Self-Assembled NBR/Nomex Nanofibers as Lightweight Rubbery Nonwovens for Hindering Delamination in Epoxy CFRPs

Abstract

Still today, concerns regarding delamination limit the widespread use of high-performance composite laminates, such as carbon fiber-reinforced polymers (CFRPs), to replace metals. Nanofibrous mat interleaving is a well-established approach to reduce delamination. However, nanomodifications may strongly affect other laminate thermomechanical properties, especially if achieved by integrating soft materials. Here, this limitation is entirely avoided by using rubbery nitrile butadiene rubber (NBR)/Nomex mixed nanofibers: neither laminate stiffness nor glass-transition temperature ( T g ) lowering occurs upon CFRP nanomodification. Stable noncrosslinked nanofibers with up to 60% wt of NBR were produced via single-needle electrospinning, which were then morphologically, thermally, spectroscopically, and mechanically characterized. NBR and Nomex disposition in the nanofiber was investigated via selective removal of the sole rubber fraction, revealing the formation of particular self-assembled structures resembling quasi-core–shell nanofibers or fibril-like hierarchical structures, depending on the applied electrospinning conditions (1.10 and 0.20 mL/h, respectively). Mode I and Mode II loading tests show a significant improvement of the interlaminar fracture toughness of rubbery nanofiber-modified CFRPs, especially G I (up to +180%), while G II enhancement is less pronounced but still significant (+40% in the best case). The two nanofibrous morphologies (quasi-core–shell and fibril-like ones) improve the delamination resistance differently, also suggesting that the way the rubber is located in the nanofibers plays a role in the toughening action. The quasi-core–shell nanofiber morphology provides the best reinforcing action, besides the highest productivity. By contrast, pure Nomex nanofibers dramatically worsen the interlaminar fracture toughness (up to −70% in G I ), acting as a release film. The achieved delamination resistance improvements, combined with the retention of both the original laminate stiffness and T g , pave the way to the extensive and reliable application of NBR/Nomex rubbery nanofibrous mats in composite laminates.

New Application Field of Polyethylene Oxide: PEO Nanofibers as Epoxy Toughener for Effective CFRP Delamination Resistance Improvement

“…Regarding Mode I, a similar behavior was found for CFRPs reinforced with rubbery nanofibers made of NBR mixed with Nomex 25 or PCL: 39 the toughening action was so effective that multiple crack paths and even a carbon fabric break occurred. In the latter work, PCL-only nanofibers were tested too, revealing a low attitude for increasing the interlaminar properties.…”

“… 24 Thus, localized interlaminar toughening would be preferred for retaining the overall outstanding properties of high-performance FRP laminates. 25 …”

“…Matrix toughening of brittle thermosetting epoxy resins by adding thermoplastic or rubber polymers is a common practice for contrasting the formation and propagation of microcracks. − Usually, the modification affects the resin bulk, potentially lowering thermal and mechanical composite properties, such as the glass transition temperature ( T g ) and elastic modulus and strength . Thus, localized interlaminar toughening would be preferred for retaining the overall outstanding properties of high-performance FRP laminates …”

“…A wide range of thermoplastic polymers have been tested as nonwoven interleaves for increasing interlaminar fracture toughness. Most of them are polyamides (mainly Nylon 6 and Nylon 66) and the polyester poly­(ε-caprolactone) (PCL). ,,− Specialty polymers, such as polyvinylidene fluoride (PVDF), and aromatic ones, such as polysulfone (PSU) and polyetherimide (PEI), have also been explored. ,, Recently, new elastomeric nanofibers based on nitrile butadiene rubber (NBR) were developed , and used for increasing the interlaminar fracture toughness of epoxy-based carbon FRP (CFRP) laminates ,, as well as for enhancing their damping. , These nanofibers enhance the energy release rate ( G ) up to +480%, an enhancement higher than the one usually achieved by interleaving conventional thermoplastic membranes, whose improvement is no more than 150% in general. ,,− …”

“…Researchers have made a great effort to enhance the toughening effect of well-established polymeric nanofibers by adding nano-reinforcements, such as carbon nanotubes, ,, or by combining different polymers, for example, producing core–shell nanofibers, , polymer-impregnated nanofibers, , and blended ones, such as the above-mentioned rubbery nanofibers. ,, However, searching for new uses of “basic” and well-established materials is equally important for reducing costs and complexity of manufacturing advanced materials such as nanofibers with integrated high-performance nano-reinforcements or core–shell ones.…”