PLASTICS

Definition:

A plastic (organic polymer) is defined as a solid, the fundamental element of which is manufactured synthetically or semi-synthetically from monomer organic molecules. Plastics can be comprised of both linear chains, as well as branched and networked chains. The chain length of the individual polymer molecules varies between several thousand molecule units to over one million monomer organic molecule units.

Due to the synthetic nature of their manufacture in contrast to natural materials, materials such as steel, porcelain or glass could be classed as "plastics". However, they are not regarded as such! Rather, the word plastics was created as a generic term for a limited group of materials manufactured artificially, that consist of macromolecular organic compounds and whose structural make-up is in principle similar to that of many products in the animal and plant kingdoms.

A plastic workpiece consists of millions of very long, intertwined molecular chains (polymers) that are comprised of constantly recurring basic units (monomers). For example, the plastic polyproylene consists of polypropylene units that recur many times.

One outstanding feature of plastics is that its industrial properties, such as mouldability, hardness, elasticity, resistance to fracture, temperature resistance, thermal deformation resistance and chemical resistance can be greatly varied depending on the choice of base material, the manufacturing process and the admixture of additives.

Introduction:

Plastics can be divided up into four main groups:

Thermoplastics

Thermoplastics are plastics that consist of long linear molecules. By adding energy, these materials are rendered soft and formable to plastic as often as needed, and finally melt. They can be shaped as required by means of various primary shaping and reshaping processes. The part in question maintains its shape after it has cooled down. This process is thus reversible. The reason for this behaviour is filamentous, linear macromolecules.

Most plastics used today belong to this group. They are used just as commonly for simple consumer goods and packaging etc. as they are for technical components in the automobile and electrical industries or in the construction industry, particularly for roofing sheets, window profiles and pipes.

Two or more thermoplastics can be mixed in order to create new properties (polyblend).

Duroplasts

Duroplasts are polymers that are produced in a hardening process from a melt or the release of the components by a cross-linkage reaction. This irreversible reaction is in most cases caused by heating. The heating of duroplasts does not result in plastic deformability, rather merely in their decomposition. Hardened duroplasts are mostly hard and brittle, and can be worked only mechanically in the further production process. The reason for this behaviour is the three-dimensionally cross-linked macromolecules.

Due to their mechanical and chemical resistance even at high temperatures, they are frequently used for electrical installations.

Elastomers

The elastomers include all types of cross-linked caoutchouc. The cross-linkage is brought about, for example, by vulcanisation with sulphur, by means of peroxides, metal oxides or irradiation. The elastomers display wide-meshed cross-linkage, and are therefore flexible. They do not become soft when heated, and are not soluble in most solvents. They are therefore used for hygiene articles or chemical gloves.

Thermoplastic elastomers

Thermoplastic elastomers (TPE or elastoplasts) are elastomers that behave like traditional representatives of the group at room temperature, but become deformable when heated. Most are copolymers that consist of a "soft" elastomer and a "hard" thermoplastic component. The properties of the elastoplasts lie between those of duroplasts and thermoplastics. Examples are the block copolymers developed by Shell from 1965, made of styrenes and polyolefines.

One great advantage of these elastic plastics is the possibility of being able to weld them in order to create watertight compounds with them."