Small Bubbles in Parts

Sometimes small bubbles are apparent throughout parts. These bubbles may or may not be localized.

Part with small bubbles due to moisture contamination.

Potential Causes

Moisture Contamination of Components
Moisture (H₂O) will react with the isocyanate group in a prepolymer to produce carbon dioxide gas (CO₂). CO₂ bubbles will manifest themselves as small bubbles in the part. Moisture can come from many sources, including the prepolymer, curative, catalyst, adhesive, primer or additional additives. Of primary concern are polyol curatives (such as 1,4-butanediol, or BDO) and pigments which are hygroscopic, meaning they readily absorb atmospheric moisture. With moisture contamination of the raw materials, the bubbles are seen throughout the part, and when you have moisture contamination of the adhesive or primer, the bubbles are generally seen only at the bond line.

Moisture Contamination of the Mold
Molds which have not been heated for a sufficient time above the boiling point of water could have atmospheric moisture on their surface. Molds may also have water condensation in humid environments if they are allowed to cool down and are not completely reheated. Also, the improper application of water-based mold release agents can lead to water being on the mold. With water contamination of the mold, bubbles usually appear on the surface of the part, especially on the lower horizontal surfaces.

Inadequate Degassing
If the prepolymer is not degassed prior to mixing, and the batch is not degassed prior to pouring, small entrapped bubbles will be the result.

Poor Pouring Technique
Considerable splashing during pouring will introduce
bubbles into the batch. This is especially an issue with low-viscosity systems where splashing is more pronounced.

Failed Seal in the Processing Machine
When using a meter-mix dispensing machine, degassing usually occurs in the component storage tanks. With this system, materials are pumped from the storage tanks directly to the mixing head, where they are mixed, so there is little opportunity for incorporation of air into the mixture. However, seals in the mix head can fail, which can introduce air into the stream, and thereby negate the effect of the original degassing.

Solutions

Store All Products Under Dry Nitrogen
Storing as many materials under dry nitrogen as possible will reduce the likelihood of the absorption of atmospheric moisture. With some products, there is also the added benefit of potentially reducing oxidation due to the presence of air.

Degas All Components Before and After Mixing
Degassing all components prior to and after mixing will remove any entrained air in the components and the mixed system. Also, degassing raw materials will distill out any water in the material, which will negate the NCO/H₂O reaction which forms CO₂.

Ensure that the Mold is Properly Heated
Heating the mold above the boiling point of water [212°F (100°C)] will remove moisture that has condensed on the mold, as well as any remnants of water-based mold release left on the mold.

Maintain the Mix Head Seals
A regular maintenance program for machinery is always a wise practice. Consult your machine supplier for their maintenance recommendations.