By Juan J. Caprari* Contamination problems originate in the formulation concepts currently handled, which tend to achieve good behavior in service and a good economic equation for the product supplier. From this it follows the great importance of an adequate selection of raw materials in order to reduce the emission of volatile organic compounds (VOCs) responsible, in part, for the deterioration observed in the different layers of the atmosphere.
Definitions
What is defined as a Volatile Organic Compound and what are its main sources of emission? After application, a paint is transformed into a solid film that coats the surface to be protected and is composed of resins, plasticizers, pigments, inert pavers, fillers, additives and dryers. Some of these components are retained throughout the life of the coating while others are gradually phased out.
During the film formation process there is the emission of a large number of volatile organic components such as toluene, benzene, xylene, methylene chloride, trichloroethane, etc. According to ASTM standards, a volatile organic compound (VOC) is considered to be any hydrocarbon solvent emitted that is likely to participate in atmospheric photochemical reactions, producing what is known as photochemical "smog". VOC levels are expressed in grams of solvents per liter of paint, and it is possible to calculate them by the expression:
where:
A = Density of the paint (g.1-1).
B = Volatile content by weight (including water) in the paint (g).
C = Water content by weight in the paint (g).
D = Content of solvents excluded in the paint (g).
Dw = Density of water (g.1-1). 193
Dex, = Density of excluded solvents (g.1-1).
Excluded solvents (D) are those that participate in the formula but are ecologically inactive, which is why it is necessary to discount them together with the water content of the formulation.
When it is stated that the content of VOCs in a conventional lacquer is 720g.1-1 it is equivalent to saying that when a liter of lacquer is applied on a surface, 720g of VOCs evaporate and contribute to atmospheric pollution. This single mention might seem at first of little relevance; however, in 1991 Munn estimated that in the Los Angeles, California area, 360 tons per day of hydrocarbon solvents were emitted due only to the activity of the paint industry and their use for decorative or protective purposes.
Such is the importance attached to this factor that measures have been taken in developed countries whose rigor is constantly increasing. After many years of study, based on the results obtained in numerous determinations and the composition of the solvents used and marketed in the area, the regulatory commissions of New York and New Jersey have determined the VOC limits for the different coatings used in the painting and architectural maintenance of both cities (Table 7.1).
The differences that can be seen in the different regulations are due to the different criteria adopted by those responsible for the specification, although the tendency is that these values are more restrictive and are taken in "boca de lata". This position is reasonable given that the VOC limit sets the maximum values allowed in the product but does not take into account subsequent viscosity adjustment operations that differ according to the application method used. Fundamentally, it is a question of maintaining these values at the minimum allowed including dilution.
Thus the VOCs mentioned in Table 7.1, with variations between 450-680 for New York and 350-600 for New Jersey, are reduced in the military specifications (MIL) to limit values between 144 and 420g.1-1 (Table 7.2). Table 7.3 indicates new materials defined as hazardous (year 1990).
Pollution on the ecosystem
At a symposium held in March 1994 between marketing experts from different companies in the European Economic Community, Sykes proposed to the European Commission for Environmental Protection a decrease in VOCs for water-based paints for architectural use, starting from the 1992 values with a reference point in 1996 and the end in the year 2000 (Table 7.4).
The limits are based on the verification of the ecological effects produced by the emission of pollutants that, in addition to what is expressed in the aforementioned ASTM definition, alter other natural mechanisms when a decrease in thickness and extension of the ozone layer is verified, an increase in the greenhouse effect, the production of acid rains and the deterioration of water sources (underground, lakes, rivers and streams), which affects flora and fauna.
Production of photochemical "smog"
Photochemical "smog" is the main phenomenon where the hydrocarbon solvents used have a great influence. It develops in the lower atmosphere and its composition depends fundamentally on the emission of nitrogen oxides generated by human activity. A study carried out in 1995 on the sources of nitrogen production in the city of Paris (Figure 7.1), allowed to determine that 73.5% corresponded to emissions from cars, buses, light trucks, etc., 6.1% to electric power generating stations, 12.2% to residential activity and minor or third-order auxiliary services (dry cleaners, bakeries, laundries) and 8.2% to industrial activity, where the painting industry is included in all its aspects.
In the presence of ultraviolet light and hydrocarbons, nitrogen oxide reacts as shown in Figure 7.2. All reaction products are compounds likely to cause acute irritation in the eyes, respiratory system or allergic reactions of greater or lesser magnitude, especially due to an increase in the concentration of ozone.
To the presence of hydrocarbons due to the poor combustion of engines and evaporation of fuels in service stations, the evaporation of solvents used in cleaning operations and those emitted during the formation of the film in the painting operation is added, keeping the high concentration of ozone constant in an atmospheric region where it does not naturally exist and where it participates cyclically in other reactions and in its own regeneration.
A high concentration of ozone in low areas of the atmosphere is harmful since it affects the health of the human being, while in the upper atmosphere it causes the deterioration of the ozone layer. In this way it is affected in the atmosphere, while its destruction in the upper layer nullifies the regulatory effect of the intensity of UV radiation that reaches the Earth's surface.
The problem is particularly important in those areas where, due to geographical location, high levels of UV radiation, low wind and high temperatures, there is an atmospheric inversion preventing pollutants from escaping quickly from the area where they originate. Even when critical periods take place in relatively short periods, the reaction rate of hydrocarbons is the most important factor in increasing the concentration of irritants.
Therefore, an adequate choice of solvent mixture contributes not only to achieve a final film of good characteristics and resistance but also to avoid the damage that could be caused to the ecosystem. In this sense, aliphatic structure solvents are less aggressive than aromatic ones.
* Cidepint (Center for Research and Development in Technology in Paints.
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