Vulcanization silica-rubber bonding

This compound also uses Neoprene W type polychloroprene rubber. The compound, though coloured black with a few parts of carbon black is mainly filled with silica, which is a reinforcing mineral filler. The silica also helps to bond the rubber to the steel cords other ingredients added for this purpose include cobalt naphthenate. A commonly used system for this purpose is one comprising resorcinol and hexamethylene tetramine (HMT) which acts as a formaldehyde donor to form a phenolic resin in situ, but this is not suitable for Neoprene compounds because resorcinol is a fast accelerator for Neoprene vulcanization and interferes with its processing safety. 

Bifunctional sulfidic silanes, such as TESPT and TESPD, acted as a chemical bonding agents at the vulcanization stage between silica and rubber matrix due to their bifimctional characteristics. On the one end of the alkoxy group, it reacts with the silanol group on the silica surface via hydrolysis mechanism [ 175 to 178]. The other end of the sulfur reacts with the rubber chain at the vulcanization stage [179] and covalently bonds with silica and ties rubber chains [180]. 

RTV silicone adhesive is a rubberlike polymer called polydimethsiloxane. RTV stands for room temperature vulcanizing, or simply a rubber that cures at room temperature. Silicone rubber adhesives are made using a complicated process that turns elemental silicon metal made from sand (silica) into a rubbery polymer. When cured, silicone rubber adhesives/sealants have excellent resistance to heat (500—600 °F) and moisture that makes them exceptionally suited for outdoor weathering applications such as sealant and caulking compounds in the construction industry. Because of its exceptional properties, silicone adhesive has been used in some exotic applications such as the soles of the boots worn by the first astronauts to walk on the moon. Silicone adhesive/sealants are used to seal windows, doors, and portholes on the space shuttle and many satellite missiles. A special silicone adhesive is used to bond the heat shield tiles on space shuttles. 

One end of such molecules is expected to specifically react with silanols on silica surface, whilst the other end is expected to eventually interact with the vulcanization system (essentially sulfur based) in order to provide chemical bonding with the rubber network. It worth underlining again the fundamental difference between carbon black and silica reinforcement no chemistry is needed with the former but is essential with the latter. 

The silica modification with a bi-functional organosilane (either TESPT of TCPTS for instance) and the subsequent development of rubber-filler bonding during vulcanization is essentially considered as follows ... 

Vulcanization (Figure 6.5) the tetrasulfane group (with TESPT silan-ated silica) is broken and forms rubber-filler covalent bonds with the... 

---