Crill are particles that are too small to be visible under an ordinary microscope but that are of great importance for fibre bonding in paper. A method for measuring crill, which was developed thirty years ago, is now being made available for industrial use.
"It's only during the past three years, while working in one of our research projects, that we realized how important crill really is. The content of crill was found to be the variable that showed the strongest covariation with strength," says Lars Wallbäcks at Innventia, a research company in Sweden.
When measuring crill, you measure how "hairy" the fibres are. The more crill found on and around the fibres, the better binding ability they have, which in turn results in a stronger paper. In order to improve paper strength, it is therefore very useful to be able to measure the amount of crill in a pulp. With the aid of a crill sensor, it is possible to determine the amount refining required for optimizing the amount of crill in the pulp. This is also important from an economic point of view, since fibre refining is a very energy intensive process in paper production.
With the support of Forska & Väx (Research & Growth), a VINNOVA Programme, Innventia and Eurocon Analyzer, a measurement technology company, are developing a crill sensor for on-line application in pulp and paper mills. The aim is to make it possible to produce improved paper and pulp quality at lower energy costs.
The basic method for measuring crill was developed in the early 1980s by Thorulf Pettersson at Innventia. However, at that time, no importance was given to the small threads that surround the fibres. The crill sensor has been used in laboratory and pilot environments over the years, but it is only in the last three years, during the Advanced Fibre Management Project, that there has been a realization of how important crill is, when it comes to paper strength properties.
The crill measurement technique has been significantly improved since it was first invented. Jointly with Eurocon, it is now being further developed, since it is being integrated with their analysis platform, called PulpEye. Unlike sensors based on conventional image analyses, the crill sensor makes measurements of crill rapidly and frequently, which means that it is very suitable for online measurements in paper production.
"It's great that this invention can be utilized on a larger scale and that pulp and paper producers can now use this measurement technique to improve their processes and products," says Thorulf Pettersson at Innventia.
Crill consists of very thin particles (fibrils), which are about one hundred times thinner than the fibres. Despite the fact that crill represent only approx. 1% by weight of the particles in a suspension, it may contribute to as much as 50% of the free surface. This large surface area per weight gives an indication of the importance of crill in fibre bonding and for the strength of a paper.