Vulcanization shortening

Zinc oxide is the most important zinc compound. The principal industrial use of zinc oxide is as a catalyst to shorten the time of vulcanization in the production of mbber. The compound also is used as a white pigment in paints, cosmetics, and photocopy paper, hi everyday life, ZnO is also a common sunscreen. 

Inorganic accelerators and metallic oxides (usually zinc) are essential for satisfactory cure. Organic accelerators, introduced in the early 1920s, notably shortened vulcanization time. 

As natural rubber is vulcanized, the disulfide bonds shown in Figure 8.11 shorten the chains of the rubber and increase the rate at which the chain will contract. The greater the number of disulfide bonds, the greater is the hardness of the natural rubber. Hardness affects the seal s ability to compress as well as its performance in thermal cycling events. 

The photooxidation of vulcanized ethylene-propylene-diene monomer/montmor-illonite nanocomposite as well as EPDM/nanocomposite with stabilizers was recently reported [105]. The photooxidation products were not changed in the presence of the nanofiller. However, the presence of MMt was observed to dramatically enhance the rate of photooxidation of EPDM with a shortening of the oxidation induction time, leading to a decrease in the durability of the nanocomposites. On the other side, it was observed that addition of stabilizers, either Tinuvin P or 2-mercaptobenzimidazole, was efficient in inhibiting the degradation effect of MMt. 

Sodium hexamethylene-l,6-bisthiosulflde dihydrate, when added to the vulcanization system, breaks down and inserts a hexamethylene-1,6-dithiyl group within a disulfide or polysulfide crosslink. This is termed a hybrid crosslink. During extended vulcanization periods or accumulated heat history due to product service, polysulfidic-hexamethylene crosslinks shorten to produce thermally stable elastic monosulfidic crosslinks. At levels up to 2.0 phr, there is little effect on compound induction or scorch times, nor on other compound mechanical properties (Rubber Chemicals, 1998). 

Reclaiming is a procedure in which the scrap tire rubber or vulcanized rubber waste is converted, using mechanical and thermal energy and chemicals, into a state in which it can be mixed, processed, and vulcanized again. The principle of the process is devulcanization [12]. In devulcanization, it is assumed that the cleavage of intermolecular bonds of the chemical network, such as carbon-sulfur and/or sulfur-sulfur bonds, takes place, with further shortening of the chains occuring [14]. 

It is also accepted — and there is much experimental evidence — that during vulcanization once formed polysulfidic crosslinks are desulfurated by crosslink shortening and finally leading to mono- and disulfidic crosslinks. The sulfur removed from the polysulfidic crosslinks is recycled into the vulcanization process and is able to sulfurate more alkene sites to form additional crosslinks. Zinc complexes derived from the zinc oxide and the accelerator intermediately formed may well be responsible for this desulfuration reaction [126 129]. 

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