Chemical Reaction Coatings
Chemical reaction coatings are usually the most durable. They cure by polymerization (cross linking) reactions between at least two chemical entities. These chemical entities are mixed together just prior to application. After mixing, the cure molecule attaches to the base molecule, and then another base molecule attaches to another spot on the cure molecule, and so on, until nearly all of the molecules are linked together as a single molecular chain. This process may take several days or even several weeks before the film develops all of its properties. Chemical reaction coatings cover a vast range of chemistries such as epoxies, polyurethanes, polyureas, polyaspartics, polysiloxanes etc.
They are generally packaged in two separate containers that are mixed in ratios range from 1:1 to 100:1 or more, depending on the material’s chemistry to initiate the reaction. Components must be combined in the specified proportions in the manner specified by the supplier to achieve a film with optimum properties. Sometimes, an “induction period” is required after mixing and before application to permit the reaction to get started. After mixing, there is always a “pot life” during which the coating must be applied, before the reaction has advanced so far that the coating cannot be properly applied.
Because chemical reaction coatings are thermosetting, they have excellent chemical and solvent resistance. They are difficult to topcoat when fully cured, because topcoat solvent cannot bite into the films. Thus, a topcoat is best applied while the undercoat still has some residual tack. If a completely cured thermosetting coating is to be top coated, it is necessary to first spray a thin (e.g., 2 mil wet film thickness) tie coat (tack coat) of the topcoat and allow it to cure to a tacky state. Some products cannot be top coated at all and must be used as a stand-alone single coat system.
Areas of concern on the application of chemical reaction coatings:
- Coating components are the correct ones
- Mixing ratio is correct
- Materials are fully and properly mixed
- Induction time (if required) is observed
- Pot life is not exceeded
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