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Synthesis and Characterization of Cross-Linked Poly(Sodium Acrylate)/Sodium Silicate Hydrogels.

INTRODUCTION

Aqueous solutions of soluble silicates, such as salts of a weak silicic acid and a strong base, for example, sodium, are alkaline. Therefore, as a result of the reaction of an alkaline silicate with an acidic substance, for example, orthophosphoric acid, the hardening process of water glass is caused by gelation of silicic acid. Esters (most often acidic derivatives of glycerine and ethylene glycol), alcohols, phenols, and aldehydes are also used as hardening substances. Water glass has a lower silicate module, that is, the more alkaline it is, the harder it becomes to coagulate. This is due to the fact that the sodium ions limit the mobility of water molecules, stabilizing the sol. Gelation of the water glass solution occurs when a significant amount of hydrogen ions, which weaken the effect of sodium ions and cause the precipitation of silicic acid molecules, is found in the intermiceral medium. Acids are the fastest-acting coagulation factor. To control the gelation process, substances susceptible to slow hydrolysis in the alkaline solution, such as esters of polyhydric alcohols, should be used [1-5]. Salts containing cations of different metals may also be used. Silanol groups of alkali silicates react with di--and trivalent cations, forming silanolmetal bridges, which then polymerize with the release of hydrogen, producing silicate-metal polymers. Another modification of this solution is a two-step reaction in which silicic acid precipitates first, and then reacts with cations of the corresponding metal, forming silicatemetal polymer [6]. Another method of obtaining gels with water glass is based on the use of the properties of vinyl monomers and soluble silicates solutions. As a result of simultaneous precipitation of silica gel and cross-linking of vinyl polymers, mineral-organic hydrogels with increased elasticity and durability are created. Another fact in favor of using acrylic polymers is their ability to absorb water. Additional advantages are nontoxicity, high thermal and chemical resistance, and mechanical strength. These properties are used in such fields as medicine (lenses, dressings, and drug release), agrotechnics (water storage) and construction (waterproofing panels, fillers for fire-retardant panes) [7-11]. However, the problem here is that acrylate monomers are very sensitive to changes in environmental conditions. If pH ranges 2.5-5, the polymerization degree decreases with the increase in pH, reaching a minimum between pH 6 and 7. Above pH = 7, the polymerization rate increases, reaching a maximum at pH = 10, and then drops again in the pH range of 11-12. A low degree of polymerization at pH = 6-7 results from the lower value of the polymerization degree for the anion than for the undissociated acid. The increase in this value above pH = 7 is explained by the reduction in the degree of polymer chain termination caused by the repulsion of polyions with the same charge [12-15].

Hydrogels based on acrylic polymers are most often formed as a result of the homopolymerization of acrylic acid or its copolymerization with acrylamide. As the radical polymerization of vinyl monomers is an exothermic process, the use of concentrated solutions is very dangerous. It is recommended to use solutions with a concentration of about 25 wt% or less. Due to this fact, the polymerization reactions of vinyl monomers are generally carried out in an aqueous solution, at room temperature and in the presence of redox initiators that lower the reaction temperature. The structural integrity and stability of hydrogels are provided by cross-linking agents through physical interactions and chemical bonds. Cross-linked polymers become insoluble, can only absorb solvent, as a result of which they swell. Di vinyl and acrylate systems, for example, N,N'-methylenebisacrylamide and glycol acrylate, are used as cross-linking agents [12,16-18].

This article presents the results of the research on obtaining transparent silicate-acrylate hydrogels with enhanced fire-retardant properties, in which to obtain a homogenous, durable and transparent hydrogel, in addition to sodium acrylate, aqueous sodium silicate, the redox initiator system and cross-linking agent, sodium polyacrylate is added to the polymerization mixture. In comparison to the known methods of vinyl systems' polymerization, the method of the present invention has the following advantages:

* the system does not require the use of any chemicals harmful to health or the environment and

* the gel durability is higher than that of acrylic polymers, which can be explained by the homogeneous structure of the silicate-acrylate gel-forming mixture.

It is estimated that in addition to the fire-resistant glazing, other insulation materials used in construction, such as mineral wool, wood, or other combustible materials, impregnated with a developed gel will increase their insulating properties and, above all, will obtain fire-resistant properties, which will contribute to the improvement of the safety of constructed objects.

MATERIALS AND METHODS

Materials

Sodium water glass R-145 (WG) with silicate modulus M = 2.50 was used as the silicate species source. Midafen R-100 (MR-100) was used as the polymer filler preventing precipitation of silica during the preparation of the silicate-acrylic polymerization mixture. The monomer was a 20 wt% aqueous solution of sodium acrylate (ANa) synthesized in a stoichiometric reaction by neutralization of acrylic acid (AA) with a 22 wt% aqueous solution of sodium hydroxide (NaOH) and then diluted to the required concentration. The polymerization reaction was carried out according to a radical mechanism, where potassium persulphate (KPS) and sodium thiosulphate (NTS) were the redox initiators and N,N'-methylenebisacrylamide (NNMBA) was the cross-linking monomer. All reagents were used without further purification. Table 1 comprises description of all used materials.

Synthesis of Cross-Linked Poly(Sodium Acrylate)/Sodium Silicate Hydrogels

In the first step, 5, 10, and 20 wt% of Midafen R-100 was distributed in sodium water glass by vigorous stirring on a magnetic stirrer for 10 min. To the base mixture prepared in such a manner, a 20 wt% aqueous solution of sodium acrylate was added at a mass ratio of 1:1 and 1:2 and again was vigorously stirred on a magnetic stirrer for 5 min to obtain a transparent solution. The cross-linking agent (NNMBA) and system of redox initiators (KPS/NTS) were added in an amount of 0.3 wt% to the sample weight. The polymerization mixture prepared in such a manner gelled within 15 min according to the free-radical mechanism. The composition of the poly(sodium acrylate )/sodium silicate hydrogels is shown in Table 2.

Methods

An Anton Paar Physica MCR-301 rheometer was used for the measurement of the polymerization kinetics. The gelation time of the tested hydrogels was measured based on the sharp increase in viscosity (about 2000 mPas; determined based on our previous research). Studies were conducted in a plate-plate system, at a constant shear rate of 1 [s.sup.-1] and the gap width of 0.1 mm. All the measurements were performed at room temperature.

The zeta potential of the tested silicate-polymer hydrogels was measured with the use of a Zetasizer Nano ZS device. The appropriate amount of dry hydrogel was weighed and its 0.1 wt% solution in distilled water was prepared. The mixture was vigorously stirred to disperse the polymer and the solution prepared in such a manner was measured. Each sample was measured three times.

An Excalibur BIO-RAD 6000 spectrometer equipped with an ATR optical disk was used to identify silicate and polymer functional groups in the analyzed hydrogel samples. The spectra were recorded at a resolution of 1 and 25 scans in the range of 400-4,000 [cm.sup.-1].

NMR measurements, which among others provided information about the chemical environment of the polymer molecules, were performed on a Magritek Rock Core Analyzer at magnetic field of 0.05 T. Relaxation times [T.sub.1] and [T.sub.2] were registered using Inversion Recovery (IR) and Carr-Purcell-Meiboom-Gill (CPMG) sequences, respectively. The inter-experiment delay for both measurements and maximum [T.sub.1] delay for IR were set to 5 s. About 50,000 echoes with echo time of 100 ps were registered for CPMG. The acquired data, after background correction, were calculated into relaxation times' distributions with the Laplace Inverse Transform (ILT) using the Lawson and Hanson algorithm applied in the Prospa software.

Thermogravimetry (TG) and differential scanning calorimetry (DSC) were used for thermal measurements. Tests were carried out using a NETZSCH STA 449 F3 thermal analyzer and aluminum oxide crucibles, in [N.sub.2]/[O.sub.2] atmosphere. Samples were heated up to 815[degrees]C at a heating rate of 5[degrees]C/min. TG was used to determine the weight loss of a polymer sample resulting from the thermal degradation of the polymer. This method in combination with the quadrupole mass spectrometry (QMS) also gave information on the polymer chain structure of the tested polymer-silicate hydrogels. With the use of the DSC method, enthalpy changes ([DELTA]H) of the tested samples occurring due to endothermic (-[DELTA]H) or exothermic (+[DELTA]H) transitions and glass temperature ([T.sub.g]) were determined.

The fire test was performed in a research stand specially built for this purpose. It consists of two main components, that is, a furnace equipped with a removable torch fueled with propane-butane and infrared camera (NEC Thermo Gear G100, measuring range up to 500[degrees]C). The furnace was built in such a way that it can fulfill temperature rise in accordance with the standardized fire curve. Samples of two dimensions can be examined in this device, 20/30 cm for the so-called average temperature using a burner panel (Fig. le), and 10/10 cm for the so-called point temperature using the point burner (Fig. 1d). Additional elements are two thermocouples (inside and outside the furnace) (Fig. 1b), exhaust system (Fig. lc), and an infrared camera (Fig. 1a).

The preparation of samples consisted of the "cast in place" gel casting method, it is direct polymerization of the polymer-silicate mixture between glass panes with dimensions of 10/10 cm. Thickness of a glass pane was 0.4 cm, whereas the filling gel had a thickness of 1 mm. To avoid gel drying and its possible flowing out while heating, glass forms were wrapped with a silicone seal and extra strengthened with an aluminum tape. Fire insulation was tested for both the single-layer and multi-layer system during the preparation of the samples, respectively, single and double layer. The analysis of the results was focused on the assessment of fire insulation (I) of the samples, as in each trial, fire resistance (E) and fire load capacity (R) were maintained. Times needed for reaching the 180[degrees]C limit point temperature were compared and thermograms with respect to the temperature distribution in time were analyzed. Figure 2 presents an exemplary sample before, during, and after the test, that is, after reaching 180[degrees]C on the side not exposed to fire.

RESULTS AND DISCUSSION

Rheological Measurements

Figure 3 shows the dependence of the apparent viscosity from the polymerization time for the tested silicate-polymer hydrogels. It should be remembered that a number of factors influence the kinetics of the polymerization reaction. The most important are the concentration of initiators (the higher the initiator concentration, the faster the reaction occurs), concentration of monomer, pressure, and temperature. In the case of the studied systems, although the measurement was initially carried out at room temperature, that is, 20[degrees]C, due to the exothermic nature of the radical polymerization reaction, the temperature increased over time to about 60[degrees]C. Comparing the polymerization time of samples with a different polymer content in the base mixture (i.e., 5, 10, and 20 wt%), no correlation was found (although each measurement was repeated three times). However, when comparing the mass ratio of sodium acrylate to water glass, it was found that the same sample with a different monomer content polymerized faster when the content was lower.

To better understand the polymerization phenomena of the tested silicate-polymer hydrogels, flow curves were also determined (Fig. 4). Their course changed with the polymerization time (the measurement was made immediately after the preparation of the polymerization mixture, after 5 min and after 10 min). For both mass ratios of monomer to water glass, flow curves exhibited hysteresis of shear stress loops at increasing and decreasing shear rates. All curves showed pseudo-antithixotropy or pseudo-thixotropy (Fig. 5) associated with the successive building of the 3D structure (cross-linking) within the solution over time. In the case of the 1:1 mass ratio of the base mixture to the monomer, the cross-linking reaction was slow and therefore the system showed pseudo-thixotropic properties at the initial stage. After about 2 min, the strength of the cross-linking reaction was higher than the possibility of mechanical destruction of the structure. Therefore, the system irreversibly changes to the pseudoantithixotropic. However, this structure is flexible and allows for a pseudoplastic flow (shear thinning) until the end of the measurement (10 min). In the case of the 1:2 mass ratio of the base mixture to the monomer, the cross-linking reaction occurs rapidly. The structure breaks down during shearing and that forces the slip between the rheometer plates. Because in this case the linear shear gradient is not observed, the measurement results do not determine the typical flow (deformation). In other words, the material becomes nonuniform and incoherent during shearing. Because the cross-linking reaction continues, it probably further strengthens the detached elements of the structure that begin to flow independently in the medium increasingly saturated from the cross-linking monomer. For this reason, after reaching the minimum of pseudo-antithixotropy, the system begins to pass to the pseudo-thixotropy area. In this case, the medium between the detached elements of the 3D structure has different rheological properties and can cause a typical flow characteristic of suspensions or emulsions. In other words, the system does not change into a homogeneous 3D gel during shearing, but into emulsion in which gel is the dispersed phase and water is the dispersive phase [19-25].

Zeta Potential

Zeta potential (electrokinetic potential) occurs at the interface of the diffusion layer and stationary ions associated with the micelle, in this case with the tested polymer. It determines surface phenomena occurring at the interface, such as peptization and coagulation. The particle coagulation process takes place within the range of potential values from -30 to +30 mV and the system becomes unstable. On the other hand, values above and below this limit indicate the predominance of repulsive forces and system stabilization, that is, peptization of the dispersed phase (micelles) [18, 26]. Table 3 shows the measured values of the zeta potential ([zeta]).

Measured zeta potential values have a negative value and are in the range of -39.82 to -49.35 mV, that is, much lower than -30 mV. However, no special correlation between the content of water glass, polyacrylate, and the zeta potential value was found. In general, it can be concluded that the tested hydrogel solutions are stable systems. The composition of the tested silicate-polymer hydrogel has an influence on such a high negative value of the zeta potential. This means that both the anionic polymer (dissociation of carboxyl groups) and the alkaline sodium silicate contributed to the basic character of the tested superabsorbents [27-29].

IR Spectra

The IR spectra analysis was difficult due to the composition of the analyzed system, because sodium silicate solutions, as well as derivatives of polyacrylic acid, give bands with similar wavenumber, so they can overlap. It was also found that individual samples of hydrogels differ only in the intensity of bands and their small shifts. All the samples were characterized by the presence of polyacrylate chain fragments, silicate structures, and amine bonds (at about 770 [cm.sup.-1] and 2,300 [cm.sup.-1]), derived from the cross-linking monomer. However, the analysis of bands coming from the silicate structures derived from the sodium silicate, associated with vibrations of hydroxyl groups, water molecules and bands coming from the vibrations of Si--O bonds in silicon units, turned out to be the most interesting. Their location depends mainly on the polymerization degree of the silicate unit. Stretching vibrations of hydroxyl groups within the range of 3,200-3,450 [cm.sup.-1], derived from water molecules and-OH groups, can be combined with both, the silicate structures and polyaciylate chain. No visible bands at 1100 [cm.sup.-1], derived from the highest polymerized silicate structure, that is, Si[O.sub.2], were observed. The band at about 1,000 [cm.sup.-1] corresponds to the asymmetric stretching vibrations of Si-O groups, while bands at about 460 [cm.sup.-1] are derived from the O-Si-O bending vibrations. Bands corresponding to the vibrations coming from cyclosilicate units (at about 620 [cm.sup.-1]) were also observed [30-37].

[Sup.1]H NMR

Figure 7 shows distributions of relaxation times [T.sub.1] and [T.sub.2] for the examined samples 1:1 and 2:1. It can be noticed that both relaxation times are shorter for sample 2:1. A decrease of 169 ms is observed for longitudinal relaxation time [T.sub.1] and of 161 ms for transverse relaxation time [T.sub.2] (see values of relaxation times at maximum in Table 4). This difference reaches about 57.4% and 55.8% for [T.sub.1] and [T.sub.2], respectively. Any significant difference was noticed for [T.sub.1]/[T.sub.2] ratio between the samples. Longer relaxation times for sample 2:1 may indicate the presence of larger pores in this sample. This can also be influenced by the higher proportion of silicate structures in sample 2:1. We should also consider that paramagnetic Na ions present in the sample can also affect the values of relaxation times. However, this impact would be pronounced in the case of transverse relaxation time, while similar changes for both times are observed for the samples. Therefore, we could assume that change in porous structure may be the main cause of the observed effect [38].

Thermal Analysis

Figure 8 shows the exemplary changes in mass loss (TG) and enthalpy changes in the tested silicate-polymer hydrogel samples. The individual samples contain a similar amount of water. The relationship between the amount of polymer in the sample and the loss of water mass were observed. The more polymer was in the base mixture, the more water evaporated during the TG measurement. Probably, syneresis phenomenon occurs during heating and separating water from the polymer. The results of the weight loss of individual samples of hydrogels are presented in Table 5 (loss of water--silicates at this temperature can only undergo polymorphic changes). On the basis of the obtained thermograms, it was also possible to determine glass-transition temperature ([T.sub.g]) for the 1:1 composition, whose range was determined between 150[degrees]C and 180[degrees]C (Table 5). Due to the presence of sodium silicate, these values are significantly higher than for the pure polyaciylic acid (about 105[degrees]C). Such inclusions always increase the [T.sub.g] range to higher temperatures. In turn, exothermic peaks were found mainly at 400-480[degrees]C. It means that exothermic processes related to polymerization and cross-linking of polymers occur in this temperature range (Table 6) [18,39-43].

Analyzing the results of QMS measurements (Fig. 9), no changes in 13, 14, and 28 mass lines were observed. That means that any short fragments of the polymer chain, like CH-(13), C[H.sub.2]-(14) i C=0 (28), were not isolated during the measurement. At about 100[degrees]C, H20 molecules escaped from some samples, as demonstrated by mass line 18. Water in molecular form coming from the polymerization/re-crystallization process was released in the higher temperature range (400-500[degrees]C). In this temperature range, the presence of NH[4.sub.sup.+] ions derived from the decomposition of the NNMBA cross-linking monomer was also possible. During the analysis of mass lines 44 and 45, it was found that at temperatures between 400[degrees]C and 500[degrees]C, the tested hydrogels decompose into C[O.sub.2] (line 44) and longer parts of polymer chains (line 48). The QMS analysis confirmed the results of the tests performed by MIR spectroscopy.

Fire Tests

Table 7 presents the time needed for reaching the point temperature limit of 180[degrees]C for individual samples, while Fig. 10 presents the thermograms made with the use of a thermal imaging camera.

An important aspect of the fire test analysis was the comparison of fire insulation times (I) between individual samples. For both types of gel filling, the longest times needed for reaching 180[degrees]C were noted for samples with a double gel layer separated by a glass pane with thickness of 4 mm. During the analysis of the effect of the hydrogel composition on fire insulation, a certain regularity was also observed. This time was higher in the case of a 1:1 mass ratio of the polymerizing mixture to water glass than that for the 1: 2 mass ratio. During the fire test, the resulting intumescent layer was more compact and less porous in the case of a 1:1 mass ratio, which better blocked the temperature increment on the untreated side. The optimal composition was 10%/1:1.

While analyzing temperatures of the tested samples during the fire test, their steady increment during the measurement was observed. The temperature distribution on the outer surfaces of the samples reflects the thermal phenomena occurring in the gel layer. This can be particularly observed in the individual stages of point heating. The gel turns into a liquid at high temperatures, water evaporates, and then a solid heat-insulating layer is formed. The same phenomena occur during layer heating (thermograms not shown here). The temperature increase during the combustion of samples followed the standard temperature-time fire curve determined by the PN-EN 1991-1-2 standard [44], which does not take other fire parameters, that is, pre-flash phase, fire load, and ventilation conditions into account. Figure 11 shows the temperature-time relationship of the selected samples subjected to the fire test, which are described in Table 7.

Despite different limit times of the insulation fire point (180[degrees]C), curves are characterized by a similar course. The curves are inflected at around 120[degrees]C. Initially, there is a rapid linear increase in the outer temperature of the samples, and then the increase proceeds much slower. This fact suggests that the fire-retardant operation of a laminar pane consists of several stages in which different mechanisms take place:

* at the first stage, heat is absorbed to approximately 120[degrees]C, what is characterized by a linear rapid increase in temperature;

* at the second stage, water evaporates quickly from gel at about 120-220[degrees]C, which is the first insulation barrier that discharges significant amounts of heat;

* at the third stage, depending on the viscosity and other properties of gel, evaporation of water can be slowed down and can occur up to 500[degrees]C;

* at the fourth stage, a silicate insulation layer, constituting a hard and opaque pumice which is the second and the ultimate insulating barrier, is created. Depending on its porosity, the temperature increase on the outer side of the sample may vary [45-47].

In the case of multilayer panes, these mechanisms can overlap each other, extending the fire insulation function.

SUMMARY

The article presents the results of the research on obtaining transparent silicate-polymer hydrogels with increased fire-resistance. An innovative aspect of this work was the development of a polymerization method where in the strongly alkaline environment of sodium silicate, sodium acrylate (which under these conditions normally precipitates) polymerizes. This problem was solved by using the so-called base mixture consisting of a solution of sodium water glass and a suitable polymer, which was the polymerization medium for the monomer.

The aim of the rheological tests was to investigate the gelation kinetics of silicate-polymer hydrogels for different applications. Samples of gels polymerized within over a dozen minutes. Comparing the gelation time of samples with a different polymer content in the base mixture, no correlation was found. However, when comparing the mass ratio of sodium acrylate to water glass, it was found that the same sample with a different monomer content polymerizes faster when its content is lower. Spectroscopic and thermal analysis proved that polymer combines with sodium silicate by hydrogen bonds. 'H NMR studies showed shorter relaxation times and hence, presumably smaller pores in the hydrogel structure, for sample 2:1 (with a higher silicate content). During thermal degradation, only small fragments of the polymer chain and water molecules are released. The cross-linking substance, that is, N, N'-methylenebisacrylamide, is also degraded. Importantly, this hydrogel is environmentally friendly, because no toxic gases were found. Fire tests confirmed that the obtained material meets the requirements set out in the relevant standards for fire tests of construction materials. Due to the restrictive conditions of this test, it was assumed that all the tested compositions of hydrogels, in particular those with a composition of 10%/l: 1, can be used as fire-retardant layers.

ACKNOWLEDGMENTS

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

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Joanna Mastalska-Poptawska (ID), (1) Piotr Izak, (1) tukasz Wojcik, (1) Agata Stempkowska, (2) Zuzanna Goral, (1) Artur T. Krzyzak, (3) Iwona Habina (3)

(1) Faculty of Materials Science and Ceramics, AGH University of Science and Technology, Mickiewicza 30 Av., 30-094 Krakow, Poland

(2) Faculty of Mining and Geoengineering, AGH University of Science and Technology, Mickiewicza 30 Av., 30-094 Krakow, Poland

(3) Faculty of Geology, Geophysics and Environmental Protection, AGH University of Science and Technology, Mickiewicza 30 Av., 30-094 Krakow, Poland

Correspondence to: J. Mastalska-Popiawska; e-mail: jmast@agh.edu.pl

DOI 10.1002/pen.25111

Published online in Wiley Online Library (wileyonlinelibrary.com).

Caption: FIG. 1. Elements of the fire research stand.

Caption: FIG. 2. Exemplary sample before (a), during (b and c) and after the fire test (d).

Caption: FIG. 3. Dependence of viscosity on polymerization time for the tested silicate-polymer hydrogels.

Caption: FIG. 4. Flow curve of the 1:1 10 wt% system measured directly after the sample preparation.

Caption: FIG. 5. Energy of the 3D structure during cross-linking of the silicate-polymer hydrogel with 10 wt% of Midafen R-100.

Caption: FIG. 6. MIR spectra of the silicate-polymer hydrogels with the 1:2 mass ratio.

Caption: FIG. 7. (a, c) relaxation [T.sub.1] and [T.sub.2] times distributions; (b, d) comparison of [T.sub.1] and [T.sub.2] values at maximum for samples 1:1 and 1:2.

Caption: FIG. 8. TG/DSC thermogram of the 10%/1:1 sample.

Caption: FIG. 9. QMS analysis of the 10%/1:1 silicate-polymer hydrogel (from the upper left corner mass line 18, 28, 44 and 48 respectively).

Caption: FIG. 10. Thermograms of the 10%/1:1 sample (1 layer of gel).

Caption: FIG. 11. Temperature vs. time of hydrogel samples with 10 wt% of Midafen R-100.
TABLE 1. Materials.

Abbreviation   Description                   Source

WG             Sodium water glass            "Rudniki" Chemical Plant,
                                               Poland
mr-100         Midafen R-100                 Lubrina LLC, Poland
AA             Acrylic acid                  Acros Organics, Belgium
NaOH           Sodium hydroxide              Stanlab SJ, Poland
NNMBA          N,N'-methylenebisacrylamide   Acros Organics, Belgium
KPS            Potassium persulphate         Acros Organics, Belgium
NTS            Sodium thiosulphate           POCH LLC, Poland

TABLE 2. Compositions of the poly(sodium acrylate)/
sodium silicate hydrogels.

Sample symbol   WG (wt%)   MR-100 (wt%)   20% ANa (wt%)

5%/1:1           47.50         2.50           50.00
10%/1:1          45.00         5.00
20%/1:1          40.00        10.00
5%/1:2           63.35         3.35           33.30
10%/1:2          60.00         6.70
20%/1:2          53.35        13.35

Sample symbol   KPS (wt%)   NTS (wt%)   NNMBA (wt%)

5%/1:1            0.15        0.15         0.30
10%/1:1
20%/1:1
5%/1:2
10%/1:2
20%/1:2

TABLE 3. Zeta potential values of the tested
silicate-polymer hydrogels.

Sample symbol   Zeta potential (mV)

5%/1:1          -39.82 [+ or -] 1.25
10%/1:1         -43.88 [+ or -] 0.98
20%/1:1         -49.35 [+ or -] 1.54
5%/1:2          -41.48 [+ or -] 2.05
10%/1:2         -43.54 [+ or -] 1.37
20%/1:2         -49.00 [+ or -] 1.52

TABLE 4. TI and [T.sub.2] relaxation times and
diffusion coefficient from NMR measurements.

Sample symbol   [T.sub.1]   [T.sub.2]   [T.sub.1]/     D (le-9
                  (ms)        (ms)      [T.sub.2]    [m.sup.2]/s)

5%/1:1            294.0       288.0        1.02         0.662
5%/1:2            350.6       348.3        1.00         0.677

TABLE 5. Mass loss ([DELTA]m) and glass temperature
([T.sub.g]) determined on the basis of TG/DSC thermograms.

Sample     [DELTA]m (wt%)   [T.sub.g] ([degrees]C)
symbol

5%/1:1           34                  175
10%/1:1          37                  180
20%/1:1          47                  155
596/1:2          41                   --
10%/1:2          44                   --
20%/1:2          47                   --

TABLE 6. Temperatures of the exothermic processes
related to polymerization and cross-linking.

                 Temperature of the exothermic processes

Sample     [T.sub.1]     [T.sub.2]     [T.sub.3]     [T.sub.4]
symbol    ([degrees])   ([degrees])   ([degrees])   ([degrees])

5%/1:1        419           443           455           474
10%/1:1       416           443           464         472/477
20%/1:1       407           445           467           --
5%/1:2        413           --            465           475
10%/1:2       414           --            --            --
20%/1:2       --            440           --            --

TABLE 7. Fire insulation of the tested samples.

Sample symbol    1 layer of gel (min)   2 layers of gel (min)

5%      1:1             10.27                   25.06
        1:2              9.28                   17.36
10%     1:1             12.51                   29.03
        1:2             11.58                   21.40
20%     1:1             11.02                   25.30
        1:2             10.50                   19.29
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Author:Mastalska-Poptawska, Joanna; Izak, Piotr; Wojcik, tukasz; Stempkowska, Agata; Goral, Zuzanna; Krzyza
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:4EXPO
Date:Jun 1, 2019
Words:5584
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