Catalyzed Finishes – How They Work

There are many different types of catalyzed finishes; lacquers, conversion varnish, polyurethanes, polyesters, and even some vinyl sealers. Each of these finishes has its own particular set of characteristics with different strengths and weaknesses.

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A catalyst is a chemical that increases the rate of a chemical reaction. The particular chemical reaction that takes place in wood finishes is called cross-linking. Think of the molecular structure of a finish as strands of spaghetti. The catalyst caused the ends to join together to form long chains of molecules. The long molecular strands make the finishes harder and more resistant to water and chemicals.

Catalyzed finishes are broken into two types; pre-catalyzed and post-catalyzed. As the name implies, the pre-cat already has a catalyst mixed into it and since the catalyst is rather weak, it has a pot life in the range of months. The most common pre-catalyzed finishes are pre-cat lacquer, vinyl, and occasionally a conversion varnish. Post Cat products require that you add the catalyst to the finish right before you spray. The catalysts used in these products are much stronger (hotter) than those used in pre-cats and once added they have a pot life ranging from hours to minutes. All of the above-mentioned finishes are available in a post-catalyzed version.

When discussing these products we will be talking more in generalities than specifics because an individual manufacturer’s formulations can influence several critical factors.

Pot life

The time in which you have to use the finish after the catalyst is added. After this time the finish will either no longer crosslink or it will harden in your equipment. Generally the hotter the catalyst the shorter the pot life and the harder the finish. Every manufacturer states the pot life for their particular products. The pot life generally decreases with an increase in temperature

Applied Wet Mil Thickness

How much of a wet film can you lay down at one time. Can vary from 3 mils for some types of vinyl to 8-15 ml for polyesters. Follow your manufacturer’s recommendations to achieve the proper dry film thickness and reduce runs

Total System Maximum Dry Mil Thickness

Some manufacturers recommend a maximum limit to the amount of dry film accumulated on a piece of wood. Exceeding this amount can lead to finish failure such as cracking

Induction Time 

Also known as the sweat in time. Some products require that after mixing the components, you let them sit for a while before spraying. This lets the chemical chain reaction get fully underway before atomizing. Not all finishes require an induction time and for those that do, it is usually temperature-dependent

As opposed to shellac or nitrocellulose lacquer, once dry catalyzed finishes do not melt into the previous coat; so to get an acceptable intercoat adhesion you must sand between coats.

The exception is if you are applying a wet-on-wet coat, in which the first coat has not dried so the solvent in the following coat will burn into the first coat causing it to chemically bond. Your timing when applying wet on wet coats is critical. If the first coat is too wet it will run, too dry and the second coat will not burn in. Wet-on-wet coats are frequently used when spraying polyester so that there is no halo if you burn through a layer when polishing.

Pre-Catalyzed Lacquers

Pre-catalyzed lacquers come in both a water white and amber version. Since the clear lacquers contain an amount of nitrocellulose, they can yellow over time; some more than others. There are pigmented versions of the pre-cats that are non-yellowing. The pot life of these products can range from a few months to over a year. As these products age past their pot life, they generally lose their ability to crosslink rather than gel up or harden in the can. There are versions of pre-cat lacquers that are thermosetting, that is they do not soften when exposed to heat. Once cured, the thermosetting versions of these products will even resist being re-dissolved by their own lacquer thinner. While they have improved performance characteristics and a higher percentage of solids by volume than nitrocellulose, they spray and have similar dry times. Pre-cat lacquers work well over most stains, except those that contain a lot of oil. They may also have difficulty adhering to oily woods like rosewood or teak. Vinyl or non stearated sanding sealers are usually recommended as a first coat. Some pre-cat lacquers can be used as a self-sealing product. A vinyl sealer will help some to meet KMBA kitchen and bath moisture resistance standards. Many of this product group will meet HAPs and VOC standards.

Pre-cat lacquers are a good choice for fast drying, medium performance applications, where yellowing is not an issue.

Post-Catalyzed Lacquers

As the name implies, you add the catalyst just before spraying. Most of these products have a higher percentage of solids by volume and better water, chemical, and scratch resistance than the pre-cat lacquers. Most will also yellow over time because of their nitrocellulose content. Generally, these products have an 8-to-12 hour pot life and will gel and harden in the can once the pot life is exceeded. The pot life is accelerated by an increase in temperature.

Post-catalyzed lacquers, like the pre-cats, work well over most stains, except those that contain a lot of oil. They may also have difficulty adhering to oily woods like rosewood or teak. Vinyl, catalyzed vinyl or catalyzed non stearated sanding sealers are usually recommended as a first coat. Some post-cat lacquers can be used as a self-sealing product. A vinyl sealer is usually not necessary to meet KMBA kitchen and bath moisture resistance standards, however, to meet the AWI quality standards a vinyl sealer is required. There may or may not be recoating windows when using this type of product. Many of this product group will meet HAPs and VOC standards.

Post-catalyzed lacquers are a good choice for fast drying, medium-to-high performance applications, where yellowing is not an issue.

Conversion Varnish

Conversion varnishes are a post-catalyzed alkyd resin-based finish and have most of the same specifications as post-catalyzed lacquer with only a few exceptions.

CV will often have a slightly higher percentage of solids by volume than the lacquers and may take a little longer to dry. It also provides some additional wear and chemical resistance over the lacquer products. Usually, these products have an 8-to-12 hour pot life and will gel and harden in the can once the pot life is exceeded. The pot life is accelerated by an increase in temperature.

There are versions of CV that have non-yellowing formulations and still others that will yellow even in the absence of sunlight. Some pigmented versions are more prone to yellowing than pigmented pre-cat lacquers.

Conversion varnish works well over most stains, except those that contain a lot of oil. They may also have difficulty adhering to oily woods like rosewood or teak. Vinyl, catalyzed vinyl, or catalyzed non stearated sanding-sealers are usually recommended as a first coat. Some post-cat lacquers can be used as a self-sealing product. A vinyl sealer is usually not necessary to meet KMBA kitchen and bath moisture resistance standards However, to meet the AWI quality standards a vinyl sealer is required. There may or may not be recoat windows when using this type of product. Many of these product groups will meet HAPs and VOC standards, however, some use xylene as a reducer which is on the HAPs list.

Conversion varnish is a good choice for moderately fast-drying, high-performance non-yellowing applications.

A Note About Stearated Sealers

To achieve improved sanding some sealers add Zinc Stearate to their formulation. Zinc Stearate is a soap-like material that acts as a lubricant during the sanding process. Most post-catalyzed lacquers and conversion varnishes use an acid-based catalyst. If an acid cure top coat is put on top of this type of sealer, a chemical reaction takes place between the zinc and acid causing “blooming”, and a haze forms in the coating after the finish is fully cured. This haze may not appear for weeks or even months.

Catalyzed Polyurethane

Two-component polyurethanes consist of a polyisocyanate hardener and a resin, either an acrylic or polyester. Once combined, they result in a highly cross-linked finish. The isocyanate hardener can be either aromatic or aliphatic. Aromatic polyurethanes are prone to darkening and yellowing on exposure to sunlight but are faster drying. Aliphatic poly- urethanes do not yellow and retain their gloss better than aromatics. Polyurethane can be either clear or pigmented and is offered in a wide range of colors and gloss levels.

2K Poly has a higher percentage of solids by volume than conversion varnishes and is a little slower to dry. They provide excellent wear, water, and chemical resistance properties. Usually, these products have a 4-to-6 hour pot life and will gel and harden in the can once the pot life is exceeded. The pot life is accelerated by an increase in temperature. When mixing, the resin should be stirred as the catalyst is added to avoid shocking the mixture and forming a gel where the two components meet. Some of these products have an induction or sweat in time.

2K Poly usually works well over most stains, including those that contain oils, however, check the manufacturer’s recommendations. Some require a barrier coat over solvent-based stains. Most 2K products have their own sealers, some of which may contain zinc stearate to improve sanding. Since there is no acid catalyst there is no reaction with the zinc, there often is a recoat window with these products, and sometimes a “no sooner than” and a “no later than” window.

Recommended wet film and maximum dry film thickness often exceed that of conversion varnish. Some catalysts are moisture-sensitive. Some have a very limited shelf life once opened.

Two-component polyurethanes are a good choice for applications where a very hard, durable, chemical-resistant finish is needed. 2K Polys are also very flexible finishes. See the warning about isocyanates after the section on Polyester.

Polyester

Polyester is a three-component finish: the resin, the promoter (cobalt), and the catalyst (methyl ethyl ketone peroxide, or MEKP). When mixing these three items exercise extreme caution as the promoter may come into contact with the catalyst, creating a violent reaction. Polyester has a higher percentage of solids by volume than conversion varnishes and is slow to dry. It provides excellent wear, water, and chemical resistance properties; however, the finish can be brittle, especially in the cold, and is subject to impact damage.

Polyester has a pot life rated in minutes and will harden in the can once the pot life is exceeded. If this stuff hardens in your gun you will have to buy a new gun. The pot life is also greatly accelerated by an increase in temperature. I have heard some people recommend refrigerating the resin on hot days. If you are going to shoot this product with any regularity it would be wise to invest in a plural component gun that mixes the components in the air stream as they are sprayed.

Polyester is very selective about sticking to stained surfaces. For stained or oily woods it is recommended that a barrier coat is applied before sealing. Some manufacturers sell a barrier coat product, others recommend using a 2K Poly.

Polyester sealers have very good grain filling characteristics and can be applied at a very high wet ml thickness. These products are best applied wet on wet so there is no haloing if during sanding you burn through a coat. The dry film thickness of polyester can be several times that of conversion varnish.

There are two different types of polyester; direct gloss and paraffinated. Direct gloss usually uses a separate sealer, whereas the paraffinated is a self-sealing product. The paraffinated product has the shortest pot life of the two and requires the largest diameter fluid tip. Both of these products are designed to be shot wet on wet with very critical time windows. Once applied, the direct gloss may require a little polishing to remove minor surface contaminants and achieve the perfect gloss. The paraffinated product requires the surface to be “ground down” with a fairly coarse (280) sandpaper and then up through the grits before polishing and buffing. Always wait the specified time before starting the grinding or polishing process to allow the materials to harden appropriately. This will allow you to obtain a higher gloss with less shrink back.

Both of these products can be used to simply fill the grain and then be top-coated with a 2K Poly in various sheens. In any case, follow the manufacturer’s instructions implicitly and mix materials exactly as instructed. Do not deviate and once again remember that warm temperatures greatly accelerate drying.

Use polyesters to obtain that deep wet look. This finish is not very flexible and can be easily damaged by impact.

Isocyanates

There are two different schools of thought on isocyanates. Most European countries do not consider them a problem; they consider formaldehyde emissions to be the greatest risk. In the USA it is just the opposite. This is where the problem comes in; depending on the country of origin of the manufacture you will get different health warnings.

Some people can become sensitized to isocyanates. Exposure to them can bring on upper and lower respiratory reactions that are similar to asthma. Skin reactions such as rashes are also possible. Once you become sensitized, you will always have a reaction. Isocyanates are present in the finish up until the time that a full cure is achieved. This means that you can be exposed when the finish is in liquid or vapor form, as well as by coming in contact with the sanding dust and overspray. Once the finish is cured, the risk is gone.

The first line of defense is to limit physical exposure. An enclosed spray room with a booth that has sufficient airflow to remove the vapors and overspray is the best place to start. By enclosing the spray operation you limit outside exposure to vapor, as well as the dust and overspray. Protective clothing with rubber gloves is the second line of defense. 

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Goggles, or even better, a full face respirator prevents the vapors from getting in through the eyes. 

Finally, we come to the respirator. While there are cartridge-type respirators that will filter out isocyanates, the danger lies in the fact that isocyanates are odorless so you can’t tell by smell when the cartridges are spent.

UV Curable Coatings

UV curable coatings have been around for over 40 years. Simplistically, they are coatings that use ultraviolet light to cure. Sometimes a simple mercury vapor lamp is used as the light source. Most of the UV wood finishes are based on epoxy acrylates, polyesters, and urethane monomer resins. Photoinitiators are added to the formulas. They start a chain reaction when exposed to UV light which results in the polymerization of the resins.

Initially, UV curable coatings were 100% solids. Today, waterborne and even solvent-based UV curable coatings are available. These lower solids formulas are used to achieve thinner surface films, reduce toxic monomer content, provide a mechanism to formulate different sheen levels, and speed up the cure rates. UV coatings can be formulated as stains, clears, and pigmented coatings, in all of the various glosses.

With UV the cure rate is almost instantaneous. This means that very high production rates can be achieved. The product exits the light zone cured and is ready to pack and stack. As a result UV applications are typically conducted on automated lines. This also helps reduce worker exposure to the skin-irritating resins and powerful light sources. Once applied, the limited open time of the chemicals before cure helps reduce flammability concerns. The normally high solids contents will reduce the VOCs to practically zero. UV coatings remain a liquid until exposure to UV light. This means that the coatings can sit in reservoirs for several days and still be ready for use. The need for frequent equipment cleaning is eliminated. Unreacted material can be cleaned from equipment without the use of solvents. Not only does this significantly reduce labor, but hazardous waste disposal costs as well.

UV coatings are usually applied by spray, flow coating, or roller coating. Their application is predominantly on two-dimensional objects, such as flooring and panels. UV curing equipment can be configured for use on moldings and even three-dimensional objects like pieces of furniture, but changing configurations and adding light sources can be expensive and time-consuming. UV curing equipment typically has a lower energy cost and a smaller equipment footprint than conventional ovens.

UV coatings offer higher chemical, impact, and abrasion resistance than most of the typical catalyzed coatings.

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.

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.

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 and DIN EN ISO .

Bandow–Wolff drying test

The modified Bandow–Wolff method, as specified by ISO -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 -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 -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.

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.

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.

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.

Solvent resistance

Table 6 presents the outcomes of the solvent resistance experiment. This evaluation was based on the EN ISO -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.

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.

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.

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.

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.

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.

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 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.

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