Polyethylene, Chemistry tutorial


The word 'polyethylene' signifies repeating units of ethylene. Such invisibly small parts of ethylene (that is, monomers) are the building blocks for polyethylene throughout production. Polyethylene is the thermoplastic polymer that can be melted to a liquid and remolded as it returns to the solid state. The IUPAC name of polyethylene is poly (methylene). The general formula for polyethylene is ~ (CH2 - CH2)n ~. Polyethylene is chemically synthesized from molecules which have long chains of ethylene monomer.

Classification of Polyethylene:

Polyethylene is categorized into some different categories based generally on its density and branching. The mechanical properties of polyethylene significantly based on variables like the extent and kind of branching, the crystal structure and the molecular weight. There are three main classes of polyethylene. These are low density polyethylene (LDPE), high density polyethylene (HDPE) and linear low density polyethylene (LLDPE).  

1) High density polyethylene (HDPE):

It is chemically the closest in structure to pure polyethylene. It comprises mainly of unbranched molecules having very few ?aws to mar its linearity. By an extremely low level of defects to hinder organization, a high degree of Crystallinity can be accomplished, resultant in resins which encompass a high density (relative to other kinds of polyethylene). Some resins of this kind are copolymerized having a very small concentration of 1-alkenes in order to decrease the Crystallinity level slightly. High density polyethylene resins in general have densities falling in the range of around 0.94-0.97 g/cm3. Because of its very low level of branching, high density polyethylene is at times termed to as linear polyethylene (LPE).

2) Low density polyethylene (LDPE):

The low density polyethylene (LDPE) is so named as these polymers have substantial concentrations of branches which hinder the crystallization method, resultant in relatively low densities. The branches mainly comprise of ethyl and butyl groups altogether by some long chain branches. Because of the nature of the high pressure polymerization method via which low density polyethylene is produced, the ethyl and butyl branches are often clustered altogether, separated through lengthy runs of unbranched backbone. Long chain branches take place at random intervals all along the length of the major chain. The long chain branches can themselves in turn be branched. The various branches characteristic of low density polyethylene molecules inhibit their capability to crystallize, reducing resin density relative to the high density polyethylene. Low density polyethylene resins generally encompass densities falling in the range of around 0.90-0.94 g/cm3.  

3) Linear low density polyethylene (LLDPE):

Linear low density polyethylene (LLDPE) resins comprise of molecules having linear polyethylene backbones to which are linked short alkyl groups at arbitrary intervals. These materials are formed by the copolymerization of ethylene having 1-alkenes. The branches most generally encountered are ethyl, butyl or hexyl groups however can be a variety of other alkyl groups, both linear and branched. A typical average separation of branches all along the main chain is 25 to 100 carbon atoms. Linear low density polyethylene resins might as well have small levels of long chain branching; however there is not similar degree of branching complexity as is found in the low density polyethylene. Chemically such resins can be thought of as a compromise between the linear polyethylene and low density polyethylene, therefore the name. The branches obstruct crystallization to certain extent, reducing density relative to high density polyethylene. The outcome is a density range of around 0.90-0.94 g/cm3.

Production process of Polyethylene:

Polyethylene manufacturing procedure is categorized to 'High pressure' and 'Low pressure' operations. The high pressure is usually recognized as producing conventional low density polyethylene (LDPE) whereas the low pressure makes high density polyethylene (HDPE) and linear low density polyethylene (LLDPE).

1) High pressure production process:

The polyethylene products as well known as low density polyethylene (LDPE) resins are made exclusively by high pressure free radical polymerization. The chemistry engaged in their production is deceptively simple, requiring little more than a suitable source of free radicals and conditions of high pressure and temperature. The free radicals initiate the polymerization procedure whenever the monomers have been forced to close proximity via high pressure. Termination of chain growth takes place whenever the free radical on a growing chain is transferred to the other chain or is quenched via the other radical. In practice, many competing side reactions take place which result in branching and premature chain termination. The nature of the product is controlled through the initiator concentration, pressure, temperature, availability of vinyl co monomers, and the presence of the chain transfer agents.

Polyethylene was first made by the high pressure process in the year 1930. It was discovered that ethylene gas could be transformed into a white solid by heating it at very high pressures in the presence of small quantities of oxygen:

Ethylene + 10 ppm Oxygen → (1000-3000 bar/80-300°C) → Polyethylene

The polymerization reaction that takes place is a random one, producing a broad distribution of molecule sizes. By controlling the reaction conditions, it is probable to choose the average molecule size (or molecule weight) and the distribution of sizes around this average (that is, molecular weight distribution). The chains are highly branched (at intervals of 20 to 50 carbons).  This new plastic was termed 'polythene' and was produced in a density range of around 0.915 to 0.930g cm3. It is nowadays termed as LDPE and consists of its single biggest usage in blown film.

2) Low Pressure Production process:

The polymerization of ethylene at much lower pressures as well yields in the manufacture of polyethylene.

Ethylene → (10-80 bar/70-300°C, Al catalyst) → Polyethylene

HDPE is produced via three kinds of process that are slurry process, solution process and gas phase process. All operate at relatively low pressures (10 to 80 atm) in the presence of a Ziegler-Natta or inorganic catalyst. In all three methods hydrogen is mixed by ethene to control the chain length of the polymer. The product from this procedure was found to be much stiffer as compare to previous products and had a density range of around 0.940 to 0.970g cm3. The increased stiffness and density were found to be due to a much lower level of chain branching. The latest HDPE was found to be comprised of very straight chains of ethylene having a much narrower distribution of molecular weights (or chain lengths) and a potentially very high average chain length.

The low pressure procedure was as well applied to the production of LLDPE. LLDPE is made up by copolymerizing of ethylene by a small amount of the other monomer, generally butene, hexene or octene.  The most general process employed in industry is to polymerize ethylene by means of the fluidized reactor bed.

A fluidized reactor bed comprises of metallic catalyst particles which are 'fluidized' via the flow of ethylene gas. This signifies that the catalyst particles are suspended in the ethylene fluid as ethylene gas is pumped from the bottom of the reactor bed to the top.

Before ethylene is sent to the fluidized bed, it should first be compressed and heated. Pressures in the range of 100 to 300 pounds per square inch (psi) and a temperature of100°C are required for the reaction to carry on at a reasonable rate. The catalyst is as well pumped by the ethylene stream to the reactor. This is due to the reason polyethylene molecules remain stuck to the catalyst particle from which they were produced therefore incorporating the catalyst in the polyethylene product, therefore the need to refill the consumed catalyst.

The translation of ethylene is low for a single pass via the reactor and it is essential to recycle the unreacted ethylene. Unreacted ethylene gas is removed off the top of the reactor. After purification, ethylene gas is then recompressed and recycled back to the reactor. Granular polyethylene is gradually eliminated from the bottom of the reactor as soon as reasonable conversions have been accomplished. Generally, residence time of 3 to 5 hours yields in a 97% conversion of ethylene. Polyethylene comes out of the reactor as granular powder, which is then melted and flows via a film extruder. Whatever the kind of polyethylene produced, the end product is generally available in the form of small pellets, varying in shape (at is, spherical, rectangular and cylindrical) based on the manufacturer's equipment. Throughout the manufacture of polythene products, it is melted to flow via a film extruder.

LDPE is the preferred packaging material because of its limp feel, toughness, transparency and the capability to quickly take up the shape of the contents of the bag. The garbage bag is just one of many broadly practical utilizes of plastic bags. Polyethylene film, made by blown film extrusion, is generally employed for packaging of foodstuffs and other products. The thickness of the film produced tends to be making 20 to 200 μm.

Uses of Polyethylene:

1) The LDPE or LLDPE form is favored for film packaging and for electrical insulation.  

2) HDPE is blow-molded to make containers for household chemicals like washing-up liquids and drums for the industrial packaging. 

3) HDPE is as well extruded as piping.

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