Long-Term Use of Preservatives in Paints and Coatings
Today, several obstacles come into play when selecting a suitable preservative for water-based paints and coatings. These (stemming from EU regulations) include the compliance of biocidal actives and products with the Biocidal Products Regulation (BPR), changing eco-labelling rules, and new labelling requirements for actives with specific concentrations set under the 2nd ATP to CLP (Classification, Labelling and Packaging).
Long-Term Use of Preservatives in Paints and Coatings
Today, selecting appropriate preservatives for water-based paints and coatings presents various challenges. These include (EU-sourced regulations): compliance of biocidal actives and products with the Biocidal Products Regulation (BPR), changing eco-labelling requirements, and new labelling obligations related to actives with specific concentrations defined under CLP (Classification, Labelling, Packaging) 2nd ATP.
In addition, various technical issues must be considered: due to the high number of microorganisms and the wide variety of raw materials, a single microbial active may be insufficient to meet increasing protection demands, even when used at acceptable rates.
Protection Requirements
With the transition from solvent-based systems to water-based systems in paints and coatings, the need to protect water-based paints and coatings from microbial contamination has become much more important. Many water-based systems, particularly paints and coatings, are susceptible to microbiological contamination and spoilage. Water-based systems require in-can preservatives that provide protection throughout shelf life as well as in-can preservatives that provide protection during manufacturing. Not all preservatives on the market may be suitable for a given formulation due to the variable physiochemical and biological properties of the formulations.
Such incompatibility can lead to serious spoilage caused by microbial growth (colour change, structural changes, gas formation and bad odours), and in most cases the resulting batches or final products cannot be brought to market. Consequently, such waste of resources contradicts the sustainable use of products, in other words their safe and ecological use throughout their life cycle.
Legal Barriers
As mentioned previously, in addition to certain technical issues, regulatory matters are also important in selecting appropriate preservatives. These are listed in points below.
1. Biocidal Products Regulation
In the European Union and its 28 member states (including Switzerland, Norway and Iceland), EU Biocidal Products Regulation 528/2012 (BPR) regulates the use of all biocidal active substances and products. The BPR requires the authorisation of active substances and products according to 22 product types contained therein (in-can preservation, etc.). The active substance authorisation process is ongoing and may not be completed before 2024. However, some actives are already listed and products containing these actives must be registered quickly. In summary, the BPR also requires additional financial resources of approximately EUR 400,000 per product for EU-wide registration in addition to human resources.
2. CLP and Classification, Labelling and Packaging Regulation
As of 1 June 2015, labelling obligations arising from GHS become mandatory for mixtures in the EU. CLP (1272/2008/EC, Classification, Labelling and Packaging) is a systematically regulated body of law relating to the implementation of GHS requirements. In addition, CLP introduces labelling obligations with respect to sensitising substances in mixtures and covers substances causing sensitivity at specific concentration limits below 0.1%. The new labelling concentration limit is determined as 1/10 of the specific concentration limit. If a mixture contains additions above this limit, the label on the packaging must include the statement "(Name of sensitising substance) may cause allergic reaction". Actives such as CIT/MIT, BIT and OIT are sensitising substances, and care must be taken during the use of these actives to ensure that the above-mentioned ratio is not exceeded and that labelling is carried out accordingly.
General Overview
The obligations arising from BPR and CLP will create a market environment with highly stringent regulations in the biocide sector. As a result of the reduced number of available biocidal actives, the banning of formaldehyde use in consumer products, the presence of stricter regulations and high product support costs, there are limitations in the use of biocidal actives such as CIT/MIT.
Assessment of Preservatives
In still-accepted cases, in-can preservatives consisting of heterocyclic N,S compounds, for example the combination of 5-chloro-2-methyl-2H-isothiazol-3-one (CIT) and 2-methyl-2H-isothiazol-3-one (MIT), are frequently formulated with formaldehyde-releasing agents. These agents are known for their broad spectrum and high efficacy, their activity in the vapour phase, their rapid activity characteristics and for providing microbiologically safe in-can systems. However, the number of sectors using these products is declining steadily.
On the other hand, combinations such as benzisothiazolone (BIT) and methylisothiazolinone (MIT) are formaldehyde-free systems known as "soft" in-can preservation systems, which do not contain volatile organic compounds and formaldehyde or formaldehyde donors. In such cases, the MIT-BIT combination can be described as a rather modern preservative combination due to its broad-spectrum efficacy against bacteria, yeasts and moulds and its appropriate eco-toxic profile.
However, this combination is also known for providing long-term protection, but there are shortcomings in terms of activity rate (see Figure 1).
Experimental Studies
Bis(3-aminopropyl)dodecylamine (BDA) is an effective biocidal active that has been known for years. One of its main characteristics is its very rapid activity rate compared to its broad and balanced spectrum against bacteria, yeasts and moulds at low concentrations.
Following extensive research, parmetol® MBX was developed; bis(3-aminopropyl)dodecylamine (BDA), which has excellent synergistic properties, was combined with today's standard "soft" preservative combination methylisothiazolinone (MIT) and benzisothiazolone (BIT).
The product can significantly reduce the required contact time to just a few minutes. Table 1 shows the formulations used in the microorganism count test.
Table 1. Formulations Tested
The dilutions of preservatives were prepared using sterile hard water in accordance with European test standards for chemical disinfectants and antiseptics. A 50 ml portion of the final solutions was taken and 0.5 ml microorganism suspension (initial microorganism count approximately 105 cfu/ml) was added to each and mixed.
During the 60-minute test period, the conventional MIT/BIT mixture showed no effect even at high concentrations. In contrast, with the MIT/BIT/BDA combination, a clear effect could be observed within 5 minutes. The graphs in Figure 1 show the results of the microorganism count reduction test of both formulations against Staphylococcus aureus and the microorganism suspension described above.
Figure 1. Microorganism Count Test: MIT/BIT/BDA and MIT/BIT
In light of these test results, Schülke developed a completely new approach regarding the use of the in-can preservative during the manufacturing process of paints and coating products: the recommended sequence for preservative addition (see Figure 2). The BIT, MIT and BDA combination is added to the process water as the first component in the manufacturing process of any adhesive or putty system.
The rapid activity characteristic prevents spoilage caused by microbial growth in the process water and in all raw materials where there is a contamination risk.
Figure 2. New approach in the use of in-can preservatives
As a result, the risk of microbiological contamination will be reduced throughout the entire manufacturing process. A biocide that can provide long-term protection and has rapid activity characteristics enables safer manufacturing processes and the production of well-protected final products. Due to the marked synergistic effect between BDA, MIT and BIT, lower usage rates, reduced downtimes that may result from contamination problems in operations and reduced risk of final product recalls, as well as savings in raw materials and energy production, also provide added value to customers.
As stated above, it has been determined that the MIT, BIT and BDA combination has a synergistic nature due to the fact that it affects microorganisms through different pathways. MIT and BIT, representatives of the N,S compound molecular group, are two balanced electrophiles. Due to their electrophilic character, these molecules react with thiol groups in the amino acid cysteine (an amino acid that makes up proteins). At least 4 very important enzymes in the reaction centre of bacteria and moulds carry cysteine. If these reactions occur with these enzymes, permanent damage to the microorganism can result. It has been proven that isothiazolones are actively transported into cells where they not only react but also cross-link functional groups in cellular molecules, thus destroying the cell's metabolic balance and structure.
In addition to electrophilic active molecules, BDA, which is a long-chain alkylamine, reacts with membrane-active (lytic) amines in its structure and damages the outer cell wall, thereby changing the membrane's permeability; thus, osmotic lysis occurs. In addition, this lysis enables other actives such as MIT and BIT to enter the weakened cell.
In light of these explanations, MIC value tests were carried out in association with DGHM (German Institute for Hygiene and Microbiology); synergistic effects were determined using the formula described in the method explained by Kull, Eisman, Sylwetrowitz and Mayer (1961: 538-541).
QA = % concentration of component A in the product, its individual effect, the cut-off point (MIC value of component A),
Qa = % concentration of component A in the product, its effect in the mixture, the cut-off point,
QB = % concentration of component B in the product, its individual effect, the cut-off point (MIC value of component B),
Qb = % concentration of component B in the product, its effect in the mixture, the cut-off point.
Table 2. MIC values for different active combinations of MIT, BIT and BDA
Based on these calculations; if SI (Synergy Index) is greater than one, an antagonistic effect is indicated. If the total equals one, it indicates additive requirement. However, if SI is less than one, it indicates the presence of a synergistic effect; as the SI value decreases, the synergistic effect increases. According to the data in Table 2, it can be seen that the combination of MIT, BIT and BDA is indeed synergistic. Additional tests were conducted to prove the performance of the MIT, BIT and BDA combination. A "challenge test" was applied to determine the sensitivity of water-based formulations to microbiological spoilage and the protective effectiveness of the in-can preservative system used.
In this test, different concentrations of the preservatives to be tested are added to samples without preservatives. An unpreserved sample is also tested to control growth. Two days after the addition of preservatives, samples are inoculated with a microorganism cocktail from available sources (0.2 ml, titre 1010 cfu/ml). Subsequent evaluations are carried out and samples are reinoculated weekly. The bacterial mixture consists of: Kocuria rhizophila (ATCC 9341), Staphylococcus aureus (ATCC 6538), Enterobacter gergoviae (ATCC 33028), Escherichia coli (ATCC 11229), Klebsiella pneumoniae (ATCC 4352), Pseudomonas aeruginosa (ATCC 9027), Pseudomonas fluorescens (ATCC 17397), Pseudomonas putida (ATCC 12633). Yeasts used: Candida albicans (ATCC 10231), Aspergillus brasiliensis (ATCC 16404). Mould used: Penicillium funiculosum (ATCC 36839).
Table 3. Challenge test for dispersion paint, inoculation using bacterial, yeast and mould cocktail.
Test samples are inoculated weekly and the cycle is completed weekly using agar plates (tryptone-soya-agar (TSA) for bacteria and sabouraud-dextrose-agar (SA) for yeasts and moulds). In the first cycle (sterility test), agar plates with and without TLSH neutraliser are used to see how much prior contamination may be present. After three days of incubation at 25°C, analysis is performed to determine whether microbiological growth has occurred. Samples showing negative results in terms of final product safety are examined for an additional two days and retested. The protective effect of different product concentrations is determined by semi-quantitative evaluations such as −, +, ++ and +++ in terms of bacteria, yeasts and moulds. Microbiological growth is examined. Generally, the test continues for a maximum of six weeks; therefore, a maximum of six inoculations are performed. When growth is +++, the test is terminated.
Results
At 0.15%, the MIT, BIT and BDA combination provided protection throughout the challenge test period, whereas the MIT and BIT combination at the same rate showed moderate bacterial and mould growth after 3 cycles (weeks).
Summary
Because N,S compounds and long-chain alkylamines act against microorganisms through different pathways, the synergistic effect of the combination of benzisothiazolone (BIT), methylisothiazolinone (MIT) and bis(3-aminopropyl)dodecylamine (BDA) can be demonstrated. While the synergistic effect leads to reduced usage rates, BDA provides rapid activity, and the combination of MIT, BIT and BDA provides long-term protection. In addition, it is also an alternative for products/formulations free of VOC or AOX. Improving manufacturing processes and final products in an ecological and safe manner using fewer resources, ensuring sustainability throughout the entire supply chain, is very meaningful.
We conducted numerous tests to clearly demonstrate that our sustainable approach is an alternative to general biocide combinations such as BIT and MIT or formaldehyde and formaldehyde donors in terms of process and product hygiene.
parmetol® MBX with the MIT-BIT-BDA combination is a next-generation preservative that provides solutions to the needs of today and tomorrow.
Author: M.A., B.Sc. Matthias Hentz - Business Development Manager - Schülke & Mayr GmbH
Translator: Durhan Tezcan - Business Development Manager - Schülke & Mayr GmbH
References
1. Siegert, W.; Gückel, A. and Carstens, S. 2011, June: 35-40, International Journal for Applied Science.
2. Roden, K. The Preservation of Personal Care Products 2009, Society of Cosmetic Scientists (Singapore).
3. Kull, F. C.; Eisman, P. C.; Sylwestrowicz, H. D. and Mayer, R. L. Applied Microbiology. 1961, 9:538-541.
4. Paulus, W. Directory of Microbicides for the Protection of Materials: A Handbook 2005, Kluwer Academic Publishers.
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