Temperatures, maximum service

In some appHcations the high heat stabiHty of the micropowder can be utilized over a reasonably wide temperature range. A maximum service temperature is normally 260°C, provided the crystalline melting point is between 320 and 335°C. Exposure above 300°C leads to degradation and possible evolution of toxic decomposition products. 

Maximum Service Temperature. Because the cellular materials, like their parent polymers (204), gradually decrease in modulus as the temperature rises rather than undergoing a sharp change in properties, it is difficult to precisely define the maximum service temperature of cellular polymers. The upper temperature limit of use for most cellular polymers is governed predominantly by the plastic phase. Fabrication of the polymer into a... 

Material Maximum service temperature, F Important properties... 

In the case of a crystalline polymer the maximum service temperature will be largely dependent on the crystalline melting point. When the polymer possesses a low degree of crystallinity the glass transition temperature will remain of paramount importance. This is the case with unplasticised PVC and the polycarbonate of bis-phenol A. 

Over the years many attempts have been made to provide some measure of the maximum service temperature which a material will be able to withstand without thermal degradation rendering it unfit for service. Quite clearly any figure will depend on the time the material is likely to be exposed to elevated temperatures. One assessment that is being increasingly quoted is the UL 746B Relative Temperature Index Test of the Underwriters Laboratories (previously known as the Continuous Use Temperature Rating or Index). 

Details of the function and service conditions of the component part are ascertained, this including the expected lifetime and maximum service temperature. 

The hydrogenated materials have a higher maximum service temperature than the conventional SBS materials and, because of the absence of double bonds in the chain, better weathering properties. Known as SEBs these materials now find use in adhesives, sealants and roofing membrane compounds. These also find use in highly filled automotive acoustic barriers and in medical, sports and leisure applications. 

The maximum service temperature is about 60°C lower than that of PTFE for use under equivalent conditions. Continuous service at 200°C is possible for a number of applications. The polymer melts at about 290°C. 

The copolymers have been used in the manufacture of extruded pipe, moulded fittings and for other items of chemical plant. They are, however, rarely used in Europe for this purpose because of cost and the low maximum service temperature. Processing conditions are adjusted to give a high amount of crystallinity, for example by the use of moulds at about 90°C. Heated parts of injection cylinders and extruder barrels which come into contact with the molten polymer should be made of special materials which do not cause decomposition of the polymer. Iron, steel and copper must be avoided. The danger of thermal decomposition may be reduced by streamlining the interior of the cylinder or barrel to avoid dead-spots and by careful temperature control. Steam heating is frequently employed. 

The so-called flow temperature cannot be considered to be either the processing temperature or the maximum service temperature. It is obtained using the highly arbitrary Rossi-Peakes flow test (BS 1524) and is the temperature at which the compound is forced down a capillary of fixed dimensions by a fixed load at a specified rate. It is thus of use only for comparison and for quality control purposes. Since the rates of shear and temperatures used in processing are vastly different from those used in this test, extreme caution should be taken when assessing the result of flow temperature tests. 

Specific gravity Average thermal expansion > coefficient J Transition point Softening point Maximum service temperature Bending... 

A typical phase diagram for such polymers is given in Fig. 18.9. With such crystdline polymers the melting point replaces the as the factor usually determining the maximum service temperature of thermoplastics and minimum service temperature of rubbers. However, being more complicated than amorphous polymers it is more difficult to make generalisations about properties. The following remarks may, however, be pertinent for crystalline polymers ... 

The maximum service temperature for which a polymer may be used in a given application depends largely on two independent factors ... 

Polyether ether ketones (PEEK) have been developed using polyethersulphone technology. These materials crystallise, unlike the polysulphones, and have higher maximum service temperatures. They also have better resistance to hydrolysis at elevated temperatures than the polymides. 

A method commonly used to derive a maximum service temperature limit is the UL rating here, a material is aged at various temperatures and a property, say tensile strength, monitored. The... 

It can therefore be seen that the UL rating is not an adequate method of assessing maximum service temperature if a material is to support stress or undergo strain in service. 

Low maximum service temperature Not resistant to oxygen, ozone and light Not oil or solvent resistant... 

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