LIQUID CRYSTAL COPOLYESTERS

This invention relates to liquid crystal co¬ polyesters having the high mechanical properties of liquid crystal polyesters and melting points low enough to allow the copolyesters to be melt-processed into use¬ ful articles using commercially available equipment. Background of the Invention

Liquid crystal polyesters that are all-aro- matic have excellent mechanical properties. Examples of these polyesters are the copolyesters prepared from terephthalic acid, ^• isophthalic acid, 2,6-naphthaline- dicarboxylic acid and hydroquinone. U.S. Patents 3,160,602 and 3 _^ 778,410 describe processes that can be used to prepare these copolyesters. It has been diffi¬ cult to use these copolyesters because the melting points of the polymers have been so high that the polymers can not be melted and formed into useful articles in conven¬ tional processing equipment. We have found that certain all-aromatic copoly- esters prepared from terephthalic acid, isophthalic acid, 2,6-naphthalene dicarboxylic acid and hydroquinone have melting points that are low enough to permit the copolyesters to be processed into useful articles, such as fibers and molded articles, in conventional equip¬ ment.

The copolyesters of our invention are prepared from terephthalic acid, isophthalic acid, 2,β-naphthal- enedicarboxylic acid, and a diacyl ester of hydroquin- one and can be defined as copolyesters having molecular weights suitable for forming fibers and containing the following divalent radicals :

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In our copolyesters the range of terephthali acid is from 20 to 80 mole percent, based on the total moles of terephthalic acid and isophthalic acid combin Since the range of terephthalic acid is based on the s of the moles of terephthalic acid and isophthalic acid at 20 mole percent terephthalic acid our copolyesters have 80 mole percent isophthalic acid and at 80 mole percent terephthalic acid our copolyesters have 20 mol percent isophthalic acid.

In preferred copolyesters the range of terep thalic acid is from 30 to 70 mole percent, based on th total moles of terephthalic acid and isophthalic acid combined.

Also in our copolyesters the amount of 2,6- naphthalenedicarboxylic acid is from 15 to 60 mole per cent, based on the total moles of terephthalic acid, isophthalic acid, and 2,6-naphthalenedicarboxylic acid In preferred copolyesters the range of 2,6- naphthalene-dicarboxylic acid is from 20 to 50 mole percent.

The precise manner in which the melting poin of the polyesters of our invention are unexpectedly lower than the melting points of similar polyesters is illustrated in the accompanying Figure.

In the Figure the amount of 2,6-naphthalene- dicarboxylic acid, based on the total moles of tereph¬ thalic acid, isophthalic acid and 2,6-naphthalenedicar- boxylic acid, has been plotted on the abscissa. The temperature in degrees Centigrade has been plotted on the ordinate. Melting points have been plotted for the copolyesters of our 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 isoph- thalic acid. Suitable curves have been drawn through the data points for copolyesters containing the same amount of terephthalic acid and isophthalic acid. For example, the lowermost curve drawn through the solid square data points shows the melting points of copoly- esters containing 40 mole percent terephthalic acid and 60 mole percent isophthalic acid.

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

The copolyesters of our invention were pre¬ pared by an acidolysis procedure whereby terephthalic acid, isophthalic acid, 2,6-naphthalenedicarboxylic acid and a diester of hydroquinone are reacted under an increasing temperature ranging up to 3^0-380 C. and a decreasing pressure to form a high molecular weight polymer. As a specific example, the following procedure was used to prepare a copolyester from hydro¬ quinone 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) terephthal 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 was placed in a 500 ml. flask equipped with a stirrer, short dis tillation column and an inlet for nitrogen. The flas was evacuated and purged three times with nitrogen an dried at 100-110 C. for 30 minutes at 0.3 mm pressur before being immersed in a bath at 275°C. After the mixture was stirred for 30 minutes at 280°C, the tem perature was raised to 300°C. for 30 minutes and then to 325°C. for 30 minutes. Finally the temperature wa raised to 355°C. for 25 minutes and a vacuum of 0.5 m was applied. The polymerization was complete within to 30 minutes. The tough, fibrous, opaque polymer ha a melting point of 334°C.

The other copolyesters containing different amounts of 2,6-naphthalenedicarboxylic acid, terephth alic acid and isophthalic acid were prepared by a sim lar 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 tempe ature below that at which the particles will become tacky and tend to fuse together. Since this thermal treatment may give polymers with increased crystallin and melting points, compared to melt phase polymeriza tion, melt phase polymerization is generally preferre Solid-phase polymerization is preferred, however, if the melting point is above 380 C.

The melting points of the copolyesters of t invention were determined with a differential scannin calorimeter.

The accompanying Figure shows that the melting points of the polyesters of the invention containing 20 to 80 mole percent terephthalic acid and 15 to 60 per¬ cent 2,6-naphthalenedicarboxylic acid are unexpectedly 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 per- cent 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 the polymer is not useful for melt processing. When even small amounts of 2,6-naphthalenedicarboxylic acid are added the melting point falls rapidly and when 20 mole per¬ cent 2,6-naphthalenedicarboxylic acid is added, the melting point falls to 370 C. As the amount of 2,6- naphthalenedicarboxylic acid is further increased, the melting point continues to fall and reaches a minimum value of 350°C. at approximately 35 mole percent 2,6- naphthalenedicarboxylic acid. As the amount of 2,6- naphthalenedicarboxylic acid is increased, the melting point rises. Although the details of the reduction in melt¬ ing 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 poly¬ esters 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-naphthalenedicar- boxylic acid is used. Melting points for the polyesters containing small amounts of 2,6-naphthalenedicarboxylic acid often cannot be obtained because it is not possible

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to prepare these polyesters. The polyesters melts so high the polymer becomes solid in the reaction flask prior to forming a high molecular weight polyester. A wide variety of diesters of hydroquinone can be used to prepare the copolyesters of this inven¬ tion. Examples of diesters of hydroquinone include the diacetate, dipropionate, dibutyrate and dibenzoate. The diacetate and dipropionate are preferred.

The copolyesters of this invention can con- tain minor amounts of other naphthalenedicarboxylic acid isomers in addition to the 2,6- isomer. Also, minor amounts of dicarboxylic acids other than terephthalic acid and diols other than hydroquinone can be used. The copolyesters of this invention can also 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 copoly- esters of this invention are insoluble in typical sol¬ vents used for determining inherent viscosity. Although the inherent viscosity of the copolyesters of the inven¬ tion 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 about 5 ₅ 000. In most cases copolyesters of the invention have molecular weights above 8,000 and can have molecular weights as high as 20,000 and in some instances the molecular weights can range up to 25 ₃ 000 or even higher.

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