Polyethylene is a type of thermoplastic polymer, which means that this material becomes liquid when it reaches its melting point and solid when it reaches the freezing point.
Polyethylene is the chemical synthesis of ethylene, which is usually formed from a combination of crude oil and natural gases. Some of its informal names are polythene or polyethylyne, in addition it is also called PE for short. Polyethylene is mostly used to make plastic compounds to be used in pure form. This material, although used in a wide range of different uses.
polyethylene is a family of thermoplastics obtained by polymerization of ethylene gas (C2H4). Through the catalyst and polymerization method of this material, various properties such as density, melt flow index (MFI), crystallinity, degree of branching and crosslinking, molecular weight and molecular weight distribution can be controlled in them. Low molecular weight polymers are used as lubricants. Medium molecular weight polymers are miscible waxes with paraffin, and finally polymers with a molecular weight of more than 6,000 have the highest consumption in the plastics industry.
Polyethylene has a very simple structure, so it is simpler than all commercial polymers. A polyethylene molecule is a long chain of carbon atoms with two hydrogen atoms attached to each carbon atom.
What are the types of polyethylene?
Ethylenes are classified based on their density. Which is involved in the amount of density, the size of the polymer chain and the type and number of branches in the chain.
What is HDPE or Heavy Polyethylene?
This polyethylene has a branchless polymer chain. Therefore, the intermolecular strength in the high chain and its tensile strength is higher than other polyethylenes. The reaction conditions and the type of catalyst used in the production of heavy polyethylene HDPE are effective. For the production of polyethylene without branches, the polymerization method with Ziegler-Nata catalyst is usually used.
What is LDPE or Lightweight Polyethylene?
This polyethylene has a branched chain. Therefore, LDPE chains can not bond well with each other and have weak intermolecular strength and lower tensile strength. This type of polyethylene is usually produced by radical polymerization. The characteristics of this polymer are flexibility and the possibility of degradation by microorganisms.
What is LLDPE or Low Density Linear Polyethylene?
This polyethylene is a linear polymer with a number of short branches and is usually formed by copolymerizing ethylene with long-chain alkenes.
MDPE or polyethylene is medium density and MDPE is commonly used in the production of plastic pipes and piping fittings.
What is the use of polyethylene?
Polyethylene has many applications in the production of plastic appliances used in the kitchen and food industry. LDPE is used in the production of lightweight plastic containers as well as plastic bags. LDPE is used in the production of milk and liquid containers and all kinds of plastic kitchen utensils. MDPE is commonly used in the production of plastic pipes and piping fittings.
LLDPE is used in the production of a variety of flexible plastic devices such as bendable pipes due to its high degree of flexibility. Recently, many researches have been done in the production of long-chain polyethylenes with short branches. These polyethylenes are originally HDPE with a number of side branches. These polyethylenes have a combination of HDPE strength and LDPE flexibility.
Sometimes, instead of hydrogen atoms in a molecule (polyethylene), a long chain of ethylene is attached to carbon atoms, called branched polyethylene or light polyethylene (LDPE); Because its density has decreased due to more volume occupation. In this type of polyethylene, ethylene molecules are randomly attached to each other to form a very irregular shape. Its density is between 0.910 / 925. It is produced under high pressure and temperature, often using the free radical polymerization of vinyl radicals. Of course, Ziegler-Natta polymerization can also be used to prepare it.
When there are no branches in the molecule, it is called linear polyethylene (HDPE). Linear polyethylene is harder than branched polyethylene, but branched polyethylene is easier and cheaper to make. The shape and form of this polymer is very crystalline. Linear polyethylene is a normal product with a molecular weight of 200,000-500,000, which is polymerized under relatively low pressures and temperatures. There is also polyethylene whose density is between the density of these two polymers, ie in the range of 0.926 to 0.940; And it is called semi-heavy or medium polyethylene.
Polyethylene with a molecular weight between 3 and 6 million is called very high molecular weight polyethylene or UHMWPE and is produced by metallic catalyst polymerization. The material is more difficult to process, but its properties are excellent. When the polymer is completely crosslinked by radiation or the use of chemical additives, the polyethylene will no longer be soft. This material will be a real hard heat with good tensile strength, electrical properties and good impact strength over a wide range of temperatures when baked during or after molding.
It is used to make very strong fibers to replace Kevlar (a type of polyamide) in bulletproof vests; Also, its large plates can be used instead of ice skating rinks. By copolymerizing ethylene monomer with a branched alkyl monomer, a copolymer with short hydrocarbon branches is obtained, which is called low-density linear polyethylene, or LLDPE, and is often used to make objects like plastic films.
What is the history of polyethylene?
The word polymer is derived from the Greek word (poly) meaning several and (mer) meaning single or part. Polymers are also mistakenly called resins, elastomers, and plastics.
Plastic, on the other hand, is just an adjective used for cases that have the ability to deform under pressure and is often mistakenly used as a key word for the plastics industry and its products.
The word polymer was first coined by the renowned chemist Renalt in 1835, and the first commercial use of polymeric materials began in 1834 with the discovery of rubber. Hence, the first synthetic plastic called cellulose nitrate was discovered in 1862 and entered the market in 1868.
Nylon became popular in 1938, polyethylene in 1942, polypropylene in 1957, polybutylene in 1974, and liquid crystal polymers for making electronic components in 1985.
Polymers are divided into three types: natural, natural modified and synthetic polymers. The first modern industrial plastics became popular about 100 years ago, but in recent decades there has been increasing and varied growth in industry.
About 60 very important polymers have been introduced to the market so far, with more than 2,000 derivatives, and are still growing. Polyolefins are soft heat polymers with almost similar properties and close formulations. The most popular types are polyethylenes, polypropylenes and polybutylenes, which are more widely used in the polyethylene pipe industry.