Pure effect of fiber length on paper strength

Fiber length is the fiber property which has been measurable for a long time. Therefore it is very often used as a measure of pulp properties. There are a vast number of experiments where changes in fiber processing have been reported as correlations between fiber length and paper properties of interests. This has caused some misunderstanding for the role of fiber length. Even though many process changes influence on fiber length, it usually is not the primal reason for the changes in paper properties. The effect of fiber length on paper strength has been studied extensively. The results, concerning for example the tensile index, vary a lot. Some results show that the tensile index of paper increases with fiber length and some studies show the opposite results. In most cases the discrepancies can be explained by the distinctive ways of fiber length manipulation. For example if the fibers are first cut and then refined, the results do not show the pure effect of fiber length. This is due to different behavior, and hence development, of short fiber pulps in relation to long fiber pulps during refining. In this paper we have made such experiments, that we can see the effect of fiber length as purely as possible and try to clarify the effect of fiber length alone to the strength properties. We produced fiber populations with narrow length distribution by a defined cutting procedure, thus keeping the other fiber properties like fiber width and fiber wall thickness almost unchanged. Highly oriented long fiber fraction sheets were prepared using a dynamic sheet former. Dried sheets were cut in cross direction with a paper cutter applying cutting intervals of 0, 1, 2 and 5 mm. After cutting, the paper strips were hot disintegrated. Laboratory sheets were prepared from these disintegrated pulps. The modulus of elasticity is independent of fiber length whereas the tensile index increases with it. The difference between these strength properties is that the tensile index contains also the effect of inelastic changes in paper during the testing. Thus it turns out that also stretch of paper depends on fiber length. The elastic properties of the paper are governed at the length scale of the distance between the nearest bonds along the fiber, i.e. at the length of fiber segments. The factors affecting elastic properties of paper are the orientation and elastic modulus of the fiber segments. Inelastic changes is paper occur due to breakage of fibers or bonds. In such case the fiber length plays a role, because longer fibers enforce weak areas better than shorter ones. For that reason the tear strength is strongly dependent on fiber length, as is well known.