Cleaning of artworks is deemed to be one of the most important and critical actions undertaken by conservators-restorers owing to its irreversible nature. Cleaning processes include the removal of undesired deposits from the surface of the treated artwork for stability and aesthetical requirements [1]. For a better and safer cleaning performance, the cleaning action should be limited to the interface between the deposits and the surface to be treated. Using traditional poultices in combination with neat or blended solvents was the most common technique for surface cleaning/. However, in some cleaning treatments, this approach proved to be inadequate as it may damage the original materials and/or leave residues behind [2–4]. To address these issues, the University of Delaware Programme developed gels as a carrier for cleaning agents and thus introduced the so-called ‘solvent-gels’ to the conservation-restoration community as a more controllable and selective cleaning tool [5]. Since then, conservation scientists have been exploring and developing variable types of gels to meet the different cleaning requirements [6–9]. In these studies, the gels are loaded with the desired cleaning agents; their controller release when the gels are applied on the surface to be treated permits dissolving/swelling of the deposits to be removed.
In the last decade, agar and agarose gels were extensively exploited by conservators-restorers during cleaning of variable types of works of art such as: paper, stone, ceramic, textile, plaster, and wall paintings [10–15]. Agarose is a nonionic polysaccharide and is one of two components of agar gels. The other component is agaropectin which contains more sulphate groups than agarose and has a lower molecular weight. Agarose provides the gelling power for agar gels because of its high molecular weight. It is extracted from marine red seaweeds and is composed of ß-1, 3 linked-D-galactose, α-1, 4-linked 3,6-anhydro-L-galactose (see Fig. S1-a). When agarose is heated in water above 85° C, random coils start to form. Upon cooling, the random coils transform into single and double helices in the initial gelation stage (Gel I). Further cooling leads to the final gelation stage where thick bundles of double helices are formed (Gel II) (see Fig. S1-b). This gelation process is thermoreversible and creates the 3D network of rigid agarose gel due to the formation of hydrogen bonds among the bundles of double helices [16, 17].
Despite of the advantage of those gels, allowing the inclusion of a broad range of cleaning agents [10, 18], one of their limitations is being (too) rigid and the lack of conformity to complex structured surfaces, which makes them more suitable for flat ones such as paper. This limitation may lead to uneven cleaning results due to the incomplete contact between the gel and the surface to be treated [19, 20]. To obviate this problem, the gel can be covered with glass plates to enhance the contact between the gel and the surface to be treated [10]. Another way to resolve this issue is by applying the gel by means of a brush in the sol-gel phase when it is still semi-solid. However, this approach can provoke detachment of fragile fragments from the treated surfaces as in the case of fragile paint layers. Moreover, since the gel is applied in the warm state (c. 40–45° C), it may cause damage to heat-sensitive materials [15].
On the other hand, polyvinyl alcohol-borax (PVA-B) systems, which consist of PVA polymer crosslinked with borax, can serve as an alternative cleaning tool for conservators-restorers. PVA is characterized by a large number of hydroxyl group along its backbone. Inter- and intramolecular hydrogen bonds are formed among those hydroxyl groups. Thus, the number of hydroxyl group has a significant influence on PVA properties. In aqueous solutions of PVA, heat is required to improve the dissolution process especially in the case of PVA with a high degree of hydrolysis (containing more hydroxyl groups) [21, 22]. Usually, sodium tetraborate decahydrate is used as a crosslinker for PVA-borax hydrogels. When it is dissolved in water, it dissociates to form equal amounts of boric acid (B(OH)3), B(OH)4− and Na+ (Eq. 1) [23].
Equation 1
Na2B4O7.10H2O ↔ 2 B(OH)3 + 2 B(OH)4− + 2 Na+ + 3H2O
When a PVA solution is mixed with borax in the proper concentrations, the crosslinking process occurs in two steps: a) the tetrahydroxy borate anion reacts with a diol of a PVA chain to form a monodiol complex; b) the other two hydroxyl groups, attached to the boron atom in the borate, react with another adjacent diol. Thus, the crosslinking process is called didiol complexation Fig. S1-c [23, 24].
PVA-B systems are characterized by a viscoelastic behaviour; as such, from a rheological point of view, they are not considered real gels. However, for simplicity, we will refer to them as hydrogels and this is also the term broadly used within the conservation-restoration community. The flexibility of these systems eases their conformity to surfaces with a rough morphology. Yet, they can be peeled off from the surface as one piece by the simple use of tweezers [25]. PVA-B systems can be loaded with organic solvents and chelators. The concentrations to be loaded depend on the hydrolysis degree of PVA polymer [24, 26]. Nonetheless, there are some limitations regarding the surfaces they can be applied to. It has been reported that some porous surfaces (e.g. some types of wood, paper, plaster) are not easily cleaned as the gel may adhere to them and leave residues [27].
In the broader sense of gel applications, the concept of blending two or more polymers inside the 3D complex of the final gel is largely known. The blending facilitates tailoring gels with specific properties in addition to producing gels with improved characteristics that can circumvent the possible limitations conveyed by gels made using unblended polymers [28, 29]. In the conservation-restoration field, a number of gel blends have been developed. For instance, poly-2-hydroxyethyl methacrylate/polyvinyl pyrrolidone, polyvinyl alcohol/polyvinyl pyrrolidone, polyvinyl alcohol-borax blended with polyethylene oxide¸ and polyvinyl alcohol-borax complexed with carbomer. In all the above-mentioned hydrogel examples, the addition of the second polymer was done to optimize the properties of the resulting hydrogel to meet the criteria of specific cleaning needs [30–33].
In a previous study, we blended PVA-B with agarose and the resulting hydrogel proved to be efficient in removing deteriorated consolidants from ancient Egyptian wall paintings [34]. The PVA-B/AG double network (DN) hydrogel exhibited a good workability during application and was able to host different solvents such as acetone, 1-butanol, 1-pentanol, methyl ethyl ketone as cleaning agent. In this paper we investigate in greater detail various characteristics of the PVA-B/AG double network hydrogel such as a) its chemical structure, b) liquid phase retention, c) mechanical strength, d) rheological properties, and e) self-healing behaviour. The measurements described below focused on hydrogel formulations consisting of 3% PVA-B in water, 4% PVA-B in water, and 3/1% PVA-B/AG in water. We also investigate the effects of adding increased levels of agarose to PVA-B systems on their rheological behaviour. Moreover, PVA-B and PVA-B/AG hydrogels were loaded with different cleaning agents to study their influence on the characteristics of these two gel systems. Three organic solvents with different polarities were chosen for this reason: ethanol (with polarity = 0.654), acetone (0.355) and xylene (0.074). In addition, other cleaning agents such as the chelating agent EDTA and the surfactant Ethomeen C25 were studied.