# Evaluation of Impact Energy Absorption by Natural Fiber Composites in Motorcycle Helmets
## 2 Experimental Method for Impact Absorption  
This section describes how the experiment was carried out and the results of the impact absorption test. The experiments were conducted comparing only pure PU and not EPS, as EPS cannot be mixed with fibers, only layered. The choice of coconut, sisal, and sugarcane fibers, derived from Brazil's vast biodiversity, illustrates the efficient use of the country's renewable natural resources. Furthermore, the incorporation of these fibers into polymeric composites aimed to improve mechanical characteristics, such as impact resistance, to promote the alignment of materials technology with the principles of environmental sustainability [14; 15; 16]. Coconut fiber is characterized by its mechanical strength and toughness, properties that stem from its distinctive fibrous nature. Its ability to absorb a significant amount of energy before failure, attributed to its unique microscopic structure and lignocellulosic composition, emphasizes the potential of this fiber in improving the performance of reinforced composites, aligning technological innovation with environmental sustainability [17]. The integration of these fibers in various industrial sectors reveals their potential as a viable alternative to synthetic materials, offering a solution that combines superior mechanical strength, lightweight, and biodegradability. The sisal plant, also known as Agave sisalana, is a predominant species in tropical regions with a global distribution. The fibers obtained from the leaves of this plant are known for their remarkable heterogeneity in their physical-chemical and mechanical properties, such as diameter, length, and tensile strength, which can vary significantly even within a single leaf. However, the use of sisal fiber in composites faces challenges related to its propensity for moisture absorption. This factor can lead to fiber swelling, negatively affecting the mechanical strength and durability of the composites. Therefore, to optimize the performance of sisal fiber as reinforcement, surface treatment, such as alkalization, is essential for promoting compatibility and adhesion between the fiber and the polymer matrix, ensuring a more effective interfacial interaction [18; 19]. Sugarcane bagasse fibers are intrinsically hydrophilic, meaning they absorb moisture from the environment, resulting in swelling and deterioration of their mechanical properties. To mitigate such issues, various chemical modification techniques have been explored. Processes such as alkalization, treatment with potassium permanganate, acetylation, silanization, benzoylation, treatment with acetone, and acrylation alter the polar groups of the fibers, imparting hydrophobic characteristics and improving their compatibility with polymer matrices. These modifications facilitate the formation of more stable and durable composites, as documented in the scientific literature, highlighting them as effective strategies for overcoming the integration barriers of natural fibers in engineering applications [20]. Recent advances in PU processing methods allow for the production of forms with varying densities and rigidities, making PU valuable in applications that require materials with impact resistance, hardness, and elasticity. Furthermore, its properties of moisture, wear, and corrosion resistance solidify its position as a highly relevant research material in the field of advanced composites [21]. The versatility of PU is amplified by its ability to integrate additives and fillers, which enhance its mechanical and thermal properties through interfacial interactions between the fibers and the polymer matrix. This capability grants PU extensive applicability across various industrial sectors, benefiting from its viscoelasticity, which provides rigidity while allowing for energy dissipation through frictional losses.