In order to obtain a paint film with satisfactory performance, the molecular weight of the thermoplastic polymer is usually very high. To achieve the solution viscosity required for construction, it is necessary to add the largest organic solvent. The organic solvent volatilizes into the air after film formation, causing air pollution. And waste the most resources. This kind of coating is mainly used for outdoor construction, cannot be baked, and has special requirements for the performance of the paint film. The main varieties used are nitrocellulose paint, halogenated polymer paint (such as chlorinated rubber paint, perchlorethylene paint, vinylidene chloride copolymer resin paint, organic jaw paint), thermoplastic acrylic resin paint.
8. 2. 2. 2 Other factors affecting the performance of the paint film
The paint film is a thin film coated on the substrate. The substrate determines the degree of deformation of the paint film, and when the paint film has good adhesion, the substrate absorbs the energy of external impact and reduces the effect on the paint film. A thinner film is more impact resistant than a thicker one, but the thinner the film, the poorer the hiding power.
The paint film of pre-painted exterior wall panels is often compromised with a paint film with a thickness of 20~25μm, which can increase the hardness of the paint film without cracking. In order to reduce the swelling of the paint film on the inner wall of fish cans by fish oil, it is necessary to use phenolic varnish with a higher degree of cross-linking to coat pre-coated flat plates with thousand-made fish cans. This kind of paint film is very brittle due to its high degree of cross-linking. In order not to crack when processing pre-coated flat panels, the film thickness should be 5 µm or less.
The flexibility of paint films tends to deteriorate over time, especially with air-dried paints, because there is always some solvent left in the paint film. The Tg of most paint films is always close to or slightly higher than room temperature, and the solvent will evaporate slowly at this time. Solvents generally have a plasticizing effect, so when the solvent volatilizes, the V increases, and the flexibility of the paint film decreases.
Hardening with time is also often observed in baked crosslinked paint films. When a polymer is heated to a high Tg and then cooled rapidly (cooling), it tends to be less dense than when it is cooled gradually. Rapidly cooled ones have more free volume to freeze than slowly cooled ones, and thus have more opportunities for molecular motion. During storage, even if the temperature is relatively low, the molecules in the cold paint film will gradually move, making the free volume shrink and the density increase. There is no chemical change in this process, so it is called physical aging. Cracking is more likely during post-processing as free volume decreases and density increases. This phenomenon may occur on the paint film on the metal that has been baked at high temperature and cooled rapidly after coming out of the oven. This may also be a common cause of brittleness in the aging of baked lacquer films. The maximum mold growth was found at 30°C for polyol/MF paint films baked at 180°C and then cooled at 30°C. The aging rate (i.e. the speed of mold maximum growth) decreased with time. The samples Re-heating at 180"C and re-cooling to 30°C, the modulus returns to a lower value and re-enters physical aging.
