The main physical properties that make synthetic silica an effective general-purpose matting agent are particle size, porosity and density.
Synthetic silica is not solid particles, in the microstructure is spongy porous particles, there are a large number of pores, pores filled with air. As a result, the density of synthetic silica is very low. Synthetic silica with high porosity contains more silica particles per unit mass than those with low porosity. Since the reduction in gloss is a particle effect, the matting effect of high porosity silica is better for a given particle size.
Used in emulsion polymerization. One of the disadvantages of this type of sugar surfactant is that it has not been approved by the U.S. Food and Drug Administration (FDA).
In most cases, the emulsifier is physically adsorbed on the surface of the latex particles, and there is a dynamic equilibrium with the water phase. Except for a small amount that acts on the transfer reaction during the polymerization process, most of the surfactants are not in the form of covalent bonds. The latex particles are connected. The radiotracer method can quantitatively measure the distribution of sulfur-containing emulsifiers. Succinate can be measured with 35S , about 15% of the emulsifier is fixed on the latex particles, ptfe wax micronized
When under certain special pressures, the emulsifier can desorb and its stability disappears. This can occur during certain sudden changes in the polymerization process, temperature changes during freeze-thaw cycles, and changes in the ionic strength of the water phase. In addition, when the dry film is exposed to high humidity, the surfactant will be re-coated by water.
In order to avoid these situations, different methods have been discussed . One is to use photosensitive surfactants, which can be destroyed during polymerization and film formation. The second is to permanently anchor on the latex surface with polymeric surfactants. Some works mention the positive effect of this method, which can improve emulsion stability and reduce the water sensitivity of films and coatings. L25. A lot of work has been done on how to strengthen the adhesion between latex particles and surfactants. Commercially available sodium alkyl allyl succinate, such as Trem LF-40 (product of Corning), and its hydride have begun to be used in the polymerization of vinyl acetate.
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