The project leader ICECHIM by the team specialized in dyes chemistry conducted activities related to experimentation technology of thermochromic systems with transition in the range 25-75°C, in order to verify the technology reproducibility and to achieve the required amount of thermochromic complex which after encapsulation will be used in pilot testing of thermochromic coatings. Experiments were conducted in laboratory conditions in order to optimize the encapsulation of the thermochromic systems in water-insoluble polymer shells of melamine-formaldehyde resins by varying the parameters of the synthesis and increasing the efficiency of encapsulation.

        The optimized technology comprises the following steps: polycondensation of melamine with formaldehyde and prepolymer stabilization; thermochromic complex emulsification and stabilization; the encapsulation of the thermochromic complex; crosslinking of the shell; filtration, washing and drying of the thermochromic capsules. Verification of the technology reproducibility was made by obtaining several batches of different microencapsulated thermochromic complexes with different transition temperatures.

        The main task carried out by the UPB partner was the technology experimentation for the synthesis of the thermotropic systems with the transition in the 25-75°C range.

        In the first step the solution for some deficiencies of the technology elaborated during the year 2015 was looked for. These deficiencies were: a relatively high viscosity of the N-isopropylacrylamide-acrylic acid copolymer aqueous solution; agglomeration of the polymer from the aqueous solution after a few transparent-opaque cycles (20°C-50°C). In the first case the best results were obtained by adding isopropanol as the chain transfer agent in the reaction medium in order to decrease the molecular weight of the copolymer. In the second case the solution was the employment of the hydrogen peroxide-ascorbic acid redox initiating system. The proposed technological process was then experimented at a larger scale (about 1 Kg), by employing experimental conditions as close as possible to the industrial ones, including unpurified monomers. The main steps of the technological process are the following:preparation and degassing of the water/isopropanol solution of monomers and ascorbic acid;degassing of the aqueous solution of hydrogen peroxide;polymerization;packing of the polymer aqueous solution.

        The final concentration of the polymer solution was 5%, while the overall conversion of monomers was over 98%. The copolymerization process was carried out under inert atmosphere at 20-25°C for 20 hours. A mass balance was carried out for each technological step, and then the overall mass balance. The cost of the 5% polymer solution was also estimated.

        The research group of Polymer Department - ICECHIM developed laboratory experiments to optimize the encapsulation conditions of the thermotropic system in a translucent shell of water-insoluble polymer by performing the synthesis at room temperature. In this sense, a significant number of samples were synthesized in order to accomplish the encapsulation in two variants, namely: “in situ” polymerization of vinyl/acrylic monomers dispersed in water/organic solvents and internal phase separation technique. Based on the latest version, was developed the encapsulation technology for thermotropic hydrogels comprising the following steps: achieving double emulsion water-oil-water; evaporating the solvent; decanting – washing - drying the thermotropic encapsulated system.

        In this stage, at CHIMCOLOR, pilot technologies of obtaining thermochromic / thermotropic coating materials with hypsochromic / thermothropic transition in the range 25-75°C, for three primary colors (red, yellow and blue) and three types of film-forming materials, as well as for a washable paint containing a thermotropic microencapsulated system have been accomplished. All technologies have proven reproducible at pilot scale, and on this basis mass balance, specific consumption and estimated costs of the final products were estimated.

        Strength, color and integrity of coating materials were tested for various media in correlation with the response to heating and reversibility of the process in successive cycles of heating-cooling for washable paints, colorless and fireproof lakes. CHIMCOLOR has drawn up a feasibility study for the thermosensitive film-forming materials.