Mechanical Properties of Injection Molded Long FiberPolypropylene Composites, Part 1: Tensile and FlexuralProperties

K. Senthil Kumar,1 Anup K. Ghosh,1 Naresh Bhatnagar2

1Centre for Polymer Science and Engineering, Indian Institute of Technology, New Delhi, India

2Department of Mechanical Engineering, Indian Institute of Technology, New Delhi, India

Innovative polymers and composites are broadening therange of applications and commercial production ofthermoplastics. Long fiber-reinforced thermoplasticshave received much attention due to their processabilityby conventional technologies. This study describes thedevelopment of long fiber reinforced polypropylene(LFPP) composites and the effect of fiber length andcompatibilizer content on their mechanical properties.LFPP pellets of different sizes were prepared by extru-sion process using a specially designed radial impreg-nation die and these pellets were injection molded todevelop LFPP composites. Maleic-anhydride graftedpolypropylene (MA-g-PP) was chosen as a compatibi-lizer and its content was optimized by determining theinterfacial properties through fiber pullout test. Criticalfiber length was calculated using interfacial shearstrength. Fiber length distributions were analyzed usingprofile projector and image analyzer software system.Fiber aspect ratio of more than 100 was achieved afterinjection molding. The results of the tensile and flexuralproperties of injection molded long glass fiber rein-forced polypropylene with a glass fiber volume fractionof 0.18 are presented. It was found that the differences inpellet sizes improve the mechanical properties by 3–8%.Efforts are made to theoretically predict the tensilestrength and modulus using the Kelly-Tyson and Halpin-Tsai model, respectively. POLYM. COMPOS., 28:259–266,2007. © 2007 Society of Plastics Engineers

INTRODUCTION

In recent years, there has been a rapid growth in thedevelopment of long glass fiber reinforced thermoplasticcomposites and in their end use applications, especially inautomotive applications. Their high end-use applicationsassociated with lower manufacturing costs (utilizing inex-

pensive processing methods such as extrusion and injectionmolding) has spurred more research activities on thermo-plastic matrix composites. Composites with thermoplasticsas matrix offer property advantages such as enhancedtoughness, unlimited shelf-life, recyclability, etc. Long fiberreinforced thermoplastic pellets, prepared by wire coating,cross head extrusion, or pultrusion process has recentlyreceived much attention [1, 2]. In this process, the fiberlength is equal to the pellet size, and the aspect ratio isgenerally maintained over 100. Since these pellets are fedinto the injection molding machine for developing long fiberreinforced composites, pellets can also be referred as initialfeedstock fiber.

A significant amount of research work is being reportedthese days on injection molding of long fiber reinforcedthermoplastic pellets to make long fiber composites [3–8].To improve the adhesion between glass fiber and thermo-plastic resin, compatibilizers like maleic anhydride graftedpolypropylene (PP), acrylic acid grafted PP, dendritic poly-mers, and phenol grafted PP are generally used in smallproportions [9]. Reliable control of the degree of adhesionbetween fiber and matrix is required for using the compositeas a structural material. The interactions at the interfaceregion in semicrystalline thermoplastic composites dependon a number of factors such as matrix morphology, fibersurface condition, presence of residual stresses, and moduliiof the fiber and matrix. A majority of these characteristicsare affected by processing conditions including moldingtemperature, cooling rate, holding time/temperature, andannealing conditions as reported in the literature [10, 11].The interfacial bond strength of glass fiber thermoplasticcomposite can be determined by single fiber pull out test onsingle filament thermoplastic composite [12–15]. Many au-thors [16–18] reported on the determination of the fiberlength after injection molding by image analysis and opticalmicroscopy on fiber samples removed from the molded barsafter high temperature ashing. Most significant fiber break-

Correspondence to: Anup K. Ghosh; e-mail: anupkghosh@gmail.comDOI 10.1002/pc.20298Published online in Wiley InterScience (www.interscience.wiley.com).© 2007 Society of Plastics Engineers

POLYMER COMPOSITES—2007

age usually occurs in the initial stages of wet-out anddispersion of the reinforcement as reported by Erwin [19]and Franzen et al. [20]

The present work aims to develop long glass fiber rein-forced PP composites utilizing the fast and inexpensiveprocessing methods such as extrusion and injection mold-ing. First, the long fiber reinforced polypropylene (LFPP)pellets were prepared in a single screw extruder using radialimpregnation die. These impregnated strands were pellet-ized for different sizes and injection molded with optimumprocess parameters. Fiber volume fraction in the impreg-nated strand and in the injection molded long glass fiberreinforced PP was maintained as 0.18. An attempt is madeto study the effect of fiber length on the mechanical prop-erties of the injection molded long fiber PP composites.Although, the final fiber length after injection molding de-cides the mechanical properties of the composite, correla-tions are done in terms of initial feedstock fiber length. Thisis to find out the optimum pellet size that is to be consideredas feed for injection molding. The effect of fiber lengths onthe tensile and flexural properties was studied and comparedwith the theoretical values predicted by the Kelly-Tyson andHalpin-Tsai models [21, 22].

EXPERIMENTAL PROCEDURES

Materials

The continuous glass fiber rovings used were of gradeR099688 with a linear density of 2,400 tex, obtained fromSaint Gobain, India. The nominal fiber diameter was 12 �m.The polypropylene (PP) used was Repol H350FG suppliedby Reliance Industries, India, with a melt flow index of 35.The level of fiber matrix interaction in this system wasincreased using maleic-anhydride grafted polypropylene(MA-g-PP) as compatibilizer obtained from Vin Industries,India. The amount of grafting in the compatibilizer was0.88% and the MFI was 56.

Processing

A Klockner Windsor single screw extruder with screwdiameter of 30 mm and an effective cylinder length of 63cm was used. The set point temperatures were 170–230°C.Glass fiber rovings were impregnated with PP melt at ascrew speed of 10 rpm using radial impregnation die con-nected to the extruder. Schematic diagram of the fabricationof long glass fiber reinforced PP pellets is shown in Fig. 1.

Radial Impregnation Die

The process in the present study is based on the principleof impregnation [23]. The die has a through and throughopening for the fiber to feed in and out (Fig. 1). The fiber ispulled out in the downward direction and as a result shear

stress is generated between the upward resin movement anddownward fiber movement. This stress impregnates thereinforcement with the matrix.

The main purpose of the melt impregnation is to encap-sulate every single fiber by molten polymer through thefiber roving. The radial die is designed to provide increasedresidence time for the fibers in the melt by allowing it topass through an ‘S’ shaped path in the die. This createstension in the fiber, by means of passing it through rollers,and disperses the fibers providing good wettability. Theglass fiber roving is allowed to pass through the pins, whichhave three apertures for the melt to impinge on glass fiberroving. The melt impinges on the fiber roving radially.

Continuous strands were collected from the exit of thedie section and the strand was chopped into pellets ofdifferent lengths (namely 3, 6, 9, and 12 mm) for injectionmolding. These pellets have fiber length equal to the pelletsize.

Injection Molding

The chopped pellets were then injection molded usingL&T Demag (PFY 40 LNC 4P) injection molding machinewith the process parameters listed in Table 1. A generalpurpose screw with a compression ratio of 4:1 was used forprocessing. The nozzle used was 3 mm in diameter and themold was equipped with a rectangular runner and a fan typegate. Long fiber reinforced composites, with and withoutcompatibilizer, were prepared. To have the required inter-action between the glass fiber and the PP matrix, MA-g-PPcompatibilizer was added in different weight percentagesand its contents were optimized using the data from fiber

FIG. 1. Schematic diagram of developing long fiber pellets.

TABLE 1. Injection molding parameters for LFRP

Injectionpressure(MPa)

Holdingpressure(MPa)

Backpressure(MPa)

Screwspeed(rpm)

Temperatureprofile (°C)

88 88 2.0 30–35 190–230

260 POLYMER COMPOSITES—2007 DOI 10.1002/pc

pullout test and longitudinal tensile strength test. Test spec-imens for the evaluation of mechanical properties wereprepared. Glass fiber impregnated with PP melt was pre-pared with a compatibilizer content of 2, 4, 6, 8, and 10wt%.

Fiber Pullout Test

Fiber pull out test was performed to optimize the contentof compatibilizer in glass fiber reinforced PP composites.Single glass fiber impregnated strands of 120 � 5 mm2

dimension were considered for this test. The sample consistsof a fiber, which is partly embedded in the matrix. The freefiber end (say 60 mm) was pulled out of the matrix atconstant crosshead speed of 1 mm/min in a Universal test-ing machine.

Evaluation of Mechanical Properties

The dimensions of dumb-bell shaped tensile test pieceswere in accordance with the ASTM D638 Type I specimen.The test was carried out at a crosshead speed of 5 mm/minand for an extensometer gauge length of 50 mm. Flexuralproperties were measured in accordance with the proceduresin ASTM D790 at a crosshead speed of 25 mm/min and fora span width of 50 mm.

Determination of Fiber Length Distribution

There are number of methods available for determiningfiber length distribution (FLD) [16–18], all of which requiresuccessful isolation of the fibers from the composite mate-rial. In the present work, the FLD was studied by burningthe matrix at 600°C in a muffle furnace for a period of 6 h.Fiber content after molding was determined. For determi-nation of FLD, the extracted fibers were separated usingdetergent solution to reduce the surface tension. Lengths ofatleast 500 fibers were measured using profile projector andimage analyzer software system.

RESULTS AND DISCUSSION

Optimization of MA-g-PP Content

The result of fiber pull out test is shown in Fig. 2. Atdebonding, the load (Pm) shows a sudden decrease afterwhich the fiber is pulled out at a gradually decreasing loador in a stick-slip mode. The overall interfacial shear strength(�) is calculated by

� �Pm

�dfL(1)

where df is the fiber diameter and L is the embedded fiberlength.

The length of the fiber that allows the ultimate strengthof the fiber to be reached is called the critical fiber length.Critical fiber length (lc) is calculated using the equation,

lc ��fudf

2�(2)

where, �fu is the ultimate fiber stress.Figure 2 shows the effect of change in the interfacial

shear strength (�) and the longitudinal tensile strength due tothe addition of different wt% of compatibilizer. With in-crease in the compatibilizer content, the interfacial shearstrength increases upto 4 wt%. Further addition of compati-bilizer showed no influence on the interfacial shear strength.On addition of compatibilizer, the maleic anhydride groupsopen up the ring and forms an active bonding with thehydroxyl groups present in the glass fiber surface. The otherend of the compatibilizer interacts with PP matrix. Excessaddition of compatibilizer may not have any significanteffect on the adhesion of glass fibers on PP melt. This isbecause the interface gets saturated with a certain quantityof compatibilizer, as observed in the case of 2 wt% MA-g-PP addition in the present study. Further addition ofcompatibilizer may not have affected the interfacial inter-action between the glass fibers and the PP matrix. Longitu-dinal tensile strength of the pultruded strands showed max-imum value at 2 wt% addition of compatibilizer and thevalue decreased gradually on further addition. This increaseis attributed to the increased adhesion between the glassfiber and the PP matrix. However, on further addition ofcompatibilizer, the content of low molecular weight com-ponent increases in the matrix, which makes the longitudi-nal tensile properties in the composite to decrease.

Figure 2 also shows the effect of change in the criticalfiber length, lc with increase in the compatibilizer content inPP matrix. Since interfacial shear strength is indirectlyproportional to critical fiber length, a lower value for lc of1.60 and 1.55 mm were obtained at 2 and 4 wt% MA-g-PPcontent, respectively. Figure 3 shows the scanning electron

FIG. 2. Effect of MA-g-PP content on interfacial shear strength (�),longitudinal tensile strength, and critical fiber length (lc).

DOI 10.1002/pc POLYMER COMPOSITES—2007 261

microscopy pictures of LFPP impregnated strands with andwithout compatibilizer content to prove the fiber resin ad-hesion. It is clear that due to the addition of 2 wt% MA-g-PP, the adhesion between glass fiber and PP resin hasincreased (Fig. 3b), whereas this kind of fiber resin adhesionis not seen in Fig. 3a. Based on the data on the critical fiberlength and the longitudinal tensile strength, the 2 wt%MA-g-PP content was found to be the optimum concentra-tion of compatibilizer which became the basis of the workthereafter.

Assessment of Fiber Length Distributions

Injection molded long fiber-reinforced PP composites ofdifferent feedstock fiber lengths of 3, 6, 9, and 12 mm wereconsidered for the analysis. The measurements of FLDsrelate to the observations from specimens taken from theexact centre of the plaque moldings. In injection moldedparts, the number average fiber length (ln) and the weightaverage fiber length (lw) were obtained using the relations,

ln ��nili

�li(3)

lw ��nili

2

�nili(4)

where li is the length of the ith fiber in the sample and ni isthe sample frequency with the length increment range li�1

� li. The number average fiber length is always the smallervalue and is strongly influenced by the amount of fibers andfragments present, while the weight average is influenced bythe fraction of long fibers present. Weight average value ismore meaningful for mechanical behavior prediction. Inother words, mechanical properties are strongly influencedby the volume of fibers of a given length than by the numberof fibers.

Figure 4 shows the photograph of glass fibers (re-moved from the burn out test carried at 500°C) takenfrom a profile projector (magnification of 30�) of 6 mmfeedstock fiber length LFPP composite. The image was

analyzed with Image Pro Plus software and the glass fiberlengths were measured using standardized calibrationprocedure. Figure 5 shows the FLD of LFPP compositewith different feedstock fiber lengths of 3, 6, 9, and 12mm. LFPP composites show a normal distributionskewed towards the longer fiber length. The numberaverage and weight average fiber lengths for all the fourcomposites are tabulated in Table 2.

It has been observed that the average fiber length afterinjection molding increases with increase in the feedstockfiber length upto 9 mm. The weight average fiber length for9 mm LFPP composite and 12 mm LFPP composite werefound to be 1.40 and 1.32 mm, respectively. The mechanicalproperties obtained for LFPP composites with feedstockfiber lengths of 9 and 12 mm are almost same. Since theapplied shear and the amount of fiber breakage due tofiber-fiber, fiber-matrix, and fiber-machinery surface aresame for all the pellets, the LFPP composite with 9 mmfeedstock fiber length may be considered as the optimumpellet to achieve maximum mechanical properties whenprocessed in standard injection molding machine. Furtherincrease in the feedstock fiber length did not increase theaverage fiber length in the final composite.

Macromechanical Analysis

The results of the tensile strength and tensile modulusas a function of initial feedstock fiber length for LFPPcomposites prepared with and without compatibilizer areshown in the Figs. 6 and 7, respectively. With increase infiber length, there is no significant increase in tensilestrength in the LFPP composite prepared without com-patibilizer. But for the LFPP with compatibilizer, thetensile strength increases by almost 23% on an average.For LFPP composite with compatibilizer, the tensilestrength increases by 3% approximately with increase inthe feedstock fiber length.

FIG. 3. SEM micrographs of impregnated strands; (a) without compati-bilizer; (b) with 2 wt% compatibilizer (magnification �1400).

FIG. 4. Photograph of glass fibers separated from 6 mm LFPP pellets.

262 POLYMER COMPOSITES—2007 DOI 10.1002/pc

Similarly, the result on tensile modulus reveals that withincrease in the feedstock fiber length, the modulus valuesincrease by an average of 10%. Approximately 5% increasein modulus was observed for the compatibilized LFPP com-pared with that in uncompatibilized composites. Tensilemodulus versus aspect ratio of resulting fibers in injectionmolded LFPP composites is plotted in Fig. 8. With 10%increase in the aspect ratio of fibers, the modulus valueincreases by 8%. The increase is 6% steep in the case ofcompatibilized composite.

Theoretical Prediction

Although, the amount of fiber content in a composite isgenerally specified in terms of weight fractions, for macro-mechanical analysis of properties of composite, fiber vol-

ume fraction is considered. This is due to the fact that thestructure–property relationships are linear in volume frac-tion. The fiber volume fraction (Vf) is calculated from thefiber weight fraction (Wf) using the equation,

Vf �

Wf

�f

Wf

�m�

Wm

�m

(5)

where, �f is the fiber density (2.54 g/cc), �m is the matrix (PP)density (0.85 g/cc) and Wm is the weight fraction of the matrix.

Based on the Kelly–Tyson model [22], the tensilestrength of a polymer composite reinforced with discontin-uous fibers is given as

FIG. 5. Fiber length distribution of different feedstock fiber length; (a) 3 mm; (b) 6 mm; (c) 9 mm; (d) 12 mm.

TABLE 2. Average fiber length of different LFPP composites

LFPP composites ofdifferent feedstockfiber length (mm)

Amount of fibersbetween 1–2 mm

(%)Amount of fibers

2 mm (%)

No. avg. fiberlength [ln]

(mm)

Wt. avg. fiberlength [lw]

(mm)Aspect ratio

of fibers

3 36.4 2.0 0.906 1.038 86.56 41.8 4.6 0.950 1.163 96.99 72.8 8.0 1.236 1.410 117.5

12 69.8 7.2 1.167 1.327 110.5

DOI 10.1002/pc POLYMER COMPOSITES—2007 263

�uc � �0� �li�lc

�lii

df� �

lj�lc

�ff�1 lc

2lj�� � Em�f�1 f� (6)

where � is the interfacial shear strength (measured by thefiber pull out test), df is the fiber diameter, lc is the criticalfiber length, li and lj are the actual fiber lengths below andabove lc, respectively (determined by the FLDs), Vi and Vj

are the corresponding volume fractions, �f is the fiberstrength, Em�f �m, the matrix strength at fiber failurestrain, Vf is the total fiber volume fraction, and �0 the fiberorientation factor. The fiber orientation factor is consideredas 0.375 for random in-plane orientation of the fibers [17].

Kelly–Tyson model has been used as a method to inves-tigate whether the measured changes in fiber length andorientation at constant fiber content are sufficient to explainthe trends observed in the strength of composites as shownin Fig. 6. Figure 9 shows the comparison between themeasured and modeled data of tensile strength with refer-

ence to the initial feedstock fiber length for LFPP compositeprepared with and without compatibilizer. The deviation ofthe experimental value from the theoretical value is �10%.The model shows that the nature of the matrix is not theonly factor contributing to the ultimate strength. The influ-ence of the fiber–matrix adhesion, type of the fiber, the fiberorientation, and the differences in the FLDs are responsiblefor ultimate strength of a composite.

In fiber-reinforced plastic, the applied load is transferredfrom the matrix to the fiber through interfacial shear stress,with a maximum value at the fiber ends and zero at thecenter. Thus, the tensile stress in the fiber is zero at the endsand maximum at the center. However, the maximum tensilestress along the length of the fiber does not exceed thetensile stress in the matrix. To accommodate this, Halpin-Tsai equation [23] was used to model the stiffness for thesecomposites, since it takes care of the aspect ratio of fibers.The equations are given as,

Ei � Em�1 � �i�if

1 �if� (7)

FIG. 6. Effect of feedstock fiber length on the tensile strength of com-posite.

FIG. 7. Effect of feedstock fiber length on the tensile modulus of com-posite.

FIG. 8. Effect of aspect ratio of fibers on the tensile modulus.

FIG. 9. Comparison of measured and modeled data of tensile strengthwith reference to feedstock fiber length.

264 POLYMER COMPOSITES—2007 DOI 10.1002/pc

�i ��Ef/Em� 1

�Ef/Em� � �i(8)

�1 �2lw

df

�2 � 2

where, E1 and E2 are the longitudinal and transverse modu-lii. �1 and �2 are the factors, which take care of the fiberlength corrections in the longitudinal and transverse direc-tions, respectively. The above equation is applicable forunidirectionally reinforced laminate. For random in-planeorientation of the fibers, especially for injection-moldedplaques, fiber orientation factor such as �o was consideredand the composite modulus (Ec) was calculated as

Ec � �0E1 � �1 �0�E2 (9)

The above equations are derived by assuming that thefiber cross-section is circular, is arranged in a square arraymanner, and is uniformly distributed throughout the matrix.

Figure 10 shows the comparison between the modeledand measured values of tensile modulus with reference tothe initial feedstock fiber length, for the LFPP compositewith and without compatibilizer. The deviation of the ex-perimental value from the theoretical value is observed to be�5%. The equation agrees well with the experimental data.

Figures 11 and 12 show the effect of feedstock fiberlength on the flexural strength and flexural modulus, respec-tively. The trend is similar to that of the tensile behavior.With the increase in feedstock fiber length, the increase inflexural strength is not significant, but the modulus in-creases by 8% on an average. The increase in the case ofLFPP composite with 12 mm feedstock fiber length is foundto be marginal when compared with that in LFPP compositewith 9 mm feedstock fiber length. There is not much influ-

ence of the fiber length beyond 9 mm, while processingLFPP composite in a standard injection-molding machine.This is due to the fiber breakage during injection molding.Fiber breakage during injection molding is small, providedthe process is operated under specified processing condi-tions. To prevent fiber breakage, the machine should have ascrew compression ratio of more than 15 and nozzle diam-eter of more than 6 mm [20].

Table 3 shows the trend for coefficient of variation fortensile modulus and flexural modulus. The coefficient ofvariation signifies the repeatability of the experiment andthereby the homogeneous microstructure of the injectionmolded plaques. The coefficient of variation is defined asthe ratio of standard deviation and average value and isexpressed in percentage. The value of coefficient of varia-tion is found to be �5%, which indicates good repeatability.

FIG. 10. Comparison of measured and modeled data of tensile moduluswith reference to feedstock fiber length.

FIG. 11. Effect of feedstock fiber length on flexural strength of LFPPcomposites.

FIG. 12. Effect of feedstock fiber length on flexural modulus of LFPPcomposites.

DOI 10.1002/pc POLYMER COMPOSITES—2007 265

CONCLUSION

Continuous impregnation of glass fiber roving was car-ried out with PP melt in a single screw extruder with aspecially designed radial impregnation die. Fiber pullouttest was carried out to optimize the content of compatibi-lizer in the PP matrix. MA-g-PP (2 wt%) was optimized toget better interfacial properties between the glass fiber andthe PP resin. Long glass fiber reinforced PP pellets wereprepared at different fiber lengths and were injectionmolded to form long glass fiber reinforced PP compositeswith and without compatibilizer. It can be concluded thatthe FLD is optimum when the feedstock fiber length in-creases up to 9 mm. Because of the fiber attrition duringprocessing in injection molding, the FLD in the moldingremains almost similar for 12 mm feedstock fiber length asthat of the 9 mm feedstock fiber length. Differences in initialfeedstock fiber length improve the tensile and flexural prop-erties of the composites by 3–8%. Addition of optimumcompatibilizer content improves the properties to the extentof 6–13%. Predictions of composite strength using theKelly-Tyson model and tensile modulus using the Halpin-Tsai equations correlate well with the experimental dataobtained over the range of fiber lengths. The study con-

cludes that in a standard injection-molding machine, 9 mmlong feedstock fiber length provides the improved perfor-mance in terms of mechanical properties.

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TABLE 3. Coefficient of variation (%) of the tensile modulus andflexural modulus on LFPP composites

Injection moldedsamples

Coefficient of variation (%)

Tensilemodulus

Flexuralmodulus

PP 3.8 12.4LFPP compositea

Without compatibilizer3 mm 5.1 5.16 mm 2.0 3.19 mm 3.8 2.012 mm 3.0 0.8

With compatibilizer3 mm 3.7 14.46 mm 4.1 2.19 mm 3.2 1.512 mm 6.2 7.0

aValues in mm indicate feedstock fiber length.

266 POLYMER COMPOSITES—2007 DOI 10.1002/pc