A new experimental procedure has been proposed for estimating the level of surface interaction activity of silica fillers in silicone rubber vulcanizates. The method is based on a simultaneous effect of the constant tensile strain and ammonia. The sample of silica-filled silicone rubber vulcanizate is deformed in an ammonia atmosphere for 72 h. The ammonia cleaves the specific silica—PDMS hydrogen attachments. The bonds are formed in new positions in the deformed polysiloxane network between silica surface free silanols and PDMS after evaporation of ammonia. The resulting permanent deformation (tensile set) provides a measure of silica surface reactivity (interaction activity) with respect to a given silicone elastomer. It has been proven that the ammonia-induced tensile set is governed by the same mechanism as the increase of toluene swelling under an ammonia atmosphere. The results indicated that water plays an important catalytic role in the ammonia-induced cleavage of silica-PDMS attachments. It is recommended to use an NH4OH atmosphere to obtain the maximum cleavage when estimating the contribution of polymer-filler bonding (hydrogen or physical) to the total crosslink density in silica-filled silicone rubber.
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Vondráček et al. (1990) studied this question.