Synthetic polyisoprene rubbers characteristics

Comparative studies on the thermal degradation of polyisoprene, natural rubber (cis-polyisoprene) and gutta percha (t r tns-polyisoprene) all of which have the same chemical composition have shown that they differ in their thermal degradation characteristics. Studies under vacuum at 290-380 "C have shown that the decomposition of natural rubber (NR) is initiated at comparatively low temperatures at a considerable rate, whereas its decomposition rate at higher temperatures (above 330 "C) is to some extent slower than that of gutta percha and polyisoprene (Figure 2.1). The half-life temperature for synthetic polyisoprene is 320 C [1, 2]. 

Synthetic polyisoprene, isoprene rubber (IR), was introduced in the 1950s as odorless rubber with virtually the same properties as natural rubber. Isoprene rubber product and processing properties are better than natural rubber in a number of characteristics. MW and MWD can be controlled for consistent performance and processing properties. 

In this context rheo-optical FTIR spectroscopy has proved a valuable technique to study the phenomenon of strain-induced crystallization on-line to the deformation process of the elastomer under investigation. Whith the aid of an appropriate absorption band which is characteristic of the threedimensional order in the crystalline phase the onset and progress of strain-induced crystallization during elongation and its disappearance upon recovery can be unambigously monitored simultaneously to the mechanical measurements. Representative for several rubber-like materials which have been investigated by this technique in our laboratory the results obtained with sulfur-crosslinked (1.8 % S) natural rubber (100% 1,4-ds-polyisoprene) and a radiation-crosslinked synthetic polyisoprene (93% 1,4-ds-isomer) lall be discussed in some detail here. 

Vulkanox HS is an antioxidant with relatively weak staining and discoloration characteristics, it provides outstanding heat protection in natural rubber (MR), synthetic polyisoprene (IR), polybutadiene (BR), styrene butadiene rubber (SBR), nitrile rubber (NBR) and ethylene propylene rubber (EPIBl), but is less suited>le for polychloroprene (CR). Its performance is improved further in combination with Vulkanox MB-2 antioxidant, which is especially effective in thiuram cured articles with low levels or sulfur or without sulfur. Vulkanox HS antioxidant also yields good protection against rubber poisons. 

For these processing operations rubber compounds undergo high shear deformation and the flow characteristics of synthetic polyisoprenes can be used to advantage. 

SYNTHETIC c/5-POLYISOPRENE CURE CHARACTERISTICS RELATIVE TO NATURAL RUBBER IN A CONVENTIONAL BLACK COMPOUND... 

From the practical standpoint, two minor alterations in cure system should be considered if the cure characteristics of a synthetic polyisoprene need to be altered to match exactly that of a natural rubber compound. A secondary accelerator (e.g. 0T-0-2phr of TMTD) may be added to a sulphur/sulphenamide system to reduce time to optimum cure. An increase of between 5 and 15 %, pro rata, in both sulphur and accelerator will improve the degree of cure, as indicated by maximum torque value on a Monsanto rheometer. 

CREEP CHARACTERISTICS OF NATURAL RUBBER AND SYNTHETIC POLYISOPRENES (CREEP RATES IN PERCENTAGE PER DECADE UNDER A 4 1 SHEAR TO COMPRESSION RATIO AT 23 °C)... 

This viewpoint is supported by increase of the cohesion strength of the mbber compounds due to the modification of rubbers with polar compounds, for example, grafting of maleic anhydride to lithium polyisoprene [5] or by the treatment of SKI-3 mbber with n-nitrosodiphenylamine [6]. Modification of the Indian mbber is made similar to synthetic mbbers by pol5mier-like reactions, for example, by epoxidation [7], maleinization, hydroxylation and using some other techniques. Every of the enumerated reactions imparts a mbber some additional properties that are characteristic for the chemical properties of either functional group. The most extensive possibilities for proceeding of the chemical reactions are inherent for... 

High cw-polyisoprene, either natural or synthetic, will remain an essential raw material for the rubber industry because of its combination of high strength and high resilience characteristics coupled with a broad-based utility. Therefore, it is important for rubber technologists to appreciate both the natural and synthetic polymers and to know how to use each to its best advantage. 

From the point of view of the physical characteristics of rubber compounds, their processing and vulcanisate properties, the significant differences between natural rubber and c/5-polyisoprene and among different sources of the synthetic polymers relate to ... 

The rheological characteristics of synthetic high c/ -polyisoprenes can be used equally to advantage in internal mixing operations. If polyisoprene is substituted for natural rubber at the same nominal batch weight, the batch... 

It should be noted that in this comparison, although the cure system was not specifically designed to give favourable compression set, the synthetic polymers all have lower set values than natural rubber. This again is a characteristic of synthetic high c/5-polyisoprenes. 

Heat ageing characteristics of synthetic high c/5-polyisoprenes tend to be rather better than those of natural rubber. Changes in tensile strength and elongation at break after air oven ageing are less and this is illustrated in this study (Table 8). 

The load deflection characteristics of these types of component are often critical for their application. Natural rubber and the high cis-polyisoprenes have load/deflection properties which are not dissimilar, however, the uniformity of the synthetic polymer is always greater than for the natural product and reproducibility of properties is assured. Since engineering components such as railway locomotive suspension units are often complex and extremely expensive to produce, this uniformity of product ensures a lower incidence of rejection. 

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