The idea: count reactive groups, not grams
Two-component coatings cure when reactive groups pair off: one NCO with one OH or NH, one epoxide ring with one amine hydrogen. An equivalent weight is the mass of material that carries one reactive group. Once you know the equivalent weights of both sides, the mix ratio follows directly.
Isocyanate systems (polyurethane, polyurea)
The isocyanate equivalent weight comes from % NCO, measured by ASTM D2572: EW(NCO) = 4,202 ÷ %NCO. For a polyol, the equivalent weight comes from the hydroxyl number: EW(OH) = 56,100 ÷ OH no. For an amine B-side, use its amine equivalent weight.
The isocyanate index compares the groups actually supplied: index = (NCO equivalents ÷ active-H equivalents) × 100. An index of 105 means 5% excess isocyanate, a common cushion against stray moisture.
Isocyanate mix-ratio calculator
Parts of A-side per 100 parts B-side, by weight.
OH number → equivalent weight
Convert a polyol’s hydroxyl number into its equivalent weight.
Epoxy systems
Epoxy resins are characterized by EEW (epoxide equivalent weight, from ASTM D1652) and hardeners by AHEW (amine hydrogen equivalent weight = molecular weight ÷ number of active N–H hydrogens). The stoichiometric amount of hardener in parts per hundred resin (phr) is phr = AHEW × 100 ÷ EEW.
Epoxy phr calculator
Stoichiometric hardener per 100 parts resin, by weight.
Weight vs. volume
Equivalent-weight math gives a ratio by weight. Plural-component equipment usually meters by volume, so divide each part by its density to convert. Spray polyurea is commonly formulated to 1:1 by volume for this reason.
What happens off-ratio
- Too much isocyanate: harder, more brittle film; excess NCO slowly reacts with moisture.
- Too little isocyanate: soft, tacky or cheesy areas, lower chemical resistance.
- Off-ratio epoxy: unreacted resin or amine, blush, softness and poor chemical resistance.