9
Mechanical Properties of Injection Molded Long Fiber Polypropylene Composites, Part 2: Impact and Fracture Toughness K. Senthil Kumar, Naresh Bhatnagar, Anup K. Ghosh Department of Mechanical Engineering, Centre for Polymer Science and Engineering, Indian Institute of Technology, New Delhi, India This study describes the effect of fiber length and compatibilizer content on notched izod impact and fracture toughness properties. Long fiber polypropyl- ene (LFPP) pellets of different sizes were prepared by extrusion process using a new radial impregnation die, and subsequently, pellets were injection molded as described in previous publication [1]. The content of glass fiber reinforcement was maintained same for all compositions. Maleic-anhydride grafted polypropylene (MA-g-PP) was chosen as a compatibilizer to increase the adhesion between glass fiber and PP matrix and its content was maintained at 2 wt%. Notched izod impact property was studied for LFPP composites prepared with and without compatibilizer for different pellet sizes. Failure mechanism due to sudden impact was analyzed with scanning electron micrographs and was correlated with impact property of LFPP composites. Fracture and failure behavior of injection molded LFPP composite were studied and relationship between frac- ture toughness and microstructure of LFPP composite was analyzed. The microstructure of the composites was characterized by the dimensionless reinforcing effectiveness parameter, which accounts for the influ- ence of fiber layer structure, fiber alignment, fiber vol- ume fraction, fiber length distribution, and aspect ratio. Matrix stress condition factor and energy absorption ratio were determined for LFPP composites prepared with and without compatibilizer. Failure mechanism of both the matrix and fiber, revealed with SEM images, were discussed. POLYM. COMPOS., 29:525–533, 2008. ª 2008 Society of Plastics Engineers INTRODUCTION The Part 1 of the present work [1] dealt with the de- velopment of long fiber pellets and long fiber composites, optimization of compatibilizer content, and determination of fiber length distribution in the injection molded sam- ples. LFPP pellets have the fiber length equal to the pellet size (the aspect ratio is generally maintained over 100). Since these pellets are fed into the injection molding machine for developing long fiber-reinforced composites, these pellets can also be referred as initial feedstock fiber. Although the final fiber length after injection molding decides the mechanical properties of composite, correla- tions here are done with initial feedstock fiber length. This is to find out the optimum pellet size, to be consid- ered for injection molding. Long fiber-reinforced thermoplastic composites used in structural applications are subjected to impact and/or other loading conditions requiring high fracture toughness. Therefore, it is important to assess their material perform- ance well in advance. This paper is emphasized on the analysis of impact and fracture toughness of long fiber-re- inforced polypropylene composites prepared with and without compatibilizer. The effect of fiber length on notched impact properties of long fiber thermoplastic composite materials at low temperature was investigated by Marvin [2]. A detailed analysis of impact failure mechanism is carried out by scanning electron microscopy (SEM) images to identify the factors affecting the impact strength of composite [3, 4]. Thomason et al. [5] dis- cussed the influence of fiber length and concentration on the impact property of injection molded long fiber poly- propylene composites. The author also correlated the ex- perimental property with fiber pullout model and fiber strain energy model. Since glass fiber reinforced thermo- plastics are principally related to stiffness and toughness, it becomes necessary to evaluate the fracture mechanics data, which provides material constants. Fracture tough- ness variations of injection molded long fiber-reinforced thermoplastics were studied [6, 7]. Since the mode of test- ing for both the fracture toughness and tensile strength is same, a relationship between them was attempted. A lin- ear relation between them indicates that the material vari- ables that determine the tensile strength also determine the fracture toughness [8, 9]. Fiber properties and matrix properties are the two major contributions for failure of Correspondence to: Anup K. Ghosh; e-mail: [email protected] DOI 10.1002/pc.20369 Published online in Wiley InterScience (www.interscience.wiley.com). V V C 2008 Society of Plastics Engineers POLYMERCOMPOSITES—-2008

Mechanical properties of injection molded long fiber polypropylene composites, Part 2: Impact and fracture toughness

Embed Size (px)

Citation preview

Mechanical Properties of Injection Molded LongFiber Polypropylene Composites, Part 2:Impact and Fracture Toughness

K. Senthil Kumar, Naresh Bhatnagar, Anup K. GhoshDepartment of Mechanical Engineering, Centre for Polymer Science and Engineering,Indian Institute of Technology, New Delhi, India

This study describes the effect of fiber length andcompatibilizer content on notched izod impact andfracture toughness properties. Long fiber polypropyl-ene (LFPP) pellets of different sizes were prepared byextrusion process using a new radial impregnation die,and subsequently, pellets were injection molded asdescribed in previous publication [1]. The content ofglass fiber reinforcement was maintained same for allcompositions. Maleic-anhydride grafted polypropylene(MA-g-PP) was chosen as a compatibilizer to increasethe adhesion between glass fiber and PP matrix and itscontent was maintained at 2 wt%. Notched izod impactproperty was studied for LFPP composites preparedwith and without compatibilizer for different pelletsizes. Failure mechanism due to sudden impact wasanalyzed with scanning electron micrographs and wascorrelated with impact property of LFPP composites.Fracture and failure behavior of injection molded LFPPcomposite were studied and relationship between frac-ture toughness and microstructure of LFPP compositewas analyzed. The microstructure of the compositeswas characterized by the dimensionless reinforcingeffectiveness parameter, which accounts for the influ-ence of fiber layer structure, fiber alignment, fiber vol-ume fraction, fiber length distribution, and aspect ratio.Matrix stress condition factor and energy absorptionratio were determined for LFPP composites preparedwith and without compatibilizer. Failure mechanismof both the matrix and fiber, revealed with SEMimages, were discussed. POLYM. COMPOS., 29:525–533, 2008.ª 2008 Society of Plastics Engineers

INTRODUCTION

The Part 1 of the present work [1] dealt with the de-

velopment of long fiber pellets and long fiber composites,

optimization of compatibilizer content, and determination

of fiber length distribution in the injection molded sam-

ples. LFPP pellets have the fiber length equal to the pellet

size (the aspect ratio is generally maintained over 100).

Since these pellets are fed into the injection molding

machine for developing long fiber-reinforced composites,

these pellets can also be referred as initial feedstock fiber.

Although the final fiber length after injection molding

decides the mechanical properties of composite, correla-

tions here are done with initial feedstock fiber length.

This is to find out the optimum pellet size, to be consid-

ered for injection molding.

Long fiber-reinforced thermoplastic composites used in

structural applications are subjected to impact and/or

other loading conditions requiring high fracture toughness.

Therefore, it is important to assess their material perform-

ance well in advance. This paper is emphasized on the

analysis of impact and fracture toughness of long fiber-re-

inforced polypropylene composites prepared with and

without compatibilizer. The effect of fiber length on

notched impact properties of long fiber thermoplastic

composite materials at low temperature was investigated

by Marvin [2]. A detailed analysis of impact failure

mechanism is carried out by scanning electron microscopy

(SEM) images to identify the factors affecting the impact

strength of composite [3, 4]. Thomason et al. [5] dis-

cussed the influence of fiber length and concentration on

the impact property of injection molded long fiber poly-

propylene composites. The author also correlated the ex-

perimental property with fiber pullout model and fiber

strain energy model. Since glass fiber reinforced thermo-

plastics are principally related to stiffness and toughness,

it becomes necessary to evaluate the fracture mechanics

data, which provides material constants. Fracture tough-

ness variations of injection molded long fiber-reinforced

thermoplastics were studied [6, 7]. Since the mode of test-

ing for both the fracture toughness and tensile strength is

same, a relationship between them was attempted. A lin-

ear relation between them indicates that the material vari-

ables that determine the tensile strength also determine

the fracture toughness [8, 9]. Fiber properties and matrix

properties are the two major contributions for failure of

Correspondence to: Anup K. Ghosh; e-mail: [email protected]

DOI 10.1002/pc.20369

Published online in Wiley InterScience (www.interscience.wiley.com).

VVC 2008 Society of Plastics Engineers

POLYMER COMPOSITES—-2008

composites. To find out the failure mechanism of fiber

and matrix in detail, SEM analysis has been done and the

microstructure of composite was correlated with the rein-

forcement efficiency factor (R). The various energy

absorbing mechanism such as fiber debonding and pullout

for fibers and crazing and shear yielding on polypropylene

matrix were observed by SEM images [10–12].

The present work aims at the impact and fracture

toughness analysis of injection molded long fiber-rein-

forced polypropylene composites. Figure 1 shows the

block diagram of the work interested in this part of the

manuscript. The purpose of this study is to examine the

effect of long glass fiber reinforcements on these mechan-

ical properties and to correlate this relationship with

microstructural properties such as fiber length, fiber orien-

tation, and fiber content. The description of the results is

supported by extensive studies of the failure mechanisms

using SEM.

EXPERIMENTAL

The materials and processing of injection molded long

fiber-reinforced polypropylene composites are explained

in Part 1 [1]. Test specimens for the evaluation of impact

properties and fracture toughness were prepared by injec-

tion molding long fiber pellets (Table 1).

Specimen Preparation

Notched izod impact strength was determined by test-

ing injection molded specimens of materials in accord-

ance with the ASTM D 256. Impact testing was carried

out by Izod Impact Testing machine. In this test, a 12.7-

mm wide rectangular bar with a 2.5-mm milled notch was

clamped in a sample vise with the notch facing the direc-

tion of the impact. A pendulum is raised to a fixed height

and then released to break the sample. The notch serves

to concentrate the stress of impact and direct the fracture

to that area. The energy required to break the specimen is

calculated from the difference in pendulum height at the

beginning and end of the flow through swing. Five speci-

mens were tested and their standard deviation values are

presented.

Rectangular strips of dimensions 63.5 � 12.7 � 3

mm3 were injection molded. Each specimen was then

notched to produce a single edge notched test (SENT)

specimen. Edge notches were inserted midway along the

length of the specimens in two steps: first a saw cut was

made, which was then sharpened with a razor blade. The

notches are in transverse direction to the melt flow direc-

tion as shown in the Fig. 2. Fracture toughness measure-

ments, Kc, were carried out on these specimens with a

gauge length of 45 mm, in an Instron Universal Testing

Machine at a crosshead speed of 5 mm/min. All tests

were performed at room temperature.

Calculation of Fracture Toughness, Kc

Griffith introduced an energy approach to crack propa-

gation in which, a flaw is unstable when the strain energy

FIG. 1. Block diagram showing the work aimed in this manuscript.

TABLE 1. Injection molding parameters for LFPP.

Injection

pressure

(MPa)

Holding

pressure

(MPa)

Back

pressure

(MPa)

Screw

speed

(rpm)

Temperature

profile

(8C)

88 88 2.0 30–35 190–230

526 POLYMER COMPOSITES—-2008 DOI 10.1002/pc

change that emerges due to an increment of a crack

growth is greater than the surface tension of the material.

Linear elastic fracture mechanics (LEFM) relates the

energy concept referred to as G, to the stress intensity

factor K. The critical value of the stress intensity factor

Kc at which the crack grows unstable is called as fracture

toughness. Kc can be calculated as

Kc ¼ scyffiffiffia

p(1)

where, sc the apparent maximum stress, a the crack

length, and y is the geometry factor of the testing speci-

men that includes the ratio of the crack length to the

width (w) of the specimen. Harris [13] obtained an

expression for the geometry factor y, in which the single

edge notch test (SENT) specimen ends are clamped and

the distance between the clamped ends is three times the

specimen width (w).

y ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

25pða=wÞ20� 13ða=wÞ � 7ða=wÞ2

s(2)

The y function given by Eq. 2 was used for calculating

the fracture toughness Kc in Eq. 1.

RESULTS AND DISCUSSION

Impact Strength

Figure 3 shows the results of notched izod impact

strength of injection molded LFPP composites with and

without compatibilizer at 238C. It can be seen from the

Fig. 3 that the impact strength increases with respect to

increase in feedstock fiber length up-to 9 mm and

decreases for 12 mm pellets sized LFPP composite. The

trend is same for both LFPP composite with and without

compatibilizer. As mentioned earlier, the average fiber

length after injection molding decides the mechanical

properties of composites. The average fiber length

increases up-to a feedstock fiber length of 9 mm and it

remains almost same for 12 mm pellet size. There is no

increase in average fiber length for 12 mm feedstock fiber

length. Therefore, the mechanical properties of injection

molded LFPP composite for a feedstock fiber length of 12

mm has shown no improvement.

The impact strength of LFPP composite with 40 wt%

fiber increases by �50% when compared with that of the

polypropylene. This is because of the addition of fibers

which restricts the failure behavior of composites. Simi-

larly, for LFPP composite with compatibilizer, the impact

strength increases by 105%. This proves that the interfa-

cial adhesion between glass fiber and polypropylene ma-

trix is improved by addition of optimum content of com-

patibilizer. Among LFPP composite, there is an increase

of 15%, on an average, in impact strength with increase

in feedstock fiber length, whereas for composite with

compatibilizer, the increase is about 11%. Impact strength

of LFPP composite with 12-mm feedstock fiber length

decreases, due to the less retention of final fiber length af-

ter injection molding.

The major energy absorbing mechanisms when a dis-

continuous fiber reinforced composite fractures from an

existing notch are fiber pullout, debonding, and fiber frac-

ture. When a sudden impact load is applied, the matrix

gets deformed and fracture takes place at the crack tip.

The matrix transfers the stress by shear to the fiber

through the fiber–matrix interface. If the stress transfer is

more than the interfacial shear strength, the composite

fails by fiber debonding. Transfer of stress may still be

possible to a debonded fiber through frictional forces

along the interface. The composite may fail by fiber frac-

ture if the fiber stress level exceeds the local fiber

strength. Fibers that have fractured away from the crack

interface will be pulled out of the matrix, which may also

FIG. 3. Effect of feedstock fiber length on impact strength of composites.

FIG. 2. Fracture toughness specimen dimensions (in mm).

DOI 10.1002/pc POLYMER COMPOSITES—-2008 527

involve energy dissipation. Fiber pullout occurs due to the

poor adhesion between fiber and matrix.

Scanning electron micrographs of impact tested sam-

ples for LFPP composite with and without compatibilizer

were given in the Fig. 4. The figure explains that the

LFPP composite fails in a brittle fashion, since the load-

ing rate is very high in this testing. The high and low

value impact strength resulted samples were chosen for

scanning the morphology in order to analyze their failure

behavior. Figure 4 also reveals that the fiber fails due to

debonding, fracture, and pullout behavior. For LFPP com-

posite without compatibilizer (Fig. 4a and b), failure by

fiber pullout behavior is dominant near the notch. This

shows that less energy has been required for the fiber to

pullout due to poor adhesion between glass fiber and

polypropylene matrix. For compatibilized composite fiber,

fracture behavior is observed more. When compatibilizer

is added (Fig. 4c and d), the interfacial shear strength is

high, which could transfer the stress from matrix to fiber

and more energy is absorbed by the fiber before it fails.

This is observed only in the presence of compatibilizer

content. The improved adhesion between glass fiber and

polypropylene matrix due to the presence of compatibil-

izer requires more stress for the fiber to fail, and hence

improved impact strength in case of composite with com-

patibilizer. But, increase in feedstock fiber length does

not have any significant effect on the failure behavior of

composite. This is because there is no significant increase

in final fiber length after injection molding. Therefore, it

is very difficult to find out the change in failure behavior

FIG. 4. Scanning electron micrographs of impact tested samples: (a) A-3, (b) A-9, (c) B-3, and (d) B-9.

TABLE 2. Fracture toughness values of different LFPP composites.

Injection molded samples

Fracture

stress (MPa)

Fracture

toughness

(MPaHm)

Tensile

strength

(Mpa)

PP 14.46 2.11 32.65

LFPP composite without

compatibilizer

3 18.60 2.72 34.16

6 17.80 2.60 34.74

9 17.34 2.53 33.27

12 17.37 2.54 33.72

LFPP composite with

compatibilizer

3 18.34 2.68 42.13

6 18.22 2.66 44.50

9 20.21 2.95 45.45

12 19.14 2.80 45.60

528 POLYMER COMPOSITES—-2008 DOI 10.1002/pc

of LFPP composite unless there is significant increase in

average fiber length after injection molding.

Fracture Toughness

A fracture toughness value of LFPP composite with and

without compatibilizer was tabulated in Table 2 along with

fracture stress and their tensile strength. The tensile

strength of the composite is explained in Part 1 of the pub-

lished work [1]. Crack length is maintained as 6.35 mm for

all samples. Geometry factor y is calculated using the Eq. 2.

Table 2 shows that the fracture toughness value of LFPP

composite with and without compatibilizer. There is no sig-

nificant increase in fracture toughness value with increase

in initial feedstock fiber length. The increase in fracture

toughness for LFPP composite with compatibilizer is mar-

ginal when compared with that of the LFPP composite

without compatibilizer. Since the mode of testing for frac-

ture toughness and tensile strength of composite is same, a

linear relationship between fracture toughness and tensile

strength of composite was observed and shown in Fig. 5.

This explains that the material variables, such as the type of

FIG. 5. Fracture toughness versus tensile strength of LFPP composites (a) without compatibilizer and (b)

with compatibilizer.

DOI 10.1002/pc POLYMER COMPOSITES—-2008 529

fiber, fiber length, fiber content, fiber orientation, matrix na-

ture, fiber–matrix adhesion, that determine the tensile

strength and fracture toughness are same. The trend line for

LFPP composite without compatibilizer fits to an agree-

ment of 66% (Fig. 5a) and LFPP composite with compati-

bilizer confirms to an agreement of 92% (Fig. 5b). The vari-

ation in the slope of the straight line and the intercept value

determines the processing variables and due to some exper-

imental error. On comparison the linear relationship

between fracture toughness and tensile strength for LFPP

composite with compatibilizer fits well.

Structure–Toughness Correlation

The microstructure across the thickness of the injection

molded fiber reinforced composites was strongly inhomo-

geneous. SEM analysis of LFPP composite showed a three-

layered structure as shown in Fig. 6. The two outer shell

layers (S) show a fiber orientation predominantly parallel to

the flow direction and the intermediate core layer (C) con-tains fiber oriented in transverse direction to the melt flow

direction. This kind of layered fiber orientation is formed

due to the difference in the injection velocity of the flow

front. The velocity of fluid flow in the core region is higher

than the velocity at the wall. Therefore, the fiber orientation

in the core region will be different from the fiber orientation

in the shell region. Moreover, there exists two kinds of

interaction i.e. fiber–fiber and fiber–mold interactions when

a fluid flows and fills the mold. In the shell layer, the fiber–

fiber interaction will be less and fiber–mold interaction will

be more and there exists a high shear between mold and

fiber. Because of this, the fibers are aligned towards the

melt flow direction. In the core region, shear force between

the fiber and mold is zero, and the fibers are aligned in

transverse direction to the melt flow direction due to the

higher fiber–fiber interaction. T is the total thickness of the

sample. The thickness of the shell and core layer is in the

ratio of 1:2. Incorporation of long fibers of an aspect ratio

greater than 100 increases C/T ratio [14].

The fracture toughness Kc of a thermoplastic composite

can increase or decrease with increasing amounts of stiff

and strong fibers in a polymer matrix. The tendency and

degree of variation in toughness are mainly a function of

the initial fracture toughness of the base polymer and sev-

eral microstructural effects related to the fibers and fiber/

matrix interface.

According to microstructural efficiency concept devel-

oped for injection molded thermoplastic composites, the

relative fracture toughness (the ratio of fracture toughness

of composite to the fracture toughness of the matrix, Kc/

Km) is linearly related to the microstructural efficiency

factor, M.

Kc

Km

¼ M ¼ aþ nR (3)

where a is the matrix stress condition factor, n is the

energy absorption ratio, and R is the reinforcing effective-

ness parameter. R can be calculated from

R ¼ 2S

Tfp;effðSÞ þ

C

Tfp;effðCÞ

� �Vf

l

d

� �max

lnlw

(4)

where, fp,eff values correspond to the fiber orientation fac-

tor, fp, in a way that both expresses the hindrance effect

FIG. 6. SEM images of LFPP composite (�70).

530 POLYMER COMPOSITES—-2008 DOI 10.1002/pc

of fibers due to their local orientation in different layers

C/T and 2S/T with respect to the actual crack direction.

Using the Friedrich’s nonlinear correlation [8] between fpand fpeff, the value of fp,eff (for core 0.05 and for shell

0.95) was found considering fp as 0.4. The value of R was

calculated by maintaining constant Vf as 0.167 and the

values are tabulated in Table 3 with lmax and ln/lw ratio.

Detailed analysis of fiber length distribution is explained

in Part 1 of the published work [1].

Figures 7 and 8 show the plot between microstructure

efficiency factor and reinforcing effectiveness parameter

for LFPP composite without and with compatibilizer,

respectively. LFPP composite without compatibilizer has

y intercept of 1.4 and a negative slope of 0.01 (Fig. 7).

This intercept value indicates that the measured value of

Km value is small when compared with the real stress

state conditions in the matrix of the composite. Negative

slope means that the loss of energy dissipation by the ma-

trix is larger than the increase of energy dissipation by

the fibers during breakdown of composite. This happens

for very ductile matrices and it holds good for PP. But,

for LFPP composite with compatibilizer, the y intercept

has a value of 0.9 and a positive slope of 0.02 (Fig. 8).

This explains that the fracture toughness value of matrix

is high when compared with that of the real stress state

conditions in the matrix of the composite and/or the fiber

hinders the matrix deformation. Positive slope of 0.02

gives the idea that the loss of energy dissipation by the

matrix is smaller than the increase of energy dissipation

by the fibers during breakdown of composite. This shows

that the composite has become brittle by adding compati-

bilizer and further increase in interfacial shear strength

between the glass fiber and polypropylene matrix favors

positive energy absorption ratio.

Figure 9 explains the relative change in both micro-

structure efficiency factor (M) and relative modulus (E*¼ Ec/Em) of LFPP composites with Vf ¼ 16.7%. Without

compatibilizer the trend is following a negative slope and

with compatibilizer the trend is of positive slope. There-

fore, the addition of compatibilizer enhances the fracture

toughness marginally.

Fracture Toughness––Failure Mechanism

Microscopic failure mechanisms occurring in discontin-

uous long fiber reinforced polypropylene are shown sche-

matically in the Figure 10. They can be grouped into PP

matrix related and glass fiber related failure mechanisms.

Failure initiates first at the fiber ends and stress concentra-

tion in these damage zones gives rise to crazing phenom-

ena in PP matrix. For LFPP composite without compati-

bilizer, fiber debonding and fiber pullout behavior are tak-

ing place more common, because of poor interfacial bond

strength between ductile PP and glass fibers. From the fig-

FIG. 8. Effect of reinforcement on microstructure efficiency for LFPP

composite with compatibilizer.

FIG. 9. Microstructure efficiency factor versus relative modulus of

LFPP composites with Vf ¼ 16.7%.

TABLE 3. Reinforcing effectiveness parameter values for different

feedstock fiber length.

Feedstock fiber

length (mm) lmax (mm) ln/lw R

3 2.4 0.873 14.50

6 2.6 0.817 14.78

9 3.2 0.876 19.50

12 2.9 0.879 17.70

FIG. 7. Effect of reinforcement on microstructure efficiency for LFPP

composite without compatibilizer.

DOI 10.1002/pc POLYMER COMPOSITES—-2008 531

ure, it is evident that the mode of failure in these compo-

sites is ductile. For LFPP composite with compatibilizer

fiber fracture occurs more along with matrix shear yield-

ing mechanism. In these compatibilized composite, since

most of the applied load is transferred to the fibers by the

matrix through the interface, contribution of fiber is more.

FIG. 10. SEM images of (a) PP, (b) LFPP composite without compatibilizer, and (c) LFPP composite with

compatibilizer (�1,500).

532 POLYMER COMPOSITES—-2008 DOI 10.1002/pc

These fibers fail in a brittle manner and hence fiber frac-

ture is more than fiber pullout and debonding. At the time

of fiber fracture, the matrix shear yields and the compos-

ite fails. The final crack path emerges by connection of

the different craze planes due to fiber debonding, pullout,

and fracture events with concomitant matrix deformation

(shear yielding, plastic deformation). With respect to

feedstock fiber length, there is no significant change in

failure mechanism or fracture toughness values, since the

fiber length after injection molding is marginal. But with

little addition of compatibilizer, it is possible to have

good interaction between fibers and matrix and in turn the

failure behavior changes.

Table 4 shows that the coefficient of variation for

impact strength and fracture toughness on LFPP compo-

sites with and without compatibilizer. The coefficient of

variation signifies the repeatability of the experiment and

thereby the homogeneous microstructure of the injection

molded plaques. It is defined as the ratio of standard devi-

ation and average value and expressed in percentage. The

value of coefficient of variation is, on an average, not

larger than 10%.

CONCLUSION

The variations of impact strength and fracture tough-

ness were examined for LFPP composite with and without

compatibilizer, with different initial feedstock fiber length.

In conclusion, it can be stated that the addition of compa-

tibilizer does improve the impact strength values and mar-

ginally on fracture toughness values. The effect of glass

fiber length on impact strength is highly significant and

the effect of glass fiber length on fracture toughness is

quite marginal. Scanning electron micrographs also sup-

ported the major energy absorbing mechanism such as

fiber fracture and fiber pullout occurring on LFPP compo-

sites with and without compatibilizer. A linear relation-

ship between fracture toughness and tensile strength of

composites explains that the material variables that deter-

mine the fracture toughness and tensile strength are same.

The ratio between shell and core to part thickness was

also found out using SEM images. Structure–toughness

correlation was analyzed for LFPP composite with and

without compatibilizer. For LFPP composite without com-

patibilizer, the loss of energy dissipation by the matrix is

more than the increase of energy dissipation of fibers dur-

ing breakdown of composite. Matrix is very ductile in

this composition and poorly adhered with glass fiber. Ma-

trix fails by crazing phenomena. For LFPP composite

with compatibilizer, strong adhesion with glass fiber

increases the energy dissipation by fibers than that of the

matrix during breakdown of composite. The final break-

down emerges by different craze planes due to fiber

debonding, pullout, and fracture with concomitant matrix

deformation (shear yielding, plastic deformation).

REFERENCES

1. K. Senthil Kumar, N. Bhatnagar, and A.K. Ghosh, Polym.Compos., 28(2), 259 (2007).

2. M.J. Voelker, Polym. Compos., 12(4), 119 (1991).

3. A. Hassan, R. Yahya, A.H. Yahaya, and A.R.M. Tahir,

J. Reinforced Plast. Compos., 23(9), 969 (2004).

4. T.P. Skourlis, S.R. Mehta, C. Chassapis, and S. Manoo-

chehri, Polym. Eng. Sci., 38(1), 79 (1998).

5. J.L. Thomason and M.A. Vlug, Compos. A, 28, 277 (1997).

6. S. Hashemi and M. Koohgilani, Polym. Eng. Sci., 35(13),1124 (1995).

7. F. Gonzalez and C. Chassapis, J. Reinforced Plast. Compos.,20(10), 810 (2001).

8. J.K. Kocsis, Compos. Sci. Technol., 48, 273 (1993).

9. J.K. Kocsis, T. Harmia, and T. Czigany, Compos. Sci.Technol., 54, 287 (1995).

10. K. Friedrich, D.E. Spahr, J.M. Schultz, and R.S. Bailey,

J. Mater. Sci., 25, 4427 (1990).

11. K. Friedrich and J.K. Kocsis, Compos. Sci. Technol., 32,

293 (1988).

12. K. Friedrich, Compos. Sci. Technol., 22, 43 (1985).

13. D. Harris, J. Basic Eng., 3, 49 (1967).

14. J.K. Kocsis, Polypropylene Structure, Blends and Compo-

sites, Chapman and Hall, London, 166 (1995).

TABLE 4. Coefficient of variation (%) of the impact strength and

fracture toughness on LFPP composites.

Injection molded samples

Coefficient of variation (%)

Impact

strength

Fracture

toughness

PP 14.8 2.7

LFPP composite without compatibilizer

3 8.7 5.3

6 7.0 10.6

9 7.8 10.3

12 9.1 5.8

LFPP composite with compatibilizer

3 2.2 4.6

6 7.6 3.2

9 8.9 3.3

12 3.9 7.0

DOI 10.1002/pc POLYMER COMPOSITES—-2008 533