The Influence of Certain Accelerators on the Aging of Rubber

Among the accelerators which are at present on the market two are very much used, viz., diphenylguanidine as an accelerator of medium activity, and tetramethylthiouram disulfide as a rapid accelerator. Diphenylguanidine is used most of the time in proportions between 0.5 to 1 per cent, based on the rubber, and with about 3 per cent of sulfur. Tetramethylthiouram disulfide is used in proportions of between 0.2 and 0.5 per cent, based on the rubber, with about 1.8 per cent of sulfur. In this way “nervy” mixtures are obtained which have excellent physical properties; but it must be recognized that these two accelerators are, each in its class, the ones which impart to mixtures the poorest aging qualities. For example, the mixture: Pale crepe 100 Zinc oxide 5 Sulfur 3 Palm oil 1 Stearic acid 0.5 Diphenylguanidine 0.7 vulcanized 1 hour at 3 kilograms pressure, i. e., under conditions which lead to the best physical properties for the proportions used, was investigated.

An Investigation of the Causes of Spots Which Appeared Spontaneously on Rubberized Fabrics

The opportunity was offered to examine a white rubberized fabric on which rosecolored spots had developed spontaneously. This fabric was to be used in the manufacture of white footwear, and was constructed of two plies of fabric, a white cloth and an unbleached cretonne cemented together by a rubber solution. On certain pieces of the plied fabric and on certain lots of the finished footwear, there gradually appeared patches of a rose color, which were especially prevalent on the unbleached fabric and on the white edge of the footwear. Since both unblemished cloth and cloth in various stages of spotting were available for examination, an investigation was made of the differences between the good and the bad fabrics, and of the causes of the rose-colored spots. An unbleached sample of cloth on which the rose-colored blemishes were intense was first examined. In this case extraction with boiling water gave a liquid with a slight acid reaction. Another portion of the same cloth, treated with 20 per cent acetic acid, yielded a dye, the reactions of which indicated that it belonged to the triphenylmethane group of dyes, since it resembled fuchsin and rosaniline. The presence of an aromatic amine in the aqueous extract could not be established.

The Evaluation of Raw Rubber

The purchase of supplies for a rubber factory often involves a decision as to the relative merits of different samples of raw rubber. In view of the variety of needs the method of arriving at a decision must be left to the wisdom of the technical staffs concerned; but when a large portion of the output from the East is involved, affecting producers and manufacturers everywhere, it is desirable to discuss fully the methods employed to ensure that the final decision is as wise as its importance warrants. A problem affecting the whole of the rubber industry has now arisen in the East in connection with the preparation of smoked sheet. Firewood for smoking is becoming more difficult to obtain, and already some estates have been compelled to consider the production of an alternative form of rubber. As the Tropics become more cultivated, the acuteness of the problem will increase, and unless some change in the method of production occurs, sheet will tend to be replaced by crepe. It is presumed that there is a demand for sheet apart from crepe and that each form of rubber serves a purpose which the other cannot fulfil; but there is no clear and authoritative indication from manufacturers as to the differences which make both forms of rubber essential. it is of interest to point out that apart from form, color, odor, and reaction to swelling agents, there is no test which would distinguish between high grade sheet and crepe rubber unless both are prepared from the same latex, in which case sheet has a slightly higher content of mineral matter (Bull. Imp. Inst., 21, 303 (1923)) and a greater tendency to absorb moisture from the atmosphere (de Vries, “Estate Rubber,” 1920, p. 340), and possibly better aging properties (Martin, Trans. Inst. Rubber Industry 2, 354 (1927)). It is not proposed to discuss the relative merits of sheet and crepe although the subject is an illustration of the difficulty of evaluating raw rubber so as to obtain results which are in agreement with the ideas and experience permeating the industry. To the planter there appears to be a demand for sheet as well as for crepe, and he purposes to meet that demand if it is at all possible.

The Influence of Zinc Oxide on the Action of the Principal Accelerators of Vulcanization. The Preparation of Transparent Vulcanized Rubber

IT HAS been known for a long time that the action of accelerators of vulcanization proceeds to a conclusion and in a regular way only in the presence of certain metallic oxides, of which zinc oxide is the most widely used. This substance behaves also as a reinforcing pigment, improving the physical properties of vulcanized rubber. We are not at the present time concerned with the second property, we are only concerned with ascertaining under what conditions zinc oxide promotes the action of accelerators. Zinc oxide makes rubber opaque. With certain articles like molded nipples and vulcanized crepe soles, this opacity offers a serious inconvenience and we have studied how it is possible to obtain transparent vulcanizates. To study these questions, we have carried out some vulcanization tests with the principal accelerators on the following three mixtures:

The Influence of the Principal Accelerators of Vulcanization on the Preservation of Rubber

In the April and October, 1929, issues of le Caoutchouc & la Gutta-Percha the authors have shown that accelerators of vulcanization can be classified according to: (1) their precocity, (2) their temperature of normal action (characteristic temperature), and (3) their rapidity of action at the characteristic temperature. The “plateau of vulcanization” was studied for each accelerator. In order to complete the work it has been necessary to study the influence of the principal accelerators of vulcanization on the preservation of rubber. This article gives the result of these experiments. Mixtures of “pure gum” type were prepared, containing the principal accelerators of vulcanization, taking as a basis the following mixture:

An Attempt at a Rational Classification of the Principal Accelerators of Vulcanization. II. A Study of the Time Required for Vulcanization Giving the Maxima Physical Properties at Different Temperatures

We have attempted to classify the principal accelerators of vulcanization according to their chief characteristics which are the following: (1) Time necessary for the fixation of mixtures at different temperatures. (2) Times required at different temperatures to obtain vulcanization giving the maxima physical properties. (3) Aging. The first of these 3 items has been treated in Caoutchouc et Gutta-Percha (April, 1929) and we have shown how accelerators can be classified by their “precocity” of action; that is to say, by the rapidity with which they “fix” mixtures at different temperatures. The mixture being fixed, vulcanization continues and the physical quality of the rubber improves. It thus reaches a maximum, then diminishes upon too prolonged heating. In this second study we have attempted to determine the following points: (1) At a given temperature, how long a period of vulcanization is required to impart to the mixtures their maxima physical properties? (2) Within the range of temperatures, which temperature gives the optima physical properties?

An Attempt at a Rational Classification of the Principal Accelerators of Vulcanization

AT the present time there are many accelerators on the market for the vulcanization of rubber, but it is often difficult to choose among them those best suited for a required purpose. Accelerators are often classed as slow medium, rapid and ultra rapid. These brief terms are entirely unsatisfactory for characterizing clearly the properties of these properties, and it frequently happens that two accelerators which have been placed together in one class behave in reality in very different ways and are not entirely replaceable one by the other. The object of this study is to establish a rational classification of the principal accelerators of vulcanization, which is based not only on their activity but also on their other important characteristics. The following points will be considered in their order: (1) The time required for the fixation of mixtures at different temperatures. (2) The time required to bring about vulcanization giving the maxima mechanical properties at different temperatures. (3) Aging. (4) These three points of view will be completed by a study of the plasticizing power and of the influence of different charges on the action of the accelerators.