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Effect of irradiation on the mechanical properties of high-density polyethylene reinforced with metallic fibres
Surface and Coatings Technology 158 – 159 (2002) 404–407
Effect of irradiation on the mechanical properties of high-density polyethylene reinforced with metallic fibres b ´ W. Bare´ a, C. Albanoa,b,*, J. Reyesa, N. Domınguez a
´ Escuela de Ingenierıa ´ Quımica, ´ Universidad Central de Venezuela, Facultad de Ingenierıa, Caracas, Venezuela b ´ Laboratorio de Polımeros, IVIC, Apdo. 21827, Caracas 1020 A, Venezuela
Abstract In recent years, substantial efforts have been undertaken to investigate polymeric composites, which could represent a good alternative for obtaining other kinds of materials with desirable properties. Usually, the preparation of polymeric composites involves chemical treatment of the fillers to improve interaction between them and the polymer. In many cases, however, incompatibilities arise between the host matrix and the filling materials, such as organic, glass and metallic fibres. A possible way to overcome this difficulty is irradiation of the material involved so as to improve the curing of adhesives and resins. In this work, the effects of irradiation on the mechanical properties of high-density polyethylene (HDPE) reinforced with metallic fibres are presented. The samples were irradiated with a 60 Co source at doses between 10 and 70 kJ kgy1 (kGy) using an irradiation rate of 4.8 kJ kgy1 hy1 in an oxygen atmosphere at room temperature. Afterwards, the tensile and impact properties were studied and compared with the values for pure HDPE. The results show that, at lower irradiation doses, the properties of the material are positively affected, reaching highest values at approximately 10 kJ kgy1 . For the pure HDPE, Young’s modulus increases up to 100% independently of the irradiation dose, while for the polymer containing metallic fibres this parameter increases to approximately 200%. Furthermore, the resistance to impact increases by up to 600% due to the presence of the metallic fibres. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Polymer composites; Irradiation; Impact resistance; Elongation at break; Young’s modulus; Strength at failure
1. Introduction Polymers are increasingly used in the manufacture of different kinds of objects, such as car accessories, mechanical parts, pipes, etc., in substitution for metals and woods, which are become increasingly expensive. Nevertheless, the poor mechanical properties of polymers restrict their application in fields such as construction and design, where they could replace the existing heavy materials as light and cheap structures. It is possible to improve the mechanical properties of plastics by introducing filling materials, such as glass fibres, organic compounds and metals w1–4x. On the other hand, irradiation of the reinforced polymer introduces additional changes in the physical, morphological and mechanical properties of the prepared compound. It *Corresponding author. Tel.: q58-504-1636; fax: q58-504-1350. E-mail address: [email protected] (C. Albano), ´ bareucv [email protected] (W. Bare).
is known that irradiation of polymeric materials promotes cross-linking between chains and between crystallites, and therefore must be reflected in the mechanical behaviour of the material concerned w5x. For instance, it has been reported that irradiation increases the Young’s modulus, tensile and impact strength, elongation at failure, hardness, abrasion and fatigue resistance w6x. All these effects are related to a greater number of intercrystalline linkages induced by the irradiation. With respect to the polymeric matrix, irradiation can also produce radicals, double bonds and chain scission. The latter is probably due to the destruction of a previously formed alkyl radical. Another effect of irradiating a polymer results in the linking together of independent chains. This can cause the formation of gel, the content of which increases with increasing irradiation dose w5x. From the combined effect of introducing metallic fibres into the polymeric matrix and irradiating the material afterwards, it can be expected that the properties of the polymeric matrix will considerably change.
0257-8972/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 2 5 9 - 1
W. Bare´ et al. / Surface and Coatings Technology 158 – 159 (2002) 404–407
405
Fig. 1. Impact resistance of HDPE alone at different irradiation doses.
In this work, the mechanical properties of HDPE filled with long metallic fibres and irradiated with a 60 Co source at different doses are presented. 2. Experimental method High-density polyethylene (Resilin 2908) with a MFI of 6.3 g miny1 at 190 8C was used as the polymeric matrix. The metallic fillers were long fibres of carbon steel and copper with a diameter of 0.2=10y4 m, first
Fig. 4. Strength at failure of HDPE charged with Cu fibres at different irradiation doses.
placed unidirectionally, where the average loading density of the metallic fibres was 2.67 fibres cmy2 for copper and 1.67 fibre cmy2 for carbon steel. In a second case, the fibres were stitched: a definite quantity of fibres was placed in one direction and another quantity of fibres of the same metal was alternately added, so as to obtain angles of 908. In this case, the average loading density was 3.33 and 2.26 fibres cmy2 for copper and carbon steel, respectively. The samples were then prepared by compression moulding at 140 8C and 10 000 kPa. The samples prepared were then irradiated with 60Co at doses between 10 and 70 kJ kgy1 at a rate of 4.8 kJ kgy1 hy1. The irradiation process was carried out under an oxygen atmosphere at ambient temperature. In addition, the mechanical properties of the pure polymer (without metallic fibres) were determined in order to compare the values with those of the filled material. Afterwards, the tensile and impact properties were studied. 3. Results and discussion
Fig. 2. Impact resistance of HDPE charged with Cu fibres at different irradiation doses.
The results of the analysis show that the inclusion of metallic fibres in a polymeric matrix improves the
Fig. 3. Impact resistance of HDPE charged with carbon steel fibres at different irradiation doses.
Fig. 5. Strength at failure of HDPE charged with carbon steel fibres at different irradiation doses.
406
W. Bare´ et al. / Surface and Coatings Technology 158 – 159 (2002) 404–407
Fig. 6. Elongation at break of HDPE alone at different irradiation doses.
mechanical behaviour of the material. For example, the impact resistance increased by up to 600% compared to the value for pure HDPE, as can be observed in Figs. 1–3. Furthermore, this property increases with increasing irradiation dose. In Fig. 2, the resistance to impact of samples reinforced with copper fibres is reported. The values for samples with unidirectionally placed fibres are higher than those for samples with stitched fibres. This can be attributed to the fact that cross-linked
Fig. 7. Elongation at break of HDPE filled with Cu fibres at different irradiation doses.
Fig. 8. Elongation at break of HDPE filled with carbon steel at different irradiation doses.
Fig. 9. Young’s modulus of HDPE alone at different irradiation doses.
fibres introduce more failure sites in the material, in addition to residual tensions generated by the texture of the metallic fibres. This tendency is confirmed by samples filled with carbon steel fibres (see Fig. 4). Due to the natural hardness of this type of metallic fibre, the samples containing steel show higher resistance than those containing copper. It can also be observed that the irradiation affects the properties of the material, improving them at lower doses (10 kJ kgy1). In addition, the tensile strength changed considerably compared to the values obtained with the polymer without charge (Figs. 4 and 5) Depending on the orientation and quantity of the fibres in the matrix, these
Fig. 10. Young’s modulus of HDPE filled with Cu fibres at different irradiation doses.
Fig. 11. Young’s modulus of HDPE filled with carbon steel fibres at different irradiation doses.
W. Bare´ et al. / Surface and Coatings Technology 158 – 159 (2002) 404–407
properties are affected to a greater or lesser extent. During tensile experiments, it could be observed that the polymeric matrix failed (due to fracture) when the fibres were unidirectionally placed, but the fibres did not break: they slid out. This was attributed to the fact that the interface between the polymer and the fibres was not good enough to hold the patterns together. For the stitched fibres, both the polymeric matrix and the fibres broke together. For all cases, it required more strength to break the samples with stitched fibres than for samples with unidirectional fibres. Furthermore, it can be noted that the elasticity decreased, since the quantity of polymer is lowered by addition of the metal (Figs. 6–8). Increasing the quantity of metallic fibres means that the elasticity of the resulting material will be lower than that of the pure polymer. The Young’s modulus increased by up to 100%, independently of the irradiation dose used and the kind of metal (Figs. 9 and 10). In the particular cases of impact resistance (Figs. 2 and 3), strength at failure (Fig. 4), elongation at break (Fig. 6) and Young’s modulus (Figs. 10 and 11), it can be observed that the improvement in properties is more pronounced at lower irradiation doses (approx. 10 kJ kgy1). This demonstrates that some kind of slight interaction exists between the metal and the polymeric matrix due to the irradiation, and at lower doses the HDPE presents a certain degree of physical crosslinking, as found by Chapiro w7x. In order to observe clearly the effects of the irradiation on the metallic fibres, much higher irradiation doses would be required.
407
In this case, the outer electrons of the metal would reach a higher degree of excitation and would probably interact with the free radicals produced in the polymeric matrix by irradiation. 4. Conclusions From the results obtained it can be concluded that the introduction of metallic fibres into a polymer is a possible method of increasing its application field, since it is easy to obtain polymer materials due to their low cost. In addition, irradiation of HDPE filled with metallic fibres contributes to an improvement in the mechanical properties of the compound. Since irradiation affects the polymeric matrix, and eventually also the metallic fibres, it would be of great importance to investigate in more detail the resulting physical, morphological, interfacial and superficial changes in the materials concerned. References w1x R. Thomson, M. Shah, A. Mouritz, Composite Struct. 42 (1998) 107. w2x A. Guz, Proceedings of ICCEy6, Orlando, Florida, 1999, p. 241. w3x A. Seiby, S. Zadjali, Proceedings of ICCST2, Durban, South Africa, 1998, p. 477. w4x S. Zaginaichenko, Z. Matysina, D. Schur, Proceedings of ICCST3, Durban, South Africa, 2000, p. 46. w5x J. Schultz, Polymer Material Science, Prentice Hall Inc, 1974, p. 472. w6x M. Stern, Proceedings of ICCE ’99, Cincinnati, Ohio, 1999. w7x A. Chapiro, Radiation Chemistry of Polymeric Systems, Interscience, London, 1962.
Effect of irradiation on the mechanical properties of high-density polyethylene reinforced with metallic fibres b ´ W. Bare´ a, C. Albanoa,b,*, J. Reyesa, N. Domınguez a
´ Escuela de Ingenierıa ´ Quımica, ´ Universidad Central de Venezuela, Facultad de Ingenierıa, Caracas, Venezuela b ´ Laboratorio de Polımeros, IVIC, Apdo. 21827, Caracas 1020 A, Venezuela
Abstract In recent years, substantial efforts have been undertaken to investigate polymeric composites, which could represent a good alternative for obtaining other kinds of materials with desirable properties. Usually, the preparation of polymeric composites involves chemical treatment of the fillers to improve interaction between them and the polymer. In many cases, however, incompatibilities arise between the host matrix and the filling materials, such as organic, glass and metallic fibres. A possible way to overcome this difficulty is irradiation of the material involved so as to improve the curing of adhesives and resins. In this work, the effects of irradiation on the mechanical properties of high-density polyethylene (HDPE) reinforced with metallic fibres are presented. The samples were irradiated with a 60 Co source at doses between 10 and 70 kJ kgy1 (kGy) using an irradiation rate of 4.8 kJ kgy1 hy1 in an oxygen atmosphere at room temperature. Afterwards, the tensile and impact properties were studied and compared with the values for pure HDPE. The results show that, at lower irradiation doses, the properties of the material are positively affected, reaching highest values at approximately 10 kJ kgy1 . For the pure HDPE, Young’s modulus increases up to 100% independently of the irradiation dose, while for the polymer containing metallic fibres this parameter increases to approximately 200%. Furthermore, the resistance to impact increases by up to 600% due to the presence of the metallic fibres. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Polymer composites; Irradiation; Impact resistance; Elongation at break; Young’s modulus; Strength at failure
1. Introduction Polymers are increasingly used in the manufacture of different kinds of objects, such as car accessories, mechanical parts, pipes, etc., in substitution for metals and woods, which are become increasingly expensive. Nevertheless, the poor mechanical properties of polymers restrict their application in fields such as construction and design, where they could replace the existing heavy materials as light and cheap structures. It is possible to improve the mechanical properties of plastics by introducing filling materials, such as glass fibres, organic compounds and metals w1–4x. On the other hand, irradiation of the reinforced polymer introduces additional changes in the physical, morphological and mechanical properties of the prepared compound. It *Corresponding author. Tel.: q58-504-1636; fax: q58-504-1350. E-mail address: [email protected] (C. Albano), ´ bareucv [email protected] (W. Bare).
is known that irradiation of polymeric materials promotes cross-linking between chains and between crystallites, and therefore must be reflected in the mechanical behaviour of the material concerned w5x. For instance, it has been reported that irradiation increases the Young’s modulus, tensile and impact strength, elongation at failure, hardness, abrasion and fatigue resistance w6x. All these effects are related to a greater number of intercrystalline linkages induced by the irradiation. With respect to the polymeric matrix, irradiation can also produce radicals, double bonds and chain scission. The latter is probably due to the destruction of a previously formed alkyl radical. Another effect of irradiating a polymer results in the linking together of independent chains. This can cause the formation of gel, the content of which increases with increasing irradiation dose w5x. From the combined effect of introducing metallic fibres into the polymeric matrix and irradiating the material afterwards, it can be expected that the properties of the polymeric matrix will considerably change.
0257-8972/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 2 5 9 - 1
W. Bare´ et al. / Surface and Coatings Technology 158 – 159 (2002) 404–407
405
Fig. 1. Impact resistance of HDPE alone at different irradiation doses.
In this work, the mechanical properties of HDPE filled with long metallic fibres and irradiated with a 60 Co source at different doses are presented. 2. Experimental method High-density polyethylene (Resilin 2908) with a MFI of 6.3 g miny1 at 190 8C was used as the polymeric matrix. The metallic fillers were long fibres of carbon steel and copper with a diameter of 0.2=10y4 m, first
Fig. 4. Strength at failure of HDPE charged with Cu fibres at different irradiation doses.
placed unidirectionally, where the average loading density of the metallic fibres was 2.67 fibres cmy2 for copper and 1.67 fibre cmy2 for carbon steel. In a second case, the fibres were stitched: a definite quantity of fibres was placed in one direction and another quantity of fibres of the same metal was alternately added, so as to obtain angles of 908. In this case, the average loading density was 3.33 and 2.26 fibres cmy2 for copper and carbon steel, respectively. The samples were then prepared by compression moulding at 140 8C and 10 000 kPa. The samples prepared were then irradiated with 60Co at doses between 10 and 70 kJ kgy1 at a rate of 4.8 kJ kgy1 hy1. The irradiation process was carried out under an oxygen atmosphere at ambient temperature. In addition, the mechanical properties of the pure polymer (without metallic fibres) were determined in order to compare the values with those of the filled material. Afterwards, the tensile and impact properties were studied. 3. Results and discussion
Fig. 2. Impact resistance of HDPE charged with Cu fibres at different irradiation doses.
The results of the analysis show that the inclusion of metallic fibres in a polymeric matrix improves the
Fig. 3. Impact resistance of HDPE charged with carbon steel fibres at different irradiation doses.
Fig. 5. Strength at failure of HDPE charged with carbon steel fibres at different irradiation doses.
406
W. Bare´ et al. / Surface and Coatings Technology 158 – 159 (2002) 404–407
Fig. 6. Elongation at break of HDPE alone at different irradiation doses.
mechanical behaviour of the material. For example, the impact resistance increased by up to 600% compared to the value for pure HDPE, as can be observed in Figs. 1–3. Furthermore, this property increases with increasing irradiation dose. In Fig. 2, the resistance to impact of samples reinforced with copper fibres is reported. The values for samples with unidirectionally placed fibres are higher than those for samples with stitched fibres. This can be attributed to the fact that cross-linked
Fig. 7. Elongation at break of HDPE filled with Cu fibres at different irradiation doses.
Fig. 8. Elongation at break of HDPE filled with carbon steel at different irradiation doses.
Fig. 9. Young’s modulus of HDPE alone at different irradiation doses.
fibres introduce more failure sites in the material, in addition to residual tensions generated by the texture of the metallic fibres. This tendency is confirmed by samples filled with carbon steel fibres (see Fig. 4). Due to the natural hardness of this type of metallic fibre, the samples containing steel show higher resistance than those containing copper. It can also be observed that the irradiation affects the properties of the material, improving them at lower doses (10 kJ kgy1). In addition, the tensile strength changed considerably compared to the values obtained with the polymer without charge (Figs. 4 and 5) Depending on the orientation and quantity of the fibres in the matrix, these
Fig. 10. Young’s modulus of HDPE filled with Cu fibres at different irradiation doses.
Fig. 11. Young’s modulus of HDPE filled with carbon steel fibres at different irradiation doses.
W. Bare´ et al. / Surface and Coatings Technology 158 – 159 (2002) 404–407
properties are affected to a greater or lesser extent. During tensile experiments, it could be observed that the polymeric matrix failed (due to fracture) when the fibres were unidirectionally placed, but the fibres did not break: they slid out. This was attributed to the fact that the interface between the polymer and the fibres was not good enough to hold the patterns together. For the stitched fibres, both the polymeric matrix and the fibres broke together. For all cases, it required more strength to break the samples with stitched fibres than for samples with unidirectional fibres. Furthermore, it can be noted that the elasticity decreased, since the quantity of polymer is lowered by addition of the metal (Figs. 6–8). Increasing the quantity of metallic fibres means that the elasticity of the resulting material will be lower than that of the pure polymer. The Young’s modulus increased by up to 100%, independently of the irradiation dose used and the kind of metal (Figs. 9 and 10). In the particular cases of impact resistance (Figs. 2 and 3), strength at failure (Fig. 4), elongation at break (Fig. 6) and Young’s modulus (Figs. 10 and 11), it can be observed that the improvement in properties is more pronounced at lower irradiation doses (approx. 10 kJ kgy1). This demonstrates that some kind of slight interaction exists between the metal and the polymeric matrix due to the irradiation, and at lower doses the HDPE presents a certain degree of physical crosslinking, as found by Chapiro w7x. In order to observe clearly the effects of the irradiation on the metallic fibres, much higher irradiation doses would be required.
407
In this case, the outer electrons of the metal would reach a higher degree of excitation and would probably interact with the free radicals produced in the polymeric matrix by irradiation. 4. Conclusions From the results obtained it can be concluded that the introduction of metallic fibres into a polymer is a possible method of increasing its application field, since it is easy to obtain polymer materials due to their low cost. In addition, irradiation of HDPE filled with metallic fibres contributes to an improvement in the mechanical properties of the compound. Since irradiation affects the polymeric matrix, and eventually also the metallic fibres, it would be of great importance to investigate in more detail the resulting physical, morphological, interfacial and superficial changes in the materials concerned. References w1x R. Thomson, M. Shah, A. Mouritz, Composite Struct. 42 (1998) 107. w2x A. Guz, Proceedings of ICCEy6, Orlando, Florida, 1999, p. 241. w3x A. Seiby, S. Zadjali, Proceedings of ICCST2, Durban, South Africa, 1998, p. 477. w4x S. Zaginaichenko, Z. Matysina, D. Schur, Proceedings of ICCST3, Durban, South Africa, 2000, p. 46. w5x J. Schultz, Polymer Material Science, Prentice Hall Inc, 1974, p. 472. w6x M. Stern, Proceedings of ICCE ’99, Cincinnati, Ohio, 1999. w7x A. Chapiro, Radiation Chemistry of Polymeric Systems, Interscience, London, 1962.