Introduction: In recent years, many efforts have been made to use natural polymers, especially cellulose and lignin, as renewable and environmentally friendly materials. Lignin with phenylpropane units has great potential for reaction, modification, and use in various applications. This material is recovered in large quantities from the pulp and paper industry every year. Lignosulfonate (LS) is obtained from the sulfite pulping process. It is water-soluble and has a high molecular weight. Lignosulfonate can be used as an emulsion stabilizer, dispersant in oil well drilling, cement compounding and paper coating. Given the importance of lignin, it is essential to investigate the capabilities of lignin in the manufacture of paper and cardboard. Therefore, in this research, the effect of using lignin on the properties of packaging paper was studied.
Method: First, long-fiber kraft pulp sheets were crushed into small pieces. The fibers were soaked for 24 hours, then they were defibrated using a laboratory disintegrator. After moisture content determination, the fibers were refined according to the TAPPI standard to achieve freeness about 250 ± 25 ml. Hand-sheets with a basis weight (g/m²) of 60 ± 5 was made from the refined fibers, according to the TAPPI standard. Finally, after making the hand-sheets and before press, the hand-sheets surface was modified by using spray method the sodium hydroxide and then 3% hydrogen peroxide. After cold pressing, the surface of the hand-sheets was sprayed with a lignosulfonate solution. The concentration of the coating solution was 2.5 and 5%. The hand-sheets were fixed by using plate and rings to prevent wrinkles. Then, the strengths, physical and barrier properties of the control and coated papers, including tensile, tear, burst, density, thickness, ash content, surface roughness, water absorption, and air resistance, were investigated.
Results: The results showed that the highest tensile, burst and tear strengths index in hand-sheets is related to the modified hand-sheets samples with oxidative surface treatment and coated with lignosulfonate 5%. Also, statistical analysis of the data of the strength's properties indicated that the difference in the data is significant at the 95% confidence level. The highest hand-sheets thickness is related to the hand-sheet sample without oxidative surface treatment and coated with lignosulfonate 2.5%. The highest apparent density of the hand-sheets is related to the sample with oxidative surface treatment and coated with lignosulfonate 5%. Also, the highest ash content and the lowest surface roughness value in the hand-sheet samples are related to the modified hand-sheet sample with oxidative surface treatment and coated with lignosulfonate 5%. The lowest water absorption is related to the control hand-sheet sample and then the sample without oxidative surface treatment and coated with lignosulfonate 5%. The highest value of air resistance in the hand-sheets is related to the sample with oxidative surface treatment and coated with lignosulfonate 5%. Also, statistical analysis of the physical and barrier properties' data of hand-sheets showed that the difference in data is significant at the 95% confidence level.
Conclusion: The aim of this research is to coat modified fibers with lignosulfonate and investigate the properties of handmade paper. In this process, hydrogen peroxide was used as an oxidizing agent in an alkaline environment. The results showed that the surface oxidation process caused the creation of functional groups such as COOH on the surface of cellulose fibers. Also, this surface agent increased the reactivity of fibers and lignosulfonate, so that in the samples with the oxidation process, the amount of lignosulfonate absorption and, as a result, the percentage of ash in the paper increased.