Thermal Behavior of Silicone Rubber–Based Ceramizable Composites Characterized by Fourier Transform Infrared (FT-IR) Spectroscopy and Microcalorimetry Rafał Anyszka,a,* Dariusz M. Bieli nski,a,b Marcin Je˛drzejczykc a
Institute of Polymer and Dye Technology, Ło´dz´ University of Technology, 90-924 Ło´dz´, Poland Institute for Engineering of Polymer Materials and Dyes, Division of Elastomers and Rubber Technology, Institute for Engineering of Polymer Materials and Dyes, Harcerska 30, 50-820 Piasto´w, Poland c Institute of General and Ecological Chemistry, Ło´dz´ University of Technology, 90-924 Ło´dz´ Poland b
Ceramizable (ceramifiable) silicone rubber–based composites are commonly used for cable insulation. These materials are able to create a protective ceramic layer during fire due to the ceramization process, which occurs at high temperature. When the temperature is increased, the polymer matrix is degraded and filler particles stick together by the fluxing agent, producing a solid, continuous ceramic phase that protects the copper wire from heat and mechanical stress. Despite increasing interest in these materials that has resulted in growing applications in the cable industry, their thermal behavior and ceramization process are still insufficiently described in the literature. In this paper, the thermal behavior of ceramizable silicone rubber–based composites is studied using microcalorimetry and Fourier transform infrared spectroscopy. The analysis of the experimental data made it possible to develop complete information on the mechanism of composite ceramization. Index Headings: Ceramization; Ceramifiable composites; Silicone rubber; Thermal properties; Fourier transform infrared spectroscopy; FT-IR spectroscopy.
INTRODUCTION Ceramizable (ceramifiable) silicone rubber–based composites are novel materials for fire protection application. In spite of their quite short presence in the global market, they have begun to be commonly used because of their relatively small price and extraordinary properties. In the case of fire, ceramizable silicone composites are able to create a solid ceramic phase protecting against flames, heat, and mechanical stresses. Generally they are used in the cable industry to maintain electrical circuits for up to 120 min in the presence of heat and flames. The ceramizable composites are a dispersion type of materials whose continuous phase is polydimethylsiloxane (PDMS) containing a number of vinyl groups that facilitate boosting the peroxide curing reaction (usually up to 1 mol%). PDMS is the best elastomer for compounding ceramizable composites because it is able to create amorphous silica during its thermal decomposition in the presence of oxygen.1,2 The silica obtained can undergo the sintering process with the surface of refractory fillers or fluxing agent (amorphous oxide ceramics with a relatively low softening point temperature) particles leading to increased mechanical and barrier properties of a ceramic shield created as a result of ceramization. The dispersion phase is a mix of reinforcing silica, refractory fillers, and fluxing agent particles, which strongly support the ceramization process. Received 18 February 2013; accepted 9 August 2013. * Author to whom correspondence should be sent. E-mail: 800000@edu. p.lodz.pl. DOI: 10.1366/13-07045
Volume 67, Number 12, 2013
The fluxing agent particles melt at elevated temperature and stick refractory mineral filler particles together, creating physical links between them. The state of art in the field of ceramizable silicone rubber– based composites was described in several papers, including the mechanisms of thermal decomposition of a silicone matrix.1–4 Also the characterization of the ceramic phase created after the ceramization process was carefully described, but the process of ceramization has never been exhaustively presented in the literature.5,6 To date, Fourier transform infrared (FT-IR) analysis of smoke produced during thermal transformation that leads to ceramization of ceramizable composites has never been reported in the literature. To fill this gap, FT-IR spectra of volatile products of the thermal destruction of the polymer matrix were analyzed, and the kinetics of thermal decomposition of the composites was studied by microcalorimetry. The thermal degradation of simple ceramizable composite (CC) was compared to that of silicone rubber (SR) and fumed silicareinforced silicone rubber (SR-S).
EXPERIMENTAL For the infrared analysis of smoke being produced from the composite samples subjected to pyrolysis, a Nicolet 6700 spectrometer equipped with a gas cell (length 8 cm, diameter 4 cm) was used (Fig. 1). Samples of a mass of circa 0.1 g were heated from 25 to 700 8C with a temperature rate of 5 8C /min in the presence of a passive carrier gas (argon, flow rate of 60 cm3/min). Obtained spectra show measurements at intervals of 25 8C (resolution 4 cm 1, 60 scans for every spectrum). For each measurement heating was finally stopped at 700 8C and the temperature was maintained at a constant until the end of scanning. Microcalorimetric studies were made with a Pyrolysis Combustion Flow Calorimeter (PCFC; Fire Testing TechnolTABLE I. Composition of the silicone rubber mixes: SR, pure silicone rubber; SR-S, silicone rubber filled with reinforcing silica; and CC, ceramizable composite. Ingredient (phr) Silicone HTV rubber Fumed silica Fluxing agent (B2O3) Mica (phlogopite) TiO2 2,2-dichlorobenzoyl peroxide 50% paste
0003-7028/13/6712-1437/0 Ó 2013 Society for Applied Spectroscopy
SR
SR-S
CC
100 – – – – 2.5
100 50 – – – 2.5
100 50 10 15 5 2.5
APPLIED SPECTROSCOPY
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FIG. 3. Cured SR filled with 50 phr of reinforcing fumed silica (SR-S) spectra at various temperatures.
FIG. 1. Scheme of the FT-IR/pyrolysis tester.
ogy, UK) in an active gas atmosphere (O2/N2 = 20/80, gas flow rate of 100 cm3/min), according to the ASTM D73092007 standard. Silicone rubber–based samples (Table 1) were made with a Brabender-Plasticorder laboratory mixer (Germany). Rotors were spun at 30 rpm during component incorporation and at 75 rpm during homogenization (15 min). Samples were vulcanized using a laboratory press at 130 8C during 15 min.
1) A multiplet in the range 3000–2840 cm 1 responsible for C-H stretching vibration in volatile compounds, which is proved by a strong band at 1264 cm 1 originating from CH3 deformation vibrations in Si-CH3 groups. 2) A weak band at 1410 cm 1 originating from antisymmetric vibrations of the C-H in SiCH3 groups, which confirms the presence of silanes and siloxanes in the volatiles.7–9 3) Strong bands at 920–724 cm 1 corresponding to Si-C stretching vibrations in the following groups: Si(CH3)2, 855, 815–800 cm 1; -SiCH3, 765 cm 1; and -Si(CH3)3, 840 cm 1.7
Infrared Spectroscopy. Smoke, containing liquid and solid matter, was created as a result of thermal destruction of the studied composites. Their infrared spectra are very similar (Figs. 2–4). The spectra of the smoke produced from SR and SR-S samples are similar, but they differ in intensity. Only the spectrum of the CC differs from the others by additional bands originating from the fluxing agent micro-particles carried by argon and present in the smoke (BO33 , B4O72 ). The spectra in the wavenumber range 3200–500 cm 1 show the following peaks:
In the first stages of polymer decomposition, absorption bands at 805–864 cm 1 appear. Among them, the highest belongs to a -O-Si-CH3 group of linear polysiloxane particles, but with increasing temperature, the intensity of the band at 815 cm 1, originating from a -O-Si-CH3 group of cyclic particles, also becomes higher.7 It shows that a temperature increase promotes the cyclization process, which corresponds with the mechanism presented by Hamdami et al.2 In the wavenumber range 1200–900 cm 1, IR bands connected with Si-O-Si and Si-O-C group vibrations appear. The band at 1033 cm 1 is characteristic of Si-O group vibrations in cyclic siloxanes (trimers probably). Also, bands at 609 cm 1 originate from Si-O vibrations in disiloxanes.7 The spectra of the CC contain a characteristic strong multiplet in the wavelength range 1470–1260 cm 1, originating
FIG. 2. Cured silicone rubber (SR) spectra at various temperatures.
FIG. 4. Ceramizable composite (CC) spectra at various temperatures.
RESULTS AND DISCUSSION
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FIG. 5. FT-IR spectra for smoke produced at 375 8C.
from products of fluxing agent decomposition, in the case of BO33 in the range 1380–1290 cm 1, and B4O72 in the range 1380–1330 cm 1 (Fig. 4).7 It seems likely that micro-particles of fluxing agent could be carried by argon to gas cells during thermal treatment. To simplify interpretation, chosen spectra (obtained at a temperature of 375 8C) were gathered in separate figures (Fig. 5). It is interesting that transmittance for near-infrared radiation significantly decreases in a temperature range corresponding to thermal degradation of the composites and accompanying heat emission. Probably solid and liquid bodies, present in the smoke as a result of thermal degradation of samples, are responsible for a scattering effect. Even for almost pure SR, which degrades mostly to cyclic, short-chain siloxanes in an inert atmosphere,10 this effect is clearly visible. So it is possible to estimate thermal stability of the composites studied, based
on FT-IR data, providing information on their decomposition products present in carrying gas. Microcalorimetry. The evolution of the heat release rate (HRR) as a function of temperature was determined for every sample (Fig. 6) using the microcalorimetry method. The HRR value is the highest for ‘‘pure’’ SR. Addition of 50 parts per hundred of rubber (phr) of fumed silica significantly decreases the HRR value of the material (SR-S), but also decreases the thermal stability understood as a temperature of the beginning heat release for the sample studied. This temperature value is closely correlated with initiations of the polymer matrix destruction, which result in significant changes of composite properties. Estimated values of the temperature of the beginning heat release, represented by ‘‘a’’ (for the CC sample), ‘‘b’’ (for the SR-S sample), and ‘‘c’’ (for the SR sample) in Fig. 5, are given in Table 2. CC represents the lowest value of HRR, but its thermal stability is also the lowest.
FIG. 6. Heat release rate (HRR) of the composites studied.
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TABLE II. Temperatures of thermal degradation beginning and maximal value of HRR.
SR SR-S CC
Temperature of thermal degradation onset
Temperature of maximal HRR value
500 8C 450 8C 350 8C
660 8C 720 8C 520 8C
On one hand, thermal stability of the composites is lower than unfilled SR, but on the other hand, a sample filled with 50 phr of silica has a higher value of temperature of the maximal HRR rate, which makes it better material from the flammability point of view (Table 2). The performed studies demonstrate that the addition of mineral particles to a SR matrix decreases its HRR value; however, it simultaneously adversely affects the thermal stability of the material. The decrease of thermal stability facilitates ceramization of the materials. It already can be achieved partially for SR-S due to sintering mechanisms of mineral filler particles in the presence of a SR matrix, as proposed by Xiong et al.11 This study shows that FT-IR spectroscopy can be a very useful method to measure thermal properties of polymer composites. When combined with microcalorimetry, FT-IR spectroscopy can accurately describe thermal degradation processes of polymer matrices filled with different types of mineral powders. Due to the qualitative FT-IR analysis, it turns out that boron oxide species are present in smoke obtained as a result of thermal decomposition of the composites. It is very important from the point of view of the efficiency of the ceramization process if the fluxing agent (B2O3) microparticles are being removed from the ceramic phase into the volatiles or not. Using FT-IR spectroscopy, it is possible to choose the type of fluxing agent whose particles do not escape into the smoke during ceramization, but stick to refractory filler particles, thus causing enhanced mechanical properties of the ceramic layer being created. A combination of FT-IR and microcalorimetry is very useful for qualitative and quantitative analysis of the thermal behavior of ceramizable composites. The HRR value is strongly correlated with the increase of the IR signal intensity in the near-infrared. The absorbance rises with the increasing wavenumber, which is likely to be associated with the heat emission intensity and shows that it is possible to use the FT-IR method to estimate the temperature of the maximum value of HRR to materials studied.
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CONCLUSIONS The thermal stability of the SR-S mix and CC is significantly lower in comparison to unfilled SR. Probably mineral fillers (especially containing nano-sized particles) play a catalytic role in thermal decomposition of the SR matrix. The admixing of silica slightly reduces the thermal stability of SR, which decreases significantly with additional incorporation of fluxing agent. It starts melting at significantly lower temperatures, facilitating the ceramization process of the composites. However, the addition of mineral fillers strongly decreases the HRR value, especially the maximal HRR value, which drops from over 500 W/g for unfilled SR up to slightly over 200 W/g for SR-S and even up to circa 120 W/g for CC. The temperature of the maximal HRR value of materials increases after addition of fumed silica, which is very important in relation to the flammability of the composites studied. ACKNOWLEDGMENTS This work was supported by the EU Integrity Fund, project POIG 01.03.0100-067/08-00. 1. D.M. Bielinski, R. Anyszka, Z. Pe˛ dzich, J. Dul. ‘‘Ceramizable Silicone Rubber-Based Composites’’. Int. J. Adv. Mat. Mfg. Char. 2012. 1(1): 1722. 2. S. Hamdani, C. Longuet, D. Perrin, J.-M. Lopez-Cuesta, F. Ganachaud. ‘‘Flame Retardancy of Silicone-Based Materials’’. Polym. Degrad. Stabil. 2009. 94(4): 465-495. 3. L.G. Hanu, G.P. Simon, Y.B. Cheng. ‘‘Thermal Stability and Flammability of Silicone Polymer Composites’’. Polym. Degrad. Stabil. 2006. 91(6): 1373-1379. 4. L.G. Hanu, G.P. Simon, J. Mansouri, R.P. Burford, Y.B. Cheng. ‘‘Development of Polymer-Ceramic Composites for Improved Fire Resistance.’’ J. Mat. Process. Technol. 2004. 153–154: 401-407. 5. Z. Pe˛ dzich, A. Bukanska, D.M. Bielinski, R. Anyszka, J. Dul, G. Parys. ‘‘Microstructure Evolution of Silicone Rubber-Based Composites during Ceramization at Different Conditions’’. Int. J. Adv. Mat. Mfg. Char. 2012. 1(1): 17-22. 6. Z. Pe˛ dzich, D.M. Bielinski. ‘‘Microstructure of Silicone Composites after Ceramization’’. Composites. 2010. 3: 249-254. 7. G. Socrates. FTIR tables. In: Infrared Characteristic Group Frequencies Tables and Charts. Chichester, UK: John Wiley and Sons, 1994. 2nd ed. 8. A. Zenasni, V. Jousseaume, P. Hollinger, L. Favennec, O. Gouhant, P. Maury, G. Gerbaud. ‘‘The Role of Ultraviolet Radiation during Ultralow k Films Curing: Strengthening Mechanisms and Sacrificial Porogen Removal.’’ J. Appl. Phys. 2007. 102(9): 094107. 9. A.K. Singh, C.G. Pantano. ‘‘Surface Chemistry and Structure of Silicon Oxycarbide Gels and Glasses’’. J. Sol-Gel Sci. Technol. 1997. 8(3): 371376. 10. G. Camino, S.M. Lomakin, M. Lazzari. ‘‘Polydimethylsiloxane Thermal Degradation. Part 1. Kinetic Aspects’’. Polymer. 2001. 42(6): 2395-2402. 11. Y. Xiong, Q. Shen, F. Chen, G. Luo, K. Yu, L. Zhang. ‘‘High Strength Retention and Dimensional Stability of Silicone/Alumina Composite Panel under Fire.’’ Fire Mater. 2012. 36(4): 254-263.
Institute of Polymer and Dye Technology, Ło´dz´ University of Technology, 90-924 Ło´dz´, Poland Institute for Engineering of Polymer Materials and Dyes, Division of Elastomers and Rubber Technology, Institute for Engineering of Polymer Materials and Dyes, Harcerska 30, 50-820 Piasto´w, Poland c Institute of General and Ecological Chemistry, Ło´dz´ University of Technology, 90-924 Ło´dz´ Poland b
Ceramizable (ceramifiable) silicone rubber–based composites are commonly used for cable insulation. These materials are able to create a protective ceramic layer during fire due to the ceramization process, which occurs at high temperature. When the temperature is increased, the polymer matrix is degraded and filler particles stick together by the fluxing agent, producing a solid, continuous ceramic phase that protects the copper wire from heat and mechanical stress. Despite increasing interest in these materials that has resulted in growing applications in the cable industry, their thermal behavior and ceramization process are still insufficiently described in the literature. In this paper, the thermal behavior of ceramizable silicone rubber–based composites is studied using microcalorimetry and Fourier transform infrared spectroscopy. The analysis of the experimental data made it possible to develop complete information on the mechanism of composite ceramization. Index Headings: Ceramization; Ceramifiable composites; Silicone rubber; Thermal properties; Fourier transform infrared spectroscopy; FT-IR spectroscopy.
INTRODUCTION Ceramizable (ceramifiable) silicone rubber–based composites are novel materials for fire protection application. In spite of their quite short presence in the global market, they have begun to be commonly used because of their relatively small price and extraordinary properties. In the case of fire, ceramizable silicone composites are able to create a solid ceramic phase protecting against flames, heat, and mechanical stresses. Generally they are used in the cable industry to maintain electrical circuits for up to 120 min in the presence of heat and flames. The ceramizable composites are a dispersion type of materials whose continuous phase is polydimethylsiloxane (PDMS) containing a number of vinyl groups that facilitate boosting the peroxide curing reaction (usually up to 1 mol%). PDMS is the best elastomer for compounding ceramizable composites because it is able to create amorphous silica during its thermal decomposition in the presence of oxygen.1,2 The silica obtained can undergo the sintering process with the surface of refractory fillers or fluxing agent (amorphous oxide ceramics with a relatively low softening point temperature) particles leading to increased mechanical and barrier properties of a ceramic shield created as a result of ceramization. The dispersion phase is a mix of reinforcing silica, refractory fillers, and fluxing agent particles, which strongly support the ceramization process. Received 18 February 2013; accepted 9 August 2013. * Author to whom correspondence should be sent. E-mail: 800000@edu. p.lodz.pl. DOI: 10.1366/13-07045
Volume 67, Number 12, 2013
The fluxing agent particles melt at elevated temperature and stick refractory mineral filler particles together, creating physical links between them. The state of art in the field of ceramizable silicone rubber– based composites was described in several papers, including the mechanisms of thermal decomposition of a silicone matrix.1–4 Also the characterization of the ceramic phase created after the ceramization process was carefully described, but the process of ceramization has never been exhaustively presented in the literature.5,6 To date, Fourier transform infrared (FT-IR) analysis of smoke produced during thermal transformation that leads to ceramization of ceramizable composites has never been reported in the literature. To fill this gap, FT-IR spectra of volatile products of the thermal destruction of the polymer matrix were analyzed, and the kinetics of thermal decomposition of the composites was studied by microcalorimetry. The thermal degradation of simple ceramizable composite (CC) was compared to that of silicone rubber (SR) and fumed silicareinforced silicone rubber (SR-S).
EXPERIMENTAL For the infrared analysis of smoke being produced from the composite samples subjected to pyrolysis, a Nicolet 6700 spectrometer equipped with a gas cell (length 8 cm, diameter 4 cm) was used (Fig. 1). Samples of a mass of circa 0.1 g were heated from 25 to 700 8C with a temperature rate of 5 8C /min in the presence of a passive carrier gas (argon, flow rate of 60 cm3/min). Obtained spectra show measurements at intervals of 25 8C (resolution 4 cm 1, 60 scans for every spectrum). For each measurement heating was finally stopped at 700 8C and the temperature was maintained at a constant until the end of scanning. Microcalorimetric studies were made with a Pyrolysis Combustion Flow Calorimeter (PCFC; Fire Testing TechnolTABLE I. Composition of the silicone rubber mixes: SR, pure silicone rubber; SR-S, silicone rubber filled with reinforcing silica; and CC, ceramizable composite. Ingredient (phr) Silicone HTV rubber Fumed silica Fluxing agent (B2O3) Mica (phlogopite) TiO2 2,2-dichlorobenzoyl peroxide 50% paste
0003-7028/13/6712-1437/0 Ó 2013 Society for Applied Spectroscopy
SR
SR-S
CC
100 – – – – 2.5
100 50 – – – 2.5
100 50 10 15 5 2.5
APPLIED SPECTROSCOPY
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FIG. 3. Cured SR filled with 50 phr of reinforcing fumed silica (SR-S) spectra at various temperatures.
FIG. 1. Scheme of the FT-IR/pyrolysis tester.
ogy, UK) in an active gas atmosphere (O2/N2 = 20/80, gas flow rate of 100 cm3/min), according to the ASTM D73092007 standard. Silicone rubber–based samples (Table 1) were made with a Brabender-Plasticorder laboratory mixer (Germany). Rotors were spun at 30 rpm during component incorporation and at 75 rpm during homogenization (15 min). Samples were vulcanized using a laboratory press at 130 8C during 15 min.
1) A multiplet in the range 3000–2840 cm 1 responsible for C-H stretching vibration in volatile compounds, which is proved by a strong band at 1264 cm 1 originating from CH3 deformation vibrations in Si-CH3 groups. 2) A weak band at 1410 cm 1 originating from antisymmetric vibrations of the C-H in SiCH3 groups, which confirms the presence of silanes and siloxanes in the volatiles.7–9 3) Strong bands at 920–724 cm 1 corresponding to Si-C stretching vibrations in the following groups: Si(CH3)2, 855, 815–800 cm 1; -SiCH3, 765 cm 1; and -Si(CH3)3, 840 cm 1.7
Infrared Spectroscopy. Smoke, containing liquid and solid matter, was created as a result of thermal destruction of the studied composites. Their infrared spectra are very similar (Figs. 2–4). The spectra of the smoke produced from SR and SR-S samples are similar, but they differ in intensity. Only the spectrum of the CC differs from the others by additional bands originating from the fluxing agent micro-particles carried by argon and present in the smoke (BO33 , B4O72 ). The spectra in the wavenumber range 3200–500 cm 1 show the following peaks:
In the first stages of polymer decomposition, absorption bands at 805–864 cm 1 appear. Among them, the highest belongs to a -O-Si-CH3 group of linear polysiloxane particles, but with increasing temperature, the intensity of the band at 815 cm 1, originating from a -O-Si-CH3 group of cyclic particles, also becomes higher.7 It shows that a temperature increase promotes the cyclization process, which corresponds with the mechanism presented by Hamdami et al.2 In the wavenumber range 1200–900 cm 1, IR bands connected with Si-O-Si and Si-O-C group vibrations appear. The band at 1033 cm 1 is characteristic of Si-O group vibrations in cyclic siloxanes (trimers probably). Also, bands at 609 cm 1 originate from Si-O vibrations in disiloxanes.7 The spectra of the CC contain a characteristic strong multiplet in the wavelength range 1470–1260 cm 1, originating
FIG. 2. Cured silicone rubber (SR) spectra at various temperatures.
FIG. 4. Ceramizable composite (CC) spectra at various temperatures.
RESULTS AND DISCUSSION
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FIG. 5. FT-IR spectra for smoke produced at 375 8C.
from products of fluxing agent decomposition, in the case of BO33 in the range 1380–1290 cm 1, and B4O72 in the range 1380–1330 cm 1 (Fig. 4).7 It seems likely that micro-particles of fluxing agent could be carried by argon to gas cells during thermal treatment. To simplify interpretation, chosen spectra (obtained at a temperature of 375 8C) were gathered in separate figures (Fig. 5). It is interesting that transmittance for near-infrared radiation significantly decreases in a temperature range corresponding to thermal degradation of the composites and accompanying heat emission. Probably solid and liquid bodies, present in the smoke as a result of thermal degradation of samples, are responsible for a scattering effect. Even for almost pure SR, which degrades mostly to cyclic, short-chain siloxanes in an inert atmosphere,10 this effect is clearly visible. So it is possible to estimate thermal stability of the composites studied, based
on FT-IR data, providing information on their decomposition products present in carrying gas. Microcalorimetry. The evolution of the heat release rate (HRR) as a function of temperature was determined for every sample (Fig. 6) using the microcalorimetry method. The HRR value is the highest for ‘‘pure’’ SR. Addition of 50 parts per hundred of rubber (phr) of fumed silica significantly decreases the HRR value of the material (SR-S), but also decreases the thermal stability understood as a temperature of the beginning heat release for the sample studied. This temperature value is closely correlated with initiations of the polymer matrix destruction, which result in significant changes of composite properties. Estimated values of the temperature of the beginning heat release, represented by ‘‘a’’ (for the CC sample), ‘‘b’’ (for the SR-S sample), and ‘‘c’’ (for the SR sample) in Fig. 5, are given in Table 2. CC represents the lowest value of HRR, but its thermal stability is also the lowest.
FIG. 6. Heat release rate (HRR) of the composites studied.
APPLIED SPECTROSCOPY
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TABLE II. Temperatures of thermal degradation beginning and maximal value of HRR.
SR SR-S CC
Temperature of thermal degradation onset
Temperature of maximal HRR value
500 8C 450 8C 350 8C
660 8C 720 8C 520 8C
On one hand, thermal stability of the composites is lower than unfilled SR, but on the other hand, a sample filled with 50 phr of silica has a higher value of temperature of the maximal HRR rate, which makes it better material from the flammability point of view (Table 2). The performed studies demonstrate that the addition of mineral particles to a SR matrix decreases its HRR value; however, it simultaneously adversely affects the thermal stability of the material. The decrease of thermal stability facilitates ceramization of the materials. It already can be achieved partially for SR-S due to sintering mechanisms of mineral filler particles in the presence of a SR matrix, as proposed by Xiong et al.11 This study shows that FT-IR spectroscopy can be a very useful method to measure thermal properties of polymer composites. When combined with microcalorimetry, FT-IR spectroscopy can accurately describe thermal degradation processes of polymer matrices filled with different types of mineral powders. Due to the qualitative FT-IR analysis, it turns out that boron oxide species are present in smoke obtained as a result of thermal decomposition of the composites. It is very important from the point of view of the efficiency of the ceramization process if the fluxing agent (B2O3) microparticles are being removed from the ceramic phase into the volatiles or not. Using FT-IR spectroscopy, it is possible to choose the type of fluxing agent whose particles do not escape into the smoke during ceramization, but stick to refractory filler particles, thus causing enhanced mechanical properties of the ceramic layer being created. A combination of FT-IR and microcalorimetry is very useful for qualitative and quantitative analysis of the thermal behavior of ceramizable composites. The HRR value is strongly correlated with the increase of the IR signal intensity in the near-infrared. The absorbance rises with the increasing wavenumber, which is likely to be associated with the heat emission intensity and shows that it is possible to use the FT-IR method to estimate the temperature of the maximum value of HRR to materials studied.
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CONCLUSIONS The thermal stability of the SR-S mix and CC is significantly lower in comparison to unfilled SR. Probably mineral fillers (especially containing nano-sized particles) play a catalytic role in thermal decomposition of the SR matrix. The admixing of silica slightly reduces the thermal stability of SR, which decreases significantly with additional incorporation of fluxing agent. It starts melting at significantly lower temperatures, facilitating the ceramization process of the composites. However, the addition of mineral fillers strongly decreases the HRR value, especially the maximal HRR value, which drops from over 500 W/g for unfilled SR up to slightly over 200 W/g for SR-S and even up to circa 120 W/g for CC. The temperature of the maximal HRR value of materials increases after addition of fumed silica, which is very important in relation to the flammability of the composites studied. ACKNOWLEDGMENTS This work was supported by the EU Integrity Fund, project POIG 01.03.0100-067/08-00. 1. D.M. Bielinski, R. Anyszka, Z. Pe˛ dzich, J. Dul. ‘‘Ceramizable Silicone Rubber-Based Composites’’. Int. J. Adv. Mat. Mfg. Char. 2012. 1(1): 1722. 2. S. Hamdani, C. Longuet, D. Perrin, J.-M. Lopez-Cuesta, F. Ganachaud. ‘‘Flame Retardancy of Silicone-Based Materials’’. Polym. Degrad. Stabil. 2009. 94(4): 465-495. 3. L.G. Hanu, G.P. Simon, Y.B. Cheng. ‘‘Thermal Stability and Flammability of Silicone Polymer Composites’’. Polym. Degrad. Stabil. 2006. 91(6): 1373-1379. 4. L.G. Hanu, G.P. Simon, J. Mansouri, R.P. Burford, Y.B. Cheng. ‘‘Development of Polymer-Ceramic Composites for Improved Fire Resistance.’’ J. Mat. Process. Technol. 2004. 153–154: 401-407. 5. Z. Pe˛ dzich, A. Bukanska, D.M. Bielinski, R. Anyszka, J. Dul, G. Parys. ‘‘Microstructure Evolution of Silicone Rubber-Based Composites during Ceramization at Different Conditions’’. Int. J. Adv. Mat. Mfg. Char. 2012. 1(1): 17-22. 6. Z. Pe˛ dzich, D.M. Bielinski. ‘‘Microstructure of Silicone Composites after Ceramization’’. Composites. 2010. 3: 249-254. 7. G. Socrates. FTIR tables. In: Infrared Characteristic Group Frequencies Tables and Charts. Chichester, UK: John Wiley and Sons, 1994. 2nd ed. 8. A. Zenasni, V. Jousseaume, P. Hollinger, L. Favennec, O. Gouhant, P. Maury, G. Gerbaud. ‘‘The Role of Ultraviolet Radiation during Ultralow k Films Curing: Strengthening Mechanisms and Sacrificial Porogen Removal.’’ J. Appl. Phys. 2007. 102(9): 094107. 9. A.K. Singh, C.G. Pantano. ‘‘Surface Chemistry and Structure of Silicon Oxycarbide Gels and Glasses’’. J. Sol-Gel Sci. Technol. 1997. 8(3): 371376. 10. G. Camino, S.M. Lomakin, M. Lazzari. ‘‘Polydimethylsiloxane Thermal Degradation. Part 1. Kinetic Aspects’’. Polymer. 2001. 42(6): 2395-2402. 11. Y. Xiong, Q. Shen, F. Chen, G. Luo, K. Yu, L. Zhang. ‘‘High Strength Retention and Dimensional Stability of Silicone/Alumina Composite Panel under Fire.’’ Fire Mater. 2012. 36(4): 254-263.
