Factors determining the performance of thermoplastic polymer/wood composites; the limiting role of fiber fracture

• Good interfacial adhesion often results in the fracture of the fibers.
• The inherent strength of wood flour determines the performance of the composites.
• Composite strength correlates with the initiation stress of local deformation.

Thermoplastic polymer/lignocellulosic fiber composites were prepared with a considerable range of matrices and fibers in an internal mixer. Tensile properties were determined on bars cut from compression molded plates. Local deformation processes initiated around the fibers were followed by acoustic emission testing supported by electron and polarization optical microscopy. The analysis of results proved that micromechanical deformation processes initiated by the fibers determine the performance of the composites. Debonding usually leads to the decrease of composite strength, but decreasing strength is not always associated with poor adhesion and debonding. The direction of property change with increasing wood content depends on component properties and interfacial adhesion. Good interfacial adhesion often results in the fracture of the fibers. Depending on their size and aspect ratio, fibers may fracture parallel or perpendicular to their axis. At good adhesion, the maximum strength achieved for a particular polymer/wood pair depends on the inherent strength of the fibers, which is larger for perpendicular than parallel fracture. Inherent fiber strength effective in a composite depends also on particle size, larger particles fail at smaller stress, because of the larger number of possible flaws in them. A very close correlation exists between the initiation stress of the dominating local deformation process and composite strength proving that these processes lead to the failure of the composite and determine its performance.

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