Complexities of Crosslink Density in Filled Elastomers

Abstract

Abstract This work was undertaken to learn more about the complicated role of fillers in reinforcing siloxane polymers and attempt to clarify why silica fillers cause the crosslink density to increase significantly more in siloxane polymers than in hydrocarbon polymers. To carry out this study it was necessary to decide upon a method for determining the effective number of crosslinks. A simple mathematical expression applicable to both filled and unfilled elastomers was developed for use with a compression modulus technique for swollen vulcanizates. The fillers studied with polydimethylsiloxane (containing a small amount of methylvinylsiloxane) were a fume silica, wet process silica, diatomaceous earth, and two ground quartz silicas. The effect of concentration of network chains on tensile strength, modulus, and dynamic mechanical properties was compared for fume silica and diatomaceous earth filled stocks. All of the types of silica studied caused the crosslink density to increase several times over that of the unfilled vulcanizate. This may be contrasted with a factor of less than 2 for a filled (carbon black or silica filler) hydrocarbon polymer. The difference in behavior of the two polymer systems was attributed to a pronounced interaction between the siloxane polymer and the silica surface. The data indicated that filler structure has a more pronounced effect on crosslink density than surface area. It was found that the log of concentration of network chains gives a straight line when plotted as a function of volume fraction filler.