Natural fibers are a sustainable alternative to synthetic fibers due to their high weight‐specific Young's moduli and strengths. However, the mechanical properties of natural fibers are very sensitive to their moisture content. Therefore, chemical treatments are often applied to natural fibers to lower their water absorption and enhance fiber‐matrix interaction. The aim is to study the effects of fiber modifications with sodium hydroxide, silane, and siloxane on the water uptake and tensile properties of flax fiber composites produced via prepreg technology. In addition, the effect of moisture on the composites' tensile properties was investigated by conditioning one part of the tensile specimens according to DIN EN 2823 (at 70°C and 85% relative humidity). The NaOH treatment was the only modification that had positive effects on the Young's modulus and tensile strength in the unconditioned and conditioned state. The increase of the tensile modulus and strength are most likely due to changes in flax fiber composition, crystallinity of the cellulose and the rougher fiber surface of NaOH modified fibers. This shows that chemical treatment of natural fibers may improve the performance level of natural fiber composites and prevent a loss in their mechanical properties in humid environments.Highlights: Flax fiber modifications with sodium hydroxide, silane, and siloxane. Flax fiber composite production via prepreg technology. Water uptake after conditioning at 70°C and 85% relative humidity. Tensile tests before and after conditioning. SEM images of modified flax fibers.

Bio-based alternatives for petroleum-based thermosets are crucial for implementing sustainable practices in fiber-reinforced polymer composites. Therefore, the mechanical properties of diglycidyl ether of bisphenol a (DGEBA) cured with either l-arginine, l-citrulline, $\gamma$-aminobutyric acid, l-glutamine, l-tryptophan, or l-tyrosine were investigated to determine the potential of amino acids as bio-based curing agents for epoxy resins. Depending on the curing agent, the glass transition temperature, Young’s modulus, tensile strength, and critical stress intensity factor range from $98.1$${}^{\circ }\mathrm{C}$ to $188.3$${}^{\circ }\mathrm{C}$, $2.6$$\mathrm{G}$$\mathrm{Pa}$ to $3.5$$\mathrm{G}$$\mathrm{Pa}$, $39.4$$\mathrm{M}$$\mathrm{Pa}$ to $46.4$$\mathrm{M}$$\mathrm{Pa}$, and $0.48$$\mathrm{M}$$\mathrm{Pa}$$\mathrm{m}$${}^{0.5}$ to $1.34$$\mathrm{M}$$\mathrm{Pa}$$\mathrm{m}$${}^{0.5}$, respectively. This shows that amino acids as curing agents for epoxy resins result in thermosets with a wide range of thermo-mechanical properties and that the choice of curing agent has significant influence on the thermoset’s properties. After collecting the results of dynamic mechanical analysis (DMA), tensile, flexural, compression, and compact tension tests, the functionality f, cross-link density ${\nu }_{\mathrm{C}}$, glass transition temperature $T_{\mathrm{g}}$, Young’s modulus $E_{\mathrm{T}}$, compression yield strength ${\sigma }_{\mathrm{Cy}}$, critical stress intensity factor in mode I $K_{\mathrm{IC}}$, fracture energy $G_{\mathrm{IC}}$, and diameter of the plastic zone $d_{\mathrm{p}}$ are correlated with one another to analyze their inter-dependencies. Here, the cross-link density correlates strongly positively with $T_{\mathrm{g}}$, $E_{\mathrm{T}}$, and ${\sigma }_{\mathrm{Cy}}$, and strongly negatively with $K_{\mathrm{IC}}$, $G_{\mathrm{IC}}$, and $d_{\mathrm{p}}$. This shows that the cross-link density of DGEBA cured with amino acids has a crucial influence on their thermo-mechanical properties and that the thermosets considered may either be stiff and strong or tough, but hardly both at the same time.

The compression molding process of sheet molding compound (SMC) is an economical manufacturing process for lightweight parts. However, molding defects, such as fiber matrix separation, and fiber re-orientation, may develop during the molding process in confined regions, such as ribs and bosses. Hence, the mechanical properties of the composite depend on the local fiber architecture. Consequently, this work compares the predictive capabilities of tensor-based and directly modeled process simulation approaches regarding compression force, fiber volume content and fiber orientation on the example of honeycomb structures molded from SMC. The results are validated by micro-computed tomography and thermal gravimetric analysis. The fiber orientation in the honeycomb varies between individual samples because a sheet molding compound is macroscopically heterogeneous and thus the fiber architecture is strongly influenced by random events. Tensor-based fiber orientation models can not reliably predict fiber volume content and fiber orientation in the part’s thickness direction if there is a lack of scale separation. Therefore, directly modeled process simulations should be preferred in cases in which fiber length and mold dimensions prohibit scale separation. The prediction of fiber volume content is a difficult task and no simulation can predict the severity of fiber matrix separation precisely in all cases.

Bio‐based alternatives for epoxy resin curing agents are a necessity for fiber reinforced polymer composites to become more sustainable. Here, the precise knowledge about the optimal curing cycle and its influence on the thermoset's mechanical properties are imperative. Therefore, the influence of the maximum curing temperature on the network structure, crystal morphology and mechanical properties of diglycidyl ether of bisphenol A (DGEBA) cured with l‐arginine was investigated with the goal to derive structure–property relationships and a favorable curing cycle. The maximum curing temperature used can be categorized into two regimes: first, the temperature range in which the thermoset reaches complete curing and second, the temperature regime in which the thermoset is fully cured and the thermal degradation starts to diminish its mechanical properties. An optimized curing regimen for achieving high flexural strength accompanied by adequate fracture toughness entails subjecting the material to a curing cycle comprising a duration of 1 h at a temperature of 150°C, followed by an additional 2 h at a temperature of 170°C. This study represents a pioneering effort in optimizing the curing process of amino acid‐cured epoxy resin, specifically focusing on achieving the most favorable mechanical properties as a result.

Bio‐based alternatives for petroleum‐based epoxy resin curing agents, such as maleopimaric acid (MPA), are indispensable for sustainable fiber reinforced polymer composites with thermosetting matrices. However, previous investigations disregarded the importance of choosing the right stoichiometric ratio R between the anhydride groups in the rosin‐based curing agent and the epoxy groups in the resin. Therefore, the influence of R on the curing kinetics and mechanical properties of an epoxy resin cured with a rosin‐based anhydride is studied. Here, Fourier‐transform infrared spectroscopy (FT–IR) indicates that for R ⩾ 0.9 unreacted anhydride groups are present in the thermoset. Consequently, the network density decreases and the glass transition temperature T_g drops by about 40 °C. On the other hand, the steric hindrance of unreacted functional groups for R ⩾ 0.9, increases the flexural modulus and the reduced network density improves fracture toughness. The results indicate that the best R for overall high mechanical performance and good processability is preferably low (R ⩽ 0.7). Here, a low R results in a high T_g and good processability due to a low viscosity. However, the latency of the mixtures is low and therefore, the mixtures are not fit for processing via prepreg technology.

The precise characterization of new bio-based thermosets is imperative for the correct assessment of their potential as matrix material in fiber-reinforced polymer composites. Therefore, the mechanical properties of diglycidyl ether of bisphenol a (DGEBA) cured with l-arginine were investigated to determine whether the bio-based thermoset possesses the required mechanical properties for application as a matrix material. The cured thermoset is called Argopox. The mixture of amino acid and epoxy resin was prepared via three-roll milling and cured in the presence of an urea-based accelerator. The tensile, compression, flexural and toughness properties of Argopox were characterized at $T=$$-40$${}^{\circ }\mathrm{C}$, 22 ${}^{\circ }\mathrm{C}$ and 80 ${}^{\circ }\mathrm{C}$ to determine the temperature-dependence of the thermoset’s mechanical properties in its service temperature range. The glass transition temperature $T_{\mathrm{g}}$ was analyzed via dynamic mechanical analysis (DMA) and is approximately 119 ${}^{\circ }\mathrm{C}$. The tensile, compression and flexural strength at 22 ${}^{\circ }\mathrm{C}$ are about 56 $\mathrm{M}$$\mathrm{Pa}$, 98 $\mathrm{M}$$\mathrm{Pa}$ and 85 $\mathrm{M}$$\mathrm{Pa}$, respectively. The critical stress intensity factor $K_{\mathrm{IC}}$ and fracture energy $G_{\mathrm{IC}}$ at 22 ${}^{\circ }\mathrm{C}$ are roughly $1.1$$\mathrm{M}$$\mathrm{Pa}$m^0.5 and 510 $\mathrm{J}$m⁻, respectively. Consequently, Argopox possesses mechanical properties that reach performance levels similar to that of materials which are already used as matrix for fiber reinforced composites.