In-can preservatives

In general, in-can preservation is a much bigger problem for water­borne paint compared to solventbased paints. In solventbased coatings, the organic solvents are not conducive to growth of micro­organisms. Microbial growth in liquid paint may often result in problems such as gassing, obnoxious odor, staining, pH drift and viscosity drop in paints containing cellulosic thickeners. Bacteria and fungi are the major reasons for such in-can problems. An in-can preservative should have high water solubility and stability over a wide range of pH values. Once the film is dry, it should degrade or evaporate from the film without any side effect on film performance.

Some cost-effective broad spectrum in-can preservatives include isothiazolinone derivatives (Figure 5.22) such as benzisothiazo — linone, methylisothiazolinone and 5-chloroisothiazolinone. They are effective against bacteria, fungi and yeasts. Formaldehyde­releasing compounds such as o-hydroxymethyl compounds (hemi — formals), 1,3-oxazolidines and N-hydroxymethyl chloracetamide are also effective in-can preservatives that can be used alone or in combination with other biocides. Alkyl and halogen substituted phenolic compounds such as sodium salts of o-phenylphenol have also been used as effective preservatives. Owing to their potential physiological harmfulness, their use has been restricted.

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