Nano-silica and SiO 2 /CaCO 3 nanocomposite prepared from semi-burned rice straw ash
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nanocomposites (CS1, CS2 and CS3). The results show that all the diffraction peaks of nano-CaCO 3 core particles (C) can be related to a pure calcite phase. The SiO 2 /CaCO 3 nanocompos- ites CS1, CS2 and CS3 samples have similar diffraction peaks which indicate that the shell layer of SiO 2 is semi-crystalline. It can be noticed that the intensities of the peaks at 2-Theta Scale at 23 and29.5 are increased in the case of CS1, CS2 and CS3 compared to CaCO 3 core particles (C). The XRD analysis results are in a good agreement with the results obtained from FT-IR.
3.4. Zeta potential
Fig. 9 shows the result of zeta potential versus pH values (6– 10) of the produced silica nano-particles, CaCO 3 core particles (C) and SiO 2 /CaCO 3 nanocomposites (CS1, CS2 and CS3). The result shows that the negative surface charges on the pro- duced silica nano-particles increased gradually to more nega- tive values with increasing the pH value (Tsai, 2004). At pH = 10 zeta potential is 38.4 mV. The results show that zeta potential of CaCO 3 core nano-particles at pH = 10 is 17.4 mV, while zeta potentials of the prepared composites CS1, CS2 and CS3 are 20.2, 22.8 and 28.1 mV, respec- tively. It is observed that zeta potential values of the prepared SiO 2 /CaCO 3 nanocomposites are more negative than core par- ticles and these negative values increased gradually with the increase of SiO 2 :CaCO 3 molar ratio. Table 5 summarizes the obtained results of SiO 2 /CaCO 3 nanocomposites using XRD, TEM, XRF and Zeta potential techniques.
3.5. Properties of handsheets loaded with the prepared nano- fillers
The physical, mechanical and optical properties of reference handsheets (unloaded and commercial PCC loaded hand- sheets) and handsheets loaded with silica nano-particles (S), and SiO 2 /CaCO 3 nanocomposites (CS1, CS2 and CS3) fillers are summarized in Table 6. 3.5.1. Retention The results shown in Table 6 reveal that silica nano-particles (S) and SiO 2 /CaCO 3 nanocomposites (CS1, CS2 and CS3) have a higher retention than the commercial PCC; the percent- age of increase reaches 34.9% for silica nano-particles, while it varies from 6.1% to 14.4% for SiO 2 /CaCO 3 nanocomposites. The presence of cationic polyacrylamide enhances silica nano-particles and SiO 2 /CaCO 3 nanocomposite aggregation to optimum size. Agglomerated fillers are sufficiently large in size to be retained within the sheet and fill the fibres’ gaps. The long-chain cationic polyacrylamide adsorption on the sur- faces occurs through either bridging or charge neutralization. Charge neutralization occurs in the vicinity of zero potential (i.e., the isoelectric point) where the aggregation of fine parti- cles (i.e., fines, fillers) is at its maximum point since charge neu- tralization promotes the bridging through depressing the double layer thickness (Khosravani et al., 2010). Silica nano- particles and SiO 2 /CaCO 3 nanocomposites neutralize the poly- electrolyte charges more aggressively than the other anionic components of pulp stock. Adsorption of silica nano-particles on SiO 2 /CaCO 3 nanocomposite particles screens out repulsive
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