All about polyethylene PE

Polyethylene is one of the common types of polymers that are light, resistant, with different crystal structures, but in general, it is semi-crystalline. It is one of the most widely used plastics in the world and is used in various applications such as films, hoses, pipes, etc.

In the following article, we will talk about the types of polyethylene and their applications, the properties and characteristics of each of them, such as light and heavy polyethylene and mesh, polyethylene process conditions such as injection temperature, etc.

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

Types of polyethylene:

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

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

Branched polyethylenes: LDPE (light or low density polyethylene) and LLDPE (linear light polyethylene)

Linear polyethylenes: HDPE (heavy or high density polyethylene) and UHMWPE (high molecular weight polyethylene)

Reticulated polyethylene: PEX or LXPE

In some cases, polyethylene is divided into other categories such as MDPE (medium density polyethylene), VLDPE (very low density polyethylene).

HDPE heavy polyethylene

Heavy duty polyethylene is an economical thermoplastic with a linear structure and low degree of branching. Its polymerization temperature is low and around 300 degrees. Also, the pressure of the polymerization reactor is low (about 80 bar). HDPE is produced by two methods of slurry and gas phase polymerization. This polyethylene is flexible, semi-transparent, resistant to weather conditions and has appropriate hardness at very low temperatures.

Properties of heavy polyethylene HDPE:

• Melting point 120-140 degrees Celsius
• Density 0.970-0.940 grams per cubic centimeter
• Excellent resistance to most chemical solvents
• Very good resistance to alcohols
• Moderate resistance to oil and grease
• Poor resistance to hydrocarbons
• Higher tensile strength than other types of polyethylene
• Suitable processability
• Excellent electrical insulation properties
• Very little water absorption
• Ability to comply with food, drug and health laws

Disadvantages of heavy polyethylene HDPE:

• Prone to cracking
• Less stiffness than polypropylene
• Shrinkage in the above format
• Poor UV resistance
• Low heat resistance

Applications of heavy polyethylene HDPE

• Packaging industry: HDPE heavy polyethylene is used in packaging including boxes, trays, milk and fruit juice bottles, barrels due to its high impact resistance.
• Consumer goods: due to its relatively low price and easy processability, heavy polyethylene is a widely used material in everyday consumer goods such as ice boxes, toys, household appliances, garbage bags, etc.
• Fibers and textiles: due to high tensile strength, HDPE is widely used in making ropes, fishing nets, nets related to agricultural functions, industrial or even decorative fibers, etc.
• Pipes and fittings: heavy polyethylene is one of the most widely used materials in the production of plastic pipes and fittings due to the aforementioned properties and high toughness.
• Other applications: cables, manhals and plastic tanks, automotive industry, fuel tanks, etc.

light polyethylene LDPE

Light polyethylene is a semi-hard, semi-transparent polymer. Compared to HDPE, it has more side branches. The pressure of its production reactor is high (3000) and it is produced by free radical process. Two techniques are used to produce LDPE: autoclave or tubular reactor

Properties of light polyethylene:

• Melting point 115-105 degrees Celsius
• Density 0.940-0.910 grams per cubic centimeter
• Good resistance to alcohol 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
• It is transparent in thin film form

Disadvantages of light polyethylene LDPE :

• Prone to cracking
• Low strength, low stiffness
• High gas penetration
• It has carbon dioxide penetration
• Low UV resistance
• High flammability

Applications of light polyethylene LDPE:

Light polyethylene is mostly used in making containers, some bottles, plastic bags and various laboratory equipment. The most important application of LDPE is in plastic bags.

• Packaging: Due to its relatively low price and good flexibility, LDPE is used in the packaging of medicine bottles and squeeze bottles (SQUEEZE BOTTLE), lids, bags, plastic films for food packaging, etc.
• Pipe and connection: light polyethylene is used in some types of pipes and connections.

linear light polyethylene LLDPE 

Linear light polyethylene is a polymer that is used from the polymerization of ethylene with 1-butene monomer and low amounts of 1-hexene and 1-octene, using Ziegler Nata and metallocene catalysts, and is structurally similar to LDPE.

The structure of LLDPE is a linear main chain with very short non-identical branches.

Properties of linear light polyethylene LLDPE:

• High flexibility with high impact
• Milky and clear color
• Excellent resistance to strong and weak buffers
• Good chemical resistance
• Good water vapor penetration resistance
• Good impact resistance
• Good cracking resistance

Application of LLDPE:

They are suitable for various film applications such as stretch films, agricultural films and packaging. Ashless and residual polyethylene films burn in carbon dioxide. Polyethylene films do not contain softeners and heavy metals. They are physiologically safe. They are odorless and pollution free.

High molecular weight polyethylene UHMWPE

High molecular weight polyethylene has a molecular weight approximately 10 times higher than HDPE heavy polyethylene. It is made by metallocene catalyst.

UHMWPE has excellent mechanical properties, such as high wear resistance, high impact resistance and low friction coefficient

It is an almost inert polymer, and for this reason, it is used in environments with high corrosiveness at moderate temperatures. Even at high temperatures, it is resistant to a wide range of solvents (except aromatic and halogenated hydrocarbons, strong oxidizing substances 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 require high wear resistance.

PEX or XLPE network polyethylene

Heavy cross-linked polyethylene PEX or XLPE is a type of polyethylene designed by existing methods with cross-linked cross-links for sensitive applications.

The most common method of producing PEX network polyethylene is the reaction of polyethylene with peroxides under pressure, which causes the production of free radicals. These produced free radicals cause crosslinks in polyethylene. This type of polyethylene is used in the production of multi-layer pipes (PEX-AL-PEX) that have high heat resistance and also in the production of electrical insulation cables. The polyethylene network used in the wire and cable industry is usually called XLPE.

The question raised here is that when the polyethylene becomes networked (with a high gel percentage), it will not be melted and the process will be impossible due to the existing network structure created in it. How are these materials used in making pipes or cables?

To solve this problem, the act of networking or creating transverse links should be done after shaping. The usual method is to use a catalyst. In this way, polyethylene is usually prepared to react with its own catalyst through a series of chemical reactions. These materials are mixed with the composition of the manufacturer’s percentage (usually 95% prepared polyethylene and 5% catalyst) and put into the extruder to be shaped. After the tube or product is removed from the extruder, due to the presence of the catalyst, the baking and meshing process must be completed. This operation will be done in the presence of moisture and water vapor. There are two conventional methods for this, first, exposure to normal air and using moisture in the air (it takes time), which most PEX-Al-PEX pipe manufacturers use. Second, the use of steam room, which most cable manufacturers use these methods.

Characteristics of polyethylene network PEX

• Good high and low temperature resistance
• Resistance to hydrolysis
• High electrical and insulating properties
• High wear resistance
• No effect on drinking water
• High extrusion speed
• Good mechanical strength

Comparison of the properties of polyethylene types:

Polyethylene molding process

Polyethylene can also be shaped in different ways such as injection molding, blow molding, extrusion, rotary molding, etc.

The most common molding process for LDPE is extrusion. Of course, LDPE can be molded by injection and rotational methods.

UHMWPE high molecular weight polyethylene is processed by compression molding, ram extrusion, etc. And it cannot be processed by methods such as injection, normal extrusion or blowing because it does not flow and does not flow even at temperatures higher than its melting point.