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An Introduction to Green Chemistry

Author: Bianca Gonçalves


Excerpt from Bianca Gonçalves’ master’s thesis, An Introduction to Less Toxic Approaches for the Cleaning of Paintings: The Particular Case of Acrylics. Presented in July 2018 at ENSAV La Cambre, Brussels, Belgium.


1. Notions of Green Chemistry


The environmental pollution created by solvents and surfactants can be decreased by using Green Chemistry (GC). GC is a method of introducing innovative solutions to chemical-related problems and applying sustainability to molecular design. The definition of green chemistry can be simply given as:


“Green Chemistry is the utilization of a set of principles that reduces or eliminates the use or generation of hazardous substances in the design, manufacture and application of chemical products.” [1]


In theory, GC is framed by 12 Principles which guide chemists in this “design” [2] :

1- Prevention - “Prevention is better than cure”.


2- Atom Economy - Synthetic methods should be designed to maximize the incorporation of all materials used in the process into the final product which means, preventing waste on a molecular level.


3- Less Hazardous Chemical Synthesis - Wherever practicable, synthetic methods should be designed to use and generate substances that possess little or no toxicity to human health and the environment.


4- Designing Safer Chemicals – Chemicals should be designed in order to preserve the efficacy of their functions while reducing toxicity.


5- Safer Solvents and Auxiliaries - The use of auxiliary substances (e.g. solvents, separation agents, etc.) should be made unnecessary wherever possible and, when used, innocuous.


6- Design for Energy Efficiency - Energy use in chemical processes should be recognized for its environmental and economic impact and should be minimized. If possible, synthetic

methods should be conducted at ambient temperature and pressure.


7- Use of Renewable Feedstocks - A raw material or feedstock should be renewable rather than depleting whenever technically and economically practicable.


8- Reduce Derivatives - Unnecessary derivatization (blocking group, protection/deprotection, temporary modification of physical/chemical processes) should be avoided whenever

possible.


9- Catalysis - Catalytic reagents (as selective as possible) are superior to stoichiometric

reagents.


10- Design for Degradation - Chemical products should be designed so that at the end of their period of use they break down into innocuous degradation products and do not persist in the environment.


11- Real-time Analysis for Pollution Prevention - Analytical methodologies need to be further developed to allow for real-time, in-process monitoring and control prior to the formation of hazardous substances.


12- Inherently Safer Chemistry for Accident Prevention - Substances and the form of substances used in a chemical process should be chosen to minimize the potential for chemical accidents, including releases, explosions, and fires.





2. Art Conservation and Green Chemistry

Initiation to Gels


Upon close analysis, it can be noted that the trajectory the conservation field has taken very much follows the 12 principles of GC mentioned above. In 2018, Wolbers [3] highlighted 6 of the principles that have been frequently heeded in art conservation during the last decade, where most of them are linked to the use of gels:


1. Waste Minimization at Source: We can minimize waste by simply using solvents as gels instead of in their free forms, as this method would greatly reduce their evaporation. Take as an example the opening of a joint in a wooden panel, which had been glued together with PVAc (personal experience in 2017 in La Cambre). The use of a pure solvent (in this case, acetone) was inefficient because the solvent evaporated too quickly and was largely wasted. It took almost 3 days and a liter of acetone to open the joint. In contrast, about 50 ml of gelled acetone and less than one day was sufficient to open the second joint of the same painting. Additionally, gels stay stable for years, but the proportions in a mixture of free solvents will rapidly change if the container is left open. By using free solvents, we also increase our carbon footprint by requiring more fume extractors and personal safety equipment such as gloves and filters for masks.



Figure 1 - Agar gel cleaning a contemporary art painting.

2. Use of Catalysts in place of Reagents – One good way to use catalysts in CR is to add enzymes to aqueous solutions. The solutions can then be used to break down biopolymers and, as Wolbers says in his reflections, enzymes “represent green chemistry at its best” because they are “renewable, degradable, [and] inherently water-based”. In the case of the method described above, a solvent treatment is replaced by an aqueous one (e.g. removing of oil coatings). Enzymes are not the focus of this thesis, but they are surely worth further study, especially here in La Cambre. Another way to apply this second principle is through the use of polyfunctional materials. These polymers help us maximize the effect of a product while using less material [4] (yes, again, gels!). Polymers that form gels cannot only change solvents’ properties by increasing their viscosity, but also serve as cleaning systems themselves [5] .


3. Improved Atom Efficiency - Gels’ polymers do not only help replace reagents with catalysts; they also increase “atom efficiency”. As Wolbers writes, this is “not exactly ‘atom efficiency’ as in the strictest green chemistry sense”, [6] but it is similar to GC because we are increasing the polymers’ cleaning power by using molecules that have more than one use.


4. Use of Non-Toxic or Less Toxic Reagents - Again, toxicity is decreased by using water-based systems. Using less toxic products should always be part of our conservation treatments. It is not just our own health that is at stake but also that of our colleagues, who work nearby in spaces that are normally limited and enclosed [7] . We should not allow solvents to be the paradigm, and we cannot look for alternatives only when they fail. In fact, solvents should be avoided, and our current “alternative solutions” have to become the paradigm.

The two other parameters mentioned by Wolbers, the use of renewable materials and the use of solvent-free or recyclable and environmentally benign solvent systems, must be integrated into CR. These are the main concerns of this thesis, and analyzing the principles allows us to introduce eco-responsibility criteria for the choice of products in CR. Environmentally benign materials that come from sustainable sources must be embraced and incorporated into our day-to-day work. Bio-sourced and biodegradable materials must be given priority during the selection process. Solvents like D-Limonene, isopropyl palmite (IPP) and isopropyl myristate (IPM) are good examples of greener options that can be substituted for apolar mineral spirits [8] . In regard to emulsifiers and gels, Pemulen or Velvesil are two products that act not only as thickeners, but also as surfactant-free methods for creating emulsions. When both water and an apolar solvent are needed to clean a painting (e.g. cleaning of acrylics), emulsions allow us to integrate these immiscible solvents. Thus, gels are an excellent way to both improve some CR treatments and to comply with the principles of green chemistry.


Figure 2 - Cleaning with agar gel

Important Factors of using the Gels

Green chemistry will always be a guide that helps us achieve sustainability in conservation. It not only provides us with materials to use that are less toxic, but also shows us how to reduce our reliance on toxic materials that we cannot eliminate (for now). Gels are currently one of the ways to achieve this reduction in reliance, but they also have their drawbacks. One important factor in the appropriateness of a gel’s use is the condition of the painting being treated. Another is the fact that gels can leave residues; thus, mechanical action and the use of solvents are inevitable, so problems related to free solvents will still be present. Nonetheless, there are innumerous types, forms, and classifications of gels with different kinds of properties and methods of application. Even though gels are a good way to decrease toxicity, we must also be open to new ideas that discourage their use in the same way that we are now trying to discourage the use of solvents and surfactants. The goal is to become healthier and greener, but we cannot forget the ethics related to preserving the artwork.




Notes

[1] Anastas, Paul; Warner, Green Chem. Theory Pract., 30.

[2] All principles comme from: Anastas and Warner, “12 Principles of Green Chemistry - American Chemical Society.”

[3] Wolbers, “Gels, Green Chemistry, Gurus and Guides.”

[4] The shorter the list of ingredients, the smaller the possibility of residues.

[5] Wolbers, “Gels, Green Chemistry, Gurus and Guides,” 4.

[6] Wolbers, 5.

[7] Wolbers, Le Nettoyage Des Surfaces Peintes. Méthodes Aqueuses, 163–67.

[8] Wolbers, “Gels, Green Chemistry, Gurus and Guides,” 7.


Bibliography

Anastas, Paul; Warner, John. Green Chemistry: Theory and Practice. Oxford University Press, 1998.

Anastas, P. T., and J. C. Warner. “12 Principles of Green Chemistry - American Chemical Society.” Green Chemistry: Theory and Practice, 1998, 30. https://www.acs.org/content/acs/en/greenchemistry/what-is-green-chemistry/principles/12-

principles-of-green-chemistry.html#articleContent_columnbootstrap_0.

Wolbers, Richard. “Gels, Green Chemistry, Gurus and Guides.” In Gels in the Conservation of Art, edited by Lora V. Angelova, Bronwyn Ormsby, Joyce H. Townsend, and Richard Wolbers, 3–10. London: Archetype Publications Ltd, 2017.———. Le Nettoyage Des Surfaces Peintes. Méthodes Aqueuses. Institut n. Eyrolles, 2001.

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