Research & Development

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August 2009

Properties of sodium silicate bonded sand hardened by microwave heating

Male, born in 1962, professor. He is currently the advisor of doctoral candidates in Huazhong University of Science and Technology, and his research interests are mainly focused on green casting technology of sodium silicate bonded sand and precise forming technology of Mg/Al alloys.

E-mail: fanzt@mail.hust.edu.cn

Received: 2008-11-10; Accepted: 2009-03-10

*Fan Zitian

Wang Jina, *Fan Zitian, Zan Xiaolei and Pan Di(State Key Laboratory of Material Processing and Die and Mould Technology, Huazhong University of Science and Technology, Wuhan

430074, China)

Abstract: The sodium silicate bonded sand hardened by microwave heating has many advantages, such as low sodium silicate adding quantity, fast hardening speed, high room temperature strength, good collapsibility and certain surface stability. However, it has big moisture absorbability in the air, which would lead to the compression strength and the surface stability of the sand molds being sharply reduced. In this study, the moisture absorbability of the sodium silicate bonded sand hardened by microwave heating in different humidity conditions and the effect factors were investigated. Meanwhile, the reasons for the big moisture absorbability of the sand were analyzed. Some measures to overcome the problems of high moisture absorbability, bad surface stability and sharply reducing strength in the air were discussed. The results of this study establish the foundation of green and clean foundry technology based on the microwave heating hardening sodium silicate sand process.

Key words: microwave heating hardening; sodium silicate bonded sand; moisture absorbabilityCLC number: TG221 Document code: A Article ID: 1672-6421(2009)03-191-06

Green and clean production is the developing trend of the foundry industry in the 21st century. Some experts

consider the sodium silicate bonded sand has the greatest potential to achieve the green casting production [1, 2]. The key technology is to solve the two puzzles of the sodium silicate bonded sand processing, one is the bad collapsibility and the other is the diffi culty of reclamation of used sand [2-4]. Studies and practices have shown that the most effi cient method is to lower the adding quantity of the sodium silicate binder [3-5]. The preliminary experimental results showed that the bonding potential of sodium silicate binder could be exerted adequately under the microwave heating hardening. Thus the adding quantity of sodium silicate binder is reduced signifi cantly [6].

Microwave heating has some advantages, such as rapid heating rates, uniform heating, energy-saving and clean production [7-11]. The fundamental of microwave heating is shown in Fig.1 [12, 13]. Once switches 1 and 2 were connected to contacts 3 and 4 respectively, the upper and lower plates of the capacitor would be negatively and positively charged, forming an electric field between the two plates. The polar water molecules in the beaker moved directionally along the direction of this applied electric field. At the same time certain orderly arrays were formed during their random

thermal motions, and the water in the cup was polarized macroscopically under the influence of the outside electric field. When the switches changed contacts, 1 to 4 and 2 to 3, an electric field with same intensity but in the opposite direction would be established between the upper and lower plates. Then the polar water molecules in the beaker would also be polarized in the opposite direction. If the switches reversed the connections rapidly, the water molecules in the beaker would align rapidly in alternate directions, so molecular friction would happen and part of the electrical energy would be converted to heat energy due to the molecules’ random thermal motion, and raise the system temperature accordingly.

Because the wavelength of the microwaves could be from 1 to 1,000 mm, with frequencies ranging from 300 GHz to 300 MHz [6], the microwave field could change the direction of the applied electric fi eld with a very high frequency. This would make the dielectric polar molecules swing rapidly, and a uniform heating effect would be achieved in a short time [14]. During the microwave heating process, a great deal of the water molecules in the sodium silicate bonded sand samples could evaporate fastly. Therefore, a high strength of the sodium silicate bonded sand could be achieved owing to the dehydrating hardening.

Fig. 1: The fundamental of microwave heating

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As far as the hardening methods are concerned, the adding quantity of sodium silicate binder is 5%– 6% (by weight of the sand) when the sodium silicate bonded sand was hardened by CO2, it is reduced to 2.5% – 3.5% by ester hardening [14], while the adding quantity of sodium silicate binder is only 1.5% – 2% for microwave heating hardening. This will basically solve the puzzles of the sodium silicate bonded sand process mentioned above [1]. Therefore, this microwave heating hardening process would open a new avenue to green and clean foundry production.

The main problem of the microwave heating hardening sodium silicate bonded sand process is the strong moisture absorbability in the air, which would lead to the sharply reduction of the surface stability and compression strength of the sand samples. Therefore, studying and testing the moisture absorbability of sodium silicate bonded sand hardened by microwave heating has significant theoretical and practical meaning for the production application of the microwave heating hardening sodium silicate bonded sand process.

1 Experimental materials and methodsThe raw sand was coated with sodium silicate binder to form the specimens. In the experiment, the raw sand was Dalin scrubbed sand (50/100 meshes) with a sediment percentage of less than 0.3%, the module of the sodium silicate binder is 2.3, and the Baume degree is 47 ºBe. The specimens were heated in a microwave oven with a dehumidifi cation device and three power levels 700 W, 1,400 W and 2,000 W. The samples were fabricated manually using a cylindrical mold. Each specimen was cylindrical in shape with 30 mm in diameter and 30 mm in height. The compression strength and residual strength testing of the sand samples were carried out on lever-type universal strength tester. The surface stability was characterized using a rotating-screen surface performance tester. An electronic analytical balance, with an accuracy of 0.001 g, was used to quantify the absorbed moisture of the sand samples [15]. The microstructure of the sand samples’ bonding bridge was determined on selected specimens using scanning electron microscopy (SEM).

For comparison the ester hardening sand samples were also made. In the ester hardening process, the addition of the organic ester was 10% of the sodium silicate. The sand grains were mixed with the ester and sodium silicate binder, and then the samples were made and holding for 24 h until hardened completely. In the microwave heating process, the samples were put into the microwave oven together with the mold and heated for several minutes; then they were taken out after eliminating moisture for 2 minutes. The room temperature strength, residual strength, surface stability and moisture absorbability of the hardened samples were tested. Room temperature strength (vb) is the compression strength of samples cooled to the room temperature after microwave heating, while the residual strength (v800ºC) is the compression strength of samples tested after the sand samples are put into a furnace and held at 800℃ for half an hour, and then cooled to

the room temperature in the air. The testing method for surface stability is as follows: the

original weight (M0) of the sand sample after microwave heating was measured fi rst, then the sample was put into the rotating-screen surface performance tester for 1 minute before getting a fi nal weight (M1), and the surface stability of sand sample, , was obtained by

= (M1 / M0) × 100 % [16].The moisture absorbability of a sand sample is expressed

by the quantity of moisture absorbed by the sand sample in a certain period of time. After measure the original weight (M0), for comparison, the samples were put into a humidistat (humidity at 98% – 100%) and the air (humidity at 80% – 85%) for several hours, separately. Finally, re-measured the weight (M2) of the samples. Then the quantity of moisture absorbed by the sand sample is ∆M = M2 – M0. Meanwhile, the compression strength and surface stability of the sand samples were tested.

2 Results2.1 The room temperature strength,

collapsibility and surface stability(1) The room temperature strength and collapsibilityThe room temperature strengths and residual strengths of

sand samples under different microwave power, heating time and binder adding quantity are shown in Table 1, and that for samples hardened by ester is shown in Table 2 for comparison.

As shown in Table 1, the room temperature strengths of the sand samples with 2% binder, heated for 120 s with 700 W or heated for 90 s with 1,400 W by microwave are equal to or higher than the strengths of the sand samples with 3% binder hardened by ester, and the similar strength vb can also be obtained by adding 1.5% binder heated for 90 s with 2,000 W hardened by microwave. Moreover, the room temperature strength of sand samples with 2% binder heated for 90 or 120 s with 1,400 or 2,000 W by microwave far surpassed the strength of the sand samples with 5% binder hardened by ester. Therefore the sand with 1.5% – 2% sodium silicate binder hardened by microwave heating could meet the requirements of the foundry production in the hardened strength standard.

Table 1 shows that the residual strength of sand samples hardened by microwave heating is greatly affected by the quantity of added binder. However, the effects of the heating power and time on the residual strength are slight.

Tables 1 and 2 reveal that the collapsibility of sodium silicate sand hardened by microwave heating is superior to that of sand hardened by ester on the premise of same room temperature strength. The residual strength of sodium silicate bonded sand with 2% binder hardened by microwave heating is only 1/3 – 1/4 of sodium silicate bonded sand with 3% binder hardened by ester. The former is close to the residual strength of resin bonded sand [16-18]. Consequently, the collapsibility problem of sodium silicate bonded sand could be resolved basically by applying the microwave heating

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Table 1: Room temperature strength σb and residual strength σ800℃ of sodium silicate sand hardened by microwave heating under different microwave power N, heating time t and sodium silicate binder adding quantity α

Table 3: The effect of quantity of sodium silicate added α on the surface stability φ of sand samples hardened by microwave heating and the comparison with that of hardened by ester

Table 2: The room temperature strength σb and residual strength σ800℃ of sodium silicate bonded sand hardened by ester with different quantities of added sodium silicate binder α

N (W) 700 1,400 2,000

t (s) 30 60 90 120 30 60 90 120 30 60 90 120

σb

(MPa)

α = 1% 0.04 0.13 0.38 0.89 0.07 1.42 0.95 0.83 0.19 1.17 0.70 0.66

α = 1.5% 0.05 0.12 0.81 1.20 0.12 1.11 1.2 1.61 1.14 1.46 1.78 1.96

α = 2% 0.05 0.06 1.31 1.72 0.07 0.28 2.31 2.50 0.01 1.78 2.50 2.74

σ800℃ (MPa)

α = 1% 0.01 0.01 0.02 0.01 0.01 0.01 0.02 0.02 0.01 0.01 0.01 0.02

α = 1.5% 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.02 0.03 0.03 0.02 0.03

α = 2% 0.06 0.06 0.06 0.07 0.06 0.07 0.06 0.06 0.06 0.07 0.06 0.07

α (%) 3 4 5

σb (MPa) 1.72 1.89 2.06

σ800℃ (MPa) 0.20 0.39 1.05

hardening process.(2) Surface stabilityThe surface stability of sand samples affects the casting

quality greatly because poor surface stability would generate cast defects, such as burning-on and sand hole.

Table 3 shows the effect of the quantity of added binder on the surface stability of sand samples hardened by microwave heating, as well as the comparison with that of hardened by ester. The sand samples hardened by microwave heating were heated for 120 s with 1,400 W.

Table 3 shows that the surface stability of sand samples hardened by microwave heating improved with an increase in the quantity of added binder, and the surface stability with 2% binder hardened by microwave heating is close to that of sodium silicate sand with 3% binder hardened by ester. Both surface stabilities are more than 97.5 %, this indicating that both these two methods could meet the requirement of foundry production.

Microwave heating hardened Ester hardened

α (%) 1 1.5 2 2.5 3 3

M0 (g) 32.633 30.821 31.099 30.837 31.114 35.604

M1 (g) 30.512 29.154 30.422 30.390 30.816 34.892

φ (%) 93.50 94.59 97.82 98.55 99.04 98.00

2.2 Moisture absorbability and its effect on the sand samples’ strength and surface stability

(1) Moisture absorbability of the sodium silicate bonded sand hardened by microwave heating

The moisture absorbability is the quantity of moisture absorbed by the sand samples in a certain period of time under defi nite humidity conditions. Practice indicates that the moisture absorbability of the sodium silicate bonded sand hardened by heating is high. The experimental sand samples were heated for 120 s with a 1,400 W power microwave, and for comparison, the hardened sand samples were put into the humidistat (humidity at 98% – 100%) and air (humidity at 80% – 85%), respectively.

Figures 2 and 3 show the absorbed moisture quantity of sand samples with different amounts of sodium silicate binder (1%, 1.5% and 2% by weight) in humidistat and in air as a function of the standing time.

As shown in Figs. 2 and 3, the absorbed moisture quantity Fig. 2: Absorbed moisture quantity of sand samples in humidistat

of sand samples in humidistat and in air both increase as the standing time prolonged. For the same standing time, the more the added binder, the more the absorbed moisture of the sample is. Under the same conditions of time and added binder, the

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Fig. 3: Absorbed moisture quantity of sand samples in air

Fig. 4: Compression strength of sand samples in humidistat

Fig. 5: Compression strength of sand samples in air

sand samples in the humidistat absorb more moisture than the ones in the air. The maximum water uptake is up to 0.336 g (in humidistat) and 0.138 g (in air) in 24 hours.

(2) Effects of moisture absorbability on the strength of sodium silicate bonded sand samples

Figures 4 and 5 show the compression strength of sodium silicate sand samples as a function of the standing time. As shown in Fig.s 4 and 5, the compression strengths of sand samples drop dramatically both in humidistat and in air as the standing time increases. The strengths of sand samples in the

humidistat drop to nearly zero in 24 h and the ones in the air fall to 20% of the original strength.

(3) Effects of moisture absorbability on the surface stability of sodium silicate sand samples

Table 4 shows the surface stability of sodium silicate bonded sand samples held for 24 h in humidistat and air. The sand samples were heated for 120 s by microwave with 1,400 W, and quantity of added binder was 1.5%.

As shown in Table 4, the surface stability of the sand samples held for 24 h are greatly reduced, only 52.08% for samples in the humidistat and 80.00% for samples in the air.

3 DiscussionsThe moisture absorbability of the sodium silicate sand is strong because of the existence of the sodium ion and hydroxyl. The Na+ in the sodium silicate would induce chemical adsorption of water molecules. The adsorbed water would attack the silicon-oxygen bond due to the catalysis of OH-. The silicate mesh structure would be disconnected, and the cohesive strength of the sodium silicate binding fi lm would be greatly reduced. Moreover, the water molecule has characteristics of big polarity and small volume, so the capability of permeation and diffusion is strong. When the water molecules enter into the binding film to hydrate with Na+ and damage the mesh structure, they also quickly permeate and diffuse to the binding interface between the sodium silicate and the sand grain. The adhesion strength is reduced significantly because the adsorption of water molecules substituted physical adsorption of the sodium silicate and the sand grains. The loss of cohesive strength and adhesion strength causes the strength of the sand samples to be greatly reduced. The bonding bridge microstructure and fracture morphology of sodium silicate sand sample before and after absorbing moisture were observed using SEM.

The bonding bridge of the sodium silicate sand hardened by microwave heating before absorbing moisture is very smooth and compact (Fig. 6(a)). Therefore, the sand grains are bonded fi rmly and its strength is high enough. Figure 6(b) shows the sand sample’s bonding bridge after absorbing moisture. The sodium silicate binder re-hydrates after absorbing plenty of water, so the bonding bridge is eroded. At the same time, the cohesive strength of sand sample is severely reduced, some cracks appear on the bonding bridge, and signifi cantly reduce the bonding strength of sodium silicate bonded sand.

Figures 7(a) and 7(b) show the fracture morphology of sodium silicate bonded sand hardened by microwaves. Before

Table 4: The surface stability of sand samples in different humidity circumstances

Circumstance Humidistat Air

M0 (g) 31.882 31.728

M1 (g) 16.605 25.382

φ (%) 52.08 80.00

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Fig. 6: The bonding bridge microstructure of sodium silicate sand sample hardened by microwave heating before (a) and after absorbing moisture (b)

Fig. 7: The fracture morphology of sodium silicate sand hardened by microwave heating before (a) and after absorbing moisture (b)

absorbing moisture, the fracture surface has much remaining hardened binder. But after absorbing moisture, the fracture surface is very clean. This is because the adsorption of water molecules substitutes for physical adsorption of the sodium silicate and the sand grains. The adhesive strength of the sand sample is greatly reduced. Therefore the sand samples’ surface stability was damaged signifi cantly after absorbing moisture.

Some researchers [19] compared the nanometer grade surface topography of sodium silicate colloidal particles hardened by microwave heating and by CO2. It was found that the sodium silicate colloidal particles hardened by microwave heating are very flat with uniform and compact colloidal particles with diameters from 42 to 52 nanometers, and the colloidal particles hardened by CO2 are rough with sparse and far bigger colloidal particles with diameters from 102 to 115 nanometers. Accordingly, the microwave heating process could exert the bonding potential of sodium silicate binder thoroughly and achieve higher bonding strength.

The main reasons for high moisture absorbability of sodium silicate sand hardened by microwave heating are the re-hydrating ability of the sodium silicate binder in the humid

circumstances and the more tiny colloidal particles which would bring large surface and strong moisture absorbability. Therefore the moisture absorbability of sodium silicate bonded sand hardened by microwave heating is higher than that of the sodium silicate bonded sand hardened by CO2 or by ester.

The key technology to put the microwave hardening process into practical application is to reduce the moisture absorbability of the microwave-hardened sand. There are two probable ways:

(1) Develop a water-proof coating of sodium silicate bonded sand to fi lm the surface and seal the hardened sand samples. An alcohol coating fi lm would be a moisture-resistant coating layer to shield against the moisture in the air. If the coating contains the composition of water repellent, it could improve the moisture resistance of sand samples.

(2) Develop a new modifi ed sodium silicate binder to form a hardened ceramic bonding fi lm. The ceramic bonding fi lm presents good moisture-resistant performance. Primary study indicates that adding lithium silicate or an inorganic addition to the sodium silicate could improve the moisture resistance capability of the sodium silicate bonded sand.

(a) (b)

(a) (b)

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This work was fi nancially supported by the National Nature Science Foundation of China under grant No. 50575085.

4 Conclusions(1) The sodium silicate sand hardened by microwave heating

could meet the requirement of room temperature strength in casting production even though the quantity of added sodium silicate is from 1.5% – 2%, and the sand collapsibility is also very good.

(2) The surface stability of the sodium silicate bonded sand hardened by microwave heating gets worse as the binder adding quantity lowers. When the quantity of added binder is 2%, the surface stability of sodium silicate bonded sand hardened by microwave heating is close to that of sodium silicate bonded sand hardened by ester with 3% quantity of added binder.

(3) The moisture absorbability of the sodium silicate bonded sand hardened by microwave heating is quite high. It increases with the increase of the standing time and the quantity of added binder under the certain humidity. It also increases with the ambient humidity for the same quantity of added binder and standing time. The strength and surface stability of the sand samples drop greatly as the quantity of moisture absorbed by the sand samples increases.

(4) The key technology to put the microwave hardening process into practical application is to reduce the moisture absorbability. This needs more effort in developing a water-proof coating of sodium silicate bonded sand and a new type moisture-resistant modifi ed sodium silicate binder.

References[1] Fan Zitian, Dong Xuanpu, and Lu Xun. The Water-glass Sand

Process Principle and Application. Beijing: China Machine Press, 2004: 1-5. (in Chinese)

[2] Fan Zitian, Wang Jina, and Liu Jun. Performance changes and answer strategies of the sodium silicate reclaimed sand hardened with ester after recurrent use. Foundry, 2007, 56(11): 1203-1206. (in Chinese)

[3] Fan Zitian, Huang Naiyu, and Dong Xuanpu. In house reuse and reclamation of used foundry sands with sodium silicate binder. International Journal of Cast Metals Research, 2004, 17(1): 51-56.

[4] Andrade R M, Cava S, Silva S N, et al. Foundry sand recycling in the troughs of blast furnaces: a technical note. Journal of Materials Processing Technology, 2005, 159: 125-134.

[5] Che Guangdong, Liu Xiangdong and Li Jinfu. Effects of water-glass tensile strength and collapsibility of water-glass cured by microwave heating. China Foundry Machinery and Technology. 2006 (6): 18-19. (in Chinese)

[6] Li Huaji and Xie Weidong. Study on the materials used for making models of sodium silicate bonded sand core heating by microwave energy. Journal of Chongqing University (Natural Science Edition), 2002, 25(1): 116-119. (in Chinese)

[7] Menezes R R, Souto P M, and Kiminami R H G A. Microwave hybrid fast sintering of porcelain bodies. Journal of Materials Processing Technology, 2007, 190: 223-229.

[8] Millos C J, Whittaker A G, and Brechin E K. Microwave heating – A new synthetic tool for cluster synthesis. Polyhedron, 2007, 26: 1927-1933.

[9] Mou Qunying and Li Xianjun. Applications of microwave heating technology. Physics, 2004, 33(6): 438-442. (in Chinese)

[10] Benitez R, Fuentes A, and Lozano K. Effects of microwave assisted heating of carbon nanofi ber reinforced high density polyethylene. Journal of Materials Processing Technology, 2007, 190: 324-331.

[11] Basak T. Role of metallic, ceramic and composite plates on microwave processing of composite dielectric materials. Material Science and Engineering A, 2007, 457: 261-274.

[12] Tierney J P and Lidström P. Microwave Assisted Organic Synthesis. Oxford: CRC Press, 2005: 13-15.

[13] Pozar D M. Microwave Engineering. New York, Wiley Press, 1998: 2-4.

[14] Wang Jina, Fan Zitian and Wang Huafang. An improved sodium silicate binder modifi ed by ultra-fi ne powder materials. China Foundry, 2007, 4(1): 26-30.

[15] Casalino G, De Filippis L A C and Ludovico A. A technical note on the mechanical and physical characterization of selective laser sintered sand for rapid casting. Journal of Materials Processing Technology, 2005, 166: 1-8.

[16] Zhou X, Yang J Z, and Qu G H. Study on synthesis and properties of modified starch binder for foundry. Journal of Materials Processing Technology, 2007, 183: 407-411.

[17] Wang Y J, Cannon F S, Salama M, et al. Characterization of hydrocarbon emissions from green sand foundry core binders by analytical pyrolysis. Environmental Science Technology, 2007, 41: 7922-7927.

[18] Ding G L, Zhang Q X and Zhou Y H. Strengthening of cold-setting resin sand by the additive method. Journal of Materials Processing Technology, 1997, 72: 239-242.

[19] Xiao Bo, Xu Zhengda, Wang Xiuping, et al. A new method for the investigation of binding properties of silicate-sand. Journal of Hubei Polytechnic University, 1995, 10(12): 6-9. (in Chinese)