Protective Properties of Thermal-Insulation Coatings under Conditions of Long-Term Exposure to Natural Climatic Factors

Abstract

The purpose of this study was to analyze the stability of the protective properties of thermal-insulation coatings under long-term exposure to natural climatic factors. An analysis of the changes in the decorative characteristics was carried out using a CD-6834 spectro-guide sphere gloss spectrophotometer; for the mechanical tensile testing of the polymer composites, an AGS-X series tensile testing machine, TRAPEZIUM X software, and a PSO MG4 device were used to determine the adhesion strength of facing and protective coatings. The results showed that in most cases, only full-scale climatic tests to determine the qualitative indicators of epoxy coatings (ECs), such as indicators of the viability and exothermicity of epoxy compositions, changes in the viscosity of epoxy binders, and the tensile strength and thermal conductivity, allowed us to evaluate the effects of changing the epoxy coating properties in full-scale conditions. When analyzing changes in the characteristics of the polymer samples after exposure to climatic factors, it was found that the compositions based on epoxy resin ED-20, modified epoxy resin Etal-247, active thinner Etal-1, and hardener Etal-45M demonstrated the best elastic and strength characteristics.

1. Introduction

Paint coatings are usually multifunctional. They are made from liquid or pasty substances or mixtures that form solid films as a result of physical and chemical processes. According to their functional purpose, they can be decorative, weather-resistant, protective and conservational, water-resistant, chemical-resistant, and heat-resistant [1,2,3].

In recent years, research has been carried out on the creation of thermally insulating and reinforcing coatings. The range of the coefficient of thermal-insulation coatings is 0.05 to 0.14 (W/m·K).

Coatings are referred to by the type of film former: for example, phthalic, melamine, phenolic, epoxy, polyester, polyurethane, acrylic, and other binders based on synthetic compounds. Paints and varnishes can be used for concrete, metal, brick, stone, and asbestos products. For example, the authors of [4] showed that it is possible to evaluate the effectiveness of epoxy resins on a composite in terms of the bending modulus, tensile strength, elastic modulus, and impact strength. In [5,6], it was reported that one could evaluate the properties of epoxy resins as polymer coatings for concrete and reinforced-concrete structures. The authors of [7,8,9,10] evaluated the efficacy of epoxy resins as decorative coatings based on fibrous silica, which has anti-corrosion properties; the mechanical, electrochemical, and strength characteristics of epoxy resin composite coatings loaded with graphene nanoplates; and the influence of synergistic environmental conditions on the thermal properties of cold-curing epoxy resin. The authors of [11] carried out a review of recent (as of 2021) thermal barrier coatings. The evaluations presented in previous works encouraged the completion of the present study.

This article deals with coatings based on epoxy binders. Our focus was the creation of thermal-insulation coatings for concrete, metal, and brick enclosing structures.

It was assumed that thermal-insulation materials based on epoxy binders are used as primers, while acrylic coatings act as the outside layer. Epoxy coatings are convenient in that the same composition of epoxy, depending on certain modifications, can be used as an active thinner or hardener; since the base is epoxy resin, the physical and chemical characteristics of the composition are predictable in terms of the operational properties in real climatic conditions.

Epoxy coatings (ECs) can also be used for decorative purposes (as the outside layer), to increase the strength of concrete (as primers), and to protect against physically and chemically aggressive environments [6].

The purpose of the proposed work was to analyze the stability of the protective properties of epoxy thermal-insulation coatings under long-term exposure to natural climatic factors.

The scientific novelty of this work lies in the proposed method for predicting the durability of ECs, i.e., a non-destructive technique for monitoring changes in the properties of coatings under field test conditions at a test site over a period of 2 years, taking into account the seasonal temperature variation and the humidity of the environment. Currently, the durability of epoxy coatings is mostly tested in laboratories by artificially creating environmental factors.

Methods for the non-destructive quality control of thermal-insulation coatings have been developed based on modeling the interaction of the coating material with the environment and construction material. Process modeling involves the logical, analytical, physical, and graphical description of processes, which makes it possible to analyze and evaluate the dynamics of development depending on specific conditions.

2. Materials and Methods

2.1. Materials

As the object of our study, we used samples of unfilled type 2 epoxy coatings (according to the GOST 11262–2017 government standard), which were fabricated using components manufactured by the Epoxy Research Production Center Epital JSC (Saransk, Russia) [12]:

(1)

Epoxy resin ED-20 (GOST 10587-84) with an epoxy group mass fraction of 20–22.5% [13];

(2)

Modified epoxy resin Etal-247 (TC (technical condition) 2257-247-18826195-07) with an epoxy group mass fraction of 21.4–22.8% [13];

(3)

Active diluent Etal-1 (TC 2225-027-00203306-97), which is a tri-functional epoxy resin with an epoxy group mass fraction of 14.5–18.5% [14];

(4)

Hardener Etal-45M (TC 2257-045-18826195-01), which is a mixture of aromatic and aliphatic di- or polyamines modified with salicylic acid [14].

2.2. Methods

Environmental testing was carried out using the automatic control station of Ogarev Mordovia State University (Saransk, Russia). The samples were placed on test benches in the environmental and meteorology laboratory. Measurements of meteorological parameters (temperature, relative air humidity, total solar radiation, ultraviolet radiation, atmospheric pressure, wind speed and direction, the amount of precipitation, and pollutant content) were carried out automatically every 20 min around the clock [3].

The analysis of the changes in the decorative characteristics was carried out using a CD-6834 spectro-guide sphere gloss spectrophotometer (Mettler-Toledo Ltd., Boston, UK), as well as a direct scanning method using a personal computer “Statistical analysis of the color components of paint and varnish coatings” (Bangkok, Thailand) [3,15].

The values for inserts (cyan, magenta, yellow, and black) and brightness were measured using a color palette with 256 colors. The processing of the experimental data was carried out using a standard software package, and distribution curves were built that characterize the spectral density and the distribution surfaces of the color components, showing what color each pixel of the sample area under study had.

For the mechanical tensile testing of polymer composites, a tensile testing machine of the AGS-X series with TRAPEZIUM X software (345-47652-00) was used (Shimadzu, Tokyo, Japan). The frequency of fixing stress and strain values was 0.01 s. The tests were carried out in accordance with GOST 11262-2017 “Plastics. Tensile test method” at a temperature of 23 ± 2 °C and a relative air humidity of 50 ± 5%. The rate of expansion of the jaws of the tensile tester was 2 mm/min. In parallel, at least 6 samples were tested, having the shape of “eights” (type 2 according to GOST 11262). Additionally, the adhesive strength was determined by the tear-off method using a PSO MG4 adhesion meter (Technocentre Ltd., Moscow, Russia). The essence of the method is to determine the ultimate resistance of the hardened coating to separation from the base. Adhesion strength (adhesion) is determined by the force of detachment of the hardened coating sample from the substrate applied to the sample through a metal disk with an anchor glued to the surface of the sample. As a sample of the test coating, a disk with a diameter of 30 mm was used, the peel force was increased, force was increased at a rate of not more than 1 MPa/s, perpendicular to the plane of the surface to be painted so that destruction occurred within 90 s from the start of the application of the peel force [16].

The thermal conductivity coefficient was determined using the method of stationary heat flow using the ITS-1 device (Saransk, Russia). The principle of operation of the device is based on the creation of a stationary heat flow passing through the investigated flat sample. Based on the magnitude of this heat flux, the temperature of the opposite faces of the sample and its thickness, the thermal conductivity of the sample λ is calculated automatically by the recorders:

• Thermal conductivity measurement range—0.02–1.5 W/(m·K);
• Thermal resistance measurement range—0.01–1.5 m²·K/W;
• The limits of permissible relative error in measuring thermal conductivity and thermal resistance are ±5%;
• Thickness of the measured sample—10–25 mm.

The studied sample had the shape of a rectangular parallelepiped, the front faces of which were a square with dimensions of 150 mm × 150 mm. sample thickness within 10–25 mm depending on the type of the test sample [17].

The method of preparation of epoxy coatings is standard for paints and varnishes: dosage of epoxy resin, hardener, microspheres, and thinner; mixing the components in a laboratory mixer using a laboratory scale; mixing is carried out at a speed of 60–70 rpm. and for 10–15 min. The resulting composition is passed through a mesh filter and fed to the bottling. To obtain a high-quality coating, it is necessary to ensure a high level of physical and mechanical properties (hiding power, density, layer thickness). Therefore, it is recommended for primed surfaces that the material consumption should not exceed 400 g/m² for the application of the first layer. For rough surfaces, the consumption rate for the 1st layer increases by 20%–30% [18,19,20].

The influence of the nature of the base (concrete, brick, wood, plaster) on porosity, adhesive strength, vapor permeability, and thermal conductivity was determined on the stands of the test site, and epoxy coatings were applied to the surface of the base and physical, and the mechanical properties were determined.

3. Results

Results of Experimental Studies

Regardless of their functional purpose and nature, all liquid coatings are more or less affected by climatic factors. The complexity of climate impact can be overcome by studying the change in the properties of materials in the time frame of climatic seasons, allocated on the basis of climate characteristics.

Table 1. The composition of the studied epoxy polymers.

Composition No.	Component Content, % of the Total Mass of the Epoxy Binder
	ED-20	Etal-247	Etal-1	Etal-45M
1	66.67	-	-	33.33
2	60	-	6.7	33.33
3	50	-	16.67	33.33
4	-	67.8	-	32.2

shows the ratios of the components of the studied compositions, in

Table 2. Indicators of viability and exothermicity of epoxy compositions.

Controlled Indicators	Type of Epoxy Polymers
	ED-20 + Etal-45M	(90% ED-20 + 10% Etal-1) + Etal-45M	(75% ED-20 + 25% Etal-1) + Etal-45M	Etal-247 + Etal-45M
Viability, min.	70	105	151	161
Peak temperature of exotherm, °C	50	50	46	41
Time to reach peak temperature, min.	105	91	92	94

—indicators of the viability and exothermicity of epoxy compositions.

The epoxy compositions presented in Table 1 have a higher viability compared to traditionally cured analogs widely used in construction hardener polyethylene polyamine (PEPA), which is extremely important for improving the manufacturability of the compositions used. In addition, to reduce the viscosity of the developed compositions, the ED-20 epoxy resin was replaced with the low-viscosity Etal-247 resin, as well as by introducing aliphatic epoxy resin of the Etal-1 brand as a flexibilizer into the binder [21,22,23,24].

Reducing the viscosity of epoxy binders is possible due to the introduction of solvents or heating the mixture. Reducing the viscosity of the binder by joint or separate heating of the mixture components leads to a significant decrease in its viscosity; however, an increase in the temperature of the components is almost always accompanied by a significant increase in their reactivity and, as a result, a decrease in the viability of the mixture. The introduction of solvents is widely used to reduce the viscosity of epoxy binder formulations. Thus, the introduction of a 5% solvent (ketones, ethers) into the composition of the mixture without additional thermal treatment leads to a decrease in the viscosity of the epoxy composite to 60%; however, a decrease in strength indicators, in this case, reaches 35%–40% [25,26].

The chemical method of modifying epoxy binders is carried out by introducing active diluents into the composition, which are involved in the curing process and are embedded in the structure of the polymer network. As the most promising group of chemical modifiers for epoxy resins, flexibilizers should be considered, which impart a certain elasticity to a rigid cross-linked polymer matrix, reducing strength fluctuations under sudden temperature changes, impact strength, film flexibility, and peel strength due to a decrease in internal stresses during curing.

The modification of the epoxy binder was carried out by replacing part of the epoxy resin ED-20 with the active diluent Etal-1; the proportion of the latter in relation to the mass of the resin part varied from 5% to 50%. The decrease in the viscosity index depending on the content of Etal-1 is shown in Figure 1.

The effect of the introduction of an aliphatic diluent in the amount of 5%–25% is expressed in a decrease in the viscosity of the epoxy resin by 22%–67%; the decrease in the ultimate strength of epoxy coating, in this case, is in the range from 8% to 25%. Based on the modification efficiency index (see

Table 3. Evaluation of the effectiveness of modifying the epoxy resin ED-20 with an aliphatic diluents of the brand Etal-1.

Controlled Indicators	The Content of Etal-1, % by Weight of the Resin Part
	5	10	15	20	25	50
Relative viscosity reduction, %	21.8	44.2	48.7	61	66.3	87.1
Relative reduction in tensile strength, %	7.62	10.4	17.2	24.1	22.9	65.5
Modification efficiency, rel. units	2.8	4.2	2.8	2.5	2.9	1.3

Note: The effectiveness of the modification is the ratio of the relative reduction in viscosity and the relative reduction in tensile strength.

), the best compositions were selected and modified with the aliphatic diluent Etal-1 in the amount of 10 and 25% of the total mass of the resin part [25].

From the analysis of the data presented in Table 2 and in Figure 1, it was found that the introduction of the aliphatic diluent Etal-1 is accompanied not only by a significant decrease in viscosity but also leads to an increase in the viability of epoxy mixtures by 1.5–2.3 times (

Table 4. The effectiveness of the modification of epoxy resins, taking into account the pot life index.

Controlled Indicators	Type of Epoxy Binder
	ED-20 + Etal-45M	(90% ED-20 + 10% Etal-1) + Etal-45M	(75% ED-20 + 25% Etal-1) + Etal-45M	Etal-247 + Etal-45M
Relative viscosity reduction, %	-	44.17	66.26	88.22
Relative reduction in tensile strength, %	-	10.43	22.93	45.55
Relative value of viability (compared to composition based on unmodified ED-20 resin), rel. units	1	1.5	2.15	2.29

).

Important technological indicators of epoxy binders are also the level of the peak heating temperature of polymer mixtures during the curing process and the time it takes to reach it. It is known that the nature of curing depends significantly on the type of hardener used [4,6,7].

Epoxy oligomers can be cured with or without heat; respectively, hardeners of “hot” and “cold” types are distinguished.

A comparative study of the effect of the PEPA hardener on the viscosity, pot life, temperature of the exothermic reaction of the epoxy resin and the tensile strength of epoxy coatings based on them was carried out for four previously selected compositions (Table 1) based on unmodified ED-20 resin, modified Etal-247 and ED-20 containing Etal-1 in the amount of 10 and 25%. The viscosity and pot life values for two groups of studied compositions cured by PEPA and Etal-45M, respectively, are shown in Figure 2.

To form a more complete picture of the ongoing processes, it is advisable to evaluate the viability of the epoxy resin together with the analysis of the kinetic curves of the temperature of the mixture (Figure 3). As the main characteristic parameters of kinetic curves, it is customary to consider the peak temperature of exothermy and the time it takes to reach it [21,23,24].

The change in pot life and exothermicity achieved by changing the hardener can be explained by the difference in the types of hardeners used. Etal-45M is a mixture of aliphatic and aromatic amines. Despite the fact that aromatic amines are “hot” type hardeners, the presence of salicylic acid in Etal-45M allows the latter to cure epoxy resins at room temperature. The lower activity of aromatic amines contained in Etal-45M is reflected in the increased values of the viability of its mixture with Etal-247 and, probably, may be the reason for a significant difference in strength indicators compared to the composition cured by PEPA (Figure 4,

Table 5. EP strength indicators depending on the type of resin part and hardener.

Controlled Indicators	Hardener Grade	Resin Part View
		ED-20	(90% ED-20 + 10% Etal-1)	(75% ED-20 + 25% Etal-1)	Etal-247
Change in tensile strength compared to the control composition based on ED-20 + PEPA, rel. units	Polyethylene polyamine (PEPA)	1.00	1.14	1.03	0.98
	Etal-45M	1.12	1.00	0.86	0.68

) [13,14,25].

Along with indicators of strength and chemical resistance, an important property of protective and decorative (mainly floor) coatings is resistance to abrasion loads.

The assessment of the resistance of a polymer product to the action of abrasion loads can be carried out on the basis of changes in various properties of the abradable surface-mass, thickness, volume, strength, color, and gloss [2,15,18,26]. To determine abrasive wear, the following relationship can be used (Equation (1)):

$$J=J_1·p$$

(1)

where $J_1$–wear at $p=1 \mathrm{kg}/{\mathrm{cm}}^2$; $p$–external pressure.

The determination of the wear resistance of the EC was carried out using a Taber 5155 rotary abrasimeter. The decrease in the mass of samples as a result of repeated exposure to abrasion load was determined by Equation (2):

$$\mathrm{L}=\mathrm{A}-\mathrm{B}$$

(2)

where A and B are the mass of the sample, respectively, before and after abrasion (mg).

According to the results of the studies (Figure 5), the weight loss of modified EC samples does not exceed 830 mg after 1000 cycles, which is almost four times lower than the maximum allowable value regulated by GOST 32017-2012.

The climatic exposure was recorded by placing a series of samples on the test site at the beginning of each climatic season (1 March, 1 June, 1 September, 1 December) and recording changes in sorption and strength parameters of the samples every 3 months of field exposure. The total exposure time of the samples in each series was 21 months. Depending on the season in which the exhibition began, the sample series was labeled SPRING, SUMMER, AUTUMN, and WINTER, respectively. In the course of exposure after each season, changes in the physical–mechanical and decorative properties of the polymer samples were recorded.

The process of mass change in the samples can be divided into two sections: the first 3 months, during which, regardless of environmental conditions, the mass increases (Figure 6a), and the remaining 9 months, during which the mass can either decrease and increase or remain unchanged (Figure 6b). It is quite obvious that the increase in mass in the initial period (Figure 6a) is evidence of structural relaxation processes taking place during this period. The increase in mass at this site is presumably due to the solidification of unreacted reactive groups of the EC as a result of the mutual reorientation of the macromolecules of the polymer matrix and the elimination of the initial structural disequilibrium [3].

The data presented in Figure 6b suggests that the change in polymer mass is almost independent of the type of binder used—the decisive role belongs to environmental factors. An exception to the rule in this situation would be a formulation modified with 25% Etal-1 (Figure 7).

Graphs of the change in the mass of samples of the composition (90% ED-20 + 10% Etal-1) + Etal-45M relative to the common time axis are shown in Figure 8a. At the same time, despite the differences in weight gain at the initial stage of exposure, at further time intervals, the curves of weight changes for samples of different groups are almost parallel. Of additional interest are the samples of the SUMMER, AUTUMN, and WINTER series at the time of 12, 15, and 21 months relative to the common time axis. Almost the same values of weight gain of the samples make it possible, in fact, to exclude this indicator when assessing the physical and mechanical properties of the polymer material. The stabilization of the mass values of samples of different series also indirectly indicates the potential equalization of the EC properties in the long term [1,22].

It has been established that for all types of aggressive media, a linear increase in the limiting moisture absorption is observed with an increase in the content of the diluents Etal-1 (Figure 7 and Figure 8).

The change in the mass of samples of the composition Etal-247 + Etal-45M relative to the common time axis is shown in Figure 9. The data obtained indicate a high sorption–desorption capacity of the polymer of the composition under consideration.

Figure 10a shows the total seasonal levels of solar radiation and precipitation for four exposure seasons. As can be seen from the presented graphs, the total amount of solar radiation absorbed by the EC samples practically did not differ depending on the moment of exposure. The deviation of the amount of solar radiation from the average value at the end point does not exceed 2.4%. However, due to differences in the sequence of seasons, the levels of solar radiation for samples with the same exposure duration differed significantly for the ages of 3, 6, and 9 months. The highest dose of solar radiation corresponds to the summer season; the amount of solar radiation that affects the samples during the spring months differs slightly from it [23]. In turn, the values for the autumn and winter months are 2.5 and 6 times less, respectively.

A different situation is observed for the amount of atmospheric precipitation-the discrepancy in the values for the same season in different years is 25–100% (Figure 10b).

An analysis of the maximum and minimum temperatures of the EP (EC) surface (

Table 6. The minimum and maximum values of the surface temperatures of the samples of the studied compositions, depending on the month of the study.

Months, Year		Type of Epoxy Polymer				Average
		Etal-247 + Etal-45M	(90% ED-20 + 10% Etal-1) + Etal-45M	(75% ED-20 + 25% Etal-1) + Etal-45M	ED-20 + Etal-45M
Minimum surface temperature, °C
2021	January	−31.33	−30.98	−31.05	−31.39	−31.19
	February	−23.78	−23.59	−25.18	−24.97	−24.38
	March	−19.68	−19.86	−19.93	−19.65	−19.78
	April	−5.69	−5.88	−5.80	−5.48	−5.71
	May	−0.47	−0.59	−0.52	−0.57	−0.54
	June	4.56	4.54	4.66	4.83	4.65
	July	4.94	4.76	4.51	4.52	4.68
	August	2.65	2.73	2.72	2.80	2.73
	September	0.89	0.74	0.82	0.82	0.82
	October	−8.78	−8.71	−9.02	−8.84	−8.83
	November	−10.73	−10.85	−10.91	−10.66	−10.79
	December	−23.54	−24.09	−23.78	−23.53	−23.73
2022	January	−28.01	−28.75	−29.56	−28.04	−28.59
	February	−24.5	−24.3	−23.82	−25.37	−24.49
	March	−17.98	−17.01	−18.61	−18.73	−18.01
	April	−7.13	−6.98	−7.45	−7.09	−7.16
	May	−0.45	−0.48	−0.41	−0.46	−0.45
	June	5.78	5.16	5.72	5.64	5.57
	July	3.62	3.89	3.41	3.23	3.53
	August	1,11	1.13	1.48	1.71	1.35
	September	0.65	0.61	0.57	0.70	0.63
Maximum surface temperature, °C
2021	January	3.84	4.93	5.50	3.44	4.43
	February	14.28	11.90	11.04	12.64	12.46
	March	28.85	29.73	30.56	28.72	29.47
	April	45.75	47.43	46.32	42.98	45.62
	May	58.17	61.03	58.91	57.37	58.87
	June	62.64	62.50	64.10	58.98	62.05
	July	57.62	60.09	62.08	60.91	60.17
	August	55.80	57.92	57.29	56.87	56.97
	September	57.54	58.18	56.85	56.54	57.27
	October	42.52	43.25	39.58	40.22	41.39
	November	11.85	12.09	11.90	11.84	11.92
	December	2.69	4.93	3.01	3.05	3.42
2022	January	3.45	3.15	4.61	4.13	3.83
	February	13.78	10.83	12.76	13.41	12.69
	March	29.01	28.78	27.65	30.47	28.97
	April	47.76	49.50	47.83	44.72	47.45
	May	56.80	58.92	56.29	55.87	56.97
	June	60.18	63.92	62.85	62.80	62.43
	July	59.01	62.18	60.87	59.54	60.4
	August	55.45	56.02	55.69	55.15	55.57
	September	54.05	54.13	54.76	54.58	54.38

) indicates the presence of minor discrepancies depending on the type of polymer binder. As a result, when identifying the most aggressive months in terms of temperature effects, it is advisable to use averaged values for each month.

The limiting values of the ambient air temperature and the corresponding values of the surface temperature of the polymeric material, depending on the month of exposure, are shown in Figure 11.

The list of quantitative data obtained, characterizing the impact of the environment on the properties of the epoxy polymer, allows us to proceed to the analysis of changes in elastic-strength indicators during outdoor exposure.

An analysis of the data presented in Figure 12a indicates that the relative humidity of samples at the age of 3 months is practically independent of the exposure start time [2,5]. At the same time, the change in the strength parameters of the samples for each of the series of samples is different (Figure 12b), which indicates the absence of a direct relationship between weight gain and tensile strength.

4. Discussion

The discrepancy in strength characteristics depending on the moment the exposure starts can be described based on the main operating environmental factors. Analyzing the change in the strength parameters of samples of the composition Etal-247 + Etal-45M of various series relative to the common time axis (Figure 13), it should be noted that, despite the same external influence, the change in the properties of samples of this composition in almost all areas is multidirectional.

For EC, the decrease in strength over 21 months did not exceed 10%; the maximum decrease in strength (about 20%) was recorded in the first three months of exposure, while no significant changes were recorded in the remaining time period. The data obtained for the unmodified composition ED-20 + Etal-45M (Figure 14) confirm the hypothesis of seasonal differences in the course of structural relaxation processes and the elimination of the initial nonequilibrium [4,9,15,21,24]. So, for the compositions, the initial period of exposure of which fell on seasons with a high level of solar radiation (spring and summer), a sharp drop in strength (more than two times) was recorded after 3 months. At the same time, the decrease in the strength characteristics of the series of samples corresponding to the other two seasons (autumn and winter) occurred more evenly by 15%–25% per season. Moreover, if for the EC compositions of the SPRING and SUMMER series, this level was reached already after 6 months of exposure, for the WINTER series this was after 9 months, and then for the AUTUMN series this was only by the end of the experimental study, which also allows suggests the key role of solar radiation in the aging process of the epoxy composition ED-20 + Etal-45M.

The change in the strength of samples of the epoxy composition modified with 10% Etal-1 for the SPRING and SUMMER series practically does not differ from the control composition. A similar decrease in strength to the level of 20–25 MPa was recorded 3 months after exposure. In the future, there are only minor deviations from the obtained values-no more than 6% of the initial value.

It should be noted that regardless of the moment the exposure starts, a sharp drop in strength indicators is observed only in the summer months, which is confirmed by the graphs in Figure 14. As in the case of samples of the composition (90% ED-20 + 10% Etal-1) + Etal-45M, restoration of the EP (EC) strength parameters was recorded mainly during the autumn months.

Determining changes in the tensile strength of samples modified by Etal-1, depending on the season of the beginning of exposure, the coefficient of thermal conductivity, coating porosity, vapor permeability, and the coating gloss level of the EP (EC) was determined in parallel. Epoxy coatings were applied to the bases (substrates): steel (1); concrete (2); wooden (3); brick (4); plastered surfaces (5). The obtained values of the thermal conductivity coefficient are given in

Table 7. Physical and mechanical properties of epoxy coatings from epoxy compositions formed on substrates of the type 1–5.

Properties	Substrate Type
	1	2	3	4	5
Thermal conductivity, W/m·K	0.1	0.09	0.09	0.08	0.08
Porosity of epoxy coatings, %	3	3.5	3.5	3.5	3.5
Vapour permeability, mg/m·Pa	0.001	0.001	0.001	0.001	0.001
Gloss level of coatings, %	30	35	35	36	36

[17].

According to the composition of the studied epoxy polymers, the polymers presented in Table 1 have higher viability. To reduce the viscosity of the developed compositions, the epoxy resin ED-20 was replaced with the low-viscosity resin Etal-247, as well as by introducing aliphatic epoxy resin Etal-1 as a flexibilizer into the binder [21,22,23,24,27]. It has been determined that a decrease in the viscosity of epoxy binders is possible due to the introduction of solvents or heating the mixture.

The comparative analysis carried out showed that the compositions cured by Etal-45M have many valuable parameters that are unattainable for compositions based on PEPA. These should include the following:

• Increased viability of epoxy resin (2–10 times);
• A significantly lower peak temperature of the epoxy resin exotherm (by 2–4 times) and, as a result, the possibility of mixing in large volumes;
• Comparable values of mechanical tensile strength of epoxy coatings.

Depending on the season of the beginning of exposure, the process of changing the mass of samples can be divided into two sections, the first 3 months, during which, regardless of environmental conditions, an increase in mass occurs (Figure 6a), and the remaining 9 months, during which the mass can both decrease and increase, and remain unchanged (Figure 6b).

According to the total solar radiation, the largest dose of solar radiation corresponds to the summer season; the amount of solar radiation that affects the samples during the spring months differs slightly from it [23]. In turn, the values for the autumn and winter months are 2.5 and 6 times less, respectively.

Analysis of the data presented in Figure 12a of the Etal-247 + Etal-45M composition indicates that the relative humidity of samples at the age of 3 months is practically independent of the time of exposure [2,5]. At the same time, the change in the strength parameters of the samples for each of the series of samples is different (Figure 12b), which indicates the absence of a direct relationship between weight gain and tensile strength.

The change in the strength of the samples of the epoxy composition modified with 10% Etal-1 for the SPRING and SUMMER series practically does not differ from the control composition. The decrease in strength to the level of 20–25 MPa was recorded after 3 months from the start of exposure. In the future, there are only minor deviations from the obtained values-no more than 6% of the initial value. The main drop in strength indicators is observed only in the summer months. It should be noted that the restoration of the epoxy polymer strength indicators was recorded mainly during the autumn months.

According to the coefficient of thermal conductivity, the coating based on epoxy resins showed compliance with GOST 7076-99 and, depending on the substrate, steel (1); concrete (2); wooden (3); brick (4); plastered surfaces (5) is in the range from 0.1 to 0.08 W/m·K; porosity is in the range from 3 to 35%; is in the range of vapor permeability 0.001 mg/m·Pa; degree of gloss of coverings from 30 to 36%.

5. Conclusions

Methods for non-destructive quality control of thermal insulation coatings are proposed to be developed on the basis of modeling the processes of interaction of the coating material with the environment and construction material. Process modeling involves the creation of a logical, analytical, physical, graphical or any other description of a process that corresponds to the real one and allows you to analyze and evaluate the dynamics of its development depending on specific conditions. When analyzing changes in the characteristics of polymer samples after exposure to climatic factors, it was found that compositions based on epoxy resin ED-20 and modified epoxy resin Etad-247, active thinner Etal-1, hardener Etal-45M demonstrate the best elastic and strength characteristics. The stability of the indicators under consideration allows us to conclude that the use of Etal-247 resin as a base leads to the creation of the most climatically resistant epoxy coatings.