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Powder Technology 210 (2011) 323–327

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Powder Technology
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p ow t e c

Large scale and environmentally friendly preparation of micro-submicron spherical silica and their surface effect in resin materials
Changchun Ai a, Yong Xiao a, Wen Wen b, Liangjie Yuan a,?
a b

College of Chemistry and Molecular Science, Wuhan University, Wuhan 430072, China College of Chemical Engineering, Wuhan Textile University, Wuhan 430074, China

a r t i c l e

i n f o

a b s t r a c t
Amorphous micro-submicron spherical silica powders with different particle sizes and surface properties were prepared by turbulent ?ow cycle method and characterized by inductively coupled plasma atomic emission spectrometry (ICP-AES), inductively coupled plasma mass spectrometry (ICP-MASS), X-ray diffraction (XRD), particle size distribution (PSD), speci?c surface area (SSA) analysis and scanning electron microscopy (SEM). The effects of particle size and surface property of different spherical silica on the ?uidity of resin materials were conducted. The result showed that the spherical silica with the characterization of D50 = 2.5 μm and the ratio of experimental speci?c surface area and calculated speci?c surface area is 2.38, played a better role in the ?ow ability of epoxy materials. So it may be a better choice for the preparation of epoxy materials used in IC packaging area and other high-tech ?elds. ? 2011 Elsevier B.V. All rights reserved.

Article history: Received 20 January 2011 Received in revised form 25 March 2011 Accepted 3 April 2011 Available online 9 April 2011 Keywords: Turbulent ?ow cycle Spherical silica Surface effect Epoxy materials

1. Introduction Silica (SiO2) is one of the basic raw materials in aerospace, information industry, automobile, metallurgy, chemical industry, construction, energy and other areas. In order to improve the mechanical and electrical properties of resin materials as electronic information materials, many kinds of silica powder were considered and the effects of their particle shape, particle size distribution and crystal type were also investigated [1–4]. Due to the controllability of particle size, speci?c surface area, high purity, crystal type, low content of radioactive elements, spherical SiO2 is widely used in the photo-electronic information materials, special engineering plastics and daily chemicals. As raw materials used in electronic packaging materials, the technical indications of spherical SiO2, such as particle size distribution, morphology and speci?c surface area (SSA), impurities and radioactive element content, linear expansion coef?cient (CET), tap density, electrical conductivity and pH value of water extract solution should be strictly controlled. In a word, the batch preparation technology of spherical silica particles is of dif?culties. And so far, spherical silica powder is prepared by the method of high temperature melting sputtering [5,6], vapor phase synthesis [7], the combustion and atomization of metal silica, sol-gel process [8] and the hydrolysis of TEOS, TMOS [9–11] or silicon tetrachloride [12], etc. The former methods are of high-energy

consumption and high emissions of carbon dioxide. The raw materials TEOS, TMOS are expensive and silicon tetrachloride will cause heavy chemical pollution. In this paper, an environmentally friendly and low cost method was designed for the preparation of spherical silica, by which the spherical shape, particle size and surface area of silica powder can be well controlled. Further investigation on the effect of particle size and surface property in resin material was carried out and a method of choosing spherical silica used in epoxy materials was obtained.

2. Experimental procedures 2.1. Preparation of spherical silica The spherical hydration silica (SiO2·nH2O) was prepared in the reactor with turbulent ?ow cycle assembly parts (Fig. 1) from silica sol (20 wt.% SiO2), sodium silicate solution (20–25 wt.% SiO2 in Na2O·3.0SiO2) and diluted sulfuric acid (15–20 wt.%). The speed of the turbulent ?ow cycle ranged from 1500 to 2500 rpm. The pH value of reaction solution is about 5.0. 20 kg of spherical SiO2·nH2O can be obtained for each batch at room temperature and normal pressure. After washing with pure water, drying with cyclone separation and surface treating at the suitable temperature, ?ve kinds of silica powders SS1, SS2, SS3, SS4 and SS5 were obtained. The method is low cost and environmentally friendly because the ?ltrate solution from the washing process can be reused by membrane ?ltration technology and Na2SO4 concentrated from the ?ltrate solution has become a kind of valuable by-product.

? Corresponding author at: College of Chemistry and Molecular Science, Wuhan University, Wuhan 430072, China. Tel.: + 86 2768752800; fax: + 86 2768754067. E-mail address: ljyuan@whu.edu.cn (L. Yuan). 0032-5910/$ – see front matter ? 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2011.04.003

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C. Ai et al. / Powder Technology 210 (2011) 323–327

Fig. 1. The diagram of turbulent ?ow cycle assembly parts.

2.2. Characterization Thermogravimetric analysis (TGA) of the spherical hydration (SiO2·nH2O) silica was carried out by NETZSCH STA 449C at the heating rate of 10 K·min? 1 in air. The morphologies of silica powder were observed by scanning electron microscopy (SEM, FEI QUANTA 200). The powder X-ray diffraction patterns (XRD) of the products were conducted by Shimadzu XRD-6000 diffractometer with Cu Kα1 radiation (λ = 1.54056 ?) at the scanning speed of 4°·min? 1 (2θ). The scanning scope is 10–40° at tube voltage of 40 kV and tube current of 40 mA. The element contents of the products were analyzed by inductively coupled plasma atomic emission spectrometry (ICPAES, model IRIS, TJA, USA) and the uranium element content was measured by inductively coupled plasma mass spectrometry (ICPMASS, Agilent, model 7500a, USA). The electric conductivity, pH value and Na+ K+ Ca2+ Mg2+ Cl? contents of the extracted solutions of all products were measured by DDS-12A Digital Conductivity Meter (Yulong, China), pHS-3C Digital pH Meter (Shiji-Fangzhou, China) and DX-120 Ion Chromatograph Meter (Dionex, USA). Speci?c surface area (SSA) and total pore volume (TPV) were measured and carried out with Gemini 2360 analyzer (Micromeritics, USA) by the Brunauer–Emmett–Teller (BET) method. The particle size distributions of the products were measured by Mastersizer 2000 (Malvern, UK). The true densities and bulk densities were tested by the pycnometric method and vibration method. 2.3. Preparation of epoxy materials ?lled with spherical silica The chemical structure of epoxy resin and hardener used in this study were exhibited in Scheme 1.

The silica powders SS1, SS2, SS3, SS4, SS5 and SS6 (SS6 sample was obtained by mixing with SS1, SS3 and SS5 according to the close packing theory [13,14]) were treated with silane coupling agent (γGlycidoxypropyl trimethoxysilane, WD-60; 1 wt.% of silica) in the high-speed mixer. All the epoxy resins and hardeners are composed of the same equivalent weight ratio (1:1) of epoxy and hydroxyl group and the content of catalyst is 1 wt.% (TPP) of epoxy resin. Epoxy resin, hardener (PF8010), silica powder and micro capsulated catalysts (TPP-MMA) were well mixed at 120 °C [15,16]. The ?ller loading is 73 wt.% in biphenyl epoxy (NC3000) materials and they were signed with N1, N2, N3, N4, N5 and N6 respectively. The ?ller loading is 65 wt.% in o-cresol formaldehyde novolac epoxy (EOCN1020) materials and 70 wt.% in 3,3′,5,5′-tetramethyl-4,4′-diglycidyl biphenyl epoxy (YX4000) materials. Materials made from EOCN1020 were signed with E1, E2, E3, E4, E5 and E6 respectively and materials made from YX4000 were signed with Y1, Y2, Y3, Y4, Y5 and Y6 respectively. The spiral ?ow of these epoxy materials were measured by a reaction injection molding machine and a spiral mold. The materials were preheated at 120 °C and the molding temperature was 175 °C. The injection pressure is (70 ± 2) kg·cm? 2 and the pressure hold time is 10 s. SS1, SS2, SS3, SS4 and SS5 samples (50 wt.% of total) were treated with silane coupling agent in the same way and mixed with alicyclic epoxy resin (UVR6110), hardener (MeHHPA) and imidazole catalyst at room temperature by ARE-310 model Planetary Centrifugal Mixer (Thinky, Japan). These alicyclic epoxy materials were signed with A1, A2, A3, A4 and A5 respectively, and the viscosities were measured by NDJ-8S Rotational Viscometer at 25 °C. 3. Results and discussion There exist two types of water in SiO2·nH2O: one is the adherent water, the other is the bound water formed by silanol groups (≡ Si– OH) bonding with water. There is a clear stage of weight loss in the TG curve of SiO2·nH2O (Fig. 2) from room temperature to 300 °C due to the loss of adherent water. The small weight loss stage from 300 to 700 °C is corresponding to the removing of bound water. The weight loss is approaching to zero above 800 °C. The calcined temperature has a great impact on the surface structure of the silica powder. The surface property of the powder calcined at different temperatures can be signi?cantly observed in H2O media at room temperature. The powder calcined at 700 °C for 2 h could absorb amount of water and immediately became very viscous silica gel. While the powder obtained at 900 °C for 2 h was

O O H H2 C

O O H2 C n

O O CH 3

O O H2 C CH 3

O O CH 3 C n H2

(4, 4'-diglycidyl biphenyl novolac epoxy resin, NC3000)
O H3 C O H3 C CH 3 O CH 3 O

(o-cresol formaldehyde novolac epoxy resin, EOCN1020)
O O O O

(3,3 ,5,5 -tetramethyl-4,4 -diglycidyl biphenyl epoxy resin, YX4000)
OH H2 C OH H2 C n OH

(3,4 - epoxy cyclohexyl methyl -3,4 - epoxy cyclohexyl carboxylate, UVR6110)
O H3 C O O

(Phenolic resin, PF8010)

(methylhexahydrophthalic anhydride, MeHHPA)

Scheme 1. Chemical structure of epoxy resins and hardeners.

C. Ai et al. / Powder Technology 210 (2011) 323–327

325

Weight loss (%)

0 10 20 30 20 240 460 680

↓ 12.1% ↑
900 1120

Temperature (°C)
Fig. 2. TG curve of SiO2·nH2O in air.

dif?cult to hydrate and the ability of surface adsorption decreased greatly. The absorption capacity of the powder obtained at 1100 °C for 2 h was almost lost. Generally, when the calcined temperature is above 900 °C, the stable Si–O tetrahedral structure is formed and there is no internal stress in the particle. But in fact, the silanol functional groups still exist in the powder and can be examined by IR spectra. So in our experiments, in order to control the amount of surface hydroxyl groups, improve the tap density and reduce the moisture absorption of powders, the hydration silica (SiO2·nH2O) was calcined at 1100 °C for 2 h. SEM images (Fig. 3a) present that all the products have good spherical shapes with smooth surface, monodisperse and uniform distribution, which result in their good ?uidity. All samples are amorphous like according to the XRD patterns of the products (Fig. 3b) and in the nature of low coef?cient of expansion. Fig. 3c shows the particle size distribution of all samples. The D50 of the spherical silica SS1, SS2, SS3, SS4 and SS5 is 1.0 μm, 2.5 μm, 3.7 μm, 6.7 μm and 12.0 μm, respectively. The distributions belong to normal distribution and the diameter of the largest particle (Dmax) in each kind of powder is less than 45 μm and can be effectively controlled. The content of SiO2 is above 99.95% and uranium is 0.1 ppb (see Supplementary S1). Data indicates that the silica products possess high purity and very low α rays interfering signals. The electric conductivity (EC) of the extract solutions of all samples are lower than 2 μs·cm? 1, pH values are about 6.0, and the contents of Na+ K+ Ca2+ Mg2+ Cl? are all below 0.5 ppm. What's more, the moisture of all the samples is below 0.03% by weight loss method. As raw materials used in IC area, silica powder with low content of harmful ions and low moisture can enhance the moisture resistance and protect the circuits from corrosion even in high humidity environment. The true speci?c gravity of the spherical silica is 2.18–2.2 g·cm? 3 by pycnometric method, and the bulk densities are 0.73, 0.98, 1.05, 1.23, 1.28 g·cm? 3 assigned to SS1, SS2, SS3, SS4 and SS5. The multipoint speci?c surface area (MSSA), singlepoint speci?c surface area (SSSA) and total pore volume (TPV) of ?ve products tested by BET method are listed in Table 1. For all samples the data of MSSA, SSSA and TPV are very small except SS1. It is indicated that the surface of spherical silica are very smooth and the spherical particles are monodispersed, compact and solid. As for SS1 sample, there may exist tiny interfaces between the particles, so the data of MSSA, SSSA and TPV are much larger than that of the other samples. That is to say, the SS1 sample with a smaller bulk density (0.73 g· cm? 3) is not compact enough. Since the samples are close to true spherical (as shown in SEM), the data of the speci?c surface area can be calculated by the following formula (1) assuming all spherical silica being compact and solid. The calculated results (SSA) and the ratios of measured MSSSA and calculated speci?c surface area (SSA) are also listed in Table 1.

Fig. 3. SEM images (a), XRD pattern (b) and particle size distribution (c) of spherical silica.

SSA =

6 × 1000 d?ρ

?1?

where: d is the D50 of spherical silica (μm); ρ is the true density of spherical silica (2.2 g·cm? 3). The level of experimental data of speci?c surface area closing to the calculated speci?c surface area is expressed by the ratio of

326 Table 1 The surface properties of spherical silica samples. Sample D50 (μm) SS1 SS2 SS3 SS4 SS5 1.0 2.5 3.7 6.7 12.0 MSSAa m2·g? 1 23.52 2.59 2.23 0.65 0.30 SSSAb m2·g? 1 23.44 2.54 2.20 0.64 0.32 TPVc mL·g? 1 0.0152 0.0018 0.0015 0.0005 0.0003

C. Ai et al. / Powder Technology 210 (2011) 323–327

140 130
SSAd m2·g? 1 2.73 1.09 0.73 0.41 0.23 Ratio

A series materials Y series materials E series materials N series materials

4500 4000 3500 3000 2500 2000 1500 1000 500

120

Viscosity (mPa.s)

Spiral Flow (cm)

(exp/cal) 8.62 2.38 3.05 1.58 1.30

110 100 90 80 70 60 50 0 2 4 6 8 10 12

a Experimental data of BET multipoint speci?c surface area; bexperimental data of BET single-point speci?c surface area; cexperimental data of total pore volume; dcalculated speci?c surface area.

experimental speci?c surface area and calculated speci?c surface area (ratio (exp/cal)). The ratio (exp/cal) represents the surface property of the spherical silica powders and the surface property usually plays an important role in the performance of silica in polymer materials. It can be inferred from the ratios that SS2, SS3, SS4 and SS5 samples are compact spheres with little smaller internal surface area, but SS1 is relatively of much internal surface area. The characterizations mentioned earlier have proved that all of the spherical silica samples obtained by turbulent ?ow cycle method have good sphericity with very smooth surface, high purity, low α rays, small speci?c surface area and low moisture absorption. Thus they can be used as raw material for advanced electronic information materials. Spherical silica powders play a key role in determining the mechanical and electrical performance of the epoxy materials [17,18]. The effects of particle size and surface property of the spherical silica on the ?uidity of resin materials were studied. The spiral ?ow of N series (N1, N2, N3, N4, N5 and N6), Y series (Y1, Y2, Y3, Y4, Y5 and Y6) and E series (E1, E2, E3, E4, E5 and E6) epoxy materials and the viscosity of alicyclic epoxy materials (A series: A1, A2, A3, A4 and A5) were measured and exhibited in Table 2 and Fig. 4. As shown in the table, when the ?ller loading was 73 wt.% in the biphenyl epoxy system, the spiral ?ow of N2 reached up to 116 cm, which was much longer than that of the other four kinds of materials. This phenomenon was also observed in the other two solid epoxy resin systems: the spiral ?ow of Y2 was longer than Y1, Y3, Y4 and Y5 and the spiral ?ow of E2 was also longer than the other materials ?lled with SS1, SS3, SS4 and SS5. In alicyclic epoxy system, the rational viscosity of A2 was only 580 mPa·s and much lower than that of the other four kinds of materials. That is to say the material made from SS2 exhibited excellent performance in mobility and viscosity in different epoxy system. It is no doubt that this phenomenon is resulted from the special properties of the spherical silica: particle size (D50 = 2.5 μm) and special surface property (ratio (exp/cal) = 2.38). It seems to imply that the spherical silica with the characterization of D50 = 2.5 μm and ratio (exp/cal) = 2.38 may have better ?uidity in epoxy materials. In other words, the SS2 can be a better ?ller in epoxy materials. In order to further improve the ?uidity of biphenyl epoxy materials, sample SS6 was mixed from SS1, SS3 and SS5 samples by

Particle Size (D50) of Silica (μm)
Fig. 4. The ?uidity properties of different epoxy materials.

simulating with Matlab software according to Dinger–Funk–Alfred particle size distribution model [19,20]. The spiral ?ow of N6 ?lled with SS6 is 114 cm, which is much longer than that of N1, N3, N4 and N5 and close to that of N2. It is proved that SS2 as single component of ?ller in epoxy materials can achieve the desired ?ow properties. Our experiments for liquid alicyclic epoxy system also illustrated that, as ?ller, the SS2 can effectively adjust the viscosity of the liquid epoxy materials. The relationship between the rotational viscosities of the liquid epoxy composite materials and the mass content of SS2 is shown in Fig. 5. It can be found that the viscosities of the alicyclic epoxy materials decrease with the increasing of mass content of SS2 in ?llers. The viscosities reach the lowest value when the mass content of SS2 is 40% and SS1 (SS3) is 10%. So the viscosities can be controlled by the content of SS2 sample.

4. Conclusions In summary, large scale micro-submicron spherical silica were prepared by turbulent ?ow cycle method with an environmentally friendly process. Due to the special particle size distribution and surface property, the spherical silica SS2 (D50 = 2.5 μm and ratio (exp/ cal) = 2.38) exhibited better surface effects than the other spherical powders in epoxy resin materials and can effectively adjust the ?uidity of resin materials. So the spherical silica SS2 can be widely applied in photo, electronic information materials and special engineering plastics.

Table 2 The ?uidity properties of epoxy materials. Filler Spiral ?ow (cm) N series SS1 SS2 SS3 SS4 SS5 SS6 55 116 61 83 85 114 Y series 89 137 127 104 96 140 E series 59 88 78 73 69 92 Viscosity (mPa·s) A series 1090 580 730 1120 4200

Fig. 5. The viscosities of the alicyclic epoxy materials ?lled with SS1, SS2 and SS3 (SS2 mixed with SS1 as ?ller A, and SS2 mixed with SS3 as another ?ller B, the total ?ller loading is 50 wt.%).

C. Ai et al. / Powder Technology 210 (2011) 323–327

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Acknowledgements The authors are grateful to the National S&T Major Project (No.02), “863” Project and the National Natural Science Foundation of China (No. 20671074, 21071112). Appendix A. Supplementary data Supplementary data to this article can be found online at doi:10.1016/j.powtec.2011.04.003. References
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