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 rosin in 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 coal or copal resins, in old pine tree stumps, etc.
Kraft soap can be reneutralized in the presence of concentrated sulfuric acid to restore the acidic forms abietic acid, palmiric acid and 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.
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 Finland may 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.