I'd like to know how we can increase toughness of polyurethane foam. At this moment, I use polyol, polyisocyanate, surfactant, water, and some catalyst. I changed type and M.W. of polyol, weight ratio between polyol and polyisocyanate, etc, but useless.
The hardener, to be used for such purpose, could be a highly viscous polyol-based additive. These are usually special polyether or polyester polyols with high functionality for cross-linking & copolymerization. A polyether polyol or a polyester polyol means that the terminal groups of the chains are (OH) & this is ensured during the polymer manufacture. By interaction with the isocyanate groups of the PU foam, the polyol acts as a crosslinker and a copolymer is made or a toughened PU foam is produced.
If this does not work "for sure" , then the "made" or "purchased" PU foam does not have the functional groups with which the hydroxyl groups react (i.e. the PU is NOT largely –NCO groups terminated).
I am agree with Dr. Nizar Matar. In addition to his opinion I would like to suggest you it also depends on what kind of diisocyanate you are using. If you are thinking for harder foam and change of diisocyanate is not issue for you, use only aromatic diisocyanates. You can prefer trimer based aromatic diisocyanates.
I'm using polymeric type MDI at the moment. Thanks for your comment, Dr. Vikas Gite.
Chemically you might be able to use above colleagues' suggestions.
I am not a chemist but simply I propose to try to use different percentages of H2O amount and measure the material strength for different water % content that were used in the mix. I did it before and it worked!! Then, generate for example, stress-strain curves for different % H2O that were used in the mix or/and measure Hardness for each PU water % content and notice the difference.