[jimmo]

Hey guys, I'm trying to clear up something regarding 2k urethane and/or epoxy coatings. According to PPG, they claim that if you allow these products to drop below 60 degrees that the crosslinking process is interrupted and will never reinitiate. I've spoke with a chemist whom claimed that it is inacurate and once these types of products are brought back to a warmer temperature it will resume...even that the urethane products continue to cure in lower temperatures. I work as an automotive painter and am attempting to get a better understanding of the chemistry behind the products, so I apologize if this question sounds dumb. Any insight is appreciated.

[Mr Peanut]

For urethane pre-polymers the cross linking reaction usually involves isocyanate-polyol reactions. Ordinarily, this process will slow when cold and resume when warmed. However, the isocyanate functionality also reacts with water. Cooling the system may allow these unwanted, competing reactions to prevail.

Urethane cross linking reactions may not necessarily involve the standard isocyanate-polyol chemistry described above. It is conceivable that a pre-polymer could be used which also has functionality not typically associated with "vanilla" urethane technology. For example, some “sporty” urethane pre-polymers have built in UV, e-beam, or free radical initiated polymerization functionalities (usually acrylic, sometimes vinyl). These specialized systems might loose their "activity", say through destruction of an initiator by the environment while in cold state where the activation energy for the wanted reaction is absent.

Same goes for epoxy systems. Standard epoxy chemistry allows for resuming the reaction at warm-up. In fact, aerospace epoxies are often supplied premixed and frozen. Again however, as is often the case in polymer technology, the epoxy system may in fact be a mixed system just like the urethane systems that I described above.

[jimmo]

Thanks for the reply,

Please bear with me as my chemistry knowledge is quite limited at this point. My last chemistry class was about 10 years ago, I've been hanging around the forum for the last few months & have just started reading recently to get a better understanding. So let me just make sure I understand your response:

In a basic isocyanate reaction the temperature won't mess up the way it cures. Certain additives (the systems you speak of) in these coatings might also be designed to react and the reaction can not take place at lower temperatures, thus the reaction never occurs (or reacts differently) and the product is not fully cross-linked as intended.


very interesting to hear about the epoxy being shipped premixed & frozen. again, in the paint industry we've always been advised never to freeze any products & try to reuse once activated. I always wondered if it was just to keep you pouring it away and ordering more or if it truly compromised the product.

[Mr Peanut]

In a basic isocyanate reaction the temperature won't mess up the way it cures.

It might. It depends on the presence of other interfering chemicals, such as water, that may still be able to react when the wanted reaction is retarded by the cold. Water is very mobile in these systems and can reach the isocyanate groups in the mix with sufficient energy to react when the wanted reactant - the polyol - is inhibited by the cold.

The isocyanate-alcohol reaction looks like this:
 R-NCO + HO-R' ==> R-ure-R'

If these molecules have two reactive groups in their structures (diisocyanates and diols) the reaction forms a long chain (a polymer):
....R'-ure-R-ure-R'-ure-R-ure-R'..

When there are more than two of the reactive groups in the structures the long chains "hitch-up" with one another forming a network (a cross linked polymer).

Isocyanates are very reactive - particularly with water (which is everywhere) - and that is a problem for shelf like. Isocyanates are also fairly nasty health wise. These problems can be reduced by making a urethane pre-polymer with other types of reactive groups present in the molecule. (A pre-polymer is a polymer that is low molecular weight and therefore can be poured, brushed and sprayed.) Upon use, an initiator is added which starts these special groups reacting to cross link the chains. Alternatively, a special additive called a sensitizer may be included in the mix which starts the reaction when it is activated by UV light. There are countless other strategies. For coatings, these special groups usually fall into the category of "acrylic" or simple "vinyl" functionalities. They polymerize with each other by a mechanism that is different from that of the isocyanate-polyol reaction. The initiators used in coatings that do not use radiation to cure are typically organic peroxides. These peroxides are reactive and are relatively short lived once added to the mix. If the polymer is too cold for the crosslinking reaction to proceed effectively before the peroxide is destroyed by other factors, the polymer may fail to cure altogether.

Epoxies are less tricky than urethanes in terms of water reactivity but unwanted side reactions can occur once mixed. Remember that the premixed and frozen adhesives are stored in an air-tight, water-tight tube.

Finally, many coatings (particularly water based technology) consist of a "Base-Stabilized-Emulsion". These polymers frequently have a built in organic acid functionality. Low levels of a base (usually ammonia) allow a stable suspension of these polymers in a water vehicle. Freeze the water and the emulsion breaks. Call the waste dealer.

[jimmo]

The isocyanate-alcohol reaction looks like this:
 R-NCO + HO-R' ==> R-ure-R'

If these molecules have two reactive groups in their structures (diisocyanates and diols) the reaction forms a long chain (a polymer):
....R'-ure-R-ure-R'-ure-R-ure-R'..

It may take a while before I figure out exactly what that means, I'll have to do some more reading and try again! 

 Thanks again for your reply's Mr. Peanut