I have read that for the preparation of a polyurethane elastomer, a difunctional polyol is used, where this elastomer requires crosslinkings in its structure presented by hydrogen bonds. Can I use a trifunctional polyol in place of the difunctional polyol to synthesize an elastomer polyurethane, replacing the hydrogen bonds by covalent bonds?
Hi there, I think yes, to obtain elastomers the presence of some degree of crosslinking in the polymer is necessary, and using of trifunctional monomers is the most common way crosslinking. By the way, one should determine the optimum amount of the trifuntional monomer added into the system to get desired elasticity.
Difunctional polyols (including diols served as chain extenders) produce linear polyurethanes while those polyols having functionalities above 2 give polymers with covalent crosslinking sites. The linear PU elastomers are thermoplastics, so they can be shaped by conventional plastic processes such as extrusion and injection molding. The crosslinked PU elastomers are only manufactured by the casting process, i.e. mixed reactants (mainly NCO-prepolymer and diol/triol mixture) are poured into the mold where the material is cured at elevated temperature (80-100 deg C) for several hours. As a result of their crosslinked microstructure, cast elastomers exhibit improved heat stability and compression set when compared to TPU. However, TPU are melt-processable and supplied in a wide range of material grades, so they are much more popular in plastic industry.
A tri-funtional poly-ol would create cross-linking, but a more common method would be to use a di-functional poly-ol and then use a di-amine as the chain-extender to create a polyurethane-urea network.
Yes you can use a trifunctional polyol but the chances of getting all three groups to equally and predictably react is not good. So you are more likely to get difunctional reaction with only limited trifunctional reaction.
Bifunctional polyols are "chain extenders" to produce the necessary MW to achieve properties (see statement above). Normally chainextenders called Molecules like ethylene glycols. This are short chain diols influneceing the "hard" segment. Crosslinkers are trifunctinoal monomers like triethanolamine, glycerine, trimethylolpropane.
You may look at the following refernces:
B.W.Naber, W.Cholewcynski: Polyurethane Soling Materials, Gupta Publishers, 2009
G.Woods, The ICI Polyurethanes Book, Wiley 1987
With regards, Hans-Joachim
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