Resistance of Wood to Chemical Degradation

Strength loss and chemical changes in wood exposed to. acid and base solutions were measured, using wood from 11 hardwoods and two softwoods. Species used included: Douglas-fir (Pseudotsuga menziesii), Caribbean pine (Pinus caribaea), white oak (Quercus alba), teak (Tectona grandis), lignum-vitae (Guaiacum officinale), vera (Bulnesia arborea), copaia (Jacaranda copaia), Cedro granadino (Cedrela tonduzii), yekoro (Ocotea schomburgkiana), Mora (Amarilla tinctoria), marishiballi (Licania buxifolia), acapu (Vouacapoua americana), and bannia (Swartzia bannia). Small beams (l/2 by 1/2 by 9 inches) were water-saturated and a resin sealer was applied to the ends. Samples were soaked in 20?C. and 50?C. solutions of hydrochloric acid and sodium hydroxide, each at two concentrations (2 percent and 10 percent). Treatment periods were 32 days at 20?C. and 4 days at 50?C. Controls were soaked in water at 20?C. Following treatment, beams were washed with water overnight and mechanically tested to determine strength loss. Percent lignin, holocellulose, alpha-cellulose, extractives, and solubility in 1-percent NaOH were determined. Nearly all species resisted 2-percent HC1 with little loss in strength, and several underwent only a moderate loss in 10-percent HCl at 20?c. Only a few species retained 50 percent of original wet strength after treatment with 2-percent NaOH or as much as 30 percent of original strength after treatment with 10-percent NaOH. Differences between species in resistance could not be attributed simply to their classification as softwoods or hardwoods, although the two softwoods were generally among the most resistant to each treatment. Hardwoods relatively resistant to acid were not necessarily resistant to attack by alkali, and vice versa. The most significant chemical change after treatment with either acid or alkali was a reduction in hemicellulose content. Other chemical changes, including hydrolysis of alpha-cellulose in acid solutions and alkali-induced swelling, probably also contributed to strength loss. Attempts to predict resistance to chemical attack from original composition data were not particularly successful. No support was found for a long-held contention that high alpha-cellulose is the best chemical indicator of acid resistance. The relatively high resistance to attack by either acid or alkali shown by the softwoods gives indirect support to the belief that low pentosan content favors resistance to chemical attack. Cell-wall compactness or inaccessibility appears to be a major factor influencing resistance to acid degradation. Resistance to caustic soda appears to be strongly influenced by factors other than initial accessibility.

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