In the 21st century, with more understanding of the importance

An improved sodium silicate binder modified by ultra-fine powder materials *WANG Ji-na 1, FAN Zi-tian 1, WANG Hua-fang 2, DONG Xuan-pu 1, HUANG Nai-yu 1 (1. State Key Laboratory of Plastic Simulation and Die & Mold Technology, Huazhong University of Science and Technology, Wuhan 430074, P. R. China; 2. Wuhan Library of Chinese Academy of Science, Wuhan 430074, P. R. China) Abstract: This paper presents a new method of modifying sodium silicate binder with ultra-fine powders. The sodium silicate binder modified by ultra-fine powder A and the organic B can reduce the addition amount of the binder. The results indicate that the 24 h has increased by 39.9% at room temperature and the residual has decreased by 30.7% at 800, compared to the conventional An available material to improve the moisture resistance was also found by adding about 2% more inorganic C, and it can increase the moist by 20%. In the end, the microanalyses are given to explain the modifying machanism, i. e., the ultra-fine powder A can refine the sodium silicate binder to avoid holes in the binder bond, which can increase the 24 h at room temperture, and can lead to more cracks in the bond after the molding sand is heated to 800. This is because of the stress caused by the new eutectic complex of modified sodium silicate binder. Key words: sodium silicate binder modification; ultra-fine powder materials; bonding ; moisture resistance; collapsibility CLC number: TG221 Document Code: A Article ID: 1672-6421(2007)01-026-05 In the 21st century, with more understanding of the importance of environmental protection, the application of green products for foundry becomes one of the most urgent tasks. In the sand casting process, sodium silicate sand is considered to be the most possible green and clean molding sand, essentially with no toxic gas emission [1]. However, the sodium silicate sand has obvious shortcomings such as poor collapsibility, moisture resistance and reclaimability, which limit the development of the sodium silicate sand process [2]. Many years of research has shown that there are inner links between these shortcomings; for example, more than 4% addition of sodium silicate will lead to poor collapsibility, so the shakeout and reclaimation of the used sand become difficult. And the sodium silicate reclaimed sand has less moisture resistance because of more Na + in the mixture [3]. When the bonding is not sufficiently high after curing, the sodium silicate sand tends to get damp in a humid environment, so the of the sand mold becomes too low for casting. The basic rule is that the amount of the water glass must be kept below 4%, to maintain enough 24 h for a reasonable collapsibility and moisture resistance [4]. The development of new technology such as the ultra-fine powder has opened new way for modifying Ultrafine powders are defined as materials with diameters ranging *WANG Ji-na Female, born in 1982. Doctoral candidate. Research interest: process of modifying sodium silicate bonded sand. E-mail: wjn6048@163.com Received date: 2006-04-14; Accepted date: 2006-08-20 from several nm to several µm. There are many ways of making ultra-fine powders in different industries, with different taxonomies. These powders can be classified as follows: (1) fine powder: particle radius is about 3-20 µm; (2) ultra-fine powder: particle radius is about 0.2-3 µm; (3) ultra tiny powder: particle radius is between the nanometer grade and 0.2 µm [5]. The ultrafine powders have some special characteristics such as tiny size, larger specific surface area, and higher surface activity. If its radius reaches the grade of nanometer material, the nature of the material would greatly change. The physical properties of nanometer materials such as acoustics, electricity, optics, magnetism, thermodynamics and mechanical behavior are very different from the conventional powders [6]. In this paper, a new modified method is introduced and a kind of special ultra-fine powders is used to improve the performance of the sodium silicate binder. This powder material has many traits, such as certain solubility in the liquid sodium silicate, expanding when absorbing water, suspending in the liquid sodium silicate, favorable ability of ionic interchange, and it would absorb more basic ion. Other materials are also added to improve the binding and moisture resistance of the 1 Sample preparation and test methods There are special requirements for the ultra-fine powder material used for modifying The material would not only be able to partially dissolve in the basic liquid sodium silicate or suspend in the sodium silicate solution by a dispersant agent, but also be able to modify the In fact, there is 026

February 2007 only a small fraction of the powders would dissolve into the liquid, and most of them are diffused into the liquid by the disperser. To meet these requirements, three kinds of powders after hyperfine disposal were chosen to modify the sodium silicate. The diameter of the powders is about 2 µm. Some disperser was added in the liquid in order to improve the suspendability. Experiments were carried out to test the performance of different modified sodium silicate, including the compression at room temperature and residual. The additions of the ultra-fine powders were based on the amount of the A specific fraction of ultra-fine powders was added to the sodium silicate, and the mixture was then put on the magnetic stirring apparatus and stirred for 20 minutes. And the powders were dispersed into the sodium silicate completely and equably. The sand samples were made using different modifying sodium silicate, and compared with the conventional sodium silicate sand samples. The ultra-fine powder A is a kind of mineral material with sandwich structure, containing Mg 2+, Al 3+ and many silicon-oxy bonds. The material B is a kind of organic matter which can be dissolved into the sodium silicate infinitely. It can improve the dispersibility and suspension property of ultra-fine powder A in the The material C is a kind of inorganic powder which contains crystal water. It is tend to dehydrate and effloresce in the dry atmosphere. Under high temperature, it will lose crystal water and become water-free salt. And it can be dissolved into the water and The raw sand was standard sand (50/100 meshes). The sand samples were made using cylinder corebox with a diameter of 30 mm and height of 30 mm. The modulus of the sodium silicate was about 2.2-2.3, and the density of the unmodified sodium silicate was about 45 Be. The organic ester was adopted as the solidified agent of the sodium silicate sand. The addition level of the ester was 0.3 wt% of the raw sand, and the sodium silicate was 3 wt% of the raw sand. The samples were tested after the stipulated time using a lever-type omnipotent tester. Compression at room temperature and the residual were tested. The compression was tested on sample A placed for about 24 hours. And the residual was tested after exposing the sample at 800 for an hour and then cooling it to the room temperature in the furnace. As the test environment such as ambient temperature and humidity were not always the same,the test results obtained under the same mold sand and the same mixture ratio would have variations. Therefore, every experiment was compared with the conventional sodium silicate sand under the same condition. 2 Results and discussion 2.1 The effect of ultra-fine powder A on the silicate sand Based on the material structural analysis, the ultra-fine powder A was chosen to modify the And its effect on bonding of the sodium silicate sand was studied. The results are shown in the Table 1. organic material B 1.5% powder A (0% B) 1.5% powder A+0.5%B 1.5% powder A+1.0%B 1.5% powder A+1.5%B 1.5% powder A+2.0%B Research & Development Table 1 The of sodium silicate sand with different adding amount of ultra-fine powder A, MPa ultra-fine powder A 0.5% powder A 1.0% powder A 1.5% powder A 2.0% powder A 2.5% powder A Compression 0.47 0.42 0.47 26, relative humidity: 94%. Table 2 The of sodium silicate sand with different adding amount of organic B, MPa Compression 0.48 0.67 0.70 1.61 1.75 1.95 1.95 2.03 1.86 1.63 1.72 1.83 2.28 1.72 1.67 Residual 1.36 1.14 1.20 0.89 0.92 As shown in Table 1, the optimal addition amount of powder A is about 1.5% (BOB), with a high 24 h and good collapsibility. It can be seen from the results that the ultra-fine powder A could improve the binding and reduce the residual of the When the addition of powder A is about 1.5% compared with common sodium silicate, the 24 h can be increased by 21%, and the residual is lowered by 34.5% for the modified sodium silicate. 2.2 The effect of organic material B on the silicate sand Addition of organic B which can be dissolved into the sodium silicate can improve the transparency of the sodium silicate liquid and the fluidity of the foundry sand. The powder B was added on the base of the optimum addition of 1.5% powder A. Then the effect of organic material B on the of the sodium silicate sand was tested. The results are shown in Table 2. Residual 1.40 0.75 0.97 0.95 1.22 26, relative humidity: 89%. As shown in Table 2, when the adding amount of the powder A is 1.5%, and the adding amount of powder B is about 1%, the high and good collapsibility can be obtained. A combined addition of the materials A and B to sodium silicate is better than the individual addition of ultra-fine powder A. Using 1.5% ultra-fine powder A and 1% organic material B to modify the sodium silicate can increase the 24 h by 027

39.9%. On the other hand, the residual is decreased by 30.7%. The reduction in residual is less than that of only 1.5% powder A is used, but the increase in the 24 h is significantly bigger than that. Therefore, further experiment should be carried out on the basis of adding 1.5% powder A and 1% organic material B. 2.3 The effect of inorganic material C on the silicate The effect of addition of inorganic material C on the of sodium silicate sand is shown in Table 3. In the experiment, also it was found that when adding 1.5% powder A and 1% B, the sand was hardened slowly under the moist condition, and the final was not very high. This is because the moisture absorbability reduced the hardening and the speed. Through experiment, it was found that inorganic material C could improve the moisture resistance of the sodium silicate sand. Inorganic material C is a kind of dehydrated material which could form crystal hydrate with water. It could be dissolved in the Adding powder C to the modified sodium silicate can obviously improve the moisture resistance of the sodium silicate sand as well as the collapsibility. Adding overfull material C could increase the hardening and moist, but the residual is also increased obviously (namely, the collapsibility is worse). In general, the optimum adding amount of material C is 2%. Table 3 The of sodium silicate sand with different adding amount of ultra-fine powder C, MPa microstructures of the samples. The reason that modified sodium silicate could increase the and decrease the residual can be discussed by analyzing the microstructure. In the modified cases, the organic material B and the inorganic material C are dissolved into the sodium silicate solution and react with the sodium silicate, but no reaction occurred for powder A. So the modified sodium silicate after hardening only revealed the diffusion of powder A in the sodium silicate mixture in the SEM field of view. Figure 1 shows the diffusion of ultra-fine powder A in the sodium silicate binder. It can be seen that most ultra-fine powder A is about 2 µm deep and 10 µm long, with a few of them at about 0.1 µm (100 nm). These powders can improve the binding capability of the sodium silicate because of their high surface activation. The powder A has a sandwich structure, so some liquid sodium silicate entered into the inter space. Therefore, powder A could refine the sodium silicate micelle and enhance the. But the powder particles are not fine enough, so the increase in hardened is limited. From the above analysis, it can be seen that the best result of ultra-fine powder diffusing in the sodium silicate is mutual dispersion with each other which needs adequate fine powder. If the dimensions of powder A all reaches nanometer grade, the interlayer spacing of the powders is about 1 nm. The size of the colloidal sodium silicate particles are less than 1 nm. By means of certain mixing mode such as high-speed stirring, the effect of the mutual dispersion becomes more obvious. Additionally, it imposes energy on sodium silicate, which solves the problem of aging. When working along both lines, the of the sodium silicate sand would be increased significantly. ultra-fine powder C Compression Wet Residual 100 nm powder 1.5% powder A+1%B+2%C 1.5% powder A+1%B+3%C 1.5% powder A+1%B+4%C 0.50 1.10 1.15 1.70 2.20 2.32 2.36 1.40 1.68 1.78 2.00 1.60 1.60 1.80 Layer of the powder 17, relative humidity: 88%. Disposed material C could protract the hardening time of the On the other hand, it improves dispensability of ultra-fine powder A in the sodium silicate and the moisture resistance of It hasn t affected the ultra-fine powder A to improve the collapsibility of the 2.4 Microanalysis The above test results indicate that an optimal condition is reached when about 1. 5% powder A, 1% B and 2% C are added, high 24 hour and good collapsibility, good moisture resistance can be achieved. The microstructure of the binder on the sodium silicate sand particles was observed for analysis of the powder behavior in modifying the Figure1 to Fig.7 show the 10.0 µm Fig. 1 The diffusion of the ultra-fine powder A in the sodium silicate binder Figure 2 and Fig.3 show the microstructure of the binding bridge. In Fig.2, there are many cracks and holes in the bridge because of dehydrating shrinkage of the binder. But fewer cracks or holes are observed in Fig.3. The result is that the interaction between the sodium silicate and ultra-fine powder A increases its plasticity, thus diminishing the possibility of cracks. And the action between layers refined the micelles, namely the sodium silicate with high module or the silicate colloid which has already generated but has not dissolved into the sodium silicate inserts into the interlamination of the powder A. Thereby the disperse state of substance which was inserted was changed. So the possibility of producing holes is diminished while the 24 h of modified sodium silicate is increased. 028

February 2007 Fig. 2 The bonding bridge of common sodium silicate Research & Development Figure 6 and Fig.7 show the microstructure of binder fracture after being heated to 800 for an hour and cooled to the room temperature in the stove. It can be seen from Fig.6 that there are only gas cavities in the bond fracture of the common sodium silicate. In the Fig.7, there are not only more gas cavities, but also irregular external structure which is different from the bonding film (slippery) of the common Preliminary analysis indicates that there possibly exists stress that rooted in the eutectic complex which is generated by the ultra-fine powder and sodium silicate under high temperature. That's because the sodium silicate bonding film react with Mg 2+ or Al 3+ in powder A and generate high melting-point phase or glass phase eutectic complex. Its expansibility is disaccorded with the vitreous body, which results in great depressing of mechanical intensity and heat stability of vitreous body, so the residual is reduced. hole Fig. 3 The bonding bridge of modified sodium silicate Figure 4 and Fig.5 show the microstructure of bonding bridge after being heated to 800 for an hour and then cooled to the room temperature in the stove. The bonding bridge of common sodium silicate is very compact without flaws, so the is high. But there are cracks in Fig.5 which decreased the residual and improve the collapsibility. Fig. 6 The microcosmic fracture of bonding bridge after being heated to 800 for an hour of hole crack stress zone Fig. 4 The bonding bridge after being heated to 800 of Fig. 5 The bonding bridge after being heated to 800 of modified sodium silicate Fig. 7 The microcosmic fracture of bonding bridge after being heated to 800 for an hour of modified sodium silicate The improvement of moisture resistance depended on the inorganic material C chiefly. The inorganic material C can be dissolved into the sodium silicate infinitely, it didn't achieve the stable structure of the crystal water after the sodium silicate hardened. So it will combine the redundant water in the sodium silicate and form stable crystal hydrated mass, which can be enchased into the bonding film of the The crystal mass has definite, so it could increase the normal temperature and moisture resistance. For the moisture resistance, the SEM photo can t show the crystallization water of material C after the samples are heated to 800, so the microstructure analysis isn t given. 029

But the moisture has increased by 20% as shown in Table 3. 3 Conclusions Some conclusions can be drawn as follows: (1) The ultra-fine powder A is able to increase the 24 h and reduce the residual. Adding 1.5% powder A can increase the 24 h by 21% and decrease the residual by 34.5%, which is the best result. (2) The organic material B can help the ultra-fine powder A to improve the properties of the sodium silicate, not only 24 h but also residual. The 24 h increased by 32.6% and the residual decreased by 38.2% when 1.5% ultrafine powder A and 1% organic material B are added. (3) The disposed inorganic material C can extend the hardening time and improve the diffusion of the ultra-fine powder A in sodium silicate so as to enhance the moisture resistance without affecting powder A s ability to improve the collapsibility of (4) The better properties of the modified sodium silicate can be reached after the particle of the powder A is milled to nanometer size, thus adding amount can be reduced. When the humidity is high, material B had better not to be added. And References [1] [2] [3] [4] [5] [6] when the humidity is low, the material C had better not to be added. FAN Zi-tian, WANG Hua-fang, et al. Dry reusing and reclaiming of used sodium silicate sand. China Foundry, 2005, 2 (1): 38-43. FAN Zi-tian, DONG Xuan-pu, XUN Lu. The Water-glass Sand Process and Application. Beijing: China Machine Press, 2004. ZHU Chun-xi, LU Chen. The Basic Theories of Water-glass Sand. Shanghai: Shanghai Jiaotong University Press, 2000. FAN Zi-tian, HUANG Nai-yu, DONG Xuan-pu. In-house reuse and reclamation of the used foundry sands with sodium silicate binder. International Journal of Cast Metals Research, 2004, 17 (1): 51-56. ZHANG Guo-wang, HUANG Shen-sheng. Application of ultrafine powder preparation technology and its development. Express Information of Mining Industry, 2002, 397(1): 1-3. ZHANG Li-xin, LIU You-zhi. Properties, preparation and application of ultra-fine powder. Journal of North China Institute of Technology, 2001, 22(1): 38-41. The subject is supported by National Natural Science Fund of China: 50575085. 030