Plastics, thermoplastics processing

Polyesters are known to be produced by many bacteria as intracellular reserve materials for use as a food source during periods of environmental stress. They have received a great deal of attention since the 1970s because they are biodegradable, can be processed as plastic materials, are produced from renewable resources, and can be produced by many bacteria in a range of compositions. The thermoplastic polymers have properties that vary from soft elastomers to rigid brittie plastics in accordance with the stmcture of the pendent side-chain of the polyester. The general stmcture of this class of compounds is shown by (3), where R = CH3, n = >100, and m = 0-8. 

Multiblock Copolymers. Replacement of conventional vulcanized mbber is the main appHcation for the polar polyurethane, polyester, and polyamide block copolymers. Like styrenic block copolymers, they can be molded or extmded using equipment designed for processing thermoplastics. Melt temperatures during processing are between 175 and 225°C, and predrying is requited scrap is reusable. They are mostiy used as essentially pure materials, although some work on blends with various thermoplastics such as plasticized and unplasticized PVC and also ABS and polycarbonate (14,18,67—69) has been reported. Plasticizers intended for use with PVC have also been blended with polyester block copolymers (67). 

Thermoplastic elastomers have now been available for over 30 years and the writer recalls organising a conference on these materials in 1969. In spite of considerable publicity since that time these materials still only comprise about 5-10% of the rubber market (equivalent to about 1-2% of total plastics consumption). It is important to appreciate that simply being a thermoplastic material (and hence being processed and reprocessed like a thermoplastic plastics material) is not enough to ensure widespread application. Crucially the material must have acceptable properties for a potential end-use and at a finished product price advantageous over other materials. 

Plastics are long chain, high molecular weight polymers which can be formed under application of heat and pressure into desired shapes and contours by a variety of processing techniques. Plastic resins are classified either as thermoplastics, if the material can be reprocessed under heat and pressure, or as thermosets if the material, once formed, cannot be reprocessed. 

This book focuses on the relationships between the chemical structure and the related physical characteristics of plastics, which determine appropriate material selection, design, and processing of plastic parts. The book also contains an in-depth presentation of the structure-property relationships of a wide range of plastics, including thermoplastics, thermosets, elastomers, and blends. 

The major bulk processed thermoplastic using calendering is PVC sheets and films including blends and copolymers. A sample recipe to produce PVC sheet might include a plasticizer such as a dialkyl phthalate, pigment, filler, lubricant, and stabilizer. 

POLYMERS] (Vol 19) - [ELASTOMERS, SYNTHETIC - SURVEY] (Vol 8) - [ACRYLIC ESTERPOLYMERS - SURVEY] (Vol 1) -of amorphous thermoplastics [PLASTIC PROCESSING] (Vol 19) -effect of supercritical fluid on [SUPERCRITICAL FLUIDS] (Vol 23)... 

This is not of course the complete story. Most often, thermoplastic elastomers are processed on plastics machinery and it will be convenient, and sensible, that test pieces are produced in the same way. The thermoplastics processability tests are also likely to be more relevant and, certainly, curemeter tests are irrelevant. 

Sevruk VD, Prokunin AN, Fridman ML (1980) Regularites in extension of molten thermoplastics. In Fridman ML (ed) Rheology in polymer processing. NPO Plastic, Moscow, pp 84-99... 

Compression molding is an old and common method of molding thermoset (TS). It now processes TS plastics as well as other plastics such as thermoplastics (TP), elastomers (TS and TP), and natural rubbers (TS). By this method, plastic raw materials are converted into finished products by simply compressing them into the desired shapes... 

Thus, prepolymer processing contrasts sharply with thermoplastic processing. Thermoplastics are polymerized to completion during the plastics manufacture. The fabricator apphes heat and pressure by means of extrusion, injection molding, or calendering to produce the finished article. 

We have seen the development of new and improved processing equipment such as the screw injection machine. It is interesting to com-sider that in the early days of thermoplastics, they were processed largely in equipment which had been developed for the rubber industry. Today there is an increasing amount of rubber, generally in powder or crumb form, being processed in plastics screw injection machines, both in this country and in Europe. 

Gachter, R. and Muller, H. (1996) Plastics Additives Handbook Stabilizers, Processing Aids, Plasticizers, Fillers, Reinforcements, Colorants for Thermoplastics, 4 ecL, Munich, Carl Hanser Verlag Pritchard, Geoffrey (1998) Plastics Additives An A-Z Reference, London, Chapman Hall... 

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