In the fourth stage of the project, the three partners have carried out activities related on alternatives for obtaining encapsulated thermotropic systems of hydrogel-type, physical-chemical characterization of thermochromic systems obtained in an earlier stage of the project, obtaining laboratory technology for thermotropic systems with transition in the 25-75°C range, assessing physical-chemical, mechanical and transition properties of thermosensitive film forming materials, selection and characterization of thermotropic coatings which shows the transition in the 25-75°C range.

    The project leader – ICECHIM by the research team specialized in dyes chemistry, conducted activities related to physico-chemical characterization of the thermochromic systems developed in a preceding stage, as well as evaluating of the thermochromic film forming materials transition properties. Regarding thermochromic compositions previously studied it was considered the fact that embedding in film-forming materials can bring changes in the behavior of thermochromic systems depending on interactions that can occur between components, characteristics of the material used for encapsulation or presence of certain species in the film-forming material, the type of solvent used, etc. As a rule, in order to obtain thermochromic coating materials, there are three important requirements, namely: achieving optimal contrast between the two states (before and after the transition), optimal sensitivity to the appearance of a temperature difference and high stability to light and weathering of the colors obtained.

     Physico-chemical characterization of thermochromic systems includes recording of reflectance spectra and color measurements in dynamic conditions as a function of the temperature both on thermochromic mixtures and on compositions encapsulated in melamine-formaldehyde resins - set in an earlier stage as optimally materials for the encapsulation process. Hypsocromic systems with transitions between 25-75°C are formed by Crystal Violet Lactone and derivatives thereof together with phenols, fatty alcohols, fatty acid esters and mixtures thereof. On the basis of the spectral reflectance, fluorescence data or thermal analysis, it was determined the optimum molar ratio between the phenolic component and the dye, and from the maximum absorbance reached, while maintaining the characteristics of the thermochromic optimum range of temperature, it was determined the optimum concentration of the dye in the mixture. Thus, it was established an optimal molar ratio dye-polyphenol situated in the range 1:1-1:4 depending on the components and a maximum concentration of the dye in the thermochromic mixture of 20% by weight.

     Regarding thermochromic mixtures encapsulated in melamine-formaldehyde resins, particle size distribution was determined by DLS in aqueous dispersion, emphasizing for all encapsulated mixtures an asymmetric distribution situated around 5-6 μm. Differential thermal analysis reveals very high thermal stability of the thermochromic capsules up to 200°C and the presence of thermal effects proper to solvent and co-solvent present in the thermochromic mixture.

The study of alternatives for obtaining thermotropic embedded systems and selection of thermotropic embedded systems was the subject of one of the activities developed by the team specialized in polymeric materials within ICECHIM. In order to microencapsulate thermotropic aqueous systems, it is imperative that in terms of forming a protective layer to use materials that present an initial interaction with the aqueous core and a good dispersibility in the solvent used to prepare the initial dispersions.

     A variant for encapsulation was to create a shell by sol-gel processes in both acidic and basic media. As precursors for sol-gel processes were used different alkoxysilanes and as surfactants were used oleic acid derivatives. Also, it was studied retention of water belonging to the initial phase of preparation after the organic solvent is removed. In good agreement with previously published data, after removal of the solvent most of the water evaporates due to the porous structure of the silica.

     An option for obtaining a protective layer was the anionic polymerization of cyanobutylacrylate (BCA). Making an external shell of PBCA was carried out by "in-situ" polymerization in the presence of water containing thermosensitive polymer. Thus, some microemulsions prepared by UPB were modified by BCA polymerization at water-solvent interface.

     The main task carried out by the UPB partner within the year 2015 was as follows: Elaboration of the technology for the synthesis of the thermotropic systems with the transition in the 25-75°C range.

      A technology for the synthesis of the N-isopropylacrylamide – acrylic acid copolymer with 10 mole-% acrylic acid was elaborated. This copolymer has been proven within the research works carried out within the project during the previous years as being the most promising from the point of view of the transparent – opaque transitions in aqueous solution triggered by temperature variations. The intention was that the final product of this technology be the aqueous solution, which can be further used, as such or diluted, within the encapsulation process. With this aim in view, the copolymerization of the two co-monomers was carried out in demineralised water as the solvent, at 24 - 26°C, under nitrogen atmosphere, by employing the potassium persulfate – potassium metabisulfite redox initiating system. The total weight concentration of the two co-monomers in solution was 5%, while the reaction time was 24 hours. The overall monomer conversion was over 98%. The main steps of the technological process are the following: the acrylic acid distillation in order to remove the inhibitor; the preparation and degassing of the aqueous solution of monomers and potassium metabisulfite; preparation and degassing of the aqueous solution of potassium persulfate; the polymerization; packing of the polymer aqueous solution. A mass balance was carried out for each technological step, and then the overall mass balance.

     At this stage CHIMCOLOR conducted activities related to the assessment of physical-chemical and transition properties of thermosensitive film forming materials by testing coatings on the behavior of the materials to environmental factors and natural and artificial light. Also, there were tested their fastness properties, color and coating materials integrity on various substrates correlated with response to thermal excitation and reversible process during successive cycles of heating and cooling for both aqueous varnishes, and lacquers selecting optimal products.