Plastics, poor high temperature resistance

TPEs prepared from rubber-plastic blends usually show poor high-temperature properties. This problem could be solved by using high-melting plastics like polyamides and polyesters. But, often they impart processing problems to the blends. Jha and Bhowmick [49] and Jha et al. [50] have reported the development and properties of novel heat and oil-resistant TPEs from reactive blends of nylon-6 and acrylate rubber (ACM). The properties of various thermoplastic compositions are shown in Table 5.4. In this kind of blend, the plastic phase forms the continuous phase, whereas... 

Polypropylene is somewhat similar to HDPE in general properties. It exists as a homopolymer and a copolymer with ethylene and other hydrocarbons. It can also be blended with polyisobutylene. PP is one of the lowest density plastics, translucent to natural milky white with a highly crystalline structure. PP homopolymer has poor low-temperature resistance but this has largely been overcome by copolymerisation with ethylene. 

Phosphate plasticizers such as tritolyl phosphate and trixylyl phosphate are generally used where good flame resistance is required, such as in insulation and mine belting. These materials, however, are toxic and give products with poor low-temperature resistance, i.e., with a high cold flex temperature (typically, -5°C). 

Mechanical and chemical resistance is equally important with the high-temperature-resistant plastics. This class consists of hydrogen-poor amorphous polymers with a rigid skeleton poly(phenylenes), poly(phenylene oxides), poly(xylylenes), aromatic polyamides, polyamides, poly(benzimi-dazoles), etc. High-temperature-resistant tibers are also made from many of these polymers (see also Chapter 38). 

Fluoroelastomers combine excellent chemical resistance (e.g. oxidizing acids, and alkalis) and high-temperature resistance (i.e., up to 275-300°C for short periods of time) excellent oxidation resistance good resistance to fuels containing up to 30% aromatics mostly poor resistance in solvents or organic media by contrast with fluorinated plastics. 

Polypropylene has a chemical resistance about the same as that of polyethylene, but it can be used at 120°C (250°F). Polycarbonate is a relatively high-temperature plastic. It can be used up to 150°C (300°F). Resistance to mineral acids is good. Strong alkalies slowly decompose it, but mild alkalies do not. It is partially soluble in aromatic solvents and soluble in chlorinated hydrocarbons. Polyphenylene oxide has good resistance to ahphatic solvents, acids, and bases but poor resistance to esters, ketones, and aromatic or chlorinated solvents. 

Polymeric plasticizers are generally polyesters with a relatively low molecular weight. They rue used where resistance to high temperatures and freedom from migration and extraction are required. Polymeries are more difficult to incorporate, have poor low-temperature properties, and are expensive. 

Solution systems of low- and intermediate-molecular-weight nylon resins can be coated on paper, metal foil, or plastics, and when heat-activated will act as adhesives for these substrates. Modified nylons have fair adhesion to metals, good low- and high-temperature properties, and good resistance to oils and greases, but poor resistance to solvents. ... 

These resins have many of the same features of polyethylene plus high transparency, tenacity, reliance, and increased resistance to oils, greases, and solvents. Commercial ionomers are nomigid, unplasticized plastics. Outstanding low-temperature flexibility, resilience, high elongation, and excellent impact strength typify the ionomer resins. Limitations of ionomers include low stiffness, susceptibility to creep, low heat distortion temperature, and poor ultraviolet resistance unless stabilizers are added. Most ionomers are very transparent. 

It is important to know how the filled plastics will behave after a long period in service for example under long-time exposure to high temperature under the hood of a car, immersed in hot water in a clothes or dishwasher, on exposure to the elements, on exposure to chemicals such as motor oil and brake fluid, and to fatigue by ( chcal loads. Plastics tend to creep (cold-flow) when exposed to continuous loads for long periods of time. They typieally have poor resistance to heat and UV hght, and require additives to eombat these defects. 

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