polypropylene composites

Journal of Materials Processing Technology 128 (2002) 33±37

Interlaminar fracture properties of ®bre reinforced natural rubber/polypropylene composites R. Zulki¯i*, L.K. Fatt, C.H. Azhari, J. Sahari Department of Mechanical and Materials Engineering, The National University of Malaysia, Selangor Darul Ehsan, 43600 UKM Bangi, Malaysia Received 7 January 2001; received in revised form 18 April 2001; accepted 21 December 2001

Abstract The effect of the addition of natural rubber (NR) on the interlaminar fracture properties of thermoplastic composites which is a measure of the resistance of the material to delamination crack propagation, has been studied. NR±polypropylene (PP) composites were prepared with increasing amounts of NR in PP at 5±20% compositions reinforced with 5% E-type chopped strand glass-®bre mat. Panels were produced by compression moulding technique and these have been tested for mode I interlaminar fracture using double cantilever beam specimen testing method. Tests have been undertaken at cross-head displacement rate of 5 mm/min for all the specimens. It was found that, as the amount of NR was increased, the interlaminar fracture toughness, GIC of the composite materials decreased. Scanning electron microscopy of the fracture surfaces is used to help explain these ®ndings. The crack propagation areas showed all the ®bres were bare with no matrix covering them. These were seen at 200 magni®cation. The smooth clean surface of the ®bres is the result of poor interfacial debonding. No ®bre bridging is observed between the ®bres and the failure mainly occurred at the ®bre±matrix interface as seen using scanning electron microscopy. This shows that the addition of rubber made the adhesion between the interface weaker. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Polypropylene; Natural rubber; Interlaminar fracture; NR-PP composites

1. Introduction The attractive properties of natural rubber (NR)±polypropylene (PP) blends [1,2] have lead to on-going studies on these materials as a matrix for glass ®bre reinforced plastics composite. One of the most important aspects of study in this thermoplastic NR composite materials has been concerned with the transfer of the inherent toughness of the thermoplastic, with toughness in this mode being frequently assessed by using a double cantilever beam (DCB) test [3±6]. Delamination is caused by a crack propagating in a plane between the individual plies. When a laminate is subjected to out-of-plane stresses, failure is likely to develop in the relatively weak resin between the ®bres or at the ®bre± matrix interface, which is the weakest component of the composite. Delamination is the most predominant and lifelimiting failure mechanism in composite materials, hence limiting the use of composites for structural applications.

*

Corresponding author. Present address: The Queen's University of Belfast, School of Aeronautical Engineering, David Kier Building, Stranmillis Road, BT9 5AG Belfast, UK. E-mail address: [email protected] (R. Zulkifli).

There are three basic modes of deformation by which a load can operate on a crack. Each will effect a different crack surface displacement. In mode I, or the opening mode, displacements of the crack surfaces are perpendicular to the crack plane and open the crack. In mode II, or the shearing mode, the surfaces slide over each other in the direction parallel to the crack. Lastly, in mode III, or the tearing mode, the surfaces slide over each other in the out-of-plane direction. Delamination can be considered using fracture mechanics approach with the critical strain energy release rate (fracture toughness) as a criterion [7]. The interlaminar fracture toughness of the material is a measure of the material's resistance to delamination crack propagation and is a key parameter in describing the damage tolerance of laminated composite materials. The main objective in this mode I testing programme is to ®nd the effect of the addition of NR on the interlaminar fracture properties of a glass ®bre reinforced plastics composite. 2. Materials The material used was an E-type chopped strand mat glass ®bre reinforced thermoplastic NR composite (Fig. 1).

0924-0136/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 0 0 9 7 - 3

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R. Zulkifli et al. / Journal of Materials Processing Technology 128 (2002) 33±37

3. Test method

Fig. 1. Cross-section of the laminated composite.

The matrix was made from a combination of PP and an increasing percentage of NR. Grade G600 PP supplied by Polypropylene Malaysia Sdn. Bhd, ENR50 NR supplied by Guthrie Sdn. Bhd and commercial grade glass ®bre were used throughout the study. The amount of NR added to the PP was from 5 to 20%. The blends of the matrix are as follows: 95% PP/5% NR, 90% PP/10% NR, 85% PP/15% NR and 80% PP/20% NR. The panels were prepared by a compression moulding technique at a temperature and pressure setting of 220 8C and 200 kN m 2, respectively. In order to initiate delamination, an aluminium foil was placed at the laminate mid-thickness during moulding. The thickness of the foil was suf®ciently thin so as to minimise disturbance of the composite during manufacture. The length of the aluminium foil from the loading line was chosen to be 50 mm so that the in¯uence of the hinges can be neglected. All the specimens were nominally of 20 mm width and 6.0 mm thickness. The specimens had piano hinges blocks bonded to the starter defect end to permit load introduction. The hinge and the specimen were lightly abraded using a sandpaper since the load required to delaminate the specimens in these tests is quite low. Bonding of the loading tabs was done immediately after surface preparation. The bonding adhesive used was a room temperature cure epoxy adhesive. In order to minimise the errors in the applied moment arm, the distance from the hinge pin to the centre line of the specimen arm of the hinge did not exceed 10 mm. The detailed geometry of the DCB specimen is shown in Fig. 2.

Six specimens were tested for each amount of NR added. The tests were conducted according to the European Structural Integrity Society (ESIS), Protocol for mode I interlaminar fracture testing of composites [8]. All the tests were conducted on a SINTECH 6 universal testing machine, with TestWorksTM (advanced software for materials testing) for machine control and data acquisition. A 5 kN load cell was used since the maximum load never exceeded 4 kN. The specimen was loaded at a cross-head rate of 5 mm/min. At this loading rate, crack propagation can be followed visually and recorded reasonably well. Load and displacement data were recorded continuously by a personal computer at a rate of 10 points/s. Fig. 3 shows schematically the loading arrangement. To monitor the position of the crack tip, both sides of the specimen were coated with white correction ¯uid to enhance the visibility of the propagating crack. At intervals of 5 mm, the delamination length, measured visually on a scale marked using the correction ¯uid, was noted together with the corresponding load and displacements. The point on the load±displacement plot at which the crack ®rst propagated from the insert was also recorded. When crack has propagated at least 60 mm from the starter ®lm, the test was stopped and the specimen was then completely unloaded. 4. Data analysis The propagation values of the interlaminar fracture toughness were determined using the corrected beam theory. The simple beam theory expression for the compliance of a perfectly built-in DCB specimen is as follows: GIC ˆ

3Pd 2Ba

where P is the load, d the displacement, B the specimen width and a the crack length. The formula above will underestimate the compliance because the beam is not perfectly built-in, hence the beam

Fig. 2. Specimen geometry, DCB.

R. Zulkifli et al. / Journal of Materials Processing Technology 128 (2002) 33±37

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Fig. 3. DCB test method.

must be treated as containing a slightly longer crack, a ‡ D, and D may be found experimentally by plotting the cube root of compliance, C1/3, as a function of crack length as shown in Fig. 4. GIC is then given by GIC ˆ

3Pd 2B…a ‡ D†

where P is the load, d the displacement, B the specimen width, a the crack length and D the value of intersection between the plot line and the x-axis. Fig. 5. Typical graph of load (P) against displacement (d).

5. Results and discussion Stable crack growth was observed over all the specimens for the different amount of NR added and no crack jump phenomenon occurred during the test, which can be observed. All the sample specimens show the same pattern of graph of load against displacement as shown in Fig. 5. From the values of maximum load and the load at the point of non-linearity, the interlaminar fracture toughness, GIC at recorded crack length was calculated. The average values of GIC determined for different combinations of PP and NR are shown in Table 1. The interlaminar fracture tests

on the DCB specimens resulted in a linear loading curve up to initiation from the insert. From the results, it can be seen that as the amount of rubber in PP increased, the value of interlaminar fracture toughness decreased greatly. Even the addition of 5% NR, the value of interlaminar fracture toughness will drop to more than half of the original ®gure. As the amount of NR reached 20% of the total matrix, the value of GIC becomes one-third of its original value. The surfaces of the delamination areas were subsequently examined by scanning electron microscopy. Figs. 6±8 show the micrographs of the delamination area for specimens added with 5, 10 and 20% of NR. The propagation areas show that all the ®bres are bare with no matrix covering them as seen at a higher magni®cation. The smooth clean surface Table 1 Average GIC for all compositions of PP and NR

Fig. 4. Graph of C1/3 versus crack length (a) for corrected beam theory.

Compositions

Average GIC (kJ/m2)

95% 90% 85% 80%

1.415 1.257 1.170 1.056

PP/5% NR PP/10% NR PP/15% NR PP/20% NR

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R. Zulkifli et al. / Journal of Materials Processing Technology 128 (2002) 33±37

Fig. 6. Fracture surface for the 95% PP/5% NR specimen.

In the absence of any compatabiliser, as in this study, rubber and PP forms immiscible blends. This probably aggravates the already inef®cient bonding of ®bre to the PP matrix in the absence of a coupling agent. Since the dominant mode of crack propagation is by ®bre±matrix debonding, this could explain the lowering of GIC with increasing rubber content. Further studies with compatabilised matrix blends and inclusion of a matrix ®bre coupling agent should con®rm this. If ®bre bridging occurs as the delamination progress along the length of the beam, there will be an increase in the energy required to propagate the delamination further because ®bre bridging resulted in a signi®cant increase in GIC values [9,10]. However, values of GIC obtained from the delamination growth from the insert is unaffected by ®bre bridging. Results obtained shows very little scatter in the experimental data. Davies et al. [11] conducted a series of mode I DCB tests on IM6/PEEK composites. Their results indicated that the mode I propagation values of GIC are dependent on specimen thickness. This thickness effect is related to the contribution of multiple cracking and ®bre bridging and results in higher values being obtained on thicker specimens. The fact that the values of GIC obtained in the current experiments show very little scatter suggests that the test set up is reliable. 6. Conclusion

Fig. 7. Fracture surface for the 90% PP/10% NR specimen.

The addition of NR in the PP as a matrix for polymer composite was found to reduce the value of mode I interlaminar fracture toughness properties. Increasing the amount of NR resulted in the decreasing value of interlaminar fracture toughness. For only 5% of NR, the value dropped to more than half of the original ®gure. No ®bre bridging is observed between the ®bres and the failure mainly occurs at the ®bre±matrix interface as seen using scanning electron microscopy. This shows that the addition of rubber made the adhesion between the interface weaker.

References

Fig. 8. Fracture surface for the 80% PP/20% NR specimen.

of the ®bres is the result of poor interfacial bonding. No ®bre bridging is observed between the ®bres and the failure mainly occurs at the ®bre±matrix interface as seen using scanning electron microscopy. This shows that the addition of rubber made the adhesion between the interface weaker.

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