Cross-linking rubber systems

One of the general understanding about the carbon reinforcement of rubbers, which is vaguely understood but widely accepted, is to consider such a system where well-dispersed and discontinuously connected carbon particles are strongly adhered to the matrix cross-linked rubber. 

However, in the model Figure 18.3, we cannot give any reasonable answers to questions 2 and 3. It is well established that the matrix cross-hnked rubber in the filled system is almost the same as the unfilled cross-linked rubber. It means that the stress-strain relation of the matrix rubber in the filled... 

Cross-linked polymers. In cross-linked polymer systems (Fig. 14.2), polymer chains become chemically linked to each other resulting in a network. Network structures are formed when the average functionality of a mixture of monomers is greater than 2. Network polymers can also be made by chemically linking linear or branched polymers. For example, in a tire, the rubber polymer chains are interconnected with sulfur linkages in a process called vulcanization (Fig. 14.11). 

The thermoplastic elastomers (TPE) are a new class of the polymeric materials, which combine the properties of the chemically cross-linked rubbers and easiness of processing and recycling of the thermoplastics [1-8], The characteristics of the TPE are phase micrononuniformity and specific domain morphology. Their properties are intermediate and are in the range between those, which characterize the polymers, which produce the rigid and elastic phase. These properties of TPE, regardless of its type and structure, are a function of its type, structure and content of both phases, nature and value of interphase actions and manner the phases are linked in the system. 

During vulcanization, sulphur can migrate from the ebonite layer to the rubber and may result in the formation of an interlayer of highly cross-linked rubber having poor physical properties. Formation of such a layer may be avoided if a second adhesive coat consisting of the rubber mix without sulphur is applied. Since ebonite softens at higher temperatures (e.g. 80 °C in the case of a natural rubber system), the use of ebonite bonding is restricted to service temperatures near ambient. 

It is important to observe that the weighting coefficients are fixed at the time of the polymer-wall reaction(they are quenched variables ). In general, they may correspond to the equilibrium configuration of the system at a slab width Lq L. Thus, the calculated properties are bound to depend on the preparation conditions . There is a strong analogy with the elasticity of cross-linked rubbers, as discussed by Deam and Edwards[27]. 

Of somewhat more theoretical than practical interest is the fact that certain bifunctional sulphur-containing systems can cross-link diene rubbers. These include bis-thiols such as 1,3-dimercaptobenzene and bis-thiol acids. The fact that the corresponding monofunctional materials did not cross-link rubber has been considered as part of the evidence demonstrating that conventional vulcanization involves covalent cross-linking. (Other compelling evidence is that diene rub-... 

Radiation cross-linked rubbers appear somewhat weaker mechanically than those prepared by conventional accelerated sulphur systems and they have not yet become of industrial importance. In principle the process has the attraction of providing a vulcanizate which could be free of extractable, perhaps toxic, material. 

At the University of Wisconsin since 19 6, studies of viscoelasticity have evolved from concentrated polymer solutions to undiluted amorphous polymers, dilute solutions, lightly cross-linked rubbers, glassy polymers, blends of different molecular weights, copolymers, cross-linked rubbers with controlled network structures, and so forth. It became evident that each type of system required a different approach. Moreover, in amorphous polymers, the terminal, plateau, and transition zones had to be described separately. Both dynamic (sinusoidal) and transient measurements such as creep and stress relaxation have been utilized. The inderlying theme of this work is the relation of macromolecTilar dynamics—modes of motion of polymer molecules— to mechanical and other physical properties. 

This paper discusses an attempt to prepare and characterize a standardized crosslinked rubber. By careful control of preparations and cure conditions, it should be possible to obtain specimens whose properties are reproducible from batch to batch. Distribution of such materials to those doing research on the viscoelastic behavior of cross-linked polymeric systems will stimulate a rapid advance in our understanding of such systems. Moreover, even in cases where research studies are not intended, they can serve as useful materials for apparatus calibration or for round robin tests. 

Examples of Cure Systems in NR, SBR, and Nitrile Rubber. Table 6 offers examples of recipes for conventional, semi-EV, and EV cure systems ia a simple, carbon black-filled natural mbber compound cured to optimum (t90) cure. The distribution of cross-links obtained is found ia Figure 9 (24). 

Cure Systems of Butyl Rubber and EPDM. Nonhalogenated butyl rubber is a copolymer of isobutjiene with a small percentage of isoprene which provides cross-linking sites. Because the level of unsaturation is low relative to natural mbber or SBR, cure system design generally requites higher levels of fast accelerators such as the dithiocarbamates. Examples of typical butyl mbber cure systems, thein attributes, and principal appHcations have been reviewed (26). Use of conventional and semi-EV techniques can be used in butyl mbber as shown in Table 7 (21). 

The Goodyear vulcanization process takes hours or even days to be produced. Accelerators can be added to reduce the vulcanization time. Accelerators are derived from aniline and other amines, and the most efficient are the mercaptoben-zothiazoles, guanidines, dithiocarbamates, and thiurams (Fig. 32). Sulphenamides can also be used as accelerators for rubber vulcanization. A major change in the sulphur vulcanization was the substitution of lead oxide by zinc oxide. Zinc oxide is an activator of the accelerator system, and the amount generally added in rubber formulations is 3 to 5 phr. Fatty acids (mainly stearic acid) are also added to avoid low curing rates. Today, the cross-linking of any unsaturated rubber can be accomplished in minutes by heating rubber with sulphur, zinc oxide, a fatty acid and the appropriate accelerator. 

Rubber base adhesives can be used without cross-linking. When necessary, essentially all the cross-linking agents normally used in the vulcanization of natural rubber can be used to cross-link elastomers with internal double carbon-carbon bonds. A common system, which requires heat to work, is the combination of sulphur with accelerators (zinc stearate, mercaptobenzothiazole). The use of a sulphur-based cross-linking system with zinc dibutyldithiocarbamate and/or zinc mercaptobenzothiazole allows curing at room temperature. If the formulation is very active, a two-part adhesive is used (sulphur and accelerator are placed in two separate components of the adhesive and mixed just before application). 

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