Rubber

Rubber compounds rely on silicon chemistry in two quite different ways. In organic rubber — natural rubber, SBR, EPDM — silanes are coupling agents that bond reinforcing fillers such as silica into the polymer network. In silicone rubber, silicon is the polymer itself, and silanes, siloxanes and resins decide how it cures and how strong it becomes.

Coupling silica into organic rubber

Silica reinforces rubber well but does not bond to it on its own. Polysulfide silanes react with the silica surface during mixing and with the rubber during vulcanisation, tying the two together. In tyre treads this is what allows silica to lower rolling resistance while keeping wear resistance. Disulfide grades are less prone to scorch during mixing; mercapto silanes are more reactive still; carbon-black-supported grades are solids that are easier to dose.

Curing silicone rubber

Condensation cure. One-part RTV silicone sealants are built on hydroxyl-terminated polydimethylsiloxane and cure with moisture through a crosslinker: acetoxy silanes cure fast and release acetic acid; oxime silanes cure neutral; alkoxy silanes such as methyltrimethoxysilane release alcohol and are the least corrosive.

Addition cure. Liquid and high-consistency silicone rubbers based on vinyl-terminated polymers cure through platinum-catalysed reaction with Si–H crosslinkers, without by-products. Vinyl MQ resin reinforces these systems while keeping them transparent.

Processing aids

Fumed silica makes silicone rubber strong but can stiffen a stored compound — "crepe hardening". Structure-control agents such as hexamethyldisilazane, diphenylsilanediol and low-viscosity silanol fluids treat the filler during compounding and keep the rubber workable. Octaphenylcyclotetrasiloxane is the source of phenyl units for low-temperature and radiation-resistant phenyl silicone rubber.

Choosing

• Organic rubber with silica: tetrasulfide for general use, disulfide where scorch safety matters, mercapto for maximum reactivity.
• RTV sealant: choose the crosslinker by the cure by-product the substrate tolerates — acid, oxime or alcohol.
• Transparent or high-strength silicone rubber: reinforce with vinyl MQ resin.

For a recommendation for your formulation, a COA, an SDS or a quotation, contact our technical sales team.