GB2167514A

GB2167514A - Fasteners fabricated from thermotropic liquid crystalline polymers

Info

Publication number: GB2167514A
Application number: GB08518262A
Authority: GB
Inventors: Frank C Jaarsma; Ralph R Miano
Original assignee: Celanese Corp
Current assignee: Celanese Corp
Priority date: 1984-07-26
Filing date: 1985-07-19
Publication date: 1986-05-29
Also published as: GB2167514B; DE3526727A1; FR2583834A1; CA1249939A

Abstract

A fastening device comprising a headed shank e.g., a rivet, screw, bolt or nail etc., is fabricated (moulded or extruded) from a thermotropic liquid crystalline polymer, and is characterized by a shear strength of at least about 10,000 psi (e.g., 10,000 to 50,000 psi and higher), a tensile strength of at least 20,000 psi (e.g., 20,000 to 100,000 psi and higher), a linear co efficient of thermal expansion of from about 0 to -3 x 10<-5> (in/in DEG C), and extremely high chemical resistance. The fastening devices exhibit a highly oriented skin. The thermotropic liquid crystalline polymer utilized has an inherent viscosity of between about 1.0 and about 15 dl./g. when dissolved in a concentration of 0.1 percent by weight of pentafluorophenol at 60 DEG C and may be fabricated from a wholly aromatic polyester, an aromatic-aliphatic polyester, a wholly aromatic poly(ester-amide), an aromatic-aliphatic poly(ester-amide), an aromatic polyazomethine, an aromatic polyester carbonate, etc. and mixtures thereof. <IMAGE>

Description

SPECIFICATION Fastening device Background and objects of the invention This invention relates to fastening devices and, more specifically, novel rivets, screws, bolts, nails and the like. This invention provides novel fastening devices ofthe kind mentioned, in which such devices are made of a thermotropic liquid crystalline polymer.

The fastening devices of this invention find utility and provide advantages in an impressive number of end uses. The noteworthy properties of devices fabricated in accordance with the invention overcome and offer solutions to many fastening problems. One particular problem in encountered in the aircraft industry.

A most troublesome and severe problem confronting the aircraft industry today is effective fastening of aircraft structural skin materials, especially high-strength aluminum alloy sheets, titanium sheets and graphite fiber reinforced composite sheets. The problems include corrosion and/or exfoliation of the aircraft structural skin materials starting at the fastener-receiving holes. While not residing exclusively in it, most of the problems can be attributed to the rivet per se.

For example, when fastening aluminum sheets, titanium or titanium/columbium alloy rivets have been substituted for monel in order to save weight, but the titanium or titanium alloys were found to increase the corrosion rate of aluminum especially in salt spray environments and accelerated galvanic corrosion of the aluminum takes place which is apt to cause looseness of the joint. Furthermore, titanium may absorb hydrogen from the galvanic action and hydrides may form thus causing failure of the rivet.

If titanium sheets are to be fastened, aluminum rivets cannot be used although they are easy to upset and are light in weight because they corrode too rapidly in the more noble titanium. Monel rivets are used for high-strength levels of shear but, as mentioned, they are heavy compared to the weight of the sheets and they are harder to upset and distort the thin sheets when utilized. When titanium rivets are utilized, they also cause unacceptable sheet distortion with thin sheets and they are appreciably difficult to upset, thus requiring heavier rivet guns and bucking bars which result in greater operator fatigue (which is an important factor) and are difficult to upset in remote places where bucking bars of less than ideal shape have to be used.

Metal cladding of the rivets or painting of the exterior surfaces of the aircraft does not eliminate this difficulty. When the holes for the fasteners are drilled or punched in the structural members or plates to receive the fasteners, the cladding no longer provides the desired protection for the end-grain of the high-strength base metal alloy which is then exposed in the walls ofthefastener-receiving holes. Moisture seeps or is drawn into the fayed surfaces or between the fasteners and the walls of the holes and countersunk openings where the end-grain of the structural material or sheets was exposed by the drilling (or punching), for the reception of the fasteners.

With paint coatings, when applied to the skin surfaces, moisture penetration may be retarded or postponed to some degree but once the coating becomes aged it begins to crack or flake around the fastener heads. This allows the moisture direct access to the critical end-grain areas in the walls of the holes around the fasteners. In either case, although painting has been previously recommended, since it afforded some degree of temporary protection, moisture was found to eventually penetrate, and corrosion and exfoliation occurred.

Fastener installation with a wet zinc chromate primer, or uncured fuel tank sealant has also been used but this did not produce the desired results, particularly, it does not solve the problems of nonelectrical continuity and temperature variations.

Resistance to chemical solvents is of utmost importance for plastic fasteners. For example, methylethylketone (MEK) is used prior to painting of aircraft and similar components and paint strippers are even more of a concern due to their corrosive and solubilizing effect on conventional plastic fastening materials.

U.S. Patent Nos. 3,642,312 and 4,107,805 attempt to overcome the corrosion problem encountered when utilizing aluminum rivets by making a rivet of high-strength aluminum alloy and then applying a soft coating of pure aluminum to the exterior of the formed rivet. The resulting rivets, however, as well as the method of making them leave much to be desired in that the coating methods, in addition to being onerous, do not provide the desired anticorrosion characteristics.

Use of plastic fastening means per se as well as plastic clad metal fasteners have been contemplated by the art. For example, U.S. Patent 2,510,693 provides for a reinforced plastic fastening member which are fabricated from a plastic material having a fibrous reinforcing medium therein. The devices therein are formed from stock which can be in the form of a solid or a hollow rod of a plastic material which has reinforcing continuous fibers running substantially longitudinally therein and which extend continuously into the head of such fastening devices.

U.S. Patent 3,076,373 discloses wire-reinforced plastic filament fasteners such as staples, etc. to overcome a corrosion problem. These comprise steel or non-ferrous metal reinforcing means contained within a plastic body. The plastic body is a thermoplastic material with cold flow properties such as nylon and propylene which is co-extruded through suitable extrusion dies to provide the desired cross-sectional shape and also to molecularly orient the plastic in order to give increased tensile strength in the linear direction of the filament as well as to effect the firm bond with the reinforcing wires.

U.S. Patent 3,252,569 discloses a thermoplastic coated nail in which a thermoplastic material such as nylon or an acrylic resin is bonded to core wires by passage through suitable extrusion dies in order to orient long chain-like molecules of the thermoplastic material and impart an orderly arrangement in parallel relation to the extrusion axis rather than the haphazard arrangement of the unoriented thermoplastic material.

It is, therefore, an object of the present invention to minimize or obviate problems of the type discussed above.

A further object of the invention is to provide a fastening device which minimizes weight and bulk but provides advantageous physical and chemical properties.

Another object is to provide a fastening device which are easily fabricated and yet exhibit desired physical and chemical properties.

Brief description of the drawings.

Figure 1 depicts the shear failure mode, conducted by shear strength testing, of a fastening device.

Figure 2 depicts the cross-section of the shank of an extruded fastening device which presents the polymer structure of the extrudate in accordance with this invention.

Figure 3 depicts the cross-section of the shank of an injection molded fastening device which presents the polymer structure of the polymer molded in accordance with this invention.

Summary ofthe invention It has been found that a fastening device, e.g., a rivet, bolt, etc. can be fabricated from a thermotropic liquid crystalline polymer the resulting article is characterized by a shear strength of at least about 10,000 psi (e.g., 10,000 to 50,000 psi and higher), a tensile strength of at least 20,000 psi (e.g., 20,000 to 100,000 psi and higher), a linear coefficient of thermal expansion which is essentially negative, i.e., of from about 0 to -3 x 10-5 (in/in C) and extremely high chemical resistance to a variety of deleterious materials. Advantageously, the fastening device exhibits a highly oriented skin from which the advantages accrue.The thermotropic liquid crystalline polymer utilized has an inherent viscosity of between about 1.0 and about 15 dl.lg. when dissolved in a concentration of 0.1 percent by weight of pentafluorophenol at 60"C and may be fabricated from a wholly aromatic polyester, an aromatic-aliphatic polyester, a wholly aromatic poly(ester-amide), an aromatic-aliphatic poly(ester-amide), an aromatic polyazomethine, an aromatic polyester carbonate, etc. and mixtures thereof. Preferably, the liquid crystalline polymer is a wholly aromatic polyester, a wholly aromatic poly(ester-amide), or an aromatic-aliphatic poly(ester-amide), or mixtures thereof.

Description of the preferred embodiments The polymer from which the fastening devices of the present invention is formed must be a thermotropic liquid crystalline polymer which is ofthe requisite molecularweightto be capable of being shaped by injection molding or melt extrusion or the like. Such thermotropic liquid crystalline polymers have been known in the art but have not prior to the present invention been recognized to be suitable for forming the presently claimed fastening devices which evidence the surprising physical and chemical properties discussed hereinlater.

As is known in polymer technology a thermotropic liquid crystalline polymer exhibits optical anisotropy in the melt. The anisotropic character of the polymer melt may be confirmed by conventional polarized light techniques whereby cross-polarizers are utilized. More specifically, the anisotropic nature of the melt phase may conveniently be confirmed by the use of a Leitz polarizing microscope at a magnification of 40X with the sample on a Leitz hot stage and under a nitrogen atmosphere. The amount of light transmitted changes when the sample is forced to flow; however, the sample is optically an isotropic even in the static state. On the contrary typical melt processable polymers do not transmit light to any substantial degree when examined under identical conditions.

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(ester-amides), aromatic polyazomethines, aromatic polyester-carbonates, and mixtures of the same. In preferred embodiments the thermotropic liquid crystalline polymer is a wholly aromatic polyester, a wholly aromatic polyester-amide), or an aromaticaliphatic poly(ester-amide). 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 about 10 mole percent.

Representative wholly aromatic polyesters which exhibit thermotropic liquid crystalline properties include those disclosed in the following United States Patents which are herein incorporated by reference: 3,991,013; 3,991,014; 4,066,620; 3,067,851; 3,075,262; 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; 4,299,756; 4,294,955; 4,318,841; 4,337,190; 4,337,191; and 4,355,134. As discussed hereinafter the wholly aromatic polyester of U.S.Patent 4,161,470 is particularly preferred for use in the present invention.

Representative aromatic-aliphatic polyesters which exhibit thermotropic liquid crystalline properties are copolymers of polyethylene terephthalate and hydroxybenzoic acid as disclosed in PolyesterX-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: I Preparation and Properties of p-Hydroxybenzoic 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 commonly assigned United States Patent Nos. 4,318,842 and 4,355,133 which are herein incorporated by reference.

Representative wholly aromatic and aromatic-aliphatic poly(ester-amides) which exhibit thermotropic liquid crystalline properties are disclosed in United States Patent No. 4,272,625 and in commonly assigned United States Patent Nos. 4,330,457; 4,351,917; 4,351,918; 4,341,688; 4,355,132; and 4,339,375 which are herein incorporated by reference. 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 are disclosed in United States Patent Nos. 3,493,522; 3,493,524; 3,503,729; 3,516,970; 3,516,971; 3,526,611; 4,048,148; and 4,122,070. Each of these patents is herein incorporated by reference in its entirety. Specific examples of such polymers include poly(nitrilo-2-methyl-1 ,4-phenylenenitriloethylidyne-1 ,4- phenyleneethylidyne); poly-(nitrolo-2-methyl-1 ,4-phenylenenitrilomethylidyne-1 ,4-phenylenemethylidyne); and poly(nitrilo-2-chloro-1 ,4-phenylenenitrilomethylidyne-1 ,4-phenylenemethylidyne).

Representative aromatic polyester-carbonates which exhibit thermotropic liquid crystalline properties are disclosed in United States Patent Nos. 4,107,143 and 4,284,757, and in commonly assigned United States Patent No. 4,371,660 which are herein incorporated by reference. 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 commonly is selected for use in the formation of the fastening device of the present invention which possesses a melting temperature within the range that is amenable to melt extrusion while employing commercially available equipment. For instance, thermotropic liquid crystalline polymers commonly are selected which exhibit a melting temperature somewhere within the range of approximately 250 to 400oC.

The thermotropic liquid crystalline polymer selected preferably also exhibits an inherent viscosity of at least 2.0 dl./g. when dissolved in a concentration of 0.1 percent by weight in pentafluorophenol at 60"C. (e.g., an inherent viscosity of approximately 1.0 to 15.0 dl./g.).

The 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 approximately 350"C. This polyester consists essentially of the recurring moieties I and II wherein:

The polyester comprises approximately 10 to 90 mole percent of moiety I, and approximately 10 to 90 mole percent of moiety II. In one embodiment, moiety II is present in a concentration of approximately 65 to 85 mole percent, and preferably in a concentration of approximately 70 to 80 mole percent, e.g., approximately 73 mole percent.In another embodiment, moiety II is present in a lesser proportion of approximately 15 to 35 mole percent, and preferably in a concentration of approximately 20 to 30 mole percent. In addition, at least some of the hydrogen atoms present upon the rings optionally may be replaced by substitution selected from the group consisting of an alkyl group of 1 to 4 carbon atoms, an alkoxy group of 1 to 4 carbon atoms, halogen, phenyl, substituted phenyl, and mixtures thereof. Such polymer preferably has an inherent viscosity of approximately 3.5 to 100 dl./g. when dissolved in a concentration of 0.1 percent by weight in pentafluorophenol at 600C.

The particularly preferred wholly aromatic poly(ester-amide) or aromatic-aliphatic poly(ester-amide) for use in the present invention is disclosed in commonly assigned United States Patent No. 4,330,457 which is capable of forming an anisotropic melt phase at a temperature below approximately 400"C. The poly(ester-amide)s there disclosed consist essentially of recurring moieties I, II, Ill, and, optionally, IV wherein:

II is

where A is a divalent radical comprising at least one aromatic ring or a divalent trans-1, 4-cyclohexylene radical; Ill 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 IV is fO - 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 substitution selected from the group consisting of an alkyl group of 1 to 4 carbon atoms, an alkoxy group of 1 to 4 carbon atoms, halogen, phenyl, substituted phenyl, and mixtures thereof, and wherein said poly(ester-amide) comprises approximately 10 to 90 mole percent of moiety I, approximately 5 to 45 mole percent of moiety Ill, and approximately 0 to 40 mole percent of moiety IV.The preferred dicarboxy aryl moiety Ills:

and the preferred moiety Ill is:

and the preferred dioxy aryl moiety IV is:

Such polymer preferably has an inherent viscosity of approximately 1.0 to 15 dl./g. when dissolved in a concentration of 0.1 percent by weight in pentafluorophenol at 600C.

When forming the articles of the present invention by injection molding, conventional injection apparatus can be used. Suitable injection apparatus include the Stubbe automatic injection molding machine model SKM 50145.

The temperature and pressure conditions selected for injection molding the molten thermotropic liquid crystalline polymer will be influenced by the melting temperature of the polymer and its viscosity as will be apparent to those skilled in the art. Typically, injection molding temperatures approximately 5"C. to 50"C.

above the polymer melting temperature and pressures of approximately 2000 to 20,000 psi are selected. The term "Melting Temperature" as used herein is meant that at which the polymer has sufficiently iow viscosity for adequate flow in the molding process.

In the injection molding of liquid crystalline polymer, melt flows into the cavity in laminar fashion, with each strata in the melt flowing at a uniform rate without substantial inter-strata mixing. When this flow strikes the downstream dead-end of the cavity and rebounds therefrom, laminar flow at this end ceases due to interference from the rebound molecular wave patterns. Since the mechanical properties of molded liquid crystalline polymer are directly related to the degree of ordered arrangement of molecules in each strata, this reverse flow may cause points of structural weakness in the molded article, detracting from its mechanical properties.

If desired, an outlet may be placed at the downstream end of the cavity which allows molten material to flow out of the cavity, thus preventing undesired rebound molecular wave motion (i.e., patterns) within the cavity. In a preferred embodiment, a passage communicates with the downstream outlet so as to receive the molten material. This technique is disclosed in commonly assigned United States Serial Nos. 414,558 and 414,560, both filed September 3, 1982 and which are herein incorporated by reference.

Once the molded article is removed from the mold, the stub of material that solidifies in the passage may be shorn from the article. It is preferred to remove more than the stub of material and remove at least some material from the shank per se. Articles molded and trimmed in accordance with this latter embodiment exhibit mechanical properties superior to those of articles molded and the stub removed conventionally.

The fastening devices of the present invention can also be formed by melt extruding an elongated member, chopping same to a desired length and subsequently forming a head on the shank. The shank may also be extruded as a tubular elongated member. When forming the melt extruded elongated member, from which the shank is cut, conventional melt extrusion apparatus can be used wherein an extrusion die is selected having a size and shape which corresponds to the cross-sectional configuration of the elongated member to be formed with the exception that the orifice dimensions will be larger than the dimensions of the resulting elongated member in view of drawdown of the molten polymer which occurs immediately following extrusion.Polymers other than thermotropic liquid crystalline polymers are recognized to be incapable of melt extrusion to form articles of the cross-sectional area herein discussed wherein the profile will accurately correspond to the die shape. Accordingly, the thermotropic liquid crystalline polymers do not exhibit any substantial elastic recoil upon exiting from the extrusion die as do conventional polymers which are melt extruded.Suitable extrusion apparatus are described, for example, in the "Plastics Engineering Handbook" of the Society of the Plastics Industry, Pages 156 to 203,4to Edition, edited by Joel Frados, Van Nostrand Reinhold Company, 1976. The elongated members of the present invention optionally may be formed in accordance with the teachings of commonly assigned United States Patent No. 4,332,759 of Yoshiaki Ide, entitled "Process for Extruding Liquid Crystal Polymer".

The temperature and pressure conditions selected for extruding the molten thermotropic liquid crystalline polymer will be influenced by the melting temperature of the polymer and its viscosity as will be apparent to those skilled in the art. Typically, extrusion temperatures approximately 0 to 30"C. above the polymer melting temperature and pressures of approximately 100 to 5,000 psi are selected. In order to induce relatively high molecular orientation coextensive with the length of the elongated member, the extrudate is drawn while in the melt phase immediately adjacent the extrustion orifice and priorto complete solidification. The extent of such drawdown is influenced by the takeup speed under which the elongated member is wound or otherwise collected on an appropriate support or collection device.The resulting draw ratio is defined as the ratio of the die cross-sectional area to that of the cross-sectional area of the fully solidified extrudate. Such draw ratios commonly range between 4 and 100, and preferably between approximately 10 and 50 while utilizing the equipment described in the Examples.

In addition to the drawdown, appropriate cooling must be applied to the extrudate of thermotropic liquid crystalline polymer intermediate the extrusion orifice and the point of collection. Appropriate fluid media, e.g., a gas or a liquid, may be selected to impart the desired cooling. For instance, the extrudate may be simply contacted by a stream of air or other gas or preferably immersed in a circulating bath of water or other liquid which is maintained at an appropriate temperature to impart the cooling required for solidification.

The resulting cross-sectional configuration is substantially uniform and can be monitored by use of a laser or other appropriate sensing device to insure the quality control demanded, for example, by the aircraft industry. The elongated members suitable for use as shanks possess a diameter or cross-section of at least about 1/16 inch up to 1/2 inch or more, if desired with about 1/8 inch to about 1/4 inch being the range diameters utilized most often.

When the cross-sections of the molded or extruded fasteners, which have the desired properties in accordance with this invention, are closely examined, a highly oriented skin and a relatively less oriented core are usually observed. This can be detected with either cross-polarized or scanning electron micrographs or other microscopy means. It is found that the desirable properties of the fasteners are directly correlated to the thickness of the skin.

Referring to the drawings, FIGURE 1 locates a sheat failure mode on a shank 2 of a rivet 3 to which this oriented skin phenomena pertains most particularly. FIGURES 2 and 3 show in a cross-sectional view of the shank 2, the outer "true" skin 11,the innerskin layers 12 and an unoriented core 13. The innerskin layers 12 are comprised of macro fibrils 14 of about 5 micrometer, fibrils 15 of about 0.5 micrometer and micro fibers 16 of about 0.05 micrometer.

As used herein, the term "skin" means the actual surface skin and the inner oriented areas as well. The structural model for the inner oriented areas is a hierarchical fibrillartexture ranging from about 5 micrometer down to about 5 nanometer. The finest substructural unit observed, on the molecular level, are micro fibrils on the order of about 50 by 5 nanometerforthe polymers used herein. This microfibrillartexture is associated into fibrillar units (fibrils) that are an order of magnitude larger in size; that is, about 0.5 micrometer across. Macro units (macro fibrils), about 5 micrometer in size, are also observed. Accordingly, this hierarchy of textures characterizes the fibrillar, fiber-like, elongated and highly oriented structure of the skin which affords the desired physical and chemical properties.

Several mechanisms are believed responsible for this skin-core formation. These are: (a) yield stress of the melt; (b) radial distribution of the total shear strain; and (c) elongational strain after the exit of the die. The thickness of the skin based on the above mechanisms should be at least 5 percent of the cross-section of a suitable fastener and preferably at least 10 percent of the cross-section. Thus, a fastener having a 1/4 inch shank will have an oriented skin of at least 0.0125 inch (12.5 mil) and preferably about 0.025 inch (25 mil) to be suitable for use. Accordingly, orientation can be increased by drawing the extruded fastener stock; by manipulating molding conditions, e.g., using a "mild" set of conditions, such as relatively low melt and mold temperatures and injection pressures and speeds as found in a molding profile. Considering the estimate of skin thickness and also the sensitivity of orientation to elongational strains, the mechanism (c) caused by velocity rearrangement seems most responsible for the skin-core morphology.

The head of the fastening device is formed by conventional techniques using sonic or ultrasonic energy with, in some instances, the application of heat.

If the fastening device is in the configuration of a bolt, screw, nails and the like, threads, serrations, and other modifications to the shape can be accomplished using techniques conventional with the shaping of plastic articles.

If desired, physical properties such as shear strength, tensile strength, and elongation, of the solidified previously formed fastening devices of thermotropic liquid crystalline polymer may optionally be enhanced by heat treatment at a temperature below the melting temperature of the thermotropic liquid crystalline polymer for a time sufficient to increase the melting temperature of the polymer by at least 10"C. For instance, the elongated member may be heated below the melting temperature while present in a nitrogen or other atmosphere for up to 24 hours at an elevated temperature within 50"C. of the polymer melting temperature.

In accordance with the present invention, if the fastening devices are rivets, conventional means may be used to form the desired heads. For example, the forming of flush rivet heads may be accomplished by high pressure welding where the material is only softened by the application of ultrasonic energy and not actually melted. Protruding rivet heads may be formed by staking or by stake welding where the head is added as a separate piece. It is also possible to expand the rivet in the hole by directing ultrasonic energy beyond the surface of the protruding rivet or boss and softening the body of the rivet.

Conventional ultrasonic equipment can be used, for example, twenty KHz or forty KHz can be used advantageously. The desired construction of fixtures and apparatus utilized, e.g., ultrasonic horns are known in the art.

In practice, the rivet head may be formed by providing a projection of the rivet or boss through the the workpieces to be secured introduce sonic or ultrasonic energy onto the projection or the boss. The energy, when suitably introduced, will change the shape of the projection or boss and, therefore, secure the workpieces.

More specifically, the projection or boss should have a height above the base workpiece so as to provide sufficient material to form a head of the desired configuration. The ultrasonic energy is introduced into the projection or boss by placing a coupler member adjacent a tip of the projection and, therafter, ultrasonically vibrating the coupler member at a high frequency to beat it downwardly against the tip of the projection.

Ultrasonic energy is thus coupled into the projection or boss to heat it and render itflowable so that its tip is peened over by the beating of the coupler member.

The physical properties of the thermotropic liquid crystalline polymer fastening device are considered to be unique and to be totally unattainable with other polymers which are capable of undergoing the shaping operation, i.e., injection molding or melt extrusion described herein.

The shear strength of the fastening devices of thermotropic liquid crystalline polymer is extremely high and is at least 10,000 psi (e.g., 10,000 to 50,000 psi and higher). Such shear strength can be conveniently determined in accordance with the standard procedure of ASTM B565-76. Accordingly, the fastening devices of the present invention exhibit a remarkable tendency to withstand shear strain of the magnitude which would severely damage fastening devices formed from conventional thermoplastic materials.

The tensile strength of the fastening devices of thermotropic liquid crystalline polymer is also high and is at least 20,000 psi (e.g., 20,000 to 100,000 psi and higher). Such tensile strength can be conveniently determined in accordance with the standard procedure of ASTM D 638 with strain gauge at about 23"C.

The chemical resistance of the fastening devices of thermotropic liquid crystalline polymer of the present invention is exemplified by the data set forth in the following Tables IA and IB. The resin from which the fastening devices are fabricated was exposed for 30 days to hydrochloric, chromic, nitric and sulfuric acids; sodium hydroxic and a plating solution at various temperatures.

TABLE IA Liquid crystalline polymer (1) chemical resistance 30-day exposure Tensile strength Tensile modulus Flexural strength Flexural modulus Change dimension % x 10 PSI x 106PSI x 10 PSI x 106PSI weight% Hydrochloric acid (37%) &commat; Room Temperature 26 1.5 23 1.5 NC(2) &commat; 190 F 26 1.5 23 1.4 NC Chromic acid (70%) &commat; Room Temperature 27 1.1 23 1.4 NC &commat; 190 F 27 1.4 24 1.4 NC Nitric acid (70%) &commat; Room Temperature 26 1.5 23 1.4 NC &commat; 190 F 26 1.5 23 1.4 NC Sodium hydroxide (50%) &commat; Room Temperature 28 1.2 22 1.2 NC &commat; 190 F 28 1.5 21 1.3 NC Lating solution &commat; Room Temperature 26 1.7 24 1.6 NC &commat; 190 F 25 1.2 24 1.5 NC Control (air) &commat;Room Temperature 24 1.1 23 1.4 NC &commat; 190 F 25 1.5 23 1.3 NC (1) See Example 1 for Composition of Matter (2) Change less than # 1% TABLE IB Liquid crystalline polymer (1) chemical resistance 30-day exposure Tensile strength Tensile modulus Flexural strength Flexural modulus Change Dimension % x 10 PSI x 106PSI x 10 PSI x 106PSI weight% Sulfuric acid Room temperature 37% 25 1.0 24 21 1.4 NC 70% 24 1.0 24 1.3 NC 93% 27 0.9 24 1.3 NC Air (Control) 22 1.0 23 1.4 NC 150 F 37% 26 1.5 21 1.4 NC 70% 27 1.6 21 1.4 NC 93% 28 1.4 20 1.2 NC Air (Control) 25 1.4 21 1.3 NC 250 F 37% 27 1.5 23 1.2 NC 70% 27 1.5 23 1.0 NC 93% 26 1.5 20 1.0 1.9(2) Air (Control) 27 1.5 20 0.9 NC 375 F 70% 25 1.5 23 1.2 NC 93% 27 1.6 22 1.1 1.0(2) Air (Control) 26 1.4 22 0.9 NC (1) See Example 1 for Composition of Matter (2) Weight Gain Additionally, the fastening devices of thermotropic liquid crystalline polymer in accordance with the present invention have been found to exhibit a highly satisfactory linear coefficient of thermal expansion property unlike the metallic devices presently utilized and other plastic fastening devices which, in general, are found unsuitable for use because of this failing. Advantageously, the materials exhibit a linear coefficient of thermal expansion which is about 0 and may be negative in value. Typical values for the unfilled polymers from which the fasteners of this invention range from about 0 (and be slightly positive in value) to about -3 x 10-5 (in/in"C). Illustrative values are set forth below in comparison with competing materials where "C" represents the linear coefficient of thermal expansion.

C x 10-5rinlin C) Liquid Crystalline Polymer (Example 1) -0.3 to -2.4 Aluminum 2.4 Titanium/Columbium 0.8 Nylon 7.2 Rynite 530 PET resin 2.4 - 3.6 Ultem 2300 polyetherimide resin 2.9 Ryton R-4 polyphenylene sulfide resin 1.9 Various fillers and/or reinforcing agents may be included in a total concentration of about 0% to about 50% and over by weight of the resulting molded compound. Representative fibers which may serve as reinforcing media therein include glass fibers, asbestos, graphitic carbon fibers, amorphous carbon fibers, synthetic polymeric fibers, aluminum fibers, aluminum silicate fibers, oxide of aluminum fibers, titanium fibers, boron fibers, magnesium fibers, rock wool fibers, steel fibers, tungsten fibers, cotton wool, and wood cellulose fibers, etc.If desired, the fibrous reinforcement may be preliminarily treated to improved adhesion ability to the liquid crystalline polymer which ultimately serves as a continuous matrix phase. Representative filler materials include calcium silicate, silica, clays, talc, mica, polytetrafluoroethylene, graphite, aluminum trihydrate, sodium aluminum carbonate, barium ferrite, etc.

The present invention prefers a thermotropic liquid crystalline polymer matrix having glass fibers incorporated therein for reinforcement to prepare the molding compound utilized in the fastening devices.

The following examples are presented as specific illustrates of the claimed 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 melt extruded (injection molded) fastening devices 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 8.4 dl./g. when dissolved in a concentration of 0.1 percent by weight in pentafluorophenol at 60"C., and a differential scanning calorimetry melting temperature peak of 289"C.

Example 2 Example 1 was substantialy repeated with the exception that a wholly aromatic poly(ester-amide) which exhibits thermotropic liquid crystalline properties was substituted for wholly aromatic polyester of Example 1 and different extrusion conditions were employed. More specifically, the wholly aromatic poly)esteramide) was formed in accordance with the teachings of commonly assigned United States Patent 4,330,457, issued May 18, 1982, and was derived from 60 mole percent of 6-hydroxy-2-naphthoic acid, 20 mole percent of terephthalic acid, and 20 mole percent of p-aminophenol. The wholly aromatic poly(ester-amide) exhibited an inherent viscosity of 4.41 dl./g. when dissolved in a concentration of 0.1 percent by weight in pentafluorophenol at 60"C., and a differential scanning calorimetry melting temperature peak of 284"C.

Example 3 This example presents a comparison of tensile and shear properties of typical unfilled liquid crystalline polymers with metals used for fasteners in the aircraft industry. The materials of Examples 1 and 2 are compared with aluminum and titanium/columbium alloys as follows. The specific gravities of the material are also included. When these are considered with the other properties, the liquid crystal polymer variants compare favorably. When chemical resistance and resistance to galvanic corrosion are considered, the liquid crystal polymers can be deemed superior for many uses.

TABLE II Fastener material Astm Titaniuml Property test Example 1 Example 2 Aluminum columbium alloy Tensile Strength, Kpsi D638 24 26 65 95 Double Shear MPSI B565-76 15 19 30 49 Specific Gravity D-742 1.40 1.40 2.7-2.8 6.2 Strength (Double Shear) to 10.7 13.6 10.9 7.9 Weight Ratio Example 4 Injection molded 1/4 inch diameter rods of four competitive materials were double shear tested. The results indicate a clear advantage for the liquid crystal polymer materials of the present invention.The competitive materials evaluated included Nylon - a polyamide resin (33% glass fiber and nylon), Rynite 530 (30% glass fiber) - a polyethylene terephthalate resin manufactured by E.l. duPont de Nemours and Company, Ultem 2300 (30% glass fiber) - a polyetherimide resin manufactured by General Electric Corporation, and Ryton R-4 (40% glass fiber) - a polyphenylene sulfide resin manufactured by Phillips Petroleum Corporation. The results are shown below with the liquid crystal polymer of Examples 1 and 2 for comparison. It should also be remembered that in addition to the indicated shear strength advantage, liquid crystal polymer offers a chemical resistance advantage over all but the Ryton and a tensile strength advantage over each material.

Material Shear strength (PSI) Nylon 14,200 Rynite 530 PET resin 9,890 Ultem 2300 polyetherimide resin 13,210 Ryton R-4polyphenylesulfide resin 12,070 Liquid Crystalline Polymer (Example 1) 17,542 Liquid Crystalline Polymer (Example 2) 18,108 Example 5 Linear coefficients of thermal expansion were determined on rivets molded from the liquid crystalline polymer of Example 1. The experiments were conducted by attaching a 1 inch extensometer to the shaft of the rivet and placing the guage in an oven. The oven temperature was raised from room temperature to 150"C in steps of approximately 25"C with the oven rising at about 2"C per minute between steps.

The following thermal expansion coefficients were obtained: Sample Thermal expansion C x 10-5 (inchlinch/ C) 1/4" Round Head Rivet -0.9 3/16" Flat Head Rivet -1.0

Claims

1. A one-piece fastening device ofthermotropic liquid crystalline polymer comprising a head and a relatively rigid axial shank adapted to be inserted in the aperture of a workpiece characterized by a shear strength of at least about 10,000 psi, a tensile strength of at least 20,000 psi and a substantially negative linear coefficient of thermal expansion.

2. The fastening device of claim 1 wherein said fastening device has a highly oriented skin.

3. The fastening device of claim 1 wherein a second head is positioned on said axial shank.

4. The fastening device of claim 1 wherein threads are disposed on the exterior surface of said shank.

5. The fastening device as set forth in claim 1 wherein said head and shank are hollow along the linear axis of said fastening device.

6. The fastening device of claim 1 wherein the thermotropic liquid crystalline polymer has an inherent viscosity of between about 1.0 and about 15 dl./g. when dissolved in a concentration of 0.1 percent by weight of pentafluorophenol at 60"C.

7. The fastening device of claim 1 wherein the thermotropic liquid crystalline polymer is selected from the group consisting of a wholly aromatic polyester, an aromatic-aliphatic-polyester, a wholly aromatic poly(ester-amide), an aromatic-aliphatic poly(ester-amide), an aromatic polyazomethine, an aromatic polyester-carbonate, and mixtures thereof.

8. The fastening device of claim 1 wherein the thermotropic liquid crystalline polymer is selected from the group consisting of a wholly aromatic polyester, a wholly aromatic poly(ester-amide), an aromaticaliphatic poly(ester-amide), and mixtures thereof.

9. The fastening element of claim 1 wherein the thermotropic liquid crystalline polymer is melt processable poly(ester-amide) capable of forming an anisotropic melt phase at a temperature below approximately 400"C. consisting essentially of recurring moieties I, II, Ill, and, optionally, IV wherein: I is

II is

where A is a divalent radical comprising at least one aromatic ring ora divalent trans-1,4-cyclohexylene radical; Ill is 4 Y - Ar - Z +, where Ar is a divalent radical comprising at least one aromatic ring, Y is 0, 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 IV is 4 O - Ar' - 0 +, 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 substitution selected from the group consisting of an alkyl group of 1 to 4 carbon atoms, an alkoxy group of 1 to 4 carbon atoms, halogen, phenyi, and mixtures thereof, and wherein said poly(ester-amide) comprises approximately 10 to 90 mole percent of moiety I, approximately 5 to 45 mole percent of moiety II, approximately 5 to 45 mole percent of moiety Ill, and approximately 0 to 40 mole percent of moiety IV.

10. The fastening device of claim 1 wherein the thermotropic liquid crystalline polymer is a melt processable wholly aromatic polyester capable of forming an anisotropic melt phase at a temperature below approximately 400"C. consisting essentially of the recurring moieties I, II, and Ill which may include substitution of at least some of the hydrogen atoms present upon an aromatic ring wherein:: I is

II is a dioxy aryl moiety of the formula -O - Ar - O where Ar is a divalent radical comprising at least one aromatic ring, and Ill is a dicarboxy aryl moiety of the formula

where Ar' is a diva lent radical comprising at least one aromatic ring, with said optional substitution if present being selected from the group consisting of an alkyl group of 1 to 4 carbon atoms, an alkoxy group of 1 to 4 carbon atoms, halogen, a phenyl group and mixtures of the foregoing, and wherein said polyester comprises approximately 10 to 90 mole percent of moiety I, approximately 5 to 45 mole percent of moiety II, and approximately 5 to 45 mole percent of moiety Ill.

11. The fastening device of claim 1 wherein the thermotropic liquid crystalline polymer is a melt processable wholly aromatic polyester capable of forming a thermotropic melt phase at a temperature below approximately 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 II is

with said optional substitution if present being selected from the group consisting of an alkyl group of 1 to 4 carbon atoms, an alkoxy group of 1 to 4 carbon atoms, halogen, phenyl, and mixtures of the foregoing, and wherein said polyester comprises approximately 10 to 90 mole percent of moiety I, and approximately 10 to 90 percent of moiety II.

12. The fastening device of claim 1 characterized by a negative linear coefficient of thermal expansion of from about 0 to about -3.0 x 10-5 (in/inoC).