This study deals with the development and optimization of hybrid composites integrating microcrystalline cellulose and short basalt fibers in a polypropylene (PP) matrix to maximize the mechanical properties of resulting composites. To this aim, the effects of two different coupling agents, endowed with maleic anhydride (MA-g(grafted)-PP) and acrylic acid (AA-g-PP) functionalities, on the composite properties were investigated as a function of their amount. Tensile, flexural, impact and heat deflection temperature tests highlighted the lower reactivity and effectiveness of AA-g-PP, regardless of reinforcement type. Hybrid formulations with basalt/cellulose (15/15) and with 5 wt. % of MA-g-PP displayed remarkable increases in tensile strength and modulus, flexural strength and modulus, and notched Charpy impact strength, of 45% and 284%, 97% and 263%, and 13%, in comparison with neat PP, respectively. At the same time, the thermo-mechanical stability was enhanced by 65% compared to neat PP. The results of this study, if compared with the ones available in the literature, reveal the ability of such a combination of reinforcements to provide materials suitable for automotive applications with environmental benefits.

The disposal of fibre reinforced composite materials is a problem widely debated in the literature. This work explores the ability to restore the mechanical properties of thermally conditioned basalt fibres through chemical treatments. Inorganic acid (HF) and alkaline (NaOH) treatments proved to be effective in regenerating the mechanical strength of recycled basalt fibres, with up to 94% recovery of the strength on treatment with NaOH. In particular, HF treatment proved to be less effective compared to NaOH, therefore pointing towards a more environmentally sustainable approach considering the disposal issues linked to the use of HF. Moreover, the strength regeneration was found to be dependent on the level of temperature experienced during the thermal treatment process, with decreasing effectiveness as a function of increasing temperature. SEM analysis of the fibres’ lateral surfaces suggests that surface defects removal induced by the etching reaction is the mechanism controlling recovery of fibre mechanical properties. In addition, studies on the fracture toughness of the regenerated single fibres were carried out, using focussed ion beam (FIB) milling technique, to investigate whether any structural change in the bulk fibre occurred after thermal exposure and chemical regeneration. A significant increase in the fracture toughness for the regenerated fibres, in comparison with the as-received and heat-treated basalt ones, was measured.

The presence of a secondary phase between the fibres and the matrix has proved to be a good approach for interphase tailoring for enhanced load transfer. In this regard, an optimization of the low-temperature hydrothermal growth process of ZnO-nanorods on different basalt substrates as a function of different growth times was performed. Scanning electron microscope and X-ray diffraction analysis revealed the best results in terms of homogeneity and uniformity of the ZnO nanostructures for the longest growth time, i.e., 5 h for basalt fabrics and 120 minutes for single basalt fibres. The presence of ZnO nanostructures induced a hydrophobic behaviour with contact angles up to 116° for 4- and 5-h growth processes. Fibre/matrix adhesion was characterized by single fibre pull-out tests, showing a 16% increase in interfacial strength. ZnOs added also photocatalytic properties to basalt fibres, enabling a selective removal of organic pollutant equal to 37%.

The present research is focused on the synthesis of hexagonal ZnO wurtzite nanorods for the decoration of commercially available electrospun nylon nanofibers. The growth of ZnO was performed by a hydrothermal technique and for the first time on commercial electrospun veils. The growth step was optimized by adopting a procedure with the refresh of growing solution each hour of treatment (Method 1) and with the maintenance of a specific growth solution volume for the entire duration of the treatment (Method 2). The overall treatment time and volume of solution were also optimized by analyzing the morphology of ZnO nanostructures, the coverage degree, the thermal and mechanical stability of the obtained decorated electrospun nanofibers. In the optimal synthesis conditions (Method 2), hexagonal ZnO nanorods with a diameter and length of 53.5 nm ± 5.7 nm and 375.4 nm ± 37.8 nm, respectively, were obtained with a homogeneous and complete coverage of the veils. This easily scalable procedure did not damage the veils that could be potentially used as toughening elements in composites to prevent delamination onset and propagation. The presence of photoreactive species makes these materials ideal also as environmentally friendly photocatalysts for wastewater treatment. In this regard, photocatalytic tests were performed using methylene blue (MB) as model compound. Under UV light irradiation, the degradation of MB followed a first kinetic order data fitting and after 3 h of treatment a MB degradation of 91.0% ± 5.1% was achieved. The reusability of decorated veils was evaluated and a decrease in photocatalysis efficiency was detected after the third cycle of use.