Natural rubber behavior

Anorin-38 has also shown an interesting effect as a multifunctional additive (a single additive to replace many of the conventional additives) for natural rubber (NR). It showed excellent blending behavior and compatibility with NR. Aorin-38 enhances the tensile properties and percent elongation, decreases fatigue, acts as an antioxidant and antiozonant, and positively affects many of the other properties, apart from acting as a process aid and a cure enhancer [183-186]. 

Compatibility and various other properties such as morphology, crystalline behavior, structure, mechanical properties of natural rubber-polyethylene blends were investigated by Qin et al. [39]. Polyethylene-b-polyiso-prene acts as a successful compatibilizer here. Mechanical properties of the blends were improved upon the addition of the block copolymer (Table 12). The copolymer locates at the interface, and, thus, reduces the interfacial tension that is reflected in the mechanical properties. As the amount of graft copolymer increases, tensile strength and elongation at break increase and reach a leveling off. 

The results of mechanical properties (presented later in this section) showed that up to 20 phr, the biofillers showed superior strength and elongation behavior than CB, cellulose being the best. After 30 phr the mechanical properties of biocomposites deteriorated because of the poor compatibility of hydrophilic biopolymers with hydrophobic natural rubber(results not shown). While increasing quantity of CB in composites leads to constant increase in the mechanical properties. Scanning electron micrographs revealed presence of polymer-filler adhesion in case of biocomposites at 20 phr. 

Synthetic and natural rubbers are amorphous polymers, typically with glass transition temperatures well below room temperature. Physical or chemical crosslinks limit chain translation and thus prevent viscous flow. The resulting products exhibit elastic behavior, which we exploit in such diverse applications as hoses, automotive tires, and bicycle suspension units. 

Figure 15. Behavior under strain of an unvulcanized tire ply (conventional recipe) based on NR (natural rubber 100%), 1R (synthetic cis-7,4-polyisoprene 100%), BP/1R (a 50/50 blend of IR and txans-butadiene-piperylene copolymer).

Their crystallization behavior compares with natural rubber, as follows (1) their rate of crystallization is more rapid and (2) their amount of crystallinity is temperature dependent, but considerably less strain dependent. These experimental rubbers have excellent green strength and building tack. 

The main conclusions of the strain induced crystallization behavior of high trans polybutadiene based rubber and natural rubber are (1) the rate of crystallization is extremely rapid compared to that of NR (2) the amount of strain induced crystallization is small compared to that of NR, especially at room temperature and (3) for the high trans SBR s relative to NR, crystallization is more sensitive to temperature at low extension ratios, and crystallization is less sensitive to strain. 

Solvent Resistance. One of the distinct advantages of a crystalline thermoplastic elastomer over an amorphous one should be its superior solvent resistance, since the latter types are generally soluble. Table III shows the swelling behavior of the H2-BIB triblocks in toluene at 25°C. It can be seen that the maximum swelling obtained was in the case of the H2-BIB-34, which had the lowest end-block content. Furthermore, the equilibrium swelling ratio of 3-26 obtained for this polymer is considerably less than the value of 5 or 6 generally exhibited by a well-vulcanized natural rubber. 

For a moderately crossllnked network, equation (13) predicts a declining stress with lamellae formation from the amorphous melt. A stress Increase can be achieved with this model only by reorientation of the chain axis to the directions perpendicular (or nearly so) to the stress direction. If then this model is suitable for lightly crystalline materials, its behavior is in good accord with the observations of Luch and Yeh (6) on stretched natural rubber networks. They reported simultaneous lamellae formation and declining network stress. 

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.

The characteristic property of elastomers is their rubber-elastic behavior. Their softening temperature lies below room temperature. In the unvulcanized state, i.e. without crosslinking of the molecular chains, elastomers are plastic and thermo-formable, but in the vulcanized state—within a certain temperature range — they deform elastically. Vulcanization converts natural rubber into the elastic state. A large number of synthetic rubber types and elastomers are known and available on the market. They have a number of specially improved properties over crude rubber, some of them having substantially improved elasticity, heat, low-temperature, weathering and oxidation resistance, wear resistance, resistance to different chemicals, oils etc. 

When we compared the viscosities of solutions of natural rubber and of guttapercha and of other elastomers and later of polyethylene vs.(poly)cis-butadiene, with such bulk properties as moduli, densities, X-ray structures, and adhesiveness, we were greatly helped in understanding these behavioral differences by the studies of Wood (6) on the temperature and stress dependent, melting and freezing,hysteresis of natural rubber, and by the work of Treloar (7) and of Flory (8) on the elasticity and crystallinity of elastomers on stretching. Molecular symmetry and stiffness among closely similar chemical structures, as they affect the enthalpy, the entropy, and phase transitions (perhaps best expressed by AHm and by Clapeyron s... 

Chain flexibility also effects the ability of a polymer to crystallize. Excessive flexibility in a polymer chain as in polysiloxanes and natural rubber leads to an inability of the chains to pack. The chain conformations required for packing cannot be maintained because of the high flexibility of the chains. The flexibility in the cases of the polysiloxanes and natural rubber is due to the bulky Si—O and rxv-olelin groups, respectively. Such polymers remain as almost completely amorphous materials, which, however, show the important property of elastic behavior. 

The cDNAs of the cA-prenyltransferase of H. brasiliensis was successfully identified and expressed in E. coli. The in vitro polymerization of IPP after initiation with FPP using the expressed c/x-prenyltransferase resulted in low degrees of polymerization [267, 268]. After addition of rubber particles to this polymerization, the molecular weight increased tremendously [269], It can be concluded that the rubber particles are essential for rubber biosynthesis. Katarina Cornish established a detailed structural model of the in vivo synthesis of natural rubber in the rubber particle monolayer membrane and partially explained this behavior (see Fig. 12) [251],... 

Oommen, Z., S. Thomas, C. K. Premalatha, and B. Kuriakose, Melt rheological behavior of natural rubber/polyfmethyl methacrylate)/natural rubber-g-poly(methy methacrylate) blends. Polymer, 38(22), 5611-5621 (1997). 

Figure 17 (69) shows the mechanical behavior of different synthetic elastomers. They were found to be generally less efficient than natural rubber in promoting polymerization because of reduced stress during mastication due to greater softening by monomer addition. Nitrile rubber crumbed with methyl methacrylate, styrene and acrylonitrile. 

Becker25 improved the biaxial extension apparatus of Blatz and Ko in such a way that pairs of different values of Xj and X2, i.e., general biaxial extensions, can be obtained. Here, pairs of two rectangular tracks are connected to the pulling rods in asymmetric fashion, as shown in Fig. 7. With the use of this apparatus he obtained valuable information about the behavior of dW/d/j and dW/d/2 for natural rubber vulcanizate. His principal results are presented in Section V. 

Under Behavior Towards Metals and Other Substances. AN very definitely attacks shellac, baked oils and natural rubber compounds if applied as a coating. The only materials that we know of which can be used as coatings for AN and AN solutions are certain polyvinyl chloride coatings and a number of epoxy resins. The Ordnance Department used acid-proof black paint, shellac, baked oil and rubber paints. All were quite unsuccessful over long periods of time... 

Both modifications affect the analysis of dilute solution behavior, and it is difficult to judge how much the e/e0 term is actually needed. In any case, as the authors themselves point out (41), the e/e0 term makes an entirely negligible contribution to solvent activity in concentrated solutions. For example, simple calculations yield a contribution of approximately l%ina 10% solution of natural rubber in benzene at 30° C (M = 500000, [t/]g = 250, y=0.4). It is therefore clear that thermodynamic measurements can furnish no evidence for or against continued collapse in concentrated solutions. 

Price,C., Allen,G., de Candia,F., Kirkham,M.C., Subramaniam,A. Stress-strain behavior of natural rubber vulcanized in the swollen state. Polymer (London) 11, 486-491 (1970). 

Bristow, G.M. Relation between stress-strain behavior and equilibrium volume swelling for peroxide vulcanizates of natural rubber and cis-1,4-polyisoprene. J. Appl. Polymer Sci. 9, 1571-1578 (1965). 

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... 

I11 addition to the primary effect of the great length of the molecule, the details of the distribution of functional groups along tlie polymer chain modify the behavior of these materials. This leads to differences in their applications. For example, natural rubber exists in the rubbery state at... 

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