What are the effects of cold weather on urethane?

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Cold Urethanes

As winters icy fingers slowly grip the country, it’s a good time to review the effect of cold temperatures on the resin systems you are using. Almost all ambient cure resin systems, from silicones and urethanes to epoxies, are sensitive to the ambient temperature. As the temperature drops, the speed of the chemical curing reaction is slowed. Apart from delaying the time to reach full cure, additional problems can appear. Urethanes for example can have dull spots or even wrinkling occurring where condensation has formed on the slowly curing surface. This is caused by the isocyanate in the polyurethane curing agent reacting preferentially with the surface moisture rather than the base resin.

Particularly in cold and wet conditions (otherwise known as Cape Town winter), ambient cure epoxy systems will experience amine blushing, which is also known as amine blooming or carbamation. This develops on the laminate surface as an oily, waxy or milky film, although it is sometimes amber to clear in color. The amine component in the hardener reacts with moisture and atmospheric carbon dioxide to form the salts of ammonium carbamate. These amine compounds are often hygroscopic and very efficient scavengers of carbon dioxide (CO2) from the air. In indoor environments the CO2 levels may be increased 2 to 3 times compared to outdoor environments, while the presence of gas burning motors such as gas heaters and fork lifts will raise CO2 levels even higher. Gas burners may also produce copious quantities of water vapor resulting in exacerbated blushing problems. The water soluble salts interfere with adhesion if over coated with epoxy or paints, as they act like a strong release agent.

If amine blush is detected it should be removed by thoroughly washing the surface with warm to hot water and a Scotchbrite scourer. Note that peelply is also an effective deterrent against amine blush contaminating secondary bonding surfaces. Ideally, the composites workshop should be kept at the same temperature all year long, which will lead to predictable processing results.