2.1 effect of resin ratio on coating fold

Using deta as curing agent, epoxy resin E-51 and thermoplastic acrylic resin were compounded into 9 kinds of coatings according to different proportions. The thickness of the coating was controlled to 1mm by using the casting automatic film dryer, and the curing temperature was set to 90 ℃. Through the observation of the coating surface by the ultra depth of field digital microscope, it is found that when the ratio of M (E-51): m (thermoplastic acrylic resin) is 1:9 And 9:1, there is no wrinkle on the coating surface, while the wavelength of the coating wrinkle formed by the other seven samples is shown in Figure 1.

It can be seen from Figure 1 that folds can be formed on the coating surface when the content of acrylic resin in the two-component resin is 20% - 80%, and the wrinkle effect is most obvious when the ratio of M (E-51): m (thermoplastic acrylic resin) is 3:2, and the maximum wrinkle wavelength is 1.726mm. The mechanism of the above phenomena is discussed by phase structure analysis.

Take pure epoxy E-51 and m (E-51): m (thermoplastic acrylic resin) as 3:2, 1:1 and 1:5 epoxy / acrylic resin mixture, respectively, after curing and drying, it will brittle fracture under liquid nitrogen condition, and the micro morphology of its cross section is shown in Figure 2.

In the process of coating drying, the upper surface is heated first, and a large amount of heat is released during the reaction, which promotes the reaction, forming a "upper surface layer" with a certain modulus. The modulus of the upper surface layer is related to the epoxy curing agent used in the reaction and the reaction temperature, especially the phase structure formed. It can be seen from Fig. 2 (a) that the pure epoxy resin system is a homogeneous compact structure. Since there is no other phase structure, the curing shrinkage of the coating is the same and no surface wrinkles can be formed. In the bicontinuous phase structure, the resin components are evenly dispersed, the volume shrinkage of epoxy resin produces internal stress when curing, and the flexibility of acrylic resin provides space for the formation of wrinkles when the internal stress is balanced, which is conducive to the generation of wrinkles. When the acrylic resin in the blend system is the main body (Fig. 2 (d)), although the epoxy resin generates internal stress during curing, the modulus of the acrylic resin is relatively low, and the internal stress during curing is easily offset by the elastic deformation of the acrylic resin, so the surface wrinkles cannot be formed. It can be seen from Fig. 2 (b) and Fig. 2 (c) that there are two continuous phases in the mixed resin. The epoxy resin enrichment phase is adjacent to many acrylic resin phases, but the proportion of the two continuous phases is different when the content of acrylic resin is different. The most continuous phases appear in the blend system containing 40% acrylic resin, so the coating fold wavelength formed under this ratio is the largest Wrinkle effect is the most obvious.

2.2 effect of coating thickness on coating wrinkles

As mentioned before, there will be a curing rate gradient in the thickness direction of the wrinkle coating. It can be inferred that the thickness of the coating has a great influence on the formation of wrinkles. Therefore, control m (E-51): m (thermoplastic acrylic resin) = 1:4, and prepare the coating with deta as the curing agent. Set the curing temperature to 90 ℃, use the coating machine to coat different thickness of the paint film, use the digital display screw micrometer to measure the actual thickness of the coating, and use the over depth of field digital microscope to measure the corresponding coating wrinkle wavelength, the results are shown in Figure 3.