2.1 Materials
A mixture of the three Musa textilis genotypes (MT07, MT11, and CF01 clones) that previously showed ideal physical, mechanical, and thermal properties of intensive-use composite materials was used (details in Valverde et al. 25). This material proceded from a pilot plantation of INTA (Instituto Nacional de Tranferencia Agrícola, the institution that authorized and donated the natural fiber), located in Guápiles, Limón, Costa Rica (10°15' N, 83°46' W), under a tropical rainforest climate (Af) according to Köppen-Geiger classification, with an average temperature of 25°C and rainfall of 4000 mm yr− 1. The manipulation of plants and extraction of fiber was carried out according to The IUCN Policy Statement on Research Involving Species at Risk of Extinction and the Convention on the Trade in Endangered Species of Wild Fauna and Flora were both complied with in the collection and use of plant samples for this study. Specifically, M. textilis is a species introduced in Costa Rica, and its use has been scientific and commercial, in turn, classified as "near threatened" status in the IUCN Red List Assessment. On the other hand, recycled thermoplastic polyethylene (PE) and polypropylene (PP) used in the experiment was donated by IPS recycling company.
2.2 Composite preparation
M. textilis fibers were mechanically defibrated and dried (˂20% humidity) according to Gölthenboth and Mühlbauer methodology 26; then, M. textilis fiber and thermoplastics were homogenized and ground to produce a sieve with an average size of 9 mm. Regarding M. textilis fiber used as a reinforcing agent, four loadings were considered: 0 (as control), 10, 20, and 30%. As a result, three profiles with dimensions of 20 x 125 mm x 200 cm were produced for each thermoplastic-fiber loading combination (n = 32 per thermoplastic). Mechanical mixing with a single 100 cm long mixing barrel and profiling with an industrial extruder that was adjusted and profiling with an industrial extruder with a starting temperature set for PE of 275°C and PP of 277°C; the temperature increased by 370°C for PE and 380°C for PP during the profiling phase.
2.3 Physical, mechanical, and specific properties
Profiles were conditioned for 72 hours at 23°C and 50% relative humidity according to ASTM D618-21 27 standard. Consequently, they were cut and sized according to the ASTM standard dimensions for each physical-mechanical test under a sampling of 10 replicates per variable. The physical properties evaluated were density (ρ), thickness swelling (Ts), water absorption (Wa), and weight increase (Wi). On the other hand, mechanical properties considered the dynamic modulus of elasticity (MOEf), modulus of elasticity (Ef), flexural stress (σf), and tensile stress (σT). Finally, specific elastic modulus (Efm) and flexural modulus (σTm) were measured under specific properties.
The ASTM D792-20 28 stand was used for ρ measurement. In contrast, Ts, Wa, and Wi were evaluated with the ASTM D570-22 29 standard. Regarding MOEf was determined using Sylvatest Duo equipment, Vi, based on the time of flight of the wave and the distance between the transducers with a wave frequency of 22 kHz (Eq. 1).
$${\text{M}\text{O}\text{E}}_{f}={{V}_{i}}^{2}\bullet \rho \bullet {10}^{-6}$$
1
Where MOEf is the dynamic modulus of elasticity in flexion in MPa, ρ is the sample density in kg m− 3 and Vi is the wave velocity in m s− 1.
The bending test was performed under ASTM D790-17 30 standard. a JBA-855 universal testing machine (STX Radial Ambient, SL) with a deflectometer module. Then, Ef and σf were calculated according to the equations specified within ASTM standards (Equations 3 and 4, respectively). Finally, σT was tested in a universal testing machine model H10KT (Tinius Olsen Co.) using type I samples with the dimensions and equation specified in ASTM D638-22 31 (Eq. 4).
$${E}_{f}= \frac{{L}^{3}\bullet M}{4\bullet b\bullet {d}^{3}}$$
2
$${\sigma }_{f}= \frac{3\bullet F\bullet L}{2\bullet b\bullet {d}^{2}}$$
3
$${\sigma }_{T}=\frac{W}{{A}_{o}}$$
4
Where Ef is the modulus of elasticity in MPa, σf is the flexural stress in MPa, σT is the tensile stress in MPa, M is the slope of the tangent to the initial straight-line portion of the load-deflection curve in N mm− 1, L is the support span in mm, F is the load at a given point on the load-deflection curve the force applied in N, b is the sample width in mm, d is the sample depth in mm, W is the load in N, and Ao is the original cross-sectional area in mm2.
The specific properties Efm (Eq. 5) and σTm (Eq. 6) were estimated using Hull 32 methodology based on the physical-mechanical test results.
$${E}_{fm}=\frac{{E}_{f}}{\rho }$$
5
$${\sigma }_{Tm}= \frac{{\sigma }_{T}}{\rho }$$
6
Where Efm is the specific elastic modulus in MPa, σTm is the specific tensile stress in MPa, Ef is the flexural modulus in MPa, σT is the tensile stress in MPa and ρ is the sample density in kg m− 3.