Amorphous thermoplastics, creep

Alterations by moisture exposure are weak the shrinkage and coefficient of thermal expansion are rather low, typical of amorphous thermoplastics creep resistance is good, the more so as the glass fibre content increases. 

PROPERTIES OF SPECIAL INTEREST High-performance thermoplastic of relatively low flammability. Amorphous, high-creep resistance and stable electrical properties over wide temperature and frequency ranges. Transparent with good thermal and hydrolytic resistance. High alkaline stability. 

The anisotropy of creep behaviour of oriented amorphous thermoplastics appears to have received even less attention than that of oriented semicrystalline thermoplastics. This may well be associated with the fact that early measurements based on standard tests demonstrated a marked... 

Polyetherimides (PEI) are a newer class of amorphous thermoplastics with high-temperature resistance, impact strength, creep resistance, and rigidity. They are transparent with an amber color. The polymer is sold under the trade name of Ultem (General Electric) and has the stracture shown in Eig. 2.20. It is prepared from the condensation polymerization of diamines and dianhydrides. ... 

Fig. 5.21. Schematic illustration of craze formation in the creep test on transparent amorphous thermoplastics. Visible crazes occur at a certain time and strain dming the creep test. These times are indicated in the creep curves measured at different stresses. The connecting line of these points provides a curve which describes the strain limit at which craze formation occnrs as a function of time or deformation rate respectively. An extrapolation of the cnrve towards great times yields the critical limiting strain for craze formation...

Consult or plot, from data available in the creep chart, the creep modulus curve of the material of interest for the temperature at which the part will be used, extrapolating where necessary. Like creep rupture, creep modulus varies greatly with temperature. In addition it is subject to another variable—stress level. For each material grade, creep modulus data are tabulated in the creep chart, first by test temperature and second by applied (test) stress. For very rigid plastics, such as thermosets, pilled thermoplastics, and amorphous thermoplastics at room temperature and below, the creep modulus curves show minor variation with applied stress. However, for the more flexible and ductile plastics, the creep modulus curves will vary significantly and systematically with stress level. The higher the stress, the lower the creep modulus. This is a consequence of viscoelasticity. 

In thermoplastic matrices, the intrinsic characteristics derived from the linear chain structure have to be considered, mainly the capability of flow under stress at high temperatures. Thus, in amorphous and semicrystalline matrices, the service temperatures will be determined by Tg or Tg and T, respectively. Without reinforcement, creep is a major problem at temperatures lying in the interval Tg < T < r , but when the matrices are... 

Hot-melt thermoplastic elastomer systems (23. 24) are also effective coating materials. These materials are generally based on copolymers that are comprised of hard (crystalline or glassy) and rubbery (amorphous) segments contained in separate phases. The hard-phase regions form physical cross-links below their crystallization or vitrification temperature, and the system therefore has elastomeric properties. The moduli and low-temperature characteristics of these materials can be tailored to compare reasonably well with silicone rubbers at -40 C. However, they are limited in high-temperature applicability because of enhanced creep or flow due to softening. 

These thermoplastic polymers are generally characterized by their toughness in respect to resistance to heat and UV exposure. Of importance also are their transparency, warp resistance, excellent flexural recovery, high elastic limits, and good electrical and mechanical properties, including outstanding creep resistance. They have as well excellent radiation and oxidation resistance. Their thermal expansion rate is close to Uiat of metal. This amorphous material is subject to environmental stress cracking, however, particularly in the presence of aromatic or aliphatic hydrocarbons. 

Sky pel is the registered trademark of a polyester-based thermoplastic engineering elastomer manufactured by SK Chemicals [9]. It can be processed in a wide variety of products from ultra small precision parts to spread sheets by injection, extrusion, and others. Skypel is a block copolymer consisting of hard crystalline phase and soft amorphous phase, and the relative proportions of the two phases determine the material properties. Skypel , ranging over a broad hardness spectrum, offers excellent features such as toughness, resilience, resistance to creep, impact and flexural fatigue, flexibility at low temperatures, thermal stability at elevated temperatures, and resistance to many industrial chemicals, oils, and solvents. Table 9 shows some typical properties of Skypel . 

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