Cold Milling Research Articles

Rubber-Thermoplastic Compositions. Part VII. Chlorinated Polyethylene Rubber-Nylon Compositions

Abstract Nylon resins and CPE rubber can be melt-blended to give compositions which have useful properties. If the rubber is cured by dynamic vulcanization, high strength oil resistant thermoplastic elastomers result. Cold milling of melt-mixed compositions results in nylon-reinforced rubber which can be mixed with curatives and press cured or statically vulcanized to give reinforced rubber compositions which resemble vulcanized rubber-short fiber composites. The reason for the high integrity of either the statically or the dynamically vulcanized composition appears to be that chemical bonds form between the rubber and plastic.

The formation of fine crystal structure during cold milling of gear wheels

The formation of fine crystal structure during cold milling of gear wheels

Mastication of Rubber. II. Interpolymerization of Natural Rubber and Neoprene on Cold Milling

Abstract The softening of elastomers on cold milling results from scission of the polymer molecules by the applied shearing forces. The ruptured chains are free radicals, which can undergo mutual combination, interaction with oxygen and various additives, and branching (grafting) on to other polymer molecules. A general method of producing graft and block interpolymers between elstomers is therefore indicated, namely, to cold-mill the polymers together in the absence of small molecules which can terminate the polymeric radicals in order that the radicals may cross-terminate or graft onto the polymer molecules of the other type. A survey of several pairs of the commercially important elastomers, natural rubber, butadiene-styrene, Neoprene, and butadiene-acrylonitrile, has shown that cold milling does effect interlinking. Detailed results for the rubber-Neoprene system are reported in this communication. Experimental verification of polymer interlinking was obtained from the solubility properties of the milled elastomers. Cold milling of Neoprene under nitrogen produces gel, whereas of natural rubber does not, but the milling of mixtures gives gels containing natural rubber. Also, the solubilities and precipitation of the milled mixtures cannot be accounted for by these properties of the individual polymers. Finally, Neoprene-natural rubber mixtures, after and not before cold-milling, can be cross-linked by magnesium oxide, with rubber bound into the vulcanizate.

Combination of Rubber and Carbon Black on Cold Milling

Abstract Although the structure of carbon black is imperfectly known, x-ray analysis has shown it to consist of layers of condensed rings of carbon atoms. Unsaturation and discontinuities in the layers are likely to provide sites for attack of free radicals, and thus make carbon black a radical acceptor of a special polyfunctional type. Assuming combination of rubber radicals and carbon black, a particle could terminate more than one sheared rubber chain. Furthermore, rubber chains attached to a carbon black particle could also undergo scission by shear and be terminated by combination with other particles. The result anticipated from this picture is a network of rubber and carbon black held together by chemical bonds. In rubber solvents, such a network would be insoluble and merely become swollen gel. Insolubilization of rubber on milling with carbon black has been reported on occasion, but the evidence is not sufficiently extensive to draw reliable conclusions as to its cause. This paper reports a systematic investigation of the occurrence of rubber-carbon black gel on cold milling. This gel has been shown to form, and the conditions for its formation and its properties have been interpreted as strong evidence for the above hypothesis of chemical-bond attachment of rubber and carbon black.

The Mooney Viscosity and Gel Plasticity Relationship for GR-S-Xylene Gels

Abstract 1. A new method for measuring plasticities of highly viscous GR-S-solvent gels is described. 2. The coefficient and the standard error of estimate for the regression of plasticities of 35 per cent GR-S-xylene gels on Mooney viscosity (ML 4/212) of the rubber have been calculated. 3. An explanation for an observed shift from the relationship established for Mooney viscosity and gel plasticity caused by hot or cold milling of the polymer is suggested.

Physical properties of fractions of GR‐S and their vulcanizates

AbstractA large‐scale precise fractionation of GR‐S (X‐55) was performed using the fractional precipitation technique at 25°C. 9 fractions, each weighing approximately 150 g. and comprising approximately 11% by weight of the original unfractionated sample, were obtained and a detailed study was made showing the effect of number‐average molecular weight and related sol–gel properties on various physical and chemical properties of the crude and vulcanized fractions. Using a Santocure tread‐type recipe, preliminary compounding work indicated the sulfur ratios required to give vulcanizates with 300% modulus values of 1000 p.s.i. All fractions were recompounded using these indicated optimum sulfur values. Generally, as the number‐average molecular weight increased, better stress–strain and quality index data were obtained. These and other physical properties of the vulcanizates of these fractions are discussed in detail. As found in past fractionation studies pertaining to unsaturated high polymers, the higher molecular weight fractions, obtained from the unfractionated sample which was totally soluble in benzene, undergo considerable gelation when isolated and vacuum dried at 25°C. in the dark. As the indicated molecular weight of the fraction increased, higher per cent gel contents were obtained. Compared to unfractionated butadiene‐styrene copolymers of similar gel contents, the gel portions of the higher molecular weight fractions had unusually high swelling indices. indicating qualitatively that the average molecular weights between points of effective cross linking in the three‐dimensional gel structure were higher than those encountered in the past in unfractionated samples of similar gel contents. Other data are given pertaining to these fractions, such as the bound styrene content, per cent shrinkage of the compounded unvulcanized stocks, and the rate of gel breakdown with cold milling.

Structural Changes during the Processing of Rubber

Abstract The effect of mastication, heating in air, oxygen, steam, and carbon dioxide gas, and the influence of light upon the plasticity of crude rubber have been investigated. Cold milling causes a permanent change in the consistency of crude rubber, whereas heating in air or steam produces a temporary effect. In absence of air the rubber exhibits a maximum amount of disaggregation at each given temperature. At low temperatures the increase in plasticity proceeds at practically the same rate whether the rubber is heated in oxygen or in carbon dioxide. Ultra-violet light affects concurrently both the plastic and elastic properties of the rubber. The correlation between elasticity and double refraction of crude rubber has been shown. The anisotropic properties of rubber parallel the recovery values. In this investigation no agency has been found which causes such far-reaching and profound destruction of the elastic properties of the rubber as prolonged physical mastication. The interrelation between plasticity and structure of crude rubber has been studied and the fundamental difference between actual mastication and heat breakdown has been emphasized. Prolonged milling, heating in air, or in oxygen destroy the protein framework of the rubber. Heating in steam or in an inert gas leaves it more or less intact.