Not All Urethane Is Equal: Quick Urethane Chemistry

 

Urethane has gained its reputation as one of the toughest and most versatile polymers known to date.  From the mining, automotive, and oil industries all the way over to the skateboard wheel and gym equipment manufacturers; urethane has proven itself as a reliable material for many decades. What exactly is urethane, though, and are all urethanes the same?

 

For those of you that would like the quick answer to this question, it is a definite no. Not all urethanes are the same, and I am not just talking about different hardness, shapes, or colors either.  I am talking about on a molecular level.  Urethane’s physical properties such as  resiliency, tensile strength, rebound, and modulus are all subject to change based on the chemistry of the urethane that is being used. What has to be realized is there are many different “grades” of urethane such as polyester resins, PPG and PTMEG polyether’s, as well as MDI based systems.  Now I know this is somewhat of a foreign language to most, but we’ll break this down a bit and try to get some clarity.

 

MDI’s and TDI’s are what are known as diisocyanates or the monomers of polyurethanes. MDI stands for methylene diphenyl diisocyanate and TDI stands for toluene diisocyanate.  Both MDI and TDI diisocyanates combined make up 90% of the overall diisocyanate market and their major use is in the manufacture of polyurethanes (1). In order to make a polyurethane prepolymer, one has to react a diisocyanate (MDI or TDI) with a polyol. A polyol is what could be referred to as the “backbone” of a polyurethane molecule and is where most of a urethane’s physical properties are derived from. For example, lets come back to a type of urethane we mentioned earlier; a PTMEG polyether.  PTMEG (polytetramethylene ether glycol) is a polyol or backbone that is used in high performance urethane systems. To produce a PTMEG polyether prepolymer, PTMEG is reacted with TDI’s (see reaction below).

polyurethane reaction chemistry visual.

Now, realize that this reaction of a diisocyanate with a polyol produces what is a called a prepolymer.  A prepolymer is essentially just a syrupy or waxy resin that has to be reacted with a curing agent in order to produce a urethane elastomer (urethane elastomers are what urethane parts are made from). There are many different types of curing agents that can used to produce urethane elastomers. These curing agents can also modify the physical properties of the urethane.  Different curing agents can be mixed in different ratios with prepolymers to achieve different hardness ranges and different physical properties such as tensile strength and rebound.  A very common curing agent widely used across the polyurethane industry is 4,4’-methylene-bis(2-chloroaniline) commonly known as MOCA (you can see it reacting with a prepolymer in picture above).  This compound reacts with a prepolymer such a PTMEG polyether to create a high-performance polyurethane elastomer which is excellent for dynamic applications such as rollers.

 

I know is this is a big gulp of chemistry, but the important takeaway from this very brief urethane chemistry lesson is to be careful when selecting who manufactures your polyurethane parts because not all urethane is equal.  Be prepared to ask the questions about the materials (prepolymer and curing agents) that are being used to manufacture your parts.  Realize that polyurethanes are a family of compounds and they all have different attributes that can be favorable or not favorable when costs are being cut (some materials are designed to be cheap). Finally, if you’re uncertain just ask. If you’re unsure about anything, give us a call at 248-486-6000 or email us for any questions you may have regarding urethane elastomer systems or urethane parts in general.

I also highly recommend checking out this article if you are interested in a more in depth lesson on MDI and TDI based systems:  https://catalogimages.wiley.com/images/db/pdf/0471958123.01.pdf

References:

  1. MDI, TDI and the Polyurethane Industry. (n.d.). Retrieved from https://catalogimages.wiley.com/images/db/pdf/0471958123.01.pdf

Polyurethane vs Rubber

A very frequently asked question is which is the better choice: polyurethane or rubber? With this debate on polyurethane vs rubber, there is often a lot of industry bias associated. Polyurethane molding companies will say polyurethane is better whereas rubber molding companies will say rubber is better.  Coming from a company that does both, the simple answer to this question is it all depends on the application.

For some applications polyurethane is the correct choice and for others rubber is the correct choice.  For example, if you were debating on choosing a polyurethane seal vs a rubber seal for an application in an exhaust manifold that had an operating temperature of 500F, you would most definitely be choosing silicone (technically a type of rubber) over polyurethane.  However, if you had a more dynamic application such as with bushings, spacers, or bumpers, then you would be much better off choosing polyurethane (depending on chemical exposure).

The key takeaway is that polyurethane and rubber have their own specific niches that they fit into.  One may perform better in certain situations than the other and vice versa.  What needs to be realized is that there are many different grades of rubber and polyurethane that are designed for specific applications, and it is often best to consult with professionals to decide which material is best for your specific application.

Please reference our technical data page for more information regarding polyurethane and rubber.  If you have any questions, do not hesitate to give us a call at (248) 486-6000 or send us a message. We have an experienced staff that is always willing to lend support.