The active ingredient of ASA is an oily monomer. The product is almost always delivered as a light amber oil that must be kept very dry until emulsification. ASA is added to the process in the form of an aqueous emulsion, in which the stabilizer is usually cationic starch and a small amount of surfactant to aid in emulsification. The key goals in using ASA are to avoid hydrolysis and distribute it well in the process. Hydrolysis is minimized by preparing the emulsion as late as possible, usually only seconds before the material is added to the thin stock. Particle size analysis can show whether one has achieved the desired narrow size distribution of droplets, usually with an average size near or below one micrometer. Because of the instability of the emulsion, testing must be done at the point of use.
The HORIBA LA-300 has proven to be extremely popular for this application because of its portability, fast analysis times, and ease of use. The example data shows an ASA emulsion tested in the field. The small size and low weight of the instrument allow it to be transported to various locations for routine testing of the emulsion preparation equipment.
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