Thermoplastics deformation

Binder phase continuous solid carbon matrix formed during the thermoplastic deformation of those coal macerals that become plastic during carbonization formed from the thermoplastic deformation of reactive (vitrinite and liptinite) and semi-inert (semi-fusinite) coal macerals of metallurgical bituminous coals (ASTM D-5061). 

Recent studies [91-93] have shown that, for one component of the system undergoing thermoplastic deformation, the continuum percolation model can be used to predict the changes in the system with respect to a traditional pharmaceutical... 

The transition from crystalline to melt state, which is normal for crystalline polymers, is not observed with cellulose under normal conditions. It appears that the secondary bonds giving rise to the crystalline state are too strong and too numerous to be broken by a rise in temperature. Thermal degradation (beginning at ca. 180 °C) precedes melting under atmospheric pressure conditions. Nevertheless, a plastic deformation interpreted as melting has recently been reported for cellulose fibers exposed to laser radiation in a highly confined (pressurized) space [43]. The fracture surface of a thermoplastically deformed cellulose disc is shown in e Fig. 10. 

Thermoplastically deformed cellulose using laser radiation on a compressed disc of cotton fibers. (Schroeter [43])... 

Essentially, thermoplasts and rubber-reinforced thermoplasts deform by the same mechanism. Since the rubber phase is dispersed, it cannot, of course, directly contribute to the large deformations observed. The deformation must therefore involve the matrix and the rubber particles then assure that the stress peaks are more evenly distributed. The main effect appears to be the occurrence of many crazes, whereby a smaller contribution is made by cold flow. 

Thermotropic polyesters can be shaped by mouldings and film forming by thermoplastic deformation. Filaments and fibres having extraordinary mechanical properties can be produced by spinning from the melt if the polyesters do not decompose under melt condition. 

The reason for the decrease in the polymer percolation threshold was suggested to be that the polymer experiences a sintering phenomenon that results in an almost continuum medium inside the US tablets. The drug particles are practically surrounded by the inert polymer, which diminished the contact with the dissolution medium and therefore slows down the release rate. So, in the case in which a component experiences a thermoplastic deformation, the continuum percolation model can be employed to explain the changes in the system with respect to a tablet obtained by a traditional... 

So far we have considered only deformation which takes place at constant rate and temperature, but plastic deformation, like other aspects of the mechanical behaviour of polymers, has a strong dependence upon the testing rate and temperature. Typical behaviour is illustrated in Fig. 5.29 for a glassy thermoplastic deformed in tension. At a given strain-rate the yield stress drops as the temperature is increased and a, falls approximately linearly to zero at the glass-transition temperature when the polymer glass becomes a rubber. If the strain-rate is increased and the temperature held constant the yield stress increases (cf. time-temperature superposition (Section 5.2.7)). 

From the point of view of technology, it is convenient to classify polymers as thermosetting and thermoplastic. The former set by chemical crosslinks introduced during fabrication and hence do not change appreciably in their deformability with changes in temperature. Thermoplastics, on the other hand, soften and/or melt on heating and can therefore be altered in shape by heating... 

Thermoplastic coatings based on vinyl plastisols and acryUc latexes resist severe deformation duting fabrication and exhibit good exterior durabdity. 

Because of increased production and the lower cost of raw material, thermoplastic elastomeric materials are a significant and growing part of the total polymers market. World consumption in 1995 is estimated to approach 1,000,000 metric tons (3). However, because the melt to soHd transition is reversible, some properties of thermoplastic elastomers, eg, compression set, solvent resistance, and resistance to deformation at high temperatures, are usually not as good as those of the conventional vulcanized mbbers. AppHcations of thermoplastic elastomers are, therefore, in areas where these properties are less important, eg, footwear, wine insulation, adhesives, polymer blending, and not in areas such as automobile tires. 

In thermoplastics and some thermosetting resins, displacement strains are not likely to produce immediate failure of the piping but may result in detrimental distortion. Especially in thermoplastics, progressive deformation may occur upon repeated thermal cycling or on prolonged exposure to elevated temperature. 

Deformation of a polymer melt—either thermoplastic or thermosetting. Processes operating in this way include extrusion, injection moulding and calendering, and form, in tonnage terms, the most important processing class. 

The first five of these techniques involve deformation and this has to be followed by some setting operation which stabilises the new shape. In the case of polymer melt deformation this can be affected by cooling of thermoplastics and cross-linking of thermosetting plastics and similtir comments can apply to deformation in the rubbery state. Solution-cast film and fibre requires solvent evaporation (with also perhaps some chemical coagulation process). Latex suspensions can simply be dried as with emulsion paints or subjected to some... 

Traditional rubbers are shaped in a manner akin to that of common thermoplastics. Subsequent to the shaping operations chemical reactions are brought about that lead to the formation of a polymeric network structure. Whilst the polymer molecular segments between the network junction points are mobile and can thus deform considerably, on application of a stress irreversible flow is prevented by the network structure and on release of the stress the molecules return to a random coiled configuration with no net change in the mean position of the Junction points. The polymer is thus rubbery. With all the major rubbers the... 

The range of blends now available comprises a broad spectrum of materials superior in many respects, particularly heat deformation resistance, to the general purpose thermoplastics but at a lower price than the more heat-resistant materials such as the polycarbonates, polyphenylene sulphides and polysulphones. At the present time the materials that come closest to them in properties are the ABS/ polycarbonate blends. Some typical properties are given in Table 21.1. 

The friction and wear of plastics are extremely complex subjects which depend markedly on the nature of the application and the properties of the material. The frictional properties of plastics differ considerably from those of metals. Even reinforced plastics have modulus values which are much lower than metals. Hence metal/thermoplastic friction is characterised by adhesion and deformation which results in frictional forces that are not proportional to load but rather to speed. Table 1.7 gives some typical coefficients of friction for plastics. 

The mechanism of droplet deformation can be briefly summarized as follows. The factors affecting the droplet deformation are the viscosity ratio, shear stress, interfacial tension, and droplet particle size. Although elasticity takes an important role for general thermoplastics droplet deformation behavior, it is not known yet how it affects the deformation of TLCP droplet and its relationship with the processing condition. Some of... 

As reviewed thermoplastics (TPs) being viscoelastic materials respond to induced stress by two mechanisms viscous flow and elastic deformation. Viscous flow ultimately dissipates the applied mechanical energy as frictional heat and results in permanent material deformation. Elastic deformation stores the applied mechanical energy as completely recoverable material deformation. The extent to which one or the other of these mechanisms dominates the overall response of the material is determined by the temperature and by the duration and magnitude of the stress or strain. The higher the temperature, the most freedom of movement of the individual plastic molecules that comprise the... 

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