A MELT-PROCESSABLE COPOLYESTER

Abstract: The invention provides a melt-processable copolyester comprising: at least one structural unit (I) of formula wherein n is an integer of from 1 to 6; at least one structural unit (II) of formula -O-X-O- (2) wherein X is a group selected from groups of the formulae and at least one structural unit (III) of formula -O-Y-CO- (3) wherein Y is a group selected from groups of the formulae the proportion of the structural unit (III) being 10 to 80% of the structural units [(I) + (II) + (III)] in total, the structural unit (I) being substantially equimolar to the structural unit (II), and the inherent viscosity (ln n rel )/C, measured at a concentration of 0.16 g/dl in pentafluorophenol at 60°C, being not less than 0.5 dl/g.

Description: This invention relates to a novel copolyester which is melt-processable at a temperature below about 300 °C and is capable of giving molded articles having excellent heat resistance and hydrolysis resistance. Aromatic polyesters such as polyethylene terephthalate and polybutylene terephthalate have been hitherto widely used in molded articles such as fibers, films, etc., owing to their high crystallinity, high softening point and excellent mechanical strength. However, they are not necessarily satisfactory in respect of heat resistance. With a recent increase in demand for higher performance of materials accompanied by development of higher technologies, a variety of polyesters having various novel performances have been developed, and they are commercially available. Above all, a thermotropic liquid crystal polyester, which exhibits optical anisotropy in the melt phase, attracts attention since it has excellent properties. The thermotropic liquid crystal polyester greatly differs from conventional polyesters used industrially widely such as polyethylene terephthalate, polybutylene terephthalate, etc. In the thermotropic liquid crystal polyester, its molecular chain does not bend easily even in the melt phase or maintains a rod-like shape, its molecule does not tangle much with another when melted, it exhibits a specific flow behavior of one directional orientation by a small shear stress, and when it is cooled in the oriented state, it solidifies in the oriented state. Thus, the thermotropic liquid crystal polyester has excellent melt-processability, mechanical property and heat resistance. As this thermotropic liquid crystal polyester, a wholly aromatic polyester derived from p-hydroxy benzoic acid and 6-hydroxy-2-naphthoic acid as main components is known as shown in U.S.Patent 4,161,470. Said wholly aromatic polyester has excellent mechanical property and heat resistance. Since, however, 6-hydroxy-2-naphthoic acid as its material is expensive and the resulting polymer is expensive, the increase in demand therefor is prevented. On the other hand, as shown in U.S.Patent 3,778,410, a copolyester obtained by polycondensation of polyethylene terephthalate under acidolysis with p-acetoxy benzoic acid has a low melt-processing temperature of about 240 - 260 °C. Therefore, it is melt-moldable by an ordinary injection molding machine used for molding of polyetylene terephthalate, polybutylene terephthalate, etc., and it is known as a relatively less expensive thermotropic liquid crystal polyester. However, the above polyester has defects that its heat resistance is not satisfactory, since the heat distortion temperature thereof is about 65 - 70 °C and that its hydrolysis resistance is inferior to those of wholly aromatic polyesters. Summary of the invention It is an object of this invention to provide a thermotropic liquid crystal polyester excellent in melt-processability. It is another object of this invention to provide a thermotropic liquid crystal polyester excellent in mechanical property. Further, it is another object of this invention to provide a thermotropic liquid crystal polyester excellent in heat resistance and hydrolysis resistance. It is yet another object of this invention to provide a thermotropic liquid crystal polyester excellent in economy. According to this invention, there is provided a melt-processable copolyester comprising: structural units of formula (I) wherein n is 2 and structural units of formula (I) wherein n is 6, the structural units wherein n is 6 representing 5 to 95 mole % of all structural units of formula (I) a structural unit of formula (II) -O-X-O- (II) wherein X is at least one selected from the group consisting of and a structural unit of formula (III) -O-Y-CO- (III) wherein Y is at least one selected from the group consisting of the proportion of the structural unit (III) being 10 to 80% of the structural units [(I) + (II) + (III)] in total, the structural unit (I) being substantially equimolar to the structural unit (II), the inherent viscosity (1nη rel )/C, measured in a concentration of 0.16 g/dl in pentafluorophenol at 60°C, being not less than 0.5 dl/g. In the copolymerisation polyester of this invention, the structural units of formula (I) are residues of 1,2-bis(phenoxy)ethane-4,4'-dicarboxylic acid and 1,6-bis(phenoxy)hexane-4,4'-dicarboxylic acid. No limitation is imposed on the process for the production of 1,6-bis(phenoxy)hexane-4,4'-dicarboxylic acid. It may be produced, for example, by the following process. First, methyl p-hydroxy benzoic acid or ethyl p-hydroxy benzoic acid and 1,6-hexanedichloride of 1,6-hexanedibromide are reacted in an aprotic polar solvent such as dimethylformamide in the presence of an alkali such as sodium carbonate to produce methyl 1,6-bis(phenoxy)hexane-4,4'-dicarboxylic acid or ethyl 1,6-bis(phenoxy)hexane-4,4'-dicarboxylic acid. Then, the above product is saponified with an alkali such as potassium hydroxide, and the resultant product is acidified with hydrochloric acid, etc., to give 1,6-bis(phenoxy)hexane-4,4'-dicarboxylic acid. The 1,2-bis(phenoxy)ethane-4,4'-dicarboxylic acid can be produced nearly in the same way was as in the process for the production of 1,6-bis(phenoxy)hexane-4,4'-dicarboxylic acid. In this invention, the structural unit (I) represents two species in which n=2 and n=6, and comprises 5 to 95 mole % of the species in which n=6. The structural units of formula (II) are dihydroxy compound moieties such as hydroquinone, 4,4'-dihydroxybiphenyl and 4,4'-dihydroxydiphenyl ether and the structural unit (II) may comprise one component or more selected from these. The structural unit of formula (III) is comprised of at least one of p-hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid moieties. The copolyester of this invention is melt-processable at a temperature below 300 °C, and makes it possible to obtain molded articles having excellent mechanical properties, heat resistance and hydrolysis resistance by the use of an ordinary injection molding machine used for molding engineering plastics. In this invention, the proportion of the structural unit (III) is 10 to 80 mole %, preferably not less than 40 mole %, of the units [(I) + (II) + (III)] in total. In the case when the proportion is not more than 10 mole % or not less than 80 mole %, the resultant copolyester has poor melt-processability and does not make it possible to achieve the objects of this invention. The copolyester of this invention has an inherent viscosity (1nη rel )/C, measured in a concentration of 0.16 g/dl in pentafluorophenol at 60°C, of not less than 0.5 dl/g, preferably 1.0 to 10 dl/g. When the inherent viscosity is less than 0.5, the resultant molded articles have low strength, and when it is more than 10, the melt-processing is made difficult. The copolyester according to this invention is not only superior to usual aromatic polyesters such as polyethylene terephthalate in heat resistance and hydrolysis resistance, but it is also superior even to that liquid crystal polyester formed from polyethylene terephthalate and p-acetoxybenzoic acid which is described in U.S. Patent 3,778,410 in heat resistance and hydrolysis resistance. And it is less expensive and has better melt processability than that liquid crystal wholly aromatic polyester formed from p-hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid which is described in U.S. Patent 4,161,470. The copolyester of this invention can be produced according to conventional polycondensation processes for producing polyesters. For example, it can be obtained by reacting dicarboxylic acids or esters thereof which form structural units (I), an aromatic hydroxycarboxylic acid or its ester which forms a structural unit (III) and an aromatic hydroxy compound or its ester which forms structural unit (II) such that the proportion of the unit (III) to the units [(I) + (II) + (III)] in total and the proportion of the units (I) and (II) come under the predetermined ranges. Typical examples of the process of the production thereof include the following processes (a) and (b). (a) Acetic acid-removing polycondensation reaction: reaction of 1,2-bis(phenoxy)ethane-4,4'-dicarboxylic acid, 1,6-bis(phenoxy)hexane-4,4'-dicarboxylic acid, p-acetoxybenzoic acid and hydroquinone diacetate; reaction of 1,6-bis(phenoxy)hexane-4,4'-dicarboxylic acid, 1,2-bis(phenoxy)ethane-4,4'-dicarboxylic acid, 6-acetoxy-2-naphthoic acid and hydroquinone diacetate; reaction of 1,6-bis(phenoxy)hexane-4,4'-dicarboxylic acid, 1,2-bis(phenoxy)ethane-4,4'-dicarboxylic acid, p-acetoxydibenzoic acid, 6-acetoxy-2-naphthoic acid and 4,4'-diacetoxydiphenyl ether; (b) Phenol-removing polycondensation reaction: reaction of a diphenyl ester compound of 1,6-bis(phenoxy)hexane-4,4'-dicarboxylic acid, a diphenyl ester compound of 1,2-bis(phenoxy)ethane-4,4'-dicarboxylic acid, 6-hydroxy-2-naphthoic acid and hydroquinone; or reaction of a diphenyl ester compound of 1,6-bis(phenoxy)hexane-4,4'-dicarboxylic acid, a diphenyl ester compound of 1,2-bis(phenoxy)ethane-4,4'-dicarboxylic acid, p-hydroxybenzoic acid, 6-hydroxy-2-naphthoic acid and 4,4'-diacetoxydiphenyl ether. Among the above processes, the process (a) is more desirable since the polycondensation reaction in the process (a) proceeds without any catalyst. And yet, in the above process (b), metal compounds such as stannous acetate or tetrabutyltitanate can be used as a catalyst for the polycondensation reaction. The copolyester obtained by this invention is excellent in mechanical properties, melt-processable at a temperature below 300°C, capable of giving molded articles having excellent heat resistance and hydrolysis resistance and industrially better than conventional thermotropic liquid crystal polyesters, e.g., those described in U.S. Patents Nos 4,161,470 and 3,778,410. The copolyester of this invention may contain a filler, stabilizer, glass fiber, fire retardant and other additives. This invention will be explained in detail according to Examples hereinbelow. The inherent viscosity (1n n rel )/C of a copolyester in each of Examples was determined according to the following method. Sample solution: Pentafluorophenol solution having a concentration 0.16 g/dl of a sample. Viscometer: Ubbelode capillary viscometer having a flow time of 286 seconds for a flow of pentafluorophenol only. Measurement temperature: 60°C ± 0.01 °C According to the above method, the flow time of a sample solution was measured to obtain n rel, and the inherent viscosity (1nη rel )/C was calculated. The intrinsic viscosity was determined by the following method. Sample solution: Phenol/tetrachloroethane (=60/40 by weight) solution having a concentration 0.5 g/dl of a sample. Viscometer: Ubbelode capillary viscometer having a flow time of 40 seconds for a flow of phenol/tetrachloroethane (60/40 by weight). Measurement temperature: 25°C ± 0.01 °C According to the above method, the flow time of a sample solution was measured to obtain n sp/C. And the viscosity at a concentration of zero (0) was extrapolated, and the intrinsic viscosity [η] was determined. For thermal analysis, the use of TG/DTA 200 made by Seiko Electronics, measurement was made on 2 to 20 mg of a sample in 300 ml/min. of dry air at a temperature elevation rate of 10°C/min. Td (°C) stands for a thermal decomposition temperature at which a sample starts to decompose. The temperature (Tm) indicating an endotherm of a polymer by melting was measured with regard to 10mg of a sample in a nonclosed aluminium container in nitrogen gas current (30 ml/min.) at a temperature elevation rate of 20 °C/min by a differential scanning calorimeter (DSC), model SSC/560S made by Seiko Electronics. The apparent melt viscosity was measured by using a flow tester CFT-500 made by Shimadzu Corporation. The tester had a 1 mm⊘ x 2 mm die and 100 kg of a load. Synthesis Examples The following are processes employed to produce 1,6-bis(phenoxy)hexane-4,4′-dicarboxylic acid and 1,2-bis(phenoxy)ethane-4,4′-dicarboxylic acid used in the production of copolyesters in Examples. Synthesis Example 1 Synthesis of 1,6-bis(phenoxy)hexane-4,4′-dicarboxylic acid ("BHB-1") (I) : Methyl p-hydroxybenzoate (91.2 g), 46.6 g of 1,6-hexanedichloride, 600 ml of dimethylformamide and 34.9 g of sodium carbonate were charged into a 1 liter autoclave with a stirrer, thermometer, pressure gauge and nitrogen gas-introducing tube. The autoclave was closed, the charged materials were stirred and the temperature elevation was started. The reaction was carried out at 120 °C for 7 hours. Thereafter, the temperature was elevated to 130 °C and the reaction was continued for 7 hours. During the latter reaction, the pressure inside the autoclave was 5 kg/cm². The reaction liquid was cooled, a precipitate was filtered and a separated crude methyl 1,6-bis(phenoxy)hexane-4,4′-dicarboxylic acid was washed with water, and then with methanol to give 85.6 g of white plate-like crystals. 50 g of the white plate-like crystals was charged into a glass separable flask with a stirrer, thermometer and nitrogen gas-introducing tube, 900 ml of ethyl alcohol and 52 g of sodium hydroxide were added. The mixture was treated under heat at 80 °C for 4 hours to saponify the methyl 1,6-bis(phenoxy)hexane-4,4′-dicarboxylic acid. 2,000 ml of water and 100 ml of 35 % hydrochloric acid were added to the above treated liquid for neutralization to give crude 1,6-bis(phenoxy)hexane-4,4′-dicarboxylic acid. The treatment by washing with water and filtration were repeated to give 37 g of high-purity 1,6-bis(phenoxy)hexane-4,4′-dicarboxylic acid ( to be referred to as "BHB-1" hereinbelow). Synthesis Example 2 Synthesis of 1,2-bis(phenoxy)ethane-4,4′-dicarboxylic acid (BEB-1) p-Hydroxybenzoic acid (110.5 g), 66.0 g of sodium hydroxide and 700 ml of water were charged into a 1 liter autoclave with a stirrer, thermometer, pressure gauge, nitrogen gas-introducing tube and dropping funnel, and the temperature was elevated with stirring. When the temperature of the mixture liquid reached 70 °C, the heating was stopped, and 39.6 g of 1,2-ethylenedichloride was added dropwise over 1 hour with maintaining the above temperature. After the addition was finished, the autoclave was closed and the temperature was elevated to 120 °C to continue the reaction for 6 hours. During the reaction, the pressure inside the autoclave was 3.7 to 5.4 kg/cm². After the reaction liquid was cooled, 150 ml of 35 % hydrochloric acid was added, and a precipitate was filtered. The separated crude 1,2-bis(phenoxy)ethane-4,4′-dicarboxylic acid was washed with water and then with methanol to give 60.5 g of 1,2-bis(phenoxy)ethane-4,4′-dicarboxylic acid (to be referred to as "BEB-1" hereinbelow). Example 1 p-Acetoxybenzoic acid (108 g, 0.6 mole), 38.8 g (0.2 mole) of hydroquinonediacetate, 53,8 g (0.15 mole) of "BHB-1" and 15.1 g (0.05 mole) of "BEB-1" were charged into a 300 ml reaction vessel with a stirrer, thermometer, pressure gauge, nitrogen gas-introducing tube and condenser, the purging with nitrogen was carried out 3 times and the temperature was elevated with sowly stirring and with introducing a small amount of nitrogen into the vessel. The temperature of the resultant mixture was elevated stepwise to carry out reactions at 240 °C for 1.9 hours and at 250 °C for 0.2 hour. The stirring was speeded up and the reaction was carried out at 270 °C for 2.0 hours. At this stage, 55 g of acetic acid was distilled off. Then, the pressure of the reaction vessel was gradually reduced, and while the pressure was maintained at a vacuum of 0.5 Torr (0.5 x 133.322 Pa), the reaction product was stirred at 300 °C for 1.9 hours, at 320 °C for 20 minutes and at 350 °C for 20 minutes to complete the polymerization. The resultant polymer ("A-1") had an inherent viscosity of 1.98. This polymer, which was subjected to a differential scanning calorimeter (DSC), showed a sharp melt-endotherm at about 277 °C, and it showed an endotherm at about 275 °C when it was resubjected to the DSC. This polymer in the melt phase exhibited optical anisotropy. The optical anisotropy was determined by placing a sample of the polymer on a Metler hot stage of a Nikon polarizing microscope Optiphoto-POL. Examples 2 - 5 The procedure of Example 1 was repeated by changing the proportions of p-acetoxybenzoic acid, hydroquinone diacetate, "BHB-1" and "BEB-1", to prepare polymers "A-2" to "A-5". Table 1 shows the inherent viscosities, temperatures (Tm) which exhibit melt-endotherm by DSC and thermal decomposition temperatures (Td) of the resultant polymers. All of these polymers in the melt phase exhibited optical anisotropy. Comparative Example 1 Terephthalic acid (1,495 g, 9.0 moles) and 838 g (13.5 moles) of ethylene glycol were charged in this order into a 5 ℓ reaction vessel with a stirrer, thermometer, pressure gauge, nitrogen gas-introducing tube and condenser, and the stirring at a low speed was started. The purging with nitrogen was carried out 3 times and then the pressure inside the vessel was adjusted to 2 kg/cm² by nitrogen. With stirring at 200 rpm, the temperature elevation was started. With taking care that the internal pressure did not exceed 2.5 kg/cm², a dehydration condensation reaction was carried out at 215 to 240 °C to obtain an initial condensate. 1.3 g of triphenylphosphate and 0.4 g of germanium dioxide was added to the initial condensate, and the mixture was stirred for 5 minutes and then transferred to a 5 ℓ polycondensation vessel. The temperature was elevated with stirring, and when it reached 280 °C, the pressure was gradually reduced and adjusted to 0.5 Torr (0.5 x 133.322 Pa) for about 30 minutes. A polycondensation reaction was carried out with maintaining the degree of vacuum at not more than 0.5 Torr (0.5 x 133.322 Pa), and at the time when a predetermined torque was reached, the reaction was stopped to give a polyethylene terephthalate "B-1". The "B-1" had an intrinsic viscosity of 0.62 and a terminal carboxyl group concentration of 18.7 eq/10⁶ g (eq = equivalent). Table 1 shows Td and Tm (a temperature exhibiting melt-endotherm by DSC) of the "B-1". Comparative Example 2 The "B-1" in an amount of 76.8 g and 108 g of p-acetoxybenzoic acid were charged in to a 300 ml reaction vessel with a stirrer, thermometer, pressure gauge, nitrogen gas-introducing tube, distillation head connected to a condenser,etc., and the purging with nitrogen was carried out 3 times. The mixture was stirred under nitrogen atmosphere at 275 °C for about 1 hour to carry out an acetic acid-removal reaction, and then the polycondensation was carried out at a vacuum degree of not more than 0.5 Torr (0.5 x 133.322 Pa) for 4 hours to give a polyester "B-2" having an intrinsic viscosity of 0.59 and a terminal carboxyl group concentration of 149 eq/10⁶ g. This polymer in the melt phase exhibited optical anisotropy. The optical anisotropy was determined by placing a sample of the polymer of a Metler hot stage of a Nikon polarizing microscope Optiphoto-POL. Table 1 shows Td and Tm of the polymer "B-2". Example 6 Thermal stabilities of the copolyester "A-1" obtained in Example 1, the polyethylene terephthalate "B-1" obtained in Comparative Example 1 and the copolyester "B-2" obtained in Comparative Example 2 were compared by measuring apparent melting viscosities of the samples of the above polymers which had been held inside a die for a predetermined period of time, by the use a flow tester CFT-500 made by Shimadzu Corporation. Table 2 shows the results of the above measurements. It is clear that the "A-1" is superior to the "B-1" and "B-2" in thermal stability. Copolyester Temperature [°C] Hold time [second] Apparent viscosity [poise] Viscosity maintenance ratio [%] A-1 290 120 1154 100.0 300 1282 111.1 600 1369 118.6 900 1179 102.2 A-1 280 120 2345 100.0 300 2380 101.5 600 2319 98.9 B-1 280 120 5351 100.0 300 4822 90.1 600 3589 67.1 B-2 280 120 215 100.0 300 196 91.1 600 154 71.6 Example 7 6-Acetoxy-2-naphthoic acid (55.2 g, 240 mmols), 15.5 g (80 mmoles) of hydroquinone diacetate, 14.3 g (40 mmoles) of the "BHB-1" and 12.1 g (40 mmoles) of the "BEB-1" were charged into a 300 ml reaction vessel with a stirrer, thermometer, pressure gauge, nitrogen gas-introducing tube and a distillation head connected to a condenser, the purging with nitrogen was carried out 3 times and the temperature was elevated while the mixture was being slowly stirred and a small amount of nitrogen was being introduced into the vessel. The temperature of the resultant mixture was elevated stepwise to carry out reactions at 240 °C for 3.0 hours and at 250 °C for 1.5 hours. The stirring was speeded up and the reaction was carried out at 260 °C for 4.5 hours. Then, the pressure of the reaction vessel was gradually reduced, and while the pressure was maintained at a vacuum of 0.15 Torr (0.15 x 133.322 Pa), the reaction product was stirred at 280 °C for 2.0 hours and at 300 °C for 2 hours to complete the polymerization. The resultant polymer ("A-6") had an inherent viscosity of 1.21. This polymer, which was subjected to a differential scanning calorimeter (DSC), showed a sharp melt-endotherm at about 232 °C, and it showed an endotherm at about 235 °C when it was resubjected to the DSC. This polymer in the melt phase exhibited optical anisotropy. The optical anisotropy was determined by placing a sample of the polymer on a Metler hot stage of a Nikon polarizing microscope Optiphoto-POL. Examples 8 - 11 The procedure of Example 1 was repeated by changing the proportions of p-acetoxybenzoic acid, 6-acetoxy-2-naphthoic acid, hydroquinone diacetate, "BHB-1" and "BEB-1", to prepare polymers "A-7" to "A-10". Table 3 shows the inherent viscosities, temperatures (Tm) which exhibit melt-endotherm by DSC and thermal decomposition tempreatures (Td) of the resultant polymers. All of these polymers in the molten state exhibited optical anisotropy. Example Copolyester No. Material formulation (mol) Copolyester inherent viscosity Tm (°C) Td (°C) BHB-1 BEB-1 HQDA ANA ABA 7 A-6 10 10 20 60 1.21 232 435 8 A-7 8.5 8.5 17 66 1.15 274 436 9 A-8 7.5 7.5 15 70 1.06 296 438 10 A-9 7.5 7.5 15 21 49 2.36 244 416 11 A-10 7.5 7.5 15 14 56 2.85 296 420 Note ANA: 6-Acetoxy-2-naphtoic acid Example 12 Thermal stability of each of the copolyesters "A-8" and "A-9" obtained in Examples 9 and 10 was compared with that of the other by measuring apparent melting viscosities of the samples of the above polymers which had been held inside a die for a predetermined period of time, by the use of a flow tester CFT-500 made by Shimadzu Corporation. Table 4 shows the results of the above measurements. It is clear that the thermal stability of each of the "A-8" and "A-9" is superior to those of the "B-1" and "B-2" shown in Table 2. Copolyester Temperature [°C] Hold time [second] Apparent viscosity [poise] Viscosity maintenance ratio [%] A-8 280 120 9950 100.0 300 10820 108.7 600 10040 100.9 A-9 280 120 3730 100.0 300 3540 94.9 600 3430 92.0 Example 13 p-Acetoxybenzoic acid (37.8 g, 210 mmoles), 14.2 g (52.5 mmoles) of 4,4′-diacetoxybiphenyl, 9.4 g (26.3 mmoles) of the "BHB-1" and 7.93 g (26.3 mmoles) of the "BEB-1" were charged into a 300 ml reaction vessel with a stirrer, thermometer, pressure gauge, nitrogen gas-introducing tube and a distillation head connected to a condenser, the purging with nitrogen was carried out 3 times and the temperature was elevated while the mixture was being slowly stirred and a small amount of nitrogen was being introduced into the vessel. The temperature of the resultant mixture was elevated stepwise to carry out reactions at 240 °C for 3.5 hours and at 250 °C for 1.5 hours. The stirring was speeded up and the reaction was carried out at 280 °C for 1.5 hours. Then, the pressure of the reaction vessel was gradually reduced, and while the pressure was maintained at a vacuum of 0.3 Torr (0.3 x 133.322 Pa), the reaction product was stirred at 300 °C for 2.0 hours and at 320 °C for 3.0 hours to complete the polymerization. The resultant polymer ("A-11") had an inherent viscosity of 2.47. This polymer, which was subjected to a differential scanning calorimeter (DSC), showed a sharp melt-endotherm at about 271 °C, and it showed an endotherm at about 275 °C when it was resubjected to the DSC. This polymer in the melt phase exhibited optical anisotropy. The optical anisotropy was determined by placing a sample of the polymer on a Metler hot stage of a Nikon polarizing microscope Optiphoto-POL. Examples 14 - 19 The procedure of Example 13 was repeated by changing the proportions of p-acetoxybenzoic acid, 6-acetoxy-2-naphthoic acid, 4,4′-diacetoxydiphenylether, 4,4′-diacetoxybiphenyl, "BHB-1" and "BEB-1", to prepare polymers "A-12" to "A-17". Table 5 shows the inherent viscosities, temperatures (Tm) which exhibit melt-endotherm by DSC and thermal decomposition tempreatures (Td) of the resultant polymers. All of these polymers in the melt phase exhibited optical anisotropy. Example 20 Thermal stability of each of the copolyesters "A-11" and "A-17" obtained in Examples 13 and 19 was compared with that of the other by measuring apparent melting viscosities of the samples of the above polymers which had been held inside a die for a predetermined period of time, by the use of a flow tester CFT-500 made by Shimadzu Corporation. Table 6 shows the results of the above measurements. It is clear that the thermal stability of each of the "A-11" and "A-17" is superior to those of the "B-1" and "B-2" shown in Table 2. Example Copolyester No. Material formulation (mol) Copolyester inherent viscosity Tm (°C) Td (°C) BHB-1 BEB-1 DABP DADE ABA ANA 13 A-11 8.5 8.5 17 66 2.47 271 410 14 A-12 15 5 20 60 1.30 246 412 15 A-13 8.5 8.5 17 66 1.12 272 419 16 A-14 15 5 20 60 1.45 225 425 17 A-15 15 5 20 60 1.46 220 426 18 A-16 7.5 7.5 15 56 14 2.52 202 424 19 A-17 7.5 7.5 15 56 14 2.79 204 423 Note: DABP: 4,4'-diacetoxybiphenyl DADE: 4,4'-diacetoxydiphenyl ether Copolyester Temperature [°C] Hold time [second] Apparent viscosity [poise] Viscosity maintenance ratio [%] A-11 280 120 15350 100.0 300 12570 81.9 600 12200 79.5 A-17 280 120 2240 100.0 300 1990 88.8 600 1980 88.4

Claims: A melt-processable copolyester comprising: structural unit (I) of formula wherein n is 2 and structural units of formula (I) wherein n is 6, the species wherein n is 6 representing 5 to 95 mole% of the total units of formula (I) at least one structural unit (II) of formula -O-X-O- (2) wherein X is a group selected from groups of the formulae and at least one structural unit (III) of formula -O-Y-CO- (3) wherein Y is a group selected from groups of the formulae the proportion of the structural unit (III) being 10 to 80% of the structural units [(I) + (II) + (III)] in total, the structural units (I) being substantially equimolar to the structural unit (II), and the inherent viscosity (1nη rel )/C, measured at a concentration of 0.16 g/dl in pentafluorophenol at 60°C, being not less than 0.5 dl/g. A copolyester according to claim 1 wherein the inherent viscosity is not more than 10.0. A copolyester according to claim 1 or claim 2 wherein the proportion of the structural unit (III) to the structural units [(I) + (II) + (III)] in total is 40 to 80 mole %. A copolyester according to any one of the preceding claims wherein X in formula (2) for the structural unit (II) is and Y in the formula (3) for the structural unit (III) is A copolyester according to any one of claims 1 to 4 wherein X in formula (2) for the structural unit (II) is and Y in the formula (3) for the structural unit (III) is or a mixture of and A copolyester according to any one of claims 1 to 5 wherein X in formula (2) for the structural unit (II) represents at least one of and and Y in formula (3) for the structural unit (III) represents at least one of A process for preparing a copolyester as claimed in claim 1 which comprises reacting dicarboxylic acids or esters thereof which form structural units (I) wherein n is 2 and 6, at least one aromatic hydroxycarboxylic acid or ester thereof which forms a structural unit (III) and at least one aromatic hydroxy compound or ester thereof which forms a structural (II) in such amounts that the structural units of formula (I) wherein n is 6 represent 5 to 95 mole % of all structural units of formula (I) and the proportions of the unit (III) being 10 to 80 mole% of the structural units [(I) + (II) + (III)] in total, the structural units (I) being substantially equimolar to the unit (II).

Inventors: HIROTA, TOSHIZUMI POLYMER RES. INST. OF MITSUBISHI, TANISAKE, HIROKA POLYMER RES. INST. OF MITSUBISHI, YAMAMOTO, KOJI POLYMER RES. INST. OF MITSUBISHI
Applicants: MITSUBISHI GAS CHEMICAL COMPANY, INC.
Application year: 1988
Main classification: C08G63
IPC codes: C08G63