STRAIN CRYSTALLISATION

Epoxidized natural mbber is stiU a strain crystallising mbber and therefore retains the high tensile strength of natural mbber. However, as can be seen from Table 5, ia other respects they have very Httie ia common. The epoxidation renders a much higher dampiag mbber, a much-improved resistance to oil swelling (iasofar as a 50 mol % modified natural mbber has similar oil resistance to a 34% nitrile mbber), and much-reduced air permeabiUty. This latest form of modified natural mbber therefore widens the appHcations base of the natural material and enables it to seek markets hitherto the sole province of some specialty synthetic mbbers. 

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. 

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. 

In consequence of strain crystallisation even vulcanisates without fillers have high tensile strength. The elasticity and low-temperature flexibility values are considerably inferior to those of NBR. NIR has been commercially available for several years but the various grades have not yet acquired much economic importance. 

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. 

Polymers below the glass transition temperature are usually rather brittle unless modified by fibre reinforcement or by addition of rubbery additives. In some polymers where there is a small degree of crystallisation it appears that the crystallines act as knots and toughen up the mass of material, as in the case of the polycarbonates. Where, however, there are large spherulite structures this effect is more or less offset by high strains set up at the spherulite boundaries and as in the case of P4MP1 the product is rather brittle. 

Figure 8.9. Diagram of the structure of a drawn polymer fibre. The Young s modulus of the crystallised portions is between 50 and 300 GPa, while that of the interspersed amorphous tangles will be only 0.1-5 GPa. Since the strains are additive, the overall modulus is a weighted average of...

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. 

Density is also found to increase in this region, thus providing additional evidence of crystallisation. Certain synthetic elastomers do not undergo this strain-induced crystallisation. Styrene-butadiene, for example, is a random copolymer and hence lacks the molecular regularity necessary to form crystallites on extension. For this material, the stress-strain curve has a different appearance, as seen in Figure 7.12. 

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. 

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. 

In principle, any of the low temperature tests can be used to study crystallisation effects by conditioning the test pieces at the low temperature for much longer times than is usual. In fact, most of the standard methods include a clause to the effect that the method can be used in this way. In the temperature retraction test, it is suggested that the greater degrees of applied elongation are used when the effects of crystallisation are to be considered, because crystallisation is more rapid in the strained state. 

Leuconostoc mesenteroides and Leuconostoc ssp. are found in fermented foods of plant origin [11]. The occurrence of these bacteria in sugar refineries is responsible for problems in filtration processes because of increased viscosity by the presence of soluble dextran [12,13]. Furthermore, dextran retards the rate of crystallisation of sucrose and adversely affects the crystal shape. The occurrence of dextran in the matrix of dental plaque results from certain Streptococcus strains [14]. The principle organism, Streptococcus mutans, is able to produce water-soluble glucan (named dextran) and water-insoluble... 

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]. 

Critical Oxygen Index (COI), 853 Critical size, 704-705 Critical spherical nucleus, 710, 711 Critical strain, 867, 868 Critical stress energy factor, 474 Critical surface tension of wetting, 232 Critical temperature, 655 Cross-linked polymers, 29 Cross-linking, 148 Cross model, 731 Cross polarisation, 376, 377 Crystallinity, 728, 732, 815 Crystallites/Crystallisation, 690, 725 of rigid macromolecules, 739 Cyclical chain length, 782... 

The stress-strain curve for unfilled NR exhibits a large increase in stress at higher deformations. NR displays, due to its uniform microstructure, a very unique important characteristic, that is, the ability to crystallise under strain, a phenomenon known as strain-induced crystallization. This phenomenon is responsible for the large and abrupt increase in the reduced stress observed at higher deformation corresponding, in fact, to a self-toughening of the elastomer because the crystallites act as additional cross-links in the network. This process can be better visualized by using a Mooney-Rivlin representation, based on the so-called Mooney-Rivlin equation ... 

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