Molding, aromatic polyesters

Aromatic polyester Unsaturated polyester Alkyd molding compounds ... 

Aromatic polyesters, commercially important molding resin materials, show a low degree of flammability and produce high percentages of char on exposure to a flame or on heating to pyrolysis conditions (9). 

Compared with other polymeric materials. LCPs have very high unidirectional properties. Iei/nt7 1 (Celanese Corp.t resins are primarily aromatic polyesters based on p-hydroxybenzoic acid and hydroxynaphthoic acid monomers. Xytlar " (Celanese Carp.) injection molding resins are polyesters based on terephthalic acid. />. p -dihydruxybiphenyl and p-hydroxybenzoic acid Differences in monomers are primarily responsible for the differences in specific properties and end uses. The fibrous nature of the polymers imparls good impact strengths. 

It was, however, observed that such systems under appropriate conditions of concentration, solvent, molecular weight, temperature, etc. form a liquid crystalline solution. Perhaps a little digression is in order here to say a few words about liquid crystals. A liquid crystal has a structure intermediate between a three-dimensionally ordered crystal and a disordered isotropic liquid. There are two main classes of liquid crystals lyotropic and thermotropic. Lyotropic liquid crystals are obtained from low viscosity polymer solutions in a critical concentration range while thermotropic liquid crystals are obtained from polymer melts where a low viscosity phase forms over a certain temperature range. Aromatic polyamides and aramid type fibers are lyotropic liquid crystal polymers. These polymers have a melting point that is high and close to their decomposition temperature. One must therefore spin these from a solution in an appropriate solvent such as sulfuric acid. Aromatic polyesters, on the other hand, are thermotropic liquid crystal polymers. These can be injection molded, extruded or melt spun. 

The first LCP to be launched commercially was Dartco s Xydar, introduced in 1984 [11]. Xydar injectionmolding resins are aromatic polyesters based on terephthalic acid, p-hydroxybenzoic acid, and p,p -dihydroxybiphenyl. Xydar has a high melting point (close to 400°C), which necessitates certain modifications to processing equipment. It also has a high melt viscosity making it difficult to mold in... 

Unsaturated polyesters are produced in the form of dissolved resins in polymerizable monomers (usually styrene). They are also known in the form of molding resins or as hardened products. One should distinguish between them and the saturated aliphatic and aromatic polyesters. Among the latter are polyethylene terephthalate and polybutylene terephthalate. 

Aromatic Polyesters Engineering thermoplastics prepared by polymerization of aromatic polyol with aromatic dicarboxylic anhydride. They are tough with somewhat low chemical resistance. Processed by injection and blow-molding, extrusion, andftiermoforming. Drying is required. Used in automotive housings and trim, electrical wire j acketing, printed circuit boards, and appliance enclosures. 

The best known product made from aromatic polyester, however, is the blow-molded soda bottle, where the growth has been particularly dramatic during the past few years.This application accounts for 70-80% of the resin consumption and exploits the excellent barrier properties of the resin to carbon dioxide. It also requires the resin to be processed under conditions that would yield an amorphous (transparent) polymer with good gas barrier properties. The number-average molecular weight of the grade of resin used in bottles is about 80,000 (that used in fibers and films is about 45,000). 

In 1972, Cottis and coworkers at Carborundum patented wholly aromatic polyesters based on p-hydroxybenzoic acid (HBA), A,A -dihydrox-ybiphenyl (DHB), and terephthalic acid (TPA), one of which was later commercialized as EKKCEL 1-2000 [1]. In 1974, Kuhfuss and coworkers at Eastman Kodak reported a new polyester based on HBA and polyfethylene terephthalate) (PET), which was later marketed under the code of X-7G. X-7G is the first thermotropic liquid crystalline polymer to be fabricated by injection molding or melt spinning [2]. However, then Eastman Kodak withdrew its plan of marketing of X-7G and changed the target with a wholly aromatic polyester commercialized as TITAN (THERMX ) in 1996. As described later, it was acquired by DuPont in 2003. 

Polyarylate polymers are aromatic polyesters derived from aromatic dicarboxylic acids and diphenols. In contrast to liquid crystalline aromatic polyester (derived from dicarboxylic hydroxy acids), polyarylates exhibit amorphous character on molding Tg is ca 198°C. Polyarylates present a competitive cost(performance profile in the context of amorphous engineering thermoplastics, delivering an excellent balance of mechanical properties, particularly practical heat resistance which substantially exceeds that of polycarbonates (qv). 

Schaefgen and coworkers disclosed the use of substituted hydroquinones for the preparation of thermotropic liquid crystalline all-aromatic polyesters and demonstrated that fibers, films, and plastics having high mechanical properties could be obtained on extrusion or molding. ... 

The previous paper in this series described the preparation and properties of highly aromatic polyesters that have turbid melts, have melt viscosities highly dependent upon composition and shear rate, and that give unusually anisotropic molded articles. Because these unusual properties are reminiscent of the behavior of nonpolymeric nematic liquid crystalline materials, further work has been done to synthesize and characterize polymers containing other moieties known to lead to liquid crystallinity in nonpolymeric materials. The copolyesters produced were derived by the acidolysis reaction previously described from poly-(ethylene terephthalate) (PET) and a variety of dicarboxylic acids and acetylated difunctional phenols. Some of the copolymer compositions were varied to determine the limits of composition that give the turbid melts characteristic of liquid crystallinity, but which can be melted before decomposition. This paper describes the preparation and the physical and magnetic properties of these polymers. 

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