- September 01, 1996, Bentley, David J., Jr.
Treating is an important operation in the converting industry. Flexible packaging as practiced today would not exist without this significant factor, which allows the use of most polyolefin substrates. Considering the importance of treating, it is surprising how many people do not understand what is happening to a web during this process.
The approach taken by some in the converting industry comes close to elevating treatment to the status of magic. They see the web before it enters the treating station and compare it to the web after it exits. Visually, there is no difference. Something magical must therefore have occurred during the passage of the web through the treater. The simplest explanation is that the treater is a magical box. The web moving through the treater provides the box with an opportunity to work its magic.
Maybe the reason some people do not understand treating is their lack of a background in chemistry. One who does not understand chemistry may consider that entire field of science as magical. Fortunately, there is an excellent way to understand treating, and it doesn't require a background in chemistry.
Before examining the alternate explanation, however, we should give a brief chemical description of treating. Polyolefin films are chemically inert because of their nonpolar nature. This inertness makes it impossible for inks, coatings, and adhesives to adhere. Surface treatment - spark gap treating, metal electrode treating, bare roll treating, or flame treating - oxidizes the surfaces of the polyolefins to provide reactive chemical sites by forming carbonyl or other groups. The inks, coatings, and adhesives can then form bonds at these sites.
For those who do not understand chemistry, let us consider instead a mechanical approach. Velcro is a common method of fastening. It consists of two opposing pieces of fabric. One piece has a dense pile, and the other piece has many tiny hooks. When pressed together, the hooks intertwine in the dense pile and form a closure that is commonly used in garments and other applications. The bond is mechanical, because the hooks latch onto, or grab, parts of the opposing pile.
Carrying this analogy to treatment, one can visualize an ink, coating, or adhesive as the dense pile component of the Velcro. The ink, coating, or adhesive has an adherable surface. All it needs is the second component with which to bond. This second component must have a very specific requirement - the "hooks." A plain piece of fabric will not bond to the dense pile of the Velcro.
Untreated polyolefins are similar to the plain fabric. They will not bond with the inks, coatings, or adhesives. Surface treatment changes the polyolefins into the necessary second component of the Velcro analogy by modifying the surface to provide the "hooks."
There is a second aspect to surface treatment that can be comprehended without an understanding of either chemistry or mechanics. Some substrates used in flexible packaging contain slip additives or other compounds that migrate to the surface with time. Such materials will usually interfere with adhesion. Treatment of a web containing these surface contaminants will eliminate or reduce them. Surface treating by the techniques mentioned simply burns away the contaminant. Inks, coatings, and adhesives can then adhere to the uncontaminated surface in the usual Velcro-like fashion.
Some converters in-line-treat all the plastic films they process. In doing this, they are increasing the Velcro-like nature of the web surface and ensuring that they eliminate any undesirable elements that have migrated to the surface. Their action also demonstrates a facet of treatment for which the Velcro analogy does not hold. Treatment levels can dissipate over time, unlike the hooks on the Velcro that are always present. In-line treating immediately before use in a converting operation assures the maximum amount of hook-like surface.
The next time you see a treater in operation, you might want to think about Velcro. This is undoubtedly the best way to visualize what is happening as the web passes through the magical-looking apparatus known as a treater.
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.