Avoiding Surface Contamination

Many readers of this column undoubtedly have undertaken projects in their living quarters that involved painting. All cans of paint come with many items of small print on the label. These ensure the user obtains the best results from the product. Because the print is so small and extensive, the user often has the temptation to ignore the directions. This can lead to unsatisfactory results.

One very important statement usually has a title such as “surface preparation.” This instruction on one supplier's can says, “Surfaces to be painted must be clean, dry, and free of wax, grease, scaling paint and mildew.”

Excluding the references to scaling paint and mildew, the same instructions regarding surface preparation before painting also are important considerations when using an adhesive, coating, or ink.

Users of adhesives, coatings, inks, and other products also must read the instructions that may come in very small print regarding the use of these materials. All paper, film, and foil surfaces must be clean, dry, and free of contaminants before applying an adhesive, coating, or ink to them. Failure to follow this warning can be disastrous due to absence of adhesion initially or later.

The best way to minimize the problem of surface contamination is to know the most common causes. Eliminating dirt from accumulating on a surface is rather easy. Dust and dirt always will appear on a surface unless it undergoes periodic wiping from time to time. Good housekeeping practices during storage and use of a substrate often will prevent any dirt from depositing on a material. Avoid having doors or windows open to the outside, avoid fans blowing in the area of the substrate use, store substrates in uncontaminated areas that have little exposure, etc.

In the case of substrates, the best practice is simply to keep the material covered until ready for use. If this isn't possible, operators should discard a few outer wraps from the roll of substrate. Minimizing static electricity also is important, since the accumulation of a charge on a substrate could attract particles in the area to adhere to the surface.

Keeping a substrate dry follows many steps identical to those mentioned above regarding dirt. Again, good housekeeping practices during storage and use usually will prevent contamination of a surface by water. Covering is important here, too.

Avoiding wax, grease, or other foreign materials may be more difficult. These contaminants can come from various sources. Good housekeeping such as covering also can eliminate some obvious sources. Unfortunately, subtle sources also exist.

One source some people may not suspect resides within the substrate itself. If a substrate such as a plastic film contains plasticizer or a substrate such as a foil has oil on its surface, these can contribute to poor adhesion of an adhesive, coating, or ink.

The oil on the foil often is the result of the processing of the metal to prepare the foil. In the case of the plastic film, the existence of plasticizer or other foreign material is not quite so obvious. Plasticizers, waxes, or other materials may be ingredients in the formulation of the plastic film added to help processing or to offer another property or characteristic.

If such materials stay embedded deeply within the film, they may not cause a problem. If they migrate to the surface and accumulate there, they can provide serious adhesion problems when applying an adhesive, coating, or ink on top of them. This migration frequently happens simply with the passage of time or the exposure of the substrate to elevated temperatures. Unfortunately, a clearly visible physical clue may not exist that indicates the presence of the surface contamination.

Depending on the age of the plastic film in question and its exposure to higher temperatures during storage, the ingredients not bound firmly within the material might or might not have migrated. If they have migrated, adhesion will be poor initially. If they have not migrated, they might migrate after the converting step and detract from initially good adhesion at a later date.

Like painting, converting requires clean and uncontaminated surfaces. Wise converters will consider this in their use of any substrate.

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 dbentley@unm.edu.


Subscribe to PFFC's EClips Newsletter