The potential of recycling carbon fiber reinforced polymers (CFRP) as a sustainable solution for waste management is yet to be fully understood. This study reports on the evolution of mechanical, and chemical properties of reclaimed carbon fibers when recycled multiple times via pyrolysis and partial oxidation. The performed work aims at filling the knowledge gap related to repetitive recycling when moving towards a circular flow of resources. A recycling process existing at industrial scale is used to ensure the relevance and usefulness of the results in the current industry scene. Two sets of three identical model composites are recycled using distinct recycling parameters, and their properties are characterized at the end of each recycling cycle. Results show that recycling can lead to an increase in stiffness but can have a negative impact on strength of recovered fibers. Mechanical behaviour shows recovered fibers suitable for secondary applications with medium performance requirements after two recycling cycles. The findings highlight the importance of understanding the material properties evolution during recycling processes. This research contributes to the development of sustainable waste management strategies and a more environmentally friendly future.

This study examines the electrical and thermal performance of carbon fiber prepreg composite laminates for de-icing and anti-icing applications, relevant to wind turbine blades and aircraft wings. Both undamaged and damaged plates with various lay-ups, co-cured on a glass fiber composite sandwich, were analyzed. Various electrode attachment techniques were evaluated, with embedded stainless steel strips proving the most effective for uniform current distribution. Electrical resistance was measured by applying voltage and normalizing results to the effective conductive area. The resistance decreased with increasing temperature, demonstrating semiconductor behavior. Fiber orientation also significantly affected electrical resistance and temperature rise, favouring electrical current flow along the fiber directions. Different repair methods for damaged laminates were considered, revealing that repairing multiple layers in one step yields better results than a multi-step approach. A numerical analysis by means of the finite element method using ANSYS software was carried out to simulate the electrical and thermal performance. The difference between simulations and experiments was consistently within 5 % accuracy, confirming the model's reliability. In all, the findings provide valuable insights towards optimized electrical performance of carbon fiber composites in aerospace and energy applications.

Furfural-derived dimethyl 5,5′-sulfanediyldi(furan-2-carboxylate) (dm-SFA) and biobased fumarate (dm-FA) enable the preparation of partially biobased resins (UPRs) with commercial-level performance. Novel unsaturated polyesters (UPs) were synthesized via titanium-catalyzed polycondensation using the dm-SFA monomer, unsaturated dm-FA, and two commonly used diols: 1,2-propanediol and diethylene glycol. With the goal of achieving comparable or improved material properties to a fossil-based commercial UPR, the prepared UPs were cross-linked using fossil-based styrene as a reactive diluent with a UP-to-styrene ratio of 62:38 w/w. The most optimal material properties were achieved by using diethylene glycol as the sole diol (FA₆₀:SFA₄₀-DEG₁₀₀; NOROX), resulting in a UPR with a high glass transition temperature (98.0 ± 0.4 °C, 2% lower compared to the commercial reference), excellent tensile strength (71.9 ± 3.7 MPa, 76% higher compared to the commercial reference), and overall comparable material properties to the commercial reference. As UPRs are mostly utilized as binders in the composite industry, resin processability was also taken into consideration, keeping the resin viscosity at a moderate level (3.74 Pa·s).

This study investigates the fracture toughness of adhesive joints between carbon fiber-reinforced polymer composites (CFRP) and boron-alloyed high-strength steel under Mode I and II loading, based on linear elastic fracture mechanics (LEFM). Two adhesive types were examined: the excess resin from the prepreg composite, forming a thin layer, and a toughened structural epoxy (Sika Power-533), designed for the automotive industry, forming a thick layer. Modified double cantilever beam (DCB) and end-notched flexure (ENF) specimens were used for testing. The results show that using Sika Power-533 increases the critical energy release rate by up to 30 times compared to the prepreg resin, highlighting the impact of adhesive layer thickness. Joints with the thick Sika adhesive performed similarly regardless of whether uncoated or Al–Si-coated steel was used, indicating the composite/Sika interface as the failure point. In contrast, the thin resin adhesive layer exhibited poor bonding with uncoated steel, which detached during sample preparation. This suggests that, for thin layers, the resin/steel interface is the weakest link. These findings underline the importance of adhesive selection and layer thickness for optimizing joint performance in composite–metal hybrid structures.

This article explores the viscoelastic properties of polyethylene terephthalate glycol samples created by fused filament fabrication, emphasising the anisotropy introduced during fabrication. The samples were fabricated with filament direction within samples aligned along the principal axis or perpendicular. A group of samples was loaded with constant stress for 5 h, and a recovery phase with no applied stress was observed. Another group of samples was loaded for 20 h without an additional deformation recovery phase. The continuous constant stress application results on the sample were analysed, and an overall effect of anisotropy on the samples was observed. Several models describing viscoelastic deformation were considered to adhere to experimental data, with the Prony series and general cubic theory models used in the final analysis. The models could describe experimental results up to 50% and 70% of sample strength, respectively. The analysis confirmed the nonlinear behaviour of printed samples under constant stress and the significant effect of anisotropy introduced by the 3D printing process on the material’s elastic properties. The viscoelastic properties in both directions were described using the same parameters.

This feasibility study encompasses the experimental findings of utilizing lignin as a potential multi-functional epoxy resin modifier for man-made cellulosic fiber composites. Two types of lignin at different concentrations are used (with no chemical alteration) to modify the epoxy resin. The modified resin's potential for use in natural fiber-reinforced composites is evaluated through the characterization of mechanical and thermal properties. The influence of moisture on the stability of the mechanical performance is also investigated through the characterization of conditioned samples (RH=100%, T=50℃) against reference material. Preliminary results show that the addition of any type of lignin at low concentrations has a marginal effect on the overall system performance although the effect of the type of lignin remains hidden within the causes of self-agglomerations. The most notable difference concerning the lignin type used can be seen in the Tg-values for 5 wt% lignin addition.

Due to the high interest in the use of glass/epoxy laminates in aerospace applications, aviation, and as cryogenic tanks, it is crucial to understand the behavior of composites under challenging environmental conditions. Polymer composites are exposed to low temperatures, including cryogenic temperatures, which can lead to the initiation of microdamage. This paper investigates damage initiation/accumulation and its influence on the properties of cross-ply woven glass fiber epoxy composites at low temperatures compared to room temperature conditions. To evaluate the influence of a low-temperature environment on the mechanical performance of glass fiber reinforced epoxy composite (GFRP) laminates, three types of test campaigns were carried out: quasi-static tensile tests and stepwise increasing loading/unloading cyclic tensile tests at room temperature and in a low-temperature environment (−50 °C). We demonstrated that the initial stiffness of the laminates increased at low temperatures. On the other hand, there were no observed changes in the type or mechanism of developed damage in the two test conditions. However, the reduction in stiffness due to the accumulated damage was more significant for the laminates tested at low temperatures (~17% vs. ~11%). Exceptions were noted in a few formulations where the extent of damage at low temperatures was insignificant (<1%) compared to that at room temperature. Since some of the studied laminates exhibited a relatively minor decrease in stiffness (~2–3%), we can also conclude that the formulation of matrix material plays an important role in delaying the initiation and formation of damage.

Recently, considerable effort has been made to study cellulose/epoxy composites. However, there is a gap when it comes to understanding the post-conditioning anomalous effect of moisture uptake on their mechanical and dynamic-mechanical properties, and on their creep behavior. In this work, up to 10.0 wt% microcrystalline cellulose (MCC) was incorporated into epoxy resin by simple mixing and sonication. Epoxy/MCC composites were fabricated by casting in rubber silicone molds, and rectangular and dog-bone test specimens were produced. The moisture uptake, dynamic mechanical, chemical, tensile, and creep behavior were evaluated. The incorporation of MCC increased the water diffusion coefficient. The changes in storage modulus and glass transition temperature, combined with Fourier-transform infrared spectroscopy analysis, evidenced that water sorption in epoxies causes both plasticization and additional resin crosslinking, although the latter is prevented by the addition of MCC. The creep strain of the composites increased by 60% after conditioning, indicating that plasticization induced by water sorption plays an important role in the long-term properties of the composites.

Polyoxymethylene (POM), an engineering polymer commonly used in tribological applications, is often reinforced with fossil-based fibers such as carbon and/or glass fibers to improve its properties. To find more sustainable solutions, in this study, the tribological performance of POM/short cellulose fiber composites at different sliding conditions is investigated. An improvement in the wear coefficient of roughly 69% is observed at the harshest conditions of 5 MPa and 1 m · s−1 with only 10 wt.% cellulose fibers. The friction behavior is furthermore stabilized through fiber addition, as the unfilled polymer did not show a steady state. No signs of thermo-oxidative degradation are found after tribological testing. This study presents promising results for sustainable wear-resistant polymer materials in tribological applications.

Structural battery composites require a structural electrolyte to work. The structural battery electrolyte has a bicontinuous microstructure which enables its dual roles: mechanical load transfer and ion transport between the electrodes. These structural electrolytes are difficult to characterise mechanically via bulk tests. For this reason, no extensive characterisation of the mechanical properties of the structural battery electrolyte has been performed to date. In this study, we highlight the many challenges of these types of tests, including the complexity of sample manufacturing, preparation and testing. We further demonstrate a method to prepare test samples and to perform mechanical tests on the structural battery electrolyte. The executed test campaign provides measures of Young's modulus (approximately 412 MPa) and Poisson's ratio (0.34), as well as tensile (4.85 MPa) and compressive strength (32.66 MPa) and strain to failure (2.49 % and 28.11 % in tension and compression, respectively). In addition, cure shrinkage is investigated and found insignificant. These results are crucial for the further development of structural battery composites as they allow for accurate prediction of their internal stress states.