[puzzledman]

Hi. I am new to forum. I hope this does not sound like a really dumb question.
When formulating a reactive two component polyurethane or polyurea, why would you incorporate a plasticizer rather than select a different mol weight polyol in the first place?
Surely a plasticizer will always effect the final performance, bleed out etc when careful selection or the correct polyol in the first place will give you a better product?
Thanks.

[Biopolmonkey]

I do not know for certain, but a colleague of mine makes polyurethanes and it is a fickle synthesis. I would guess for that reason plasticisers are preferred.

[marquis]

Different molecular weights of polymers frequently require a large variation in molding parameters (mold temperature, mold pressure,etc.).  Adding a plasticizer usually affects these characteristics much less.

The second reason is money.  Plasticizers are usually cheaper than the polymer.  You can change the characteristics of the plastic (softness for example) and lower the cost of the compound.

As for your thought about the plasticizer bleeding out.  Yes, that often happens.  You need to decide if plasticizer bleed is a problem in your application.

Good luck

[typhoon2028]

Polyurea is stiff.  Plasticizer may be added to give a softer feel.

What kind of urethane system are you running?  (which type of polyol? which type of isocyanate)
Are you making a foamed product?

Also consider the following from my foam making experience:

The urethane and urea need to be matched correctly.  Making a urethane with the incorrect polyol and isocyanate could result in poor phase mixing with the urea.  An example from my experience was: 

"using a standard 3000 MW general purpose polyol (high PO content) and a MDI resulted in foam collapse due to incompatibility of the urea phase and urethane phase" "Using identical surfactant and other support addives, but changing to a high "EO" polyol of the same MW produces a more compatible system (ie good foam)."

I am probably totally off base on what you are talking about, but what I am trying to convey is that there may be more going on.  The "just change the polyol" approach may not be the best.

You should describe more of the system you are using.