Mechanical behavior of recycled polypropylene reinforced by coconut fibers using X-ray tomography and digital image correlation

Abstract

Natural fibers has recently gained attention due to low environmental impact, low cost and easy availability. In this study, morphological and mechanical characterization s were carried out on Brazilian coconut fibers in the 'as received' and superficially treated conditions (NaOH, heated and NaOH followed by heating), o n coconut fiber mats manufactured by compres sing, polypropylene (PP), recycled polypropylene (RPP) and composites involving these materials. Scanning Electron Microscopy (SEM) and X - ray tomography (XRT) were performe d for morphological analysis and uni directional tensile test following by D igital I mage C orrelation (DIC) and infrared thermography were used for mechanical analysis . SEM analysis showed that the chemically treated coconut fibers have sharper external cell walls and circular particles with diameters of approximately 10 μm . The fibers hav e different interfaces within the composite: fiber - matrix, fiber - latex and fiber - fiber. XRT presented the main constituents of natural coconut fiber with an area of about 57 to 60% of internal voids and a mat composed by multidirectional fibers involved by latex . Coconut fibers distribution is related to the injection direction during composites manufacturing. The volume fraction of coconut fibers in composites and impurities in PP and RPP were determined in 3D direction. Some coconut fibers not fully invol ved by matrix demonstrate the incompatibility between fiber and matrix. The initial modulus (modulus of elasticity) and tensile strength decreased with increasing diameters for the four conditions of coconut fibers . DIC analysis showed heterogeneous strain s fields on coconut fibers and on mats; the displacement fields showed the rupture process of coconut fiber. Poisson’s ratio on the manufactured materials was determined through the transverse and longitudinal strains found in the elastic region . The cocon ut fibers and the recycling of polypropylene influenced on the mechanical properties of the polymers. The DIC identified strain fields and yielding formation mechanisms on PP and the infrared thermography indicates that the adiabatic process onset during t he plastic deformation was formed during the alignment of polymer chains. The heating zones were governed by post - yeld regime and presented parabolic shapes with localized peak temperature. A numerical model could predict the stress - strain curve until the maximum stress on the materials of this work. Through numerical parameters obtained by this model, it was possible to determine each damage curve. Critical damage was determined and the composites fabricated by RPP have more time for rupture propagation. D amage evolution using strain fields from DIC demonstrated that the longitudinal strains are predominant in relation to the transverse strain on the damage process. Damage evolution from load/unload tensile testing presented changes in modulus of elasticity and a representative hysteresis. A critical damage value of the composite in accordance with the literature was found. Constitutive relationships between the longitudinal deformation and damage were represented by polynomial equations. The values of longi tudinal deformation related to the onset of damage using strain fields from DIC and from load/unload are similar. Rupture micrographs of the composite manufactured by PP show a ductile fracture and the presence of protrusions at homogeneous regions. RPP pr esented fragile and ductile fractures areas as well as heterogeneous regions of rupture. Rupture micrographs of a ruptured fiber inside the composite shows microcracks around the rupture, similar to tensile testing on individual fibers.