Rubber molecule

Rubber products may be protected against ozone attack by the use of a highly saturated rubber molecule, the use of a wax inhibitor which will "bloom" to the surface, and the use of paper or plastic wrappings to protect the surface. Despite these efforts, rubber products still crack more on the West Coast than on the East Coast of the United States. 

Another interesting innovation is that developed by the Malaysian Rubber Producers Research Association. In this case the coupling agent is first joined to a natural rubber molecule involving an ene molecular reaction. The complex group added contains a silane portion which subsequently couples to filler particles when these are mixed into the rubber. 

Several other elastic materials may be made by copolymerising one of the above monomers with lesser amounts of one or more monomers. Notable amongst these are SBR, a copolymer of butadiene and styrene, and nitrile rubber (NBR), a copolymer of butadiene and acrylonitrile. The natural rubber molecule is structurally a c/i -1,4-polyisoprene so that it is convenient to consider natural rubber in this chapter. Some idea of the relative importance of these materials may be gauged from the data in Table 11.14. 

The strueture of cis-1,4-polybutadiene is very similar to that of the natural rubber molecule. Both materials are unsaturated hydrocarbons but, whereas with the natural rubber molecule, the double bond is activated by the presence of a methyl... 

Like NR, SBR is an unsaturated hydrocarbon polymer. Hence unvulcanised compounds will dissolve in most hydrocarbon solvents and other liquids of similar solubility parameter, whilst vulcanised stocks will swell extensively. Both materials will also undergo many olefinic-type reactions such as oxidation, ozone attack, halogenation, hydrohalogenation and so on, although the activity and detailed reactions differ because of the presence of the adjacent methyl group to the double bond in the natural rubber molecule. Both rubbers may be reinforced by carbon black and neither can be classed as heat-resisting rubbers. 

The close structural similarities between polychloroprene and the natural rubber molecule will be noted. However, whilst the methyl group activates the double bond in the polyisoprene molecule the chlorine atom has the opposite effect in polychloroprene. Thus the polymer is less liable to oxygen and ozone attack. At the same time the a-methylene groups are also deactivated so that accelerated sulphur vulcanisation is not a feasible proposition and alternative curing systems, often involving the pendant vinyl groups arising from 1,2-polymerisation modes, are necessary. 

Today the common practice is first to dissolve the rubber in the styrene monomer and then to polymerise the styrene in the usual way. By this process the resultant blend will contain not only rubber and polystyrene but also a graft polymer where short styrene side chains have been attached to the rubber molecules. This gives a marked improvement in the impact strengths that can be obtained. 

Coran and Patel [33] selected a series of TPEs based on different rubbers and thermoplastics. Three types of rubbers EPDM, ethylene vinyl acetate (EVA), and nitrile (NBR) were selected and the plastics include PP, PS, styrene acrylonitrile (SAN), and PA. It was shown that the ultimate mechanical properties such as stress at break, elongation, and the elastic recovery of these dynamically cured blends increased with the similarity of the rubber and plastic in respect to the critical surface tension for wetting and with the crystallinity of the plastic phase. Critical chain length of the rubber molecule, crystallinity of the hard phase (plastic), and the surface energy are a few of the parameters used in the analysis. Better results are obtained with a crystalline plastic material when the entanglement molecular length of the... 

There were essentially three reasons for this opposition. Firstly, many macromolecular compounds in solution behave as colloids. Hence they were assumed to be identical with the then known inorganic colloids. This in turn implied that they were not macromolecular at all, but were actually composed of small molecules bound together by ill-defined secondary forces. Such thinking led the German chemist C. D. Harries to pursue the search for the rubber molecule in the early years of the twentieth century. He used various mild degradations of natural rubber, which he believed would destroy the colloidal character of the material and yield its constituent molecules, which were assumed to be fairly small. He was, of course, unsuccessful. 

Vulcanisation is the term used for the process in which the rubber molecules are lightly crosslinked in order to reduce plasticity and develop elasticity. It was originally applied to the use of sulfur for this purpose, but is now used for any similar process of cross-linking. Sulfur, though, remains the substance most widely used for this purpose. 

Sulfur reacts very slowly with rubber, and so is compounded with rubber in the presence of accelerators and activators. Typical accelerators are thia-zoles and a typical activator is a mixture of zinc oxide and a fatty acid. The chemistry of the vulcanisation reactions is complicated, but generates a three-dimensional network in which rubber molecules are connected by short chains of sulfur atoms, with an average of about five atoms in each chain. 

This thermodynamic behaviour is consistent with stress-induced crystallisation of the rubber molecules on extension. Such crystallisation would account for the decrease in entropy, as the disorder of the randomly coiled molecules gave way to well-ordered crystalline regions within the specimen. X-Ray diffraction has confirmed that crystallisation does indeed take place, and that the crystallites formed have one axis in the direction of elongation of the rubber. Stressed natural rubbers do not crystallise completely, but instead consist of these crystallites embedded in a matrix of essentially amorphous rubber. Typical dimensions of crystallites in stressed rubber are of the order of 10 to 100 nm, and since the molecules of such materials are typically some 2000 nm in length, they must pass through several alternate crystalline and amorphous regions. 

Carbon blacks are the most widely used fillers for elastomers, especially vulcanised natural rubber. They cause an improvement in stiffness, they increase the tensile strength, and they can also enhance the wear resistance. Other particulate fillers of an inorganic nature, such as metal oxides, carbonates, and silicates, generally do not prove to be nearly so effective as carbon black. This filler, which comes in various grades, is prepared by heat treatment of some sort of organic material, and comes in very small particle sizes, i.e. from 15 to 100 nm. These particles retain some chemical reactivity, and function in part by chemical reaction with the rubber molecules. They thus contribute to the crosslinking of the final material. 

Finer dispersion of silica in THF provides higher surface area to interact with the rubber molecules and the resultant nanocomposites show better mechanical properties than their macrocounterparts. Figure 3.13 illustrates these results. 

The scavenging mechanism states that antiozonants function by migrating towards the surface of the rubber and, due to their exceptional reactivity towards ozone, scavenge the ozone before it can react with the rubber [60]. The scavenging mechanism is based on the fact that all antiozonants react much more rapidly with ozone than do the double bonds of the rubber molecules. This fact distinguishes antiozonants from antioxidants. 

From these results he assigned a cyclooctadiene formula to rubber, and concluded, in the tone of the times, that the rubber molecules combined through the action of "partial valence" into much larger aggregates (39). 

In the manufacture of tyre rubber, 5% of sulphur is used as a crossUnking agent. The probable structures of vulcanised rubber molecules are depicted below ... 

How does the presence of double bonds In rubber molecules Influence their structure and reactivity ... 

Rubber molecules are synthesized from one APP molecule, which initiates the reaction, and the rubber polymer (cw-l,4-polyisoprene) is then polymerized by sequential condensations of the non-allylic IPP (magnesium cations are a required cofactor) with release of a diphosphate at each condensation. After initiation and elongation, a termination event occurs in which the rubber molecule is released from the enzyme. Despite the similar process, remarkable differences exist between plant species with respect to enzymatic reaction mechanisms and product molecular weight. 

The prevulcanization of natural rubber in latex form has also been a subject of much investigation. The cross-linking mechanism is not yet fully understood, but the water apparently plays a major role in it. Irradiation results in the cross-linking of the rubber molecules and in coarsening of the latex particles. A process of cross-linking of natural rubber latex has been developed to the point that it can be used for an industrial-scale application. The irradiation is performed in aqueous media by electron beam without a prorad (sensitizer) at a dose of 200 kGy (20 Mrad) or in the presence of n-butyl acrylate at considerably lower doses, typically 15 kGy. The cross-linked film exhibits physical properties comparable to those obtained from sulfur cured (vulcanized) film. As an alternative, the addition of a variety of chloroal-kanes makes it possible to achieve a maximum tensile strength with radiation doses of less than 5 Mrad (50 kGy). ... 

Kinetics in Non-Polar Media. Polymerization of vinyl monomers in non-polar solvents, i.e., hydrocarbon media, has been almost entirely restricted to the organolithium systems (7), since the latter yield homogeneous solutions. In addition, there has been a particularly strong interest in the polymerization of the 1,3-dienes, e.g., isoprene and butadiene, because these systems lead to high 1,4 chain structures, which yield rubbery polymers. In the case of isoprene, especially, it is possible to actually obtain a polymer with more than 90% of the eis-1,4 chain structure (7, 8, 9), closely resembling the microstructure of the natural rubber molecule. 

After modification of the organoclay by stearic acid, the interlayer spacing increases from 2.98 to 3.96 nm (Fig. 38). So, the enhancement of the d-space has taken place due to the intercalation of stearic acid into the galleries and this preintercalation seems to make penetration of the rubber molecules easier and pave the way for nanostructure formation of the final composites. 

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