Preparation And Properties Study Of Silicone Elastomer Microspheres Coated By Nano-SiO₂ Particles

Silicone elastomer microspheres coated by nano-SiO₂ particles were prepared by Pickering emulsion polymerization with polymethylhydrosiloxane?PMHS?as raw material,vinyl terminated polydimethylsiloxane?Vi-PDMS?as cross-linking agent,hydrophilicity nano-SiO₂ particles and trace cetyltrimethylammonium bromide?CTAB?as emulsifier,chloroplatinic acid as catalyst.Soely hydrophilic nano-SiO₂ particles can not be used as emulsifier for silicone oil,so it was modified in situ using trace CTAB.It was found that when the mass ratio of oil to water?Oil/H₂O?is 1:1,the mass concentration of SiO₂ relative to the oil phase?SiO₂/Oil?is greater than 0.5%,and the concentration of CTAB aqueous solution is 2.0×10^-4 mol/L,a relatively stable Pickering emulsion can be obtained.Then the platinum catalyst was added to initiate the hydrosilylation reaction to prepare silicone elastomer microspheres coated by nano-SiO₂particles uniformly on the surface.Single factor experiments showed that the vinyl group conversion rate was biggest when the catalyst amount?platinum element accounts for the total mass of the oil phase?was 20 ppm,the reaction time was 80 min,and the reaction temperature was 80°C.The results of response surface analysis showed that the catalyst amount was 24.2ppm,the reaction time was 89.3 min and the reaction temperature was 81.6?were the optimal synthesis conditions for the silicone elastomer microspheres.The effect of emulsifying conditions such as oil to water mass ratio,SiO₂ content and the molar ratio of silicon hydrogen bond to vinyl group?n?Si-H/Vinyl??on the morphology of silicone elastomer microspheres has been studied,the results showed that when the Oil/H₂O exceeds2:1 and less than 1:2,the silicone microspheres can not be obtained,and there are more lumps.The size of the silicone microspheres decreased with the increasing of silica content.When the SiO₂ content reached 3.0%,the average diameter of the microspheres was 5.2?m.The best morphology of silicone microspheres obtained when the value of n?Si-H/Vinyl?is1.67.Silicone microspheres oil absorption decreases and then increases with the value of n?Si-H/Vinyl?increases.The olefin was grafted to the PMHS main chain by hydrosilylation reaction.By controlling the alkyl chain length and the molar ratio of olefin to silicon hydrogen bonds,alkyl-modified PMHS with different alkyl graft degree and different alkyl chain length were obtained.The experimental results showed that the optimal graft conditions were as follows:catalyst dosage?platinum element accounts for the total mass of the olefin?was 2 ppm,reaction time was 5 h,reaction temperature was 100?.By adjusting the experimental conditions,the emulsion was the most stable when the Oil/H₂O was 1:1,SiO₂/Oil was 4.0%and the concentration of CTAB was 2.0×10^-4 mol/L and the best morphology of alkyl modified silicone microspheres were obtained when the value of n?Si-H/Vinyl?was 2.0.Oil absorption experiments showed that with the alkyl graft degree and alkyl chain length increases,the oil absorption of the microspheres gradually increased,indicating alkyl modification can enhance the lipophilicity of the microspheres greatly.Unmodified and modified silicone microspheres were used to encapsulate vitamin E?VE?and the maximum encapsulation amount was measured.Then the effects of the alkyl graft degree and the alkyl chain length on the encapsulation yield and efficiency were investigated.The results showed that with the increase of alkyl graft degree and alkyl chain length,the encapsulation yield and efficiency of microspheres were significantly increased.Finally,the effects of light intensity and temperature on the stability of encapsulation VE and free VE were studied.Experiments resuilt showed that strong light and high temperature have a greater effect on free VE,while the use of microspheres encapsulation can significantly increase the stability of VE.