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Elemental sulphur is the most widely used vulcanisation agent in the rubber industry and is effective in elastomers containing some degree of unsaturation. Ground sulphur is most widely used, often referred to as rhombic sulphur or rubber makers’ sulphur. The molecular structure of rhombic sulphur comprises an eight membered ring and is crystalline in nature. It has a melting point of 115 °C and is soluble to a limited degree in elastomers; for example, around 1% w/w in natural rubber at room temperature, increasing to a level of the order of 7% at 100 °C. The relatively low solubility of sulphur in rubber at ambient temperature is the cause of so-called ‘sulphur bloom’. It appears as an off-white powdery coating on the surface of the uncured compound due to migration from the bulk compound when the limit of solubility is exceeded. If present in excess it has an unfavourable effect on the building tack of green components. Sulphur bloom can also occur in vulcanisates but here the disadvantage is largely cosmetic.

Sulphur bloom can be prevented by substituting rubber makers’ sulphur with so called insoluble sulphur. This is a crystalline, polymeric form of sulphur [1] and is insoluble in solvents and elastomers. It should be processed at temperatures not exceeding 110 °C, preferably 105 °C, in order to prevent excessive conversion into the rhombic form. During vulcanisation it is converted into rhombic sulphur allowing the vulcanisation process to proceed as normal. The following advantages are claimed with regard to the use of insoluble sulphur [1, 2]:

·         elimination of sulphur bloom,

·         prevention of sulphur migration between green components during storage,

·         reduced bin-scorch during the storage of green compounds.

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Trích đăng từ sách Rubber Technologist’s Handbook, Sadhan K. De và Jim R. White, Smithers Rapra Technology, 2001, trang 167 – 168

Nguồn: www.books.google.com.vn

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