Development of Mechanically Stable Polymer-Based Silica Aerogel

Transcription

1 Development of Mechanically Stable Polymer-Based Silica Aerogel Development of Mechanically Stable Polymer-Based Silica Aerogel This paper is invited with permission from the SPE ANTEC 2010 Conference in Orlando, Florida, USA Eunji In and Hani E. Naguib * Department of Mechanical and Industrial Engineering, and Department of Materials Science and Engineering, University of Toronto, Toronto, Ontario M5S 3G8 Canada Received/Accepted: 23 February 2011 Summary Silica aerogels have attracted attention for many applications due to their unique properties such as low density (0.003 g/cm), mesoporosity (pore size 2-50 nm), high thermal insulation and high surface area ( m 2 /g). However, their fragility and environmental sensitivity restricts the use of monolithic silica aerogel. In this paper, silica aerogel that is crosslinked with diisocyanate is introduced and the effects of polymer concentration on aerogel properties, especially mechanical strength are discussed. Fracture of silica aerogel mainly occurs at the interface of secondary particles that are formed during aging. It is believed that if the surface of silica aerogel is covalently bonded to nanocast polymer coating, the interparticle necks become wider and can reinforce the structure of the aerogel. In this study, several characterizations are performed to investigate the properties of the aerogel. First, bulk density is measured to visualize the change in density with increase in crosslinker concentration. Brunauer Emmett and Teller (BET) illustrated the pore size, bulk density and surface area. The critical temperature up to which polymer crosslinked aerogel decompose was investigated by Thermogravimetric Analysis (TGA). The phase change in the aerogel was studied with Differential Scanning Calorimetry (DSC) testing. Using Scanning Electron Microscope (SEM), the nanoscale pore size and pore distribution throughout the aerogel surface were investigated. Furthermore, compression tests were performed to study the effect of crosslinking polymer on mechanical strength over non-crosslinked framework. Keywords: Silica aerogel, crosslinking, di-isocyanate, mechanical properties, Thermogravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC), Scanning Electron Microscope (SEM). * Author to whom correspondence should be addressed. Tel: (416) , Fax: (416) , naguib@mie.utoronto.ca Smithers Rapra Technology, 2011 Cellular Polymers, Vol. 30, No. 1,

2 Eunji In and Hani E. Naguib Introduction Silica aerogels have unique properties that include low density (0.003 g/cm) and mesoporosity (pore size 2-50 nm), with a high thermal insulation value (0.005 W/mK) and high surface area ( m 2 /g) [l]. Due to these properties, silica aerogels attract many applications such as thermal insulation systems, acoustic barriers, catalysis, and biomedical applications. These specialized properties of aerogels arise from the sol-gel process whereby the colloidal system of liquid converts into gel, where the pore liquid is replaced by air without altering the network structure and shrinkage or collapse of the solid 3-D network [2]. Despite their unique properties, fragility and environmental sensitivity restricts the use of monolithic silica aerogel. Fracture of silica aerogel mainly occurs because of their structure. The mesoporous structure of silica aerogel consists of fully dense amorphous silica particles (1-2 nm), which assembles into balllike secondary particles (5-10 nm). These secondary particles accumulate by dissolution and re-precipitation of silica gel during aging and connect through neck regions, creating a pearl necklace-like structure with large voids [3]. These voids between entangled particle strands are responsible for low density and low thermal conductivity of the aerogels [4,5]. However, when an external load is applied, fracture occurs at the interface of secondary particles while primary particles remain intact [6]. Recently, Meador et al. have suggested that by covalently bonding the polymer to the surface of silica aerogel will enhance the mechanical, thermal and/or electrical properties of the material [7]. Especially with silicate precursors, the functional organic group can be bonded through a hydrolytically stable Si-C group [2]. By formation of covalent bonds, the structure can be reinforced by widening the interparticle necks while minimally reducing porosity [8]. Some research has shown to improve the mechanical strength of the silica aerogel monoliths by crosslinking polystyrene [9], polymethyl methacrylate (PMMA) [10], epoxies [10] and polyurethane [11]. The purpose of this study is to investigate the effect of crosslinking the diisocyanate with silica aerogel and change in its chemical and mechanical properties. In polyurethane chemistry, a polyfunctional isocyanate reacts with a polyfunctional alcohol to form carbamates. When silica aerogel is crosslinked with di-isocyanate, the Si-OH groups on the surface of silica plays a role of functional polyol as shown Eq. (1) [12]. (1) 2 Cellular Polymers, Vol. 30, No. 1, 2011

3 Development of Mechanically Stable Polymer-Based Silica Aerogel Experimental Experimental Materials For fabrication of native silica aerogel, the following solvents were utilized. Tetramethylorthosilicate (TMOS) was used as a silica precursor, 3-aminopropyltriethoxysilane (APTES) was applied as base catalyst and both were purchased from Sigma Aldrich. The acetonitrile (CH 3 CN) solvent was obtained from EMD Chemicals. The polymer crosslinkers Desmodur N3200, 1, 6-hexamethylene di-isocyanate (HDI) based oligomers were donated by Bayer Corporation. Preparation of Isocyanate Crosslinked Aerogels Native silica aerogel was prepared by sol-gel process by mixing two separate solutions, solutions A and B. In solution A, both TMOS and APTES were combined together with CH 3 CN in order to obtain a 2.0 mol/l of total silane. The use of APTES eliminated the need for use of base-catalyst such as ammonium hydroxide. Solution B was prepared with equal amounts of CH 3 CN and water to have 10.0 mol/l of H 2 O in solution B. The solutions were cooled with dry ice/acetone bath at -78 C prior to mixing to avoid premature gelation. The 14 ml polypropylene test tubes (BD Falcon), 17 mm diameter, were used as molds by cutting the round bottom and sealing with layers of parafilm. The beaker with solution A was placed on a stir plate and solution B was poured into solution A. The beaker containing the mixture of solution A and B were capped with parafilm immediately, stirred vigorously, and solution was poured into molds where it was allowed to gel and age for 24 h. The wet gels were taken out from the mold and placed into fresh CH 3 CN with five times the volume of the gel. The solvent was exchanged three times for 24 h intervals to remove excess water and condensation byproducts. To crosslink the silica aerogel with di-isocyanate, a bath of di-isocyanate in acetonitrile with different weight percents of 0, 20, 40 and 50 wt% were prepared. The wet gels were then transferred into the di-isocyanate bath and settled with slow agitation for 24 h. Subsequently, the gel is put in fresh solvent and cured at 71 C in oven for 72 h. After taking the gel out from the oven, it was cooled to room temperature and replaced with clean solvent four times every 24 h to remove unreacted monomers still remaining within the wet gel. These gels were placed in temperature controlled pressure chamber for supercritical drying. The pressure was set at ~1480 psi and cycle of releasing and reapplying the pressure was done five times every 2 h. Cellular Polymers, Vol. 30, No. 1,

4 Eunji In and Hani E. Naguib After 10 h of pressurizing with CO 2 gas, the temperature was increased from 25 C to 45 C, which causes the pressure to increase to ~3200 psi where it reaches the supercritical state of CO 2. CO 2 gas was then slowly vented and di-isocyanate crosslinked silica aerogel was obtained. Characterization of Crosslinked Aerogels The bulk density of aerogels was calculated by simply dividing the mass. Infrared spectra of aerogels were analyzed with a Perkin-Elmer FTIR Spectrometer. The samples were powdered, pelletized with spectra grade KBr and the spectra were recorded in the region cm -1 at room temperature. The specific transmittance peaks were observed and compared with silica aerogel and crosslinked aerogel. Physical characterization of native and crosslinked aerogel samples were conducted by Thermogravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC), Surface Area (SA) measurement using Brunauer- Emmet-Teller (BET) method and Scanning Electron Microscope (SEM). TGA was performed using a TA instruments Model Q50. Samples were tested at a temperature ramp of 10 C min -1 up to 800 C in a static nitrogen atmosphere. The DSC was carried out by TA instrument Model Q2000 to test the heat required to increase the temperature of the sample during physical transformation. The surface area measurement was carried out by Autosorb-1 Surface Area/Pore Distribution analyzer (Quantachrome Instruments Corp.) using BET method. Aerogel morphology was characterized by utilizing an SEM (Hitachi S-5200). The samples were cooled in liquid nitrogen before fractured and gold coated to enhance conductivity before imaging under SEM. The compression mechanical testing was performed according to ASTM D695 [13], on a Shimadzu universal testing machine at room temperature. The displacement rate of the crosshead was 1 mm per minute. Results and Discussion Macroscopic Characterization The effect of crosslinking on silica aerogel is presented in the following section. Figure 1 shows native and diisocyanate crosslinked aerogels with different concentrations of di-isocyanate (20, 40 and 50 wt%). In general, di-isocyanate crosslinked aerogels are opaque. Non-crosslinked aerogels exhibit translucent, glassy appearance with small cracks inside of structure. With increase in polymer concentration, samples loose its glossiness and 4 Cellular Polymers, Vol. 30, No. 1, 2011

5 Development of Mechanically Stable Polymer-Based Silica Aerogel Figure 1. Photograph of aerogel samples of native, diisocyanate crosslinked with 20, 40 and 50 wt% (from left to right) Figure 2. The effect of polymer concentration on bulk density of aerogel monoliths become dull. The bulk density of the noncrosslinked silica aerogel (r b ) is found to be gcm -3. However, we observed that there is slight increase in the bulk density of aerogel samples (r b = gcm -3 ) as the concentration of polymer increases as shown in Figure 2. This is possibly due to the amount of organic polymer attached to the silica backbone which results in a wider neck region and the void space potentially occupied by air decreases. This Zhang et al. also found Cellular Polymers, Vol. 30, No. 1,

6 Eunji In and Hani E. Naguib through NMR spectroscopy of crosslinked aerogel that shows ~80-86% void space compared to ~89-92% for silica aerogel [12]. Microscopic Characterization SEM image of crosslinked aerogels is illustrated in Figure 3. According to Figure 3a, we can visualize that the primary particles cluster together to form secondary particles that arranges them in pearl-necklace like structure. As discussed earlier, when crosslinked with diisocyanate, the polymer accumulates on the ball-like neck regions of the secondary particles and the neck become wider while primary particles remain the same to result in mesoporous structure (Figure 3b). The BET surface area analysis show that as the polymer is crosslinked to the silica framework, the total surface area decreases from 400 m 2 g -1 to m 2 g -1. (a) (b) Figure 3. SEM images of interior of fractured crosslinked silica aerogel monolith a) pearl-like structure and b) wide necking region of secondary particles 6 Cellular Polymers, Vol. 30, No. 1, 2011

7 Development of Mechanically Stable Polymer-Based Silica Aerogel Chemical Characterization By addition of 1,6-hexamethylene diisocyanate (HDI), which is a major chemical component of Desmodur N3200, there is slight change in the infrared absorption data. HDI chemical component is shown below. Desmodur N3200 Figure 4 shows a FTIR graph for native and crosslinked with 20 wt% polymer concentration samples. Both FTIR spectrums represent that there are similar IR peaks. First, the absorption band around 3416, 3390 cm -1 shows the presence of hydroxyl group and H-O-H bending of water molecules in the sample. Also, the absorption band of around 692 cm -1 indicates the Si-O functional groups. However, on crosslinked aerogel spectrum, a few more bands are found such as a broad band near 1740 cm -1, which corresponds to the carbonyl stretch assigned to the diazetidinedione group of N3200 and another band around 1690 cm -1 to represent the isocyanaturate ring carbonyl stretch. Figure 4. Infrared analysis of pure silica and Desmodur N3200 crosslinked aerogels Cellular Polymers, Vol. 30, No. 1,

8 Eunji In and Hani E. Naguib Thermogravimetric (TGA) and Differential Scanning Calorimetry (DSC) Characterization Figure 5 shows the effects of polymer concentration on its thermal gravimetric analysis (TGA). When the samples were exposed to temperature ramp from ambient temperature to 800 C, a weight loss was observed up to 200 C. Native silica aerogels lost ~5% of their weight in first 200 C, which is believed to be because of evaporation of remaining acetonitrile. Aerogels crosslinked with Desmodur N3200 with different concentrations also show ~2.7-7% weight loss corresponding to solvent evaporation. However, with the crosslinker, there is decomposition at ~350 C under nitrogen, which indicates that we lose all of the crosslinking agent at about that temperature. This can be supported by an experiment that Zhang et al. have done on TGA study of polyurea derived from the reaction of N3200 with water, where it started to lose weight at ~300 C under nitrogen [12]. Differential Scanning Calorimetry (DSC) was done to study the change in heat flow when increasing the temperature. Figure 6 shows that there are few endothermic peaks that illustrate the phase transition on aerogel samples. First, in native silica aerogel, there is a small drop in heat flow at ~145 C. This could be considered as glass transition temperature (T g ) of the silica framework. When crosslinking with 20 wt% diisocyanate, this T g, increases up to ~165 C. Furthermore, another endothermic peak is found from DSC curves Figure 5. Thermogravimetric analysis (TGA) of native and crosslinked silica aerogel with decomposition of diisocyanate at ~350 C 8 Cellular Polymers, Vol. 30, No. 1, 2011

9 Development of Mechanically Stable Polymer-Based Silica Aerogel Figure 6. DSC graph of native and crosslinked silica aerogel with increase in glass transition temperature as temperature increases. This is considered as melting temperature (T m ) of the aerogel samples. The T m, of native silica aerogel is at ~180 C whereas for crosslinked aerogel, phase transition occurs at ~255 C. Mechanical Characterization The effect of polymer concentration on compression strength of silica aerogels is shown in Table 1. The native silica aerogel contained cracks within the structure and they started to chip into small particles under compressive load. Therefore, the value of elastic modulus was unable to obtain. On the other hand, the cylindrical crosslinked aerogel samples decreased in height and increased in diameter as compressive load is applied. The value of modulus decreases with increase in polymer concentration for aerogel samples with 2.0 mol/l total silane and 10.0 mol/l H 2 O content. It does not coincide with our hypothesis that the mechanical strength will increase with incorporation of polymer. This could be resulted from crack that propagated in the sample during fabrication. When compressive force is applied, this crack could become stress concentration point and result in earlier brittle fracture on the sample. However, when looking at other samples with different silane and water content, 3.0 and 15.0 respectively, we can see that the modulus of elasticity increases from 256 MPa to 285 MPa for 40 wt% and 50 wt% polymer concentration. This is about 110% increase in modulus by increasing 10 wt% of polymer concentration. Cellular Polymers, Vol. 30, No. 1,

10 Eunji In and Hani E. Naguib Table 1. Relationship between modulus of elasticity and polymer concentrations Weight Percent (%) Modulus of Elasticity (MPa) 2.0/ / N/A N/A N/A 40 N/A Conclusions In this study, the effects of crosslinking di-isocyanate with silica aerogel were examined. Several conclusions can be made from results: 1. The density increased 15.3% at 20 wt% and 24% at 50 wt% crosslinking. This is due to accumulation of polymer material on silica framework and it could possibly be reduced by increasing the number of washing cycles to remove unreacted monomers. 2. The morphology of silica aerogel illustrated widening of the neck region after crosslinking and the surface area decreases as void space decreases. 3. When crosslinking was properly carried out, the corresponding functional group bands of di-isocyanate were seen by FTIR. 4. The aerogel loses its weight due to evaporation of solvent, acetonitrile, under 200 C. Decomposition of polymer occurs at ~350 C. 5. Both T g and T m, increase significantly with crosslinking. 6. The mechanical strength of the silica aerogel can be improved by crosslinking with di-isocyanate. Although there is significant increase in mechanical strength of the aerogel samples, properties such as transparency and low-density of native silica aerogel is lost. In the future, crosslinking with other polymers will be carried out to minimize the change in appearance and density while reinforcing the structure of the aerogel. Acknowledgments The authors would like to acknowledge the financial support from NSERC Canada, Canada Foundation for Innovation, Canada Research Chairs Program, and the Government of Ontario: Ministry of Research and Innovation. 10 Cellular Polymers, Vol. 30, No. 1, 2011

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12 Eunji In and Hani E. Naguib 12 Cellular Polymers, Vol. 30, No. 1, 2011