By: Julián A. Restrepo*
The use of solvents and materials not toxic to the environment is one of the proposals of Green Chemistry. For this it is necessary to start considering the use of renewable raw materials.
Currently, conventional commercial chemicals are being replaced by those that are "environmentally friendly." The chemical industry has undergone a major change in its legislation: previously, only special emphasis was placed on the degree of toxicity of products on the market. Now, the emphasis is on the three major gaps that had the legislation and the management of chemical plants [1]:
Knowledge: Not just from the general public and governments, but in the chemical industry itself.
Safety: Hazards when prioritizing and setting toxicity limits.
Technology: Development of safer and more environmentally friendly alternatives.
"Green Chemistry" aims to respond to this need for change through a more rational use of our resources and knowledge [2]. This is a new philosophy based on a set of principles (12 in total), which contemplate, basically, the reduction or elimination of the use and generation of hazardous substances in the design, manufacture, transport, use and disposal of chemical products.
One of the most important objectives of "sustainable development" is to reduce the negative effects of the substances we use and generate [3], which inevitably leads us, among others, to the search for renewable raw materials. The role of chemistry is essential to ensure that the next generation of chemicals, raw materials and energy sources are more sustainable than today.
The global demand for environmentally friendly processes and chemicals requires the development of innovative and cost-effective technologies for pollution prevention [4].
A careful analysis of the principles of "Green Chemistry" shows that the solvent is present, in a more or less direct way, in most of its principles.
Its presence in the eighth point: "Safer solvents and auxiliaries" is evident. However, they also appear in principles such as the prevention of discharges, less dangerous synthesis and with a lower energy cost; use of renewable sources of raw materials and safer chemical processes, for example [2].
The environmental problem of paints
A solvent-based paint is composed of a vehicle (resins + organic solvents), pigments and additives. Of these components of the formulation, the organic solvent is the one that becomes VOC (volatile organic compound), after the drying of the paint, and it is precisely this component that has led to the development of alternative technologies, which seek to reduce the content of solvents in conventional coatings.
In the case of solvent-based paints, we have that current formulations involve the loss of a component during film formation. This is necessary to dissolve the resin and other components of the formulation, and generally has the lowest cost of the components.
Since the solvent allows the resin to dissolve, but does not remain in the coating (it does not constitute solids), we could consider it a "useful fool". So we have that, after the evaporation of the solvent (generation of VOCs), the paint suffers a "decrease in volume" and the loss of a low-cost component. But the main problem is that the component that "is lost from the paint" becomes an air pollutant.
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Technically, the best solvent, from an environmental point of view, is the one that does not exist [2], therefore, the ideal would be not to emit a component of the formulation into the air (loss of money), and more taking into account that this is polluting and presents a risk of toxicity. A good alternative is that if a solvent is required, it should be water, which is non-toxic and still economical.
Use of alternative solvents
"Alternative solvents", also called "green solvents" or "new solvents", are a series of solvents that are considered as alternatives to the current ones, to reduce the impact caused by the use of organic solvents. Among the "alternative solvents", there are:
a) Reactive solvents: They are solvents with low relative volatility, and during film formation they have the ability to react with other components of the paint, so they do not evaporate into the environment (they do not constitute VOCs), such as reactive coalescents (in water-based paints) and reactive diluents (in epoxy paints).
b)Benign solvents: "Benign solvents" are those environmentally friendly solvents whose group includes solvents free of chlorinated compounds, with low toxicity and low RIM (maximum incremental reactivity), compared to conventional solvents.
c)"Neoteric Solvents" [2]: The term neoteric designates new, modern, recent and in this case refers to a new generation of solvents, developed according to the philosophy of "Green Chemistry". They are a series of solvents that have a lower toxicity, are safer and less polluting than conventional solvents. These include both new fluids with adjustable properties, as well as compounds rarely used as solvents today, but which have significant potential for future applications.
Some alternative solvents
Among the "alternative solvents", from the point of view of "Green Chemistry", it is important to mention the "neoteric solvents", among which we have mainly:
Solvents from renewable raw materials [2]: Most of the solvents currently used come from petroleum. A reasonable alternative is to replace it with others from plant biomass.
Many of these solvents have low toxicity, low volatility, are not corrosive, or carcinogenic. Examples include alcohols such as biomethanol and bioethanol; there are also esters of soybean oil and ethyl lactate, obtained from lactic acid.
The water: Including the so-called "universal solvent" (in fact water is the solvent of nature) and the oldest of the known solvents, in this group is at least paradoxical. From an environmental point of view, water is the best choice as a solvent, since it is not flammable or toxic.
The biggest drawback of using it as a solvent is the low or no solubility of the resins and many of the components used in the formulation of paints. To solve this problem, surfactants are often used to "reconcile" and stabilize resins and other components in the aqueous medium.
Supercritical fluids [2]: A supercritical fluid is one that lies above its critical pressure and temperature and whose phase is neither liquid nor gaseous, but shares the properties of both: It flows like a gas and is able to dissolve substances like a liquid. Under these conditions, they possess some very interesting properties.
Many of them are inert and non-toxic, which makes it possible to classify them as "green solvents"; in addition, they are relatively inexpensive and their properties are adjustable by pressure variations. Examples of its use are in the production of decaffeinated coffee, using liquid CO2 (in a supercritical state) and some studies to use them as propellants in spray paints (N2O and CO2, for example).
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Fluorinated solvents [2]: These are organic solvents that instead of being hydrocarbons (C-H bonds), are fluorocarbons (C-F bonds), and that have interesting properties: They are chemically inert, thermally stable, are not flammable, nor toxic.
Fluorinated solvents have a particular application in the synthesis of resins: for example, the amines of fluorinated chains are drastically reduced in their basicity. However, fluorinated alcohols have a much higher acidity than alcohols, which makes them useful in some applications. The low solvent capacity of fluorinated liquids makes it necessary to modify the compounds to be dissolved in them.
Ionic Liquids (LI): An ionic liquid is made up of ions, such as salts, and is characterized by virtually zero volatility. Some of the ionic liquids have low combustibility, excellent thermal stability, are liquids in a wide temperature range and possess good solvation properties for many substances.
"Ionic liquids" have been gaining interest in many fields, including organic chemistry, electrochemistry, catalysis, physico-chemistry and engineering, even having applications of magnetic ionic liquids.
The biggest advantage of "ionic liquids" as "green solvents" is their low volatility, so they are not considered VOCs. Its application in the formulation of paints today is little (or practically nil), since its cost is relatively high.
That is, although "ionic liquids" have no risk of evaporation into the atmosphere, their repercussions as discharges are not completely clear, since studies on their toxicity and biodegradability are still very few.
Final comments
1. "Neoteric solvents", including water, are valid alternatives to conventional organic solvents, presenting advantages such as lower toxicity, volatility and flammability [2].
We all have an idea of which solvents are safer and less polluting, and it is clear that it is preferable to use water or alcohols instead of chlorinated solvents or hydrocarbons. In general, the selection of a solvent for its "green" character is a complex process and its choice depends on numerous factors, among which is the application.
2. Some "alternative solvents" show that they do not present a risk of pollution to the air, but instead (bearing in mind their low volatility), they present a risk of contamination for water sources. It is clear that from the context of "sustainable development" it is not intended to "change one problem for another". In any case, the use of a material that acts as a solvent must be completely non-toxic.
3. Finally, it is important to note that the rapid development of "green chemistry" is due to the fact that environmentally friendly products and processes prove to be economically profitable in the long term and become a wise bet to face the future challenge of "sustainable development".
References
[1] Kröger, A.; "Green Chemistry: The Changes of an Industry", August 2007. Publication on the Internet.
[2] Mayoral, J.A.; "Use of alternative solvents", publication on the Internet: www.unia.es/ nuevo_inf_academica/ bisualizar_file_adjunto.asp? ID = 1606.
[3] Horváth, I. and Anastas, P. T.; Chem. Rev., Vol. 107, No. 6, 2007.
[4] Anastas, P. T. and Warner, J.C.; "Green Chemistry: Theory and practice"; Oxford University Press; Oxford, 1998.
* INVESA S.A. / Universitat Jaume. Castellón, Spain. [email protected]
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