Influence of moderately warm and extremely cold climate on properties of basalt plastic armature

Rods of basalt plastic armature (BPA) of a periodic profile with diameters of 6, 8, 10, 16 and 20 mm on the basis of an epoxy matrix were exposed for 30 months on open stands in a moderately warm marine climate of Gelendzhik and 28 months on similar stands in an extremely cold climate of Yakutsk. For initial and exposed BPA samples, changes of mechanical parameters were determined. After the exposure in Yakutsk, an increase of ultimate compression strength by 4–12% is found, depending on the diameter of rods. After the exposure in Gelendzhik, this indicator decreases by 10–17%. Using methods of thermomechanical analysis and dynamic mechanical analysis, linear thermal expansion coefficient and glass transition temperature of epoxy matrix were measured. In an initial state, a transition from a glassy state to a highly elastic one is discovered: an α1-transition at 118 °C and an α2-transition at 149 °C. After climatic action, there were detected a shift of α1-transition to low temperatures and a shift of α2-transition to higher temperatures. Reasons for the change of temperatures of transitions are weakening of a molecular layer and post-curing of the epoxy matrix in a surface layer. These effects are accompanied by an increase of the linear thermal expansion coefficient, moisture diffusion coefficient and maximum moisture saturation after the climatic action on BPA. Fractographic studies discovered presence of pores in a structure of BPA with sizes up to 10–20 μm, a quantity and size of which increase by 20–40% after the climatic action. In general, the studied BPA has high climatic stability and can be used for a long time under the extreme climatic conditions.

Samples of basalt plastics based on a polyester matrix sorbed up to 2% of normal and sea water at room temperature according to [13]. The tensile strength, bending strength and impact strength were reduced by 50e60%, especially for samples, in which the basalt fibers were treated with solutions of H 2 SO 4 and NaOH before production of composites.
Similar mechanical properties of basalt plastics reinforced with finely chopped roving, fabrics or uniaxially oriented continuous fibers, are compared with carbon plastics and the glass-reinforced plastics based on unsaturated polyesters, epoxyphenolic and phenol-formaldehyde matrices [14]. It is shown that during 12 months of climatic tests under conditions of the South Caucasus there was observed a decrease of strength, depending on a type of polymer matrix and magnitude of applied tensile stress. It is noted that aging processes develop on surfaces of the samples, but no experimental proof of this statement is given.
To improve the mechanical properties and stability of BPA, heat-resistant polymer matrices [15,16] with modifying finely dispersed additives are used. The properties and structure of the polymer matrix in boundary layers with the basalt fiber are studied by methods of electron microscopy, dynamic mechanical analysis, differential scanning calorimetry, thermogravimetry and other sensitive methods [15,16].
An urgent problem of use of BPA in construction is confirmation and justification of retention of properties at a high level during long-term operation in a variety of climatic conditions. However, in all the works listed [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16], there is no information about a change of properties of BPA for construction purposes under the action of natural climatic conditions of different zones. In [17], the strength of basalt, carbon and glass fibers was studied in accelerated tests simulating the natural weather conditions, but conclusions obtained were not verified by direct climatic tests.
In this regard, an experimental evaluation of the climate resistance of BPA was carried out after the exposure in the extremely cold (Yakutsk) climate and moderately warm marine one (Gelendzhik). The main task of the presented work is to identify the effects of climatic aging in BPA at early stages of exposure using sensitive physical methods. The following components were used to produce BPA.
Epoxy resins are reactive oligomers that transform into an infusible and insoluble state only under the action of hardeners.
Iso-MTHPA is used as the hardener of "hot" curing of epoxy resins and compositions based on them. It ensures high physical and mechanical characteristics of cured systems, excellent waterproof properties, good electrical characteristics, and the climatic stability.
The main characteristics of iso-MTHPA are shown in Table 3.
Since the curing of epoxy resins by anhydrides proceeds extremely slowly, accelerators are usually introduced into these systems. A choice of 2,4,6-tris(dimethylaminomethyl)phenol (UP 606/2) as an accelerator is due to its high catalytic activity associated with presence in this compound of three tertiary amino groups and an acidic phenolic hydroxyl at the same time: However, it manifests itself at rather high temperatures (above 100 C). At processing temperatures, processability of the binder in the presence of the accelerator is maintained. Characteristics of UP-606/2 curing accelerator are presented in Table 4.
A fragment of chemical knots of a spatial network in the anhydride curing:  An ester group formed as a result of curing is resistant to the action of organic and many inorganic acids, but is destroyed by alkalis, but thermal stability and electrical insulation properties are higher than with the use of amine hardeners.

Environmental conditions
To assess an effect of solar radiation, temperature, humidity, precipitation and other aggressive climate factors, the rods of BPA of the periodic profile with diameters of  A mass fraction of titrated nitrogen,%, Not less than 8 14 Density at a temperature of 25 C, kg/m 3 6e12 10    Yakutsk. An average annual air temperature in Gelendzhik is 14.8 C, in Yakutsk minus 8.8 C. The city of Yakutsk is in a zone of a sharply continental climate with very cold winters and a relatively hot and short summer. The minimum recorded temperature is e64.4 C. An annual amplitude of temperatures between the highest and lowest values exceeds 100 C. In winter, in Yakutsk at a temperature below e40 C, all water vapor being in the air crystallizes and forms fogs. An average wind speed is 1.8 m/s.
The average annual relative humidity in Gelendzhik and Yakutsk differs slightly (71% and 68% respectively). During a year, precipitation in Gelendzhik is greater than in Yakutsk by a factor of 3.3 (796 mm and 237 mm, respectively). A doze of annual total solar radiation in Gelendzhik is 5004 MJ/m 2 , which is by 36% higher than in Yakutsk (3680 MJ/m 2 ).

Dynamic mechanical analysis
According to [18,19,20], the glass transition temperature T g of the polymer matrix is a sensitive indicator of the climatic aging of polymer composite materials (PCM).
The method of dynamic mechanical analysis (DMA) was used to determine this parameter. With the help of an analyzer DMA 242D Netzsch, a dynamic elastic modulus E 0 and dynamic loss modulus E 00 were measured in a temperature range from 25 C to 200 C at a rate of 3 C/min. Samples in a form of strips with sizes of 40 Â 4 Â 1 mm were cut from a central part of BPA along an rod axis. Measurements were made with a frequency of forced oscillations of 0.5 Hz, and a dynamic load of 5 N at an amplitude of oscillations up to 200 mm. A single-cantilever attachment scheme was used, in which one end of the sample was rigidly fixed in a gripper, and the other end of the sample was clamped in the movable cantilever. A calculation of the parameters E 0 and E 00 was carried out using formulas where N is an axial force, L is a length of free part of the sample, B is a width of the sample, H is a thickness of the sample, A is the amplitude of oscillations, d is an angle of a phase shift between the stress and strain of the sample.

Thermal expansion
Control of thermal expansion of the samples of BPA was carried out during heating over a wide range of temperatures using a thermomechanical analyzer TMA PTLT 600 of a company Linseis. In a module of this analyzer, a measuring indenter was pressed against the sample with a constant load of 0.5 N applied to an end of the sample with the diameter from 6 mm to 20 mm and a height of 10 mm. Movement measurements of the indenter were controlled with an accuracy of 10 À4 mm. The temperature in a measuring cell was automatically increased at a rate of 3 C/min between 25 C and 200 C. Results were processed using Linseis Data Evaluation software for Windows, version 3.00. In the sort of measurement used, the thermomechanical analyzer operated as a highly sensitive linear dilatometer. The movement of the indenter in a direction of action of a small load over a wide range of temperatures from 200 C to 300 C was negligibly small, and when the temperature was increased, micromovements caused by the thermal expansion of the sample were recorded.

Sorption and diffusion of water
It was proved in [21] that the moisture diffusion coefficient D and the maximum moisture content M N are parameters sensitive to the climatic aging of PCM. To assess possible physicochemical and structural transformations during the exposure of BPA in the open climatic conditions, kinetics of moisture transfer was studied.
The samples were previously dried in an oven at 60 C over silica gel for 6 days, then they were held at the same temperature and relative humidity of 98 AE 2% in a desiccator above a surface of water. At the same time, measurements of masses of the samples were periodically carried out using analytical scales with an accuracy of 10 À4 g and their geometric sizes with an accuracy of 0.001 mm. After 63 days of humidification, a process of moisture sorption was stopped and the samples were dried in the oven at the temperature of 60 C during 35 days.

Fractography
To

Mechanical properties
Compression and bending tests are the main and most common methods of control of the mechanical properties of BPA [14,15,16,17]. A typical example of load diagrams of the samples with the diameter of 8 mm is shown in Fig. 6.
Presence of periodic relief on the surface of BPA makes it difficult to determine the compression strength s c and the buckling strength s b due to large spreads of these parameters (Fig. 6b). When analyzing the load diagrams, it is necessary to take into the initial samples and the samples exposed in the cold climate (Table 6) revealed an increase of this parameter by 4e12%, depending on the diameter of the rods. After the exposure in the moderately warm climate, the ultimate compression strength is reduced by 10e17%.    Hz. The glass transition temperature T g of the epoxy matrix of BPA is defined as the minimum point on the graph dE ' =dT and as the maximum point on the graph E 00 similar to [22]. The glass transition temperature determined by these two criteria is shown in Table 7.

Dynamic mechanical analysis
The transition of the epoxy matrix from the glassy state to the highly elastic one occurs in a temperature range 90e190 C both in the initial state and after the climatic tests in Gelendzhik and Yakutsk. During this time of action, rigidity of the armature remains high, and the polymer matrix is in the glassy state.
Analysis of the dynamic mechanical characteristics of BPA reveals asymmetry of the transition of the epoxy matrix from the glassy state to the highly elastic one (the atransition). Regularities of a change of mobility of kinetic elements under the influence of environmental factors are reproduced when the oscillation frequency changes by two orders of magnitude: from 0.5 Hz to 50 Hz (Figs. 8 and 9).
In the initial state, a double minimum of dE 0 =dT was revealed: the a 1 -minimum at 118 C and the a 2 -minimum at 149 C (Fig. 7). Such a double transition is characteristic of epoxy polymers. The detected transitions have a well-pronounced relaxation character: with an increase of the frequency from 0.5 Hz to 50 Hz, the temperatures of a 1 -minimum and a 2 -minimum shift, respectively, to 129 C and 154 C. The temperatures of these transitions are reproduced to within 2 C in terms of the maxima of E 00 in Fig. 9. After the exposure of BPA in the open climatic conditions, multiplicity of the transition was more pronounced (Figs. 7, 8, and 9). A tendency was found to shift the a 1 -transition to low temperatures, and the a 2 -transition to higher temperatures. Quantitative effects of these shifts are presented in Table 7.
According to data of Table 7, the effect of the decrease of the temperature of the a 1 -transition does not exceed 3 C after the exposure in Gelendzhik and Yakutsk.
A probable cause of this effect is weakening of intermolecular interaction in the surface layer of the composite. A check of action of the plasticizing effect of moisture shows that the initial and exposed samples had practically the same insignificant amount of moisture (0.1%e0.2%). Therefore, a decrease of the glass transition temperature is not associated with the effect of this factor.
As a result of the climatic action, the temperature of the a 2 etransition increased by 3e5 C after 30 months of exposure in Gelendzhik and by 10e15 C after the exposure in Yakutsk (Figs. 8 and 9, Table 7).  samples. When testing the initial BPA samples, the failure occurred according to a "panicle" type, and after the exposure according to a "cut" type, which indirectly confirms the increase of the adhesive strength of the "fiber-matrix" compound.

Thermal expansion data
The temperature dependences of a relative thermal expansion DL=L 0 along a direction of reinforcement of BPA samples with a diameter of 20 mm and a height of 10 The temperature dependences of DL=L 0 show the fairly good reproducibility. With an increase of the temperature from the room temperature to 120 AE 5 C, the initial  Table 8. A calculation of CLTE b was performed under an assumption of a linear increase of DL=L 0 at T < T g and T > T g .
A reason for the observed increase of DL=L 0 and b is predominance of an effect of weakening of internal stresses of the polymer matrix at the interface with the basalt  fiber over an effect of the post-curing of the polymer matrix of BPA. Similar patterns were previously observed in the climatic aging of glass-reinforced plastic VPS-7 on the basis of epoxy binder EDT-10P [20].

Sorption and diffusion of water
In BPA with the diameters of 6e20 mm, a variation of the sizes of the samples from 2 to 100 mm made it possible to reveal the effects of the influence of the periodic screw profile and the damaged surface layer formed during rod cutting on the maximum moisture content M N and the moisture diffusion coefficient D. satisfactorily approximated by the second Fick law in a one-dimensional approximation with constant boundary conditions similar to [21]: where c is a concentration of moisture per unit volume of sample; c 0 is an initial moisture concentration at t/0; t is the time; R is a radius of the sample, h is a sample length, D is the coefficient diffusion. Parameters of Fick's desorption in the onedimensional approximation (a maximum mass change, the diffusion coefficient) for cylindrical samples are found using a relationship [21]: where MðtÞ is a moisture content of a model segment of a length l at a moment of l is a characteristic length of a diffusion path, n k ¼ ð2k þ 1Þ: In Table 9 [19]. Similar effects were observed earlier for other PCMs based on the epoxy polymers [21].
A significant increase of the values of M N and D with a decrease of the length of the samples is caused by influence of a defective end edge formed when cutting the samples [21]. In pores and defects of the edge, more moisture is sorbed than in undamaged parts of the sample. Therefore, the shorter the sample, the greater the role of edge effects, and the parameters M N and D increase in these samples.
A more detailed analysis of influence of the edge effects on the characteristics of moisture transfer in BPA after the exposure in the climatic conditions will be presented in another report. Table 9 show that M N and D increase with a decrease of the diameter of the samples. The surface relief screw layer of BPA sorbs more moisture than inner tightly packed (by the basalt fibers) layers of the rods. With the decrease of the diameter of the rod, part of the surface relief layer in the volume of the samples increases, which leads to the increase of M N and D.

Results presented in
The parameters M N and D increase to a greater extent after the exposure in Yakutsk than in Gelendzhik. The exposure in the climatic conditions has an unequal effect on the characteristics of the moisture transfer of BPA of different sections. For example, for the samples of the length of 100 mm and the diameter of 20 mm after 30 months of the exposure in Gelendzhik, the moisture diffusion coefficient with the sorption increases by 36% after 30 months of the exposure in Gelendzhik and by 71% after 28 months of the exposure in Yakutsk. For the rods of the same length and with the diameter of 6 mm, a similar increase is 120% and 160%, respectively (Table   9). Thus, a general conclusion can be drawn that the characteristics of moisture transfer are the parameters sensitive to physicochemical aging processes in the surface layer of BPA.

Optical microscopy, fractography
According to data of fractographic analysis, the structure of BPA after the exposure in the climatic conditions does not undergo significant changes. As an example, Fig. 12 shows fragments of microsections of BPA with the diameter of 20 mm in the initial state, after 30 months of the exposure in Gelendzhik and after 28 months of the exposure in Yakutsk. In the initial state, the structure of BPA is quite monolithic. A dense chaotic structure of reinforcing basalt fibers is traced. Rare pores with a size of up to 10e20 mm are noted. After the exposure in the climatic conditions, the amount and size of pores increases by 20e40%. A slight increase of porosity is caused by thermal deformations of BPA with daily and seasonal changes of temperature. This increase of porosity explains the increase of the maximum moisture saturation and the coefficient of moisture diffusion after the climatic action.

Conclusions
In 6. The fractographic studies discovered the presence of pores with the sizes up to 10e20 mm in the structure of BPA. The quantity and size of pores increase by 20e40% after the climatic exposure.
7. The studied BPA has the high climatic stability and can be used for the long time under the extreme climatic conditions.