Rubbers mechanical crystallisation

Stretching (GR-S, the co-polymer of butadiene and styrene) have a low strength unless they are mixed with carbon black. Obviously, the snapping of the chain molecules into the lattice is necessary to make the chains cohere sufficiently. On the other hand, the percentage of crystallites should not be too large, otherwise the material becomes hard and loses its rubbery properties. Polyethylene, which in the unstretched condition is from 55 to 75% crystalline, surpasses the tolerable limit. The fair average seems to be about 30%, which can be concluded from the unique mechanical properties of natural rubber, which crystallises to this extent on stretching. 

Natural rubber displays the phenomenon known as natural tack. When two clean surfaces of masticated rubber (rubber whose molecular weight has been reduced by mechanical shearing) are brought into contact the two surfaces become strongly attached to each other. This is a consequence of interpenetration of molecular ends followed by crystallisation. Amorphous rubbers such as SBR do not exhibit such tack and it is necessary to add tackifiers such as rosin derivatives and polyterpenes. Several other miscellaneous materials such as factice, pine tar, coumarone-indene resins (see Chapter 17) and bitumens (see Chapter 30) are also used as processing aids. 

Amorphous stereotactic polymers can crystallise, in which condition neighbouring chains are parallel. Because of the unavoidable chain entanglement in the amorphous state, only modest alignment of amorphous polymer chains is usually feasible, and moreover complete crystallisation is impossible under most circumstances, and thus many polymers are semi-crystalline. It is this feature, semicrystallinity, which distinguished polymers most sharply from other kinds of materials. Crystallisation can be from solution or from the melt, to form spherulites, or alternatively (as in a rubber or in high-strength fibres) it can be induced by mechanical means. This last is another crucial difference between polymers and other materials. Unit cells in crystals are much smaller than polymer chain lengths, which leads to a unique structural feature which is further discussed below. 

If rubber is investigated under conditions giving rise to increased intermolecular cohesion e. g, if it partly crystallises on cooling or at high extensions) the theoretical treatment becomes more complicated and extremely difficult since then, besides entropy relations, energetic factors also come into the picture. The mechanical behaviour then also becomes more and more analogous to that of fibrous proteins and cellulose where intermolecular cohesion plays an intrinsic part. 

Let us now see what happens if isotropic rubber is frozen. Crystallisation occurs, but the crystallites are now randomly orientated. This can be most readily demonstrated with raw unvulcanised rubber. Rubber thus frozen is hard and stiff and has lost its elastic character. With regard to a general understanding of the mechanical... 

The fact that carbon black brings the mechanical properties of non-crystallising materials to a level comparable with those of natural rubber, can be explained by the positive heat of wetting. 

Figure 3.405. Influence of mechanical hysteresis on the profile of crack tip and of fracture surface for crystallisable rubbers [1184] (a) stretching (b) compression.

Hence, the results stated in the present section demonstrated accuracy in the description of mechanical behaviour for PCP samples, crystallised at different tension degrees, within the frameworks of the rubber high-elasticity concept. Such a description is... 

A new binary accelerator system uses l-phenyl-2-4-dithiobiuret (DTB) as a secondary accelerator for 2-(4-morpholinothio)-benzothiazole (MBS) for the sulphur vulcanisation of natural rubber. Particular reference is made to the processing characteristics, mechanical properties and swelling behaviour. It was found that that irrespective of the concentration of DTB, all the cure reactions followed first order kinetics. Mechanical properties of the system were analysed for different DTB loading, and the strain crystallising nature of NR was found to be not affected by DTB. Based on processing characteristics, mechanical properties and swelling behaviour, the optimum dosage of DTB was found out. 24 refs. 

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