Influence of veneer roughness, lathe check, and annual ring characteristics on glue-bond performance of Douglas-fir plywood Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/b5644v338

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  • Current lathe technology, smaller diameter logs, smaller core diameters, and the sale of higher grade veneer for use in engineered wood products are all factors contributing to plywood manufacturers using rougher veneer with different lathe check characteristics. When rough veneer is encountered, plywood manufacturers typically increase the adhesive spread rate in an attempt to achieve sufficient bonds between veneer surfaces. However, the effectiveness of this practice has not been clearly established. Little is known about how veneer roughness and lathe check characteristics interact to determine glue-bond quality or how lathe checks propagate under load while contributing to glue-bond failure. It was hypothesized that veneer roughness, lathe check, and annual ring characteristics interact to determine plywood glue-bond quality (i.e., wood failure percentage and load at failure). This study investigated the influence of veneer roughness, lathe check, and annual ring characteristics on Douglas-fir Pseudotsuga menziesii) plywood glue-bond performance. The study also investigated the differences in glue-bond quality when samples were tested in a dry and wet (PS 1 boil method) state and prepared such that lathe checks were pulled open or closed. To evaluate differences in test conditions on standard glue-bond samples, 120 blanks were cut from a Douglas-fir plywood panel and kerfed accordingly to produce 60 open and 60 closed specimens. Out of these specimens, half of the open samples and half of the closed samples were tested in a wet condition and the other half in a dry condition. On each sample, ultimate failure load and percent wood failure were recorded. A two-way analysis of variance (ANOVA) test performed on the load at failure results indicated that each factor (wet or dry conditioning and open or closed lathe checks) had a statistically significant influence in load at failure, as did the interactions of factors. Further analysis of the interactions using multiple range testing indicated a statistically significant difference between all four groups (dry open, dry closed, wet open, and wet closed). In terms of load at failure, dry closed exhibited the highest average load value, followed by dry open, wet closed, and wet open, respectively. A two-way ANOVA test indicated that each factor (i.e., wet or dry conditioning and open or closed lathe checks) did not have any statistically significant influence on percent wood failure nor did the interactions. In addition, multiple range testing indicated no statistically significant difference between all four groups. To investigate the influence of veneer characteristics on glue-bond quality, ninety veneer sheets, 12-inches by 12-inches, were separated into three visual roughness categories; smooth, intermediate and rough. Using a laser scatter/optical imaging system, fifteen mathematical roughness measurements were determined for five randomly selected 1-inch by 1-inch areas per sheet. The scanned veneers were placed as center plies in 3-piy, 3-layer plywood panels and pressed using typical mill lay-up procedures. Glue-bond specimens were prepared and tested in accordance to PS 1-95 to evaluate adhesive bonding of the 1-inch² Results from an ANOVA test showed that there was a statistically significant difference (p-value < 0.000 1) for average load at failure between visual roughness categories. Two sample t-tests indicated a statistically significant difference between average load at failure between smooth and intermediate (p-value < 0.000 1), smooth and rough (p-value <0.0001), and intermediate and rough (p-value = 0.043). Analysis of multiple range tests indicated a statistically significant difference for load at failure between smooth and intermediate, and smooth and rough, but found no significant difference between load at failure for intermediate and rough. Intermediate samples had the highest average load, followed by the rough and smooth, respectively, indicating that visual veneer roughness may not be a primary factor in determination of load at failure. Results from an ANOVA test showed that there was a significant statistical difference (p-value < 0.000 1) for average percent wood failure between visual roughness categories. Multiple range tests indicated a statistically significant difference between all three visual roughness groups for average percent wood failure. In addition, two sample t-tests showed a statistically significant difference for average percent wood failure between smooth and intermediate (p-value < 0.000 1), smooth and rough (p-value <0.0001), and intermediate and rough (p-value 0.01). Smooth samples had the highest average percent wood failure, followed by intermediate and rough, respectively. Using stepwise and all possible combination best-fit regression techniques, load at failure was found to decrease as lathe check frequency increased. In addition, load at failure was influenced by the number of growth rings per inch, percent latewood in the test area, earlywood/latewood ratio, distance of second lathe check to the saw kerf, and two distinct mathematical veneer roughness measures. Stepwise and best-fit regression analysis showed that percent wood failure was influenced mainly by mathematical veneer roughness measures, but was also affected by the number of growth rings per inch, percent latewood in the test area, and percent latewood at the tight-side glue-line. Specimen failure typically occurred by lathe checks propagating in a tangential-radial mode, radial-tangential mode or by glue-line failure attributed to peeling forces and/or severe surface roughness at the glue-line. In addition, both mathematical veneer roughness measures and veneer characteristics of latewood angle, percent latewood, lathe check frequency, growth rings per inch, number of latewood bands, average lathe check depth, and earlywood/latewood width ratio were found to influence elastic properties of glue-bond samples. These results suggest that plywood manufacturers can improve glue-bond quality by monitoring and adjusting for the key veneer characteristics of roughness, lathe check occurrence, and annual ring orientation that were found significant in the study. In particular, by reducing the frequency of lathe checks, higher loads at failure can be obtained and by reducing veneer roughness, percent wood failure can be increased.
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