Creation of Biocidal Coatings using the Stabilization of Silver Nanoparticles in Aqueous Acrylic Dispersions

This article proposes a method for silver nanoparticle (AgNP) stabilization in polymer coatings obtained from aqueous acrylic dispersions. The main objective of the study was to improve the biocidal properties of coatings using AgNPs due to the preservation of their nanoscale state. Two types of AgNP solutions with fundamentally different stabilization mechanisms were synthesized and compared. Two mechanisms were determined: an aqueous electrostatic mechanism with sodium docusate stabilizer (AOT) and a steric, propylene glycol with polyvinylpyrrolidone (PVP) stabilizer. The results showed that both solutions were unstable and prone to precipitation as early as the first day after synthesis. However, the saturation of the propylene glycol AgNP solution with ammonium hydroxide to pH < 9 allowed the strengthening of the electrostatic factor of aggregative stability, providing optimal conditions for mixing with acrylic dispersions stabilized by anionic surfactants. The obtained AgNPs in the acrylic dispersions stabilized over time, and when they became film-forming, stable AgNPs (~20–30 nm) occurred on the surface. As a result, the developed coatings using AgNPs synthesized in propylene glycol in the presence of non-ionic PVP and modified with ammonium hydroxide, demonstrated a high inactivation of bacteria colony-forming units (CFU) (> 60%) within 60 min of contact compared to aqueous AgNP solutions using anionic surfactants as stabilizers, where the decrease in CFU was around 25%.

Acrylic dispersion; Aggregative stability; Biocidity; Coatings; Silver nanoparticles

The prevention of biological corrosion that affects almost all existing types of materials, regardless of their composition, method of production, and use, is currently one of the most important problems. An effective way to protect building structures from bio-damages and prevent the evolvement of pathogenic microorganisms in the environment is to form coatings with different compositions on the surface of materials (Tokach and Goncharova, 2016; Bondarenko et al., 2018; Bessmertny et al., 2019; Elnaggar et al., 2019), among which paint and varnish materials can be identified as the most techno-economically beneficial (Tarasova, 2018).

A significant reduction in the number of toxic impurities in the composition of various paints, in combination with uncontrolled sanitary conditions in production, increases the risk of microbial contamination (Karamah and Sunarko, 2013; El-Sakhawy et al., 2016). The storage of paintwork materials in liquid form leads to the discoloration of surfaces; changes in rheological characteristics, pH, and coagulation; reduction in molecular weight; destruction of dispersions; and the release of putrid odors and gas. In hardened coatings, it causes visible algae or fungi fouling, the appearance of a green or gray color, and cracking. However, uncontrolled microbial growth can be reduced or prevented using biocides.

Until recently, the list of substances used for these purposes was quite expansive and included a number of mercury compounds and sulfur- and nitrogen-containing cyclic organic compounds (dithiocarbonate, thiophthalamide derivatives, benzimidazole, and trialkyl tin compounds).

The above-mentioned biocides are highly effective, but their actions reduce biological activity not only in the volume of paint, but also in the environment. In fact, they may be attributed to ecocides. As the United Nations Environment Program was implemented, the list of banned carcinogens continues to grow, which ultimately will drive Russian biocides out of the market.

Colloidal solutions of metallic silver nanoparticles (AgNPs) are promising, as they can meet most of the requirements of the above-mentioned biocides with sufficient control of stabilization (selection and concentration of surfactants, degree of conversion) and environmental conditions (pH, viscosity, electrolytes) (Wang et al., 2018; Adur et al., 2018; Deshmukh et al., 2019).

Nowadays, many ways to obtain nanoscale substances with high reactivity and varying degrees of stabilization exist (Yuwono et al., 2010; Aripin et al., 2017; Helmiyati, 2019; Kusdianto et al., 2019).

The preparation of colloidal dispersions of AgNPs via reduction in polyatomic alcohol is a dynamically developing direction. In this method, the solvent acts as a reducing agent in contrast to the traditional methods of borohydride and biochemical synthesis. Diols, such as 1, 2-ethanediol (ethylene glycol) or 1, 2-propanediol (propylene glycol) are usually used as solvents. Polyols are soft reducing agents that can reduce silver ions to a null-valence state. The molecular mass used to stabilize AgNPs is 40,000, and the molar ratio (R) of its units with the metal lies in the range of 20–40. If R < 1, then the resulting dispersion is unstable. As a result of this reaction, the alcohol is oxidized to the corresponding carbonyl compound, which subsequently reduces the silver to the null-valence state:

                                                CH₂OH–CHOH–CH₃ ? C₂H₅CHO + H₂O                                                                 (1)

                                C₂H₅CHO + AgNO₃ ? Ag⁰ + HNO₃ + 0,5C₂H₅COCOC₂H₅                                                      (2)

Song et al. (2014) proved that the use of polyvinylpyrrolidone (PVP) stabilizers with low molecular weight (M_w < 29000) leads to aggregative particle instability, resulting in the aggregation of the particles to micron agglomerates. The researchers also found that AgNPs stabilized by PVP with M_w = 8000 tend to form associates, which indicates that they are not sterically stabilized. However, Chou et al. (2004) discovered that with the introduction of carbonate ions into the system, the aggregation of the nanoparticles can be stopped. Therefore, the reduced steric barrier of PVP with low molecular weight can be compensated by increasing the electrostatic barrier. In controlling both effects, it is possible to achieve nanoparticle stabilization in the composition of the polymer dispersion for the subsequent formation of biocidal coatings.

To achieve the biocidal effect of acrylic-based coatings, it is most efficient to use AgNPs synthesized as a result of polyol synthesis using PVP of relatively low molecular weight (Mw < 10,000). Low molecular weight PVP weakly inhibits the surface of AgNPs, and as a result, their activity is preserved. The addition of ammonium hydroxide to a pH of < 9 enhances the electrostatic aggregate stability factor, providing optimal conditions for mixing with acrylic dispersions stabilized by anionic surfactants. This allows AgNPs to maintain their dimensions (~20–30 nm) until the polymer dispersion is dry and is also found on the surface of formed coatings. As the final result, the aforesaid coatings demonstrate high bacteria CFU inactivation (> 60%) within 60 min of contact compared to aqueous AgNP solutions using anionic surfactants as stabilizers, where the decrease in CFU is around 25%. 

This research was conducted in the framework of the State Task of the Russian Federation Ministry of Education and Science No. 7.872.2017/4.6. “Development of Principles for the Design of Ecologically Positive Composite Materials with Prolonged Bioresistance”, 2017–2019.

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