[whiterose]

In  chemistry lab I m working on the production of polyurethane sealants from  polyurethane preppolymers. During the sealant production from the prepolymer, it is suggested to add a polyisocyanate(based on diisocyanate) of about 0.5% percent to the sealant formulation. I quite dont understand the role of this extra isocyanate as there is already diol and diisocyanate reacted in the polyurethane prepolymer. Any comment on this would be very much appreciated!!!

[Honclbrif]

Sounds like its acting as a crosslinker to toughen up your final polymer.

[fledarmus]

That really depends on the identification of the polyisocyanate and the prepolymer. It might be a tri- or tetraisocyanate, which would, as Honclbrif said, provide crosslinks that tie your polymer chains together into nets. It might also contain groups with different properties between the isocyanates, such as a highly hydrophobic segment in a mostly hydrophilic polymer or vice versa, or possibly a highly crystalline region in a mostly non-crystalline polymer or vice versa, to change other properties of the polymer. Or if your prepolymer has very long segments, it may simply be a very small segment added to provide more rapid reaction and curing of your prepolymer.

[whiterose]

Sounds like its acting as a crosslinker to toughen up your final polymer.

That really helped me make up my mind. So if this is the case, does this crosslinking prevent the flexibility of my product? thnx a lot for the comment.

[whiterose]

That really depends on the identification of the polyisocyanate and the prepolymer. It might be a tri- or tetraisocyanate, which would, as Honclbrif said, provide crosslinks that tie your polymer chains together into nets. It might also contain groups with different properties between the isocyanates, such as a highly hydrophobic segment in a mostly hydrophilic polymer or vice versa, or possibly a highly crystalline region in a mostly non-crystalline polymer or vice versa, to change other properties of the polymer. Or if your prepolymer has very long segments, it may simply be a very small segment added to provide more rapid reaction and curing of your prepolymer.

 Thank you for all the explanations. The one I m using is 'polyisocyanate based on diphenyl methane diisocyanate'. I couldnt make sure if it is then a polyisocyanate or a diisocyanate( which wouldnt cause the crosslinking). Clarification on this would be very much appreciated as well.

[fledarmus]

 The one I m using is 'polyisocyanate based on diphenyl methane diisocyanate'. I couldnt make sure if it is then a polyisocyanate or a diisocyanate( which wouldnt cause the crosslinking).

Yes, this description is a little vague - do you have an MSDS of the material, or a brand name, that you might be able to search for more information?

Depending on what your prepolymer looks like, this may be a way of adding rigidity to your polymer structure. The benzene rings from the methylene diphenyl group can stack across the polymer chains, giving small regions of crystallinity in an otherwise amorphous chain.

[typhoon2028]

There are many types of MDI.  MDI has many isomer mixtures.  I doubt a MSDS would be of any help.

MDI is probably chosen because it is not too volitile and easier to handle compared to TDI.  I do not have much experience with aliphatic isocyanates.

The pre-polymer probably has not had all of the hydroxyl groups fully reacted.  Adding more isocyanate in a later step will react more of the remaining hydroxyl groups.

This is common in PU foam production.