Wood fibers are the base upon which most of the world’s paper is made. But as a pulp or paper maker, how much do you really know about the details regarding morphology and structure of wood fibers? ABB’s Papermaking Fiber Guide contains a wealth of material and information covering this subject matter – and much more. 

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Note: This text is a brief extract of Chapter 12 from the ABB Fiber Guide, which is entitled: Measurement of Fiber Properties.

For any analysis of papermaking fibers, new techniques today give much faster, more reliable and statistically significant measurement of their morphology and properties. This provides powerful new tools to pulp and paper makers. For example, if we measure the length of 20,000 individual fibers, and the average fiber length is 2.5 mm, then we have measured a total length of 50 m of fibers in a single analysis which is incredibly much greater than earlier methods allowed. The new measuring methods can also add important detail to the data, and eliminate errors caused by older capillary tube methods that did not allow fiber deformations and fiber length to be measured independently of each other. The older capillary tube analysis only gave a projected fiber length.

In a more modern fiber analyzer like the L&W Fiber Tester Plus, the highly-diluted suspension is made to flow between two glass plates. The distance between the glass plates is very small and limits the possibility for the fibers to move in one direction (Z), but it allows the fibers to move freely in the other two (X-Y) directions. Two-dimensional images allow us to measure fiber length and deformations separately, if the fibers are well aligned in a plane (Figure 61). Conversely, three-dimensional appearance of the fibers and orientation across the image plane will cause errors.

If the distance between the glass plates is greater than the fiber length, this error will be greater. Low flow speed in the measurement cell gives a laminar flow pattern, but at very high flow rates, the flow will become turbulent.

A problem that can occur with capillary cell measurements, and also with glass plates that have very narrow gaps between them, is that fibers can become stuck there. Using a dynamic measurement gap solves this problem in the L&W Fiber Tester Plus, since the gap is 3 mm before measurement, then decreases to 0.5 mm during measurement and increases again after measurement.

A typical image of eucalyptus pulp fibers from bleached chemical pulp can be seen in Figure 62. Note that basically all the fibers have a curved or kinked shape. A single vessel cell can also be seen at the far left in the image. This image is a greyscale image, before compensation for the background.

Fibers have complicated structures, which vary widely based on species, growing conditions, pulping techniques and level of refining. In order to measure fiber properties, we have to define typical properties which are possible to measure. An obvious parameter is fiber length. However, fiber length is not simple to measure, but a model that works quite well is to consider the fiber as a rectangle with a width and a length.

For example, in the L&W Fiber Tester Plus, the area (A) and perimeter (P) are measured for each detected object (fiber).

Length (L) and width (W) are calculated from the following equations:

A = L × W
P = (2 × L) + (2 × W)

Where: A = measured area of detected object; P = measured perimeter of detected object; L = calculated length of detected object; W = calculated width of detected object.

The length is roughly half the perimeter, and the width is then calculated from this length and the area. All pixels in the image are used to calculate average length and width of the object.

When beating fibers, some thin parts of the fiber wall are partly loosened. These are called fibrils and are at the limit of what is possible to see in a measuring cell wide enough to let fibers flow through. By filtering the image, it is possible to see larger fibrils (Figure 71). By measuring the fiber with and without this filter it is possible to get a value of the degree of fibrillation of each fiber. As a value, the fibrils part of the area or perimeter of the whole fiber is used. By getting the value for an individual fiber, it is possible to study how different fiber dimensions affect the fibrillation during the beating. When a mix of hardwood and softwood is beaten, we often see fibrillation on the long, wide softwood fibers but not on the thin, short hardwood fibers.

In the pulp, there is also tiny thinner fibrillar material called crill. In the beating process, crill develops very similar to both secondary fines and fibrils, hence crill is also important for fiber bonding and strength properties in the dry paper. Crill particles can be only a few 0.1 μm wide, and 100 μm long and most of these cannot be seen with normal visible light. However, special testing methods on the Fiber Tester Plus allow crill to be measured at different wavelengths of light.