...polyurethane works by bonding with oxygen to produce a polymer. Whereas oil base paint deposits a layer of pigments as the "oil" evaporates. Once the poly is exposed to oxygen in the air it starts to polymerize and the process is irreversible. - Scott Bryan Sorry, but this chemist must disagree with this incomplete description of the polyurethane curing?
--Begin chemistry lesson---
First some background:
What makes a
polyurethane (PU) polymer unique is the existence of carbamate cross links.
Most common compounds used to make PU finishing materials is OH terminated polymer and NCO terminated cross linker (usually called isocyanate). Carbamates can also be created with combination of chlorinated esters combined with amine or amide. Amine/Amide are less common in wood finishes as they tend to be high viscosity, produce hard crystalline polymers, and produce hydrochloric acid (HCL) as a byproduct. The amine/amide PU curatives are more often found in PU metal primers as the HCL can help clean/bond to metal. When you do find amine/amide in a wood finish, it usually is added to speed up cure on natural (pine) resin, and is called a PU varnish blend.
Oil based PU wood finish material is usually a blend of
drying oil, conventional PU polymer/curative blend, and solvents.
All WW know that a drying oil (tung, linseed, etc) finish reacts/polymerizes by an oxidation (oxygen) reaction. But few know that during transition stage of curing, drying oils create OH terminations. Some of these OH sites react with PU curatives to bind the harder polymer from drying oil with softer PU polymer, into final polymer matrix.
PU polymer/curative reaction is not due oxidation. Intended cure mechanism for a single part PU, is isocyanate capped with a solvent material that inhibits the NCO termination from reacting with OH, until has been spread into a thin film on a surface and evaporates. But if the air contains moisture (H2O - and it always does), it replaces the blocking solvent, and opens the isocyanate to start the curing reaction. Hence, excess solvent evaporation and water exposure are the mechanism(s) that typically gel your can of PU finish.
The comparison to oil based house paints needs more detail as well:
Modern oil based house paints contain
alkyd resins, pigments, fillers, and solvents. These paint grade alkyd s are usually short oils , containing modified
Drying Oils ( tung, linseed, etc). They cross link via similar oxidation method as other drying oil finishes. BTW - Many alkyd resins produce water as byproduct when cross linked, which is why (non-PU) oil based paints will not cure in colder temperatures, especially with high humidity.
--End chemistry lesson---
Hopefully, now you can understand how canned air products are designed to prevent and slow these reactions? They do this by containing; DRY nitrogen, or argon; containing no moisture. They also recommend expelling enough gas to displace the existing (wet) air in the can before sealing. However, this does not stop the solvent evaporation. When the PU can is nearly empty, the dry gas can still absorb solvents that are preventing the PU from cross linking. Which is why Stop Loss bags or smaller containers with less surface area and space are useful; as you reduce amount of solvent evaporation.
Hope this helps. Thanks for reading to end.
Cheers!
- CaptainKlutz