PSI - Issue 82

Jet Best et al. / Procedia Structural Integrity 82 (2026) 98–106 J. Best et al. / Structural Integrity Procedia 00 (2026) 000–000

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(a)

(b)

Fig 2. PES film: (a) 0.5 mm; (b) 0.05 mm thickness cured in EC IN2 epoxy resin at 140 ° C for 3h.

Results are presented in Fig. 1. Easy Composites IN2 resin cured under scheme [1], 120 ° C for 3h, exhibited the highest storage modulus of 2293 MPa. The glass transition temperature rises with exposure to higher curing temperatures, indicating that the resin may not be fully cured under the manufacturer’s recommended curing scheme [3]. In this analysis, the glass transition temperature of the scheme [1] sample was lower than that of scheme [2], however, scheme [1] still exhibited the greatest stiffness (Carbas et al., 2014). At this stage, it was decided to use the curing scheme [1] during composite sample production. This scheme was later amended to 140 ° C for 3h in an attempt to further encourage PES/epoxy phase separation. 3.2. Phase-separation of PES films Three tests were conducted to analyse the phase separation behaviour of PES in EC IN2 resin. (1) PES 0.5 mm thickness, curing at 120 ° C for 3h Sample (1) showed little to no sign of epoxy penetration or phase separation. Sample (2) showed signs of discoloration throughout the film, indicating some epoxy penetration, but the structure of the film was still clearly intact. Sample (3) showed strong signs of epoxy penetration, with the entire film becoming translucent in colour, however, it still appeared not to have formed a homogenous mixture with the epoxy matrix. Sample (3) was observed under a microscope (Fig. 3). The distinct structure of the film separate from the surrounding epoxy, leads to the conclusion that phase-separation of the PES film was not achieved in this epoxy system under this curing regime. These results are not in line with those of (Cheng et al., 2019), where it was observed that a PES film would rapidly phase-separate in an E51 epoxy matrix once curing temperatures of 120 ° C were reached. It is possible that the elevated curing temperature of 140 ° C, with which EC IN2 is not designed to be used, caused a rapid solidification of the matrix, preventing phase-separation of the PES films as they are “locked” in-place by the rapidly hardening epoxy matrix. Further raising of curing temperature to 150 ° C resulted in discoloration and deterioration of the IN2 resin. No alternate epoxy solutions were available at hand; thus, it was decided to go ahead with films of 0.05 mm thickness and the curing scheme of 140 ° C for 3h in composite sample production, despite phase separation not being achieved. The 0.05 mm films had an average areal weight of 10 g/m 2 and were perforated by hand to aid in resin flow during infusion. (2) PES 0.5 mm thickness, curing at 140 ° C for 3h (Fig. 2 (a)) (3) PES 0.05 mm thickness, curing at 140 ° C for 3h (Fig. 2 (b))