The Low Temperature Thermally Induced Reversible Color Coatings

In this paper, development of theory and technology on thermoindicator paint and thermocolor was briefly reviewed, and the writer systematically introduced the preparation of thermocolour microcapsules by complex coacervation/phase separation techniques and application of those microcapsules in paint.In the process of preparation of thermochromatic coatings, two kinds of changeable-color pigment constitution were adopted. One is the microcapsules product, in which Gelatin/Arabic gum serviced as shell materials, and Crystal Violet Lactones/2,2-Bis (4-hydroxlynhenyl) Propane dissolved in dodecanol as core material. In the preparation of microcapsules, complex coacervation/phase separation techniques were employed. Complex coacervation is based on two oppositely charged polymers. Gelatin (Base processed) carries a higher proportion of negative charges at neutral pH and it has an isoelectric at pH of about 3.5-4.5. On the other hand, the Arabic gum carries negative charges. The other is CoCl2.C6H12N4.10H2O as thermocolor. In the preparation of microcapsules, the influence of concentration and the rate of mass of Gelatin to Arabic gum, the mass of water used for dilution, the value of pH, mshell, mcore, and the quantity of formaldehyde as curing agent were studied. The investigations clarify that in the solution of the 10%(mass concentration) Gelatin/Arabic gum as shell material, the liquid core material (Crystal Violet Lactones /2,2-Bis (4-hydroxlynhenyl) Propane dissolved in dodecanol) was stirred at high speed, maintaining at 50-60C until the diameter of core particles to less than 100m . The pH adjustment is the desolvation agent of HC1 (1mol/L), when the value of acid reduce to 3.5-4.5, the phase separation process in gelatin solution take place, and then added 37%(mass concentration) formaldehyde, remaining 30 minutes at 0-5 C with gentle stirring. Finally, graduately rising the temperature at the rate of 1 C /min to 50-60 C. The cross-linking polymer of gelatin with formaldehyde end up and the microcapsules could be subsequently hardened. Being washed up and filtered, the microcapsules product was manufactured, which could be added into water-base constructional paint. At the same time, the dependence of droplet size on stirring speed and polymer concentration was discussed.As a kind of modification method, microencapsulate was used to incorporation of pigments into droplets, which partly tackled the difficulties that the oily pigments is hard to whether dissolve into or disperse into water-based coatings, moreover, the shell can prevent the core from the effect of harmful chemical agent in the environment. Finally, the feature of the two kinds of paint respectively added with microcapsules and CoCl2.C6H12.N4.10H2O was compared. Obviously, beside lower color-changing temperature, the former have better behaviors in thermo-sensibility, and much shorter color-recovering time.This study provides a kind of feasible method that disperses oily pigments into water-based paint, which enriches the preparation technology of water-borne low-temperature thermoindicator paint. Moreover, technical progress in this field is going on. That microencapsulate of pigments may be one of the most important method could be expected in the future.