Heat distortion temperature polyesters

Polyarylate It is a form of aromatic polyester (amorphous) exhibiting an excellent balance of properties such as stiffness, UV resistance, combustion resistance, high heat-distortion temperature, low notch sensitivity, and good electrical insulating values. It is used for solar glazing, safety equipment, electrical hardware, transportation components and in the construction industry. 

Polyesters exhibit excellent physical properties. They have high tensile strength, high modulus, they maintain excellent tensile properties at elevated temperatures, and have a high heat distortion temperature. They are thermally stable, have low gas permeability and low electrical conductivity. For these reasons, polyesters are considered engineering polymers. 

Typically, polyester resins are used for high-end applications that require excellent electrical and thermal resistance. When dimensional stability under load is more critical, glass fibers are incorporated to increase the heat distortion temperature and the stiffness of the part. Examples of glass fiber reinforced parts include electrical housings, electrical adapters, computer components, telephone housings, and light bulb sockets. When impact modified, polybutylene terephthalate can be injection molded to make car bumpers. 

Known for many years, epoxy oligomers made from tetrabromobisphenol A are still used as the flame retardant in polycarbonates because they minimally affect the heat distortion temperature and even show a positive effect on impact strength. About 6—9 wt% of the epoxy oKgomer is required for achieving V-0 rating and a thermotropic liquid crystal polyester helps to improve melt flow, so that thin-waUed parts can be molded [38]. Antimony trioxide is not normally used in combination with halogen-containing additives in PC, because it causes loss of clarity. 

The glass reinforcement content is relatively low, at 20 wt%, but stiffness and impact strength are high (with a modulus about 50% higher than glass reinforced TS polyester and 3-4 times higher than RTFs). Heat distortion temperature is 300°C at 1.8 MPa and continuous service temperature is 170-190°C. Molding conditions are similar to those for TS polyester SMC. The recommended mold temperature is about 145°C and pressure is 5-8 MPa. 

Terephthalate polyesters are based on terephthalic acid, the para isomer of phthalic acid. The properties of cured terephthalic-based polyesters are similar to those of isophthalic polyesters, with the terephthalics having higher heat distortion temperatures and being somewhat softer at equal saturation levels. 

Aromatic Polyester Estercarbonate A thermoplastic block copolymer of an aromatic polyester with polycarbonate. Has higher heat distortion temperature than regular polycarbonate. 

Consider unsaturated polyesters as illustrations of some of the earlier discussed principles. They are not outstanding in their thermal stability, but they are extensively used in composite fabrication and have heat distortion temperatures in the range 55-120°C depending on molecular structure. Above this temperature a significant decrease in the modulus is noticed and accelerated creep is observed. Resins cannot be used for load bearing applications within 20-30°C of the Tg, or even 50°C in situations involving substantial solvent or moisture pickup, and standard polyesters are not usually used above about 100°C, even though they do not decompose chemically until around 150°C. 

The most common form is the nematic, a bimdle of parallel, long, rodlike molecules. Additional cooling to the primary transition temperature, T leads to solidification into a solid crystalline phase (small crystallites). In the region between Tj and T , a liquid of very low viscosity prevails, in contrast to the high melt viscosity. The aromatic polyesters have high heat distortion temperatures (HDT). Vectra, composed of para-hydroxy-benzoic acid (PHBA) and para-hydroxy-naphtoic acid (PHNA), has an HDT of 180 C-240 C. Xy-dar, composed of PHBA, tera-phthalic acid and biphenol, has an even higher HDT of 260 C-350 C. 

The only effects on the thermal properties seen from the incorporation of a fire retardant additive occurs in the case of high-impact polystyrene (HIPS) where, as shown in Table 8.4, the incorporation of a fire retardant leads to a decease in expansion coefficient and, in the case of the polyesters, where the incorporation of a fire retardant produces a small improvement in heat distortion temperature. 

The mechanical properties of the resole phenoUcs are comparable with those of orthophthaUc polyesters. Typical values are given in Table 3.4. However the high heat distortion temperature and fire resistance are leading to greater use of these materials. Low shrinkage compared with polyesters is a characteristic of this resin. 

Polyester resins derived from orthophthalic anhydride have good all-round properties, whereas isophthalic acid gives a tougher resin with some improvement in chemical resistance. Resins derived from terephthalic acid have properties similar to their isophthalic counterparts, but with a slightly greater heat distortion temperature (HDT). 

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