Polymer Technology

Raw Material:

The first polyethylene plant built by ICI used ethylene produced by dehydration of ethanol. Modern ethylene production plants are based on the thermal cracking of hydrocarbon feed stock at 800-900°C The polymerization grade ethylene is 999 vol% pure The main impurities of importance to free radical polymerization are oxygen and water. Oxygen could cause inhibition of low temperature initiators, or uncontrolled initiation of reaction at higher temperatures Water can form ethylene hydrate in the cooler parts of high-pressure process, completely blocking the pipes. Hydrogen is poison only for Phillips catalyst.

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The main types of processes used to manufacture polyethylene are:

1.High-pressure autoclave.
2.High pressure tubular
3.Gas-phase fluidized bed
4.plurry phase autoclave/loop
5.Solution autoclave

Modern polyethylene production processes offer the possibilities of a versatile range of products. High-pressure processes can produce LLDPE in addition to the normal range of LDPEs and ester copolymers. As well as HDPE some low-pressure plants can also produce LLDPE and VLDPE

Following Table gives the technical applicability of processes for polyethylene manufacture.

+ Suitable; 0 Technically feasible with some limitations; – Unsuitable

From the above Table following conclusions can be drawn.
1. LDPE, EVA and Acrylate copolymers can be manufactured only by high-pressure processes.
2. HDPE is preferably manufactured by low-pressure catalytic process.
3. LLDPE can be manufactured by both processes.
4. Gas-phase fluidized bed is the most popular process.
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Polyethylene has simplest structure of all the polymers but involves very complex chemistry. Polyethylene is available in different types. There are many different types of manufacturing processes also. It is the major commercial plastics sold and used throughout the world.
Polyethylene was discovered in 1933 at lei UK

Three major types:

LOPE Low density polyethylene (density 0915 – 0.93 g/cm3) HDPE High density polyethylene (density 0.94 – 0.98 g/cm3) LLDPE Linear low density polyethylene (density 0.92 – 0.94 g/cm3)

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LDPE has random long-branching structure, with branches on branches. The short branches are not uniform in length but are mainly 4 to 2 carbon atom long. The molecular mass distribution is broad.
HDPE is free of both long and short branching The molecular weight distribution depends upon catalyst type but is typically of medium width.
LLPDE has branching of uniform length, which is randomly distributed along a chain. Fairly narrow molecular weight distribution.
LDPE and LLDPE are flexible and in the form of films are transparent with only slight milkiness. HDPE is white opaque solid that is more rigid and forms films, which have a more turbid appearance and a crisp feel.
There are some less common types of polyethylene such as HMWPE (High Molecular Weight Polyethylene), HMWPE (Ultra High Molecular Weight Polyethylene). HMWPE is HDPE with MFI in the range of 0.01 – 0.1. UHMWPE have extremely high molecular mass (3 – 6×106) The viscosity is too high to be measured by MFI test. This material is abrasion resistant, chemically inert and tough.
Polyethylene is also copolymerized with different monomers. Vinyl acetate copolymers (VLDPE) are produced in largest quantities and improve flexibility. Copolymerization reduces crystallinity Following Table gives principal types of ethylene copolymers

Principal types of ethylene copolymers

Comparison of typical properties of polyethylenes

Different product densities and rheological characteristics (MFI) mean different end-uses and applications. The Melt Flow Index or Melt Index of a polyolefin is essentially inversely proportional to its molecular weight. Examples of different applications of polyethylene according to its melt index and density are shown in following figure.

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