2.29

2.29Continuous Molding ofThermoplastic CompositesA. G. GIBSON

University of Newcastle upon Tyne, UK

2.29.1 INTRODUCTION 1

2.29.2 THE IMPREGNATION PROBLEM 2

2.29.3 PROCESS MODELING 5

2.29.4 IMPREGNATION PROCESSES 6

2.29.4.1 Melt Impregnation 72.29.4.1.1 Extrusion compounding 72.29.4.1.2 Strand impregnation processes 72.29.4.1.3 Belt press processes 9

2.29.4.2 Powder Processes 102.29.4.2.1 Aligned fiber techniques 102.29.4.2.2 The Radlite process 12

2.29.4.3 Fiber Co-mingling 122.29.4.4 Solvent-based Processes 132.29.4.5 Reactive Techniques 14

2.29.5 CONTINUOUS PROCESSES FOR THERMOPLASTIC COMPOSITES 14

2.29.5.1 Thermoplastic Pultrusion 142.29.5.1.1 Reinforced extrusions and moldings 162.29.5.1.2 Roll forming 16

2.29.5.2 Thermoplastic Tape Laying and Filament Winding 162.29.5.3 Reinforced Thermoplastic Pipe (RTP) 17

2.29.6 REFERENCES 18

2.29.1 INTRODUCTION

Thermoplastic matrix composites offer thefollowing advantages over their thermosettingcounterparts:(i) a rapid fabrication cycle which avoids the

need for a curing reaction,(ii) cleaner processing,(iii) precursor materials with infinite shelf

life,(iv) a broad selection of tough matrix mate-

rials, and(v) recyclability.

These are the factors driving the developmentof thermoplastic composite technology, whichis now one of the most rapidly expanding sec-tors of the composite market. Surprisingly,until the 1980s, the difficulty of achieving fullimpregnation and wetting of the reinforcementwith thermoplastic resin prevented the cost-effective manufacture of continuous fiber rein-forced thermoplastic composites, and confinedcommercial activity mainly to short fiber injec-tion molding materials. Once the full potentialof thermoplastic matrix composites was rea-lized, however, numerous promising processing

1

methods were developed and refined, with theresult that several manufacturing routes,described in Figure 1, are now available(Leach, 1989; Gibson andManson, 1992; Cogs-well, 1992).Precursors for thermoplastic composites can

be broadly divided into two types: consolidatedand nonconsolidated (requiring further proces-sing to produce wetting of the fibers by thematrix) (see Chapters 2.16, 2.26 and 2.27, thisvolume).Each product form has particular advantages

and drawbacks. Fully consolidated material,e.g., has optimum mechanical properties but isspringy, hard, and elastic, which renders it dif-ficult to handle, shape, or lay up into complexshapes. The nonconsolidated materials, by con-trast, are much easier to handle and can, forinstance, be woven into fabrics that have gooddrape properties. Their disadvantage is thatthey need to be compressed or worked in someway to consolidate them, and this results insignificant reduction in bulk, which is difficultto accommodate in some molding processes.The processes that have shown the most im-

pressive growth to date are direct melt impreg-nation and fiber co-mingling. The former isused for long fiber injection molding materials,unidirectional thermoplastic prepregs, andglass mat thermoplastics. Co-mingled productsare frequently converted into woven fabrics,processed by vacuum bag consolidation orpress molding.Although thermoplastic composites have

been available since the 1980s, their penetrationinto the wider composites markets was initiallyrather slow, and some of the anticipated ad-vantages were more difficult to realize than hadbeen envisaged. The fast, chemistry-free proces-sing cycle, for instance, was not free of pro-blems. The concept of rapidly heating, forming,and cooling is attractive, but some difficulties

needed to be overcome to put it into place. Thetemperatures to which thermoplastics must beheated for processing, for instance, are higherthan with thermosets, which presents problemswith equipment design and material compat-ibility. Moreover, if a part is to be formed inmatched steel tooling, it cannot be demoldedwhile the resin is still liquid, so how should it becooled? One answer is to preheat the composite,then combine the forming and cooling opera-tions. Hot stamping of glass mat thermoplasticsand other materials is now one of the mostactive areas of thermoplastic composite techno-logy.The underlying principle with all thermo-plastic composites is to consider the completeprocess cycle, rather than simply the prepara-tion of pre-impregnated materials.Early research concentrated on high perfor-

mance applications and led to the developmentof aromatic polymer composites which, inmany ways, outperformed their thermosettingcounterparts (Cogswell, 1992). It was soon rea-lized, however, that thermoplastic technologies,based on medium cost polymers, such as poly-propylene, polyesters, and polyamides, wascapable of penetrating a much broader rangeof markets and applications.Problems with processing technologies have

now been overcome and the green require-ments of clean processing and recyclability areof increasing importance, so the thermoplasticroute, based on some of the technologies re-viewed here, is assured of an increasingly im-portant role in the future.

2.29.2 THE IMPREGNATION PROBLEM

The principles of manufacturing with ther-moplastic composites have been discussedwidely by, e.g., Gibson and Manson (1992),

Figure 1 Alternative fabrication routes for the processing of thermoplastic matrix composites.

Continuous Molding of Thermoplastic Composites2

Chang and Lees (1988), Cogswell (1992), andMantell and Springer (1992). The key problem,illustrated schematically in Figure 2, lies in theeffective impregnation (see Chapter 2.17, thisvolume) of fiber tows (or a fiber network) witha thermoplastic resin. The viscosity of conven-tional thermoplastics under melt processingconditions is usually in the range502000 Pa.s, which contrasts with thermosetsin their noncross-linked state where the viscos-ity seldom exceeds 50 Pa.s. In the case of the2400 tex roving shown in Figure 2, continuousimpregnation would involve wetting the surfaceof all the glass fibers. This surface, ifunwrapped and laid flat, would be equivalentin area to a continuous strip 300mm wide. Theviscous resin would have to be spread out to athickness roughly equivalent to the interfiberspacing, i.e., approximately 9 microns thick.The impregnation of a bed of fibers, Figure 3,

can be described in terms of Darcys law which,for 1-D flow, gives

u dxdt KZdp

dx1

where u is the flow velocity of the resin, thepressure gradient is dp/dx, K is the permeabilityof the fiber array, and D is the polymer meltviscosity. Observing that the pressure gradientin this case is simply P/x and integrating Equa-tion (1) for the boundary conditions in Figure 3gives the impregnation time

timp ZX2

2KP2

This relationship, although approximate, servesto demonstrate the problems involved in im-pregnating fibers arrays with thermoplastic

melt. The importance of the impregnationlength can be seen from the squared term in X.The principle on which all effective impregna-tion processes operate is to manipulate the vari-ables in Equation (2) to attain an economicallyshort impregnation time. For processes withpolymermelts the most straightforwardmethodof achieving this is clearly to minimize X.The pressure, which drives the flow of poly-

mer, contains components due to the externalmechanically applied pressure, Pm, and the ca-pillary pressure, Pc, so that

P Pm Pc 3

The capillary pressure, discussed in greater de-tail elsewhere (Connor, 1995), results from thechange in surface energy when the polymer wetsthe surface of the reinforcement. Unfortu-nately, capillary pressure values for polymer/fiber systems are generally relatively small, inthe range 440 kPa, as observed by Ahn et al.(1991), so the impregnation time would be verylong if one relied on capillary effects alone toachieve impregnation (Lee and Springer, 1987).In most impregnation processes the capillarypressure is small enough to be neglected incomparison to the applied pressure needed todrive the process at an economic rate.Permeability is a key parameter in impregna-

tion processes. Assuming the fiber bed to con-sist of an array of parallel fibers, thepermeability can be estimated from the mod-ified CarmanKozeny equation

K r2f

4c

1 vf3v2f

4

Values of the Kozeny constant, c, are differentfor flow parallel and perpendicular to the fibers

Figure 2 The problem of impregnating a roving, containing a large number of small diameter fibers, with aviscous thermoplastic resin.

The Impregnation Problem 3

and they are also dependent to some extent onthe particular resin/fiber system and on theexperimental method used, as observed by Wil-liams et al. (1974). The applicability of thisapproach to thermoplastic melts, which displaynon-Newtonian characteristics, has been exam-ined by Astrom (1992) and Astrom et al. (1992).Improved models for the permeability of

composite fiber arrays have been proposed byGebart (1992) and Gutowski et al. (1987),amongst others, but in all cases the permeabil-ity is predicted to be proportional to the squareof the fiber radius. This is exploited in theproduction of special grades of glass fiber forthermoplastic composites, which employ signif-icantly larger fiber diameters than the 10 mi-crons that is widespread for thermosets. Glassfibers for continuous fiber reinforced thermo-plastics are often supplied with 18 mm diameter,sometimes up to 25 mm, for which Equation (4)predicts permeability improvements of morethan a factor of six. It should be borne inmind, however, that the strength of glass fibersdepends on diameter, and larger fibers oftenhave strengths as low as 1400MPa, comparedto about 1800MPa for 10 mm fiber.Unfortunately carbon fibers, which are

79 mm in diameter, are not similarly amenableto manipulation, so impregnation processesgenerally run more slowly than those for glass.Applied transverse stress significantly affects

the permeability by forcing the fibers closertogether. This deformation has been likenedto that of a nonlinear spring, and has been

studied by several workers, including Kimet al. (1991) and Toll and Manson (1995).Strictly speaking the K value in Equations(1)(3) should be treated as pressure-depen-dent. One simple functional form of pressuredependence that can conveniently be assumedfor modeling purposes is

KP K0 exp PmP0

5

where K0 is the permeability at low or negligiblepressure (from Equation (4) or similar) and P' isa constant with the units of pressure.The detrimental effect of compression on

permeability was not understood in the earlydays, and it took some time before it wasrealized that high levels of transverse pressureare counterproductive to impregnation. De-spite the presence of pressure in the denomina-tor of Equation (2) the most successfulimpregnation processes employ only modestpressure. In practice it is often found that thepressure which gives the highest value of theproduct, KP is of the order of 12MPa.Viscosity, of course, appears in Equation (3)

and benefits can be obtained by keeping this toa minimum, as discussed later. Viscosity can belowered by several means, which include the useof local high temperatures, low molecularweight thermoplastics, and, in certain pro-cesses, solvents, or plasticizers (Cogswell et al.,1981a, 1981b; Cogswell, 1992; Goodman andLoos, 1990).

Figure 3 Impregnation of a fiber array by resin under an externally applied pressure.


The pronounced effect of the squared term inflow length, already noted, provides the mostpowerful means of reducing the impregnationtime. Clearly it is desirable that the polymer andfiber components of a thermoplastic compositeshould be intimately mingled together prior toimpregnation in a manner which minimizes X.Most techniques for the preparation of well-wetted composites with thermoplastic matrices,therefore, involve two stages:(i) Stage 1: The intimate mechanical min-

gling of the fiber and matrix, and(ii) Stage 2: Melting of the thermoplastic

polymer with the application of a modest levelof pressure to promote impregnation.The three most important methods of redu-

cing X are pin impregnation, powder impreg-nation, and commingling. These, and others,will be discussed in the next sections.

2.29.3 PROCESS MODELING

Equations (1)(4) can be used, along withappropriate values of the impregnation length,X, as the basis of simple process models, as

reported by Chang and Lees (1988) and Gibsonand Manson (1992). In the present case, theeffect of pressure will be included through theincorporation of Equation (5). Figure 4 showsthe predicted relationships between impregna-tion time and pressure for a range of thermo-plastic composite systems of interest, calculatedfrom the data given in Table 1. The four fiber/resin combinations chosen all relate to commer-cially interesting systems. X values of practicalinterest for flow processes transverse to thefibers are generally in the region 100300 mm.Each material combination therefore producesa band of typical impregnation times as afunction of increasing applied pressure. Atzero applied pressure the impregnation processis driven solely by wetting effects, so theimpregnation times are long. As the appliedpressure increases, the impregnation timedecreases rapidly until the effect of the fibertow compressibility on permeability begins toshow. Eventually, at high values of appliedpressure, the impregnation time begins toincrease again, because of the pressure effect.It can be seen therefore that high, applied pres-sures are generally counterproductive in theimpregnation process.

Figure 4 Model prediction for impregnation time (log scale) vs. applied pressure for various thermoplasticcomposite systems of interest, using model parameters given in Table 1.

Table 1 Parameter values used in the process model.

Viscosity, Z (Pa.s) vf rf (mm)

Glass/polypropylene (low MW) 10 0.3 9Glass/thermoplastic 100 0.3 9Carbon/thermoplastic 100 0.6 3.5Carbon/PEEK 500 0.6 3.5

Capillary pressure, Pc=0.04MPa; Kozeny constant, c=9; P'=2 bar; impregnation length,X=100300 mm.

Process Modeling 5

In the case of glass fiber with low molecularweight polypropylene, melt viscosity values aslow as 10 Pa.s can be achieved, leading to verylow impregnation times. With other polymers,however, it is often difficult to obtain materialswith viscosities lower than about 100 Pa.s,which gives impregnation times an order ofmagnitude larger. It can be seen that changingfrom glass to carbon fiber results in more thanan order of magnitude increase in impregnationtime. This is due partly to the smaller fiberdiameter and partly to the fact that higherfiber volume fractions are of interest in thecarbon fiber case. Finally, the most difficultrange of systems to melt impregnate can beseen to be the ones based on carbon fiber withhigh viscosity polymers, such as PEEK, wherethe impregnation times extend to more than onehour. In the latter case, special measures, be-yond conventional melt impregnation, are oftenrequired to achieve well-impregnated product.Individual categories of impregnation pro-

cess can each be characterized by a processwindow of achievable pressure and impregna-tion time. Figure 5 shows the same materialimpregnation data as Figure 4, with processwindows for different techniques superposed.In each case the upper edge of the processwindow indicates the practical limit that canbe achieved in terms of impregnation time. Itcan be seen that the most rapid impregnationtimes are achievable with pin impregnation,which will be described in the next section. Thistechnique is not applicable, however, to thematerials of highest viscosity. The other con-tinuous processes shown are the belt press,

which extends the applicable pressure and com-pounding extrusion, traditionally used forinjection molding materials, which is applicableonly to short fiber products.

2.29.4 IMPREGNATION PROCESSES

In this review of materials systems and pro-cesses, the need for compatibility and a stronginterface between the matrix resin and the fibersis, in most cases, an important requirement thatsignificantly influences properties and applica-tions. The advent of fibers specifically tailoredfor use with engineering thermoplastics is asignificant advance and improvements are con-tinuing in this area.The glass/polypropylene system is worthy of

special note, as this is one of the most commer-cially significant material combinations (Gib-son, 1994). Although pure polypropylene isnonpolar, and adheres poorly to glass, the ad-dition of an internal coupling agent in the formof a functionalized polyolefin with maleic oracrylic acid groups grafted on to the main chaincan confer a considerable improvement in in-terlaminar properties (Adur et al., 1989; Con-stable et al., 1989; Constable and Adur, 1991;Rijsdijk et al., 1993). Functionalized additivesof the Polybond2 type are most effective whenused in conjunction with glass fibers treatedwith a suitably compatible size. Another bene-fit, specific to polypropylene, is the ability toclip the high molecular weight tail from thepolymer by melt processing with an organic

Figure 5 Impregnation time (log scale) vs. pressure, as in Figure 4, with process windows for differentimpregnation techniques superposed.


peroxide, producing resins of very low viscositythat perform extremely well in the melt impreg-nation process, as seen in Figures 4 and 5.With all reinforcements to be used with ther-

moplastics it should be remembered that theprocessing temperatures are much higher thanthose employed for thermosets, so the size coat-ing on the glass should be appropriately tem-perature-resistant, in addition to beingcompatible with the polymer.

2.29.4.1 Melt Impregnation

2.29.4.1.1 Extrusion compounding

Until about 1980 the main examples of ther-moplastic matrix composites were short fiberreinforced injection molding materials (seeChapter 2.30, this volume), manufactured bythe process of extrusion compounding, shownin Figure 6. This is still one of the most effectivemethods of producing molding pellets andaccounts for more than half the current tonnageof glass fiber reinforced thermoplastics.Significant improvements have taken place

over the years in the design of compoundingextruders. These include the facility for down-stream addition of the reinforcement, whichreduces work by allowing the polymer to befully melted before it is brought into contactwith the fibers. Nevertheless, significant levelsof fiber breakage are virtually unavoidable ifthe fibers are to be fully wetted by the meltcompounding route.The residual fiber length in injection molding

compounds is usually 100600 mm, with aweight average fiber length of about 250 mm.This compares to critical fiber length values ofabout 200 mm for nylon and 400 mm for poly-propylene. While the addition of short glassfibers to polymers such as polypropylene ornylon undoubtedly results in useful increasesof stiffness and strength the reinforcing effi-ciency is only of the order of 40%, as observed

by Bader and Bowyer (1973). Several studieshave been carried out on the breakage of fibersduring compounding, including those of Fran-zen et al. (1989) and von Turkovich and Erwin(1983).One interesting argument in relation to fiber

length preservation is that, in processes which,by mixing, impose a fully random fiber orienta-tion distribution, the fiber aspect ratio is con-trolled ultimately by the ability of randomlyoriented fibers to pack in space. Evans andGibson (1986) showed that for random fiberorientation there is a simple inverse relationshipbetween the maximum achievable fiber aspectratio (l/d)max and fiber volume fraction, vf sothat

l

d

max

kvf

6

This is consistent with the results of the experi-mental studies of Milewski (1973, 1978) onrandom fiber packing (for an experimentallydetermined value of k=5.3), and also describesthe residual fiber length after melt compound-ing in many cases. Equation (2), therefore,places a fundamental restriction on the fiberlength that can be retained in processes thatrandomize the orientation of the fibers.

2.29.4.1.2 Strand impregnation processes

As early as the 1960s, attempts were beingmade to overcome the limitations of the com-pounding extruder. Several workers realizedthat it was undesirable to process the reinforce-ment through the extruder along with the poly-mer, as this caused problems with machine wearas well as with fiber breakage. The most pro-mising early attempts to overcome the restric-tions of melt compounding involved thepassage of fiber tows through cross-head diessimilar to those used for wire-coating extrusion,as shown in Figure 7. The process patented by

Figure 6 The extrusion compounding process for the manufacture of short fiber reinforced thermoplasticinjection molding granules.

Impregnation Processes 7

Bradt (1959, 1962) employed a technique simi-lar to this, as did those of Moyer (1976) andHattori et al. (1977), in which a feedstock basedon a polymer melt or aqueous latex was com-bined with glass fibers to form fiber concen-trates. Experiments on cross-head extrusionwere also reported by Bader and Bowyer(1973).Cross-head extrusion, followed by chopping

of the strand into pellets for molding, seemed tooffer a number of advantages over the com-pounding extruder, including reduced machinewear and lower power consumption. However,in the early continuous strand process impreg-nation was incomplete, with the melt rarelypenetrating beyond the outer layers of the pel-let. In hindsight, it can be seen that the reasonfor this is the pressure dependence of the per-meability mentioned earlier: the external pres-

sure exerted on the tow by the polymer meltserved to lock the individual fibers together,severely hindering the penetration process.With only partial impregnation, the task of

wetting the fibers at the core of each pellet wastransferred from the extruder to the injectionmolding machine, and this caused some ma-chine wear and often produced unwetted orundispersed fibers in moldings. Nevertheless,some longer fibers were often retained intact,which produced parts with improved impactproperties, so early processes of the strand im-pregnation type achieved some success in nichemarkets.The problem of achieving a substantially

completely wetted product by the melt routewas solved in the 1980s with an improved meltpultrusion technique for long fiber pellets(Cogswell et al., 1981a, 1981b). The Vertonprocess, which pointed the way towards manycurrent thermoplastic impregnation processes,addressed the problem of reducing the flowlength for impregnation by opening and flatten-ing the fiber tow.Patents for the new process taught the im-

portance of spreading the fiber tows and ofensuring that the melt viscosity of the polymerwas as low as possible. Impregnation devices,Figure 8, are quite simple and resemble super-ficially the baths used in the processing ofthermosetting composites. Pin impregnationworks, first by using cylindrical spreader sur-faces to open up and spread out the roving.When this is accomplished the thickness of thefiber tow can be reduced to just a few fiberdiameters, resulting in a flow length of theorder of 100200 mm. Following this, a film ofpolymer melt is entrained between the movingfibers and the pin. This generates a modestpressure that drives the melt into the fibers, asshown. The process usually requires severalpins to achieve good impregnation.The flow mechanisms operating in pin

impregnation have been studied by a numberof workers, including Chandler et al. (1992),

Figure 7 Cross-head die, or wire coating extrusion process for the continuous manufacture of partiallyimpregnated injection molding materials.

Figure 8 The pin impregnation process for contin-uous pultrusion of well-impregnated unidirectionaltows: (a) impregnation bath and (b) mechanism of

impregnation.


Peltonen et al. (1992), Bijsterbosch and Gay-mans (1993a, 1993b), and Bates and Charrier(1999). As might be expected, there have beenseveral variants of the process, some of whichhave been patented. Benefits have been var-iously claimed, for instance, for rotating thepins, varying the pin surface profile, injectingthe resin through slots in the pin, or through aporous pin material. Commercial versions ofthe process generally employ many impregna-tion cells, side by side, fed from a commonextruder, as in Figure 9. In an alternative con-figuration a single broad impregnation cavitymay be employed, through which many sepa-rate rovings are passed and the pins may bereplaced by curved, wavelike, or angled sprea-der surfaces (Hawley, 1982).Although many thermoplastics have been

processed by melt impregnation, nylon 66 andpolypropylene have been found to work veryeffectively. The molecular weight needed forgood mechanical properties in thermoplasticmatrix composites is substantially lower thanthat required in conventional thermoplasticproducts, permitting the use of thermoplasticmelts of much lower viscosity than would nor-mally be encountered: 30 Pa.s and even as lowas 10 Pa.s have been advocated (Cogswell et al.1981a, 1981b). Low viscosity grades of poly-propylene, which are particularly suitable forthis process, may be prepared by compoundingthe resin with an organic peroxide. It has alsobeen suggested that viscosity can be lowered inthe vicinity of the impregnation pins by heatingthem to a higher temperature than the rest ofthe bath.In most processing operations a compromise

has to be achieved between line speed, degree of

wetting, and polymer viscosity. Nevertheless,line speeds in excess of 30mmin71 are oftenencountered in processes for nylon or polypro-pylene injection molding pellets. As might beexpected, higher fiber volume fractions, or theuse of carbon fibers instead of glass, necessitatethe employment of lower line speeds, because ofthe lower permeability.Pin impregnation is used most commonly for

the manufacture of long fiber injection moldingpellets. Variants of the process, however, arereadily adaptable for making continuous ther-moplastic prepregs, such as Plytron2 (glass/polypropylene) and APC (carbon/PEEK).

2.29.4.1.3 Belt press processes

A belt press enables the processes of pressing(with consolidation and elimination of air) andcooling to be carried out in a continuous opera-tion. These processes are mainly used in themanufacture of glass mat thermoplastic(GMT) prepregs, for subsequent processingby hot stamping or pressing (see Chapter 2.27,this volume). In the Azdel-Laminate process,Figure 10, the reinforcement can take the formeither of chopped strands or continuous fiberswirl mat. A film of matrix polymer, mostfrequently polypropylene, is inserted orextruded between the layers of mat prior toentering the belt press. In one version, recycledin-house scrap is incorporated into the centrallayer of the GMT.The moving belt is a stainless steel band

and, within the press, there are stations for bothheating and cooling the product, while exerting

Figure 9 Arrangement of impregnation cells on a production unit for long fiber injection moldingmaterials.


a through-thickness pressure to ensure that acertain level of impregnation takes place as thepolymer is melted, followed by consolidation asit cools. The outstanding advantage of the beltpress is that, despite its relatively high capitalcost, it makes possible the continuous manu-facture of thermoplastic prepregs. Because theindividual rovings are not spread out to mini-mize the flow length, belt processes generally donot achieve full wet-out of the prepreg by thepolymer melt. When the reinforcement is in the

form of strands or bundles, e.g., the flow ofpolymer may often be sufficient only to sur-round the bundles, without fully impregnatingindividual strands. The key to the success of thistype of process is the recognition that, in manycircumstances, including GMT manufacture,complete impregnation is not necessary.Belt press processes are also beginning to be

investigated for processing woven continuousfiber materials, but again it is difficult toachieve complete impregnation by this route.

2.29.4.2 Powder Processes

2.29.4.2.1 Aligned fiber techniques

Processes of this type rely on incorporatingpolymer, in the form of powder, into the inter-stices between the fibers, as shown inFigure 11(a), in order to reduce considerablythe flow length required for impregnation. Thismethod is particularly attractive when the poly-mer is available in the form of powder. One ofthe earliest references to powder impregnationis the patent of Price (1973). The hot tubeprocess, as it was known, was used successfullyfor some years as an alternative to melt impreg-nation for long fiber injection molding pellets.Several others, including Chabrier et al. (1986),Muzzy et al. (1989, 1990), de Jager (1987),Hartness (1988), Iyer and Drzal (1990), Ogdenet al. (1992), Ye et al. (1992), Hugh et al. (1992),Baucom and Marchello (1994), and Wagnerand Colton (1994), subsequently discussed orpatented versions of the process. Ramani et al.(1992), Texier et al. (1993), and Vodermeyeret al. (1993) described similar processes basedon aqueous suspensions of powder. These var-ious processes have been reviewed in greaterdetail by Miller and Gibson (1996).

Figure 10 The melt process for the manufacture of stampable glass mat thermoplastics (GMTs).

Figure 11 Powder impregnation: (a) powder im-pregnated roving and (b) schematic of a powder

impregnation process.


Fiber tows can be impregnated continuouslywith powder by various means, the simplest ofwhich is through the action of pins or rollers ina bed or suspension of the powder. The mech-anism of initial dispersion of the powder withinthe strand is rather similar to that describedpreviously for melt impregnation. The strand isopened up and spread by the action of pins,then the powder is mechanically forced betweenthe fibers by the trapping of particles betweenthe moving fibers and the surface of the pin.There are several versions of the powder

impregnation process, one of which is shownin Figure 11(b). It is possible, e.g., to employgas fluidization or mechanical vibration to agi-tate the powder. Gas fluidization presents diffi-culties in terms of the finer polymer particlesbeing blown out of the bed. It is usually neces-sary to trap these particles and return them tothe bed. There can also be an explosion hazardif air is used as the fluidizing medium; the use ofinert gas as a substitute significantly increasesthe cost of the process. Some processes employelectrostatic charge to attract the polymer tothe fibers, while others rely simply on mechan-ical forces. It is possible to employ a venturi oran air knife to open up the rovings prior toinsertion of the polymer particles.The polymer powder should ideally be in the

form of near-round particles. There are econo-mic advantages in using polymers which areavailable as powder direct from the polymeriza-tion process and these, in addition, tend to bemore regular in shape than those produced bygrinding processes such as cryogrinding. Inprinciple it might be seen as desirable for thepowder particle size to be small, and similar tothe fiber diameter, but this is often difficult to

achieve, as particle size tends to be limited byeconomic considerations. In practice, powderswith diameters up to 150 mm have been usedwith success.One key problem is control of the resin to

fiber ratio. In some versions of the process anexcess of powder is incorporated at the impreg-nation stage. The surplus is then removed bymeans such as scraping, mechanical vibration,or the use of a wipe-off die (following meltingand consolidation). One convenient method ofcontrolling resin/fiber ratio is to use a powderbed in which the impregnation pins are on aninclined ladder, as shown in Figure 11. The pinsabove the powder level have the effect of re-moving powder, so polymer pick-up can bemaintained accurately by controlling the levelof powder in the bed.Processing the impregnated strand into a con-

solidated laminate involves two further steps.First, the polymer particles are melted using ahot air or infrared oven. Following that, it isthen necessary to consolidate the material. Var-ious models have been proposed to describe theconsolidation process by, among others, Leeand Springer (1987), Connor et al. (1995), andHaupert and Friedrich (1995). Generally, it isagreed that the main mechanism of consolida-tion in powder impregnated tows involves flowof resin along the fibers, since the axial perme-ability is about an order of magnitude greaterthan that in the transverse direction. Accordingto Connor et al. (1995) the melted particles formbridges between adjacent fibers, as shown inFig-ure 12. Lateral pressure must then be applied topromote consolidation, so that each bridgespreads along the fiber, displacing air, until itmeets a neighboring one coming the other way.

Figure 12 The melt bridge model for powder impregnation (Connor et al., 1993) shows bridges betweenfibers which flow axially under applied pressure, to achieve impregnation.


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In continuous processing, consolidation maybe achieved by drawing the moving tow over anumber of pins or cylindrical surfaces in muchthe same way as discussed above for strandimpregnation. This process has been describedby Miller et al. (1998). Alternatively the towmay be pulled into a convergent die. In cyclicmolding processes the consolidation is achievedby direct pressing in the case of matched diemolding or through the use of combinations ofvacuum and external pressure in the case ofautoclave or vacuum bag molding.The consolidation step can be carried out in-

line with powder impregnation, but more fre-quently it is desirable to process the tows into aconvenient form, such as woven fabric, prior tofinal fabrication. Two methods have been pro-posed to ensure that the powder remains withinthe roving during subsequent handling. Ganga(1986) recommended that the impregnated towbe coated with a thin-walled tube of polymerfilm, whereas Muzzy and Colton (1995) de-scribe the fusion of the powder to keep it inplace. In the latter case the flexibility of the towis retained.

2.29.4.2.2 The Radlite process

The Radlite process for GMT, Figure 13,employs paper-making technology to dispersepolymer powder and glass fibers in an aqueousfoam. This is then deposited on a continuousmoving filter and the aqueous componentremoved by vacuum. After drying, the compo-site paper is passed through a belt press,where the polymer is melted and the compositeconsolidated by the application of moderatepressure. By virtue of the foam dispersionstep, GMTs made by the Radlite process gen-

erally contain more completely dispersed fibers,compared with those made by the melt route,where the fibers tend to remain in bundles. Thepaper-making deposition process also results insome preferential fiber orientation in themachine direction.

2.29.4.3 Fiber Co-mingling

A further method of achieving initial inti-macy between fires and matrix is by the randomintermingling of fiber and matrix materials inthe form of parallel fibers (Coldicott et al., 1989;Hamada et al., 1993; Miao et al., 1994; Svens-son et al., 1998). Co-mingling, Figure 14(a), canbe achieved with either continuous or discon-tinuous fibers. In the processes employed byVetrotex (St. John, 1995) and Trevira Neckel-mann (1999) glass and thermoplastic fibers aremingled continuously to form a single tow. Itshould be remembered that, even when the twodifferent fiber types are dispersed in a comple-tely random manner, fiber-rich and resin-richareas will naturally be present, so the flowlength needed to achieve complete impregna-tion may be several times the interfiber spacing,as shown in Figure 14(b). The co-mingled sys-tems used most widely are glass/polypropylene,glass/PET (and the copolymer, PETG), andglass polyethylene.The largest commercial tonnage of co-

mingled material processed is glass/polypropy-lene. It is recognized, however, that the inter-laminar properties achievable with glass/polypropylene may not be sufficient for somestructural applications. Although the technol-ogy exists to achieve good coupling betweenpolypropylene and glass (Constable et al.,1989), the relatively low glass transition tem-

Figure 13 The Radlite process for the manufacture of GMT sheet from thermoplastic powder and fibers.


perature limits the interlaminar shear strengthachievable at room temperature. Since bothcompressive strength and flexural strength aredetermined to a large extent by the interlaminarproperties, the limitations on these propertiesrestrict the load-bearing applications in whichpolypropylene-based composites may be used.The limitations of polypropylene have stimu-

lated interest in cost-effective matrix materialswith higher glass transition temperatures, no-tably polyesters and polyamides. Recently, co-mingled materials have been developed basedon PET and its amorphous copolymer PETG(Trevira Neckelmann, 1999), the latter beingfavored in certain types of process because ofthe high melting point of PET (285 8C).The co-mingling processes employed by

EMS-Chemie (1999) and Schappe Techniques(France) to process carbon fibers and nylon 12(McDonnell et al., 1999), and by others toprocess various materials (Ives and Williams,1988; Coldicott et al., 1989) employs stretch-breaking and twisting to combine two differenttypes of fiber. The technique is well known inthe textile industry as a method for combiningdifferent fiber types. Stretch-breaking is per-formed by passing the fiber tow over two setsof rollers, the second rotating at a higher speedthan the first. Although it might be expectedthat stretch-broken reinforcement, being dis-continuous, would give products of inferiorstrength it should be borne in mind that thedistance between breaks is very large comparedwith the critical fiber length. It is also arguedthat the stretch-breaking process acts mainly onthe statistical weak points in the fiber, whichwould in any case have broken on loading ofthe composite, and that the slight lack of align-ment of the twisted structure gives improvedinterlaminar properties. Often it has been foundthat well-consolidated composites from thestretch-breaking route have properties similarto or only slightly inferior to those of contin-uous fiber materials.

Co-mingled materials may be processed inthe form of continuous rovings, by pultrusion,filament winding, or braiding. Alternativelythey may be woven into fabrics for subsequentfabrication operations. As in the case of powderimpregnated prepregs, the fabrication step forco-mingled composites requires both melting ofthe thermoplastic fibers and consolidationunder pressure, to cause the reinforcing fibersto be impregnated and wetted by the matrix.The mechanisms of impregnation have beeninvestigated and models for the process havebeen proposed by, amongst others, Bernet et al.(1999), VanWest et al. (1991), Klinkmuller et al.(1995), Cain et al. (1997), Miller et al. (1998),and Wilks et al. (1999).An important factor in developing successful

co-mingled systems is control of the diameter,properties and structure of the polymer fibers,which are different from those in textile fibers.For co-mingling it is desirable that the poly-meric and reinforcing fibers should have similardiameter, which requires the polymer fibers tobe of lower diameter than they would be inconventional textiles. It is also important thatthe polymer fibers should not contract exces-sively or move around on melting so, unliketextile fibers, generally they are processed underconditions that minimize molecular orienta-tion. Finally, fibers for co-mingling generallycontain polymer of lower molecular weight, inorder to promote flow and impregnation.

2.29.4.4 Solvent-based Processes

The purpose of solvent processing is to reducethe viscosity of the thermoplastic in order topromote flow. Solvent processes have been de-scribed by Turton and McAinsh (1974), Good-man and Loos (1990), and Savadori and Cutolo(1993). Solvent-based technologies, such asthose employed by Ten Cate (1998), are gener-

Figure 14 Fiber co-mingling: (a) co-mingled fibers tow and (b) resin and fiber-rich regions resulting fromrandom dispersal.


ally applicable to amorphous, rather than crys-talline polymers, as the latter are not usuallyeasy to dissolve under normal conditions. Poly-mers that can be conveniently processed bysolution techniques include thermoplastic poly-etherimide (PEI), polysulfone, polyether sul-fone, and polyphenyl sulfone.Solutions of thermoplastic polymer can be

used to impregnate either unidirectional towsor woven fabrics. The most significant technicalproblem is in handling the complete removal ofthe solvent after wet-out of the prepreg, sinceresidual solvent can be deleterious to the prop-erties of the composite.When choosing composites based on amor-

phous resins, their potential susceptibility tospecific solvents needs to be borne in mind.Nevertheless, such materials have been used ina range of engineering applications (Ten Cate,1998). The most successful are those based onPEI, which has excellent fire smoke and toxicitycharacteristics. Applications of the Ten CateCetex range of PEI-based composites includeaircraft interiors and external fuselage ice im-pact protection.An alternative to the solvent technique is the

use of a plasticizer, which may either be allowedto remain in the structure after processing, withsome detriment to properties, or be removed.One such technique, appropriate to PEEK orPPS composites has been described (Cogswell,1992; Cogswell and Staniland, 1985; Cogswelland Measuria, 1988) where it was claimed to bebeneficial for the impregnation of carbon fiberfibers with minimum damage to the fibers.

2.29.4.5 Reactive Techniques

An alternative method of avoiding the diffi-culties associated with high viscosity is to pro-cess the matrix resin in a condition where themolecular weight is low, and to follow theimpregnation step with a polymerization orchain extension reaction. In situ polymerizationhas been considered in the case of monomerssuch as methyl methacrylate, lactams, and sys-tems leading to polyketones (Cogswell, 1992).Recently, an improved system has been devel-oped for the anionic ring-opening polymeriza-tion of caprolactam and laurolactam, theprecursors for nylons 6 and 12 respectively(EMS-Chemie, 1999). This has led to interestin in situ polymerized polyamide 12 composites(OMairtin et al., 1999; Luisier et al., 2000). Inaddition to the use of chemical initiators and/orheat to promote polymerization, light andUV-initiated polymerization have also beenconsidered.

In certain cases it has been found beneficialto carry out the impregnation with a low mo-lecular weight thermoplastic, then increase themolecular weight by chain extension. Somethermoplastics, e.g., PPS and PEEK, increasetheir molecular weight at high temperaturesand this may be used to advantage, althoughchain extension may sometimes be accompa-nied by less desirable cross-linking reactions. Inthe case of PEEK (Ward et al., 1987; Cogswell,1992), it has been found possible to impregnatespecially-sized carbon fiber with a low viscosityversion of the resin using a process similar tothe pin impregnation process described inFigure 8. The molecular weight is then substan-tially increased by the action of a reagent pre-viously applied to the fibers. This reagent isclaimed to promote chain extension initially inthe important region close to the resinfiberinterface. This sequence of melt impregnationand chain extension results in a thermoplasticcomposite with unique properties, due to thecombination of excellent impregnation, highmolecular weight resin and a good fibermatrixinterface. The successful development and ex-ploitation by ICI, and later Fiberite, of thetechnology for the manufacture of aromaticpolymer composite (APC), as the carbon/PEEK system is referred to, is one of the mostnotable achievements to date in the area ofthermoplastic matrix composites.

2.29.5 CONTINUOUS PROCESSES FORTHERMOPLASTIC COMPOSITES

The processing routes available for ther-moplastic matrix composites have been sum-marized in Figure 1. This section will dealspecifically with those techniques that involvecontinuous or near continuous movement ofthe reinforcement through the process, namely:(i) thermoplastic pultrusion,(ii) thermoplastic filament winding (includ-

ing tape laying), and(iii) helical tape winding.The other cyclic processes are dealt with in

Chapter 2.28, this volume.

2.29.5.1 Thermoplastic Pultrusion

Thermoplastic matrix composites offer somepotential advantages over their thermosettingcounterparts for the production of sectionalproducts by pultrusion. Perhaps the greatestbenefit is the elimination of the cure reaction,which appears to offer the prospect of muchfaster speeds. A number of studies have taken


bibiHighlight

bibiHighlight

place recently with the aim of making thermo-plastic pultrusion a viable commercial tech-nique (Hawley, 1982; Larock et al., 1989;Astrom and Pipes, 1991; Devlin et al., 1992;Michaeli and Jurss, 1996; Miller et al., 1998).One option is to conduct he process in-line withone of the impregnation processes describedpreviously (melt impregnation, powder impreg-nation or fiber co-mingling). Alternatively, itmay be operated with a pre-impregnated pre-cursor material.The pultrusion process, shown in Figure 15,

has the following stages:(i) in-feed,(ii) melting of the resin in the precursor

material,(iii) consolidation,(iv) forming, and(v) cooling.Careful control of the spatial arrangement

and tension of the in-feed is essential for agood quality product with regular spatial dis-tribution of the reinforcement. The meltingstage should make allowance for the low ther-mal diffusivity of the in-feed materials. Toachieve melting throughout the section of thein-coming product it is necessary to separate allthe tows or tapes of material prior to heating, tomaximize the area available for heat transfer.For line speeds above about 1mmin71, infra-red (IR) heating, or a mixture of this and hot airis desirable. IR allows a high rate of energytransfer to be achieved, and the heating effectpenetrates some distance into the material. Afrequently used design of IR oven for pultru-sion preheating consists of upper and lowershells which can be opened up in the case of aline stoppage, to allow the energy input to theproduct to be halted. This avoids the risk ofigniting the polymer.In the case of powder impregnated or co-

mingled precursors it is necessary to carry out

a consolidation operation to achieve wet-out.This is best accomplished by the use of pins,similar to those described for melt impregna-tion. Pulling the pretensioned tows over thepins results in the necessary lateral pressure tocause the powdered or co-mingled material toconsolidate and wet-out. This stage is not ne-cessary if fully consolidated precursor is used.Following wet-out, the strands must be

brought together carefully, usually in a gather-ing die, to form the shape of the product. Twodifferent process configurations have been ad-vocated for the forming/cooling stages, asshown in Figure 16. In the two-die process,a heated consolidation die is followed by acooled shaping die. The convergence in thehot die generates a substantial pressure, whichassists in consolidation of the product. The diegeometry may also result in a certain amount ofwipe-off of resin. While the two-die system hasbeen found to operate successfully at lowspeeds its drawback is the very high pull-forcesthat are developed due to the resin viscosity andthe relatively large surface area of the die. Thepull-force is almost proportional to velocity, sothe line speeds are often significantly lower thanthose in conventional thermoset pultrusion(about 1mmin71), and the high interfacialstresses in the dies can lead to problems withsurface appearance.In the second configuration in Figure 16(b),

the contact time between the product and theforming devices is substantially reduced, result-ing in much lower pulling forces and higherspeeds, up to 20mmin71. A short gatheringand wipe-off die is followed by a cooling bath,containing a series of product consolidationdevices. The down-stream configuration resem-bles the forming dies sometimes used in theextrusion of thermoplastic sections. The conso-lidation devices used may either be short,water-cooled forming dies or, when the product

Figure 15 Schematic of the thermoplastic pultrusion process.

Continuous Processes for Thermoplastic Composites 15

section permits it, rollers. With dies, it is desir-able that they are split and spring-loaded, soonly moderate pressure is applied to the movingproduct. The best surface finish is achieved withlow-die temperatures.

2.29.5.1.1 Reinforced extrusions and moldings

Coating thermoplastic pultrusions by meansof a cross-head die has been advocated to im-prove surface appearance. Likewise, the use ofpultrusions to strengthen thermoplastic injec-tion moldings (Hawley, 1982) and thermoplas-tic profiles has been considered, but practicaldifficulties are encountered in achieving suf-ficient fiber content, within the shape envelopeof the component, and in ensuring that theprepreg is fully bonded to the thermoplasticmatrix.

2.29.5.1.2 Roll forming

A semicontinuous process (Cogswell, 1992),similar to that used in metal working, was

proposed for forming profiles such as top hatsections, from thermoplastic prepreg sheet. Theprocess changes the shape, but not the thick-ness, of the section and requires a stand ofseveral rolls.

2.29.5.2 Thermoplastic Tape Laying andFilament Winding

Processes where the final shape of the part isbuilt up by the accurate placing of plies, as inFigure 17, are known collectively as fiber place-ment techniques (see Chapter 2.31, thisvolume). Techniques such as tape laying, bynumerically controlled multi-axis machinesare well-established in thermoset prepreg pro-cessing. Thermoplastic prepregs do not adherein the cold state, but fiber placement processescan be very effective provided the plies arebonded to one another as they are placed.Processes which achieve consolidated parts viathis route, without the use of autoclaves orfurther heat treatment, have considerablepotential, as they utilize the capability of thematerial to be rapidly formed and solidified.

Figure 16 Alternative die configuration for thermoplastic pultrusion (a) two die process and (b) toolingconfiguration for rapid pultrusion (Miller et al., 1998).

Figure 17 Schematic of the tape-laying and consolidation process for thermoplastic matrix composites.


With all consolidation processes of this type(as with conventional welding of thermoplas-tics) the requirement is that the surfaces to bejoined should be heated to a temperature abovethe melting point of the thermoplastic and heldtogether for sufficient time to enable a bond tobe established throughmolecular diffusion. Thepractical requirements can be illustrated byconsidering the situation where two compo-nents, initially at temperatures T1 and T2 arebrought together with the aim of achieving aweld. The temperature of the interface betweenthe components, Ti, will be given approxi-mately by

Ti= (T1+T2)/2 (7)

Bond formation requires that the interfacialtemperature should exceed the melting pointof the polymer. In many thermoplastic place-ment processes the component being laid downis heated to a temperature just a little above themelting point. With the substrate at ambienttemperature it can be seen that the thermalcondition for bonding is not fulfilled. Thereare three ways of addressing this problem:(i) to raise the temperature of the material

being placed to a level much higher than themelting point,(ii) to increase the substrate temperature,

and(iii) to add extra heat to the bond region.All three methods have been examined. Solu-

tion (i) alone is generally insufficient to solvethe problem, since heating the polymer muchabove its melting point will cause degradation,unless the it is blanketed with inert gas toprevent oxidation or unless the period at thehigh temperature is very short. Most processesrely on a combination of (i) and (ii) to solve theproblem, or on adding extra energy (iii) near tothe bonding point. Sources of heating that havebeen investigated include hot gas, includingsometimes a flame, focused IR energy andlaser energy. In addition to achieving the neces-sary thermal conditions it is usually necessaryto apply a degree of local consolidation pres-sure to eliminate air and fulfill the condition formolecular diffusion across the bond line. Whenthe process is operated under the optimumthermal conditions it is possible to producecomplex structures with good interlaminarproperties and low thermally induced stresses.High precision multiaxis machines for fiberplacement are now available from a numberof manufacturers, including AutomatedDynamics (1999).Large structures may be built up using tape-

laying, but difficulties have been found some-times in achieving full consolidation. Processing

factors have been discussed in a number ofpublications, including Grove (1988), Groveand Short (1988), Beyeler and Gucieri (1988),and Beyeler et al. (1988).The manufacturing problems for thermo-

plastic filament winding are similar to thoseencountered in tape laying and a range of solu-tions have been proposed. Some employ a com-bination of mandrel heating and hot air toachieve the necessary thermal conditions,whereas others use an unheated mandrel, theadditional energy being supplied by focusedradiation or hot gas. Some processes rely solelyon the tension of the fiber tow to apply con-solidation pressure to the bond region, whileothers lay down the in-coming tow with a rolleror hot shoe to apply additional consolidationpressure.As with pultrusion, thermoplastic filament

winding can be operated with precursor mate-rial from any of the sources described above.Processes have been reported where the precur-sor is melt impregnated (Moyer, 1976; Cogswellet al. 1981a, 1981b; Fahrer, 1997; Fisher andGibson, 1998), powder impregnated (Astromand Pipes, 1990; Wagner and Colton, 1994;Romagna et al., 1995; Haupert and Friedrich,1995) and co-mingled (Fedro et al., 1989; Elliottet al., 1999). Recently, considerable success hasbeen achieved in high speed winding, at linearspeeds of 50m/min or more, with co-mingledglass/PETG melted and wetted out in-line withthe process, making use of controlled windingtension to achieve the necessary consolidationforce. This version of the process offers possi-bilities for the rapid manufacture of gas bottlesfor a range of applications (Elliott et al., 1999).One of the advantages of thermoplastic fila-

ment winding is that, once consolidated andcooled, the material has immediate mechanicalstrength, unlike its thermosetting counterpart,which requires to be cured. This offers thepossibility of following severely nongeodesicpaths, of making shapes with re-entrant orconcave curvature and of removing the mandrelaltogetherall of which would be difficult orimpossible with thermosets. Thermoplastic fila-ment winding now has the possibility of beingused in large scale and mass production envir-onments, which will result in considerably en-hanced use of thermoplastic matrix compositesin this type of application.

2.29.5.3 Reinforced Thermoplastic Pipe (RTP)

Pressurized pipework is an excellent load-bearing application for thermoplastic matrixcomposites. Because the main loads in thepipe wall are all tensile, the significance of

Continuous Processes for Thermoplastic Composites 17

impregnation and bonding between the layerswithin the structure is greatly reduced. Indeedsome lack of bonding may even be regarded asadvantageous, as it increases the axial flexibilityof the pipe. Continuous fiber reinforced ther-moplastic pipes (RTPs) offer considerable per-formance enhancement, compared tounreinforced thermoplastic pipe, while retain-ing the advantages of light weight, ease of in-stallation and corrosion resistance. As shown inFigure 18, an RTP comprises an inner liner andouter cover of thermoplastic (usually mediumdensity polyethylene), with an even number ofload-bearing +558 reinforcing plies, usually ofaramid fiber.Because RTPs are required in long lengths, a

continuous winding process is preferred overconventional filament winding. The most fa-vorable technique at present is continuous he-lical winding of reinforcing tape, as shown inFigure 18. The tape comprises yarns of reinfor-cing fiber, encapsulated in a polymeric matrix.The advantage of this configuration is that thetape acts as a carrier, permitting many reinfor-cing yarns to be applied simultaneously, with-out the need for a large number of rotatingbobbins on the winding assembly. Dependingon the manufacturer and the application thereinforcement may or may not be bonded to theliner and covers.At present the most attractive reinforcement

for RTP is aramid fiber, the reason being that,unlike glass or carbon, aramid can performsatisfactorily in tension in the unwetted state.In contrast to the other materials described inthis chapter the yarns in the reinforcing tapegenerally are not impregnated by the thermo-plastic resin. Use of nonimpregnated fibers inthis case has certain advantages, including flex-ibility. Lengths of several hundred metres ofRTP may continuously manufactured andcoiled for transport.

On the basis of strength per unit cost, non-impregnated aramid/PE tape is less expensivethan impregnated glass/PE. Glass fiber reinfor-cement would be competitive in this applica-tion, however, if impregnation could be carriedout at the same time as the winding operation,and a process has been developed to do this(Fisher and Gibson, 1998).At present the main applications of RTP are

in high pressure transport of natural gas andoilfield fluids (Frost, 1999). In the mediumterm, however, RTPs have a wide range ofother potential applications and the future ap-pears excellent for processes to manufacturethis type of product.

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Comprehensive Composite MaterialsISBN (set): 0-08 0429939

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