Thermoplastic and Thermosetting Behaviour

It may also be mentioned that a number of commercial polymers are produced by chemical modification of other polymers, either natural or synthetic. Examples are cellulose acetate from the naturally occurring polymer cellulose, poly(vinyl alcohol) from poly(vinyl acetate) and chlorosulphonated polyethylene (Hypalon) from polyethylene. 

In all of the examples given so far in this chapter the product of polymerisation has been a long chain molecule, a linear polymer. With such materials it should be possible for the molecules to slide past each other under shear forces above a certain temperature such that the molecules have enough energy to overcome the intermolecular attractions. In other words above a certain temperature the material is capable of flow, i.e. it is essentially plastic, whereas below this temperature it is to all intents and purposes a solid. Such materials are referred to as thermoplastics and today these may be considered to be the most important class of plastics material commercially available. 

Whilst the term thermosetting plastics arose out of the fact that early products of this type were cross-linked by subjecting the intermediate-stage materials to elevated temperature, the term is also widely used where cross-linking takes place at normal ambient temperatures. 

The vulcanisation of natural rubber, a long chain polyisoprene, with sulphur involves a similar type of cross-linking. 

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Plastics are mosdy solid and stable at ordinary temperatures, and at some stage of their manufacture they are plastic , Le. soft and capable of being shaped. The shaping process is done by the application of heat and pressure, and it is the behaviour of the material when heated that distinguishes between the two classes of plastics thermoplastics and thermosetting plastics. 

In addition to copolymerization described above, there is considerable interest in the control of polymer architecture. As this may have a considerable influence on the behaviour of polymers as materials. Thus, polymers may be linear (Fig. 1.3a), or branched (Fig. 1.3b) or may for more complex structures such as a star arrangement (Fig. 1.3c) or more sophisticated dendrimer arrangements. Some of these more complex architectures pose considerable challenges to the organic chemist in terms of reagents and equipment (Hadjichristidis et al. 2000), while others, for example, the introduction of cross-links, can be achieved using technically quite simple methodologies. Cross-linked polymers are an important class of materials in themselves and provide another classification, namely thermoplastics and thermosets The former are those which melt and flow, the latter are materials which cannot melt or dissolve and are built up of cross-linked polymer chains. 

Polycarbonate is perhaps the most notoriously notch-sensitive of all thermoplastics, although nylons arc also susceptible to ductileAjrittle transitions in failure behaviour caused by notch sharpening. Other plastics such as acrylic, polystyrene and thermosets are always brittle - whatever the crack condition. 

The studies in the literature indicate that several different vegetable fibers have been mixed with glass fibers and incorporated into a number of polar and non-polar polymers, including thermoplastics, thermosets and even natmal rubber. The influence of fiber content and relative amounts, fiber length, modification of both fibers and matrices and the use of compatibilizers on the mechanical and thermal behaviour of these systems was described. 

Structural concepts for tissue-compatible and biodegradable polymers, thermoplastic elastomers, and thermosets with shape memory capabilities will be introduced. Their thermal and mechanical properties and degradation behaviour will be explained. An important precondition for the shape memory effect of polymers is elasticity. An elastic polymeric material consists of flexible segments, so-called network chains, which are connected via netpoints or junctions. The permanent shape of such a polymer is determined by the netpoints. The network chains take a coil-like conformation in unloaded condition. If the polymer is stretched, the network chains become extended... 

Glass fibres dominate this field either as long continuous fibres (several centimetres long), which are hand-laid with the thermoset precursors, e.g., phenolics, epoxy, polyester, styrenics, and finally cured (often called fibre glass reinforcement plastic or polymer (FRP)). With thermoplastic polymers, e.g., PP, short fibres (less than 1 mm) are used. During processing with an extruder, these short fibres orient in the extrusion/draw direction giving anisotropic behaviour (properties perpendicular to the fibre direction are weaker). 

Adhesives, as all plastics, are viscoelastic materials combining characteristics of both solid materials like metals and viscose substrates like liquids. Typically, the adhesive shear stress vs. shear strain curve is non-linear. This behaviour is characteristic especially for thermoplastic adhesives and modified thermosetting adhesives. Thermosetting adhesives are, by their basic nature, more brittle than thermoplastic adhesives but, as discussed earlier, are often modified for more ductile material behaviour. 

The behaviour described in the previous section occurs in thermoplastic polymers, where the single molecules are bondedtogether by thermosensitive intermolecular attractive forces (van der Waals, dipole-dipole, hydrogen bonding). Crosslinked polymers (thermosetting polymers) show a different behaviour owing to primary bonds between main chains. These types of bonds are not thermosensitive and they limit viscous flows. The segments of chain, included between transversal bonds, are obstructed in their movement. 

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