Final Report: Triggered-Release Biocidal Nanocomposite CoatingsEPA Contract Number: EPD04047
Title: Triggered-Release Biocidal Nanocomposite Coatings
Investigators: Myers, Andrew
Small Business: TDA Research Inc.
EPA Contact: Richards, April
Project Period: March 1, 2004 through August 31, 2004
Project Amount: $70,000
RFA: Small Business Innovation Research (SBIR) - Phase I (2004) RFA Text | Recipients Lists
Research Category: Nanotechnology , SBIR - Nanotechnology , Small Business Innovation Research (SBIR)
Biocidal additives are crucial components of paints. They are needed to prevent the growth of bacteria in the can before it is opened and used, and to protect paint from attack by algae and fungi after it is applied and dried. Architectural coatings, which account for more than 50 percent of the volume of the U.S. coatings market, are designed for protection and aesthetics. Antimicrobial coatings are designed to kill fungi by slowly and steadily releasing biocides from the dried film. Unfortunately, this mechanism also is responsible for the ultimate deactivation of biocidal activity; once the biocides have leached out or washed out of the coatings, all protection against fungi is lost. The protection of the coatings by the biocide typically lasts only about 18 months, and less than that in hot, humid environments.
The loss of biocidal protection has several negative environmental affects. Once coatings have lost their biocidal nature they become supports for mold growth, introducing allergens to the surrounding area and defacing the painted surface. To maintain antifungal protection, new coatings must be applied after the biocides have leached out from the original surface. This nonspecific leaching can contaminate the surrounding areas with biocides, and the repetitive process of coating reapplication emits volatile organic compounds (VOCs) to the atmosphere. The need for continued repainting also creates significant economic, labor, and solid waste burdens. A system that prolongs biocidal coating lifetimes and decreases the environmental impacts of the current system would benefit both the environment and property owners.
Because of increased environmental legislation, coatings technology is moving toward low-VOC systems such as waterborne latexes. Biocides are even more critical in these cases because the coatings are water-based and contain carbon-based polymers and surfactants that are excellent food sources for bacteria, algae, and fungi. The regulatory process for biocide approval is lengthy and expensive. Many active biocides are commercially available, but they typically are used in combination with one another because each biocide is active against only a small number of fungal or algal strains. Instead of trying to develop a new, universal biocide that would protect against a wide variety of microbial threats over a long period of time, the goal was to develop a way to make existing biocides stay in the coating for a longer period of time, while still remaining active.
In Phase I, TDA Research, Inc. (TDA) developed a new antimicrobial coating system that prolonged biocidal activity by immobilizing the biocidal additive on a nanoparticle. Biocides are needed primarily when the coatings are attacked by bacteria, and are largely wasted if they are continuously but slowly emitted. TDA’s additive retains the biocide in the coating for a longer period of time than traditional approaches. Coatings that retained biocidal activity for twice as long, for example, would require repainting only one-half as often, leading to a decrease in VOC emissions during painting and in the wastes produced. An imbedded biocide also would prevent the formation of biocide-resistant bacteria that can form from a traditional slow-release biocide. A biocide that operates by leaching kills bacteria within a “zone of inhibition,” but often only weakens bacteria outside that zone.
There has been great excitement over the addition of nanoparticles to coatings. Nanoparticles—particles with dimensions of 100 nm or less—have a very high surface area and can impart useful properties to the coatings. Unfortunately, bare inorganic nanoparticles are essentially incompatible with, and will not disperse in, polymers such as paints. Therefore, the surface of the nanoparticles must be modified (coated with organic groups) to make them more chemically “like” and compatible with the surrounding organic polymer. Through the use of appropriate surface chemistries, not only can the nanoparticle surfaces be made compatible with the polymers, but the nanoparticles also can be used as carriers for organic groups that have specific functional uses. For example, antioxidants, plasticizers, corrosion inhibitors, and biocides can be attached to the surface of the nanoparticles. By tailoring the nanoparticle surface, it is possible to develop a drop-in additive that mixes easily with the paint and can provide biocidal activity and other desirable properties to the coatings. TDA has developed a wide range of surface-modifying chemistries for modifying nanoparticle surfaces. These methods were used to produce additives for acrylic latex architectural coatings that make the coatings more durable and able to resist attack by fungi or other bacterial contaminants for longer periods of time.
The goal of this research project was to demonstrate the effectiveness of new antifungal nanocomposite coatings prepared from surface-modified nanoparticles and waterborne latex films. To accomplish this goal, nanoparticles that have good compatibility with a waterborne latex and demonstrate biocidal properties were synthesized. Once prepared, the nanoparticles were incorporated into waterborne latex coatings for testing and evaluation. TDA prepared its own coating formulation instead of mixing nanoparticle biocides into a commercially available, off-the-shelf coating (commercial coatings already contain biocides, which would have interfered with measuring biocidal activity). Latex coatings are complicated, balanced mixtures of many components that can be upset easily by new additives such as nanoparticles. Methods to test the feasibility of the concept within the 6-month timeframe of a Phase I research project were needed. Accelerated American Society for Testing and Materials (ASTM) tests for biocidal coatings were used for this purpose. Finally, economic and commercialization assessment was conducted to determine the feasibility of incorporating biocidal nanoparticles into latex coatings.
Many obstacles must be overcome in the development of a prolonged-lifetime biocidal coating. The initial hurdle was nanoparticle synthesis. Although many methods were developed to add groups to TDA’s nanoparticles, biocides represented a new challenge. Furthermore, dispersing nanoparticles in a waterborne latex also was challenging. Appropriate short-term testing could not be accomplished in a single test, and therefore required a combination of methods. Fortunately, all of these obstacles were surpassed, and a nanocomposite biocidal coating was developed that showed antimicrobial activity in the dry coating.
During this project, nanoparticles were prepared that contained biocides and were compatible with a waterborne latex coating. Biocides were selected that were currently used as dry film preservatives. TDA’s nanoparticles were added to a latex coating based on a flat, exterior, waterborne formulation. This allowed for evaluation of the performance of TDA’s nanoparticle biocides without interference from traditional biocides found in commercially available, off-the-shelf paints. Current biocidal coatings contain 0.4 to 1 percent biocides, and TDA’s nanoparticles were mixed in concentrations that mirrored those levels.
Antimicrobial tests, based on ASTM D5590, were carried out at an independent testing laboratory. ASTM D5590-94 is an accelerated test used to determine the relative resistance of coating films to fungal growth. In this test, mold growth is monitored by placing the sample in a moist incubating chamber and monitoring growth for a 4-week period. Three fungal bacteria types are specified in the method, Penicillium funiculosum (ATCC #11797), Aspergillus niger (ATCC #6275), and Aureobasidium pullulans (ATCC #9348). Several samples showed no fungal growth after 4 weeks (see Figure 1). Other samples showed fungal growth that was inhibited on the coating itself, while not on the agar medium surrounding the sample. In control experiments when no biocides were incorporated into the coatings, there was extensive fungal growth on both the samples and the agar medium (see Figure 2).
Figure 1. A coating with TDA’s biocidal nanocomposites that showed zero fungal growth.
Figure 2. A “negative control” coating sample without any biocides.
Long-term tests also were initiated during this research project. Waterborne exterior coating samples were prepared and submitted for outdoor weathering tests by TDA’s commercial partner. These tests will measure biocidal activity in real-world conditions and will take approximately 18 months. These results should be available during Phase II.
TDA’s nanoparticles are produced from inexpensive starting materials and therefore can be economical value-added additives for the coatings market. TDA’s nanoparticles are made from an inexpensive mineral product and organic groups that can be selected to provide compatibility between the nanoparticles and a coating. By the nature of the starting materials, these nanoparticles also are very inexpensive. It is anticipated that TDA’s base nanoparticles could be produced for approximately $2-3 per pound. Biocides are more expensive, ranging from $4-6 per pound, and a biocidal nanoparticle would have to account for the cost of each of the components. The low loading levels necessary for both the nanoparticles and the biocides, however, decreases the economic costs of nanoparticle addition.
Long-lived and effective biocides are an elusive goal for the coatings industry. Commercially available biocides leach out of dry coatings, eventually rendering the coatings ineffective at preventing mold growth. This is an area that affects both exterior and interior coated surfaces. Biocides are important in controlling the spread of harmful microorganisms; they help coatings maintain their integrity (and therefore protecting the surface that has been painted) and uphold the aesthetic appearance of architectural structures.
TDA was successful in preparing biocidal nanoparticles that were added to waterborne latex coatings. The coatings showed resistance to fungal growth in accelerated tests.