# Evaluation of Impact Energy Absorption by Natural Fiber Composites in Motorcycle Helmets
## 2 Experimental Method for Impact Absorption
### The Manufacture of Composites  
The production of polyurethane (PU) foams was conducted using a 1:1 mass ratio of polyol to isocyanate, achieving an expansion rate of 20 times the initial volume and a density of 1.3 g/cm\({}^{3}\), as specified by the manufacturer (AVIPOL(r)). Specific natural fibers--coconut, sugarcane bagasse, and sisal--were incorporated during the production process. These fibers were chosen for their mechanical properties, widespread availability in Brazil, and their frequent improper disposal as solid waste, thereby highlighting an opportunity for sustainable utilization [27].  
All fibers used in this study originate from the state of Rio de Janeiro, Brazil, and were sourced from local suppliers, without being tied to a specific brand or manufacturer. The natural fibers underwent an alkaline treatment to improve their affinity and adhesion to the polymeric matrix and remove lignin, waxy substances, and other surface impurities before their incorporation into the PU. This treatment was performed in a 10% sodium hydroxide solution (PA, VETEC(r)) with continuous agitation at 1300 rpm for 15 min in a controlled-temperature environment (25 \({}^{\circ}\)C \(\pm\) 5 \({}^{\circ}\)C). After treatment, the fibers were rinsed with distilled water until a neutral pH was achieved, then dried in an oven at 70 \({}^{\circ}\)C for 24 h to ensure complete moisture removal. The fibers were subsequently ground using a Wiley-type knife mill equipped with four blades and sieved with an electromagnetic sieve shaker featuring a 10 mm mesh to ensure particle size uniformity, Figure 2[28; 29].  
Figure 2: Fiber treatment: (**a**) stabilization with distilled water; (**b**) moisture removal.  
To prepare the test specimens, it was necessary to calculate the polyvinyl chloride mold's volume, considering the PU's expansion. The cylindrical mold, with a diameter of 10 cm and thickness of 1 cm, had its volume calculated using the formula \(V=\pi\cdot\left(\frac{d}{2}\right)^{2}\cdot h\), resulting in 78.54 cm\({}^{3}\). After division by the PU expansion factor, an initial volume of 3.93 cm\({}^{3}\) was obtained, corresponding to 5.11 g of material for the formulation. The composites were prepared with 5%, 10%, and 15% fiber proportions, limited experimentally to 15% due to restrictions identified in higher fiber volumes as reported by [PERSON] et al. [30], due to uncontrolled expansion of the polymer. This standardization ensured consistency and efficacy in fabricating the test specimens.  
All stages of the composite fabrication were conducted in a controlled-temperature environment (25 \({}^{\circ}\)C \(\pm\) 5 \({}^{\circ}\)C), adhering to a total curing time of 48 h as recommended by the manufacturer prior to the impact absorption tests. The testing was performed at the Product Testing Laboratory (Laboratorio de Ensaios em Produtos) (LAENP) of the National Institute of Technology in Brazil. To assess the variability of the results, ten replicates of each composite were produced, except for the 15% sisal composite, for which thirty replicates were formulated based on preliminary statistical analysis.