Final Report: Triggered-Release Biocidal Nanocomposite CoatingsEPA Contract Number: EPD05054
Title: Triggered-Release Biocidal Nanocomposite Coatings
Investigators: Myers, Andrew
Small Business: TDA Research Inc.
EPA Contact: Richards, April
Project Period: April 1, 2005 through June 30, 2006
Project Amount: $225,000
RFA: Small Business Innovation Research (SBIR) - Phase II (2005) Recipients Lists
Research Category: Nanotechnology , SBIR - Nanotechnology , Small Business Innovation Research (SBIR)
Biocidal additives are a crucial component of paints. They are needed to prevent the growth of bacteria in paint cans before the cans are opened and used, and to protect paint from attack by algae and fungi after it is applied and dried. 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 coating, all protection against fungi is lost. Biocide protection of the coating typically lasts approximately 18 months, and less in hot, humid environments.
The loss of biocidal protection has several negative environmental effects. Once paint 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 continual repainting consumes large amounts of chemicals. The indiscriminate leaching can contaminate the surrounding areas that have biocides, and the repetitive process of coating reapplication emits volatile organic compounds (VOCs) into the atmosphere. The need for continued repainting also creates significant economic, labor, and solid waste burdens.
Because of increased environmental legislation, coatings technology is moving toward low-VOC systems such as water-borne latexes. Biocides are especially critical in these cases because the coatings are water-based and contain carbon-based polymers and surfactants that are an excellent food source for bacteria, algae, and fungi. The regulatory process for biocide approval is lengthy and expensive. Many active biocides are commercially available but they are typically used in combination with one another because each biocide is active against only a small number of fungal or algal strains. Rather than trying to develop a new, universal biocide that would protect against a wide variety of microbial threats over a long period of time, TDA Research, Inc.’s (TDA) objective was to develop a process that would allow existing biocides to remain in the coating for a longer time while continuing to be active.
In this Phase II project, TDA developed a new antimicrobial nanocomposite coating system that prolonged biocidal activity by combining biocidal additives with the surface-modified nanoparticles. These nanocomposite coatings retain the biocide in the coating longer than traditional approaches. A coating with prolonged bioactivity would not need to be repainted as often, leading to a decrease in VOC emissions during painting and fewer wastes produced.
There has been great excitement over the addition of nanoparticles to coatings. Nanoparticles, which are particles with dimensions of 100 nm or less, have a very high surface area and can impart useful properties to 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 the 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, such as biocides. By tailoring the nanoparticle surface, it is possible to develop a drop-in additive that mixes easily with paint and can provide biocidal activity and other desirable properties to the coating.
Phase II Objectives
The overall objective of this Phase II project was to develop the antifungal nanocomposite coatings TDA identified in the Phase I project. These coatings were prepared from surface-modified boehmite nanoparticles, commercially available biocides, and water-borne latex coatings. To achieve this objective, TDA needed to synthesize nanoparticles that offered good compatibility with water-borne latex and would contain biocides. Once prepared, the nanoparticles had to be incorporated into water-borne latex coatings for testing and evaluation. A specific coating formulation was prepared instead of mixing TDA’s nanoparticle biocides into a commercially available, off-the-shelf coating. (Commercial coatings contain biocides, which would have interfered with the measurement of biocidal activity.) Latex coatings are complicated, balanced mixtures of many components that can be easily upset by new additives such as nanoparticles. Thus, one Phase II objective was to develop nanoparticles that were compatible with standard coating components without requiring significant changes in the overall coating formulation. TDA also needed to verify and further probe the bioactivity of the nanocomposite coatings through accelerated American Society for Testing and Materials (ASTM) tests for biocidal coatings as well as real-time outdoor exposure tests. Finally, an economic and commercialization assessment was planned to determine the feasibility of incorporating biocidal nanoparticles into latex coatings.
TDA’s Phase II project focused on three key technical objectives: biocidal nanoparticle preparation, water-borne latex coating nanocomposite formulation, and antimicrobial coating testing. Many obstacles must be overcome in the development of a prolonged-lifetime biocidal coating. Not only do the biocidal nanoparticles need to be prepared by synthetically reasonable and economic methods, but they need to be dispersed in a water-borne latex. The issue of biocide regulation also needs to be addressed, as new biocides are subject to lengthy and costly regulatory approval. The formulation of a nanocomposite coating is not trivial because of the high surface area of most nanoparticles, and the development of a nanocomposite coating that would be attractive to a commercial partner in the coatings industry was a significant focus of the Phase II effort. Appropriate testing of the coating formulation, dry film properties, and biocidal activity was necessary. Bioactivity was measured in both short- and long-term, tests. Fortunately, TDA overcame these obstacles and developed a biocidal nanoparticle additive that could be added as a drop-in additive to a standard water-borne latex coating formulation. The resulting nanocomposite coatings showed longer biocidal activity than traditional biocidal coatings (without nanoparticles), better coating protective properties, and biocidal effectiveness in a real-time, extended outdoor exposure study.
The first task consisted of nanoparticle preparation. While the Phase I project focused on a triggered-release mechanism for biocide activity, the Phase II project found better results using a “controlled release” process. This simplified the synthesis of biocidal nanoparticles, leading to nanoparticles that were easy to prepare from inexpensive starting materials. TDA was able to extend the syntheses to a variety of commercially available biocides, allowing the use of biocides that had received prior regulatory approval.
The nanoparticles prepared in this project also were designed for compatibility with a standard water-borne exterior coating formulation. Current biocidal coatings contain 0.2 to 1 percent active biocides, and the nanoparticles developed by TDA were mixed in concentrations that mirrored those levels. By carefully selecting the components of the nanoparticles, TDA prepared nanoparticles that could be added to their standard coating formulation as a drop-in additive. The large surface areas and high surface energies of nanoparticles can easily disrupt the delicate balance of surfactants and coatings additives that comprise a water-borne latex formulation. The nanoparticles developed by TDA did not upset this balance; in fact, biocidal coatings made with TDA nanoparticles were stable in the can (as a wet, uncured coating) for more than 13 months. These properties are required for ultimate commercialization; biocidal nanoparticle additives that can be added easily to an existing coating formulation—without requiring significant changes in the formulation or interrupting the integrity of the formulation—are much more likely to be considered by a coatings company.
The biocidal nanocomposites were tested for resistance to fungal growth in accelerated tests according to ASTM D5590, which is an accelerated test 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 over 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). These tests were carried out in the biology department at Colorado State University in Pueblo, Colorado. Several samples showed no fungal growth after 4 weeks, as shown in Figure 1. Specifically, all of the final generation of biocidal nanoparticles led to coatings that showed complete resistance to fungal growth during the 4-week test. The longer time frame of the Phase II project also allowed more time for extended length tests. Real-time, extended outdoor exposure tests were carried out at TDA and with a coating company, who partnered with TDA in this effort. In a field trial in the Midwest, the samples showed better resistance to microbial contamination than standard coatings after 18 months of exposure.
TDA also measured the longevity of bioactivity of the samples by washing dry film samples with water. Because traditional biocides are designed to slowly leach out of a coating, the continuous washing with water accelerates this leaching and aggressively ages the samples. Positive blank samples (containing identical concentrations of biocides but no nanoparticles) lost biocidal activity after 1–2 weeks of leaching. The final generation of nanoparticles, however, produced coatings that resisted all fungal growth by A. pullulans after 1 month of washing and showed only light to moderate fungal growth (between 10–60% of the surface was covered) when exposed to P. funiculosum and A. niger.
One additional improvement of the nanocomposite coatings developed by TDA was a decrease in water vapor permeation. Nanocomposites often have higher barrier properties than the unmodified host resin, and these latex coatings were no exception. the prepared nanocomposite coatings that consistently gave lower water vapor permeation rates than the blank coating. Water vapor permeation decreases up to 66 percent were seen with TDA nanocomposite coatings. Lower water permeation rates indicate better coating protection of the underlying surface. Not only do the biocidal nanocomposites created by TDA prevent fungal growth for longer periods than traditional biocides, but the nanocomposite coatings are better protecting films.
Long-lived and effective biocides have been an elusive goal for the coatings industry. Commercially available biocides leach out of dry coatings, eventually rendering the coating ineffective at preventing mold growth, which affects both exterior and interior coated surfaces. Biocides are important in controlling the spread of harmful microorganisms; they help coatings maintain their integrity, therefore protecting the surface that has been painted, and they uphold the aesthetic appearance of architectural structures.
In summary, TDA views this project as a way to develop a biocidal product with environmental, health, and safety advantages that would meet a pressing need and be commercially attractive to the architectural coatings industry. The proposed product will not only improve the environment, it will make a better paint. Longer lived biocidal protective coatings are a goal of coatings companies, but the technology could be extended to other polymer systems and applications. This would create a large new market for the nanoparticles developed by TDA.
TDA was successful in preparing biocidal nanoparticles that were added as a drop-in additive to standard water-borne latex coatings. The coatings were stable in the can, the dry films showed prolonged resistance to fungal growth in accelerated and real-time tests, and the coatings’ water vapor permeability decreased.