PROCESS FOR FABRICATING ORIENTED POLYBENZAZOLE FILMS

Abstract: Films containing polybenzazole polymer dissolved in a solvent can be mechanically stretched to provide biaxial orientation. The resulting dope film can be coagulated. The polymer film has improved properties in the direction in which stretching occurs.

Description: PROCESS FOR FABRICATING ORIENTED POLYBENZAZOLE FILMS This invention relates to films containing polybenzazole polymers and processes for making them. Polybenzazole polymers are typically synthesized by polymerization in non-oxidizing dehydrating acid solutions to form viscous dopes containing the acid and the polymer, ϋniaxially oriented film may be synthesized from dope by extruding onto a rotating drum that draws the film in the machine direction and rotates it down into a water bath to coagulate. Biaxially oriented films are synthesized by extruding the dope as a tube, which is blown or forced over a mandrel to impart some biaxial orientation, and is then immersed in water to coagulate it. Examples of such processes are described in: Chenevey, U.S. Patent 4,487,735 (December 11, 1984); Lusignea et al., U.S. Patent 4,871,595 (October 3. 1989); Chenevey, U.S. Patent 4,898,924 (February 6, 1990); Harvey et al., U.S, Patent 4,939,235 (July 3, 1990); Harvey et al., U.S. Patent 4,963,428 (October 16, 1990); Lusignea et al., U.S. Patent 4,966,806 (October 30, 1990); and Fujiwara, Japanese Kokai 63( 1988)-74612 (published April 5, 1988). Improvements are needed in the processes for making polybenzazole films. Films made by the previously described processes tend to have very inconsistent gauge thickness. The biaxial orientation is less than complete, so that films still tend to have j- better properties in the machine direction and poorer properties in the transverse direction. A further disadvantage of existing processes is that they inherently create a multi-layered film. Each 10 layer may contain the same polymer, but molecules within the crystalline domains are oriented in a different direction in each layer. The different layers are subject to delamination from each other. 15 An object of the invention is to find an improved process that can improve any one of: the guage control of the polybenzazole film; the control over degree of orientation in the polybenzazole film; or the Q delamination resistance of the polybenzazole film. One aspect of the present invention is a process for making a polybenzazole film or sheet that 5 has the steps of: (1) extruding a dope, that contains a solvent and a polybenzazole polymer, to form a dope film or sheet; 0 (2) stretching the dope film or sheet in the transverse direction at a temperature and a rate at which the dope film or sheet does not tear; and (3) coagulating the stretched dope film or sheet, whereby a polymer film or sheet is formed, characterized in that the dope film is stretched mechanically using a device that grips the transverse edges of the dope film or sheet and draws the transverse edges apart from each other. A second aspect of the present invention is a film or sheet that contains a lyotropic liquid crystalline polybenzazole polymer and that has a tensile strength of at least 35 Ksi (241 MPa) in both the machine direction and the transverse direction, characterized in that the crystalline domains on the faces of the film or sheet are not oriented in opposite uniform angles of + or - θ with respect to the machine direction. As used in this application, the term "transverse" means approximately at right angles with respect to the machine direction in which the dope film or sheet was extruded. 1 Ksi = 1000 psi; and 1 Msi = 1,000,000 psi. The mechanically stretched films and sheets may be used in structural materials or electronic components as described in U.S. Patent 4,871,595 (October 3. 1989) and 11 Ency. Poly. Sci. & Eng., Polybenzothiazoles and Polybenzoxazoles, 601 (J. Wiley & Sons 1988). The present invention uses dopes containing polybenzazole (PBZ) polymer. The polybenzazole polymer is preferably polybenzoxazole (PBO) or polybenzothiazole (PBT) or copolymers thereof, dissolved in a solvent. PBO, PBT and random, sequential and block copolymers of PBO and PBT are described in references such as Wolfe et al. , Liquid Crystalline Polymer Compositions, Process and Products, U.S. Patent 4,703,103 (October 27, 1987); Wolfe et al. , Liquid Crystalline Polymer Compositions, Process and Products, U.S. Patent 4,533 . 692 (August 6, 1985); Wolfe et al., Liquid Crystalline Poly(2,6-Benzo- thiazole) Compositions, Process and Products, U.S. Pat¬ ent 4,533,724 (August 6, 1985); Wolfe, Liquid Crystal¬ line Polymer Compositions, Process and Products, U.S. Patent 4,533,693 (August 6, 1985); Evers, Thermoxada- tively Stable Articulated p-Benzobisoxazole and p-Benzo- bisthiazole Polymers, U.S. Patent 4,359,567 (November 16, 1982); Tsai et al., Method for Making Heterocyclic Block Copolvmer, U.S. Patent 4,578,432 (March 25, 1986); 11 Ency. Poly. Sci. & Eng., Polybenzothiazoles and Polybenzoxazoles, 601 (J. Wiley & Sons 1988) and W. W. Adams et al., The Materials Science and Engineering of Rigid-Rod Polymers (Materials Research Society 1989). The polymer may contain AB-PBZ mer units, as represented in Formula 1(a), and/or AA/BB-PBZ mer units, as represented in Formula 1(b) Ka) AB Kb) AA/BB wherein: Each Ar represents an aromatic group. The aromatic group may be heterocyclic, such as a pyridinylene group, but it is preferably carbocyclic. The aromatic group may be a fused or unfused polycyclic system, but is preferably a single six-membered ring. Size is not critical, but the aromatic group preferably contains no more than 18 carbon atoms, more preferably no more than 12 carbon atoms and most preferably no more than 6 carbon atoms. Examples of suitable aromatic groups include phenylene moieties, tolylene moieties, biphenylene moieties and bis-phenylene ether moieties. Ar1 in AA/BB-mer units is preferably a 1 ,2,4,5-phenylene moiety or an analog thereof. Ar in AB-mer units is preferably a 1 ,3,4-phenylene moiety or an analog thereof. Each Z is independently an oxygen or a sulfur atom. Each DM is independently a bond or a divalent organic moiety that does not interfere with the synthesis, fabrication or use of the polymer. The divalent organic moiety may contain an aliphatic group, which preferably has no more than 12 carbon atoms, but the divalent organic moiety is preferably an aromatic group (Ar) as previously described. It is most preferably a 1 ,4-phenylene moiety or an analog thereof. The nitrogen atom and the Z moiety in each azole ring are bonded to adjacent carbon atoms in the aromatic group, such that a five-me bered azole ring fused with the aromatic group is formed. The azole rings in AA/BB-mer units may be in cis- or trans-position with respect to each other, as illustrated in 11 Ency. Poly. Sci. & Eng., supra, at 602. The polymer preferably consists essentially of either AB-PBZ mer units or AA/BB-PBZ mer units, and more preferably consists essentially of AA/BB-PBZ mer units. The polybenzazole polymer may be rigid rod, semi-rigid rod or flexible coil. It is preferably rigid rod in the case of an AA/BB-PBZ polymer or semi-rigid in the case of an AB-PBZ polymer. It is preferably a lyotropic liquid-crystalline polymer. Azole rings within the polymer are preferably oxazole rings (Z = 0), so that the polymer is a polybenzoxazole polymer. Preferred mer units are illustrated in Formulae 2 (a)-(h). The polymer more preferably consists of mer units selected from those illustrated in 2(a)-(h), and most preferably consists of identical mer units selected from those illustrated in 2(a)-(d). Each polymer preferably contains on average at least 25 mer units, more preferably at least 50 mer units and most preferably at least 100 mer units. The intrinsic viscosity of rigid AA/BB—PBZ polymers in ethanesulfonic acid at 25°C is preferably at least 10 dL/g, more preferably at least 15 dL/g and most preferably at least 20 dL/g. For some purposes, an intrinsic viscosity of at least 25 dL/g or 30 dL/g may be best. Intrinsic viscosity of 60 dL/g or higher is possible, but the intrinsic viscosity is preferably no more than 40 dL/g. The intrinsic viscosity of semi¬ rigid AB-PBZ polymers is preferably at least 5 dL/g, more preferably at least 10 dL/g and most preferably at least 15 dL/g. The polymer or copolymer is dissolved in a solvent to form a dope solution. Some polybenzoxazole and polybenzothiazole polymers are soluble in cresol, but the solvent is preferably an acid capable of dissolving the polymer. The acid is preferably non- -oxidizing. Examples of suitable acids include polyphosphoric acid, methanesulfonic acid and sulfuric acid, and mixtures of those acids. The acid is prefer¬ ably polyphosphoric acid or methanesulfonic acid, and is more preferably polyphosphoric acid. The dope should contain a high enough concentration of polymer for the polymer to coagulate to form a film of the desired thickness without substantial flaws. When the polymer is lyotropic liquid- crystalline, then the concentration of polymer in the dope is preferably high enough to provide a dope having liquid crystalline domains. The concentration of the polymer is preferably at least 7 weight percent, more preferably at least 10 weight percent and most preferably at least 14 weight percent. The maximum concentration is limited primarily by practical factors, such as polymer solubility and dope viscosity. The concentration of polymer is seldom more than 30 weight percent, and usually no more than 20 weight percent. Suitable polymers or copolymers and dopes can be synthesized by known procedures, such as those described in Wolfe et al., U.S. Patent 4,533,693 (August 6, 1985); Sybert et al., U.S. Patent 4,772,678 (September 20, 1988); Harris, U.S. Patent 4,847,350 (July 11, 1989); and Ledbetter et al., "An Integrated Laboratory Process for Preparing Rigid Rod Fibers from the Monomers," The Materials Science and Engineering of Rigid-Rod Polymers at 253-64 (Materials Res. Soc. 1989). In summary, suitable monomers (AA-monomers and BB- monomers or AB—monomers) are reacted in a solution of nonoxidizing and dehydrating acid under nonoxidizing atmosphere with vigorous mixing and high shear at a temperature that is increased in step-wise or ramped fashion from no more than 120°C to at least 190°C. Examples of suitable AA-monomers include terephthalic acid and analogs thereof. Examples of suitable BB- monomers include 4,6-diaminoresorcinol, 2,5- diaminohydroquinone, 2,5-diamino-1,4-dithiobenzene and analogs thereof, typically stored as acid salts. Examples of suitable AB-monomers include 3-amino-4- hydroxybenzoic acid, 3-hydroxy-4-aminobenzoic acid, 3-amino-4-thiobenzoic acid, 3-thio-4-aminobenzoic acid and analogs thereof, typically stored as acid salts. The present invention converts dopes into polymer films by a three-step process. In the first step, the dope is extruded as a dope film or sheet which is relatively thick and is relatively narrow in the transverse direction, as compared with the final desired film. The extrusion may optionally be through a slit die, such as a coathanger die or T-die, or through a tubular die, such as the counter-rotating die described in the previously-named references. An example of extrusion techniques is described in Chenevey, U.S. Patent 4,487,735 (December 11, 1984). The extrusion die preferably does not contain opposing surfaces that move in relation to each other during extrusion to orient different layers of the film in different discrete directions, in the manner taught in U.S. Patents 4,487,735 and Harvey et al., U.S. Patent 4,939,235 (July 3, 1990). In other words, the die preferably does not subject the faces of the dope film to transverse shear forces during extrusion. The opposing surfaces moving with respect to each other are reported to orient the nematic polymer molecules at the surface of the film in opposite uniform angles defined as + or - θ. The angle θ is reportedly broadened by c - later steps in the process, so that the film or sheet contains discrete layers with molecules oriented in different directions. Those layers are undesirable because they increase the susceptibility of the film or sheet to delamination. The preferred dies also do not 10 apply the twisting force and orientation that is applied by an anular die in which one surface rotates. The extruded dope film is preferably left as a continuous sheet, rather than cutting into discrete 15 sections. The dope film should not be coagulated. The dope film preferably does not contain discrete layers with different orientations. Q Optimal extrusion temperature varies according to many factors such as the solvent, polymer rigidity, molecular weight and concentration, and dope viscosity. It should be high enough that the dope flows through the die well, and low enough that the dope is stable and 5 handlable. It may be between 0°C and 300°C. The extrusion temperature for liquid crystalline dopes of about 14 weight percent cis-polybenzoxazole in polyphos¬ phoric acid is preferably at least 100°C, more preferably at least 130°C, and most preferably at least 0 170°C. It is preferably no more than 260°C, more preferably no more than 230°C and most preferably no more than 200°C. The dope film or sheet should be thick enough that it can be stretched as desired without leaving flaws after coagulation. The optimum thickness varies considerably depending upon the desired thickness of the final product and the desired stretch from the second step of the process. For most purposes, the dope film or sheet is preferably at least 1 mil thick, more preferably at least 5 mil thick and most preferably at least 25 mil thick. The dope film or sheet is preferably no more than 250 mil thick and more preferably at most 100 mil thick. (Ordinarily, the term "film" refers to an article no more than 10-15 mil thick, and the term "sheet" refers to an article at least 10-15 mil thick. In the interest of brevity, this Application shall use the term "film" to refer to both film and sheet.) It may optionally be desirable to extrude the dope film between two layers of a stretchable polymer film which is inert with respect to the dope under process conditions. Examples of a suitable polymer film include Teflon™ fluorocarbon film and amorphous polyester film. The outer layers protect the dope from the atmosphere and prevent it from adhering itself or other objects if the dope is stored after extrusion. In the second step, the dope film is mechanically stretched in at least the transverse direction. A mechanical device grips the transverse edges of the dope film and pulls the transverse edges apart. The dope film may also be stretched in the machine direction and/or in any other direction. If the dope is stretched in multiple directions, then the stretching may be in a single direction at a time or may be simultaneous in two or more directions. Ordinarily, stretching increases the tensile strength and modulus of the finished film in the direction stretched, but slightly decreases those same properties perpendicular to the directions stretched. Individual sheets of dope film may be stretched in a batch fashion using commercially available equip¬ ment such as the T.M. Long™ Film Stretcher, available from T.M. Long Co. Mechanical stretching is preferably carried out on a tentering apparatus, which continuously stretches a continuous dope film. The dope film is tentered in a manner similar to known thermoplastic polymers. Ordinarily, the dope film travels continuously through a tentering device. Near the front of the device, gripping means such as clips grip the transverse edges of the dope film. As the dope film travels through the tentering device, the gripping means move further apart, drawing the transverse edges of the dope film further apart. The tentering device may have zones, in which no stretching occurs, before and/or after the stretching zone. Examples of tentering equipment and procedures are described in numerous literature references, such as Young, U.S. Patent 2,473,404 (June 14, 1949); Minich, U.S. Patent 2,334,022 (November 9, 1943); Milne, U.S. Patent 2,618,012 (November 18, 1952); Tomlinson, U.S. Patent 3,571,846 (March 23, 1971); Levy et al., U.S. Patent 4,104,769 (August 8, 1978) and Kwack, U.S. Patent 4,862,564 (September 5, 1989). Tentering devices are also commercially available from sources such as Marshall & Williams Co., Bruckner Machinery Corp., and Crown Products. The materials that come in contact with dope should be inert with respect to the dope under process conditions. Some tentering machines are known which can simultaneously stretch film in the machine direction and the transverse direction. Alternatively, equipment that stretches film in the machine direction, such as a series of rollers moving at different speeds, may be placed on the film line before or after the tentering apparatus. Stretching transversely improves the transverse tensile properties of the film after the dope is coagulated. The transverse tensile strength and modulus increase with greater stretch, and the machine direction tensile strength and modulus decline slightly with greater transverse stretch (except when the dope is also stretched in the machine direction) . The extruded film is preferably stretched to at least 1.5 times its original transverse width, more preferably at least 2 times, highly preferably at least 3 times, and more highly preferably at least 4 times and most preferably at least 5 times. The same preferred stretch applies to stretching in the machine direction. The maximum stretch is limited by the strength of the dope film or sheet. In most cases, it will be no more than about 50 times the original width of the dope film. Stretching should be carried out at a temperature and at a rate at which the dope film can stretch without tearing. Liquid crystalline dope films containing polymer dissolved in polyphosphoric acid may exhibit a phase change similar to a glass-transition temperature. The temperature of stretching is preferably not below 5°C to 10°C below the glass- transition temperature, and is more preferably at least the glass transition temperature. The maximum tempera¬ ture of stretching should be low enough that the dope film is stable. It is preferably low enough to minimize sagging of the dope film during stretching. The temperature of stretching is preferably no more than 105°C above the "glass-transition temperature" of the dope, more preferably no more than 55°C above and most preferably no more than 5°C above. For liquid crystalline dopes containing poly¬ phosphoric acid (82 percent to 83 percent P2O5) and 14 weight percent rigid rod polybenzoxazole or polybenzothiazole, the temperature for tentering is preferably at least 20°C, more preferably at least 50°C and most preferably at least 75°C. It is preferably at most 175°C, more preferably at most 150°C and most preferably at most 125°C. The optimal rate of stretching varies widely depending upon a number of factors such as polymer structure, molecular weight and concentration, solvent acid, and dope temperature. It can best be determined by experiment. In the third step, the stretched dope film is coagulated to yield a polybenzazole film. Ordinarily the stretched dope film is coagulated by contacting the film with a liquid that is a nonsolvent for the polymer and a diluent for the solvent. The coagulating liquid may be organic, such as methanol, but it is preferably aqueous. The aqueous coagulant may be basic or slightly acidic, but is preferably neutral. If the dope was extruded and stretched between water-impermeable polymer face films, then the films should be stripped off of at least one side before coagulating. If the dope was extruded and tentered between water-permeable films, then the entire structure may be immersed to initially coagulate the film, although at least one face sheet is preferably stripped off after the initial coagulation in order to facilitate washing out residual solvent. If the solvent contains a volatile component, such as methanesulfonic acid, then the polymer may optionally be concentrated by evaporating the solvent. Preferably, the volatile component is not stripped off to the point that the polymer coagulates. Instead, the concentrated dope is still preferably contacted with a nonsolvent to coagulate the polymer. After coagulation the polybenzazole film is preferably washed for a period of time to substantially remove residual solvent. It is preferably dried under restraint to prevent it from curling or shrinking. The coagulated film may optionally be densified by heat treatment. The heat treatment is preferably under pressure. It is preferably at a temperature of at least 150°C, and more preferably at a temperature of at least 250°C. The previously described steps may be integrated into a single continuous process in which the dope is extruded to form an extruded film, the extruded film passes into a tentering machine, and the tentered film is coagulated within the tentering machine or afterwards. The continuous process might optionally further contain an apparatus to stretch the dope film in the machine direction before or after tentering. Alternatively, one or more of the steps may be broken apart from the others. For instance, the extruded film may be taken up on a roll and stored until ready for tentering. The tentered film may be stored or further processed before coagulation. Two or more layers of tentered film may be pressed together to form a thicker sheet before coagulating. Reinforcing fibers may be pressed into a tentered film before coagulation to form a prepreg. A tentered film may be placed over a mold or form before coagulation. The film recovered from the present process contains polybenzazole polymer as previously described, and preferably consists essentially of polybenzazole polymer. It may be very thin, for instance suitable for membrane purposes, or thicker to be suitable for structural purposes. It is preferably at least 0.1 mil thick, and more preferably at least 0.2 mil thick. It preferably has an average thickness variation of no more than 5 percent, more preferably no more than 0.5 percent. The optimal properties of the finished polybenzazole film will vary depending upon its intended use. The average tensile strength of the polymer film in the transverse direction is preferably at least 35 Ksi (240 Mpa), more preferablyat least 50 Ksi (345 Mpa), " more highly preferably at least 70 Ksi (480 MPa), and most preferably at least 100 Ksi (690 MPa). The average tensile strength of the polymer film in the machine direction has similar preferred embodiments in a relatively balanced film. The average tensile modulus of a relatively balanced film in the machine and transverse directions is preferably at least about 3 Msi (21 GPa), more preferably at least about 5 Msi (34 GPa) and most preferably at least about 7 MPa (48 GPa). Films have been made having even higher moduli in either the transverse or machine direction, but not in both. One reason that mechanical stretching is advantageous is that the degree of stretch in each direction may easily be selected. For instance, the film may be stretched in the transverse direction with essentially no stretch in the machine direction. Alternatively, the film may be stretched in the machine and transverse directions in almost any ratio that the dope and equipment will permit, from essentially equal to much greater stretch in the machine or transverse directions. Because mechanical stretching permits a wide variety of stretching, the films made using mechanical stretching may have a wide variety of tensile properties. The tensile strength and/or tensile modulus of the film may be at least as high in the transverse direction as it is in the machine direction. For instance, the average tensile strength and/or tensile modulus in the machine and transverse directions of a film with balanced properties preferably differ by no more than 10 percent, and more preferably differ by no more than 5 percent. Alternatively, the tensile strength or modulus may be substantially higher in either the machine direction or the transverse direction in a film with unbalanced properties. Films made using a slit die and the current process are structurally different from films made using rotating anular dies and blown bubble in the prior art process. . As previously explained, films that are made by extruding a tube from a rotating or counter-rotating die and blowing the tube contain discrete layers of nematic polymer that are oriented in a uniform angle of either + or - θ with respect to the machine direction of the polymer. Those discrete layers are theroized to give the film more biaxial tensile properties. However, the laters also make the film more susceptible to delamination. See Lusignea, "Film Processing and Applications for Rigid- Rod Polymers", The Materials Science and Engineering of Rigid-Rod Polymers at 256 (Materials Research Society 1989). For instance, films made by the processes disclosed in U.S. Patents 4,898,924; 4,939,235; 4,963,428; and 4,966,806 contain polymer oriented at an angle +θ on one face, polymer oriented an angle -θ on the other face, and a middle layer in which the polymer is substantially non-oriented. Films which are extruded through a slit die and mechanically stretched according to the preferred process of this invention have biaxial tensile properties that are equivalent to the properties realized by the prior art. However, the polymer of films of the present invention is not oriented in a uniform preferred angle of + or -θ. Some areas of the film do not contain a plurality of discrete layers, but are more isotropic in the z direction. This distinction may be advantageous in minimizing curling and/or delamination of the film with changing temperature. The advantage in delamination resistance tends to be greater when the dope film is stretched less and lesser when the dope film is stretched more. The preferred films of the present invention are anisotropic in the transverse and machine directions, unlike films that are coagulated from an isotropic dope. The preferred films are also stronger than typical films coagulated from isotropic dopes. 5 The films are useful as coatings or structural materials or membranes are electronic substrates. The process of the present invention is more 10 specifically illustrated in the following Examples. Illustrative Examples The following examples are given to illustrate _ j c the invention and should not be interpreted as limiting the Specification or the Claims. Unless stated otherwise, all parts and percentages are given by weight. 0 Example 1 - Batch Mechanical Stretching A solution of polyphosphoric acid (85 weight percent P2O5) containing 14 weight percent cis- -polybenzoxazole (about 25-40 intrinsic viscosity in 5 methanesulfonic acid at 25°C) is extruded through a 6-inch wide slit die with a 0.005 inch gap using a ram extruder. The barrel temperature in the extruder is 180°C and the die temperature is 160°C. The speed of 0 extrusion is 6.5 cm3/min. The dope film is taken up between two films of skived polytetrafluoroethylene (PTFE) that are 0.003 inch thick. The resulting product is a sandwich containing 0.006 to 0.007 inch of PBO between the PTFE films. Several 4-3/8 inch by 4-3/8 inch samples are cut from the film. The samples are placed one at a time in a T.M. Long mechanical stretcher having an air temperature of about 140°C and a plate temperature of about 125°C. The samples are held for 30 seconds to equilibrate temperature, and then are stretched in the transverse direction or the machine direction or both at a rate of 2 inch/sec. until a desired stretch ratio is achieved. The samples are cooled with an air gun and recovered. The PTFE films are stripped off of each side of each sample. Each dope film is clamped on a steel hoop and immersed in deionized water for 48 hours. Each sample is then dried on the hoop at 300°C for 1.5 hour. Dog-bone shaped test samples are cut from each sample and tested for tensile strength and tensile modulus in the transverse direction. The test method is set out in ASTM D 882-83. The tensile measurements are made using an Instron™ tensile tester. The results are set out in Table 1. The batch stretching is recognized as a good bench-scale approximation of performance in a continuous tentering apparatus. Transverse Stretch Ratio 2.0:1 2.5:1 3.0:1 3.5:1 Example 2 - Batch Mechanical Stretching of PBO Film A dope containing 14 weight percent rigid rod cis-polybenzoxazole (I.V. 20 to 40 dL/g) and polyphos¬ phoric acid (About 82 to 84 percent P2O5) is extruded through a six-inch slit die with a barrel temperature of 180°C and a die temperature of 160°C. The film crosses and air gap. Each dope film is sandwiched between two face sheets of 15 mil amorphous polyester (PETG) (K0DAR™ 6763 film). The multilayer structure is taken up upon a roll at a variable speed to provide variable draw upon film across the air gap. The conditions for extrusion are set out in Table 2(A) . The dope film is cut into samples of about 4 inches by about 4 inches Each sample is stretched either unixially or biaxially on a T.M. Long™ film stretcher at a rate of 2 inches/sec with a plate temperature of about 105°C and an air temperature of about 90°C. The stretching ratio in the machine and transverse directions are set out in Table 2(A) and Table 2(B). For instance, 1 by 2 means that the sample was stretched to 2 times its original width in the transverse direction and was not stretched in the machine direction, whereas 3 by 3 means that the sample was stretched to 3 times its original width in the transverse direction and to 3 times its original length in the machine direction. Stretched samples are cooled and coagulated. The samples are coagulated by immersing the entire multilayer structure for about 4 hours, then stripping off the face sheets and immersing in water for 24 hours. Dog bone shape test samples are cut in both the transverse and machine directions and tested for tensile strength and modulus as described in Example 1. The results are set out in Table 2(B). 10 2120 650 0.010 12.15 4 by 4® ® - stretch in machine and transverse directions simultaneously © - stretch in machine and transverse directions sequentially Sample Stretch Ratio l l by 2 2 2 by 2® 3 2 by 2® 4 1 by 3 5 1 by 3 i 6 1 by 3 7 3 by 3® 8 2 by 3.5® 9 3 by 5® ! 10 4 by 4® ® - stretch in machine and transverse directions simultaneously <D - stretch in machine and transverse directions sequentially 0 Example 3 - Tentering of PBO film A solution containing 14 weight percent polybenzoxazole in polyphosphoric acid (84 percent P2O5) 5 is extruded through a 6-inch slit die with a 0.020-inch gap. The extruder temperature is 160°C, and the die temperature is 180°C. The extrusion speed is 9.4 cm3/min. The extruded dope travels across an air gap and is taken up between two 0.015 in sheets of amorphous 0 polyester film to form a multilayered structure. The multilayered structure is tentered at a temperature of 85°C to 87°C and a rate of 5 ft./min. to stretch it 4 times in the transverse direction. The stretched film is coagulated and dried as described in Examples 1 and 2. It is heated at 300°C for 1-1/2 hours before removing from the drying hoops. The tensile properties are tested as described in Example 1. The tensile strength of the film is about 42.6 Ksi in the machine direction and about 69.9 Ksi in the transverse direction. The tensile modulus of the film is about 2.03 Msi in the machine direction and about 4.16 Msi in the transverse direction. Example 4 - Tentering of PBO film The procedure of Example 3 is followed except as follows: (1) The extruder temperature is about 160°C and the extrusion speed is about 10.2 cm3/min; (2) The film is drawn to three times its original length in the machine direction before tentering by drawing between a slow draw roller moving at 4 ft./ in. and a fast draw roller moving at 12 ft./min. at a temperature of about 90°C; and (3) The resulting film is tentered in the transverse direction to 5 times its width at the completion of the machine direction stretch. The tensile strength of the film is about 55.7 Ksi in the machine direction and about 65.1 Ksi in the transverse direction. The tensile modulus of the film is about 3-71 Msi in the machine direction and about 4.36 Msi in the transverse direction. Example 5 - PBO Film A solution containing 14 weight percent polybenzoxazole in polyphosphoric acid (83 to 85 percent P2O5) is extruded through a 6-inch slit die with a 0.030-inch gap. The extruder temperature is 160°C, and the die temperature is 160°C. The extrusion speed is 10.2 cm3/min. The extruded dope travels across an air gap and is taken up between two 0.003 inch thick sheets of skived PTFE to form a multilayered structure. The multilayered structure is cut into 3.5 inches by 3-5 inches squares. The PTFE sheets are removed an each dope film sample is stretched simultaneously in the machine and tranverse directions. The stretch ratio, temperature and speed are shown in Table 3 using an Iwamoto biaxial stretcher Model No. BIX-703. Each sample is heated for 90 seconds to the proper temperature before stretching and is quenched in water at room temperature after stretching. The samples are clamped on 16 inch diameter steel hoops, immersed in deionized water for at least hours, dried at 95°C for 4 hours, and heated at 300°C for 1.5 hours. TABLE 3(A) Stretch Stretch Polymer Sample Stretch Film Ratio Temp Speed (°C) (mm/sec) Thickness (mm) 11 4 by 4 101 40 690-760 12 4 by 4 139 60 580-710 13 4 by 4 135 20 690-760 14 5 by 5 135 20 610-660 15 5 by 5 135 20 660-810 The tensile properties of the films are measired and set out in Table 3(B).

Claims: Claims : 1. A process for making a polybenzazole film or sheet comprising that has the steps of: (1) extruding a dope, that contains a solvent and a polybenzazole polymer, to form a dope film or sheet; (2) stretching the dope film or sheet in the transverse direction at a temperature and a rate at which the dope film or sheet does not tear; and 0 (3) coagulating the stretched dope film or sheet, whereby a polymer film or sheet is formed, characterized in that the dope film is stretched mechanically using a device that grips the transverse 5 edges of the dope film or sheet and draws the transverse edges apart from each other. 2. A process as described in Claim 1 wherein the dope is extruded in Step 1 through a die that does 0 not apply transverse shear forces to the faces of the dope film. 3. A process as described in any of the r preceeding claims wherein the polybenzazole polymer is a lyotropic liquid-crystalline polymer in a concentration in the dope suitable to form liquid crystalline domains. 4. A process as described in any of the preceeding Claims wherein the dope film is stretched in r - the transverse direction to at least three times its original width. 5. A process as described in any of the preceeding claims wherein the dope film is stretched to 10 at least three times its original length in the machine direction. 6. A process as described in any of the preceeding claims wherein the dope film is stretched in 5 Step (2) using a tentering equipment. 7. A process as described in any of the preceeding claims wherein the solvent in the dope is polyphosphoric acid. 8. A film or sheet that contains a lyotropic liquid crystalline polybenzazole polymer and that has a tensile strength of at least 241 MPa (35 Ksi) in both 5 the machine direction and the transverse direction, characterized in that the crystalline domains on the faces of the film or sheet are not oriented in opposite uniform angles of + or - θ with respect to the machine direction. 0 9. The invention as described in any on the preceeding claims wherein the polybenzazole polymer is a lyotropic liquid crystalline polybenzoxazole or polybenzothiazole polymer. 10. The invention as described in any of the preceeding claims wherein the polybenzazole polymer contains mer units represented by any of the Formulae: (e) AMENDED CLAIMS [received by the International Bureau on 11 May 1992 (11.05.92); original claims 1 and 2 amended; other claims unchanged (1 page )] 1. A process for making a polybenzazole film or sheet comprising that has the steps of: (1) extruding a dope, that contains a solvent and a polybenzazole polymer, to form a dope film or sheet; (2) stretching the dope film or sheet in the transverse direction at a temperature and a rate at which the dope film or sheet does not tear; and (3) coagulating the stretched dope film or 0 sheet, whereby a polymer film or sheet is formed, characterized in that the dope is extruded in Step ( 1) through a die that does not apply transverse shear forces to the faces of the dope; and in that the dope 5 film is stretched mechanically in Step (2) using a device that grips the transverse edges of the dope film or sheet and draws the transverse edges apart from each other. 0 2. A process as described in Claim 1 wherein the dope is extruded in Step (1) through a slit die. 3. A process as described in any of the c preceeding claims wherein the polybenzazole polymer is a STATEMENT UNDER ARTICLE19 In response to the International Search Report dated March 17, 1992, the Applicant submits the attached substitute pages, containing amendments to Claims 1 and 2. The amendment to Claim 1 adds to the claim that dope is extruded through a die that does not apply transverse shear forces to the faces of the dope. The amendment to Claim 2 removes a similar limitation from that Claim. The amendment is supported in the original Specification on pages 10 and 11.

Inventors: DOLLINGER SUSAN E, KIM PETER K, PIERINI PETER E, VERMEULEN ROBBERT M, WESSLING RITCHIE A, DOLLINGER, SUSAN, E., KIM, PETER, K., PIERINI, PETER, E., VERMEULEN, ROBBERT, M., WESSLING, RITCHIE, A.
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1991
Main classification: C08G75
IPC codes: B29C55, B29D7, B29K79, B29K81, B29L7, C08G73, C08G75, C08J3, C08J5