Rubber strain crystallising

In a (stereo-speeifie) rubber spontaneous crystallisation occurs under strain here also small regions in whieh ehain parts lie parallel, aet as nuclei.These nuelei, which improve the strength eonsiderably, do, however, not grow out into a continuous phase they disappear upon stress release. 

When polyurethanes are stretched about 150%, the nearly-straight, short, soft segments crystallise. This increases the tensile strength and abrasion resistance of polyurethane rubbers. A similar strain-crystallisation phenomenon, which occurs in natural rubber at about 500% strain, limits the extension of rubber bands. Both the polyurethane soft segments and natural rubber have crystal melting points in the region 25-60 °C. In the unstretched state, the chain disorder prevents crystallisation. 

Neither SBR nor cis-PB contain any appreciable amount of crystallinity and in this respect they differ from natural rubber, which crystallises slowly at ambient temperatures. Crystalhsation is a disadvantage before vulcanisation but is a maj or advantage when subjected to stress and strain in a tyre. The higher the apphed stress, the greater is the crystallinity and... 

A comparison was made of the room temperature strain crystallisation of naturally occurring cis-1, 4-polyisoprenes with varying non-rubber contents, i.e. NR... 

More positively there is no substantial evidence that SBR is likely to be replaced extensively by other rubbers in non-tyre applications, this being assured largely by the relatively low cost of the polymer. No new general purpose rubbers appear to be on the horizon at the present time. There may be some replacement where up-grading of product specifications for such properties as ozone and heat resistance requires the use of ethylene-propylene rubbers. On the other hand the advent of the new SBR polymers such as the low styrene strain-crystallising materials described in the previous section with their improved tack and green strength may well, if their initial promise is realised, capture some of the markets currently held by the more expensive natural materials. 

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. 

When a rubbery polymer, such as natural rubber, is stretched the molecules become aligned. This orientation leads to crystallisation. The effect of this so-called strain-induced crystallisation is to make the extended polymer stiffer than the unstrained polymer. Such crystallisation is not permanent but disappears when the sample is allowed to retract and regain its original dimensions. 

One effect of this strain-induced crystallisation is that there is a characteristic upswing in the plot of stress against strain for natural rubbers, as illustrated in Figure 7.11. 

Carbon black increases the tensile strength of an SBR vulcanisate to its 10 to 20-fold (which would, otherwise, be very low), as well as its abrasion resistance. Natural rubber can, because of its stereospecific (cis) chain structure crystallise under strain, and, therefore, reach higher values of its tensile strength for a good abrasion resistance carbon black is also of importance with NR. 

Raman and IR spectroscopic studies dealing with the qualitative and/or quantitative determination of rubber compounding ingredients, i.e., the elastomer itself [22, 26-31], fillers [32, 33], vulcanisation chemicals and other additives [34-37], are not included here. The same applies to studies dealing with the crosslinking of elastomers by means of chemicals other than sulfur or peroxide [38-41], self-crosslinking of elastomers blends [42-44], crystallisation (strain-induced) [45-48] and oxidation/ageing [49-53]. 

No. 18, 1996,p.4009-24 FOURIER TRANSFORM RAMAN STUDY OF THE STRAIN-INDUCED CRYSTALLISATION AND COLD CRYSTALLISATION OF NATURAL RUBBER... 

Two NR samples (cured and uncured) were studied. In all studies, the samples were stretched to 500% elongation. The Fourier-transform Raman spectrum of NR is presented as a function of time of cold soaking at -25C and of strain with respect to laser polarisation. Under both sets of conditions, changes occur in the spectra that can be attributed to crystallisation. Difference spectra showing only those bands due to crystallisation (i.e. spectra of crystalline NR) are presented, which allows the crystallisation process to be discussed with respect to the conditions under which crystallites are formed. A combination of Fourier-transform Raman and Fourier-transform IR depolarisation spectra was used to deduce preliminary assignments for some of the vibrational bands of natural rubber. 40 refs. 

When rubber crystallises upon stretching, e.g., NR, obviously there is structure change and non-linear behaviour is expected. Even with non-crystallising gum rubbers, nonlinear behaviour is observed at large deformations. With the form given in equations (6.20-6.22), the internal structure is affected by strain but is independent of strain rate. Tbe internal structure of rubber is affected by the motion of chain segments and their relaxation times. Therefore, non-linearity should appear in both time-dependence and... 

Titanium-polymerised rubber, CBll, is the most branched. However, the branches are relatively short. The rubber gives strain-softening and strain-induced crystallisation upon stretching [30]. 

Previonsly, it was noticed with the matrix rnbber [33] that in the presence of the larger amonnt of the crystalline particles there was an indication of more strain-induced crystallisation. Among the samples that we examined, the three rnbbers of the VCR series were the only ones, which gave an indication of strain-indnced crystallisation in milling. This means that in order for the gum rubber and the compound to have strain-induced crystallisation during milling the material must be under sufficient tension, i.e.. Region II. 

Another importance of linear viscoelasticity is that it provides a reference for non-linear behaviour that is, the latter is expressed as a deviation from the former with use of appropriate parameters. First is the universal parameter, the elongation ratio, a, which reduces the time-scale of nonlinear behaviour to that of linear behaviour. Next is the modulus shift factor, T(a), which indicates the degree of strain-hardening or strainsoftening. Finally, comparison of linearised elongation data with that of shear data indicates, if they disagree, the presence of strain-induced crystallisation or strain-induced association. All these deviations from linearity are related to the structure of rubber. 

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... 

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