Coating Matters | Film Split Defects

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When there are two rolling surfaces moving in the same direction, the fluid needs to decide which surface to follow. But how is this decision made?

Film split instabilities are caused in a variety of coatings and with a variety of coating techniques. Most commonly found in forward roll coating, film split is confusion by the fluid as to which way to go. When there are two rolling surfaces moving in the same direction, the fluid needs to decide which surface to follow. But how is this decision made?

The Reynolds equation governs this decision:

P/ΔX = 12µ[((U2+U1)/2)/H2 – Q/H3]

P = pressure

X = distance

µ = viscosity

U1 & U2 = roll speeds

Q = volumetric flow rate

H = separation between roll surfaces

How does the fluid follow this equation? Imagine the fluid entering the roll gap—the rolls are turning in the same direction (away from the fluid and in the direction of the web path). The inlet side of this nip point develops increased pressures. As the substrate and fluid exit the nip point, the diverging rolls develop low pressures. At this point of divergence between the two rolls, the fluid needs to decide which way to go—onto the substrate or follow one of the two rolls. When the fluid follows both rolls, the resultant coating defect is called film split.

How do we encourage the fluid to choose wisely? A couple of factors play into the decision process. If the rolls are operating at the same speed, then the film split occurs when the volumetric flow rate is greater than the separation between the roll surfaces multiplied by the roll speed by a factor of approximately 1.3. This factor increases slightly if the fluid is viscoelastic in nature. What if the rolls are not operating at the same speed? The fluid will follow the faster roll. However, if the fluid is shear thinning, this effect is less pronounced.

So ultimately, how does this affect coating equipment operation? The math leads to a series of setup parameters and some concepts for defect resolution. For thinner coat weights, a small roll-to-roll gap is required. The coating thickness will be affected by mechanical tolerances in the rolls (including roll deflection) and variations in substrate caliper. For thicker coat weights, either higher pump rates or lower line speeds will be required. In all cases, as the line speed increases, film split is more evident. Especially in the case of viscoelastic fluids, where the internal elongational parameters will allow for only so much movement before separation, film split can occur more as the coating becomes thinner.

The Reynolds equation shows that the effects of pressure on the exit of the roll-to-roll nip point are a function of gaps and viscosity (in addition to roll speed and flow rate). As the gap increases, the divergent angle increases and a common coating defect called ribbing occurs. To eliminate ribbing, mechanical methods need to be considered. The roll radius and the gap between the rolls create the subsequent divergent angle and separation that confuse the fluid. If one of the roll radius is reduced and the gap is increased, there is less confusion by the fluid. Also, if the resultant meniscus is pinned to one side, it will tend to flow to that side. Anything to stabilize the flow and encourage proper fluid transport behavior.

The effects of film split in roll coating have not all been determined and calculated, but the fundamental equations are known. Utilizing these ideas of pressure, gap, viscosity, roll speed, and flow rate will allow you to analyze the coating defects at hand and develop a reasonable approach for a solution.

Mark D. Miller, author of PFFC's Coating Matters column, is a fluid coating expert with experience and knowledge in the converting industry accumulated since 1996. Mark holds a Bachelor's degree in Chemical Engineering from the Univ. of Wisconsin-Madison and a Master's degree in Polymer Science & Engineering from Lehigh Univ. and a Juris Doctor from Hamline Univ. Mark is a technical consultant and CEO of Coating Tech Service LLC. He has worked in web coating technologies and chemical manufacturing operations and is a certified Six Sigma Black Belt trained in both DMAIC and DFSS disciplines. Coating Tech Service provides process troubleshooting and project management for precision coated products. Mark has extensive process knowledge in high precision coating applications including thin film photo voltaic, Li-Ion battery, and optical systems technology. Mark has been integral to new developments and technology that minimize product waste and improve process scalability.

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