Wholly Aromatic Polyester Fiber-Reinforced High Performance Thermoplastic and Process for Preparing Same

This invention relates to self-reinforced polymer composites and processes for making the same, and more particularly to novel self-reinforced polymer composites comprising crystalline or partially crystalline high performance thermoplastic polymer and a melt processable wholly aromatic polyester which serves as the reinforcing agent and to processes for making the same.

Fiber-reinforced polymer composites are well known and widely used. Polymers of improved strength and increased stiffness can be obtained by the use of an appropriate reinforcing fiber. Probably the most widely used reinforcing fibers are glass, carbon and aramid (or "Kevlar" which is a registered trademark of the E. I. du Pont de Nemours & Co., Wilmington, Del.).

The base polymers used in making reinforced polymer composites such as those described above include a wide range of thermoplastics, such as polystyrene and copolymers thereof, polyamides, polycarbonates, polyetherimide, polyether etherketone (PEEK) and polyesters such as polybutylene terephthalate. These polymers are thermoplastics and are either amorphous or semi-crystalline. They may be called flexible chain polymers, since individual monomer units in the polymer chain are free to rotate with respect to each other so that the polymer chain may assume a random shape. By way of illustration, F. N. Cogswell, Intern. Polymer Processing, Vol. 1, no. 4, pages 157-165 (1987) discloses carbon fiber reinforced PEEK.

More recently developed are self-reinforced polymer composites comprising long, continuous, predominantly unidirectionally oriented fibers of a melt processable wholly aromatic polyester in a matrix of a thermoplastic flexible chain polymer. Such polymer composites are described in commonly assigned, U.S. Pat. No. 4,728,938 of Avraam Isayev et al., issued Mar. 1, 1988. As described therein, the fibers of the wholly aromatic polyester, which may also be termed a thermotropic liquid crystal polymer (LCP) are long continuous fibers formed in situ by mixing the matrix of base polymer with the wholly aromatic polyester in a suitable mixing and extrusion apparatus, as for example, an extruder-static mixer setup, or a twin screw extruder. Polymer composites specifically disclosed therein are polycarbonate/LCP composites containing from 2.5 to 50 weight percent of LCP, and polyetherimide/LCP composites containing from 5 to 30 percent by weight of LCP.

Copending commonly assigned U.S. patent application of Avraam Isayev (the inventor herein) and Suresh Swaminathan, Ser. No. 07/050,705, filed May 14, 1987, now U.S. Pat. No. 4,835,047, describes composites of polyetherimide (PEI) and a wholly aromatic polyester or LCP, in which the LCP content varies from 40 to 95 percent by weight. PEI is an amorphous high performance thermoplastic polymer having a glass transition temperature of 215.degree. C. The LCP is present in fibrous domains in a matrix of PEI. Mechanical properties of these composites in general are superior to those described in U.S. Pat. No. 4,728,938 cited supra. These composites of PEI and an LCP are also described in A. I. Isayev and S. Swaminathan, "Thermoplastic Fiber-Reinforced Composites Based on Liquid Crystalline Polymers," Proceedings of the Third Annual Conference on Advanced Composites, pages 259-267, 15-17 Sept. 1987, Detroit, Mich., published by ASM International.

Applicant has now found that outstanding physical and mechanical properties are obtained in wholly aromatic polyester fiber-reinforced composites of high performance thermoplastic polymers which are at least partially crystalline.

This invention provides self-reinforced polymer composites comprising (a) from about 60 to about 5 percent by weight, based on total polymer weight, of an at least partially crystalline high performance thermoplastic polymer and (b) from about 40 to about 95 percent by weight, based on total polymer weight, of a melt processable wholly aromatic polyester, said polyester being essentially in the form of continuous, predominantly unidirectionally oriented fibers which are formed in situ in a matrix of said thermoplastic polymer. The amount of wholly aromatic polyester is preferably from about 50 to about 95 percent by weight, based on total polymer weight. Polymer composites of this invention are characterized as "self-reinforced" because the fibers of wholly aromatic polyester of LCP are formed in situ rather than being added, as is the case with conventional fiber reinforcing materials such as glass and carbon.

An "at least partially crystalline high performance" polymer, as the term is used herein, denotes a thermoplastic polymer having at least some degree of crystallinity in the unoriented state at ambient temperature and having a melting point of at least about 200.degree. C., a minimum melt processing temperature of at least about 250.degree. C., long term thermal stability at temperatures up to at least about 220.degree. C., and short term thermal stability at temperatures up to at least about 300.degree. C. Polymer characterized as "at least partially crystalline" as used herein are those which are characterized as either crystalline or semicrystalline in the literature.

The required starting materials for preparing the novel polymer composites of this invention are an at least partially crystalline high performance polymer and a melt processable wholly aromatic polyester. Novel polymer compositions according to this invention are prepared by mixing from about 60 to about 5 percent by weight of base polymer with from about 40 to about 95 percent by weight of a melt processable wholly aromatic polyester at a temperature at which both polymers are melt processable, extruding the resulting blend in the melt phase, cooling the blend, and recovering a polymer composite according to the invention.

Polyether etherketone (PEEK) is a preferred high performance base polymer for composites of this invention. PEEK is variously described as crystalline and semicrystalline and is available from Imperial Chemical Industries PLC (ICI) of London, England (in the United States from ICI Americas, Inc. of Wilmington, Del.) under the trademark "Victrex". According to "Modern Plastics Encyclopedia 1984-1985", published by McGraw-Hill, Inc., New York, 1984, pages 59 and 468-469, PEEK is a crystalline polymer having a melting point of 334.degree. C., a processing temperature range of 680.degree.-750.degree. F. (360.degree.-400.degree. C.) for injection and 660.degree.-720.degree. F. (350.degree.-380.degree. C.) for extrusion, and having long term thermal stability in service up to about 450.degree. F. (230.degree. C.) and short term thermal stability in service up to about 600.degree. F. (315.degree. C.). Although PEEK is characterized in some references as a crystalline polymer (including Modern Plastic Encyclopedia 1984-1985, published by McGraw-Hill, Inc., 1984, at page 59), Cogswell cited supra, at page 158, suggests that PEEK is not 100 percent crystalline, i.e., that some amorphous phase is present. According to Jones et al, Polymer, vol. 26, pp 1385-(at page 1385), 1985, maximum achievable crystallinity of PEEK is about 48 percent. More typical crystallinity is less than 30 percent. Other high performance polymers which can be used as base polymers for composites of this invention include polyphenylene oxide (PPO), polyphenylene sulfide (PPS), and polymethylpentene (polymer of 4-methyl 1-1-pentene).

The polyester starting materials are melt processable wholly aromatic polyesters such as those described in U.S. Pat. Nos. 3,991,014; 4,067,852; 4,083,829; 4,130,545; 4,161,470; 4,318,842 and 4,468,364 and in G. W. Calundann et al., "Anisotropic Polymers, Their Synthesis and Properties", reprinted from the Robert A. Welch Conferences on Chemical Research, XXVI Synthetic Polymers, Nov. 15-17, 1982, Houston, Tex., pp. 247-291. The melt processable or thermotropic, polyester may also be described as a liquid crystal polymer (LCP) since it exhibits anisotropy even in the melt phase.

The wholly aromatic polyester must be matched or paired with the base polymer so that the two have overlapping processing temperatures. That is, the melting point of the wholly aromatic polyester must be within the melt processing temperature range of the base polymer. Also, the wholly aromatic polyester must have a viscosity lower than that of the base polymer under melt processing conditions (e.g., temperature and shear rate).

One series of particularly suitable polymer compositions or composites according to the present invention are those made from polyether etherketone as the base polymer and a wholly aromatic polyester thermotropic liquid crystal polymer having a melting point of about 275.degree. C. and supplied by Celanese Research Company, Summit, N.J. under the designation "Vectra A950". This polymer is believed to consist essentially of about 25-30 mole percent of 6-oxy-2-naphthoyl moieties, as described for example in U.S. Pat. No. 4,161,470 and in example 4 of U.S. Pat. No. 4,468,364.

The amount of wholly aromatic polyester in the final product is from about 40 to about 95 percent by weight, preferably from about 50 to about 95 percent by weight, based on the combined weight of the base polymer and the wholly aromatic polyester.

The wholly aromatic polyester, or LCP, is incompatible with the base polymer over the entire composition range from 0 to 100 percent by weight LCP. Therefore, the two are present as separate phases in blends. When the percentage of wholly aromatic polyester is as specified above and suitable high strain mixing conditions are used, the wholly aromatic polyester is present in the form of long continuous fibers in a matrix of the base polymer. The term, "high strain mixing conditions" herein includes a combination of shear and elongation.

Surprisingly, continuous fibers of the polyester are formed during mixing of the polyester with the base polymer (e.g., PEEK), even at high polyester loadings. For example, even in polymer composites containing 70 percent by weight of wholly aromatic thermotropic polyester and conversely 30 percent by weight of PEEK, the product consists essentially of long polyester fibers in a matrix of base polymers, provided that proper mixing conditions are observed. More will be said subsequently about proper mixing conditions.

Additional materials are not required but may be present. Thus, it is within the scope of the invention to prepare a mixed composite polymer by inclusion of an additional reinforcing fiber, such as glass, carbon, or aramid, in addition to the wholly aromatic polyester. The additional reinforcing fiber may be incorporated into either the base polymer or the polyester. The additional reinforcement provided by the additional fiber is not necessary in most cases, but where a very high stiffness (or very high strength) reinforced polymer composite is desired, such can be attained according to the present invention without the high loadings of conventional reinforcing fiber required in presently known conventional polymer/fiber composites.

Other additives, such as pigments and fillers, coupling agents, flame retardants, lubricants, mold release agents, plasticizers and ultraviolet stabilizers, may be mixed with the base polymer and wholly aromatic polyester as desired. The use of such additives is well known in the polymer processing art.

The base polymer and the wholly aromatic polyester are mixed at ambient temperature to form a physical mixture. Any additional ingredients which are desired in the final product may also be mixed in at this time. The physical mixture is then dried under conventional conditions, e.g., at temperatures of about 100.degree. C. to about 150.degree. C. for approximately 6 to 24 hours, in a vacuum oven. The dry blended polymers (and additives, if any) are then thoroughly mixed at a temperature above the melting points of both polymers in a suitable mixing apparatus which will give thorough high strain mixing sufficient to cause fiber formation. The blend may be melt processed at a temperature within the range of about 300.degree. C. to about 400.degree. C. Preferred melt processing conditions also lead to development of a high degree of crystallinity in the base polymer, which in turn improves toughness of the composite.

The mixing apparatus may be, for example, a single screw extruder in series with a suitable static mixer. Other high strain mixing apparatus may also be used. The blend is then extruded in the form of a strand, which upon solidification, may be chopped into pellets.

Preferred mixing apparatus includes an extruder, static mixer and extrusion die through which blends of the base polymer and the liquid crystal polymer are extruded. Good results have been obtained by using a single screw extruder having four (4) heating sections in series with a heated six-element Koch mixer (a static mixer), with an adapter between the extruder and the Koch mixer, and a discharge die having a 1/16 inch (in diameter) opening on the outlet side of the Koch mixer. The processing temperature is the temperature at which both polymers are melt processable, i.e., a temperature at which the base polymer is either melted or sufficiently soft to be processed in ordinary mixing apparatus and at which the wholly aromatic polyester is above its melting point. The ingredients are brought up to processing temperature at the beginning of the mixing operation and are thereafter maintained in the desired temperature range. In the case of the preferred apparatus, the ingredients are brought up to temperature near the feed end of the single screw extruder and are thereafter maintained at appropriate processing temperature by appropriate controls of the various independently adjustable heating sections.

The preferred product polymer composition or blend is a self-reinforced polymer composite in which PEEK is the matrix and the wholly aromatic polyester is in the form of predominantly unidirectionally oriented long continuous fibers or strands, oriented in the direction of extrusion. Fiber diameters are predominantly less than 10 microns, primarily in the range of about 1 micron to about 10 microns, although fibers of other diameters can be obtained. The polymer composite is characterized as self-reinforced because the wholly aromatic fibers are formed in situ during the mixing process rather than being fed to the mixing apparatus as solid fibers. The proportions of ingredients in the polymer composite are essentially the same as in the feed.

The product polymer composite may be further processed as desired. For example, the polymer composite may be pelletized and then formed into shaped articles, tapes, films or fibers. This shaping may be accomplished by conventional means such as extrusion, injection molding, etc. Molded composite articles may be formed by injection molding. Films may be formed by conventional means such as melt extrusion or casting. Fibers may be formed by conventional melt spinning techniques. Polymer composites of this invention are especially suitable for injection molding.

Products of the present invention exhibit exceptional mechanical properties, including tensile modulus, tensile strength and notched Izod impact strength. Mechanical properties, especially tensile modulus and tensile strength, are significantly higher than those of the polyetherimide/wholly aromatic polyester composites described in U.S. Pat. No. 4,728,938, in which the amount of wholly aromatic polyester may range from 5 to about 30 percent by weight, based on total polymer weight. Tensile properties of composites based on PEEK and those based on polyetherimide are similar. Impact properties of composites of this invention are either similar or superior to those of composites based on PEI. Mechanical properties of the present polymer composites, for the most part, are well above the values which would be predicted from the Rule of Mixtures. The discussion of the Rule of Mixtures can be found in Lawrence E. Nielsen, "Mechanical Properties of Polymers and Composites," vol. 2, Marcel Dekker, Inc., New York 1974; pages 455 and 465 are of particular interest. Also surprising and unexpected in the fact that PEEK/wholly aromatic polyester blends of this invention are in the form of composites in which the wholly aromatic polyester is in the form of long, continuous, predominantly unidirectionally oriented fibers. Blends of polycarbonate with the same wholly aromatic polyesters did not exhibit a fiber structure even at 25 percent by weight of the wholly aromatic polyester.

Composites of the present invention are anisotropic. That is, they exhibit better tensile properties, e.g., higher secant modulus, higher tensile strength and greater elongation in the fiber or flow direction than they do in the transverse or cross direction. Tensile properties of composites of this invention are much improved over those of the unreinforced base polymer in the fiber direction. Differences in the cross direction are less notable.

Polymer composites of this invention are also characterized by high heat resistance and good electrical properties which remain stable over a wide range of temperatures and frequencies. Polymer composites of this invention also have good flame resistance.

Polymer composites of this invention are especially useful in high performance applications where high tensile strength, high modulus and good impact resistance are required or at least highly desirable. These products are particularly useful in various electrical, electronics, aerospace and automotive applications. In particular, polymer composites of this invention are useful in automotive and aerospace applications as replacements for present composite components which are produced by sheet molding compound technology Products of this invention can be produced at faster rates and with less power consumption, resulting in lower product costs, compared to conventional composites in which fibers are prepared in advance. The additional step involving fiber preparation, the cost of machinery and the time required to prepare fibers are avoided.

Self-reinforced polymer compositions having a high degree of toughness (which is measurable by the Izod impact test) can be obtained by appropriate control of crystallization conditions. Such control affects the toughness of the base polymer, which in turn affects the toughness of the polymer composite. Polymer composites of this invention are appreciably tougher than the corresponding base polymers. Suitable crystallization conditions for achieving toughness in an at least partially crystalline polymers are known in the art. This is not possible in the case of an amorphous polymer such as polyetherimide. This can be used to advantage to obtain high toughness composites in accordance with this invention.

This invention will now be further described in detail with reference to the specific example that follows. It will be understood that this example is by way of illustration of the invention and not by way of limitation of the scope thereof.

Polyether etherketone (PEEK) used in these experiments was supplied by ICI Americas, Inc., Wilmington, Del., under the name "VICTREX 380G".

The melt processable wholly aromatic polyester used in the examples was a thermotropic polymer supplied by the Celanese Research Company, Summit, N.J. under the designation "Vectra A950". This polymer has a melting point of 275.degree. C. and is believed to consist essentially of about 25-30 mole percent of 6-oxy-2-naphthoyl moieties and 70-75 mole percent of p-oxybenzoyl moieties.

The apparent viscosity ratio of PEEK melt to "Vectra A950" melt was more than 10 at 350.degree. C. at any shear rate.

Mixtures of polyether etherketone ("Victrex 380G")(PEEK) and wholly aromatic polyester ("Vectra A950")(LCP) were prepared by dry mixing pellets of the two polymers at ambient temperature to form a physical mixture, and drying this mixture at 100.degree. C. for 24 hours in a vacuum oven. Compositions ranged from 100 percent PEEK to 100 percent LCP. (Blends ranged in composition from 2.5 percent to 90 percent LCP by weight.) The dried and blended pellets were fed to a static mixer apparatus which comprised, in series from inlet to outlet, a 3/4" Killion single screw extruder (screw L/D 24:1) driven by a one horsepower motor, a 6-element Koch Static Mixer (Model No. KMB-150), and a 1/16 inch (in diameter) discharge die, with transition sections between the extruder and the static mixer and between the static mixer and the die. The screw extruder had three temperature zones, with Zone 1 being at the feed section and Zone 3 at the screw tip. The temperature of Zone 1 was controlled at 540.degree. F. (282.degree. C.) whereas the other two zones were kept at 590.degree. F. (350.degree. C.) The Koch static mixer temperature was controlled by 4 temperature controllers, all maintained at 590.degree. F. (350.degree. C). The screw extruder was operated at 30 RPM. The shear rate was 225 sec. .sup.-1. As the blend exited the static mixer, it was cooled in a room temperature water bath located just after the exit region. The solidified extrudate was cut into pellets approximately 4 mm in length with a pelletizer.

These pellets were then fed to a BOY 15S reciprocating screw injection molding machine with a maximum shot size of 36 cm.sup.3. The following process conditions were used for molding of all the blends:

Samples of the injection molded blends described herein were observed in a Scanning electron Microscope (SEM) model ISI-SX-40 (International Scientific Instruments) and were found to be in the form of fibers of predominantly 3 to 5 microns in diameter. These fibers were oriented essentially in the direction of molding, were well distributed across the surface of the material, and were nearly continuous in length.

Injection molded samples of each polymer blend were subjected to impact and stress-strain tensile tests.

Impact tests were carried out according to ASTM method D 235 C, using dumbell shaped samples (standard tensile bars) 6.3 cm in length and having notches 0.125 inch (about 0.32 cm) in width, and using 2.0 lb and 5.0 lb. pendulums. Impact strengths, in joules per meter (J/m) and foot-pounds of force per inch (ft-lb/in) of notch, were found to be as shown in TABLE I below.

Tensile properties, i.e., tensile modulus (in gigapascals, or GPa), tensile strength (in megapascals, or MPa) and elongation to break (percentage based on original length) were measured on a Monsanto tensile tester (Model T-500) with a crosshead speed of 0.18 inch/min. The test specimens were mini-tensile bars. Results for the mini-tensile bars are given in TABLE I below. In this table, tensile modulus was measured at 1 percent strain, tensile strength was measured at maximum stress.

While in accordance with the patent statutes, a preferred embodiment and best mode has been presented, the scope of the invention is not limited thereto, but rather is measured by the scope of the attached claims.