The film-forming material in blocked polyurethane coatings is similar to two-component polyurethane coatings, consisting of polyisocyanates and polyhydroxy resins. The main difference is that the polyisocyanates have been blocked by phenol or other substances with active hydrogen atoms. This allows the two components to be mixed without reacting, forming a single-component coating with excellent storage stability. Chemical reation process of blocked polyurethane coatings is as follow:
The NCO groups in the curing agent are highly reactive and can react with moisture in the air at room temperature, causing deterioration. To make storage easier and reduce material waste, a blocking agent can be used to create a stable blocked curing agent at room temperature. When heated, the blocked curing agent can return to its original structure and react with components containing hydroxyl groups.
The formulation of blocked polyurethane coatings typically involves introducing a blocking agent into common solvent-based and water-dispersible polyurethane curing agents. This process helps seal excess NCO groups. There is variation in the de-blocking temperature for different blocking agents. The choice of blocking agent can be tailored to specific applications. Common blocking agents and their de-blocking temperatures are listed in the table below.
|De-blocking Temperature (°C)
|Methyl ethyl ketoxime
Blocked polyurethane coatings have the advantage of being a one-part system, requiring no equipment modifications during application and curing. They provide film properties similar to two-component coatings and offer wide adjustability. Therefore, blocked polyisocyanate is more convenient and widely used in industrial applications.
However, it is necessary to cure or bake the coating at high temperatures, making them unsuitable for use on wood and plastic surfaces. The blocking agent used in these coatings tends to evaporate during curing process, which may lead to atmospheric pollution.
Due to its single-component nature, superior electrical insulation performance, as well as corrosion resistance and weather resistance, blocked polyurethane coatings are extensively utilized in industrial applications. This includes electrical insulation coatings, powder coatings, electrophoretic coatings, enamel wire coatings, coil coatings, and more.
Blocked polyurethane coatings are employed as insulating coatings due to their excellent insulation properties, solvent resistance, water resistance, and mechanical performance. Common electrical materials such as copper, aluminum, sand steel sheets, etc., can withstand baking. Moreover, enamel wires coated with polyurethane paint often exhibit the characteristic of "self-soldering tin." Under high temperatures, the urethane bonds degrade, exposing the copper or aluminum wire. This eliminates the need to remove the paint layer, making it easy to solder, known as solderable wire enamel.
Coil coatings offer high production efficiency, and the volatile solvents released during coating can be concentrated and used as energy for combustion without polluting the atmosphere. Coated coils (steel or aluminum) can be post-formed without cracking, providing flexibility for end-users (such as construction panel manufacturers and household appliance factories). This eliminates the need for end-users to establish paint workshops in the factory, avoiding fire hazards and management complications.
Common primers include high molecular weight epoxy resins with melamine resin and chromic acid. If a closed TDI adduct is used as a primer with a hydroxyl resin, the flexibility of the coating is enhanced. Common topcoats for coil coatings are polyester/melamine systems. Blocked polyurethane coatings can also be used as coil coatings.