Understanding relative humidity vs temperature


It wasn't until I started working with moisture-cured urethanes that I really understood how relative humidity (RH) and temperature (T) affected the amount of moisture in the air. Everyone that uses urethanes or polyaspartics should spend a moment studying the relationship below - it will save you plenty of headaches!

I have included two charts - US units on the left, and metric units on the right - so you can easily refer to the one that makes most sense to you.

Image Source: Engineering toolbox and Cleaning Technologies group


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

In fact, if you look at the 50% RH curve and read the amount of water at 60 deg F for example, and compare that to the amount of water at 80 deg F, you'll see 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 10 deg C compared to 30 deg C.

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.

The crosslinking will be faster, the working time less, and, importantly, the re-coat window will be shorter.

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 in 60 deg F temperature, but you find you can't keep a wet edge at  50% RH at 90 deg F.

Next time you are using a MCI urethane or a polyaspartic, just take 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 some would say: "I paid for my education".

Have you seen this in practice when rolling out your favourite urethane or polyaspartic?