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Coatings Psychrometric Chart

Polyurethane coatings cure or polymerize to high molecular weight films by reacting with moisture in the air.  This reaction takes place before the solvent is completely evaporated.

Under optimum conditions of 77 degrees F (25C) and 50% relative humidity there are 70 grains of water in the air to react.

Any climate conditions that do not have a minimum of 42 grains of moisture in the air will retard cure and result in reduced coating properties. 

A simple psychometric chart is provided below to illustrate how the grains of moisture can be determined when the percentage of relative humidity and dry bulb temperature are known.

Moisture levels below 42 grains will severely retard the cure of polyurethane coatings beyond the usual 7-10 days at optimum conditions.

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