Tải bản đầy đủ - 0trang
5 Future trends: Key issues in improving the properties of natural rubber (NR)
Chemistry, Manufacture and Applications of Natural Rubber
1. Coran, A. Y., ‘Vulcanization’, in Mark, J. E., Erman, B. and Eirich, F. R., Science
and Technology of Rubber, 2nd edn, San Diego, CA, Academic Press, 1994,
2. Bateman, L., Moore, C. G., Porter, M., Saville, B., ‘Chemistry of vulcanization’,
in Bateman, L., The Chemistry and Physics of Rubber-like Substances, London,
MacLaren & Sons, 1963, 449–561.
3. Coran, A. Y., ‘Vulcanization. Part VII. Kinetics of sulfur vulcanization of natural
rubber in presence of delayed-action accelerators’, Rubber Chem Technol, 1965,
4. Chapman, A. V. and Porter, M., ‘Sulphur vulcanization chemistry’, in Roberts,
A. D., Natural Rubber Science and Technology, Oxford, Oxford University Press,
5. Coleman, M. M., Shelton, J. R. and Koenig, J. L., ‘Sulfur vulcanization of hydrocarbon
diene elastomers’, Ind Eng Chem Prod Res Develop, 1974, 13, 154–166.
6. Trivette, C. D. Jr., Morita, E. and Maender, O. W., ‘Prevulcanization inhibitors’,
Rubber Chem Technol, 1977, 50, 570–600.
7. Coran, A. Y., ‘Chemistry of the vulcanization and protection of elastomers: a review
of the achievements’, J Appl Polym Sci, 2003, 87, 24–30.
8. Ghosh, P., Katare, S., Patkar, P., Caruthers, J. M., Venkatasubramanian, V. and
Walker, K. A., ‘Sulfur vulcanization of natural rubber for benzothiazole accelerated
formulations: from reaction mechanisms to a rational kinetic model’, Rubber Chem
Technol, 2003, 76, 592–693.
9. Heideman, G., Datta, R. N., Noordemeer, J. W. M. and Baarle, B. V., ‘Activators
in accelerated sulfur vulcanization’, Rubber Chem Technol, 2004, 77, 512–541.
10. Nieuwenhuizen, P. J., Ehlers, A. W., Haasnoot, J. G., Janse, S. R., Reedijk, J.
and Baerends, E. J., ‘The mechanism of zinc(II)-dithiocarbamate-accelerated
vulcanization uncovered: theoretical and experimental evidence’, Am Chem Soc,
1999, 121, 163–168.
11. Stein, R. S., ‘Determination of the inhomogeneity of crosslinking of a rubber by
light scattering’, J Polym Sci, Polym Lett Ed, 1969, 7, 657–660.
12. Shibayama, M., ‘Spatial inhomogeneity and dynamic fluctuations of polymer gels’,
Macromol Chem Phys, 1998, 199, 1–30.
13. Ikeda, Y., Higashitani, N., Hijikata, K., Kokubo, Y., Morita, Y., Shibayama, M.,
Osaka, N., Suzuki, T., Endo, H. and Kohjiya, S., ‘Vulcanization: new focus on a
traditional technology by small-angle neutron scattering’, Macromolecules, 2009,
14. Bastide, J., Duplessix, R., Picot, C. and Candau, S., ‘Small angle neutron scattering
and light spectroscopy investigation of polystyrene gels under osmotic deswelling’,
Macromolecules, 1984, 17, 83–93.
15. Higgins, J. S. and Benoit, H. C., Polymers and Neutron Scattering, Oxford, Clarendon
16. Shibayama, M., ‘Universality and specificity of polymer gels viewed by scattering
methods’, Bull Chem Soc Jpn, 2006, 79, 1799–1819.
17. Shibayama, M., Tanaka, T. and Han, C. C., ‘Small-angle neutron scattering study on
poly(N-isopropyl acrylamide) gels near their volume-phase transition temperature’,
J Chem Phys, 1992, 97, 6829–6841.
18. Horkay, F., McKenna, G. B., Deschamps, P. and Geissler, E., ‘Neutron scattering
Network control by vulcanization for sulfur cross-linked NR
properties of randomly cross-linked polyisoprene gels’, Macromolecules, 2000, 33,
19.Karino, T., Okumura, Y., Zhao, C., Kataoka, T., Ito, K. and Shibayama, M., ‘SANS
studies on deformation mechanism of slide-ring gel’, Macromolecules, 2005, 38,
20.Archer, B. L., Barnard, D., Cockbain, E. G., Dickenson, P. B. and McMullen, A.
I., ‘Structure, composition and biochemistry of Hevea latex’, in Bateman, L., The
Chemistry and Physics of rubber-like substances, London, MacLaren & Sons,
21. Wititsuwannakul, D. and Wititsuwannakul, R., ‘Biochemistry of natural rubber and
structure of latex’, in Steinbuchel, A., Biopolymers, Vol. 2, Weinheim, Wiley-VCH,
22.Karino, T., Ikeda, Y., Yasuda, Y., Kohjiya, S. and Shibayama, M., ‘Nonuniformity
in natural rubber as revealed by small-angle neutron scattering, small-angle X-ray
scattering, and atomic force microscopy’, Biomacromolecules, 2007, 8, 693–699.
23. Candau, S., Bastide, J. and Delsanti, M., ‘Structural, elastic, and dynamic properties
of swollen polymer networks’, Adv Polym Sci, 1982, 44, 27–71.
24.Dusek, K. and Prins, W., ‘Structure and elasticity of non-crystalline polymer
networks’, Adv Polym Sci, 1969, 6, 1–102.
25. Okabe, S., Nagao, M., Karino, T., Watanabe, S., Adachi, T., Shimizu, H. and
Shibayama, M., ‘Upgrade of the 32 m small-angle neutron scattering instrument
SANS-U’, J Appl Crysallogr, 2005, 38, 1035–1037.
26. Okabe, S., Karino, T., Nagao, M., Watanabe, S. and Shibayama, M., ‘Current status
of the 32 m small-angle neutron scattering instrument, SANS-U’, Nucl Instrum
Methods Phys Res, Sect A, 2007, 572, 853–858.
27. de Gennes, P. G., Scaling Concepts in Polymer Physics, Ithaca, NY, Cornell
University Press, 1979.
28. Mallam, S., Hecht, A. M., Geissler, E. and Pruvost, P., ‘Structure of swollen
poly(dimethyl siloxane) gels’, J Chem Phys, 1989, 91, 6447–6454.
29. Mallam, S., Horkay, F., Hecht, A. M., Rennie, A. R. and Geissler, E., ‘Microscopic
and macroscopic thermodynamic observations in swollen poly(dimethylsiloxane)
networks’, Macromolecules, 1991, 24, 543–548.
30. Horkay, F., Hecht, A. M., Mallam, S. and Geissler, E., ‘Macroscopic and
microscopic thermodynamic observations in swollen poly(vinyl acetate) networks’,
Macromolecules, 1991, 24, 2896–2902.
31. Onuki, A., ‘Scattering from deformed swollen gels with heterogeneities’, J Phys
II France 1992, 2, 45–61.
32. Shibayama, M., Takahashi, H. and Nomura, S., ‘Small-angle neutron scattering
study on end-linked poly(tetrahydrofuran) networks’, Macromolecules, 1995, 28,
33. Shibayama, M., Isono, K., Okabe, S., Karino, T. and Nagao, M., ‘SANS study on
pressure-induced phase separation of poly(N-isopropylacrylamide) aqueous solutions
and gels’, Macromolecules, 2004, 37, 2909–2918.
34. Wu, W., Shibayama, M., Roy, S., Kurokawa, H. Z., Coyen, L. D., Nomura, S. and
Stein, R. S., ‘Physical gels of aqueous poly(vinyl alcohol) solutions: a small-angle
neutron-scattering study’, Macromolecules, 1990, 23, 2245–2251.
35. Soni, V. K. and Stein, R. S., ‘Light scattering studies of poly(dimethylsiloxane)
solutions and swollen networks’, Macromolecules, 1990, 23, 5257–5265.
36.Debye, P. and Bueche, A. M., ‘Scattering by an inhomogeneous solid’, J Appl
Phys, 1949, 20, 518–525.
Chemistry, Manufacture and Applications of Natural Rubber
37. Ikeda, Y., Yasuda, Y., Hijikata, K., Tosaka, M. and Kohjiya, S., ‘Comparative
study on strain-induced crystallization behavior of peroxide cross-linked and sulfur
cross-linked natural rubber’, Macromolecules, 2008, 41, 5876–5884.
38. Fujimoto, K., ‘Inhomogeneous structure, fracture and fatigue phenomena of rubber
(in Japanese)’, Nippon Gomu Kyokaishi, 1964, 37, 602–619.
39. Park, C. R., ‘Relations between vulcanization and reinforcement’, Ind Eng Chem,
1939, 31, 1402–1406.
40.Donnet, J. B., Bansal, R. C. and Wang, M. J., Carbon Black, New York, Marcel
41.Kohjiya, S., Katoh, S., Shimanuki, J., Hasegawa, T. and Ikeda, Y., ‘Nano-structural
observation of carbon black dispersion in natural rubber matrix by three-dimensional
transmission electron microscopy’, J Mater Sci, 2005, 40, 2553–2555.
42.Kohjiya, S., Katoh, A., Suda, T., Shimanuki, J. and Ikeda, Y., ‘Visualization of
carbon black networks in rubbery matrix by skeletonization of 3D-TEM image’,
Polymer, 2006, 47, 3298–3301.
43.Kato, A., Kohjiya, S. and Ikeda, Y., ‘Nanostructure in traditional composites of
natural rubber and reinforcing silica’, Rubber Chem Technol, 2007, 80, 690–700.
45. Tosaka, M., Murakami, S., Poompradub, S., Kohjiya, S., Ikeda, Y., Toki, S., Sics,
I. and Hsiao, B. S., ‘Orientation and crystallization of natural rubber network as
revealed by WAXD using synchrotron radiation’, Macromolecules, 2004, 37,
46. Tosaka, M., Kohjiya, S., Murakami, S., Poompradub, S., Ikeda, Y., Toki, S., Sics,
I. and Hsiao, B. S., ‘Effect of network-chain length on strain-induced crystallization
of NR and IR vulcanizates’, Rubber Chem Technol, 2004, 77, 711–723.
47. Ikeda, Y., Yasuda, Y., Makino, S., Yamamoto, S., Tosaka, M., Senoo, K. and
Kohjiya, S., ‘Strain-induced crystallization of peroxide-crosslinked natural rubber’,
Polymer, 2007, 48, 1171–1175.
48.Determination of crystallinity index of IR-3-Z4-St0 was not done, because the
significant WAXD patterns of ZnO were recognized due to the large amount and
the absence of stearic acid.
49. Suzuki, T., Osaka, N., Endo, H., Shibayama, M., Ikeda, Y., Asai, H., Higashitani,
N., Kokubo, Y. and Kohjiya, S., ‘Nonuniformity in cross-linked natural rubber as
revealed by contrast-variation small-angle neutron scattering’, Macromolecules,
2010, 43, 1556–1563.
The effect of strain-induced crystallization
(SIC) on the physical properties of
natural rubber (NR)
S. T o k i, Natural Metal and Materials Technology
Abstract: The strain-induced crystallization (SIC) of vulcanized and
un-vulcanized natural rubber (NR) has been considered a key element of
the superiority of NR. Since filled NR compounds show SIC more than
pure NR and un-vulcanized NR, the mechanical properties of NR and NR
compounds at large strain – such as stress relaxation, stress–strain relation,
tear resistance and green strength – are related to SIC. Both practical
and academic interests have driven research on SIC in NR for about 90
years, but the contribution of SIC to mechanical properties still remains
controversial. Recent synchrotron X-ray analysis makes it possible to
analyze the time-resolved phenomena of SIC and mechanical properties.
Key words: crystallization, tear resistance, stress relaxation, green strength,
The strain-induced crystallization (SIC) of vulcanized and un-vulcanized
natural rubber (NR) was first reported by Katz using X-ray diffraction in
1925 (Katz, 1925a, 1925b, 1925c). Katz’s work led to further research into
the contribution of SIC to the mechanical properties of NR. Since then, SIC
has been considered as an essential cause of the mechanical superiority of
NR; the superior tear resistance of vulcanized NR, compared to vulcanized
synthetic rubbers, is an example. Stress relaxation and the decrease in volume
of vulcanized NR occur simultaneously, a phenomenon known to result from
SIC (Gent, 1954a,b). The stress in the stress–strain relation of un-vulcanized
NR is larger than the stress in any un-vulcanized synthetic rubbers; this is
also thought to be related to SIC.
SIC is also an important contribution in NR processing. In rubber goods
manufacturing, green strength (modulus and toughness of un-vulcanized
rubber) is particularly important during processing, as un-vulcanized
rubber is blended, extruded and formed before it is finally vulcanized. Unvulcanized NR is tough and strong, due to its high viscosity and SIC. Filled
NR compounds show SIC at smaller onset strains than pure NR, since rubber
© 2014 Woodhead Publishing Limited
Chemistry, Manufacture and Applications of Natural Rubber
chains may deform to compensate for the lack of deformation of fillers in
NR compounds. Furthermore, fillers may accelerate SIC, since the amount
of crystalline fraction is larger than the calculated value of the volume effect
of the fillers (Poompradub et al., 2005; Gonzalez et al., 2008a, 2008b; Toki
et al., 2008a; Weng et al., 2010).
SIC presents technical reasons to favor the practical use of NR and NR
compounds. In an aeroplane landing, tire temperature climbs immediately
due to the wide area and high speed of deformation as contact with the
ground is made. Almost all airplane tire components are composed of NR
compounds, since the SIC of NR is key to withstanding these sudden increases
in temperature and deformation. Key parts of truck and passenger tires are
also composed of NR compounds.
However, the question of whether SIC is an essential or minor element
in tensile strength is still controversial. The theory of rubber elasticity,
established in the 1940s, suggests that the elasticity of rubber originates from
entropy. Mathematical network models using the theory of rubber elasticity
were able to simulate the stress–strain relation of NR without reference to
SIC (Guth and James, 1941; James and Guth, 1943; Treloar, 1975; Arruda
and Boyce, 1993; Boyce and Arruda, 2000). This chapter discusses the
mechanical properties of NR such as stress relaxation, stress–strain relations,
tear resistance and green strength, and how they relate to the SIC of NR in
the vulcanized and un-vulcanized state.
Temperature-induced crystallization (TIC) and
strain-induced crystallization (SIC)
The temperature-induced crystallization (TIC) of un-vulcanized NR is well
known as the term ‘cold crystallization’, since raw NR readily shows whitening
and crystallization at sustained low temperatures. After vulcanization, which
creates three-dimensional network structures, the sulfur content of NR
determines the extent of TIC; vulcanized NR with a very low sulfur content
(less than 0.5 parts per hundred rubber sulfur) may show TIC (Bekkedahl and
Wood, 1941) much more readily than ordinary sulfur-vulcanized NR. Cold
crystallization has been studied by measuring the decrease in NR volume
under low temperatures. Crystallization of un-vulcanized NR in a 24-hour
period has been measured in a temperature range from –50°C to 2°C; the
maximum rate of cold crystallization is at –25°C, with a time to crystallization
of around 2.5 hours. The maximum crystalline content of cold crystallizing
NR is around 27% by density (Wood and Bekkedahl, 1946; Treloar, 1975).
The broad range of crystallization temperatures and the low value of the
maximum crystalline content suggest that the high molecular weight, broad
molecular weight distribution and the gel component of NR may decrease
the mobility of molecules, impeding crystal formation. Wide-angle X-ray
The effect of SIC on the physical properties of NR
diffraction (WAXD) studies have confirmed the presence of crystals in cold
crystallizing un-vulcanized NR, although the microstructure of crystals – as
may be determined by small angle X-ray scattering (SAXS) – has not yet
been reported. Therefore, un-vulcanized NR creates crystals without a certain
order of lamellar structure under static conditions at low temperatures.
SIC has been observed at high temperatures (75°C and 100°C) (Flory,
1947; Toki et al., 2005). Flory found SIC occuring with a rise in melting
temperature (Tm), attributing this rise to decreased entropy due to deformation
(Flory, 1947; Yamamoto and White, 1971; Tosaka, 2009); this in turn
suggests that higher deformation induces higher Tm of SIC. SIC takes place
over a duration of 60–100 nanoseconds (Mitchell and Meier, 1968; Tosaka
et al., 2006), in contrast to the several hours required for TIC. At the high
altitudes of intercontinental flight, airplane tires are exposed to temperatures
of –50°C and low air pressure for more than six hours. In such cases, NR will
crystallize, but not by TIC; NR compounds might be vulcanized with more
than 2 phr of sulfur, a few phr of stearic acid, a few phr of anti-oxidants,
and about 40 phr of carbon black, all of which hinder TIC. Vital parts of
the tire – especially the fiber cord coating rubber – may already undergo
strain when the tire is formed and vulcanized; at higher altitudes, where air
pressure inside of the tire is high and the air pressure on the outside reduces
significantly, the tire would be expanded by air pressure, increasing strain on
the coating gum. NR compounds in an airplane tire might thus be crystallized
by SIC. The comparison of TIC and SIC on crystal structure and crystallites
is discussed (Che et al., 2013b).
Stress relaxation and SIC
When vulcanized NR is stretched and fixed at certain extension, it shows
stress relaxation. The stress relaxation of vulcanized rubber has been studied
extensively with reference to birefringence (Treloar, 1941), dilatometry
(Gent, 1954a), and X-ray diffraction (Toki et al., 2005; Rault et al., 2006).
The saturation values of birefringence of vulcanized NR at fixed strain are
approximately proportional to the changes in density by different extensions
at 0°C, suggesting that the orientation and crystallization of rubber chains
occur in deformed state (Treloar, 1941). The stress in deformed vulcanized
NR at low temperature becomes zero in a comparatively short timescale.
The changes of stress and volume of peroxide vulcanized NR sample by
extension at –26°C are measured simultaneously as shown in Figs 5.1 and 5.2
(Gent, 1954a). The sample was a long square bar in the dilatometer and was
stretched in the dilatometer at –26°C. At all extension ratios l = 1.5, 2.0, 2.5,
and 3.0, stress was relaxed to zero, and volume decreased significantly.
The Avrami equation can be used to analyze crystal growth in stretched
vulcanized NR. Initial polyhedral crystal growth is replaced by aciform