Optically Anisotropic Melt Forming Copolyesters

1. Field of the Invention

This invention provides novel fiber-forming, melt-spinnable wholly aromatic copolyesters prepared from hydroquinone, hydroxybenzoic acid or 6-hydroxy-2-naphthoic acid, 3,4'-dicarboxydiphenyl ether and optionally isophthalic acid or resorcinol. These copolyesters are useful for preparation of filaments having high tenacity and/or high modulus. They are also useful for extrusion molded or injection molded products and for preparation of tough films.

2. Description of the Prior Art

Aromatic copolyesters capable of forming optically anisotropic melts are well known in the art. These polymers have yielded heat-treated fibers with properties that are especially useful in tire cords or drive belts. The provision of novel copolyesters for these purposes is deemed a worthwhile objection.

The present invention is directed to melt-spinnable copolyesters of fiber-forming molecular weight that exhibit optical anisotropy in the melt and consist essentially of Units having the structural formulas: ##STR1## where X and Y are the same and are selected from --O-- or ##STR2## said Units being present in the proportions of from about 10 to 60 mol % of Unit I, from about 15 to 45 mol % of Unit II, and from about 15 to 45 mol % of Unit III.

In each case the number of dioxy units in the copolyester is substantially equal to the number of dicarbonyl units. Melt-spun and heat-strengthened filaments of such polyesters as well as films and molded or extruded articles from such polyesters are included in the invention.

Unit I in the copolyesters of the invention is 4-oxybenzoyl or 6-oxy-2-naphthoyl. Unit II is 1,4-dioxyphenylene, Unit III is 3,4'-dicarbonyldiphenyl ether (DCDPE), and Unit IV is isophthaloyl or 1,3-dioxyphenylene.

The number of dioxy units present in the copolyester is substantially equal to the number of dicarbonyl units. Mole % is calculated on the basis of total moles of units present, e.g. [I+II+III].

A suitable precursor for Unit I is 4-hydroxybenzoic acid, generally employed in the form of the monoacetate or 6-hydroxy-2-naphthoic acid, also employed as the monoacetate. Hydroquinone is a suitable precursor for Unit II, generally employed as the diacetate. Unit III is provided by 3,4'-dicarboxydiphenyl ether. Unit IV is ordinarily derived from isophthalic acid or resorcinol in the form of the diacetate.

The precursor reactants are generally combined in proportions corresponding to the molar proportions of the units desired in the copolyester products except that it is preferred to use a molar excess, indicated in the examples as (%) of the more volatile diacetates of hydroquinone and resorcinol.

Conventional polymerization techniques may be employed such as described in U.S. Pat. No. 4,118,372 and more particularly in the examples described below. In general, a mixture of monomers is heated with stirring, under nitrogen in a 250 ml 3-necked flask or polymerization tube in a Wood's metal bath or other suitable heating medium to approximately 310.degree.-380.degree. C. Polymerization is continued for up to a total of 0.5 to one hour or longer if necessary until a polymer or fiber-forming molecular weight is obtained. Usually a vacuum is applied to obtain a final product with high molecular weight. The copolyesters of the invention exhibit optical anisotropy in the melt as described in U.S. Pat. No. 4,118,372.

The copolyesters of the invention are spun into filaments by conventional melt-spinning at temperatures below decomposition temperatures, usually less than 360.degree. C. In the examples below, filaments were prepared by melt-spinning into a quenching atmosphere and collected at a windup speed specified in the example. Melt pumping speed is adjusted to give the approximate linear density (tex) shown in the tables at the stated windup rate.

As used herein, the term "as-spun fiber" refers to a fiber which has not been drawn or heat treated after extrusion and normal windup.

Following collection, samples of undrawn (as-spun) monofilament are heat-treated in essentially relaxed condition in an oven as taught in Luise U.S. Pat. No. 4,183,895. Heating is in a nitrogen atmosphere. The temperature is typically increased in stages from room temperature to a final temperature, which is usually that needed for the optimum development of high tenacity and break elongation.

Molecular weight growth during heat treatment can increase the flow temperature of the filaments (see U.S. Pat. No. 4,118,372), making possible heat treatment temperatures in excess of the original polymer flow temperature. The maximum heat-treatment temperature should be close to or above the initial flow temperature. Higher molecular weights favorably affect the development of high tenacity and break elongation. Higher spin stretch factor also favors the development of high tenacity, break elongation and modulus.

The heat treated fibers of this invention are useful for a variety of applications such as in ropes or in nonwoven sheets, and in reinforcement of plastic composites.

Inherent viscosity (n.sub.inh), a measure of molecular weight, was computed from n.sub.inh =(1n n.sub.rel)/C where n.sub.rel is the relative viscosity and C is solution concentration in grams of polymer per deciliter of solvent. Relative viscosity is the ratio of polymer solution flow time to solvent flow time in a capillary viscometer at 30.degree. C. The solvent employed was a special mixture coded TM4PP consisting of 7.5% trifluoroacetic acid, 12.5% perchlorethylene, 17.5% methylene choride, 50% 4-chlorophenol, and 12.5% dichloroetrafluoroacetone hydrate. The concentration was 0.5 g polymer per deciliter of solvent.

Monofilament tensile properties were measured in accordance with A.S.T.M. 2101 Part 33 (1980) using a recording stress-strain analyzer at 70.degree. F. (21.1.degree. C.) and 65% relative humidity. Gauge length was 1.0 in (2.54 cm), and rate of elongation 10%/min. Results are reported as T/E/Mi where T is break tenacity in dN/tex, E is elongation-at-break expressed as the percentage by which initial length increased, and Mi is initial tensile modulus in dN/tex. Linear density is reported in tex units. Average tensile properties for five filament samples are reported.

The same general procedure was used in all the examples. It should be understood that the best values reported below are believed to be representative of what can be obtained. The data presented do not constitute all the runs performed involving the indicated reactants. Unfamiliarity with the reaction requirements of the system, use of impure reactants or inappropriate heat treatment conditions may have caused the variation in results such as lower tenacity, elongation or modulus to be obtained.

The monomer ingredients are added in substantially the same molar ratios as desired in the final polymer except that an excess (usually 4 to 7%) of acetylated dihydric phenol is generally used. The resultant polymer is identified, for example, as HQ/DCDPE/HBA (35/35/30) meaning it contained 35 mole % of 1,4-dioxyphenylene units, 35 mole % of 3-oxy-4'-carbonyldiphenyl ether units and 30 mole % of 4-oxybenzoyl units. Excesses of acetates are not included in these percentages.

The 3-necked flask or polymer tube was fitted with: (1) a glass stirrer extending through a pressure-tight resin bushing, (2) a nitrogen inlet, and (3) a short column leading to a water- or air-cooled condenser with a flask for collecting acetic acid by-product. An attachment for application of vacuum was provided at the end of the condenser. An electrically heated Wood's metal bath or a boiling liquid vapor bath mounted for vertical adjustment was used for heating. The reaction mixture was heated to increasing temperatures with stirring at atmospheric pressure under nitrogen purge until essentially all the acetic acid had evolved. Then vacuum was applied and pressure was reduced gradually from atmospheric to generally less than 1 mm of mercury (133.3 Pa). Heating under vacuum at less than 1 mm mercury pressure was then continued until viscosity had increased to a level believed satisfactory for melt-spinning. The cooled and solidified polymer was comminuted, and a portion was molded into a cylindrical plug for melt spinning.

A polymer with a composition HQ/DCDPE/HBA (35/35/30) was prepared by mixing the following in the polymerization vessel:

8.64 g HQ diacetate (0.0445 mole including 6% excess),

10.84 g DCDPE (0.042 mole), and

6.48 g HBA acetate (0.036 mole)

The mixture was heated in the polymerization vessel from 225.degree. C. to 330.degree. C. in 63 minutes at atmospheric pressure under nitrogen purge, and then was raised from 330.degree. to 350.degree. C. during eleven minutes under vacuum while reducing the pressure from 25 mm to 0.5 mm. The resulting polymer softened on the thermal gradient hot bar at 337.degree. C. and fibers could be pulled from the bar at 335.degree. C. Inherent viscosity in TM4PP was 0.94.

The polymer which formed an optically anisotropic melt was melt spun through a single-hole spinneret with 0.23 mm hole diameter at 358.degree. C. and a windup speed of 600 ypm. The monofilament was heat treated in a nitrogen-purged atmosphere in an essentially relaxed condition starting at a temperature approximately 100.degree. C. below the maximum oven temperature. The temperature was increased at a rate of 20.degree. C. per hour until the maximum temperature of the heat treatment was reached. It was then held at this temperature for 16 hours. The following tensile properties were obtained for the as-spun fiber and fibers heat-treated at the indicated maximum temperatures:

A polymer with a composition HQ/DCDPE/HBA (40/40/20) was prepared by mixing the following in the polymerization vessel:

9.00 g HQ diacetate (0.046 mole including 5% excess),

11.35 g DCDPE (0.044 mole), and

3.96 g HBA acetate (0.022 mole)

The temperature was raised from 220.degree. C. to 350.degree. C. in 61 minutes at atmospheric pressure under nitrogen, then was raised from 350.degree. to 355.degree. C. in 3 minutes under vacuum while reducing the pressure from 25 mm to about 0.5 mm. The resulting polymer on the thermal gradient hot bar softened at 330.degree. C. and at 350.degree. C. fibers could by pulled from the bar. Inherent viscosity in TM4PP was 0.72.

The polymer was melt spun through a single-hole spinneret with 0.23 mm hole diameter at a spinning temperature of 350.degree. C. and a windup speed of 600 ypm. The filament was heat-treated in a nitrogen purge as in Example 1. Tensile properties were as follows:

A polymer with a composition HQ/DCDPE/HBA (45/45/10) was prepared by mixing the following in the polymerization vessel:

9.17 g HQ diacetate (0.047 mole including 5% excess).

11.61 g DCDPE (0.045 mole), and

1.80 g HBA acetate (0.010 mole)

The temperature was raised from 200.degree. C. to 345.degree. C. in 65 minutes at atmospheric pressure under nitrogen, then was held at 345.degree. C. for 10 minutes under vacuum while reducing the pressure from 25 mm to 2 mm. The resulting polymer on the thermal gradient hot bar stuck at 320.degree. C. and at 335.degree. C. fibers could be pulled from the bar. Inherent viscosity in TM4PP was 0.67.

The polymer was melt spun through a single-hole spinneret with 0.23 mm hole diameter at a spinning temperature of 335.degree. C. and a windup speed of 600 ypm. The filament was heat-treated in a nitrogen purge as in Example 1. Tensile properties were as follows:

A polymer with a composition HQ/DCDPE/IA/HBA (20/15/5/60) was prepared by mixing the following in the polymerization vessel:

4.07 g HQ diacetate (0.021 mole including 5% excess)

3.87 g DCDPE (0.015 mole)

0.83 g IA (0.005 mole)

10.80 g HBA acetate (0.060 mole)

The temperature was raised from 200.degree. C. to 370.degree. C. in 37 minutes at atmospheric pressure under nitrogen, then was held at 365.degree. C. for 8 minutes under vacuum while reducing the pressure from 25 mm to 0.5 mm. The resulting polymer on the thermal gradient hot bar softened at 330.degree. C. and at 360.degree. C. fibers could be pulled from the bar.

The polymer which formed an optically anisotropic melt was melt spun through a single-hole spinneret with 0.23 mm hole diameter at a spinning temperature of 380.degree. C. and a windup speed of 600 ypm. The filament was heat-treated in a nitrogen purge as in Example 1. Tensile properties were as follows:

A polymer with a composition HQ/RQ/DCDPE/HBA was prepared by mixing the following in the polymerization vessel:

3.06 g HQ diacetate (0.016 mole including 5% excess)

1.02 g RQ diacetate (0.0053 mole including 5% excess)

5.16g DCDPE (0.020 mole)

10.80 g HBA (0.060 mole)

The temperature was raised from 200.degree. C. to 365.degree. C. in 48 minutes at atmospheric pressure under nitrogen, then held at 365.degree. C. for four minutes under vacuum while reducing the pressure from 20 mm to 3 mm. The resulting polymer on the thermal gradient hot bar softened at 315.degree. C. and at 370.degree. C. fibers could be pulled from the bar. Inherent viscosity in TM4PP was 1.18. In the thermooptical test of Schaefgen U.S. Pat. No. 4,118,372, the polymer was optically anisotropic above the 371.degree. C. flow temperature.

The polymer was melt spun through a single-hole spinneret with 0.23 mm hole diameter at a spinning temperature of 395.degree. C. and a windup speed of 350 ypm. The filament was heat-treated in a nitrogen purge as in Example 1. Tensile properties were as follows:

A polymer with a composition HQ/DCDPE/HBA (20/20/60) was prepared by mixing the following in the polymerization vessel:

4.07 g HQ diacetate (0.021 mole including 5% excess)

5.16 g DCDPE (0.020 mole)

10.8 g HBA acetate (0.060 mole)

The temperature was raised from 200.degree. C. to 355.degree. C. in 37 minutes at atmospheric pressure under nitrogen, then was raised from 355.degree. to 360.degree.-5.degree. C. during 18 minutes under vacuum while reducing the pressure from 20 mm to 0.5 mm. The resulting polymer on the thermal gradient hot bar softened at 330.degree. C. and at 365.degree. C. fibers could be pulled from the bar. Inherent viscosity in TM4PP was 0.81.

The polymer which formed an optically anisotropic melt was melt spun through a single-hole spinneret with 0.23 mm hole diameter at a spinning temperature of 360.degree. C. and a windup speed of 600 ypm. The filament was heat treated in a nitrogen purge as in Example 1. Tensile properties were as follows: