Resin acid

Resin acids are protectants and wood preservatives that are produced by parenchymatous epithelial cells that surround the resin ducts in trees from temperate coniferous forests. The resin acids are formed when two- and three-carbon molecules couple with isoprene building units to form mono-, sesqui-, and diterpene structures. Resin acids have two functional groups, carboxyl group and double bonds. Nearly all have the same basic skeleton: a 3-ring fused system with the empirical formula C₁₉H₂₉COOH.

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Natural occurrence

Pines contain numerous vertical and radial resin ducts scattered throughout the entire wood. The accumulation of resin in the heartwood and 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 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.

Chemical characteristics

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:

Abietic-type acids

• abietic acid
  • abieta-7,13-dien-18-oic acid
  • 13-isopropylpodocarpa -7,13-dien-15-oic acid
• neoabietic acid
• dehydroabietic acid
• palustric acid
• simplified formula C₂₀H₃₀O₂, or C₁₉H₂₉COOH
• represents the majority 85-90% of typical tall oil.
• structurally shown as (CH₃)₄C₁₅H₁₇COOH
• molecular weight 302

Pimaric-type acids

• pimaric acid
  • pimara-8(14),15-dien-18-oic acid
• levopimaric acid
• isopimaric acids
• 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 pulp grade chemical cellulose using the kraft chemical pulping processes releases these resin acid compounds. The Kraft process is conducted under strongly basic conditions of sodium hydroxide, sodium sulfide and sodium hydrosulfide which 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 sulphuric 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.

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 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.