The drawback of biobased polymer matrix composites (PMCs) is their limited temperature stability, resulting from degradation, which restricts their processability in established composite manufacturing processes requiring elevated temperatures. These key issues not only affect the mechanical properties but ultimately limit the utilization of flax fibers as fiber reinforcement in PMCs. In this study, kinetic models for the thermal degradation of flax fibers and PA11 are derived and combined with a model for thermo-chemical fiber degradation. Selective degradation of the fibers and mechanical testing establishes a link between degradation and the accompanying deterioration of the mechanical performance. The deterioration of flax fiber mechanical properties under concurrent thermal and thermo-chemical degradation is primarily governed by the thermos-chemical contribution (chain scission) up to 3% thermal degradation, beyond which the influence of thermal degradation becomes evident. Even 1% thermal degradation of flax fibers results in a pronounced reduction in their mechanical performance. In contrast, equal degradation values enhance the PMCs' strength, which may be attributed to improved fiber-matrix interactions. Compiling results into processing maps establishes a framework for designing tailored processing of temperature-sensitive materials, offering transfer opportunities to individual processing conditions and heat treatments, enabling broader research on bio-based PMCs.

Vitrimers represent a class of polymers featuring dynamic covalent crosslinks that can undergo dynamic network rearrangements and hold a notable promise as recyclable thermosets with crack-healing capabilities. In particular, vitrimers with aromatic disulfide hardeners have emerged over the past few years. In these high-glass-transition polymers, the healing regime coincides with the onset of thermal degradation, posing a fundamental challenge for reconciling adequate network mobility with thermal degradation prevention. Therefore, the present study explores the repair capabilities of glass fiber-reinforced polymers (GFRP) with a vitrimer matrix to understand how the interplay of temperature, time, and pressure affects the repair. A transferable framework for estimating a favourable time-temperature regime for vitrimer healing is introduced, based on thermal degradation and dynamic bond-exchange kinetics. The suggested optimized conditions are applied to repair different GFRP samples subjected to quasistatic intralaminar crack opening, and the extent of apparent mechanical property recovery is used to assess the repair effectiveness. Pressurized reconsolidation is required to restore the microstructure and repair cracks. When applying the prescribed repair conditions to the specific damage in short-beam samples, remarkable stiffness and strength recovery ratios of 93 % and 80 %, respectively, are achieved. Conversely, deviations from the prescribed regime due to improper dwell temperatures and times considerably reduce the recovery ratios of mechanical properties, limiting them to their residual values. However, the double cantilever beam repair conducted within the favourable regime shows that repair effectiveness depends strongly on the fracture surface morphology, revealing inherent limitations in the repair capabilities of the investigated laminate system.

Vitrimers are a class of polymer networks that hold promise as recyclable thermosets with self-healing capabilities, enabled by dynamic molecular-level rearrangements. However, achieving the desired network rearrangements usually demands thermal treatments at elevated temperatures substantially above the glass transition temperature Tg while maintaining these harsh conditions for prolonged dwell times. Therefore, the present paper examines the effects of thermo-oxidative degradation on the dynamicity of a disulfide-based epoxy vitrimer. First, comparison with a non-disulfide-containing reference indicates that disulfide bond degradation is the predominant early-stage degradation mechanism. The thermo-oxidative degradation process was described using model-free kinetics fitted to thermogravimetric data, which was subsequently used to selectively control the degradation state of the vitrimer samples as a function of temperature and exposure time. FTIR identified the presence of a highly oxidized carbonyl surface layer, while DMTA confirmed a drop in the primary Tg[jls-end-space/]. Stress-relaxation testing indicates a temporary, favorable effect of decreased crosslink density: increased bond exchange rates, which in turn facilitate shorter dwell times for healing and shape reconfiguration. This manifests as shifts in the initiation of macroscopic flow, reducing the (re)processing temperature regime. In the long run, cleavage of the dynamic S-S crosslinks becomes predominant, adversely compromising the dynamic properties of these systems, as evidenced by incomplete relaxation and reduced macroscopic flow capabilities. These insights into the distinct effects of thermo-oxidative aging provide a critical foundation for evaluating the long-term viability after high-temperature exposure in an oxygen environment and have important implications for designing appropriate (re)processing regimes for disulfide-based epoxy vitrimers.

The present article introduces a high-performance epoxy vitrimer to target structural composite applications. By utilizing a reactive diluent derived from biobased feedstock, the maximum glass transition is tailored to maintain a sufficient temperature window for reprocessing, avoiding the degradation of permanent bonds. Different fractions of permanent cross-links are imbued into the network structure, and the hybrid network is elucidated by creep and stress relaxation. The creep behavior at service temperatures below 150 °C remains unaffected, while slower bond exchange dynamics and higher extrapolated topology freezing temperatures T_v are reported for an increasing number of permanent cross-links. Comprehensive studies of physicochemical, thermo-rheological, and curing reactions are carried out and summarized in a conversion-temperature phase diagram first reported for a vitrimer. The vitrimers show great malleability, even with permanent cross-link fractions above the theoretical limit for a percolated network formation, and we demonstrate recycling by comminuting and subsequent reconsolidation. These findings provide valuable guidance for enhancing material and process development of high-performance vitrimer resins and lay the groundwork for advancing composites built on vitrimer matrix systems.

Vitrimers are a class of polymer networks featuring dynamic covalent crosslinks that can undergo associative bond exchange. These dynamic polymer networks hold a notable promise as recyclable thermosets with self-healing capabilities, provided by network rearrangements at the molecular level, allowing for macroscopic flow. When the relaxation time is sufficiently short, vitrimer material behaves like a thermoplastic even though it is covalently crosslinked. However, temperature-induced malleability remains limited by the high-viscous nature of vitrimers. Therefore, the present article explores the cure dependence of structural relaxation and vitrimer transition phenomena of a dynamic disulfide-containing epoxy vitrimer. Stress relaxation measurements reveal that intermediate cure states—accompanied by lower crosslinking density—exhibit bond exchange and segmental relaxation rates exceeding those of fully cured networks over an order of magnitude. This enhanced dynamics facilitates lower viscosities and residual times during high-temperature malleability processing. Advanced methods combining rheology, cure kinetics, and thermomechanical analysis are utilized to assess the cure dependence of the vitrimer transition temperature (Formula presented.). We outline a distinct cure dependence of (Formula presented.), which may be approximated by a linear correlation, suggesting that viscoelastic flow can be initiated at significantly lower temperatures in undercured networks. These findings provide valuable guidance for enhancing material and processes, contributing to opening doors to the “melt processing” of this family of materials.

Tactix 742 (a trifunctional aromatic epoxy resin) is a benchmark epoxy monomer elucidating the highest T_gepoxy resin system; however, its manufacturing relies on petroleum-based toxic chemistries. In the realm of sustainability, potential replacement of Tactix 742 with a renewable platform possessing similar thermomechanical properties as well as processing character is critical. The resin system of renewable resveratrol triglycidyl ether with aerograde hardener 4,4-diaminodiphenylsulfone shows an ultrahigh T_gof 324 °C and mechanical properties comparable to the industrially used petro-based tris(hydroxyl phenyl)methane epoxy monomer. The results highlight the untapped potential that biobased molecules show in innovating existing high-performance plastic formulations while potentially increasing sustainability potential in the future. Resveratrol triglycidyl ether-4,4-diphenyldisulfone (RTE-DDS) formulation shows T_g, stiffness, strength, and processing behavior similar to that of the industrially used petro-equivalent formulation (T742-DDS) with slightly lower modulus and strength in both tensile and flexural testing. Fracture toughness of the biobased formulation is 61% higher than the petro-based formulation. CFRP manufacturing yields high-quality composite materials tested in compression, interlaminar shear, and in-plane shear modes. Fiber volume content is slightly lower than ideal due to the formulations high viscosity and the choice of vacuum bagging and autoclave manufacturing technique, but low void content of 0.55% ± 0.26 allows for accurate characterization of CFRP laminates. Conclusions are drawn regarding the future potential of resveratrol epoxy monomer in high-performance epoxy CFRPs.

The development of epoxy resin formulations from renewable feedstocks has been thoroughly explored in the chemical literature. A simple one-pot chemical reaction involving sustainable phenolic molecules and epichlorohydrin results in the production of renewable epoxy monomers. These monomers can be cured with amines or anhydrides to yield cross-linked thermosetting resins. Although a wide variety of recipes exist, there is a notable gap in the application of these sustainable resin formulations to engineering contexts. This gap is primarily due to the lack of comprehensive, standardized analyses of these resin recipes, which impede their potential use in advanced composite applications. In this study, we reveal a high-performance resin formulation utilizing epoxidized phloroglucinol derived from brown algae in combination with an aerospace-grade amine hardener. The resin processing and thermomechanical properties are investigated using ASTM standard tests including tensile strength, flexural strength, fracture toughness, and interlaminar shear strength. Given the detailed comparative analysis, the partially renewable resin recipe outperforms petroleum derived analogues.

The present study describes the development of cure kinetics and chemo-rheological models for an epoxy vitrimer based on polyimine exchange to elucidate the potential in terms of processing and accurate process selection. Reaction kinetics is investigated using differential scanning calorimetry. A good agreement between the model and data can be demonstrated for different stoichiometries by selecting a parallel reaction approach consisting of an nth-order and an autocatalytic approach. The suggested chemo-rheological model captures the intrinsically high viscosity of the resin over a broad temperature and curing range, even after the gelation point. The Di-Benedetto equation represents the glass transition temperature advancement with cure while combining the rheological Winter-Chambon criterion and the kinetic model determines the degree of cure at the gelation. These results give important advice for improved process modeling of vitrimeric resins, facilitate accurate process selection, and pave the way towards the development of composites based on the matrix system investigated in this work.