CA2215265A1 - Thermoplastic moldable composite sheet containing hollow microspheres - Google Patents

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

CA 0221~26~ 1997-09-12 "THERMOPLASTIC
MOLDABLE COMPOSITE SHEET CONTAINING HOLLOW MlCROSPHERESn.

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 0 microspheres, and a process for making the same.

BACKGROUND OF T~E IN~'ENTION
Compression molding of high modulus (typically glass) fiber reinforce~ resin sheet or bulk molding compound materials is routinely used 15 in the automotive and aerospace industry, for example, for fabrication of structural and semistructural articles. ~Iigh modulus generally pertains to fibers having tensile strengths of greater tllall 3()()0 mpcl and tensile modulus of 80 gpa. EP Application 0 341 977, puhlished Nov. 15, 1989 discloses a thermoplastic preform sheet material consisting of higll modulus reinforcing 2 o fibers and thermoplastic resin globules whicll can be used for compressio molding of long fiber reinforced thermoplastic parts.
Molded articles intended for automotive etld use must show a combination of useful properties to satisfy end use needs. These properties include stiffness, good surface after paintillg, illlp.lCt resistance, and many 25 other properties. The ability to fabricate parts having reduced density, wllile retaining other needed property characteristics of structural and semistructural composites, is a highly desirable goal, since tllis often leads to fuel saving and other advantages in use. One parlicular need in the engineered thermoplastics molding industry is a mealls to form a moldecl 3 0 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 knowll in the plastics industry today, and are sold commercially for incorp0ratioll hllo cngilleerillg reshls. I lollowmicrospheres provide the benefits of convelltiollal soli(J spheres wllile 35 significantly lowering the weight of finished engilleerillg grade plastic compounds and molded parts. Generally llollow nlicrosplleres are sold as a CA 0221~26~ 1997-09-12 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. l~hermosetting resins generally have a low viscosity and conventional compounding means are suitable for achieving a good distribution of microsplleres in the final molded part. The 0 higher melt viscosity and molding pressures of many engineering thermoplastic resins, however, require more intensive shear for achievillg good mi~in,~
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 hlends with very short reinforcing fibers due to attrition during blending and breakup during injection cycle. It would be highly desirable to find a tecllllique for creating long fiber reinforced thermoplastic parts containing uniformly distributed microspheres which could be flow molded at reduced pressure, an(3 would retain long reinforcing fiber length during processing to the molde(l part. It would also be highly desirable to have a molding compound alld technique for molding parts with integral molded ribs showing minilnal rib read-tllrougll 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 molde(l article having a thermoplastic matrix in combinatioll with long reinforcing fibers.
The invention also provides for a very uniforlll distriblltion of llollow glass microspheres which can be processed at mo(lerate m(31dillg pressures, minimi7.ing attrition of the microspheres and long fibers. The invention also CA 0221~26~ 1997-09-12 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 5 fibers have a length of from about ().5 Clll to about 8 cm, and diameters fromabout 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 slurrv. The chopped reinforcing fibers consist of bundles of numerous aligned reinforcing fibers adllere(l with 0 a sizing agent, with uncrimped thermoplastic resin staple fibers of fine clenier and short cut length from about 1 mm to 5 Clll. The hollow glass microspheres have a <liameter distribution from I - 100 microns, with an average size in the range of 30 to 70 microns, an(3 a bulk density of 0.2 - ().7g/cc having a crush strength of at least 500 psi. The thermoplastic fibers may 15 be pretreated with a finishing agent to aid tlleir dispersion in water, as may the reinforcing fibers.
Uniquely, the slurry hlend of reinforcing fibers and resin fibers provides a means to capture the fine glass microsplleres during processing into sheet form on paper making equipment. Alternatively, the reinforcing 20 fibers and thermoplastic resin fibers can be formed hlto a sheet, followed byspray 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 tlle thermoplastic fibers, whicll binds the reinforcing fibers and microspheres together in a bonded porous sheet 25 form consisting of randomly dispersed, in-plane, reinforcing fit-er ancl hollow microspheres bonding together with films an(l globules of thermoplastic resin. Layers of the sheet have the characteristic of being very uniform in the distribution of microspheres witllin the reinforcing fiber network, ancl being porous enough that the slleet can be quickly reheated by passing hot air 30 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 calculatecl from the individual components, and showing excellent surf.lce properties. Parts having ribs molded with microspheres show mucll reduced rib "read tllrougll"
35 compared to parts Witllout microspheres.

CA 0221~26~ 1997-09-12 WO 96130088 PCT/US95rO3874 .

BRIE~ DESCRIPTlON Ol~ TIIE 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 5 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 I HE INVENTION
Referring to FIG. 1 the wet laying process used in making products of this invention utilizes paper making equipn1ent and generally includes a highly agitated mix tank 10, an agitated supply tank ~2, 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. Tl1e slurry is pulped via pump 11 from the higl1ly 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. T he slurry is drained tl1rougl the wire in the usual manner and dewaterecl hy p.lSSillg over suclion slots 1 in the dewatering section. The dewatered si-eet 19 is then wound in damF7 form on driven spool 20. The sheet wound on the spool 20 is unwound in layers 17 and air dried as schematically ShOWIl in I~IG. 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 waterfrom the dewatering box 16a and pipe 16c via pump 16b as substalltially shown in ~IG 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 througl1 nip rolls 50 and then wound up on spools by windup 42.
The composite preform layers of this al-plication are formed on conventional papermakil1g e4uil)mel1t where tl~e rorming section of the machine is preferably of the type known as an inclil1ed wire, i.e., tl-e slurry is drained through a forming wire WhiCIl iS inclil1e(3 at an angle (usually 15 to 45 degrees) into the forming slurry. Sllch e(luipmel1t is specifically designecl to CA 0221~26~ 1997-09-12 be capable of forming fibrous mats at moderate to high slurry dilutions and is ideal for processing long fibers into nonwoven mats. ~lowever, other m~chine styles can be used as well, such as cylindcr formers and others.
Sheet drying can be accomplished via a combination of thru-air drying, 5 contact drying or radiant heat. The maximum temperature reached by the fibrous mat must be sufficient to activate melting of tlle resin fibers to achieve sufficient mat strength for good processahiltiy.
An alternative embodiment of the presellt invention is shown in ~ig. 3. In operation glass and thermoplastic fibers are dispersed in water 10 in highly agitated mix tank 10. The slurry is plltlll~e~l via pump 11 from highly agitated mix tank 10 to an agitated supply tank 12. ~eed 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 15 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 2 o means 62. Feed supply is pumped hy means Or pulrlp ~4 to a spray nozzle ~)6 located over preform sheet 19. Return pipe f,~ connects back to supply tank 60. A vacuum slot is positioned opposite spray nozzle ~6 under the supported preform sheet 19 to remove excess water from sheet 19.
The preform sheet is dried by passillg it through a circular air 25 dryer 30 at a temperature sufficient to both dry and rnelt bond the sheet by melting the thermoplastic fiber. Sheet 19 is therl passed through nip rolls 5() and wound up on spools by windup 42. The process as sllown in ~ig. 3 is the preferred composite preform layer process.
The reinforcing fiber may be chosen from any high modulus 3 o fiber with a melting point substantially above the thermoplastic resin fiberincluding, but not limited to, glass fibers, carbon fiber, glass wool fibers, and aramid fibers such as Kevlar~' whicll is availal)le from E.I. du Pont de Nemours and Co., etc. The reinforcing fiber rmay have a sizing to enhance dispersion in water and further enhancing wetting and bonding to the 35 thermoplastic in the final composite. ~ typical sizing for glass consists of a film-forming agent, such as a low molecular weight polyester or polyurethane CA 0221~26~ 1997-09-12 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 5 thermoplastic organic materials including, but not limited to, polypropylene, polyesters, co-polyesters, polyamides, polyethere~llerketones, polyetherketoneketones, liquid crystal polymers, etc. Optionally, the fiber may be sized with a dispersing aid such as, but nOt limited to, long chain alkylphosphates, condensation pro~ucts of tallow alcohol~s with polyethylene 10 oxides, an~ 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 sol(l 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 micl 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. l ypically several preform layers will be sheeted and stacked to produce a thickness which is suitable for 20 molding. Several layers of the resulting slleets can l~e layered togetller an~l placed in a mold an(3 made into a plaque to evaluate pllysical 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 mol(3ing cycle times from the 25 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. ~-lot air is passed through the preform to remelt the thermoplastic resin. Tlle preform is quickly transferred to a male/female compression tool of the appropriate design.
30 The tool is closecl, flowing the ~reform ancl forming the part. The tool temperature is selected according to consi~leratiolls of cooling rate, cryst:~lli7~tion, ancl mold flow to optimize formation.
The following examples in whicll p.lrts and percentages are l~y weight unless otherwise indicated further illustra~e tlle invention.

CA 0221~26~ 1997-09-12 The following example illustrates the process of forming a ~ preform sheet consisting of reinforcing fiber, thermoplastic resin fiber, and hollow rnicrospheres.
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 lengtll 0.25 inch 0 are added and dispersed witll a lligll speed agitator for 2 minutes to create a uniforlll fiber dispersion;
c) 0.6 pounds of 1 inch wet chop 13 micron diameter glass (Owens Corning Ford type 133A) i.s 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 tlle pulper agitated for an addition 10 minutes.
The pulper is then pumped to a moderately agitated machine supply tank 2 o 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 illCh Bruderhaus inclined wire minipaper making machine being further diluted at the headbox to a consistency of 0.07~o. The head box stock is drained througll the forming wire in the usual manner and dewatered by passing over suction slots. The wire speed is S fpm, and the resulting slleet is captured on a take up roll in damp form. The sheet is then laid out hl layers and air dried to an ambient moisture level substantially as disclosed heretofore in the discussion of the apparatus of ~IG. 1. Dry sheet weight is approximately 0.05 pounds per sq. ft. Microscopic cross sectional analysis shows that the wet C}lOp glass fiber bundles are substantially dispersed capturing the llollow microspheres in a uniform blend with the resin and reinforcing fibers.

EXAMPL,E 2 The following formulations are prepared from PQ type C~G
"Extendospheres' polyester staple fiber, ancl glass fil!er.

CA 0221~26~ 1997-09-12 Polyester Fiber Glass Fiber Microspheres 1. 68 Ibs.(30.84 kg) 40 Ibs.(18.14 kg) 34 Ibs.(15.42 kg) 2. 61.2 Ibs.(27.76 kg) 20 Ibs.(9.07 kg) 35.1 Ibs.(15,92 kg)3. 70.2 Ibs.(31,84 kg) 20 Ibs.(9.07 kg) 26 Ibs.( l 1.79 kg) 4. 80.7 lbs.(36.60 kg) 40 Ibs.(18.14 kg) 42.9 Ibs.(19.46 kg) The ingredients are dispersed in a conventional agitated feed tank. The feed tank is filled to the 20(~0 gallon level ancl tlle polyester and glass fiber added 0 and agitated for 10 minutes. The tank is tllen filed to the 7000 gallon level, the hollow glass spheres are added and the t~nk 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 25~ 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 "Extendosplleres". The resulting uniform, and intimately mixed water dispersion of glass all(l 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 foot . Vacuum slots are used for dewatering prior tu 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. 1 he dryer provides ~ sufficient heat to completely melt the polyester fiber and melt bond the glass fibers and microspheres into a uniform hoIllogelleous sheet product.
Microscopic examination of the mat in cross section shows excellent dispersion of the glass filaments and hollow microsplleres.

3 o 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 ec3uivalent to 15 psi is then placed on the mold and it is heated to 2G9~C as measured by a thermocouple placed in CA 0221~26~ 1997-09-12 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, tlle cooling water to the platens is turned on, and the mold and contents are a11Owed to cool to room 5 temperature under 600 psi pressure. The composite plaque is then removed from the mold and the density is calcula~ed by weighing the plaque and carefully measuring its dimensions.

COMPARISON OF CALCULATED AND ACTUAL DENSITY FOR
SAMPLE WITH EQUA~ GLASS CONTENT, Wl r~-i AND WIT~IOUl HOLLOW GLASS SPHERES
~Glass ~Microspll. %PE I Calc. Density Actual 1. 20% 30% 50~ 1.18g/cc 1.32 g/cc 2. 20~o 0% 80% 1.51 g/cc 1.50 g/cc 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 2 o cut length thermoplastic polyethyleneterep}lthalate 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 alld dispersed for 10 minutes under high agitation to create uniform slurry.
Subsequently 48.0 pounds of 0.75 incll 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~o consistency at the head box. At 3 o the head ~ox, a sheet was formed havillg 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 (Irailled through the forming wire in the usual manner and dewalered l~y passing over suction slots. The forming wire speed was 25 fpm resulting in a damp ~rcform layer.
Glass microspheres were thell added to the sheet in the following manner. A slurry suspension of 3M SCOTCiILITE type B37 CA 0221~26~ 1997-09-12 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. Tlle microsplleres were observed to collect primarily on the surface, but also to penetrate the sheet. The final percent weights of resin fiber, glass fiber, and microsphercs were 72.6%, 17.2~, and 10.2~fo respectively, leading to a calculated volume percent in a consolidated plaque made from this sheet of 57.6~, 7.4~o, and 30% respectively.
The sheet was bonded by passing througll a honeycomb circular oven having an air pressure drop across the sheet of about 5 inches of water and an air temperature of 28~ C for a residcnce time of 20 seconds.
In the dryer excess moisture leaves the sheet and tlle resin fiber componellt ismelted, forming globules.
The preform sheet was then molde(l as follows. A clicker die is used to convert the rolled sheet into a form suitable for thermal compression molding. 30 diced slleets were stacked as a batt which gives a part of ahout 1.0 pounds per square foot on thermal compression flow molding. Before thermal compression molding, the batt is placed h~ 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. I he 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, tlle prcform flows to fill the tool.
3 o The final mold pressure was 2000 psi and tl~e dwell time was 60 seconds.
The part was then removed. A control was molded in the same way without the microspheres being applied.
The (Icnsity of the control and lnicrosp1lere 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 CA 0221~26~ 1997-09-12 WO 96/30088 PCT/US9~/03874 per cubic centimeter versus the control 1.55 gram per cubic centimeter, or a 15% reduction in density.

Tvpe %Resin ~GIass %Micro Theory Measured Control Spheres Density Density Density S-60 73 Z0.5 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 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 10 1.0 inch cut length, 43 pounds of PQ Corporation Iype 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 thell passed through a rotary forced 5 hot air oven with an air temperature of 288~C nleltirlg the polyester thermoplastic fiber and resulting in a bonded nonwoven preform sheet of glass fiber, hollowspheres, and resin globules. Similarly, a second preform sheet contz~ining no microspheres was prepared according to U.S. Patent 5,194,106 consisting of 40% by weight glass fiber and 60% by weight PET
2 o 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 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 CA 0221~26~ 1997-09-12 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 preheatillg and n-olding consisting of five 5 9.75 inch radius circular sheets without microsplleres 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 lO non-microsphere containing sheets facing the smootll 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 ~)0 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 0.00000l inches. Surface deformation due to rib read-through was observed to be 0.00()45 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.

A preform sheet was prepared On a modified rotary inclined wire wet lay machine having a rotary througll air dryer similar to Example 5.
The stock formulation was 15% glass fiber lype OCE~ 133A having a 1.0 inch cut length, 23% PQ Corporation Type CG microsplleres~ and 65~ polyester thermoplastic fiber cut to 0.25 inch staple lengtll. I he total weight of these solid ingredients was 184 pounds. The ingredients were slurried in 5000 gallons of water under moderate agitation for approx;mately 30 minutes, then fed to the forming section of the wet lay machine in the usual manner, 3 o and dewatered over vacuum slots. The damp sheet was then passed tllrough a rotary forced hot air oven with an air temperature of 288~C melting the polyester thermoplastic fiber and resultillg hl a bollded nonwoven preform sheet of glass fiber, hollowspheres, and resin globules.
A pie pan shape part havillg hlterior ribs was molded USillg 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 matclled mctal tool was machined to CA 0221~26~ 1997-09-12 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 170~ C 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 cllarge weigtlt 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 lo 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 tlowec3 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 ad(3itional. 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 0.000001 inches. Surface deformation due to rib read-through was observed to be ().0001 inclles or less.

Claims (8)

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 fur 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.