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Thermoplastics are organic long chain polymers that usually become soft when heated and can be molded under pressure. They are linear or branched chain polymers with little or no crosslinking. The growth of thermoplastics is attributed to certain attractive properties of the product such as lightness in weight, chemical corrosion resistances, toughness and ease of handling. One of the growing fields that uses commodity plastics is pipe production. Engineering thermoplastics include polymers with special properties such as high thermal stability, good chemical and weather resistance and other useful characteristic. This type of plastic includes polycarbonates, polyether-sulfones, polyacetals, nylons and thermoplastic polyesters.

By : ssking wie

XII. THERMOPLASTICS

Thermoplastics are organic long chain polymers that usually become soft when heated and can be molded under pressure. They are linear or branched chain polymers with little or no crosslinking. The growth of thermoplastics is attributed to certain attractive properties of the product such as lightness in weight, chemical corrosion resistances, toughness and ease of handling. One of the growing fields that uses commodity plastics is pipe production. Engineering thermoplastics include polymers with special properties such as high thermal stability, good chemical and weather resistance and other useful characteristic. This type of plastic includes polycarbonates, polyether-sulfones, polyacetals, nylons and thermoplastic polyesters.
Among the plastics used to reduce weight in their application are polyesters, poly-propylene, polyvinyl chloride, poly-urethane, polyethylenen, nylon and ABS (acrylonitrile butadiene styrene). These will, in general, be used as reinforced plastics composites. Graphite reinfroced plastics currently are the material of choice. The highest growth rate among those plastics is reinforced polyesters, and followed by polypropylene and polyurethanes. Plastic in the enviroment is not biodegradable and can produce pollution problems, and about 80 % is thermoplastic type. Remolding thermoplastics is possible by an injection process. Cracking of plastics have been tried using a fluidized bed reactor of sand. When polyethylene was cracked at 740 °C, the main products were methane, ethylene, BTX and other aromatics and wax. When polystyrene was cracked at the same temperature, 71,6% of styrene was recovered. Another consideration is to produce photodegradable plastics. The last choice is direct recycling at the injection molding machine or central mixing of recycled material with virgin material.

12.1. POLYETHYLENE

Polyethylene PE, n(CH2–CH2)n is the most extensively used thermoplastic. The availability of the monomer, the ease of processing the polymer, its low cost, its resistance to chemicals, and its flexibility are strong influences. Polymerization of ethylene was introduced in 1930 at high pressure condition. Karl Ziegler discovery of new titanium catalysts apened the polymerization of ethylene at lower pressures. The two most widely used grades of polyethylene are low density polyethylene LDPE, which has branched chains, and high density polyethylene HDPE, which is predominantly linear. Low density polyethylene is produced by a free radical initiated polymerization at high pressure, while high density polyethylene is produced by a low pressure process with a metallic oxide catalyst of the Ziegler type. The main difference between the two grades of polyethylene is that LDPE is more flexible because of its lower crystallinity, caused by the presence of
branches of two or four carbons along the backbone of the polymer. HDPE is more closely packed because of the absence of branches and thus the molecules become closer and less permeable to gases. Several processes can produce polymers with a wide range of densities.

Physical and chemical properties.

The most important property of the polyethylene is the molecular weight and its distribution. Methods used to determine molecular weights are numerous, one is by viscosity determination. The melt viscosity and melt flow index MFI, measure the extent of polymerization. A polymer with a high melt flow index has a low melt viscosity, a lower molecular weight and lower impact tensile strength. Polyethylene is permeable to most gases, the higher density polymers are less permeable than the lower density ones. Polyethylene in general has a low degree of water absorption and is not attacked by dilute acids and alkalis, however it is attacked by concentrated acids. Its overall chemical resistance is excellent. Polyethylene are affected by hydrocarbons and chlorinated hydrocarbons and swell alowly in these solvents. Tensile strength of polyethylene is relatively low, but impact resistance is high, and it is used in insulation due to the excellent electrical resistance properties.

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