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Three-dimensional visualization and characterization of paper machine felts and their relationship to their properties and dewatering performance, TAPPI Journal July 2021

ABSTRACT: Polymeric felts are commonly used in the papermaking process on the paper machine wet end, in the press section, and in the dryer section. They provide an important function during paper manufacturing, including as a carrier or support; as a filter media assisting with water removal on the paper machine; in retention of fibers, fines, and fillers; and in some applications, such as tissue and towel, to impart key structural features to the web. These felts can have highly interwoven complex internal structures comprised of machine direction and cross-machine direction yarns of varying sizes and chemical compositions. Here, we present a non-intrusive three-dimensional (3D) image visualization method using advanced X-ray computed tomography (XRCT). This method was used to characterize the complex 3D felt structure and determine the water removal characteristics of some commonly used paper machine felts. The structural features analyzed include porosity; specific pore-yarn interfacial surface area; 3D pore size distribution; 3D fiber or yarn-size distribution; and their variations through the thickness direction. The top, middle, and bottom layers of the felt have very different structures to assist with water removal and impart paper properties. The size distribution of the yarns, as well as the pores in the different layers of the felt, are also inherently different. These structural features were non-intrusively quantified. In addition, variation in the structural characteristics through the thickness of the felts and its potential role in papermaking is explored. In addition to the 3D structural characteristics, permeability characteristics and water removal characteristics, including rewetting of select felt samples, have also been experimentally determined. It is interesting to observe the relationship between key structural features and permeability and water removal characteristics. These relationships can provide additional insights into press felt design, as well as ways to improve product properties and the dewatering efficiency and productivity of the paper machine.

Journal articles
Magazine articles
Uruguay Prepares to Open a Gateway to the World, Paper360º July/August 2021

Uruguay Prepares to Open a Gateway to the World, Paper360º July/August 2021

Journal articles
Magazine articles
The Paper360° 2021 TOP 50 POWER LIST, Paper360º July/August 2021

The Paper360° 2021 TOP 50 POWER LIST, Paper360º July/August 2021

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Thinking Beyond the Basement, Paper360º July/August 2021

Thinking Beyond the Basement, Paper360º July/August 2021

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TAPPI Journal Summaries,Paper360º July/August 2021

TAPPI Journal Summaries,Paper360º July/August 2021

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Magazine articles
Escalating Pressure for Uncoated Woodfree Producers in Europe Raises Essential Questions, Paper360º July/August 2021

Escalating Pressure for Uncoated Woodfree Producers in Europe Raises Essential Questions, Paper360º July/August 2021

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TAPPI News,Paper360º July/August 2021

TAPPI News,Paper360º July/August 2021

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The Paper360° Top 75, Paper360º September/October 2020

The Paper360° Top 75, Paper360º September/October 2020

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The Commissioning Dilemma: Pay Me Now or Pay Me Later, Paper360º September/October 2020

The Commissioning Dilemma: Pay Me Now or Pay Me Later, Paper360º September/October 2020

Journal articles
Magazine articles
Open Access
Flow rheology of light foams generated from aqueous solutions of polyvinyl alcohol, TAPPI Journal January 2023

ABSTRACT: Recent studies have shown that foam-assisted application of additives into a wet web has advantages over the conventional way of adding the chemicals into the pulp suspension before forming, e.g., increased mechanical retention as well as high dosage giving increased wet strength without impairing the sheet uniformity. To engineer processes utilizing this new technology, the complex flow behavior of applied foams must be quantified. At the minimum, the foam viscosity and the slip velocity at the solid surfaces need to be known to build practical models that can be used in analyzing and upscaling unit processes of the foam-assisted application.In this study, the rheological behavior was quantified for foams having polyvinyl alcohol (PVOH), a widely used strength additive chemical, as the surfactant. The foam density was varied between 100 g/L and 300 g/L, and the concentration of the PVOH solution was varied between 0.5% and 6.0% (w/w). The foams were generated with a commercial foam generator, and the rheological properties of the foams were measured by using a horizontal pipe bank. At the outlet from the generator, the volumetric flow rate, the absolute pressure, and the bubble size distribution of the foam were measured. In the measurement pipe section, the viscous pressure gradient and the slip velocity were measured, after which the foam was discharged to ambient air pressure. The viscosity and the dynamic surface tension of the PVOH solutions were quantified with commercial laboratory devices. In the viscosity analysis, the apparent shear rate was calculated from the volumetric flow rate, and the resulting apparent viscosity was translated to real material viscosity data by applying the Weissenberg-Rabinowitsch correction. The results indicated that PVOH foams can be described with high accuracy as shear-thinning power-law fluids where the detailed behavior depends on the foam density and the PVOH concentration. Slip flow, as usual, increased with increasing wall shear stress, but it was also dependent on the PVOH concentration, the air content, and the bubble size. For both the foam viscosity and the slip flow, a correlation was found that described the quantitative behavior of all the studied foams with good accuracy.

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