Troubleshooting Polyurethane Air Entrapment

Urethanes are commonly used to encapsulate electronic circuits because of their moisture resistance and other positive attributes. These benefits are important, however sometimes issues arise that must be remedied immediately to achieve the best results.

Bubbles Appearing in Parts

On occasion, small bubbles do become visible throughout the parts and may not be localized.

Causes of Bubbles

  1. Moisture Contamination of Components

Moisture reacts with the isocyanate in a polymer, producing carbon dioxide gas. This carbon dioxide gas accrues as small bubbles. Moisture can come from a variety of sources such as the prepolymer, catalyst, adhesive and primer. When you have moisture contamination of either the adhesive or primer, bubbles are visible only at the bond line.

  1. Moisture Contamination of Mold

Molds that aren’t heated for an extended period over the boiling point of water may experience atmospheric moisture on the surface. In humid atmospheres, molds can have water condensation if they are cooled and not reheated. When water-based mold release agents are applied improperly, this can lead to water on the mold. When this occurs, bubbles typically appear on the surface.

  1. Insufficient Degassing

When the prepolymer isn’t degassed before mixing AND the batch isn’t degassed before pouring, small bubbles will form.

  1. Incorrect Pouring Technique

If there is a lot of splashing while pouring, bubbles will form in the batch. This is a major problem when it comes to low-viscosity systems where splashing occurs frequently.

  1. Seal in the Processing Machine is Not Working

Degassing happens in the component storage tanks and when using a meter-mix dispensing machine, components are pumped from the storage tanks to the mixing head. Since they are pumped directly into the mixing head, it’s highly unlikely that air will end up into the mixture.

Fixing Air Bubbles

Place Materials Under Dry Nitrogen

Atmospheric moisture is much less likely to be present when nitrogen gas replaces the air in container holding the material. If you can’t get a hold of nitrogen, use a heat gun or hairdryer for roughly 10 seconds prior to putting the lid on.

Degas Components

In the components and the mixed system there could be entrained air so it’s crucial to degas components prior to and after mixing. Moisture can also lead to foaming so again, degas the raw materials.

Heat Mold Efficiently

Any moisture that has condensed on the mold will be removed when the mold is heated above the boiling point of water.

About Resin Designs Urethanes

Resin Designs creates several custom products including the Nexus 52101 which is ideal for automotives, bonding plastics, encapsulants and potting. The Entex 51011 is both soft and compliant, and suitable for high vibration environments. Fill out our contact form to get in touch with us or give us a call at 781-935-3133.