Nano-silica and SiO 2 /CaCO 3 nanocomposite prepared from semi-burned rice straw ash
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affected while the handsheet brightness, whiteness and opacity were improved compared to commercial PCC.
compared to unloaded (BK) and commercial PCC loaded handsheets. The reasons for these behaviours are the smaller particle size, larger surface and higher light scattering of silica nano-particles and SiO 2 /CaCO 3 nanocomposites (CS1, CS2 and CS3) comparing to micron size of PCC (Hubbe and Gill, 2004).The incident light on SiO 2 /CaCO 3 nanocomposite was refracted on air/silica interface and SiO 2 /CaCO 3 interface, thus led to imparting SiO 2 /CaCO 3 nanocomposites higher refractive indices than CaCO 3 alone (Bala et al., 2007). SiO 2 / CaCO 3 nanocomposites increased the optical properties than silica nano-particles. SiO 2 shell thickness has a little effect on brightness and whiteness while it has a significant effect on opacity. In addition, CS3 shows the highest handsheet opacity. The handsheet opacity increased from 4.6% to 7.3% with increasing SiO 2 shell thickness from CS1 to CS3, compared with PCC loaded handsheets. This may be due to high reten- tion of CS3 than CS1 and CS2. 3.5.5. Morphology of handsheets Figs. 10 shows the surface of handsheets loaded with commer- cial PCC reference, silica nano-particles (S) and SiO 2 /CaCO 3 nanocomposite CS1, respectively. Fig. 10a shows bad PCC dis- tribution and more particles were attached on fibre surfaces than on fibre gaps. The high retention of silica nano-particles, high negatively charged surface and the use of high molecular weight polyacrylamide caused aggregation of silica nano-parti- cles as large agglomerates (Fig. 10b). The aggregation of silica nano-particles within paper sheets enhanced optical properties but reduced tensile and burst indexes comparing to commer- cial PCC as shown in Table 6. Fig. 10c shows that CS1 nano- composite is uniformly distributed. From the properties of handsheets (Table 6) it was found that the nanocomposite CS1 shows higher mechanical properties than CS2 and CS3. This filler has less negative surface charge and smaller SiO 2 shell thickness than CS2 and CS3. Thus, it may be able to form less aggregate particles to cover more fibre surface and densely fill the fibre gaps.
Acknowledgement
This work was supported by Egyptian Science and Technology Development Fund (STDF) under Grant no. ID 737.
References
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4. Conclusions
Silica nano-particles and SiO 2 /CaCO 3 nanocomposite were successfully prepared from sodium silicate solution that was obtained from the semi-burned rice straw ash (SBRSA). The thermal treatment of SBRSA at 800 C for 2 h concentrated SiO 2 from 60.2% up to 82.9%. XRD patterns showed that the precipitated silica nano-particles have a semi-crystalline phase and Zeta potential 38.4 mV at pH 10. While the nega- tive surface potential of the prepared nanocomposites changed from 20.2, 22.8 to 28.1 mV with increasing SiO 2 :CaCO 3 molar ratio. TEM images, XRD, FT-IR with zeta potential measurements indicated that the amount of SiO 2 increased with increasing SiO 2 :CaCO 3 molar ratio at the coating process. The results showed that the retention of silica nano-particles increased due to the high negatively charged surface which caused aggregation in the matrix. Thus, enhanced optical properties but it slightly reduced mechanical properties com- paring to commercial PCC. The retention of the prepared SiO 2 /CaCO 3 nanocomposite also increased with increasing SiO 2 :CaCO 3 molar ratio. On the other hand, the results revealed that the mechanical properties were not significantly
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