Chemistry. Polyvinylsiloxane materials are a modification of the original condensation silicones. Both are based on the polydimethyl siloxane polymer, however the presence of differing terminal groups accounts for their different curing reactions. Polyvinylsiloxane based on cross-linking polyaddition reaction of vinyl terminated polysiloxane polymer with methylhydrogen silicone cross-linking agent in the presence of platinium catalyst. As the dimensional stability of the polyvinyls is so much improved and the setting reaction is sufficiently different from the conden- sation curing silicones. Several authors have reported hydrogen gas bubble formation on the surface of gypsum dies poured immediately from polyvinylsiloxne impressions. A side reaction of the hydrides on the base polymer can produce hydrogen gas if moisture or residual silanol groups are present. Manufacturers have now eliminated the possibility of this side reaction by proper purification and accurate proportioning of the materials, and by the addition of palladium to the pastes as a hydrogen absorber. It is no longer necessary to wait for one hour before pouring these impressions.Addition silicones (polyvinylsiloxanes) have a moderately low-molecular weight silicone that contains silane groups. Since addition silicones do not produce a volatile by-product during polymerization, very small dimensional changes occur on setting. Hydroxide groups in many products produce hydrogen gas, resulting in small bubbles on the model surface if pouring is not delayed by 30-60 minutes. Many of these addition silicones contain catalysts like palladium that absorb this hydrogen. Newer addition silicones have been formulated to be more hydrophilic.The base and accelerator pastes also contain fillers. Amorphous silica or fluorocarbons are used as fillers to add bulk and improve the properties of the paste. The filler is also normally silanated to increase the bond strength between filler and polymer, which better allows it to function as a cross - link. Colouring agents are added to distinguish the base and catalyst pastes and to aid evaluation of mixing.