Use of Silver Ion as an Antibacterial Agent in Decorative Paints

Silver in the form of silver chloride on a TiO₂ carrier has been demonstrated to be an effective antimicrobial agent in dry paint films against MRSA, Pseudomonas aeruginosa and E. coli bacteria, even at very low concentrations. Laboratory testing showed that "wiping" of the dry paint film had no adverse effect on the activity of the silver. As a result of increasing public awareness regarding hygiene stemming from high-profile hospital-acquired infection cases such as MRSA, there is now considerable interest in decorative paints and coated surfaces that provide hygienic benefits [1]. The ability to inhibit bacterial growth and reproduction has become increasingly important today in areas where good hygiene practice is critical. While some bacteria can survive on dry surfaces for intervals ranging from several hours to several days, some bacterial spores can remain viable for years.

Bacteria can contact painted surfaces through various routes, such as accumulated dust and skin contact. Contact of bacteria with water can provide suitable conditions for the growth of existing bacteria.

Hygienic coatings are preferred as part of the approach to creating cleaner environments. In this sense, paint films with antimicrobial properties that are easily washable provide extra protection for wall coatings. Today, there are numerous antimicrobial paint products containing triclosan (2,4,4'-trichloro-2'-hydroxydiphenyl ether), an organic compound. Triclosan has been documented as an effective biocide in this respect. However, the use of such a compound as a bactericide also has some disadvantages. A recent report [2] showed that exposure of E. coli to triclosan promoted mutant bacteria that altered cell wall chemistry. Some ammonium compounds are also commonly used for this purpose. However, similar changes in cell wall structure have been observed in Pseudomonas aeruginosa using this additive. With thickening of the cell wall, bacteria became impermeable to the biocide and biocidal efficacy was reduced. In this respect, the use of silver-containing coatings is described as a safer and much less likely-to-develop-resistance alternative. In aqueous systems, silver is effective against bacteria even at very low concentrations [3,4]. Other reports demonstrate the positive effects of silver in textiles [5] and on the interior surfaces of refrigerators [6]. It also demonstrates its use in the medical field for treating burns and various infections. However, there is very little research on the effectiveness of silver ions in paint used in interior wall coatings.

3. Results and Discussion 3.1. Comparison of the Effectiveness of Silver and Triclosan on Dry Paint Film

The objective was to determine how effective silver ions are over a 24-hour period on a typical paint film sample and to compare their effectiveness with samples obtained from varying concentrations of triclosan - chlorinated diphenyl ether. Paint 1 containing 0.26 mM silver, containing 0.26 mM triclosan, and serving as a reference containing no bactericidal additive, was applied to flexible Lanata panels and allowed to dry for 5 days at 20°C and 65% relative humidity. Dry paint films were subsequently inoculated with MRSA, Pseudomonas aeruginosa and E. coli at 20°C and 65% relative humidity (RH) and bacterial populations were monitored at specific time intervals over 24 hours. As can be seen from Graph 1, the standard dry paint film did not contribute to a reduction in MRSA bacterial populations over 24 hours. However, with paint film 1 containing the silver additive, the MRSA population was significantly reduced compared to the standard even after just 6 hours. Triclosan was tested at three different concentrations; however, at all three concentrations, triclosan showed no real activity against MRSA. Similar trends to MRSA were observed in Pseudomonas aeruginosa and a significant reduction in bacterial population was observed after 12 hours (Graph 2). [caption id="attachment_120867" align="aligncenter"] Graph 1. Effectiveness of silver and triclosan in Paint A formulation over 24 hours on MRSA population[/caption] [caption id="attachment_120868" align="aligncenter"] Graph 2. Effectiveness of silver and triclosan in Paint A formulation over 24 hours on Pseudomonas aeruginosa population[/caption] [caption id="attachment_120869" align="aligncenter"] Graph 3. Effectiveness of silver and triclosan in Paint A formulation over 24 hours on E. coli population[/caption]  

3.2. Effect of Dry Film Thickness on Silver Activity

The following study was conducted to determine whether the activity of silver is affected by dry film thickness. Paint A containing 0.39 mM silver was applied at i) 200 microns wet (100 microns dry) and ii) 400 microns wet (200 microns dry) film thicknesses and allowed to dry under the same environmental conditions (25°C, 65% RH). Dry paint films were subsequently inoculated with MRSA at 20°C and 65% relative humidity over a 24-hour period to determine whether silver exhibited any bactericidal activity. As can be seen from Graph 4, silver was found to effectively reduce bacterial population over a 24-hour period and very little difference in activity was found between a coating of 100 microns (dry film) and 200 microns (dry film). This study demonstrates that a single coat application of silver-containing paint would be sufficient to obtain an effective antimicrobial coating. However, in practice, two coats of paint are recommended to achieve complete coverage. [caption id="attachment_120870" align="aligncenter"] Graph 4. Effectiveness of MRSA population over 24 hours at different film thicknesses with and without silver in Paint A formulation[/caption]  

3.3. Effect of Abrasion Test on Silver Effectiveness

Today, especially in high-traffic areas, paint films that resist wiping are preferred to be able to clean soiling that may occur on walls. For this purpose, the following study was conducted to determine whether wiping had any adverse effect on silver activity. 3.3. Effect of Abrasion Test on Silver Effectiveness Today, especially in high-traffic areas, paint films that resist wiping are preferred to be able to clean soiling that may occur on walls. For this purpose, the following study was conducted to determine whether wiping had any adverse effect on silver activity. [caption id="attachment_120871" align="aligncenter"] Graph 5. Effectiveness of Paint A formulation after application following rubbing with abrasive and soft cloth on MRSA population over 24 hours[/caption]   Silver chloride in dry paint films has been demonstrated to be effective against MRSA, Pseudomonas aeruginosa and E. coli even at very small concentrations. Dry film thickness was found to have no significant effect on effectiveness. However, in practical applications, it is recommended that at least 2 coats of paint be applied to ensure complete coverage. Laboratory testing of the dry paint film showed that wiping did not cause any adverse effect on the antimicrobial effectiveness of silver.

References 1. D. Raynar, MRSA: an infection control overview, Nursing Standard, 17, (45), 2. McMurry et al, FEMS Microbiol Lett, 3. J. Kusnestov, N. Elomaa, P. Martikainen, Wat. Res., 35, p4127, 2001. 4. J. Keheler, J. Bashant, L. Johnson, Y. Li, World J. Microb. Biotech., 18, p133, 2002. 5. J. Kiwi, T. Yuranova, A.G. Rincorn, PRA Third International Conference Dedicated to Hygienic Coatings and Surfaces, Paper 18, 2005. 6. H. Studer, PRA Third International Conference Dedicated to Hygienic Coatings and Surfaces, Paper 16, 2005. 7. Anon (2000), Antimicrobial Products – Test for antimicrobial Activity and Efficacy, Japanese Industrial Standard JIS Z 2801: 2000. 8. ISO 22196:2011 Measurement of antibacterial activity on plastics and other non-porou surface.

   

Arzu Soyalp

Research and Development Manager Marshall Boya ve Vernik San. A.Ş.