Two-Component Solventborne Polyurethane Coatings
A polyurethane coating is comprised of two major components:
- A co-reacting polyol or base resin, which can be acrylic, polyether or polyester and is commonly referred to as the A-Component. These products are available under the tradename Desmophen.
- A polyisocyanate, sometimes referred to as a hardener, activator or B-Component. Look for these products under the tradename Desmodur N (HDI based) or Desmodur Z (IPDI based).
Other ingredients in a formulation - typically incorporated into the polyol component - can include pigments, solvents, leveling agents, thickening agents, catalysts and other modifiers included to enhance the coating. The polyisocyanate-based activator is moisture-sensitive and should be maintained under a dry nitrogen blanket. This prevents viscosity increase and/or solidification, which could result in decreased flow and potential paint defects. Reducing solvents used with polyurethane coatings should be "dry" (urethane grade, or less than 500 ppm moisture content) and alcohol-free to prevent the reaction of water or alcohol with the polyisocyanate.
A polyurethane paint film is formed when the A and B components chemically
react and cross-link. This may occur at room temperature or under
elevated baking conditions. Desired coating properties can be tailored
depending upon the choice of resin raw materials.
Mixing or Metering
The A and B components of a polyurethane formulation can either be hand "batch" mixed or proportioned by automatic meter/mix equipment. For small production volumes, hand mixing is an adequate approach. However, for high-volume production, the recommended approach is accurate and consistent precision mixing just prior to the spray gun.
In either case, the ingredients must be stored in dedicated vessels. For high-volume production, components A and B are pumped to the spray booth in separate lines. At this point, the two components are brought together in specific ratios and then thoroughly mixed by an in-line static mixer prior to delivery to the spray gun. Common materials utilized for storage vessels, piping, and related fittings are stainless steel and Teflon. Bayer offers component mixing and spray support to customers utilizing their technologies. Additional information can be obtained from traditional suppliers of paint handling and spray equipment.
To receive additional information about two-component solventborne coatings, visit our literature request section.