- Part 1: Conditions of the Reverse Roll Coater
- Part 2: Two Basic Approaches to Reverse Roll Coating
- Part 3: Factors that Influence Defects in Metered Roll Coating
- Part 4: Position of the Wetting Line Affects Roll Coating Thickness
- Part 5: Controlling the Position of the Wetting Line in Reverse Roll Coating
- Part 6: Affects of Non Newtonian Flow and Viscoelasticity on Reverse Roll Coating
- Part 7: Using Hydraulic Force to control the Capillary Number and Metering Roll Ratio
- Part 8: Mechanical Design of the Reverse Roll Coater
Non Newtonian Flow vs. Viscoelasticity
In Part 5 we discussed how the metering roll ratio and the capillary number control the position of the wetting line in reverse roll coating. Other complications that are likely to be encountered are the rheological considerations of non Newtonian (typically shear thinning) flow and viscoelasticity.
Viscoelasticity occurs when a liquid possesses the ability to extend and relax in response to the application and subsequent removal of a shearing force. It is a liquid possessing a property normally associated with a solid. Both non Newtonian and viscoelastic properties can alter the stable region.
Studies have indicated that non Newtonian materials behave similarly to Newtonian fluids in most respects, but the stability region with respect to ribbing can be narrowed, or even eliminated altogether in the worst cases.
Viscoelastic coatings, on the other hand, can differ significantly in appearance from the Newtonian liquids, depending on the degree of elasticity. At low levels, ribbing is accentuated, but the same instability to cascade observed with the Newtonian model is also present. At high levels, however, ribbing is extremely pronounced at low metering ratios, while the cascade normally occurring at higher metering ratios is replaced by a randomly mottled, streaky coating. This is particularly significant since most polymer coatings exhibit some degree of elasticity. Pressure sensitive adhesive, for example, is a viscoelastic material. Most pressure sensitives can, therefore, be expected to show an increased tendency toward ribbing at low metering roll ratios. However, these coatings also exhibit cascading at higher ratios, indicating a moderate level of elasticity.