Polyurethane paint, also known as polyurethane enamel, refers to a coating that contains a significant amount of urethane bonds in its film.
The resin of polyurethane paint differs from other resins (such as polyester, polyether, etc.). While polyester resin contains only ester bonds apart from hydrocarbon groups, and polyether contains only ether bonds, polyurethane resin, in addition to urethane bonds, may also contain numerous ester bonds, ether bonds, unsaturated double bonds from oils, and acrylic ester components. At times, the quantity of acrylic esters may even exceed that of urethane bonds. However, conventionally, it is still referred to as polyurethane paint, but in reality, it is an approximation of a segmented copolymer.
The resin of polyurethane paint is not polymerized from urethane monomers like acrylic paint. Instead, the resin of polyurethane paint is formed by the combination of polyisocyanate (primarily based on diisocyanate) and polyol.
It’s important to note that the resin of polyurethane coatings is not polymerized from urethane methacrylate monomers but is formed by the combination of polyisocyanate (primarily based on diisocyanate) and polyol.
When it comes to selecting solvents for polyurethane coatings, it is essential to consider not only the common properties such as solubility and evaporation rate but also the specific characteristics of the NCO group within the paint. Therefore, attention should be given to the following two aspects:
As the solvent should not contain substances that react with the NCO group, alcohol and ether-alcohol solvents should be avoided. Although hydrocarbon solvents are stable, they have low solubility and are often used in combination with other solvents. Esters are the most commonly used solvents. For instance, ethyl acetate and butyl acetate are frequently used, and historically, acetic acid solvents (ethylene glycol diethyl ether acetate) have also been employed. The latter has strong solubility and a suitable evaporation rate, making it highly suitable for use.
In two-component polyurethane coatings, the molecular weight of the two components is often not high, and they are typically low oligomers. These components exhibit high polarity and sensitivity to trace oil contamination. Due to differences in surface tension between the two components, the low molecular weight leads to slow film setting (gelation) after application, resulting in a still-flowing liquid state. At this stage, driven by the gradient in surface tension, pinholes are easily formed, necessitating the addition of an anti-shrinkage agent.
Polyurethane paints often suffer from foaming issues, requiring the addition of defoaming agents. These defoaming agents are typically categorized into non-silicone resin-based and organosilicon-based additives. Resin-based additives consist of thermoplastic copolymers, such as copolymers of ethylene vinyl ether and acrylic esters. These additives, being incompatible with polyurethane paint, disrupt the surface layer of small air bubbles in the coating, causing them to coalesce into larger bubbles over time.
Aliphatic polyurethane clear coatings find extensive use in vehicles and other applications. However, they are prone to cracking and peeling under exposure to sunlight's ultraviolet (UV) radiation. To mitigate this, it is necessary to incorporate light stabilizers, including two main components: ultraviolet absorbers and hindered amine light stabilizers (HALS). The combination of these two components can significantly extend the lifespan of the clear coating.
In polyurethane coatings, the reaction of polyisocyanates with water, especially in the presence of atmospheric moisture, can generate urea and carbon dioxide. This reaction leads to swelling of the coating container, foaming of the film, and an increase in viscosity, ultimately resulting in gelling. Therefore, it is crucial to control the moisture content in the coating. This is particularly important when manufacturing moisture-cured coatings and solvent-free coatings, where measures must be taken to eliminate the moisture introduced by pigments, solvents, and other components.