Photocrosslinking of Silicones. Part 13. Photoinduced Thiol-Ene Crosslinking of Modified Silicones

Abstract

The photoinduced radical crosslinking of α,ω-ene-terminated silicones (vinyl, vinyloxy, allyl, norbornenyl, etc.) with siloxanes containing pendant mercapto groups has been investigated by means of calorimetry, Raman and swelling measurements. The reaction enthalpy depends on the type of olefin. The values vary from 99.5 kJ/mol (hexenyl) to 209 kJ/mol (norbornenyl). The crosslinking rate is influenced by the type of α,ω-ene terminated silicone used and differs by a factor of 4.5 from the norbornenyl to the allyl product. The final degree of conversion of the double bonds is generally high. Using Raman techniques, one can show that nearly 100% of the thiol groups are consumed. The crosslinking is inhibited and terminated by oxygen. The inhibition period depends on the light intensity, the oxygen pressure, and the type of olefin. Using values of the inhibition time and reaction rate, one can estimate the relative efficiency of several radicals. The application of a modified swelling test allows us to estimate the formation of the network density as a function of exposure time, thiol content and type of silicone. The network density of the vinyl product is much higher than other products. Norbornenyl and allyl derivatives formed networks with the lowest crosslink density.