Abstract |
- 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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