Porous Structure of Never-Dried Pulp Fibers Analyzed by Nitrogen Adsorption Method

Porous structures of never- dried pulp fibers were investigated by nitrogen adsorption method. A distinct bent-point in the desorption isotherms was observed at a 45% relative vapor pressure for both softwood and hardwood never- dried pulp fibers. The singular reduction in nitrogen adsorption volume was likely attributed to the presence of mesopores formed via lignin removal in wood cell walls during pulping, which was indicated from the results of nitrogen adsorption/desorption isotherms for partially delignified wood powders. The specific surface area of a sufficiently delignified softwood powder was ~150 m2/g, slightly lower than 200 m2/g. These results indicate that 4×4=16 individual cellulose microfibrils (each of which is 3–4 nm wide) form one unit of cellulose fibril bundle and each bundle is surrounded by lignin thin layer in softwood cell walls. On the contrary, partially delignified hardwood powders had extremely small specific surface areas, whereas a hardwood pulp fiber showed a fairly large surface area. The specific surface areas of delignified hardwood powders were drastically increased after extraction with alkali to remove xylan. Thus, not only lignin but also xylan plays a significant role in the formation of pore structures for the hardwood pulp fiber and the delignified wood powders. The results of pore size distribution analysis using BJH and NLDFT techniques showed that the sizes of principal mesopores present in unbeaten pulp fibers were 3.2–3.7 nm and were increased by disintegration process.