Heavy duty topcoats

The use of thermoplastic acrylics in high build, heavy duty anti-corrosive systems is reported by Steed who describes top coat formulations based on thermoplastic acrylic resins, chlorinated rubber, solution vinyl resins as well as blends of two or more of these. 

Volume solids of up to 40% at application viscosity may be achieved using a thermoplastic acrylic such as Plexigum PM685 (a BA/MMA bead copolymer powder) plasticised with a chlorinated paraffin such as Cereclor S52 (4.25 1) This binder combination was used to formulate a finishing coat as shown  

Hydrogenated Castor wax thixotrope (e.g. Thixatrol, Crayvallac MT) is added to the formulation under specific conditions of dispersion and temperature to provide the formulation with sufficient shear thiiming behaviour to allow application to vertical surfaces without sagging. 

A blend of volatile and less volatile solvents is used. The former to provide rapid evaporation to give initial film set whilst the slower one allows sufficient wet edge time to allow overlaps. 

A calculated PVC/CPVC (L) value of around 0.55 indicates that this paint will be a low-gloss or eggshell finish. 

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They are exceptionally useful in primer plastic applications where drying speed, compatibility with various kinds of topcoats, and high adhesive strength is required. Phenoxies are used in automotive and marine primers as well as in heavy-duty maintenance primers. Important use is as a vehicle for coating formulations. 

Heavy-duty coating systems generally comprise two primer coats and two topcoats, modern system sometimes consist of one primer coat and two topcoats. The total dry film thickness (DFT) of such anticorrosive paint systems is 150 - 200 pm, each layer has a minimum DFT of 40-50 pm. Due to their excellent adhesion, the first and second primers prevent corrosion of the metal surface. The pigments and extenders allow the primers to react with ions (CD and SO4") that diffuse into the film from the atmosphere. The pigmented organic film also forms a barrier against humidity that may otherwise initiate a corrosive process. 

Chlorinated rubber is used in topcoats for heavy duty maintenance paints because of its low water permeability. It is also used in tie coats on polyolefin plastics. Chlorinated rubber dehydrochlorinates and requires stabilizers similar to those used with PVC. Some metal salts, especially those of iron, tend to promote degradation of chlorinated rubber and so it degrades when applied over rusty steel. Chlorinated ethylene/vinyl acetate copolymers have been developed that can be used to replace chlorinated rubber in at least some applications (183). 

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