Coating Matters | I'm Done Coating...Now What?

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I am not a curing expert. Now that we have that out of the way, I would like to discuss why curing is important. Just like coating technology and web handling, if you don’t have at least a rudimentary understanding of curing technologies, you will not be able to solve many issues associated with coating.

There are many ways to cure a liquid onto a substrate in roll-to-roll manufacturing. Oven dying is very common; UV curing (or ultra-violet light curing) is another option; e-beam (or electron beam curing) is a third option. While these three techniques don’t cover all curing technologies, they provide a basis of understanding that will help a coating technician or engineer.

Oven drying

Ovens are utilized to dry the fluid by evaporating the solvent or water that the polymer is suspended in. Solvent flashes off faster than water, but there are governmental controls on the amount of solvent that can be removed on a regular basis. As you may already know, there are three ways to heat a fluid:

  • Convection—This is the most common heating method in which the web is either supported or unsupported as air is passed over the product.
  • Conduction—Contact with heated rollers can dry the web enough to solidify the product.
  • Radiation—Usually radiative heat is not the method but the assistance for drying the fluid.

UV curing

Light energy can be utilized to initiate the reaction of a polymer. This light energy can be supplemented with oven drying after the reactive end groups of the polymers initiate, but sometimes the UV light banks stretch out along the web to control the extent of reaction. The two functions UV light curing can propogate are the cross-linking of polymer solution coating or the polymerization of liquid monomer coating.

E-Beam curing

E-Beam curing is similar to UV curing, in that the electrons that energize the fluid initiate the polymerization of the fluid into solid. The difference is that the energy is not light, and the system is one unit that hits the fluid and is not typically found in banks along a web path.

When the fluid being coated is not solvent- or water-based, there are some additional methods that are used.

Chilling

When coating 100% solid hot melt polymer, sometimes the only required curing step is to cool the polymer to room temperature. This cooling drum after the coating station can be utilized for solidifying gelatin-based coatings also.

Dewatering

For porous substrates, removing water from a particulate coating is required. This step can be an operation in addition to traditional drying techniques.

All the curing techniques typically follow immediately after the coating system applies the fluid to the substrate. This curing step is described as drying, curing, coalescing, and sometimes even knitting. The whole concept is that the liquid becomes a solid on the substrate.

The concept of curing has a major trade-off: The coating step is easier if the fluid is lower viscosity and the fluid layer is laid down thick, which is the exact opposite of what makes curing easier. Curing works better (and faster) with less solvent (i.e., higher viscosity) and when the fluid is laid down as a thin layer.

This dichotomy of coating to curing is further complicated by the fact that there are stresses associated with both unit operations in the roll-to-roll process. Both stress measurement techniques and scanning electron microscope images of coated cross sections provide a great deal of insight into the extent of cure that is occurring in your product.

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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