Problem definition and objectives: A significant challenge in using calcium carbonate is its tendency to precipitate in papermaking systems due to particle size. Cationic polyacrylamide (CPAM) is commonly used to address this issue. However, excessive CPAM use can adversely impact the environment. Additionally, calcium carbonate may reduce inter-fiber bonding, leading to decreased mechanical strength in paper. To mitigate these issues and enhance paper mechanical properties, chemical modification of calcium carbonate has been proposed. In this study, calcium carbonate was treated with glycerol. Glycerol, due to its hydroxyl functional groups, reacts with calcium ions to form stable complexes. Furthermore, as a byproduct of biodiesel production, glycerol is readily available and cost-effective, offering economic and environmental advantages for papermaking. This research investigates the effects of glycerol on filler retention, fiber retention, and mechanical strength of paper. Moreover, it explores glycerol as an environmentally friendly alternative to CPAM while evaluating its positive environmental impact.
Methodology: To prepare the glycerol-calcium carbonate complex, ground calcium carbonate (GCC), glycerol, raw starch, polyvinyl alcohol (PVA), and carboxymethyl cellulose (CMC) were used. Glycerol and calcium carbonate were combined at different molar ratios, and the mixture was stirred for 90 minutes at 50°C to form the complex. Next, a 0.7% PVA solution was prepared separately and added to the glycerol-calcium carbonate mixture, followed by 50 minutes of stirring. Subsequently, pre-cooked raw starch and CMC were added at 1.2% and 1% (based on the dry weight of the pulp), respectively, and stirred for 20 minutes. Finally, borax was introduced at concentrations of 0.01, 0.005, and 0.001 g/mL of distilled water, with gentle stirring to ensure homogeneity. To adjust the pH of the pulp slurry to around 8, sodium hydroxide was added. Ground calcium carbonate, both unmodified and modified, was added at levels of 20%, 35%, and 47% (based on the dry weight of the pulp). Paper samples with a basis weight of 60 g/m² were prepared. The microstructural features, particle size, and chemical structure of the calcium carbonate samples were analyzed using field-emission scanning electron microscopy (FESEM) and Fourier-transform infrared spectroscopy (FTIR).
Results: Broad and strong peaks observed at 3370 cm⁻¹ and 3500 cm⁻¹ indicated the stretching of O-H groups. The ash content of papers treated with the complex containing 0.05% borax and 32% calcium carbonate was comparable to those treated with 0.03% CPAM. Similarly, the ash content of papers containing the glycerol-calcium carbonate complex with 0.3% borax and 49% calcium carbonate was equivalent to those containing 0.06% CPAM. Using the glycerol-calcium carbonate complex enabled the production of paper with a high calcium carbonate content, achieving filler retention levels 5% to 40% higher than control samples. This approach facilitated the addition of 5% to 7% more filler in handmade paper without CPAM, consequently reducing chemical pulp consumption.
Conclusion: The glycerol-calcium complex increased hydrogen bonding within the paper structure. In contrast, conventional methods of adding calcium carbonate using CPAM reduced inter-fiber bonding. This improvement enhanced the tensile strength index of samples treated with the complex. This study demonstrates that the glycerol-calcium carbonate complex is a viable alternative to cationic polyacrylamide (CPAM) in the paper industry. It offers improved mechanical properties and filler retention in paper production, aligning with sustainability goals.