6790
Ind. Eng. Chem. Res. 2008, 47, 6790–6795
Transferred Arc Production of Fumed Silica: Rheological Properties Ramona Pristavita, Richard J. Munz,* and Tony Addona Department of Chemical Engineering, McGill UniVersity, 3610 UniVersity Street, Montreal, Quebec, H3A 2B2, Canada
Different grades of fumed silica can be obtained using an environmentally friendly thermal plasma process. In the present work, we studied the changes in the powder quality by varying the quench conditions used for the production of the powder and by agglomerating the product. Tests done before and after the agglomeration experiments showed that the agglomeration had no effect on the powder’s rheological or other properties. We concluded that the inferior rheological properties of the plasma produced fumed silica were due to the lack of the free hydroxyl groups on the surface of the particles. Posttreatment of plasma produced powder showed that the surface chemistry was modified and the free hydroxyl groups were introduced on the surface of the particles. 1. Introduction Fumed silica is considered to be unique in industry because of its unusual particle characteristics. Its extremely fine particle size, its large surface area, its high purity, and its chain-forming tendencies set it apart. The most important characteristic of fumed silica is the presence of hydroxyls on the surface of the particles, in the form of isolated hydroxyl groups, hydrogenbonded hydroxyl groups, geminal hydroxyl groups, and siloxane groups (Figure 1). Common applications of the powder include its use as a thickening and thixotropic agent in paints, thermosetting resins, and printing inks; a reinforcing filler in rubbers; a free-flow and anticaking agent in the processing of dry materials; and a thermal insulator (especially in the high temperature range up to 1000 °C). At the present, fumed silica is produced industrially by the vapor phase hydrolysis of silicon tetrachloride in a hydrogen/oxygen flame. The combustion process results in silicon dioxide molecules that condense to form particles. The particles collide, attach, and sinter together forming a three-dimensional branched-chain aggregate. Once the aggregates cool below the fusion point of silica (1710 °C), further collisions result in mechanical entanglement of the chains, termed “agglomeration”. Along with the main product, this process also generates a large quantity of HCl(g) as byproduct. Currently, there is interest in the use of thermal plasma technology for the production of fumed silica. The plasma process does not involve chlorine, it uses cheap raw material (quartz instead of SiCl4), and it has no negative impact on the environment, because no byproducts, such as HCl, are generated. This process consists of the reduction of quartz to SiO(g) followed by oxidation back to SiO2. Usually, a reducing agent (C, Si, H2, CH4, or NH3) is used to increase the rate of quartz decomposition. The oxidation is done with steam, air, or mixtures of both. Previous research conducted by Addona et al.1-3 demonstrated the technical feasibility of making fumed silica using a transferred arc system, but was unable to demonstrate the special rheological properties of the powder. The current study considered two hypotheses to explain this deficiency: (1) the lack of a physical agglomerator, which is present in the industrial process and was thought to cause a tangling of the chain structure of the product and an increase in its thickening and * To whom correspondence should be addressed. E-mail:
[email protected].
thixotropic properties, and (2) the surface chemistry of the plasma produced fumed silica was different from the commercial material. 2. Experimental Section In order to produce the samples of fumed silica needed, a reactor system, originally used by Addona,4 was refurbished and modified to allow more reliable and flexible operation. This reactor was operated near the optimum operating conditions identified by Addona. Seventeen plasma experiments were carried out to examine the effect of quenching by steam, air, and a mixture of steam and air. The plasma power ranged from 5 to 10 kW, and the production rate of powder was from 0.32 to 0.51 g/min. The first seven experiments were pioneering tests in which not all the important parameters could be measured and controlled. The analysis of the data was thus based on the latter experiments. For these experiments the plasma power and product production rate were essentially constant at 7.7 kW and 0.43 g/min. The run time was from 90 to 120 min, and only the quench conditions were changed. There was a slight reduction in production rate with air quench (experiments 14 and 16) because a backflow of oxygen made the conditions within the reactor less reducing and thus reduced the decomposition rate of SiO2 to SiO(g). A summary of the conditions is given in Table 1. An agglomerator was designed and built5 to test the hypothesis that agglomeration of plasma produced material in a way similar to the commercial material would provide the desired rheological properties. The design was based on the commercial plant but downscaled to treat small amounts of powder. The commercial agglomerator consisted of a series of aluminum pipes, each 12 in. in diameter and 16 ft long. There were 16 of these pipes, connected in series by U bends. The operation of the laboratory-scale agglomerator was different from the commercial unit in two aspects. (1) The temperature used was ambient temperature, which was not felt to be a problem since the commercial unit operated at only a few hundreds of degrees, well below the point where the physical properties of the fumed silica would be changed. (2) The other difference was the mass loading ratio of powder to gas, which was limited to very low values by the small quantities of experimental powder produced. Commercial material was used to test the system and identify suitable operating conditions and procedures. The material used for the agglomera-
10.1021/ie701724s CCC: $40.75 2008 American Chemical Society Published on Web 08/06/2008
Ind. Eng. Chem. Res., Vol. 47, No. 17, 2008 6791
Figure 1. Typical groups on the surface of fumed silica particles: (a) isolated hydroxyl group, (b) siloxane group, (c) hydrogen-bonded hydroxyl groups, and (d) geminal hydroxyl groups. Table 1. Summary of Experiments quench flow expt time (min) current (A) voltage (V) air (slpm) steam (g/min) 8 9 10 11 12 13 14 15 16 17
120 118 120 111 110 120 120 120 90 110
170 175 170 170 170 175 168 170 166 170
40 41 43 46 47 47 48 48 47 45
27 33
62 110 50 62 60 19 115
30 27
22
tion tests was the material from experiments 11 and 16, as representative of steam and air quenched material. As noted above, the agglomerator operated at much lower solids/gas loading ratios than the commercial plant (0.025 vs 0.14). To verify if the loading ratio has any effect on the particle agglomeration and on the powder’s rheological properties, a small-volume (200 mL) mechanical blender was also used to agglomerate the powders. For these tests the same loading ratio as the commercial process was used. The necessary amount of powders was poured into the blender and mixed with the air for 1 min. 3. Results and Discussion The products from each experiment were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), BET specific surface analysis, Fourier transform infrared (FTIR) DRIFT analysis, and rheological tests. A commercial product, Aerosil 200 from Degussa, was used as a standard for comparison. Examples of XRD diffraction patterns are shown in Figure 2 for both the commercial material and the plasma produced material. It is clear that the patterns are very similar. The lack of any distinct peaks shows that both materials are amorphous. SEM analysis was used to examine the morphology of the fumed silica agglomerates. In Figure 3, in both cases (a, the commercial product, and b, the experimental powder) an accelerating voltage of 2.0 kV and the upper detector were used on uncoated samples. The working distance used was 6 mm. The particles shown in Figure 3 consist of large agglomerates of the individual nanoparticles since SEM cannot resolve individual particles. The images of both types of material are very similar, and this was typical of many different fields examined. TEM was used to provide information about the primary particle shape and size and also about the aggregate structure. The TEM pictures showed that fumed silica was built up of many primary particles (Figure 4). The primary particles form
Figure 2. XRD diffraction patterns of (a) commercial fumed silica and (b) plasma produced fumed silica.
a network. Practically, no isolated particles were observed; the particles lined up with each other to form irregular chains. This chainlike structure is a very important property of commercial fumed silica. The plasma produced material had a slightly broader size distribution of individual particles (it contained some particles as large as 50 nm) than did the commercial material, which consisted primarily of particles about 12 nm in diameter. This wider size distribution of the primary particles in the case of the experimental powders is thought to be due to the fact that the plasma reactor and quench zone conditions (SiO(g) concentration, mixing intensity, etc.) have not been optimized. Specific surface area measurements were made on all samples using BET. The results ranged from 105 to 263 m2/g compared to 200 m2/g for Aerosil 200. With a steam quench, the specific surface area increased with increasing steam flow rate, showing that smaller particles were formed at higher quench rates as expected.
6792 Ind. Eng. Chem. Res., Vol. 47, No. 17, 2008
Figure 3. SEM pictures of (a) commercial fumed silica and (b) plasma produced fumed silica.
The two most important characterization tests were the rheological tests and the diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy tests. Standard rheological tests were carried out for all plasma produced materials and compared to Aerosil 200. For the rheological tests, 2% by weight suspensions in Clearco Pure Silicone Fluid 1000 cSt were prepared. This fluid is a 100% polydimethylsiloxane that is clear, colorless, and odorless. The mixing time was 10 min, which was the time required to produce a suspension. The mixer used was a normal kitchen blender for which the blades were modified to fit to a small volume cup (30 mL of suspension was prepared for each test). The tests were done using an ASTM Standards Test (D2196-05) and a Nametre 1000 Rotary-B Viscometer, at 21 °C. All parameters except the powder sample were kept constant in these tests. In all cases apparent viscosity values decreased with increasing shear rate (Figure 5). The experimental powders all showed lower maximum viscosities and a lower decrease in viscosity with increasing shear rate than the commercial material. This was also observed by Addona.4 It is important to recall that, when added in a liquid system, fumed silica provides thickening and thixotropic properties, which depend largely on the tendency of the fumed silica aggregates to link together into chains through hydrogen bonding. To form these bonds, the oxygen from the hydroxyl groups must not be involved in another hydrogen bond. For the same particulate structure, the viscosity of the system will be higher if the fumed silica surface has more isolated hydroxyl groups than other types of groups. Conversely, the viscosity values will be low if the isolated hydroxyl groups are absent from the silica surface or if they are already involved in hydrogen bonding with another molecule. The slight drop in apparent viscosity with increasing shear rate is an indicator of
Figure 4. TEM pictures of (a) commercial fumed silica and (b) plasma produced fumed silica.
Figure 5. Apparent viscosity as a function of rheometer shear rate. Table 2. Important IR Peaks in the Silica Spectrum OH (isolated) OH (bridged) SiOH (combination oscillation) SiO
3750 cm-1 3000-3800 cm-1 4550 cm-1 1500-1900 cm-1
the presence of small amounts of isolated hydroxyl groups since it is the breaking of the hydrogen bonds that lowers the viscosity. It was found that the maximum apparent viscosity of plasma produced samples increased with powder specific surface area, but even when the specific surface area of our samples was greater than that of Aerosol 200, the viscosities of plasma produced material was lower. The small values of the apparent viscosity of the plasma produced samples is indicative that these
Ind. Eng. Chem. Res., Vol. 47, No. 17, 2008 6793
Figure 6. Comparison of IR Spectra of Aerosol 200 and typical plasma produced material.
hydroxyl groups are already involved in a hydrogen bond with another molecule, most probably with a water molecule. The experimental fumed silica powders and Aerosil 200 were also analyzed using DRIFT spectroscopy. The samples were diluted in KBr powder, a transmitting matrix. The sample was ground with the KBr, until an evenly dispersed mixture was obtained. The mixture was then placed in the sample holder. The sample concentration was 5% by mass of fumed silica in KBr. The most important peaks in the silica spectrum are presented in Table 2. The FTIR spectra of Aerosil 200 and the experimental powders were different. The differences are in the 3000-3800 cm-1 regions of the spectra (see Figure 6) and are believed to be due to the differences in the type and amounts of surface hydroxyl groups present. The peak at around 3750 cm-1 is due to the free hydroxyl groups. This peak is present in the case of Aerosil 200 but was very weak or not present in the plasma produced fumed silica powders. Thus the experimental powders have much less free surface hydroxyl groups, and this can explain the reason for the lower viscosity values obtained for the experimental powders during the rheological tests. Thermogravimetric tests in the temperature range 50-800 °C confirmed the DRIFT analyses. These tests showed that the overall mass loss for plasma produced samples was on the order of 8% compared to 4% for Aerosil 200. Most of this loss was at temperatures below 120 °C, suggesting the loss of adsorbed water. Aerosil 200 lost weight at around 450 °C which, according to Khalil,6 is due to the loss of the free hydroxyl groups, in the form of water molecules. The conclusion again is a lack of free hydroxyl groups in the plasma produced fumed silica powders. After agglomeration, the powders were again analyzed using SEM, TEM, DRIFT spectroscopy, and viscosity tests. Figure 7 shows an SEM image of the plasma produced fumed silica after the agglomeration test. Some of the agglomerates are larger than in the untreated material (see Figure 3b). The TEM samples were prepared by dispersing the powder in ethanol and then spraying it on a grid. Since we wanted to determine the effect of agglomeration, the samples subjected to agglomeration were only lightly dispersed. Thus, in Figure 8, the dark areas are due to the presence of many layers of particles on the grid. We can also observe some areas where the sample is more dispersed. In these areas we can observe that the agglomeration had no effect on the particle fine structure; the chainlike structure of the initial product has been preserved. Since the agglomeration used here is purely a physical process, no changes in surface chemistry or fine structure were
Figure 7. SEM picture of plasma produced fumed silica after agglomeration test.
Figure 8. TEM pictures of plasma produced fumed silica after agglomeration experiments.
Figure 9. IR spectra plasma produced fumed silica before and after agglomeration.
expected and none were found. We can also observe the same wide size distribution of the primary particles observed before agglomeration (Figure 4b). Figure 9 presents DRIFT spectra of the experimental powders, before and after agglomeration. These spectra show no significant change in the type and amount of groups present on the particle surface (i.e., the agglomeration process does not significantly alter the surface chemistry of fumed silica), as was expected. This is due to the fact that the agglomeration took place at room temperature (21 °C), which did not allow any modification in the particle surface chemistry. This kind of
6794 Ind. Eng. Chem. Res., Vol. 47, No. 17, 2008
Figure 10. Apparent viscosity of the same experimental powder before and after agglomeration.
modification required higher temperatures and humid air. The air used for the agglomeration tests was supplied from gas cylinders and contained less than 4 ppm of water. The same rheological tests described above were carried out after agglomeration. The values of apparent viscosity before and after agglomeration were very close (see Figure 10), and a hypothesis test based on three samples showed that, at the 95% confidence level, the initial viscosities of the samples before and after agglomeration were the same as were the variances in these viscosities. This analysis supports the hypothesis that the enhanced rheological properties of fumed silica are not due to agglomeration but are due to the presence of hydroxyl groups on fumed silica, which are not present to a sufficient degree in the plasma produced powders. As has been noted above, the experimental evidence was strong that the free hydroxyl groups are not produced in the transferred arc plasma process, at least at the conditions examined, in the amount needed to give plasma produced fumed silica its desired rheological properties. An obvious difference in the chemistry of the flame process and the plasma process is the lack of chlorine during the quench, and there is some evidence that the presence of chlorine may lead to the formation of these free hydroxyl groups.7 To examine this further, we modified an existing flow reactor to permit the treatment of plasma produced powder with HCl (the form of chlorine present in the quench of the flame process) at a temperature of 500 °C. Samples of 0.2 g of plasma produced fumed silica were placed in an alumina crucible inside a quartz tube in a muffle furnace. A mixture of two gas streams at atmospheric pressure consisting of water-saturated nitrogen at 22 °C and nitrogen containing 15 ppm by volume HCl was allowed to flow over the sample at 500 °C for 1 h. After that the furnace was allowed to cool with the gas still flowing. The total gas flow rate was 0.0333 mol/min with mole fractions of 0.9804, 0.019 60, and 1.28 × 10-7 of nitrogen, water, and HCl, respectively. During the time at high temperature, each mole of fumed silica was exposed to 11.8 mol of water vapor and 0.002 23 mol of HCl. The samples were analyzed using DRIFT spectroscopy, and as is shown in Figure 11 the treatment produced the free hydroxyl groups which had been absent before. The rheological tests performed on the treated samples showed that the product quality was improved by the posttreatment (Figure 12).The material was now essentially identical to the commercial product Aerosil 200.
Figure 11. IR spectra of commercial fumed silica and HCl treated plasma produced fumed silica.
Figure 12. Apparent viscosity of the same experimental powder before and after treatment.
4. Conclusions The thermal plasma production of fumed silica in a transferred arc consists of the decomposition of quartz to SiO(g) and oxygen followed by an oxidizing quench back to SiO2. The particles formed have diameters on the order of 10-20 nm and are linked in a three-dimensional branched-chain aggregate. Previous work by Addona and Munz1 demonstrated the technical feasibility of producing fumed silica using this method, but was unable to demonstrate the special rheological properties of the powder. The most important characteristic of fumed silica is the presence of hydroxyls on the surface of the particles, in the form of isolated hydroxyl groups, hydrogen-bonded hydroxyl groups, and siloxane groups. In the present work, we studied the changes in the powder quality by varying the quench conditions used for the production of the powder and by agglomerating the obtained particles. The fumed silica was agglomerated by conveying in a length of tubing with sharp bends. The powder was characterized using BET, viscosity tests, FT-IR, TEM, SEM, XRD, and thermogravimetric analysis. The product was compared to a commercial product (Aerosil 200). Tests done before and after the agglomeration experiments showed that the agglomeration had no effect on the powder’s rheological or other properties. We concluded that the inferior rheological properties of the plasma produced fumed silica were due to the lack of the free hydroxyl groups from the surface of the particles. A posttreatment of plasma produced powder showed that the surface chemistry was modified and the free hydroxyl groups were introduced on the surface of the particles.
Ind. Eng. Chem. Res., Vol. 47, No. 17, 2008 6795
The plasma process looks promising for the production of different grades of fumed silica with the addition of a small amount of HCl to the quench or perhaps an optimized posttreatment with a small amount of HCl. In the flame hydrolysis process HCl is generated as byproduct (1 mol of silica produces 4 mol of HCl) and must be recycled in an integrated process to generate the SiCl4 used as raw material. The on-site generation of the SiCl4 is preferred because the material is dangerous to transport. Avoiding the production and use of large quantities of HCl makes the plasma process much simpler and potentially reduces capital and operating costs. Acknowledgment The financial contributions of the Natural Sciences and Engineering Research Council of Canada and SITEC Sec are gratefully acknowledged. Literature Cited (1) Addona, T.; Munz, R. J. Silica decomposition using a transferred arc process. Ind. Eng. Chem. Res. 1999, 38 (6), 2299–2309.
(2) Addona, T.; Proulx, P.; Munz, R. J. Mathematical modelling of silica anode decomposition. Plasma Chem. Plasma Process. 2000, 20 (4), 521– 553. (3) Addona, T.; Munz, R. J. Silica nanopowder generation via a transferred arc thermal plasma process: the effect of oxidation zone parameters on powder properties. 15th International Symposium on Plasma Chemistry, Orleans, France, July 9-13, 2001, Symposium Proceedings; 2001; Vol. VII, pp 2699-2704. (4) Addona, T. The Study of a Novel Plasma Process For The Production of Fumed Silica. Ph.D. Thesis, Department of Chemical Engineering, McGill University, Montreal, 1998. (5) Pristavita, R. Transferred Arc Production of Fumed Silica: Rheological Properties. M.Eng. Thesis, Department of Chemical Engineering, McGill University, Montreal, December, 2006. (6) Khalil, A. M. Thermal Treatment of Aerosil 200 Silica: Induced Surface Porosity and Surface Chemistry Relative To The Heat of Immersion In Water. Surface Technol. 1981, 14, 383–390. (7) Khavryutchenko, V.; Khavryutchenko, Al. Fumed Silica Synthesis: Influence of Hydrogen Chloride on the Fumed Silica Particle Formation Process. Macromol. Symp. 2003, 194, 253–268.
ReceiVed for reView December 18, 2007 ReVised manuscript receiVed June 16, 2008 Accepted June 25, 2008 IE701724S
