density polyethylene] ⇒ (low density polyethylene = soft polyethylene).
② Food safety (conforms to Food Sanitation Act.)
③ Has excellent cryogenic properties (does not get soft.)

Flexibility is one of PE's properties, and it can be either a benefit or a drawback. PE is a semi-transparent to opaque plastic of milky white color with a lead-like feeling. It has a relatively low specific gravity along with excellent water resistance and electrical insulation (especially high-frequency insulation) and resists acid and alkali well. In addition, it is hygienically non-toxic and does not lose flexibility at a very low temperature but has a strong impact resistance. While PE hardly allows water to pass through it, permeability of carbonic acid gas, organic solvents, spices, etc. is very high.

PE is chemically very stable and does not get dissolved by any solvent at room temperature. And, because it is completely non-toxic, it is hard to glue with adhesives or print on its surface.

As is common to both polyethylene and crystalline plastics, the greater the crystallinity (crystalline density) is, the higher the melting point, softening point, yield strength, stiffness, chemical resistance, hardness, etc. are. In the mean time, its transparency decreases or the elasticity declines. Machining also becomes difficult, and currently this relationship is often called Ml (Melt Index) as a standard. MI is also known as the MFI.

As such, PE's properties change greatly depending on density, Ml, and distribution of molecular weight. PE has shortcomings of large mold shrinkage, vulnerability to repetition, bad heat-resistance, etc.

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