Multiaxially reinforced liquid crystal polymer sheet

Abstract: A melt consolidated, multiaxially oriented liquid crystal polymer ("LCP") sheet or other article formed from LCP particles. One embodiment includes a multi-component LCP sheet in which particles of a higher melting LCP are imbedded in a matrix of a lower melting LCP. These LCP components are formed into sheet at a temperature between the melting point of the lower melting matrix LCP and the higher melting reinforcing LCP, so that the imbedded LCP particles may maintain their shape, orientation, and mechanical characteristics while the matrix LCP component flows around the particles and forms a solid sheet. In another embodiment, LCP particles are melt consolidated to form a single article in which a significant amount of LCP orientation is retained. A sheet of this invention may be formed by a variety of processes, and may be made from recycled scrap LCP.

Description: Background Of The Invention This invention relates to the field of polymer sheet or film, particularly thermotropic liquid crystalline polymer ("LCP") sheet having one or more LCP components. A variety of thermotropic liquid crystal polymers and films made therefrom are known in the art. For example, U.S.-A-4,161,470 discloses a polyester of 6-hydroxy-2-naphthoic acid and p-hydroxy benzoic acid capable of undergoing melt processing, U.S.-A-4,184,996 discloses melt processible thermotropic wholly aromatic polyesters, and U.S.-A-4,279,803 discloses a polyester of phenyl-4-hydroxybenzoic and 4-hydroxybenzoic acid and/or 6-hydroxy-2-naphthoic acid. Other patents disclosing LCPs include U.S.-A-3,991,013, 3,991,014, 4,057,597, 4,066,620, 4,067,852, 4,075,262, 4,083,829, 4,093,595, 4,118,372, 4,130,545, 4,219,461, 4,267,289, 4,276,397, 4,330,457, 4,339,375, 4,341,688, 4,351,917, 4,351,918, 4,355,132, 4,355,133, 4,371,660, 4,375,530, 4,460,735, 4,460,736, 4,473,682, 4,489,190, 4,581,443, 4,671,969, 4,673,591, 4,726,998, 4,752,643, 4,770,777, 4,772,421, 4,857,255, 4,898,924, and 4,913,867. Due to the particular molecular structure of thermotropic liquid crystal polymers, LCP film can be molecularly oriented in the melt phase, uniaxially, biaxially, or otherwise. After the extruded LCP cools and solidifies, the molecular orientation is maintained. When the orientation is uniaxial, this results in a film having anisotropic properties, such as enhanced mechanical properties in the direction(s) of orientation (e.g. high strength and stiffness). However, the film's properties in other directions are often much poorer. In producing LCP film/sheet, molded articles, and fibers, there is a certain amount of scrap LCP left over after processing. Due to the expensive nature of this material, and in keeping with modern environmental theory, it is desirable to recycle this material. Recycling scrap is especially difficult if the scrap contains a mixture of materials. For example, in processes for producing the coextruded LCP film described in US-A-5,248,530, one would expect the scrap to contain at least two liquid crystal polymer components. Summary Of The Invention In general, the present invention comprises a multiaxially reinforced liquid crystal polymer sheet comprising oriented liquid crystal polymer ("LCP") particles wherein the direction of orientation of the particles varies randomly, producing an isotropic LCP sheet. In detail, the present invention relates to a multiaxially reinforced liquid crystal polymer sheet comprising: (a) particles comprising a first anisotropically oriented liquid crystal polymer; and (b) a polymeric matrix comprising a second liquid crystal polymer; wherein said second liquid crystal polymer has a lower melting point than said first liquid crystal polymer; wherein said particles are randomly distributed and randomly oriented in said matrix. These LCP components are formed into said sheet at a temperature between the melting point of the lower melting matrix LCP and the melting point of the higher melting reinforcing LCP, so that the higher melting LCP particles may maintain their shape, orientation, and mechanical characteristics while the matrix LCP component flows around the particles and forms a solid sheet. The imbedded particles tend to reinforce the matrix polymer. The LCP starting materials may comprise particles derived from a coextruded film or fiber containing both the higher and lower melting liquid crystal polymers, or a blend of lower-melting LCP particles and higher-melting LCP particles. These particles may include chopped film, chopped fiber, flake, and the like. A sheet according to this aspect of the invention may be formed by a variety of processes, provided that the process used is performed at a temperature between the melting points of the LCP components so that one component melts to form the matrix and another component remains in particulate form. Such processes include: melt extrusion through a die; consolidation of a layer of scrap LCP particles under heat and pressure, e.g., in a press; and, fluxing the scrap material in a roll mill, extruder, or mixer followed by calendering. In detail, the present invention also relates to a process for preparing the sheet which process comprises preparing a mixture comprising said particles and said second liquid crystal polymer; heating said mixture to a temperature at least as high as the melting point of said second polymer, but not as high as the melting point of said first polymer, so that said second polymer melts but said first polymer remains solid; and, forming said heated mixture into said sheet. It is an object of the present invention to recycle scrap LCP into a useful form. Other objects and advantages of the present invention will be apparent to those skilled in the art from the following description and the appended claims. Detailed Description Of The Preferred Embodiments In a preferred embodiment of the present invention, LCP film comprising two coextruded LCP components having different melting points is ground into particles, e.g. flakes, and these particles are processed at a temperature between the two melting points to make a sheet according to the present invention. (The equivalent terms "sheet" and "film" are used interchangeably throughout this specification.) By processing the particles at such a temperature, the lower melting LCP will melt and flow to form a matrix, but the higher melting LCP particles will maintain their structural integrity, including any orientation or anisotropy. The resulting sheet of this invention comprises particles of higher melting LCP imbedded in a matrix of lower melting LCP. The imbedded particles are randomly distributed and oriented in the matrix, and serve to provide multiaxial reinforcement for the matrix material. The LCP sheet of this invention may include more than two components. For example, if three LCP components are present there may be two types of imbedded particles in a lower melting LCP matrix, or one higher melting LCP may be imbedded in a blend of two lower melting components, depending upon the LCP melting points and the processing temperature chosen. More than three components are also possible. One or more non-LCP components may also be included. Virtually any LCP may be included as a component in the sheet of the present invention. Although a higher melting LCP that is to be imbedded in a matrix material must be used in the form of particles, e.g., flakes, fibrils, chopped strand, and the like, the polymer matrix may be made from any form of material since it is to be melted. In one variation of the preferred embodiment described above, additional neat matrix resin is added to the recycled scrap material to dilute the higher melting component, or the matrix resin may be entirely non-recycled material, i.e. neat resin, which is mixed with particles of a higher melting LCP in any desired ratio. In this way, the proportion of the components may be adjusted as desired. The reinforced multiaxially oriented LCP sheet of the present invention exhibits balanced physical properties and coefficient of thermal expansion due to the isotropic distribution of the imbedded particles, even if the matrix material is anisotropic. The sheet has the good physical and chemical properties expected of an LCP, such as chemical resistance, good tensile strength, and stiffness. Examples of liquid crystal polymers useful in the practice of this invention include the VECTRA® polyester resins available from Hoechst Celanese Corporation, having its headquarters in Bridgewater, New Jersey. VECTRA® A polyester (melting point about 280°C) comprises 73 mole % of repeating monomer units derived from 4-hydroxybenzoic acid ("HBA") and 27 mole % of monomer units derived from 6-hydroxy-2-naphthoic acid ("HNA"). VECTRA® E polyester (m. p. about 350°C) comprises 60 mole % of repeating units derived from HBA, 4 mole % derived from HNA, 18 mole % derived from terephthalic acid ("TA") and 18 mole % derived from biphenol. VECTRA® C polyester (m. p. about 320°C) comprises 80 mole % of repeating units derived from HBA and 20 mole % derived from HNA. VECTRA® B polyester comprises 60 mole % of monomer units derived from HNA, 20 mole % derived from TA, and 20 mole % derived from acetaminophen. There are several different grades of each type of VECTRA; the grades vary in their molecular weight distributions. The melting points stated for the several types of VECTRA may vary, depending upon the specific grade of polymer. Other liquid crystal polymers may also be used in the practice of this invention; for example, XYDAR® polyester (a polymer product comprising monomers units derived from HBA, TA and 4,4'-biphenol sold by Amoco Corp.). The reinforced LCP sheet of this invention may be made by a variety of methods. In one preferred process, LCP flakes comprising at least two LCP components having different melting points are spread in a layer and hot pressed into a sheet at a temperature between the melting points of the two polymers. In another preferred process, the LCP components are mixed in an extruder and extruded through a sheet die at a temperature between the melting points of the two polymers; alternatively, the polymers may be calendered into a sheet at a temperature between the melting points of the two polymers, rather than extruded through a die. Where the hot calendering process is employed, the polymers may be fluxed in a roll mill, mixer, extruder, or any suitable device known in the art. The sheet of this invention may also preferably be made using an isobaric double belt press to uniformly apply heat and pressure. The belts can have a non-stick surface, or a release film may be fed into the press on each side of the LCP. The LCP components may be formed into a sheet according to this invention by any sheet-forming process known in the art provided that the process causes the lower melting component, but not the higher melting component, to melt, and does not cause undesirable chemical reactions or significant degradation of the polymers. Although not necessary in the practice of the present invention, post-processing steps may be performed if desired. For example, after the reinforced sheet has been formed it optionally may be calendered, stretched, coated or laminated with other materials, and so forth. As long as the imbedded LCP particles are not melted or separated from the matrix by post-processing, the sheet will retain its properties. Although the invention has been described in terms of sheet, the same multiaxially reinforced material can easily be used to make molded articles. The LCP components can be pressed into molded articles of almost any shape at a temperature between the melting points of the polymers. As with sheet, additional LCP or non-LCP components may be included. The sheets of this invention may be laminated to other materials such as metals (e.g. copper, aluminum, silver, etc.) and other plastics to provide a laminate having an isotropic LCP layer. These laminates may be particularly useful in the electronic or electrical area as dielectric layers in circuit boards and other devices; in these applications, the balanced coefficient of thermal expansion of the sheet is especially important. LCP sheet at least 127 µm (5 mils) thick having balanced properties is especially useful in rigid circuit boards; these films are difficult to produce using conventional orientation processes, e.g., blown film processes, but can be produced without great difficulty according to the present invention. The sheets of the present invention also can be used for any other applications where an LCP sheet having balanced properties is needed, whether as part of a laminate or alone. Although all the potential uses of this invention cannot be listed here, the properties described herein will lead those skilled in the arts to find many other uses for the sheet of this invention. The following Example is presented to illustrate the present invention, but the scope of this invention is not limited to this Example. EXAMPLE A 254 µm (10-mil) thick coextruded LCP film comprising VECTRA® A polyester and VECTRA® C polyester was chopped into pieces approximately 1 cm square and processed according to the process used in Example I, except that the press temperature was 300°C. VECTRA® A polyester melts at about 280°C and VECTRA® C polyester melts at about 320°C. By this method, a consolidated sheet about 1.016 mm (40 mils) thick was obtained; unmelted VECTRA® C particles were clearly visible in the VECTRA® A matrix.

Claims: A multiaxially reinforced liquid crystal polymer sheet comprising: (a) particles comprising a first anisotropically oriented liquid crystal polymer; and (b) a polymeric matrix comprising a second liquid crystal polymer; wherein said second liquid crystal polymer has a lower melting point than said first liquid crystal polymer; wherein said particles are randomly distributed and randomly oriented in said matrix. The sheet according to claim 1 wherein at least one of said liquid crystal polymers is a polyester having repeating units derived from 4-hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid. The sheet according to claim 1 wherein said first liquid crystal polymer is a polyester in which a majority of the repeating units are derived from 4-hydroxybenzoic acid, or 6-hydroxy-2-naphthoic acid,or a combination thereof. The sheet according to claim 1 wherein said second liquid crystal polymer is a polyester in which a majority of the repeating units are derived from 4-hydroxybenzoic acid, or 6-hydroxy-2-naphthoic acid,or a combination thereof. A metal laminate comprising the sheet according to any one of claims 1 to 4 bonded to a metal sheet. A metal laminate according to claim 5 wherein said metal sheet comprises copper, aluminum, or silver. A process for forming the sheet set forth in any one of claims 1 to 4 comprising: preparing a mixture comprising said particles and said second liquid crystal polymer; heating said mixture to a temperature at least as high as the melting point of said second polymer, but not as high as the melting point of said first polymer, so that said second polymer melts but said first polymer remains solid; and, forming said heated mixture into said sheet. A process according to claim 7 wherein said preparing step comprises coextruding an oriented film or fiber comprising both said first liquid crystal polymer and said second liquid crystal polymer; and subsequently chopping or grinding said film or fiber into particles. A process according to claim 7 wherein said preparing step comprises chopping or grinding an article comprising said first liquid crystal polymer to form said particles, and mixing said particles with said second liquid crystal polymer to form said mixture. A process according to claim 7 wherein said forming step comprises extruding said heated mixture through a die to form said sheet. A process according to claim 7 wherein said heating and said forming are accomplished in one step by pressing or calendering said mixture at said temperature to form said sheet.

Inventors: JESTER RANDY D, JESTER, RANDY D.
Applicants: HOECHST CELANESE CORP, HOECHST CELANESE CORPORATION
Application year: 1993
Main classification: C08L67
IPC codes: B32B27, B32B15, C08L101, C08L67, C08J11, B29D7, C09K19, H05K1, C08J5, B29C55