(604f) Effective and Industrially Relevant Compounding of Natural Fiber-Reinforced Thermoplastic Composites Via Solid-State Shear Pulverization

Conventional composite materials are most often made with glass fibers due to their high stiffness/strength to cost ratio, while carbon-fibers are also used frequently in advanced applications for their unparalleled mechanical properties.  Recently, fibers derived from natural plants have been also explored as a technical composite filler material of choice due to a combination of practical advantages such as low density, low cost, and low wear to processing equipment. In addition, natural fibers uniquely bring about their environmental benefits such as sustainable production, CO₂neutrality, and minimal energy embodiment [1]. Accordingly, natural fiber composite materials have gradually gained prominence in the manufacturing industry.  Specifically, flax- and hemp-reinforced composites possess advantages beyond mechanical performance and environmental benefits, including acoustic, optical, and haptic properties [2].

While melt compounding process with twin-screw extrusion is a conventional way to mass-produce short fiber composites, a specialized solid-state compounding technique called solid-state shear pulverization (SSSP) offers additional capabilities, especially when natural fibers are used as reinforcement [3]. Due to its low-temperature processing, unwanted thermal degradation of the natural fiber component is avoided, allowing for a wider range of thermoplastic matrix options. The SSSP process relies on mechanochemistry, rather than molten viscous flow, to not only effectively disperse the fillers and prepare homogeneous material, but also favorably de-bundle natural fibers from their technical to elementary fiber formats. This fiber hierarchy transition is expected to yield a high aspect ratio filler state, leading to superior physical properties in the composites [4].

The paper presents the results of the first processing-structure-property study in which SSSP is applied to short flax fiber materials and polyamide 6, a model bio-based plastic composite system. Mechanical property measurements were conducted on injection-molded specimens.  Thermal characterization involved differential scanning calorimetry and thermogravimetric analysis. Fiber structure was probed with optical and electron microscopy. Fiber morphology and its interaction with the matrix polymer play a key role in dictating the mechanical and other physical performance of the composite.

[1] A. K. Mohanty, M. Misra, and L. T. Drzal, eds, Natural Fibers, Biopolymers, and Biocomposites, Boca Raton, CRC Press (2005).

[2] L. Pil, F. Bensadoun, J. Pariset, and I. Verpoest, Composites Part A 83, 193 (2016).

[3] K. Khait, S.H. Carr, and M.H. Mack, Solid-State Shear Pulverization, Boca Raton, CRC Press (2001).

[4] H. L. Bos, M. J. A. Van Degen Oever, and O. C. J. J. Peters, Journal of Material Science 37, 1683 (2002).