Chemical Property
Chemical functional groups located on the surface of cellulose fibres determine the electrostatic reactivity and bonding potential of paper pulp. Carboxyl content represents the concentration of acidic groups, primarily uronic acids and carboxymethyl groups, expressed in millimoles per kilogram of dry fibre. High levels of these groups increase the anionic charge of the furnish, which influences the retention of wet-strength resins and cationic sizing agents.
The measurement defines the limit of chemical modification achieved during pulping or bleaching.
Analytical Method
Determination of these acidic sites usually involves ion exchange followed by back titration or conductometric analysis. While carboxyl content varies between wood species and pulping processes, bleached kraft pulps typically show lower values than mechanical or unbleached grades. Total charge density remains a primary factor in predicting the efficiency of drainage aids on the paper machine.
Excessively low concentrations may result in poor internal sizing or weak fibre-to-fibre bonding. Chemical interactions at the wet end depend on the accessibility of these groups to large polymers.
Performance Influence
Surface chemistry dictates how a sheet interacts with aqueous coatings and printing inks. High carboxyl content improves the hydrophilicity of the substrate, facilitating faster wetting but potentially causing excessive swelling of the fibre wall. Control of this property ensures consistent performance in high-speed converting processes where moisture stability is required.
Pulps with optimized charge profiles support the development of high-strength packaging materials.
