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Article: Process Heating Challenge: Drying a Wet Laminated Coating

Process Heating Challenge: Drying a Wet Laminated Coating

Do you know what these packaging products have in common?

  • Popcorn bags
  • Blister pacs for medication or gum
  • Foil-lined packages for baked goods or laundry detergent

All of these products rely on a process called wet lamination, in which two substrates (usually made of paper, foil or film) are combined into a single web. A water-based adhesive coating is applied to one substrate, which is then nipped with another substrate. The combined web is then passed through a heated oven to remove the moisture from the coating.

In many lamination processes, the coating is dried and then laminated. However, some applications require wet lamination for the coating to be partially absorbed into a permeable web like paper.

The drying process for wet lamination can be a challenge, especially when one substrate is a non-permeable material like foil or film. If you’re not careful in planning your process and choosing the right dryer, you can end up with bubbles and blistering in the coating.

The challenge of wet lamination

The difficulty with drying a wet laminated product is that the wet coating is trapped between the two webs. That means the moisture needs to pass through one of the substrates to be removed from the coating. As a result, the drying rate slows down significantly as compared to most drying processes where the coating is directly exposed to the heat.

The problem is compounded when one of the webs is non-permeable film or foil, so no moisture can come out from that side. The coating has to be dried in such a way that all the moisture comes out from the paper side. If you try to dry too fast by applying too much heat, the moisture boils in place and the coating starts to blister, resulting in a defective product.

The trick to drying a wet laminated coating

The goal is to reduce the rate of heat transfer and increase the residence time to match the rate that the moisture can be safely removed through the paper web. At the same time, you want to use as much heat as you can in order to minimize the dryer size.

Many computer models, which focus strictly on heat load, fail to account for the slower drying time needed for the moisture to pass through the web. To get a good result when drying a wet laminated coating, experience and testing become very important. It’s essential to run trials in order to figure out the rate at which the moisture can be removed through the permeable side without blistering the coating.

Choosing the right oven for wet lamination

Depending on how much heat the product can handle without damage, you can successfully dry a wet laminated coating using the following types of process heating ovens. In all cases, the product is heated on both sides to expedite drying process while keeping the dryer length as short as possible.

Hot air convection.  This option offers the most gentle heating when the product can’t handle very high temperatures. Lowering the air temperature and velocity slows the drying rate to prevent product defects.

IR with room temperature impingement air.  If your product can handle a bit more heat, another option is to use an IR oven that also has room-temperature impingement air. The addition of unheated air blowing through the heaters onto the coating helps to distribute the air, preventing the web from overheating and the coating from blistering. You’ll need to reduce the IR emitter temperature and/or increase the velocity of the impingement air to successfully dry the wet laminated coating.

Combination IR and hot air.  Another possibility is to have a combination dryer where IR heaters are alternated with hot air nozzles. The IR provides a good burst of energy to the web, and then the nozzles help the energy to be dissipated into the web and the coating.

Don’t neglect testing

For a challenging application like wet lamination, make sure you choose a vendor that will spend the time to develop and test the precise details of the heating process to ensure a good quality product. The best way to do this is to work with a vendor that has a well-equipped pilot lab.  This allows you to test your process with actual samples on several types of ovens to determine which one works best.