Chemical modification of polysaccharides with hydrophilic polymers for CaCO3 crystal growth modification and filler retention, for paper applications

[摘要] Polysaccharides were modified with selected polymers via the grafting technique. Bothanionic and cationic polysaccharides were prepared. Random and crosslinked graftcopolymers were also prepared. The percentage grafting was determined by gravimetricanalysis and results were confirmed by cross-polarization magic angle spinning carbon-13nuclear magnetic resonance microscopy (CP/MAS 13C NMR). These modified biodegradablepolymers were then used to flocculate precipitated calcium carbonate (PCC). The effects ofpH, percentage grafting, crosslinker concentration and polysaccharide concentration on PCCflocculation were evaluated. Furthermore, the effects of anionic and cationic starch, eitheradded to PCC sequentially or simultaneously, on PCC flocculation were also investigated.Generally, anionically modified starch showed excellent flocculation properties, which aredesirable for the end application of PCC retention.The effect of polyacrylic acid (PAA) and polyacrylamide (PAM) modified cellulose fibers oncalcium carbonate crystal nucleation and growth modification was investigated. When theheterogeneous crystallization of CaCO3 was carried out in the presence of modified cellulosefibers the CaCO3 crystals were found to be residing on the surface of the fibers. Themorphologies of the crystallized CaCO3, polymorph and fiber surface coverage were differentfor cellulose materials grafted with polymers of different functionalities, meaning that there isinteraction between the crystal growth modifier and the growing nuclei.The effect of the modified starch on the crystallization of calcium carbonate gave usefulinsight into designing CaCO3 filler morphologies. It was found that the filler size,morphology and surface properties of fillers can be tailor-made by choosing suitable CaCO3crystallization conditions as well as a suitable crystal growth modifier. The crystallizedCaCO3 had a negatively charged surface. Results of fluorescence studies showed that thePAA modified starch (polymeric additive used) resided on the surface of the crystals. Thusthe presence of the polysaccharide on the surface of a filler could be advantageous forstrengthening fiber–filler bonding in paper applications.Anionic starch materials were also used to prepare anionic-starch-coated starch particles. Boththe anionic starch and anionic-starch-coated starch particles were evaluated for PCC retentionand other properties of hand sheets. When anionic-starch-coated starch particles were usedthere was generally an improvement in the PCC retention, while the other paper propertiesremained desirable. The success achieved with the use of anionic-starch-coated starchparticles now opens the way for the further preparation and testing of various modified starchparticles, for optimization of filler retention.

[发布机构] Stellenbosch University