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.

Processing parameters of melt mixing (one of the most conventional techniques in polymer processing) play a significant role in the quality and properties of the resulting material, especially when nanoreinforcements are involved. The current study investigates varying processing temperature, rotation speed and elements of the screw extruder, aiming to enhance mechanical properties of polyethylene (PE) nanocomposites by improving dispersion of nanoparticles from a commercial masterbatch in two grades of PE. The study investigates the effect of a common compatibilizer (MAPE) and shearing forces at varying amounts of graphene nanoplatelets (GNPs) in polyethylene. A comparison is made on mechanical properties, morphology, and changes in the microstructure. Results show that increasing amounts of GNPs lead to expected continuous increase of mechanical properties with reference to the base polymer. Addition of MAPE did not result in significant improvement in the performance of the studied systems. Use of stronger shear forces resulted in mostly negative impact on the properties.

In various structural applications polymer composites are exposed to sub-zero and even cryogenic temperatures which may initiate of microstructural damage. To anticipate these events, one needs to understand the behavior of composites in a sub-zero environment. This study focuses on damage initiation and accumulation, and its influence on the properties of cross-ply glass fibers epoxy composites at sub-zero temperatures. The effect of bromine modification of epoxy, and the dissolution in an organic solvent on the mechanical performance of the produced composite is also investigated. To evaluate the influence of a sub-zero environment on the mechanical performance of glass fiber epoxy laminates, tensile tests in a sub-zero environment of unconditioned specimens were carried out. The quasi-static tensile tests were performed to measure the elastic modulus of the composites while loading-unloading experiments were performed to monitor the initiation (and accumulation) of microstructural damage and its influence on the stiffness of glass fiber epoxy laminates. The results of cryogenic damage and fracture in the laminates are discussed with a focus on the degradation of properties of glass fiber crucial for their use in structural applications: strength and stiffness.

The effect of graphene nanoplatelets (GNPs) on the long-term performance of wood fiber/high-density polyethylene (HDPE) composite is investigated by using short-term creep tests with an efficient, faster data analysis approach. Previously, it was shown that the addition of GNPs at 15 wt% into HDPE reduces the viscoplastic (VP) strain developed during 2 h creep by ~50%. The current study shows that 25 and 40 wt% wood content in HDPE reduce the VP strains developed during 2 h creep time by >75% with no noticeable effect of the increased wood content. However, further addition of GNPs results in more than 90% total reduction in the VP strains. The current study shows that the development of the VP strains in the hybrid composites follows Zapas model. Viscoelastic (VE) response of these composites is nonlinear and thus is described by Schapery's model. Parameters for VP and VE models are obtained from the creep experiments and were validated in a separate loading-unloading test sequence. Results show a very good agreement between experiments and predictions for the studied materials as long as the micro-damage is not present.