Eucalyptus bark residue application for Poly(Vinyl Chloride) composite production: Influence of fiber size and content

Brazil is the world’s largest producer of short fiber cellulose, generating large amounts of eucalyptus bark residue (EBR). Aiming to obtain composites known as wood plastic composites (WPC), the effect of ground EBR addition to a poly(vinyl chloride) (PVC) matrix was studied, considering different granulometries and matrix/load proportions. The influence of fiber content addition and particle size range was analyzed in terms of mechanical and thermal properties of the PVC-EBR fiber composites obtained. Finally, by comparing these properties with those reported in the literature, the viability of EBR application as filler/reinforcement in a WPC with PVC matrix was verified. The main objective of the present study was to evaluated the influence of EBR fiber size and content in the WPC with PVC matrix, aiming to reduce the costs and improve its mechanical and physical properties. The processing method for prepared the composites were two-roll milling and subsequent hot pressing. The residue was characterized via chemical and thermogravimetric analyses, scanning electron microscopy (SEM), and aspect ratio determination. Composite evaluation involved density, tensile and flexural tests, impact resistance, heat deflection temperature (HDT), moisture absorption, and SEM of tensile fractured specimens. Tensile and flexural moduli were improved with fiber addition attaining 46% and 58% increases, respectively, with better results for smaller particle size fibers; impact resistance and elongation at break, however, were reduced, attaining 48% and 5% of the control sample’s properties. SEM images reveal fiber detachment and pull-out due to their low matrix adhesion. EBR fibers cause more void formation due to low interface adhesion, which results in poor stress transference from the matrix to the fiber, in addition to EBR acting as stress concentrators in the PVC matrix; therefore, impact fracture occurs with lower energy levels. Regions with fiber detachment and pullout from the matrix are visible, clearly demonstrating the low adhesion properties of the phases and also in accordance with the observed decrease in mechanical properties for both fibers. SEM images indicate that fibers can be considered foreign particles inside the PVC matrix, acting as stress concentrators. Also, since fibers have diameters larger than the voids caused by crazing, planar density is reduced in the direction perpendicular to chain stretching leading to low strain at break values. Composites of fiber with finer particle showed fewer voids, possibly indicating a more efficient adhesion for fibers. This could be due to a higher penetration of polymeric chains in the rugosity of finer fibers, leading to higher values of tensile strength. The addition of EBR content increased especially flexural and tensile moduli whilst elongation at break and impact resistance are reduced. The reduction in tensile strength due to poor fiber-matrix interfacial adhesion, though significant, allowed the use of composites with higher fiber additions. The use of EBR is coherent with the concept of circular economy. Thus, higher fiber additions relate directly to money savings since this residue costs less than PVC, and also this is an environmentally correct destination for this residue since WPC is long-term usable allowing for recycling.