In the third stage of the project, the three partners conducted activities related to a comparative analysis of the systems studied, their properties evaluation, selection and characterization of the systems with transition temperature in the range 25-750C, respectively the production technologies for thermochromic systems, and their encapsulation in order to obtain thermochromic coatings materials.

   The project leader INCDCP - ICECHIM carried out activities related to the assessment of physico-chemical and transition properties of thermosensitive materials, selection and characterization of the systems showing a hypsochromic transition in the range 25-750C and the development of their manufacturing technology. As was shown during other research stages, thermochromic compositions considered for obtaining film-forming materials must meet three important requirements, namely: achieving optimal contrast between the two states (before and after the transition), optimal capacity transition to thermal stimuli and high stability of the color obtained to light and weathering. Of particular importance is the selection of those compositions which have a hypsochromic transition to the temperature elevation, for reasons related to the reducing of heat absorption when the outdoor temperature increases. The reversibility of the process must be maintained over many heating - cooling cycles as the outdoor temperature could be significantly above or below the transition temperature.

   The main components of the thermochromic compositions established during investigations are: the color former, the color developing agent and solvent of the thermochromic mixture. During investigations were selected triphenylmethane lactones as color formers and various bisphenols as color developers, while C12-C16 fatty alcohols and fatty acid esters of polyhydric alcohols, and mixtures thereof were selected as solvents. It should be noted that the solvent determine the transition temperature regardless of the nature and quantity of the other components in the mixture.

   The selected systems present hypsochromic transitions in the range 25-750C are made of Crystal Violet Lactone and its derivatives together with bisphenols, in fatty alcohols or fatty acid esters for transition temperatures above 500C. The molar ratios established as optimal for all thermochromic mixtures were determined by the JOB method, from both the absorption/reflective spectra and from fluorescence emission.

   The data obtained by optical methods lead to the establishment of optimal ratio between components and it was verified by calorimetric measurements. For all optimal compositions were selected additives used as light stabilizers and the manufacturing technology of thermochromic mixtures with transition temperature in the range 25-750C has been carried out.

   With regard to encapsulation of the thermochromic compositions, have been tried many options for both compositions comprising fatty alcohols and esters, by establishing the optimal systems of melamine-formaldehyde and urea-formaldehyde resins. The main reason is due to the very weak interactions between polymers obtained and composition, which does not affect the thermochromic equilibrium and color of the compositions obtained. The capsules obtained by these variants retain systems integrity, providing a shell that resists well to mechanical stress without breaking and which does not allow interactions of the core with the external environment.

   Encapsulation technology determined to be optimal comprises the following steps: polycondensation reaction of melamine with formaldehyde, stabilizing the melamine-formaldehyde resin, obtaining of the final solution of encapsulation, melting and mixing the thermochromic complex with the pre-formed resin solution, neutralizing the pre-formed resin solution to provide an aqueous dispersion of encapsulated thermochromic microcapsules and isolating the particles by centrifugation, filtration, washing and drying.

   The main tasks carried out by the UPB partner within the year 2014 were as follows: a) comparative analysis of the thermotropic systems studied and b) selection and detailed characterization of the best systems with the transition in the 25-75°C range from the point of view of performance and application

    The thermotropic systems developed during the 2012-2013 phases were compared from the point of view of the transparent-opaque transition temperature, stability, degree of opacity, resistance to freezing and commercial availability/price of the polymer. It resulted that the most promising thermotropic systems are those based on a) Pluronic L31, especially because it is a cheap and commercially available polymer (industrial product), and forms systems with acceptable properties and b) N-isopropylacrylamide – acrylic acid copolymer, due to the formation of solutions with a high level of opacity (increased degree of whiteness) and good stability, even in the presence of co-solvents which ensure a good resistance to freezing.

   The systems with the best properties, i.e. those based on Pluronic L31 and NIPAM-AAc copolymer, selected within the Task III.1, were subjected to additional investigations consisting in the examination of the behavior of the aqueous solutions of these polymers in the presence of additives/co-solvents mixtures, others than those already employed during the 2012-2013 phases. The systems studied in the case of Pluronic L31 consisted in 5 wt.-% polymer dissolved in aqueous solutions containing the following mixtures as additives: a) DMF-DMSO; b) DMF – ethylene glycol (EG); c) dioxane – EG; d) 1,2-propyleneglycol (PG)-DMSO; e) dioxane – nBuOH; g) DMF – nBuOF; h)PG 20%- EG 20% - surfactant (SDS or Tergitol NP-9). In the case of NIPAM-AAc copolymer, the properties of the 1 wt.% polymer solutions with the following additives: a) dioxane -DMSO; b) dioxane – nBuOH; c) dioxane – EG; d) dioxane – PG; e) DMF – surfactants (SDS, Tergitol NP-9); f) dioxane – Tergitol NP-9; g) PG – SDS, were studied.

   At this stage CHIMCOLOR partner carried out activities related to the assessment of physico-chemical and transition properties of thermosensitive materials by performing tests on the behavior of the microcapsules in the field of forces in atritors used to obtain coatings materials. The fastness properties, color, and integrity of the film-forming materials were tested for various substrates and in correlation with the response to thermal excitation and reversibility of the thermochromic process in successive cycles of heating and cooling. In this step has been accomplished the obtaining technology of thermochromic film-building materials with hypsochromic transition in the range 25-750C for three primary colors (red, yellow and blue) and three types of film-forming materials.