Microscopy of Abrasive-Planed and Knife-Planed Surfaces in Wood-Adhesive Bonds
Keywords:Planed surfaces, wood-adhesive bonds, microscopy, yellow-poplar, Douglas-fir
Fluorescence microscopy (FM) disclosed no differences in wood cell structure between abrasive-and knife-planed Douglas-fir joints under constant conditions. However, after a one-cycle soak-dry exposure, formation of checks along the rays were visible in both abrasive- and knife-planed samples by fluorescence microscopy. For this same exposure, scanning electron microscopy revealed many radial cracks in the S2 layer and ruptures between the S1 and S2 layers in abrasive-planed samples. Knife-planed samples had few ruptures between the S1 and S2 layers and very few cracks in the S2 layer.
Previous work showed that, although knife planing gave much smoother surfaces at the cellular level than did abrasive planing, both surfaces resulted in high strength bonds. When those bonded samples were subjected to a soak-dry treatment, however, strength of abrasive-planed samples was much lower than that of knife-planed samples.
The substantially intact S2 layers in knife-planed samples, as revealed here, apparently retain considerable strength, while rupturing and cracking in the abrasive-planed samples explain the loss of bond quality reported in earlier work.
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