A Critical Evaluation of Mechanical and Fire Performance of Flax Fiber Epoxy Resin Composites

In the present work, the mechanical behavior of flax fiber reinforced polymers (FFRPs) intended for racing applications is evaluated when subjected to different environmental conditions. A significant drop of mechanical performance in the presence of water (both 100% relative humidity and water submersion) is observed, highlighting also the fact that panels themselves already contain a fraction of water, probably absorbed onto the flax fibers prior their impregnation with the resin, that, where removed, may influence the mechanical behavior. Moreover, the flame behavior of the FFRP composite is also assessed in comparison with the widely applied carbon fiber reinforced polymers (CFRPs) to highlight the effect of the different reinforcement. Both FRPs are produced with the same flame retarded resin to highlight the contribution of the different reinforcement. The evaluation of the flame behavior of the FFRP panels shows that it completely burns during the cone-calorimetric test, involving in the fire both the matrix and the reinforcement with a stronger and faster heat release than the corresponding CFRP based on the same resin. The above observations seem thus to discourage their use in critical conditions, where the decrease of mechanical performance and the event of fire incidental condition may dramatically and negatively affect the final application

Optimization of Pyro-gasification of Carbon Fiber Reinforced Polymers (CFRPs)

This work focuses on the optimization of pyro-gasification process of carbon fiber reinforced polymers (CFRPs) with the aim of recovering carbon fibers (CFs) with properties suitable for the production of new more sustainable composites with high performances. In particular, the pyro-gasification process is carried out on cured CFRPs panels based on both epoxy (EC) and vinyl ester (VC) matrices, which are the two most used resins for CFRPs. The matrix degradation is evaluated via sample's weight loss measurement and the recovered CFs obtained after different time of treatment are analyzed to identify convenient pyro-gasification conditions to avoid damaging of the recovered CFs. The obtained results highlight the importance of the thickness of the composites to be treated for the identification of the more suitable pyro-gasification conditions

Experimental Investigation of the Effect of Transient Overvoltages on XLPE-insulated HVDC Cables

HVDC cables are subject to several types of impulses superimposed on the rated DC voltage during their service lifetime. Temporary Overvoltages (TOVs) and Superimposed Switching impulses (SSIs) are considered some of the most challenging due to the relatively long impulse duration. This paper aims at investigating experimentally the effect of TOVs and SSIs on XLPE insulation for extruded HVDC cables. 0.15-mm-thick DC-XLPE specimens, aged by applying TOVs and SSIs, are characterized using dielectric analyzer and Fourier Transform InfraRed spectroscopy (FTIR) to detect the aging effects on the insulation. Results show an increase in the imaginary part of permittivity, ε", accompanied with the appearance of additional dipolar polarization losses peaks. The amplitude and frequency of the aforementioned peaks vary with the amplitude and the number of applied TOVs and SSIs. An increase in electrical conductivity is also noticed with aging. FTIR results show absorbance peaks in the aged specimens likely due to the intramolecular bonds rupture accompanied with the formation of aging products. In summary, SSIs and TOVs cause a noticeable reduction of insulating properties in XLPE specimens. The higher the peak of the transient, the greater the aging effect

A Thermoplastic Elastomeric Nanofibrous Membrane as CFRP Modifier to Boost Both Delamination and Damping Performance

In the present work, thermoplastic elastomeric nanofibers made up of a homogenous blend of nitrile butadiene rubber (NBR) and Ppolycaprolactone (CL), with 80% wt of rubbery component, are used to modify a carbon fiber reinforced polymer (CFRP) laminate with the aim of improving its delamination and damping behavior at the same time. Since the nanofibrous membrane is not chemically cross-linked, the fibrous morphology is lost during composite curing owing to its melting. Nonetheless, the nanomodified CFRP displays an impressive ability to improve the delamination resistance in mode I and also an enhanced damping capacity at low temperature. The use of nanofibrous membranes allows for modification of specifically selected areas, thus maximizing the toughening and damping behavior where most required, without necessarily affecting the whole bulk of the resin. Both PCL and NBR components contribute to the final performance; however, the very high amount of rubber leads to a membrane difficult to handle whose final performance in CFRP modification is not superior to membranes up to 60% wt NBR that are instead more stable and easier to deal with. Overall, the proposed results are nonetheless very promising, taking into account also that the improved delamination resistance in mode I and enhanced damping are obtained without significantly sacrificing the weight and overall dimension of the obtained composite

Rubbery nanofibers by co-electrospinning of almost immiscible NBR and PCL blends

The paper presents a simple method for producing rubbery nanofibers without additional crosslinking required. Electrospinning's fast solvent evaporation is able to induce in liquid NBR/PCL pairs thermoplastic elastomeric (TPE) structure that holds stable over time without any further processing. The slight relative solubility of the polymers pair promotes a homogeneous blend formation with low Tg regions of blended NBR/PCL, avoiding phase separation. Such nanofibers show also a PCL-like crystal phase that is surprisingly higher than plain PCL nanofibrous counterpart and seemingly promoted by NBR/PCL interaction at molecular scale. The obtained nanofibrous-TPE morphology is reproducible, stable with time up to at least two years and is detected in a wide range of blend compositions (up to 80%wt NBR). Such a morphology reflects in good mechanical properties, which are analysed with a fitting model taking into account nanofibrous structure. Its impressive fitting ability helps interpretation of tensile tests behavior, carried out via normalization of force data with respect to sample mass, highlighting the contribution of liquid rubber in improving both elastic modulus and properties at failure. Such rubbery nanofibers represent a cost-effective powerful tool for the production of advanced self-damping composite materials with improved overall mechanical properties

A New Wood Surface Flame-Retardant Based on Poly-m-Aramid Electrospun Nanofibers

Poly(meta-phenylene isophtalamide) (PMIA) was processed via electrospinning to provide nanofibrous membranes with randomly and aligned fibers. Mechanical performance of such membranes was evaluated, applying a normalization procedure that takes into account the peculiar morphology of such complex substrate where voids can sum up to almost 80% of the sample volume. Random and aligned fibers membranes are applied onto wood panels to test their fire resistance in cone calorimetry when coated in polyaramidic thin nanofiber mats. Tests highlighted that random fibers provide a better fire protection, increasing Time to Ignition and decreasing the Fire Performance Index. Another important parameter affecting the performance is the adhesive system used to apply the nanofibers onto wood that is able to significantly modify the fire performance of the polyaramidic-coated wood panels. POLYM. ENG. SCI., 2019. (c) 2019 Society of Plastics Engineer

Polyamide Nanofibers Impregnated with Nitrile Rubber for Enhancing CFRP Delamination Resistance

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

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

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

Is Graphene Always Effective in Reinforcing Composites? The Case of Highly Graphene-Modified Thermoplastic Nanofibers and Their Unfortunate Application in CFRP 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

Rubber-enhanced polyamide nanofibers for a significant improvement of CFRP interlaminar fracture toughness

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