Thermoset droplet curing performance in the microbond test

Users of the microbond test largely assume that the properties of a microbond resin droplet are essentially equivalent to those of a macroscale specimen. However, there currently does not exist a standardised methodology for determining the cure state of droplet specimens used in the microbond test. In this paper, we present a technique for microbond test users to understand better the properties of thermoset droplet specimens. A novel sample preparation technique involving curing epoxy droplets on thin steel filaments allowed for high-throughput determination of microbond droplet cure state using a conventional benchtop spectrometer. Parity between steel filament and glass fibre microbond samples was confirmed by infrared microspectroscopy. It is shown that cure schedules used in the manufacture of composite parts produced microbond droplets with degrees of cure lower than that of bulk matrix specimens subjected to an identical thermal history. For commercial resin systems, testable microbond droplets could only be produced when a room temperature pre-curing time of at least 2 hours was introduced. It is concluded that microbond testing be supported by some method of droplet cure state characterisation to ensure that interfacial effects are not artefacts of droplet sample preparation.