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The use of recycled Carbon Fibers (rCFs) into polylactic acid (PLA) matrix to produce new reinforced 3D printing filaments was assessed. Filaments containing 5 and 10 wt% of rCFs were manufactured and, in order to compare the effect of the partial oxidized surface of recycled CFs on the adhesion with PLA matrix, filaments of pure PLA and with the same concentration of washed virgin Carbon Fibers (vCFs) were also produced. As expected the addition of CFs impacts the 3D printed thermal and mechanical properties, leading to the reduction of thermal expansion up to 85% and to a substantial increase of the storage modulus up to 7.1 GPa for PLA/vCF10. Furthermore this study underlines that the use of rCFs leads to a thermo-mechanical performance well comparable with that of the analogue virgin fibers based materials, supporting the suitability of recycled CFs for 3D printing applications. Indeed, when the fiber aspect ratio is not a crucial detrimental point, the quality of recycled CFs has now achieved performances that make them a better alternative to the pristine fibers, in particular when economic aspects are also taken into account.

This work is focused on the mechanical characterization and fracture surfaces analysis of thermosetting polymers reinforced with short, randomly oriented, recycled carbon fibres (rCFs). This work aims at evaluating fibre/matrix adhesion between recycled CFs-reclaimed via pyrolysis followed by controlled oxidation of the pyrolytic char-and different polymer matrices, namely epoxy and vinyl ester resins. The latter is the main focus in this work, being amongst the most widely used thermosetting resins in SMC processes, which are the typical target for short rCFs. The evaluation of the properties of this new recycled carbon fibre reinforced polymer (rCFRP) has been via thermogravimetric analysis, dynamic mechanical analysis, stress/strain tests in tensile mode, and a subsequent analysis of the fracture surfaces by means of images analysis obtained by macrophotography, Optical Microscopy and Scanning Electron Microscopy. The comparison amongst the results allowed to evaluate the influence of the polymer nature and of the adhesion quality between fibres and polymeric matrix, mainly on the mechanical properties of the rCFRPs.

Coupling bio-based epoxy resins and recycled carbon fibers: New advances in more sustainable CFRPs

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 (Tg) 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 GI (up to +180%), while GII 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 GI), acting as a release film. The achieved delamination resistance improvements, combined with the retention of both the original laminate stiffness and Tg, pave the way to the extensive and reliable application of NBR/Nomex rubbery nanofibrous mats in composite laminates.

Graphene (G) can effectively enhance polymers’ and polymer composites’ electric, thermal, and mechanical properties. Nanofibrous mats have been demonstrated to significantly increase the interlaminar fracture toughness of composite laminates, hindering delamination and, consequently, making such materials safer and more sustainable thanks to increased service life. In the present paper, poly(ethylene oxide) (PEO), polycaprolactone (PCL), and Nylon 66 nanofibers, plain or reinforced with G, were integrated into epoxy-matrix Carbon Fiber Reinforced Polymers (CFRPs) to evaluate the effect of polymers and polymers + G on the laminate mechanical properties. The main aim of this work is to compare the reinforcing action of the different nanofibers (polyether, polyester, and polyamide) and to disclose the effect of G addition. The polymers were chosen considering their thermal properties and, consequently, their mechanism of action against delamination. PEO and PCL, displaying a low melting temperature, melt, and mix during the curing cycle, act via matrix toughening; in this context, they are also used as tools to deploy G specifically in the interlaminar region when melting and mixing with epoxy resin. The high extent of modification stems from an attempt to deploy it in the interlaminar layer, thus diluting further in the resin. In contrast, Nylon 66 does not melt and maintain the nanostructure, allowing laminate toughening via nanofiber bridging. The flexural properties of the nanomodifed CFRPs were determined via a three-point bending (3PB) test, while delamination behavior in Mode I and Mode II was carried out using Double Cantilever Beam (DCB) and End-Notched Flexture (ENF) tests, respectively. The lack of a positive contribution of G in this context is an interesting point to raise in the field of nanoreinforced CFRP.

Two novel protocols for the chemical valorization of polyhydroxybutyrate (PHB) were developed, aiming at the production of two bio-based molecules: methyl 3-hydroxybutyrate (MHB) and methyl 3-methoxybutyrate (MMB). Optimized reaction conditions were applied to pure PHB and PHB inclusions inside bacterial cells as starting materials. MHB was synthesized through a single-step catalytic methanolysis, while MMB was synthesized through a three-step process: thermolytic distillation to give crotonic acid (CA), esterification to give methyl crotonate (MC), and oxa-Michael addition of MeOH. The obtained MHB and MMB were tested as solvents for the recovery of PHB itself both from freeze-dried single strain cultures (SSC) and mixed microbial cultures (MMC) with low to medium contents of PHB (22-57 wt %). High PHB recovery was achieved: up to 96 ± 1% through MHB and up to 98 ± 1% through MMB. Extraction from MMC slurry (with a PHB content of 39% on dry weight) was also performed, recovering 77 ± 2% using MHB and 92 ± 2% using MMB. High purities and excellent molecular weights and polydispersity indexes of extracted PHB were obtained with both MHB and MMB. Solubility in water, octanol/water partition coefficients (log Kow), and aerobic ready biodegradability of both solvents were also evaluated.

Delamination is the most severe weakness affecting all composite materials with a laminar structure. Nanofibrous mat interleaving is a smart way to increase the interlaminar fracture toughness: the use of thermoplastic polymers, such as poly(ϵ-caprolactone) and polyamides (Nylons), as nonwovens is common and well established. Here, electrospun polyethylene oxide (PEO) nanofibers are proposed as reinforcing layers for hindering delamination in epoxy-based carbon fiber-reinforced polymer (CFRP) laminates. While PEO nanofibers are well known and successfully applied in medicine and healthcare, to date, their use as composite tougheners is undiscovered, resulting in the first investigation in this application field. The PEO-modified CFRP laminate shows a significant improvement in the interlaminar fracture toughness under Mode I loading: +60% and +221% in GI,Cand GI,R, respectively. The high matrix toughening is confirmed by the crack path analysis, showing multiple crack planes, and by the delamination surfaces, revealing that extensive phase separation phenomena occur. Under Mode II loading, the GIIenhancement is almost 20%. Despite a widespread phase separation occurring upon composite curing, washings in water do not affect the surface delamination morphology, suggesting a sufficient humidity resistance of the PEO-modified laminate. Moreover, it almost maintains both the original stiffness and glass transition temperature (Tg), as assessed via three-point bending and dynamic mechanical analysis tests. The achieved results pave the way for using PEO nanofibrous membranes as a new effective solution for hindering delamination in epoxy-based composite laminates.

The present work reports the use of recycled carbon fibers (rCF), obtained from pyro-gasification treatment of carbon fibers reinforced polymers (CFRP), to produce a thermoplastic composite filament for additive manufacturing, in particular fused deposition modeling (FDM) process. Polylactic acid (PLA), a thermoplastic biobased and biodegradable polymer, is used as matrix for the composite filament, as it is the most common plastic used in FDM due to its good mechanical properties, stiffness, and strength. Upon production process optimization, filaments with rCF loadings of 5 and 10% wt are produced and analyzed. A particular attention is devoted to the evaluation of the production process on the carbon fibers (CFs) length and the study of the thermal and mechanical properties of the obtained composite materials.

Nanofibrous mats provide substantial delamination hindering in composite laminates, especially if the polymer (as rubbers) can directly toughen the composite resin. Here, the well-known Nylon 66 nanofibers were impregnated with Nitrile Butadiene Rubber (NBR) for producing rubber/thermoplastic membranes for hampering the delamination of epoxy Carbon Fiber Reinforced Polymers (CFRPs). The starting polyamide mats were electrospun using two different solvent systems, and their effect on the mat's thermal and mechanical properties was investigated, as well as the laminate Mode I delamination resistance via Double Cantilever Beam (DCB) tests. Plain Nylon 66 mats electrospun from formic acid/chloroform perform better than the ones obtained from a solvent system containing trifluoroacetic acid, showing up to + 64% vs + 53% in interlaminar fracture toughness (GI), respectively. The effect of NBR coating benefits both nanofiber types, significantly raising the GI. The best results are obtained when interleaving medium-thickness and lightweight mats (20 µm, 9–10 g/m2) with 70–80 wt% of loaded rubber, achieving up to + 180% in GI. The work demonstrates the ability of NBR at improving the delamination hindering of common polyamide nonwovens, paving the way to the use of NBR-coated Nylon 66 nanofibers as effective interleaves for GI enhancement and overall composite safety improvement.

Delamination is the main responsible for structural failure of composites having a laminar structure. In the present work, polyamide (Nylon 66) nanofibers, even impregnated with uncrosslinked nitrile butadiene rubber (NBR), are interleaved into epoxy-based carbon fiber reinforced polymer (CFRP) laminates with the aim to counteract the delamination phenomenon. The performance of nano-modified composites using both the nanofibrous mat types, that is, Nylon 66 and NBR-impregnated Nylon 66 membranes, is investigated. Mode I loading tests show a significant improvement of the interlaminar fracture toughness of rubber-modified CFRPs, especially in the G(I,)(R) (up to +151%). The improvement in the G(I,)(C) is less pronounced, but still significant (up to +80%). The achieved results are very encouraging and pave the way to the use of such Nylon-NBR hybrid mats for hindering delamination.