- October 11, 2011, By Mark Miller, Contributing Editor
One of the most common issues in coating is bubbles. Not solids. Not liquids. Bubbles.
Where do bubbles come from, and how can we avoid the defects they create? Well, first of all, the bubbles are air. Air can be introduced in a variety of points in the coating process, but there are two main types of air: air entrainment and air entrapment. These very similar words have very different causes.
Air that is introduced into the fluid prior to the coating head is entrained. I remember the term by thinking about the fact that the bubble is already on the “train track” before it enters the coating head. You are probably already thinking this, but it is easier to keep air out than to remove it once it is in the system.
So how does air enter the system prior to the coating head? One way is through excessive agitation of the liquids in the mixing tank. This can be reduced through proper mixing techniques and preparing batches ahead of time to allow settling prior to the coating run.
It is also possible to place a nitrogen blanket over the fluid to remove air. If the liquid moves from one vessel into another, before the coating head, both vessels need to have air reduction measures.
The fluid moves, right? So when the fluid passes a seal, make sure the seal does not leak and cause air entrainment. Lastly, make sure the fluid moves uphill as much as possible. Any bends in the pipe/tubing will allow air to settle. This curved settlement area for the air will slowly leak into the coating head, showing up as a random and difficult to diagnose bubble problem. If a curve in the tube or pipe is necessary, a relief valve will help.
Temperature and pressure also can be your friend. If the process and materials allow, you can heat the fluid up and pull a vacuum. These process conditions will help reduce bubble formation prior to the fluid traveling to the coating head. Ultrasonic measures also have been utilized to reduce bubbles, but these have had mixed results.
Air that is trapped between a fluid and a solid (substrate) is entrapped. This can occur for many reasons, but if you read my article on “What the Coating Bead Can Tell You,” http://pffc-online.com/coating_matters/coating-bead-message-0822 you would understand that the surface energy of the fluid and the substrate play a major role in the reduction of air entrapment.
Vacuum again can be used to reduce bubble formation. At the point of interaction of the fluid and substrate, the vacuum can remove air from the interface, allowing the fluid to adhere more substantially to the substrate.
In an extreme case of air interfering with coating, a coating defect appears that is referred to as “herringbone” (where there is a periodic cross-web defect that resembles the angled look of the v-shaped weaving pattern). This coating defect usually means that the coating head and substrate are not in proper position to seal the fluid to the substrate.
As the coating head is positioned for a better seal, the air introduced becomes more of a minor defect that may be noticeable only on the final coated product. Even with full coating, however, the improper seal of the fluid to the substrate may lead to fluid flow changes in the coating bead or build-up and streaking of the coating fluid on the substrate.
The interaction of fluid, substrate, and vacuum all are affected by the speed of the line. The faster you go, the more difficult it is to avoid air.
You’ll never look at a bubble the same way again!
Roll-to-roll coating industry expert Mark Miller, owner of Coating Tech Service, has 14+ years of slot die coating experience and troubleshooting. Contact him at 612-605-6019; firstname.lastname@example.org; http://www.coatingtechservice.com.