In the study, polylactide-based (PLA) composites modified with natural particles (wood, bamboo, and cork) and with different levels of infilling (100%, 80%, and 60%) obtained by fused deposition modeling were tested. The effect of fiber type, infill level and crystallization rate on the mechanical properties were investigated by using tensile, flexural, and impact tests. The materials were subjected to mechanical tests carried out at 23 and 80 °C. Differential scanning calorimetry were employed to analyze crystallization behavior of composite. Furthermore, hydrothermal degradation was performed, and its effect on the properties was analyzed. The addition of natural fillers and different levels of infilling result in a similar level of reduction in the properties. However, the addition of natural fillers resulted in a slightly lower drop than the lowered infilling rate − 40% and 50% for tensile strength, respectively. Moreover, it was found that, composites made of PLA are more sensitive to high temperatures than to water. The decrease in Young's modulus of PLA at 80 °C was 90%, while after 28 days of hydrodegradation ~ 9%. The addition of fibers reduced this decrease at elevated temperatures. Importantly, in the case of a brittle material such as PLA, the impact strength has been improved by 50% for composites with cork particles and other lignocellulosic composites remained at the same level as for resin. Generally, the thermal treatment of composites increased the degree of crystallinity of the materials, as reflected in the higher results of mechanical tests.

Due to the ubiquity and single-use character of plastic products, their production represents a burden to the environment. Therefore, an increasing interest in biodegradable and bio-compostable materials has been observed in the recent years. Bio-based materials are becoming more and more popular, especially in applications where biodegradability provides an advantage for customers and environment. However, biodegradable materials are more expensive compared with durable plastic materials, so to reduce costs and in order to improve their mechanical properties, biocomposites are created by reinforcement with natural fibers: cellulose, hemp, jute, cotton, etc. This paper is focused on the investigation of the selected group of biocomposites based on poly (3-hydroxybutyrate-co-3-hydroxyvalerate) with the addition of 15 wt% of various fillers (nanocellulose, walnut shell flour, eggshell flour, and tuff). Thus far, there is limited information concerning comparison of the different natural fillers introduced into the poly(3-hydroxybutyrate-co-3-hydroxyvalerate) matrix. Here, the following mechanical properties were evaluated: the tensile strength, modulus of elasticity, strain at break, flexural modulus, and flexural stress at 3.5% strain. The tensile test was performed at various temperatures (− 24, + 23, and + 60 °C), followed by samples conditioning in water and compost. Thermal behavior of the biocomposites was studied by means of differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The study showed that the value of the elasticity modulus of each composite was higher in comparison with neat poly (3-hydroxybutyrate-co-3-hydroxyvalerate), at each of the above temperatures. The factor responsible for the enhancement of the mechanical properties of composites (enhancement of the stiffness of the material) was the increase in crystalline phase content in composites. Interestingly, at − 24 °C, all of the analyzed composites exhibited over twofold increase in tensile strength which was accompanied by an almost 30% increase in elastic modulus. This phenomenon likely resulted from an increase in tensile strength and an increase in internal stress at interfaces within the components of the composites during tests. It was also observed that both conditioning in water and degradation in the compost heap led to a considerable decrease in mechanical properties of the examined composites. The scanning electron microscopy analysis, carried out to assess the distribution of particles and the adhesion of fillers to the matrix, revealed that the size and the shape of the particles affected the mechanical properties of the composites.

Biodegradable composites based on poly (3-hydroxybutyrate-co-3-hydroxyvalerate), reinforced with 7.5% or 15% by weight of wood fibers (WF) or basalt fibers (BF) were fabricated by injection molding. BF reinforced composites showed improvement in all properties, whereas WF composites showed an increase in Young’s modulus values, but a drop in strength and impact properties. When compared with the unmodified polymer, composites with 15% by weight of BF showed an increase of 74% in Young’s modulus and 41% in impact strength. Furthermore, the experimentally measured values of Young’s modulus were compared with values obtained in various theoretical micromechanical models. The Haplin-Kardas model was found to be in near approximation to the experimental data. The morphological aspect of the biocomposites was studied using scanning electron microscopy to obtain the distribution and interfacial adhesion of the fibers. Additionally, biodegradation tests of the biocomposites were performed in saline solution at 40 °C by studying the weight loss and mechanical properties. It was observed that the presence of fibers affects the rate of water absorption and the highest rate was seen for composites with 15% by weight of WF. This is dependent on the nature of the fiber. After both the first and second weeks mechanical properties decreased slightly about 10%.

The key goal of this study was to characterize polytetrafluoroethylene (PTFE) based composites with the addition of bronze particles and mineral fibers/particles. The addition of individual fillers was as follows: bronze—30–60 wt.%, glass fibers—15–25 wt.%, coke flakes—25 wt.% and graphite particles—5 wt.%. Both static and dynamic tests were performed and the obtained results were compared with the microscopic structure of the obtained fractures. The research showed that the addition of 60 wt.% bronze and other mineral fillers improved the values obtained in the static compression test and in the case of composites with 25 wt.% glass fibers the increase was about 60%. Fatigue tests have been performed for the compression-compression load up to 100,000 cycles. All tested composites show a significant increase in the modulus as compared to the values obtained in the static compression test. The highest increase in the modulus in the dynamic test was obtained for composites with 25 wt.% of glass fibers (increase by 85%) and 25 wt.% of coke flakes (increase by 77%), while the lowest result was obtained for the lowest content of bronze particles (decrease by 8%). Dynamic tests have shown that composites with “semi-spherical” particles are characterized by the longest service life and a slower fatigue crack propagation rate than in the case of the long glass fibers. In addition, studies have shown that particles with smaller sizes and more spherical shape have a higher ability to dissipate mechanical energy, which allows their use in friction nodes. On the other hand, composites with glass fiber and graphite particles can be successfully used in applications requiring high stiffness with low amplitude vibrations.

The composites based on polylactide (PLA) and poly (3‐hydroxybutyrate‐co‐3‐hydroxyvalerate) (PHBV) with the addition of antibacterial particles: silver (Ag) and copper oxide (CuO) are characterized. Basic mechanical properties and biodegradation processes, as well as biocompatibility of materials with human cells are determined. The addition of Ag or CuO to the polymers do not significantly affect their mechanical properties, flammability, or biodegradation rate. However, several differences between the base materials are observed. PLA‐based composites have higher tensile and impact strength values, while PHBV‐based ones have a higher modulus of elasticity, as well as better mechanical properties at elevated temperatures. Concerning biocompatibility, each of the tested materials support the growth of fibroblasts over time, although large differences are observed in the initial cell attachment. The analysis of hydrolytic degradation effects on the structure of materials shows that PHBV degrades much faster than PLA. The results of this study confirm the good potential of the investigated biodegradable polymer composites with antibacterial particles for future biomedical applications.