Preservation of Coatings with Silver
The combination of silver or silver with isothiazoline gives a new and very effective preservative system. By using this combination, the amount of sensitizing isothiazoline can be significantly reduced and, if an excess of benzisothiazoline is used, the well-known photosensitivity of silver compounds is reduced. This combination is a highly effective and safe new preservative system for coatings.
The combination of silver or silver with isothiazoline gives a new and very effective preservative system. By using this combination, the amount of sensitizing isothiazoline can be significantly reduced and, if an excess of benzisothiazoline is used, the well-known photosensitivity of silver compounds is reduced. This combination is a highly effective and safe new preservative system for coatings.
Labeling limits such as R43 for “sensitization” and N for “dangerous for the environment,” and pressures to choose from positive lists like the Blue Angel, are reducing the choice and applications of the existing biocides. Additionally, numerous preservatives will no longer be available once the Biocidal Products Directive comes into force and regulatory pressure continues to lower the limits for the labeling of single preservatives. Existing combinations of known actives have already reached their limits, and further novel optimization is unrealistic.
Labeling limits such as R43 for “sensitization” and N for “dangerous for the environment,” and pressures to choose from positive lists like the Blue Angel, are reducing the choice and applications of the existing biocides. Additionally, numerous preservatives will no longer be available once the Biocidal Products Directive comes into force and regulatory pressure continues to lower the limits for the labeling of single preservatives. Existing combinations of known actives have already reached their limits, and further novel optimization is unrealistic.
Silver, the Safe Biocide
The germicidal activity of silver has been known since ancient times. Bacteria are inhibited by Ag+ions in concentrations down to 10-6 mol/L, but are also attacked at lower levels.1 A further advantage of silver is its very broad and universal spectrum of activity coupled with its low human toxicity. For example, silver ions are used for the preparation of drinking water, i.e., for the cleaning of water in camping equipment.2 Because of this, silver and silver compounds are very effective against microbes, but at the same time safe for humans.
Silver - a Preservative in Paint
So far, the use of silver or silver compounds in coatings has been very limited because of concerns over possible silver-related discoloration caused by the reaction of silver with sulfur-containing ingredients, or through the action of light (photo-reduction of ionic silver to metallic silver). In addition to unacceptable discoloration effects, possible side reactions also lead to a reduction of the potential to preserve the paint through the reduced bioavailability of the silver. Additionally, silver has been considered as too expensive to be used as a preservative in paint. Because of these features, in most cases paint composition has been adapted to the use of silver compounds as a preservation system.
Side reactions with paint ingredients are not unique to silver and are also known for conventional preservatives. Furthermore, labeling limits, i.e., for sensitization (R43 sensitizing), may reduce the possible use level of an active below the effective preservation limit. Combining different actives helps to overcome this hurdle by adding the preservation power of the different biocides; staying below the labeling limits of the single actives; and without the negative influence on the paint’s performance.
This concept of combining two or more well-known actives can also be applied to silver-based preservatives, i.e., the combination of silver plus at least one other well-known conventional biocide.
In our test we used 1,2-Benzisothiazolin-3-one (BIT) as the second biocide in combination with silver. BIT is very resistant against high temperatures and most chemicals and also has a high (500 ppm) labeling limit, and Blue Angel approval for use in low-emission wall paints (up to 200 ppm).
Side reactions with paint ingredients are not unique to silver and are also known for conventional preservatives. Furthermore, labeling limits, i.e., for sensitization (R43 sensitizing), may reduce the possible use level of an active below the effective preservation limit. Combining different actives helps to overcome this hurdle by adding the preservation power of the different biocides; staying below the labeling limits of the single actives; and without the negative influence on the paint’s performance.
This concept of combining two or more well-known actives can also be applied to silver-based preservatives, i.e., the combination of silver plus at least one other well-known conventional biocide.
In our test we used 1,2-Benzisothiazolin-3-one (BIT) as the second biocide in combination with silver. BIT is very resistant against high temperatures and most chemicals and also has a high (500 ppm) labeling limit, and Blue Angel approval for use in low-emission wall paints (up to 200 ppm).
Kill kinetics of silver compounds in a phosphate buffer solution.
Kill Kinetics of Silver Compounds and BIT
In our experiments we determined the efficacy of different silver compounds as preservatives. For this we used an in-house-developed kill kinetics test to determine the speed of kill of a mixed bacterial in a defined matrix.
For this study, 10 ml of a mixture of bacteria from aliquot parts of Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli and, Enterobacter aerogenes were made in a 0.1mol phosphate buffer solution, resulting in a concentration of approximately 1 x 107 CFU/ml (colony forming units). One ml of this solution was added to 19 ml of a phosphate buffer/preservative solution, resulting in a starting concentration of approximately 1 – 2.5 x 106 CFU/ml. The number of colonies was determined at 0.5 h, 1 h, 2 h, 3 h, 5 h, 7 h and 24 h.
Figure 1 is a time plot of CFU following treatment with different silver compounds. With the addition of 80 ppm of silver as silver nitrate, silver chloride, or silver chloride on TiO2, recovery of bacterial species is still observed after 24 h at a level of 100 to 300 CFU. The addition of dioctylsulfosuccinate (DOSS), which is a long-established synergist with silver3, demonstrated a complete bacterial kill after 24 h.
For this study, 10 ml of a mixture of bacteria from aliquot parts of Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli and, Enterobacter aerogenes were made in a 0.1mol phosphate buffer solution, resulting in a concentration of approximately 1 x 107 CFU/ml (colony forming units). One ml of this solution was added to 19 ml of a phosphate buffer/preservative solution, resulting in a starting concentration of approximately 1 – 2.5 x 106 CFU/ml. The number of colonies was determined at 0.5 h, 1 h, 2 h, 3 h, 5 h, 7 h and 24 h.
Figure 1 is a time plot of CFU following treatment with different silver compounds. With the addition of 80 ppm of silver as silver nitrate, silver chloride, or silver chloride on TiO2, recovery of bacterial species is still observed after 24 h at a level of 100 to 300 CFU. The addition of dioctylsulfosuccinate (DOSS), which is a long-established synergist with silver3, demonstrated a complete bacterial kill after 24 h.
Kill kinetics of Benzisothiazolin in a phosphate-buffer solution.
The kill kinetics of BIT are demonstrated in Figure 2. The initial effect at a low dosage level (80 ppm) is similar to silver, but after 24 h there is no bacterial recovery. At higher dosage levels, quicker kill rates are seen; for example, at 1000 ppm a complete kill is observed in less than 5 hours.
Kill kinetics of silver compounds with BIT in a phosphate buffer.
Combining BIT with a silver compound, i.e., silver nitrate, at a total concentration of actives of 80 ppm (silver + BIT) bacterial numbers are reduced to non-detectable levels within 24 h. By using a combination of Ag plus BIT in the ratio 1:2 bacterial numbers are reduced to non-detectable levels in 7 h (Figure 3).
Kill kinetics of silver + BIT + dioctylsulfosuccinate.
Adding further DOSS to silver chloride on TiO2 and BIT, as shown in Figure 4, leads to further enhancement of the speed of kill, with a total kill being achieved after 2 hours!
Color of silver compounds exposed to natural daylight after 1 minute.
Stability of Silver Compounds Against Light
A common feature for all silver-based preservatives is their “depot-effect”. By including the silver within a matrix, the amount of silver released and solvated, and therefore, bioavailable, is controlled. Additional benefits are that photo-reduction and the inter-action with other paint ingredients is much reduced.
In a further series we checked the sensitivity of silver compounds against light, with and without BIT. In these tests, silver nitrate became black in a few minutes, while silver chloride, silver chloride on TiO2 and silver nitrate plus BIT stayed white (Figure 5).
In a further series we checked the sensitivity of silver compounds against light, with and without BIT. In these tests, silver nitrate became black in a few minutes, while silver chloride, silver chloride on TiO2 and silver nitrate plus BIT stayed white (Figure 5).
Color of silver compounds exposed to daylight after 24 hours.
However, after 24 h all silver compounds except silver nitrate plus excess BIT became colored (Figure 6). Silver nitrate plus BIT stayed white for weeks (the silver nitrate became black again after a few days).
Sensitivity Against Light and Efficacy of Silver plus BIT
As is shown in Figures 3 and 4, the combination of silver compounds with BIT will give an enhancement in the efficacy against microorganisms. Choosing the right combination (BIT excess), one can also suppress the problematic reaction of the silver compounds with light.<p>
Based on these effects, the dosage levels of combined preservatives can be reduced against the dosage level of single actives, which will lead to a lower risk for labeling, and also reduce cost. With these findings, traditional problems in the use of silver in the preservation of paints are history.
Based on these effects, the dosage levels of combined preservatives can be reduced against the dosage level of single actives, which will lead to a lower risk for labeling, and also reduce cost. With these findings, traditional problems in the use of silver in the preservation of paints are history.
This paper was presented at the Nürnberg Congress held during the European Coatings Show, Nürnberg, Germany, May, 2007, and organized by the Vincentz Network. See events@coatings.de.
Looking for a reprint of this article?
From high-res PDFs to custom plaques, order your copy today!
