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Kinds of Polyethylene and Polypropylene

Polyethylene takes various forms. The typical kinds of polyethylene are high-density polyethylene (HDPE), liner low-density polyethylene (LLDPE) and low-density polyethylene (LDPE)

Schematic model of the structure of polyethylene (molecular weight distribution not included)

In addition to the above types, polyethylene comes in an increasing variety of kinds, such as VLDPE (very low density polyethylene), ULDPE (ultra-low-density polyethylene), polyethylene elastomer, EVA (ethylene vinyl acetate copolymer) and EMAA ethylene-methacrylic acid copolymer). VLDPE and ULDPE are made according to the same formula as LLDPE, and have lower density than LLDPE. EVA and EMMA are high-pressure co-polymers, which are produced by high-pressure radical polymerization, in the same way as LDPE is.

High-density polyethylene (HDPE) has a higher density than any other kind of polyethylene. It has a wide range of applications in products or fields that require high stiffness, while some other physical properties of a product also have to be optimized. HDPE is broadly divided into the two types: HDPE produced by a Ziegler catalyst in single-stage polymerization (partially including HDPE produced by a metallocene catalyst) and HDPE produced by a Ziegler catalyst in a multi-stage polymerization or a Cr or Phillips-type catalyst. The former type, whose molecular weight distribution is relatively narrow, has applications in injection moldings or flat yarns, and the latter type, whose molecular weight distribution is wide, is used to make film products (e.g. a checkout bag at a supermarket or an ultra-thin film), hollow plastic products (e.g. a gasoline tank or a cooking oil bottle), and pipes.

Next, LLDPE or linear low-density polyethylene is produced by a catalyst for polymerization as HDPE is. "Because LLDPE has such a low density, it is in high demand in products the require flexibility, transparency, and low-temperature sealability." Traditionally, LLDPE produced by a Ziegler catalyst was often used to produce general-purpose transparent films. But thanks to the successful practical use of a single-site catalyst or a metallocene catalyst, LLDPE has been gaining popularity for use in films and laminates since the middle of the 1990s. LLDPE features a narrow molecular weight distribution, narrow composite distribution, high strength, low extraction and heat sealability. Furthermore, only a small volume of a metallocene catalyst is required to produce LLDPE. As a result, THP now offers product lines that do not contain any additive agents, which were used in polyethylene to maintain heat stability throughout the molding process.

On the other hand, LDPE, which is produced by high-pressure radical polymerization, has a disadvantage in strength compared to LDPE, but is superior in extrusionability and formability. In addition, LDPE contains much less foreign substances or FE (fish eyes) because there is no catalyst used for polymerization. Therefore, LDPE has a wide range of application in films, extrusion moldings, and laminates.

Kinds of polyethylene

Kind

Catalyst and process

Features

Example of use

High-density polyethylene (HDPE)

Produced by a Ziegler catalyst in single-stage polymerization

High density and medium molecular weight distribution

Injection moldings, and flat yarns

Produced by a Ziegler catalyst in multistage polymerization

Cr catalyst

High density and wide molecular weight distribution

Films, hollow products, and pipes

Linear low-density polyethylene (LLDPE)

Produced by a Ziegler catalyst

Low density and medium molecular weight distribution

General-purpose films

Produced by a metallocene catalyst

Low density, narrow molecular weight distribution, and narrow composite distribution

Films (including the original fabrics of dry laminates) and lamination

Low-density polyethylene (LDPE)

Produced by radical polymerization, and tubular method

Low density and long-chain branching

Films and injection moldings

Produced by radical polymerization, and auto clave method

Low density, long-chain branching, and wide molecular weight distribution

Laminates


Polypropylene also comes in a variety of kinds such as homo polypropylene (homo-PP), random polypropylene (random-PP) and block polypropylene (block-PP). Homo-PP is polymerized by only propylene. Random-PP is co-polymerized by propylene and a small volume of ƒ¿-olefin other than propylene (e.g. ethylene) in order to add flexibility, transparency, and low temperature sealability. Block-PP (also known as HIPP) is co-polymerized by homo-PP and ethylene-propylene rubber in order to keep the balance of stiffness and shock resistance. Homo-PP and Random-PP are used in a wide variety of applications such as package films, and block-PP in the injection molding of automotive parts.

THP carefully selects and identifies the polyethylene or polypropylene supplied at home and abroad, and uses the one that best fits the purpose of a product and the needs of our customers for performance and cost effectiveness.


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Technical information,@Other pages
1.Polyethylene, Polypropylene and THP's Products 2.Kinds of polyethylene and polypropylene
3.Structure and Physical Properties of Polyethylene and Polypropylene 4.THP's Design and Development