Natural rubber 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...

Thermodynamic Analysis. As reported previously, the storage modulus G of PDMS networks with tetrafunctional crosslinks is independent of frequency between 10 3 and 1 Hz (21). This behaviour which is entirely different from that of vulcanized natural rubber or synthetic polyisoprene networks, was attributed to the lack of entanglements, both trapped and untrapped, in these PDMS networks. Figure 4 shows that G of a network with comb-like crosslinks is also frequency independent within an error of 0.5%. For comparison, two curves for PDMS having tetrafunctional crosslinks are also shown. The flat curves imply that slower relaxations are highly unlikely. Hence a thermodynamic analysis of the G data below 1 Hz can be made as they equal equilibrium moduli. 

From a theoretical point of view, the equilibrium modulus very probably gives the best characterization of a cured rubber. This is due to the relationship between this macroscopic quantity and the molecular structure of the network. Therefore, the determination of the equilibrium modulus has been the subject of many investigations (e.g. 1-9). For just a few specific rubbers, the determination of the equilibrium modulus is relatively easy. The best example is provided by polydimethylsiloxane vulcanizates, which exhibit practically no prolonged relaxations (8, 9). However, the networks of most synthetic rubbers, including natural rubber, usually show very persistent relaxations which impede a close approach to the equilibrium condition (1-8). 

Time-crosslink density superposition. Work of Plazek (6) and Chasset and Thirion (3, 4) on cured rubbers suggests that there is one universal relaxation function in the terminal region, independent of the crosslink density. Their results indicate that the molar mass between crosslinks might be considered as a reducing variable. However, these findings were obtained from compliance measurements on natural rubber vulcanizates,... 

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.

It seems probable that when governmental subsidies are relaxed in the United States and certain other countries where synthetics are produced on emergency funds, natural rubber will be in a fairer position relative to synthetics, and latex rubber may increase its popular lead. In 1952 the world consumed 1,860,000 long tons of latex rubber, and in 1953 about 80,000 more than this. The Far East still produces over 90% of the natural rubber of the world, and nearly half of that percentage comes from Malaya. Africa ships about 3%, and Latin America about 2%. 

Fig. 15. dWIdlj measured by Becker at 10-min relaxation for natural rubber vulcanizate. These values are normalized by shear modulus, G. [Reproduced from Becker, C. W. J. Polymer Sei., Part C, 16, 2893 (1967), a part of Fig. 4 and a part of Fig. 5.]... 

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... 

Relaxation studies on natural rubber (uncrosslinked and crosslinked)19) show that the experimental T2 curve is narrower near the T2 minimum than predicted by the BPP theory. Although the depth of the Tj minimum in some polymers, for example, the one due to the methyl group reorientation in uncured diglycidyl ether... 

Andreis M, Koenig JL (submitted to Rubber Chem. Technol. for publ.) Solid state carbon-13 NMR studies of vulcanized elastomers. VI. Relaxation in sulfur vulcanized natural rubber... 

A good elastomer should not undergo plastic flow in either the stretched or relaxed state, and when stretched should have a memory of its relaxed state. These conditions are best achieved with natural rubber (ds-poIy-2-methyl-1,3-butadiene, ds-polyisoprene Section 13-4) by curing (vulcanizing) with sulfur. Natural rubber is tacky and undergoes plastic flow rather readily, but when it is heated with 1-8% by weight of elemental sulfur in the presence of an accelerator, sulfur cross-links are introduced between the chains. These cross-links reduce plastic flow and provide a reference framework for the stretched polymer to return to when it is allowed to relax. Too much sulfur completely destroys the elastic properties and produces hard rubber of the kind used in cases for storage batteries. 

Several NMR relaxation studies using carbon-black-filled natural rubber (NR), EPDM and butadiene (BR) rubbers have shown that a layer of immobilised, tightly bound rubber is formed on the carbon black surface [20, 62, 79, 87, 89] (Figure 10.9). 

Katz demonstrates by X-ray diffraction that natural rubber is amorphous in the relaxed state and crystalline upon stretching Meyer and Mark show that the crystallographic and the chemical evidence for the chain concept are in agreement... 

In Fig. 12, G and 6 are plotted against cure time at various frequencies. For w = 10 rad/sec, there is no relaxation peak of 6" or cross-over point of G and G". This is due to the fact that at 140 C, the relaxation times of natural rubber at various states of cure are all longer than 0.1 sec (i.e., 1/w). However, there is a relaxation peak and cross-over point of G ... 

Caffrey and Bilderback have made a similar study for natural rubber. Using a Vidicon camera they concluded that the amorphous halo disappears while the preferentially oriented powder pattern appears at the same time. Holl et all., have studied the reversibility of this process in more detail. Thus in Fig. 51 the variation of the modulus and the draw ration are compared with selected Vidicon patterns. corresponds to the onset of the crystallization, 3- to the maximum in crystallisation and 3-i to the melting of the last crystallites upon relaxation. Note that and Xj, occurr, at different draw ratios. This is obviously due to the nucleation process which demands a certain overdrawing while the melting occurs at the equilibrium melting temperature. 

Fig. 51. Change of modulus (0 and draw ratio (X) for natural rubber The Vidicon pictures show the onset of crystallization during stretching (X,), the maximum of the crystallization (X ) and the melting of the last crystallites upon relaxation (X.,)...

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). 

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