Thermosensitive energy saving systems with tailored solar reflecting/absorbing properties for construction structures – THERMOSOLAR

Stage III – 2014

Implementation of laboratory technologies for thermochromic systems –experimental model implementation

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.

Stage IV – 2015

Implementation of laboratory technologies for thermotropic systems–experimental model implementation.

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.