system
Given the following post, respond as if you're commenting on Reddit:
human
What exactly happens when Polyurethane foam is formed?

I am currently doing my mechanical engineering dissertation on making structures using polyurethane foam, I have no background in chemistry and reading through many papers on polyurethane I find them hard to follow sometimes. I have searched through many papers to try and find what is going on but I cannot. Basically I have seen studies whereby the PU foam is formed within a vacuum, however in my experiments I have tried to form PU foam within a bag  and it does not expand anything like it does when open to the air. Can anyone explain what is going on and why air is needed? Or am I missing something ? I am using 2 part pour\-able foam. 

TL:DR Polyurethane 2 part foam is not expanding properly when not exposed to air, why?
gpt
Polyurethanes are actually a group of related polymers, so there's no single answer. But typically the foaming comes from the reaction forming a gas that then creates bubbles in the material as it sets. CO2 is the most common gas.

Some reactant mixtures will not release any gas, or they might release more gas if exposed to the atmosphere. It's possible your mix gets its foaming from isocyanate reacting with water to release CO2, and in that case perhaps atmospheric water can add more foaming.