Polyethylene

Polyethylene (PE) is an organic polymer made up of monomer subunits that have been polymerized. Polyethylene is made up of identical ethylene polymers. More than 500 ethylene subunits can be found in a typical polyethylene molecule.

Polyethylene sheet is the world’s most extensively used plastic. It may be molded into any shape, suitable for flexible, hard, and robust items. It’s used to line tubes and tanks and wrap  pipes to protect them from corrosive substances.

Chemical Structure of Polyethylene

• The length of the chain and the branching level significantly impact the plastic’s characteristics.
• The quantity of side-chain branching influences how near molecules can be brought together. More hard and solid polymers are produced via tightly packed chains.
• At times, the molecules will lie next to each other. A crystalline alignment is formed as a result of this process.

Plastics with a high crystalline arrangement content are more rigid and robust, and brittle. UHMWPE chains are 10–20 times longer than HDPE chains and contain fewer branching. Compared to lesser density PEs, this allows for creating many more crystalline regions.

What is the process for making polyethylene?

The polymerization technique is used to create a polyethylene sheet. Hydrocarbon fuels are distilled into lighter groups known as monomers of polyethylene, then exposed to a catalyst to initiate the polymerization process.

Many carbon chains are generated at the same time during polymerization. The lengthy chains are mobile when PE is molten, but they entangle and lock together as it cools.

The most frequent polymerization method is coordination polymerization, which uses metal chlorides and oxides. However, radical polymerization can also be used.

Polyethylene sheet comes in various forms, grades, and formulations, each with its qualities. PE’s density is determined by the manufacturing method. Low-density (LDPE) is produced one way, while high-density (HDPE) is produced.

The density of the plastic can be tweaked to make medium density (MDPE) and ultra-high molecular weight (UHMWPE) plastics. The shape and length of the carbon chains and how closely they compact determine the qualities of each variety of PE. Branched polyethylenes, linear polyethylenes, and cross-linked polyethylenes are the three most frequent forms of polyethylene sheet.

Types of polyethylene

High-density polyethylene (HDPE)

The degree of branching in HDPE is modest. It is the most durable and inflexible form. It’s made at a low temperature of 70°C to 300°C at 10-80 bar pressure.

It is made from gasses such as methane, ethane, propane, or a crude oil and gasoline blend.

It has a linear structure with limited branching, resulting in a more robust molecular structure, a high tensile strength, and a melting point of 210 to 270°C, and is used to make milk jugs, bottles, tubs, containers, and water pipes.

Low density polyethylene(LDPE)

Because of its significant short- and long-chain branching, LDPE has a lower tensile strength and greater elasticity. It has many short branches and is made up of 4,000-40,000 carbon atoms. It’s a semi-rigid, translucent polymer with a higher degree of branching on both the short and long sides.

Molten LDPE has unique and desirable flow properties as a result of this.It’s used in rigid containers and plastic film applications, including bags and film wrap.

 Low-density polyethylene  (LLDPE)

LLDPE has a structural resemblance to LDPE. It has a primarily linear polymer but contains a lot of small branching.It uses metallocene catalysts in a basic polymerization method that involves ethylene monomer, one butane, and lower amounts of 1 hexene and one octene.

It has a stronger tensile strength than LDPE, as well as greater impact and puncture resistance. LLDPE is robust, inflexible plastic. Larger goods, such as coverings, storage bins, and some types of containers can benefit from these capabilities.

Thermoplastic and thermosetting polymers

The physical response of polymers to heat can be used to classify them. Polyethylene is a thermoplastic that, when changed, can become a thermoset material (cross-linked polyethylene). 

Plastics soften and harden when heated and cooled, known as thermoplastics. Because the heating and chilling may be repeated and the thermoplastic can be reformed, this is the more popular type of plastic.

Thermosets are plastics that soften and can be shaped when heated but then solidify permanently. When reheated, they will decay.

Applications

The primary application of polyethylene was for electrical insulation. Its adaptability and application areas steadily expanded as time went on. Polyethylene can now be found in cling foil, packaging, bottles, and various plastic bags. Pipes, tubes, and gasoline tanks are formed or reinforced using it. 

Containers and lids, bearings, filters, covers,  gaskets, reinforcements, and bullet-proof vests are all made of more rigid polyethylene.

Injection molding of polyethylene in its melted state produces an extruded, cooled, and ready-formed product for end-use. Polyethylene’s elasticity and dependability are advantageous in blow molding and rotational molding.

Conclusion

One of the most widely used engineering plastics is polyethylene. The Chemical industry likes it because of its chemical resistance and ease of manufacture. The key to its adaptability is its molecular structure.