Semi-crystalline thermoplastics

Property Amorphous thermoplastic Semi-crystalline thermoplastic... 

The polymers used in injection moulding can be divided into three main classes, depending on their structure and properties. These classes are amorphous thermoplastics semi-crystalline thermoplastics rubbers. 

An important subdivision within the thermoplastic group of materials is related to whether they have a crystalline (ordered) or an amorphous (random) structure. In practice, of course, it is not possible for a moulded plastic to have a completely crystalline structure due to the complex physical nature of the molecular chains (see Appendix A). Some plastics, such as polyethylene and nylon, can achieve a high degree of crystallinity but they are probably more accurately described as partially crystalline or semi-crystalline. Other plastics such as acrylic and polystyrene are always amorphous. The presence of crystallinity in those plastics capable of crystallising is very dependent on their thermal history and hence on the processing conditions used to produce the moulded article. In turn, the mechanical properties of the moulding are very sensitive to whether or not the plastic possesses crystallinity. 

Friedrich, K. Crazes and Shear Bands in Semi-Crystalline Thermoplastics. Vol. 52/53, pp. 225-274. 

The true value of the chloropolymer (I) lies in its use as an intermediate for the synthesis of a wide variety of polytorgano-phosphazenes) as shown in Figure 1. The nature and size of the substituent attached to the phosphorus plays a dominant roll in determining the properties of the polyphosphazene. Homopolymers prepared from I, in which the R groups are the same or, if different, similar in molecular size, tend to be semi-crystalline thermoplastics. If two or more different substituents are introduced, the resulting polymers are generally amorphous elastomers. (See Figure 1.)... 

Polymers with blocks containing different tactcities can be produced, e.g., atactic PP (amorphous)/isotactic PP (semi-crystalline) can be made using metallocene catalysts. They behave in a manner similar to SBS thermoplastic elastomers. 

Ogata, N., Yasumoto, H., Yamasaki, K., Yu. H., Ogihara, T., Yanagawa, T., Yoshida, K. and Yamada, Y. (1992). Evaluation of interfacial properties between carbon fibers and semi-crystalline thermoplastic matrices in single fiber composites. J. Mater. Sci. 27, 5108-5112. 

Friedrich, K. and Karger-Kocsis, J. (1989). Unfilled and short fiber reinforced semi-crystalline thermoplastics. In Fractography and Failure Mechanisms of Polymers and Composites, (A.C. Roulin-Moloney ed.), Elsevier Appl. Science, London, pp. 437-494. 

All three transitions are shown by a semi-crystalline thermoplastic with a chain length long enough to extend the rubbery region to above the melting point. 

Only Tg and are present in an amorphous thermoplastic and in a non-vulcanised rubber. A semi-crystalline thermoplast with short chains only shows Tg and Tm- In a (very exceptional) fully crystalline polymer Tg is not present... 

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... 

PE, PP and PA are semi-crystalline polymers melting and solidification go accompanied by a (though gradual) volume jump. PS, PVC and PC are amorphous thermoplastics upon solidification they show no volume jump, but only a bend in the V-T relation. 

APC-2 is a semi-crystalline thermoplastic prepreg. The surface roughness characterization for two different batches of APC-2 prepreg are reported in Table 7.2. Batch I is a 152.4 mm (6in.)-wide prepreg sheet, whereas batch II is 304.8-mm wide. In addition, the... 

Figure 1.27 Schematic of a pvT diagram for semi-crystalline thermoplastics.

Due to the increase in density upon solidification of semi-crystalline thermoplastics, the thermal conductivity is higher in the solid state than in the melt. In the melt state, however, the thermal conductivity of semi-crystallinepolymers reduces to that of amorphous polymers as can be seen in Fig. 2.2 [40],... 

Furthermore, it is not surprising that the thermal conductivity of melts increases with hydrostatic pressure. This effect is clearly shown in Fig. 2.3 [19]. As long as thermosets are unfilled, their thermal conductivity is very similar to amorphous thermoplastics. Anisotropy in thermoplastic polymers also plays a significant role in the thermal conductivity. Highly drawn semi-crystalline polymer samples can have a much higher thermal conductivity as a result of the orientation of the polymer chains in the direction of the draw. 

A decrease in thermal diffusivity, with increasing temperature, is also observed in semicrystalline thermoplastics. These materials show a minimum at the melting temperature as demonstrated in Fig. 2.18 [24] for a selected number of semi-crystalline thermoplastics. It has also been observed that the thermal diffusivity increases with increasing degree of crystallinity and that it depends on the rate of crystalline growth, hence, on the cooling speed. 

Table 2.6 Constants for Carreau-WLF (Amorphous) and Carreau-Arrhenius (Semi-Crystalline) Models for Various Common Thermoplastic...

Table 2.6 presents constants for Carreau-WLF (amorphous) and Carreau-Arrhenius models (semi-crystalline) for various common thermoplastics. In addition to the temperature shift, Menges, Wortberg and Michaeli [50] measured a pressure dependence of the viscosity and proposed the following model, which includes both temperature and pressure viscosity shifts ... 

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