Preparation of fumed silica compacts for thermal insulation using wet processing method

Transcription

1 Received: 22 July 2017 Accepted: 9 August 2017 DOI: /ijac ORIGINAL ARTICLE Preparation of fumed silica compacts for thermal insulation using wet processing method Xueyuan Tang Fujian Key Laboratory of Advanced Materials, College of Materials, Xiamen University, Xiamen, Fujian, China Correspondence yu_heart@xmu.edu.cn Funding information National Natural Science Foundation of China, Grant/Award Number: No , No ; Research Foundation, Grant/Award Number: Abstract This paper describes the preparation of fumed silica compacts for thermal insulation, using wet processing method. A series of thermal insulation compacts based on fumed silica powder and glass fibers were prepared. The influence of the mass ratio about fumed silica and glass fibers on the fracture strength and thermal conductivity was investigated. The results showed that the fracture strength increased first and then decreased with the mass ratio increasing. The thermal conductivity decreased linearly with the mass ratio increasing. When the compact was pressed under 6 MPa with a mass ratio of 5:1, it exhibited excellent thermal insulation at room temperature with a thermal conductivity of 0.042W/mK. Moreover, the compact was hydrophobic, after being modified by KH-570. KEYWORDS fumed silica compacts, hydrophobic modification, thermal insulation, wet processing method 1 INTRODUCTION In the last few years, high-performance thermal insulation materials attracted more and more attention because of the elevating energy consumption. 1-6 The fumed silica is a kind of promising thermal insulating material for its low thermal conductivity, which is attributed to its high porosity and nanoscale porous structure. Recently, a few researches have concentrated on the preparation of fumed silica compacts for thermal insulation. 2,7-9 In order to improve its mechanical reliability, the glass fibers were introduced. So far, most of the fumed silica compacts are prepared by the dry powder processing method. 8,9 It is difficult to form large compacts because of the high forming pressure, and it is difficult to homogeneously disperse the glass fibers in the compact by the dry powder processing method. The most important thing is that the dry powder processing would generate a lot of dust which may harm our health. 10 However, such problems could be avoided, using the wet powder processing method. Until now, there are few researches on the wet processing method. Especially, the surface modification of the compacts and the influence of glass fibers content on the properties of the compacts have rarely been reported. In this study, thermal insulation compacts based on fumed silica powder and glass fibers were prepared, using the wet processing method. The influence of mass ratio (fumed silica and glass fibers) on the fracture strength and thermal conductivity has been studied. In addition, the influence of surface modification on the hydrophobic property of the silica compact was also studied. 2 EXPERIMENTAL PROCEDURE 2.1 Fabrication of thermal insulation compact Fumed silica powder with specific surface area of 200 m 2 g 1 was used as the starting material for thermal insulation compacts (Shengsen, Chifeng, China). The average particle size of fumed silica was about 30 nm as shown in Figure 1. Figure 1 also shows the fumed silica has a three-dimensional nanoscale chain-like aggregated structure with spaces of several tens nanometers. A commercial glass The American Ceramic Society wileyonlinelibrary.com/journal/ijac Int J Appl Ceram Technol. 2018;15:

2 TANG AND YU 233 The coupling agent c-methacryloxypropyl trimethoxysilane (KH-570, Xiangqian, Nanjing, China) was used to modify the surface of compacts. The KH-570 was first dissolved in HCl solution with a ph value of 4 at 80 C. Then the compacts were immersed in the above solution for 3 h and then dried at 90 C. 2.2 Characterization of the samples FIGURE 1 TEM image of fumed silica powder fiber was added to reinforce the strength of the compact (Xinghui, Yancheng, China). Figure 2A shows the SEM image of glass fibers. It is evident that the surface of the fibers is smooth with a uniform diameter distribution. The statistical distribution (Figure 2B) shows the fiber diameter varies from 9.4 to 10.2 lm with an average value of 10 lm. Deionized (DI) water is added to increase the cohesiveness of the compact. Figure 3 shows the schematic diagram of preparing thermal insulation compact. A certain amount of fumed silica and deionized (DI) water were mixed together by rotary agitator (JJ-1 A, Liangyou, Changzhou, China) for 5 minutes. Then a certain amount of glass fibers were added slowly to the silica solution and the mixture was stirred for 15 minutes. After it was homogenized, the mixture was poured into the mold and then uniaxially pressed to form the shape of the board. The boards were dried at 120 C. Finally, the compacts with different mass ratio were prepared. The mass ratio of fumed silica and glass fibers is 1:1, 2:1, 3:1, 5:1, 10:1, and 15:1, respectively. Scanning electron microscope (SEM, XL30, Philips-FEI, Eindhoven, the Netherlands) and transmission electron microscope (TEM, JEM-2100 CX, Jeol, Japan) were used for examination of morphology and microstructure. The strength of the insulation samples was compared, using the three-point bending test (Universal testing machine, Sun2500, Galdabini, Italy). Three-point fracture strength was measured on specimens with 30 mm in width, and 100 mm in length that were machined using a conventional metal machining tool. The testing was completed using a crosshead speed of 10 mm/min with an inner span length of 60 mm at room temperature. The apparent density and porosity of the compact were determined, using the Archimedes method. Three samples were measured for each composition. The thermal conductivity of the insulation samples in the temperature range from 20 to 200 C was determined, using the transient hot-wire method (Thermal conductivity instrument, JW-III, Aoda, China). 11 Water contact angles (WCA) were measured by the sessile-drop method (Contactangle goniometer, DSA10, Kruss, Germany). FTIR spectra were recorded on Nicolet Avator 360 (Nicolet Instrument Corporation, Madison, WI) with KBr plates for liquid samples and KBr disks for solid samples. 3 RESULTS AND DISCUSSION Figure 4 shows the SEM images of the mixture from glass fibers and fumed silica. The uniform fumed silica layer (A) (B) FIGURE 2 SEM image (A) and diameter distribution (B) of glass fibers

3 234 TANG AND YU FIGURE 3 Schematic diagram of preparing thermal insulation compact [Color figure can be viewed at wileyonlinelibrary.com] FIGURE 4 SEM images of the mixture from glass fibers and fumed silica FIGURE 5 Relationship between three-point fracture strength and mass ratio [Color figure can be viewed at wileyonlinelibrary.com] would prevent the heat transfer through the direct contact of the fibers. Figure 5 shows the relationship between three-point fracture strength and mass ratio under different pressure. The mass ratio appearing later all represent the mass ratio of fumed silica and glass fibers. The pressure of 1 MPa, 4 Mpa, and 6 MPa was applied on the compact, respectively. The results indicate that the fracture strength of the compacts is affected by both of the mass ratio and the compressive stress. The fracture strength of the samples pressed under different pressure shows the same trend. It first increases and then decreases with the increasing of the mass ratio, as shown in Figure 5. Figure 5 also shows the strength reaches the maximum when the mass ratio is 5:1. When the mass ratio is low, there are so many glass fibers that the fibers are not well coated by the fumed silica. Therefore, the fibers will aggregate,leading to a low fracture strength. However, when the mass ratio is too high, there are only a few glass fibers, which have no obvious effect on the strength of the compact. In addition, the fracture strength increases significantly with pressure increasing. But when the pressure is more than 4 MPa, the strength changes little. So in this experiment, the pressure of 6 MPa was applied to prepare the compacts. Figure 6 shows the apparent density and porosity of the compacts pressed under 6 MPa. The apparent density decreases and the porosity increases with the increasing of the mass ratio, which caused by the low density of fumed silica. Figure 7 shows the relationship between thermal conductivity and mass ratio at 25 C for the specimens pressed under 6 MPa. The results indicate the thermal conductivity almost decreases linearly with the mass ratio increasing. When the mass ratio is 15:1, the thermal conductivity reaches the lowest value, about 0.037W/mK. There are only a few fibers in the compact and the porosity is very high (as shown in Figure 6), which may lead to less heat

4 TANG AND YU 235 FIGURE 6 Apparent density and porosity of the compacts pressed under 6 MPa FIGURE 9 FT-IR spectra of the compacts without (a) and with (b) KH-570 modification [Color figure can be viewed at wileyonlinelibrary.com] FIGURE 7 ratio Relationship between thermal conductivity and mass FIGURE 10 with KH-570 Water contact angles of the compacts modified FIGURE 8 SEM image of the cross-section for the compact with mass ratio of 5:1 transfer. But when the mass ratio is too high, the strength of compacts is very low, as Figure 5 shown. According to the above results, the strength of the compact reaches the maximum when the mass ratio is 5:1. Therefore, the thermal insulation compacts with the mass ratio of 5:1 were prepared under 6 MPa pressure, and its thermal conductivity is about 0.042W/mK. Figure 8 shows the SEM image of the cross-section for the compact with mass ratio of 5:1 pressed under 6 MPa. As can be seen, the glass fibers disperse and arrange well in the compacts without aggregation. And the fibers are coated uniformly by the fumed silica to form a fine interface, which could improve the fracture strength of the composites. However, the compact easily absorbs water due to a large number of hydrophilic groups of hydroxyl on its

5 236 TANG AND YU surface, which will cause the weight increasing. In addition, the thermal conductivity of the compact will be significantly affected by the absorbed water, because the thermal conductivity of water is 20 times more than that of air. In order to avoid this phenomenon, the surface modification should be carried out. Figure 9 shows the FT-IR spectra of the compacts modified with and without KH-570 modification. The peaks of 1088, 800, and 476 cm 1 correspond to the stretching, bending, and out of plane of Si O bonds, respectively. 11 The broad band at about 3450 cm 1 is assigned to -OH stretching vibration, the intensity of which is a little lower after modification. 11 The peak of 694 cm 1 and 851 cm 1 is attributed to the stretching vibration band and bent vibration band of Si CH 3 bond, indicating the Si CH 3 has been introduced into the surface of compacts after the modification. And the new peaks appears at 1741 cm 1 and 2947 cm 1, which are associated to the stretching vibrations of C=O and the stretching vibrations of C H bonds of CH 3, respectively. 12,13 These results indicate the hydrophilic groups of hydroxyl on the surface are mostly replaced by the groups of -CH 3. Figure 10 shows the water contact angles of the compacts modified with KH-570. The average value of the water contact angles is 150.4, which also demonstrates modification with KH-570 is successful. 4 SUMMARY In this study, the glass fiber reinforced fumed silica porous compacts were successfully fabricated by the wet pressing of the powder mixture. The influence of the mass ratio about fumed silica and glass fibers on the fracture strength and thermal conductivity was investigated. The compact with a mass ratio of 5:1 exhibited excellent thermal insulation at room temperature. In addition, the compact modified with KH-570 showed hydrophobicity. The average water contact angles value was REFERENCES 1. Wakili KG, Bundi R, Binder B. Effective thermal conductivity of vacuum insulation panels. Build Res Info. 2004;32: Abe H, Abe I, Sato K, et al. Dry powder processing of fibrous fumed silica compacts for thermal insulation. J Am Ceram Soc. 2005;88: Wiener M, Reichenauer G, Braxmeier S, et al. Carbon aerogelbased high-temperature thermal insulation. Int J Thermophys. 2009;30: Fricke J, Emmerling A. Aerogels-recent progress in production techniques and novel applications. J Sol-Gel Sci Tech. 1998;13: Jelle BP, Gustavsen A, Baetens R. The path to the high performance thermal building insulation materials and solutions of tomorrow. J Building Phy. 2010;34: Baetens R, Jelle BP, Gustavsen A. Aerogel insulation for building applications: a state-of-the-art review. Energy Build. 2011;43: Ryu JS, Oh KL, Lee BW. High performance of fumed silica insulation board for green building. 2nd International Conference on Green Buildings Technologies and Materials (GBTM 2012), Wuhan, China. Adv Mater Res. 2013;689: Abe I, Sato K, Abe H, et al. Formation of porous fumed silica coating on the surface of glass fibers by a dry mechanical processing technique. Adv Powder Tech. 2008;19: Li TW, Kondoa A, Kozaw T, et al. Effect of fumed silica properties on the thermal insulation performance of fibrous compact. Ceram Int. 2015;41: Merget R, Bauer T, Kupper HU, et al. Health hazards due to the inhalation of amorphous silica. Arch Toxico. 2002;75: Shokri B, Abbasi MF, Hosseini SI. FTIR analysis of silicon dioxide thin film deposited by metal organic-based PECVD. In: Proceedings of 19th international symposium on plasma chemistry society, Bochum, Germany, August 26-31, Pantoja M, Benito BD, Velasco F, et al. Analysis of hydrolysis process of g-meth- acryloxypropyltrimethoxysilane and its influence on the formation of silane coatings on 6063 aluminum alloy. Appl Surf Sci. 2009;255: Rodriguez MA, Liso MJ, Rubio F, et al. Study of the reaction of c-methacryloxy- propyltrimethoxysilane (c-mps) with slate surfaces. J Mater Sci. 1999;34: ACKNOWLEDGMENTS Financial support from the Natural Science Foundation of China (No and No ), and the Research Foundation ( ) are acknowledged. How to cite this article: Tang X, Yu Y. Preparation of fumed silica compacts for thermal insulation using wet processing method. Int J Appl Ceram Technol. 2018;15: ORCID