Monday, February 06, 2012

Polyurethanes - A Love/Hate Relationship

I both love and hate "polyurethanes".

Polyurethanes (PU) are a condensation polymer [1], usually formed [2] by reacting a diiscocyante and a diol:
The reaction can then continue from either end, leading to a polymer. The problem with this polymer is that the urethane linkage is actually the least important aspect of the polymer - the R1 and the R2 are where the action is. It's R1 and R2 that determine whether you have a bowling ball, a car bumper, a kitchen sponge [3], a pressure-sensitive adhesive, a chair cushion [3], a floor coating or any of thousands of other products. Polyurethanes are without a doubt the most versatile class of polymers that we have, and that is why I love them.

But their name, "polyurehtane" causes significant confusion because it is a generic name that is generic only at the molecular level. While all condensation polymers, such as polyesters, polyureas, polyamides... have the same issue where their generic name focuses on the functional group that forms between them and not what is actually between the named functional group, this seems to be especially prevalent with polyurethanes, probably because they are so common. I've lost track of all the times clients and colleagues have told me that "they tried polyurethanes and they didn't work". If they had told me they had tried polyethylenes or polypropylene or polystyrenes or polycarbonates and they didn't work, I would be inclined to eliminate them from further consideration because the properties of a single grade of those polymers is not too terribly different from the properties of any other grade. But it is impossible to eliminate polyurethanes from consideration just because a few examples didn't work. As a result of this confusion, I usually have to explain what I explained in the paragraph above in order to get the clients/colleagues approval to still consider polyurethanes for their situation, and that is why I hate them.

One last thought: I've mentioned before that when someone tries to play "stump the polymer guy" by asking what a particular piece of plastic is made of, polyurethane is always an excellent answer. It may not be correct, but because of the versatility noted above, it is very likely not a stupid answer. And sometimes, appearing to be "not stupid" is just enough.

[1] Depending on your definition of "condensation", you could argue that PU's aren't a condensation polymer since there is no by-product formed.
[2] I say "usually", as there are plenty of isocyanate-free urethanes, but most of them have not made too much of an inroad into the marketplace. I suspect price to be a huge factor.
[3] This is actually a foamed polyurethane which typically is prepared by adding a small amount of water to the mix. I won't go into the details here.


milkshake said...

I wonder if anyone tried polyuretanes with short oligo-ethylene oxide or polyethylene glycol (hydroxy-terminated) as the diol component. The PEG part could provide interesting ambiphilic properties.

Anonymous said...

In a system containing a strong hydrophobic entity,like your proposed PEG may be, the chances of the isocyanate group reaction are reduced considerably and this not speaking about any steric hindrance possible effect on the hydroxy (end?) groups. C2 to C4 even C5 up to about C12 are known and eventually used. Such polyurethane precursor systems while possible are probably difficult to convert in meaningful end products.
Interesting are NOPs (natural oil polyols) that are more likely candidates.