THERMOTROPIC LIQUID CRYSTALLINE POLYMER BLEND WITH REDUCED SURFACE ABRASION

Abstract: A thermotropic liquid crystalline polymer matrix having wollastonite fibres incorporated therein for reinforcement to prepare a moulding compound. The wollastonite fibres have an average aspect ratio of greater than 3 to 1, and may be coated on their surface with a coupling agent to produce better bond­ ing between the fibres and the thermotropic liquid crystalline polymer. The surface abrasion of an article produced from a blend of wollastonite with a thermotropic liquid crystalline polymer is reduced as compared to that of unfilled polymer.

Description: The present invention relates to reinforced thermotropic liquid crystalline polymers, and more particularly to moulding compounds and moulded or extruded articles produced from such reinforced thermotropic liquid crystalline polymers. Many thermotropic liquid crystalline polymers have poor surface characteristics, particularly with respect to abrasion. One attempt to reduce such abrasion has been the incorporation of various fillers and reinforcing agents. For example, the use of various fillers is disclosed in our United States Patent No: 4,067,852, which teaches that fillers and/or reinforcing agents may be included in a total concentration of 1 to 60% by weight of the resulting moulded compound. Representative fibres which may serve as reinforcing media therein include glass fibres, asbestos, graphitic carbon fibres, amorphous carbon fibres, synthetic polymeric fibres, aluminium fibres, aluminium silicate fibres, oxide of aluminium fibres, titanium fibres, magnesium fibres, rock wool fibres, steel fibres, tungsten fibres, cotton wool and wood cellulose fibres. If desired, the fibrous reinforcement may be preliminarily treated to improve its adhesion to the liquid crystalline polymer which ultimately serves as a continuous matrix phase. Representative filler materials disclosed in that Patent include calcium silicate, silica, clays, tale, mica, polytetrafluoroethylene, graphite, aluminium trihydrate carbonate and barium ferrite. United States Patent 4,267,304 discloses the use of reinforcing fillers such as glass fibres, asbestos, aluminium silicate fibres and synthetic polymer fibres, as well as the use of particulate filler such as kaolin, chalk, silica and glass ballottini. Thus, the incorporation of various fillers and reinforcements into liquid crystalline polymers has been investigated as shown above. However, although the surface abrasion can be reduced by blending mineral fillers such as talc and mica into liquid crystalline polymer, mechanical properties such as tensile strength and impact strength are significantly reduced in such blends. Although reinforcements such as carbon fibre also reduce surface abrasion, they are must more expensive than mineral fillers. Accordingly, the above prior art gives no hope of achieving liquid crystalline polymer blends having a combination of excellent mechanical properties, excellent surface properties and excellent economical properties. surprisingly, it has nevertheless been found, in accordance with the present invention, that blends having such a desirable combination of properties can, after all, be produced using a particular fibrous material with a particular aspect ratio. This material is wollastonite having an aspect ratio of at least 10 to 1. Wollastonite is not mentioned in any of the above Patents. Wollastonite is a naturally-occurring calcium silicate found in metamorphic rocks having the chemical formula CaSiO₃. The reference in U.S. 4,067,852 to calcium silicate is not equivalent to a reference to wollastonite since the term "calcium silicate" does not specify the chemical stoichiometry (CaSiO₃, Ca₂SiO₄ and Ca₃SiO₅, for example, are all calcium silicates). Furthermore, commercially-prepared calcium silicate could be a powder, while wollastonite particles are acicular; that is, they are shaped like small needles or spines. Thus, the wollastonite can function as a fibrous reinforcing agent rather than just as a filler. Wollastonite is commercially available as products which range from fine particle size products (e.g. with an aspect ratio of 3:1) to high aspect ratio products (20:1). The use of wollastonite in combination with liquid crystalline polymers is mentioned in European Patent Application No. 81302836.2 of Bailey et al which describes the use of inert fillers at greater than 10% by volume and preferably higher than 20% by volume, to obtain a reduction in anisotropy ratio. The inert fillers in that Patent application include particulate or fibrous fillers which do not react with the polymer of the composition but may have been surface treated to make them more compatible with the polymer matrix. Wollastonite is used in one example at a concentration of 70% by weight (53.7% by volume). However, no information is given concerning the size or aspect ratio of the wollastonite. (By "aspect ratio" is meant the ratio of length to diameter of the fibres). Moreover it is made clear that the preferred class of fillers is glass fibres in the form of short fibres (one specific example uses three millimeter glass fibres) or glass mats. There is nothing in the above-mentioned European Patent application to lead the reader to suppose that a moulding compound in accordance with the present invention might be worth producing or that it could have the desirable combination of properties mentioned above. The present invention comprises a thermotropic liquid crystalline polymer matrix having wollastonite fibres incorporated therein for reinforcement to prepare a moulding compound, such that the wollastonite fibres have an average aspect ratio of at least 10 to 1. These fibres may be coated on their surface with a coupling agent to produce better bonding between the fibres and the thermotropic liquid crystalline polymer. In a preferred embodiment, the wollastonite fibres also are treated with a titanate coupling agent. The invention surprisingly provides a number of advantages. The thermotropic liquid crystalline polymer blend of the invention is less subject to surface abrasion. It suffers no concomitant large reduction in mechanical properties, such as tensile strength and impact strength. The reinforcing agent is relatively inexpensive and so does not increase the overall cost of articles made from the polymer blend. The resulting moulding compound can be used to prepare moulded or extruded articles which possess a reduced tendency for surface abrasion while maintaining the mechanical properties at essentially the same level as that of unfilled thermotropic liquid crystalline polymer. Thus, the liquid crystalline polymer blend of the present invention can be used for various plastics applications requiring reduced surface abrasion. The wollastonite fibres used in the present invention have an average aspect ratio of at least 10 to 1, and preferably at least 20 to 1. Suitable wollastonite fibres are available commercially from various sources. A particularly useful wollastonite fibre has an average diameter of about 3.5 µm (microns) and an average aspect ratio of about 20 to 1. While beneficial results will be seen from the use of any amount of wollastonite fibres, it is preferred that the loading fall between 10% and 70% by weight, and more preferably between 10% and 40% by weight. While not essential, it is preferred that the wollastonite fibres be treated with a titanate coupling agent. One coupling agent which has been used successfully is isopropyl tri(dioctylpyrophosphate) titanate, although other coupling agents may give similarly beneficial results. Wollastonite already treated with coupling agents or other surface treatments can be purchased from commercial suppliers. Representative classes of polymers from which the thermotropic liquid crystalline polymer suitable for use in the present invention may be selected include wholly aromatic polyesters, aromatic-aliphatic polyesters, wholly aromatic poly(ester-amides), aromatic-aliphatic poly(esteramides), aromatic polyazomethines, aromatic polyestercarbonates and mixtures of the same. In preferred embodiments the thermotropic liquid crystalline polymer is wholly aromatic polyester, a wholly aromatic poly(esteramide) or an aromatic-aliphatic poly(ester-amide) and mixtures of the same. In such wholly aromatic polyester and wholly aromatic poly(ester-amide) each moiety present within the polymer chain contributes at least one aromatic ring. Also, it is preferred that naphthalene moieties be included in the thermotropic liquid crystalline polymer, e.g., 6-oxy-2-naphthoyl moiety, 2,6-dioxynaphthalene moiety, or 2,6-dicarboxynaphthalene moiety, in a concentration of not less than about 10 mole percent. The particularly preferred naphthalene moiety for inclusion in the thermotropic liquid crystalline polymer is the 6-oxy-2-naphthoyl moiety in a concentration of not less than 10 mole percent. Representative wholly aromatic polyesters which exhibit thermotropic liquid crystalline properties include those disclosed in the following United States Patents 3,991,013; 3,991,014; 4,066,620; 4,067,852; 4,075,262; 4,083,829; 4,093,595; 4,118,372; 4,130,545; 4,146,702; 4,153,779; 4,156,070; 4,159,365; 4,161,470; 4,169,933; 4,181,792; 4,183,895; 4,184,996; 4,188,476; 4,201,856; 4,219,461; 4,224,433; 4,226,970; 4,230,817; 4,232,143; 4,232,144; 4,238,598; 4,238,599; 4,238,600; 4,242,496; 4,245,082; 4,245,084; 4,247,514; 4,256,624; 4,265,802; 4,267,304; 4,269,965; 4,279,803; 4,294,955; 4,299,756; 4,318,841; 4,337,190 and 4,337,191 and 4,355,134. As discussed hereafter the wholly aromatic polyesters of U.S. Patents Nos: 4,161,470 and 4,256,624 are particularly preferred for use in the present invention. Representative aromatic-aliphatic polyesters which exhibit thermotropic liquid crystalline properties include coplymers of polyethylene terephthalate and hydroxybenzoic acid as disclosed in Polyester X-7G-A Self Reinforced Thermoplastic , by W. J. Jackson, Jr., H.F.Kuhfuss and T.F.Gray, Jr., 30th Anniversary Technical Conference, 1975 Reinforced Plastics/Composites Institute, The society of the Plastics Industry, Inc., Section 17-D, pages 1-4. A further disclosure of such copolymers can be found in "Liquid Crystal Polymers: 1. Preparation and Properties of p-Hydoxybenzoic Acid Copolymers, " Journal of Polymer Science , Polymer Chemistry Edition , Vol. 14, pages 2043 to 2058 (1976), by W.J.Jackson, Jr. and H.F.Kuhfuss. See also United States Patent No: 4,138,842 and our United States Patent No: 4,355,133. Representative wholly aromatic and aromatic-aliphatic poly(ester-amides) which exhibit thermotropic liquid crystalline properties include those disclosed in United States Patents Nos: 4,272,625; 4,330,457; 4,339,375; 4,341,688; 4,351,917; 4,351,918 and 4,355,132. As discussed hereafter the poly(ester-amide) of United States Patent No: 4,330,457 is particularly preferred for use in the present invention. Representative aromatic polyazomethines which exhibit thermotropic liquid crystalline properties include those disclosed in United States Patents Nos: 3,493,522; 3,493,524; 3,503,739; 3,516,970; 3,516,971; 3,526,611; 4,048,148 and 4,122,070. Specific examples of such polymers include poly(nitrilo-2-methyl-1,4-phenylenenitriloethylidyne-1,4-phenyleneethylidyne); poly(nitrilo-2-methyl-1,4-phenylenenitrilomethylidyne-1,4-phenylene-methylidyne) and poly(nitrilo-2-chloro-1,4-phenylenenitrilomethyidyne-1, 4-phenylene-methylidyne). Representative aromatic polyester-carbonates which exhibit thermotropic liquid crystalline properties include those disclosed in United States Patents Nos: 4,107,143, 4,284,757 and 4371660. Examples of such polymers include those consisting essentially of p-oxybenzoyl units, p-dioxyphenyl units, dioxycarbonyl units and terephthoyl units. A thermotropic liquid crystalline polymer which may commonly be selected for use in the present invention is one which possesses a melting temperature within a range such that it is amenable to melt extrusion or moulding while employing commercially-available equipment. For instance, thermotropic liquid crystalline polymers may be selected which exhibit a melting temperature somewhere within the range of 250 to 400°C. The thermotropic liquid crystalline polymer selected preferably also exhibits an inherent viscosity of at least 1.0 dl./g. when dissolved in a concentration of 0.1 percent by weight in pentafluorophenol at 60 C. (e.g. an inherent viscosity of 1.0 to 15.0 dl./g.) A particularly preferred wholly aromatic polyester for use in the present invention is that disclosed in United States Patent No: 4,161,470 which is capable of forming an anisotropic melt phase at a temperature below 350°C. This polyester consists essentially of the recurring moieties I and II wherein: I is and II is The polyester comprises 10 to 90 mole percent of moiety I and 10 to 90 mole percent of moiety II. In one embodiment, moiety II is present in a concentration of 65 to 85 mole percent, and preferably in a concentration of 70 to 80 mole percent, e.g., 73 mole percent. In another embodiment, moiety II is present in a lesser proportion of 15 to 35 mole percent and preferably in a concentration of 20 to 30 mole percent. In addition, at least some of the hydrogen atoms present upon the rings optionally may be replaced by substituents selected from alkyl groups of 1 to 4 carbon atoms, alkoxy groups of 1 to 4 carbon atoms, halogen, phenyl and mixtures thereof. Such polymer preferably has an inherent viscosity of 3.5 to 10 dl./g. when dissolved in a concentration of 0.1 percent by weight of pentafluorophenol at 60°C. Another particularly preferred wholly aromatic polyester for use in the present invention is that disclosed in United States Patent No: 4,256,624 which is capable of forming an anisotropic melt phase at a temperature below 400°C. This polyester consists essentially of the recurring moieties, III, IV and V which may include substitution of at least some of the hydrogen atoms present upon an aromatic ring wherein: III is IV is a dioxy aryl moiety of the formula -O-Ar-O-where Ar is a divalent radical comprising at least one aromatic ring; and V is a dicarboxy aryl moiety of the formula where Ar' is a divalent radical comprising at least one aromatic ring; with said optional substituents if present being selected from alkyl groups of 1 to 4 carbon atoms, alkoxy groups of 1 to 4 carbon atoms, halogen, a phenyl group and mixtures of the foregoing, and wherein said polyester comprises 10 to 90 mole percent of moiety III, 5 to 45 mole percent of moiety IV and 5 to 45 mole percent of moiety V. A particularly preferred wholly aromatic poly(ester-amide) or aromatic-aliphatic poly(ester-amide) for use in the present invention is disclosed in United States Patent No: 4,330,457, which is capable of forming an anisotropic melt phase at a temperature below 400°C. The poly(ester-amide(s)) theredisclosed consist essentially of recurring moieties VI, VII, VIII and, optionally, IX wherein: VI is VII is where A is a divalent radical comprising at least one aromatic ring or a divalent trans -1,4-cyclohexylene radical; VIII is -Y-Ar-Z-, where Ar is a divalent radical comprising at least one aromatic ring, Y is O, NH or NR and Z is NH or NR, where R is an alkyl group of 1 to 6 carbon atoms or an aryl group; and IX is -O-Ar'-O-, where Ar' is a divalent radical comprising at least one aromatic ring; wherein at least some of the hydrogen atoms present upon the rings optionally may be replaced by substituents selected from alkyl groups of 1 to 4 carbon atoms, alkoxy groups of 1 to 4 carbon atoms, halogen, phenyl and mixtures thereof, and wherein said poly(ester-amide) comprises 10 to 90 mole percent of moiety VI, 5 to 45 mole percent of moiety VII, 5 to 45 mole percent of moiety VIII and 0 to 40 mole percent of moiety IX. The preferred dicarboxy aryl moiety is: the preferred moiety VIII is: and the preferred dioxy aryl moiety IX is: Such polymer preferably has an inherent viscosity of 2.0 to 10 dl./g. when dissolved in a concentration of 0.1 percent by weight in pentafluorophenol at 60°C. The wollastonite fibres may be incorporated into the liquid crystalline polymer by a melt admixture technique to form an injection moulding compound. In addition, any of the commonly-used blending techniques can be also used in conjunction with commercially-available equipment. The resulting blend can be melt processed to form a variety of shaped articles, e.g., moulded three-dimensional articles. The blend of the present invention is particularly suited for moulding applications and can be moulded by standard injection moulding techniques commonly utilised when forming moulded articles. The melt blend can also be melt extruded if desired. The following Examples are presented as specific illustrations of the invention. It should be understood, however, that the invention is not limited to the specific details set forth in the Examples. EXAMPLE 1 A wholly aromatic polyester which exhibits thermotropic liquid crystalline properties was selected for use in the formation of an injection moulded article in accordance with the present invention. The wholly aromatic polyester was formed in accordance with the teachings of United States Patent No: 4,161,470 and consisted of 73 mole percent of recurring p-oxybenzoyl units and 27 mole percent of recurring 6-oxy-2-naphthoyl units. The wholly aromatic polyester exhibited an inherent viscosity of 6.0 dl./g. when dissolved in a concentration of 0.1 percent by weight in pentafluorophenol at 60°C., and had a differential scanning calorimetry melting temperature peak of 289°C. The above wholly aromatic polyester was used to prepare the various samples used for testing. Blending with the various fibres at 15 weight percent of fibre was accomplished on a co-rotating twin screw extruder (28mm ZSK, manufactured by Werner-Pfleiderer. The extrusion was accomplished at a cylinder temperature of 570°F. (299°C.), a percent torque between 20 and 50, using 150 rpm, without a vacuum when unfilled wholly aromatic polyester was being extruded, and a vacuum of 27 inches (686 mm)(91,4kPa) when fibres were being added. The unfilled wholly aromatic polyester was put through the same blending process as the fibre containing wholly aromatic polyester to ensure that all samples had been subjected to the same treatment. The composition of samples A-F is shown in Table I: All of the wollastonite fibres were purchased from Nyco, Inc., Willsboro, N.Y. The commercial designations are as follows: After extrusion, samples of each of the above were dried in a vacuum oven overnight at 120°C., then injection moulded at 300°C. on an 0.8 oz. Arburg injection moulding machine under the following moulding conditions: The resulting bars had configurations of standard moulding bars according to ASTM specifications for the tests set forth below. To illustrate the improvements resulting from the use of wollastonite fibres, the bars made from Samples A-F were tested according to ASTM D-638 for tensile strength and modulus. The results are set forth in Table II: As can be seen, a coating with a silane coupling agent is actually detrimental to tensile strength and modulus, whereas a titanate agent has a very beneficial effect. In addition, it is readily seen, especially in the case of tensile modulus, that a blend including high aspect ratio wollastonite fibres results in a significant improvement over that obtained with low aspect ratio wollastonite. EXAMPLE 2 In order to demonstrate the effect on mechanical properties of varying the loading level of wollastonite fibres in liquid crystalline polymer, the following tests were performed. Appropriate samples for performing each of the ASTM tests set forth below were prepared by adding G.Wollastokup KR38-2 wollastonite fibres average aspect ratio 20:1; coated with a titanate coupling agent to the thermotropic liquid crystalline polymer described in Example 1 (but with an inherent viscosity of 4.34 dl./g. when dissolved in a concentration of 0.1 percent by weight in pentafluorophenol at 60°C). Tensile strength and tensile modulus were tested in accordance with ASTM D-638. Flexural strength and flexural modulus were tested in accordance with ASTM D-790. Notched Izod impact strength was tested according to ASTM D-256. The results of these tests are shown in Table III: As can be seen from a review of the data in Table III, the following general trends are indicated: 1) tensile modulus is unaffected by increasing amounts of wollastonite fibres; 2) flexural modulus and strength increase slightly with increasing levels of wollastonite fibres; and 3) tensile strength and notched Izod impact strength decrease with increasing wollastonite fibre loading. EXAMPLE 3 The following Example demonstrates the surprising advantages resulting from the use of wollastonite fibres as compared to the use of mineral fillers such as mica, talc or calcium carbonate. Samples were prepared as in Example 1 by adding 15 weight percent of mica, talc, calcium carbonate or G Wollastokup KR38-2 wollastonite fibres (average aspect ratio of 20 to 1; coated with titanate coupling agent) to the thermotropic liquid crystalline polymer of Example 1. The results obtained when tested as in Example 1 are set forth in Table IV: The data in Table IV clearly illustrate that wollastonite fibres produce superior mechanical properties when blended with thermotropic liquid crystalline polymers. EXAMPLE 4 That the surface abrasion is reduced by the addition of wollastonite is shown by the following Example. The surfaces of injection moulded four-inch (10.2 cm) discs were abraded using a Taber abraser. The test procedures and equipment are described in ASTM D-1044. A 1000 g load was used with a CS-17 wheel. Surface abrasion was measured by the amount of sample weight loss after a given number of cycles of revolution of the abrasive wheels. The samples tested were prepared as in Example 1. After 10,000 cycles, the unfilled polymer lost a total of 458 mg of material. The blend of 15 weight percent of G Wollastokup KR38-2 with the polymer showed a weight loss of only 111 mg of material after 10,000 cycles, thus demonstrating the benficial effect of wollastonite on surface wear characteristics.

Claims: A moulding compound comprising a thermotropic liquid crystalline polymer matrix having a mineral filler incorporated therein, characterised in that the mineral filler is wollastonite fibres having an average aspect ratio of at least 10 to 1. The moulding compound of claim 1 wherein the wollastonite fibres have an average aspect ratio of at least 20 to 1. The moulding compound of claims 1 or 2 wherein the wollastonite fibres are treated with a titanate coupling agent prior to being blended with the thermotropic liquid crystalline polymer to produce the moulding compound. The moulding compound of any of claims 1-3 wherein the wollastonite fibres are present in the moulding compound in an amount of 10 to 70% by weight. The moulding compound of claim 4 wherein the wollastonite fibres are present in the moulding compound in an amount of 10 to 40% by weight. The moulding compound of any of claims 1-5 wherein the thermotropic liquid crystalline polymer has an inherent viscosity of between 1.0 and 15 dl./g. when dissolved in a concentration of 0.1 percent by weight of pentafluorophenol at 60°C. The moulding compound of any of claims 1-6 wherein the thermotropic liquid crystalline polymer is selected from wholly aromatic polyesters, aromatic-aliphatic polyesters, wholly aromatic poly(ester-amide)s, aromatic-aliphatic poly(ester-amide)s, aromatic polyazomethines, aromatic polyester-carbonates and mixtures thereof. The moulding compound of claim 7, wherein the thermotropic liquid crystalline polymer is a melt processable poly(ester-amide) capable of forming an anisotropic melt phase at a temperature below 400°C, consisting essentially of recurring moieties VI, VII, VIII and optionally IX wherein: VI is VII is where A is a divalent radical comprising at least one aromatic ring or a divalent trans -1,4-cyclohexylene radical; VIII is -Y-Ar-Z, where Ar is a divalent radical comprising at least one aromatic ring, Y is O, NH or NR and Z is NH or NR, where R is an alkyl group of 1 to 6 carbon atoms or an aryl group; and IX is -O-Ar'-O-, where Ar' is a divalent radical comprising at least one aromatic ring; wherein at least some of the hydrogen atoms present upon the rings may optionally be replaced by substituents selected from alkyl groups of 1 to 4 carbon atoms, alkoxy groups of 1 to 4 carbon atoms, halogen, phenyl and mixtures thereof, and wherein said poly(ester-amide) comprises 10 to 90 mole percent of moiety VI, 5 to 45 mole percent of moiety VII, 5 to 45 mole percent of moiety VIII and 0 to 40 mole percent of moiety IX. The moulding compound of claim 7, wherein the thermotropic liquid crystalline polymer is a melt processable wholly aromatic polyester capable of forming an anisotropic melt phase at a temperature below 400°C. consisting essentially of the recurring moieties III, IV and V which may include substitution of at least some of the hydrogen atoms present upon an aromatic ring wherein: III is IV is a dioxy aryl moiety of the formula -O-Ar-O-where Ar is a divalent radical comprising at least one aromatic ring, and V is a dicarboxy aryl moiety of the formula where Ar' is a divalent radical comprising at least one aromatic ring, with said optional substituents if present being selected from alkyl groups of 1 to 4 carbon atoms, alkoxy groups of 1 to 4 carbon atoms, halogen, phenyl groups and mixtures of the foregoing, and wherein said polyester comprises 10 to 90 mole percent of moiety III, 5 to 45 mole percent of moiety IV and 5 to 45 mole percent of moiety V. The moulding compound of claim 7, wherein the thermotropic liquid crystalline polymer is a melt processable wholly aromatic polyester capable of forming a thermotropic melt phase at a temperature below 350°C. consisting essentially of the recurring moieties I and II which may include substitution of at least some of the hydrogen atoms present upon an aromatic ring wherein: I is and II is with said optional substituents if present being selected from alkyl groups of 1 to 4 carbon atoms, alkoxy groups of 1 to 4 carbon atoms, halogen, phenyl and mixtures of the foregoing, and wherein said polyester comprises 10 to 90 mole percent of moiety I and 10 to 90 mole percent of moiety II. A moulded article produced from the moulding compound of any of claims 1 - 10. An extruded article produced from the moulding compound of any of claims 1 - 10.

Inventors: EICKMAN, NANCY C.
Applicants: CELANESE CORPORATION
Application year: 1984
Main classification: C08K7
IPC codes: C08L77, C08L79, C08K9, C08L101, C08L67, C08K7, C08K3, C08L69