Composites Manufacturino 6 (1995) 35-43 1995 Elsevier Science Limited

Printed in Great Britain. All rights reserved 0956-7143/95/$10.00

Experiments on compression moulding and pultrusion of thermoplastic powder impregnated towpregs

Karthik Ramani*, Harshad Borgaonkar and Chris Hoyle Advanced Materials Design and Composites Processing Laboratory, School of Mechanical Engineering, 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 flexibility with thermoplastic composites. The effect of pressure, temperature, mould closing rate and time on the consolidation of poly(ether ketone ketone) (PEKK) powder impregnated glass fibre towpregs in compression moulding were studied. A design of experiments approach was used. Isothermal pultrusion experiments using nylon 11 and PEKK powder impregnated glass fibre tows were conducted. These experiments were repeated at different temperatures and pulling speeds. The microstructural changes during coating and consolidation of the powder impregnated tows were studied. Temperature and mould closing rate were observed to be the significant parameters affecting consolidation in compression moulding. Superposition of the pulling force-pulling speed data at different processing temperatures was demonstrated in pultrusion.

(Keywords: pultrusion; compression moulding; powder impregnation; coating)

INTRODUCTION

Economical processes to manufacture composites must be developed to use them in a wide range of applications. Thermoplastic matrix composites offer excellent impact resistance, environmental stability and recyclability. However, resin impregnation is one of the difficult subprocesses in manufacturing continuous fibre-reinforced thermoplastic matrix composites due to the high melt viscosities exhibited by thermoplastics. Melt impregnation has drawbacks such as the possibility of polymer degradation due to the long residence times at high temperatures. Solvent-based techniques are environ- mentally hazardous and expensive. Powder impregnation is attractive since it ensures low residence times of the polymer during heating and localized melt flow of the polymer during consolidation, resulting in better fibre wetting and less fibre damage. Slurry-based powder impregnation techniques are simple, environmentally safe, and have been known for several years l'z. In this investigation, a slurry-based powder technique developed earlier 3 was used to produce towpregs for compression moulding and pultrusion.

The effect of processing variables (pressure, temperature, mould closing rate and time) on the consolidation of poly(ether ketone ketone) (PEKK) powder impregnated towpregs is studied in compression moulding using a design of experiments approach. Effect of temperature and pulling speed on pulling force is studied in pultrusion of nylon 11 and PEKK powder impregnated towpregs.

*To whom correspondence should be addressed

Characteristics of powder impregnated towpregs and the effects of these characteristics on consolidation behaviour are presented in the background section. Experimental apparatus for compression moulding, pultrusion and impregnation processes are then described. Micro- structural changes in the powder impregnated tows prior to and after consolidation, flexural test and void content results from the compression moulding study, and results on pulling force as function of pulling speed and temperature are detailed and discussed.

BACKGROUND

The powder impregnated tow is heated to fuse the powder on the tow before consolidation. The microstructure of the coated tow during heating is developed through interparticle and particle-fibre interactions. The coated tows are then consolidated to form the composite product by application of heat and pressure in subsequent processing. The optimum degree of coating prior to processing varies for different processes. Well-coated tows are preferred in continuous processes like filament winding and puitrusion, while partially coated flexible tows are preferred for processes such as weaving, so that handling the towpregs will be easier.

The stages of fibre coating were studied using an optical microscope with a heating stage and an environmental scanning electron microscope (ESEM) for nylon I I impregnated glass fibre tows and PEKK impregnated glass fibre tows 4. The stages and mechanisms of fibre

COMPOSITES MANUFACTURING Volume 6 Number 1 1995 35

Experiments on compression moulding and pultrusion: K. Ramani et al.

Figure ! ESEM pictures showing the stages of polymer flow

coating were identified and are summarized in Figures 1 and 2. The polymer flow in the powder impregnated towpreg during consolidation depends upon the extent of polymer coating and flow prior to consolidation, particle size, particle size distribution, and on the flow characteristics of the polymer. Hence the effect of processing parameters such as pressure and temperature on the consolidation of powder impregnated towpregs is different from that of other material forms such as melt impregnated prepregs. This was the motivation for studying the influence of processing parameters on the consolidation behaviour of powder impregnated towpregs.

Compression moulding

Previous studies on thermoplastic consolidation have focused on the use of intermediate material forms such as commingled fibres 5, prepregs 6, tapes v and, more recently, power impregnated towpregs 8,9. Vodemayer et al. 1° calculated the optimal particle size of spherical powders in order to minimize the flow length, based on geometrical models. However, the wetting behaviour and the extent of coating were not considered.

In the present work, the influence of four key processing parameters on the degree of consolidation of powder impregnated towpregs in compression moulding was

36 COMPOSITES MANUFACTURING Volume 6 Number 1 1995

Experiments on compression moulding and pultrusion: K. Ramani et al.

Initial Stage-Powder on Fibers. First Stage-Smoothing of Particles.

Second Stage-Coalescence of Particles.

F

I I) (i

Third Stage-Droplet Formation and Bridging.

Fourth Stage-Film Formation with or without Bridging.

Figure 2 Different s tages of po lymer flow on the fibres

investigated. These parameters are consolidation time, pressure, temperature and displacement rate. Full factorial experiments were used to determine the effect of each parameter and parameter interaction upon consolidation.

Puhrusion Extensive experimental and analytical research has

been done on the pultrusion of thermosets owing to the ease of fibre impregnation with thermosets and availability of the materials 11 - 19. Thermoplastic pultrusion has been investigated only recently. Models for thermoplastic pultrusion using melt impregnated prepregs have been presented in recent years 2°-21. These models included analysis of pulling force, heat transfer in the die, and pressure due to compaction of the prepregs in the die.

The pulling force in thermoplastic pultrusion depends on processing parameters (temperature, pulling speed), material parameters (type of polymer, fibre volume fraction, degree of impregnation) and design parameters (die size and profile, die taper). Pulling force is a very important parameter and must be studied in order to understand and design the process. Viscous resistance of the polymer layer between the fibres and the die wall, resistance due to compaction of the composite in the tapered section of the die, and frictional resistance between the die wall and the dry fibres, contribute to the total pulling resistance 21.

Pultrusion with thermoplastic powder impregnated tows offers an attractive method to integrate impregnation and pultrusion in one process. Pultrusion with the powder impregnated tows involves coalescence of the polymer particles, followed by film formation in the preheating stage and local (longitudinal and transverse) flow of the polymer during consolidation. In the present investigation, the effect of tow temperature and pulling speed on pulling force in isothermal pultrusion of thermoplastic powder impregnated tows is studied.

EXPERIMENTAL INVESTIGATION

Material system The thermoplastic resins used in this investigation were

nylon 11 and PEKK. Volume-average particle sizes determined using a Climet RL series Hydrocell liquid particle sensor + were 26.3 and 71.3/~m for nylon 11 and PEKK, respectively. The fibres used were continuous E-glass (Owens Corning 497) with a silane-based sizing. A slurry, consisting of the thermoplastic powder, a suspension agent (Hercules Klucel M) and a wetting agent (3M FC-430), was prepared for impregnating the fibres. The impregnated fibre tows were wound on the creels for pultrusion, and around a frame for compression moulding as shown in Figure 3. PEKK impregnated tows (55% fibre volume fraction) were used for the compression moulding experiments. The pultrusion experiments were conducted with PEKK impregnated tows 165% fibre volume fraction) and nylon 11 impregnated tows (55% fibre volume fraction).

Compression moulding The coated tows were wound to a 25 mm width on a

frame to form a fibre charge as shown in Figure 3. A frame with strain gauges was used to apply a known tension to the fibres before consolidation, in order to avoid fibre waviness and form a unidirectional composite. The wound fibres were placed in the consolidation apparatus, consisting of a matched die, a nitrogen chamber and a mounting mechanism, as shown in Figure 4. The apparatus was mounted on an MTS tensile testing

Mandrel

Figure 3

Winding Frame Wind ing f rame used for conso l ida t ion exper iments

I MTS Machine t I I

Load Cell

l I /Nitr°genChamber I I

i ~ ~ . / Upper Die ~ JHeater

I . . . . -"~='- - ='= . . . . . . . . . J . ~ 1 I r , ~ ).m.i L

L.. Frame . . . . Lower Die / / I t

I I Towpreg/ I I ~ Actuator

Figure 4 Conso l ida t ion a p p a r a t u s

COMPOSITES MANUFACTURING Volume 6 Number 1 1995 37

Experiments on compression moulding and pultrusion: K.

Table 1 Experimental matrix used in consolidation experiments

Test Time Press Rate Temp Strength (GPa) % Void

1 Low Low Low Low 0.246 23.2 2 High Low Low Low 0.610 19.7 3 Low High Low Low 0.101 29.9 4 High High Low Low 0.934 15.6 5 Low Low High Low 0.273 28.8 6 High Low High Low 0.579 26.9 7 Low High High Low 0.963 14.5 8 High High High Low 0.349 12.4 9 Low Low Low High 1.135 17.0

10 High Low Low High 0.494 15.3 11 Low High Low High 0.429 20.3 12 High High Low High 0.852 12.3 13 Low Low High High 0.462 16.1 14 High Low High High 1.704 5.2 15 Low High High High 1.246 7.9 16 High High High High 1.851 4.1

Table 2 Values of low and high parameters used in consolidation

Parameter Low High

Time (s) 750 1500 Press (MPa) 2.6 5.2 Rate (mm s- 1) 0.207 0.415 Temp (C) 295 325

machine to control and record the load and the displacement of the die during consolidation. A thermo- couple was placed on the tow surface to monitor the composite temperature during consolidation. The wound fibres were consolidated to form a 25 mm x 203 mm x 3 mm unidirectional composite that was 70% fibre by weight.

The consolidation apparatus was used to determine the effects of consolidation time (time), pressure (press), displacement rate (rate) and temperature (temp) on the properties of the composite. A predetermined process cycle was used for all the experiments. The fibre charge was heated to a desired temperature in the die without applying pressure. The die was then closed using a constant displacement rate. The pressure was held constant during consolidation. The die was cooled at the end of the cycle until the polymer was sufficiently solidified before removing the composite plaque from the die.

A full 24 factorial study, consisting of 16 individual experiments, was conducted to determine the effect of each of the four process parameter interactions (Table 1). High (+) and low ( - ) values of each of the process parameters were chosen based upon preliminary con- solidation experiments (Table 2). The values of each parameter were chosen to produce plaques with varying degrees of consolidation. The results, Xe, of the plaques were quantified through mechanical testing and determination of void content. Sixteen plaques were cut to 25 mm x 127 mm x 3 mm and tested in three-point bending to determine flexural modulus and strength. Void content of each sample was determined using Archimedes' principle. The effects, R j, were evaluated using two

Ramani et al.

different results criteria (Xe), strength and void content. The results using strength and void content are listed in Table 1. The results of each of the 16 experiments were recorded in an analysis of variance (ANOVA) table (Table 3). The effect of each variable and the interaction of one variable with another can be determined from this table.

The effect of a single parameter or parameter interaction, j, is determined by R j:

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

The sign of an interaction was determined by multiplying the signs of the individual parameters making up the interaction. The magnitude, IRjl, determines the significance of the parameter or the parameter interaction, j. The sign of Rj shows the influence of the parameter or parameter interaction level on the flexural strength or the void content.

Pultrusion A pultrusion line consisting of a creel stand to mount

the powder impregnated tows, a radiant tunnel preheater, a hot gas preheater, a die and a pulling mechanism was used to conduct the experiments. A I00 mm long heated die was used to produce composite tapes with a rectangular cross-section of 13 mm x 0.15 mm. The die assembly was mounted on a slide parallel to the pulling direction, and a load cell was used to measure the pulling force, as shown in Figure 5. A personal computer

Table 3 ANOVA table of the effects in compression moulding

Parameter/Parameter interaction Strength % Void

Time 3.647 65.9 Press 1.771 23.1 Rate 3.811 37.5 Temp 5.973 72.5 Time-press - 0.033 5.3 Time-rate 0.815 - 19.8 Press-rate 2.267 53.3 Time temp 1.069 - 16.4 Press temp -0.079 5.3 Rate-temp 3.017 20.7 Time-press-rate - 4.481 - 35.2 Time-press-temp 1.273 - 1.6 Time-rate-temp 5.177 34.7 Press-rate-temp 1.445 - 18.0 Time-press-rate-temp - 0.451 4.0

Hot Gas Impregnated Fiber Tows

I ~ Acquisition J

Figure 5 Schematic diagram of the experimental pultrusion facility

38 COMPOSITES MANUFACTURING Volume 6 Number 1 1995

Experiments on compression moulding and pultrusion: K. Ramani et al.

Table 4 Experimental matrix used in isothermal pultrusion experiments

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

14 18 21

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

The temperatures of the radiant oven and the hot gas were adjusted to achieve the desired tow temperature at the die entrance, and the die was maintained at the same temperature as the tows. The experiments using PEKK impregnated tows were conducted at four different temperatures and seven different pulling speeds. Experiments using nylon 11 impregnated tows were conducted at three different temperatures and seven different pulling speeds (Table 4). For each experiment, the die temperature, radiation oven temperature and the hot gas torch temperature were changed to maintain isothermal conditions. Once isothermal conditions and a constant pulling force were observed, each experiment was conducted for 5 min and the average pulling force was recorded.

on the surface of the tow. In this case, interparticle coalescence dominates the fibre coating process, causing formation of a polymer sheath around the tow. The cross-section of the PEKK pultruded tape shows that polymer flow through the tow thickness occurs to consolidate the tow (Figure 9).

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

RESULTS AND DISCUSSION

Microstructural evolution The consolidation process consists of fibre bed

compaction and polymer flow within the fibre bundle. ESEM pictures show that the polymer particles undergo morphological changes upon heating. They change from a randomly shaped solid phase to a rounded liquid phase that ultimately spreads along the length of the fibre (Figures 1 and 2). The increase in temperature causes a reduction in the surface tension and viscosity, resulting in the phase change. Optical microscope pictures of the preheated powder impregnated tow surface show several channels available for the coalesced droplets to flow between the fibres with minimal resistance (Figures 6 and 7). Pictures of the consolidated tow surface show the channels between the fibres bridged by a smooth polymer film (Figure 6).

Scanning electron microscope (SEM) pictures of the preheated nylon 11 tow cross-section show that the polymer is distributed through the thickness of the tow (Fi,qure 8). Volume-average particle size (20/~m for nylon 11) of the order of the fibre diameter (10pm) results in uniform impregnation through the thickness of the tow. In addition, the low melt viscosity of nylon 11 above its melting point enhances polymer flow. The cross-section of the consolidated pultruded tape shows uniform distribution of the polymer through the thickness of the tape (Figure 8).

SEM pictures of the preheated PEKK tow cross- section show that a sheath of polymer is formed, while the core of the tow is not impregnated (Figure 9). Large volume-average particle size (71.3 ILm for PEKK) compared with the fibre diameter (10/tin) results in dense packing

Stage I Tow (before Heating)

Figure 6 Changes in tow prepreg microstructure during consolidation

COMPOSITES MANUFACTURING Volume 6 Number 1 1995 39

Experiments on compression moulding and pultrusion. K. Ramani et al.

Pressure

riUer Molten Polymer Droplets

Stage 1. Surface Tension and Capillary Driven Flows before Application of Pressure.

Stage 2. Spreading, Coalescing of Droplets and Local Squeeze Flow due to Compaction.

Pressure

Stage 3. Completely Coated and Consolidated Tow.

Figure 7 Morphological changes of powder impregnated tows during consolidation

the local longitudinal flow and the transverse flow depends on the average particle size, the uniformity of impregnation and the viscosity-temperature relationship of the polymer.

Compression moulding Using void content and strength criteria, it was found

that temperature was the most significant, displacement rate and consolidation time were equally significant, and pressure was the least significant of the processing parameters. The signs of all effects were positive, showing that the high (+) level of each variable positively influenced consolidation. The most significant positive interactions were pressure<lisplacement rate and time- displacement rate-temperature. The most significant neyative interaction was time-pressure-displacement rate and the most insignificant interaction was among all four parameters. The experiment conducted with high levels of all the parameters produced the laminate with the highest degree of consolidation.

Figure 8

r~!~iiiii/!~!I~I~

Cross-sections of nylon 11 tows before and after consolidation Figure 9 Cross-sections of PEKK tows before and after consolidation

40 COMPOSITES MANUFACTURING Volume 6 Number 1 1995

Experiments on compression moulding and pultrusion. K. Ramani et al.

160

140

~" 12o

~ 100-

~ 80

60

40

• y : : , , 0 "° ' "

O ." ~ 210 ° c ., g .-.A... 230°C

/v " ' I I t t

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

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

The local longitudinal flow and transverse flow should be maximized by using the optimum set of processing parameters to minimize cycle time. For a given towpreg permeability, the flow rate can be maximized by increasing the pressure gradient or decreasing the viscosity. Increasing the applied pressure causes greater fibre compaction and a decrease in permeability of the towpreg 23, explaining the relative insignificance of pressure on the consolidation.

The viscosity of the polymer can be reduced by increasing the temperature and increasing the shear rate. In compression moulding, the shear rate of the polymer can be increased by increasing the displacement rate. Using the squeeze flow approximation, the average shear rate was calculated as the ratio of the displacement rate and the displacement. The viscosity of the polymer (PEKK) was measured with a capillary rheometer at high ( + ) and low ( - ) processing temperatures. Viscosity-shear rate data measured at a fixed temperature were fit to a power-law expression. The viscosity of the polymer during the initial stages of compression moulding was then obtained by substituting the calculated shear rate into the power-law expression. The average ratio of the viscosity for high shear rate (+) and low shear rate ( - ) was 0.78. The average ratio of viscosity at high temperature (+ ) and low temperature ( - ) was 0.93. This reduction in the viscosity of the polymer due to increase in the shear rate and temperature explains the significance of the temperature and the displacement rate on the consolidation.

The tows occupy a greater volume before consolidation than after, since the particles keep the fibres apart prior to melting. Significant volume reduction of the tows occurs during consolidation. This volume reduction is due to squeeze flow, transverse flow and coating of the

fibres. Polymer flow is enhanced by the reduction in the viscosity of the polymer and coating of the fibres is driven by reduction in the surface tension. The displacement rate effect occurs over a very short distance during mould closing. The resulting squeeze flow occurs over a short time compared with the time taken for completing consolidation.

Pultrusion Average pulling force values were plotted as a function

of pulling speed at different temperatures for nylon 11 impregnated tows and for P E K K impregnated tows as shown in Figures 10 and 11. Power-law constants (A), power-law exponents (n) and correlation coeffcients (R) for each of the curve fits are listed in Table 5. The correlation coefficients were observed to be close to 1. This shows a good power-law fit to the data at all the temperatures. A reference temperature - 190°C for nylon II and 340°C for P EK K - was selected. An arbitrary pulling force value in the observed range was then selected. The pulling speeds were calculated at the reference temperature (V,e0 and at the desired temperature (Vr) from the corresponding curve-fit equations for the chosen pulling force. A shift factor (a r) for a temperature (T) was then calculated using

aT = VrojVT

The shift factors were calculated for three different pulling forces and averaged. Hence one average shift factor for a temperature (T) was obtained. The shift factors for nylon 11 and P EK K are listed in Table 6. Log {pulling force) was plotted as a function of log (aT. V) for nylon 1 1 and P E K K as shown in Figures 12 and 13, respectively. All the points on the plots lie on the line represented by the reference temperature curves for both nylon 11

90

80

~ 7 0 -

u

5O-

40

.gt °'" n ,

~ / A, - 380 °C

I I I I 0 5 l 0 15 20 25

Pul l ing Speed (mm/sec)

Figure I! Pulling force as a function of pulling velocity tPEKK impregnated glass fibre tows)

COMPOSITES M A N U F A C T U R I N G Volume 6 Number 1 1995 41

PEKK Nylon 1 !

Temperature (°C) A n R Temperature (°C) A n R

340 49.96 0.198 0.944 190 89.6 0.168 0.86 350 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 0.904 380 40.67 0.214 0.992 - - -

Table 6 Shift factors for PEKK and nylon 11

PEKK (reference temp. 340°C)

Z 2"

t., O t~ 2'

gg °-

"~ 2' a.

o .z 2'

Nylon 11 (reference temp. 190°C)

Temperature (cC) Shift factor (at) Temperature (°C) Shift factor (at)

350 0.6085 210 0.452 360 0.570 230 0.229 380 0.447 - -

-0.5

O

Figure 12

A

~o

O

A 190 *C O 210 *C a 230 *C

I ' 1 I 0 0.5 1 1.5

Log(a 'v )

Master curve for nylon 11 (reference temperature 190°C)

impregnated tows and P E K K impregnated tows (Figures 12 and 13). The resultant master curves shown in Figures 12 and 13 clearly indicate the validity of the superposition method. Thus, by conducting a limited number of experiments for a particular material system and using the superposition method, one can predict pulling force over a wider range of temperatures and pulling speeds. Superposition of shear stress-shear rate curves for different temperatures was demonstrated in a similar manner for polymers by Mendelson 24. Validity of superposition in pultrusion indicates that the viscous resistance of the polymer film between the die wall and the outer fibres is the major component of the pulling resistance.

Some divergence of the data from the master curve was observed for nylon 11 at 190°C and high pulling speeds (20 mm s - 1) (Figure 12). This divergence from the master

Z -.-. 2

O a, 2

"~ 2 a~

,d 2- _- 340 *C O 350*(2 zx 360 *C A 380 *C

Experiments on compression moulding and pultrusion: K. Ramani et al.

Table 5 Power-law coefficients for P E K 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 Master curve for PEKK (reference temperature 340°C)

curve may be attributed to the shear thinning of nylon 11 and a corresponding reduction in the pulling force. Divergence of the data from the master curve was not observed at the higher temperatures for nylon 11 and at all the processing temperatures for PEKK.

1.4

C O N C L U S I O N S

Surface tension, capillary action and squeeze flow contribute to the consolidation behaviour of the powder impregnated tows. Localized longitudinal flow along the length of the fibres dominates the initial stages of consolidation of the thermoplastic powder impregnated tows, while transverse flow is necessary to complete the consolidation. The localized polymer flow during the consolidation depends on the extent of coating prior to consolidation, fibre volume fraction, particle size, particle

42 COMPOSITES MANUFACTURING Volume 6 Number 1 1995

Experiments on compression moulding and pultrusion. K. Ramani et al.

size d i s t r ibu t ion , and the flow character is t ics of the polymer . Po lymer sheath fo rma t ion due to in te rpar t ic le

coalescence occurs when the par t ic le sizes are large c o m p a r e d with the fibre d iameter .

T e m p e r a t u r e and d i sp lacement rate were observed to be t h e most significant factors affecting conso l ida t ion of the the rmoplas t i c powder impregna t ed towpregs based on the void conten t and the s t rength cri teria. Exper imenta l ly based pull ing ve loc i ty - t empera tu re super- pos i t ion of the pul l ing force curves for i so the rmal pu l t rus ion of p o w d e r impregna t ed towpregs was demon- strated. Val id i ty of the supe rpos i t ion process indicates that the m a j o r con t r i bu t i on to the pul l ing resistance is due to the viscous resis tance of the po lymer film between the die wall and the ou te r fibres.

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

The au thors would like to thank E.I. D u Pon t de N e m o u r s and C o m p a n y for supply ing P E K K powder , This research was s u p p o r t e d by The N S F - E R C for Intel l igent Manufac tu r i ng Systems.

R E F E R E N C E S

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152 8 Yang, H. and Colton, J.S. Polym. Compos. 1994, 15, 34 9 Yang, H. and Colton, J.S. Polym. Compos. 1994, 15, 42

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Composites in Multiphase Systems' (Ed. C.D. Han), American Chemical Society, Washington, DC, 1984

15 Batch, G.L. and Macosko, C.W. '42nd Ann. Conf., Composites Institute, Society of the Plastics Industry Inc.', 1987

16 Gibson, A.G., Lo, C.Y., Lamb, D.W. and Quinn, J.A. Plast. Rubber Process. Applic. 1989, 12, 191

17 Viola, G., Portwood, T., Ubrich, P. and DeGroot, H.R. 35th lnt. SAMPE Syrup. 1990, 35, 1968

18 Lackey, E. and Vaughan, J.G.J. Rein[i Plast. Compos. 1994, 13, 188

19 Batch, G.L. Ph.D. Thesis, University of Minnesota, 1989 20 Lee, W.I. and Springer, G.S.J. Compos. Mater. 1991, 25, 1632 21 Astrom, B.T. and Pipes, R.B. PoO,m. Compos. 1993, 14, 173 22 Astrom, B.T and Pipes, R.B. PoO,m. Compos. 1993, 14, 184 23 Gutowski, T.G. SAMPE Quart. 1985, 16, 58 24 Mendelson, R.A. Polvm. Eng. Sei. 1968, 8, 235

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