Stability Of System In The Mixed Silica Sol And Potassium (Sodium) Silicate And Its Thermodynamics And Kinetics From Microcalorimetry

The stability (when aged at ambient temperature) of mixtures (as a one-component coating binder) of silica sol and potassium silicate (or potassium sodium silicate) was investigated. Some of the mixtures were characterized by using isothermal heat conduction microcalorimetry and by measuring their particle size and pH value. Experimental results showed better stability for the mixture containing small nano-sized colloidal silica particles (less than 19.0 nm in diameter) and for that in a larger total enthalpy change (1.6234-3.3882 J) for a mixing reaction when it has gone to completion. And the stability was affected by the molar ratio of silica to alkali metal oxide in the potassium silicate (or potassium sodium silicate), the percentage by weight of the silica sol (53,65,75, and 85 wt% fraction) in the mixture, operating conditions when mixing, raw material specifications, temperature, pH value, SiO2 concentration, K+ and Na+ ions, stabilizers such as organic silicon and silane coupling agent, polymer emulsion, a reasonable collocation between thickener and dispersing agent, etc. When appropriate stabilizers were added into a mixture, the mixture exhibited a significantly longer shelf life. Moreover, the shelf life of the binder which was prepared by mixing the mixture and stabilizers as well as styrene-acrylic emulsion can reach more than 7 months at ambient temperature. Selected additives and pigments and fillers, the coating from preliminary preparation formed a smooth and hard film.Isothermal heat conduction microcalorimetry is a novel characterization method for silica polymerization, and was adopted to investigate the polymerization processes of silica when the combination of silica sol and potassium (sodium) silicate was stirred at 25.0°,35.0°, and 45.0℃. Microcalorimetric results indicated that chemical reactions occurred immediately in the mixed silica sol and potassium (or sodium) silicate were not a acid-base neutralization reaction but the dissolution and complex polymerization of silica with heat evolved, which was affected by temperature, the percentage by weight of the silica sol in the mixture, and K+ and Na+ ions. The silica polymerization was characterized by reaction orders which were rapidly and continued changing from low to high all the time. And when the reaction order for the oligomerization of silica in the mixed silica sol and potassium silicate was 3.0, the maximum rate constant occurred at 25.0℃(k = 1.22 x 10﹣4 mol﹣2 dm6 s﹣1). The two temperature regions (25.0°-35.0℃region with a faster rate and 35.0°-45.0℃region with a lower rate) reflected a two-stage oligomerization of silica monomers with different oligomers formed in a two-step anionic mechanism. The enthalpy change was greater at each higher temperature. The formation of circular and large silica particles was favored at high temperature, and the formation of linear and branched-chain oligomers was done at low temperature. The mixture of the silica sol and potassium silicate was more stable at the low temperature than that at the high temperature. The formation of nano-sized colloidal silica particles can be divided into three stages. In the first phase, when the silica sol was mixed with the potassium (sodium) silicate, the pH value of both silica sol and potassium (sodium) silicate changed, both the small and large colloidal particles in the silica sol and potassium (sodium) silicate dissolved, this was reflected at sectionl in power-time curve in microcalorimetric experiments. The second stage, followed by silica monomer polymerization, continuously created dimers, trimers, etc. oligomers and growing particles of silica, became smaller reaction rate continuously until the reaction completed, this is section2 in the power-time curve. At this stage, the two original peaks of the silica sol on particle size distribution by intensity changed, "active silica" in the potassium (sodium) silicate then redeposited onto the particles of rearrangement of the silica sol to form a distinct particle size distribution from original that of the silica sol and potassium (sodium) silicate. In the mixture of the silica sol and potassium silicate, the more the silica sol fraction by weight in the mixture, the higher the peak height of power-time curve, the greater the enthalpy change, the smaller size the silica particles to form, otherwise the larger size the silica particles to do. Strengthening interparticle siloxane bonds resulted from small size particles can develop into greater coalescence and stronger and more fibrillar Si-O-Si chain structure network with great tensile strength and good water resistance, but gels formed by the small size particles will be cracking during drying. The third stage, that is section3 in the power-time curve, was the process of aggregation between silica particles occurred with much less heat evolved. The size and distribution observed by particle size measurement is the back section2 and the all section3 in the power-time curve. Size statistics reports by intensity or volume can be qualitatively designated as three parts for elementary particles in less than 100 nm size in diameter, large colloidal silica particles grown from the elementary particles in more than 100 nm to several hundred nm sizes and silica monomers and oligomers in 1000 nm around and above sizes. Microcalorimetric experiments, particle size and pH value measurements can be combined to observe comprehensively and completely processes in the mixed and aged silica sol and potassium (sodium) silicate, and to provide a theoretical guidance for the formulation and development of coatings.