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In this study, the mechanical pulps were prepared under different preheating time and sulfonation conditions. The pulping conditions chosen yield a gradient of pulps ranging from thermomechanical pulp (TMP) through chemithermomechnical pulp (CTMP) with a low degree of sulfonation to CTMP with a high degree of sulfonation. The increase of the sodium sulfite charge resulted in a gradient of sulfur content which was shown to be related to the introduction of strong acidic groups as well as to the decrease of the carbohydrate content at the surface revealed by Electron Spectroscopy for Chemical Analysis (ESCA). These results together with Scanning Electron Microscopy (SEM) investigations confirm that the plane of rupture in the fiber has been transferred to a more lignin-rich area of the middle lamella. Despite the lower carbohydrate content at the surface, the water wettability of fibers was increased as revealed by dynamic contact angle. This is explained by the polar contribution of sulfonated lignin to the surface energy of the fibers. Inverse Gas Chromatography (IGC) analysis confirmed the differences in the surface properties of the pulps.

Surface Properties of Mechanical Pulps Prepared under Various Sulfonation Conditions and Preheating Time