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3.17 Automobile Bumpers☆
3.17 Automobile Bumpers☆ M Nomura, Idemitsu Petrochemical Co. Ltd., Chiba, Japan SM Shanmuga Ramanan, Ponjesly College of Engineering, Nagercoil, India S Arun, National Institute of Technology Meghalaya, Shillong, India r 2018 Elsevier Ltd. All rights reserved.
3.17.1 3.17.2 3.17.3 3.17.4 3.17.5 References Further Reading
3.17.1
Introduction Transition of PP Materials Used in Bumpers of Automobiles Composites Technology Supporting Bumper Materials Technology Considering the Environment Conclusion
460 460 461 466 468 468 468
Introduction
Polypropylene (PP) composites have excellent flowability, mechanical characteristics, weatherability, and chemical resistance, and are economical considering cost aspects. Such composites are used widely as an important raw material especially in automobile parts. After accumulating various technologies for improving PP composites over many years, high-performance PP composites with superior physical properties have finally been developed today. Fig. 1 shows the performance of PP composites in terms of contradicting properties, namely rigidity and impact resistance. It is known that by chemically combining elastomers or inorganic fillers with PP, a material with an extremely wide range of physical properties, such as the physical properties of ABS or modified PPO including even the physical properties of PC can be obtained. The automobile bumper is one of the major applications of PP composites. An overview of the materials for automobile bumpers is reported in this chapter.
3.17.2
Transition of PP Materials Used in Bumpers of Automobiles
Today, PP composites are being used in the bumpers of the majority of automobiles made in Japan. Fig. 2 shows the transition of PP bumper materials. PP elastomer (rubber) composite with improved impact resistance was first used in wraparound bumpers as
Fig. 1 Rigidity/impact resistance properties of PP composites. ☆ Change History: August 2015. S.M. Shanmuga Ramanan and S. Arun added Abstract, Keywords, expanded text with additional review materials and updated the list of references.
460
Comprehensive Composite Materials II, Volume 3
doi:10.1016/B978-0-12-803581-8.03962-X
Automobile Bumpers
461
Fig. 2 Development of PP bumper materials.
an alternative to steel used in bumpers in second half of the 1970s. Subsequently in the 1980s, adequate rigidity was demanded in the material for bumpers because of the trend of using increasingly large-sized bumpers, and in addition to elastomer, talc started being used in composites. In other words, three-element type bumper materials (PP/rubber/filler) made their appearance during this period. Furthermore, PP bumpers began to be used extensively in luxury vehicles; paint became necessary for aesthetic design (quality of appearance), and technologies that contributed to paintability (improved paint film adhesion) were added. Upon entering the 1990s, large bumpers integrated with the body became the norm for automobiles. To keep up with this trend, highperformance bumper materials with high flowability, high rigidity, and high impact strength properties co-existing with other contradictory properties were developed.
3.17.3
Composites Technology Supporting Bumper Materials
The technology for combining elastomer with PP is indispensable for using PP as the bumper material because the impact strength of PP by itself, particularly the impact strength at low temperature, is inadequate. Conversely, this combination has a disadvantage in that it reduces the flexural modulus considerably. Fillers can be dispersed in the PP matrix based on their size and surface treatment (Mnif et al., 2010). To compensate for this disadvantage, the three-element type bumper (PP/elastomer/filler) material was developed. Fig. 3 shows the direction of modified technologies in relation to impact strength and flexural modulus of PP. Although inorganic fillers reduce the impact strength, they improve the flexural modulus significantly. On the other hand, elastomers reduce the flexural modulus but significantly improve the impact strength. If elastomer and filler are used in balanced proportions in PP, both impact strength and flexural modulus can be improved compared to PP alone. Based on this technology, High-performance (high impact strength and high rigidity) three-element type bumpers are being developed. Generally, talc is used as the inorganic filler in this three-element type PP bumper material. Talc is an inorganic compound in tabular crystal form that is very effective in improving the rigidity of PP. Compared to other fillers in tabular crystal form it has various advantages, such as little degradation in appearance and small reduction in impact strength. Talc is said to be the best filler for obtaining a proper balance of the physical properties of PP bumper material. Front bumpers were made of Thermoplastic Olefin Elastomer (TPO ) to take advantage from its elasticity, whereas rear ones are often made from talc-filled TPO to decrease production costs (Szostak, 2011). Moreover, by adding talc, several essential properties of the bumper can be improved such as: (i) (ii) (iii) (iv)
Shrinkage ratio and heat shrinkage ratio become smaller; The coefficient of linear expansion reduces, and the dimensional stability of the product improves; Cooling time reduces, and molding cycle improves; and Fabricability, including paintability, improves.
On the other hand, defects such as the following need to be considered:
462
Automobile Bumpers
Fig. 3 Rigidity/impact resistance properties of modified PP materials.
Fig. 4 Effect of talc particle diameter on PP properties.
(i) Weld strength degrades, (ii) Flow marks stand out, and (iii) Drying becomes necessary during molding. Fig. 4 shows the effect of particle diameter of talc on the flexural modulus and impact strength of the composite. It is noted from Fig. 4 that as the particle size (average particle diameter) of talc decreases, the impact strength improves significantly. The flexural modulus is also seen to improve. In view of these reasons, fine talc with an average particle diameter of about 1 mm is used in the bumper material. As shown in Fig. 2, together with the increase in size of bumpers, the volume of talc added to the material is also increasing. Currently, bumper materials tend to use 20% by weight of talc. Fig. 5 shows the results of measurement of the degree of orientation of PP lamella by wide angle X-ray diffractiometry (WAXD). This figure shows the intensity profiles in the direction of the azimuthal angle of the PP crystal (040) surface. It is noted that the intensity profile jumps in the following order: Talc A (3.8 m) 4 Talc C (7.6 m) 4 Talc E (16.6 m), and the PP lamellae are oriented in the ND direction (thickness direction). This is a phenomenon that is considered to have its origin in the surface structure of the
Automobile Bumpers
463
Fig. 5 Orientation of PP lamella measured by wide-angle X-ray diffractometry.
Fig. 6 Structural factors of PP materials.
talc. This phenomenon cannot be observed in other fillers in sheet form. Therefore, the constraining zone of the talc interface is considered to increase as the particle diameter decreases, resulting in an improvement in rigidity. On the other hand, PP has a number of structural factors as shown in Fig. 6. In recent years, thanks to advances in catalyst technology and polymer technology, it has become possible to control structural factors accurately. One such instance is the ability to improve significantly the flexural modulus by enhancing stereoregularity. Fig. 7 shows the values of flexural modulus for PP with high stereoregularity (highly crystalline PP) filled with fine-grained talc. Generally, the flexural modulus improves significantly when compared to PP containing talc of large particle diameter. This technology is one of the many important technologies that support highly rigid bumper materials developed in recent years. On the other hand, the selection of elastomer is also a very important technology. Studies are being carried out not only considering impact aspects but also considering various other aspects such as weld appearance, appearance of molded product, and fabricability (paintability). Ethylene–Propylene–Rubber (EPR), Ethylene–Propylene–Diene Methylene-linkage (EPDM), and Styrene–Ethylene–Butylene Styrene–Rubber (SEBS) are some of the elastomers being used, with EPR being the most commonly used elastomer. For the same type of EPR, as the diameter of the dispersed particles becomes smaller, the impact strength increases. Fig. 8 shows the relationship between temperature and dependence on impact strength of EPR-modified PP. It is observed that as the percentage of EPR increases, the brittleness/ductility transition temperature shifts to the low-temperature side. As has been
464
Automobile Bumpers
Fig. 7 Flexural properties of talc-filled PP.
Fig. 8 Variation of impact strength with temperature for EPR-modified PP.
pointed out for most resins, when the distance between particles becomes constant, the impact strength improves rapidly. This is true even in the case of PP/EPR systems. However, the distance between particles is not the primary factor on which design of the material is decided. Depending on the characteristics of the base PP and the characteristics of the elastomer, the properties of the
Automobile Bumpers
465
Fig. 9 EPR–PP matrix interface.
material vary significantly. In practice, the design of the material is based on the extensive databases maintained by each company. General considerations are: (i) The smaller the diameter of dispersed rubber particles, the higher the impact strength exhibited by the material. (ii) For the same particle diameter, the larger the molecular weight of rubber, the higher the impact strength. (However, materials with large molecular weight are likely to have dispersed particles with large diameter; therefore, the mixing method needs to be considered carefully.) (iii) For the same molecular weight, the lower the Tg, the higher the impact strength. In recent years, the importance of the PP/elastomer interface has been pointed out. Fig. 9 is an electron micrograph showing the area in the vicinity of the PP/elastomer interface. The condition wherein EPR has fused into the PP can be confirmed from this electron micrograph. The proportion of EPR in PP has a significant effect on the interface condition. EPR with a large C3 composition has high compatibility with the PP matrix. As seen in the interface condition of Fig. 9, even when the molecular weight is high, the diameter of the dispersed particle tends to become small, exhibiting high impact strength. Both control of elastomer and the structural factors of PP shown in Fig. 6 have a large effect on the impact strength. High flowability is being demanded since bumpers are becoming larger, but if the molecular weight is merely reduced, the proportion of low molecular weight component increases, causing the impact strength to reduce considerably. To prevent this, the molecular weight distribution is made narrow and the average molecular weight is reduced. However, to achieve a balance in the impact strength, the increase in the molecular weight of the components is also restricted. Furthermore, the copolymer part of the PP is also controlled. Fig. 10 shows an electron micrograph of ethylene–propylene copolymer. In the example of poor morphology of the copolymer, the PE composition is high so impact strength is low. On the other hand, in the instance of good morphology of copolymer, the PE composition is low, moreover, the randomness of EPR is high, and excellent impact strength is exhibited. Table 1 shows the balanced physical properties of high-performance PP bumper material designed by combining the techniques mentioned above. Compared to conventional bumper materials, the flowability, rigidity, and surface hardness have improved significantly, indicating the superiority of this material.
466
Automobile Bumpers
Fig. 10 Ethylene–propylene copolymer.
Table 1
Mechanical property of bumper materials
Elongation MFR (MI) Flexural modulus Rockwell hardness Brittleness temperature 30 1C IZOD impact Strength Heat sag (120 1C) Coefficient of linear expansion
% g 10 min MPa R-scale 1C Jm 1 mm 105
1
High-performance bumper material
Standard bumper material
R-RIM
500 18 1500 65 40 70
500 9 1000 35 40 70
100 – 500 204 30 NB
5 6.5
9 6.5
7.5 5
Clark et al. (1991) prepared PP composites with continuous glass fiber and the mechanical properties were found to be enhanced. Cheon et al. (1995) made a bumper system using composite material by sheet moulding compound (SMC) with randomly chopped glass fiber composites and the mechanical properties were found to be increased compared to the pure polymer. Minaudo et al. (1997) developed an injection moldable thermoplastic rear bumper system with pole impact protection. Cheon et al. (1999) developed the bumper beam for a passenger car using glass fiber epoxy composite, which was found to be increased in performance compared to the existing bumpers. Gilliard et al. (1999) developed an I-section beam with 40% chopped glass fiber glass mat thermoplastic (GMT). It was observed that the I-section bumper design has improved static load and dynamic impact of mineral filled/chopped glass fiber GMT. Rawson (1999) evaluated the performance of polyolefin in comparison with engineering thermoplastics for blow molded bumper beams for midsize vehicles and it was observed to be significant compared to the existing bumper. Arun and Kanagaraj (2013) reinforced various concentration of Multi walled carbon nanotubes (MWCNT) such as 0, 0.5, 1.0, 1.5, and 2.0 wt% with PP and the mechanical properties was found to be increased up to 1.5 wt% beyond which it was found to be decreased. The maximum mechanical properties such as tensile strength, flexural strength, flexural modulus, and Young’s modulus were found to be enhanced by 42.8, 33.7, 17.4, and 16%, respectively compared to pure PP.
3.17.4
Technology Considering the Environment
Various kinds of technology related to automobile bumpers are being developed considering the global environment. The production and sale of controlled chlorofluorocarbons have been abolished totally with the aim of protecting the ozone layer. The general practice used until now for painting the PP bumper material has been to use 1,1,1-trichloroethane (TCE) as the solvent for cleaning the bumper during the pretreatment stage. During TCE cleaning, the rubber component (EPR) near the surface is eluted and micropores are formed, which contribute to the ‘anchoring effect’ that improves the adhesion of paint.
Automobile Bumpers
467
Fig. 11 Electron micrograph of PP bumper cleaned using non-TCE method.
Fig. 12 Sandwich injection molding technique for recycling of bumper materials.
With the total abolition of TCE, new water-based technologies for cleaning the bumper became necessary. In other words, since achieving an anchoring effect similar to the TCE cannot be anticipated in new cleaning methods, improvements were made to the material surface. PP bumper materials compatible with new cleaning methods have been developed in which: (i) the resistance to swelling of EPR in paint has been improved, and (ii) low molecular weight component containing polar groups is used as the paint modifier. Fig. 11 shows the electron micrograph of the cross-section of a painted bumper that has been cleaned by the new cleaning method. It can be observed that the EPR eluted from the side of the resin to the side of the paint contributes to the anchoring effect. Lightweight vehicle bodies are also being demanded because of the strengthening of regulations for exhaust gas emissions. Therefore, research is being carried out not only on material aspects but also on the structural aspects of bumpers. By using the gas injection method wherein nitrogen gas is injected during injection molding, a hollow part is formed. This new technology has led to the advent of lightweight bumpers with improved rigidity that also has the property of preventing burrs and deformation. Henceforth, the development of bumper systems incorporating new processing techniques is likely to proliferate.
468
Automobile Bumpers
The development of recycling technologies is another aspect of technology that considers the environment. In addition to the use of bumper materials in other parts, various technologies for recycling bumper materials for use in bumpers again have also been proposed. The key to these new technologies is the technique of removal of urethane paint, which becomes the foreign matter. In addition to separation techniques by pulverization or by using mesh, new techniques for decomposing paint film in the mixing stage have also been developed and part of these techniques have been put into practice. The sandwich injection molding technique shown in Fig. 12 is also used for recycling bumper material for use in bumpers again without removal of the paint film. The recycled material is used as the core material in the product and the surface is coated with virgin material (new material). This method is also being used in practice.
3.17.5
Conclusion
Various technologies related specifically to automobile bumper materials have been discussed. Bumper material may be said to be the product of assimilation of PP composite technologies. This material is designed taking into account a very large number of performance factors such as moldability and appearance, and naturally mechanical properties. In recent years, there has been an increasing need to design materials considering environmental effects. With advances in automobiles, the use of new materials is likely to increase. Technologies that are user-friendly to the environment, such as the design of lightweight products and recycling of materials, are likely to be developed and become more sophisticated in the future.
See also: 3.14 Development of Composite Chassis for Motorsports. 3.16 Design, Certification and Field Use of Lightweight Highly Chemically Resistant Bulk Liquid Transport Tanks. 3.18 Composites in Sports Applications
References Arun, S., Kanagaraj, S., 2013. Effect of reinforcement and processing methods in PP/MWCNTs nanocomposites. Advanced Materials Research 747, 575–578. Cheon, S.S., Choi, J.H., Lee, D.G., 1995. Development of the composite bumper beam for passenger cars. Composite Structures 32, 491–499. Cheon, S.S., Lee, T.S., Lee, D.G., 1999. Impact energy absorption characteristics of glass fibre hybrid composites. Composite Structures 46, 267–278. Clark, C.L., Bals, C.K., Layson, M.A., 1991. Effects of fibre and property orientation ‘C’ shaped cross sections, SAE Technical Paper 910049. Gilliard, B., Bassett, W., Haque, E., et al., 1999. I-section bumper with improved impact performance from new mineral-filled glass mat thermoplastic (GMT) composite, SAE Technical Paper 1999−01−1014. Minaudo, B.P., Rawson, J., Montone, M., 1997. Development of a one-piece, injection moulded, thermoplastic rear bumper system with pole impact protection. SAE Technical paper 970483. Mnif, N., Massardier, V., Kallel, T., Elleuch., B., 2010. New(PP/EPR)/nano-CaCO3 based formulations in the perspective of polymerrecycling. Effect of nanoparticles properties and compatibilizers. Polymers for Advanced Technologies 21 (12), 896–903. Rawson, J., 1999. Performance evaluation of polyolefins vs. engineering thermoplastic for blow moulded bumper beams for mid−size vehicles-part. SA Technical Paper 1999− 01−1015. Szostak, M., 2011. Recycling of pp/epdm/talc car bumpers. Chemicke Listy 105 (15), s307–s309.
Further Reading Luda, M.P., Ragosta, G., Musto, P., et al., 2002. Natural ageing of automotive polymer components: Characterization of new and used poly(propylene) based car bumpers. Macromolecular Material Engineering 287 (6), 404–411. Luda, M.P., Ragosta, G., Musto, P., et al., 2003. Regenerative recycling of automotive polymer components: Poly(propylene) based car bumpers. Macromolecular Materials and Engineering 288 (8), 613–620. Pegoraro, M., Severini, F., Di Landro, L., Braglia, R., Kolarik, J., 2000. Structure and morphology of a novel ethylene-propylenecopolymer and its blends with poly(propylene). Macromolecular Materials and Engineering 280−281, 14–19. Pugh, S., 1990. Total Design. Wokingham, England: Addison-Wesley Publishing Company, pp. 5−6.
3.17.1 3.17.2 3.17.3 3.17.4 3.17.5 References Further Reading
3.17.1
Introduction Transition of PP Materials Used in Bumpers of Automobiles Composites Technology Supporting Bumper Materials Technology Considering the Environment Conclusion
460 460 461 466 468 468 468
Introduction
Polypropylene (PP) composites have excellent flowability, mechanical characteristics, weatherability, and chemical resistance, and are economical considering cost aspects. Such composites are used widely as an important raw material especially in automobile parts. After accumulating various technologies for improving PP composites over many years, high-performance PP composites with superior physical properties have finally been developed today. Fig. 1 shows the performance of PP composites in terms of contradicting properties, namely rigidity and impact resistance. It is known that by chemically combining elastomers or inorganic fillers with PP, a material with an extremely wide range of physical properties, such as the physical properties of ABS or modified PPO including even the physical properties of PC can be obtained. The automobile bumper is one of the major applications of PP composites. An overview of the materials for automobile bumpers is reported in this chapter.
3.17.2
Transition of PP Materials Used in Bumpers of Automobiles
Today, PP composites are being used in the bumpers of the majority of automobiles made in Japan. Fig. 2 shows the transition of PP bumper materials. PP elastomer (rubber) composite with improved impact resistance was first used in wraparound bumpers as
Fig. 1 Rigidity/impact resistance properties of PP composites. ☆ Change History: August 2015. S.M. Shanmuga Ramanan and S. Arun added Abstract, Keywords, expanded text with additional review materials and updated the list of references.
460
Comprehensive Composite Materials II, Volume 3
doi:10.1016/B978-0-12-803581-8.03962-X
Automobile Bumpers
461
Fig. 2 Development of PP bumper materials.
an alternative to steel used in bumpers in second half of the 1970s. Subsequently in the 1980s, adequate rigidity was demanded in the material for bumpers because of the trend of using increasingly large-sized bumpers, and in addition to elastomer, talc started being used in composites. In other words, three-element type bumper materials (PP/rubber/filler) made their appearance during this period. Furthermore, PP bumpers began to be used extensively in luxury vehicles; paint became necessary for aesthetic design (quality of appearance), and technologies that contributed to paintability (improved paint film adhesion) were added. Upon entering the 1990s, large bumpers integrated with the body became the norm for automobiles. To keep up with this trend, highperformance bumper materials with high flowability, high rigidity, and high impact strength properties co-existing with other contradictory properties were developed.
3.17.3
Composites Technology Supporting Bumper Materials
The technology for combining elastomer with PP is indispensable for using PP as the bumper material because the impact strength of PP by itself, particularly the impact strength at low temperature, is inadequate. Conversely, this combination has a disadvantage in that it reduces the flexural modulus considerably. Fillers can be dispersed in the PP matrix based on their size and surface treatment (Mnif et al., 2010). To compensate for this disadvantage, the three-element type bumper (PP/elastomer/filler) material was developed. Fig. 3 shows the direction of modified technologies in relation to impact strength and flexural modulus of PP. Although inorganic fillers reduce the impact strength, they improve the flexural modulus significantly. On the other hand, elastomers reduce the flexural modulus but significantly improve the impact strength. If elastomer and filler are used in balanced proportions in PP, both impact strength and flexural modulus can be improved compared to PP alone. Based on this technology, High-performance (high impact strength and high rigidity) three-element type bumpers are being developed. Generally, talc is used as the inorganic filler in this three-element type PP bumper material. Talc is an inorganic compound in tabular crystal form that is very effective in improving the rigidity of PP. Compared to other fillers in tabular crystal form it has various advantages, such as little degradation in appearance and small reduction in impact strength. Talc is said to be the best filler for obtaining a proper balance of the physical properties of PP bumper material. Front bumpers were made of Thermoplastic Olefin Elastomer (TPO ) to take advantage from its elasticity, whereas rear ones are often made from talc-filled TPO to decrease production costs (Szostak, 2011). Moreover, by adding talc, several essential properties of the bumper can be improved such as: (i) (ii) (iii) (iv)
Shrinkage ratio and heat shrinkage ratio become smaller; The coefficient of linear expansion reduces, and the dimensional stability of the product improves; Cooling time reduces, and molding cycle improves; and Fabricability, including paintability, improves.
On the other hand, defects such as the following need to be considered:
462
Automobile Bumpers
Fig. 3 Rigidity/impact resistance properties of modified PP materials.
Fig. 4 Effect of talc particle diameter on PP properties.
(i) Weld strength degrades, (ii) Flow marks stand out, and (iii) Drying becomes necessary during molding. Fig. 4 shows the effect of particle diameter of talc on the flexural modulus and impact strength of the composite. It is noted from Fig. 4 that as the particle size (average particle diameter) of talc decreases, the impact strength improves significantly. The flexural modulus is also seen to improve. In view of these reasons, fine talc with an average particle diameter of about 1 mm is used in the bumper material. As shown in Fig. 2, together with the increase in size of bumpers, the volume of talc added to the material is also increasing. Currently, bumper materials tend to use 20% by weight of talc. Fig. 5 shows the results of measurement of the degree of orientation of PP lamella by wide angle X-ray diffractiometry (WAXD). This figure shows the intensity profiles in the direction of the azimuthal angle of the PP crystal (040) surface. It is noted that the intensity profile jumps in the following order: Talc A (3.8 m) 4 Talc C (7.6 m) 4 Talc E (16.6 m), and the PP lamellae are oriented in the ND direction (thickness direction). This is a phenomenon that is considered to have its origin in the surface structure of the
Automobile Bumpers
463
Fig. 5 Orientation of PP lamella measured by wide-angle X-ray diffractometry.
Fig. 6 Structural factors of PP materials.
talc. This phenomenon cannot be observed in other fillers in sheet form. Therefore, the constraining zone of the talc interface is considered to increase as the particle diameter decreases, resulting in an improvement in rigidity. On the other hand, PP has a number of structural factors as shown in Fig. 6. In recent years, thanks to advances in catalyst technology and polymer technology, it has become possible to control structural factors accurately. One such instance is the ability to improve significantly the flexural modulus by enhancing stereoregularity. Fig. 7 shows the values of flexural modulus for PP with high stereoregularity (highly crystalline PP) filled with fine-grained talc. Generally, the flexural modulus improves significantly when compared to PP containing talc of large particle diameter. This technology is one of the many important technologies that support highly rigid bumper materials developed in recent years. On the other hand, the selection of elastomer is also a very important technology. Studies are being carried out not only considering impact aspects but also considering various other aspects such as weld appearance, appearance of molded product, and fabricability (paintability). Ethylene–Propylene–Rubber (EPR), Ethylene–Propylene–Diene Methylene-linkage (EPDM), and Styrene–Ethylene–Butylene Styrene–Rubber (SEBS) are some of the elastomers being used, with EPR being the most commonly used elastomer. For the same type of EPR, as the diameter of the dispersed particles becomes smaller, the impact strength increases. Fig. 8 shows the relationship between temperature and dependence on impact strength of EPR-modified PP. It is observed that as the percentage of EPR increases, the brittleness/ductility transition temperature shifts to the low-temperature side. As has been
464
Automobile Bumpers
Fig. 7 Flexural properties of talc-filled PP.
Fig. 8 Variation of impact strength with temperature for EPR-modified PP.
pointed out for most resins, when the distance between particles becomes constant, the impact strength improves rapidly. This is true even in the case of PP/EPR systems. However, the distance between particles is not the primary factor on which design of the material is decided. Depending on the characteristics of the base PP and the characteristics of the elastomer, the properties of the
Automobile Bumpers
465
Fig. 9 EPR–PP matrix interface.
material vary significantly. In practice, the design of the material is based on the extensive databases maintained by each company. General considerations are: (i) The smaller the diameter of dispersed rubber particles, the higher the impact strength exhibited by the material. (ii) For the same particle diameter, the larger the molecular weight of rubber, the higher the impact strength. (However, materials with large molecular weight are likely to have dispersed particles with large diameter; therefore, the mixing method needs to be considered carefully.) (iii) For the same molecular weight, the lower the Tg, the higher the impact strength. In recent years, the importance of the PP/elastomer interface has been pointed out. Fig. 9 is an electron micrograph showing the area in the vicinity of the PP/elastomer interface. The condition wherein EPR has fused into the PP can be confirmed from this electron micrograph. The proportion of EPR in PP has a significant effect on the interface condition. EPR with a large C3 composition has high compatibility with the PP matrix. As seen in the interface condition of Fig. 9, even when the molecular weight is high, the diameter of the dispersed particle tends to become small, exhibiting high impact strength. Both control of elastomer and the structural factors of PP shown in Fig. 6 have a large effect on the impact strength. High flowability is being demanded since bumpers are becoming larger, but if the molecular weight is merely reduced, the proportion of low molecular weight component increases, causing the impact strength to reduce considerably. To prevent this, the molecular weight distribution is made narrow and the average molecular weight is reduced. However, to achieve a balance in the impact strength, the increase in the molecular weight of the components is also restricted. Furthermore, the copolymer part of the PP is also controlled. Fig. 10 shows an electron micrograph of ethylene–propylene copolymer. In the example of poor morphology of the copolymer, the PE composition is high so impact strength is low. On the other hand, in the instance of good morphology of copolymer, the PE composition is low, moreover, the randomness of EPR is high, and excellent impact strength is exhibited. Table 1 shows the balanced physical properties of high-performance PP bumper material designed by combining the techniques mentioned above. Compared to conventional bumper materials, the flowability, rigidity, and surface hardness have improved significantly, indicating the superiority of this material.
466
Automobile Bumpers
Fig. 10 Ethylene–propylene copolymer.
Table 1
Mechanical property of bumper materials
Elongation MFR (MI) Flexural modulus Rockwell hardness Brittleness temperature 30 1C IZOD impact Strength Heat sag (120 1C) Coefficient of linear expansion
% g 10 min MPa R-scale 1C Jm 1 mm 105
1
High-performance bumper material
Standard bumper material
R-RIM
500 18 1500 65 40 70
500 9 1000 35 40 70
100 – 500 204 30 NB
5 6.5
9 6.5
7.5 5
Clark et al. (1991) prepared PP composites with continuous glass fiber and the mechanical properties were found to be enhanced. Cheon et al. (1995) made a bumper system using composite material by sheet moulding compound (SMC) with randomly chopped glass fiber composites and the mechanical properties were found to be increased compared to the pure polymer. Minaudo et al. (1997) developed an injection moldable thermoplastic rear bumper system with pole impact protection. Cheon et al. (1999) developed the bumper beam for a passenger car using glass fiber epoxy composite, which was found to be increased in performance compared to the existing bumpers. Gilliard et al. (1999) developed an I-section beam with 40% chopped glass fiber glass mat thermoplastic (GMT). It was observed that the I-section bumper design has improved static load and dynamic impact of mineral filled/chopped glass fiber GMT. Rawson (1999) evaluated the performance of polyolefin in comparison with engineering thermoplastics for blow molded bumper beams for midsize vehicles and it was observed to be significant compared to the existing bumper. Arun and Kanagaraj (2013) reinforced various concentration of Multi walled carbon nanotubes (MWCNT) such as 0, 0.5, 1.0, 1.5, and 2.0 wt% with PP and the mechanical properties was found to be increased up to 1.5 wt% beyond which it was found to be decreased. The maximum mechanical properties such as tensile strength, flexural strength, flexural modulus, and Young’s modulus were found to be enhanced by 42.8, 33.7, 17.4, and 16%, respectively compared to pure PP.
3.17.4
Technology Considering the Environment
Various kinds of technology related to automobile bumpers are being developed considering the global environment. The production and sale of controlled chlorofluorocarbons have been abolished totally with the aim of protecting the ozone layer. The general practice used until now for painting the PP bumper material has been to use 1,1,1-trichloroethane (TCE) as the solvent for cleaning the bumper during the pretreatment stage. During TCE cleaning, the rubber component (EPR) near the surface is eluted and micropores are formed, which contribute to the ‘anchoring effect’ that improves the adhesion of paint.
Automobile Bumpers
467
Fig. 11 Electron micrograph of PP bumper cleaned using non-TCE method.
Fig. 12 Sandwich injection molding technique for recycling of bumper materials.
With the total abolition of TCE, new water-based technologies for cleaning the bumper became necessary. In other words, since achieving an anchoring effect similar to the TCE cannot be anticipated in new cleaning methods, improvements were made to the material surface. PP bumper materials compatible with new cleaning methods have been developed in which: (i) the resistance to swelling of EPR in paint has been improved, and (ii) low molecular weight component containing polar groups is used as the paint modifier. Fig. 11 shows the electron micrograph of the cross-section of a painted bumper that has been cleaned by the new cleaning method. It can be observed that the EPR eluted from the side of the resin to the side of the paint contributes to the anchoring effect. Lightweight vehicle bodies are also being demanded because of the strengthening of regulations for exhaust gas emissions. Therefore, research is being carried out not only on material aspects but also on the structural aspects of bumpers. By using the gas injection method wherein nitrogen gas is injected during injection molding, a hollow part is formed. This new technology has led to the advent of lightweight bumpers with improved rigidity that also has the property of preventing burrs and deformation. Henceforth, the development of bumper systems incorporating new processing techniques is likely to proliferate.
468
Automobile Bumpers
The development of recycling technologies is another aspect of technology that considers the environment. In addition to the use of bumper materials in other parts, various technologies for recycling bumper materials for use in bumpers again have also been proposed. The key to these new technologies is the technique of removal of urethane paint, which becomes the foreign matter. In addition to separation techniques by pulverization or by using mesh, new techniques for decomposing paint film in the mixing stage have also been developed and part of these techniques have been put into practice. The sandwich injection molding technique shown in Fig. 12 is also used for recycling bumper material for use in bumpers again without removal of the paint film. The recycled material is used as the core material in the product and the surface is coated with virgin material (new material). This method is also being used in practice.
3.17.5
Conclusion
Various technologies related specifically to automobile bumper materials have been discussed. Bumper material may be said to be the product of assimilation of PP composite technologies. This material is designed taking into account a very large number of performance factors such as moldability and appearance, and naturally mechanical properties. In recent years, there has been an increasing need to design materials considering environmental effects. With advances in automobiles, the use of new materials is likely to increase. Technologies that are user-friendly to the environment, such as the design of lightweight products and recycling of materials, are likely to be developed and become more sophisticated in the future.
See also: 3.14 Development of Composite Chassis for Motorsports. 3.16 Design, Certification and Field Use of Lightweight Highly Chemically Resistant Bulk Liquid Transport Tanks. 3.18 Composites in Sports Applications
References Arun, S., Kanagaraj, S., 2013. Effect of reinforcement and processing methods in PP/MWCNTs nanocomposites. Advanced Materials Research 747, 575–578. Cheon, S.S., Choi, J.H., Lee, D.G., 1995. Development of the composite bumper beam for passenger cars. Composite Structures 32, 491–499. Cheon, S.S., Lee, T.S., Lee, D.G., 1999. Impact energy absorption characteristics of glass fibre hybrid composites. Composite Structures 46, 267–278. Clark, C.L., Bals, C.K., Layson, M.A., 1991. Effects of fibre and property orientation ‘C’ shaped cross sections, SAE Technical Paper 910049. Gilliard, B., Bassett, W., Haque, E., et al., 1999. I-section bumper with improved impact performance from new mineral-filled glass mat thermoplastic (GMT) composite, SAE Technical Paper 1999−01−1014. Minaudo, B.P., Rawson, J., Montone, M., 1997. Development of a one-piece, injection moulded, thermoplastic rear bumper system with pole impact protection. SAE Technical paper 970483. Mnif, N., Massardier, V., Kallel, T., Elleuch., B., 2010. New(PP/EPR)/nano-CaCO3 based formulations in the perspective of polymerrecycling. Effect of nanoparticles properties and compatibilizers. Polymers for Advanced Technologies 21 (12), 896–903. Rawson, J., 1999. Performance evaluation of polyolefins vs. engineering thermoplastic for blow moulded bumper beams for mid−size vehicles-part. SA Technical Paper 1999− 01−1015. Szostak, M., 2011. Recycling of pp/epdm/talc car bumpers. Chemicke Listy 105 (15), s307–s309.
Further Reading Luda, M.P., Ragosta, G., Musto, P., et al., 2002. Natural ageing of automotive polymer components: Characterization of new and used poly(propylene) based car bumpers. Macromolecular Material Engineering 287 (6), 404–411. Luda, M.P., Ragosta, G., Musto, P., et al., 2003. Regenerative recycling of automotive polymer components: Poly(propylene) based car bumpers. Macromolecular Materials and Engineering 288 (8), 613–620. Pegoraro, M., Severini, F., Di Landro, L., Braglia, R., Kolarik, J., 2000. Structure and morphology of a novel ethylene-propylenecopolymer and its blends with poly(propylene). Macromolecular Materials and Engineering 280−281, 14–19. Pugh, S., 1990. Total Design. Wokingham, England: Addison-Wesley Publishing Company, pp. 5−6.