Dual-Cure Propargyl Novolac-Epoxy Resins: Synthesis and Properties

Abstract

A partially propargylated oligomeric phenolic novolac (PPN) was synthesised by the Williamson's reaction of a novolac with propargyl bromide and was characterised. Reactive blending of the PPN resin with epoxy resin resulted in a dual cure thermoset. The curing occurred through the phenol-epoxy reaction at about 135°C together with a Claisen rearrangement and the addition polymerisation of propargyl ether groups at around 235°C. The phenol-epoxy reaction could be catalysed by triphenyl phosphine, without affecting the curing of the propargyl ether groups. The cure characterisation was done by DSC and DMA. The effect of the phenol-epoxy ratio on the adhesive properties of the PPN-epoxy blend between aluminium adherends was evaluated. The system exhibited moderately good lap shear strength which was optimised for a phenol:epoxy equivalent ratio of 2:1. Good retention of the properties was observed at 100°C. On a comparative scale, the diglycidyl ether of bisphenol-A was better than the novolac epoxy, in improving the adhesion. Addition of conventional matrix tougheners was not conducive to improved adhesion. The system formed a good composite with glass fabric. The mechanical properties of the glass laminates were independent of the phenol-epoxy stoichiometry. The cured system possessed good thermal stability and a Tg>300°C. The epoxy adversely affected the Tg and the thermal stability.