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Composites Manufacturino6 (1995) 35-431995 E lsev ier Science L im ited

Prin ted in Great Br i ta in . Al l r igh ts reserved0956-7143 /95 /$10 .00

E x p er i men t s o n co mp res s i o n mo u l d i n g an dp u l t r u s i o n o f t h e r m o p l a s t i c p o w d e ri m p r e g n a t e d t o w p r e g s

K a r th ik R a m a n i* , H a r s h a d B o rg a o n k a r a n d C h r i s H o y leAdvanced Materials Design and Composites Processing Laboratory, School of MechanicalEngineering, Purdue University, West Lafayette, IN 47907, USA(Received I July 1994; revised 22 September 1994)

Powder impregnation techniques have been developed to increase design and manufacturing flexibilitywith thermoplastic composites. The effect of pressure, temperature, mould closing rate and time on theconsolidation of poly(ether ketone ketone) (PEKK) powder impregnated glass fibre owpregs in compressionmoulding were studied. A design of experiments approach was used. Isothermal pultrusion experimentsusing nylon 11 and PEKK powder impregnated glass fibre tows were conducted. These experiments wererepeated at different temperatures and pulling speeds. The microstructural changes during coating andconsolidation of the powder impregnated tows were studied. Temperature and mould closing rate wereobserved to be the significant parameters affecting consolidation in compression moulding. Superpositionof he pulling force-pulling speed data at different processing emperatures was demonstrated in pultrusion.

(Keywords: pultrusion; com pressionmoulding; powd er m pregnation;coating)

INTRODUCTION

Economical processes to man ufact ure composites must

be developed to use them in a wide range of applications.Thermoplastic matrix composites offer excellent impactresistance, envir onment al stability and recyclability.However, resin impregnation is one of the difficultsubprocesses in manufacturing cont inuo us fibre-reinforcedthermoplastic matrix composites due to the high meltviscosities exhibited by thermoplastics. Melt impregnationhas drawbacks such as the possibility of polymerdegradat ion due to the long residence times at hightemperatures. Solvent-based techniques are environ-mentall y hazar dous and expensive. Po wder impregnati onis attractive since it ensures low residence times of thepolymer during heating and localized melt flow of thepolymer during consolidation, resulting in better fibrewetting and les s fibre damage. Slurry-based powderimpregnation techniques are simple, environmentallysafe, and have been kno wn for several years l'z. In t hisinvestigation, a slurry-based powder techniqu e developedearlier 3 was used to produce towpregs for compressionmoulding and pultrusion.

The effect of processing variables (pressure, temperature,mould closing rate and time) on the consolidation ofpoly(ether ketone ketone) (PEKK) powder impregnatedtowpregs is studied in compression moulding using adesign of experiments approach. Effect of temperature

and pull ing speed on pulling force is studied in pultrusio nof nylon 11 and PEKK powder impregnated towpregs.

* To w h o m co r re sp o n d en ce sh o u ld b e ad d res sed

Characteristics of powder impregnated towpregs and theeffects of these characteristics on consoli datio n behaviourare presented in the background section. Experimentalapparatus for compression moulding, pultrusion andimp regnat ion processes are then described. Micro-structural changes in the powder impregnated tows priorto and after consolidation, flexural test and void contentresults from the compression mouldi ng study, and resultson pulling force as function of pulling speed andtemperature are detailed and discussed.

BACKGROUND

The powder impregn ated tow is heated to fuse the powder

on the tow before consolidation. The microstructure ofthe coated tow during heating is developed throughinterparticle and particle-fibre interactions. The coatedtows are then consolidated to form the composite productby application of heat and pressure in subsequentprocessing. The opt imu m degree of coating prior toprocessing varies for different processes. Well-coatedtows are preferred in co ntin uous processes like filamentwinding and puitrusion, while partially coated flexibletows are preferred for processes such as weaving, so thathandling the towpregs will be easier.

The stages of fibre coati ng were studied using an opticalmicroscope with a heating stage and an environmentalscanning elect ron microscope (ESEM) for nyl on I Iimpregnated glass fibre tows and PEKK impregnatedglass fibre t ows 4. The stages and mechanisms of fibre

C O M P O S I T E S M A N U F A C T U R I N G Vo lu m e 6 N u m b e r 1 1 9 9 5 3 5

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Experiments on com pression m oulding and pultrusion: K. Ramaniet al .

Figure ! ESEM pictures showing the stages of polymer flow

coating were identified and are summarized in Figures 1and 2. The polymer flow in the powder impregnatedtowpreg during consolidation depends upon the extentof polymer coating and flow prior to consolidation,particle size, particle size distri bution, and on the flowcharacteristics of the polymer. Hence the effect ofprocessing parameters such as pressure and temperatureon the consol idation of powder impregnated towpregs is

different from that of other material forms such as meltimpregn ated prepregs. This was the motivat ion forstudying the influence of processing parameters on theconsolidation b ehaviour of powder impregnated towpregs.

Compress ion mould ing

Previous studies on thermoplastic consolidation havefocused on the use of intermediate material forms suchas commingl ed fibres 5, prepregs 6, tapes v and, morerecently, power impreg nated towpregs 8,9. Vodemaye r etal. 1 calculated the optimal particle size of sphericalpowders in order to minimize the flow length, based ongeometrical models. However, the wetting behaviour and

the extent of coating were not considered.In t he present work, the influence of four key processing

parameters on the degree of consolidation of powderimpregnated towpregs in compression moulding was

3 6 C O M P O S I T E S M A N U F A C T U R I N G Vo l u m e 6 N u m b e r 1 1 9 9 5

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Experiments on compression mould ing an d pultrusion: K. Ramanie t a l .

Initial Stage-Powde r on Fibers. First Stage-Smoothing of Particles.

Second Stage-Coalescence of Particles.

F

I

I ) ( i

Third Stage-Droplet Formation andBridging.

Fourth Stage-Film Formation with or withoutBridging.

Figu re2 D i f f e r e n t s t a g e s o f p o l y m e r f l o w o n t h e f i b re s

investigated. These parameters are consolidation time,pressure, t emperature and displacement rate. Full factorialexperiments were used to determine the effect of eachparameter and par ameter interaction upon consolidation.

PuhrusionExtensive experimental and analytical research has

been done on the pultrusion of thermosets owing to the

ease of fibre impreg nation with thermosets and availabilityof the mat erials 11 - 19. Thermoplasti c pul trusi on has beeninvestigated only recently. Models for thermoplasticpultrusion using melt impregnated prepregs have beenpresented in recent years 2-21. These models includedanalysis of pulling force, heat transfer in the die, andpressure due to compaction of the prepregs in the die.

The pulling force in thermoplastic pultrusion dependson processing parameters (temperature, pulling speed),material parameters (type of polymer, fibre volumefraction, degree of impregnation) and design parameters(die size and profile, die taper). Pull ing force is a veryimportant parameter and must be studied in order tounder stan d and design the process. Viscous resistance ofthe polymer layer between the fibres and the die wall,resistance due to compaction of the composite in thetapered section of the die, and frictional resistancebetween the die wall and the dry fibres, contribute to thetotal pulling resistance 21.

Pultrusion with thermoplastic powder impregnatedtows offers an attractive method t o integrate impregnat ionand pul trusi on in one process. Pultr usion with the powderimpregnated tows involves coalescence of the polymerparticles, followed by film formati on in the preheat ingstage and local (longitudinal and transverse) flow of the

polymer during consolidation. In the present investigation,the effect of tow temperat ure an d pulling speed o n pullingforce in isothermal pultrusion of thermoplastic powderimpregnated tows is studied.

EXPERIMENTAL INVESTIGATION

Material systemThe thermoplasti c resins used in this invest igation were

nylon 11 and PEKK. Volume-average particle sizesdetermined using a Climet RL series Hydrocell liquid

particle sensor + were 26.3 and 71.3/~m for ny lon 11 andPEKK, respectively. The fibres used were continuousE-glass (Owens Corning 497) with a silane-based sizing.A slurry, consisting of the thermoplastic powder, asuspension agent (Hercules Klucel M) and a wet ting agent(3M FC-430), was prepared for impregnating the fibres.The impregnated fibre tows were wound on the creelsfor pultrusion, and around a frame for compressionmoul ding as shown in Figure 3.PEKK impregnated tows(55% fibre volume fraction) were used for the compressionmoulding experiments. The pultrusion experiments wereconducted with P EK K impregnated tows 165% fibrevolume fraction) and nylon 11 impregnated tows (55%fibre volume fraction).

Compression mouldingThe coated tows were wound to a 25 mm width on a

frame to form a fibre charge as shown in Figure 3. Aframe with strain gauges was used to apply a knowntension to the fibres before consolidation, in order toavoid fibre waviness and form a uni directi onal composite.The wound fibres were placed in the consolidationapparatus, consisting of a matched die, a nitrogenchamber and a moun ting mechanism, as shown in Figure4. The appara tus was moun ted on an MTS tensile testing

M a n d r e l

F i g u r e3W in d i n gF r a m e

W i n d i n g f r a m e u s e d f o r c o n s o l i d a t io n e x p e r i m e n t s

I M T S M a c h i n e t I I

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l I / N i tr g e n C h a m b e r I I

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L.. Fram e . . . . L ower Die // I t

I I To w p r e g / I I~ Ac tua to r

F i g u r e4 C o n s o l i d a t i o n a p p a r a t u s

C O M P O S I T E S M A N U F A C T U R I N G Vo l u m e 6 N u m b e r 1 1 9 9 5 3 7

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Experiments on compression mo ulding a nd pultrusion:K .

Ta b l e 1 E x p er im en ta l m a t r ix u sed in co n so l id a t io n ex p e r im en t s

Tes t Tim e P ress R a te Tem p S t r en g th (G P a) % Vo id

1 Low Low Low Low 0 .246 23 .22 High Low Low Low 0 .610 19 .73 Low High Low Low 0 .101 29 .94 High High Low Low 0 .934 15 .6

5 Low Low High Low 0 .273 28 .86 High Low High Low 0 .579 26 .97 Low High High Low 0 .963 14 .58 High High High Low 0 .349 12 .49 Low Low Low High 1 .135 17 .0

10 High Low Low High 0 .494 15 .311 Low High Low High 0 .429 20 .312 High High Low High 0 .852 12 .313 Low Low High Hig h 0.462 16.114 High Low High High 1 .704 5 .215 Low High High High 1 .246 7 .916 High High High High 1 .851 4 .1

Ta b l e 2 Va lu es o f l o w an d h ig h p a ram e te r s u sed in co n so l id a t io n

P a r a m e t e r L o w H i g h

Tim e (s) 750 1500Press (M Pa) 2 .6 5 .2Rat e (m m s- 1) 0 .207 0.415Te m p ( C ) 2 95 3 25

m a c h i n e t o c o n t r o l a n d r e c o r d t h e l o a d a n d t h ed i s p l a c e m e n t o f t h e d i e d u r i n g c o n s o l i d a t i o n . A t h e r m o -c o u p l e w a s p l a c e d o n t h e t o w s u r f a c e t o m o n i t o rt h e c o m p o s i t e t e m p e r a t u r e d u r i n g c o n s o l i d a t i o n . T h e

w o u n d f i b re s w e r e c o n s o l i d a t e d t o f o r m a 2 5 m m x2 0 3 m m x 3 m m u n i d i r e c t io n a l c o m p o s i t e t h a t w a s 7 0 %fi b re b y we i g h t .

T h e c o n s o l i d a t i o n a p p a r a t u s w a s u s e d t o d e t e r m i n ethe effec ts o f cons o l id a t io n t im e ( t ime) , p ressur e (press) ,d i sp l acemen t ra t e ( ra t e ) an d t emp era t u re ( t emp ) o n t h ep r o p e r t ie s o f t h e c o m p o s i t e . A p r e d e t e r m i n e d p r o c e sscy c l e was u sed fo r a l l t h e ex p e r i men t s . Th e f i b re ch a rg ew a s h e a t e d t o a d e s i r e d t e m p e r a t u r e i n t h e d i e w i t h o u tap p l y i n g p re ssu re . Th e d i e was t h en c l o sed u s i n g ac o n s t a n t d i s p l a c e m e n t r a t e . T h e p r e s s u r e w a s h e l dc o n s t a n t d u r i n g c o n s o l i d a t i o n . T h e d i e w a s c o o l e d a t t h een d o f t h e cy c l e u n t i l t h e p o l y me r was su ff i c i en tl ys o li d if i ed b e f o r e r e m o v i n g t h e c o m p o s i t e p l a q u e f r o m t h edie.

A fu l l 2 4 fac t o r i a l s t u d y, co n s i s t i n g o f 1 6 i n d i v i d u a le x p e r i m e n t s , w a s c o n d u c t e d t o d e t e r m i n e t h e e f f e c t o fe a c h o f t h e f o u r p r o ce s s p a r a m e t e r i n t e r a c t io n s(Table 1).H i g h ( + ) a n d l o w ( - ) v a l u es o f e a c h o f t h e p ro c e s sp a r a m e t e r s w e r e c h o s e n b a se d u p o n p r e l i m i n a r y c o n -s o l i d a t i o n e x p e r i m e n t s(Table 2) .T h e v a l u e s o f e a c hp a r a m e t e r w e r e ch o s e n t o p r o d u c e p l a q u e s w i t h v a r y i n gd e g r e e s o f c o n s o l i d a t i o n . T h e r e s u l ts , X e , o f th ep l a q u e s w e r e q u a n t if i e d t h r o u g h m e c h a n i c a l t e s t in g a n dd e t e r m i n a t i o n o f v o i d c o n t e n t . S i x t e en p l a q u e s w e r e c u t

t o 2 5 m m x 1 27 m m x 3 m m a n d t e s te d i n t h r e e - p o i n tb e n d i n g t o d e t e r m i n e f l e x u r a l m o d u l u s a n d s t r en g t h . Vo i dc o n t e n t o f e a c h s a m p l e w a s d e t e r m i n e d u s i n g A r c h i m e d e s 'p r inc ip le . The effec ts ,R j , w e r e e v a l u a t e d u s i n g t w o

Ramani et a l .

d i ffe ren t r e su l t s c r i t e r ia (Xe ), s t r en g t h a n d v o i d co n t en t .Th e re su l t s u s i n g s t ren g t h an d v o i d co n t en t a re l i s t ed i nTable 1.Th e re su l t s o f e ach o f t h e 1 6 ex p e r i m en t s we rer e c o r d e d i n a n a n a l y s i s o f v a ri a n c e ( A N O VA ) t a b l e(Table 3).Th e e ffec t o f e ach v a r i ab l e an d t h e i n t e rac t i o no f o n e v a r i a b l e w i t h a n o t h e r c a n b e d e t e r m i n e d f r o m t h i stab le .

T h e e f fe c t o f a s i ng l e p a r a m e t e r o r p a r a m e t e ri n t e rac t i o n , j , i s d e t e rm i n ed b yR j :

= T , x o f j + - T , x o r j

T h e s i g n o f a n i n t e r a c t io n w a s d e t e r m i n e d b y m u l t i p l y i n gt h e s ig n s o f th e i n d i v i d u a l p a r a m e t e r s m a k i n g u pt h e i n t e r a c t i o n . T h e m a g n i t u d e ,IRj l , d e t e rmi n es t h es i g n if i can ce o f t h e p a ram e t e r o r t h e p a ram e t e r i n t e rac t io n ,j . Th e s i g n o fR j s h o w s t h e i n f l u e n ce o f th e p a r a m e t e r o rp a rame t e r i n t e rac t i o n l ev e l o n t h e f l ex u ra l s t ren g t h o rt h e v o i d co n t en t .

PultrusionA p u l t ru s i o n l i n e co n s i s t i n g o f a c ree l s t an d t o m o u n t

t h e p o w d e r i m p r e g n a t e d t o w s , a r a d i a n t t u n n e l p r e h e a t er,a h o t g a s p reh ea t e r, a d i e an d a p u l l i n g mech an i sm wasu s e d to c o n d u c t t h e e x p e r i m e n ts . A I 0 0 m m l o n g h e a t e dd i e w a s u s e d t o p r o d u c e c o m p o s i t e t a p e s w i t h arec t an g u l a r c ro ss - sec t i o n o f 1 3 m m x 0 .1 5 mm . Th e d i ea ssemb l y was mo u n t ed o n a s l i d e p a ra l l e l t o t h e p u l l i n gd i rec t i o n , an d a l o ad ce ll was u sed t o mea su re t h e p u l l i n gfo rce , a s sh o wn i nFigure 5 . A p e r s o n a l c o m p u t e r

Ta b l e 3 AN OV A tab le o f the effects in com pression mould ing

P aram ete r /P a ram e te r i n t e r ac t i o n S t r en g th % Vo id

Tim e 3.647 65.9Pre ss 1.771 23.1Rat e 3 .811 37.5Te mp 5.973 72.5Tim e-press - 0 .033 5 .3Tim e-rat e 0 .815 - 19 .8Press-ra te 2 .267 53 .3Tim e tem p 1.069 - 16.4P ress t em p -0 . 0 7 9 5 .3Ra te- tem p 3 .017 20 .7Tim e-pre ss-ra te - 4 .481 - 35 .2Tim e-p res s - t em p 1 .2 7 3 - 1 .6

Tim e- ra t e - t em p 5 .1 77 3 4. 7Pre ss-ra t e- te mp 1 .445 - 18 .0Tim e-p res s - r a t e - t em p - 0 .4 51 4 .0

Hot GasImpregnatedFiber Tows

I ~ Acquisition J

Figur e 5 S ch em at i c d i ag ram o f t h e ex p e r im en ta l p u l t ru s io n f ac il it y

3 8 C O M P O S I T E S M A N U F A C T U R I N G Vo lu m e 6 N u m b e r 1 1 9 9 5

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Experiments on compression m oulding and pultrusion: K. Ramaniet a l .

Table 4 Experimental matrix used n isothermal pultrusion experiments

Pulling speed (mm s 1) 2 5 8 11Temperature PEKK (C) 340 350 360 380Temperature nylon 11 (C) 190 210 230

14 18 21

equipped with a data acquisition system was used tomonitor and record the pulling force, radiant oventemperature, hot gas temperature and die temperature.

The temperatures of the radiant oven and the hot gaswere adjusted to achieve the desired tow temperature atthe die entrance, and the die was maintained atthe same temperature as the tows. The experimentsusing PEKK impregnated tows were conducted at fourdifferent temperatures and seven different pulling speeds.Experiments using nylon 11 impregnated tows wereconducted at three different temperatures and sevendifferent pulling speeds (Table 4).For each experiment,the die temperature, radiati on oven temperature and thehot gas torch temperature were changed to maintainisothermal conditions. Once isothermal conditions anda constant pulling force were observed, each experimentwas conducted for 5 min and the average pull ing forcewas recorded.

on the surface of the tow. In this case, interparticlecoalescence dominate s the fibre coating process, causingformation of a polymer sheath around the tow. Thecross-section of the PEKK pultruded tape shows thatpolymer flow through the tow thickness occurs toconsolidate the tow (Figure 9).

It is clear that local longitudinal flow occurs in theinitial stages of the consolidation while transverse flowis necessary to complete the consolidation. The extent of

RESULTS AND DISCUSSION

Microstructural evolutionThe consolidation process consists of fibre bed

compaction and polymer flow within the fibre bundle.

ESEM pictures show that the polymer particles undergomorphological changes upon heating. They change froma randomly shaped solid phase to a rounded liquid phasethat ultimately spreads along the length of the fibre(Figures 1and 2). The increase in temperature causes areduction in the surface tension and viscosity, resultingin the phase change. Optical microscope pictures of thepreheated powder impregnated tow surface show severalchannels available for the coalesced droplets to flowbetween the fibres with minimal resistance (Figures 6and7). Pictures of the consolidated tow surface show thechannels between the fibres bridged by a smoot h polymerfilm (Figure 6).

Scanning electron microscope (SEM) pictures of thepreheated nylon 11 tow cross-section show that thepolymer is distributed through the thickness of the tow(Fi,qure 8) .Volume-average parti cle size (20/~m for nyl on11) of the order of the fibre diameter (10pm) results inuniform impregnation throug h the thickness of the tow.In addition, the low melt viscosity of nylon 11 above itsmelting point enhances polymer flow. The cross-sectionof the consolidated pultruded tape shows uniformdistribution of the polymer through the thickness of thetape (Figure 8).

SEM pictures of the preheated PEKK tow cross-

section show that a sheath of polymer is formed, whilethe core of the tow is not impregnated (Figure 9).Largevolume-average particle size (71.3 ILm for PEKK) comparedwith the fibre diameter (10/tin) results in dense packing

Stage I Tow (before Heating)

Figure6 Changes n tow prepreg microstructure during consolidation

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Experiments on compression mould ing an d pultrusion. K. Ramanie t a l .

P r e s s u r e

r iUer Mol t en Po lymer Drop le t s

Stage 1. SurfaceTension andCapillary Driven Flows beforeApplication of Pressure.

Stage 2. Spreading,CoalescingofDroplets and Local Squeeze Flowdue to Com paction.

Pres su re

Stage 3. CompletelyCoated andConsolidatedTow.

F i g u r e 7 M o r p h o l o g i c a l c h a n g e s o f p o w d e r i m p r e g n a t e d t o w s d u r i n gc o n s o l i d a t i o n

the loca l longi tudinal f low and the t ransverse f lowdepends on the average par t ic le s ize , the uniformity ofimpregna t ion and the v i scos i ty - tempera tu re re la tionsh ipo f the po lymer.

Compression mouldingUsing void content and s t rength cr i te r ia , i t was found

that tempera ture was the most s ignif icant , d isp lacementra te and consol ida t ion t ime were equal ly s ignif icant , andpressure was the leas t s ignif icant of the process ingparam eters . The s igns of a ll e ffec ts were posit ive , showingthat the h igh (+ ) level of each var iab le posi t ive lyinf luenced consol ida t ion . The most s ignif icant posi t ivein terac t ions were pressure

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Experiments on compression mou lding an d pultrusion. K. Ramanie t a l .

16 0

140

~" 12o

~ 100-

~ 8 0

6 0

40

y : : , ,0 " ' "

O ." ~ 210 c. , g . - .A . . . 230C

/v"' I I t t

0 5 10 15 20 25S p e e d ( m m / s )

Figure l0 Pulling force as a function of pulling velocity [nylon IIimpregnated glass fibre tows)

T h e l o c a l l o n g i t u d i n a l f l o w a n d t r a n s v e r s e f l o w s h o u l db e m a x i m i z e d b y u s i n g th e o p t i m u m s e t o f p r o c e s s i n gp a r a m e t e r s t o m i n i m i z e c y c l e t im e . F o r a g iv e n t o w p r e gp e r m e a b i l i t y, t h e f l o w r a t e c a n b e m a x i m i z e d b yi n c r e a s i n g t h e p r e s s u r e g r a d i e n t o r d e c r e a s i n g t h ev i s c o s i t y. I n c r e a s i n g t h e a p p l i e d p r e s s u r e c a u s e s g r e a t e r

f i b re c o m p a c t i o n a n d a d e c r e a s e in p e r m e a b i l i t y o f t h eto w p r e g 2 3, e x p la in in g th e r e l a t i v e i n s ig n i f i c a n c e o fp r e s s u r e o n t h e c o n s o l i d a t i o n .

T h e v i s c o s i ty o f t h e p o l y m e r c a n b e r e d u c e d b yi n c r e a si n g t h e t e m p e r a t u r e a n d i n c r e a s in g t h e s h e a r r a t e .I n c o m p r e s s i o n m o u l d i n g , t h e s h e a r r a te o f t h e p o l y m e rc a n b e i n c r e a s e d b y i n c r e a s i n g t h e d i s p l a c e m e n t r a t e .U s i n g t h e s q u e e z e f l o w a p p r o x i m a t i o n , t h e a v e r a g e s h e a rr a t e w a s c a l c u l a t e d a s t h e r a t i o o f t h e d i s p l a c e m e n t r a t ea n d t h e d i s p l a c e m e n t . T h e v i s c o s i ty o f t h e p o l y m e r( P E K K ) w a s m e a s u r e d w i t h a c a p i l l a r y r h e o m e t e r a t h i g h( + ) a n d l o w ( - ) p r o c e s s i n g te m p e r a t u r e s . Vi s c o s i t y - s h e a rr a t e d a t a m e a s u r e d a t a f i x e d t e m p e r a t u r e w e r e f i t t o ap o w e r - l a w e x p r e s s i o n . T h e v i s c o s it y o f t h e p o l y m e rd u r i n g t h e i n it ia l s ta g e s o f c o m p r e s s i o n m o u l d i n g w a st h e n o b t a i n e d b y s u b s t i t u t i n g t h e c a l c u l a t e d s h e a r r a t ei n t o t h e p o w e r - l a w e x p r e s s i o n . T h e a v e r a g e r a t i o o f t h ev i s c o s it y f o r h i g h s h e a r r a t e ( + ) a n d l o w s h e a r r a t e ( - )wa s 0 .7 8 . T h e a v e r a g e r a t i o o f v i sc o s i t y a t h ig ht e m p e r a t u r e ( + ) a n d l o w t e m p e r a t u r e ( - ) w a s 0 .9 3. T h i sr e d u c t i o n i n th e v i s c o s i ty o f th e p o l y m e r d u e t o i n c r e a s ei n th e s h e a r r a t e a n d t e m p e r a t u r e e x p l a i n s t h e s i g n if i c an c eo f t h e t e m p e r a t u r e a n d t h e d i s p la c e m e n t r a t e o n t h ec o n s o l i d a t i o n .

T h e t o w s o c c u p y a g r e a t e r v o l u m e b e f o r e c o n s o l i d a ti o n

th a n a f t e r, s i n c e t h e p a r t i c l e s k e e p th e f i b r e s a p a r t p r io rt o m e l t in g . S i g n i f ic a n t v o l u m e r e d u c t i o n o f th e t o w so c c u r s d u r i n g c o n s o l i d a t i o n . T h i s v o l u m e r e d u c t i o n i sd u e t o s q u e e z e f lo w, t ra n s v e r s e f l o w a n d c o a t i n g o f th e

f i b r e s . P o l y m e r f l o w i s e n h a n c e d b y t h e r e d u c t i o n i n t h ev i s c o s it y o f t h e p o l y m e r a n d c o a t i n g o f t h e f i b r e s is d r i v e nb y r e d u c t i o n i n t h e s u r f a c e t e n s i o n . T h e d i s p l a c e m e n tr a t e e ff e c t o c c u r s o v e r a v e r y s h o r t d i s t a n c e d u r i n g m o u l dc l o s i n g . T h e r e s u l t i n g s q u e e z e f l o w o c c u r s o v e r a s h o r tt i m e c o m p a r e d w i t h th e t i m e t a k e n f o r c o m p l e t i n gc o n s o l i d a t i o n .

Pu l t rus ionAv e r a g e p u l l i n g f o r c e v a lu e s w e r e p l o t t e d a s a f u n c t i o n

o f p u l li n g s p e e d a t d i f f e r en t t e m p e r a t u r e s f o r n y l o n 11i m p r e g n a t e d t o w s a n d f o r P E K K i m p r e g n a t e d t o w s ass h o w n i n Figures 10 a n d 11 . P o w e r - l a w c o n s t a n t s ( A ) ,p o w e r - l a w e x p o n e n t s ( n) a n d c o r r e l a t i o n c o e f f c i e n ts ( R )f o r e a c h o f t h e c u r v e f i t s a r e l i s t e d i nTa b le 5 . T h ec o r r e l a t i o n c o e f f i c i e n t s we r e o b se r v e d to b e c lo se t o 1 .T h i s s h o w s a g o o d p o w e r - l a w f i t t o t h e d a t a a t a l l t h et e m p e r a t u r e s . A r e f e r e n c e t e m p e r a t u r e - 1 9 0 C f o r n y l o n

I I a n d 3 4 0 C f o r P E K K - w a s se l ec t ed . A n a r b i t r a r yp u l l i n g f o r c e v a l u e i n t h e o b s e r v e d r a n g e w a s t h e nse l e c t e d . T h e p u l l i n g sp e e d s we r e c a l c u l a t e d a t t h er e f e r e n c e t e m p e r a tu r e ( V,e0 a n d a t t h e d e s ir e d t e m p e r a tu r e( Vr ) f r o m t h e c o r r e s p o n d i n g c u r v e - f i t e q u a t i o n s f o r t h ec h o se n p u l l i n g f o r c e . A sh i f t f a c to r(a r) f o r a t e m p e r a t u r e( T ) w a s t h e n c a l c u l a t e d u s i n g

a T = Vr o j V T

T h e sh i f t f a c to r s w e r e c a l c u l a t e d f o r t h r e e d i f f e r e n t p u l l i n gf o r c e s a n d a v e r a g e d . H e n c e o n e a v e r a g e s h i f t f a c t o r f o ra t e m p e r a t u r e ( T ) w a s o b t a i n e d . T h e s h i f t f a c t o r s f o r

n y l o n 11 a n d P E K K a r e l i s t e d i nTa b le 6 . L o g {p u l l i n gf o r c e ) w a s p l o t t e d a s a f u n c t i o n o f l o g( a T. V)f o r n y l o n1 1 a n d P E K K a s sh o w n i nFigures 12a n d 13 , r e sp e c t iv e ly.A l l t h e p o in t s o n th e p lo t s l i e o n t h e l i n e r e p r e se n t e d b yt h e r e f e re n c e t e m p e r a t u r e c u r v e s f o r b o t h n y l o n 11

9 0

80

~ 7 0 -

u

5O-

40

. g t ' " n ,

~ A, - 3 80 C

I I I I

0 5 l 0 15 20 25

P u l l i n g S p e e d (mm/sec)

Figure I ! Pulling force as a function of pulling velocity tP EK Kimpregnated glass fibre tows)

C O M P O S I T E S M A N U F A C T U R I N G V o l u m e 6 N u m b e r 1 1 9 9 5 4 1

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PEKK Nylon 1 !

Temp erat ure (C) A n R Temp erat ure (C) A n R

340 49.96 0.198 0.944 190 89.6 0.168 0.86350 46.84 0.186 0.97 210 45.64 0.339 0.906

360 42.24 0.219 0.977 230 31.98 0.37 0.904380 40.67 0.214 0.992 - - -

Table 6 Shift actors for PEKK and nylo n 11

PE KK (reference temp. 340C)

Z 2"

t.,Ot~ 2'

gg-

"~ 2'.

o.z 2 '

Nylon 11 (reference temp. 190C)

Tem perature (cC) Shift factor (at) Tem perature (C) Shift factor (at)

350 0.6085 210 0.452360 0.570 230 0.229380 0.447 - -

-0.5

O

Figure 12

A

~o

O

A 190 *CO 210 *Ca 230 *C

I ' 1 I0 0.5 1 1.5

L o g ( a ' v )

M aster curve for ny lon 11 (reference temperature 190C)

i m p r e g n a te d t o w s a n d P E K K i m p r e g n a te d t o w s(Figures12 a n d 13).T h e r e s u l t a n t m a s t e r c u rv e s s h o w n i nFigures12 a n d 13 c l e a rl y i n d i c a t e t h e v a l i d i t y o f t h e s u p e r p o s i t i o nm e t h o d . T h u s , b y c o n d u c t i n g a l im i t e d n u m b e r o fe x p e r i m e n t s f o r a p a r t i c u l a r m a t e r i a l s y s t e m a n d u s i n gt h e s u p e r p o s i t i o n m e t h o d , o n e c a n p r e d i c t p u l l i n g f o r c eo v e r a w i d e r r a n g e o f t e m p e r a t u r e s a n d p u l l i n g s p ee d s .S u p e r p o s i t i o n o f s h e a r s t r e s s - s h e a r r a t e c u r v e s f o rd i f f e r e n t t e m p e r a t u r e s w a s d e m o n s t r a t e d i n a s i m i l a rm a n n e r f o r p o l y m e r s b y M e n d e l s o n 2 4. Va l id i ty o fs u p e r p o s i t i o n i n p u l t r u s i o n i n d i c a t e s t h a t t h e v i s c o u sr e s i st a n c e o f t h e p o l y m e r f il m b e t w e e n t h e d i e w a ll a n dt h e o u t e r f i b r e s i s t h e m a j o r c o m p o n e n t o f t h e p u l l i n g

res i s t ance .S o m e d i v e rg e n c e o f t h e d a t a f r o m t h e m a s t e r c u r v e w a so b s e r v e d f o r n y l o n 11 a t 1 9 0 C a n d h i g h p u l l i n g s p e e d s( 20 m m s - 1 )(Figure 12).T h i s d i v e rg e n c e f r o m t h e m a s t e r

Z-.-. 2

Oa, 2

"~ 2~

,d 2- _- 340 *CO 350*(2zx 360 *CA 380 *C

Experiments on com pression m ouldin g an d pultrusion: K. Ramaniet a l .

Tab le 5 Power-law coefficients for PEK K and nylon 11 data

2 [ I I , I I I I I-0.2 0 0.2 0.4 0.6 0.8 l 1.2

Log ( a * v )

Figure 13 M aster curve for PEKK (reference temperature 340C)

c u r v e m a y b e a t t r i b u t e d t o t h e s h e a r t h i n n i n g o f n y l o n11 and a co r res pond i ng r educ t i on i n t he pu l l i n g fo rce .D i v e rg e n c e o f t h e d a t a f r o m t h e m a s t e r c u r v e w a s n o to b s e r v e d a t t h e h i g h e r t e m p e r a t u r e s f o r n y l o n 11 a n d a ta l l t h e p r o c e s s i n g t e m p e r a t u r e s f o r P E K K .

1.4

C O N C L U S I O N S

S u r f a c e t e n s i o n , c a p i l l a r y a c t i o n a n d s q u e e z e f l o wc o n t r i b u t e t o t h e c o n s o l i d at i o n b e h a v i o u r o f th e p o w d e ri m p r e g n a t e d t o w s . L o c a l i z e d l o n g i t u d i n a l f l o w a l o n g t hel e n g t h o f t h e f i b r e s d o m i n a t e s t h e i n i t ia l s t a g e s o fc o n s o l i d a ti o n o f th e t h e r m o p l a s t i c p o w d e r i m p r e g n a t e d

t o w s , w h i l e t r a n s v e r s e f l o w i s n e c e s s a r y t o c o m p l e t e t h ec o n s o l i d a t i o n . T h e l o c a l i z e d p o l y m e r f lo w d u r i n g t h ec o n s o l i d a t i o n d e p e n d s o n t h e e x t e n t o f c o a t i n g p r i o r t ocons o l i da t i on , f i b re vo l um e f r ac t i on , pa r t i c l e s ize , pa r t i c l e

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Experiments on compression m oulding and pultrusion. K. Ramaniet a l .

s i ze d i s t r i b u t i o n , a n d t h e f l o w c h a r a c t e r i s t i c s o f t h ep o l y m e r . P o l y m e r s h e a t h f o r m a t i o n d u e t o i n t e r p a r t i c lec o a l e s c e n c e o c c u r s w h e n t h e p a r t i c l e s iz e s a r e l a r g ec o m p a r e d w i t h t h e f i b re d ia m e t e r .

Te m p e r a t u r e a n d d i s p l a c e m e n t r a te w er e o b s e r v e d tob e t h e m o s t s i g n i f i c a n t f a c t o r s a f f e c ti n g c o n s o l i d a t i o no f t h e t h e rm o p l a s t i c p o w d e r i m p r e g n a t e d t o w p r e g sb a s e d o n t h e v o i d c o n t e n t a n d t h e s t r e n g t h c r i t e r i a .E x p e r i m e n t a l l y b a s e d p u l l in g v e l o c i t y - t e m p e r a t u r e s u p e r -p o s i t i o n o f t h e p u l l i n g f o r c e c u rv e s f o r i s o t h e r m a lp u l t r u si o n o f p o w d e r i m p r e g n a t e d t o w p r e g s w a s d e m o n -s t r at e d . Va l i d i t y o f t h e s u p e r p o s i t i o n p r o c e s s i n d i c a t e st h a t t h e m a j o r c o n t r i b u t i o n t o t h e p u l l in g r e s i s ta n c e i sd u e t o t h e v i s c o u s r e s i s t a n c e o f t h e p o l y m e r f i lm b e t w e e nt h e d i e w a l l a n d t h e o u t e r f i b re s .

A C K N O W L E D G E M E N T S

T h e a u t h o r s w o u l d l i ke t o t h a n k E . I . D u P o n t d eN e m o u r s a n d C o m p a n y f o r s u p p l y i n g P E K K p o w d e r ,T h i s r e s ea r c h w as s u p p o r t e d b y T h e N S F - E R C f orI n t e l li g e n t M a n u f a c t u r i n g S y s t e m s .

R E F E R E N C E S

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Com posites in M ultiphase Systems' (Ed. C .D . Han), AmericanChemical Society, W ashington, D C, 1984

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C O M P O S I T E S M A N U F A C T U R I N G Vo lu m e 6 N u m b e r 1 1 9 9 5 4 3