The Role of Extensional Viscosity in Curtain Coating
Lehtimäki, Watti (2021)
Lehtimäki, Watti
2021
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Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi-fe2021110954543
https://urn.fi/URN:NBN:fi-fe2021110954543
Tiivistelmä
The aim of this study was to characterize different rheological additives and their effect on the extensional viscosity of barrier coating dispersions, as well as map out the role of extensional viscosity in liquid curtain stability. With the knowledge about the role of extensional viscosity, the goal is to enlarge the operating window for a curtain coater, allowing for thinner coating layers and higher machine speeds.
The apparent extensional viscosity over a variety of shear rates was measured using a capillary viscometer. Additives tested include polyvinyl alcohol, polyethylene oxide, and polyacrylamide-based specialty rheology modifiers. The tested additives showed differences in rheological behavior as well as extensional viscosity.
Extensional viscosity was successfully achieved without increasing the shear viscosity of the dispersions out of range of processability. A small addition of polyethylene oxide increased the extensional viscosity at a wide range of shear rates. Additions of polyvinyl alcohol follow predictions by the FENE-P elastic dumbbell model, showing behaviors such as critical extension rate and finite extensibility.
Extensional viscosity plays a significant role in liquid curtain stability. In the curtain breakup process, extensional viscosity slows down the expansion of a hole that would eventually break up the curtain as it resists the two-dimensional flow. It also induces elastic stress into the curtain, hindering perturbations from causing holes in the first place.
The apparent extensional viscosity over a variety of shear rates was measured using a capillary viscometer. Additives tested include polyvinyl alcohol, polyethylene oxide, and polyacrylamide-based specialty rheology modifiers. The tested additives showed differences in rheological behavior as well as extensional viscosity.
Extensional viscosity was successfully achieved without increasing the shear viscosity of the dispersions out of range of processability. A small addition of polyethylene oxide increased the extensional viscosity at a wide range of shear rates. Additions of polyvinyl alcohol follow predictions by the FENE-P elastic dumbbell model, showing behaviors such as critical extension rate and finite extensibility.
Extensional viscosity plays a significant role in liquid curtain stability. In the curtain breakup process, extensional viscosity slows down the expansion of a hole that would eventually break up the curtain as it resists the two-dimensional flow. It also induces elastic stress into the curtain, hindering perturbations from causing holes in the first place.