These bubbles are caused by the interaction of your Isocyanate and Polyol in the presence of water. To solve this problem it is best to use a Zeolite Molecular Sieve Powder to absorb the water, and to prevent it reacting with your Isocyanate. Typically these water scavengers (such as 3A SUPERSIV) are added into the formulation at addition rates of 1.5-2.5% wt, but this can vary depending on the nature of your formula and production facilities. For more information about Zeolite dosage, click here
For smaller companies, who offer have difficulty dispersing fine powders into liquids, there is a convenient alternative to Zeolite Powder, and this is called Zeolite Paste. These pastes resemble a viscous liquid (with the consistency of similar to that of honey). They are poured directly into your formulation, and stirred-in with a minimum of effort. One of the higher quality brands that you might want to consider for this is 3A SUPERSIV Paste.