Predicting paper properties

Mark Willliamson
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Recent breakthrough developments in online fiber measurement open up a fascinating possibility to manage paper properties before the sheet is formed.

Papermakers have always known that upstream furnish properties affect downstream paper quality; now it is being proven with online measurements that measure fiber morphology - and fibrillation in particular - even before the sheet is formed. This new capability represents a breakthrough in how sheet strength can be controlled, since in-mill results presented by Valmet at the recent 2015 PaperCon conference in Atlanta show that dry sheet strength tests can be predicted. The predictor is a soft sensor derived from automated online fiber measurements. That opens the door to a type of predictive control to achieve end-use tests.

Feedforward controls are not new to papermaking; the first ones were introduced in the 1970s to control basis weight by regulating fiber flow to the machine with thick stock consistency and flow measurements. That control is now mature and used regularly, as consistency measurement has improved considerably in recent years. But, until now, many sheet functional properties have been controlled indirectly by operator intervention. Today, a direct control link looks feasible.

A high-definition image from an online analyzer clearly shows fibrillation. Source: Valmet publication
Science meets papermaking practice

Sheet strength properties are related to refining energy, however stable freeness has been the operators' primary objective for adjusting refining energy. It is true that freeness measurement is influenced by the degree of fiber fibrillation created in the refiners, however the main goal of freeness control is to produce a furnish that drains consistently and runs well on the paper machine. On a scientific level, fiber physicists have known for many years that fiber fibrillation created in LC refining is the key factor in creating high bond strength between fibers and a strong sheet of paper. The physicists have used laboratory microscopy to identify this fine detail of fibers and how it related to refining and final sheet properties. But those microscopy studies do not reflect the real-time variability of sheet strength in a papermaking process. That is where the online measurements fit in and science meets papermaking practice.

Valmet's test data show a strong correlation between traditional laboratory tests and predicted strength properties using fibrillation and other fiber measurements in the same analyzer. The need to achieve acceptable sheet strength with less fiber or lower quality and variable recycled furnish calls out for this technology.

Tensile, z-direction tensile and CD tear have been predicted with models using fibrillation. Predicted values (in yellow) track lab tests (in green) very well. Source: Valmet publication
Conventional control using freeness to control specific energy may have to be updated with new and more effective models based on fibrillation. Fibrillation control represents a step above traditional horsepower days per ton and freeness control since it can make online compensation for consistency, plate wear and changes in stock chemistry to produce a paper sheet that has consistent strength. With that consistent strength achieved by the fibers, papermakers can run lower basis weights, higher ash contents and can tolerate lower-quality recycled furnish with the optimum fiber blend consuming the right amount of energy. With more cost-effective furnish, a significant return on investment is possible.

Consistent sheet strength is an important end result but other sheet properties like opacity, surface properties, bulk and porosity are related to refining, as well. That is why controls should be multi-variable, perhaps using other sheet quality measurements and freeness as constraints.

These predictive soft sensors will not replace the traditional dry sheet functional tests as they will be needed as always for quality assurance certification. However, the combination of predictive online measurements and automated lab tests could be a promising way to ensure consistent and user-compliant quality.

Soft sensors/hard measurements

Looking into the future, soft sensors followed by hard measurements could be a sure way to predict and control other paper properties. For instance, it is known that fiber coarseness influences sheet roughness, porosity and bulk as well as tensile and tear strength. Papermakers know that this linkage can be used to get the right paper quality by managing the furnish fractionation and blending. However, the fast-responding online measurement by a fiber morphology analyzer and proactive management capability would close the loop.

Fiber coarseness has an influence on paper properties. Can this linkage be modeled and predicted? Source: Canfor Pulp Ltd.
In the near future, furnish management controls should include online measurements of micro and nano-fibrillated cellulose (MFC,NFC) and cellulose fibrils (CF) made by on-site mechanical processes. In fact, Canada's FPInnovations has already announced an online measurement that is being used for quality control at the CF demonstration plant in a Kruger mill in Trois Rivières, QC. With these promising micro-fiber products used in small quantities , basis weights will come down and filler levels will go up to unprecedented levels. That will accentuate the need for stable quality. They will also have an impact on sheet physical properties like smoothness, internal bond and porosity.

With new online measurements, there are more opportunities for better quality control and an ROI. The management of paper furnish to achieve paper functional properties is indeed getting more scientific and encouraging.