Novel Biorenewable Composites Based on Poly (3-hydroxybutyrate-co-3-hydroxyvalerate) with Natural Fillers

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 degrees 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 degrees 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.