THERMOPLASTIC MOLDABLE COMPOSITE SHEET CONTAINING HOLLOW MICROSPHERES

Abstract: Molded articles showing substantially reduced rib read through are provided from preform layers of high modulus fiber reinforced composites wherein an intimate mixture of long discrete reinforcing fibers and hollow microspheres are dispersed in a thermoplastic resin matrix.

Description: "THERMOPLASTIC MOLDABLE COMPOSITE SHEET CONTAINING HOLLOW MICROSPHERES' FIELD OF THE INVENTION The present invention concerns fully densified composite articles made by compression molding of materials consisting of a thermoplastic matrix, high modulus reinforcing fibers, and hollow microspheres, and a process for making the same. BACKGROUND OF THE INVENTION Compression molding of high modulus (typically glass) fiber reinforced resin sheet or bulk molding compound materials is routinely used in the automotive and aerospace industry, for example, for fabrication of structural and semistructural articles. High modulus generally pertains to fibers having tensile strengths of greater than 3000 mpa and tensile modulus of 80 gpa. EP Application 0341 977, published Nov. 15, 1989 discloses a thermoplastic preform sheet material consisting of high modulus reinforcing fibers and thermoplastic resin globules which can be used for compression molding of long fiber reinforced thermoplastic parts. Molded articles intended for automotive end use must show a combination of useful properties to satisfy end use needs. These properties include stiffness, good surface after painting, impact resistance, and many other properties. The ability to fabricate parts having reduced density, while retaining other needed property characteristics of structural and semistructural composites, is a highly desirable goal, since this often leads to fuel saving and other advantages in use. One particular need in the engineered thermoplastics molding industry is a means to form a molded panel with ribs, having no "rib read-through." Hollow microspheres, or micro-balloons made from a variety of glass, ceramics, and carbon materials are well known in the plastics industry today, and are sold commercially for incorporation into engineering resins. I lollow microspheres provide the benefits of conventional solid spheres while significantly lowering the weight of finished engineering grade plastic compounds and molded parts. Generally hollow microspheres are sold as a distribution of particle sizes typically in the range of from about 1 to about 100 microns in diameter. Potters Industries Inc of Parsippany, N. J. and 3M Corporation of St. Paul, MN are example of producers of commercial grade microspheres. The use of hollow glass microspheres as a component of molding compounds consisting of glass reinforcement and thermosetting resin is describing in U.S. Patent 5,134,016. Thermosetting resins generally have a low viscosity and conventional compounding means are suitable for achieving a good distribution of microspheres in the final molded part. The higher melt viscosity and molding pressures of many engineering thermoplastic resins, however, require more intensive shear for achieving good mixing. Conventional means for introducing microspheres into engineered thermoplastics includes melt compounding in an extruder. While this is a useful technique, it has several limitations. For instance, the pressure required for injection molding often exceeds 10,000 psi, thereby requiring the use of more dense, heavy walled microspheres. Second, conventional screw melt compounding to pellets, following by injection molding of parts, is generally limited to blends with very short reinforcing fibers due to attrition during blending and breakup during injection cycle. It would be highly desirable to find a technique for creating long fiber reinforced thermoplastic parts containing uniformly distributed microspheres which could be flow molded at reduced pressure, and would retain long reinforcing fiber length during processing to the molded part. It would also be highly desirable to have a molding compound and technique for molding parts with integral molded ribs showing minimal rib read-through to the opposite part surface. SUMMARY OF THE INVENTION The present invention relates to products and a process which provide reduced density in a fully densified compression molded article having a thermoplastic matrix in combination with long reinforcing fibers. The invention also provides for a very uniform distribution of hollow glass microspheres which can be processed at moderate molding pressures, minimizing attrition of the microspheres and long fibers. The invention also provides for a method for rapid cycle molding of panel articles having integral ribs molded on one side which minimize rib "read through." The invention relates to molded articles formed from preform layers of high modulus fiber reinforced composites where the reinforcing fibers have a length of from about 0.5 cm to about 8 cm, and diameters from about 5 to 50 microns. An intimate mixture of the long discrete reinforcing fibers, hollow microspheres and thermoplastic matrix resin is first achieved by dispersing the components in an aqueous slurry. The chopped reinforcing fibers consist of bundles of numerous aligned reinforcing fibers adhered with a sizing agent, with uncrimped thermoplastic resin staple fibers of fine denier and short cut length from about 1 mm to 5 cm. The hollow glass microspheres have a diameter distribution from 1 - 100 microns, with an average size in the range of 30 to 70 microns, and a bulk density of 0.2 - 0.7 g/cc having a crush strength of at least 500 psi. The thermoplastic fibers may be pretreated with a finishing agent to aid their dispersion in water, as may the reinforcing fibers. Uniquely, the slurry blend of reinforcing fibers and resin fibers provides a means to capture the fine glass microspheres during processing into sheet form on paper making equipment. Alternatively, the reinforcing fibers and thermoplastic resin fibers can be formed into a sheet, followed by spray application of microspheres in such a manner as to achieve a uniform blending of the fiber components and microspheres. The resulting sheet can be thermally bonded by melting all of the thermoplastic fibers, which binds the reinforcing fibers and microspheres together in a bonded porous sheet form consisting of randomly dispersed, in-plane, reinforcing fiber and hollow microspheres bonding together with films and globules of thermoplastic resin. Layers of the sheet have the characteristic of being very uniform in the distribution of microspheres within the reinforcing fiber network, and being porous enough that the sheet can be quickly reheated by passing hot air through it. Sheets of this material can be cut to size and stacked and reheated, transferred to a compression mold and pressed into fully consolidated composite articles having a density close to that calculated from the individual components, and showing excellent surface properties. Parts having ribs molded with microspheres show much reduced rib "read through" compared to parts without microspheres. BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 and 1 A are schematic illustrations of one embodiment of a wet laying process for producing the products of this invention; FIG. 2 is a schematic illustration of an alternative embodiment for producing the products of this invention; FIG. 3 is a schematic illustration of an alternative embodiment for producing the products of this invention. DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 1 the wet laying process used in making products of this invention utilizes paper making equipment and generally includes a highly agitated mix tank 10, an agitated supply tank 12, a head box 14 of an inclined wire paper machine 16, a dewatering section 17, and a windup or driven spool 20. In operation, glass and thermoplastic fibers and hollow microspheres are dispersed in water in highly agitated mix tank 10. The slurry is pulped via pump 11 from the highly agitated mix tank to an agitated supply tank 12. Feed stock from the supply tank is then pumped by means of pump 13 to the head box 14 where dilution water is added from tank 15 reducing stock consistency by a factor of 5-10. The slurry is drained through the wire in the usual manner and dewatered by passing over suction slots 18 in the dewatering section. The dewatered sheet 19 is then wound in damp form on driven spool 20. The sheet wound on the spool 20 is unwound in layers 17 and air dried as schematically shown in FIG. 1 A. In an alternate embodiment used in large scale production, FIG 2, the slurry consistency is reduced by a factor of 8-10 with additional dilution water from the dewatering box 16a and pipe 16c via pump 16b as substantially shown in FIG 2. After sheet 19 is passed oven vacuum suction slots 18, it is then dried by passing it through a flat forced air dryer 30 at a temperature capable of both drying and then melt bonding the sheet by melting the thermoplastic fiber. Sheet 19 is then passed through nip rolls 50 and then wound up on spools by windup 42. The composite preform layers of this application are formed on conventional papermaking equipment where the forming section of the machine is preferably of the type known as an inclined wire, i.e., the slurry is drained through a forming wire which is inclined at an angle (usually 15 to 45 degrees) into the forming slurry. Such equipment is specifically designed to be capable of forming fibrous mats at moderate to high slurry dilutions and is ideal for processing long fibers into nonwoven mats. However, other machine styles can be used as well, such as cylinder formers and others. Sheet drying can be accomplished via a combination of thru-air drying, contact drying or radiant heat. The maximum temperature reached by the fibrous mat must be sufficient to activate melting of the resin fibers to achieve sufficient mat strength for good processabiltiy. An alternative embodiment of the present invention is shown in Fig. 3. In operation glass and thermoplastic fibers are dispersed in water in highly agitated mix tank 10. The slurry is pumped via pump 11 from highly agitated mix tank 10 to an agitated supply tank 12. Feed stock from the agitated feed tank 17 is then pumped by means of pump 13 to the head box 14 where dilution water is added from a water tank, not shown, reducing stock consistency by a factor of 5-10. The slurry is drained through the wire in the usual manner and dewatered by passing over suction slots 18 in the dewatering section. Glass microspheres are then added to damp preform layer 19 in the following manner. A slurry suspension of microspheres in water is prepared in supply tank 60. The suspension is constantly agitated via mixing means 62. Feed supply is pumped by means of pump 64 to a spray nozzle 66 located over preform sheet 19. Return pipe 68 connects back to supply tank 60. A vacuum slot is positioned opposite .spray nozzle 46 under the supported preform sheet 19 to remove excess water from sheet 19. The preform sheet is dried by passing it through a circular air dryer 30 at a temperature sufficient to both dry and melt bond the sheet by melting the thermoplastic fiber. Sheet 19 is then passed through nip rolls 50 and wound up on spools by windup 42. The process as shown in Fig. 3 is the preferred composite preform layer process. The reinforcing fiber may be chosen from any high modulus fiber with a melting point substantially above the thermoplastic resin fiber including, but not limited to, glass fibers, carbon fiber, glass wool fibers, and aramid fibers such as Kevlar ® which is available from E.I. du Pont de Nemours and Co., etc. The reinforcing fiber may have a sizing to enhance dispersion in water and further enhancing wetting and bonding to the thermoplastic in the final composite. A typical sizing for glass consists of a film-forming agent, such as a low molecular weight polyester or polyurethane to protect the glass, and a coupling agent, usually a silane compound such as alpha-aminopropyltriethoxysilane. Typical sizes for carbon fibers are polyvinyl alcohol or polyvinylpyrolidone. The thermoplastic fibers may be chosen from the many known thermoplastic organic materials including, but not limited to, polypropylene, polyesters, co-polyesters, polyamides, polyetheretherketones, polyetherketoneketones, liquid crystal polymers, etc. Optionally, the fiber may be sized with a dispersing aid such as, but not limited to, long chain alkyl phosphates, condensation products of tallow alcohols with polyethylene oxides, and low molecular weight polyesters. The microspheres can be glass, ceramic, or carbon, and should have a density of between 0.2 and 0.7 grams per cc. and a crush strength of at least 500 psi. Generally microspheres are sold as a distribution of diameters. Any sphere combination with diameters in the range 1 - 100 microns is generally acceptable with a range of 30 to 70 mici ons being preferred. 3M Corp. and PQ Corp. make suitable microspheres. The preform layer is usually not thick enough as-produced to be used directly to make composite parts. Typically several preform layers will be sheeted and stacked to produce a thickness which is suitable for molding. Several layers of the resulting sheets can be layered together and placed in a mold and made into a plaque to evaluate physical properties by applying heat and pressure to remelt and compress the thermoplastic resin and thereafter cooling the compacted structure under pressure. To obtain useful articles at faster molding cycle times from the preform layers, one or more compound preform layers can be heated by placing the preform in a forced convection oven capable of maintaining a 2-10 inch water pressure through the preform. Hot air is passed through the preform to remelt the thermoplastic resin. The preform is quickly transferred to a male/female compression tool of the appropriate design. The tool is closed, flowing the preform and forming the part. The tool temperature is selected according to considerations of cooling rate, crystallization, and mold flow to optimize formation. The following examples in which parts and percentages are by weight unless otherwise indicated further illustrate the invention. EXAMPLE 1 The following example illustrates the process of forming a preform sheet consisting of reinforcing fiber, thermoplastic resin fiber, and hollow microspheres. The following materials are introduced into a 150 gallon total capacity Bird Pulper: a) 50 gallons of deionized water; b) 1.5 pounds 1.5 denier thermoplastic polyethylene terephthalate uncrimped staple fibers of cut length 0.25 inch are added and dispersed with a high speed agitator for 2 minutes to create a uniform fiber dispersion; c) 0.6 pounds of 1 inch wet chop 13 micron diameter glass (Owens Corning Ford type 133A) is then added to the mixture with an additional 68 gallons of deionized water gradually, and the batch is agitated for an additional 2 minutes; and d) 0.9 pound of PQ Corporation Type CG hollow microspheres are then added and the pulper agitated for an addition 10 minutes. The pulper is then pumped to a moderately agitated machine supply tank without further dilution such that the consistency is 0.72% by weight solids. The feed stock is then pumped to the headbox of a 12 inch Bruderhaus inclined wire minipaper making machine being further diluted at the headbox to a consistency of 0.07%. The head box stock is drained through the forming wire in the usual manner and dewatered by passing over suction slots. The wire speed is 5 fpm , and the resulting sheet is captured on a take up roll in damp form. The sheet is then laid out in layers and air dried to an ambient moisture level substantially as disclosed heretofore in the discussion of the apparatus of FIG. 1. Dry sheet weight is approximately 0.05 pounds per sq. ft. Microscopic cross sectional analysis shows that the wet chop glass fiber bundles are substantially dispersed capturing the hollow microspheres in a uniform blend with the resin and reinforcing fibers. EXAMPLE 2 The following formulations are prepared from PQ type CG "Extendospheres", polyester staple fiber, and glass fiber. TABLE 1 Polyester Fiber Glass Fiber Microspheres 1. 68 lbs.(30.84 kg) 40 lbs.( 18.14 kg) 34 1bs.(15.42 kg) 2. 61.2 lbs.(27.76 kg) 20 lbs.(9.07 kg) 35.1 lbs.( 15,92 kg) 3. 70.2 lbs.(31,84 kg) 20 lbs.(9.07 kg) 26 1bs.(1 1.79 kg) 4. 80.7 lbs.(36.60 kg) 40 lbs.(18.14 kg) 42.9 lbs.( 19.46 kg) The ingredients are dispersed in a conventional agitated feed tank. The feed tank is filled to the 2000 gallon level and the polyester and glass fiber added and agitated for 10 minutes. The tank is then filed to the 7000 gallon level, the hollow glass spheres are added and the tank agitated for 15 additional minutes. The polyester fiber is 1.5 denier, 0.25 inch cut length uncrimped fiber with a commercial finish on its surface at approximately a 2% level to assist dispersion in water. The glass fiber is Owens Corning Fiberglass 133 A - AB 1.0 inch wet chop glass having a 13 micron diameter. The microspheres are PQ Corporation Type CG "Extendospheres". The resulting uniform, and intimately mixed water dispersion of glass and polymeric fiber and microspheres is pumped to the forming head of a modified rotary inclined wire machine at a rate of 121 gpm . The machine dilution loop flow is 1200 gpm . The forming wire speed was 25 fpm. The resulting sheet dry weight is 0.11 pounds per square fool . Vacuum slots are used for dewatering prior to dry bonding of the sheet in a circular gas fired through-air dryer of 3 foot diameter, where the dryer air temperature is 277° C. The dryer provides sufficient heat to completely melt the polyester fiber and melt bond the glass fibers and microspheres into a uniform homogeneous sheet product. Microscopic examination of the mat in cross section shows excellent dispersion of the glass filaments and hollow microspheres. EXAMPLE 3 The ambient dried nonwoven sheets of Example 1 are stacked to form a 7 x 7 inch batt of basis weight 1 pound/sq. ft. and dried in a vacuum oven to 0.02% by weight moisture. The dried batt is quickly transferred to a preheated 7 x 7 inch picture frame mold coated with a mold release. The mold and contents are then placed between the platen of a hydraulic press with heated platens. Contact pressure equivalent to 15 psi is then placed on the mold and it is heated to 269°C as measured by a thermocouple placed in the side of the mold. When full temperature is reached, the pressure is gradually raised to 600 psi . When the first evidence of flash is noted at roughly 600 psi, the heaters are turned off, the cooling water to the platens is turned on, and the mold and contents are allowed to cool to room temperature under 600 psi pressure. The composite plaque is then removed from the mold and the density is calculated by weighing the plaque and carefully measuring its dimensions. TABLE 2 COMPARISON OF CALCULATED AND ACTUAL DENSITY FOR SAMPLE WITH EQUAL GLASS CONTENT, WITH AND WITHOUT HOLLOW GLASS SPHERES %Glass %Microsph. %PET Calc. Density Actual 1. 20% 30% 50% 1.18 g/cc 1.32 g/cc 2. 20% 0% 80% 1.51 g/cc 1.50 g/cc EXAMPLE 4 To an agitated 7000 gallon blend tank, 5000 gallons of water were added. Approximately, 201.4 pounds of 2 denier per filament 0.25 inch cut length thermoplastic polyethyleneterephthalate fiber containing approximately 0.7% by weight spun-in Ethanox E 330 antioxidant and 0.2% spun-in carbon black was then added to the tank and dispersed for 10 minutes under high agitation to create uniform slurry. Subsequently 48.0 pounds of 0.75 inch cut length wet chop E- glass fiber, OCF type 133A, of average diameter approximately 13 microns, was added to the water and agitated for an additional 20 minutes. This slurry stock was then fed at the rate of 100 gallons per minute into the recirculating white water loop of a rotary inclined wire sheet forming machine in the usual manner providing a dilution to about 0.05% consistency at the head box. At the head box, a sheet was formed having a dry equivalent basis weight of approximately 0.09 pounds per square foot using a modified inclined rotary wire forming head. The head box stock was drained through the forming wire in the usual manner and dewatered by passing over suction slots. The forming wire speed was 25 fpm resulting in a damp preform layer. Glass microspheres were then added to the sheet in the following manner. A slurry suspension of 3M SCOTCHLITE type B37 microspheres in water was prepared by mixing 21.8 pounds of the 3M microspheres with 64.3 gallons of water in a 75 gallon mix tank. The suspension was constantly agitated. The feed supply was gravity fed to a 2 inch Moyno Co. pump having a 2-10 gpm capacity. The pulp was used to feed a Spray System Inc. 2 gpm spray nozzle at a head pressure of 40 psi placed over the moving sheet between the head box and the dryer. Sufficient quantity of microspheres were sprayed on the web to achieve the overall composition by weight described below. A vacuum slot opposite the spray and under the supported sheet removed excess water from the spray application of the microspheres. The micro.spheres were observed to collect primarily on the surface, but also to penetrate the sheet. The final percent weights of resin fiber, glass fiber, and microspheres were 72.6%, 17.2%, and 10.2% respectively, leading to a calculated volume percent in a consolidated plaque made from this sheet of 57.6%, 7.4%, and 30% respectively. The sheet was bonded by passing through a honeycomb circular oven having an air pressure drop across the sheet of about 5 inches of water and an air temperature of 288° C for a residence time of 20 seconds. In the dryer excess moisture leaves the sheet and the resin fiber component is melted, forming globules. The preform sheet was then molded as follows. A clicker die is used to convert the rolled sheet into a form suitable for thermal compression molding. 30 diced sheets were stacked as a batt which gives a part of about 1.0 pounds per square foot on thermal compression flow molding. Before thermal compression molding, the batt is placed in a convection oven and air at 280° C air circulated through the batt at about 200 cfm for 30 seconds to remelt the resin and convert the porous batt into a moldable form. The heated preform was hand transferred to a matched metal die compression mold with shear edges to contain the flowable charge. The tool temperature was held at 150° C. On applying pressure, the preform flows to fill the tool. The final mold pressure was 2000 psi and the dwell time was 60 seconds. The part was then removed. A control was molded in the same way without the microspheres being applied. The density of the control and microsphere containing moldable panel were evaluated using the density gradient tube technique. The microsphere-containing part was found to have a density of 1.32 grams per cubic centimeter versus the control 1.55 gram per cubic centimeter, or a 15% reduction in density. Tjφ§ %Resin %Glass %Micro Theory Measured Control Spheres Density Densitv Density S-60 73 20.5 6.5 1.385 1.448 1.52 S-60 70.4 23.2 6.4 1.400 1.54 1.53 B-37 73 22.9 4.1 1.36 1.472 1.52 B-37 70 26.0 4.0 1.39 1.432 1.50 S-60 69.6 15.8 14.6 1.23 1.381 1.53 S-60 65 20.7 14.3 1.26 1.404 1.55 B-37 70 20.6 9.4 1.18 1.391 1.52 B-37 72.6 17.2 10.2 1.09 1.323 1.51 B-37 65 18.4 15.6 1.005 1.187 EXAMPLE 5 A preform sheet was prepared on a modified rotary inclined wire wet lay machine having a rotary through air dryer similar to Example 2. The stock formulation was 49 pounds of glass fiber Type OCF 133A having a 1.0 inch cut length, 43 pounds of PQ Corporation Type CG microspheres, and 92 pounds of polyester thermoplastic fiber cut to 0.25 inch staple length. These ingredient were slurried in 5000 gallons of water and fed to the forming section of the wet lay machine in the usual manner, and dewatered over vacuum slots. The damp sheet was then passed through a rotary forced hot air oven with an air temperature of 288° C melting the polyester thermoplastic fiber and resulting in a bonded nonwoven preform sheet of glass fiber, hollowspheres, and resin globules. Similarly, a second preform sheet containing no microspheres was prepared according to U.S. Patent 5,194,106 consisting of 40% by weight glass fiber and 60% by weight PET resin. Pie pan shaped parts having interior ribs were molded using a 100 ton Schuller press. The molding tool was a 10 inch diameter pie pan with a 1 inch lip and 0.5 inch depth. This matched melal tool was machined to have 0.18 inch width ribs 0.5 inch in depth in a 2.5 inch grid pattern inside the pan. The bottom of the pan was a smooth mold surface. The tool temperature was held at 175° C during molding. A part was molded in the following manner. A stack of preform sheets were prepared for preheating and molding consisting of five 9.75 inch radius circular sheets without microspheres and 12 layers of the above preform sheet containing microspheres. This preform stack was heated in a forced air oven such that the pressure drop across the preform stack was 6-8 inch of water. The preform stack was heated for approximately 60 seconds and then hand transferred to the open match metal tool with the non-microsphere containing sheets facing the smooth surface of the mold. On closing and pressurizing to 2000 psi the preform material flowed to fill out the mold. After a dwell time of approximately 60 seconds the tool was opened, the part removed, and allowed to cool to room temperature. The surface smoothness opposite the ribs was then measured using a standard stylus profilometer with a resolution 0.000001 inches. Surface deformation due to rib read-through was observed to be 0.00045 inches or less. Similarly, a measurement made on parts molded from a stack of preform sheets consisting of only 40% glass/60% polyester thermoplastic resin with no microspheres showed a distortion of 0.0020 inches or greater. EXAMPLE 6 A preform sheet was prepared on a modified rotary inclined wire wet lay machine having a rotary through air dryer similar to Example 5. The stock formulation was 15% glass fiber Type OCF 133A having a 1.0 inch cut length, 23% PQ Corporation Type CG microspheres, and 65% polyester thermoplastic fiber cut to 0.25 inch staple length. The total weight of these solid ingredients was 184 pounds. The ingredients were slurried in 5000 gallons of water under moderate agitation for approximately 30 minutes, then fed to the forming section of the wet lay machine in the usual manner, and dewatered over vacuum slots. The damp sheet was then passed through a rotary forced hot air oven with an air temperature of 288° C melting the polyester thermoplastic fiber and resulting in a bonded nonwoven preform sheet of glass fiber, hollowspheres, and resin globules. A pie pan shape pai t having interior ribs was molded using a 100 ton Schuller press. The molding tool was a 10 inch diameter pie pan with a 1 inch lip and 0.5 inch depth. This matched metal tool was machined to have 0.18 inch width ribs 0.5 inch in depth in a 2.5 inch grid pattern inside the pan. One rib was machined to be slightly wider at 0.25 inches. The bottom of the pan was a smooth mold surface. The tool temperature was held at 170PC during molding. A part was molded in the following manner. A stack of preform sheets were prepared for preheating and molding consisting of 7.0 inch diameter circular sheets having a total charge weight of 338 grams. This preform stack was heated in a forced air oven such that the pressure drop across the preform stack was 6-8 inches of water and the air temperature was 283°C. The preforms stack was heated for approximately 40 seconds and then hand transferred to the open match metal tool. On closing and pressurizing to 95 tons the preform material flowed to fill out the mold. After a dwell time of approximately 3 seconds, the press tonnage was dropped to 20 tons and dwelled for 3 minutes additional. The tool was then opened, the part removed and allowed to cool to room temperature. The surface smoothness opposite the ribs was then measured using a standard stylus profilometer with a resolution O.ϋϋϋOOl inches. Surface deformation due to rib read-through was observed to be 0.0001 inches or less.

Claims: CLAIMS WHAT IS CLAIMED IS: 1. An air permeable web of randomly dispersed high modulus fibers and hollow microspheres held together by globules of thermoplastic resin which envelope fiber cross over locations and microsphere-fiber interface locations wherein some of said randomly dispersed fibers have agglomerates of solidified thermoplastic resin adhered thereto at locations along their length other than crossover and microsphere-fiber interface locations. 2. A self-supporting preform layer of high modulus reinforcing fibers and hollow microspheres intimately mixed with thermoplastic fibers comprising a) an intimate essentially uniform blend of 10 to 40 % by weight of high modulus reinforcing fibers, b) 30 to 70 % by weight thermoplastic fibers, and c) 2 to 20% by weight hollow microspheres, said microspheres having a diameter from 1 to 100 microns and a bulk density of 0.2 to 0.7 gram/cubic centimeter. 3. The preform of Claim 1 wherein said reinforcing fibers have a length from 0.5 cm to 8 cm and said hollow microspheres are glass microspheres. 4. The preform of Claim 2 wherein said thermoplastic fibers are polyethylene terephthalate fibers. 5. A fully densified composite structure comprised of a thermoplastic resin matrix reinforced with 10 to 30% by volume high modulus reinforcing fibers having a length of at least 2mm and 10 to 30 % by volume hollow microspheres having a diameter from 1 to 100 microns and a crush strength of at least 0.0479 mpa, said reinforcing fibers and said microspheres being intimately and uniformly dispersed in said thermoplastic matrix. 6. The structure of Claim 5 wherein said thermoplastic resin is polyethylene terephthalate, said reinforcing fibers are glass fibers and said microspheres are glass microspheres. 7. A process for preparing a densified, low void fiber reinforced composite structure comprising a) co-dispersing with agitation in an aqueous slurry 10 to 40 % by weight of high modulus reinforcing fibers, 30 to 70 % by weight of thermoplastic resin fibers and 2 to 20 % by weight of hollow microspheres having a diameter from 1 to 100 microns and a bulk density of 0.2 to 0.7 grams per cubic centimeter, b) forming a damp sheet of uniformly dispersed materials of paragraph a) by depositing said slurry on a porous wire screen and removing excess water from said slurry, c) heating said damp sheet to remove remaining water and to melt said thermoplastic resin fibers to cause the resin to flow and adhere said reinforcing fibers and microspheres, d) cooling the product of step c) to form a bonded sheet, e) stacking layers of the bonded sheet to achieve a desired size and weight, f) heating the stacked layers to remelt the resin, and g) consolidating the heated stack of preform layers to form a densified, low void composite by applying pressure less than the crush strength of said microspheres. 8. A process for preparing a fully densified fiber reinforced composite structure comprising a) co-dispersing with agitation in an aqueous slurry 10-50% by weight of discrete high modulus reinforcing fibers and 50-90% by weight of discrete thermoplastic resin fibers, b) forming a damp sheet of uniformly dispersed materials of paragraph (a) by depositing said slurry on a porous moving wirescreen and removing excess water from said sheet, c) passing said sheet between a spray applicator and a vacuum slot, said spray application impinging an aqueous slurry of hollow microspheres having a diameter of 1 to 100 microns and a bulk density of 0.2 to 0.7 grams per cubic centimeter into said sheet achieving a uniform penetration of the microspheres through the thickness of the sheet and increasing its dry weight up to 20%, d) heating said damp sheet to remove remaining water and to melt said thermoplastic resin fibers to cause the resin to flow and adhere said reinforcing fibers and microspheres, e) cooling the product of step c) to form a bonded sheet, f) stacking layers of the bonded sheet to achieve a desired size and weight, g) heating the stacked layers to remelt the resin, and h) consolidating the heated stack of preform layers to form a densified, low void composite by applying pressure less than the crush strength of said microspheres.

Inventors: RABE ALLEN, WEEKS GREGORY P, RABE, ALLEN, WEEKS, GREGORY, P.
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1995
Main classification: A63B37
IPC codes: B29C70, B32B5, D04H1