The curing of novolacs with paraformaldehyde

The curing of novolacs with paraformaldehyde is analyzed with several experimental techniques (DSC, IR, sol fraction) as well as statistical arguments. Results for typical novolacs show that a maximum crosslinking density is attained at a mass ratio mP/mN close to 0.20. However, at mP/mN greater than 0.10, hemiformals appear in the cured product, increasing the amount of extractable formaldehyde. The heat of reaction increases linearly from mP/mN = 0 to 0.15, attaining a maximum value of −ΔH = 98 cal/g novolac at mP/mN ∼ 0.20. The curing kinetics follows an overall second‐order law with an activation energy E = 32.5 kcal/mol and a preexponential factor A = 1.1 × 1015 min−1 (both for mP/mN > 0.1).