The spraying temperature of the automatic spraying line process determines the coating adhesion

The coating process of the product is a coexistence of physical and chemical processes. The coating process of the product is the process of the powder coating being heated to melt and adhere to the product and the curing reaction process of epoxy resin.

The spraying temperature of the automatic spraying line process refers to the temperature of the material when the powder is sprayed on the raw material. The spraying temperature is low, the viscosity of the resin after melting is high, the gelation time is long, the curing reaction speed is slow, the wetting time with the substrate is long, and the cohesion of the formed coating is high; if the spraying temperature is high, the resin melt viscosity is low, and the gelation time Short, the curing reaction speed is fast, and the wetting time with the substrate is short, then the cohesion of the formed coating will become low. Considering the full wetting of the coating and the substrate, the low viscosity and long wetting time have good wetting effects. However, low viscosity and long wetting are contradictory in the powder coating process. Because the curing reaction speed is proportional to the curing temperature, the spraying temperature is higher, the melt viscosity is low, but the time for crosslinking and curing will be shortened at the same time, so it is more important to choose a relatively reasonable temperature during the spraying process.

Choose the same spray material, spray at 120℃, 200℃ and 280℃ respectively, and then cure at 200℃. Then put the test board in a water bath at 95℃ and boil for 30 days to test its adhesion (test method according to SY/T0315). According to the test results, the spray adhesion effect is best at a temperature of 200°C. This is due to the joint effect of the adhesion between the coating and the substrate, the cohesion of the coating, and the stress generated during the thermal expansion and contraction of the coating.

The spraying temperature is low, the epoxy resin cures more uniformly, and the cohesive force of the cured coating is relatively high. After the coating undergoes thermal expansion and contraction process after boiling, the shear stress generated by the thermal expansion and contraction process is too large, when the stress is greater than the coating and the substrate When the adhesive force is high, the coating is easy to peel off from the substrate. Conversely, if the spraying temperature is increased, the epoxy resin will cure quickly, and the cohesion of the cured coating will be lower. After the coating undergoes thermal expansion and contraction, the shear stress generated by the coating will be less, and the coating will not be easily removed from the base. Peel on the material.

At the same time, increasing the temperature of the coating can increase the movement of the electron cloud, which is conducive to the formation of covalent bonds between the substrate and the coating, thereby improving the adhesion of the coating. However, the porosity of the coating, especially the porosity of the bonding surface, will increase with the increase of the spraying temperature. The increase of the porosity will affect the adhesion of the coating to the substrate, especially for the adhesion of the coating after long-term boiling. Therefore, when the temperature rises to a certain level, it is not conducive to the improvement of adhesion.