Hai
loại lưu huỳnh thường được sử dụng trong kết mạng cao su là lưu huỳnh tan và
lưu huỳnh không tan. Ưu và khuyết điểm của từng loại lưu huỳnh được mô tả bên
dưới.
…
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.
…
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
(vtp-vlab-caosuviet)
