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The effects of dryer temperature, sapwood and heartwood, and time elapsing between drying and gluing on the gluing properties of Engelmann spruce veneer Shen, Kuo-Cheng

Abstract

Although Engelmann spruce is being utilized on a commercial scale for plywood manufacture in this province, difficulties are still encountered in the manufacturing process. The most serious problems are the long drying time required at low temperatures for veneers and the frequently unsatisfactory nature of the glue bond. Complaints have also been voiced about the unique characteristics of Engelmann spruce veneer that render it susceptible to degrade in storage. Experience has shown that dried veneer stored under manufacturing conditions for only a few days will no longer form a good glue bond. The present study was initiated to investigate the causes behind these difficulties. Three factors were included: three maximum veneer-drying temperatures (358°F., 378°F. and 393°F.), sapwood and heartwood, and two periods of storage (3 and 30 days) in a fully dried condition. Two measures of glue-bond strength, breaking-load and percentage wood-failure, were used as indicators of glue-bond quality. These were determined by three methods of test: plywood- shear, glue-line cleavage, untreated and glue-line cleavage, treated. It was found that drying temperature and storage time, as studied, had no important effect upon the glue-bond quality. Sapwood veneer gave a lower breaking load but a higher percentage wood-failure than did heartwood veneer. The breaking load seemed to be a more reliable indicator for estimating the glue-bond quality than did the use of percentage wood-failure.

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