Rubber free-radical reaction

Solomon (3, h, 5.) reported that various clays inhibited or retarded free radical reactions such as thermal and peroxide-initiated polymerization of methyl methacrylate and styrene, peroxide-initiated styrene-unsaturated polyester copolymerization, as well as sulfur vulcanization of styrene-butadiene copolymer rubber. The proposed mechanism for inhibition involved deactivation of free radicals by a one-electron transfer to octahedral aluminum sites on the clay, resulting in a conversion of the free radical, i.e. catalyst radical or chain radical, to a cation which is inactive in these radical initiated and/or propagated reactions. 

Cumene Hydroperoxide is used as a catalyst for free radical reactions, such as polymerization as a vulcanization accelerator in rubber-reclaiming flotation and as a poly-... 

The alternating tendency in copolymeri/ation was established on a quantitative basis by Frank F<. Mayo (of the Stanford Research Institute) and Cheves Walling (of the University of Utah) while working in the laboratories of the U.S. Rubber Company. Their work was fundamental to the development of free radical chemistry it showed clearly for the first time the dependence of reactivity on the nature of the attacking free radical, and led directly to the concept of polar factors, working not only in copolymerization and other additions of free radicals, but in free radical reactions of all kinds. 

Over half of the remaining market for products used in the processing of rubber is made up of antioxidants, antiozonants and stabilizers, either amino compounds or phenols. Aniline is used to manufacture vulcanization accelerators, antioxidants and antidegradants. Of the latter, several are A-substituted derivatives of p-phenylenediamine and octyl dipheny-lamine. Diphenylamines terminate free-radical reactions by donating the amino hydrogen, and are used to protect a wide range of polymers and elastomers. Many synthetic rubbers incorporate alkylated diphenylamine antioxidants. Other antioxidants include aryl amine resinous products from, e.g. condensation of aniline and acetone in the presence of... 

Graft Copolymers. In graft copolymerization, a preformed polymer with residual double bonds or active hydrogens is either dispersed or dissolved in the monomer in the absence or presence of a solvent. On this backbone, the monomer is grafted in free-radical reaction. Impact polystyrene is made commercially in three steps first, solid polybutadiene rubber is cut and dispersed as small particles in styrene monomer. Secondly, bulk prepolymerization and thirdly, completion of the polymerization in either bulk or aqueous suspension is made. During the prepolymerization step, styrene starts to polymerize by itself forming droplets of polystyrene with phase separation. When equal phase volumes are attained, phase inversion occurs. The droplets of polystyrene become the continuous phase in which the rubber particles are dispersed. R. L. Kruse has determined the solubility parameter for the phase equilibrium. 

Halogenation reactions of unsaturated polymers follow two simultaneous paths, ionic and free radical. Ionic mechanisms give soluble products from chlorination reactions of polybutadiene." The free-radical mechanisms, on the other hand, cause crosslinking, isomerization, and addition products. If the free-radical reactions are suppressed, soluble materials form. Natural rubber can be chlorinated in benzene with addition of as much as 30% by weight of chlorine without cycliza-tion. " Also, chlorination of polyalkenamers, both cis and trans, yields soluble polymers. X-rays show that the products are partly crystalline. The crystalline segments obtained from 1,4-trans-polyisoprene are diisotactic poly( 0 /rw-dichlorobutamer)s while those obtained from the 1,4-cis isomer are diisotactic polyOAfieo-l,2-dichlorobutamer)s. ... 

In spite of the great discoveries by Ziegler and Natta, most synthetic polymers are still made by free-radical reactions. Some of the important homopolymers are poly (vinyl chloride), poly (methyl methacrylate), polystyrene, and low-density polyethylene. Other important polymers made by free-radical reactions contain two or more monomers, for example, the styrene-butadiene rubbers, and the acrylonitrile-butadiene-styrene plastics. Most of these polymers are not stereoregular. A few that are represent the subject of this section. 

The chemical crosslinks or chemisorptions of the polymer on to the filler surface due to free radical reaction between carbon atoms in the filler and in the rubber. 

These considerations account for the formation of a viscous liquid film on certain materials, and not on others, during frictional sliding. Moreover, the properties of the film, its viscosity and adhesiveness, will clearly depend upon the detailed Reactions initiated by mechanical rupture of the elastomer molecules. They will therefore differ from one elastomer to another and they will also depend upon the particular ingredients used in the rubber mix formulation, especially when these substances are themselves able to participate in free-radical reactions. 

Sulfur is available to the compounder in two forms amorphous and rhombic. The amorphous form, also known as insoluble sulfur is a metastable high polymer that is insoluble in rubber and most solvents. Rhombic sulfur, a ring of eight sulfur atoms, is soluble in rubber and the form normally used for vulcanization. About 1 to 3 phr (parts per 100 parts of rubber elastomer) of sulfur are used for most rubber products. Vulcanization by sulfur alone is believed to be primarily via a thermally induced free-radical reaction. The sulfur will slowly react allylic to the site of unsaturation to form polysulfide substituents. These then are subject to... 

Some inorganic fillers are used as flame retardants in rubber base formulations. Flame retardants act in two ways (1) limiting or reducing access of oxygen to the combustion zone (2) reacting with free radicals (especially HO ), thus acting as terminator for combustion-propagation reaction. The additives most widely used as flame retardants for polymers are antimony oxides and alumina trihydrate. 

Bateman, Gee, Barnard, and others at the British Rubber Producers Research Association [6,7] developed a free radical chain reaction mechanism to explain the autoxidation of rubber which was later extended to other polymers and hydrocarbon compounds of technological importance [8,9]. Scheme 1 gives the main steps of the free radical chain reaction process involved in polymer oxidation and highlights the important role of hydroperoxides in the autoinitiation reaction, reaction lb and Ic. For most polymers, reaction le is rate determining and hence at normal oxygen pressures, the concentration of peroxyl radical (ROO ) is maximum and termination is favoured by reactions of ROO reactions If and Ig. 

FIGURE 11.2s Reaction scheme for styrene-assisted free radical grafting of glycidyl methacrylate onto natural ruhher. (From Suriyachi, P., Kiatkamjomwong, S., and Prasassarkich, P., Rubber Chem. TechnoL, 77, 914, 2004.)... 

Another vulcanizing agent for diene rubbers is m-phenylenebismaleimide. A catalytic free-radical source such as dicumyl peroxide or benzothiazyldisulfide (MBTS) is commonly used to initiate the reaction [61]. Phenolic curatives, benzoquinonedioxime, and m-phenylenebismaleimide are particularly useful where thermal stability is required. 

Anionic polymerization of polystyrene takes place very rapidly- much faster than free radical polymerization. When practiced on a large scale, this gives rise to heat transfer problems and limits its commercial practice to special cases, such as block copolymerization by living reactions. We employ anionic polymerization to make tri-block copolymer rubbers such as polystyrene-polybutadiene-polystyrene. This type of synthetic rubber is widely used in the handles of power tools, the soft grips of pens, and the elastic side panels of disposable diapers. 

A radical which can exist in the free or uncombined state. Such radicals are extremely reactive with themselves and with other neighbouring molecules. Free radicals are considered to take part in many reactions of importance to the rubber industry - vulcanisation, oxidation, ageing, polymerisation, etc. 

Control of fiber friction is essential to the processing of fibers, and it is sometimes desirable to modify fiber surfaces for particular end-uses. Most fiber friction modifications are accomplished by coating the fibers with lubricants or finishes. In most cases, these are temporary treatments that are removed in final processing steps before sale of the finished good. In some cases, a more permanent treatment is desired, and chemical reactions are performed to attach different species to the fiber surface, e.g. siliconized slick finishes or rubber adhesion promoters. Polyester s lack of chemical bonding sites can be modified by surface treatments that generate free radicals, such as with corrosive chemicals (e.g. acrylic acid) or by ionic bombardment with plasma treatments. The broken molecular bonds produce more polar sites, thus providing increased surface wettability and reactivity. 

The reaction with free radicals plays an important role in the interaction of carbon black with rubber (103) and with styrene (58, 102). 

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