Presence Of Zinc Oxide Research Articles

Decomposition of Carbon Monoxide in the Presence of Zinc Oxide

Decomposition of Carbon Monoxide in the Presence of Zinc Oxide

Tetramethylthiuram Disulfide Vulcanization of Extracted Rubber. II. Structure of Radioactive Tetramethylthiuram Disulfide

Abstract Tetra-substituted thiuram polysulfides are notable because of their action as “accelerators” in rubber vulcanization and also because of their ability to vulcanize rubber without added sulfur, especially in the presence of zinc oxide, which leads to the formation of zinc dimethyldithiocarbamate (ZnDMDC) and other substances. A study of the structure of tetramethylthiuram disulfide (TMTD) is reported in this paper. This has involved the introduction of radioactive sulfur. A corollary has been the similar study of ZnDMDC, which is an “accelerator” but not a vulcanizing agent. The synthesis of radioactive TMTD (i.e., TMTD*) was accomplished by heating 4.16 grams (0.02 mole) of tetramethylthiuram monosulfide (TMTM) and 1.28 grams (0.04 atom) of radiosulfur (S*8) to 120° in a stream of nitrogen for 30 minutes. By extraction with hot alcohol and benzene, a series of crystalline fractions was secured which totaled 3.38 grams, a yield of 70 per cent. This material melted at 150–156° C, with decomposition. The excess S*8 was recovered in a second experiment. Pyrolysis of the TMTD* prepared in the above manner was carried out at 160–230° C in a modified Claisen flask. There were produced S8*, tetramethylthiourea*, and CS2* in nearly theoretical yield. The radioactivities of the thiuram disulfide and of the pyrolysis products (the CS2* being converted to phenylhydrazinium phenyldithiocarbazate* or to 1-phenylthiosemicarbazide* for study), which are recorded in Table I, show that the tracer sulfur atoms distributed themselves uniformly throughout the various molecules.

Tetramethylthiuram Disulfide Vulcanization of Extracted Rubber. IV. Behavior of Rubber as an Acid

Abstract The retarding effect of organic acids on vulcanization as well as the activating effect of certain acids on certain accelerators is well recognized. The activating and accelerating effects of bases and metallic oxides is also well recognized. In this connection, the mild activating effect of palmitic acid on vulcanization with TMTD in the presence of zinc oxide was described in the first paper of the present series. With the behavior of acids and bases during vulcanization as a background, the study of rubber itself as an acid has been undertaken. This paper sets forth a description of two reactions investigated during the study. These are the reaction of rubber with zinc palmitate and of TMTD with acids.

Natural, Thermal, and Biochemical Degradation of Ammoniated Hevea Latex to Give Heat Sensitivity

Abstract In most heat sensitizing processes ammonium salts are added to the latex, with consequent complications in manufacturing operations. A method is described which eliminates the use of ammonium salts and in which heat sensitivity of ammoniated latex in the presence of zinc oxide is obtained by natural aging, by heat treatment, or by biochemical methods. With natural aging or heat treatment, the sensitized mixture is stable at ordinary temperature provided the zinc oxide (or carbonate) is free from ionizable impurities. Biochemical degradation of the latex is readily obtained by adding trypsin. It produces sensitized mixtures which gel quickly and satisfactorily at ordinary temperature, and which are suitable for the manufacture of articles, even of great thickness in cold molds, the resulting coagulum needing no washing on removal from the mold.

Rubber, Polyisoprenes and Allied Compounds. VIII. The Formation of Dialkyl Sulfide Dihalides, and Its Bearing on the Problem of Determining the Unsaturation of Vulcanized Rubber

Abstract In determining the unsaturation of olefinic sulfides and disulfides by halogenation methods, the addition of halogen to the sulfur atom must be taken into account. The dihalides of dialkyl or dialkenyl sulfides react with aqueous potassium iodide partly to liberate iodine with regeneration of the sulfide, and partly to form halogen acid and dialkyl or dialkenyl sulfoxides. Correction for the latter reaction, which occurs extensively in the presence of alcohol or acetic acid, must be applied before reliable unsaturation values for dialkenyl sulfides can be obtained. With vulcanized rubber and iodine monochloride, the precise magnitude of the correction to be applied is uncertain, but this uncertainty can be removed by using bromine as the additive reagent. Simple rubber-sulfur vulcanizates show a loss of unsaturation corresponding to one double bond per sulfur atom combined up to at least 8 per cent of combined sulfur; vulcanizates accelerated with mercaptobenzothiazole in the presence of zinc oxide exhibit a substantially smaller loss of unsaturation. The dibromides of dialkyl sulfides readily yield a molecule of bromine to olefins, the reaction being practically exclusively additive.

Vulcanization of Latex. Differences in the Behavior of Fresh, Old, and Purified Latex

Abstract Patents granted to Schidrowitz show that when latex is vulcanized and then dried at room temperature, the product has the properties of vulcanized rubber. Films produced in this way show tensile strengths and elasticity which correspond to those of latex films vulcanized in the dry state. It is apparent, however, that when fresh latex is vulcanized under certain definite conditions, the product has fair tensile strength and elasticity only at a relative humidity of zero, but which under ordinary atmospheric conditions is brittle, seems to be overcured, and is practically without tensile strength. This tensile strength, however, is increased by additional dry vulcanization, so there can be no question of overcure. Just as soon as vulcanization has proceeded to a point where brittle films without tensile strength are obtained, the latex, when treated with acid, does not coagulate, but merely flocculates. Nor can such vulcanized fresh latex at this stage be made to coagulate coherently by other means. This form of latex is not sticky. The flocculate referred to can be obtained only by vulcanizing fresh latex in the presence of zinc oxide, and under conditions such that hydrolysis of the nonrubber substances is a minimum. It is, therefore, desirable to have recourse to ultra-accelerators and to be sure that the vulcanization temperature is not too high. By keeping fresh latex alkaline, or by purifying it, it will not flocculate. Latex that has been purified or aged may occasionally, under similar conditions, give a brittle and incoherent coagulum, whereas in other cases a normally coherent but somewhat brittle coagulum results. The nature of the coagulum is governed by the degree of purification and hydrolysis of the nonrubber substances; hence all transition stages between a flocculate and a completely coherent coagulum may occur. By adding serum from fresh latex to purified latex, the behavior of such purified latex changes in the sense that it behaves more like fresh latex. In studying experimentally the difference in behavior of fresh latex and purified latex, the first thing considered was the combination of sulfur. It was found that sulfur first dissolves in the serum, after which it dissolves in the rubber itself. Only then does vulcanization take place. This became evident from the definite acceleration of the combination of sulfur in the latex stage, when before vulcanization, latex was heated with sulfur alone. By this preparatory treatment too, dry vulcanization at room temperature was accelerated, but there was no noticeable effect on dry vulcanization at 80° and 110° C. At 30° C, about 1 per cent of the sulfur dissolved in the rubber particles, in the form of free sulfur. From this it was concluded that it is not possible to remove by mechanical means (as by clarification) excess free sulfur from vulcanized latex. No essential difference could be found between the combined sulfur of fresh latex and that of purified old latex.

Reactions during Vulcanization. II Reaction between Zinc Soaps and Mercaptobenzothiazole

Abstract Fatty acids react rapidly with zinc oxide to form zinc soaps during milling, and accordingly the action of fatty acid in the presence of zinc oxide is the same as that of an equivalent amount of the corresponding zinc soap. In accelerated stocks, the zinc soaps in the mix react with the accelerator to form the active zinc accelerator compound. The dynamic equilibrium between the accelerator and zinc soaps results in the formation of a smaller amount of the zinc accelerator compound in the case of mercaptobenzothiazole than for many accelerators, and accordingly relatively large amounts of zinc soaps are required with it. Since the zinc reacts with mercapto benzothiazole as the zinc soap, and since there is sufficient time for the reaction between zinc oxide and the fatty acid to proceed to practical completion, irrespective of the particle size of the zinc oxide, it follows that particle size of the zinc oxide is not a factor in the activation of mercaptobenzothiazole.

Natural and Synthetic Rubber. VI. the Pyrolysis of Natural Rubber in the Presence of Metallic Oxides

Abstract Pale crepe rubber pyrolyzed in the presence of zinc oxide or magnesium oxide gives the same decomposition products as in the absence of the oxides, but in different proportions. This modification is attributed to an action of the oxides upon the double bonds of the rubber molecule.

NATURAL AND SYNTHETIC RUBBER. VI. THE PYROLYSIS OF NATURAL RUBBER IN THE PRESENCE OF METALLIC OXIDES

Abstract Pale crepe rubber pyrolyzed in the presence of zinc oxide or magnesium oxide gives the same decomposition products as in the absence of the oxides, but in different proportions. This modification is attributed to an action of the oxides upon the double bonds of the rubber molecule.