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S.A. Nenakhov, A.V. Sokolov Ltd. SRC “NEOHIM”
Report, Yaroslavl,2005 Retardants based on liquid-filled sodium silicate glass are known [1-5], are widely used, and it can be assumed that these materials will provide long both technical and economic interest. Typically, these formulations contain fillers with low thermal conductivity.
By rheological characteristics these compositions can be divided into paints (relatively low viscosity) and mastics (a relatively high viscosity). In materials with mastic consistency filler concentration vary in the range of about 0,2 - 0,3 wt. shares. In the paints the concentration of filler is lower.
However, in the literature, descriptions of systematic studies of the effect of the nature and concentration of the filler in a wide range of concentration of the filler, is practically absent. The solution to this problem has been the aim of this work.
As objects of study liquid sodium silicate glass was used according to GOST 13078-81s with silicate module 3.0 and a density of 1.44 g/cm3, and fillers with low thermal conductivity (Table 1). Table. 1. The main characteristics of the fillers | № | Name | Coefficient of thermal conductance | | 1 | Perlite (group B) | 0,25 | | 2 | Vermiculite intumescent | 0,27 | | 3 | Glass microspheres MB | 0,29 | | 4 | Talc | 0,32 | | 5 | Microdolomite | 0,44 | | 6 | Basalt fiber | 0,04 | | 7 | Graphite intumescent | 0,39 | | 8 | Microvolostonite | 0,44 | The compositions were prepared by manual mixing the liquid sodium silicate glass with fillers. Mixing time was determined visually to achieve homogeneity of composition.
To carry out fire-resistant test, formulations were applied to steel plates measuring 140h80h1 mm. Coatings were dried for 3 days. The thickness of dried coating thickness was determined with gauge Mega-Chek FN, produced by NEURTEK Instruments. Coating thickness varied between 2 - 5 mm.
Coefficient of thermal conductivity of fillers and finished compositions were determined using an electronic meter heat ITP-MG4.
Fire-retardant properties of the coatings were determined by a modified method based on the technique [6], under the standard fire curve, GOST 30247.0-94 in the temperature ranges 20 - 1000 0C. In accordance with the methodology the main measured parameter is the time limit (T500), within which a steel plate under a flame retardant coating heated to a temperature of 500 0C. Feature of composite materials in mineral binder is a linear dependence of the time limit on the thickness of dry coating in a wide range of coating thickness. This fact allows using for comparative assessment of flame retardant properties of various materials of so-called reduced thickness of fireproof efficiency.  (1) where a is the time required for heating of free steel plates of used thickness, h - the thickness of dry coating. Coating based on sodium silicate solution shows the following features during the fire retardant test. First, the kinetic curve of the heating of metal under the coating consists of two stages: the first stage (20 - 100 0 C) leads to heating the coating with a sufficiently high speed. In the second stage (100 - 500 0C) the heating rate markedly reduces, and further heating takes place in a concave curve. The rate of heating at 250 0C is about 40 deg / min. Second, the sodium silicate coating upon heating undergoes significant morphological transformation: the glass is strongly foaming, at the height of the foam layer is about 40 times greater than the height of the dry initial layer. The cellular structure of the layer preserves throughout the thermal effects. Cell foams have a certain distribution of sizes: from very small (tens of microns) to of enough large size up to about 0.1 cm (Fig. 1).
Img. 1. Coating of sodium silicate glass after firing tests.
Third, the sodium silicate coating has a sufficiently high fire resistance. Thus, the coating dried for 1 day (20 0C), has  = 11.2 min / mm. To cover dried for 3 days = 6.2 min / mm. That is, with the decreasing of concentration of water, fire-retardance of coating efficiency decreases. Studies of binary systems “liquid glass / filler” showed the following. Regardless of the nature of the filler, kinetic curve of heating the metal under the coating transforms from the form characteristic of liquid glass to form characteristic of filled systems (Img. 2). 
Img. 2. Kinetics of heating the metal under the coating. Material: soda-lime glass / bead. Filling of water glass binder with fillers are always accompanied by a greater or lesser reduction of enumerated thickness fireproof efficiency of solution . This is obviously connected to the fact that, firstly, the filling of sodium silicate material with any of the used fillers reduces the effect of gas in the sodium silicate glass. Already at a small filling, nature of the morphological transformations in the coating varies: a homogeneous foam over the entire volume is terminates, the coating swells approximately in the middle of layer, at the same time, on the metal remains the cushion of material of certain thickness, above which is the gas cavity of larger or smaller sizes, which closes with the arch of swollen material (Img. 3). Secondly, all used fillers have the thermal conductivity of one or two orders of magnitude higher than that of foamed glass, formed from water glass coating, foam cells of which are filled with the gaseous product precipitated during the heating. Therefore, when replacing the air-mesh foam on the mass, full of fillers with a higher thermal conductivity than air and gases in general, there is a deterioration of the flame retardant properties of coatings.
Img. 3. The composition of "liquid glass - expanded vermiculite" after the fire tests. The dependence of the fireproofing efficiency of the reduced thickness of concentration of fillers of different nature is presented in Img. 4 (for all systems the time of drying of the coating from application to the test was 1 day). 
Pic. 4. Dependence of the thickness of fireproof efficiency on the filler concentration in the sodium silicate glass. It is seen that almost all the studied excipients form a single curve, although their nature and thermal conductivity are different: aluminosilicate materials (basalt fiber) and glass microspheres, perlite, volastonit, and other excipients form a single series. We also see that even at low filling quantity decreases quickly with increasing of filler concentration. Dependence of on concentration drop-offs at 3.2 min / mm in the concentration range from 0,2 to 0,3 wt. share and further (in the studied concentration range) remains unchanged. Thus, these data suggest that the nature of the filler (in the range of used fillers) in mixtures with liquid sodium silicate glass has little effect on fire retardant properties of the compositions, and fire-retardant efficiency is mainly determined by the concentration of filler in the composition. In the case of use in the content of mixture of fillers takes the value according to the total concentration of fillers in the system, ie the relation "reduced thickness of fireproof efficiency - the concentration of filler" subject to the above dependence for the individual components.
It is known [7] that the effective thermal conductivity of the composite material  can be described in the relation:
 (2)
Where  is coefficient of the heat conduction of matrix,  is coefficient of the heat conduction of filler,  is a volume fraction of filler. On the other hand, it is obvious that the effectiveness of fire-retardant material T500 is inversely proportional to the thermal conductivity, ie
 (3) Where A is the constant. Then, giving the same indices to the variables  , we obtain
 (4) In virtue of proportionality  and T500, the enumerated fire-retardant efficiency must also be proportional to the concentration of filler . Dependence of the thickness of fireproof efficiency on the concentration of filler in the composite material is presented in Img. 5. 
Img 5. Dependence of the thickness of fireproof efficiency on the reciprocal concentration of the filler.
Actually, the experimental data are satisfactorily rectified in the coordinates - . In our opinion, this indicates that the fire retardant properties of swelling materials filled with sodium-silicate are determined mainly by the thermal conductivity of the composite. The results for the flame retardant performance of materials filled with sodium-silicate can be easily transformed into material science solutions. Of course, at that the rheological characteristics of materials in a state of semi-products (paints, mastics) should be taken into account, as well as mechanical properties and stability of these properties in the final products - composite coatings. Literature
1. Patent # 2,038,977. 1995.
2. Patent # 2,133,241. 1997.
3. Patent # 2,079,525. 1997.
4. Patent # 2,132,311. 1999.
5. Strakhov V. L., Kroot A. M., Davydkin N. F. Fire protection of building structures. IPC "Timra, M., 2000, p. 74-81.
6. Kolganova M. N., Levitas F. A., Moscovskaya A. M. Instructions for determining thermal insulating properties of the swelling coatings on metal. VNIIPO, M., 1980.
7. Industrial polymer composite materials. Ed. M. Richardson. Trans. Babayevsky P.G. Moscow, Chemistry. 1980. Pp. 289. |
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