Understanding Relative Humidity vs Temp

It was not until I started working with moisture cured urethanes that I really understood how relative humidity and temperature affected the amount of moisture in the air. Everyone that works with urethanes or polyaspartics should spend a moment studying this relationship. It will save you much headache!

Image Source: Engineering toolbox and Cleaning Technologies group

I have included a chart showing US terminology on the left and and metric on the right so you can easily refer to the chart that utilises your units of measure.

Somewhere along the line, I had forgotten that the term "Relative Humidity" was not an absolute figure. So I was not taking into account that even though the relative humidity was 50%, there is a significant difference in the actual amount of moisture particles in the air in hot weather compared to cool weather.

Infact if you look at the 50% RH curve and read the amount of water at say 60degF and compare that to the amount of water at 80degF, you will see that there is twice as much water in the atmosphere. The same can be seen if you look at the metric chart and compare the 50% RH curve at 10degC compared to 30degC.

Where this relationship became really important was that if we have twice as much water in the atmosphere then we have twice as many reaction activators if we are using a moisture cured urethane. So the crosslinking will be much faster, the working time less and importantly the recoat window will be less.

Likewise, if you are using a polyaspartic, the reaction is greatly impacted by the amount of moisture in the atmosphere, so you may have a polyaspartic that behaves itself at 50%RH at 60degF but you can't keep a wet edge at 50%RH at 90degF.

So next time you are using a MCI urethane or a Polyaspartic, just make a note of the RH and the temperature and make the necessary adjustments to your workplan before you start rolling out the product.

I learnt this lesson the hard way, or as soem would say  "I paid for my education". Have you seen this in practice when rolling out your favourite urethane or polyaspartic?

As always... Take care and keep smiling 


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Tabish Pawaskar wrote:
29 Sep 1:35pm
I work with Urethanes and Poly Aspartic coatings. The manufacturer says that RH must be 75% or 95%, however there are no mechanisms or guidelines to measure the RH.

Can you please elaborate more on practical measurements at site that can be done to determine correct RH value?
Resin Jack
Resin Jack replied with:
29 Sep 7:27pm
Thanks for the question Tabish. I use an electronic Delfesko Positector 200 (but there are other brands) which will give you accurate climatic conditions like ambient, surface, dewpoint and RH conditions. There are manual devices that you can use and the calculate dew point also.

Does that help?

Take Care and keep smiling