Oil drying agent

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An oil drying agent, also known as siccative, is a coordination compound that accelerates (catalyzes) the hardening of drying oils, often as they are used in oil-based paints. This so-called "drying" (actually a chemical reaction that produces an organic plastic) occurs through free-radical chemical crosslinking of the oils. The catalysts promote this free-radical autoxidation of the oils with air.

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Typical oil drying agents are derived from ions of cobalt, manganese, and iron, prepared as "salts" of lipophilic carboxylic acids such as naphthenic acids, in order to give them a soap-like chemical formula and make them oil-soluble. [1]

Varieties of drying agents

In technical literature, oil drying agents, such as naphthenates, are described as salts, but they are probably also non-ionic coordination complexes with structures similar to basic zinc acetate. These catalysts were traditionally hydrocarbon carboxylate chelates of lead, but due to lead's toxicity, cobalt and other elements, such as zirconium, zinc, calcium, and iron, have replaced the lead in more popular products. Most driers are colorless but cobalt driers are a deep blue purple color and iron driers are reddish orange. These colored driers are therefore compatible only with certain darker pigmented paints where their color will be unseen.

Japan drier is a common lay term and generic product name for any oil drying agent that can be mixed with drying oils such as boiled linseed oil and alkyd resin paints to speed up "drying". The name refers to "japanning", a term for the use of drying oils as an imitation or substitution for urushiol based Japanese lacquer.

Separate drying additives for paints became necessary as zinc oxide-based paints were developed as an alternative to the lead oxide paints ("white lead") that had been previously used. Zinc oxide paints were developed in parallel with the introduction of "oil soluble driers" or "terebines" around 1885. These were lead and manganese soaps of linseed fatty acids or resin, also termed lineolates or resinates. Terebines had poor shelf life in mixed paints, as they auto-oxidised and lost their effectiveness. As a result, early factory-mixed paints, unless fresh, were a poor substitute for fresh paint mixed by a painter on site from raw ingredients. This situation lasted until the late 1940s; by then further drier developments had superseded the terebines. In 1925, stable napthenate driers were developed in Germany and commercialised in the US in the early 1930s, in parallel with the development of durable and fast-drying alkyd resin enamels. In the 1950s, metallo-organics based on synthetic acids were introduced as driers. [2]

An early work on the drying oils and oil drying agents was by Andés (1901). [3]

Antiskinning

Premature hardening of paint, called "skinning" because it forms a skin on the surface of the paint, can be inhibited by the addition of volatile ligands. These ligands bind to the oil drying agents, then lose their inhibiting action as they themselves slowly volatilize and leave the drying agent to work. One such inhibitor is methylethyl ketone oxime (MEKO), which is called an "antiskinning agent".

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Cobalt(II) naphthenate is a mixture of cobalt(II) derivatives of naphthenic acids. These coordination complexes are widely used as oil drying agents for the autoxidative crosslinking of drying oils. Metal naphthenates are not well defined in conventional chemical sense that they are mixtures. They are widely employed catalysts because they are soluble in the nonpolar substrates, such as the alkyd resins or linseed oil. The fact that naphthenates are mixtures helps to confer high solubility. A second virtue of these species is their low cost. A well-defined compound that exhibits many of the properties of cobalt naphthenate is the cobalt(II) complex of 2-ethylhexanoic acid. Often in technical literature, naphthenates are described as salts, but they are probably also non-ionic coordination complexes with structures similar to basic zinc acetate.

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<span class="mw-page-title-main">Naphthenic acid</span> Chemical compound

Naphthenic acid (NA) is a mixture of several cyclopentyl and cyclohexyl carboxylic acids with molecular weight of 120 to well over 700 atomic mass units. The main fraction are carboxylic acids with a carbon backbone of 9 to 20 carbons. McKee et al. claim that "naphthenic acids (NAs) are primarily cycloaliphatic carboxylic acids with 10 to 16 carbons"[1], although acids containing up to 50 carbons have been identified in heavy petroleum. The term naphthenic acid has roots in the somewhat archaic term "naphthene" used to classify hydrocarbons. It was originally used to describe the complex mixture of petroleum-based acids when the analytical methods available in the early 1900s could identify only a few naphthene-type components with accuracy. Today "naphthenic" acid is used in a more generic sense to refer to all of the carboxylic acids present in petroleum, whether cyclic, acyclic, or aromatic compounds, and carboxylic acids containing heteroatoms such as N and S. Although commercial naphthenic acids often contain a majority of cycloaliphatic acids, multiple studies have shown they also contain straight chain and branched aliphatic acids and aromatic acids; some naphthenic acids contain >50% combined aliphatic and aromatic acids.

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A metallic soap is a metallic salt of a fatty acid. Theoretically, soaps can be made of any metal, although not all enjoy practical uses. Varying the metal can strongly affect the properties of the compound, particularly its solubility.

Copper naphthenate is the copper salt of naphthenic acid. Naphthenic acid is a term commonly used in the petroleum industry to collectively refer to all of the carboxylic acids naturally occurring in crude oil. Naphthenic acids are primarily cycloaliphatic carboxylic acids with 10 to 24 or more carbons, although substantial quantities of non-cyclic, aromatic and heteroatom- containing carboxylic acids are also present. Copper naphthenate is most widely used in wood preservation and for protecting other cellulosic materials such as textiles and cordage from damage by decay fungi and insects. Other metal naphthenates are used as paint driers, rubber adhesion promoters, lubricant additives, and catalysts where oil solubility is required.

A finishing oil is a vegetable oil used for wood finishing.

References

  1. Ulrich Poth, "Drying Oils and Related Products" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2002. doi : 10.1002/14356007.a09_055
  2. Oil and Colour Chemists’ Association, Australia (2014). "Chapter 29: Paint Driers". Surface Coatings volume 1: raw materials and their usage. Dordrecht: Springer Verlag. pp. 352–361. doi:10.1007/978-94-011-6940-0_29. ISBN   978-94-011-6942-4.
  3. Andés, Louis Edgar (1901). Drying oils, boiled oil, and solid and liquid driers. London: Scott, Greenwood & Co.