Semi-crystal glass

If soda is replaced by potassium carbonate in glass manufacture and lead oxide is added, the light refraction properties of the glass are improved. The whole crystal glass contains at least 24% lead oxide, while the semi-crystal glass has at least 10% in all of lead, potassium and zinc oxides. Some characteristic physical properties of the different glass types are shown in Table 43.5. 

The particular physical state of polymeric fibers in which they are actually used, depends on the purpose they serve. Obviously, the fibers have to be reasonably tough and should not stretch significantly under the influence of longitudinal forces which occur in the fiber during its use. This immediately rules out the viscous and the elastic states. As for the other two, the fibers can be either semi-crystals (cellulose fibers, nylon, and terylene) or polymeric glasses (orlon). There is, however, something special about the structure of the fibers in these states. 

In some semi-crystalline polymers, two glass transitions can be distinguished a lower glass transition, Tg(L) and an upper glass transition, Tg(U). It may be assumed that Tg(L) arises from purely amorphous material, while Tg(U) arises from amorphous material which is under restraint due to the vicinity of crystallites. Frequently Tg(U) increases with the degree of crystallization. Some general rules are ... 

In view of the multitude of observed deformation mechanisms it is useful at this point to examine the effects of external variables, especially that of ambient temperature, on the deformation behavior of semi-crystalline thermoplastics. At room temperature many of these polymers are above their glass transition point and owe their strength and stiffness to the crystalline phases. The first displacements start in the relatively soft amorphous layers, but the stress-strain curve is largely determined by the presence and arrangement of the crystals. Interlamellar slip has been identified as an important mechanism, but, in addition, crystalline deformation mechanisms occur at moderate strains The corresponding stress-strain curve shows an... 

In addition, PET can be used to form semi-IPNs with other naturally functionalized triglyceride oils, such as vemonia oU (31). The procedures for PET/vemonia semi-IPNs are essentially the same as those of PET/castor ones, but with important differences. For PET/castor mixtures, the diisocyanate cross-hnker was added at 240°C and the mixture was poured into the molds rapidly before the castor gel point had been reached. In this case, PET/castor polyurethane semi-IPNs were formed, in which crystallization and gelation occurred simultaneously resulting in a single, broad glass transition temperature. For PET/vernonia, the sebacic acid was added at 280°C, which reduced the temperature to about 250°C, where the mixture was held for another 5 min, then poured into a preheated mold and allowed to cool, during which time the PET crystalhzes. In this case, PET/vernonia polyester network was formed, and the PET crystallized prior to network formation because the latter... 

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