SYNTHESIS OF SULFUR-CONTAINED MICROCAPSULES AND POTENTIAL APPLICATION IN RUBBER

Microcapsule-based material is potentially utilized in a variety of fields such as pharmaceuticals, food, biology, self-healing materials, etc. More remarkedly, in the rubber-related fields, this outstanding material is able to have a crucial role to play as an alternative of sulfur in compounding and vulcanizing process with regard to the self-healing ability after cracking. In this research, the interface polymerization was applied to generate microcapsules, whose shell was synthesized from Urea-formaldehyde pre-polymer modified by 0.25 wt% melamine containing sulfur (S) as a core substance. When the synthesizing process was carried out at 80 C and stirring rate of 300 rpm in 2 hours, the microcapsule product was spherical with the average size of 115 m and contained 60% of core content that was examined by FTIR, DLS, SEM, TGA and experimented the potential application. As a result, the amount of 8 phr of produced microcapsules utilized in NBR rubber compounds necessitated a longer time to vulcanize rubber at 160 C compared to using 5 phr free S. Besides, the mechanical strength of the microcapsules-contained product was insignificantly changed but bloom-like phenomenon on the rubber surface was markedly improved. It is noticeable that the vulcanized NBR rubber with the presence of these microcapsules are well able to heal its crack or cut when heated up to 150 C in 10 minutes while the free S-vulcanized NBR rubber is definitely unable to be self-healing in the same conditions.

Reaction of a Thiuram and Sulfur

In a previous article the occurrence of an intensive interchange between atoms of elemental sulfur when present as vulcanizing agent and the sulfur atoms of the mercapto group of mercaptobenzothiazole as accelerator was shown by the isotopic method, i.e., by the application of tagged atoms of radioactive sulfur. Thus, the existence of a reaction between the accelerator and the vulcanizing agent during the vulcanization of rubber was established. The application of isotopes as tagged atoms is an effective new method of investigating the chemical structure of substances, their reactivities, and the mechanism of chemical reactions in which they take part. This method is being successfully developed in our country, particularly by the work of Brodskii˘ and his pupils. The present study is devoted to the problem of the reaction of tetramethylthiuram disulfide (hereafter called thiuram) and elemental sulfur. Studies in this field are of great scientific and practical interest, since the mechanism of the accelerating action of thiuram is not yet clear. We assume that the chemistry of the reaction is as follows. Thiuram is decomposed during vulcanization, with the formation of a dithiocarbamate and free active sulfur. Because of this, the thiuram can vulcanize rubber without the addition of elemental sulfur to the mixture. Such vulcanizates are particularly widely used in the cable industry.

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

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.

A Temperature-Recording Micropress for Studying the Course of Vulcanization

A microscopic study of the course of vulcanization offers many possibilities, especially since, by its aid, the progressive changes which take place during vulcanization can be followed. Using the micropress described above, it is possible to observe these changes and to record the temperature at which they take place. A rubber-sulfur-litharge mixture after cure exhibits no recrystallization of sulfur even when cured for a very short time. A further microscopical study of the vulcanization of this type of rubber mixture may yield an explanation of the bloom-preventing properties of litharge. m-Dinitrobenzene and benzoyl peroxide both vulcanize rubber in the absence of sulfur. The former requires an activator such as litharge and does not recrystallize from the mixture when a cure is effected, whereas the latter does not require an activator and recrystallizes after cure in a manner very similar to sulfur.