Fluorescence microscopy was applied to understand adhesion interfaces developed within laminated composite sandwiches formed between poly(lactic acid) (PLA) and wood veneers. Composites formed with maple veneer had greater tensile bond strength when manufactured at 200 °C (10.4 N/mm2) compared to formation at 140 °C (8.7 N/mm2), while significantly lower bond strength was achieved using spruce veneers, at 5.2 and 3.5 N/mm2, respectively. Qualitative and quantitative confocal microscopy assessments revealed differing bondline thicknesses and PLA ingress within the wood ultrastructure. Forming maple veneer composites at 200 °C promoted greater PLA mobility away from the bondline to reinforce the wood–PLA interface and deliver associated greater composite bond strength. The addition of 25% wood fibre to PLA led to fibre alignment and overlap within bondlines contributing to relatively thicker, heterogeneous bondlines. Study outcomes show that the composite processing temperature impacts the adhesion interface and composite performance and will have broad application over veneer overlays, laminates and wood plastic composites (WPCs) using wood, particles or fibres with PLA.
This paper investigates biochemical, morphological and mechanical properties of a large range of plant fibres explored with the same methods. Biochemical results clearly exhibit strong differences between gelatinous, i.e. flax and hemp, and xylan type, i.e. jute and kenaf, cell walls. These differences into parietal composition have an impact on cell wall stiffness, highlighted through nanoindentation and atomic force microscopy measurements, but also on fibre individualisation, due to variations into fibre bundles cohesion. In addition, the morphology and particularly the lumen size induces apparent density differences. Moreover, the influence of fibre morphology and properties is demonstrated on UD materials. Finally, longitudinal Young's modulus of each plant fibre batches is back-calculated from UD stiffness by an inverse method; the results obtained are in accordance with the values in the literature values, proving the interest of this method to estimate longitudinal Young's modulus of short plant fibres.
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