Biocide retention variation of southern yellow pine products treated with waterborne preservatives in commercial, pilot plant, or laboratory cylinders

The among-sample retention variability of southern yellow pine (SYP) products was examined for seven sets of dimension and timber lumber treated with CCA-C in various commercial plants with retention based on x-ray fluorescence, and for seven sets of 8-foot 2 by 4's treated with ACQ-D in a pilot plant with retention measured by weight gain. In addition, among- and within-sample variability was determined for one set of small (19 mm by 19 mm by 1016 mm) SYP field stakes treated with CCA-C in a laboratory cylinder with biocide levels measured by x-ray fluorescence, and among-sample variability also determined using weight gain. For the commercially-treated lumber, as the lumber thickness increased the biocide retention (based on the outer shell as per AWPA Standard C2) variability increased. Retention variability for the small field stakes treated to ground-contact level in a laboratory cylinder was lower than the variability of the nominal 2-inch-thick lumber treated to the same target retention in either the commercial facilities or pilot plant. Biocide retention was highly correlated (r2 = 98.1%) to specific gravity for the small field stakes, but specific gravity explained less than 30 percent of the biocide retention for lumber treated in commercial operations. Within-sample variation for the small defect-free stakes averaged 60 percent of the among-sample variation, and the ends of the stakes had a slightly higher biocide retention (8.2%) than the interior sections. The ratio of chromium retention in the earlywood to that in the latewood (E/L ratio) at the ends of field stakes that were air-dried right after treatment was significantly lower than the copper or arsenic E/L ratios, suggesting that chromium migrated to the latewood with free water during air-drying to give an uneven macrodistribution. Biocides in the newer waterborne systems may also undergo this migration to give a nonuniform distribution.