The effect of pressure on the curing behavior of unsaturated polyester resins

Abstract The effect of pressure on the curing behavior of unsaturated polyester resin was investigated, both experimentally and theoretically. The resin used was a general‐purpose unsaturated polyester resin and the initiator used was t‐butyl perbenzoate. A series of isothermal runs with differential scanning calorimetry (DSC) were made at various levels of cure pressure. It was found that the rate of cure was retarded under pressure, and that the ultimate degree of cure went through a maximum at a certain pressure as the cure pressure was increased from atmospheric pressure to 6.21 MPa (900 psi). It was interpreted that pressure has two competing effects on the curing behavior of unsaturated polyester resin; one is a free volume effect that hinders the curing reaction and the other is a thermodynamic effect that favors it. Therefore, when the pressure is higher than a certain level, the free volume effect becomes predominant over the thermodynamic effect, the ultimate degree of cure diminishing as the cure pressure is increased beyond that level. Theoretical interpretation of the experimental results is given, using a mechanistic kinetic model developed in our previous publication.