Chemical sulfation of lignocellulosic pulps from commercial sources: Reactivity and properties evaluation

Lignocellulosic fibers require chemical modifications to acquire new properties. Sulfation, i.e. the chemical grafting of sulfate moieties (-OSO₃^-NH₄⁺), is widely used due to its ability to introduce charged functional groups. In this study, four different commercial pulps, namely bleached kraft pulp, bleached sulfite pulp, thermomechanical pulp (TMP), and recycled pulp derived from old corrugated containers (OCC) were used as lignocellulosic substrates for sulfation. The reaction was carried out in a solvent-free system, using a straight molten mixture of sulfamic acid and urea. Sulfation was successfully achieved for all pulp samples. The sulfation reagent system is robust, being only slightly affected by contaminants in OCC pulp or by the high recalcitrant lignin content of TMP pulp. The results show a high sulfate charge density grafted on pulp samples, ranging between 2100 and 2500 mmol.kg^-1. The water retention value increases from low values of 1-1.5 g.g^-1 before modification to impressive amounts of 30-35 g.g^-1 after sulfate grafting, enabling the formation of gel-like pulps. The thermal stability of the sulfated pulps is also significantly improved. Grafted sulfate is very effective in forming a char layer on the surface of the fibers, increasing the amount of mass residue to up to 30% at a temperature of 900 °C. These properties make sulfated lignocellulosic fibers promising for applications in a wide range of fields, such as bio-based superabsorbent polymers, ion exchange bio-resins, dispersion bio-additives, and fire-resistant biomaterial.