Polyethylene
Polyethylene is one of the simplest and cheapest polymers. Polyethylene is a waxy and inactive solid. This material is obtained from the polymerization of ethylene and is abbreviated as PE. Ethylene molecule has a C=C double bond. In the process of polymerization, the double bond of each monomer is broken and instead a simple bond is created between the carbon atoms of the monomers, and the product created is a macromolecule.
History of polyethylene production:
Polyethylene was first accidentally synthesized by the German chemist “Hans Von Pechmanv”. مکالمهها براساس جنسیت ارائه شده است. بیشتر بدانید Conversations are presented by gender. Know more These two scientists obtained a wax-like substance by heating a mixture of ethylene and benzaldehyde under high pressure.
The cause of this reaction was the presence of oxygen-containing impurities in the devices used, which acted as polymerization initiators. In 1935, Michael Perrin, another ICI scientist, developed this method and synthesized polyethylene under high pressure, which became the basic method for the industrial production of LDPE in 1939.
The use of various catalysts in the synthesis of polyethylene
An important event in the synthesis of polyethylene was the discovery of several new catalysts that enabled the polymerization of ethylene at a milder temperature and pressure than other methods. The first catalyst discovered in this field was chromium trioxide, which was discovered in 1951 by “Robert Banks” and “John Hosen” at Phillips Teprolium Company. In 1953, German chemist Karl Ziegler developed catalyst systems involving titanium halides and organic aluminum compounds.
These catalysts were usable under milder conditions than the Phillips catalysts and also produced polyethylene of an arrangement (with a regular structure). The third type of catalytic system was the use of metallocene compounds, which was produced in Germany in 1976 by “Walter Kaminicki” and “Hans Jogessin”. Ziegler and metallocene catalysts are very flexible in terms of function and are used in the process of copolymerization of ethylene with other olefins, which are the basis for the production of important polymers such as VLDPE, LLDPE, and MDPE.
Ziegler and metallocene catalysts are very flexible in terms of function and are used in the process of copolymerization of ethylene with other olefins, which are the basis for the production of important polymers such as VLDPE, LLDPE, and MDPE. Also, another type of catalyst called iminophthalate complex with group 6 metals has been considered, which show higher performance than metallocenes.
Types of polyethylene
The classification of polyethylenes is 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.
HDPE high density polyethylene
HDPE or High Density polyethylene
This polyethylene has unbranched polymer chains, so the intermolecular force in the chains is high and its tensile strength is higher than other polyethylenes. The reaction conditions and the type of catalyst used in the production of polyethylene HDPE are effective. Polymerization method with Ziegler-Natta catalyst is usually used to produce unbranched polyethylene.
LDPE low density polyethylene
LDPE or Low Density polyethylene
This polyethylene has a branched chain, so the LDPE chains cannot bond well with each other, and it has weak intermolecular forces and less tensile strength. This type of polyethylene is usually produced by radical polymerization. One of the characteristics of this polymer is flexibility and the possibility of decomposition by microorganisms.
Use of LLDPE as food coating
LLDPE or Linear Low Density polyethylene
This polyethylene is a linear polymer with a number of short branches and is usually created from the copolymerization of ethylene with long chain alkenes.
MDPE or Medium Density Polyethylene
MDPE or Medium Density Polyethylene
MDPE water supply pipes
Applications of polyethylene:
Polyethylene is widely used in the production of all kinds of plastic appliances used in the kitchen and food industry. LDPE is used in the production of light plastic containers as well as plastic bags. HDPE is used in the production of milk and liquid containers and all kinds of plastic kitchen utensils. MDPE is usually used in the production of plastic pipes and pipe fittings.
Due to its high degree of flexibility, LLDPE is used in the preparation of all kinds of flexible plastic items such as pipes with the ability to bend. Recently, many researches have been conducted in the production of polyethylene with long chains and short branches. These polyethylenes are basically HDPE with some side branches. These combined polyethylenes have the strength of HDPE and the flexibility of LDPE.