The causes of this phenomenon are as follows
① The solubility of Pu diluent is not enough to completely dissolve the PU coating, so that the oh and NCO in the PU coating can not be completely contacted to produce chemical cross-linking reaction;
② The addition of curing agent in the reactive system is not enough, there are residual resins not participating in the reaction, or the resin type does not match the composition type of hardener, so the crosslinking reaction can not be completed at room temperature;
③ When the air humidity of the construction environment is too high, or the moisture content of the coating and solvent is high, or the PU coating P diluent contains alcohol solvent, the hardener reacts with water and alcohol first, resulting in the insufficient hardening dose reacting with the resin;
④ Too high air humidity in the construction environment leads to too slow volatilization of diluent, too low temperature and blocked oxidation or polymerization reaction, thus affecting the reaction rate;
⑤ One time spraying is too thick, or the recoating interval is short, so that the lower coating is not dry. Or the lower coating is not suitable, especially the multi liquid coating;
⑥ Sandpaper moisture regain, improper sandpaper model, improper grinding method;
⑦ Due to insufficient ventilation, the vapor of diluent fills the whole space, which makes the solvent volatilize slowly and affects the drying;
⑧ The material is not clean, such as AC coating, contact with alkaline adhesive, bleach, and sweat, so that the coating has water and oil adhesion, the moisture content of the coating is too high, and the drying is slow;
⑨ But when the film is repainted, the treatment is improper, the coating is not suitable, or there is paint adhesion;
⑩ Too much BCS is added in NC coating construction;
Too much plasticizer was added to the coating.
For these reasons, there are some solutions:
① Choose the matching dilution with strong solubility and fast volatilization
② The hardener that can fully cross-linking reaction is selected, and the accuracy of the ratio is required, so as to avoid increasing or reducing the hardener as far as possible;
③ Pay attention to the humidity of the construction environment, add air dehumidification device or forced drying, increase the temperature of the construction environment, or extend the drying time;
④ Two or more times spraying is used to extend the coating interval;
⑤ Select the appropriate type of sandpaper, according to the direction of wood grain along the wood grain grinding;
⑥ Proper ventilation, but the wind speed near the coating surface should not be greater than 1 m / sel (PU coating material, the speed should be very slow;
⑦ The moisture content of the substrate should be the same as that of the local material, which should be cleaned with solvent or alkaline solution or steam;
⑧ Notice the type of old coating in advance, and pay attention not to be contaminated;
⑨ Unclean materials should be fully cleaned and dried;
⑩ PU coatings, diluents and hardeners shall not contain alcohol solvents. Attention shall be paid to the moisture content of raw materials such as pigment solvents, and the detection shall be controlled within the specified range;
No coating should not add too much BCS in high temperature and humidity environment;
In order to improve the flexibility of the coating, too much plasticizer should not be added to the coating formulation.