Thermal insulation coating based on water-based polymer dispersion

For Russia, due to its long winter period, improvement of thermal insulation properties of envelope structures by applying thermal insulation paint and varnish coating to its inner surface is considered perspective. Thermal insulation properties of such coatings are provided by adding aluminosilicate microspheres and aluminum pigment to their composition. This study was focused on defining the effect of hollow aluminosilicate microspheres and aluminum pigment on the paint thermal insulation coating based on water-based polymer dispersion and on its optimum filling ratio. The optimum filling ratio was determined using the method of critical pigment volume concentration (CPVC). The optimum filling ratio was found equal to 55%.


Introduction
Heat transmission through building envelope structures can occur in different directions depending on climate conditions of the region.The most relevant issue for countries with hot climate or long summer season is decrease of buildings heat gain caused by solar radiation.This problem is solved by covering the surface of building envelope structures with coatings that can reflect solar radiati0n [1][2][3].
It is economically reasonable to consider the possibility of using hollow alunimosilicate microspheres as filler for paint coatings, for their price is 8 to 10 times lower than that of polymer and glass microspheres.While producing paint pigment compositions, the color of microspheres has special relevance and depends on the ability of microspheres to disperse wave radiation in optical wavelengths.This ability is defined by the surface type, microsphere wall thickness and porosity.Shells of micropheres are light-gray and have a small number of closed pores due to which they provide sufficient transparency and do not change paint color and opacity, which allows saving high-priced pigments [13].

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Corresponding author: indyboot@list.ruOptical values that characterize radiation properties of materials are absorption coefficient α and emissivity coefficient ε [14].The most complex task is to obtain coatings with low emissivity coefficient values.Therefore, for a coating to be able to reflect thermal radiation it is required to add to its composition a component with high reflective capacity in IR regionaluminum pigment [15,16].
Traditional aluminum pigments are powders PAP-1 and PAP-2 (aluminum pigment powder).However, such pigments have a number of drawbacks: poor wetting properties, oxidation of aluminum particles, fire and explosion hazards, inhalation toxicity, as well as presence of active aluminum able to interact with water [17].German company Schlenk and American company Eckart developed the technology for obtaining spherical aluminum pigment particles [18].Aluminum particles are covered with a transparent silicone dioxide layer, which allows using them for water-based paint compositions.Pigments are produced in the form of ready-to-use pigment pastes.In addition to aluminum pigment and water, such pastes contain admixtures for wetting and dispersion, antifoaming agents and special admixtures to optimize rheological properties.
Use of styrene acrylic dispersions as binder for highly filled compositions is considered reasonable due to their optimum price/quality ratio.The research is aimed to define the effect of hollow aluminosilicate microspheres and aluminum pigment on the paint thermal insulation coating based on water-based polymer dispersion and to find its optimum filling ratio.

Materials and Methods
Light-gray hollow aluminosilicate microspheres (Ural Ekibastuz class) produced by Inotek company were used during the study.Grain size distribution of microspheres was 75-300 microns, with the mean diameter of 152 microns.The image of a microsphere is given in Figure 1.The task of making a paint formulation consists in selecting composition that provides the paint's stability, good technological properties and forms a film with as closed structure as possible [19].Pigment concentration at which the filming agent completely covers pigment and filler particles and occupies all space between them is called critical pigment volume concentration (CPVC).As a rule, while increasing the filling ratio of a coating its properties smoothly vary, and at the point of CPVC either sharp variation or almost no further variation of properties occurs [20].In other words, the best coating properties are obtained when the filling ratio is slightly less than the critical pigment volume concentration.Therefore, CPVC method was used for defining the optimum pigment filling ratio of a paint thermal insulation coating.

Results
The main characteristics for paint coatings meant for interiors are water absorption of the film, opacity and adhesion to surface.On the one hand, loosening of film-forming agents' structure can be observed in proximity to pigment and filler particles, which can cause degradation of coating properties.And, conversely, when tightened structures of filmformation agent are formed, the coating properties improve [20,21].It is known that CPVC of water-based styrene acrylic paints lies between 40 and 80% [22].Hence, search of optimum filling ratio was performed in this range.The research results presented in Figures 3-5 show that degradation of coating properties occurs when the CPVC of the film reaches the value of 58%.