Epoxy curing conditions: important factors of heavy-duty anticorrosive coating

The heavy-duty anti-corrosion powder coating industry, which is composed of epoxy resin as the main base material, attaches great importance to improving the product quality from the perspective of process improvement Recently, Changshu Jiafa company and other companies have made new progress in the research of curing conditions of epoxy resin and its curing agent, and confirmed that it is an important factor affecting the heavy-duty anti-corrosion powder coating Experts said that there are two kinds of hydroxyl groups with different structures in the molecular structure of hydroxyl terminated polymer curing agent, one is phenol hydroxyl group, the other is alcohol hydroxyl group formed by ring opening of epoxide When determining the hydroxyl value of the curing product, the hydroxyl value is the total sum of the two hydroxyl groups As the active group of curing epoxy resin, the activity of phenol hydroxyl is far greater than that of dry alcohol hydroxyl The former can react smoothly with epoxy group in weak base or even without catalyst, while alcohol hydroxyl needs to react in strong base or strong acid such as Lewis acid At present, the catalyst used in powder coating is weak basic anionic polymerization catalyst Under the condition of fast curing required by powder coating, alcohol hydroxyl is difficult to participate in the reaction Therefore, only the reaction of phenol hydroxyl needs to be considered when calculating the amount of catalyst In theory, the molar ratio of phenolic hydroxyl group to epoxy group is 1:1, while in the actual formula design, the amount of curing agent is less than the molar amount of phenolic hydroxyl group, and the molar ratio of phenolic hydroxyl group to epoxy group is 0.6-0.9 The reason why the amount of phenolic hydroxyl is less than that of epoxy group is that in addition to the addition reaction between phenolic hydroxyl compound and epoxy group, there are also anionic polymerization catalysts in the system to make the ring opening polymerization of epoxy group At present, in order to speed up the curing reaction, the amount of this catalyst is relatively large, so part of the epoxy group has been consumed by the anionic polymerization catalyst, which does not participate in the addition reaction, that is to say, the actual consumption of phenolic hydroxyl will be less than the theoretical value The excess of phenolic hydroxyl may bring adverse effects, such as the decrease of solvent resistance and chemical resistance of the coating There are two competitive reactions in the curing process of heavy-duty anticorrosive powder coating Increasing the amount of anionic polymerization catalyst will accelerate the curing speed, but the flexible group of anionic polymerization catalyst itself is less, and the coating film is easy to be brittle The addition reaction rate of phenolic hydroxyl compounds was significantly lower than that of anionic polymerization According to the experts of China epoxy Industry Association, in order to speed up the reaction, it is necessary to increase the reaction temperature, unilaterally reduce the reaction temperature and shorten the curing time, which will not be conducive to the improvement of the final performance of the film In addition to determining the relationship between different curing conditions and coating properties through curing condition test and obtaining the best formula, the curing temperature can also be determined by DSC curing curve of specific powder The starting temperature, the maximum exothermic temperature, the end temperature and the curing time of the powder coating are obtained according to the isokinetic heating curing curve The enthalpy value of the curing exothermic is determined The reaction activation energy and the kinetic constant of the chemical reaction are calculated from the kinetic analysis, and the time required to complete the curing reaction under different temperature conditions is calculated according to the kinetic equation It can also calculate the curing reaction rate in a certain period of time at a certain temperature At present, the moderate curing condition of pipeline heavy-duty anticorrosive powder coating is 210-240 ℃, and the curing time is 1.5-2min.