Comparison of individual and combined effect of nanosilica and cationic additives on the optical properties of the white packaging liner.

Background and objectives: Waste paper recycling utilization has been increasing in Iran and world paper industries which offers many benefits to the environment and humans. Municipal waste is also reused after recycling processes. Cardboard recycling industry has great environmental and economic importance and helps overcone the lack of wood resources and high demand for paper products. It plays an important role in the development of paper-related industries. However, recycling can be associated with the reduction in the optical properties of the paper. The use of nanoparticles in the paper industry is also expanding gradually. Nanosilica is one of the most important nanoparticles which is used as a retention aid in the paper industry. In order to reduce the consumption of long fibers and obtain the desired optical properties, the use of nanosilica alone or in combination with other materials such as cationic starch and cationic polyacrylamide is investigated. The purpose of this research is to compare the effect of individual and combined use of nanosilica additives, cationic polyacrylamide, cationic starch and long fibers on the optical properties of white liner paper pulp. Methodology: In this study, white paper pulp with the brightness of at least 78% and the gloss of at least 45% was used to prepare handmade paper sheets. Long fiber kraft chemical pulp from coniferous wood imported from Russia with the brightness of 89% was used in the laboratory paper sheet making. Nanosilica powder (NanoSiO₂) (Degussa), Germany, cationic polyacrylamide (Farinret K325 brand, Degussa, Germany), and cationic starch (LyckebyAmylex, Slovakia) were used. Independent treatments included the addition of 10% refined long fibers pulp, 6% nanosilica, 1.5% cationic starch and 0.15% cationic polyacrylamide and combined treatments included 6% nanosilica and 1.5% cationic starch and 6% nanosilica and 0.15% cationic polyacrylamide. Then 127 g.m^-2 handmade papers were prepared and the optical and microscopic properties were evaluated. Results: The results showed that by adding 10% long fibers, the brightness decreased and by using 6% nanosilica, the maximum brightness was reached. Meanwhile, the whiteness of papers with 6% nanosilica was minimum and with the dosage of only 0.15% cationic polyacrylamide, the whiteness reached at the maximum value. Opacity showed its highest value with the combination of 6% nanosilica and 0.15% cationic polyacrylamide. Also, by increasing the amount of polyacrylamide and cationic starch, individually or in combination with nanosilica, the opacity increased. The light absorption coefficient was the lowest in papers with 6% nanosilica and the light scattering coefficient was the highest in papers containing 6% nanosilica and 0.15% cationic polyacrylamide. A colorimeter was used to measure the color components and the results showed that the additives had an effect on the brightness and whiteness of the papers. Also, changes in the color spectrum and the value of color change were also observed. Additives increased the darkness and changes in different colors. Conclusion: The use of nanosilica individually and in combination with starch and cationic polyacrylamide increases the brightness of papers. Also, the use of cationic polyacrylamide individually and in combination with nanosilica leads to an increase in the whiteness and opacity of papers. The brightness factor of the papers, which is representative of the L* component, decreased with the exception of the addition of 10% long fibers and 6% nanosilica. The amount of overall color change with ΔE* was the lowest in samples containing 1.5% cationic starch and the highest in samples containing 0.15% cationic polyacrylamide. The use of some treatments can lead to a decrease in the optical properties and a decrease in the printability quality of the white liner. To solve this problem, mechanical pulp that has been decolorized or coated on the surface of the paper can be used. The presence of nano-silica particles in the structure of the paper improves the surface structure and reduces the prosity, which results in the reduction of surface roughness and less light refraction, and increases the light reflection and brightness of the paper. [ABSTRACT FROM AUTHOR]