Heat deflection temperature limitations

An all aromatic polyetherimide is made by Du Pont from reaction of pyromelUtic dianhydride and 4,4 -oxydianiline and is sold as Kapton. It possesses excellent thermal stabiUty, mechanical characteristics, and electrical properties, as indicated in Table 3. The high heat-deflection temperature of the resin limits its processibiUty. Kapton is available as general-purpose film and used in appHcations such as washers and gaskets. Often the resin is not used directly rather, the more tractable polyamide acid intermediate is appHed in solution to a surface and then is thermally imidi2ed as the solvent evaporates. 

Polyarylates have been employed for electrical and electronic components, firefighter helmets, and appHcations requiring higher heat-deflection temperatures than PC resins. Polyarylates were first used for lighting, especially for small automotive lenses and sodium light outdoor lamps. However, the inherent yellow color and heat-induced darkening have limited appHcations. 

When cured with room temperature curing system these resins have similar thermal stability to ordinary bis-phenol A type epoxides. However, when they are cured with high-temperature hardeners such as methyl nadic anhydride both thermal degradation stability and heat deflection temperatures are considerably improved. Chemical resistance is also markedly improved. Perhaps the most serious limitation of these materials is their high viscosity. 

Figure 7.8 shows the relationship between the heat deflection temperatures (HDT) and PVDF content of PVDF/PMMA blends. HDT is the starting temperature at which the polymer begins to deform under a certain stress. A minimum HDT is observed at a PVDF content of 50 wt %. The sharp increase in HDT is most likely due to the crystallization of PVDF in solid blends when the PVDF composition exceeds 50 wt %. The crystallites serve as temporary crosslinking sites to limit the mobility of polymer segments and thus to increase the heat resistance of PVDF/PMMA blends. When a blend has a PVDF content greater than 65 wt %, the material provides a heat resistance exceeding that of PMMA. 

At elevated temperatures al polymers soften, dependent on their glass-rubber transition points, Tg, and/of their melting points, Tm. These temperatures limit the practical use of plastics. To characterize the softening behaviour, in practice various types of standard tests are being carried out, resulting in values for the softening temperature , defined in different ways. The values mostly used are the ISO Heat Deflection Temperature (HDT) and the Vicat Softening Temperature (VST or... 

GP-PS is a clear, rigid polymer that is relatively chemically inert. Polystyrene, as produced, has outstanding flow characteristics and consequently is very easy to process. Its excellent optical properties, including high relractive index, make it usefiil in optical applications. However, GP-PS has a number of limitations, including its brittleness, low heat-deflection temperature (Table 15.1), poor UV resistance, and susceptibility to attack by a variety of solvents. Polystyrene is sensitive to foodstuffs with high fat or oil content it erazes and turns yellow during outdoor exposure. 

The use of two isomeric acids leads to an irregular chain which inhibits crystallization. This allows the polymer to be processed at much lower temperatures than would be possible with a crystalline homopolymer. Nevertheless the high aromatic content of these polyesters ensures a high Tg ( 90°C). The polymer is self-extinguishing with a limiting oxygen index of 34 and a self-ignition temperature of 545°C. The heat-deflection temperature under load (1.8 MPa) is about 175 C. 

P-FRs have both negative chemical and mechanical efferts in the resin, limiting their applications. For example, they are not usually used in thin applications, or in water-handling applications, because of their solubility and extraction tendencies. Moreover, as with mineral FRs, P-FRs tend to inaease modulus and heat-deflection temperature (HDT), while lowering impart strength and other mechanicals [1-1, 5-11, 5-13). 

Heat Deflection Temperature. This is increased slightly in amorphous polymers, because the fillers or fibers reduce the mobility of the polymer molecules. It may be increased tremendously in crystalline polymers, because fillers and especially fibers raise the plateau of the modulus versus temperature curve just enough to extend the pass/fail limit of the standard test by hundreds of degrees (Fig. 5.7, Table 5.18). The practical significance of this obviously depends on the judgment of the product designer. 

Heat deflection temperature— ASTM D648. This data is dangerous in the respect that it often is the only temperature data provided on a resin data sheet, which leaves the impression that it is a reliable indicator of the limit to which the product can be used (see the section on Relative Temperature Index later in this chapter). It is really nothing more than the temperature at which a given load (66 or 264 Ib/in ) will deflect a specimen an arbitrary amoinit. Other temperature tests are also used, and some are described in the following sections. Results of those tests are usually available from the resin manufacturer. 

MMT type MMT content (wt%) Styrene content (wt%) Hardness (N mm" ) Heat deflection temperature HDT ( C) Limiting oxygen index LOI (%)... 

The high-strength Basalt fiber (Plain weave, 150 g/m2) which is supplied by ASA. TEC, Austria are used in the present study. The isopthalic polyester resin used for matrix system is ISO-4503 procured from Vasavibala resins (P) Ltd., Chennai (viscosity 500-600 cps, acid value 15-19, gel time 15-25 min, heat deflection temperature 95 °C). One weight percent of accelerator (cobalt Napthanate) and 1 wt% of methyl ethyl ketone peroxide (MEKP) catalyst were mixed with the polyester resin. The type of epoxy resin used here is LY 556 and the hardener HT 951 both supplied by M/s Hindustan Ciba Geigy Limited, Mumbai, India. 

The low heat deflection temperature of PLA limits its use for several application fields, such as in packaging materials and electronic components. The introduction of rigid building blocks [63] or cross-links [64] is known, for instance, to increase the glass transition temperature and/or heat resistance of lactic acid based polymers. The effect of different amounts of comonomers in the prepolymers on the Tg and mechanical properties of poly(ester-urethane)s is demonstrated in Table 3.2. The heat resistance of poly(ester-ure-thane)s can be improved by the copolymerization of lactic acid with D,L-mandehc acid. This broadening of the operating temperature range is of clear practical importance. The incorporation of other comonomers that impede rotation and make polymer chains less mobile also causes an increase in Tg, even if the same comonomers can depress the rate of poly condensation [50]. 

Heat deflection temperature Good maximum operating temperature without stresses. However, it is severely limited in the presence of stresses. Consider using engineering plastics, e.g., acetal, PPE, polysulphone or polycarbonate. 

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