Preparation and Optimization of Thermochromic Microcapsules as a Ternary System of Crystal Violet Lactone: Bisphenol A: Decanol Encapsulated with Urea Formaldehyde Resin in a UV-Curable Primer

Coatings can achieve the property of changing color with temperature variations by adding thermochromic microcapsules, which can bring a variable surface to the substrate. Ultraviolet ray (UV)-cured primers have the advantages of a fast curing rate, low-temperature curing, and low pollution. Thermochromic microcapsules can expand the application range of UV primers. Thermochromic microcapsules were synthesized through an orthogonal test, using crystal violet lactone, bisphenol A, and decanol as the core materials in a 1:4:50 mass ratio, with urea formaldehyde resin as the wall material. The effects of the addition of batches of the urea, the mass ratio of the formaldehyde solution to the urea, the hydrophilic-lipophilic balance (HLB) value of the emulsifier, and core-to-wall mass ratio on microcapsules yields, encapsulation rates, thermochromic color differences (ΔE), and formaldehyde releases during synthesis were investigated. The results were normalized, with the thermochromic ΔE as the primary reference for analysis. The results indicate that the HLB value of the emulsifier was the key factor that affected the microcapsule performance. In a single-factor test, the HLB value was adjusted within the range of 6.00 to 10.00. It was found that when the HLB value was 10.00, the microcapsules exhibited the best comprehensive performance, with a yield of 43.29%, an encapsulation rate of 45%, a thermochromic ΔE of 4.60, and a formaldehyde concentration released of 1.310 mg/L. The 11# microcapsules with the optimal morphology and better comprehensive performance were compared with the best 14# microcapsules. Different amounts of these microcapsules were added to the UV primer to investigate the effects of the 11# and 14# microcapsules on the mechanical and optical properties of the UV primer. The main component of the UV primer was polyurethane acrylic resin, propylene glycol diacrylate, and hexanediol diacrylate. When 14# microcapsules were added to the UV primer at a concentration of 10%, the primer exhibited the best comprehensive performance, with a fracture elongation of 17.44%, a roughness of 0.15 μm, and a visible light transmittance of 83%. Microcapsule technology was used to modify UV primers, endowing them with thermochromic properties and expanding the application range of thermochromic microcapsules.