Discrete Bonding of Bio-Based Adherends

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This project uses new 3D printing and laser etching technologies to simulate and model the effect of different droplet arrays on the bond strength between strand adherends. The difficulty with a very heterogeneous material like wood is the high coefficient of variation in strength and toughness measurements requires a large number of samples to be tested. A metal substrate would meet the requirements of a uniform, homogeneous material. Part 1 uses 3D printing to deposit `adhesive' droplets in the form of hot melt glue of fixed shape and size onto a uniform surface, i.e. aluminium plates. Droplets can be pre-printed onto one or both of the plates and bonds formed by heating them on a hot plate until the adhesive melts and then letting them cool to room temperature. The strength and toughness of the bonds will be measured and related to the droplet size and spacing, and optimal target distributions identified. These will then be tested on the non-uniform wood-resin adhesive (both hot melt and  a thermoset) system of strand-based composites to validate the model. Part 2 will use laser etching to produce wooden adherends with raised bonding surfaces of controlled size and spacing. Adhesive droplet size dispersion at the interface between two wood adherends and the strength of that interface has been examined in the past by Kamke et al. (1996), Conrad et al. (2003) and Smith (2003), but the close control over droplet size and placement was limited. Smith (2005) was able to have some control over the size of the resin droplets that were printed onto a strand surface using the modified flexographic printing technique but the range of droplet sizes that could be reproducibly deposited was small. The 3D printer does not have this limitation and that should permit a much wider range of droplet sizes to be examined quickly and easily. The range of droplet sizes and inter-droplet spacing on wood strands in an industrial blender is large and with this new approach the droplet size and spacing can be carefully controlled. This will permit distributions from simple, monodisperse droplet size distributions where every droplet has the same size and distance from its neighbor to complex, random distributions that mimic those occurring in an industrial blender to be created. The next step in this process will be to bond adherends with the same or different droplet arrays together and measure the strength and fracture toughness of those bonds. Practical means of modifying the current lab spray system to produce optimal resin distributions will also be investigated. It is expected that the difference in bond strength obtained for the model bond samples and those coated in the lab-scale blender will define how much additional strength could be obtained by depositing the resin more effectively.