Figure 1
At every stage of their service lives, latex emulsions and coatings formulated from them are susceptible to contamination and degradation by a variety of microorganisms.1-5 Bacteria can spoil emulsions and paint in the storage container. After paint has been applied to a substrate and dried, it becomes vulnerable to attack by the fungi that cause mildew. To protect against the microorganisms that can cause problems, paint manufacturers must customarily use biocides.

In the early 1970s, the most popular biocides for paints were based on phenyl mercury acetate because they had the appealing ability to serve in the dual capacity of in-can preservative and dry film mildewcide.6-8 However, the toxicity and environmental concerns of mercury-based biocides resulted in the paint industry discontinuing their use in the United States in the 1980s. They were replaced by biocides based on alternative chemistries, such as 3-iodo-2-propynyl butyl carbamate (IPBC) and 2-n-octyl-4-isothiazolin-3-one (OIT).9-10 Highly effective against fungi, the new chemistries proved to be very capable mildewcides.

These non-mercurial agents, however, did not protect against the bacteria that cause spoilage during storage. As a result, manufacturers were compelled to use preservatives to protect paints in the can. Among the products employed for this purpose were formaldehyde releasers and a number of isothiazolinone compounds introduced in the early 1980s. Although these biocides performed well as in-can preservatives, they were not effective as dry film mildewcides. Consequently, for the past 20 years, paint manufacturers have been compelled to use a combination of biocides for complete antimicrobial protection.

Figure 2
Rohm and Haas Company has a novel isothiazolinone biocide that provides paint manufacturers with a highly effective, "two-for-one" alternative to conventional biocide packages and eliminates the need for separate in-can preservatives and mildewcides. ROZONE(tm) 2000 biocide contains the active ingredient 4,5-dichloro-2-n-octyl-4-isothiazoline-3-one (DCOIT) (see Figure 1), which both effectively controls bacteria that cause architectural paints and coatings to degrade in the can and prevents mildew growth after the films dry. At the same time, ROZONE 2000 industrial microbicide has low water solubility, making it very resistant to being leached from the paint film by ambient moisture. What's more, ROZONE 2000 does not have the same environmental and toxicity issues associated with mercury-based biocides.

Dual-purpose ROZONE 2000 controls a wide range of microorganisms, including fungi, algae and bacteria. As far as fungi are concerned, this isothiazolinone product inhibits the growth of many species at low use levels. Its efficacy against Aureobasidium pullulans (the fungus predominantly responsible for the development of mildew in exterior paints) is particularly noteworthy. Equally important from the stand point of in-can protection, this biocide also is very effective against the bacteria, especially the Pseudomonas species, that attack paints, joint compounds, roof coatings, exterior insulating and finishing systems (EIFS), and clear finishes in the can.

In addition to its impressive antimicrobial capabilities, ROZONE 2000 has a number of other appealing features for formulators. Because of its isothiazolinone chemistry, it is non-persistent in the environment, breaking down readily into harmless compounds. In addition, this biocide does not release formaldehyde, does not require zinc oxide for mildewcide stability and is easily incorporated into paint and coatings formulations. The new biocide can be used in both latex- and solventborne paints. With all its performance benefits, ROZONE 2000 exacts no appearance or performance penalty. This compound will not cause a paint film to yellow, will not destabilize a paint or detract from its tint retention and is EPA-registered for use in coatings.

Figure 3

Stringent Testing

The performance benefits of ROZONE 2000 were supported by stringent Rohm and Haas Company tests that demonstrated the ability of this biocide to function in the dual role as an in-can preservative and mildewcide. A number of studies had already documented its ability to control the growth of numerous fungi and algae. Rohm and Haas scientists confirmed these findings and also established the antibacterial efficacy through a series of different tests of ROZONE 2000 containing 20% DCOIT. The procedures used to evaluate ROZONE 2000 included minimum inhibitory concentration (MIC) determinations, in-can preservative efficacy tests and dry film mildewcide tests.

MIC Method and Results

The following procedure was used to establish MIC values. First, the test organisms were cultivated in three different media: nutrient broth for bacteria, potato dextrose for fungi and yeast, and Allen's medium for algae. In each case, the microorganisms were introduced to the medium and incubated at 25 deg C for seven days. An aqueous suspension of each test organism was then prepared by harvesting the growth from the medium in a test tube containing an aqueous solution of 50 ppm of Triton(tm) X-100 surfactant. The number of microorganisms in the suspensions was assessed with a hemocytometer and reported as colony-forming units (CFU) per milliliter.

The MIC values for DCOIT, the active ingredient of ROZONE 2000, were determined by the microdilution test method on microtiter plates through two-fold serial dilution. In each case, the microtiter plates were incubated at 25 deg C for five days, and the minimum quantity of biocide that inhibited the growth in a series was recorded.

The results of the testing clearly demonstrated the broad-spectrum antimicrobial efficacy of DCOIT. The MIC values of DCOIT against all organisms tested were under 20 ppm. As previous testing had shown, DCOIT was extremely effective against fungi and algae in the MIC tests, with values of generally less than 1 ppm (see Table 1). Equally impressive, DCOIT also provided efficient control of many bacterial species; MIC values for these microorganisms ranged between 2 and 20 ppm inclusive (see Table 2).

Based on these evaluations, the MICs for ROZONE 2000 against selected gram-negative and gram-positive bacteria in typical architectural paints would range between 20 and 80 ppm (see Table 3). These MIC values are superior to those claimed for the latex paint industry's most widely used in-can preservatives - formaldehyde releasers, chloromethyl isothiazolinone/methyl isothiazolinone (CMIT/MIT), and benz-isothiazolinone (BIT) - and a prominent isothiazolinone mildewcide - n-octyl isothiazolinone (OIT).

The importance of the MIC figures for gram-negative bacteria in particular is underscored by analysis of numerous paint and latex samples provided by paint manufacturers (see Table 4). Over a 10-year period, Rohm and Haas biologists examined almost 400 latices and more than 150 paint formulations that had spoiled due to microbial contamination.11

Collectively, approximately 60 to 70 percent of the bacteria identified were gram-negative (see Figures 2 and 3). The great majority of these, approximately 45% to 50% of the total contaminants identified, are Pseudomonas sp., with P. cepacia, P. putida and P. flourescens predominating. A number of other species, such as Alcaligenes sp., Citrobacter sp., Enterobacter sp., and Klebsiella sp., accounted for the remaining gram-negative species identified. It is these gram-negative bacteria, Pseudomonas in particular, that ROZONE 2000 controls so effectively.

In-Can Challenge Test Results

To ascertain the efficacy of ROZONE 2000 for in-can preservation, the challenge method based on the ASTM Test Method D-2754 was followed.12 The investigators inoculated various paint and coating formulations with a mixture of freshly harvested Pseudomonas species consisting of P. aeruginosa, P. cepacia, and P. putida. All of these bacterial species were isolated from commercial paint and coating formulations. At various intervals after inoculating the paints and coatings (1, 3 and 7 days), the investigators analyzed samples of the test formulations for the presence of the Pseudomonas species.

The efficacy of ROZONE 2000 for in-can preservation was demonstrated in a wide variety of coating formulations, including tile coatings, roof coatings, EIFS coatings, joint compounds and paints (see Table 5). At no point in the testing of the samples - even in those containing the lowest level (1lb/100 gal) of ROZONE 2000 - were investigators able to detect any of the bacteria that had been introduced into them.

This preservative performance was especially impressive in latex paints. To provide a comprehensive analysis of this biocide, the investigators studied the biocide in different kinds of latex paints - all-acrylic flat exterior, all-acrylic semi-gloss, and polyvinyl acetate (PVA) flat - under a variety of conditions. In addition to standard efficacy trials, the investigators evaluated the shelf stability of ROZONE 2000 at normal and elevated (10 days at 60 deg C) temperatures over an 18-month period (see Table 6).

Figure 4
HPLC analysis of the paints tested showed that all of the ROZONE 2000 remained in the formulations even after 1.5 years of long-term storage. Pictures of agar plates after two challenges of bacteria clearly indicated that DCOIT had completely suppressed all bacteria, thereby preventing spoilage (see Figures 4-7). In one case, a reference control (a commercial paint) containing an unidentified in-can preservative/mildewcide combination was heavily infested with bacteria, whereas paints formulated with only ROZONE 2000 and no separate in-can preservative showed no signs of contamination (see Figure 4). The performance of the biocide in this first comparison was replicated in a comparison of various paints formulated with all-acrylic and PVA binders (see Figures 5-7). Once again, ROZONE 2000 demonstrated its long-term ability to prevent in-can spoilage.

Figure 5

Dry Paint Mildewcide Performance

The ability of ROZONE 2000 to control the growth of mildew was evaluated per ASTM Test Method D-5590.13 Two samples each of paints containing biocide were brushed onto different Leneta charts and allowed to dry in open air for 48 to 72 hours. One of the two charts was then exposed to running water for 24 hours to simulate the leaching effect of weathering. One-inch squares were cut from each chart, placed on malt agar plates and individually swabbed with a fungal inoculum comprised of Alternaria sp., Penicillium sp., and Aureobasidium pullulans, three species commonly implicated in mildew. The treated squares were then incubated at 28 deg C for the remainder of the study. The plates were observed weekly for surface disfigurement due to mildew growth.

Figure 6
In addition to establishing the viability of ROZONE 2000 for in-can preservation, tests of the acrylic and PVA paints also confirmed its antifungal properties in the dried paints (see Figure 8). Just as the in-can tests established both the antimicrobial efficacy and the robust stability of ROZONE 2000 as a preservative, the dry film evaluations confirmed its antifungal properties, its shelf stability and its weatherability as a mildewcide. In this case, paints stored for a year and a half and cast on Leneta charts possessed excellent mildew resistance in trials simulating either a weathered or an unweathered state.

Figure 7

A Cost-Effective Option To Biocide Packages

ROZONE 2000 affords paint manufacturers a very effective option to conventional biocide packages for their formulations. For decades, manufacturers had to employ separate biocides to preserve paint in the can and prevent mildew growth on dry film because none of the available products had the broad-spectrum antimicrobial efficacy required to serve in both capacities. This microbicide, however, can meet both needs.

Figure 8
It matches or surpasses the anti-fungal properties of the most popular mildewcides currently employed in latex coatings formulations. At the same time, it effectively controls a wide variety of gram-positive and gram-negative bacteria for extended periods. Its ability to provide both in-can and dry film preservation greatly simplifies the task of paint manufacturers. They now can eliminate one of the biocides from their formulations, reduce the chance of adverse product interactions and achieve raw materials cost savings.

For more information on ROZONE 2000 Industrial Microbicide, contact David Sutton, Coatings Biocides Marketing Manager, Rohm and Haas Company, 100 Independence Mall West, Philadelphia, PA 19106-2399; phone 215/592.6855; fax 215/592.2178; e-mail; or visit

This paper was originally presented at the Federation of Societies for Coatings Technology's Annual Meeting Technical Program, ICE 2002 in New Orleans.


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