Analysis of intra-increment pulping and papermaking properties of loblolly pine wood

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Virginia Polytechnic Institute and State University


Loblolly pine growth increments were divided into five fractions; two earlywood a transition and two latewood growth zones. Each growth zone was analyzed for lignin, extractives, and carbohydrate content. Each fraction was digested to four different time schedules using kraft pulping process. All other cooking variables were held constant. A total of twenty pulps were prepared.

Significant differences were observed in pulp yields between intra-incremental growth zones. For the longer digestion times, fraction 1, the early springwood, produced pulp yields greater than the remaining four growth zones. For the shorter digestion periods, fractions 1, 4 and 5 showed comparable pulp yields. For a given cooking time, fraction 2 produced the lowest pulp yield. Pulp yields were shown to be dependent on cellulosic material found in the whole wood. The unusually high pulp yields for fraction 1 were explained on the basis of high level of resistance of carbohydrates to chemical degradation in that zone.

Responses to refining and papermaking properties were investigated from the partitioned growth zones. For the four digestion series, fraction 2 refined the slowest followed by fractions 1 and 3 and then fractions 4 and 5. Differences in refining characteristics were defined in terms of Canadian standard freeness and explained on the basis of differences in fiber morphology and residual lignin content in the pulps.

Significant differences were observed in the physical and mechanical properties of paper produced from the twenty pulps. The results showed that for a given digestion series, fraction 2, the late springwood zone, produced paper with superior tensile strength but with inferior bulk and tear properties. The opposite trends were observed for fractions 4 and 5, the latewood zones. Fractions 1 and 3 showed intermediate sheet properties as compared to the other growth zones. Differences in sheet properties were explained on the basis of difference in fiber properties for the intra-increment growth zones.

Sheet properties were shown to be dependent on digestion time or residual lignin content remaining in the pulps. For all intra-increment pulps, tensile properties increased whereas bulk and tear properties decreased as digestion time increased. Carbohydrate removal influenced sheet properties due to its apparent influence on fiber flexibility. Insignificant differences in sheet properties were observed when compared at identical sheet densities.

Differences in tensile strength properties were related to inherent characteristics of individual fibers. It was shown that the number of fibers per unit volume of paper is the most important attribute to strength. Of secondary importance is the strength of the individual fiber-to-fiber bonds which is influenced by residual lignin in the pulp. Using tensile energy values, the number of hydrogen bonds active in resisting tensile forces was estimated. This number was also related to the number of fibers per unit volume as well as to the residual lignin. All the above variables could be explained on the basis of intra-increment chemical and anatomical properties of wood.