YELLOWING OF OIL-BASED PAINTS
| Main Authors: | , , |
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| Format: | Article |
| Language: | English Slovak |
| Online Access: | http://www.viks.sk/chk/studcon_2_01_121_131.doc |
| Abstract: | SUMARRIES. The cause of yellowing of oil-based paints has been investigated by analysing drying oils simultaneously by iodometry (to determine the degree of oxidation) and by colorimetry. It was found that yellowing of drying oils can be attributed to co-oxidation reactions of contaminants. Yellowing level is closely related to the extent of drying and appears to be unaffected by increase in temperature, the addition of driers, or linolenate content. CONCLUSION. Our results have shown that yellowing is closely related to the extent of drying. In our opinion, yellowing of drying oils is mainly due to contaminants. At temperatures up to 60°C, parameters increasing the oxidation rate-such as higher temperature or presence of a drier-only increase the yellowing rate but have no influence on the yellowing tendency. For temperatures below 40°C, when reactions of fatty acid chains are slowed down, reactions with contaminants appear to be favoured and yellowing tendency is higher. Differences in degrees of yellowing for different oils may be attributed to various concentrations of contaminants or various sorts of contaminants. Comparative studies of yellowing are then very difficult to interpret, even with comparable drying extents. For a given oil, yellowing will increase until there are no more contaminants or until the complete drying of the oil sample is achieved, with no more radicals formed in the film. Since yellowing is also significant in poppyseed oil containing less than 1% of linolenic acid, the role generally attributed to this polyunsaturated acid in the formation of yellowing products appears to be unfounded. Solutions to avoid yellowing of oil-based paints appear to be very difficult to achieve. Only extremely purified oils or synthetic compounds may lead to colourless films, and an elevated drying temperature would be required to reduce interactions with contaminants. However, it must be noted that yellowing, although unaesthetic, is not a significant criterion of physical degradation because it involves only weak concentrations of contaminants and leaves the network unaffected. Even more, because it is a measurement of the extent of drying, it may be considered as a proof of the hardness of the film that is completely dried. Yellowing must therefore be considered as an unavoidable characteristic of drying oils and this must be kept in mind by users. |
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| ISSN: | ISSN 0039-3630 |
