Natural fibres such as kenaf, hemp, and flax, also known as bast fibres, offer several benefits such as low density, low cost, carbon dioxide neutrality, sustainability and renewability. In Europe, their composites are used intensively by almost all car manufacturers, mostly in interior applications. Compared to glass fibres, they are safe in processing, renewable, and recyclable. Other than that, their specific mechanical properties are either close to or at times higher than glass fibres. They have great potential to replace a segment of the glass fibre reinforced composites in automotive applications. The shortcomings of bast fibres are their poor mechanical strength, varying fibre characteristics (because of climate, cultivation, soil), and low processing temperature (approximately 200°C). Therefore, they cannot meet the structural and durability demands of automobile parts as do glass fibres. 

This research work aimed to improve the mechanical performance of bast fibre reinforced polymer com-posites by hybridisation with high performance natural basalt fibres. They are natural inorganic fibres with mechanical and thermal properties higher than bast fibres and close to or higher than common glass fibres (E-glass). As bast fibres, flax, hemp, and kenaf were selected for reinforcement. Polypropylene and acrylic based polyester resin were used as matrix. The target was to prepare the composites by established processing methods for the applications of natural fibre reinforced composites in the automotive sector, such as carding, resin impregnation, and compression moulding. 

The first step in the production of composites was the preparation of nonwovens by carding process and needle punching. It was challenging to card fine and brittle basalt fibres. The carding process was therefore intensively optimised so that the nonwovens could be produced without fibre damage and with homogeneously distributed fibres. For thermoplastic composites, nonwovens were prepared with poly-propylene fibres. The reference composition was a mixture of polypropylene and bast fibres (50:50). For the hybrid compositions, the bast content was gradually replaced by basalt fibres. Nonwovens were ready for compression moulding immediately after their production. Nonwovens for thermoset resin impregnation were prepared with only natural fibres. The reference composition was a mixture of flax and kenaf (50:50). For the hybrid compositions, the bast content in the reference composition was gradu-ally replaced with basalt fibres. The nonwovens were first impregnated with acrylic based polyester resin and compression moulded after drying. 

The compression moulded flat plates were analysed for their mechanical, thermal, and morphologi-cal properties. The mechanical performance was characterised by flexural, tensile, and Charpy impact analysis. Thermal analysis included thermogravimetry, differential scanning calorimetry, heat deflection temperature, and thermal conductivity. The morphological properties regarding fibre matrix interaction and fibre distribution were studied by scanning electron microscopy. 

The material characterisation showed significant improvement in mechanical performance by the addi-tion of basalt fibres. It was found that the basalt fibres not only improved the strength and stiffness of the composites, but simultaneously increased the energy absorption as well. The fracture surface analy-sis confirmed a better fibre matrix adhesion in thermoset composites than in thermoplastic composites because of chemical compatibility between thermoset and bast fibres. It was found that selecting basalt fibres with appropriate sizing could significantly influence the fibre matrix bonding and is a deciding factor in defining the optimal mechanical performance of the composite. It was shown that bonding could also be improved by polymer modification, which is, however, an expensive method. The competitive performance of thermoset composites was compared with thermoplastic composites. Furthermore, the composites were processed to the final parts at lab and industrial scale, to study their processability for the implementation of material in practical applications. This study highlights the potential of basalt fibres to improve the mechanical performance of bast fibre reinforced composites by hybridisation while stressing the importance of fibre matrix interaction to define the composite’s performance.

New technologies in the automotive industry require lightweight, environment-friendly, and mechanically strong materials. Bast fibers such as kenaf, flax, and hemp reinforced polymers are frequently used composites in semi-structural applications in industry. However, the low mechanical properties of bast fibers limit the applications of these composites in structural applications. The work presented here aims to enhance the mechanical property profile of bast fiber reinforced acrylic-based polyester resin composites by hybridization with basalt fibers. The hybridization was studied in three resin forms, solution, dispersion, and a mixture of solution and dispersion resin forms. The composites were prepared by established processing methods such as carding, resin impregnation, and compression molding. The composites were characterized for their mechanical (tensile, flexural, and Charpy impact strength), thermal, and morphological properties. The mechanical performance of hybrid bast/basalt fiber composites was significantly improved compared to their respective bast fiber composites. For hybrid composites, the specific flexural modulus and strength were on an average about 21 and 19% higher, specific tensile modulus and strength about 31 and 16% higher, respectively, and the specific impact energy was 13% higher than bast fiber reinforced composites. The statistical significance of the results was analyzed using one-way analysis of variance. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.

Composites with reinforcements based on bast fibers such as flax, hemp and kenaf offer many advantages such as weight reduction, improved specific impact, flexural, acoustic properties, and balanced performance to cost that can be achieved by properly designing the material composition. Their position is well established, especially in the nonstructural automotive applications. However, in structural applications of composites, their mechanical property profile is not comparable to the dominant reinforcements such as glass and carbon fibers. The low mechanical properties of these composites could be improved by hybridization that involves adding high-performance fibers to the bast fiber composites that could improve the low mechanical performance of the bast fiber composites. The review presented in this article provides an overview of the developments in the field of hybrid polymer composites composed of bio-based bast fibers with glass, carbon, and basalt fibers. The focus areas are the composite manufacturing methods, the influence of hybridization on the mechanical properties, and the applications of hybrid composites.

Natural fibers, such as kenaf, hemp, and flax, also known as bast fibers, offer several benefits such as low density, carbon dioxide neutrality, and less dependence on petroleum sources. Their function as reinforcement in polymer composites offers a great potential to replace a segment of the glass fiber-reinforced polymer composites, especially in automotive components. Despite their promising benefits, they cannot meet the structural and durability demands of automobile parts because of their poor mechanical properties compared to glass fibers. The focus of this research work was the improvement of the mechanical property profile of the bast fiber reinforced polypropylene composites by hybridization with natural high-performance basalt fibers and the influence of basalt fibers coating and polymer modification at the mechanical and thermal properties of the composites. The specific tensile strength of the composite with polymer tailored coating was 39% and the flexural strength was 44% higher than the composite with epoxy-based basalt fibers. The mechanical performance was even better when the bast/basalt hybridization was done in maleic anhydride modified polymer. This led to the conclusion that basalt fibers sizing and polymer modification are the deciding factors in defining the optimal mechanical performance of the composites by influencing the fiber-matrix interaction. The composites were analyzed for their mechanical, thermal, and morphological properties. The comparison of bast/basalt hybrid composite with bast/glass fibers hybrid composite showed a 32% higher specific flexural and tensile strength of the basalt hybrid composite, supporting the concept of basalt fibers as a natural alternative of the glass fibers.

With increasing limitation of petrochemical resources, there is a growing demand for the replacement of nonrenewable fiber-reinforced polymer composites by renewable polymer composites. Therefore, the aim of this research work was to improve the mechanical properties of bast (plant) fibers reinforced polymer composites without reducing their renewable material content. To achieve this goal, basalt fibers (natural mineral fibers) were used to partially substitute the amount of bast fibers in the polymer composites. The applied fibers were processed to semi-finished materials by carding and needle punching and processed further by afterwards press-molding. An intense optimization of the carding process led to the production of homogeneous fabrics based on various types of fibers (bast, basalt and PP). The homogeneity of the fabrics was confirmed by scanning electron microscopy (SEM) analysis of the composites. Several composites based on polypropylene and acrylate thermoset resin, reinforced with merely bast fibers, were prepared as reference. In the next steps, the bast fiber content of the reference compositions was partially replaced by basalt fibers. The compression-molded samples were tested for their flexural, tensile, and impact energy properties. The very positive finding was that the addition of basalt fibers not just improved significantly the strength and stiffness of the composites, but simultaneously increased the properties of the composites regarding energy absorption, a key requirement in the automotive industry. The fracture surface analysis confirmed a better fiber matrix adhesion in thermoset composites compared to thermoplastic composites. The tested renewable hybrid polymer composites have great potential to replace nonrenewable fiber-reinforced polymer composites.