Лако-красочные материалы — производство

Being a volatile component of coatings, all solvents inevitably eva­porate into the atmosphere. The presence of organic compounds in the atmosphere can lead to serious problems related to air pollution. These solvents have been termed VOCs. When emitted from paints and accumulated in the troposphere, VOCs can react with oxygen in presence of nitrogen oxides and sunlight to generate smog and ozone. Ground-level ozone and smog cause a variety of health pro­blems even at very low levels. They may cause permanent lung damage after long-term exposure. They can also damage plants and ecosystems. Smog can cause eye irritation and respiratory problems in humans and animals, and can be harmful to crops and trees. Therefore, many countries have enforced legislation to reduce emissions of both VOCs and nitrogen oxides. VOC regu­lations have been a major driver for development of many of the present coating technologies, such as waterborne, high-solid and UV-curable technologies.

The EU “Paint Directive” 2004/42/EC defines a VOC as an organic compound having an initial boiling point lower than or equal to 250 C at the atmospheric pressure of 101.3 kPa. In the EU, the VOC is calculated by the following formula, and is expressed as mass per unit volume:

Equation 4.3: VOC = (МШаШе — MExempJ / VC0ating

Where MVolatile = mass of volatiles, MExempt = mass of exempt volatiles and VCoating = volume of volatiles

In the United States, as per the Environmental Protection Agency (EPA), a VOC is defined as “any compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates, and ammonium carbonate, which participates in atmospheric photochemical reactions.”

It should be noted that unlike in the EU, the US EPA uses photoche­mical reactivity of organic compounds as a criterion to identify an organic solvent as a VOC. The US EPA has released a list of exempt solvents that have negligible photochemical reactivity. Methyl ace­tate, acetone, tert-butyl acetate, dimethyl carbonate and propylene carbonate are some examples of exempt solvents important for the coating industry. Per the US EPA, VOC concentration is calculated according to the following formula and expressed as mass per unit volume:

Equation 4.4: V°C = (мШаШе — MExempt) / (Vcoating — VExmpt)

Where VExempt = volume of exempt volatiles

Although this mass-based approach to calculate VOC concentra­tion has been used for years, in 2008, the US EPA published new VOC emission standards for 36 aerosol coating categories. In this approach VOC contents are based on ozone formation potential of individual formulation components rather than the conventionally used mass-based approach. To express the ozone formation poten­tial, each VOC is assigned a maximum incremental reactivity (MIR) value that indicates the compound’s potential to generate ground — level ozone (its photochemical reactivity) in terms of grams of ozone formation per gram of solvent. The higher the value, the greater the potential for producing ground-level ozone. This provides more practical differentiation of the environmental impact of various compounds. In the earlier mass-based regulation, a low density, high activity solvent was more beneficial to paint formulators. In MIR-based calculations, the MIR value of each component is used to calculate grams of ozone formation potential per gram of product, often referred to as product weighted MIR, expressed in grams of ozone per gram of product. MIR values of different solvents have been documented by the US EPA.

The procedures specified by the US EPA for testing paint products for compliance with VOC limits are described in Federal Reference Method 24, which employs several ASTM test standards. VOC values for waterborne or solventborne coatings are calculated by the following formula:

Equation 4.5:

YQC~(wt% of volatiles — wt% of water — wt% of exempt solvent)(density of coating)
100-Volume % of water-Volume % of exempt solvents

It is common to dilute paint during application; therefore, regula­tory limitations are based on VOC content of coatings as applied. Experimental measurement of VOCs is not easy because the amount of VOCs released depends on several parameters under which the coating is applied and cured, such as time, temperature, film thick­ness and air flow over the surface. Most of the test methods for VOCs give results with some degree of deviation. Measurement of VOCs in waterborne coatings is even more complex due to the limitations of water content analysis.