Post-curing for enhanced performance of epoxy coating systems

Post-curing for enhanced performance of epoxy coating systems

“Temperature:  friend or foe?”

 

We received an urgent call from a client several weeks ago.  A critical process tank was forced out of service.  Engineers and contractors were working around the clock on the tank and other associated systems, as needed to get it back into service ASAP.

 

We had about 36 hours to do two weeks of intake, document review, and vetting of responses in order to identify a coating system and develop an installation specification for a product that was:

 

  1. Immediately available. There was no allowance of time for materials to be manufactured or shipped to the site.

 

  1. Expected to provide at least 6-years of no-maintenance service life.

 

To make a very long, complex and mostly confidential story short, we had identified various epoxy novolac coating systems which appeared to have the chemical resistance characteristics that would get our client through six years to the next turnaround (TAR).

 

But just barely.  The temperature, chemicals, pH swing and cleaning protocols for the tank were right at the cutting edge of the performance characteristics of these systems.

 

As we were working frantically to develop an installation specification, (we needed to determine blast, SCAT testing, surface temp requirements, DFT, etc.), I remembered a project I had worked on some nearly 30 years before.

 

(As in a bad movie, please visualize the page going squiggly with squeaky music indicating it’s time for a flash-back).

 

About 30 years ago, in the days of hand-shakes, integrity and when words mattered*, I was involved in a fascinating project for an international cosmetic company.

 

I had just started working with an industrial tank lining company which had lined a series of underground storage tanks (USTs) at the client’s facility.  The tanks contained various surfactants, colorants, odorants, and other liquids used in the manufacture of lipstick, eye-shadow, blush and whatever else my wife and three daughters’ might slather on.

 

We had an exceptionally good rapport with our client and, in fact, helped them develop their confined space training program.

 

We received a call from one of their reliability engineers who told us that, due to a change in process, they were going to be changing the product in one of the underground tanks, and they called to ensure that the existing coating system was compatible with the new product.

 

Firstly, it is important to note that the client is, and remains, highly competent.   Most companies would have simply switched product and, when the coating system failed, become angry at us for the failure – but these guys were sharp.

 

We gathered information on the new product which, by the way, would be stored at temperatures above ambient.

 

This was important.  Really important.  Because in the Chicago area, once you get below several feet into the ground, the temperature hovers in the 50s.  This new product would be stored, and processed, at over 90°F.

 

There is something called the Arrhenius equation, part of which reads, and I’m paraphrasing, “For many common chemical reactions at room temperature, the reaction rate doubles for every 10 degree Celsius increase in temperature.”

 

The Arrhenius equation came to bite us, and different client, in the hindquarters, a about 20 years ago in exceedingly unusual manner.

 

AN UNEXPECTED ALCOHOL STILL – WHERE THERE SHOULDN’T HAVE BEEN ONE

 

We had lined the interior of a process tank which contained trace amounts of alcohol and mostly water.  We had done our due diligence and selected a coating for optimal long-term performance.  We were shocked, and disappointed to find, however, that just a few months after the tank was put back into operation, the coating system had started to fail.

 

Well.  Sort of.

 

This was long before the days of smart phones and texting, and we were not immediately able to get a photo from the client, so I drove out.

 

When I put my head near the manhole (taking great care not to break the plane of the manhole and violate any OSHA or safety regulations) I was stunned at what I saw.  Roughly 70 of the coating was perfect – it was still glossy.  But there was a sharp line, as if someone had taken a utility knife, about 2’ down from the top, dead center of the tank, where the coating was gone.  The tank hadn’t even started to rust – the white metal blast looked pristine.

 

It took me only a few minutes to determine the mode of failure.

 

 

TEMPERATURE AS FOE:

 

We were initially informed that the tank operated at ambient temperatures, and further that the flow-through rate was high. In speaking with one of the engineers however, it turned out that the tank operated at between 95°F and 110°F.  The engineers considered that ambient, in a facility where temperatures were often above 185°F for most processes.

 

The increased temperature caused the trace amount of alcohol in the water to evaporate (just like in a still) and condense in the ullage, or vapor space, at the top of the tank, where the concentration of alcohol was now nearly 100%.

 

The shame of it was that if we had known, all we would have needed to do to make the existing coating compatible was to post cure it for roughly 6 hours at 180°F, which is exactly what we did after conducting repairs.

 

Which brings us back to our cosmetic client and the underground storage tank…

 

 

 

TEMPERATURE AS FRIEND:

 

So, the new product in the UST for our cosmetic client 30 years ago was going to be stored at roughly 100°F – substantially higher than the normal operating temp of around 55°F, due to the tank being in the ground, where all else being equal, the temperatures are cool and stable.

 

We contacted the manufacturer and head chemist (who, by the way, was a close family friend) who called back and said all we needed to do was empty the tank, wash it down, dry it, and post cure it to 175°F for 12 hours, or 250°F for 2 hours, or anywhere in between based on a chart he was mailing to us (yes, this was even before fax machines).

 

I was new in the industry and had never heard such a thing.  He went on to explain it.

 

Epoxies, and other coating systems, will “cross-link” their polymer chains within the coating film.  A good friend of mine is a polymer scientists and physicist at NIST, and he explained it by visualizing a big pot of cooked spaghetti.  The individual strands would be the polymer chains.  When you add heat, the space between the individuals strands of spaghetti get smaller as they are compressed together.  The tighter the cross linking, the more inert the coating, the lower its permeation rate, and the more chemically resistant it becomes.

 

Me being a died-in-the-wool coatings nerd thought, and still think, this is one of the coolest things I’ve ever heard.  I asked my friend the chemist if it mattered that it had been in service for several years.  He said, and I’m quoting here, “Nope.”

 

So that’s what we did.

 

The client was thrilled – as they had been budgeting for a complete relining.

 

*As a brief side-note, we ran into a similar situation a few years ago where the coating manufacturer initially recommended complete removal of the perfectly good lining system and replacement with a different one.  I asked about the post-curing option, and they said, oh yeah, you could do that too.  Back in the day, we would work on six-digit projects on a hand shake or phone call, without a contract.  Clients knew we would do the right thing, over the expedient or unnecessary thing – and, of course, do only those things which were ethical and in their best interest.  I fear those days, in many realms, have passed.

 

 

JUST THIS PAST MONTH

 

(Picture squiggly pages again, as we return to the present)

 

Whereas post-curing was something we had done on a regular basis for decades prior, I hadn’t remembered, or even heard about any instances for decades.

 

I immediately contacted technical support from three different manufacturers, and none of them knew what I was talking about.  But it just so happened that I had worked with an old-timer, like myself, who worked for one of the coating companies.  I insisted they track him down and ask him.

 

Sure enough, he emailed me a post-curing chart from about 25 years ago.  And the document indicated that post-curing would increase chemical resistance of the coating and, coincidentally, reduce cure time – allowing the tank to get back into service faster, and be more chemically resistant.

 

I thought I had hit a home-run.  The cost to the client would be negligible – all they needed to do was turn the heating system up about 40 degrees for 12 hours.

 

Ultimately, however, the client chose a novolac from a different manufacturer.  I thought – no problem, same novolac backbone to the coating, it should also benefit from post-curing.

 

But no.  Tech support was unfamiliar with post-curing for improved chemical resistance (though they were familiar with it to reduce return-to-service time).

 

I was, and remain, confused.  It was my understanding that post-curing all epoxies made them, well, better.  And we had a direct conflict between two different manufacturers of similar materials (both 100% solids).

 

The client asked for our advice and input regarding post-curing.  While I intuited that it would provide improved performance, I could not contradict the manufacturer who said post-curing would have no impact.

 

 

Ultimately, we recommended post-curing for two reasons:

 

  1. It would reduce the overall cure-time.
  2. As my old Jewish grandmother used to say about the efficacy of chicken soup (please read with your best old-lady Jewish accent), “It couldn’t hurt.”

 

I’m hoping that the handful of people who read this blog will share their thoughts about post-curing, this specific situation and, also, pass the blog around.

 

I’m curious as to other, more enlightened, thoughts.

 

 

 

 

4 Comments
  • Garner Holdsworth
    Posted at 15:26h, 08 November Reply

    Great Article. Love your style of writing. Most of my early career (20 early years) majority of work involved lining tanks and post curing for various reasons (some of which you mentioned). Even at my age I learned something new from reading your article.

    Thank You!

    Your old friend,

    Garner

  • Warren Brand
    Posted at 17:28h, 08 November Reply

    Garner! How great to hear from you! Thank you very much for your kind words and taking the time to write. Much appreciated. Kind Regards, Warren

  • Jeff Kim
    Posted at 15:52h, 09 November Reply

    Interesting read Warren. I too have always heard that post curing epoxies will force a tighter crosslink of the polymer chains. However, I have also always heard that there is no free lunch. The trade-off for a tighter crosslink is likely to be reduced flexibility. This increased rigidity might become a liability and lead to cracking of the lining in services subject to vibration or movement caused by cargo loading and/or thermal cycling.

    As for post curing a liner that has already been in service; My concern here would be that since all linings are permeable, the moisture uptake within the coating system’s matrix would have to be evacuated in a somewhat controlled manner. The heat applied to force a post cure would have to be increased slowly to avoid the latent moisture being trapped if the coating surface tightens up too quickly.

    As you can probably tell, I’m not a scientist but I am a professional worrier.

  • Warren Brand
    Posted at 23:16h, 09 November Reply

    Hi Jeff. Thanks for taking the time to respond. I agree 100% that enhanced cross-linking will lead to a more brittle coating. This simply wasn’t an issue for this underground storage tank, but I should have mentioned it in the article. And I am familiar with the concept that all coatings are permeable. We always run into this when the issue of Holiday testing an existing coating comes up. I remain somewhat skeptical of whether or not the permeation will be problematic when Holiday testing or post-curing. How long does it take for the material to leave the coating film, for one thing? Anyway, the post-cure schedule includes a slow-ramp up. Thanks again. Warren

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