Date of Award

12-2009

Level of Access Assigned by Author

Campus-Only Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Forest Resources

Advisor

Douglas J. Gardner

Second Committee Member

David J. Neivandt

Third Committee Member

Barbara J. W. Cole

Abstract

The adhesion properties of the individual components of wood plastic composites (WPCs) are highly relevant to determining their compatibility, and in the case of wood plastic composite boards to determine their potential application as structural components. Preliminary results indicated that WPCs have low surface energy, mainly because of the migration of the lubricant added during processing to the extruded board. Different treatments were performed on the WPC surfaces to increase their wettability, and the treatments included chemical, mechanical and energetic modifications. As a result, the most promising surface treatment regime was a combination between energetic (forced atmospheric plasma treatment, FAPT) and mechanical treatment (sanding) which increased the WPC surface energy up to 40 mJ/m2 . The bondability and delamination properties of bonded WPCs and hybrid WPC-Fiber reinforced plastic (FRP) composites were studied using shear strength and fracture toughness analysis. Bonded WPCs displayed a critical strain energy release rate (Gc) for a crack at the interface 25% to 100% higher when the specimens were mechanically (sanded) and energetically (FAPT) treated. For the hybrid composite samples (WPC-FRP) after the FATP treatment, though was demonstrated than high loads were needed to produce the crack on the interface, no clear trend was found between the compliance (C) and the crack opening length and therefore it was not possible to determine their critical strain energy release rate. The reason for this behavior was most likely because of the high load rate used in the experiments. In terms of the surface analysis of WPC boards and its components, chemical analyses were performed using X-ray photoelectron spectroscopy (XPS), which demonstrated an increase of oxygen on the surfaces after the energetic treatment, and micro- and nano- analysis using atomic force microscopy (AFM). AFM analysis consisted of roughness determinations and adhesion force quantification between surfaces and silicon tips prior and post surface treatment. The AFM analyses were performed both in air and in water and it was demonstrated that interactions with the environment were dramatically avoided when the analysis was performed in water. The adhesion forces for WPC surfaces increased up to 100% after the energetic treatment. Finally, a vapor adsorption surface analysis technique using inverse gas chromatography analysis was used to determine acid-base properties of different components comprising the WPCsto evaluate a potential correlation between the work of adhesion/work of cohesion properties and the final mechanical properties of the composites. The results indicated that for the same kind of filler (assuming same shape and dimensions) mechanical properties of the composites increased when the ratio of work of adhesion/work of cohesion increased as well. For composites prepared with resins of low surface energy no clear trend was found.

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