Electrospinning of Super Tough Nylon 6,6 Fibers and Their Use in Layered Composites

Abstract

Despite their obvious advantages in applications that require a high strength-to-weight ratio, delamination problems in layered composite structures continue to be a major drawback. Interleaving the layers with films or fibers of thermoplastics has been among the viable strategies to increase the interlaminar toughness. Although a wide variety of polymers have been assessed for this purpose, polyamides (such as PA 6, PA 6,6) and polycaprolactone (PCL) come forward with their high intrinsic toughness. It is also possible to combine fibers of polymers physically or embed fibers of one into the film matrix of the other. More recently, blending the polymers at the fiber level has been suggested for the sequential activation of multiple toughening mechanisms. PA 6/PCL, PCL/rubber, and PA 6,6 /rubber blends were the three blends that have been electrospun and were interleaved in the veil (i.e. tulle) form. Super tough (ST) PA’s are commercially available blends of either different PA’s or PA with impact modifiers like ethylene propylene diene monomer (EPDM) rubber and their intrinsic toughness values can be up to 50% higher compared to those of PA’s and PCL. Electrospinning of such ST-PA’s and the interleaving of their veils forms have not been studied. In this study, ST-PA 6,6 (can also be named as ST-Nylon 6,6 or ST-N 6,6) was dissolved in various solvents in various mass ratios and the mixture suitable for electrospinning was determined after consecutive scanning electron microscope (SEM) analysis. Fixing all parameters, but the electrospinning time three different areal densities (3.5 g/m2, 7 g/m2, 10 g/m2) was spun and transferred on an epoxy-impregnated carbon fiber prepreg which was laid up with unmodified layers from bottom and top. Lay-up processes were completed in accordance with standards by a double-sided bagging-vacuuming process in a cleanroom of a fully certified facility, and the final stack was cured in an industrial scale fully controlled autoclave. After the curing process, the double cantilever beam (DCB) test was carried out in accordance with the AITM 1-0053 standard. With 3.5 g/m2 veil, Mode I toughness, GIC, in the initiation increased by 21% and for the propagation it increased by 15% compared to the unmodified composite. When interleaving was 7 g/m2, GIC initiation increased by 50% and enhancement in the propagation remained low at 18%. Increasing the veil areal density to 10 g/m2, decreased the improvements in the initiation to 35% and the enhancement in the propagation decreased to 10%. Results indicate that there is an optimum areal veil concentration for maximizing the improvements against delamination. If a better solvent can be designed for ST-PA6 and even for its potential blends with PCL, for a better adhesion between epoxy and fiber surfaces, this may lead to higher interlayer fracture toughness improvements in both initiation and propagation.