ReviewBiodeterioration of external architectural paint films – A review
Highlights
► The biodeterioration of exterior dry paint films is reviewed. ► Influence of paint formulation and climate on biofilm formation is discussed. ► Methods used for paint deterioration studies are included. ► New trends in the paint industry are discussed.
Introduction
Paint can be defined as ‘a fluid material which, when spread over a surface in a thin layer, will form a solid, cohesive and adherent film’ (Morgans, 1990). The term paint describes a variety of materials including enamels, undercoats, varnishes and lacquers. Paints and other surface coatings confer two main properties on a structure, protection and decoration. More recently, other functions have been developed, such as the new “cool paints”, which improve building insulation. All of these properties may be prejudiced by microbial growth. The microbial colonization of painted buildings causes aesthetic problems and can lead to degradation and blistering, flaking and spalling of the coating (Banov, 1978). Such biodeterioration was estimated as long ago as 1981 to cause economic losses in excess of one million US dollars per year (Winters, 1981) and modern buildings, with increased levels of thermal insulation, show a higher risk of microbial growth (Sedlbauer et al., 2011).
Microorganisms are deposited on painted surfaces from the surrounding environment. On external surfaces, rain and wind bring small fragments of plant and animal origin, spores and microbial cells, as well as minerals and air pollutants. Internally, the aerial microorganisms are derived from the external air, but the population is also influenced by human activities within the building, not only shedding of bacteria and fungi from skin and clothes, but also through respiration-induced alteration of atmospheric gases and humidity. The internal environment is often more controlled than that outside, heating, air conditioning and humidifiers/dehumidifiers exerting their influence over factors that control microbial growth and survival. Much of the literature on biofilms on internal surfaces deals with effects on human health and these paint films will not be discussed here except where the information is relevant to external surfaces.
Once they impinge on the surface, microorganisms adhere and grow at rates that depend on the nature of the coating, the painted substrate and the environmental conditions. Hygrothermal conditions are particularly important, high humidity or condensation, along with raised temperatures, favouring growth. The constituents of the coating also affect microbial development, some components being inhibitory and others stimulatory to growth. Materials such as cellulose derivatives can act as nutrients for fungal cells (Winters and Guidetti, 1976, Allsopp et al., 2004a), while organic solvents and heavy metals in pigments can adversely affect them.
The eventual formation of the microbial biofilm, whether thin (some micrometres) or thick (1 mm or more), leads to discoloration and/or changes in properties such as brightness, smoothness, hydrophobicity, heat and water retention of the paint film and, over the longer term, reduces its durability.
Section snippets
Resident microbiota
External painted surfaces support a very diverse microbial population; bacteria (including actinomycetes and cyanobacteria), algae, small animals (protozoa, rotifers and nematodes) and a wide variety of fungi (Gaylarde and Gaylarde, 1999a, Gaylarde and Gaylarde, 2000a). Some of these are involved in biodeterioration and biodegradation. For example, filamentous algae, actinomycetes and fungi can alter the painted surface by penetrating the film and attacking, or exposing to attack, the
Effects of environment on biofilm formation and survival
The climate, tropical, temperate, or cold, and the surrounding environment, urban, industrial, coastal or rural, will affect the level of natural biodegradation occurring in that region. The interactions are complex and interwoven, industrial emissions and tree cover, for example, affecting solar penetration and concomitantly temperature. Location and climate influence the microbiota and the chemical content of the air arriving at exposed surfaces. Climatic changes with season may also be
Influence of paint formulation on colonization
All paints contain the same primary constituents; a matrix or binder, pigments and extenders (which confer colour and build) and a solvent. The solvent is either organic in nature, for a solvent based paint, or water, for a water-based or latex emulsion paint. In addition to these common components there are several other ingredients, which make up about 5% of the total system. These include emulsifiers (surfactants), biocides, pigments, plasticisers, extenders (colloids) and thickeners, the
Effects of painted substrate on susceptibility
Microbial adhesion and growth on paint films is influenced by the nature of the underlying substrate. Viitanen et al. (2011) developed a mathematical model for determining risk and durability of various materials to fungal growth under different conditions, but point out that perhaps the most difficult requirement of the calculation is assessing the sensitivity of the material, a process that would require experimental testing for accurate results. Künzel (2011) states that “The ultimate goal
Instrumental methods used in the analysis of paint film biodeterioration
The resistance of coatings to biodeterioration can be evaluated by accelerated tests in the laboratory or in climate chambers, or in the field, the latter being preferred for true assessment of coatings performance (Loh et al., 2011). There are many standard tests using these methods (Allsopp et al., 2004b). The aim of this section is to review some of the more recently developed techniques that can be used after such exposure, or weathering, to detect and measure the degree of degradation of
New technologies in the coatings industry
In the last few decades, because of climatic changes, a need has been felt to develop new technologies for a more sustainable built environment. One of the strategies in the coatings industry is the development of innovative raw materials that can confer new functions on traditional materials. One such development is the use of micro or nanoparticles to help reduce energy consumption and urban pollution.
Acknowledgements
We are grateful to FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo) for funding to MAS and KL, and to Aecio Breitbach for Fig. 4.
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