Saturated Rubber Research Articles

Vulcanization of saturated ethylene-propylene rubber with tetramethylthiuramdisulphide in the presence of dicumyl peroxide

THE curing of saturated hydrocarbon polymers (polyethylene, ethylene-propylene rubber, etc.) is most frequently effected by using a combination of dicumyl peroxide (PDC) and sulphur with small amounts of tetramethylthiuramdisulphide (TMTD) and zinc oxide [1] added to activate the process. We know that spontaneous reactions between TMTD and saturated polymers take place only at temperatures above 200 ° [2], and so under peroxide vulcanization conditions (140-160 °) one would rather expect the curing efficiency to be reduced through blocking by TMTD or by products of the thermal conversion of polymeric radicals emerging in the reaction with PDC. A study of the curing of ethylene-propylene rubber (EPR) by TMTD in the presence of dicumyl peroxide should also be of interest from the point of view of elucidating the mechanism of the action of this accelerator. The total available experimental data show that the vulcanization of unsaturated rubbers in the presence of TMTD gives rise to a complex chain of reactions of thermal breakdown of TMTD and of interaction of the breakdown products with one another and with the rubber. Most authors are inclined to take the view that the curing reaction involving TMTD is mainly of a radical nature [3-5]. However, according to the suggestions recently advanced in [6] ionic reactions have a considerable role in thiuram vulcanization processes. The processes involved in the curing of a saturated polymer in the presence of TMTD and PDC are much simpler in view of the absence of any spontaneous reactions ~,f the TMTD and the conversion products of TMTD with the rubber. Their addition to EPR becomes possible only after the saturated polymer molecules have been activated as a result of their reactions with the peroxide. These reactions are of a radical nature [7], and in the case to be considered here we may expect to find mainly radical reactions of TMTD. The aim of this investigation was to study features of the peroxide vulcanization of EPR with TMTD and zinc oxide.

Mechanism of Peroxide Vulcanization of Elastomers

Abstract As is now well known, the use of an organic peroxide as a crosslinking agent for an elastomer was first reported by Ostromislensky in 1915. In this work benzoyl peroxide was used to cure natural rubber; similar acyl peroxides together with more recently discovered, less active, and more convenient dialkyl peroxides have since this time been used to crosslink a very large number of polymers. With the common unsaturated polymers the vulcanizates produced have mechanical properties rather inferior to those obtained with accelerated sulfur cures. They do, however, have the good aging and low compression set properties which are, in general, associated with sulfurless cures. The commercial use of peroxides in unsaturated elastomers is nevertheless small, being employed only where the presence of sulfur would be deleterious. In spite of its technological disadvantages peroxide crosslinking has been extensively studied since it forms a chemically simple means of introducing crosslinks into a wide variety of rubbers and leads to vulcanizates of simple structure with physically and chemically stable, carbon-to-carbon crosslinks. Interest in the industrial use of peroxides as curing agents has increased recently with the introduction of a number of fully saturated rubbers for which the usual accelerated sulfur systems are unsuitable. Such saturated rubbers are, in general, more resistant to aging, are more thermally stable, and will probably be used to increasingly greater extents as operational conditions become more and more severe. The study of curing systems available for use with such rubbers is therefore of considerable importance and organic peroxides, forming one such system, are worthy of detailed examination. Much work has been done on the technological evaluation of peroxide vulcanizates and much of this is summarized in a recent book edited by Alliger and Sjothun which is recommended as a source of references for a more detailed study of this aspect of the subject. The basic mechanism of the peroxide crosslinking of a wide variety of rubbers has, however, been neglected until relatively recently. For this reason, and for reasons of length, it is proposed to limit the present review to this latter aspect of the subject.

Adduct Rubbers: A Versatile New Family of Elastomers

Abstract A versatile family of elastomers has been prepared by free radical addition of aliphatic mercaptans to the double bonds of diene polymers. High saturation levels were readily achieved without degradation of the basic polymer chain. The technology is similar to ordinary emulsion polymerization. By varying the nature of the base polymer, the mercaptan used, and the extent of saturation, a wide range of compositions and of physical properties were attained in the adducts. In general, resistance to aging, ozone attack, heat, solvent swelling, and to permeation by gases increased with increasing extent of saturation. Outstanding performance in these qualities was achieved by adducts of polymers whose double bonds had been over 90 per cent saturated. A highly saturated methyl mercaptan adduct of polybutadiene showed (1) stress-strain and air aging properties at 400° and 500° F better than commercially available ethylacrylate copolymers and various semicommercial heat resistant elastomers; (2) permeability resistance equivalent to butyl; (3) solvent swell intermediate between neoprene and medium nitrile content butadiene-acrylonitrile rubbers, with a Tg below −30° C; (4) ozone resistance comparable to the better commercially available saturated rubbers. Increased solvent resistance was obtained by preparing adducts of the appropriate butadiene-acrylonitrile copolymers. Adducts with saturation levels up to about 85 per cent could be cured by the same procedures used for the base polymers. Activated or butyl-type curing systems were required for higher saturation levels. The relative rates at which aliphatic mercaptans add to the several types of double bonds present in emulsion diene polymers have been examined in a preliminary way.

Infrared analysis of isomerized, vulcanized and oxidized natural rubber

AbstractThe purpose of this study was to follow by i.r. analysis double bond shifts and chemical modification of natural rubber. Accurate determination of the original rubber spectrum is facilitated by the use of an internal standard. Isorubber, prepared by thermal dissociation of rubber hydrochloride, contains about 50% of double bonds in the side groups. Although a similar double bond shift is observed in the cyclization of rubber, part of the absorption, in the 11 μ region, may here be due to ring formation. Vulcanization with accelerated sulfur mixtures does not produce changes in the i.r. spectrum, unless the rubber is transformed into a derivative by overcuring with S2Cl2 or during hard rubber formation. At least three types of oxidation reactions can be distinguished spectroscopically: (1 ) degradation of the rubber (by oxidation of a rubber solution with air) with the ultimate formation of only volatile oxidation products; (2 ) gradual formation of oxygenated derivatives (oxidation with ozone), with a distinguishable structure in the spectrum; (3 ) gradual formation of an oxidized and more saturated rubber (oxidation of solid rubber with air), with less characteristic broad absorption bands.