Preparation Of Large-sized Silica Sol Particles From Sodium Silicate

The silica sol is a colloidal solution in which the nanoscale silica particles are uniformly dispersed in water.Large numbers of hydroxyl groups locate on the surface of the SiO2 nanoparticles in the sol and show notable reactivity,moreover,they could interact with many polymers after surface modification.Thanks to its significant physicochemical properties,it has been widely applied to coatings,electronics,investment casting.paper,refractories,petrochemical,and many other fields.However,the size of SiO2 nanoparticles from industrial silica sol is between 10 and 40 nm generally,which is difficult to meet the requirements of some special industries.Therefore,synthesis of large particle size silica sol has become a hotspot in the research of silica sol industry.In this thesis,sodium silicate aqueous solution was employed as raw material to explore the preparation method of silica sol with large particle size.It was investigated the effect of polyethylene glycol(PEG)on the size and morphology of SiO2 nanoparticles.The size-controllable silica gel particle and large particle size silica sol were obtained by the method of "direct polymerization of sodium silicate" and "sodium silicate direct polycondensation-tetraethyl orthosilicate hydrolysis polymerization combination".It provides information for the research and applications of silica sol controllable preparation technology.The main results and conclusions of this thesis are as follow:(1)The active silicic acid was prepared by ion exchange method using sodium silicate solution.The polycondensation process of activated silicic acid was studied under different circumstances.The results showed that the polvcondensation product was easy to gel in acidic conditions(pH 1～3).and the addition of PEG(molecular weight M_w = 200)had no obvious inhibitory effect on gelation.Differently,in the alkaline conditions(pH～10),the polycondensation silica sol was obtained with good dispersibility(particle size under given conditions～70 nm).The particle size of silica sol was reduced by the addition of PEG(particle size in research conditions～40 nm).(2)Monodispersed organic silica sol(mSiO2·nH2O nanoparticles)was prepared by hydrolysis and condensation of tetraethyl orthosilicate(TEOS),which was catalyzed by ammonia in ethanol medium.The multi-step hydrolysis-polymerization method was used to control the particle size,which was 30～50 nm under the given conditions.(3)The direct condensation method of sodium silicate was explored according to TEOS hydrolysis-condensation method.The stable globular particles were obtained by direct condensation of sodium silicate solution in ethanol/ammonia aqueous medium.The particle size could be regulated and controlled by the variation of sodium silicate concentration.Globular particles with large particle size(>100 nm)and size poor uniformity were prepared by the method above,which revealed that the polycondensation conditions need to be optimized.In addition,the polycondensation mechanism remains to be studied.(4)The feasibility of preparing homogeneous dispersion of large size silica sol was explored by combining " the direct condensation method of sodium silicate " with"TEOS hydrolysis-polymerization method".The results showed that large size particle(～300 nm)products with uniform size were prepared by this method,which provided a new way for the controllable preparation of silica sol.Compared with the direct polycondensation method of sodium silicate,this method can improve the uniformity of particles.And compared with TEOS hydrolysis-polymerization method,this method can significantly reduce the amount of TEOS(cost reduction),providing practical value.The surface properties and application performance of the prepared silica sol particles are still needed to be studied.(5)It was investigated that the effect of PEG on the preparation of silica sol by direct polycondensation of sodium silicate method.The results showed that the addition of PEG can reduce the particle size and suppress the aggregation of particles.The larger the PEG molecular weight(M_w 4000～20000),the smaller the silica sol particles prepared under the same conditions.