Thermoclad Duravin Vinyl Primers
CHEMICAL BONDING
The useful life of a coating system, whether it protects a ferrous or non-ferrous metal surface, is largely dependent on the integrity of the chemical bond achieved between the substrate and the plastic coating. Thermoclad Company manufactures a broad line of primers designed to provide optimum bond and maximum longevity for a variety of metallic and non-metallic surfaces.
The theory of chemical bonding involves the combination of a compatible multicomponent resin system offering affinity, and secondary protection for both the substrate and the coating. The usual primer design involves a choice of appropriate thermoset, thermoplastic, and coupling resins to create a bond to the substrate, yet offer a compatible surface for the coating to penetrate and react chemically. The nature of such a bond is quite dissimilar to a painted product where only a cohesive bond is achieved. It is, therefore, possible to offer a bond that can be described as 'greater than the strength of the coating', providing that the surface to be coated has been properly cleaned and pretreated.
SURFACE PRETREATMENT
Depending on the desired life of the product, a number of different cleaning and pretreatment methods may be considered. Specific recommendations should be discussed with a technical representative, however, the following methods, or a combination thereof, have been successfully employed. They are listed in increasing order of effectiveness:
- Vapor degreasing
- Grit or shot blasting
- Hot alkaline clean, water rinse sand blasting
- Solvent clean, sandblast
- Hot alkaline clean, rinse, acid pickle, rinse, dry
- Three stage iron or zinc phosphate
- Five stage iron or zinc phosphate
- Seven stage iron or zinc phosphate
The philosophy that should always be foremost in the mind of the design engineer is ... the 'cleaner' the part, the better the bond, and the more control or attention is focused on the cleaning chemicals, the more reproducible and predictable will be the end result.
Primer application should be made as soon after pretreatment as is practical to minimize the oxidation effect on the cleaned surface. The dry primer film, even before the curing step, will offer considerable protection for the 'activated' surface. Best adhesion results are obtained if the part is immediately put through the coating cycle, as this minimizes dirt and dust accumulation.
APPLICATION
DURAVIN primer may be applied by several acceptable techniques, whichever best suit the geometry of the object to be primed. Dip into a tankful of reduced primer is the most economical and practical manner of priming, however, spray or flow-coat may be more suitable for some continuous, and conveyorized systems. After the part has been totally 'wetted' by primer, a drain period and excess droplet removal is recommended to minimize subsequent blistering in the preheat oven.
All DURAVIN primers have been designed with sufficient latitude to withstand the heating time necessary to preheat a substrate to normal application and powder fusion temperatures. Dramatic mass differential of a part may on occasion create some processing problems and primer burn, but this condition can be averted by choosing suitable heating cycles.
PRODUCTS
Solvent based primer solutions are currently most common - they are individually balanced to develop properties for the intended application, such as galvanized (zinc) surfaces, steel, exotic metals, or ceramics, with further consideration for the ultimate end use requirements. The solvent system may be further adjusted to meet certain environmental demands for Flash point, evaporation characteristics, photochemical reactivity (Rule 66), or other local regulations.
Water borne DURAVIN Vinyl Primer Emulsions were pioneered by Thermoclad in the early seventies and compete in popularity with the solvent primers. These materials have been adapted to just about every application mentioned above, although more attention must be devoted to pretreatment operations. The advantages of water base systems are obvious: non-flammability and economics.
ECONOMICS
The rule of the thumb for cost estimate calculations is to add approximately 5% to the powder cost, which takes into account manufacturing losses. Theoretical extrapolation yields coverages as high as 5,000 square feet per gallon at recommended application solid levels. The recommended operating solids percentage for most DURAVIN Vinyl Primers is 6 to 8 % by weight.
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