Mechanical behavior of composite based polypropylene: Recycling and strain rate effects

Transcription

1 J. Phys. IV France 134 (2006) C EDP Sciences, Les Ulis DOI: /jp4: Mechanical behavior of composite based polypropylene: Recycling and strain rate effects N. Bahlouli 1,D.Pessey 1,S.Ahzi 1 and Y. Rémond 1 1 Institut de Mécanique des Fluides et des Solides, UMR 7507, ULP, Strasbourg, France Abstract. Recycling effects on the dynamic response of composite based polypropylene is studied in this paper. Materials used here are filled and unfilled impact modified polypropylene. Nodules of EPDM represent the elastomeric phase for the filled composite based polypropylene. Fillers are particles of talc. For the unfilled polypropylene, the elastomeric phase is nodules of EPR. Different tensile tests, until rupture, were performed from quasi static to high strain rates. Virgin specimens were recycled in order to study the degradation due to the effect of recycling under dynamic loading. Thus microstructural effects due to dynamic loading and recycling were observed by SEM. 1. INTRODUCTION A new European standard concerning the recycling of materials forces the automotive manufacturers to recycle car parts [1]. Accordingly, they are compelled to recycle 85% of the average weight of a car in 2006 and increase this value up to 95% by Therefore, the open technological challenge is to be able to re-use these recycled materials, specifically polymers for example as instrument panel and bumpers. Work on recycling of polymers and its application in the automotive industry for quasi static loading (until 10 2 s 1 ) can be found in the literature [2 7]. The recycling process results in a degradation of the matrix molecular chain in polymer composites, which in turn affect strongly the mechanical response. In this work we will point out the effect of the recycling process on the mechanical response of composite polymers (talc/epdm/polypropylene and EPR/polypropylene) during dynamic loading. The reason for this study is that most automotive applications endure loads ranging from quasi static to dynamic, which can also play a role in the polymer degradation [8-9]. Polypropylene is classically used for non structural automotive application because it has low impact properties. A high impact polypropylene with low stiffness and strength can be formed as a composite with elastomeric nodules. Therefore, rigid inorganic particles are recommended to increase the composite toughness. In a recent study, Stamhuis [10] showed that talc filler can significantly improve the impact resistance of polypropylene if it is physically blended with either an SBS or an EPDM elastomer. In order to quantify the composite morphology and analyse recycling and dynamic effects, different tensile tests are performed. Also SEM analyses are conducted on the drawn samples to better understand the effects of recycling and dynamic loading. 2. EXPERIMENTAL Two kinds of polypropylene based composites were studied in this paper: a high impact polypropylene (referenced by SABIC PP, grade 108MF97) and talc filled high impact polypropylene (referenced by SABIC PPcompound, grade 7510). The composite characteristics given by SABIC are summarized in Table 1 for the unfilled high impact polypropylene and in Table 2 for the talc filled high impact polypropylene. Moulded sheets were furnished by the University of St-Etienne where the percentage of elastomeric nodules (phase EPDR) is 22% in the first material and in the second (phase EPDM) is Article published by EDP Sciences and available at or

2 1320 JOURNAL DE PHYSIQUE IV Table 1. Characteristics of unfilled high impact polypropylene from SABIC. Properties Methods Specification range Values Units MFR 230 C/2.16kg ISO dg/min Izod 20 C ISO 180/4A kj/m 2 E-modulus ASTM D N/mm 2 Table 2. Characteristics of talc filled high impact polypropylene from SABIC. Properties Methods Specification range Values Units MFI 230 C/2.16kg ISO dg/min Ash-% 15min, 525 ISO 3451/ % Volatile SABIC QCTV 23 Max % Figure 1. Strain rate effect on stress-strain curves for virgin unfilled high impact PP. Figure 2. Strain rate effect on stress-strain curves for virgin talc filled high impact PP. of 20% and 12% of talc. To recycle these two materials, six cycles of extrusion were performed in the extrusion Biscrew machine with a flow of 10Kg/h at temperature of 230 C. Uniaxial tensile tests were performed on an INTSRON machine model 8031 and the temperature of the test is assumed constant (25 C). The tests were conducted at displacements rate from 0.01 mm.s 1 to 10 mm.s 1. Since the extensometer gauge length was 10 mm, it is assumed that the strain rates are from 0.001s 1 to 1s 1. Mechanical characteristic for virgin and recycled polymer for different strain rates: Young modulus (E), yield stress (σ y ) are obtained from the stress-strain curves (figure 1 to 4) on an average of five specimens moduli. The strain failure cannot be determined due to the limitation of the elongation of the extensometer gauge (50%). Elastic modulus is the initial slope of the stress-strain curve. Yield strength is assumed to be the maximum stress observed in each stress-strain curve. For a better understanding of the mechanism of the recycling process under the strain rate effect, microstructural evolution was observed using SEM [8]. 3. RESULTS AND DISCUSSION 3.1 Tensile tests Tensile stress-strain curves of virgin unfilled and talc-filled high impact polypropylene under different test conditions are shown in Figs. 1 and 2. It can be observed that the stress-strain relationships of both materials are non-linear even at strains lower than the yield strain. Each curve shows a maximum stress,

3 EURODYMAT Figure 3. Strain rate effect on stress-strain curves for recycled unfilled high impact PP. Figure 4. Strain rate effect on stress-strain curves for recycled talc filled high impact PP. which is assumed to be the yield strength of the material. After yielding, the engineering stress decreases steadily with strain until the fracture occurs. Figs 3 and 4 show the tensile stress-strain curves of recycled unfilled and talc-filled high impact polypropylene under different test conditions. The same observations as in figs 1 and 2 can be reported Strain rate effect From Figs1to4, it can be concluded that both virgin and recycled unfilled and talc-filled high impact polypropylene are strain rate sensitive materials. Thus, the Young s modulus, the yield stress and the failure strain are modified. As the strain rate is higher, the elastic modulus and the yield stress increase and the failure strain decreases Recycling effect From Figs 3 and 4 it can be seen that the recycling process affects strongly the strain failure and less the yield stress and the Young s modulus. The recycling process has definitly degraded the properties of the polypropylene matrix. 3.2 Morphological study The two first SEM micrographs (figs 5 a-b) show a similar cryo-fracture for undeformed virgin and recycled material. The same results are obtained when specimens were submitted at a quasi static tensile test (figs 5 c-d). However, morphological changes are observed between cryo-fractured undeformed specimens and quasi-statically loaded ones. From Figs 5 c-d, we can see that the deformed material is transformed into a fibril matrix embedded by elastomeric particles. On the other hand, when materials are tested under dynamic loading (figs 5 e-f ), a large difference is observed between the quasi static and dynamic loading and also between recycled and virgin material. Under dynamic loading (figs 5 e-f ), the molecular orientation is not developed as much as in the quasi-static case. We can see that cryo-fractured and dynamic fracture micrographs are very similar (figs 5 b-f ).

4 1322 JOURNAL DE PHYSIQUE IV a) Cryo-fracture of virgin unfilled high impact PP b) Cryo-fracture of recycled unfilled high impact PP c) Virgin unfilled high impact PP at 0.01 s-1 d) Recycled unfilled high impact PP at 0.01 s-1 e) Virgin unfilled high impact PP at 1s-1 f) Recycled unfilled high impact PP at 1s-1 Figure 5. Scanning electron micrograph.

5 EURODYMAT CONCLUSION Experimental tests in order to identify recycling and strain rate effects were performed to analyse the effects on morphology, mechanical properties and degradations, particularly in the matrix phase. For a better observation of these effects on morphology, TEM analysis will be necessary. Modelling and numericals simulations of the behaviour of these materials is underway and will be reported in an upcoming paper. Acknowledgements The authors would like to acknowledge the financial support provided by the Ministry of the Innovation and Research, France. We would like also to thank Professor Jacques Faerber for the SEM micrographs. References [1] Directive 2003/53/CE, du Parlement Européen et du Conseil, Journal official de l Union Européenne, [2] D Orazio L., Greco R., Martuscelli E. and Ragosta G., Polym. Eng. Sci., 23 (1983) pp [3] Luda M.P., Ragosta G., Musto P., Pollicino A., Camino G., Recca A. and Nepote V., Macromolecular Materials Engineering 287 (2002) pp [4] Luda M.P., Ragosta G., Musto P., Acierno D., Di Maio L., Camino G. and Nepote V., Macromolecular Materials Engineering 288 (2003) pp [5] Aurrekoetxea J., Sarrionandia M.A. and Urrutibeascoa I., J. of Materials Science 36 (2001) pp [6] Guerrica-Echevarria G., Eguiazabal J.I., Nazabal J., Polymer degradation and stability, 53 (1996) pp [7] R. Hosseinzadeh M.M. Shokrieh et L.B Lessard: Parametric study of automotive composite bumper beams subjected to low-velocity impact. [8] Dasari A., Misra R.D.K., Materials Science and Engineering 358 (2003) pp [9] Gensler R., Plummer C.J.G., Grein C. and Kausch H.-H., Polymer 41 (2000) pp [10] J.E. Stamhuis, Polym. Comp., 5, 202, (1984).