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Polyurethane Rail Road Analogy

Have you ever wondered what we mean when we talk about the curing process of a polyurethane coating, or why it is an effective water barrier once cured?   Ever needed a better way of explaining what a polyurethane coating is without needing a chemistry book?  Here is a useful analogy that should help explain what is going on in that poured out polyurethane film.  Terms and language that should help connect what is really going on will be in italics.  Additionally, this paper will briefly tie in this idea of curing to the behavior of acrylic roof coatings.

In The Pail

We can illustrate what is going on with a polyurethane coating at the molecular level by imagining a railroad yard.  Our rail yard is made up of boxcars of identical length and build.  The boxcar represents one segment of polyurethane polymer, and it is where the majority of the properties of the film (flexibility, strength, etc.) reside.  The couplings on either end of the boxcar represent the isocyanate reactive sites

In a large rail yard the tracks split off into many different spurs via side tracks.  Picture our identical boxcars (polyurethane segment) on each track connected to each other in various lengths (chains).  This boxcar-boxcar coupling is very strong, so once coupled, it doesn’t come apart.  One spur may have a chain of five boxcars, then a chain of thirteen boxcars, and maybe a chain of three boxcars, and so on, each group of chains all independent of one another.  Then another spur will have four boxcars attached to one another, followed by a separate chain of 24 boxcars, and so on.  The numbers of units are not important, just the image of random chain lengths.

Picture this rail yard spreading out for miles and miles with spur after spur of variously sized couplings of identical boxcars and you get an idea of what is inside the pail of a polyurethane coating.  In actuality, the tracks and subsequent boxcar chains would be weaving in and out of each other in random 3-dimensional ribbons of polymer chains (more like spaghetti in a bowl), but to keep things simple, will assume everything is linear and parallel, although at intervals, to help our illustration, there are switch tracks to go from one track to another.

On The Roof – The Curing Process

Once a polyurethane coating is poured out onto the roof, another component is added…atmospheric moisture.  Moisture is represented by the train engine.  So, our rail yard of identical boxcars coupled at various lengths on miles and miles of spurs are gradually peppered with train engines that weave from spur to spur using the switch tracks.  The engine’s sole job it is to link with either end of the long boxcars chains.  The first engine links with any boxcar coupling it comes across; whatever is closest to it at the time.

Likewise, water reacts with the isocyanate end group on the polyurethane chain.  This process releases carbon dioxide gas (those bubbles you sometimes see on the surface of the coating) and forms an amine.  An amine group is very reactive and reacts rapidly with another isocyanate reactive site in its vicinity

By way of illustration, the train then pulls the coupled boxcars to another line of boxcars to form a funny looking train where the engine is between two sets of boxcars.  The engine becomes a urea once it has been coupled on either side with boxcars. The ends of the two chains of boxcars can be coupled further with other engines.

The fast-reacting amine and isocyantate form a urea, a very rigid strong bond.

The number of train engines (moisture) present dictates how often the coupling occurs.  Fewer engines, the slower the coupling; more engines, the faster the coupling, or,

The less moisture, the slower the cure while more moisture speeds up the cure.

So, drier areas like Arizona will cause a polyurethane coating to cure slower than in relatively more humid areas like Georgia in the summer.  Gradually, with enough of this type of coupling, the rail yard becomes full of very large connected (crosslinked) chains.  The side tracks become full and the engines (moisture) that arrive fresh on the scene are finding it more and more difficult to find any boxcars to couple with.  In the coating, this is called the cure process.  Pretty soon, the rail yard becomes so saturated with boxcars coupled to engines that everything grinds to a halt.  The film is cured and will not allow water to pass through.

How does this curing process compare with an acrylic coating?

The big difference between a polyurethane and an acrylic coating is the fact that a polyurethane coating undergoes the chemical reaction previously described.  In other words, it cures. During this reaction, carbon dioxide is released (bubbles), and the polyurethane polymer chains become linked together with very strong urea bonds. Additionally, a secondary bonding phenomenon, hydrogen bonding, takes place between the urethane chains that are rubbing along each other like spaghetti in a bowl. This secondary bond also contributes to the coating’s incredible strength. 

In contrast, most acrylic coatings undergo a drying process and do not cure, although they are often incorrectly described as “curing systems”. In an acrylic or acrylic-latex water-based coating, droplets of polymer are suspended in an emulsion surrounded by surfactant and water.

Once the coating is applied, the water begins to evaporate, reducing the space between the droplets, and causing them to bump against other droplets (below at left). Soon, enough water evaporates that the walls of the droplets cannot maintain their shapes and break against other droplets. The polymers within the broken droplets intermix, forming a film that hardens with continued drying. This film-forming process is called coalescence (below at right).

It should be noted here that acrylic coatings can be modified performance-enhancing additives that induce chemical cross-linking within the polymer, but usually, their primary film formation process is through coalescence. 

The upside to acrylic coatings is that they are inexpensive, waterbased (no solvent), and are relatively more UV stable than an aromatic polyurethane coating like a polyurethane coating.  The downside is that acrylic roof coatings are not as strong as a cured polyurethane and will fail eventually in ponding water.  Additives and other chemistry can be employed to slow the process, but in ponding water, acrylic-based coatings eventually will begin to delaminate from their substrates.  A polyurethane coating, on the other hand, maintains its integrity under ponding water (after it cures).  A polyurethane coating is also much stronger than most acrylic roof coatings on the market, this, again, a testament to its curing ability.  A strong film will be able to withstand freeze/thaw cycles, flash cooling during a hot summer day’s rain storm, the day to day motions of a building. 

Conclusions

The key points to take with you from this article:

  • Obtain a level of understanding when you talk about a polyurethane coating as a “curing system”, in other words, what makes a polyurethane coating cure?
  • Understand that a cured polyurethane film is a strong, resilient and water proof surface.
  • Realize that acrylic roof coatings coalesce, meaning it is a “film forming” process, not curing process.
  • Acrylic coating manufacturers have the ability to use chemistry to induce a cure reaction, but this is usually as a second step, or “post add” where a small jar of contents are added to the bucket and stirred in at the job site.  Incomplete mixing will lead to spotty curing on the surface.  The only additive a one component polyurethane coating needs to cure is moisture supplied by the atmosphere.
  • Acrylic roof coatings tend to fail in ponding water.  Polyurethane coating fare better in ponded water areas.
  • The more moisture in the air, the quicker the cure rate of polyurethane coatings.  So, more humid locations will cure the polyurethane film quicker.  Drier areas will cure a polyurethane slower.
  • Although not mentioned, temperature will play a lesser role in curing. Warmer temperatures speed up cure rate and with colder weather, the cure rate slows.

For more information, visit the Center for Polyurethanes Industry.

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