Table 1. Typical Microorganisms
Microorganisms are ubiquitous in the environment. Many of them have simple requirements for growth, which can be met by a range of waterborne surface coatings in their wet state. Typical microorganisms isolated from paints or paint-related materials are listed in Table 1. Spoilage of such products, which can go unnoticed until the paint reaches the consumer, can result in a significant economic loss to the manufacturer.

The use of biocides is recommended to maintain the microbiological quality of a product and to protect it against contamination. However, with the current trend away from heavy metals and phenolic-based biocides to more environmentally acceptable ones, manufacturers may find that the use of biocides alone may not prevent the paint from becoming spoiled. If paints spoil in spite of the use of biocides, then the technical knowledge of the biocide manufacturer as well as the effectiveness of the biocide may be questioned. In fact, the issue is not the effectiveness, but the nature of these current biocides, which happen to degrade quickly in the environment. An integral strategy, which involves both the use of biocides and preventive measures to control microbial contamination, should be applied. This article outlines such an integral approach.

Figure 1. Microbial Growth Curve

How Microorganisms Contaminate and Grow in Paints

Microorganisms can contaminate the paint in different ways during the manufacturing process. Unsanitary conditions may exist, such as for the raw materials (including thickeners, extenders, pigments, emulsions, surfactants and defoamers), water (process water and recycled water), containers, and equipment (tanks, pipes, hoses, etc.). The ability of some microorganisms to attach to surfaces and form adherent biofilms is also important. Biofilms are functional consortia of microbial cells entrapped within an extensive matrix of extracellular polymer (glycocalyx) produced by them. Biofilms can be formed in water systems, processing tanks and other areas. Biofilms may be sources of contamination of the product, may cause corrosion, scaling, the reduction of heat transfer efficiency and other problems in addition to the spoilage. Microbial biofilms are usually very resistant toward biocide treatments or disinfectants.

Depending on the growth conditions (nutrients, minerals, gas composition, temperature, pH, water activity, etc.), microorganisms can reproduce very rapidly in the paint. The growth of microorganisms, such as bacteria that reproduce by binary fission in a closed system, can be plotted as the number of cells versus the time. The resulting curve is composed of four different phases (see Figure 1).

During the lag phase, the microorganisms adapt into the new environment. There is no increase in mass or cell numbers, but microorganisms are metabolically active. During the exponential phase, microorganisms grow and divide at the maximal constant rate, doubling in number at regular time intervals, possibly as little as every 20 minutes. Exponential growth can be described by the following equation:

N = N0 ekt

where N is the number of microorganisms at time t; N0 is the initial number of microorganisms, k is a growth constant that depends on environmental factors and t is the time in hours. Eventually, population growth slows down and the number of viable cells becomes constant over time. This can be due to a population growing and dying at the same rate or else a population that ceases to divide while still metabolically active.

At the stationary phase, microorganisms reach the highest cell numbers that the environment can support. Under favorable conditions, bacteria can achieve levels of 109 colony forming units/mL (CFU/mL). Cultures of other microorganisms such as protozoa, fungi or algae do not reach such high population densities. Typical levels are about 106 organisms/mL. A microbial population reaches a stationary phase for several reasons. One is nutrient limitation. Another is a decrease in oxygen availability. Population growth may also cease due to the accumulation of toxic waste products such as organic acids, which lower the pH of the medium enough to halt cell growth.

Eventually, nutrient deprivation and the buildup of toxic waste leads to the decline in the number of viable cells, which is characteristic of the death phase. The rate of death is constant. Most of a microbial population will die in a logarithmic fashion, although the death rate may decrease after the population has been drastically reduced. This is due to the extended survival of some resistant cells.

The growth of microorganisms, represented by this curve, shows that the initial number of cells is critical. The initial number (N0) will dramatically affect the shelf life of the product. For example, E.coli has a generation time of about 0.35 hr-1 at 40?C. If 1,000 cells are able to enter a product and reproduce, it will take 5.75 days for E.coli to reach a population of 1 million cells. However, if the initial number is 10,000 cells it will take as little as 6.8 hours for E. coli to reach that same number!

Figure 2

Common Signs of a Paint Contamination Problem

The most common signs and possible causes of microbial growth include offensive odor, viscosity changes, swelling of the cans and discoloration (see Table 2). However, other problems can arise not normally associated with contamination, including changes in adhesion, color acceptance, sag resistance or flow and leveling. These can result from microbial degradation of formulation additives, or changes that affect the function of the additives. Susceptibility of the dry film to future fungal attack can also be increased by breakdown of some higher molecular weight components.

Avoiding a Contamination Problem

The first part of an integral strategy to control contamination is to prevent it from occurring. Plants need to develop their own control strategy according to their own situation and capabilities. A generic integral strategy includes the following steps.
    a. Develop a flow diagram of the process
    The purpose is to provide a clear, simple picture of all the steps involved in the production of the paint product. It must include all steps in the process under the control of the manufacturing plant. It should also display other steps related to the incoming materials and distribution of the finished product. Figure 2 shows a flow diagram of a generic paint manufacturing process.

    b. Identify possible contamination sources/causes (control points)
    Once the flow diagram has been finalized, the next step is to identify all sites and materials that may cause contamination; these points are the control points. Table 3 shows an example of such control points. These points consider water (storage, processing, recycled), raw materials, equipment and finished paint.

    c. Establish control measures, monitoring and verification procedures for each identified control point
    Each control point should have control measures to assure the reduction or elimination of contamination. Control measures include the treatment of water, the establishment of standard operating procedures for sanitation and the addition of a biocide. Plant design and maintenance of equipment are crucial. For example, it is important to eliminate sharp curvatures or dead ends in pipes, to store hoses in such a way that they allow drainage of any remaining liquid, to design the plant facility and the product flow to eliminate cross-contamination between raw materials and finished product.

    Monitoring procedures for each control measure must be established. It is important to address the following points: what should be monitored, how it is going to be done (method), at what frequency, and who is responsible and how the action is documented.

    Verification procedures, such as routine microbiological testing, provide the means to show that the control measures and the control points are indeed effective. Examples of microbial testing procedures may include: traditional plating methods; "dip slides" (contact slides used on surfaces or in fluids); membrane filters to check microbial quality of water and rapid automated microbial methods (ATP determination, impedance changes). The frequency and type of test should be established according to the plant capabilities (in-house microbiology laboratory vs. outside testing services, staff expertise and experience). The results obtained through the verification procedures will allow the control measures to be adjusted if problems are encountered (see Table 4).

    d. Establish corrective actions
    If a control measure was not applied as established, a corrective action should be taken. Failure to do so may result in product spoilage. A corrective action should determine the fate of the product when one of the control measures failed. An example of a corrective action is: hold the finished product while a sterility test is performed when the control measure of, for example, chemical treatment of water failed. Add more biocide to protect the product since the recommended biocide level may have been used up because of a higher initial microbial load. Release the product if sterility tests show no microbial growth. The corrective actions include the description of what should be done if control measures failed, who is responsible for the corrective action, how to prevent it in the future, and how it is documented.

    e. Documentation and Verification
    During the production of a paint, all the steps that are designed to control the risk of contamination should be documented. This information should be gathered by the supervisor or designated personnel. Ideally, all documents should be checked by QC before the finished product leaves the plant. The information will show that control points were indeed under control. They will also serve to troubleshoot any problem, pointing out where and why it originated.

    Verification of the plan is needed to ensure that the plan is working properly. It is important to verify that the identified control points are adequate to control sites of potential contamination, that monitoring intervals are reliable, etc.


Proper implementation of an integral strategy is of paramount importance to control microbial contamination and protect the product's shelf life. Even if procedures as written are satisfactory, management must recognize that the success relies on their continuous implementation. Employees must be trained to do the work properly, and supervisors must see that the work is done properly. External auditories of the control program are also recommended (usually provided by biocide suppliers).

This strategy to control microbial contamination is a common-sense plan that can be applied to any plant. An integral control plan must be customized for a particular product, process and manufacturing facility. It can be used to improve quality and to drive down the cost of losses by reducing the chance of microbial contamination and spoilage of the finished product.

For more information on biocides, contact William Woods, International Specialty Chemicals, 2 Turner Place, Piscataway, NJ 08855-0365; phone 732/981.5121; e-mail; or Circle Number 139.