Microbial Resistance: The Protective Shield for Coatings

The presence of microorganisms causes the paints and coatings to degrade. Manufacturers dealing with contaminated paints face a huge loss of money and time.

Microbial contamination of unprotected coatings is certain. Several conditions favoring the growth of microbes are:

humidity,
moisture, or
availability of nutrients

Water-based paints are very susceptible to microbial deterioration. This is due to the availability of nutrients for microorganisms to reproduce and colonize. Hence microbes are an undesirable part of the coatings formulation. To protect the substrate from microbial attack, antimicrobial coatings come to the rescue.

Get detailed information on:

Why microbial resistance is important in coatings?
What are antimicrobial coatings and the mechanism behind them?
How does microbial growth occur in coatings?
What is the effect of microbial growth on coating properties?
What factors and challenges influence antimicrobial coatings' formulation?
Which test methods evaluate the efficacy of antimicrobial coatings?

Why microbial resistance is important in coatings?

Microbes are the smallest living organisms. Depending on their nature they can be:

unicellular - organisms that consists of a single cell
bicellular - organisms made of two cell
multicellular - organisms living in the form of colonies

However, most of the microbes are unicellular. They grow in great populations by means of reproduction.

Bacteria are the most common spoilage organisms, but fungi and yeasts are sometimes responsible for product deterioration. Spoilage of the waterborne products, which may go unnoticed until the product reaches the consumer, can result in significant economic loss. Upon drying, both water- and solvent-borne coatings are susceptible to colonization by fungi and/or algae.

Fungi can penetrate coatings, resulting in cracking, blistering and loss of adhesion, leading to decay or corrosion of the underlying substrate. Algae colonies, which seem to grow more rapidly on porous substrates such as stucco, cement and bricks, can occlude water.

Mold growing on wet and moist surfaces (L)
Algae on a polymer dispersion (R)

The freezing and thawing of this entrapped water may induce cracking or increase the permeation properties of the coating, leading to failure. The presence of water may also encourage colonization by other microorganisms, which in turn may cause bio-deterioration.

To avoid the above failures, microbial resistance in paints and coatings is important. It is known as the degree of resistance against microbes or antimicrobial activity. It is a property of an antimicrobial agent (here coating) known as efficacy against a particular species of microorganism.

In paints, there are many microorganisms that can be categorized according to their species and nature. The following table contains some examples of microbes found in paints and coatings.

Classification	Sub Category	Species
Bacteria	Gram-positive	Bacillus, Brevibacillus, Micrococcus, Staphylococcus, Kocuria genus, etc.
Bacteria	Gram-negative	Pseudomonas, Enterobacter, Proteus, Aerobacter, etc.
Fungi	Mold and mildew	Aureobasidium, Alternaria, Aspergillus, Cladosporium, and Penicillium
Algae	-	Chlorella, Chlorococcum, Oscillatoria, and Trentepohilia

Nature and types of microorganisms in paints, coating, and inks

An antimicrobial agent can have efficacy against many microbes at the same time. The range of microbes on which the antimicrobial agent shows efficacy is known as the spectrum of efficacy or activity. The antimicrobial resistance of coatings can change or differ under different circumstances. There are many types of antimicrobial agents. They show different amounts of microbial resistance against different species. This depends on their chemistry, nature, and mode of action or killing.

Antifungal and Antibacterial efficacy of different commercial antimicrobial agents

Antifungal and Antibacterial efficacy of different commercial antimicrobial agents

Anti-fungal and Anti-bacterial Efficacy of Various Commercial Antimicrobial Agents

What are antimicrobial coatings and the mechanism behind them?

Antimicrobial coatings are a class of paints and coatings characterized by their antimicrobial nature. They protect the substrates and the coatings from the attack of various microbes. For example, bacteria, fungi, algae, yeast, mold, mildew, and viruses. These coatings contain a special class of additives known as preservatives or biocides. They can further be classified into different categories, based on their specific action. This includes:

anti-bacterial agent (bactericide),
anti-fungal agent (fungicide),
anti-algal agent (algaecide), and
anti-viral agent (virucide)

These coatings are employed in areas with high humidity or those susceptible to microbial growth. For example, outdoors, indoors, public areas, health institutions, and other relevant areas. Antimicrobial coatings can be applied by brush, roller, spray, or polishing. They are available in various systems mentioned below:

water-based coating,
solvent-based coating, or
powder-based coating

Antimicrobial action of anti microbial coating (right) on a surface

Germs on the Surface Before Application of Antimicrobial Coating (Left); Killing of Germs After Application of Antimicrobial Coating (Right)

Unlocking the mechanisms of action of antimicrobial coatings

The performance of antimicrobial coatings depends on the mode of killing of the microbes. Microbes can be present in the coating system (wet system) and on the coating film (dry system). Different biocides/actives or anti-microbial agents/coatings have different modes of action. The antimicrobial mode of killing has three main categories as follows:

Repulsive mode of reduction — This category of antimicrobials is biofilm-resistant. They do not let bacteria or other microbes adhere to the coating. This makes it antimicrobial. But, this is not a killing mode, but it comes in the scope of antimicrobial coatings or surfaces.

Release killing — In this method, an antimicrobial agent migrates or leaches from the coating. It then deactivates or kills microbes. For example, bacteria or fungi. They perform different physical or chemical interactions like:
- destruction of cell walls/leakage
- destruction of cell proteins. For example, DNA
- inactivation of enzymes in the cell

Contact killing — Here, the microbes come in contact with the antimicrobial coating and get killed or inactivated. This is due to the antimicrobial present on the coating surface. The most common examples are Quaternary Ammonium Compounds (QACs), N-chloramines, and advanced polymer brushes.

Major Classification of Antimicrobial Coatings Based on the Action of Antimicrobial Surface

How does microbial growth occur in coatings?

Sources of microbial growth

Microbial growth in the wet state is usually manifested by a loss of product functionality and may include gas formation, offensive odor and changes in pH, viscosity and color. Microbial contaminants can be introduced:

Basic Source	Source Identification
Air	Factory equipment
Water	Make-up water Water
Raw materials	Powders	Fillers Extenders
Raw materials	Liquids	Polymer emulsions Defoamers Pigment dispersions Dye solutions Dispersing aids Emulsifiers
Poor plant hygiene
Poor plant design
Final containers

Growth requirements for microorganisms

Microorganisms are often a cause of illness, odor, and damage to a wide variety of material and substrates. Green mold and algae formation on surfaces is a familiar indication of a product affected by microorganisms. If neglected, these problems could lead to costly customer quality issues and down time for factory decontamination.

Requirements	Bacteria	Fungi (molds and yeast)
Light	✘	✘
Ideal pH	Slightly alkaline	Slightly acidic
Ideal Temperature	25°C-40°C	20°C-35°C
Nutrients	C, H and N sources
Trace Elements	✔	✔
Oxygen	O₂ or inorganic (SO₄, NO₃ etc.)	O₂
Water	Liquid or vapor

A low level of contamination of only a few hundred / g can reach problem proportions in just hours

Effect of pH on microbial growth

Most of the microorganisms encountered in industrial practice are in the range of 4-9 pH.

Fungal organisms are more prominent at acidic pH
Bacterial organisms are more prominent at neutral to slightly alkaline pH

Polymer emulsions generally fall in the ideal pH range for microbial growth. Find out the pH range of different polymer emulsion listed below:

Types of Polymers	Typical pH Value
Ethylene vinyl acetate Polyvinyl acetate PVA / acrylic PVA / Versatate and PVA / Acrylic Polyurethane	Acidic (pH 3.5-6.5)
Acrylic Styrene Acrylic Styrene butadiene Polyolefins	Alkaline (pH 7.0-9.5)

Also, apart from factors mentioned above, type of microorganism that can colonize the coating will depend on several other factors too, such that:

Moisture content of the surface - Surface moisture content is affected by climatic conditions (amount of rainfall, dew, humidity, temperature and time of the year) as well as by local conditions (surfaces sheltered from winds and shaded areas will contain higher moisture content)
Presence of nutrients - Nutrient sources include constituents of the coating itself (such as polymers, thickeners, etc.), partially biodegraded substances produced by other microorganisms, or simply material deposited on the coating from the atmosphere, such as dirt.
Substrate - The composition of the substrate may affect the pH of the surface, making it suitable for microbe colonization. For example, fungi favor more acidic conditions, such as those provided by wood. Some species of wood are more susceptible to colonization by fungi than others (e.g., pine is more susceptible than cedar). Algae, on the other hand, favor alkaline conditions, such as those provided by masonry.
Coating composition - the coating composition and properties (polymer type, water repellency, porosity, hardness, chalking and roughness) determine the type of microbial community that will colonize the coating.

The use of biocides is recommended to maintain the microbiological quality of a product and to protect it against contamination

What is the effect of microbial infection on coating properties?

The table below lists the impact of microbial infection on the various property changes.

Property Change	Impact Due to Microbial Infection
Viscosity change	Polymer dispersion can become thinner or thicker depending on the effect of increased concentration of acidic byproducts. Phase separation can also occur. Viscosity increase and microbial infection can also restrict the flow within the factory equipment piping, filters etc.
pH change	The metabolic by-products often are acidic in nature. The reduced pH will cause destabilization of the polymer dispersion and promote a corrosive environment both in the factory (surface of plant equipment) and once in service (corrosion of substrates).
Odor production	Bacteria are often sulfur-reducing. Other microbes have the ability to produce odors based on their biochemical reactions.
Gas production	Bacteria can produce hydrogen sulfide gas which leads to odor and gas production problems.
Color change	Microbes can change the color of the product before or after application. Sulfur-reducing bacteria generally blacken the polymer dispersion or the finished product.
Visible surface growth	Microbes lead to color and viscosity change (see above).
Corrosion	Corrosion of plant equipment and of substrates can occur from metabolic byproducts and acid production.
Change in properties (due mainly to reduction in molecular weight)	Breakdown of the polymer molecular weight and / or change of dispersion property characteristics can affect the end-use properties of paints and coatings.

There are various preservation strategies for the formulator to use to protect his or her formulation. These include:

Checking and treating the water supply
Checking raw materials
Improving plant design and hygiene
Using a broad-spectrum biocide

Biocides are particularly effective when used proactively in a formulation, however, they can also be used for clean-up of contaminated water or equipment. Proper factory maintenance strategies can prevent microbial infection from the source and reduce the need for a biocide. There exists a criterion for selecting antimicrobial additives depending on the coating type and end-use application of your product.

What factors and challenges influence antimicrobial coatings' formulation?

Factors influencing their performance

Category and nature of antimicrobial agents in coatings
The selection of antimicrobial agents and their nature is most important. For example, wet-state preservatives are effective against bacteria and fungi. They do not show any significant anti-algal effect. Similarly, a dry film preservative with a low leaching rate will perform better. This is in terms of durability and longevity as compared to the higher leaching rate bioactive.

Minimum inhibitory concentration (MIC)
MIC is the minimum concentration of a bioactive agent. It is required to exhibit a certain level of antimicrobial action on particular species of microbes. MIC is also known as the dosage of antimicrobial agent in coatings. The dosage of biocides or preservatives must be higher than the MIC levels. This ensures a good antimicrobial resistance of the coatings.

pH of the coating system
The pH of a coating system especially in the wet state can influence the antimicrobial properties of the paint or coating system. An alkaline environment has a pH between 8 to 9.5. It is considered better for paints and coatings as most of the microbes don’t grow and reproduce in this range of pH. pH lower than 8 is strictly not desired. This is because it aids the reproduction or multiplication of the microbes in paint or coating.

Rate of killing of antimicrobial agent
Any antimicrobial agent can be effective only when it has a fast-killing action. More preferably it has more than one mode of killing or acting on the target microbes. There are many modes of action. They are robust and fast which is employed in coating systems. It provides better antimicrobial properties and resistance to the coating. For example, the nano-silver or silver-based coatings are very effective. This is in terms of a shorter time-killing period and broad antimicrobial spectrum.

Presence of synergists in the formulation
A synergist is a booster additive. It can be another antimicrobial agent or any other catalytic agent in the formulation. It increases the antimicrobial efficacy of the coating.

Environmental conditions
Humidity and temperature are the two most critical factors. They affect the performance of coatings. This is in terms of antimicrobial susceptibility and performance.

Challenges arising during formulation

Challenges	Description
Regulatory and legal compliances	Most of the antimicrobial coatings face the challenge of regulations and legal compliances. The regulations can be national, regional, or global. This depends on their nature and scope.
Efficacy of antimicrobial coatings	It is important to test the efficacy of antimicrobial agents or coatings. They may fail to perform against different microorganisms if efficacy is not optimized. It is critical to add the minimum amount of antimicrobial agents in the coating system. This ensures antimicrobial efficacy against different microorganisms.
Areas of application and approval	The performance of antimicrobial coating changes with the areas of application and the environmental aspects. An interior paint or coating may work very well in rooms or airy halls. Whereas, it may not be as much efficient in areas like kitchens or bathrooms. This is due to the presence of higher humidity. Similarly, an exterior paint or coating may work well in an area where there is lower humidity or high temperatures. It would not work as well in tropical regions where flora, fauna, and humidity are higher in amount.
Reactivity of active ingredient	The solubility or reactivity of an antimicrobial agent may impact the antimicrobial efficacy of a coating. For example, the bioactive substance is highly soluble in water in a water-based paint or coating. Here, it may leach out from the coating leaving an unprotected surface behind.
Optimization of dosage	Dosage optimization of antimicrobial coatings plays a critical role in optimizing or delivering antimicrobial properties in a paint or coating system. There are various antimicrobial agents that: May need a higher dosage. For example, zinc pyrithione or diuron. This enables them to perform against various antimicrobial agents. May not need higher doses. For example, isothiazolinones or nano-silver. They deliver higher efficacy and antimicrobial action. Thus, it can be said that the antimicrobial coatings may not solve the purpose without dosage optimization of the antimicrobial agents.
Relating in-vivo and in-vitro results	Lab testing and field testing can be hard to relate to in the real world. This is because the real world and environment are hard to replicate in laboratories.

Which test methods evaluate the efficacy of antimicrobial coatings?

ASTM D2574 — It determines the resistance of emulsion paints in the container to attack by microorganisms. It is also known as the in-can challenge test.

JIS Z 2801 — It determines the anti-bacterial activity on plastic surfaces.

ISO 22196 — It determines the anti-bacterial activity on plastic surfaces. It is similar to the JIS Z 2801.

ASTM G21 — It determines the resistance of synthetic polymeric materials to fungi.

ASTM D5590 — It determines the resistance of paint films and related coatings to fungal defacement. This is done by the accelerated four-week agar plate assay.

ASTM D5589 — It determines the resistance of paint films and related coatings to algal defacement.

ASTM D3273 — It determines the resistance to the growth of mold on the surface of coatings in an environmental chamber. This is an anti-fungal test method.

ASTM E2180 — It determines the activity of incorporated antimicrobial agents. This measurement takes place in polymeric or hydrophobic materials.

Antimicrobial Efficacy Testing According to ASTM E 2180 Standard (Credits: GAIKER)

References

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