Liquid Crystal Copolyesters Containing Terephthalic Acid, Isophthalic Acid, 2,6-Naphthalenedicarboxylic Acid and Hydroquinone

This invention relates to liquid crystal copolyesters which exhibit the exceptionally high mechanical properties typical of liquid crystal polyesters but still have a melting point low enough to allow the copolyesters to be melt-processed into useful articles using commercially available equipment.

In the last four decades, a wide variety of plastics have been developed to fulfill a wide variety of uses. Starting with the commercial introduction of nylon 66 in 1938 and continuing with the introduction of polyacetal, polycarbonate, polyphenylene oxide, polysulfone and polytetramethylene terephthalate, new plastics have been continually introduced to overcome deficiencies in previous plastics. As plastics were developed to satisfy ever more demanding requirements, the aromatic character of the polymer often increased. As the amount of aromatic character increased, the melting point of the polymer increased. Ultimately, the increased melting point of highly aromatic polymers became so high that melt processing the polymer challenged the heating capacity of even the most sophisticated melt spinning and molding equipment and even approached the thermal stability limit of the polymers themselves. Thus, as the demand for polymers with greater and greater mechanical properties was met, the melting point of the polymers generally went up.

In very recent years the never ending search for polymers with greater and greater mechanical properties has resulted in a fundamental breakthrough in polymer science by development of a new class of polymers exhibiting a general overall balance of mechanical properties substantially enhanced over previous polymers. These polymers have been described by various terms, including "liquid crystalline", "liquid crystal", and "anisotropic". The polymers of this new class are thought to involve a parallel ordering of the molecular chains and are prepared from monomers which are generally long, flat, and fairly rigid along the long axis of the molecule.

As is the case of previous polymers, it was recognized that increasing the aromatic character of a liquid crystal polyester would enhance the general overall balance of mechanical properties. Also, just as with previous polymers, when the aromatic character of the liquid crystal polyesters became greater, the melting points became greater. In the case of previous polymers, the increasing melting point merely challenged the heating capacity of conventional equipment to heat the polymer in order to melt-process the polymer into useful articles. In the case of liquid crystal polyesters, increasing the aromatic character of the polyester created melting points that far exceeded the capability of conventional equipment to melt the polymer.

We have now discovered a wholly aromatic liquid crystal copolyester which has a melting point that is low enough to permit the copolyester to be melt processed into useful articles, such as fibers and molded articles, using conventional equipment. We have accomplished this objective of lowering the melting point by using the combination of a critical balance of terephthalic acid and isophthalic acid and a critical range of 2,6-naphthalenedicarboxylic acid.

Our invention, therefore, is a copolyester prepared from terephthalic acid, isophthalic acid, 2,6-naphthalenedicarboxylic acid, and a diacyl ester of hydroquinone and can be defined as a copolyester having a fiber forming molecular weight consisting essentially of the following divalent radicals: ##STR2##

In this invention the balance of terephthalic acid and isophthalic acid is critical. The range of terephthalic acid is from 30 to 80 mole percent, based on the total moles of terephthalic acid and isophthalic acid combined. Since the critical range of terephthalic acid is based on the sum of the mole of terephthalic acid and isophthalic acid, at 30 mole percent terephthalic acid the invention contemplates the use of 70 mole percent isophthalic acid and at 80 mole percent terephthalic acid the invention contemplates the use of 20 mole percent isophthalic acid.

In a preferred embodiment the range of terephthalic acid is from 30 to 70 mole percent, based on the total moles of terephthalic acid and isophthalic acid combined.

Also in this invention a critical range of 2,6-naphthalenedicarboxylic acid is used. The amount of 2,6-naphthalenedicarboxylic acid is from 15 to 60 mole percent, based on the total moles of terephthalic acid, isophthalic acid, and 2,6-naphthalenedicarboxylic acid.

In a preferred embodiment the range of 2,6-naphthalenedicarboxylic acid is from 20 to 50 mole percent.

Applicants are aware of a large volume of prior art relating to the copolyesters of this invention.

U.S. Pat. Nos. 3,778,410 and 3,160,602 disclose processes which can be used to prepare a polyester from terephthalic acid, isophthalic acid, 2,6-naphthalenedicarboxylic acid and hydroquinone.

U.S. Pat. Nos. 3,036,990 and 3,039,992 disclose polyesters prepared from terephthalic acid, isophthalic acid and hydroquinone.

Belgian Patent No. 828,935 discloses liquid crystal polyesters wherein a substantial portion of the rings in the polymer chain contain a substituent or a substantial portion of the rings in the polymer chain have chain extending linkages which are neither coaxial or parallel.

German Offenlegungsschrift No. 2,520,820 contains a disclosure similar to Belgian Patent No. 828,935.

Applicants regard the closest prior art to be U.S. Pat. Nos. 3,778,410 and 3,160,602.

The copolyesters of this invention are thought to be novel over these references because these patents are both directed to processes and contain a very broad disclosure and the claims recite the combination of a specific range of terephthalic acid, based on the sum of terephthalic acid and isophthalic acid, and a critical range of 2,6-naphthalenedicarboxylic acid, based on the sum of terephthalic acid, isophthalic acid and 2,6-naphthalenedicarboxylic acid. Although all the materials used to prepare applicants' polymer are within disclosure of these references, applicants believe these references do not disclose applicants' polyester in the sense of 35 USC 102 because applicants' claims are limited to the use of a combination of a critical range of terephthalic and a critical range of 2,6-naphthalenedicarboxylic acid.

The copolyesters of this invention are thought to be unobvious over these references because when one observes the limitations regarding the amount of terephthalic acid and 2,6-naphthalenedicarboxylic acid the melting point of the copolyester is unobviously low compared to similar polyesters wherein these limitations are not observed.

The precise manner in which the melting points of the polyesters of the invention are unobviously lower than the melting points of similar polyesters is illustrated in the FIGURE.

In the FIGURE the amount of 2,6-naphthalenedicarboxylic acid, based on the total moles of terephthalic acid, isophthalic acid and 2,6-naphthalenedicarboxylic acid, has been plotted on the abscissa. The temperature in degrees Centigrade has been plotted on the ordinate. Melting points have been plotted for polyesters of the invention, containing a quantity of terephthalic acid in the range of 20 to 80 mole percent, based on the total moles of terephthalic acid and isophthalic acid. Suitable curves have been drawn through the data points for polyesters containing the same amount of terephthalic acid and isophthalic acid. For example, in the lowermost curve drawn the solid square data points represent the melting points of polyesters containing 40 mole percent terephthalic acid, based on the total moles of terephthalic acid and isophthalic acid combined. Since the amount of 2,6-naphthalenedicarboxylic acid is plotted on the abscissa and is based on the total moles of terephthalic acid, isophthalic acid and 2,6-naphthalenedicarboxylic acid combined and since the amount of terephthalic acid is based on the total moles of terephthalic acid and isophthalic acid combined, the lowermost curve, as well as the other curves, corresponds precisely to the manner used in the claims to express the various components in the polyester.

The data for the copolyesters of the invention were obtained by preparing each of the copolyesters using a process known in the art and then determining the melting points of each copolyester.

The copolyesters of the invention were prepared by an acidolysis procedure whereby terephthalic acid, isophthalic acid, 2,6-naphthalenedicarboxylic acid and a diester of hydroquinone are contacted under an increasing temperature ranging up to about 340.degree.-380.degree. C. and a decreasing pressure to form a high molecular weight polymer. As a specific example, the following procedure was used to prepare a polyester from hydroquinone and 40 mole percent terephthalic acid and 60 mole percent isophthalic acid, based on the moles of terephthalic acid and isophthalic acid combined, and 30 mole percent 2,6-naphthalenedicarboxylic acid, based on the moles of 2,6-naphthalenedicarboxylic acid, terephthalic acid and isophthalic acid combined.

A mixture of 23.2 g. (0.14 mole) terephthalic acid, 34.9 g. (0.21 mole) isophthalic acid, 32.4 g. (0.15 mole) 2,6-naphthalenedicarboxylic acid, and 111 g. (0.500 mole) hydroquinone dipropionate is placed in a 500 ml. flask equipped with a stirrer, short distillation column and an inlet for nitrogen. The flask is evacuated and purged three times with nitrogen and dried at 100.degree.-110.degree. C. for 30 minutes at <0.3 mm pressure before being immersed in a Wood's Metal bath at 275.degree. C. After the mixture is stirred for 30 minutes at 280.degree. C., the temperature is raised to 300.degree. C. for 30 minutes and then to 325.degree. C. for 30 minutes. Finally the temperature is raised to 355.degree. C. for 25 minutes and a vacuum of 0.5 mm is applied. The polymerization is complete within 20 to 30 minutes. The tough, fibrous, opaque polymer obtained has a melting point of 334.degree. C.

The other copolyesters containing different amounts of 2,6-naphthalenedicarboxylic acid, terephthalic acid and isophthalic acid can be prepared by a similar procedure but using slightly different reaction temperatures because of differences in melting points.

Solid-phase polymerization also may be used to increase the molecular weight of the copolyesters of the invention by heating polymer particles in an inert atmosphere or under reduced pressure at a temperature below that at which the particles will become tacky and tend to fuse together. Since this thermal treatment may give polymers with increased crystallinity and melting points, compared to melt phase polymerization, melt phase polymerization is generally preferred. Solid-phase polymerization is preferred, however, if the melting point is above 380.degree. C.

The melting points of the copolyesters of the invention were determined with a Perkin-Elmer DSC-2 differential scanning calorimeter.

As can be appreciated from a consideration of the curves in the FIGURE, the melting points of the polyesters of the invention containing 40 to 80 mole percent terephthalic acid and 15 to 60 mole percent 2,6-naphthalenedicarboxylic acid are unobviously lower than the melting points of polyesters containing less than 20 or more than 80 mole percent terephthalic acid and less than 15 or more than 60 mole percent 2,6-naphthalenedicarboxylic acid. For example, consider the polyester which contains a constant value of 50 mole percent terephthalic acid and is represented by the second curve from the bottom connecting the open square data points. When there is no 2,6-naphthalenedicarboxylic acid present, the melting point is so high it cannot be measured. When even small amounts of 2,6-naphthalenedicarboxylic acid are added the melting point falls rapidly and when 15 mole percent 2,6-naphthalenedicarboxylic acid is added, the melting point falls to about 370.degree. C. As the amount of 2,6-naphthalenedicarboxylic acid is further increased, the melting point continues to fall and reaches a minimum value of about 350.degree. C. at around 35 mole percent 2,6-naphthalenedicarboxylic acid. As the amount of 2,6-naphthalenedicarboxylic acid is increased, the melting point goes up.

Although the details of the reduction in melting point have been discussed only for the polyester containing 50 mole percent terephthalic acid, the same lowering of the melting point applies to the other polyesters within the scope of this invention. For example, the melting point of the polyester containing 40 mole percent terephthalic acid is also substantially lowered when from 15 to 60 mole percent 2,6-naphthalenedicarboxylic acid is used. Although melting point data is not plotted for many of the polyesters of the invention containing less than 20 mole percent 2,6-naphthalenedicarboxylic acid, it is clear from the shape of the curve the same melting point suppression is occurring. Melting points for lower 2,6-naphthalenedicarboxylic acid containing polyesters often cannot be obtained because it is not possible to prepare these polyesters because the polyester melts so high it becomes solid in the reaction flask well prior to forming a high molecular weight polyester.

A wide variety of diesters of hydroquinone can be used to prepare the copolyesters of this invention. Examples of diesters of hydroquinone include the diacetate, dipropionate, dibutyrate and dibenzoate. The diacetate and dipropionate are preferred.

The copolyesters of this invention are described as "consisting essentially of" the various radicals. By the term "consisting essentially of" we mean that the copolyester can contain other divalent radicals, even in significant amounts, as long as the melting points of the copolyesters of the invention remain unobvious in view of the melting points of similar copolyesters not containing the critical range of terephthalic acid and 2,6-naphthalenedicarboxylic acid. For example, minor amounts of other naphthalenedicarboxylic acid isomers can be used in addition to the 2,6- isomer. Also, dicarboxylic acids other than terephthalic acid and diols other than hydroquinone can be used. By the term "consisting essentially of" we mean also that the copolyesters of this invention may contain nucleating agents, fillers, pigments, glass fibers, asbestos fibers, antioxidants, stabilizers, plasticizers, lubricants, fire-retardants, and other additives.

The inherent viscosity of the copolyesters of this invention cannot be determined because the copolyesters of this invention are insoluble in typical solvents used for determining inherent viscosity. Although the inherent viscosity of the copolyesters of the invention has not been measured, the molecular weights of the copolyesters of the invention are high enough to be in the fiber forming range. The minimum fiber forming molecular weight of the polymer is thought to be around 5,000. In most cases copolyesters of the invention have molecular weights above 8,000 and can have molecular weights as high as around 20,000 and in some instances the molecular weights can range up to 25,000 or even higher.