CA2044787A1 - Fiber-reinforced composites toughened with porous resin particles - Google Patents

Abstract

ABSTRACT
Layered, fiber-reinforced composites toughened by the use of thermosetting matrix resins including rigid particles are improved in toughness when the particles are porous and formed of a polyamide. The method of the invention is particularly effective in improving the toughness of composites based on thermoset matrix resins, and particularly epoxy resins.

Description

FIBER-REINFORCED COMPOSlTES TOUGHENED WrrH
POROUS RESIN PARTICLES
BACKGROUND OF THE INYFNTION
This invention relates to composite materials and more pardcularly to tough, impact resistant fiber-reinforced composites. Sdll more particularly, this invendon relates to methods for toughening fiber-reinforced composi~es and to particles useful in toughening such composites.
10 Fiber-reinforced composites are high strength, high modulus materials which are finding wide acceptance for use in sporting goods and in producing consumer items such as appliances. Composites are also ~lnding increased acceptability for use as structural components in automodve applicadons, as components of buildings and in aircraft.When used in structural applicadons the composites are typically formed of continuous 15 fiber filaments or woven cloth embedded in a thermosetting or thermoplasdc matrix.
Such composites may exhibit considerable strength and stiffness, and the potendal for obtaining significant weight savings makes them highly attracdve for use as a metal replacement. However, acceptance for many structural applications has been limited by the brittleness of many of the presently available composite materials. The inability of 20 such composites to wi~hstand impact while retaining useful tensile and compression strengths has been a serious problem for many years. Low impact resistance in composites may ordinarily be accomplished by using larger amounts of ~he composite.
However, this approach increases costs, reduces the weight savings that might otherwise be realized and may make them unacceptable for many uses.
25 The composites indllstry has long been involved in finding ways to overcome these deficiencies. Considerable effort has been expended over the pass t vo decades directed ~oward the development of composites with improved fiacture toughness. Inasmuch as most of the commonly employed matrix resins, as well as many of the reinforcing fibers, are generally brittle, much of that effort has gone into a search for components having 30 better toughness characteristics. As a consequence, the search for toughened matrix resins has become the subject of numerous recent patents and publicadons.
For decades, the plastics industry has used rubber modifiers to toughen rigid, frequently brittle thennoplastic and thermoset engineering resins. Most often the rubber isdispersed in the form of particles throughout the rigid resin. Various means for altering 35 the interaction between the rubber particle and the rigid phase to improve the 2 ~v~q~

effectiveness of the rubber component have also been explored. Foq exarnple, the rubber components have been modified by grafting to change compatibility wi~h the rigid phase, and adding reactive functional groups to the rubber to promote bonding to the rigid phase has also been shown to be effective. Other approaches have included the combining of 5 dissimilar resins, forming blends and alloys with improved properties.
The methods used for toughening engineering resins have been adapted for the toughening of the matrix resins commonly used in composite structures, as shown for example by Diamant and Moulton in "Development of Resin for Damage Tolerant Composites - A Systematic Approach", 29th National SAMPE Symposium, April 3 - 5,10 1984. The forming of alloys and blends by adding a more ducdle therrnoplastic such as d polysulfone to an epoxy resin forrnulation has also been shown to improve the ductility of the epoxy resin and provide enhanced toughness, according to British patent 1,306,231, published February 7, 1973. More recently, combinations of an epoxy resin with terminally functional thermoplastics were shown to exhibit enhanced toughness.
See U.S. patent 4,498,948. Still more recently, curable combinations of epoxy resins and therrnoplastics with reacdve terminal functionality were also said to improv e the toughness of specifically forrnulated matrix resins, provided that the neat resin after curing exhibits a specific phase-separated morphology having a cross-linked glassy phase dispersed within a glassy continuous phase. See U.S. paten~ 4,6~6,208. Further 2 0 improvements are said to be achieved by including a reactive rubber component which is said to be contained within the cross-linked dispersed glassy phase. See U.S. patent 4,680,076. Still more recently, the use of an infusible particle made from a rubber disp¢rsed within the phase-separated cross-linked epoxy ~esin matrL~c has been suggested for toughening composites based on such matrix resins. See U.S. pa~ent 4,783,50~.
25 Although the addition of rubber, thermoplastics and the like generally improves the ductility and impac~ resistance of neat resins, the effect on the resulting composites is not neces~a~ily beneficial. In many instances the increase in composite toughness may be only marginal, and a reduction in high temperature properties and in resistance to environmental ex~emes such as exposure to water at elevated temperatures frequently is 30 seen. Composite structures that rely on comples~ manufacturing methods or on unique resin morphologies that are difficult to reproduce for achieving improvements intoughness may require an impractical degree of control duling fabrication, adding to the production cos~ and often resulting in elTatic per~ormance and poor leliability.
An alternative approach to producing toughened composites has been the developrnent of 3 5 layered composile strucsures ha~dng layers formed of fibers imbedded in a matrix resin alternated with layers forrned of a thermoplastic resin, described in Japanese patent 3 ~ . 3~' ~

application 49-132669, published May 21, 1976. More recently, in U.S. patent 4,6S)4,31g, there were disclosed layered fiber-resin composites having a plurality of fiber-reinforced mamx resin layers inter-leafed with therrnoplastic layers adhesively bonded to the reinforsed matrix resin layers. Inter-leaf structures are ordinarily produced S by impregnating continuous fiber to form prepreg, then laying up the composite by alternating prepreg with sheets of thermoplastic ~llm. The laid-up structure is then subjected to heat and pressure, curing the ma~rix resin and bonding the layers. The patent also discloses inter-leaf layers which comprise a thermoplastic filled with a rein~orcing material such as chopped fibers, solid particles, whiskers and the like.
10 Although inter-leafed composite structures with improved toughness have been disclosed, there has been some sacrifice in other physical properties, including a reduction in glass transition temperatures together with an increase in creep at high temperatures. Further difficulties with such composites may include a loss in stiffness for many such compositions, adhesive failure that may occur between layers forrned of 15 dissimilar resins and property deterioration during use due to poor solvent resistance. In addi~ion, prepregs based on thermoplastic resin generally are lacking in tack, which complicates their fabrication into composites and increases the degree of skill needed to fabricate complex structures. This may in turn result in increased scrap losses and a need for more complex quality control procedures, increasing manufacturing costs in order to 2 O achieve an acceptable level of reliability.
Recently, the use of an infusible particle made from a rubber dispersed within a phase-separated cross-linked epoxy resin matrix has been suggested for toughening composites based on such mat~ix resins. See U.S. Patent 4,783,506. Dispersing rigid particulate modifiers in the ma~ix resin has also been disclosed in the art for toughening composite 2 5 materials, and has been described fo~ example in published European Patent Applications 0 274,899 and 0 351,025 as well as in U.S~ Patent 4,863,787, the teachings of the latter t~ee r~ferences are hereby incorporated by reference.
The compositions and methods presently available for producing toughened composites thus require further improvement. Composites having improved resistance to impact and 3n particularly those with better compressive strength after impact would be a useful advance in the ar~, and reliable methods for producing such toughened composites could find rapid acceptance, displacing the more complex and expensive manufacturing processes currently available for these purposes.

4 ?J'~,7~ J ~?

SUMMARY O~ THE INVENTION
The present invention is directed to layered composite structures comprising continuous fiber and a matrix resin formulation, and more particularly to improved layered composite structures comprising continuous fiber embedded in a matrix resin formulation 5 toughened with a particulate modifier, the improvement comprising the use of aparticulate modifier having an essentially spheroidal, spongy structure, also described as porous polyamide particles, and to a method for producing toughened layered composite by incorForating porous polyamide particles in the matrix resin within the inter-ply spacing of the layered composite before cunng. The resulting cornposite structures 10 exhibit a marked and unexpected improvement in toughness.
DETAILED DESCRI~ION
The improved composite structures of this invention comprise discrete layers forrned of continuous fiber embedded in a matrix resin, the layers being separated or spaced normally apart by laminar regions or layers comprising matrix resin filled with finely-15 divided polyamide resin in the form of pardcles having an essentially spheroidal, spongystructure, also described as porous polyamide pardcles.
_ e Matrix Resins The matrix resins useful in forming toughened composites according to the practice of this invention are the resin formulations commonly used in the fiber-reinforced ~0 composites art and may include both thermosetting and thermoplastic materials.
However~ thermosetting resins will be preferred for most applicadons and the thermoset resins disclosed to ~ useful in the practice of this invention will include those most commollly employed for making fiber reinforced composites such as epoxy resins, cyanate resins, bismaleimide resins, BT resins comprising a combination of cyanate and 2s bismaleimide resin components, mixtures of such resins and the like, as well as the widely osed cross-linkable polyester resins. Many thermose~ resins generally possess low ductility and consequently are quite brittle, and composite structures based on such resins are therefore greatly benefited when toughened according to the leachings hereof.
The preferred matrix resin forrnulation will thus be based on a therrnoset resin, and 30 particularly preferred are the well known and widely used epoxy formulations comprising in general an epoxy resin and an appropriate curing agent such as a diarnine hardener or the like. The epoxy forrnulations may optionally include an appropriate curing accelerator and such additional components as are commonly employed in the therrnoset composi~e a~.

The epoxy resins which may be employed are curable epoxy resins having a plurality of epoxy groups per molecule. Such resins are commonly employed for producing composite materials, and many are readily available from commercial sources. Examples of such resins are polyglycidyl compounds, including the reaction products of 5 polyfunctional compounds such as alcohols, phenols, carboxylic acids, aromadc amines or aminophenols with epichlorohydrin, and epoxidized dienes or polyenes. Furtherexarnples include diglycidyl ethers of diene-modi~led phenolic novolacs, cyçloaliphatic epoxides such as the reaction products of polyfunctional cycloaliphatic carboxylic acids with epichlorohydrin, cycloaliphatic epoxides, cycloaliphatic epoxy ethers and 10 cycloaliphatic e.poxy esters and the like. Mixtures of epoxy resins may also be used.
Prefe~red epoxides include Bisphenol A epoxides, epoxy novolacs, cycloaliphatic epoxy ethers and glycidyl amines. A wide variety of these epoxy resins are available fTom commercial sources under trade names such as PGA-X from Sherwin Williams Company, DEN 431 and Tactix 56 from Dow Chemical Company, Glyamine 125 from F.I.C. Corp. and RD87-160, XU MY-722 and MY-720 from Ciba-Geigy Corp.
Diamine hardeners which may be used include the aromatic diamines conventionallyemployed in for nulating epoxy resins, such as for example, 4,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl methane, 4,4'-diaminodiphenyl sulfone, 3,3'-diaminodiphenylsulfone, p-phenylene diamine, m-phenylene diamine, 4,4'-bis(aminodiphenyl) propane, 2 o 4,4'-diaminodiphenyl sulfide, trimethylene glycol bis(p-aminobenzoate) and the like, as well as their various position isomers. Also useful are the variety of polynuclear aromatic diarnine hardeners such as those disclosed for example in U.S. patents 4,579,885 and 4,517,321, and U.S. patent 4,S86,250, all incorporated herein by reference, as well as xylylene diamine, bis(arninomethyl) cyclohexane, dicyandiamide 2 5 and the like. The valious diamine hardeners may be used alone or in combination.
Suitable epoxy resin formulations may be prepared accor~ing to methods and practices well known and widely used in the resin art. Generally the matrix resin formulations will comprise greater than 2 parts by weight (pbw) diamine hardener per hundred parts by weight epoxy resin. Although the particular level selected will depend upon the 3 0 particular diamine employed, preferably at least 3 pbw and more preferably from about 6 to about 150 pbw diarnine hardener per hundred pbw epoxy resin will be used. Thearnount of each component selected will depend upon the molecular weights of theindividual components and the molar rado of reacdve amine (N-H) groups to epoxy groups desired in the final matrix resin system. For most prepreg and composite 3 5 applications, sufficient diamine hardener will be used to provide a molar ratio of N-H
groups to epoxide groups in the range of from about 0.3/1 to 1.8/1, preferably from 0.4/1 to 1.311.

6 ~ t.,~ ~ 3 ~1 The forrnulations may further include a therrnoplastic polymer to impart improved toughness of the resulting composite by increasing the ductility and impact resistance of the cured resin formulation. When dissolved in the formulation prior to curing, thermoplastics may also increase the viscosity and film strength of the uncured resin 5 thereby improving the resin processability for use in impregnadng operations, and can provide prepreg with better handling characteristics for use in composite fabrication. A
variety of thermoplasdcs are known in the art for use in combination with epoxy resins, including for example polyaryl ethers such as polyaryl sulfones and polyaryl ether sulfones, polyether ketones, polyphenylene ethers and the like, as well as polyarylates, 10 polyamides, polyamide-}mides, polyether-imides, polycarbonates, phenoxy resin3 and the like. Where the purpose for including the thermoplasdc is to improve viscosity, processability and handling characteristics, the thermoplastic selected will necessarily be soluble in the uncured epoxy resin formu~ation. The proportion of thermoplastic employed will depend in part upon the thermoplastic selected and the particular end use 15 envisioned. However, for most purposes, the formulation will cornprise from 0 to 30 pbw of thermoplastic per 100 pbw of the cornbined diamine hardener and epoxy resin components.
The epoxy formuladons may addidonally include an accelerator to increase the rate of cure. The acceleralors will be selected from those hlown and used in the epoxy resin art 20 may be employed in conventional amounts. Accelerators which may be found to be effective include Lewis acid:amine complexes such as BF3:monoethylamine, BF3:tnethanolamine, BF3:piperidine and BF3:2-methylimidazole; amines such as imidazole, l-methyl imidazole, 2-methyl imidazole, N,N-dimethylbenzylamine and the like; acid salts of ter~ary amines such as the p-toluene sulfonic acid:imidazole complex 2 s and the li~e, salts of trifluoromethane sulfonic acid such as FC-520 (obtained from 3-M
Company), organophosphonium halides, dicyandiamide, 4,4'-methylene bis~phenyl-dirnethyl urea) and l,1-dimethyl-3-phenyl urea. Mixtures of such accelerators may also be employed For some end uses it may also be desirable to include dyes, pigments, stabilizers, thixotropic agents and the like, and these and other additives may be included 30 as needed at levels commonly prasticed in the composite art. Upon curing, the matrix resin formulations, exclusive of any particula~e additives, fillers and reinforcement which may be employed, will form a substantially single, continuous rigid phase.
The Fibers Prior to curing, the matrix resin fonnulation will be combined with continuous flber 3 5 rein~orcement or structural fibers and the particulate modifier that will be used in forming toughened composites acco~ding to the practice of this invention. Suitable fibers may be 7 ~ r\ ~ j characteriæd in general terms as having a tensile strength Qf greater than lO0 kpsi and a tensile modulus of greater than two million psi. Fibers useful for these purposes include carbon or graphite fibers, glass fibers and fibers formed of silicon carbide, alurnina, titania, boron and ~he like, as well as fibers formed from organic polymers such as for 5 example polyolefins, poly~benzothiazole), poly(benzimidazole), polyarylates, poly(benzoxazole), aromatic polyamides, polyaryl ethers and the like, and may include mixtures comprising two or more such fibers. Preferably the fibers will be selected from glass fibers, carbon fibers and aromatic polyamide fibers such as the fibers sold by the DuPont Company under the trade name Kevlar. The fibers may be used in the form of continuous tows of typically from 500 to 42,000 filaments, as continuous unidirectional iape or as woven cloth.
The Particulate Modifiers The polyamide particles employed in the practice of this invention comprise a finely divided polyamide resin and have an essentially spheroidal, spongy structure. The 15 polyamide particles generally have a mean diameter between 1 and 75 microns, preferably from about l to about 25 microns, and still more preferably from about 2 to about l~ microns. The polyamide particles may be further characterized as porous, that is, as having a large internal pore volume, and powder forrned of such polyamideparticles will have a large specific surface, ordinarily greater than 5 m2/g and preferably 2 o greater than about 9 m2/g, to as great as 30 m2/g or more. The specific surface of such powders is determined according to the classical BET method. It will be understood that the porous yolyamide par~icles in other physical forms such as flake, cylindrical polyarnide particles or fibrid-like materials may also be useful in ~he practice of the invention, however such fonns are not preferre~ Pore volume may a]so be considered a 2s measure of the polyamide particle porosity, and the polyamide particles useful in the pracdcr of this invention ~pically have high pore volumes, preferably greater than about 1.5 cm3/g to as gIeat as 3.~ cm3/g or even greater.
The porous polyarnide particles useful in the practice of this invention may be formed of any rigid polyamide. The polyamide selected will have, in its ~mal particle form, 30 sufficient thermal resistance, hardness and rigidity to resist being melted, compressed or flattened under the pressures and ternperatures that will be encountered during the fabricadng and curing of the laminate. In addition, the polyamide will be selected to be substantially insoluble in the matrix resin font~uladon prior to gelation, in order to preserve the unique surface characteristics of the particle.
35 The polyamide resins that may be used will include any of the readily available nylon resins such as polycaprolactam (nylon 6), poly(hexame~hylene diamine sebacamide 8 2 ~ ` I 7 s ,3 , (nylon 6,6), polyundecanoamide (nylon 11), polydodecanoamide (nylon 12) and the like. The preparation of particles from such resins with the requisite porosity has been described in the art, for example in U.S. 4,831,061, the teachings of which are incorporated herein by reference, as well as in U.S. 2,3~9,877 and in Japanese published application 62-240325. A variety of porous polyamide particles and porous nylon-coated particulates such as titaniurn dioxide particles which may be suitable for use in the practice of this invention are available, including porous nylon 6 particles, porous nylon 11 particles and porous nylon 12 particles.
The particulate modifiers used in the fabrication of composites may comprise only the porous polyamide particles of this invention or may be mixtures of such particles Witi'l non-porous particles formed for example from a cross-linked rubber or from a rigid resin such as polystyrene, polyphenylene ethers, polysulfones or the like, including any of the wide variety of particulate modifiers that are known in the art for use in toughening composites.
The Composite ~tructures The toughened composite structures of this invention comprise discrete layers forrned of continuous fiber embedded in a matrix resin, the layers or plies being separated or spaced norrnally apart, the layer surfaces defining laminar regions or spacing layers comprising matnx resin filled with the porous polyamide pardcles. The pardcles serve to separate 2 0 the plies, and the thickness of the ply spacing will thus be directly related to the particle size.
As used herein, the tenn "pardcle size" refers to the particle dimension deterrnining the ply separation, which for small, irregular or substantially spherical particles is ordinarily the particle diameter. Inasmuch as it will not be pracdcal in most instances to obtain 2 5 pardcles uu~ifonn in size throughout, the particulate modifiers will ordinarily comprise mixtures of particles ensompassing a variety of particle sizes. The particle size may be determined by any of the variety of standard methods, such as by use of a Coulter counter or "Multisi~er" appairatus, or by a Granulometer device. Particle modifiers useiful and effective in toughening composites according to the practice of this inven~ion 30 have the majority of the particles with a mean diameter lying in the range of from 1 to about 75 microns. Mixtures of powdered particulate materials suitable for the purposes of this invention may be obtained by classifying particle mix~ures using well-lcnown methods such as scIeen classification and the like.
Use of par~culate mixtures comprising a wide variety of particle sizes may have other 3 5 detrirnental effects and therefore be less preferred. Dispersing mixtures of particles in the matrix resin forrnulation uniformly may be made more difficult by the presence of very large particles, and the coating characteristics of the filled resins will be more variable.
The presence of a small number of very large (>50 micron) particles widely dispersed in the film of unculed filled matrix resin adhered to one or both surfaces of a prepreg tends 5 to create significant peaks or high spots at the outer surface. The presence of such high spots has the effect of an apparent surface roughness, reducing the surface tack of the prepreg by preventing full and effective contact between layers in a layup operation. The reduced tack will be particularly noticeable for particle mixtures that comprise a wide distribution of particle sizes, hence, narrowly disperse particle mixtures will be 1 o preferred.
The proportion of each component employed in fabricating the toughened composites of this invention will depend in part upon the end use envisioned, as well as on the particular resin, fiber and resin particles selected. Overall, the composites will comprise from about 20 to about 80 wt% continuous fiber, the balance comprising matrix resin 15 and particles, with the particles amounting to from I to about 25 wt% based on combined weight of the particles and the matrix resin formulation. Although the level of resin particles needed to toughen the composite will lie within the stated range, the optimum level will necessarily vary depending upon the type of matrix resin, the fiber loading, the particle type and similar factors, and must therefore be determined for the particular fiber 2 o and resin system employed. In general, it will be desirable to employ the lowest level of particles that will impart the desired improvement in composite ~oughness. Although greater than optimum levels may be employed, further improvements in toughness will be marginal, and other physical properties such as hot/wet strength may be detrimentally affected. Composites having a very high fraction of the particles located in the interply 2 5 spacing are believed tO ~e most effective in providing improvements in toughness at a minimum level of particles.
Composite Fabrication Medlods ordinarily used for the production of layered composites may be readily adapted for fabricating ~e composites of this invention. Most commonly, such composites are 30 formed from impregnated tape comprising uniformly disposed, parallel f~laments of continuous fiber, or from resin-impregnated fabric woven from condnuous fiber tow.
These impregnated fiber structures, designated prepreg, may be produced by impregnating tape or fabric with matrix resin formulation in an uncured state using any convenient method including melt coating, calendaring, dip impregnation with a resin 3 5 solution or molten resin, melt pressing the tape or f~bric into a film of the matrix resin or the like.

~ t~`J

The composite will then be formed by laying up sheets or tapes of the prepreg to forrn a layered stack or lay-up, and curing the lay-up, usually with heat and under pressure.
The prepreg layers, each comprising continuous fiber and matrix resin in uncured forrn, will have their adjoining surfaces adhered upon curing to forrn a single structure having 5 discrete layers of continuous fiber ernbedded in an essentially continuous and substantially homogeneous matrix resin phase.
In forrning the toughened composites of this invention, it will be necessary to dis~ibute the porous polyarnide particles unifonnly between each of the prepreg layers. A variety of methods may be used for this purpose, and the placing of particles at a surface of the 10 prepreg may be carried out as a separate step prior to or during the lay-up operation, or integrated into the step of impregnating the tape or woven fabric. The former will be referred to as tw~step processes, while the latter are termed one-step processes.
Methods for carrying out the two-step process include physically distributing the particles by a sprinkling, spraying, spreading or similar operation on a surface of each 15 prepreg tape or sheet during the lay-up operation; dispersing the particles uniforrnly in liquisi matrix resin formulation and coating the mixture on a surface of the prepreg;
forrning a film of palticle-filled matrix resin formulation and inter-leafing the prepreg layers with the film during the lay-up operation and the like. Two-step methods based on a coating or inter-leafing step provide added matrix resin, ensuring that adequate 20 rnatrix resin is available to filll the laminar region between the plies formed by the particles.
In the alternative one-step method, the particles may be placed on a surface of the prepreg during the impregnation step by dispersing the particles into the matrix resin and then carrying out the impregnation step. In this process, a fiber structllre having a surface 2 5 layer of the filled resin may be formed, for example, by placing a film of filled resin on a sur~ace of tape or fabric or by coating the filled resin directly onto the surface. The continuous fiber is then embedded in the matrix resin by heating the fiber-and-resin structure in a melt-pressing or ironing operation. The matrix resin becomes molten and a portion ~lows into the fib~r structure, leaving behind at the tape or fabric surface matrix 3 0 resin filled with those particles too large to enter the interstices of the ~lber structure.
The one-step process may be viewed as a ~lltering operation whereby the fiber structure acts as a filter, passing matrix resin while recaining at the surface those particles larger than the openings between the fibers.
As previously noted, other than the adaptations needed to introduce the par~icles, the lay-35 up and curing steps used in preparing the toughened composite s~uc~ures will be conventional. These process steps may be carTied out using any of the variety ofconventional processing devices and equipment and employ such conventional process steps, adaptations and modifications as are ordinarily employed in the composite art.
The invention will be better understood by consideration of the following Examples, 5 which are provided by way of illustration. In the Examples, all parts are by weight, and all temperatures are given in Centigrade unless otherwise noted.
EXAMPLES
The following materials and formulations are employed in the Examples.
EPQXY 1: A mixture of tetraglycidyl derivatives of aromatic amines comprising about 40 mole % N,N,N',N'-tetraglycidyl-bis(4-amino-3-ethyl-phenyl)methane, about 47 mole % (4-diglycidylamino-3-ethyl-phenyl)-(4-diglycidylarninophenyl) methane and about 12 mole % N,N,N',N'-tetraglycidyl-bis(4-aminophenyl) methane. An epoxy obtained as RD 87-160 fiom 1 s Ciba-Geigy.
Ta~c 556: A mixtu~ of oligomeric polyglycidyl ethers of polycyclic bridged hydroxy-substituted polyaromatic compounds. An epoxy obtained as Tactix 556 fTom I)ow Chemical Company.
MY9~12 N,N,N'N'-tetraglycidyl-4,4'^methylene dianiline. An epoxy resin 2 0 obtained as MY 9612 fTom Ciba Geigy MYO~10: O,N,N-triglycidyl p-aminophenol. An epoxy resin obtained as MY 0510 from Ciba Geigy ~,~: 3.3'-diaminodiphenyl sulfone. An aromatic diamine hardener, obtained as HT-9719 from Ciba Geigy.
4.4'~DD~: 4,4' diaminodiphenyl sulfone. An aromatic diamine hardener, ob~ained as HT 96~ from Ciba-Geigy ~; N,N-dimethyl-N'-phenyl urea, cure accelerator obtained from Omicron Chemicals.
PEI: Polyether imide thermoplastic resin obtained as Ultem 1000 fsom the General Electric Company PES: Polyether sulfone thermoplastic resin obtained as Victrex 200 frvm ICI, Ltd.
ERR 42Q5: Bis(2,3-epoxycyclopentyl) ether, from Union Carbide Corporation.

12 ~ ?~

S~m: 4,4'-bis(3-aminophenoxy)diphenyl sulfone, from Mallinkrodt Chemical Company BAPP: 2,2-bis(4-(4'-aminophenoxy)phenyl) propane, from Mallinkrodt Chemical Company BF3TEA: Boron trifluoride-triethanolamine complex forrn Englehard Indus~ies Particles Porous Nvlon: Porous Nylon 12 particles, average particle size of 10 microns, BET surface area of 18.2 m2/g, and pore volume of 3.53 cm3/g.
Non-porous Nylon: Non-porous Nylon 12 particles, average particle size of 8 microns and BET surface area of 1.7 m2/g, and a pore volume of 1.10 cm3/g.

1 5 Fi~ers Ca~: Thornel(D T 40 grade carbon fiber from Amoco Perforrnance Products, Irlc. This fiber typically has a filament count of 1~,000 filarnents per tow, a yield of 0.44 g/m, a tensile strength of 810 kpsi, a tensile modulus of 42 mpsi and a density of 1.81 g/cc.
2 o In the Examples, ribbon forrned from the fiber was used to prepa~e prepr~g having fiber areal weights of 140 to 150 g/m2.
Test PrQcedu~e~
Particle sizes were determined by Granulometer, and are given as average particle size, while BET surface area was determined b,y Krypton BET technique and Pore volume 2 5 was deterrnined by mercury porosimetry.
~o~r~ssiQn Aft~Im~act Test fCAI). Ihis procedure, referred to as the CompressionAfter Impact test or CAI, is generally regarded as a standard test method in the industry.
The test specimens are panels measuring 6 X 4 in., cut from 32 ply fiber-reinforced composite sheets. The panels are first impacted by ~eing subjected to an impact of 1500 30 in-lbs/in at the center in a Gardner Impact Tester, using a 518 in. diameter indent~r; a panel thickness of 0.177 in. was assumed. The impacted panel is then placed in a jig and 13 2~

tested edgewise for residual compressive strength. The details are further described in "NASA Contractor Report 159293", NASA, August, 1980.
The methods of the following Examples are representative of those that may be employed for preparing the resin forrnulations, prepreg and composites useful in the practice of this 5 invention. The processes will be generally recognized by those skilled in the art as processes and methods commonly employed for the production of thermoset resin formulations and composites.
Example 1 An epoxy resin formulation was prepared by heating 70.5 parts by weight (pbw) of MY9612 epoxy in a flask to 110 C while shrring, and then adding 29.5 pbw of 4,4' DDS diamine hardener and tnixing at 110 C for 20 min. The mixture was cooled to 103 C and 15 g of Porous Nylon particles ~,vere added with vigorous stirring. After a total 20 min. of stirring, the resin was discharged from the reaction vessel and cooled.
A second preparation of the formulation was carried out, but omilting the particles.
Prepreg was then prepared by a two-step method, with the particulate modifier dispersed on one side thereof, substantially according to the processes described herein. The prepreg had a fiber areal weight of 149 g1m2 and a resin content of 38 wt%. The 12"
prepreg tape was then laid up into 15"xlS" laminates using a ply configuration of [+451~0/-4510]4S and then cured in an autoclave under 90 psi pressure at 355 C for 2 hr.
The resul~ng composite panel, after cooling, was used to provide test specimens for CAI
2 o evaluation. The data are surnmarized in Table 1.
Çon~rol Example A. An epoxy resin forrnulation was prepared substantially by theprocess of Example 1, but substituting Non-porous Nylon particles as the modifier.
The thermosetdng epoxy forrnulation was used to p~pare composi~es substantially as descnbed in Example 1 for further evaluadon. The composite composition and property 2 5 dala are summariæd in Table I.
Exan~h 2. An epoxy resln formulation was prepared by heating a soludon of 2~.4 pbw of MY0510 epoxy and 37.3 pbw of MY9612 epoxy in 37.5 pbw of methylene chloride to 45 C. The mixtt~e was sdned and methylene chloride was disdlled while adding 15 pbw PES polye~ler sulfone. The sdrred mixture was further heated to remove methylene chloride, finally to a reduced pressure of 28 in. and a temperature of 110 C and held at that temperature for 1 hr. The 3,3'-DDS, 71.5 pbw, was then added over a ~ min.
period and the mixture was then stirred at 100 C for 1 hr., under a vacuum of 28 in. to remove residual solvent. lhe temperature was reduced to 90 C and 0.2 pbw of the Ornicure 94 was added, stirring was continued for 5 min. and the resin was discharged.

The resin was used a base resin to prepare prepreg and composite using Porous Nylon particles by the two-step method, following substantially the process described in Example 2. The resin and particles were combined in the sigma blade mixer at about 50 C, and blended for about 2 hr. at 50-75 C to complete the dispersion of the particles.
5 The fimal prepreg tape had a fiber sma ! r~v~gili Ot' 1-~5 ~ ;`.`' a resin content of 37.3 wt%. Composite specimens vere prepared as in Example l. The composite composition and property data are summarized in Table I.
Control Example B. The resin of Example 2 was combined with Non-porous Nylon pardcles and used to provide composite for sompanson tests by following substantially 10 the sarne process. The final prepreg tape had a fiber areal weight of 145 g/m2 and a final resin content of 37 wt%.
Example 3. ERR-4205 epoxy resin, 42.0 pbw, was placed in a resin kettle and heated with stirring to 130 C before adding 8.0 pbw PEI. Heating was continued for 30 min.
until a homogeneous soludon was obtained, then 34.8 pbw of SED-m were added and 15 the mixture was cooled to 105 C and held. When the mixture was again homogeneous, 14.3 pbw BAPP were added and heating was continued at 105 C for 10 min. before cooling the mixture to 80 C, adding 0.9 pbw BF3.TEA, mixing for an additional 10 min. and then discharging.
An interleaf fonnulation was prepared from a portion of the resin by charging 2131 pbw 2 0 of the ~ozen resin to a sigma mixer, mixing the resin until the temperature reached 35 C, about 15 min., and ~hen adding 291 pbw Porous Nylon particles. The mixn~re reached a temperature of 45 C while mixing was continued for about 15 min.to disperse theparticles.
A prepreg was prepared from the filled and unfilled resins by the two-step process, and 2 5 then f~rmed into a composite specimen substantially as described herein.
Control ~xam~le C. An epoxy resin formulation with Nonporous Nylon particles wasprepared substandally by the process of Example 3 and using the same base resin formulation. A prepreg was prepared from the filled and unfilled resins by the t vo-step process, and then formed into a composite specimen substantially as described herein.
3 0 Additional exarnples of composites according to this invention were also prepared.
~x~sml~!e 4. A mixture of 800 g of Tactix 556 epoxy resin and 800 g of Epoxy-l epoxy resin was placed in a S liter resin ilask and heated to 110 C. A solution of 165 g of PEI
therrnoplastic dissolved in 3000 g of methylene chloride was added with stirring over a 1.5 hr. period, and then solvent was removed. The mixture was degassed by heating 2~ 7 and stirling the mixture at 110 C and vacuum for 45 min.to remove residual solvent before adding 590 g of 3,3'-DDS and stirring for 25 minutes to disperse the diamine.
The resin was ~hen discharged and cooled.
A sample of the resin, 86 pbw, was charged to a sigma blade mixer and allowed to warm 5 to room temperature. Porous i~ylon particles, 14 pbw, were added alld the mixture was sheared for about 60 min. to disperse the particles uniformly, giving a resin temperature of about 70 C. A film of the filled resin was prepared at a coating weight of 33 g/m2 and combined using a prepreg machine with separately-prepared prepreg tape having a fiber content of 77 wt% and a fiber areal wt. of 145 g/m2, prepared from carbon ~Iber 10 and the unfilled resin by substantially following the procedures of Example 1. The filRal prepreg tape had a flber content of 37 wt% and a fiber areal weight of 145 g/m2, with Nylon 12 particles dispersed in the resin coating on one surface. Composites were prepared substantially by the procedures of Example l to provide test panels having a panel thickness of 0.197 in. The composite property data are summarized in Table I.
Table I

Example Particlesl CAI~
No. Kpsi Porous nylon 32.7 Control A Non-porous nylon28.0 2 Porous nylon 48.8 Cont~ol B Non-porous nylon43.5 3 Porous nylon 47.8 Control C Non-porous nylon42.8 4 Porous nylon 47.6 Notes: 1. For particle composition and porosity description, see text.
2. CAI = Compression After Impact, lS00 in.-lb./in.
impact. For test procedure, see text.
It will be apparent from a consideration of the CAI test daaa presented in Table I that a variety of epoxy compositions are substan~ially improved in damage tolerance when porous particles are employed as modifiers. Compare the CAI results for Examples 1- 3, prepared using porous Nylon 12 particles, with those of Control Examples A-C made with non-porous Nylon 12 particles. The improvement is generally about 5 Kpsi, 2 ~
considered to be a surprising and very substantial increase in damage tolerance by those skilled in the art. Consider also the excellent CAI properties of the composite of Example 4.
The invention will thus be seen to be an improved fiber reinforced composite or s composition comprising discrete layers forrned of continuous structural fiber embedded in a matrix resin, the toughening of the composite being accomplished by including in the matrix resin component polyamide particles having an essentially spheroidal, spongy structure. The polyamide particles generally have a mean diameter between 1 and 75 microns, preferably from about 1 to about 25 microns, and still more preferably from 10 about 2 to about 15 microns and may be further characterized as having a large internal pore volume. Powder formed of such particles will have a large speci~lc surface,ordinarily greater than S m2/g and preferably greater than about 9 m2/g, to as great as 30 m2/g or greater as determined according to the classical BET method. The invention will also be recognized to include an improved method for toughening fiber reinforced15 composites comprising continuous structural fiber embedded in a therrnoset matrix resin having a particulate modifier dispersed therein, the improvement being the use of porous particles as defined.
Further modiflcations and variations will become apparent to those skilled in the resin formulating and composite fabricating art, and such variations and modifications will be 2 o incluW within the scope of the invention as defined by the appended claims.

Claims (9)

1. A fiber-reinforced composition comprising continuous structural fiber and porous rigid polyamide particles having a spheroidal spongy structure embedded in a thermoset matrix resin.

2. A layered, fiber-reinforced composite comprising (a) an epoxy matrix resin formulation and (b) continuous structural fiber embedded in said matrix resin said structural fiber forming a plurality of discrete plies defining layers comprising said matrix resin with porous rigid polyamide particles having a spheroidal spongy structure and a mean diameter in the range of from about 1 to about 75 microns dispersed therein.

3. The composite of Claim 1 or 2 wherein said porous rigid polyamide particles comprise from 1 to about 25 wt% of said matrix resin.

4. The composite of Claim 1 or 2 wherein said epoxy matrix resin comprises at least one epoxy resin, an aromatic diamine hardener and, optionally, a thermoplastic.

5. The composite of Claim 4 wherein said matrix resin comprises at least one epoxy resin, an aromatic diamine hardener and from about 5 to about 30 pbw, based on combined weight of epoxy resin and diamine hardener components, of a thermoplastic selected from the group consisting of polyaryl ethers and polyether imides.

6. The composite of any of Claims 1-5 comprising from about 20 to about 80 wt%
of said structural fiber.

7. The composite of Claim 6 wherein said structural fiber is carbon fiber.

8. The use of porous polyamide particles having a spheroidal spongy structure and a mean diameter of from 1 to 25 microns to toughen a layered, continuous fiber-reinforced composite structure comprising continuous carbon fiber reinforcement embedded in a cured epoxy resin formulation.

9. The composite structure of Claim 8 wherein said rigid polyamide particles comprise from about 1 to about 25 wt% of the combined weight of epoxy resin formulation and said particles.