Semicrystalline engineering thermoplastics

Novel Composites from Blends of Amorphous and Semicrystalline Engineering Thermoplastics with Liquid-Crystalline Polymers... 

Polyacetals, also referred to as polyoxymethylenes (POMs) or polyfonnalde-hydes, are a semicrystalline engineering thermoplastic polymerized as a homopolymer and copolymer. The homopolymer and copolymer have somewhat different molecular structures and performance values. The difference between performance values is narrowing with new formulations (compounds). Polyacetal engineering thermoplastics were introduced to the world in 1956 with the potential of replacing metals, aluminum, brass, and cast zinc, which polyacetals continue to do. 

TABLE 7.2 Properties of PBT Compared with Other Semicrystalline Engineering Thermoplastics... 

While there are subtle differences within the family, all of these materials are semicrystalline engineering thermoplastics, with high strength and toughness, high chemical resistance, and tiigh-temper-ature resistance. 

Aromatic-aliphatic polyesters, in which either R1 or R2 is aromatic, are generally high-melting (150-270°C) semicrystalline materials that find applications as engineering thermoplastics, films, or fibers. 

This class of polyesters consists of four major commercial polymers and their copolymers, namely PET, PTT, PBT, and PEN (see Table 2.1). They compete for engineering thermoplastics, films, and fibers markets with other semicrystalline polymers, such as aliphatic polyamides, and for some other applications with amorphous engineering plastics such as polycarbonate. The syntheses of PET and PBT, detailed in numerous reviews and books,2-5 are described in Sections 23.2.2 and 2.3.2.1. 

Thermomechanical behaviour is most probably the most widely exploited property of engineering thermoplastics. Figure 3.1 shows the behaviour of two types of thermoplastic, one amorphous and the other semicrystalline, versus temperature. We can see several steps moving from low to high temperatures ... 

Several liquid cryatalline polymers were melt blended with an amorphous (Ultem) and two semicrystalline (PEEK and PPS) engineering thermoplastics in a single screw extruder. Flat film was processed with different degrees of stretch imparted while the film was being cooled. In the case of Vectra, which was thought to be ideally suited to be blended with PPS based on thermal and rheological... 

PIT is a semicrystalline polymer synthesized by the condensation of PDO with either terephthalic acid or dimethyl terephthalate, followed by polymerization. Studies of PTT had never gone beyond academic interest until recent years because one of its raw materials, PDO, was very expensive and available only in a small volume. PTT received less attention compared with PET and PBT. However, recent breakthroughs in PDO synthesis made PTT available in industrial quantities, thus offering new opportunities in carpet, textile, film, packing, and engineering thermoplastics markets. 

The two principal types of polymerization for thermoplastics including engineering thermoplastics are polycondensation polymerization and chain-growth polymerization. Both types can usually produce hnear, branched, crosslinked amorphous and semicrystalline aromatic and aliphatic polymers [14]. 

The study of relaxation processes in Semicrystalline Polymers (qv) is a subject of continuing technological interest because of its practical importance. This is based on the observation that the stiffness of typical crystalline engineering thermoplastics at room temperature may be only one-third to one-fifth that of the same material at a low temperature. The drop in stiffness or modulus takes place in regions of temperature associated with a relaxation process. 

Isoplast engineering thermoplastic polyurethane resins of Dow [15] combine the toughness and dimensional stability of amorphous resins with the superior performance and chemical resistance associated with semicrystalline resins. Isoplast is available in impact-modified, clear and glass-reinforced resins, with a range of options to meet the most demanding applications. Table 15 gives descriptions and applications of Isoplast TPE-U products. 

A specialty engineering thermoplastic with a unique balance of properties. It provides high stiffness and strength at elevated temperatures, along with exceptional chemical resistance. It is a semicrystalline thermoplastic. Like all semicrystalline thermoplastics, it has both amorphous and crystalline components in its solid form. 

---