Microscopy of Acid-Activated Bonding in Wood

Fluorescence and scanning electron microscopy were used to reveal the effect of nitric acid on activated bonding in wood. The physical properties of the treated wood were analyzed and the feasibility of the bonding technique was evaluated. Results showed that the technique was too severe as it greatly damaged the wood. On both sides of the bond line the cells were crushed beyond identity. Below this zone was a zone of darkened wood, 20 to 50 cells, that was undistorted or partially distorted. Fractured surfaces in samples with high shear strength showed conventional wood failure, while low shear strength samples exhibited amorphous masses of destroyed wood and partially distorted cells. Longitudinal views of fractured surfaces indicated that the acid diffuses readily through cell walls, cell lumina, and intercellular spaces. Lignin and lignin-containing gap fillers applied during acid treatment did not seem to change the action of the acid on the wood. Addition of filter paper and walnut shell flour gap fillers caused deeper penetration of the acid into the wood

Structure of Pit Border in Pinus Strobus L.

Sections fro111 white pine trccs were studied by electrori lllicroscopy in a search for the organization of cell wall layers in the pit border. Depending on the developmental stage of the trachcids, or perhaps on technical imperfections, differences appeared in the pit border within the same tree species. From an electron micrograph of a mature latewood tracheicl, a diagram was reconstructed that appears to be the most representative structure for the pit border in white pine

Improving the fatigue resistance of adhesive joints in laminated wood structures

The premature fatigue failure of a laminated wood/epoxy test beam containing a cross section finger joint was the subject of a multi-disciplinary investigation. The primary objectives were to identify the failure mechanisms which occurred during the finger joint test and to provide avenues for general improvements in the design and fabrication of adhesive joints in laminated wood structures

Microscopy of Abrasive-Planed and Knife-Planed Surfaces in Wood-Adhesive Bonds

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

Surface and Subsurface Characteristics Related to Abrasive-Planing Conditions

The goal of this study was to examine the quality of abrasively planed wood surfaces when variable grit sizes, feed speeds, and depths of cut are used. Our observations show that grit size and wood structure and density seem to have larger effects on the depth and type of damage than feed speed and depth of cut. Coarser grit sizes seem to cause greater damage than finer grit sizes.Surface damage in Douglas-fir occurs at every grit size, feed rate, and depth of cut combination; the earlywood shows more severe damage than the latewood. Surface damage is more variable in hard maple and yellow-poplar than in Douglas-fir. This variability may be due to different cell types present at the surface and the angle of intersection between the surface and the rays. Similar machining conditions do not always have similar effects on the surface quality even in the same wood species. Other factors, such as moisture content, between and within species density variations or belt conditions, might also contribute to the surface quality variability, but these were not explored

Fluorescence Microscopy of Hardboards

We developed a microscopic technique and used it to explore the internal structure and resin distribution in hardboards. The technique will enable us better to understand the behavior of hard-boards in use. Glycol methacrylate (JB-4 embedding medium) proved to be satisfactory for preparing 10- to 15-μm sections of hardboards with a steel knife on a sliding microtome. This thickness of sample, when viewed in transmitted near-ultraviolet light, allowed a clear visualization of hardboard internal structure and resin distribution through the board thickness. We examined wet-formed and dry-formed hardboard samples. Wet-formed high-density and medium-density boards usually showed fibers consolidated into a compact structure and a uniform resin distribution. Dry-formed high-density boards had a compact structure and medium-density boards a less compact structure; both characteristically showed uneven resin distribution

Electron Microscopy Study of Hardboards

Wet-formed and dry-formed aspen fiber hardboards are examined by transmission electron microscopy to obtain information on the hardboard internal structure and fiber-resin interactions. These factors, when related to strength and dimensional properties of hardboards, may be helpful in determining hardboard quality and suitability for structural use.During hardboard manufacturing, the wood cells break apart at the middle lamella and come in contact again when subjected to pressure during hot-pressing. Occasionally fibers remain attached in bundles. Various stages of middle lamella degradation can be observed. When totally disintegrated, middle lamella appears as dark granular material. Voids of variable size exist in medium- and high-density wet- and dry-formed hardboards. In wet-formed boards the resin (which has high electron opaqueness and appears black) shows even distribution. In dry-formed boards the resin shows uneven distribution; it is present as large accumulations in some areas but absent in others

Resin Distribution in Hardboard: Evaluated by Internal Bond Strength and Fluorescence Microscopy

Product performance, to a large extent, depends upon the uniformity of resin deposition on or through the hardboard product. Presently, destructive testing of the hardboard, by measuring its internal bond (IB) strength, is the only method that will provide information about adhesive bond performance.The objective of our research was to compare IB test results with resin distribution patterns observed by microscopy of wet- and dry-formed medium- and high-density hardboards formed under varying conditions of pre- and post-blending variables.Using fluorescence microscopical techniques, we found that differences in resin distribution can be clearly detected. We observed that decreasing the resin solids content, mechanically increasing the fiber rubbing action with the resin, and changing the rate of resin application were effective ways for improving resin distribution in hardboard furnish. Our microscopic technique also showed that uniform distribution of the resin throughout the hardboard produced boards with the highest IB strengths.This research provides guidelines for estimating levels of IB strength based on the use of a developed fluorescence microscopical technique