Measurement of wood fiber properties for pulp and paper making

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Note: This text is a brief extract of Chapter 12 from the ABB Fiber Guide,

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. 

Get your free copy of the full ABB Fiber Guide using the form below. 

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.


Figure 61: Measurements of length and shape are shown with two-dimensional imaging technology.

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.

Figure 62: A typical image of eucalyptus fibers from bleached chemical pulp

Fiber models to simulate the complicated structure of fibers

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.

 

Fiber length and impact on the paper sheet

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.

Shape factor

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.

Kink

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.

A direction on each side of a center point is calculated. If the angle is above 20°, a kink is recorded. Data from kink measurements are: kink/mm; kink/ fiber; kink > 60° per fiber; mean segment length (average distances between the kinks); modified kink index (20°) [Kibblewhite]; and mean kink angle.

One way to detect the effects of hidden deformations is to treat the fibers chemically or mechanically and measure the result on the fibers. One such technique to measure dislocations and other weak points in spruce pulp fibers uses hydrochloric-acid-induced cleavage of the fibers, followed by subsequent analysis of length-weighted fiber length distributions.

It is possible to save raw data to be able to look at individual fibers and the computed kink angles afterwards. Kink and shape factor are often correlated. All types of deformation are included in the shape factor. The number of local deformations of the fiber may, on average, be one per fiber, meaning that the fibers in general have weak points. These are probably important in determining fiber strength. 

Fines

Fines often have a different impact on processes and products than the fibers. Primary fines are apparent before beating and include ray cells. Primary fines have poor bonding properties. Secondary fines are those that are created during beating and generally improve the strength of the sheet.

Both types of fines have a negative impact on the dewatering capacity on the paper machine. While beating softwood, a large part of the produced secondary fines are shorter than 100 μm and thinner than 30 μm. When beating hardwood, the dimensions of the produced fines have a wider span in length (meaning the variation in length of hardwood fines can be wider, compared to beating softwood fibers).

Fibrils

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.

 

Crill

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.

 

Figure 71: A fibrillated softwood fiber

This article on the Measurement of Fiber Properties is a highly condensed version of 1 of the 20 chapters included in the ABB Papermaking Fiber Guide, which contains valuable information for any professional working with wood fiber processing in pulp or paper mills.

 

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