Interface characterisation)

Background and Objective

The study outcomes will significantly enhance understandings of wood polymer composite interfaces, and how they develop during processing. Such fundamental understandings of factors controlling interfacial behaviours are a prerequisite when optimising composite performance. This study builds on an earlier, preliminary Scion-Hamburg collaboration which developed the initial knowledge and unique combination of techniques to successfully extend the collaboration and progress to these types of composites.

The study outcomes will significantly enhance understandings of wood polymer composite interfaces, and how they develop during processing. Such fundamental understandings of factors controlling interfacial behaviours are a prerequisite when optimising composite performance. This study builds on an earlier, preliminary Scion-Hamburg collaboration which developed the initial knowledge and unique combination of techniques to successfully extend the collaboration and progress to these types of composites.

Approach

In this project, we will evaluate the adhesive and cohesive bonding of polyester plastics to wood veneer in novel composite sandwich structures. We will determine the molecular, polymer and microscopic behaviours at the plastic-wood interfaces within the composite structures and relate these to mechanical properties and composite performance. Specifically, advanced microscopy and chemical analysis techniques available at the research partners Scion, the Thünen-Institute (TI) and University Hamburg (UHH) will be used to determine these interfaces and compare polymer and engineering properties. The new style of plastic-wood composites will be prepared with the plastic in either sheet or low density foamed laminate form. These differing plastic forms may be expected to provide differing interfacial interactions which manifest in molecular-, nano-, micro- and macro-scale properties. Analysis of composites will be extended to include those formed with other natural fibres and modified wood fibres to determine any contrasting interactions due to controlled changes in fibre chemistry or reinforcement provided by fibre.

Results

A microscopy methodology was developed for qualitative and quantitative assessments of bondlines formed in composite sandwich manufacture with wood veneer and PLA. The use of ca. 0.0075% fluorescent chromophore in PLA matched to the lignin autofluorescence of veneer sandwiches allowed the visualisation of PLA flow and ingress at the wood cell level. The use of higher sandwich formation temperatures promoted PLA mobility further into the wood ultrastructure, acting as anchor points for the reinforcement of bondlines. This was demonstrated with both maple and spruce composite sandwiches formed at 140 and 200 °C with the higher processing temperature resulting in greater composite bond strength. The study results reiterated that relatively higher bond strengths can be achieved by bonding wood veneer with PLA. It is the PLA/wood interface which promotes this bond strength and associated stress transfer across the PLA matrix. Maple veneer revealed greater PLA ingress and bond strength compared with spruce veneer use. Overall, the study findings and, particularly, the impact of processing temperature on laminate strength and adhesion will have broad applications across the manufacture of veneer overlays, laminate composites and WPCs using wood substrates, particles or fibre with PLA.