Formulation of the coating solution:
Adequate preparation of the coating solution is essential for achieving an effective coating within a reasonable timeframe. This solution is prepared in S S vessels equipped with a stirrer.
Various techniques are beneficial in facilitating the prompt formulation of coating solutions:
- Dispersion in hot water
- Mixing of ingredients
- Dispersion in a non-solvent
- Direct addition to room temperature
Dispersion in hot water:
The coating materials remain insoluble in hot water; however, effective lump-free dispersion can be achieved by mixing them in hot water. It is advisable to use temperatures above 80˚C, although temperatures ranging from 60 to 80˚C can also facilitate the dispersion of the polymer. Subsequently, the polymer dispersion is cooled to promote polymer hydration. This cooling process can be performed externally using jacketed vessels, or by reserving a portion of the water as cold water to be added after the polymer has been dispersed.
Mixing of ingredients:
An alternative approach to reduce polymer agglomeration involves dispersing polymer particles through dilution with additional coating excipients. It is often possible to directly incorporate combinations of dry pigments, dry plasticizers, and polymers into the processing water.
Dispersion in a non-solvent:
To prepare hydroalcoholic or organic solvent solutions, one can easily achieve this by dispersing the polymer in alcohol (which acts as a non-solvent) followed by the incorporation of water for hydration. In organic systems, the polymer can similarly be dispersed in alcohol and subsequently hydrated through the addition of methylene chloride.
Direct addition to room temperature:
This approach, although the most challenging, is frequently employed in extensive coating processes because of constraints related to equipment and heat transfer. It can be introduced directly into the process water, provided that a gradual and controlled addition of the polymer is implemented alongside effective agitation. It is essential to manage the level of agitation carefully to reduce foaming and allow adequate time for the complete hydration of the polymer.
Solutions often exhibit foaming when stirred due to the surface-active nature of these polymers. The incorporation of air from the ingredients or excessive agitation can exacerbate this phenomenon. Nevertheless, once foaming occurs, it can be mitigated through the use of deformers or by allowing time for the foam to settle.
Subjecting coating solutions to filtration is frequently advantageous, as it effectively eliminates any lumps or inadequately hydrated polymers. Typically, a 60 to 80 mesh screen can be utilized with standard commercial filtration equipment. While gravity flow is an option, employing air pressurization is recommended for expedited filtration.