Injection molding, aromatic polyesters

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 lowest cost process for preparing all-arcmatic liquid crystalline polyesters involves the reaction of aromatic carboxylic acids with acetates of aromatic hydroxy ocnpounds a recent history (2) describes the development of these ICP s. Because aoetic acid is evolved in the process and reaction temperatures are above 300°C, expensive corrosion-resistant reactors must be installed for ccranercial production. In cur latest paper (2) of this ICP series, we described a number of aliphatic-aromatic ICP s which can be produced in conventional polyester reactors and injection molded to give plastics with very high mechanical properties, heat-deflection temperatures (HDT s), and solvent resistance. These ICP s (la) were prepared by the reaction of the dimethyl ester of... 

A number of commercially significant PC copolymers are produced. In addition to the previously discussed branched PCs (for extrusion and blow-molding applications) and copolymers of BPA with tetrabromo-BPA for enhanced flame retardancy, high-Tg polyester carbonate copolymers have been produced for a number of years (Bayer Apec , GE LEXAN PPC Tg approximately 190°C). Polyester carbonate copolymers can be produced via copolymerization of BPA with diacyl chlorides. Aromatic diadds produce high-Tg copolymers, while aliphatic diadds yield lower-Tg copolymers. A lower-Tg PC aliphatic polyester copolymer (GE LEXAN SP resin) exhibits enhanced flow and ductility in comparison to standard PC and is useful for thin-wall injection molding applications requiring ductility and ease of melt processability, such as personal communication devices. GE has recently introduced two new PC copolymers, a PC-siloxane copolymer (LEXAN EXL) and a copolymer of... 

Another possibility for obtaining imide modified thermoplastic polyesters was to use as a monomer the hydroxy acid made from trimellitic anhydride and aminoethanol. Such a poly(ester-imide) was claimed for injection molding [240]. For the same use, poly(ester-imide)s containing aminophenol/trimellitic anhydride [241],imidised polyfbutylene terephthalate) [242] and a wholly aromatic poly(ester-imide) made from trimellitic anhydride, p-aminobenzoic acid, p-acetoxybenzoic acid, diacetoxybiphenyl and terephthalic and isophthalic acids are known, which showing optical anisotropy [243]. 

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 term hierarchical structure was first described for aramid by Dobb et al. [4] and for injection molded thermotropic liquid crystalline copolyester by Weng et al. [5]. It has been found that no matter which LC polyester is concerned, wholly aromatic or semi-aromatic ones. 

Could a similar phenomenon occur in the melt Were thermotropic polymers possible They were possible, and indeed the number reported to date by academic and industrial researchers is nearly beyond count. In addition, as it later turned out, aromatic thermotropic polymers were found to offer a great many more useful properties than just their now well-known tensile capabilities. These polymers are injection moldable, albeit at temperatures in the vicinity of 400° C, a temperature not compatible with common melt-spinning equipment. The rate of thermal degradation of such a polyester at 400° C makes stable fiber production particularly difficult. Moreover, most conventional injection-molding equipment requires modification to operate at the high temperatures needed to ensure reasonable processing of this polymer. 

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. 

Udipi [2] has discovered that polymer blends comprising a PC, amorphous polyester such as PETG, poly(ethylene terepthalate), and a nitrile rubber can form an alloy with superior balance of properties. Improved melt flow and spiral flow was demonstrated in injection molding applications. Udipi prepared a polymer alloy with 30% aromatic polycarbonate, 30% of PETG made of a condensation copolymer of terepthalic acid and a mixture of ethylene glycol and 1,4-cyclohexanedimethanol, and 3% nitrile rubber. This blend has a spiral flow of 27 cm, heat distortion temperature of 75°C, and Izod impact resistance of 130 J/m. 

Poly(ethylene terephthalate) (abbreviated PET or PETE) is a semi-aromatic thermoplastic polyester obtained by condensation reaction of difunctional reactants and well-known for more than 60 years. PET is commonly produced by esterification reaction between terephthalic acid and ethylene glycol with water as a byproduct or by transesterification reaction between ethylene glycol and dimethyl terephthalate with methanol as a byproduct. In order to obtain high molar masses polymers, solid-state polymerization is carried out. PET is one of the most important industrial polymers because of its excellent properties as tensile impact strength, chemical resistance, processability, clarity, thermal stability and others. The main applications of PET are fibers for textiles, films and bottles. Annual world PET production is around 60 millions tons. PET materials were manufactured using extrusion, injection molding and blow molding techniques. 

PCT displays a high deflection temperature (HDT) consistent with its high melting point which enables it to be used in glass-fiber reinforced formulations resistant to elevated temperatures. Usual formulations for injection molding contain 30-40 % of glass fiber as well as other additives, such as low molecular weight aliphatic polyesters, in order to increase the crystalizability of PCT. In some applications, thermal stabilizers and flame retardant bromide and antimony aromatic derivatives are also added (14,118). 

It is instructive to compare the melt viscosities of an all-aromatic liquid crystalline polyester with those of a similar polyester that is not liquid crystalline because of kinks in the polymer chain. Figure 7 compares the melt viscosities at 360 C of the polyterephthalate and polyisophthalate of phenylhydroquinone at the shear rates which are employed in melt processing (about 1000 sec for melt spinning and about 10,000 sec" for injection molding). The polyterephthalate has the low melt viscosities that are typical of liquid crystalline... 

De Candia F, Renzulli A, Vittoria V, Roviello A, Sirigu A (1990) Transport properties of a thermotropic liquid-crystalline polyester. J Polym Sci Part B Polym Phys 28 203-211 East AJ, Charbonneau LF, Calundann GW (1982) Poly(ester-amide) capable of forming an anisotropic melt phase derived from 6-hydroxy-2-naphthoic acid, dicarboxylic acid, and aromatic monomer capable of forming an amide linkage. US Patent 4,330,457, 18 May 1982 Fellahi S, Meddad A, Fisa B, Favis BD (1995) Weldlines in injection-molded parts a review. Adv Polym Technol 14 169-195... 

---