Semi-crystallinity softening

With an amorphous thermoplast the polymer softens over a rather short temperature interval from the glassy to the rubbery state. With a semi-crystalline polymer a certain amount of softening takes place at Tg with further T- increase the stiffness drops very gradually up to the melting point... 

The blending of short glass fibres results in an increase of the E-modulus to e g. its threefold over the whole temperature region. If the slope of the log E - T curve is small, such as with semi-crystalline polymers between Tg and T, a vertical shift causes a considerable horizontal shift (see MT 8.1.2.), so a strong increase of the softening points, which are of importance in applications at higher temperatures. 

The effect of short fibres on the softening temperature has already been discussed in 8.2.2. It appeared that their effect on amorphous polymers is minor, whereas for semi-crystalline polymers the softening points are drastically increased. Both observations are directly related to the slope of the log E - T curve. 

The difference in softening temperatures for amorphous and semi-crystalline polymers becomes also clear from Fig. 13.3, where the Young moduli of amorphous and of semicrystalline polystyrene are illustrated. For amorphous polystyrene the two HDTs appear to be 92 and 97 °C and for the semi-crystalline polystyrene 99 and 114 °C. It has to be mentioned, however, that the curves in Fig. 13.3 are the so-called 10 s moduli, i.e. measured after 10 s of stress relaxation, every point at a specific temperature. The measurements in the softening experiments are not in agreement with the determination of the standard Young modulus. 

FIG. 26.1 Schematic view of the ISO-A and ISO-B softening temperatures and the Young moduli as functions of temperature for (A) amorphous polymers, (B) semi-crystalline polymers and (C) thermo-sets. 

TABLE 26.2 Softening points in °C of amorphous and semi-crystalline polymers AT = Vicat-B- ISO-A... 

Generally all thermoplastic polymers can be processed by thermoforming, but there are significant differences regarding process windows. Amorphous polymers show a wider softening temperature range than semi-crystalline polymers, which results in a wider process window and a more stable process [7]. For this reason only few semi-crystalline polymers are used in thermoforming, for example PP, PE, C-PET and polyester. The most commonly used amorphous polymers are PVC, PS, ABS, SAN, PMMA, PC and A-PET. 

PEEK is a semi-crystalline plastic with a Tg 144°C and T 335 C. Processing temperatures are in the range 360-400°C. Unreinforced PEEK is very ductile, reaching strains in excess of 1.0 in the tensile test. like other high-softening plastics, it is based on the benzene ring... 

The term TPS describes an amorphous or semi-crystalline material composed of gelatinized or destructurized starch containing one or a mixture of plasticizo-s. TPS can be repeatedly softened and hardened so that it can be moulded/shaped by the action of heat and shear forces, allowing its processing to be conducted with the techniques commonly used in the plastic industry. The following sections are devoted to a brief description of the basics of starch extrusion and processing and to the more relevant applications of TPS [50, 57, 73-75, 79-81]. TPS or destructurized starch are also known as PLS [82], because of the inevitable presence of non-volatile plasticizers in their composition. TPS is however the predominant term used for these materials. 

In a thermoplastic, the macromolecules are not cross-linked so that the material can melt , i. e. above the glass transition temperature the material begins to soften. Thermoplastics can be amorphous or semi-crystalline. In microfluidics, amorphous pol3miers are often preferred because of their optical transparency. Amorphous polymers include polymethylmethacrylate (PMMA), polycar-... 

For a semi-crystalline plastic (curve K), while the amorphous phase in it will gradually soften at temperatures above the Tg, thus slightly increasing the elasticity of the plastic. It is only when increased energy reaches the molecules and when the temperature, Tk, is... 

In their entropy elastic region, amorphous thermoplastics are not well suited for structural applications because of their low stiffness and strength properties. The decrease in strength properties is less pronounced in semi-crystalline thermoplastics because their crystalline zones remain rigid and tough until they melt, while only the amorphous zones soften. The degree to which the properties decrease depends on the degree of crystallization. Nonetheless, semi-crystalline thermo-... 

Softening behavior plays a decisive role mainly under high thermal loads (such as when plastics substrates are welded) and is determined by the glass transition temperature in amorphous and by the melting temperature in semi-crystalline... 

From a topological point of view, the IPN s are closely related to polymer blends, block and graft copolymers, AB-crosslinked copolymers " and ionomeric blends. Some interesting hybrids exist between the IPN s and other polymer materials. The thermoplastic IPN s contain physical crosslinks rather than chemical (covalent) crosslinks. Physical crosslinks can be formed from block copolymers, ionomers or semi-crystalline polymers. When the temperature is raised above the softening point of the respective components, the material flows like a thermoplastic. At service temperatures, it behaves like an IPN, with thermoset behavior. Table 1 summarizes some of the terminology used to describe IPN structure and morphology. 

Amorphous thermoplastics exhibit a progressive softening over a wide temperature span, whereas the semi-crystalline materials rapidly change from the solid melt condition over a quite narrow temperature band. 

Semi-crystaUine Many polymers are semi-crystalline. This means they are composed of two phases, often with quite different properties. One phase is amorphous and the other crystalline. They are chemically identical. The amorphous phase exhibits a glass transition, Tg, which is always below the melting point of the crystalline phase. The amount of softening observed at Tg will be decreased in proportion to the amount of crystalline material present. 

Thermoplastic It describes polymers that can be processed by heating. The material can be repeatedly softened and formed. The chemical structure is linear (see Thermoset for comparison) and can be made up from an aliphatic or aromatic backbone. There is no chemical bonding between neighbouring chains. These polymers can be amorphous or semi-crystalline. Generally lower modulus and better impact strength than thermosets. 

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