Coaters and Primers: Should You Apply In- or Out-of-Line?

Many converting operations involve application of a coating or primer to a substrate. Two application techniques are possible: in-line or out of line. Some coatings and primers can be used either way. Others may require one or the other.

The in-line technique involves application of the coating or primer to a substrate, followed by drying to remove water or solvent if necessary. The next step is activation and use of the coating or primer, usually by combining to something else, such as a substrate or extrudate.

The out-of-line process uses application of the coating or primer to a substrate with subsequent drying to remove water or solvent if necessary. Then the substrate with applied coating or primer is rewound. In a subsequent operation, the roll is unwound. The coating or primer is activated and used by combining to a substrate or extrudate.

Consider the following two examples:

To illustrate the in-line technique, imagine an extrusion coating primer that enhances the adhesion of PE extrudate to a foil substrate. The foil unwinds at the entrance to the coating station of a multistation coater/extruder. In the coating station, it receives a coating of the primer. The coated foil progresses through an oven to remove the water or solvent vehicle. The dried primer-coated foil then passes under a curtain of molten PE extrudate that adheres the PE to the coated side. After travel around a chill roll to cool the construction, the material is rewound. The roll of foil/primer/PE now is ready for subsequent use as a component in, for example, a packaging application.

As a example of an out-of-line application, consider the use of a heat seal coating on foil that will provide adhesion to polyester. The heat seal coating is applied to one side of the foil using a standard coating unit. The coated foil travels through an oven to dry any solvent or water as necessary. The dried, coated foil is then rewound. That roll is stored until needed for use. In a subsequent operation, the coated foil is unwound and combined to a polyester film by passing the two substrates through a heated nip to effect the bond. The material is then rewound, and the foil/heat seal coating/polyester is ready for use in its intended application.

Both techniques have combined two materials. The in-line process does it in a single operation. The out-of-line process involves two separate steps.

Certain aspects of the out-of-line technique are crucial. Any coating or primer must be thoroughly dry. Retention of any vehicle in which the coating or primer was supplied could make the material tacky. When wound on itself, the tacky material will adhere to the back side and make subsequent unwinding difficult or impossible.

Any coating or primer used in an out-of-line operation must be fully compatible with the substrate on which it has initial application. If any ingredients such as plasticizers or low-molecular-weight materials migrate from the substrate to the coating or primer while it is in storage, the coating or primer can become tacky. Again, blocking in the roll will occur, and unwinding will be difficult, if not impossible.

Another possibility is that an ingredient contained in a coating or primer that has use in an out-of-line application can migrate to the surface while the coated roll is in storage. This migrating material on the surface is now in contact with the back side of the coated roll. Obviously, some material will contaminate this back side or poison it. Depending on the amount and type of contaminant, the contaminated back side may make the material unsuitable for further use.

One final point to remember: In-line coatings or primers can never be used out-of-line, but out-of-line materials may be suitable for use in-line.

David J. Bentley Jr. is a recognized industry expert in polymers, laminations, and coatings with more than 30 years of experience in R&D and technical service. Contact him at e-mail: dbentley@unm.edu.


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