Synthetic polyisoprene rubbers vulcanisation

The oxidation of diene rubbers proceeds under the influence of atmospheric oxygen even at room temperature, and results in the hardening and fragility of the surface layer. In its initial stages, the oxidative degradation of natural rubber is characterised by softening of the material and the appearance of stickiness rubber elasticity then decreases and it cracks [2]. Non-vulcanised synthetic polyisoprene is oxidised extensively even at room temperature [47]. The oxidation of rubber proceeds at its double bonds and at the single bond a to the tertiary carbon atom. 

The structural parameters, determined by infra-red spectroscopic measurements (Table 1) show that the synthetic polyisoprenes produced using the Ziegler catalyst systems are closely similar to each other and are almost as structurally pure as natural rubber. Recent studies using the NMR technique indicate that the natural rubber hydrocarbon is at least 99-6% c/.y-l,4-polyisoprene. The small stereo-irregularities present in the synthetic polymers are sufficient to cause a reduced tendency for the synthetic polymers to crystallise either at low temperature or induced by applied strains. This difference in the rate of crystallisation, or perhaps the magnitude of the crystallites formed, is suggested to influence both processing and vulcanisate properties. The alkyl/lithium catalysed rubbers... 

All the well-established systems of vulcanisation used for natural rubber can be used for synthetic polyisoprenes and usually little or no change is required in the vulcanisation recipe. Nevertheless, there are certain factors which are important and advantages to be gained by correct formulation. 

Natural rubber invariably contains fatty acids which have an activating effect in accelerated sulphur vulcanisation systems. This type of activator is absent in synthetic polyisoprenes and must be added in sufficient quantity (1 -5-2 0 phr) to ensure full crosslink development. Alternative activators of a similar chemical nature, such as zinc stearate or zinc 2-ethylhexanoate, have been used at lower levels (see Section 7). 

In gum or mineral filled compounds, synthetic polyisoprenes exhibit longer times to optimum cure than natural rubber and significant differences between the various synthetic polymers are observed with some vulcanisation systems. The lack of naturally occurring amine activators present in natural rubber becomes evident in a simple thiazole-accelerated sulphur cure system. This is very well illustrated by observing the incremental addition of diphenylguanidine (DPG) to a MBT-accelerated sulphur system (Table 5) where the lack of added amine accelerator shows a very slow cure for the synthetic polymer but, if sufficient is added, the differences between the rubbers become indiscernible. 

In addition to the slower rate of crystallisation of synthetic polyisoprenes in the raw state relative to natural rubber, which was mentioned earlier, the effect persists in vulcanisates. It has been shown by measuring the times ( 1/4) for 25 % stress relaxation at — 26 °C of vulcanisates at 150 % extension that crystallisation is an order of magnitude slower for the synthetic rubber (Table 9). 

CRYSTALLISATION QUARTER LIVES (/j/J AT -26°C OF VULCANISED NATURAL RUBBER AND SYNTHETIC POLYISOPRENE... 

The compression set of synthetic polyisoprenes is also improved by these curing techniques and, moreover, they give substantially lower compression set than natural rubber in each of the vulcanisation systems. 

In two situations in particular MAS NMR has a strong edge over IR or NMR with solubilisation [696]. The first involves highly cured samples or samples where solubilisation of the elastomer component is difficult or impossible. For example, peroxide cured rubber is difficult to devulcanise using ODCB. Here, MAS NMR with or without CP, as appropriate, provides spectra of equal quality as for samples cured to a lesser degree. The problems of incomplete or selective solubilisation of elastomeric components can be avoided. MAS NMR maybe the method of choice for peroxide-cured rubber. The second application for which MAS NMR is well suited is the aforementioned analysis of relatively small amounts of NR or synthetic c -polyisoprene in filled vulcanisates [696]. 

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 ... 

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