Determination of the extent of reaction for the system carboxyl‐terminated polybutadiene‐epoxide beyond the gel point by infrared spectroscopy

Curing reactions were investigated after the gel point, using infrared transmittance and reflection spectrophotometric techniques. In the system carboxyl‐terminated polybutadiene (CTPB)/Epon X801 or Epotuf STF‐6, catalyzed with Fe(III)‐acetyl‐acetonate (FeAA), at an equivalent ratio of carboxyl to epoxide groups, the final extent of reaction of COOH groups was found to be between 64% and 77% for eight CTPB samples. Increasing the ratio of epoxide to carboxyl groups (up to 1.6) resulted in an increase in the conversion, but the complete reaction of COOH groups could not be accomplished. The final extent of reaction also depends upon the functionality of the crosslinking agent. The lower the functionality of the epoxide, the higher the conversion of COOH groups. The use of chromium naphthenate as a catalyst for epoxide–COOH reactions gave higher conversions than FeAA. This result is related to side reactions catalyzed by the chromium naphthenate. The change in mechanical properties and the change in extent of reaction were followed simultaneously; it was found that no further development of mechanical properties occurred when the curing reaction between COOH and epoxide groups had ceased.