Polyethylene is one of the common types of polymers that is lightweight and durable with different crystalline structures but is generally semi-crystalline. It is one of the most widely used plastics in the world and is used in various applications such as films, hoses, and pipes. In the continuation of this article from PETROPROVIDERS, we will talk about the types of polyethylene and their applications, properties, and characteristics of each of them, such as light and heavy polyethylene and mesh polyethylene, polyethylene process conditions such as injection temperature.

Familiarity with the structure and types of polyethylene:

Polyethylene is made from the polymerization of ethylene monomer and its structure is as follows:

polyethylene structure

polyethylene structure

Polyethylene is classified based on its density and linear structure. The most important types of polyethylene are:

  • Branched polyethylenes: LDPE (low-density polyethylene) and LLDPE (linear low-density polyethylene)
  • Linear polyethylenes: HDPE (high-density polyethylene) and UHMWPE (high molecular weight polyethylene)
  • Mesh polyethylene: PEX or LXPE

Polyethylene is sometimes divided into other categories such as MDPE (Medium Density Polyethylene) and VLDPE (Very Low-Density Polyethylene).

High-density polyethylene or HDPE

High-density polyethylene is a cost-effective thermoplastic with a linear structure and low branching grade. Its polymerization temperature is low and about 300 degrees. The polymerization reactor pressure is also low (about 80 bar). HDPE is produced by two methods of slurry polymerization and gas-phase polymerization. This polyethylene is flexible, translucent, resistant to weather conditions, and has good hardness at very low temperatures.

The most important properties of HDPE:

  • Melting point: 140-120 ° C
  • Density or density: 0.940 to 0.970 g per cubic centimeter
  • Excellent resistance to most chemical solvents
  • Very good resistance to alcohols
  • Medium resistance to oil and grease
  • Poor resistance to hydrocarbons
  • Higher tensile strength than other types of polyethylene
  • Proper processability
  • Excellent electrical insulation properties
  • Very low water absorption
  • Ability to comply with food, medicine, and health laws

Disadvantages of HDPE:

  • Fraction
  • Less stiffness than polypropylene
  • Shrinkage in the mold
  • Poor UV resistance
  • Low heat resistance

Applications of high-density polyethylene or HDPE

high-density polyethylene

high-density polyethylene

Packaging Industry:

High-density polyethylene or HDPE is used in the packaging boxes, trays, milk and juice bottles, barrels due to their high impact resistance.

Consumer goods:

Due to its relatively low price and easy processing, high-density polyethylene is a widely used material in everyday consumer goods such as iceboxes, toys, home appliances, and garbage bags.

Fibers and textiles:

Due to their high tensile strength, HDPE is widely used in rope making, fishing nets, agricultural nets, industrial fibers, or even decorative fibers.

Pipes and fittings:

High-density polyethylene is one of the most widely used materials in the production of plastic pipes and fittings due to the said properties and high toughness.

Other important applications of HDPE:

Production of cables, plastic tanks, automotive industry, fuel tanks, and …

Low-density polyethylene or LDPE

Low-density polyethylene is a semi-hard and semi-transparent polymer. It has more lateral branches than HDPE. The production pressure of the reactor is high (3000 bar) and it is produced by the free-radical process. Two techniques are used to produce LDPE: autoclave or tubular reactor

Properties of low-density polyethylene:

  • Melting point: 105 – 115 ° C
  • Density: 0.910-0.940 g per cubic centimeter
  • Good resistance to alcohols and acids
  • Limited resistance to aliphatic and aromatic hydrocarbons, mineral oils
  • Temperature resistance up to 80 degrees (up to 95 degrees for a short time)
  • Good processability
  • High impact resistance at low temperatures
  • Good resistance to weather conditions
  • Excellent electrical insulation properties
  • Very low water absorption
  • In the form of a thin film, it is transparent

Disadvantages of low-density polyethylene or LDPE:

  • Fraction
  • Low strength, low stiffness
  • High gas penetration
  • It penetrates carbon dioxide
  • Low UV resistance
  • High flammability

High molecular weight polyethylene or UHMWPE

High molecular weight polyethylenes have a molecular weight of approximately 10 times that of HDPE. This is one of the most important types of polyethylene made by metallocene catalysts.

UHMWPE has excellent mechanical properties, such as high abrasion resistance, high impact resistance, and low friction coefficient. It is an almost inert polymer and is therefore used in environments with high corrosion at medium temperatures. It is resistant to a wide range of solvents even at high temperatures (except for aromatic and halogenated hydrocarbons, strong oxidants such as nitric acid).

These excellent properties allow UHMWPE to be used in high-performance applications. Such as pipes, liners, silos, and containers or equipment that need high abrasion resistance.

Mesh polyethylene (PEX or XLPE)

High-density cross-linked polyethylene PEX or XLPE is a type of polyethylene designed by existing methods with crosslinked crosslinking for sensitive functions.

The most common method of producing PEX is the reaction of polyethylene under pressure with peroxides, which causes the production of free radicals. These free radicals are generated by crosslinking in polyethylene.

This type of polyethylene is used in the production of multilayer pipes (PEX-AL-PEX) that have high heat resistance and also in the manufacture of electrical insulation cables. This polyethylene, commonly used in the wire and cable industry, is called XLPE.

The question here is, when does polyethylene become networked? Due to the structure of the existing network created in it, it will not melt and its process will be impossible. How is this material used in the manufacture of pipes or cables?

To solve this problem, networking or cross-linking should be done after shaping. The usual method is to use a catalyst. In this way, polyethylene is usually prepared to react with a special catalyst by a series of chemical reactions. These materials are mixed with a percentage of constituents (usually 95% polyethylene and 5% catalyst) and inserted into the extruder to form.

After the pipe or product is removed from the extruder, due to the presence of a catalyst, the cooking and networking operation must be completed. This operation will be performed in the presence of moisture and water vapor.

There are two common ways to do this, first is to expose the atmosphere to air and use the humidity in the air (it takes time); Most PEX-Al-PEX pipe manufacturers use this method. The second is the use of steam chambers, which most cable manufacturers use.

PEX Properties:

  • Good high and low-temperature resistance
  • Hydrolysis resistance
  • High electrical and insulating properties
  • High abrasion resistance
  • No effect on drinking water
  • High extrusion speed
  • Suitable mechanical strength

The process of forming all types of polyethylene

forming all types of polyethylene

forming all types of polyethylene

Polyethylene can also be formed by various methods such as injection molding, blow molding, extrusion, and rotary molding. The most common process of forming low-density polyethylene is extrusion. Of course, LDPE can also be formed by injection and rotation methods.

High molecular weight polyethylene UHMWPE is processed by compression molding and extrusion ROM. It cannot be processed by methods such as injection, conventional extrusion, or blowing; Because it does not flow even at temperatures above its melting point.

Summary of types of polyethylene and their applications

Polyethylene is a thermoplastic polymer (heat elastic) with a soft crystal structure. Polyethylene is one of the main and important plastics in the world that has mass production in the market. Obviously, due to its high spread and diversity among polymers, it has various applications and properties.

Types of polyethylene, according to their density, will be divided into various categories, including high density, low density, and low linear density. One of the most prominent features of this polymer is its widespread use in the food industry due to the non-toxicity of polyethylene.

What is your opinion about the types of polyethylene and their applications? Is this material affordable for use in the plastics industry? Is it appropriate to use this polymer to produce different products?

You can share your comments with us in the comments section. If this article was helpful to you, it’s useful for your friends too! Share for them too …

What are the most important applications of polyethylene?

Polyethylene is used to make toys, beverage bottles, food packaging, trash cans, building insulation, and wire and cable.

Is polyethylene harmful to human health?

This polymer is one of the most important and widely used polymers in the world. Polyethylene has low resistance to UV rays; Therefore, it can be dangerous when it enters the human body.

What are the problems of polyethylene?

This polymer, when heated, produces toxic gases such as activated carbon dioxide. These gases increase respiratory diseases in humans and are dangerous for the environment.

Is it correct to use polyethylene plastics?

Polyethylene plastics have good impact resistance and temperature rise. This material is used to produce durable plastics; But with excessive temperature rise, it loses its physical properties.