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Nanomaterials as Sustainable Challenges and Opportunities in Cultural heritage: Cleaning.

Authors: Souty Beskhyroun, Aline Assumpção, Bianca Gonçalves



Overview

As nanomaterial science expanded into multidisciplinary fields over the last few decades, it began to play a prominent role in cultural heritage conservation. Research on nanomaterials has indicated that they may provide promising methods for the cleaning, consolidation, and coating of different archaeological materials like paper, wood, canvas, wall paintings, and stone. As in the case of any other treatment option that was brought to conservation from a different field, nanomaterials’ efficacy, rather than their environmental and health safety, has been conservators’ primary focus in their assessments of the products. However, sustainable conservation practices have recently become more important in the field of cultural heritage.


In the case of nanomaterials, environmental benefits are often connected to the advantages of treatment outcomes; for example, a self-cleaning surface coating will reduce the effort necessary to clean a surface, saving both energy and cleaning agent. Identifying the actual impacts of nano products - both positive and negative – requires analysis of their entire life cycle from the production of the raw material to disposal [1]. Unfortunately, many of the nanomaterials that are currently used and processed depend on non-renewable resources and have the potential to create hazardous waste. Green nanotechnology, which combines green chemistry and nanotechnology, seeks to design nano products and processes that reduce or eliminate the creation and use of hazardous substances [2]. The aim of this research is to identify the green aspects of some of the nanomaterials that are already being used for cleaning, consolidation and protection procedures, and to encourage the implementation of more sustainable conservation practices in the near future.


What is Nanomaterial?

The primary objective of nanotechnology is to model, simulate, design and manufacture nanostructures and nanodevices with extraordinary properties and assemble them economically into a working system with revolutionary functional abilities. Nanotechnology offers a new paradigm of groundbreaking material development by controlling and manipulating the fundamental building blocks of matter at nanoscale, that is, at the atomic/molecular level. [3]


The application of nanotechnology in Conservation and Restoration can improve features in our work, that so far were not a concern or a working directive such as: more health conditions to art conservators (health), more action with less material (energy), less waste and less toxicity (environment). Their three-branch philosophy of developing health, reducing environmental pollution, and developing innovative technology is the future of science and continues to revolutionize any and all sectors of the industry.


A nanomaterial is an object that has at least one dimension in a nanometer scale. The nanoscale is usually defined as smaller than one-tenth of a micrometer, but the term is sometimes also used in reference to materials smaller than one micrometer. Nanomaterials can be categorized by their dimensions. [4]


As a particle’s size is reduced into the nanoscale, the surface area per unit volume increases. The material’s reactivity is consequently enhanced [5], because more active surface becomes usable for reactions and transformations. The other advantages of size reduction include the ability of solid particles to be dispersed more easily into carrier solvents, and favorable penetration through porous metrics that minimize the risk of haze formation and allow for homogeneous diffusion [4].


Nanostructured Fluids for Cleaning

The cleaning of artifacts is a delicate and irreversible task that could alter the whole appearance of artistic and historical surfaces. Cleaning is generally defined as the removal of any undesirable layer, from grime, dirt, and soil to naturally and synthetically discolored coating, adhesives, and varnishes. In the past, the removal of these layers has been performed with traditional solvents, which are difficult to control and therefore pose a risk to the substrate [4]. During the last few decades, colloid and nano scientists have carried out intensive research on the use of binary and ternary systems - composed of water, surfactant, and other additives - as solvents. Thus, the formulation of these amphiphile-based systems has been improved in the past forty years [6].


Surfactants present in these systems have become ubiquitous compounds in both industry and home life. In conservation and restoration practice they are used due to its ability to link an apolar and polar phase together. We see them in gels, emulsions, fat solutions or buffer solutions for the cleaning of paintings. It is important to keep in mind that surfactants are not chemically pure products, but rather mixtures of homologous substances whose composition varies according to synthetic processes and that they can have different effects on animals and the environment. The toxicity of surfactants depends on different things, especially as there are different types of surfactants: anionic, cationic, nonionic, amphoteric.


Skin contact during cleaning preparations is the main route of exposure to surfactants, and, according to several authors, surfactants are mainly responsible for the cutaneous effects of detergents which can lead to skin irritation [7] (dermatitis), allergies and even dermal toxicity. Surfactants are very weakly volatile primarily contaminates waters by the direct discharge of cleaning effluents into watercourses or by accumulation in sewage sludge [8] .


When a surfactant reaches the surface water or the earth, it causes phenomena such as sorption (the accumulation of foam in calm sea waters, which are very toxic for the aquatic life) and degradation, respectively . The effects of surfactants in watersheds concerns in particular aquatic toxicity, biodegradability and bioaccumulation [9] . In general, anionic surfactants produce less sorption than nonionic surfactants and significantly less than the cationic ones [10]. For applications in cultural heritage, the trend should be to minimize the number of surfactants needed, whenever possible. Careful considerations regarding the use of degradable (and biodegradable) surfactants must be taken into account in basic research for the formulation of cleaning systems or alternatives materials with the same properties as surfactants but surfactants-free.


Microemulsions and Micellar Solutions

The nanodroplet was first used as a cleaning method in cultural heritage conservation at the end of the 1980s in Florence, Italy. In this instance the droplets were used to remove contaminated wax spots from the surface of paintings in the Brancacci Chapel [11]. They proved to be effective in removing the synthetic polymers that had previously been applied to consolidate and protect the work of art [12]. These polymers included natural varnishes, modern paints[13], and aged adhesive tapes[14]. Microemulsions have been described by Lindman and Danielsson as “liquid, stable and homogeneous, optically transparent, isotropic and ‘spontaneously’ formed systems, comprising two liquids mutually insoluble; one dispersed in the other in the form of microspheres stabilized by at least a monolayer of amphiphilic molecules (surfactant micelles)”. The continuous phase can be hydrophilic (O/W) or hydrophobic (W/O) to allow for its controllable diffusion into the artifact being treated.


These emulsions create a large surface area that keeps interactions with the undesirable materials within the core of the nanodroplet and/or droplet interface, minimizing the diffusion of the solubilized material into the porous matrices. The formulation of O/W microemulsions requires a small amount of organic content - between less than 0.5% to about 15% - from both solvents and surfactants. This reduction in organic solvent use lowers the toxicity and environmental impact of the treatment while also giving the conservator more control over the process.


There were drawbacks to the sodium dodecyl sulfate (SDS) surfactant used in the first microemulsions, and these have been investigated as sustainable alternatives become more important in conservation. Several biodegradable surfactants, such as polyalkylglycosides, have been introduced in the conservation field as non-ionic surfactants with good eco-toxicological profiles [6]. On the other hand, the preparation of nanostructured materials in current green nanotechnology practices often involves non-hazardous solvents [2].


Nanostructured Gels

As a consequence of uncontrolled spreading and the penetration of cleaning fluids into artwork, several strategies have been developed to confine solvents and solutions to a controlled area [15]. The high viscosity of gel allows solvents to be released more gradually, reducing their rates of evaporation and consequently limiting their ability to penetrate the original substrate [16]. Many studies have been carried out with the aim of identifying gelled systems that are both effective and completely removable after usage [17]. For example, Bonini et al used a magnetically-responsive nanosystem to remove gels from very sensitive surfaces safely. Their nanomagnetic sponge was created by crosslinking magnetic nanoparticles through a polymer network based on polyethylene glycol and acrylamide. The resulting sponge was later loaded with a microemulsion made up of nitrodiluents and p-xylene, and then applied to a marble surface covered with an aged layer of Paraloid B72. Complete removal of the resin was achieved, and no residue of the sponge was left on the treated surface after application. The sustainable aspect of nanomagnetic gel lies in its ability to be dried and reused, which reduces toxic residues and waste. Being loaded with gel, the evaporation of the volatile organic solvent in the microemulsion is much slower than it would be in a pure solvent; thus, the toxic vapors in VOCs’ emissions issued by the solvents are drastically reduced [18].


Recent and commercially available options are the Nanorestore Gels, that are chemical hydrogels with some of the most attractive tools recently proposed for the cleaning of very fragile and water-sensitive surfaces. The University of Florence CSGI research group, mostly represented by Piero Baglioni and Rodorico Giorgi, has recently developed semi-interpenetrated network hydrogels based on pHEMA and PVP polymers. Many applications for chemical gels have been investigated by research groups in European universities, and this research has been supported by two EU projects called Nanoforart and Nanorestart. Within the project Nanorestart they made available 18 new products for the conservation of art. The study of the nanoscale will further improve the performance of restoration formulations and our understanding of degradation mechanisms.


For these reasons, green nanotechnology practices should be incorporated into the field of cultural heritage, and more research should be carried out on the topic in order to develop more sustainable conservation methods. Check our blog again soon to learn how sustainable nanomaterials can be when used as consolidants!



Bibliography

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Sustainability in Conservation is an online network providing resources and information about environmentally responsible practices  in art conservation and related fields. Within a practice that produces so much waste, we hope to inspire collaboration and awareness to make cultural heritage a more sustainable profession. 

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