Effect of Final Steaming on Distribution and Permanence of Pentachlorophenol in Air-Seasoned Southern Yellow Pine Pole Sections Pressure Treated with Pentachlorophenol-Petroleum

After-treatment methods for conditioning pressure treated wood have gained importance in recent years. With steaming becoming the most popular final conditioning method, the effects of steaming on the preservative gradient and thus on protection have been questioned. This study was conducted to evaluate the effects of final steaming on the pentachlorophenol gradient in southern yellow pine poles. Four 35-foot peeled class 4 southern pine poles were selected for treating. Aromatic gas oil with 5.1 percent by weight of technical pentachlorophenol was used as the treating solution. Before treating, the poles were cut into 17-foot sections and the moisture content, weight per cubic foot, average density, and volume were calculated. Each pole was then treated as a single pole charge, using an empty-cell schedule with a desired retention of 10.0 lbs./cu.ft. After treating, a disc was removed from the center of each pole and the disc was analyzed by extraction to determine the distribution of pentachlorophenol and solvent before steaming. One section of the pole was then stored, and the other section was returned to the cylinder for final steaming. After steaming, a disc was once again removed from the steamed section and the pentachlorophenol gradient calculated. Four months later sections were removed from both the steamed and non-steamed pole sections, and pentachlorophenol gradients calculated. It was found that final steaming removed approximately 19 percent of the pentachlorophenol and 25 percent of the solvent. The pentachlorophenol and solvent removal was uniform throughout the discs, and statistical comparison showed that final steaming did not change the preservative gradient. The analyses following storage showed that the steamed sections lost slightly more preservative and solvent than did the non-steamed sections. The loss was insignificant, however, and did not affect the protection ability of the preservative. It was also found that after the 4-month period, the pentachlorophenol gradient in both steamed and unsteamed sections showed more preservative in the outer 1/4-inch zone than before, while there was less preservative in the preceding, two zones. Changes in all other zones were insignificant.

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