Understanding Additives by Bodo Müller

Author:Bodo Müller
Language: eng
Format: epub
Publisher: Vincentz Network
Published: 2019-09-05T16:00:00+00:00

Figure 5.2: Formation and decomposition of hydroperoxides

Driers are metal salts of long-chain organic acids (metal soaps) that are soluble in organic solvents. Driers are added to oxidatively curing paints to accelerate the crosslinking reaction (solidify the coating). The organic acids necessary for salt formation are either naturally occurring fatty acids or synthetic carboxylic acids, such as naphthenic acids (see Figure 1.31) and 2-ethylhexanoic acid. The effect of driers is to decompose hydroperoxides to free-radicals at ambient temperature (Figure 5.2). Especially effective are salts of metals capable of forming two or more oxidation states [e.g. Co(II)/Co(III), Mn(II)/Mn(III)/Mn(IV) or Ce(III)/ Ce(IV)] and thus acting as redox catalysts; they are also called surface driers. The most important redox catalyst is cobalt, whose mode of action is shown in Figure 5.3.

In the first step, cobalt(II) is oxidised to cobalt(III) while in the second step cobalt(III) is reduced to cobalt(II); i.e. cobalt(II) is not consumed and thus is a true catalyst. Nowadays, the use of cobalt compounds is becoming more and more controversial due to possible toxicological effects. Cobalt-free driers are already available on the market.

However, metal salts which form only one oxidation state [e.g. Ca(II), Ba(II), Zn(II) or Zr(IV)] serve as driers, too. These have a weak catalytic effect, if any, when used alone; but they can enhance the effect of redox catalysts. This group of driers can be divided into “through-driers” (or “coordination driers”) and “auxiliary driers” (Table 5.1) [4]. “Auxiliary driers” probably derive their effect (enhancement of the action of redox catalysts) from their Lewis acid character (see Friedel-Crafts catalysts).

The mode of action of “through driers” (or “coordination driers”) is shown in Figure 5.4. The metal ions link paint resin molecules together by salt formation in a manner akin to ionomer crosslinking. Thus, “through-driers” are not true catalysts, as they are consumed.

Oxidative curing accelerated by driers never comes to an end but becomes progressively slower. In the long term, excessive crosslinking may cause embrittlement (see also Chapter 8.2.3). At some stage, embrittlement leads to cracking, as shown in Figure 5.5.

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