Interlaminar Toughening of GFRP—Part I: Bonding Improvement Through Diffusion and Precipitation

Abstract

A low concentrated polystyrene (PS) additive to epoxy is used, since it is able to reduce the curing reaction rate but not at the cost of increasing viscosity and decreasing glass transition temperature of the curing epoxy. The modified epoxy is cocured with a compatible thermoplastic interleaf during the vacuum assisted resin transfer molding (VARTM) to toughen the interlaminar of the composites. Using viscometry, the solubilities of thermoplastics (TPs) polycarbonate (PC), polyetherimide (PEI), and polysulfone (PSU) are determined to predict their compatibility with epoxy. The diffusion and precipitation process between the most compatible polymer PSU and epoxy formed semi-interpenetration networks (semi-IPN). To optimize bonding adhesion, these diffusion and precipitation regions were studied via optical microscopy under curing temperatures from 25 °C to 120 °C and PS additive concentrations to epoxy of 0–5%. Uniaxial tensile tests were performed to quantify the effects of diffusion and precipitation regions on composite delamination resistance and toughness. Crack paths were observed to characterize crack propagation and arrest mechanism. Fracture surfaces were examined by scanning electron microscopy (SEM) to characterize the toughening mechanism of the thermoplastic interleaf reinforcements. The chemically etched interface between diffusion and precipitation regions showed semi-IPN morphology at different curing temperatures. Results revealed deeper diffusion and precipitation regions increase energy required to break semi-IPN for crack propagation resulting in crack arrests and improved toughness.

Effect of a Dwell Stage in the Cure Cycle on the Interphase Formation in a Poly(ether imide)/High Tg Epoxy System

“…This means that the epoxy polymeric chains/oligomers were still in a mobile state and can dissolve/diffuse further during the second dwell curing stage. 26 While at 80% DOC, the degree of cure was higher than the one corresponding to α g , which theoretically means that the polymeric chains should be completely bound due to infinite molecular weight. However, a considerable diffusion distance was still observed at 80% DOC (Figure 5).…”

“…22,24 Likewise, an increase in droplet size with the increasing dwell temperature has been observed in the literature for an epoxy system toughened with PEI. 25 On the other hand, Bian et al 26 described the effect of cure temperature and thermoplastic content on the interphase thickness and the resultant fracture toughness of the modified epoxy system. The experimental outcome showed a higher fracture toughness for samples cured at higher temperatures, which was attributed to a higher interphase thickness.…”

Methods of modifying through-thickness electrical conductivity of CFRP for use in structural health monitoring, and its effect on mechanical properties – A review

“…The reduction in the curing rate allows for the formation of a semiinterpenetrated networks between the epoxy and compatible thermoplastic. Curing at a temperature of 120 • C also showed improvement over curing at 80 • C, which was attributed to lower viscosity of the polymers [107]. The resulting composites show improved delamination resistance with increasing depth of the semi-IPNs (semi-interpenetrated networks).…”

“…The plasma treatment changes the PA12 surface increasing the topology as well as producing more functional groups that can bond with the resin during curing [9]. Bian et al [107] work looks at the interpenetration of thermoplastics and epoxy resin by choosing a thermoplastic with a good compatibility with the resin as well as adding 5 wt% polystyrene to the epoxy to reduce the curing rate. The reduction in the curing rate allows for the formation of a semiinterpenetrated networks between the epoxy and compatible thermoplastic.…”

Effects of Laser Radiation on the Wetting and Diffusion Characteristics of Kovar Alloy on Borosilicate Glass

“…Because its excellent thermal expansion match to that of borosilicate glass, kovar or Fe-Ni-Co alloy has been widely used to join with borosilicate glass. To achieve a high joint strength and hermetic sealing [2][3], chemical bonding is necessary. For a chemical bond, there must be a continuity of the atomic structure across the interface.…”