Longer fiber length positively influences sheet strength, but it can also have a negative effect on sheet formation. Length must be measured with a minimum impact of the degree of fiber deformation and with compensation for fiber deformation. After reporting average length, the most common presentation of data is the length distribution curve. The limit for fines is below 0.2 mm according to the standards, so the curve starts at 0.2 mm.
Birch has very few fibers above 1.5 mm. CTMP from spruce has fibers as long as those from pine chemical pulp, but spruce CTMP also has a lot of fines and cut fibers. Note that the surface area below each curve is the same and each curve is built up of 75 fiber length classifications.
The shape factor (also called form factor) is an important measure of pulp quality. Shape factor is defined as the ratio of the maximum extension length of the fiber (also called projected length, which is approximately the distance between the fiber ends) to the true length of the fiber (along the fiber contour) and is often presented as a percentage.
Shape factor S = 100 × l / L, where l = projected length; L = true length.
Note: Curl is often used as an alternative to shape factor and is defined as: Curl = (L / l) – 1
A high shape factor S means straight fibers and gives in most cases good mechanical properties to the sheet. It is well correlated with tensile strength and tensile stiffness. A gently treated laboratory pulp has straight fibers, whereas there are several process stages in a mill that are potential curlers of fibers, like presses, mixers, etc.
A variation in shape factor between 81% and 85% can make a difference of 15 Nm/g in tensile index in unrefined samples from bleached Scandinavian softwood market pulp (see Chapter 14 for more details). This difference remains after refining with constant energy, even though the tensile index level has increased due to fibrillation of the fiber surface.
Fibers with a shape factor below 50% are not included in the statistics generated by the L&W Fiber Tester Plus because very few fibers have such a low shape factor. CTMP fibers of softwood after latency treatment are straighter than softwood chemical fibers. Nevertheless, softwood chemical fibers give a better sheet strength due to better bonding, since they are more flexible and contain less lignin.
Local deformations such as knees and wrinkles in the fibers are called kinks. They are detected as changes in the direction of the main axis of the fibers within a limited distance of the fiber.