Chemical Functionality
Organic acids act as non-toxic additives in the paper industry to adjust pulp slurry pH and improve the retention of sizing agents. Incorporating citric acid into the wet end helps control the deposition of calcium carbonate and other inorganic scales. This organic acid is particularly favored in food-contact grades because it complies with stringent health safety regulations.
Its natural origin makes it an attractive alternative to synthetic acidifying agents in eco-friendly paperboard grades.
Crosslinking Action
Thermal treatment of paperboard coated with starch and polycarboxylic acids creates strong ester bonds that improve moisture resistance. The use of citric acid as a bio-based crosslinker enhances the dry and wet tensile strength of paper webs. When starch-coated sheets undergo high-temperature curing, this acid reacts with the hydroxyl groups of both starch and cellulose.
This reaction establishes a three-dimensional network that resists water penetration, preventing the sheet from softening under humid conditions. If the crosslinking density is too low, the starch remains water-soluble and washes away, whereas optimal curing yields a stable and durable surface. The resulting material maintains its shape in moist environments.
Performance Limit
Acidic processing can degrade cellulose chains if the concentration is too high or if the drying temperature exceeds the safety threshold. Excess acidity leads to fiber embrittlement and reduces the tearing resistance of the finished paperboard. To avoid this degradation, mills must carefully balance the chemical dosage and use neutralizing buffer salts during the coating preparation.
