Load distortion, glass transition temperature

Except for a lew thermoset materials, most plastics soften at some temperatures, At the softening or heat distortion temperature, plastics become easily deformahle and tend to lose their shape and deform quickly under a Load. Above the heat distortion temperature, rigid amorphous plastics become useless as structural materials. Thus the heat distortion test, which defines The approximate upper temperature at which the material can be Safely used, is an important test (4,5.7.24). As expected, lor amorphous materials the heat distortion temperature is closely related to the glass transition temperature, hut tor highly crystalline polymers the heat distortion temperature is generally considerably higher than the glass transition temperature. Fillers also often raise the heat distortion test well above... 

The heat-distortion temperatures in the tables are the temperatures at which the films distorted 2% while under a load of 50 p.s.i. Except for examples in which the antiplasticizer concentration was 40 to 50%, the heat-distortion temperatures were usually within 10°C. of the glass transition temperatures, which are the temperatures at which the films distorted 0.25% while under a load of 5 p.s.i. Distor-... 

S/MA/MM). Miscibility behavior of typical matrix pairs is discussed as a function of composition, and discussion of material behavior in this work emphasizes response to elevated temperatures and to stress at temperature extremes. Thus, glass transition temperature (Tg), distortion temperature under load (DTUL), and tensile deformation properties as a function of temperature are reported for various typical blend combinations. Also discussed are the results of elevated temperature air aging studies on a typical blend compared to ABS. 

Figure I. Effect of maleic anhydride content on distortion temperature under load (DTUL) and on glass transition temperature (Tg). Key o, Si MA copolymer and a. Si MAI MM terpolymer (ca. 6% MM).

It was noted that, above the glass-transition temperature, the polymer is able to have considerable conformational freedom involving concerted motion over 50 atoms, which results in a decrease in modulus by a factor of about 10". The polymer is still far from the melt and the chain entanglements result in a material with viscoelastic properties. The rubbery nature may be seen from the tendency of the polymer to recover when a stress is applied. This recovery is a consequence of the higher order conferred on the chains when they are distorted so that when the stress is released there will be an entropic drive to return to the coiled state (Queslel and Mark, 1989). It is this entropic recovery that results in the shrinking of a loaded crosslinked elastomer (shown schematically in Figure 1.14) when it is heated. 

The structures and glass transition temperatures of several commercial polysulfones are listed in Table 4.30. The polymers have different degrees of spacing between the p-phenylene groups and thus have a spectrum of glass transition temperatures which determine the heat-distortion temperature (or deflection temperature under load), since the materials are all amorphous. 

It is one of several methods used to measure the softening temperature or temperature resistance of a plastic. It should be noted that the heat distortion point is generally lower than the glass transition temperature or melt temperature of the polymer because of the mechanical effect of the load. However, in amorphous polymers, HDT is generally the same as the glass transition temperature (Tg) of the material. The HDT test method is more reproducible with amorphous polymers than with crystalline polymers. With some polymers, it may be necessary to anneal the test specimens to obtain reliable results. 

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