Resin acids are
protectants and woodpreservatives that are produced by parenchymatous epithelial cells that surround the resin ducts in trees from temperate coniferous forests. The resinacids are formed when two- and three-carbon molecules couple with isoprenebuilding units to form mono- (volatile), sesqui- (volatile), and diterpene(nonvolatile) structures. Resin acids have two functional groups, carboxylgroup and double bonds. Nearly all have the same basic skeleton: a 3-ring fused system with the empirical formula C19H29COOH.
Pines contain numerous vertical and radial resin ducts scattered throughout the entire wood. The accumulation of resin in the heartwoodand resin ducts causes a maximum concentration in the base of the older trees. Resin in the sapwood, however, is less at the base of the tree and increases with height.
Natural resins are water-insoluble mixtures of compounds, many of which have a hydroaromatic structure. Mixtures of isomeric
carboxylic acids, such as abietic and pimaric acids, which occur in rosinin nature in solvent-free form, in the form of tree sap or wood rosin such as pine oleoresin, where they are dissolved in terpenic hydrocarbons. They can also be present as fossil coalor copalresins, in old pine tree stumps, etc.
Resin acids occur in pines in a number of isomeric forms having the molecular formula C19H29COOH and in some related structures. The most prevalent resin acids are:
**13-isopropylpodocarpa -7,13-dien-15-oic acid
*simplified formula C20H30O2, or C19H29COOH
*represents the majority 85-90% of typical
*structurally shown as (CH3)4C15H17COOH
*simplified formula C20H30O2 or C19H29COOH
*structurally represented as (CH3)3(CH2)C15H18COOH
*molecular weight 302
Production in tall oil (chemical pulping byproduct)
Commercially, the manufacture of
wood pulpgrade chemical cellulose using the kraft chemical pulping processes releases these resin acid compounds. The Kraft processis conducted under strongly basic conditions of sodium hydroxide, sodium sulfideand sodium hydrosulfidewhich neutralizes these resin acids, converting them to their respective sodium salts, sodium abietate, ((CH3)4C15H17COONa) sodium pimarate ((CH3)3(CH2)C15H23COONa) and so on. In this form, the sodium salts are insoluble and, being of lower density than the spent pulping process liquor, float to the surface of storage vessels during the process of concentration, as a somewhat gelatinous pasty fluid called kraft soap, or resin soap.
Kraft soap can be reneutralized in the presence of concentrated
sulfuric acidto restore the acidic forms abietic acid, palmiric acidand their isomers which form the resin acid component of a pulping byproduct called tall oil. Other major components include fatty acids and unsaponifiable sterols.
Resin acids, because of the same protectant nature they provide in the trees where they originate, also impose toxic implications on the effluent treatment facilities in pulp manufacturing plants. Furthermore, any residual resin acids that pass the treatment facilities add toxicity to the stream discharged to the receiving waters.
Variation with species and biogeoclimatic zone
The chemical composition of tall oil varies with the species of trees used in pulping, and in turn with geographical location. For example, the coastal areas of the southeastern United States have a high proportion of
Slash Pine("Pinus elliottii"); inland areas of the same region have a preponderance of Loblolly Pine("Pinus taeda"). Slash Pine generally contains a higher concentration of resin acids than Loblolly Pine.
In general, the tall oil produced in coastal areas of the southeastern United States contains over 40% resin acids and sometimes as much as 50% or more. The
fatty acids fraction is usually lower than the resin acids, and unsaponifiables amount to 6-8%. Farther north in Virginia, the resin acid content decreases to as low as 30-35% with a corresponding increase in the fatty acids present. Still farther north in Canada, where mills process Lodgepole Pine("Pinus contorta") Jack Pine("Pinus banksiana"), Eastern White Pine("Pinus strobus") and Red Pine("Pinus resinosa"), resin acid levels of 25% are common with unsaponifiable contents of 12-25%. Similar variations may be found in other parts of the United States and in other countries. For example, resin acid values from Scots Pine("Pinus sylvestris") in Finlandmay vary from 20 to 50%, fatty acids from 35 to 70 %, and unsaponifiables from 6 to 30%. In 2005, as an infestation of the Mountain pine beetle("Dendroctonus ponderosae"), devastated the Lodgepole Pine forests of northern interior British Columbia, Canada, resin acid levels three to four times greater than normal were detected in infected trees, prior to death. These increased levels are based on the fact that a tree uses the resins as a defense. Resins are both toxic to the beetle and the fungus and also can entomb the beetle in diterpene remains from secretions. Increasing resin production has been proposed as a way to slow the spored of the beetle in the "Red Zone" or the wildlife urban interface.
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