In-flight loss of solvent - Because this portion of the added reducer or thinner is designed to evaporate from the time it leaves the gun and before it reaches the object being sprayed, the chemical used must be very volatile. Most paint guns used in the aftermarket painting industry are external mixing guns. This means that paint and atomizing air do not mix until the paint travels out of the gun's tip. Because paint and air are mixed outside the gun, the viscosity or thickness of paint must be such that paint can be broken into small enough particles to allow the paint to lie smoothly on the vehicle.
Leveling solvents - Leveling solvents are the solvents that remain in the paint after it hits the surface of the vehicle being painted, allowing atomized droplets to reconnect and “flow out.” The solvent must remain in the coating, allowing gravity and other factors such as temperature, humidity, and airflow to help the paint surface to become flat and smooth.
Though this portion of solvent is less volatile (evaporates more slowly) than the in-flightloss solvents, it must also evaporate fast enough to allow the technician to apply
additional coats within a reasonable time. The time that must pass before applying additional coats of a coating or before the paint can be baked is referred to as flash time. If sufficient flash time is not allowed and a subsequent coat is applied, a condition called “solvent popping” will occur. Solvent popping is a condition in which solvents from below new coats are trying to escape (gas out), and get trapped under a paint film that is starting to harden. As the bubble of gas solvent breaks through the surface, a small crater is formed, causing an imperfection in the paint surface.
Final solvents - The final solvent evaporation can take up to 90 days, in some cases, to completely gas out. Solvents remain in the coating, allowing the second curing process to take place.
Thus far, we have been exploring the evaporation process of paint. However, evaporation is not how modern urethanes, polyurethanes, and epoxies cure. In fact, only 25% of the curing process is truly drying or evaporating. Paints cure by a process called crosslinking. Cross-linking is a process that forms chemical bonds between two separate
molecular chains, generally polymers. This cross-linking is added, and a much harder more durable bond is formed if some of the solvent remains in the paint as the crosslinking take place. As this three-dimensional molecular structure is formed, the resulting coating is more shear-resistant, has a higher temperature resistance, and is oil-and solvent-resistant. This process takes an extended period, and is aided when small amounts of solvent remain in the coating.
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