S1060
V.S. Chauhan, N.K. Bhardwaj
cally hydrophobic, generally inert and the softest mineral on earth. Because both sides of this structure expose an oxide surface, individual talc platelets are held together only by weak van der Walls forces (Fig. 1) (Trivedi, 1997). Compared to other silicates, talc is relatively hydrophobic due to the oxide surfaces (Trivedi, 1997; Ciullo and Robinson, 2003). The edge face is, however, hydrophilic as a result of the –SiOH and –MgOH groups where the surface potential is pH dependent (Ma¨ lhammar, 1990; Fuerstenau et al., 1988). Being hydropho- bic in nature, proper dispersion of talc might be required before its addition in papermaking slurry. This may affect the light scattering and opacity of paper through properly dis- persed particles. These considerations are used in the present study. Five samples of talc with different particle size distribution have been chosen in order to understand their dispersion chemistry and the role of dispersed talc in papermaking.
purposes. Filler also decreases the energy demand in pulp and papermaking process due to lesser usage of fibrous mass (Dong et al., 2008; Chauhan et al., 2011). Loading of higher filler con- tent in paper is always desirous by the papermaker due to decreasing cost and increasing optical properties. The filler particles added to fibers suspended in water are not easily retained in the forming sheet, as they are too small to be entrapped mechanically. Additionally both filler and fibers are negatively charged, so they repel each other (Al-Mehbad, 2004). The properties of filler are linked with the ability of the same to refract and backscatter light through the surface of the sheet. If the filler is not evenly dispersed and flocculates in small clumps, then the optical efficiency of the fil- ler is reduced. No filler is capable of yielding high light scatter- ing for the development of brightness and opacity without having any detrimental impact on wet-web strength and phys- ical properties of paper. The filler used for the development of brightness and opacity debonds fibers because of its inherent high surface area (Wilson, 2006). Filler is supposed to be well dispersed prior to its addition to papermaking slurry in order to get its impact on light scat- tering power of both filler and paper. The light scattering pri- marily depends upon the particle size and shape of filler. For the particles of same shape, the higher the particle size the low- er is the scattering coefficient. The good dispersion of filler may help in increasing the light scattering for the same type of filler. The dispersion behavior of talc powders has been reported in few literatures (Charnay and Lagerge, 2003; Goalard et al., 2006; Chauhan et al., 2012b). The role of dispersion science in pulp and papermaking process was reviewed by Rojas and Hubbe (2005). They explored the scientific principles that underlie the art of papermaking, emphasizing the state of dis- persion of the fibrous slurries during various stages of the pa- per manufacturing process. The literature on dispersion of talc filler for use in papermaking is scarce. The filler/pigment used in paper coating are dispersed well with suitable dispersing agents prior to their application on the paper surface, however they are added as such in paper. The effect of wetting and dispersion of filler in papermaking is not, so far, available in detail. The most suitable wetting agent suggested in the literature is nonionic triblock copolymer. The triblock copolymer is having a central hydrophobic chain of poly(propylene oxide) flanked by two hydrophilic chains of poly(ethylene oxide) (Lee et al., 2010). It results in a complete removal of the bubble-induced attractive forces (Wallqvist et al., 2009). The widely used dispersing agent is sodium salt of anionic poly(acrylic acid) i.e. sodium polyacrylate. Sodium polyacrylate is a polymer with the chemical formula [ CH 2 CH(COONa) ] n . It has the ability to absorb as much as 200–300 times its mass in water. Acrylate polymers gener- ally are considered to possess an anionic charge. It does not adsorb to the basal plane of talc and affects the measured forces (Wallqvist et al., 2009). Now-a-days calcium carbonate based fillers (GCC and PCC) are manufactured in situ and available in the pre-dis- persed slurry form. They are mixed with some amount of dis- persant to avoid the agglomeration of particles. This practice is not yet commercialized for talc (Mg 3 Si 4 O 10 (OH) 2 ). This may be because of a comparatively higher particle size of talc fillers which is less favorable to particle agglomeration than the lower particle size calcium carbonate fillers. It is characteristi-
2. Experimental
2.1. Materials
The bleached mixed hardwood chemical pulp was collected from an integrated pulp and paper mill in north India. The pulp furnish was 50% eucalyptus, 35% poplar and 15% bam- boo. The initial freeness of the pulp measured on Canadian Standard Freeness (CSF) tester (Tappi test method T 227 om-09) was 620 ml which was decreased to 430 ml through refining in the PFI mill following the Tappi test method T 248 sp-00. Dry powders of talc filler with five different particle sizes were sourced from a talc manufacturer in north India. The talc fillers were designated as Talc-1, Talc-2, Talc-3, Talc-4, and Talc-5 based on the decreasing particle size. The nonionic triblock copolymer having a central hydrophobic chain of poly(propylene oxide) flanked by two hydrophilic chains of poly(ethylene oxide) and nominal molecular weight of 6300 Da was used as a wetting agent to wet the surface and remove the air from the surface of talc particles. The so- dium salt of poly(acrylic acid) based anionic polymer (sodium polyacrylate) having a nominal molecular weight of 5100 Da was used as a dispersant. Both wetting and dispersing agents were procured from a chemical supplier in north India. The commercial grade medium to high molecular weight cationic polyacrylamide (CPAM) was procured from a chemical man- ufacturer in India, and used for the retention of filler and fiber fines.
Figure 1
Molecular structure of pure talc mineral.
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