A Study of the Thermomechanical Properties of Carbon Fiber–Polypropylene Composites

8/3/2019 A Study of the Thermomechanical Properties of Carbon FiberPolypropylene Composites

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1. INTRODUCTION

Carbon fibers are widely used in polymer-matrixcomposites, owing to their good mechanical, ther-mal and electrical properties. Vapor grown carbon

fibers (VGCFs) have recently gained interest, owing to

their potential low-cost production and favorable

properties. In this way, fibers can be produced at

much higher rates and lower costs (1, 2). The VGCFs

with greater economic potential are, however, small:

the length of the fibers is usually below 100 m and

the diameters near 200 nm (2). Because of this, they

are sometimes referred to as submicron (diameter)

fibers. Although from a cost-property-relation point of

view, the submicron VGCFs are commercially attrac-tive, their use poses new problems, as conventional

expertise on composite production may in some cases

no longer be applied. Moreover, as the minute dimen-

sions of the VGCFs do not allow direct determination

of mechanical and thermal properties, the entire mod-

eling process is more difficult.

In a previous work (3), the production and me-

chanical characterization of polycarbonate (PC) ther-

moplastic composites reinforced with this type of

VGCFs was described. It was shown that the pro-

duction of these composites is straightforward usingconventional processing technologies. Although the

incorporation of the VGCFs in the polymeric matrixled to better mechanical properties, the improvement

was marginal.The objectives of the present work are therefore

twofold. First, to produce VGCF-thermoplastic com-posites with thermomechanical properties significant-

ly better than those of the unreinforced polymer andto compare them with conventional composites.Second, to use micromechanical models to directly

calculate the properties of short fiber composites. Ifthese models show accurate enough, they can be

used to infer the properties of the submicron VGCFs

from those of the composite.In the previous investigation (3), it was concluded

that the presence of polycyclic aromatic hydrocar-

bons (PAHs) on the surface of the fibers could causechemical stress cracking in the polycarbonate matrix.

As a consequence, in the work described below, thefibers were heated to drive off the PAHs before incor-

poration in the matrix. Polypropylene (PP) was cho-sen, as this polymer has relatively good mechanical

properties at a moderate price and is not very sensi-tive to chemical stress cracking.

A Study of the Thermomechanical Properties of

Carbon FiberPolypropylene Composites

F. W. J. VAN HATTUM andC. A. BERNARDO

Department of Polymer Engineering, University of Minho4800 Guimara~es, Portugal

J. C. FINEGAN andG. G. TIBBETTS

Department of Physics and Physical Chemistry, General Motors R&D CenterWarren, Michigan 48090-9055

R. L. ALIG andM. L. LAKE

Applied Sciences Inc.Cedarville, Ohio 45314

Short fiber composites were produced using polypropylene as matrix and poly-acrylonitrile (PAN)-based fibers or vapor grown carbon fibers (VGCF) as reinforce-ment. The strength, stiffness, and coefficient of thermal expansion (CTE) of thecomposites were measured. The VGCF-composites showed strength and CTE thatare competitive with those of conventional PAN-fiber composites, but the stiffnesswas marginally lower. Micromechanical modeling of the PAN composite propertiesgives results consistent with the measurements. The models can be used to inferthe apparent VGCF-properties from their composites.

POLYMER COMPOSITES, OCTOBER 1999, Vol. 20, No. 5 683


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For comparison, polypropylene composites reinforcedwith PAN-based carbon fibers were also produced andinvestigated. The micromechanical models found in theliterature (46, 13) were validated on these PP-PANcomposites to calculate the elastic modulus, strength,and CTE.

2. EXPERIMENTAL

2.1 Materials and Sample Preparation

The VGCFs, Applied Sciences Pyrograf III, were pro-duced using a floating catalyst method developed byTibbetts, Gorkiewicz, and Alig (2). A catalyst such asiron pentacarbonyl was injected into the flowing hy-drocarbon gas, where it nucleates and grows the fiber. The fibers moved through the reactor with the gasstream and were collected at the exit. Montells MoplenF30G polypropylene, TENAXs PAN-based HTA 5131carbon fibers, and the VGCFs were processed in aLeistritz LSM 30.34 twin-screw extruder to obtaincomposites with a fiber volume fraction of 15%. TheVGCFs were kept in an oven at 200C for 2 hoursprior to processing, to ensure that any PAHs remain-ing on the fibers surface had vanished. The extrudatewas granulated to obtain composite granules with alength of some millimeters. Prior to further process-ing, batches with fiber volume fractions of 10% and5% were obtained by mixing the granulate with unre-inforced polypropylene. Tensile bars, adapted fromASTM D638M, were subsequently injection moldedusing a Klockner Ferromatic FM20 injection moldingmachine. The processing conditions were kept con-stant for the different materials, however higher injec-tion pressure and melt temperature had to be usedfor the PP-VGCF composites. In this way, PP-PAN-fiber and PP-VGCF composite tensile bars were ob-tained with fiber volume fractions of 5%, 10%, and15% respectively. Tensile bars of unreinforced poly-propylene were similarly produced. The processingconditions are given in Table 1.

2.2 Material Characterization

PP-PAN Composites

In the modeling work referred to above (6), the pres-ent PP-PAN composites and PAN fibers were fullycharacterized. In order to obtain the mechanical prop-

erties of the composites and the virgin material, ten-sile tests were performed according to the ASTMD638M standard, using an Instron 4505 universaltesting machine. Tensile bars were tested at a cross-head speed of 5 mm/min. The PAN-fiber propertieswere determined by testing the fibers according toASTM D 3379-89 in an Instron 1122 universal testingmachine with a 5 N load cell. Using a crosshead speed

of 0.5 mm/min and 5 different gauge lengths (5, 10,20, 40, and 80 mm), 20 samples at each gauge lengthwere tested. The diameters of the PAN-fibers weremeasured prior to testing using a laser diffractiontechnique.

In the present work, the longitudinal CTE of thecomposites was also determined, since we were par-ticularly interested in the lowest coefficient of thermalexpansion and the longitudinal CTE is usually lowerthan the transverse CTE. The coefficient of thermalexpansion in the longitudinal direction was measuredwith a Thermal Mechanical Analyzer 2940, fabricated by TA Instruments, which is capable of heating orcooling samples while applying a predetermined force.A quartz probe, placed in contact with the sample, isused to determine linear or volumetric changes, atany selected temperature. Samples of material havinga maximum height of 25 mm and maximum diameterof 10 mm were cut with parallel faces, and the ther-mal expansion coefficients were measured from 30Cto 120C.

Fiber orientation measurements were made on pol-ished cross sections of the PAN-composites cut parallelto the flow at the center of the composite tensile bars,using the method described by Bay and Tucker (7). Todetermine the fiber length, the matrix of some of thePP-PAN tensile bars was burned off in an oven, thefibers spread on a glass slide and fiber lengths meas-ured under an optical microscope using an ImageAnalysis system. The fiber volume fractions for eachsample were determined by density measurements.

PP-VGCF Composites

Because of the small fiber size in the PP-VGCF com-posites, traditional methods to determine fiber proper-ties cannot be applied. Earlier work on a different typeof longer VGCFs (diameters and lengths in the mi-crometer and centimeter range, respectively) has re-vealed the large dependence of the properties on fibershape, and the occurrence of a large number of fibershapes (8, 9). Overall, the properties showed a quali-

tative dependence similar to that observed with con-ventional carbon fibers. The determination of the com-posites strength, stiffness, CTE, and fiber volumefraction was done using the same methods as withthe PP-PAN composites. However, because of theirfiber size, conventional measurement of the fiber ori-entation state in the PP-VGCF composites was notpossible. The fiber lengths of the VGCFs were deter-mined by oxidizing the composite in air at 400Covernight. The SEM samples were prepared by firstsqueezing a glue dot onto an aluminum sample stub

F. W. J. Van Hattum, C. A. Bernardo, J. C. Finegan, G. G. Tibbetts, R. L. Alig, and M. L. Lake

684 POLYMER COMPOSITES, OCTOBER 1999, Vol. 20, No. 5

Table 1. Operational ConditionsUsed in the Injection Molding.

PPPAN PPVGCF

Injection pressure (bar) 25 40Hold pressure (bar) 20 30Hold time (s) 6 4Cool time (s) 20 20Mold temperature (C) 60 60Melt temperature (C) 230 250

Note: the pressures are those of the hydraulic system.


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and then scraping it off with forceps to leave an areaof adhesive so thin that fibers could not submerge init. Next, a small wad of fibers held in a pair of forcepswas scraped over the adhesive area to disperse andspread the individual fibers. Gold was then sputter-deposited on the stub to make it conductive. From mi-crographs of regions where the fibers were well dis-persed, the lengths of at least 50 fibers weremeasured and averaged.

2.3 Micromechanical Modeling

Micromechanical models to predict stiffness and CTEof short fiber composites have already been successfully

applied (4, 5). In addition, the authors recently derivedand successfully applied a model to predict strength in

short fiber composites (6), following the same generalmethod used for stiffness- and CTE-prediction.

As a first step, all these models derive expressionsfor the properties of a unidirectionally aligned shortfiber composite as a function of fiber length. The ex-pressions for the unidirectional stiffness are directlyderived from the matrix and fiber stiffness by the well-known Halpin-Tsai equations (10). The expressionsfor the unidirectional CTEs are given by Schaperysequations as adapted by Halpin (11). These expres-sions contain not only the underlying CTEs of matrixand fiber, but also the above unidirectional compositestiffness, thus slightly complicating the overall expres-

sions. The unidirectional strengths are given by rela-tions based on the Kelly-Tyson theory (12). This theory

Thermomechanical Properties of Carbon Fiber-PP Composites


Fig. 1. Normalized experimentalmodulus at 1% strain.

Fig. 2. Normalized experimentalstrength.


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assumes that an applied load is transferred to thefibers by means of a shear force at the fiber-matrixinterface. A critical minimum fiber length is thus need-ed to build up sufficient strength to fracture the fiber.If the fibers in a composite are shorter than this criti-cal length, fiber fracture will not occur. Hence, thestrength of such a unidirectional composite is not de-termined by the fiber strength, as one would expect,but by the strength of the interface between fiber andmatrix, the so-called interfacial shear strength.

By substituting the average fiber length found inthe composite in these expressions or, alternatively,integrating the expressions over the whole range offiber lengths, the unidirectional composite propertiescan be obtained.

The properties of the composite are then taken asan average of these unidirectional properties over alldirections, weighted by the orientation distributionfunction, which describes the fiber orientation. This isoften referred to as orientation averaging. The unidi-

rectional properties are used to construct the stiff-ness, thermal expansion and strength tensors. Thesetensors can be orientation averaged following the

method described by Advani and Tucker (13), to de-rive the final composite properties (5, 6).

In the present work, micromechanical models areused to predict the properties of the PP-PAN compos-ites. The applicability of the models is verified by com-parison with experimental data. Because of the diffi-culty of determining the orientation of the VGCF incomposites, the above models were not applied toVGCFs at this stage. However, when shown to bevalid, the micromechanical models can be used to de-termine the VGCF properties, if their orientation inthe composites is known.

3. RESULTS AND DISCUSSION

3.1 Material Characterization

The properties of the composites are given in Table2. It should be noted that the properties for the unre-inforced polymer (fiber fraction 0%) differ for the PP-PAN and PP-VGCF composites. This is due to the dif-

ferences in processing conditions used, as explainedin Section 2.1 (see also Table 1). For ease of compari-son, Figs. 1 through 3 depict, therefore, the normal-



Fig. 3. Normalized experimentalCTE.

Table 2. Composites Test Results.

Fiber Fraction 0% 5% 10% 15%

Fiber volume PPPAN 4.5 9.6 14.7fraction (%) PPVGCF 4.7 9.7 15.6

Fiber length (m) PPPAN 161.7 148.3 161.3PPVGCF 3.8 3.8 3.8

Tensile modulus at PPPAN 1343 2821 4294 4591

1% strain (MPa) PPVGCF 1646 2584 3466 3685Tensile Strength PPPAN 30.4 36.6 42.6 49.7(MPa) PPVGCF 32.9 40.3 46.5 48.5

CTE (106/C) PPPAN 114.0 31.5 20.8 16.9PPVGCF 111.3 57.4 39.8 26.2


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ized (that is, divided by the matrix value) stiffness,strength and CTE, respectively. In these Figures, thenormalized properties of the PC-VGCF composites (3)are also shown. From the Tableand the Figuresit iseasily observed that the VGCF-composites showstrength properties comparable to those of the PP-PANcomposites. Stiffness values are lower, whereas theCTEs are slightly higher. Other work (14), however,has shown that 16% (v/v) PP-VGCF composites canbe 300% stiffer than the matrix. This increase com-pared with the present work can possibly be attrib-uted to better fiber alignment and less fiber lengthdegradation. The dependence of the properties onfiber volume fraction is similar for both PP-VGCF andPP-PAN composites. From the Figuresit is clear thatthe strength and CTE (and possibly stiffness) of the

VGCF-composites are competitive with properties ofcomposites based on PAN-fibers.

3.2 Modeling

The experimental data were used to model the PP-PAN composite properties. The data used for modelingthe properties are summarized in Table 3. A typicalfiber orientation distribution through the short dimen-sion transverse to the injection direction is shown inFig. 4, for the PP-PAN composite with 15% fiber vol-ume fraction. In this Figure, a11 and a22 represent the well-known 2nd order orientation tensor components(13), giving the relative orientation of the fibers aroundthe 1-axis (flow-direction) and 2-axis (cross-flow direc-

tion), respectively. The skin-core structure, reportedfor many injection molded short fiber composites, can

be readily observed in this Figure.From the properties of fiber and matrix shown in

Table 2, stiffness, strength, and CTE for the PP-PANspecimens in the flow direction were calculatedusing the methods described in Refs 13, 6, and 5,respectively.

The predictions, together with the experimental val-ues, are given in Table 4. As already shown in Ref. 6,the predicted values for strength and stiffness for thePP-PAN composites are within 10% of the experimen-tal values. The CTE-predictions lead to greater dis-crepancies. These errors, however, are consistent withresults found in work of other authors (5). The depen-dence of all composite properties on fiber volume frac-tion, as observed experimentally, is well described bythe models.

Because of the problems related to conventional fiberorientation measurements, the current modeling can-not be applied to the submicron VGCF-composites.Alternative methods of measuring fiber orientation, likeX-ray scattering, could be considered. In that case, themicromechanical models can be inversely applied to thePP-VGCF composites to derive the apparent VGCFproperties. However, the models are based on variousassumptions, the most critical one being that the fibersare cylindrical in shape. This is a very restrictive hy-pothesis, as it is well known that because of the pro-duction method, submicron VGCFs are not cylindrical(15). Hence, strict conclusions could not be drawn withthis method. The properties derived would only corre-

spond to those of a model fiber. If used as reinforce-ment in a polypropylene matrix, this ideal fiber, as-

Thermomechanical Properties of Carbon Fiber-PP Composites


Table 3. Material Parameters Used for the Evaluation of the Model.

Material Parameters PPPAN PPPAN PPPAN PPPAN PAN0% 5% 10% 15%

Matrix modulus (GPa) 1.343Matrix Poisson ratio 0.40Matrix yield stress (MPa) 30.4Matrix stress at fiber 20.8failure strain1 (MPa)

Matrix CTE (106/C) 114.0Fiber modulus (GPa) 218Fiber diameter (m) 7.2Fiber Poisson ratio 0.26Fiber tensile strength2 4940.1(MPa) l0.1554

Fiber CTE (106/C) 0.1Critical fiber length (m) 1012Fiber volume fraction (%) 4.5 9.6 14.7Average fiber length (m) 161.7 148.3 161.3

1Calculated from fiber failure strain and matrix modulus.2Fiber length l in mm.

Table 4. Experimental and Predicted Values of the Thermomechanical Properties.

PPPAN 5% PPPAN 10% PPPAN 15%

Strength (MPa) Experimental 36.6 42.6 49.7Predicted 35.0 45.2 51.4

Modulus (MPa) Experimental 2821 4294 4591Predicted 2787 4140 5043

CTE (106/C) Experimental 31.5 20.8 16.9Predicted 51.3 32.3 24.1


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sumed cylindrical and characterized by its length anddiameter, would lead to composite properties matchingthose of the PP-VGCF properties. Nevertheless, thismethod would still allow direct comparison of the VGCFwith conventional fibers. In fact, in this way one canreadily determine what properties a conventional fibershould have to obtain composite properties equivalentto those of the PP-VGCF composites.

Furthermore, in previous work (6), the authors haveshown that in the PP-PAN composites, the interfacialshear strength rather than fiber strength dominatescomposite strength, owing to the short fiber lengths(see Section 2.3). In the PP-VGCF composites thisshould also be the case, considering the average fiber

length found experimentally (see Table 2). Therefore,using inverse modeling, an actual fiber strength valuefor the VGCFs cannot be derived. Instead, the interfa-cial shear strength between fiber and matrix can beobtained. In this way, the apparent properties of theVGCFs can be determined, allowing comparison withconventional fibers.

CONCLUSION

In the present work it has been shown that VGCF-thermoplastic composites can be produced, withstrength and CTE that are competitive with those ofconventional PAN-fiber composites. Micromechani-cal models have been successfully applied to predict

the strength and stiffness of PP-PAN composites.Modeling the CTE in the same way yielded slightlyworse results. The models can be used to infer the ap-parent VGCF-properties from their thermoplasticcomposites, if the exact orientation of the fibers in thecomposites is known.

ACKNOWLEDGMENTS

This work was supported by the European Eco-nomic Community, through the Human Capital and

Mobility Programme, under Grant Number CHCRX-CT940457. F. W. J. van Hattum acknowledges the per-sonal grant received under the same contract. J. C.Finegan would like to thank the NIST ATP for supportunder co-operative agreement number 70NANB5H1173,Vapor-Grown Carbon Fiber Composites for Automo-tive Applications. The support of Tenax Fibers, Ger-many, which kindly supplied the PAN-fibers, is alsoacknowledged.

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Fig. 4. Typical fiber orientationdistribution through-thickness, forPP-PAN 15%.

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A Study of the Thermomechanical Properties of Carbon Fiber–Polypropylene Composites