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5 Future trends: Key issues in improving the properties of natural rubber (NR)

5 Future trends: Key issues in improving the properties of natural rubber (NR)

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Chemistry, Manufacture and Applications of Natural Rubber


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Chemistry, Manufacture and Applications of Natural Rubber

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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

Center, Thailand

DOI: 10.1533/9780857096913.1.135

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,

natural rubber.



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

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