Natural rubber stress relaxation

Very frequent are the cases of stress-induced crystallizations. A typical case is that of slightly vulcanized natural rubber (1,4-m-polyisoprene) which, under tension producing a sufficient chain orientation, is able to crystallize, while it reverts to its original amorphous phase by relaxation [75],... 

Figure 7.11 Stress relaxation curve for vulcanized natural rubber showing characteristic upswing at higher stresses...

Figure 17 Calculated stress-relaxation behavior at 298 K for five uncross-linked elastomers of M = 200,000 EP, ethylene-propylene (56 44) styrene-butadiene (23.5 76.5), SB natural rubber, N butyl and dimethyl siloxane.

Natural rubber exhibits unique physical and chemical properties. Rubbers stress-strain behavior exhibits the Mullins effect and the Payne effect. It strain crystallizes. Under repeated tensile strain, many filler reinforced rubbers exhibit a reduction in stress after the initial extension, and this is the so-called Mullins Effect which is technically understood as stress decay or relaxation. The phenomenon is named after the British rubber scientist Leonard Mullins, working at MBL Group in Leyland, and can be applied for many purposes as an instantaneous and irreversible softening of the stress-strain curve that occurs whenever the load increases beyond... 

Some polymorphic modifications can be converted from one to another by a change in temperature. Phase transitions can be also induced by an external stress field. Phase transitions under tensile stress can be observed in natural rubber when it orients and crystallizes under tension and reverts to its original amorphous state by relaxation (Mandelkem, 1964). Stress-induced transitions are also observed in some crystalline polymers, e.g. PBT (Jakeways etal., 1975 Yokouchi etal., 1976) and its block copolymers with polyftetramethylene oxide) (PTMO) (Tashiro et al, 1986), PEO (Takahashi et al., 1973 Tashiro Tadokoro, 1978), polyoxacyclobutane (Takahashi et al., 1980), PA6 (Miyasaka Ishikawa, 1968), PVF2 (Lando et al, 1966 Hasegawa et al, 1972), polypivalolactone (Prud homme Marchessault, 1974), keratin (Astbury Woods, 1933 Hearle et al, 1971), and others. These stress-induced phase transitions are either reversible, i.e. the crystal structure reverts to the original structure on relaxation, or irreversible, i.e. the newly formed structure does not revert after relaxation. Examples of the former include PBT, PEO and keratin. 

Host outstanding properties of these products were high resilience and good resistance to stress decay. Resilience Is Illustrated In Figure 1 which shows the stress-strain relationship of a poly[(plvalolactone-b-lsoprene-b-plvalolactone)-g-plvalo-lactone] fiber as It was stretched 300% and then allowed to relax. The shaded area Is the work lost as the fiber was loaded and then unloaded. This area amounts to 13% of the total, which shows that work recovered was 87%. Such high resilience compares very favorably with that of chemically-cured natural rubber. 

The effects of HAF black on the stress relaxation of natural rubber vulcanizates was studied by Gent (178). In unfilled networks the relaxation rate was independent of strain up to 200% extension and then increased with the development of strain induced crystallinity. In the filled rubber the relaxation rate was greatly increased, corresponding to rates attained in the gum at much higher extensions. The results can be explained qualitatively in terms of the strain amplification effect In SBR, which does not crystallize under strain and in cis-polybutadiene, vulcanizates of which crystallize only at very high strains, the large increase in relaxation rate due to carbon black is not found (150). 

Figure 5-16. Physical stress relaxation at 25 °C in a crosslinked network made by a zinc oxide cure of a halobutyl rubber. The elastomer has been soaked in oil to speed the relaxation. The line is the fit to the data using the Chasset-Thirion equation with E =0.018 MPa, t= 1.04 s and m = 0.31. The nature of the polymer and the cure makes for a chemically stable structure otherwise, the plot would begin to curve downward at long time.

Figure 5-17. Chemical stress relaxation of Natural Rubber cured with dicumyl peroxide. [Data from Y. Takahashi and A. V. Tobolsky, Technical report No. 125, Office of Naval Research, N00014-67-A-0151-0011 (1970).]...

Caffrey et al. [47] performed similar studies on natural rubber. Again stress induced crystallization is observed and the WAXS reflections disappear, when the material relaxes. More detailed studies on the reversibility of this process were performed by Holl et al. [48],... 

When the chains are deformed during a bounce, a stress is applied and then rapidly removed. The time required for the chains to regain their original positions is measured by the relaxation time x, as defined in Chapter 13, Section 13.4. Thus, relaxation times are a measure of the ability of the chains to rotate at room temperature the butyl rubber with the bulky methyl groups will not rotate as readily as the civ-polyisoprene, so that when the deformation of chains in the sample of butyl rubber occurs, the chains do not return to their equilibrium positions as rapidly as the natural rubber i.e., X is longer. 

Geethamma, V.G. Pothen, Laly A. Rhao, Bhaskar Neelakantan, N.R. Thomas, Sabu. Tensile stress relaxation of short-coir- fiber -reinforced natural rubber composites. Journal of Applied Polymer Science, 94(1), 96-104 (2004). 

FIGURE 4 Comparison of rheological model of Eqs. (47)-(49) with experiment for natural rubber, (a) Steady-state shear viscosity, (b) Transient shear viscosity at beginning of flow, (c) Stress, relaxation following now. 

FIGURE 7 Stress relaxation in natural rubber-carbon black compound following flow, (a) NR 0 = 0.2. (b) NR 0 = 0.3. 

A more striking difference is found between strain-crystallizing and noncrystallizing elastomers when the stress is not relaxed to zero during each cycle. As shown in Fig. 33, the fatigue life of a natural rubber vulcanizate is greatly increased when the minimum strain is raised from zero to, say, 100% because the crystalline barrier to tearing at the tip of a crack does not then... 

FIGURE 24.9. Stress relaxation curves—as explained in the text—for polyisoprene (natural rubber, 1), oriented low density polyethylene (LDPE) with the draw ratio A = 1.8 (curve 2), indium (3), unoriented LDPE (4), cadmium (5), polyisobutylene (6), and lead (7). 

Figure 10.8 Stress relaxation curve of Ti02-filled natural rubber. (Reproduced with permission from ref. 29.)...

Fig. 41. Stress-strain curve of an elongation/stress-relaxation sequence of sulfiir-crosslinked natural rubber at 300 K...

---