By Juan J. Caprari* In this edition we will continue talking about the environmental aspects in the paintings, a topic that was discussed in the last edition.
7.4. Low VOC content
Most VOCs are solvents with different aggressiveness for the environment, so international regulations increasingly limit their content in commercial paints. The technological response to this new situation recognizes at least two alternatives: 1). For workshop work use powder, electrostatic or curing paints with UV radiation; 2). In works on site conventional paints of high thickness and solvent-free paints. For both uses, high solids and water-based paints can be used.
Powder paints, products totally free of solvents, applied with electrostatic procedures in closed rooms and mechanisms for recovery and classification of dust not deposited on the object to be painted, allow the almost total recycling of the product. In general, its use is limited to workshop work (household appliances, equipment for the food industry, etc.) because the formation of film and its curing is carried out by heat (150-200°C). However, as an example of its use in large works, the painting of pipes for gas and oil pipelines can be mentioned. The degree of contamination they produce is minimal because they do not contain VOCs in their composition and the small particle size dust that can escape from the system is easily controlled by the purifying equipment.
UV curing paints are products used in the formulation of printing inks, wood lacquers, paper, printed circuit protection and plastic painting. From the ecological point of view, the problem with these formulations is that they contain mixtures of monomers, oligomers and photoinitiators. They are applied by roller system or by immersion (only very rarely with aerographic torches) in very thin films and their compositions contain only small amounts of organic solvents. Its emissions are perfectly controllable by modern purifying circuits.
Since the gradual withdrawal of chlorinated rubbers and some vinyl resins from the international market for ecological reasons, the use of conventional high-thickness paints has been reduced to high-thickness alkyd paints. Chlorinated rubber is made in carbon tetrachloride solution. The final product comes in the form of a white powder that retains about 3 to 5% of this solvent, which is then gradually released into the atmosphere over the life of the coating. In the form of a film it decomposes at relatively low temperatures (60-70°C). releasing hydrogen chloride. This gas in high humidity environments is transformed into hydrochloric acid, causing metal corrosion and affecting the ecosystem.
High-thickness vinyl paints also retain solvents, which are released slowly and alter the working atmosphere indoors. Alkyd resins, on the other hand, are soluble in aliphatic hydrocarbons and in the new ecological solvents so they can be a good solution for normal and low aggressive environments or service conditions, being able to increase their resistance by mixtures with phenolic resins or pure synthetic rubbers ecologically acceptable.
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Solvent-free paints, products of components made from liquid epoxy resins, are intensively used in the fermentation industries (wine vessels), food processing equipment, drinking water production (treatment pools and storage tanks), etc. Its use is limited by:
a) Application characteristics such that, in general, it is carried out by means of spatulas and less frequently with sophisticated high pressure airless spray equipment, which limits its use to those cases in which the surface is of reduced extension or allows overlapping between areas showing few decorative properties.
b) Low resistance to the ultraviolet fraction of sunlight although the resins used have high resistance to chemical agents.
Paints with high solids content have been developed in the last 10 years more rapidly also due to ecological pressure. The relative advantage over the other paints is their great resistance to highly aggressive environments. High solids products are defined based on two fundamental parameters: solids content relative to other coatings and solids level by volume in terms of VOC emission (Figure 7.6).
Powder paints with 100% solids contain 0% VOCs and it is expected to obtain liquid paints of similar content without the limitations explained above for solvent-free products (Table 7.10).
Before ecological awareness, and for economic and market reasons, the development of paints of high bakable solids has been carried out more quickly than those of high content of air-dried solids.
This is due to the joint effort of resin and application equipment manufacturers that provided products of lower viscosity, soluble in ecological solvents and machines of greater power and speed of application, respectively.
Obtaining products with high solids content with good application characteristics, cured in the air or at lower temperatures in baking and high resistance to very aggressive media, meant solving various problems depending on the properties detailed in Table 7.11.
At present, bakable products are based on epoxy resins, polyesters, acrylics with medium solids content and high thickness alkyds, while air-drying resins use epoxy resins, acrylics modified with aldicas, acrylics and two-component urethanes. Work is also being done to reduce viscosity by modification with reactive diluents.
*Cidepint (Center for Research and Development in Technology in Paints.
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