by Juan J. Caprari*
Deterioration of the ozone layer
The deterioration of the ozone layer, that is, its decrease in thickness and extension begins in the stratosphere, above the poles (20-30 km. high). It is due, in particular, to the action of chlorofluorocarbon compounds subjected to the UV fraction of solar radiation, forming radical catalysts for ozone decomposition and, consequently, atmospheric deterioration. This problem is serious because the compounds mentioned, with a relatively long half-life (1 to 2 years), have enough time to reach and act in the upper atmosphere. Both aromatic and aliphatic solvents have a half-life of less than 24 hours and do not act on the ozone layer.
Greenhouse effect
In this respect, the opinions of various researchers are controversial. According to the most widespread theory, certain gases from substances of high molecular weight contribute to preventing the infrared rays of sunlight that reach the earth from being reflected or radiated into space. When trapped at the ground level, they gradually raise their average temperature, something that is supposed to continue due to the increase in the concentration of gases in the atmosphere (Figure 7.3).
One of the main factors is considered to be the concentration of carbon dioxide (60%) and methane (20%); the first produced by human and industrial activity and the second by biological activity (human, animal and bacteriological). Another important factor is halogenated compounds (15%); however, its control and gradual elimination will reduce its influence in the coming years.
The paint industry employs aggressive solvents in some bakable paints, in degreasing operations, and in the manufacture of organic film removers. In the latter case, the generation of new products with equal cost, low toxicity and solubility in water, high boiling point and autoignition, low vapor pressure and excellent solvent power, in addition to biodegradable, will make it possible to replace them in the short term.
These products are a very refined blend of dimethyl esters derived from adipic, glutaric and succinic acids. There is evidence to show that aromatic and aliphatic solvents do not produce methane but carbon dioxide even if in extremely low proportions in relation to the amounts that appear as a result of human activity.
Acid rain production
Acid rain is caused by the emission into the atmosphere of sulfur dioxide and nitrogen oxides, residues from the combustion of naphthas, diesel oil, gas oil, natural and packaged gas, wood and stone coal and deforestation. In the latter case, the operation is carried out by burning forests and in other cases stubble from already harvested fields. There is no evidence of any action related to the emission of solvents from paints.
The organic solvents used have different solubility in water, their presence causes different effects on the water of rivers, water tables and oceans as well as constituting a problem as serious as atmospheric pollution. The contamination of underground rivers, which in many cases are used to provide drinking water to the population, becomes a critical problem because it is potentially irreversible.
One way to classify the effects of these substances in water is by determining their toxicity and biodegradability. All organic substances exposed to natural environments are degraded by physical, chemical and/or biological mechanisms. Chemical and biological mechanisms use oxygen dissolved in water as an energy source; therefore, if the amount of pollutant provided comes from substances whose decomposition occurs with a very high demand for oxygen, its levels can decrease to values incompatible with life within the receiving channel. Consequently, it is very important to determine parameters that indicate the oxygen requirements of the same (Figure 7.4), and that are expressed by two values:
The chemical chemistry COD (Chemical Oxygen Demand) that reports on the oxygen consumption necessary for the oxidation of almost all soluble organic substances in a given medium (river, stream and sea water), except for a series of nitrogenous compounds and some hydrocarbons almost insoluble in water. It is an important parameter for the control of contamination of watercourses produced by industrial waste and/or poorly functioning effluent treatment plants. If the waste incorporated into the watercourse is of high toxicity, it is almost the only value that allows to determine the organic load of the stream and take measures for its treatment.
b. The biochemical oxygen demand BOD (Biochemical Oxygen Demand) that
reports on the amount of oxygen needed to produce aerobic microbial degradation of organic substances contained in the receptor channel. As the process of biological decomposition takes several months to complete and its speed depends on temperature, in practice BOD is taken.
The correct measurement of the boD value requires the simultaneous presence of organic matter on which the decomposition operates, aerobic or optional microorganisms that produce decomposition and dissolved oxygen so that the process can be carried out in an aerobic environment. In special cases this value can also be determined after 20 days and at 20°C, which is expressed as BOD20.
The total oxygen requirements generated by both cases can push the concentration to limits incompatible with biological needs, determining the extinction of life in the receiving course. The formation of a solvent film on the surface of the water due to its lower density also cancels the arrival of oxygen and prevents the normal aeration of the fluid. The toxic effects of solvents are assessed on fish using the concept of Limit Concentration CL50 (or Limit Concentration LC50) for 96 hours of exposure. The method allows to establish the lower and upper limits of the lethal dose of toxic capable of killing 50% of the study population in 96 hours at 20°C (Table 7.5). The lower the value of CL50 in ppm, the greater the toxicity of the solvent tested.
The German classification for toxic contaminants does not take into account the chemical or biochemical demand for oxygen but the direct action on rats, bacteria and fish. The coefficients obtained for each category determine a range of values of the WGZ index (in German Wassergefährduengzahlen) or WEN (in English water endangering number) establishing classes called WKG (in German Wassergefährdungsklasse) or WEC (in English water endangering class), which indicates the value of the limit concentrations of the substance concerned, classifying it from non-dangerous to very dangerous (Table 7.6).
According to this classification, aromatic solvents are more dangerous than their mixtures (solvesso 100, high flash naptha) or aliphatics with reduced amounts of aromatics (mineral turpentine). Naphthenic and isoparaffinic solvents are not considered dangerous. No values were found for dimethyl esters (Table 7.7).
Two control strategies have been summarized here: the monitoring of the VOCs produced by each stage (production, dilution and application) and the list of solvents classified as dangerous by the regulatory bodies established by each country, province, state or community. A third is to distinguish between the different types of solvents taking into account the potential factor of creation of photochemical oxidants, in English POCP (Table 7.8), which is evaluated considering the following parameters:
a) Molecular weight (the higher the molecular weight, the greater the POCP factor)b) Quantity emitted (the higher the emission, the higher the POCP factor) c) Chemical structure (the more unstable have the higher POCP factor)
d) Reaction with OH groups (the greater the reaction with this group, the greater the POCP factor).
The POCP factor varies widely with higher values for alkenes and aromatic hydrocarbons and lower values for chlorinated solvents. It is then possible to reduce the levels of VOCs taking into account the POCP factor; the relationship between the two is shown in Table 7.9. Accordingly, degreasing operations carried out with chlorinated solvents are less polluting than printing inks, a circumstance that should be carefully studied as it would indicate the need for a single index that would bring together all the factors listed so far or that a combination of them capable of defining the degree of aggressiveness of a solvent is used.
*Cidepint (Center for Research and Development in Technology in Paints.
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