## 1 Introduction  
In recent years, biodegradable or natural polymers have been raised as alternatives to substitute synthetic polymers since natural polymers are materials that have a short shelf life after disposal [1]. They could suffer degradation and disappear in the environment, not resulting in the accumulation of plastic waste in landfills [2]. As feedstock, these polymers can be produced naturally (microorganisms, plants, and animals) or chemically synthesized using biological materials (sugars, oils, or fats) [3]. PLA is a natural polyester produced by two different routes of polymerization: lactic acid polycondensation or the ring-opening polymerization of the lactide [4]. Lactic acid monomers are produced by sugar fermentation or starch from renewable materials such as corn and sugarcane [5]. Meanwhile, PHB is a homopolymer formed by the polymerization of 3-hydroxybutyric acid produced in the anaerobic fermentation of microorganisms, such as the bacteria _Raustonia eutropha_, and its strains, where the polymer is stored. It has a function as an energy reserve [6]. As reported by the literature, PHB is a semicrystalline polymer with high crystallinity [7], reaching 80% crystallinity. Its structure is fragile and brittle due to its arrangement within the microorganism [8]. To modify its global properties, fibers (synthetic or natural) are used as polymeric reinforcements to obtain materials with other physical characteristics [9]. As engineering materials, natural fibers are looking for a replacement with the same mechanical and thermal efficiency as synthetic fibers such as aramid, glass, and carbon fibers [10]. Above that, natural fibers could bring other advantages, such as reducing the weight of the final material because of its low density, costs in production, low price, the potential for the end-of-life recycling of the product, and increased biodegradability [11]. Initially, the natural fibers are lignocellulosic materials constituted by cellulose, hemicellulose, and lignin. The cellulose consists of a long chain of cellobiose, forming crystals from the intermolecular bonds in the macromolecular chains. These bonds give cellulose flexibility and elasticity, whereas the intramolecular covalent bonds impart rigidity to the macromolecule [12]. The hemicellulose presents ramifications in its structure, conferring amorphous properties and increasing the susceptibility to acid and alkaline attacks [13; 14]. The complex lignin structure comprises propane-phenol molecular units substituted with several functional groups [15]. From the perspective of using natural fibers as engineering materials, lignin presents an obstacle to connecting with other materials, such as polymers [16]. For this reason, superficial treatments are made in the fibers to obtain the most significant amount of cellulose possible since this component confers mechanical resistance to the whole. Among the treatments of natural fibers, the most outstanding are merecization, treatment with acids, the addition of silane, acetyl, isocyanate, or peroxide groups, and bleaching [17]. The vegetable fibers, such as sugarcane bagasse, flax, hemp, sisal, jute, coconut, kenaf, curaua, or zmie, have outstanding mechanical behaviors [18]. Mechanical tests are essential for characterizing polymer composites reinforced with natural fibers, as they assess the mechanical resistance of the material under tensile or flexural stress, i.e., its capacity to withstand loads. These tests provide information on properties such as maximum strength, the modulus of elasticity, and elongation at break, which are direct indicators of the efficiency of the reinforcement provided by the fibers [19; 20]. Furthermore, through these tests, it is possible to better understand the interaction between the polymer matrix and the natural fibers, as well as the fibers' contribution to improving the composite's mechanical properties, which is essential for structural and engineering applications.