Synthesis, Structure and Thermal Protective Behavior of Silica Aerogel/PET Nonwoven Fiber Composite

Fibers and Polymers 2014, Vol.15, No.10, 2154-2159 DOI 10.1007/s12221-014-2154-z Synthesis, Structure and Thermal Protective Behavior of Silica Aerogel/PET Nonwoven Fiber Composite Zahra Talebi Mazraeh-shahi, Ahmad Mousavi Shoushtari*, Ahmad Reza Bahramian 1, and Majid Abdouss 2 Textile Engineering Department, Amirkabir University of Technology, Tehran, Iran 1 Polymer Engineering Department, Faculty of Chemical Engineering, Tarbiat Modares University, Tehran, Iran 2 Department of Chemistry, Amirkabir University of Technology, Tehran, Iran (Received December 4, 2013; Revised April 4, 2014; Accepted April 12, 2014) Abstract: In recent years, flexible, mechanically strong and environmental friendly thermal insulation materials have attracted considerable attention. In this work, silica aerogel/polyethylene terephthalate (PET) nonwoven fiber composite with desirable characteristics was prepared via a two-step sol-gel process followed by an ambient drying method through immersing the PET nonwoven fiber into silica sol. The silica aerogel particles were characterized by FTIR, FE-SEM, TGA and nitrogen adsorption analysis. The morphology and hydrophobic properties of neat PET nonwoven fiber and its silica aerogel composite were also investigated. For studying thermal protective properties, the thermal diffusivity was calculated from temperature distribution curves. The mean pore size of 11 nm, the surface area of 606 m 2 /g and the total pore volume of 1.77 cm 3 /g for the silica aerogel particles in the composite are obtained from nitrogen adsorption analysis, indicating the aerogel can maintain its high porosity in the nonwoven composite structure. Silica aerogel particles were efficiently covered the surface of the PET fibers and completely filled the micron size pores of the nonwoven fiber leading to a stronger hydrophobicity and higher thermal insulation performance in the aerogel composite samples compared to the neat PET nonwoven. In this regard, an almost 64 % decrease in the thermal diffusivity was achieved with 66 wt% silica aerogel. Keywords: Silica aerogel, PET nonwoven composite, Thermal protection properties, Ambient pressure drying, Thermal diffusivity Introduction Silica aerogels are known as unique porous materials with highly cross-linked network structure having large specific surface area, high porosity, low density and very low thermal conductivity [1,2]. Due to these prominent properties, silica aerogels in recent years have been used in various advanced applications such as thermal insulators, catalysts, sensors, oil adsorbent [9] and drug delivery systems [3-6]. They are usually synthesized via sol-gel process under supercritical drying conditions to prevent the effects of capillary forces and porous structure destruction [7,8]. However, the supercritical drying technique as expensive and risky method has led to finding new procedures such as ambient pressure drying method in which a surface modification step is needed prior to the drying process [9-11]. On the other hand, drying technique problems and the fragility and hydrophilicity leading to undesirable physical and environmental instability have avoided the commercial use of these materials. Approaches such as incorporation of a secondary reinforcement materials including fibers, particles or organics and further condensing the silica network with molecular precursors have been used to reinforce the silica network [12-14]. In recent years, commercially flexible aerogel blankets have been fabricated by using a silica aerogel coating process onto a fibrous material [15-17]. Incorporation aerogels in to a fiber matrix such as nonwoven fabrics may support *Corresponding author: amousavi@aut.ac.ir brittle aerogel network and can certainly increase their mechanical properties. Therefore, by following this method the weakness of fragility in the aerogel for industrial applications could be overcome. In the most of studies on the flexible aerogel composite, mainly a fiber glass batting has been used as the fibrous material and limited literatures have been reported on the fibrous silica aerogel composite with the organic fiber such as PET. On the other hand, silica aerogel/pet nonwoven is more environmentally friendly than fiber glass and it can be used in more widespread applications due to its lower weight, less thickness and more flexibility. The aim of this work is to prepare a PET nonwoven/silica aerogel composite via sol-gel using ambient pressure drying method. Then the morphology, hydrophobic and thermal protective behavior of the obtained composite are investigated. Experimental Materials Tetraethoxysilane (TEOS) and trimethylchlorosilane (TMCS) were purchased from Merck Co. The Solvents used in the production of aerogels including ethanol and n-hexane were supplied from Scharlau Co. PET nonwoven fabric was purchased from Bibaft Co., Iran. PET Nonwoven/silica Aerogel Composite Preparation In order to remove finishing agents, PET nonwoven was firstly rinsed in industrial ethanol for 1 hr at 30 o C followed 2154

Silica Aerogel/PET Nonwoven Fiber Composite Fibers and Polymers 2014, Vol.15, No.10 2155 by rinsing in water and drying in air. Then, a silica sol was prepared using TEOS as precursor according to our previous work [18]. PET nonwoven fiber was immersed into silica sol after hydrolysis step and was aged for 30 min at 60 o C. Other subsequent steps for preparing the composite including gelation, aging, surface modification, washing and drying were followed according to our described work [18]. For studying the effect of silica content on the thermal diffusivity of composite, the volume ratio of initial sol mixture to the nonwoven felt is varied in the range of 1.44-2.4 resulting in composites with 66-76 wt% silica aerogel content. Characterization The surface morphology of the PET nonwoven fabric and its composite and silica aerogel particles were characterized by scanning electron microscopy (SEM, Philips XL 30) and field emission scanning electron microscopy (FE-SEM, Hitachi S4160, Japan) respectively. The specific surface area and pore size distribution were determined using Nitrogen adsorption analysis (Belsorp mini2, Japan) by BET and BJH thechniques. A spectrometer (FTIR, Thermo Nicolet Nexus 670, USA) was used to investigate the chemical bonding of the silica aerogel particles in the composite. Thermogravimetric analysis (TGA) was used to determine the thermal stability of silica aerogel particle using a TA Instruments (NETZSCH TG 209, Germany) at temperature range of 20 o C to 700 o C and a heating rate of 5 o C/min in air. The hydrophobic properties of the samples were determined by measuring water contact angel with Kruss K100-SF system. Thermal insulation property was measured using Marsh cooling method [19] in which a hot body was wrapped with fabric sample and its rate of cooling was measured. In this experiment, a brass cylinder (10 cm length, 3 cm external diameter and 2 mm thickness) with closed one end by a crock was filled with silicon oil heated to about 150 o C. Then it is left to cool down and finally the cooling curve (temperature versus time) was obtained. The thermal diffusivity of the neat nonwoven fabric and its silica aerogel/pet composite was calculated from the cooling curve and the numerical solution of the related heat transfer equations documented in Bahramian et al. work [20]. Results and Discussion The FTIR spectra of silica aerogel particles extracted from the composite are shown in Figure 1. The O-H stretching and bending vibrations are visible at 3420 and 1635 cm -1 respectively. The strong peak at 1082 cm -1 is related to Si-O- Si asymmetric stretching vibrations and the Si-O-Si symmetric stretching appears as a weak peak at 802 cm -1. The O-Si-O stretching vibration also can be seen around 459 cm -1. The absorption peak at around 951 cm -1 is due to the asymmetric stretching vibrations of Si-OH groups. The Si-C vibrations at 1256 and 847 cm -1 and the vibrations of the C-H bonds of Figure 1. FT-IR spectra of silica aerogel particle in the composite. Figure 2. Nitrogen adsorption-desorption isotherm of silica aerogel particle in the composite. the -CH 3 terminal groups at 2963 cm -1 indicate the successful surface modification of wet gel by TMCS [8,18]. Figure 2 shows the nitrogen adsorption-desorption isotherms of silica aerogel particles extracted from the silica aerogel/ PET composites. The initial increase of the gas adsorption associated with the presence of micropores in network. The hysteresis loop and type-iv adsorption isotherm according to IUPAC classification adsorption isotherms is observed for them which approve the presence of mesopores [21]. The specific surface area, mean pore size, and total pore volume of the silica aerogel particles obtained from BET and BJH techniques are 11 nm, 606 m 2 /g and 1.77 cm 3 /g respectively. The porosity and the average particle size of the samples were determined from equation (1) and equation (2) respectively. The bulk density was calculated according to equation (3) [22-24]. v P = --------------------- ( v + 1/ρ s ) (1)

2156 Fibers and Polymers 2014, Vol.15, No.10 Zahra Talebi Mazraeh-shahi et al. 6 d = ----------- ( Sρ s ) ρ b = ρ s ( 1 P) Where P is the porosity, v is the total pore volume, ρ b is the bulk density, ρ s is the theoretical density which is 2.2 g/ cm 3 for amorphous silica, d is the average particles size, S is the specific surface area. The porosity, average particle size and density of silica aerogel particle determined from equations (1)-(3) are about 80 %, 5 nm and 0.3 g/cm 3 respectively. Also the apparent density of silica aerogel particles is determined experimentally from the ratio of mass to volume from 4 measurements in which the density of 0.284±0.05 g/ cm 3 is obtained. The pore size distribution of the silica aerogel particles is shown in Figure 3 in which r p is the pore diameter and v p is the pore volume. The heat transfer through silica aerogel is vigorously affected by its porosity and mean pore size, in addition the porosity of silica aerogel depends on the pore volume of its structure according to the equation (1) [7,25]. Furthermore, the type of pore size distribution is also important by which narrow pore size distribution is more desirable because it decreases the capillary forces gradient produced in the drying process so that silica aerogel with low density and high porosity can be obtained [18]. As demonstrated in Figure 3, the silica aerogel can also maintained its highly porosity structure in the silica aerogel/ PET nonwoven fiber composite. Figure 4 shows the thermogravimetric analysis (TGA- DTA) of the synthesized silica aerogel particles. A two-step weight loss can be observed including small weight loss of about 2 % up to 170 o C which can be associated to the residual solvent evaporation. While in the second step with major weight loss of 11 % between 290-600 o C is due to sever oxidation of -CH 3 created from modification process. (2) (3) The residue weight of 88 % at 700 o C is related to SiO 2 [26]. Figure 5 shows the SEM images of PET nonwoven fabric and silica aerogel/pet nonwoven fiber composite at two different magnifications. The efficiency of fiber coating with silica aerogel is well documented in Figure 5. It clearly demonstrates that an excellent coating is achieved (Figure 1(b,d)) and also silica aerogel completely filled the micron level pores existed among the PET nonwoven fiber (Figure Figure 4. TGA curves of silica aerogel patticle. Figure 3. Pore size distributions of silica aerogel particle in the composite. Figure 5. SEM images of the PET nonwoven and its composite with different magnifications; (a) nonwoven fabric ( 156), (b) nonwoven fabric ( 1250), (c) silica aerogel/pet nonwoven composite ( 156), (d) PET nonwoven/silica aerogel composite ( 1250), and (e) FE-SEM of silica aerogel.

Silica Aerogel/PET Nonwoven Fiber Composite Fibers and Polymers 2014, Vol.15, No.10 2157 Table 1. Contact angle of fibrous samples with water Sample Dry contact angle a (deg) Wet contact angle b (deg) PET nonwoven fiber 89 37 Composite with 76 wt% silica aerogel 98 96 a Dry surface of fibrous samples and b wet surface of fibrous samples after immersing in water for 1 hr. 1(a,c)). Figure 1(e) shows the FE-SEM morphology of silica aerogel which exhibits the porous network structure. The contact angle of PET nonwoven fabric and its silica aerogel composite with water were presented in Table 1 under dry and wet surface conditions which measured before and after immersing the samples in water for 1 hr. These results obviously reveal the noticeable enhancement of hydrophobic properties of the silica aerogel/pet nonwoven fiber composite compared to the neat nonwoven fiber especially in wet conditions. This can be attributed to the presence of -CH 3 terminal groups on the surface of silica aerogel due to the surface modification of wet gel by TMCS. In wet surface conditions, when the PET nonwoven fiber immersed into water for 1 hr before the contact angle measuring, the surface of PET fibers and the pores among the nonwoven fabric could be covered by water molecules due to wicking phenomena and hydrogen bonding interactions between ester groups in PET fibers and water. Hence, the contact angle is significantly decreased compared to dry surface conditions. This is while no difference between contact angle of the composite at wet and dry conditions is observed. This clearly can demonstrate the hydrophobic properties of the composite sample. The heat transfer through fibrous materials with high porosity involves combined modes of the conduction and radiation heat transfer. The radiation heat transfer is insignificant in low temperature and can be ignored. The conduction heat transfer consists of both solid conduction through fibers and gas conduction in void space among fibers according to equation (4) [27]. K f f 2 * = ( K s ) + ( 1 f)k g (4) Where K is the thermal conductivity, f is the volume * fraction of fiber, K s is the thermal conductivity of the fiber parent material and K g is the gas thermal conductivity. Solid conduction is the least significant mode while gas conduction is the major component of heat transfer in high porous fibrous materials. The gas conduction is highly affected by the porosity and pore size of fibrous felt and the gas mean free path [27-29]. On the other hand, aerogel is known as one of the best thermal insulation materials having very low conductivity (10-15 mw/mk) even less than air (25 mw/mk) [7]. The total heat transfer through silica aerogel as a very high porous material is composed of solid and gas conduction heat transfer. Solid conduction mode in the aerogel is low due to its high porosity. Moreover, the mesopores smaller than mean free path of the gas molecules (70 nm for air molecules) cause very low gas conductivity of aerogel [25]. The basic concept of thermal insulation properties of aerogel/fibrous nonwoven composites is being related to the micron size pores of nonwoven fiber with about 90 % porosity which is filled with the aerogel having the high porosity and small mesopores of less than 50 nm. This filling of the micron size pores with silica aerogel gives rise to decrease in the gaseous thermal conductivity and consequently, the improvement of thermal protective properties in aerogel/ fibrous nonwoven composites [30]. Thermal conductivity is also calculated from the thermal diffusivity using equation (5): K = αc P ρ where C p and ρ are the heat capacity and density of the composite, respectively [31]. In this work, for studying the thermal protective properties, the thermal diffusivity of samples was determined. Figure 6 shows the temperature distributions of the silicon oil in the Marsh cooling method which reveals the thermal insulation properties of the neat PET nonwoven and silica aerogel/pet nonwoven composite with silica content of 76 wt%. The thermal diffusivity of neat PET nonwoven fabric and its silica aerogel composite which calculated by using Figure 5 and numerical solution of the related heat transfer equations [20], are 2.6 10-7 and 1.58 10-7 m 2 /s, respectively. These values clearly demonstrate that the presence of silica aerogel in the matrix of nonwoven fabric results in significant improvement of the thermal protective properties due to noticeable decrease in the thermal diffusivity of the composite compared to the neat nonwoven fabric. In this regard, the Figure 6. Temperature distributions of the silicon oil in the brass cylinder wrapped with the fibrous samples. (5)

2158 Fibers and Polymers 2014, Vol.15, No.10 Zahra Talebi Mazraeh-shahi et al. Associations Office of Amirkabir University of Technology for the financial support of this work. References Figure 7. Variation of the thermal diffusivity of silica aerogel/pet nonwoven composite with silica aerogel content. effect of volume ratio of initial sol to nonwoven felt (silica content) is investigated and the obtained results presented in Figure 7. As can be seen in Figure 7, the thermal diffusivity of the composite samples increases with increasing the silica content. This can be attributed to the effect of solid conduction heat transfer of silica aerogel particles in the applied silica content range which increases the total conduction of the composite in the higher silica content. Also according to the equation (4) and equation (5), the composite thickness has no influence on the thermal conductivity and thermal diffusivity. Conclusion The silica aerogel/pet nonwoven composites were prepared successfully via sol-gel under ambient pressure drying process. An efficient silica aerogel coating is achieved meanwhile the micron level pores existed among the PET nonwoven fiber are completely filled with the synthesized silica aerogel particles. Based on the applied procedure the aerogel maintained its high porosity in the PET nonwoven composite structure which clearly confirmed by mean pore size of 11 nm, the surface area of 606 m 2 /g and the total pore volume of 1.77 cm 3 /g for the silica aerogel particles extracted from the silica aerogel/pet composites. The silica aerogel/pet nonwoven fiber composite exhibited the noticeable enhancement of hydrophobic and thermal protection properties compared to the neat nonwoven fabric. Decreasing the silica aerogel content reduces the thermal diffusivity of the composite and the presence of 66 wt% of silica aerogel in the matrix of the nonwoven fabric led to 64 % decrease in its thermal diffusivity. Acknowledgement The authors are highly grateful to the Olympia and Scientific 1. F. Shi, L. Wang, and J. Liu, Mater. Lett., 60, 3718 (2006). 2. H. X. Zhang, X. D. He, and F. He, J. Alloys. Compds., 469, 366 (2009). 3. D. Ge, L. Yang, Y. Li, and J. Zhao, J. Non-Cryst. Solids, 355, 2610 (2009). 4. I. Smirnova, S. Suttiruengwong, and W. Arlt, J. Non-Cryst. Solids, 350, 54 (2004). 5. J. L. Gurav, I. K. Jung, H. H. Park, E. S. Kang, and D. Y. Nadargi, J. Nanomater., 2010, Article ID 409310 (2010). 6. S. O. Kucheyev, A. V. Hamza, J. H. Satcher, and M. A. Worsley, Acta Mater., 57, 3472 (2009). 7. A. C. Pierre and G. M. Pajonk, Chem. Rev., 102, 4243 (2002). 8. K. W. Oh, D. K. Kim, and S. H. Kim, Fiber. Polym., 10, 731 (2009). 9. M. L. Liu, D. A. Yang, and Y. F. Qu, J. Non-Cryst. Solids, 354, 4927 (2008). 10. A. Fidalgo, M. E. Rosa, and L. M. Ilharco, Chem. Mater., 15, 2186 (2003). 11. P. B. Sarawade, J. K. Kim, H. K. Kim, and H. T. Kim, Appl. Sur. Sci., 254, 574 (2007). 12. K. A. D. Obrey, K. V. Wilson, and D. A. Loy, J. Non-Cryst. Solids, 357, 3435 (2011). 13. A. Fidalgo, J. P. S. Farinha, J. M. G. Martinho, and L. M. Ilharco, J. Mater. Chem. A, 1, 12044 (2013). 14. Y. Liao, H. Wu, Y. Ding, S. Yin, M. Wang, and A. Cao, J. Sol-Gel Sci. Technol., 63, 445 (2012). 15. Z. Zhang, J. Shen, X. Ni, G. Wu, B. Zhou, M. Yang, X. Gu, M. Qian, and Y. Wu, J. Macro. Sci., Part A: Pure and Appl. Chem., 43, 1663 (2006). 16. J. Liu, X. Wang, F. Shi, and J. Luo, Adv. Mater. Res., 534, 106 (2012). 17. E. R. Bardy, J. C. Mollendorf, and D. R. Pendergast, J. Heat Transfer, 129, 232 (2007). 18. Z. Talebi Mazraeh-shahi, A. Mousavi Shoushtari, M. Abdouss, and A. R. Bahramian, J. Non-Cryst. Solids, 376, 30 (2013). 19. S. Debnat and M. Madhusoothanan, Ind. J. Fibre Text. Res., 35, 38 (2010). 20. A. R. Bahramian and M. Kokabi, J. Hazard. Mater., 166, 445 (2009). 21. IUPAC Recommendations, Pure Appl. Chem., 66, 1739 (1994). 22. N. Leventis, Acc. Chem. Res., 40, 874 (2007). 23. L. Zhang, J. Gu, H. Yao, and Y. Zhang, Rare Mets., 30, 552 (2011). 24. A. Soleimani Dorcheh and M. H. Abbasi, J. Mater. Process. Technol., 199, 10 (2008). 25. B. E. Yoldas, M. J. Annen, and J. Bostaph, Chem. Mater.,

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