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Traditional papermaking fillers mainly include talc, calcium carbonate, kaolin, gypsum whiskers, wollastonite and titanium dioxide and other inorganic minerals, these substances and plant fibers have poor adhesion, which may lead to a decrease in the physical strength of paper when the amount of addition is large, and the retention rate of fillers will be low.
At present, the research on fillers is mainly concentrated in the modification of fillers and the preparation of composite fillers, and among the existing methods of modification of fillers, the use of degradable and regenerated biomass materials (such as starch, cellulose and chitosan) as modifiers is a very promising research topic.
1. Cellulose and its derivatives
Nanocellulose (CNF) has the advantages of high mechanical strength, large specific surface area, large length-to-diameter ratio, good barrier, good dimensional stability, and good biodegradability. The filler is modified with CNF, the filler can be better flocculated to improve the retention rate of the filler, and the CNF can fill the gap between the fibers - the filler is added to the filler, so that the connection between the filler and the filler, the fiber and the filler is more tight, and the strength of the fiber network is improved.
MINGH et al. prepared PCC-CNF composite filler, the filler retention rate reached almost 90.0%, and found that the floc of PCC-CNF composite filler was larger and more numerous than the floc of PCC, and THE CNF well wrapped the PCC particles, increasing the adhesion area between the filler and the paper fiber, thereby improving the strength of the paper.
Cellulose is usually difficult to dissolve in water and requires a specific solvent to dissolve; carboxymethyl cellulose, while retaining cellulose characteristics, is easily soluble in water. JINGS et al. modified carboxymethyl cellulose (CMC) and alum as modifiers to precipitate calcium carbonate fillers, and found that after the addition of alum, carboxymethyl cellulose can effectively coat the precipitated calcium carbonate, so that the retention rate of fillers is improved. The modified precipitated calcium carbonate particles bind more effectively to the fibers, reducing the negative impact on the physical strength of the paper and improving the optical properties and breathability of the paper.
2. Starch and its derivatives
Starch and its derivatives can improve the strength performance of filler paper by reinforcing the binding force between fiber and fiber, wrapping the filler with starch and its derivatives, on the one hand, increasing the particle size of the filler, on the other hand, starch and its derivatives can form hydrogen bonds with pulp fibers, which can alleviate the negative impact of fillers while retaining the role of fillers.
Zhao Chen et al. used starch to modify calcium carbonate, and the fine particles of modified calcium carbonate were fully wrapped, and the particle size became larger, which could form an effective combination with pulp fibers; the tensile index of paper filled with modified calcium carbonate could be increased by up to 39.4% compared with paper filled with unmodified calcium carbonate.
Cationic starch (C-Starch) is the introduction of tertiary amino or quaternary ammonium groups into starch macromolecules to impart cationic properties to starch. Cationic starch can be combined by ion bonds to improve the retention rate of fine fibers, promote the binding of fibers and thus enhance the physical strength of paper.
Shang Wei et al. used cationic starch and diatomaceous earth as raw materials, and modified diatomaceous earth by using cationic starch-sodium hexametaphosphate complex, and the deposition rate of cationic starch on the surface of modified diatomaceous earth reached more than 98.0%, and the retention rate after filling exceeded 78.0%.
Chang Jiandong et al. used cationic starch and polydimethyldilylammonium chloride (PDADMAC) to modify the surface coating of wollastonite, so that hydrogen bonds were formed between molecules and fibers, which improved the binding force between fibers and reduced the negative impact on the physical strength of paper after filling.
3. Chitosan and its derivatives
Chitosan is generally prepared by chitin deacetyl, chitosan molecule contains a large number of active groups (such as -NH2, -OH), under acidic conditions will form a sol electrolyte solution with H+ in the solution, under alkaline conditions and OH- in the solution to generate a gel solution, showing good flocculation and adsorption performance.
Cheng Yun et al. modified the gypsum whiskers using chitosan, and found that compared with the unmodified gypsum whisker filling paper, the retention rate of the modified gypsum whisker paper sample filler was increased by 15.1%, and the tensile index was increased by 5.6%. Yu Yang et al. used carboxymethyl chitosan and aluminum sulfate to modify the surface coating of the talcum powder of the papermaking filler, and found that when the amount of carboxymethyl chitosan was used, the amount of aluminum sulfate had a greater relationship with the final performance of the filler wrapping, when the amount of aluminum sulfate was 6.0%, compared with the unmodified filled paper, the tensile index of the modified filled paper increased by 22.5%, and the whiteness and opacity were better.
Due to the high cost of chitosan, it is necessary to graft it with other substances to reduce costs, and Chen Youlun used cationic chitosan to modify calcium carbonate. After the wheat starch is modified by silane coupling agent, it is gelatinized and expanded after blending with calcium carbonate filler, and finally cationic chitosan and other active ingredients are added to grind and dry, so that the particle size of calcium carbonate filler particles is increased; after the modified calcium carbonate surface is smooth, hydrophobic and lipophilic, whiteness is good, shear resistance is strong, retention rate is high, when the amount is added to 10.0%-40.0%, the retention rate in the paper copying process is increased between 28.3% and 65.5%.
Source: Liu Qiang, Han Qing, Chen Zhuo, Zhuang Kun. Research progress on papermaking filler modification and new enhancer[J].Papermaking Science and Technology,2021,40(01):1-8+34", edited by [Powder Technology Network], please indicate the source when reprinting!