Introduction

Serendipity has played a pivotal role in numerous scientific breakthroughs.1 Examples include the discovery of penicillin, X-rays, and even the unintentional creation of Post-it notes. Similarly, the research presented here took an unforeseen direction. The primary aim was to assess the efficacy of a commercially available antibacterial agent for possible use in antifouling coatings. However, it was found that this agent considerably shortened the pot life of a polyurethane (PU) coating formulation.

Polyurethane systems are conventionally catalyzed by electrophilic compounds (Lewis acids) and nucleophilic compounds (Lewis bases).2 Organometallic compounds of tin, zinc, zirconium, bismuth, cobalt, nickel, manganese, titanium and aluminum have shown to be catalytic active Lewis acids.3,4 Within this group, dibutyl tin dilaurate (DBTL) stands out as the most significant.2,4,5 However, due to rising concerns over the toxicity of organotin compounds, there is a pressing need to explore safer alternatives.2,3 Recent research describes the potential of transition-metal complexes, especially copper-amine complexes, as alternatives to traditional tin-based catalysts.3,6,7,8 Additionally, metal-b-diketone complexes, including iron, copper, and chromium compounds have demonstrated potential in polyurethane synthesis, some even presenting reaction rates comparable to traditional tin catalysts.5

Building on the unexpected catalytic behavior of the antibacterial agent in PU coatings, this study conducts a comparison with standard catalysts. The focus was on understanding the agent's impact on the properties of PU coatings and to get insights about the underlying mechanisms that is driving its catalytic activity. Benefits and challenges of the technology are discussed to provide a comprehensive understanding of its potential for the coatings industry.

Materials and methods

Raw materials

An overview of the used raw materials is presented in Table 1. All raw materials are commercially available and are used without further modification if not stated otherwise.

Table 1 Overview of the used raw materials
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Coating properties in comparison to established catalysts

Coating formulation

Nine coatings with different concentrations of antibacterial (AB) agent as well as two coatings for each, a commercial zirconium (Zr) catalyst and a commercial tin (Sn) catalyst, were produced. The concentration for the commercial catalysts were selected in accordance with the minimum and maximum concentrations given in the technical data sheet. Additionally, a reference coating formulation without catalyst was prepared (nKat). Tables 2 and 3 show the coating recipes for the AB agent formulations and the reference coating formulations, respectively. Barium sulfate was used to balance the solid content for varying concentrations of antibacterial agent. Formulations where the content of the antibacterial agent was completely or partially replaced with barium sulfate showed an increased tendency for settling. This was counteracted with a rheological additive.

Table 2 PU-coating formulations with antibacterial agent for determining the physical properties of the coatings
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Table 3 Reference PU-coating formulations with conventional zirconium (Zr) and tin (Sn) catalysts as well as a catalyst-free formulation (nKat)
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Sample preparation

The paint samples were applied on steel sheets with dimensions of 200 × 100 × 2 mm3. The steel panels were cleaned with an aqueous surfactant solution, followed by a clear rinse with deionized water (DI water) and subsequent drying at room temperature. The coatings were applied using a 200 µm doctor blade. The coatings were left for drying at room temperature. The dry film thickness (DFT) was around 80 µm.

Pot life

The pot life was determined by measuring the time it took for the viscosity to double using a DIN 6 flow cup. Further information about the measurements can be found in the standards DIN EN ISO 9514 and DIN EN ISO 2431.

Bandow–Wolff drying test

The modified Bandow–Wolff method, as specified by ISO 9117-5, was used to determine the drying progress. The test was performed on one coated panel for each formulation. This study evaluated three stages of drying: TG1 (dust dry), TG3 (touch dry), and TG5 (assembly dry). The initial drying of the coating layer (TG1) was examined by scattering glass beads and wiping them off. To evaluate TG3 and TG5, a 2 cm × 2 cm square piece of paper was placed on the coating's surface, with varying weights applied based on the specific drying stage under examination. The test arrangement was evaluated after 1 min in accordance with the ISO standard.

Solvent resistance

Butyl acetate (BuAc) and methyl ethyl ketone (MEK) were used as test solvents to assess solvent resistance following DIN EN ISO 2812-3. A piece of absorbent cotton was soaked with the corresponding solvent, applied to the specimen's surface, and covered with a watch glass. The cotton and any excess solvent were removed after 10 min of exposure time. The degree of blistering, as specified by DIN EN ISO 4628-2, as well as any visible changes on the paint's surface, were evaluated. The test was performed on one coated panel for each formulation.

Characterization of antibacterial agent

Extraction experiments

Extraction experiments were carried out using 200 g DI water and 10 g antibacterial agent. The solutions were prepared in a glass container, which was sealed to prevent evaporation. A reference solution of copper (II) sulfate pentahydrate (CuSO4·5H2O) was prepared in DI water as well. The solutions were left to stand overnight. They were examined for any visual changes the next day and the powder was filtered and dried for further analysis.

SEM–EDX analysis

SEM–EDX (Prisma E-SEM, Thermofisher Scientific, Denmark) was used to analyze the shape and chemical composition of the antibacterial agent. The powder was fixated on carbon tape on an SEM pin and excess powder was cautiously removed with pressurized air. A 6 nm thin layer of silver was sputtered before the analysis. The surface of the samples was analyzed at different magnifications with an acceleration voltage of 18 kV.

Reaction kinetics with rheometer

Coating formulation

Table 4 presents the fundamental recipe used for the coating formulations tested with the rheometer. The reference coating contained 1 wt% of an antibacterial agent, which equates to 0.25 wt% copper. Fresh coatings were always produced one day in advance to the rheometer measurements. All other copper salts were incorporated to match this specific copper concentration. To achieve various particle sizes, the copper salts were processed using a ball mill and then sieved using a sieve tower. The particles with a diameter of around 1 µm were obtained through spray drying of a 3 wt% copper sulfate pentahydrate solution in DI water.

Table 4 Base coating formulation for kinetic study with rheometer
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Rheometer measurements

The viscosity measurements for the reaction kinetics were conducted using an HR-20 rheometer from TA Instruments, equipped with a 40 mm plate/plate setup. The gap size was set at 125 µm, and the measurements were taken at a consistent temperature of 21 °C. Following the addition of the curing agent to the formulation, the viscosity increase over time was observed with repeated viscosity measurements, using a shear rate of 500 s−1. The data displayed show the relative viscosity increase over time with the initial measurement as reference point.

Results

Coating properties in comparison to established catalysts

The following section presents the findings with respect to how the catalytic effect of the antibacterial agent compares to conventional catalysts.

Pot life

The pot life is a pivotal parameter in paint application because it determines the window of time available for application. It also gives the first indications about the significant catalytic effect of the antibacterial agent. For optimal performance, it is desirable for a coating to possess an extended pot life but short curing times. However, often it is a balance to find an acceptable pot life that leads to the desired fast curing times.

Figure 1 presents the results of the pot life measurements. In general, these results indicate a consistent decrease in pot life with increasing catalyst concentrations. For the antibacterial agent, the pot life decreased from approximately 3.5 h to below 1 h as the concentration increased from 0.01 to 0.5%. At concentrations above 2%, the pot life reached a minimum of 20 min. The behavior observed for the antibacterial additive at lower concentrations (AB0.01–0.05) show a similar behavior than the tin-catalyzed samples even though the tin-catalyzed samples seem to reduce the pot life more at already lower concentrations. Meanwhile, samples containing the zirconium catalyst displayed the highest values, aligning closely with the catalyst-free coating that had a pot life of 270 min.

Fig. 1
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Pot life results, log-scale x-axis. The catalyst-free reference coating had a pot life of 270 min

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Bandow–Wolff drying test

The level of dryness is a crucial factor of a coating. The aim is often to achieve fast and predictable drying times while maintaining high coating quality. By specifying the degree of dryness, it is possible to identify when a coating has reached the dust-dry (TG1), contact-resistant (TG3), and block-resistant (TG5) stages.9

Table 5 displays the time in hours for the different coatings to reach the various drying stages. The zirconium catalyst, used within the recommended concentration range, did not exhibit any acceleration with respect to drying when compared to the catalyst-free coating. A comparison between the tin catalyst and the antibacterial sample's catalytic effect demonstrates that even small amounts (0.01–0.05%) of the antibacterial agent showed an improved drying time over the tin catalyst. At the same time, the pot life was not significantly reduced (compare Fig. 1). The catalytic effect of the antibacterial agent increased with higher concentrations leading to a decrease in drying time. TG3 values for the samples AB0.01-AB0.2 could not be determined since the samples had already reached TG5 at the time of the measurement. But still, the results show that the samples that were catalyzed with the AB agent can compete with the samples that were catalyzed with the conventional catalysts.

Table 5 Results of Bandow–Wolff drying test. Sometimes a drying stage was skipped because measurements were not taken in time
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Solvent resistance

Table 6 presents the outcomes of the solvent resistance experiment. This evaluation was based on the EN ISO 4628-2 standard, which employs a rating system that accounts for the number of blisters, rating them on a scale from 2 to 5, and their size, rated from S2 to S5. For example, a rating of 2(S2) signifies the presence of a few small blisters. A numerical value of "0" represents an unchanged surface. Exemplary images are shown in Fig. 2.

Table 6 Results of solvent resistance
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Fig. 2
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Example images of blisters formed during exposure with MEK

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Upon examination, none of the tested samples exhibited visual changes when exposed to butyl acetate (BuAc). Similarly, both the catalyst-free and zirconium-catalyzed coatings remained unchanged after methyl ethyl ketone (MEK) exposure. In contrast, the tin-catalyzed coatings displayed blistering when subjected to MEK; more extensive blistering was evident in coatings with a higher catalyst concentration. Samples containing 0.01% and 0.05% antibacterial agent exhibited minor blisters, whereas those ranging from 0.05 to 0.5% displayed none. Interestingly, blistering reappeared in samples with concentrations between 1 and 5%. This particular behavior, where both, lower and higher concentrations induced blistering, but medium ones did not, might stem from the general characteristic of a catalyst to also promote the reverse reaction—from the product back to its initial reactants. A minimum concentration of 0.05 wt% of the antibacterial agent was essential to establish a robust network with a high molecular mass. However, when the concentration surpassed 0.5 wt%, the reverse reaction was catalyzed to such an extent that it led to a decrease in molar mass, which in turn compromised the solvent resistance of the coating. Ultimately, coatings with the antibacterial agent showed improved solvent resistance if compared to the tin catalyst at similar concentrations of active substance.

Investigation of catalytic effect of antibacterial agent

The SEM–EDX analysis in Fig. 3 might suggest that the particle size of the antibacterial agent was on the order of 5 µm, and the agent consisted of a BaSO4 carrier with smaller, copper-based compounds adhered to it. In addition to copper, sodium was detected. Given that copper compounds are recognized for their catalytic activity in PU-systems,3,5,6,7,8,10 the copper in the antibacterial agent may be instrumental for the underlying mechanism.

Fig. 3
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SEM–EDX result for the antibacterial agent as a powder. The red circle highlights a single particle of the antibacterial agent. The surrounding is mainly carbon tape

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Tests regarding the antifouling performance of the antibacterial agent showed that the copper from the agent leaches upon exposure to DI water. A comparison in Fig. 4 between SEM–EDX analysis of unexposed and a DI water exposed antibacterial powder highlighted a pronounced decline in copper concentration from roughly 25 wt% to under 10 wt% after 24 h exposure to DI water.

Fig. 4
figure 4

SEM–EDX results for filtered and dried antibacterial agent powder after extraction experiments. The orange bar displays the reference antibacterial agent that did not go through extraction experiments. The magnification was 100 × and the error bars display the 95% confidence interval for three measurements at three different locations on the samples. The area of the sample was roughly 0.5 cm2

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From these depleted samples, the importance of copper was further investigated. Figure 5 shows the results from rheometer measurements, comparing the relative viscosity increase of the coatings over time, serving as an indication for the catalytic effect. The results show a reduced catalytic effect for the coating containing the copper depleted variant of the antibacterial agent.

Fig. 5
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Rheometer kinetic study comparing the pure antibacterial agent to its copper-depleted variant

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Figure 6 shows experiments that were performed with copper(II) sulfate pentahydrate as a copper alternative to test if the catalytic effect of the antibacterial agent resulted from a synergistic effect between one or multiple components that the agent contained. However, it can be seen that the catalytic effect of the copper(II) sulfate variant was comparable with that of the non-depleted antibacterial agent sample. Importantly, the catalytic effect of the copper(II) sulfate highly depended on the particle size. Larger particles (355–600 µm) had almost no effect on the curing time. However, if the copper(II) sulfate particles had a size of less than 200 µm, the catalytic effect was higher than that of the antibacterial agent. Notably, there was a reduction in the antibacterial agent’s overall catalytic effect when compared to the results in Fig. 5. With approximately a 2 month gap between the experiments (from Figs. 5, 6), potential humidity exposure may have affected its efficacy.

Fig. 6
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Rheometer kinetic study comparing the antibacterial agent with copper(II) sulfate pentahydrate across different particle sizes

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Figure 7 presents results from tests involving various copper salts in addition to copper(II) sulfate pentahydrate. Copper(II) acetate monohydrate demonstrated a pronounced catalytic effect, outperforming copper(II) sulfate pentahydrate. However, foam formation was also observed (compare Fig. 8). Gel formation of samples with copper(II) chloride and copper(II) nitrate occurred within less than 5 min. Consequently, it was not possible to measure these samples with the rheometer. In addition to the short pot life, extensive foam formation occurred for these samples (Fig. 8). Copper(II) sulfate pentahydrate demonstrated the least amount of foam formation.

Fig. 7
figure 7

Rheometer kinetic study showcasing the catalytic effects of various copper salts. The particle size was in between 63 and 125 µm for all copper salts

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Fig. 8
figure 8

Foam formation of samples

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Discussion

It was surprising to see that the catalytic effect of the antibacterial agent outperformed the coatings that were produced with conventional tin or zirconium catalysts in terms of curing time and solvent resistance. This is a good indication that it might be possible to replace harmful tin catalysts with copper-based compounds.

Generally, the reaction mechanism might be described as the copper atom acting as Lewis acid, coordinating with the oxygen atom of the NCO group, which causes the NCO carbon to be more electrophilic and thus facilitates the reaction between OH groups and NCO groups. This reaction might be enhanced if some kind of amine compound is present. where the nitrogen atom interacts with the proton of the hydroxyl group and causes the hydroxyl oxygen to be more nucleophilic.2,3,6

Particularly interesting is the fact that the antibacterial agent as well as the tested copper salts are solids that are not expected to dissolve in the paint matrix. This is generally seen as a challenge when using such compounds as catalysts.10 The poor compatibility of copper(II) sulfate with organic systems11 is probably also the reason why—to the best of our knowledge—the use as PU catalyst has not yet been reported in the literature. Heterogenous catalysis might be considered here, even though it is not common in the field of coatings. However, it is plausible that the catalytic effect initiated on the surface of the antibacterial agent moved forward into the bulk of the coating. The correlation between particle size and catalytic effect would be in favor of such a mechanism. Smaller particles provide a larger surface area, increasing the number of accessible active sites. Heterogeneous catalysis in the context of polyurethanes was described in a patent, where metal ions were grafted on a metal oxide carrier through functionalized silanes which reacted with the OH groups on the surface of the metal oxide.12

The catalytic effect of the copper(II) acetate might have been stronger due to the increased compatibility with the polymer matrix that originated from the presence of the acetate ligands. The use of copper(II) acetate as catalyst for polyurethanes is mentioned in the academic literature3,6 as well as in patents.7,8,10 Typically, these are combined with additional ligands like amine compounds, such as ethylenediamine, triethylenetetramine,3 or dimethyl ethylenediamine (DMEDA),7 as well as diketones.5 This enhances compatibility and, as previously mentioned, the nitrogen atoms in the ligands may interact with the proton of the OH group, promoting the reaction. In contrast to using DMSO or diethylene glycole as solvents,8,10 the approach highlighted here integrated the powder like a filler or pigment using a dissolver before dispersion.

It might be worth considering that the active catalyst species was formed in-situ. The copper salts that were used contained small amounts of water. This was especially evident through the foaming that was observed during the reaction. It is known that water reacts with isocyanates to form amines and CO2.2 The so formed amines might have acted as ligands for the copper ions and thus improved the compatibility to the surrounding matrix. Figure 8 shows a more pronounced foam formation for the copper(II) acetate formulation compared to the copper(II) sulfate formulation. Thus, the improved catalytic effect of the acetate copper compared to the sulfate.

A limitation not previously discussed is that the use of highly water-soluble copper salts can lead to blistering and negatively affect other properties of coatings when exposed to humidity or moist environments. This issue may also weaken the coating's resistance to hydrolysis. Future research should investigate this aspect. Additionally, preliminary tests using different binders indicated that the effectiveness of copper(II) sulfate pentahydrate as a catalyst varies depending on the OH-polymer employed in the coating. In this research, Desmophen 1300 BA—a polyester polyol—proved effective. In contrast, tests with Desmophen 680 BA—a branched polyester polyol—were ineffective. This variation in effectiveness also warrants further exploration.

Conclusions

This study followed a serendipitous path, beginning with the discovery that an antibacterial agent surprisingly led to a reduction in the pot life of a polyurethane (PU) coating system. It was demonstrated that this antibacterial agent showed an at least comparable catalytic effect to traditional PU catalysts in reducing drying time and enhancing solvent resistance. The primary reason for this catalytic activity appears to be largely due to the copper content within the antibacterial agent. Consequently, the study investigated the catalytic effect of commonly known copper salts, such as copper(II) sulfate pentahydrate and copper(II) acetate monohydrate, showing that those exhibit a similar catalytic effect, which is dependent on particle size. Between copper(II) sulfate pentahydrate and copper(II) acetate monohydrate, the latter had a stronger tendency to act as a catalyst for the PU reaction.

The potential for heterogeneous catalysis or the in-situ formation of the catalyst from residual water in the copper salts was discussed. Although these mechanisms are speculative at this point, conducting future investigations incorporating FTIR analysis to investigate the influence of formation of urea and precise measurements on how molecular weight and glass transition temperature are affected by various concentrations and particle sizes of copper salts could unveil more details about the actual catalytic mechanism of the tested copper compounds and lead to the development of new catalysts that are less harmful than, for example, tin alternatives.

While the results of this study are specific to polyurethane systems, the replacement of tin-based catalysts is a pertinent issue in other contexts, such as silicone-based fouling release antifouling coatings. Further research is crucial to understand the mechanisms in different systems and to fully explore the potential of copper-based catalysts in solid form. However, potential drawbacks also need to be considered, such as how coatings with water-soluble copper salts manage exposure to water. Despite these challenges, pursuing copper-based alternatives to tin catalysts represents a promising avenue for future development.