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277

Recent Development in ThermoplasticComposites:A Review of Matrix Systems

and Processing Methods

I. Y. CHANGAND J. K. LEES

E. I. du Pont de Nemours and Company, Inc.

Composites CentersChestnut Run Plaza 702

Wilmington, Delaware 19880-0702

ABSTRACT

Manufacturers of composite materials have been developing high strength fiber rein-forced composites with thermoplastic matrix systems exhibiting enhanced performance infracture toughness and damage tolerance with the potential of reduced manufacturing costfor both primary and secondary structure components.A review will be made of principalthermoplastic matrix systems and processing methods used to fabricate the fiber rein-forced composites, which have been developed during recent years. Included is a com-

parison of the neat matrix resins chemical structure, thermal/tensile properties, fracture

toughness, melt viscosity, solvent resistance, morphology and melt processability. Thematrix-dominated mechanical properties of thermoplastic composites are given, includingflexural, short beam shear and compression strengths, interlaminar fracture toughnessand/or compression-after-impact performance. Innovative process development in

thermoplastic composites including thermoformable sheets, filament winding, 3D braid-ing and impregnation will also be reviewed and discussed.

INTRODUCTION

URINGTHE PAST decade major advances have been made in the develop-ment of advanced composites based on organic polymer matrix systems rein-

forced with high performance fibers as metal replacements in aerospace, auto-motive, recreational and industrial applications.As a result of continuous im-

provements in fiber/matrix properties and development of innovative fabricationtechnologies, advanced composite structures offer possibilities for major leaps indesign, manufacturing, energy conservation, product utility and diversity [1].

In the course of organic polymer matrix development the thermoset systemssuch as 350F cured epoxies reinforced with high strength boron or graphitefibers were initially evaluated for primary structure of subsonic military aircraft.

Although the thermoset systems showed superior tensile, shear and compressivestrengths, several deficiencies were uncovered with the epoxy-based composites,

Journal of THERMOPLASTIC COMPOSITE MATERIALS, J1J1. 1-,Iuly 1988

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278

namely, inferior performance in damage tolerance, hot/wet stability, and highmanufacture cost associated with the conventional hand-layup fabrication pro-cess. Significant progress has been made for improving the fracture toughnessproperty of the thermoset systems, but other problems associated with hot/wetstability and manufacture cost remain unresolved. These needs have drawn atten-tion to the potential use of thermoplastic matrix systems.Fiber reinforced thermoplastic composites were initially made via solvent im-

pregnation with amorphous polymers such as polyethersulfone (PES) andpolyetherimide (PEI). Fiber reinforced composites with semi-crystalline matrixresins including ICIs PEEK (polyether-ether-ketone) and Phillips PPS(polyphenylene sulfide) were later developed via film/fabric stacking, melt orpowder impregnating processes. Recently, new thermoplastic matrix systems in-

cluding DuPonts K-polymer (an amorphous polyimide), J-polymer (an amor-phous polyamide) and PEKK (a semi-crystalline polyether-ketone-ketone) cover-ing a wide range of end-use temperature applications have been demonstrated foradvanced composites with superior performance in mechanical properties, frac-ture toughness and damage tolerance [2-4]. Several additional thermoplasticmatrix systems with high T. have been developed by other material suppliers[5-9]. Innovative processing methods have been developed to produce a varietyof thermoplastic prepregs and preforms. This paper will review the recentdevelopment in thermoplastic material systems and processing/fabricationmethods

includingthe

developmentwork

performedat DuPont.

THERMOPLASTIC MATERIAL SYSTEMS DEVELOPMENT

Attributes of Thermoplastic Matrix Systems

The most important difference between thermoplastic and thermoset matrixsystems is best described by their distinguishable chemical characteristics. Whilethermosets are &dquo;crosslinkable&dquo; polymers, thermoplastics may be characterized as&dquo;linear&dquo; polymers which normally need no &dquo;cure&dquo; during consolidation into a

composite. The consolidated thermoplastic composites are remoldable and re-formable. The thermoplastic prepregs or preforms can be stored in any ambientenvironment with infinite shelf life unless they contain solvent which may limittheir shelf life. In contrast, the thermoset prepregs with a crosslinkable matrix

require refrigerated storage, must be consolidated through &dquo;cure&dquo; (chemical reac-tion) and are not reformable after &dquo;cure.&dquo;During the past several years,&dquo;reprocessibility&dquo; and &dquo;reformability&dquo; of fiber-reinforced thermoplastic com-posites were demonstrated with a variety of high performance thermoplasticmatrix systems characterized as &dquo;linear&dquo; polymers such as PEEK, PEKK, PPS,

J-polymer, K-polymer,etc.

High-Performance Thermoplastic Matrix Resins

Several high-performance thermoplastic resins with chemical structures (ordescriptions) are listed in Table 1. These thermoplastic polymers containing stiffaromatic or cyclic chains in the polymer backbone have relatively high Tg as




279

Table 1. High-performance thermoplastic matrix systems.




280

compared with other engineering resins such as polyethylene-terephthalate(PET), nylon 66, etc. Due to high T, and high melt viscosity of these matrixresins, a relatively high processing temperature (T,) is required for fabricationand consolidation of the composites. In particular, K-polymer (DuPont), HTX(ICI), &dquo;PAS-2&dquo; (Phillips) and &dquo;Radel&dquo;-C PES (Amoco) with T, above 200Cwere recently developed as high-performance matrix candidates for carbon fiberreinforced composites to be used on advanced military aircraft [2,5-8]. Thesethermoplastic resins have high toughness with a fracture energy, G,, ~it 1.0kJ/m2, superior to BMI (bismaleimide) or aerospace epoxies such as Narmcos5208 or Hercules 3501. The semi-crystalline thermoplastic polymers (PEEK,PEKK, HTX) are generally more resistant to organic solvents than the amor-

phous polymers such as PEI (&dquo;Ultem&dquo; 1000), PES and &dquo;PAS-2.&dquo; It has been

found that the amorphous K-polymer and N-polymer have excellent solvent re-sistance. Recently, a new class of PEI, &dquo;Cypac&dquo; X 7005, was reported to have

improved solvent resistance over other unmodified PEI resins [9].A new semi-crystalline polyetherketoneketone (PEKK) polymer is beingdeveloped at DuPont as a potential high performance thermoplastic matrixsystem for advanced composites [4]. This resin with upper use temperature be-tween those of K-polymer (polyimide) and J-polymer (polyamide) has potentialadvantages such as retention of mechanical properties in hot/wet environment orafter exposure in organic solvents. PEKK also has excellent flame resistance

with flammability rating V-O (UL-94), Limiting Oxygen Index 40%, low NBSsmoke density, and heat release rate (OSU) less than 65/65, which are requiredfor civil aircraft by a new FAA regulation in the U.S.A.

K-polymer and N-polymer, developed by DuPont with a dry T, of 250 C and350C, respectively, show excellent neat resin properties and are suitable for

high use-temperature applications [2,10]. These high-temperature thermoplasticmatrix systems are more difficult to process than most other thermoplastics as aresult of extremely high melt viscosity.As a &dquo;linear&dquo; organic polymer, eitherN-polymer or K-polymer is classified as a thermoplastic matrix despite its highmelt

viscosity.Because of its unique chemical makeup, N-polymer comprises one of the mostthermally and oxidatively stable organic polymer systems known. Three chemi-cal features give it this extraordinary thermal oxidative stability and toughness.First, its polymer chain is completely aromatic with no unstable linkages.Second, its chemistry limits branching or crosslinking which could possiblyintroduce weak links in the polyimide structure.And, third, the hexafluoroiso-propylidene group present in the 6FTA monomer is chemically inert and itsbulkiness introduces a kink in the polymer chain, helping to prevent crystalliza-tion.

A partially fluorinated polyimide thermoplastic with dry Tg at 430C and goodthermal stability has been recently commercialized as &dquo;EYMYD&dquo; L-30N byEthyl Corporation, requiring molding temperature up to 454C and pressure upto 27.6 MPa (4000 psi) [ 11 ] . Mechanical properties of the neat polymer and itsfiber-reinforced composites have not yet been published by the manufacturer orusers.




281

Other high performance matrix systems including polyimide LARC-TPI(Mitsui Toatsu) and polyamideimide &dquo;Torlon&dquo;-C (Amoco) [12] with dry Tgs of264C and 288C, respectively, also show excellent mechanical properties and

high melt viscosity requiring high processing temperature. Due to presence ofamide groups in the polymer chains, &dquo;Torlon&dquo;-C is more sensitive to moisturethan the polyimide or polyetherketone systems previously discussed. Recently,a new derivative of polyamideimide with less moisture sensitivity such asAMCOsAI-696 was reported with a dry T, of 243 C and equilibrium watersorption at 1.8% by weight of polymer. However, the flexural modulus (2.83GPa) and the tensile strength (90 MPa) of this resin are lower than those of&dquo;Torlon&dquo;-C PAI neat resin [8,12].The neat resin properties of various thermoplastic matrix systems mostly

obtained fromsources

cited in the reference listare

given in Tables 2 and 3. Theperformance comparison of matrix systems is given in Table 4.

Carbon Fiber Reinforced Thermoplastic Composites

The thermoplastic matrix systems listed in Table 1 were evaluated with highstrength continuous filament carbon fibers includingAS-4 or IM-6 by DuPont orother manufacturers. The carbon fiber composite laminates with J-polymer orPEKK were made by compression molding from the melt impregnatedAS-4

Table 2. Thermal, morphological and rheological properties of neat resins.

Glass transition temperature (dry)bMelt temperatureNominal processmg temperaturedX-ray diffractioneAt processing temperature, 10 sec shear rate

Untoughened thermoset systems




282

Table 3. Mechanical and fracture toughness properties of neat resins.

atensile or flexural modulusbTensile strengthctensile elongationdGlc from compact tension testASTM-E399.

euntoughened thermoset

Table 4. Performance comparison of matrix systems.

+ + Excellent

+ Fair.- Poor.




283

Table 5. Mechanical properties of thermoplastic composites reinforcedwith carbon fiber.ea

allnidirectional lammates mth nommal 60% fiber volume (AS-4 or equivalent)bwith quasi-isotropic specimen and 67 J/cm impact energycFlexural/compression strengths from References [6] and [7]dUntoughened thermoset systems

tows prepared via a proprietary melt impregnating process [3,4].Avimid* K orAvimidg N prepregs containing carbon fiber with K-polymer or N-polymermatrix were processed using vacuum bag layup and autoclave technology [2,10].The carbon fiber reinforced composites with other matrix systems were prepared

and tested by manufacturers of these polymer resins. Several matrix-dominatedmechanical properties of the thermoplastic composite laminates at room temper-ature (mostly obtained from various sources cited in the reference list) are givenin Table 5.

The most important feature of these high performance thermoplastic matrix

composites is their high Mode I interlaminar fracture toughness (G,,) asmeasured by the double cantilever beam test and superior damage tolerance per-formance as measured by the compression-after-impact test. The interlaminarfracture toughness of most thermoplastic matrix composites with carbon fiber is

equivalent to or greater than 1.0 KJ/M2 as compared with 0.2 KJ/M2 for the car-bon composites of untoughened aerospace epoxies (Hercules 3501-6). Similarresults of low fracture toughness were reported for the untoughened BMI com-

*DuPont Registered Trademark.,




284

posites. In the compression-after-impact test, the majority of thermoplasticmatrix composites retained at least 270 MPa stress and 0.6 % strain after 67 J/cmimpact as compared with 145 MPa residual stress and 0.34% strain for

AS-4/3501 epoxy after impact with the same amount of energy.The unidirectional laminates with the new PEKK matrix andAS-4 fiber

showed excellent mechanical properties with flexural strength 1620 MPa at23C and 1379 MPa at 93C (wet), short beam shear strength 117 MPa at 23Cand 92 MPa at 93C (wet), compressive strength 1390 MPa at 23C and 1234MPa at 82C (wet). These properties are equivalent or superior to those of AS-4/PEEK (APC-2) orAS-4/3501 epoxy [4].The carbon composites ofA vimid@ K andAvimidg N showed excellent

mechanical properties in high temperature/humid environment [2,10].A vimid@

K/IM-6 retains more than 5 % of its mechanical strengths and other propertiesat 200C with saturation of water. The carbon composite ofAvimida K is beingevaluated by a major aerospace company as a leading candidate for the advanced

military fighter program due to its superior performance at high temperature.The inherent stability ofAvimidg N results in composites having exceptional

long-term durability at temperatures where many other organic-based compos-ites exhibit rapid deterioration of mechanical properties. Our laboratory testresults show that the time it takes to lose 50 % of the original flexural strength is

approximately 50,000 hours at 260C, 5,000 hours at 316C, and 1,500 hoursat 343C.

Recently,carbon-fiber/Avimid N

high-temperature toolingtech-

nology has been developed to fabricate quality parts from prepregs with thermo-set or thermoplastic matrices. Because of high 7~, toughness and extraordinarythermal-oxidative stability ofAvimidg N, such tools offer potential for long,useful life and several other advantages [10].

Kevlar* Fiber Reinforced Thermoplastic Composites

Various thermoplastic resins including J-polymer, nylon 66, PET reinforced

Table 6. Mechanical properties of composites reinforced withKeviarO aramidor E-glass fiberb(60% fiber volume).

Finish free and zero twist yarnbFiber mth polyurethane size for J-polymer and PEKK, fiber mth epoxy size for &dquo;Epon&dquo; 828.%hree point bendmg mth span-depth ratio 32 for flex modulus, 16 for flex strength, 4 0 for short beamshear strength.




285

with Kevlarg aramid fibers were evaluated with the composites prepared fromthe proprietary melt impregnated tows developed at DuPont [3,13]. The melt

impregnated tow as a precursor can be woven into a variety of fabric construc-

tions for consolidation into porosity-free final components or for subsequentthermoforming operation. Due to inherent fiber/matrix compatibility, attractivemechanical properties of Kevlar/J-polymer composites were obtained for theunidirectional or fabric composite laminates [13]. Recently, preliminary testsalso showed good consolidation for the composite laminates prepared from themelt impregnated tows of Kevlarg aramid with PEKK matrix. The matrix-dominated mechanical properties such as flex and short beam shear strengths of

Kevlar21/J-polymer and Kevlar/PEKK are given in Table 6. The mechanical

properties of a conventional epoxy system (&dquo;Epon&dquo; 828) reinforced with Kevlarfiber are also

givenfor

comparison.E-Glass Fiber Reinforced Thermoplastic Composites

The thermoplastic matrix systems such as J-polymer and PEKK have goodcompatibility under &dquo;dry-as-molded&dquo; condition with continuous E-glass fibercontaining a conventional &dquo;polyurethane&dquo; size designed for thermoplasticmatrices by glass fiber manufacturers. The flex properties and short beam shear

strength of E-glass/J-polymer or E-glass/PEKK are equivalent or superior tothose of E-glass/&dquo;Epon&dquo; 828 at room temperature under &dquo;dry-as-molded&dquo; con-

ditions (Table 6). It was found, however, that these glass reinforced ther-moplastic composites have inferior hot-wet properties probably due to loss ofadhesion at fiber-matrix interface under hot humid environment. The unidirec-

tional composite laminates showed more than 50 % loss of initial short beamshear strength after exposure in 71 C water for 14 days and testing at 93 C .A

thermally stable size on glass fiber is required for processing at high temperaturewith the advanced thermoplastic matrix systems. Recently, the high perfor-mance S-type glass fiber with such a size has been developed and should be eval-uated with thermoplastic matrices under hot/wet test conditions.

PROCESS DEVELOPMENT IN THERMOPLASTIC COMPOSITES

Thermoplastic Melt Flow Model

The shift from using thermosets to using thermoplastics requires a shift in our

thinking regarding the processes required to make parts. In thermoset tech-

nology the key to success is control of the crosslinking reaction which directly

affects the properties in the composites. Hence,we

must deal with this issuethroughout the process stream from making prepregs to storage and shipping of

prepregs, during the layup process and finally during the autoclave process. Ourinvestment plan and process considerations are all affected by this chemical reac-tion.

In thermoplastics, the emphasis shifts from controlling the chemistry to con-




286

trolling the rheology. Thermoplastics at their processing temperature have vis-cosities of 500-5,000 pa.s compared to thermosets which are less than 100 pa.s.Hence the critical process elements must deal with the effects of this high vis-

cosity. In the impregnation step viscosity affects both fiber/resin uniformity aswell as fiber wet-out which is critical for good load transfer in the final part. In

processing the final part, viscosity affects consolidation rate, fiber wash ormovement of the fibers in the part and cycle time.The most critical phase in processing of thermoplastics is the impregnation

step. Here the fiber and resin are brought into intimate contact and thefiber/matrix distribution is essentially set. For most thermoplastic systems this isaccomplished first before proceeding to make a part.

Several methods are used for bringing the fiber and matrix together: melt im-

pregnation, comminglingof matrix fibers and

reinforcing fibers, powderim-

pregnation and solvent impregnation. Each of these has their strengths andweaknesses. In all cases the one-dimensional penetration of a thermoplastic meltinto a uniaxial fiber bundle may be modeled by Equation (1) derived from

Darcys law of flow through porous media [14]. By integrating the Darcy equa-tion (over the media thickness with the time), the following relation for the one-dimensional penetration is obtained:

where

x = penetration depthk = media permeability coeflicientP = pressuret = time under pressure

tt= media viscosity

Equation (1) shows that the penetration depth is proportional to the mediapermeability, the applied pressure and inversely proportional to the viscosity.Since the viscosity of the resin system is given and is little affected by processconditions, then all we can hope to change is the permeability through the

porosity of the fiber bundle and the applied pressure. Hence, the most favorableconditions for good flow required for uniformity occur at high porosity and highapplied pressure.Thus, attempts made to simultaneously complete the impregnation process and

part forming would lead to poorer than desired fiber-resin distribution; there-fore, it is important to establish the distribution early. In each of the non-solvent,non-melt cases this is done by matching the matrix resin size to that of the rein-

forcing fiber to obtain a good distribution in the dry state. In the case of melt

processing this is accomplished by simultaneously controlling the resin feed

pressure and the density of the yarn bundle. When solvent is used, one can in




287

effect reduce the viscosity to achieve the desired distribution. However, the

presence of the solvent, particularly polar solvents, can modify the fiber surfaceresulting in an undesirable shift and ultimately in reduction of the fiber/matrix

shear strength.The other variable that affects impregnation quality is the ability of the resinto wet the fiber. This wetting is necessary for fiber-matrix adhesion needed toachieve the desired level of stress transfer for dynamic, mechanical perfor-mance. This problem is particularly critical in the case of carbon fibers wherethe carbon fiber cross section is not round but serrated. Inadequate wetting of thefiber by the matrix resin can increase void concentration at the interface andresult in poor composite properties.Fiber wetting is effected by the resin viscosity, surface tension and the char-

acter of the fiber surface. One can alter the resin surface tension either

throughthe addition of solvents or through a change in the polarity of the fiber surface

by plasma etching, chemical treatment, ozone treatment or with oxidative attackat the fiber surface. Considerable work is being done to study the effects of thesetreatments on all the major fibers using both micro techniques, such as theWhilemy balance to study surface tension force between the resin and fiber aswell as macro studies using chemical treatment such as DuPonts high tempera-ture polyimide sizing for carbon fiber or the amino silanes frequently used with

glass fiber.A variety of characterization methods such as surface spectroscopy,thermal desorption, surface free energy, interfacial shear strength, photoelectricobservation, etc. to study the chemical and physical nature of the fiber-matrix in-terface as affected by the surface treatment were reviewed by Drzal [ 15] . Thisscience is still in its early stages and will continue to be a fruitful area for in-

vestigation, particularly in the area of the fiber-reinforced thermoplastic matrixcomposites.Proceeding forward in the process area we can use either a fully consolidated

prepreg (one in which all the air has been removed) or an unconsolidated formthat contains greater flexibility. Materials of both types can be made using anyof the four prepreg methods described earlier.At this stage in the process we

must focus on resin flow to provide a good interlaminate structure or to allow thelaminate to be reformed. The work of Cogswell and Leach [16] deals with thevarious flow processes that occur in a direct forming process in which a flat sheetis transformed to a complex shape. Here they define three types of shear, trans-verse intraply, axial intraply and interply, that take place, allowing resin flow tooccur during the shaping. This is necessary since the fibers are essentially non-

compliant in this regime. This approach represents a good model for forming byany number of processes such as match-die molding, press forming, bladder

molding or super plastic forming. The processes for thermoplastic composites

formingare described in various

publications [17-23].

Thermoformable Sheets

An alternate method for handling the manufacture of formed complex shapesis to use a thermoformable sheet with oriented discontinuous fiber structure as




288

developed at DuPont [24,25]. The sheet is designed such that the filament rein-forcement (3-15 cm length) is aligned in each layer to such a degree that con-tinuous fiber-like properties result. The discontinuous fiber reinforcement, how-

ever, does permit drawing required to form complex shapes. In this case thematrix shearing differs from the earlier case because the movement of the fibersallows axial draw to occur in the fiber structure as well as the resin. Performance

of these parts relative to parts made from continuous fibers is heavily dependenton the quality of the fiber orientation, rather than the use of discontinuous fiber.This is verified theoretically using shear lag theory where the length-to-diameterratio of the fiber (AS-4 carbon) is greater than 1000 with &dquo;critical&dquo; fiber lengthL~ well below 0.1mm. In addition, experimental work in this area, althoughlimited, confirms this expectation as shown by comparison of mechanical prop-erties between the

compositesfrom continuous

filamentand discontinuous form-

able sheets in Table 7.

Filament Winding

Other methods for making parts involve processes, such as filament windingand 3D braiding. In filament winding, as in tape lay down, we can begin with a

fully or partially impregnated prepreg. If post processing is to be used it is possi-ble to use a partially impregnated prepreg and complete the impregnation pro-cess

at thesame

timeas one

does the consolidation. In thiscase

the rate limitingstep will be the impregnation step and will depend on the quality of fiber-resindistribution and the wet-out required to meet the structural requirements. If theprepreg is fully impregnated it is possible to reduce the post processing tominutes. Here the rate limiting step is the time required to heat the part up to theflow temperature of the resin.For the most effective and low cost process it is desirable to directly con-

solidate the prepreg during the winding step such as in-situ consolidation being

Table 7.Comparison

of mechanicalproperties-continuous

filament vs.

discontinuous formable sheet.

Quasi-isotropic layup




289

developed at DuPont [26]. In this particular case the time for heat up and con-solidation is in the order of seconds. This can only be accomplished with fully

impregnatedfeed material. However, full consolidation of the feed is not re-

quired but may eventually limit the winding speed. To date little has been pub-lished on the subject but it appears the fiber winding at economically attractiverates is possible. Tubes with both thin (less than 0.25 mm) and thick (approx. 5cm) sections have been made with acceptable translation of properties.

3D Braiding

Finally in 3D structures the primary focus has been on the formation of thestructure with high fracture toughness. While this focus is possible for use in

thermosets which have low viscosity and can penetrate the 3D network when itis opened up, it requires additional technology for use with thermoplastics.Work has been done at DuPont and by a number of speciality companies to bothbraid and weave pre-impregnated yarns in order to make thermoplastic 3D struc-tures.Again the degree of impregnation plays an important role in deciding onthe process techniques required to fully consolidate the structure. For in-lineconsolidation a fully impregnated yarn is required. For off-line post consolida-tion, the cycle times will again be controlled by the impregnation flow equation.A new process with &dquo;in-line&dquo; consolidation was developed at DuPont for form-

ing3D braids with

thermoplasticmelt

impregnatedtows that can form a wide

range of structure shapes [27].A rectangular slab with 50 % fiber volume wasdemonstrated, using axialAS-4 carbon tows which was melt impregnated witha thermoplastic matrix (J-polymer) and braiding Kevlarg aramid tows melt im-pregnated with the same matrix. The ratio ofAS-4 to Kevlarg aramid was 6 to1. The consolidated structure showed good flex/shear properties with high in-terlaminar fracture toughness.

Impregnation

Because of the importance of the impregnation step to both the economics andpart quality, it is worth looking into this process step in more detail.As notedearlier there are four primary processes: (a) solvent impregnation; (b) melt im-pregnation ; (c) commingling; and (d) powder impregnation.

SOLVENTIMPREGNATION

This process is employed by several firms such asAmerican Cyanamid andTen Cate in Europe. In both of these cases the matrix resin is a polyetherimide(PEI). The benefit of this process is derived from the low viscosity of the result-

ingresin solution which allows both unidirectional

tapesand woven fabrics to be

used as feed material. However, there are two big drawbacks: the potential forresidual solvent in the prepreg, and inability to use higher performance resinsthat have excellent solvent resistance. In the case of PEI the solvent used is NMP

(N-methyl pyrrolidone). For other resins like polyamides solvents as aggressiveas formic acid would have to be used and for polyetherketone like ICIs PEEKor DuPonts PEKK, there are no good solvent systems. In addition to en-




290

vironmental restrictions, investment would be required to recover the solvent.As a result this approach is in limited use today.

MELT IMPREGNATION

The second approach &dquo;melt impregnation&dquo; is defined as a process in which thefinal prepreg form has each of the fibers coated by resin (i.e., impregnated). Thematerial may be either fully consolidated or partially consolidated.Three basic approaches are used to achieve this type of preform: (1) film

stacking with either a roll or belt type continuous process to melt the resin andwork it into the yarn bundle; (2) adding the matrix as a powder then melt it andwork it into the yarn bundle in the same way as described for the film feed

system; (3)direct melt

injectioninto the

yarnbundle. In the first two cases it is

necessary to prepare the matrix resin for use in these processes.Again both tapesand tows that are fully consolidated can be made this way. The third approachuses the matrix resin in granule form and melts it to make a partially consoli-dated tow that is flexible enough to be woven into a fabric. This flexibility differsfrom that for the fully consolidated tows.A majority of manufacturers includingDuPont, ICI and Phillips use the melt impregnation process with either of these

approaches.

COMMINGLING PROCESS

This is a relatively easy process to use.Any thermoplastic resin can be madeto work if it can be made into a fiber. Here the trick is to insure that the fiber di-

ameter of the matrix fiber is matched to that of the reinforcing fiber in order toassure good distribution of the two fibers. It also requires a texturing process to

open the yam bundle without damaging the brittle fiber. This is particularlydifficult to accomplish for a high filament count carbon tow such as 6K or 12K

AS-4 or IM-6. In addition, there is an added cost to make the fiber form of thematrix. Resins like polyester that are inexpensive in fiber form dont suffer thisadded cost, but conversion of resins like PEEK into fiber in small lots will in-

crease the resin cost by 2X. This concept has been practiced by BASF in its jointdevelopment with Concordia. Recently, the advances in commingled yarn tech-nology with a wide range of polymer matrices and reinforcement fiber types(AS-4, S2-glass, etc.) were reported [28].The commingled product cannot be used effectively in high speed process like

filament winding because of the lack of bundle integrity. It was observed duringfilament winding of a 3KAS-4/PEEK commingled tow in our laboratory that thePEEK filaments in the tow were separating from the carbon filaments as tensionwas applied for &dquo;in-situ&dquo; consolidation. The resulting tube showed a low torque

strength of 390Nm

witha

high level of voids after post consolidation viaautoclave. In contrast, a melt impregnatedAS-4/PEEK tow with good bundle

integrity was successfully processed with &dquo;in-situ&dquo; consolidation. The resultingtube showed a high torque strength of 650 Nm.On the positive side the commingled yarn flexibility which is derived from the

lack of connectivity since no fusion of the matrix has occurred makes this formmuch easier to braid and weave, hence reducing costs in this step. Some of this

I




291

gain is then lost due to the longer cycle times required to consolidate the part.Our laboratory tests showed that it took twice as long to consolidate the com-

mingledtows into a plaque as it did for the melt impregnated tows under similar

molding conditions.

POWDER IMPREGNATION

This process generally produces a flexible tape or tow in which the matrixresin is held in place but stays in a powder form in the same way as the resinstays as a separate entity in the commingling process. There are two publishedmethods for powder prepregs. The Atochem process encapsulates the thermo-

plastic resin powder in the yarn with a sheath of either the same resin or a lowermelting point resin [29]. This product can be readily woven and used in fabric

form. The disadvantage of this method is that the composite laminate producedfrom the encapsulated tows may contain excessive resin-rich areas due topresence of the sheath layer around the tows. It would be difficult to achieve a

good fiber-matrix distribution, particularly when a high fiber volume is re-

quired. The resin-rich areas are increased when a high melting-point resinsheath is used for encapsulation.A second approach is that employed by BASF wherein the powder is put in anaqueous suspension that is then used to impregnate the fibers. This yields a tackyproduct similar to a thermoset prepreg. When a woven fabric is to be used it is

possible to make the fabric then impregnate it because of the low shear viscosityof the slurry. The water is then driven off during the forming process.Again the

process requires a step to obtain the particle size necessary to match the fiberdimensions. This can result in a 50% increase in the cost of the matrix resin

except in cases such as nylon 11or PEEK where the matrix is produced as a

powder.The most important advantage of powder impregnation technology is its abil-

ity to process matrix systems with very high melt viscosity and high melt tem-

perature. BASF claims that they made consolidated laminates via powder im-

pregnationwith PMR-15 and PEEK of

high viscosity grades [30].This

technology has attracted a lot of attention because most high performance ther-moplastic matrix systems have high melt temperature with high melt viscosity.It is difficult to process these matrix systems through the melt impregnation.

Table 8. Relative cost of prepregging for thermoset versusthermoplastic tows.




292

However, there are two major concerns for the powder impregnated prepregscurrently being evaluated in the trade. First, the unidirectional carbon prepreglacks sufficient transverse

strength.Thus, it

requires extraordinarycare and ef-

fort to handle the prepreg during layup to form a complex-shaped part. Theother is the use of a relatively large amount of polymeric binding agent as highas 6 % of a polyacrylic acid by weight of matrix resin as described in a patent ref-erence [31]. This crosslinkable binding agent may remain in the prepreg duringhigh temperature processing and cause undesirable effect on the matrix resin andits composite properties. Further effort would be expected from the manufac-turer to resolve these concerns.

RELATIVE COST OF PROCESSES

By examining each of the impregnation processes in terms of the resin form

required and the potential throughput rate we can estimate the relative cost ofthese processes.As can be seen in Table 8, the added costs range from 1.8X-3Xof those for a thermoset prepreg with the melt and solvent processes having thelowest potential cost. In Table 9 we have also examined the cost of part fabrica-tion from each of these using filament winding. Thus, despite the higher costsfor the thermoplastic prepreg the part costs are 40 % below the thermoset partcost for the melt and solvent processes, 20% below or equal to the thermoset

partcost,

respectively,for the

powderor

commingling process.As can be seen there are available several cost effective methods for prepreg-ging with thermoplastics that lead to economic part manufacturing. The choiceof the specific prepreg form will be determined by the matrix resin used and the

type of part fabrication technology needed.The relative forming cost of continuous filament tape/tow versus thermoform-

able drawable sheet for various types of part are given in Table 10. It shows thatwhile the drawable sheet technology (with discontinuous filaments) is interestingas previously discussed, it is not to be used indiscriminately. For simple shapes,like a single curvature skin, the 30 % reduction in forming cost shown in Table10 will not offset the added cost of making the formable sheet. However, sec-tions like a hat section or multiple indentation shape, and a rib where the exterior

geometry must be maintained, will justify the use of this product form becauseof the 30-50% reduction in cost plus the reduction in scrap material.

FUTURE DEVELOPMENT OF THERMOPLASTIC COMPOSITES

To demonstrate manufacture of thermoplastic composite parts with the pro-jected cost reduction will be a major challenge for the aerospace industry and thematerial

suppliers.Effort will focus on

design,manufacture and

testingof

largecomplex-shape thermoplastic composite parts to be used for new advanced

weapon systems including the advanced military fighter plane and V-22 Ospreytilt-rotor aircraft. The advanced thermoplastic composites are under evaluationfor both primary and secondary structure components on next generations ofsubsonic and high-speed civil transport. Westland Helicopters in Great Britainhas taken a lead to produce a complete &dquo;tailplane&dquo; assembly comprising hori-




293

Table 9. Effect on downstream cost for filament winding.

zontal and vertical stabilizers with the carbon fiber thermoplastic composites[32].After successful testing and demonstration, it is expected that engineersand designers at Westland and other aerospace companies will move toward

using the thermoplastic composites in helicopter fuselage, wing structures, tailcones and even dynamic components [33].TheAir Force Materials Laboratory has taken a lead position and initiated

several major thermoplastic composites programs to exploit unique materialsand processing characteristic of thermoplastic matrices. Major universities andaerospace manufacturers have been jointly and actively involved with these pro-

grams including &dquo;Manufacturing Science of Complex Shape Thermoplastics&dquo;with the objective to develop, demonstrate and validate a science base for pro-cessing thermoplastic composites.A major aerospace company will continueunder anAir Force contract to work on &dquo;Design and Development of Databasesfor Thermoplastic Composites&dquo; with the projection of producing the first proto-type of the computerized thermoplastic composites data base in 1991. It isbelieved that completion of these programs will be most beneficial for us infuture development of advanced thermoplastic composites.We continue to believe along with several other major users of composites and

Table 10. Relative forming cost of parts via continuous filamenttape/tow and discontinuous drawable sheet-thermoset

(TS) versus thermoplastic (TP).




294

theAir Force Materials Laboratory that thermoplastic composites represent afuture direction. To fulfill the promise of these systems, we need to demonstratethe

viabilityof the

technologyreviewed in this

paperin real

applications,not

only to meet aerospace needs, but also to serve a wide range of industrial as wellas automotive markets.

ACKNOWLEDGEMENTS

The authors wish to thank their colleagues, Drs. R. K. Okine, J. F. Pratte, M.W. Egerton and P. Popper, for valuable discussions and/or providing test resultsof thermoplastic composites process development at DuPont. The authors wouldlike to thank Dr. R.A. Baker for his assistance in

developingthe relative cost

estimates of processing with thermoset versus thermoplastic matrix systems.

REFERENCES

1.Forney, R C "Advanced Composites The Structural Revolution," KeynoteAddress, ICCM-V

Conference, San Diego, California (1985)2 Boyce, R. J., T. P Gannett, H H Gibbs andA R Wedgewood "Processing, Properties andAp-

plications of K-polymer Composite Materials Based onAvimid K Prepregs," 32nd InternationalSAMPE Symposium,Anaheim, California, 169-184 (1987)

3. Chang,I Y.

"ThermoplasticMatrix

Composites," Composites86 RecentAdvances in

Japanand the United States, Proceedings ofthe Third Japan-US Conference on Composite Materials,Tokyo, Japan, 615-622 (1986)

4 Chang, I Y "PEKKAs a New Thermoplastic Matrix for High Pertormance Composites," TheSAMPE Quarterlv, 19(4) 29-34 (1988)

5. Cogswell, F D , D C Leach, P T McGrail, H M Colquhoun, P MacKenzie and R MTurner "Semi-Crystalline Thermoplastic Matrix Composites for Service at 350F," 32nd Inter-national SAMPE Symposium, 382-395 (1987)

6. Linstrom, M R and R W Campbell "PAS-2 High Performance Prepreg and Composites,"Ibid, 147-152 (1987)

7. Mills, S D, D M Lee,A Y. Lou, D. F. Register and M L Stone Advances in PAS-2

Thermoplastic/Carbon Fiber Prepregs and Composites," 20th International SAMPE TechnicalConference, 263-270 (1988)

8. Crowe, W C and B W Cole "Thermoplastic Composites Development atAMOCO," Ibid,248-261 (1988)

9. Peake, S L , A. Maranci and E Sturn "Thermoplastic Matrix Composites Based on

Polyetherimides," 32nd International SAMPE Symposium, 420-432 (1987)10. Gupta, D "Avimid N/Graphite Composite Tooling forAdvancedAircraft andAerospaceAp-

plications," SME Conference on Tooling for Composites, LosAngeles, California (1988)11. Jones, R J and E M Silverman "Thermal Properties of EYMYD Polyimides," 20th Interna-

tional SAMPE Technical Conference, 542-551 (1988).

12. Cole, B "Torlon-C Graphite Composites," 30th National SAMPE Symposium

,

799-808 (1985).13. Krueger, W H , S Khan, R. B. Croman and I. Y Chang "High Performance Composites of J-2

Thermoplastic Matrix Reinforced with KevlarAramid Fiber," 33rd International SAMPE Sym-posium, 181-205 (1988)

14 Larson, R. G. "Derivation of Generalized Darcy Equations for Creeping Flow in Porous Media,"Ind. Eng. Chem Fundamentals, 20 132-137 (1981).

15 Drzal, L T. "Composite Interphase Characterization," SAMPE Journal, 19(5) 7-14 (1983).




295

16 Cogswell, R. N and T. C. Leach "Processing Science of Continuous Fiber Reinforced Thermo-

plastic Composites," SAMPE Journal, 24(3):11-14 (1988).17 Lee, W. I. and S. Springer. "A Model of the Manufacturing Process of Thermoplastic Matrix

Composites," J. of Composite Materials

,

21.1017-1055 (1987)18 Gutowski, T. G. and Z. Cai. "The Consolidation of Composites," Manufacturing International

Proceedings, 13-15 (1988)19. Muzzy, J D. "Processing ofAdvanced Thermoplastic Composites," Ibid, 27-39 (1988)20 Post, L and W Van Dreumel. "Continuous Fiber Reinforced Thermoplastics," Prog. inAdv.

Materials and Process: Durability, Reliability and Quality Control, 201-210 (1985).21 Mallon, P. J , C M. OBredaiger and R P Pipes "Polymeric Diaphram Forming of Complex-

Curvature Thermoplastic Parts," 33rd International SAMPE Symposium, 47-61 (1988)22. Soll, W and T. G. Gutowski. "Forming Thermoplastic Composite Parts," SAMPE Journal,

24(3) 15-19 (1988)

23. Muzzy, J. , L Norpoth and B Varughese "Characterization of Thermoplastic Composites forProcessing," 20th SAMPE International Technical Conference, 162-173 (1988).

24. Okine, R. K., D H. Edison and N K Little "Properties and Formability of a NovelAdvanced

Thermoplastic Composite Sheet Product," 33rd International SAMPE Symposium,

1413-1425

(1987).

25. Okine, R. K. "Analysis of Forming Parts fromAdvanced Thermoplastic Composite SheetMaterials," 20th SAMPE International Technical Conference, 148-161 (1988)

26. Egerton, M. W. and M. B. Gruber. "Thermoplastic Filament Winding Demonstrating Economicsand Properties Via In-Situ Consolidation," 33rd International SAMPE Symposium, 35-46 (1988)

27. Popper, P. and R. McConnell. "A New 3D Braid for Integrated Parts Manufacture and Improved

Delamination ResistanceThe 2-Step Process," 32ndInternational SAMPE

Symposium

,

92-97

(1987)28 Handermann,A. C. "Advances in Commingled Yam Technology," 20th International SAMPE

Technical Conference, 681-688 (1988)29. U.S. Patent 4,614,678 (1986)30. Hartness, T. "Thermoplastic Powder Technology forAdvanced Composite Systems," 33rd Inter-

national SAMPE Symposium, 7-10 (1988)31. International PatentApplication PCT/US87/02867 (1987)

32. Griffiths, G R , W. D Hillier and J.A S Whiting "Thermoplastic Composite Manufacture

Technology for a Flight Standard Tailplane," 33rd International SAMPE Symposium, 308-323

(1988).33. Marsh, G. "Thermoplastics Invade Helicopters," Helicopter World, U.K., 7(3) 8-10 (1988).

BIOGRAPHIES

I. Y. Chang

Dr. Chang is a ResearchAssociate with the Composites Division (R&D) ofE. I. du Pont de Nemours & Company, Inc. He received his Ph.D. in Physical

Chemistryfrom the University of Texas.After

graduation,he worked for one

year as a Senior Research Chemist at Tracor, Inc.,Austin, Texas. He joinedDuPont as a Research Scientist and has been working on numerous research

projects in polymer and fiber. During the past several years he has been respon-sible for the development of new thermoplastic matrix systems such as

J-Polymer and PEKK for advanced composites reinforced with carbon orKevlarg fiber.




296

J. K. Lees

Dr. Lees received his Ph.D. in Physics from Carnegie-Mellon University. Hehas worked in the field of Composites, PolymerAlloys and Blend Technologywith several departments at DuPont. Currently he is the Manager of the Com-

posites Research and Development Group at DuPont.

journal of thermoplastic composite materials volume 1 issue 3 1988 [doi 10.1177_089270578800100305]...

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