Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 90 of 331

cal coreactants outlined earlier under "Solventborne Coatings." As the name im- plies, two-component coating are supplied to the end user in two separate con- tainers, and are mixed prior to application. One container (component A) holds the polyol, catalyst, solvent, and additives (rheology control agents, etc.). The second container (component B) holds the polyisocyanate, solvent, and addi- tives (UV stabilizers). Each container is individually shelf stable. The isocyanate/hydroxyl (NCO/OH) reaction begins as soon as the contain- ers are combined. The mixing ratio of the components can be based either on vol- ume or weight and is precisely calculated to achieve stoichiometry of the core- actants (NCO/OH = 1). This ratio gives a finished coating optimum mechanical performance and optimum chemical resistance. If necessary, the NCO/OH ratio may be varied to change coating properties. If the NCO/OH ratio is less than one, some OH functionality is unreacted and the coating has increased flexibility, better adhesion to substrates, and reduced solvent and chemical resistance. If the NCO/OH ratio is greater than one, some NCO is unreacted and the coating needs a longer time to dry and surface harden. The final product is harder, sol- vent and chemical resistance is increased, flexibility is decreased, and adhesion to the substrate is reduced. A rule of thumb in coatings formulation is to main- tain the NCO/OH ratio between 1.03 and 1.05. In the formulation process, dry time and pot life are determined by the amount of catalyst in component A. The amount of solvent used in the formulation de- pends upon the applications requirements, as well as on VOC regulations for the finished coating. Coating formulators will blend, adjust, and readjust the concentrations of the coating reactants until the finished product has met all the required performance goals. One-component solventborne polyurethane coatings utilize the same chem- ical reactants used in two-component coatings and are formulated by the same process except for three modifications: the polyisocyanate adduct is blocked; all chemical coreactants are blended into one container; and the coating must be heat cured. Blocked polyisocyanates utilize a chemical moiety to protect the isocyanate func- tionality from reaction during shelf storage at room temperature. The chemical moiety blocks, or caps, the isocyanate by reacting with the NCO functional groups. With the addition of heat, the blocked isocyanate breaks apart, regenerating the isocyanate functional groups (NCO) at the higher temperature (see Fig. 3). After the NCO functional group is regenerated, the coating cures by the reaction of the NCO group with the OH group (acrylic or polyester) on the polyol. One-component coat- ings are used in end use markets (such as automotive OEM) that require no mixing or metering prior to applications and that have available curing ovens. Most blocked polyisocyanates require high (>300°F) unblocking tempera- tures. This disadvantage has slowed the growth rate of one-component polyurethane coatings. Material suppliers are working on novel unblocking chemistry to lower the required temperatures. POWDER COATINGS Urethane powder coating for metallic substrates is a rapidly growing coating technology that offers significant advantages over solventborne coatings. The core- actants used in polyurethane powder coatings are: a solid blocked polyisocyanate (blocked with -caprolactam), a solid polyester resin, catalyst (dibutyltin dilau- 89

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