Amorphous thermoplastics processing

The less simple polymers (like the epoxies, the polyesters and the formaldehyde-based resins) are networks each chain is cross-linked in many places to other chains, so that, if stretched out, the array would look like a piece of Belgian lace, somehow woven in three dimensions. These are the thermosets if heated, the structure softens but it does not melt the cross-links prevent viscous flow. Thermosets are usually a bit stiffer than amorphous thermoplastics because of the cross-links, but they cannot easily be crystallised or oriented, so there is less scope for changing their properties by processing. 

Notes. Within amorphous thermoplastics some are, due to asymmetry, non crystallisable while others are usually amorphous because they are too slow to crystallise during normal processing. 

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)... 

POLYARYLATES. These are clear, amorphous thermoplastics that combine clarity, high heat deflection temperatures, high impact strength, good surface hardness, and good electrical properties with inherent ultraviolet stability and flame retardance. No additives or stabilizers are required to provide these properties. Polyarylates are aromatic polyesters that are manufactured from various ratios of iso- and terephthalic acids with bisphenol A.1 The resultant products are free-flowing pellets which can be processed by a variety of thermoplastic techniques in transparent and... 

Block copolymers of the A—B—A type where A is a thermoplast and B an elastomer can have properties at ambient temperatures which would normally be expected from a crosslinked rubber. The cause of this phenomenon are the physical crosslinks produced by the thermoplastic blocks which may be either crystalline or amorphous (glassy). Above the melting temperature of the hard phase such materials flow and can be processed by the usual thermoplastic processing techniques. 

Thermal diffusivity, defined in eqn. (2.3), is the material property that governs the process of thermal diffusion over time. The thermal diffusivity in amorphous thermoplastics decreases with temperature. A small jump is observed around the glass transition temperature due to the decrease in heat capacity at Tg. Figure 2.17 [24] presents the thermal diffusivity for selected amorphous thermoplastics. 

Polyurethane. This rubber is mainly thermoset, but thermoplastic processability can be achieved by block copolymers of amorphous polyurethane rubber with strongly hydrogen-bonded crystalline polyurethane blocks. 

The thermal behavior of polymers is of considerable technological importance. Knowledge of thermal transitions is important in the selection of proper processing and fabrication conditions, the characterization of the physical and mechanical properties of a material, and hence the determination of appropriate end uses. For example, the glass transition temperature of rubber determines the lower limit of the use of rubber and the upper limit of the use of an amorphous thermoplastic. We take up discussion of these transition temperatures in succeeding sections. 

The polyester-imides constitute a class of modified polyimide. These are typified by the structure shown in Figure 4.23. Polyether-imides form yet another class of modified polyimide. These are high-performance amorphous thermoplastics based on regular repeating ether and imide linkages. The aromatic imide units provide stiffness, while the ether linkages allow for good melt-flow characteristics and processability. 

In neat polymers the fatigue failure process is quite similar, but in certain amorphous thermoplastics, such as polystyrene, phase crazes are formed during the initiation [13] with the subsequent fatigue crack propagation phase to final failure. (Crazes resemble regions with many cracks bridged by oriented fibrils). 

Thermoplastics are uncrosslinked plastics up to their decomposition temperature. Flow or melting (Fig. 2) occurs above the softening point of the amorphous structure in amorphous thermoplastics and above the melting temperature of semicrystalline thermoplastics. In this thermoplastic state, the viscous liquid can be processed. Form strength is achieved by cooUng. Meltdown, solidification, and crystallization can be repeated any number of times. 

As in classical amorphous thermoplastic/crystallizable thermoplastic miscible blends, the chains of the thermosetting are rejected from the crystallizing front when the crystaUizable thermoplastic is crystallizing in thermosetting/thermoplas-tic blends. The curing process allows the formation of a network that is not involved... 

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