Date of Award
Bachelor of Science
Paper Science and Engineering
Dr. John Cameron
Dr. Margaret Joyce
The coating application process is an area of great concern for most mills that produce a coated media. During the roll application of the coating onto the sheet, the fluid that is passed to the sheet from the roll, goes through a process known as film splitting. Film splitting is the mechanism for the transfer of the fluid from the application roll to the sheet. Instead of smoothly transferring onto the sheet, the film splits away from the roll and then onto the substrate. Without even transfer and leveling of the coating fluid, the splitting results in stripes or ribs on the paper in the machine direction. The result on the sheet is known as ribbing.
Fort James board mill of Kalamazoo, MI is currently experiencing this phenomenon with their coated board. Dr. Kim W. Robinson, Director of Technology at Fort James, Kalamazoo, has studied the ribbing effect, and it has been noticed that the ribbing follows a wave pattern in the machine direction transversely along the rolls. Ribbing is caused from the film split that is occurring between the roll and the sheet. The film split is a function of the low shear viscosity (LSV) and high shear viscosity (HSV) of the coating fluid.
The HSV of the fluid is related to the transfer of the coating while the LSV is related to the leveling effect of the coating. Good transfer and leveling of the coating yields good coating coverage on the sheet. A measure of good coating coverage can be determined by calculating the rheology index. The rheology index is the ratio of the HSV/LSV. Good coating surface appearance has been found at a rheology index of greater than 0.25 (3).
The viscosifier in the air knife coating formulation for FJ was replaced with carboxymethyl cellulose (CMC) and hydroxyethyl cellulose (HEC). Each thickener was used at low and medium molecular weights. The low shear viscosity was then calculated using the dynamic stress simulator. Next, the high shear viscosity was calculated using the Hercules viscometer.
The results showed that none of the air knife coating formulations had a rheology index of greater than 0.25. The greatest increase in rheology index came from the low molecular weight HEC.
In conclusion, air knife coating formulations have a propensity to rib. A small rheology index of less than 0.25 is an indicator towards ribbing. The reduced rheology index is a function of the solids content within air knife formulations. In order to reach the rheology index of 0.25 or greater, it is recommended that other factors within the formulation, such as the binder or pigment, be changed along with viscosifier.
Fadden, Bradley W., "MD Ribbing Instability of Air Knife Coating Application" (1997). Paper Engineering Senior Theses. 102.