Do new measurements foretell tailor-made fibers?

Mark Willliamson

Online fiber fibrillation and crill analyzers are now being evaluated for their ability to assess fiber bonding potential in paper, board and tissue sheets.

Freeness and fiber length have been established for a long time as the primary control targets for pulping and fiber refining operations. However, the times are changing as some significant advancements in fiber quality management capabilities are just coming on stream. Recently introduced and ready-for-use fiber property measurements, now being evaluated in pulping and refining processes, indicate the microscopic fiber structure that promotes good fiber to fiber bonding. It is now possible to manage this fine structure in chemical pulp, mechanical pulp and furnish refining operations to make physical tests and end use properties more consistent. Perhaps tailor-made pulp for specific customers and a more consistent furnish for papermaking are in the offing. Ultimately, end use properties like strength, stiffness and bulk could be predicted before the paper machine.

Within the last year Metso and PulpEye have introduced online measurements that are now being tested in mills. While the optical measurement techniques are different (and vigorously debated, for sure) the expected end results are similar. That is to produce fibers with consistent bonding potential and end use properties by manipulating the pulping and LC refining processes. With analysis cycles in minutes, the online measurements are a significant improvement over infrequent lab tests, promising to be good tools for tighter process and quality control. There are also some potential energy savings by stabilizing the process and quality.

Fiber analysis using high definition images

Metso is marketing its Pulp Analyzer, which is an upgrade of its MAP platform in which fiber and shive measurements are bundled with freeness measurement. To measure fiber fibrillation the resolution of fiber image analysis module has been increased significantly from previous versions. Metso refers to the high definition fiber module as an online fiber microscope. This fibrillation increases the bonding surface area of fibers ensuring the fiber- to-fiber interlocking required to improve paper strength properties. Metso makes the point that that fiber processing and refining that cuts and separates the fibrils from the fiber body is not desirable. Stopping short of this by only roughing up the fibers is ideal.
In addition to fibrillation, Metso says the module measures fines and vessel cells. Other larger scale fiber dimensions, curl, kink, and shive measurements are also included in the Fiber-Shive module package.

The same high definition fiber measurements are also incorporated in the Pulp Expert on-line pulp analyzer, that measures other physical properties of automatically scheduled or manually entered pulp samples, and in the FS5 laboratory benchtop analyzer. Metso reports that numerous high definition measurement modules have been supplied, including those in the laboratory model. The online measurements are undergoing in-mill evaluation for their potential to be used as a fiber quality control tool. Figure 1 shows that strength calculations made by from fiber analysis data and pulp handsheet strength testing in the Pulp Expert are in close agreement. This provides the validation for strength modeling.

Metso's analyzer portfolio also includes on-line measurement of kappa, dirt count, and brightness.

Figure 1: Tensile strength validation by measuring fibrillation

Metso Pulp Analyzer
Hairy fibers

PulpEye has recently introduced its CrillEye online crill measurement. You may ask what is crill? It is a term that was coined in the 1980s by researchers at Sweden's Innventia research organization to describe how "hairy" the fibers are. PulpEye has developed the laboratory measurement into a commercial instrument. Crill is finely divided cellulosic material- finer than fibrillation – that is liberated during the refining of pulp. The crill particles are about a hundred times thinner than the fibres. In spite of the fact that only about one per cent of the weight of fibres and other particles in the furnish is crill, it can correspond to as much as fifty per cent of the total free surface area. This shows the importance of crill for the strength properties of pulp or a paper. Research studies at Innventia have shown that crill is the single variable having the strongest connection to paper strength. Lab results in Figure 2 show a strong correlation to paper tensile strength index.

Figure 2: Lab crill measurement correlates very well to pulp and paper tensile strength index. Source: Innventia.
CrillEye measurement moduleThe crill measurement is based on the comparison of two optically measured surface areas. The total area of fibres and crill is measured with UV light. Pulp Eye says that this short-wavelength light is required to resolve the small crill particles. In fact, the technique is being used to measure the microscopic particles of micro-fibrillated cellulose (MFC). The total area of fibres only is measured with IR light. The "crill variable", KFP, is a concentration independent ratio, obtained when the fibre+crill area (UV) is divided by the fibre only area (IR). PulpEye says the measurement module can also differentiate between free crill and that which remains attached to the fibers.

The crill measurement module is an adaptation of PulpEye's Kappa measurement module. The crill module can be configured with other online modules that make up the company's modular PulpEye product. The other modules include Kappa, brightness, dirt specks in the pulp slurry, fiber dimensions, shive content, freeness, consistency and pH.

Originally proven on chemical pulps, the measurement is now being used in mechanical pulp applications. PulpEye has recently announced the commissioning of its first crill measurement system at Waggeryd Cell, a BCTMP producer in Sweden. The system is now in use as a guideline for operators to achieve consistent end use properties and manage the quality of many different grades of pulp.

Proof in the pudding

Both Metso and PulpEye offer a calculation module (a soft sensor) that estimates end use tests such as tensile, burst and tear indices as well as Scott bond. The calculations involve a model that includes other online fiber measurements. At this early stage it is premature to say that end use properties can be calculated with certainty, but it is an interesting and potentially valuable possibility. Papermaking is still a complex multi-variable process that is hard to predict with fiber being a natural variable. With these advancements in fiber measurements it may be possible to at least tailor-make fiber properties for individual end users.

Will fiber fibrillation and crill measurements displace freeness as a primary control target? Maybe not in the near term, since freeness is so well entrenched. For the time being, these measurements may become valuable complementary tools that give operators a more complete picture of fiber properties and something to aim for in the final pulp quality. With an eye on creating customer-specific quality, special pulp applications and consistent fiber quality it's worthwhile for pulp and paper makers to look into the potential of these new measurements. Like all new online technologies in-mill tests and their acceptance by pulp and paper makers will be the proof in the pudding that will lead to general market acceptance. It will be interesting to follow these developments in the coming months and years.