naphthente

Naphthenates are sorts of carboxylic acids which contain saturated cyclic hydrocarbons such as cyclopentane and cyclohexane; acids of this kind are known as naphthenic acids. The acids are separated from the gas oil fraction in petroleum distilling. Naphthenates are formed through interaction of naphthenic acids in crude oil with metal ions such as calcium and sodium. Insoluble in either the oil or water phase, and with a density between that of oil and water, naphthenates tend to accumulate at the oil/water interface and act as surfactants to help stabilize emulsions.

The most widely used naphthenates are certain metallic soaps. These soaps are soluble, not in water but in many organic substances. The metallic naphthenates are yellowish-brown and, depending on metallic content, viscous liquids, sticky compounds or soft, solid substances. The metallic content can vary between about 6% and about 40%. 

 

Naphthenates are mostly used as an ingredient in paints, to accelerate the drying of the painted surface. When the binder in paint comes into contact with atmospheric oxygen, the intention is for the binder to cross-link and become a solid layer, i.e. to set. The metallic part of the siccative accelerates absorption of atmospheric oxygen, catalyses the formation of free radicals, and the binder reacts – dries – more rapidly. Most often a combination of two metallic naphthenates is used to achieve rapid drying of the surface and after-setting in depth. Calcium/lead naphthenate mixtures have been common, but since the 1970s lead has been increasingly superseded by zinc and zirconium. Lead, however, gives better after-setting at temperatures below 10ºC. Thus it is the metallic part which decides how the naphthenate functions. The naphthenate acid contributes by dissolving the substance in oils and other non-polar substances. Metallic soaps applied to oils have a gelling effect – the lubricant oil turns into lubricant grease. Naphthenates are also used in this way.

Use of naphthenates as siccatives is diminishing globally, partly due to a transition to other acids, above all 2-ethylhexane acid, which permits higher metallic content in the metallic soap, and also due to the substitution of water-based paints.

Copper and zinc naphthenates are also used to prevent microbial attacks on wood and textiles. These two naphthenates account for over 3/4 of use in Sweden, mostly as wood preservatives. Lead naphthenates are still being used in certain lubricants, in which the lead provides lubrication at high temperature (cf. lead lubrication of valve seats in petrol engines).