How do we formulate a latex, anti corrosion coating ?
The stringent demands made on anti-corrosion paints can only be fulfilled in aqueous systems if the intended materials are selected so as to be compatible each other and are optimally combined in the mixture. Water thinnable dispersions and emulsions of acrylate / styrene copolymers, air drying alkyd resins or, better, combinations of these ( hybrid systems ) or of special PVC terpolymers are suitable as binders. To ensure stabilisation of the dispersions or emulsions the pH must be adjusted to a specific value characteristic of the system, if necessary by the addition of triethylamine or ammonia or a mixture of the two.
Stabilisation and coalescence of the dispersion particles can be obtained by addition of co-solvents ( ethylene glycol ether, propylene glycol monobutyl ether, n-butanol, sec.-butanol etc. ) Many of the usual pigments and extenders can also be used in aqueous formulations, although only if they have little or no water soluble components.
Strontium or zinc chromate are suitable as anti corrosion pigments but zinc phosphate, barium metaborate or zinc molybdate / phosphate can be used if the formulation must be chromate free. Under some conditionsit is also advantageous to incorporate corrosion inhibitors ( benzoates, nitrates ) in order to counter flash rust and initial rust formation.
However with active anti corrosion pigments and a weakly alkaline formulationthis should not be necessary. In addition to the anti corrosion pigments, preferred pigments ar titanium dioxide and red iron oxide; calcium carbonate and mica are used as extenders. In general the formulation will also require thickeners, dispersing and anti foam agents, driers and rheological additives.
