WO1993017856A1

WO1993017856A1 - Low density fiber reinforced polymeric composites

Info

Publication number: WO1993017856A1
Application number: PCT/US1992/001647
Authority: WO
Inventors: Jeffrey Gamble; Daniel B. Roitman; Ritchie A. Wessling; Tsungnan Cheng
Original assignee: The Dow Chemical Company
Priority date: 1992-03-05
Filing date: 1992-03-05
Publication date: 1993-09-16

Abstract

A low density reinforced polymeric composite comprising a polymeric matrix, reinforcing fiber, a polymeric binder and a fibrillated fibrous pulp. The void volume of the polymeric composite is enhanced by the inclusion of polymeric pulp comprising polybenzoxazole (PBO), or polybenzothiazole (PBT) or copolymers thereof. A method of making fiber-reinforced composites having an enhanced void volume is also disclosed.

Description

LOW DENSITY FIBER REINFORCED POLYMERIC COMPOSITES

This invention pertains to the formation of low density fiber reinforced polymeric composites. More particularly, the invention relates to low density polymeric composites comprising a fibrillated fibers of polybenzoxazole (PBO), or polybenzothiazole (PBT) or copolymers thereof.

Porous fiber reinforced composites are known from U.S. Patents •*^■(,6 3,9*40; 4,65-4, 100; and 4,670,331.

According to the teachings of such references, polymeric composite reinforced by short fibers is first prepared by an aqueous process from reinforcing fibers and water insoluble particulate powder. The sheet formed by the aqueous process is consolidated by the application of heat and pressure to reduce the void volume of the sheet to less than 20 percent of the total volume of the sheet. Enhanced void volume composites are prepared by expanding the composite sheet in the thickness dimension, e.g., perpendicular to the plane formed by the sheet. The thickness dimension is also referred to as the Z-dimension. The expansion occurs when the composite sheets are heated above the softening point of the polymeric matrix. While the invention is not intended to be limited to any particular theory, it is believed that the consolidation of the composite mat causes the fibers to bend where the fibers overlap. Elastic strain is believed frozen into the reinforcing fibers when the composite is consolidated in the Z-dimension and then cooled below the temperature at which a heat fusible polymer begins to melt. The flexural modulus of the fibers under conditions of lofting of the composite must be greater than the flexural modulus of the continuous resin matrix. Consistent with the theory, the lofting potential of a preform of heat fusible fiber-reinforced polymeric composite laminae layered in the Z-dimension is influenced by the extent to which the fibers are bent over one another and the number of fibers in contact with other fibers. The greater the degree of consolidation of a given composite preform, the greater will be the degree of bending of fibers in the regions where fibers overlap. As a consequence of the greater strain stored in fibers bent to a greater degree, the larger will be the Z-dimension increase possible during the lofting phase. However, excessive consolidation has been found to break reinforcing fibers thereby reducing the maximum lofting attainable.

Additional factors influencing lofting potential of the composites are the length of reinforcing fibers, fiber composition, the fiber diameter and concentration of fibers. Although the mechanism is not known, it has been observed that reinforcing fibers of a shorter length e.g., 3/16 inch (5 mm), result in a greater Z-dimension expansion than substantially the same weight percent of longer fibers, e.g., 1/2 inch (13 mm). Reinforcing fibers useful in the invention have lengths from 3mm to 25 mm (1/8 to 1 inch). Composites prepared of glass fibers of larger diameter, e.g., 17 μm, are observed to loft to a greater degree than composites similarly prepared but having glass fibers of a smaller diameter, e.g., 13 μm.

Polymers useful in the preparation of articles according to the method of this invention include, for example, polyolefins, such as polyethylene, e.g., high density polyethylene, linear low density polyethylene, or polypropylene and the like; chlorinated polyethylenes; polycarbonates; ethylene/acrylic acid copolymers; polyamides, such as Nylon 6, Nylon 6.6 and the like; polyphenylene oxide resin; polyphenylene sulfide resins; polyoxymethylenes; polyesters; the so-called ABS (acrylonitrile-butadiene-styrene) resins; polyvinyl chloride; vinylidene chloride/vinyl chloride resins; polyetheretherketones; polysulfones; polyetheramides; and vinyl aromatic resins, such as polystyrene, poly(vinylnaphthalene) poly(vinyltoluene) and the like. The foregoing listing is not intended to be exhaustive and the skilled artisan will recognize the utility of other thermoplastic resins in compositions of this invention in place of any of the foregoing.

Although any of the resins listed herein are suitable, the particular choice of resin will depend upon the requirements of the end use of the object to be formed from the planar laminae of heat fusible fiber- -reinforced polymeric composite. Properties such as impact resistance, tensile strength, heat distortion temperature, barrier characteristics, flexural strength, flexural modulus, compressive strength, compressive modulus, associated dielectric constant values and the like are all affected by the choice of polymer.

Additional optional ingredients frequently included in the manufacture of composite sheet include low levels of synthetic polymeric fibers such as polyolefin pastes or pulp. Such synthetic polymeric fibers, pastes, or pulp have been found advantageous in the preparation of the polymeric composite. For example, polyarimide pulp additives are generally disclosed in French Patent 2507123-A as contributing to the cohesive properties of the composite. • Other typical polymeric fibers are polyethylene, polypropylene, polyvinylidenechloride, polyester, polystyrene and ABS (acrylonitrile-butadiene-styrene copolymer). One preferred polymeric fiber is commercially available from Himont Corporation under the trademark PULPEX E™ for polyethylene fibers and the trademark PULPEX P™ for polypropylene fibers. Fig. 1 and Fig. 2 are electron micrographs of Pulpex fibrillated polyolefin fibers.

Generally the polymeric fibers are present from about 1 to about 10 preferably from 3 to about 5 weight percent based on total solids. Fiberous pulps are observed as useful to improve the cohesive properties of the composite, particularly during the composite formation steps of forming a sheet from an aqueous slurry.

Heretofore, expansion of the composite perpendicular to the plane of the sheet (Z-direction) has been limited by the potential energy stored in the reinforcing fibers during the consolidation step. Z-direction expansion for a composite including closed cells may be enhanced by blowing agents. For open cell expanded composites the degree of expansion is limited by the flexural potential energy stored in the consolidated composite.

It would be desirable to increase the degree of expansion of fiber reinforced composites without qualitative or quantitative change to the reinforcing fibers of the composite, and without adding extraneous ingredients such as blowing agents.

Applicants have found that the Z-direction expansion of a composite sheet may be enhanced by incorporating fibrillated fibers of PBO, PBT, or copolymers thereof as an aid in the preparation of the composite.

Applicants have found that a composite sheet incorporating a fibrillated fibrous pulp of PBO, PBT, or a copolymer thereof have significantly enhanced expansion capabilities over composite sheets comprising pulps prepared from high performance fibers such as KEVLAR™ brand polyaramide fibers.

Brief Discription of the Figures

Fig. 1 and Fig. 2 are electron micrographs of fibrillated polyolefin fibers also known as polyolefin fibrillated pulp or simply polyolefin pulp.

Fig. 3 and Fig. 4 are electron micrographs of fibrillated polyaramide fibers also known as polyaramide fibrillated pulp or simply polyaramide pulp.

Fig. 5 is an electron micrograph of an unconsolidated composite prepared by the aqueous process showing KEVLAR™ polyaramide fibrillated pulp bridging between reinforcing fibers of the composite. Fig. 6 and Fig. 7 are electron micrographs of heat treated PBO fibers after freeze grinding showing a small degree of fibrilation.

Fig. 8 and Fig. 9 are electron micrographs of fibrillated PBO fibers, or PBO pulp.

Description of Preferred Embodiments

The composites of this invention are preferably prepared from sheets of heat-fusible, fiber-reinforced polymeric composite where reinforcing fibers are dispersed with substantial uniformity in the plane defined by the sheet. Sheets of heat fusible fiber- reinforced polymeric composites may be advantageously prepared according to the aqueous slurry process in which a dilute aqueous slurry of a solid, water- insoluble heat-fusible organic polymer, a reinforcing material, and a polymeric binder is formed, the slurry is flocculated, and the solids collected in the form of a continuous mat, dewatered, and dried according to the process of U.S. Patents 4,426,470 and 4,550,131 which are hereby incorporated by reference. The composite mat formed by dewatering the aqueous slurry may be consolidated by heat and pressure to form a sheet of heat-fusible, fiber-reinforced polymeric composite.

The temperature and pressure necessary to consolidate the laminae of heat-fusible, fiber- reinforced composites depends upon the characteristic of the heat-fusible polymer matrix used in the preparation of the heat-fusible, fiber-reinforced composite lamina.

Generally, a pressure up to 400 psi (2760 kPa) will be useful for the densification the composite preform. Adequate densification and reduced breakage of glass reinforcing fibers results from consolidation at pressures less than the maximum in a range from 200 to 300 psi (1380 to 2070 kPa).

After the composite is consolidated it may be expanded by heat softening the polymeric matrix. The viscosity and stability characteristics of the polymer matrix at an elevated temperature are considerations in fixing the temperature to which the consolidated composite is raised during the lofting step. Higher temperatures yield faster rates of Z-dimension expansion. This increase in rate of Z-dimension expansion is attributed to lowered viscosity of the thermoplastic matrix at increased temperature. Typically, mold temperatures during the lofting, or expansion, phase are in the range of from T_m, to 50° to 100°C higher than T_m, the melt temperature, of the thermoplastic matrix.

The temperature of the composite itself may be less than the melt temperature. During expansion, the lofting mold maintains the composite above the softening temperature of the polymer matrix. The polymer matrix must be sufficiently softened by temperature increase such that the viscosity of the polymer will no longer restrain the elastic energy stored within the strained fibers by the prior consolidation of the composite mat. Upon completion of the Z-dimension expansion, the article has acquired the net shape of the lofting mold. The lofting mold is cooled when the Z-dimension expansion of the article has filled the volume of the lofting mold. The expansion requires the composite to be sufficiently heat softened to permit the stored flexural energy to overcome the resistance of the matrix to expansion.

The practitioner will seek to avoid excessive temperatures which cause degradation of the polymer or temperatures which reduce the viscosity of the polymer matrix of the composite so as to separate the reinforcing fiber from the polymer matrix. Factors to be considered in fixing the time necessary to loft the consolidated object include the temperature stability of

10 the polymer matrix, the thermal conductivity of the composite article, the thickness of the object to be formed, the lofting temperature employed, and the flexural modulus of the reinforcing fiber, among other _._£- factors. The practitioner may empirically optimize each such factor as necessary for the desired end-use.

The composite sheets generally include from 10 to 79 percent of a high modulus reinforcing fiber, a

20 continuous polymer matrix, a polymeric binder, and a fibrillated fibrous pulp.

Fibers of glass are frequently the fibers of choice as reinforcement because of their competitive _pc- price, availability, and properties satisfactory for many applications. However other fiber reinforcement is useful in the practice of this invention. A convenient method of imparting desired additive properties to the heat fusible fiber-reinforced polymeric composite is to

30 incorporate into one or more of the lamina fibers of an additive material that possesses the desired property. For example, iron or stainless steel fibers may be incorporated to provide radiopaque properties. Additional materials which may be incorporated in the composite article in addition to, or as a replacement for, glass fibers, include metallized glass fibers, carbon fibers, graphite fibers, organic fibers, and metallized graphite fibers. Additive fibers and metal coated fibers are effective when present from about 1 to 45 percent by weight of the composite mat. The reinforcing fibers should have an aspect ratio of at least about 25, preferably at least about 80, more preferably about 100 to 2000. The average length of the conductive metal fibers or metallized glass, carbon, graphite, or ceramic fibers is advantageously from 3 to 25 mm, preferably in the range from 3 to 12.5 mm.

In addition to polymer matrix and reinforcing fibers, the composite may also contain optional components, i.e., fillers, dyes, pigments, chemical additives, UV stabilizers, thickeners, bactericides and other ingredients. The density of the consolidated heat fusible fiber-reinforced polymeric composite preform of the article may range from 0.3 to 1.6 g/cc; advantageously the density is greater than 0.7 g/cc.

Advantageously the consolidated composite has less than 20 percent void volume.

Particulate material in the form of finely divided particulate of one or more of conductive, semi- conductive, magnetic, radiopaque, electromagnetic, or absorptive additives is effective for their intended purpose or as a pigment or as a filler when present in the lamina in an amount less than 30 percent by volume. Particulate material is preferably present from 0.1 to about 25, more preferably from 0.1 to 20 volume percent, most preferably from 0.1 to about 10 volume percent of the lamina. Particulate material useful as additives include finely divided iron, iron oxide, steel, silicon, silicon dioxide, germanium selenium, carbon black, graphite, compounds of tungsten, and combinations including these particulate additives.

Polymeric binders useful to provide wet strength and adhesion of the dried aqueous laid composite are aqueous colloidal dispersions of substantially water-insoluble organic polymers having anionic or cationic bound charges in an amount sufficient to provide stabilization of the colloid, but insufficient to cause the polymer to be water-soluble.

10 Such an amount will usually be from about 0.04 to about 0.60 milliequivalent per gram of polymer. The berm "bound to the polymer" with respect to ionic groups or charges,refers to ionic groups or charges which are not -₅ desorbable from the polymer by dialysis against deionized water.

By "available" charge is meant the amount of charge anionizable group would provide to a polymer when 0 fully ionized.

By the term "pH independent groups" as applied to ionic groups is meant that the groups are predominantly in ionized form over a wide range of pH, e.g., 20f12. _r- Representative of such groups are sulfonium, sulfoxonium, isothiouronium, pyridinium, quaternary ammonium groups, sulfate and sulfonate groups.

The essentially water-insoluble organic

30 polymers have a backbone which may be natural or synthetic and may be a homopolymer or a copolymer of two or more ethylenically unsaturated monomers or be derived from such homopolymers or copolymers. Representative organic polymers are natural rubber, the synthetic rubbers such as styrene/butadiene rubbers, isoprene rubbers, butyl rubbers and other rubbery or resinous polymers of unsaturated monomers which are film-forming, preferably at room temperature or below, although in a particular instance a polymer may be used which is film- -forming at the temperature of processing. Non-film- -forming polymers may be sed in blends provided the resulting blend is film-forming. Polymers which are made film-forming by the use of plasticizers may be used. Polymers which are readily available in latex form are preferred, especially hydrophobic polymers which are prepared by emulsion polymerization of one or more ethylenically unsaturated monomers. When in latex form, such polymers advantageously have a particle size of from 500 to 5000A and preferably have a particle size of from 800 to 3000A as measured by electron microscopy.

Cationic structured particle latexes consisting of a water-insoluble, nonionic, organic polymer core encapsulated with a thin layer of a copolymer having chemically-bound pH independent cationic groups wherein the bound cationic charges are at or near the outer surface of the particles according to Bibbs et al., U.S. Patent 4,056,501. Useful anionic latexes are well known in the art and include such products as carboxylated stryrene-butadiene latexes and acrylic latexes which are prepared by emulsion polymerization.

Also useful as a binder is a salt of an ethylene acrylic acid copolymer having an acrylic acid content from about 12 to about 30 percent by weight copolymer solids. Preferably, the acid content is 20 percent by weight copolymer solids. The copolymer is conveniently dispersed in an aqueous phase when present as an aqueous ammonium dispersion or aqueous alkali metal dispersion to form their respective salts.

For example, the ethylene acrylic acid copolymer is stabilized in aqueous dispersions as characterized by the following structural diagram: fCH₂-CH₂-CH-CH₂f + M⁺OR-

C=0

I

CH

(CH₂-CH₂-CH-CH₂) + H₂0 C=0 OM⁺

wherein M⁺ is NH₄ ⁺, Na⁺, K⁺ Li⁺, etc. Preferably, the ethylene acrylic acid is dispersed an an aqueous ammonium dispersion where M⁺ is NH₄ ⁺.

A suitable binder is available through The Dow Chemical Company under the trademark PrimacorTM which is a high melt index (300 to 3,000) ethylene acrylic acid copolymer in an aqueous ammonium dispersion or an aqueous alkali metal dispersion having a variable acid content of 15-20 percent by weight copolymer solids.

When using and ethylene acrylic acid copolymer as a binder material, the pH of the aqueous medium is adjusted to disperse the binder. When the solid ingredients of the aqueous slurry are uniformly dispersed therein, the pH of the dispersion is adjusted to destabilize the system causing the composite ingredients to agglomerate such that they can be collected and formed into a sheet.

Fibrillated pulp of the present invention is formed from the fibers of polybenzoxazole (PBO), polybenthiozole (PBT), or copolymers thereof. 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 Copolymer, 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), which are incorporated herein by reference.

The polymer may contain AB-mer units, as represented in Formula 1(a), and/or AA/BB-mer units, as represented in Formula Kb)

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 about 18 carbon atoms, more preferably no more than about 12 carbon atoms and most preferably no more than about 6 carbon atoms. Examples of suitable aromatic groups include phenylene moieties, tolylene moieties, biphenylene moieties and bis- phenylene ether moieties. 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 about 12 carbon atoms, but the divalent organic moiety is preferably an aromatic group (Ar) as previously described.

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- -membered 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, which is incorporated herein by reference.

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 semirigid in the case of an AB-PBZ polymer. Azole rings within the polymer are preferably oxazole rings (Z = 0). Preferred mer units are illustrated in Formulae 2 (a)-(e).

Each polymer preferably contains on average at least about 25 mer units, more preferably at least about 50 mer units and most preferably at least about 100 mer units. The polymer may also be a random, sequential or block copolymer containing PBO or PBT mer units and mer units of other polymers, such as polyamide, polyimide, polyquinoxaline, polyquinoline or poly(aromatic ether ketone or sulfone) such copolymers are described in Harris et al., Copolymers Containing Polybenzoxazole, •Polybenzothiazole and Polybenzimidazole Moieties, International Application No. PCT/US89/04464 (filed October 6, 1989), International Publication No. WO 90/03995 (published April 19, 1990), which is incorporated herein by reference.

The polymers are spun into fibers from spinnable dopes containing polymer dissolved in a solvent acid, which is preferably polyphosphoric acid and/or methanesulfonic acid. The dope should contain a sufficient amount of fiber to be spinnable to form fibers. The optimum concentration may vary widely depending upon the polymer in the dope and its average molecular weight. In most cases, the dope preferably contains at least about 2 percent polymer and more preferably at least about 4 percent polymer.

When the dope contains a rigid rod polybenzoxazole or polybenzothiazole having an intrinsic viscosity of at least 20 dL/g at about 25°C in methanesulfonic acid (preferably saturated with methane¬ sulfonic acid anhydride), the concentration of polymer in the dope is highly preferably at least about 10 weight percent, more highly preferably at least about 12 weight percent and most preferably at least about 15 weight percent. When the dope contains a rigid rod polybenzoxazole or polybenzothiazole having an intrinsic viscosity in methanesulfonic acid of at least 20 dL/g, the maximum concentration of polymer in the dope is limited primarily by practical considerations, such as solubility and viscosity. The concentration is ordinarily less than about 20 percent and preferably no more than about 17 percent.

The dope is spun to form a fiber by a dry jet- -wet spinning process. Such processes are described in Chenevey et al, "Formation and Properties of Fiber and Film from PBZT," The Materials Science and Engineering of Rigid-Rod Polymers 245 (Materials Research Society 1989); and Ledbetter et al., "An Integrated Laboratory Process for Preparing Rigid Rod Fibers from Monomers," The Materials Science and Engineering of Rigid-Rod Polymers 253 (Materials Research Society 1989), which are incorporated herein by reference. The spun and drawn dope fiber is coagulated in a liquid which dilutes the solvent acid and is a non-solvent for the polymer. It is advantageous if the liquid is freezable. The freezable non-solvent liquid may be organic, but it is preferably aqueous. Aqueous coagulants may be basic or mildly acidic, but are preferably about neutral, at least at the commencement of coagulation. The most preferred freezable non-solvent liquid is water.

The coagulated fiber has a relatively open structure containing the coagulant liquid. Once the fiber has been dried, it has very little water regain and can not be effectively rewetted, so that grinding of a fiber which has been dried and rewetted is much less effective. From the stand point of both convenience and effectiveness, it is important to keep the coagulated fiber wet and freeze it with the coagulating non-solvent without drying. Although the fiber may be re-wet by methods such as boiling. The wet fiber suitable for freezing contains the polymer or copolymer and the freezable liquid, as previously described. The weight ratio of freezable liquid to polymer is preferably at least about 10:90 and more preferably at least about 50:50. It is preferably at most about 95:5.

The wet fiber is frozen to a temperature at which it becomes brittle. For the purposes of this application, the term "freezing" refers broadly to any

10 solidification by reduction in temperature, without regard to whether a crystalline structure or a glassy solid is formed. For fibers containing aqueous liquid, the temperature is preferably less than 0°C, more -.- preferably at most about -100°C and most preferably at most about -190°C. A convenient temperature is at about liquid nitrogen temperatures.

Once frozen, the fiber is mechanically reduced

20 to a desired length and degree of fibrillation, such as by grinding, crushing, tearing, cutting and/or chopping. The preferred techniques vary depending upon whether short fibers or pulps are desired. To obtain a pulp, it is preferred to grind, tear or crush the fiber, so that 25 extensive fibrillation occurs. Cryogenic grinding equipment is known and described in numerous references, such as U.S. Patents 2,347,464; 3,480,456; 3,921,874; 4,846,408 and 4,884,753, which are incorporated herein by reference. To obtain a short fiber, it is preferred

30 to scissor, chop or otherwise cut the fiber by a means such that little or no fibrillation occurs.

The short fiber or pulp may be returned to warmer temperatures, dried and used, for instance by impregnating with a matrix resin and curing to provide a composite.

The length of short fibers and fibrils within pulps is preferably no more than about 1/2 inch, more preferably no more than about 1/4 inch and most prefer¬ ably no more than about 1/8 inch. Pulps are preferably highly fibrillated. They preferably have an average fibrillar diameter of no more than 10 μm, more prefer¬ ably no more than about 5 μm and most preferably of no more than 1 μm. Short fibers preferably have a diameter about the same as that of the original fiber. Their average diameter is preferably more than 10 μm and more preferably at least about 15 μ . Segments of the short fiber may be partially fibrillated, but the short fiber is preferably not substantially fibrillated and most preferably essentially unfibrillated, except at the ends.

The short fibers and pulps of the present invention are preferably substantially uniform. When the average length or width of a pulp or short fiber is limited as previously described, then preferably no more than about 20 percent of the short fibers or pulp fall outside that limit, more preferably no more than about 10 percent.fall outside that limit, and most preferably no more than about 5 percent fall outside that limit. Among pulps, preferably no more than about 20 percent of the pulp is unfibrillated, more preferably no more than about 10 percent, and most preferably no more than about 5 percent. Among short fibers, preferably no more than about 20 percent of the fiber is fibrillated, more preferably no more than about 10 percent, and most preferably no more than about 5 percent. The process and resulting fibers and pulps of the present invention have several advantages over processes and resulting pulps from simply chopping or grinding a dried fiber. Dried fibers are very difficult to cut or fibrillate. Therefore, attempts to cut them cause excessive wear on grinding and cutting equipment and ordinarily yield to very inconsistent quality cut fibers or pulp, containing irregular lengths of fiber, some of which is highly fibrillated and some of which is essentially unfibrillated. On the other hand, frozen wet fibers are more brittle. They cut, grind, crush and tear more easily without excessive wear to the equipment, and the resulting short fiber or pulp product is much more uniform. The degree of fibrillation can easily be selected from uniformly highly fibrillated to essentially unfibrillated or degrees of fibrillation in between by proper selection of the cutting or grinding or other technique.

It is also possible to prepare fibrillated fibers from the fibers of PBO, PBT, or copolymers which have not been preserved in a wet condition from their spinning. To prepare pulp from previously dried fibers, the fibers are boiled in water from one-half to several hours. The fibers then cooled to room temperature, frozen, and then ground.

The invention disclosed herein is exemplified by the following examples and comparative examples but the invention is not limited by the examples. The skilled artisan will readily extend the invention disclosed and exemplified herein to equivalent elements. The invention includes all equivalents of the invention as claimed. Example 1

A sample of PBO fiber having a diameter of 15 μm having a length of about 0.37 mm (0.015 inches) is soaked in boiling water for 1 hour. The PBO fibers are prepared from polymers having as a repeating unit the following structure (a) according to Formulae 2. The boiled fibers are then cooled to room temperature in water. Wet room temperature fibers are then frozen in liquid nitrogen by immersion for about 1 minute. The fibers are then ground using a Retsch Centrifugal grinder operating at 20,000 rpm using a 0.5mm screen. A small amount of liquid nitrogen is fed into the grinding chamber before, and during grinding to maintain the grinding temperature below 0°C.

Fibrillated pulp results from the cryogenic grinding operation. When examined by electron microscope, the fibrillated nature of the fibers is disclosed as seen in Fig. 8 and Fig. 9. The fibrillated fibers have diameters less than 3 μm, preferably, less than 1 μm.

Comparative Example 2

Heat treated PBO fiber, having a diameter of 15 μm cut to a length of 0.1 cm are boiled in water 1 hour as Example_.1. Heat treatment of the fibers comprises placing the fibers under mild tension and exposing the fibers to temperatures of 500°C or more 30 seconds as taught by U.S. patent 4,554,119. The boiled fibers are cooled to room temperature in water, then frozen in liquid nitrogen, and ground as the fibers in Example 1 for an equivalent time. A small amount of localized fibrillation is observed, however, in general the fibers are not fibrillated. An example of the ground heat treated fibers is shown in Fig. 6 and Fig. 7.

Example 3

Random fiber composite of crystalline polyamide matrix and graphite fibers are prepared using PBO pulp prepared according to Example 1. In a vessel containing 30 liters of water thickened with KELZAN™ XC xanthan gum, manufactured by Kelco Company, San Diego, California, to a viscosity of about 2 centipoises are dispersed 9.0 g of PBO pulp prepared according to Example 1 and 90g of graphite fiber having a length of 6.35mm (1/4 inch) and a diameter of 7.5 μm (0.0003 inch). The fibers used are secured from Fortafil Fibers, Inc. 26185 Matlin Rd. Ramona, California 92065 USA, although similar graphite fibers are effective. The resulting slurry is stirred under high sheer for about 5 minutes. Continuing the stirring, 190.5 g of a powdered crystalline polyamide (poly (4,4p-methylene diphenylene azelamide)) neat polymer containing 1 percent talc as a nucleation aid and 1 percent IN0RGAN0X^™ as an antioxidant having a melt index of 20g per ten minutes at 310°C. The polyamide is prepared by the reaction of 4,4'diphenylmethane diisocyanate (MDI) and azelaic acid under melt conditions, then melt mixing the antioxidant and the talc. 10.5 Grams solids of a 54/45/1 styrene/butadiene/fumaric acid latex are added, then 110 g of an aqueous solution containing 0.2 percent of a cationic acrylamide flocculant sold under the trademark BETZ™ 1260 available from Betz Labs, Inc., Philadelphia, Pennsylvania, is added slowly to the mixing slurry. It is understood the flocculant designation is now discontinued, and that an equivalent flocculant is sold by Betz Labs, Inc. under the designation 1160X. After 1 minute of mixing, 5 liters of the slurry is poured into the head box of a laboratory sheet former having a square dimension of 20.3 cm by 20.3 cm (8 inch by 8 inch) available from M/K Systems, Inc. Linway, Massachusetts. The head box contains 5 liters of water at the time of the addition. The slurry is mildly agitated and dewatered. Solids are collected on a 177 μm (80 mesh) screen, wet-press, and air dried at room temperature to form a composite mat of 30 percent reinforcing fiber and 3 weight percent fibrillated PBO fiber. The composite mat having a mass sufficient to make a consolidated composite sheet of a thickness 3.175 mm (1/8 inch) is stacked in a heatable sheet mold. The composite is heated in the absence of oxygen. A vacuum bag arrangement is used drawing a vacuum to 73.5 cm mercury (29 inches of mercury vacuum). An inert atmosphere such as nitrogen would also be effective.

The major surfaces of the sheet mold in contact with the stacked composite are coated with FREK0TE™ mold release. Initially the stack is heated to 110°C for 10 minutes under 349 kPa (50 psi) to remove moisture. This stack is then removed from the sheet mold while the mold is heated to 325°C. The stack is replaced in a 325°C sheet mold under vacuum and compressed by 345 kPa (50 psi) for 2 minutes. Pressure is then increased to 2,068 kPa (300 psi) for 2 minutes. A composite sheet having a thickness of 3-175 mm results.

Next, a consolidated composite sheet was heat expanded. The vacuum on the composite sheet is released and the vacuum bag filled with nitrogen. The sheet forming mold is spaced to permit expansion of the sheet perpendicular to the major surfaces of the sheet to an expanded thickness of 12.7 mm (1/2 inch). The composite sheet was permitted to expand in the Z-direction as the sheet forming mold and the composite cooled to room temperature. The expanded composite substantially fills the mold and has a density of 0.347 g/cm (21.592 lbs/cu. ft.).

Example 4

According to the method of Example 3, a heat expanded composite sheet is prepared from a sheet comprising 15 g of fibrillated PBO fibers. A corresponding amount of graphite reinforcing fiber was removed from the composite leaving the polyamide matrix and latex content unchanged. The resulting sheet has a solids composition of 63-5 weight percent polymeric matrix, 28 weight percent graphite reinforcing fiber, and 3-5 percent latex, and 5 percent fibrillated PBO pulp. The expanded composite substantially fills the mold and has a density of 0.363 g/cc (22.583 lbs/cu. ft.).

Example 5

Following Example 3, a composite sheet is prepared having a solids composition of 63.5 weight percent polymeric matrix, 23 weight percent graphite reinforcing fiber, and 3.5 percent latex, and 10 percent fibrillated PBO pulp. The fully expanded composite sheet has a density of 0.387 g/cc (24.121 lbs/cu. ft.).

Comparative Example 6

The preparation of Comparative Example 6 follows the procedure of Example 3, except all fibrillated PBO fibers are substituted with the same weight (9 g, 3 weight percent) of aromatic polyamide pulp available from E.I. DuPont de Nemours Co., Wilmington DE 19898, under the trade designation KEVLAR™ type 979 having a nominal length of 2mm. Fig. 3 and 4 are electron micrographs of KEVLAR™ polyaramide fibrillated pulp.

Fig. 5 is an electron micrograph of an unconsolidated composite prepared by the aqueous process including KEVLAR™ polyaramide fibrillated pulp. The pulp is visible bridging between reinforcing fibers in the micrograph.

The expanded sheet substantially fills the mold. The density of the expanded sheet is 0.340 g/cc (21.175 lbs/cu. ft.).

Comparative Example 7 and 8

Following Comparative Example 6, Comparative Example 7 and 8 replace the graphite reinforcing fibers with 5 and 10 weight percent fibrillated KEVLAR™ fibers, respectively. Comparative Example 7 expands to substantially fill the available space of the mold and has a density of 0.384 g/cc (24 lbs/cu. ft.).

Comparative Example 8, comprising 10 weight percent KEVLAR™ pulp, expansion of Comparative Example 8 leaves a visible void volume in the mold. The density of the expanded composite is 0.428 g/cc (26.657 lbs/cu. ft.).

Table 1 compiles the density of the examples 3 through 8. It is shown that fibrillated PBO fibers contribute to the Z-dimension expansion of a consolidated random fiber composite sheet. TABLEI

Theoretical Density Degree of Expansion = Measured Density

*Aromatic Polyamide

Claims

CLAIMS:

1. A fiber reinforced polymeric composite heat expanded in thickness to avoid volume of from 20 to 90 percent by volume comprising a continuous matrix of a solid polymeric resin, distributed throughout said matrix from 10 to 60 percent by weight of the composite of randomly oriented reinforcing fibers having an average length from 3 mm to 25 mm, an aspect ratio of at least 40 and a fibrillated polymeric pulp of polybenzoxazole, polybenzothiazole, or copolymers thereof.

2. The fiber reinforced polymeric composite according to Claim 1 wherein the polymeric matrix is selected from the group consisting of polyethylene, high density polyethylene, linear low density polyethylene, polypropylene, chlorinated polyethylenes, polycarbonates, ethylene/acrylic acid copolymers, polyamides, polyphenylene oxide resin, polyphenylene sulfide resins, polyoxymethylenes, polyesters, the ABS (acrylonitrile-butadiene-styrene) resins, polyvinyl chloride, vinylidene chloride/vinyl chloride resins, polyetheretherketones, polysulfones, polyetheramides, and vinyl aromatic resins, such as polystyrene, poly(vinylnaphthalene), poly(vinyltoluene).

3. The fiber reinforced polymeric composite according to Claim 1 wherein the polymeric pulp also comprises pulp of an aromatic polyamide, or a polyolefin.

4. The fiber reinforced polymeric composite according to claim 1 wherein the fibrillated polymeric pulp has a fibrillar diameter of no more than 10 μm.

5. The fiber reinforced polymeric composite according to claim 1 wherein no more than 20 percent of the pulp is unfibrillated.

6. A method of making a fiber-reinforced resin composite having a void volume of from 20 to 90 percent comprising

(a) heating a densified fiber-reinforced resin composite above the softening temperature thereof, wherein said composite comprises a continuous heat fusible resin matrix;

(b) distributed throughout said matrix from 10 to 79 percent by weight of the composite of reinforcing fiber, wherein said fibers are randomly oriented in two dimensions substantially in the plane defined by said sheet and have an average length from 3mm to 25mm (0.12 to 1 inch) and an aspect ratio of at least 40; and

(c) from 1 to 10 percent by weight of the composite of a fibrillated pulp of polybenzoxazole (PBO), or polybenzothiazole (PBT) or copolymers thereof;

being prepared from a consolidation of one or more fiber reinforced resin sheets heated to a temperature above the softening temperature of the heat fusible resin whereby the composite is expanded In a direction perpendicular to that of a plane defined by said sheet.

7. The method of Claim 6 wherein the composite is heat expandable to a density less than a comparable composite comprising a fibrillated pulp other than polybenzoxazole, polybenzothiazole, or copolymers thereof.

8. The method of Claim 7 wherein the consolidated fiber reinforced resin composite includes from 1 to 10 percent of a latex binder.