CHARACTERIZATION AND MODELING OF STRENGTH ENHANCEMENT MECHANISMS IN A POLYMER/CLAY

NANOCOMPOSITE

Samit Roy *, Hongbing Lu †, Kamesh Narasimhan‡ and Farzana Hussain §

Department of Mechanical & Aerospace Engineering, Oklahoma State University, Stillwater, OK-74078.

The compressive failure of continuous fiber reinforced composites has received considerable attention in the recent years because it typically occurs at a stress level that is 40-50 % below the tensile strength of the composite. This paper is primarily focused on enhancing the compressive strength of polymer matrix composites (PMC) through a series of modifications in material composition and thermoplastic pultrusion manufacturing process variables. Low-cost commodity resins such as polypropylene (PP), suffer primarily due to low compressive strength. Enhancement of the compressive strength of pultruded thermoplastic composites is achieved by improving the yield strength of the surrounding matrix in shear and reducing fiber misalignment in the composite through optimization of manufacturing process variables. The dispersed platelets are typically one micron in length but only a nanometer in thickness. A single-screw extruder was used to facilitate nanoclay dispersion in PP. This new family of materials exhibits enhanced stiffness and strength of the matrix material, through the inclusion of exfoliated nano-scale montmorillonite particles in the fabrication of resin pre-impregnated (prepreg) glass fiber filaments. After the prepreg was pultruded to form a composite laminate, uniaxial compression tests were performed to determine compression strength of the laminate. Scanning Electron Micrographs (SEM) were taken to examine the failure surfaces. Transmission Electron Microscopy (TEM), were also employed to reveal all nano-scale platelet morphologies, namely exfoliated, intercalated and stacked structures within the samples. Dramatic improvement in compressive strength and compressive modulus were observed with relatively low nanoclay loadings. Compressive strength tests were also performed on aged pultruded PP composite with 0% and 1 % nanoclay loadings, and the data were compared with compressive strength data for unaged specimens. A significant increase (12-20%) in compressive strength was observed after the specimens have been aged for 15 months under ambient conditions. However, a 30% decrease in compressive modulus due to aging was recorded for specimens with 1% clay loading. Multi-scale simulations of nanoclay/polymer interface behavior are currently in progress in order to understand the strength enhancement mechanism.

INTRODUCTION

he use of polymer matrix composites (PMC) in aerospace and civil engineering, as well as in sports and leisure applications is rapidly increasing. PMC has found wide range of applications in the structural components where the substitution of PMC for metals has substantially improved performance and reliability. The high

tensile strength of PMC is mainly derived from the high strength of the carbon or glass fibers embedded in the matrix. Fibers typically have high strength in tension. However, their compressive strength is generally much lower due to the fact that under compression, the fibers tend to fail through buckling well before compressive fracture occurs. Also, fiber misalignment and presence of voids during manufacturing processes contribute to a further reduction in compressive strength. In fact, the overall compressive strength of a PMC is only about 50% of the

T

* Associate Professor, Department of Mechanical and Aerospace Engineering, Oklahoma State University, Stillwater, OK 74078† Associate Professor, Department of Mechanical and Aerospace Engineering, Oklahoma State University, Stillwater, OK 74078‡ Graduate Research Student, Department of Mechanical and Aerospace Engineering, Oklahoma State University, Stillwater, OK 74078§ Research Scientist, NRC, Montreal, Canada

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46th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics & Materials Conference18 - 21 April 2005, Austin, Texas

AIAA 2005-1853

Copyright © 2005 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

tensile strength. The strength of the surrounding polymer matrix plays a key role in characterizing the critical buckling load of the fibers by constraining the fibers from buckling. Consequently, high-strength polymer resins such as PEEK and PPS–although very expensive–are typically used for applications requiring high compressive strength. Low-cost commodity resins such as polypropylene (PP), suffer primarily due to low compressive strength. Because the critical buckling stress in the fiber is directly related to the stiffness and yield strength of the surrounding matrix material, any increase in these matrix properties would directly result in an improvement in the compressive strength of the composite. In this context, it has been observed that addition of small amounts of nano-structured montmorillonite (MMT) clay (~5% wt.) significantly improves the stiffness, strength, gas barrier and fire-resistance properties of most thermoplastic resins, with only small effect on flexibility. These nanoclay fillers are only slightly more expensive than glass (few dollars per pound), yet generally far less expensive than carbon fibers (~$100/lb) or carbon nano-tubes (~$50,000/lb). Additionally, the small amount of nanofillers required to enhance properties enables these materials to compete more effectively with traditional glass-fiber reinforcements.

Earlier work by Rosen,1 which assumed extensional (transverse) and shear micro buckling of the fibers imbedded in an elastic matrix, consistently over predicted the compressive strength when compared with experimentally measured values. Argon,2 was among the first to recognize that fiber reinforced composites made by normal manufacturing processes, including pultrusion, have regions of fiber misalignment. Argon,2 and later Budiansky, 3, 4 showed that fiber misalignments present in fiber reinforced composites could lead to yielding of the polymer matrix in shear. The yielding of the matrix would, in turn, result in loss of matrix stiffness that could eventually trigger fiber micro buckling with resulting kink band formation leading to final failure4. Consequently, it was recognized that the dominant compressive failure mode in aligned- fiber polymer matrix composites is localized compressive buckling, or kinking. Argon,2 proposed a simple yet elegant formula for composite compressive

strength given by 0

YC φ

σ=σ where, σY is the matrix yield strength in shear, and φo is the initial fiber misalignment

angle with respect to load direction, sometimes referred to as fiber misorientation. Further, it is likely that any increase in composite longitudinal shear stiffness (G12c) due to the presence of nanoclay particles would result in an increase in the compressive strength of the composite, as indicated by a modified form of the Argon formula proposed by Budiansky that includes both elastic and plastic buckling behavior as limiting cases, 4, 5, 6

⎟⎟⎠

⎞⎜⎜⎝

⎛+

=

Y

cc

G

γφ

σ0

12

1 (1)

where, γY = σY / G12c is the composite yield strain in longitudinal shear. The failure mechanism described in Eq.(1) is still a shear mode of fiber buckling, but unlike in Rosen’s elastic analysis, it is the local nature of the imperfections, coupled with (perfectly) plastic yielding of the matrix in shear, that results in local micro buckling and kink band formation as shown in Fig. 1. However, it should be noted that Eq.(1) assumes zero kink band angle and an elastic-perfectly plastic matrix material.

From Eq.(1), it quickly becomes apparent that improvements in the compressive strength of the composite may be achieved by: (a) improving the yield strength and stiffness of the surrounding matrix in shear, and (b) reducing fiber misalignment in the composite through optimization of manufacturing process variables, such as pull-speed, preformer temperature, nanoparticle alignment and/or dispersion. It is therefore the aim to accomplish this objective by concurrently increasing matrix yield strength and matrix shear modulus, as well as by decreasing fiber misalignment, thereby making use of synergies that may exist between these effects. There exist very little published data on the compressive strength of pultruded thermoplastic composites. Fibers in pultruded material are

general well aligned. However, the composite compressive strength is very sensitive to even slight misalignment in fiber orientation.

in

Figure 1: Compression failure of pultruded specimen showing kink bad

5, 6

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Typically, a thermoplastic resin matrix offers better structural performance than thermosetting resin matrix. Thermoplastics exhibit superior fracture toughness,7 and impact resistance, and have better post-process formability and improved reparability. With improved manufacturing techniques, such as ultrasonically activated consolidation die, it is now feasible for thermoplastic pultrusion to have higher production rates than thermoset composites. Recyclability of thermoplastics is another attractive feature. In this paper, the effect of incorporating nanoclay particles on the mechanical properties such as compressive strength and compressive modulus of pultruded E-Glass/PP unidirectional composite will be presented and discussed.

I. EXPERIMENTAL ANALYSIS A. Materials

Marlex HGL-350 Polypropylene Homopolymer (antistatic) was used as neat resin (density 902 Kg/m3 and melt flow rate 35 g /10 min ). Material systems evaluated in the experimental studies are unidirectional E-Glass fibers melt impregnated with polypropylene with and without nanoclay particles using a single-screw extruder. To pultrude unidirectional composites shapes using either E-Glass/PP or nanoclay/E-Glass/PP, pre-preg tapes were fed to the pultrusion machine. Subsequently, 2.54 cm wide and 1.27 cm thick composite beams for different nanoclay content (0%, 1%, 2%, 3%, 4%, 5% and 10% by weight) were pultruded. The pultruded samples were machined according to ASTM D695 specifications to obtain compression test specimens. The clay used was organically modified montmorillonite clay, 1.28 E, supplied by Nanocor.

B. Processing of fiber–reinforced composite 1. Thermoplastic Pultrusion Process

The use of thermoplastics as matrix material for polymer composites has gained popularity recently due to their high-performance properties compared with thermoset resins.8 Pultrusion is among the most rapid and efficient processes to fabricate composites when low-cost and/or high volume is required.9, 10 The versatility of this process enables the production of profiles with many different sectional shapes, in what has largely contributed to the rapid increase of pultruded composites in the construction industry. It remains one of the most efficient and inexpensive methods for producing continuous fiber composites. In the case of the novel process utilized by the pultrusion machine at Oklahoma State University (OSU), the pre-preg is guided through a heated pre-former and then through an ultrasonically activated final consolidation die. A schematic of the ultrasonic pultrusion machine is shown in Fig. 2. The ultrasonic vibrations directed into the pre-heated pre-preg by means of a waveguide helps stimulate the flow of resin in the pre-preg. Pressure is also applied to the pre-preg within the die to consolidate the pre-preg to the chosen profile. The ultrasonic vibration at low power input (maximum power: 50 watts) does result in some incidental heating in the pre-preg, but the increased flow rates of the resin indicates a reduction in viscosity that is far greater than expected in relation to the temperature increase. The net effect of the ultrasonic die is to reduce effective resin viscosity as well as friction at the composite/die interface, thereby significantly reducing void content and pull-force requirement. Pultruded E-Glass/PP samples that are 2.54 cm wide and 1.27 cm thick are shown in Fig. 3 for 0%, 1%, 2% and 3% nanoclay loading by weight. The change in color of the samples due to the presence of clay is noticeable.

Figure 3: Pultruded E-Glass/PP samples with different nanoclay loadings

Figure 2: Schematic of pultrusion machine with ultrasonic consolidation die

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2. Process parameter control The most critical processing parameters to be controlled are process speed and pre-former temperature profile.

For any fixed die length the process speed will determine the residence time of material. It also depends on the sectional thickness of the part. In this research, E-Glass/PP composites have been pultruded successfully at speed up to 5 mm/sec. Initially pulling speed was maintained as low as 0.5 mm/sec to ensure correct consolidation of the thermoplastic composite. The pre-former die consists of three temperature zones. Temperature set points as measured by the thermocouples at these three zones in the pre-former die were 220ºC, 300ºC and 325ºC respectively. The pre-former die temperature profile determines the rate of heating of material, and influences the resin viscosity and degree of cure attained during residence in the pre-former. Subsequently, the composite passes through the final consolidation die, where ultrasonic vibrations, with an energy of 22-25 Watts is directed into the pre-heated pre-preg by means of a waveguide helps stimulate the flow of resin in the pre-preg. Pressure of around 35-70 KPa was applied to the pre-preg within the die to consolidate the pre-preg to the chosen profile. Finally, the pultruded laminates were sectioned using a precision cutting saw that travels at the same speed as the part being pultruded, and examined in an optical microscope in order to determine the size and distribution of voids and unwetted fibers.

3. Mechanical Test Method Two separate sets of compression tests were performed. In the first set, ASTM D695 procedure was used to determine the compressive properties of neat PP and nanoclay modified PP resin samples. The tests were carried out using compression-molded cylindrical test specimens that were 12.7 mm in diameter and 25.4 mm in length. From applied load and displacement data, stress vs. strain curves was plotted using the Instron fast track data acquisition software. In the second set of tests, compressive strength and compressive modulus of pultruded E-Glass/PP with different loadings of nanoclay particles were determined by performing compressive tests using a standard 245 KN servo hydraulic MTS machine in displacement mode. Again, ASTM D-695 test method was used to determine the

pressive strength and modulus of elasticity of the E-Glass/PP nanocomposites. Compression tests were carried out using pultruded test specimens that were 25.4 mm long, 25.4 mm wide and 12.7 mm deep. A test specimen after failure is shown in Fig. 4 for 10% nanoclay loading, with a close-up view of the resulting kink band shown in Figure 1. At least four replicate test specimens were used for each

nanoclay loading.

com

Figure 4: Pultruded Compression test specimen after failure with 10% nanoclay loading

II. RESULTS AND DISCUSSION OF TEST DATA

A. Morphology 1.Transmission Electron Microscopy Transmission Electron Microscopy (TEM) was applied to characterize the hierarchical morphology present in the nanocomposites. Fig. 5 a) shows a bright field TEM micrograph of Polypropylene with 3% clay loading in a dog bone sample. Fig. 5 b) show selected area diffraction pattern for 3% polypropylene based nanocomposites. The diffraction pattern shows several reflections with varying degrees of intensity. Fig. 5 a) and 5 b) reveal the presence of aggregates, but there are regions where completely delaminated sheets are dispersed individually, showing the dark lines of 10-Å thickness. It appears that the clay exists as expanded aggregates made up of 2 to 10 platelets. The size of these aggregates and the number of platelets in them increase with the percentage of clay within the PP matrix. Indeed the TEM pictures show all possible platelet morphologies, namely exfoliated, intercalated and stacked structures within the samples. There appears to be

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Figure 5 a): Bright field TEM micrograph of Polypropylene with 3% clay loading in a dog bone sample showing the irregular lamellar morphology of the polymer microstructure with nano size clay particle.

Figure 5 b): Selected area diffraction pattern. Scale: 225 nm

reasonably good compatibility of the surface treated clay with the polypropylene resin. Fig. 6 a) shows a typical arrangement of a group of such glass fibers present in a thin film of E-

glass/Polypropylene with 3% nanoclay loading. Nano-sized clay particles present in the composites were also found randomly distributed in the polypropylene within the individual spacing of E-glass. Fig. 6 b) shows a bright field micrograph indicating the existence of a nano sized clay particle embedded in the matrix of polypropylene and in contact with the E-glass fibers. This provides evidence of particle-fiber interaction in which the clay particles are actually in contact with the fiber wall. The orientation of the clay with respect to the fiber surface is unclear. A close-up view in Fig. 7 reveals the existence of nanoclay agglomerate, as well as intercalated particles, and an exfoliated particle.

B. Mechanical Properties 1. Compression test

Compression strength data for neat polypropylene (PP) resin for different nanoclay loading are tabulated in Table 1. The bar chart in Fig. 8 shows the progressive improvement in the normalized compressive strength of nanoclay reinforced PP for different nanoclay content. A 134% increase in compressive strength is observed for PP resin with 10% clay loading. Stress verses strain curves for nanoclay reinforced E-Glass/PP composite are shown in Fig. 9 for different nanoclay loadings, and the compressive strength data are summarized in column 3 of Table 2. Because our primary objective was to obtain strength data, the data shown in Fig. 9 were not corrected for machine compliance. The bar chart in Fig. 10 shows the progressive

improvement in the normalized compressive strength of pultruded E-Glass/PP composite with different nanoclay content. Each bar represents the average of four test specimens, with a standard deviation of roughly 5 %. A 122 percent improvement in strength for 10% clay content is observed compared with baseline specimens with zero nanoclay content. A similar dramatic enhancement is measured for normalized compressive modulus (stiffness) as shown in Fig. 11, where 10% clay reinforcement leads to a 110 % improvement in modulus. From these charts it is apparent that a significant improvement in compressive strength (87%) as well as modulus (99%) is attainable for E-

Figure 6 a): Bright field TEM micrograph of compression molded E-glass-Polypropylene with 3% clay loading sample.

Figure 7: Bright field TEM micrograph of Polypropylene with 3% clay loading Fiber reinforced composite sample. Fine structure of polymer is evident in the micrograph. Scale: 375 nm

Figure 6 b): TEM bright field micrograph showing how polymer is entrapped between two fibers. Positions of nanoclay particles (3% nanoclay) within polymer can be seen.

134%

94%

67%57%42%

26%

0

0.5

1

1.5

2

2.5

3

1 2 3 4 5 6 7

Wt % of clay loading

Nor

mal

ized

C

ompr

essi

ve

stre

ngth

0% 1% 2% 3% 4% 5% 10%

Figure 8: Normalized Compressive Strength vs. Clay content for neat Polypropylene

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Ffo

Glass/PP system with only 5% nanoclay content while using purely mechanical (un-optimized) means of nanoclay dispersion, namely, a single-screw extruder and sonication at the die.

Examination of TEM micrographs of pultruded samples indicate presence of unexfoliated clay agglomerates, which, when fully exfoliated would presumably yield even better mechanical properties. As evidenced in Fig. 10, any negative effect of the ultrasonic treatment on fiber alignment is small compared with the large increase in matrix strength due to the presence of nanoclay particles. Further, the test data indicate that it is feasible to pultrude nanoclay reinforced E-Glass/ PP laminates with up to 27% fiber volume fraction and up to 10% nanoclay content

without a dramatic increase in resin viscosity that could clog the consolidation die and/or result in poor wetting of the fibers.

Table 1: Compression test data for neat Polypropylene resin and nanoclay-modified resin (without aging)

Sample Clay (weight %)

Average compressive strength (MPa)

% Increase compressive strength

Average standard deviation

PP 0.0 26.45 - 0.15 PP 1.0 32.30 26 2.17 PP 2.0 36.78 42 0.72 PP 3.0 40.71 57 1.76 PP 4.0 43.98 67 1.29 PP 5.0 51.22 94 1.86 PP 10.0 61.45 134 3.55

33%

0

0.5

1

1.5

2

2.5

1 2

W

Nor

mal

ized

C

ompr

essi

ve

stre

ngth

0% 1%

Figure 10: NormalizeContent for Pultruded

0

5 0

10 0

15 0

2 0 0

2 5 0

0 . 0 0 2 0 . 0 2 2 0 . 0 4 2 0 . 0 6 2

S t r a i n( mm/ mm)

P P E- gl a ss- 0 % c l a y

P P E- gl a ss- 1% c l a y

P P E- gl a ss- 2 % c l a y

P P E- gl a ss- 4 % c l a y

P P E- gl a ss 5 % c l a y

P P E- gl a ss- 10 % c l a y

igure 9: Comparison of stress vs. strain curves from compression test r E-glass/PP pultruded nanocomposite with different nanoclay loadings

39%

48% 67%87%

122%

3 4 5 6 7

t % of clay loading

2% 3% 4% 5% 10%

110%99%92%

72%69%62%

0

0.5

1

1.5

2

2.5

1 2 3 4 5 6 7

Wt % of clay loading

Nor

mal

ized

M

odul

us

of

Elas

ticity

0% 1% 2% 3% 4% 5% 10%

Figure 11: Normalized Compressive Modulus vs. Clay Content for Pultruded E-Glass/MMT/PP Composite

d Compressive Strength vs. Clay E-Glass/MMT/PP

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C. Changes in Nanocomposite Mechanical properties with Aging The effect of ageing on mechanical properties of the PP/Glass fiber pultruded composites for different nanoclay

loadings was also investigated. Compression tests were performed for neat PP, and PP with nanoclay loading of 1 weight %. The specimens were aged for approximately 15 months under ambient conditions. Three replicate specimens were used for each set of tests. The mechanical property data obtained for aged samples is tabulated in Tables 2 and 3. The testing followed standard ASTM (D695) procedures for reinforced plastics with sample dimensions of 50.8 mm x 12.7 mm x 12.7mm. Photographs of three test specimens (after failure) are shown in Figure 12 for the 0% clay loading, with kink bands clearly visible. Average values for of three specimens are presented in Tables 2 and 3. The test results were compared with compression test data obtained for specimens prior to aging. Compressive strength data comparison for pultruded PP composite with different nanoclay loading is shown in Table 2. The modulus comparison data for pultruded PP composite with different nanoclay loading is shown in Table 3.

The comparison of the aged specimen with specimen prior to ageing reveals a significant improvement in the compressive strength for both neat PP pultruded composite and pultruded PP composite with nanoclay loading of 1 weight %. But the modulus data comparison indicates a discrepancy, with a 30% drop in the modulus for aged pultruded PP composites with nanoclay loading of 1 weight %.

Table 2: Comparison of compressive strength for neat polypropylene pultruded composite and nanoclay-modified pultruded composite before and after aging

Sample Clay weight (%)

Average compressive data (unaged) (MPa)

Average compressive data (after aging) (MPa)

% Change

PP 0.0 105.26 126.07 19.77

PP 1.0 139.78 157.42 12.62

Table 3: Comparison of compressive modulus for neat polypropylene pultruded composite and nanoclay-modified pultruded composite before and after aging

Sample Clay weight (%)

Average modulus data (unaged) (GPa)

Average modulus data (after aging) (GPa)

% Change

PP 0.0 4.16 6.08 46.26

PP 1.0 7.4 5.73 -29.09

Figure 12: Pultruded neat PP composite (after 15 months of ageing) showing kink bands due to compressive failure

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III. SUMMARY AND CONCLUSIONS The determination of compressive strength of pultruded composites is of considerable importance as these

materials are generally weak in compression. This area is one of the least understood fields in composites, especially nanocomposites, because of the various parameters that affect the compressive behavior as well as difficulty in determining the strength experimentally. Unidirectional E-glass fiber reinforced pultruded composite were prepared in which the matrix was polypropylene with layered silicate nanoclay reinforcement. Thermoplastic pultrusion process was used to prepare the composites. The compressive properties exhibited significant increase with increasing clay loadings (up to 10 weight t %). The exact strengthening mechanism of the nanoclay reinforcement is not well understood. Multi-scale simulations of the nanoclay/polymer/glass-fiber interactions would be necessary to adequately address this issue. TEM studies indicate a favorable interaction between the silicate clays and the glass fibers, which are also silicon based, indicative of enhanced matrix-fiber adhesion. The clays are also presumed to decrease the CTE mismatch, significantly reducing residual stresses and leading to higher quality laminates. Additional characterization experiments to directly obtain resin and composite shear properties are currently underway for additional verification of compressive failure data. Experimental verification of predicted shear strains within the kink band will be attempted using digital image correlation technique. Compressive strength tests were also performed on aged pultruded PP composite with 0% and 1 % nanoclay loadings, and the data were compared with compressive strength data for unaged specimens. A significant increase (12-20%) in compressive strength was observed after the specimens have been aged for 15 months under ambient conditions. However, a 30% decrease in compressive modulus due to aging was recorded for specimens with 1% clay loading. Multi-scale simulations of nanoclay/polymer interface behavior are currently in progress in order to understand the strength enhancement mechanism.

IV. ACKNOWLEDGMENT This work was financially supported by the NASA-EPSCoR and NSF-Nanonet Research Initiation Grants.

Special thanks are due to Dr. Shamsuzzoha at University of Alabama for his kind cooperation with TEM characterization, and Dr Scott Taylor of Vybron Composites for fabricating pre-preg tapes. We also acknowledge Nanocor Inc. for supplying us with the surfactant modified nanoclay particles.

REFERENCES

1Rosen, B. W., “Fiber Composite Materials,” American Society of Metals, 1964, pp. 37- 75. 2Argon, A.S., Treatise on Material Science and Technology, Academic Press, New York, 1972, pp. 79,114. 3 Budiansky, B., “Computers and Structures,” Vol. 16, No. 1-4, 1983, pp. 3. 4 Budiansky, B., and Fleck, N.A., “Journal of Mechanics and Physics of Solids,” Vol. 41, No. 1, 1993, pp. 183. 5 Creighton, C.J., and Clyne, T.W., “Composites Science and Technology,” Vol. 60, No. 4, 2000, pp. 525. 6 Creighton, C.J., Clyne, T.W., and Sutcliffe, M.P.F., “Composites: Part A,” Vol. 32, No. 2, 2000, pp. 221. 7 Lu, H., Roy, S., Sampathkumar, P., and Jin Ma, “Characterization of the Fracture Behaviour of Epoxy Nanocomposites,”

Proceedings of 17th Annual Technical Conference [CD-ROM], American Society for Composites, West Lafayette, (October 21-23, 2002).

8 Taylor, S., and Thomas, W., “High Speed Pultrusion of Thermoplastic Composites,” Proceedings of the 22nd International SAMPE Technical Conference, Boston, (November 6-8, 1990).

9 Carsson, A., and Astrom, B.T., “Composites Science and Technology,” Vol. 29, No. 5-6, 1998, pp. 585. 10 Liskey, A.K., “Advanced Materials & Processes,” Vol. 135, No. 2, 1989, pp. 31. 11 Maier, C., and Calafut, T., Polypropylene The Definitive User's Guide and Data book, Plastics Design Library, New York,

1998, pp. 268.

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