by Miguel Ángel Hernández Cano*
Architectural coatings are all those coatings applied in residential, commercial or institutional buildings. These coatings have two functions. On the one hand, through color, they contribute to the aesthetics of a building or to the decoration of structural components, while on the other hand, they give protection against external influences such as moisture, sunlight, mechanical or chemical damage.
The ageing of the outer coating depends on the quality of the substrate and the primer, as well as on environmental factors such as light, heat (cold), humidity and oxygen which together generate a synergistic effect. In indoor coatings, decoration is the main function and its aging is often due to humidity.
Architectural painting
A paint for architectural purposes is formulated with the main components such as water, binders and pigments; auxiliary components or additives include dispersants and wetting agents, rheological modifiers and thickeners, defoamers, preservatives and biocides, solvents, coalescing solvents and film-forming auxiliaries. Other additives in a coating include neutralizing and defoaming agents; a large number of specialties such as corrosion inhibitors, mattes, waxes, cross-linking metals, uv absorbers and optical bleaches are added.
It is common for an architectural coating to carry between 10 and 20 different ingredients. Biocides can be specific to control the growth of algae (algaecides), fungi (fungicides), or bacteria (bactericides). It is only applied in the formulation of water-based paints where the resins used are a source of carbon for microorganisms. Water-based latex paints today dominate the architectural coatings market (80%) so the demand for biocides also rises.
The substrate
Architectural coatings are applied on different substrates such as wood, metal, plastic and concrete, which must be treated before applying the coating and help slow down the aging process. In wood, moisture is the main problem; it must be dried and treated with sealants and primers, corrosion products must be removed on metals, and primers with corrosion inhibitors can be added. In concrete, the moisture present causes efflorescence and can be avoided just by cleaning and drying the surface to be painted; in plastics there is usually a low adhesion of the coating which leads to delamination, which is avoided with the use of adhesion promoters. The substrate can be prepared with primers or fillings that provide adhesion properties and leave the substrate ready for the architectural coating to give a protective and decorative finish.
Definition of biocidal product
It is important to clarify that a biocide is a substance that inhibits microbial growth and that it is used in architectural coatings to protect the coating in packaging and also to prevent microbial attack when the coating is already applied on a substrate. Microbial development includes aerobic and anaerobic bacteria, fungi, yeasts and algae that adapt to living in these environments and generate serious damage to the final product, the packaging or equipment and facilities.
Types of biocidal products
Many biocides are classified as broad-spectrum which means they control bacteria, fungi and other microbes over a wide temperature range. Others have a narrower spectrum of activity and are used to control specific types of microorganisms.
Major biocides used as preservatives (in-can protection) of architectural coatings include bromohydroxyacetophenone, bromonitropropanediol (bronopol), dodecylguanidine hydrochloride, glutaraldehyde, isothiazolinones (MIT, CIT, BIT), dibromonitropropionamide (DBNPA), and tionas. However, the mixtures of these biocides give a greater application with successful results in the face of microbial attack. Examples include mixtures of CIT and MIT, MIT and BIT, or CIT, MIT and Bronopol. The concentrations of these mixtures are lower and their efficiency increases over a greater range of microorganisms. This is a more suitable form of microbial control and product protection.
This means that the coating should not degrade easily on the packaging for a considerable period at a pH and temperature range. This type of biocide determines the shelf life of the coating. Other biocides must be able to function more easily and kill fast-growing microbial populations; these are called rapid annihilators. Other biocides are used for coating protection when it comes into contact with the substrate (dry-film protection). Some examples include Octylisothiazolinone, Dichlor octyl isothiazolinone, Carbendazim, Diuron, Zinc pyrithion, IPBC, Terbutrin, Cybutrine.
All coating applications require biocide as a preservative but the addition of biocide as an annihilator is only done in situations where there is contamination in a tank or container or in products very susceptible to microbial attack such as paints that carry starch or pigmented coatings.
Perhaps the most important aspect in the protection of an architectural coating is that biocide technology is based on the formulation of a single product that contains a combination of different activities, in addition to improving stability and reducing the doses of the active ingredients. In latex paints it has been seen that the viscosity decreases due to the effect of reducing and oxidizing agents and not by the presence of the biocide, coupled with this the thickeners and stabilizers of the paint, such as hydroxyethyl cellulose are degraded with the effect on the decrease of viscosity.
Mechanism of action
The mechanism of action of biocides is based on the reaction of the active in the membrane of microorganisms where proteins are oxidized, the permeability of the membrane is altered, cross-links are generated that make the membrane rigid and the intracellular pressure is altered with which the microorganism dies. Oxidizing biocides tend to oxidize all kinds of organic matter and are consumed quickly in heavily contaminated systems. Non-oxidant biocides, on the other hand, have a more harmful effect, can penetrate the cell and alter DNA, RNA or cellular defense systems, which is why they are the most recommended in these coatings.
Effectiveness of a biocidal product
Each biocidal product has a minimum inhibitory concentration against a specific microorganism that defines the ability to inhibit microbial growth, although in most applications biocides are applied not against a microorganism, but against bacterial populations or communities of bacteria and algae, bacteria and fungi or bacteria, fungi and algae. Therefore, to increase the inhibition interval, biocides are produced as mixtures of two or three compatible actives that together target bacteria, algae, fungi, yeasts and even bacterial spores.
So, we can affirm that they are broad-spectrum biocides under the concept of mixture, whose production involves the addition of substances that stabilize the mixture and protect it from agents that inactivate the biocide. It is in this sense that many patent developments have been made proposing the stabilization of mixtures.
Stability in an application
When choosing a biocide for an application in an architectural coating, the factors that affect the stability of the biocide in the application are taken into account. The main factors are temperature, pH, redox potential, the presence of oxidants and reducers as well as free radicals. Nucleophilic attacks against certain biocides such as isothiazolinones are a common problem in certain latex paints where the presence of polymerization initiators, such as a residual, attacks the S-N bond that is oxidized and inactivated. (Figure 1)
pH usually affects the biocidal activity of isothiazolinones; at alkaline pH the CIT is sensitive to a temperature of 40°C, when the test is run in an adequate shelf time (50 weeks). This result may already vary in the particular application, but at room temperature this stability can be extended. (Graph 2).
MIT isothiazolinone remains stable at 40°C for 12 months, while BIT undergoes an approximate degradation of 20% at the end of this period. For its part, MIT remains stable in its biocidal activity when it is in a medium with oxidants at different pH, but not when there are reducers in the medium; at acidic pH it loses activity. And if the concentration of oxidants and reducers increases, the reducers accentuate their inactivating effect at acidic pH. (Figure 3)
The presence of free radicals in some applications (paints) is another factor that inactivates isothiazolinones. Some polymerization initiators in latex paints such as ammonium persulfate or sodium m-bisulfite produce free radicals. Fortunately, there are patent developments that propose protection against free radicals, including, among others, 3,3-thiodipropionic acid, diethylhydroxylamine, L-ascorbic acid and glutaraldehyde which is also a biocide and is compatible with BIT. In latex paints, the viscosity usually decreases in the presence of oxidizing and reducing agents and not by the presence of the biocide. In particular, thickeners and stabilizers such as hydroxymethyl cellulose are degraded by these agents which explains why the viscosity drops. Also the presence of nucleophiles is a negative factor of the biocidal activity of isothiazolinones; degrade the biocide. They can be counteracted by stabilizing with copper salts, orthoesters and epoxies. Measuring efficiency
The method used in measuring the efficiency of a biocidal product for packaging protection consists of the weekly inoculation of a known concentration of microorganisms (bacteria, fungi and yeasts) to a coating sample, for a period of three weeks. A weekly monitoring is carried out by sowing in plate and is evaluated at the end taking as a criterion of passing a zero growth of microorganisms.
The method used to measure the efficacy of a biocide in application as a film protector consists of preparing an involved substrate (filter paper) and then applying the coating film that leads the biocide to be tested (different concentrations). This film is challenged with different microorganisms (fungi and algae), mainly the most common in the environment where the coating will be applied; it is allowed to incubate for a time interval of 28 days at different temperatures depending on the microorganism in question. The result is obtained, observing the substrate that was not attacked by microorganisms with the lowest concentration of biocide. All substrates that could not be protected show bacterial, algae and fungal growth on the surface. It is important to use a control substrate, which does not carry biocide, as it helps to determine the result in an obvious way.
Characteristics of a biocidal product
The most important characteristics of a biocide in an architectural coating are based on the substrate, whether it is indoor or outdoor. Interior coatings must use an effective mold biocide and show low solubility in water, have been shown to be safe against people susceptible to allergies such as children and pregnant women (be classified as <low voc or zero voc > coatings), have passed a risk assessment, check that the active components are allowed by local laws and that there are no restrictions on application on.
On the other hand, biocides in outdoor coatings must be broad-spectrum against fungi and algae that are embedded by air, show low solubility in water, have good stability to UV light and present long-lasting protection.
Another important aspect of biocides is that they comply with their permission before the FDA (in the United States) and the allowed limits of biocide in each application. In Europe, biocides for architectural coatings are regulated by the BPD (Biocidal Products Directive) in group 2 classified as "Preservatives: biocides for paints and coatings", divided in turn into preservatives in type 6 packaging and type 7 film protectors. It is also important to properly apply the biocide to achieve the planned protection; mix correctly in the manufacturing process, in mills, tanks or towers where an additional dose is applied after manual cleaning.
In some applications the biocide must be added directly or in a dilution with dosing pumps that add continuously to maintain a constant concentration. Another factor to take into account is the functionality of the biocide at specific temperature and pH conditions; the activity level time of the biocidal product for effective control.
*Thor Químicos de México, S.A. de C.V. [email protected]
