“LIQUID FOAMS OF ETHYLENE VINYL ACETATE RESIN/S FOR THE RESTORATION OF PAINTINGS ON CANVAS”

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Technical field of the invention

The present invention relates to a new process for the restoration of paintings on canvas, such as to be executable also on-site, avoiding the transfer of the painting, comprising the well-known method of painting lining wherein a new liquid foam of ethylene vinyl acetate (EVA) resin/s is interposed as an adhesive between the back of the canvas of the painting to be restored and the new canvas replacing the structural functions of the original support, forming, also in the course of time, an adhesive layer between the two canvasses such as to comply with all the requirements of restoration and conservation of the painting: long-lasting, sufficient adhesive strength, no chemical reactivity, no structural interaction, high compatibility, durability, applicability and reversibility.

State of the art

As hinted above, the lining process is a traditional restoration technique envisaging the application of a new canvas on the back of a painting on canvas to replace the structural functions of the original support, which is too deteriorated and damaged. It is a highly invasive intervention and requires great care in the selection of the materials to be used and the operation mode. Plentiful examples actually exist wherein a bad lining caused more damages than benefits, very often irreversibly harms the pictorial layer, the readability and the structural consistency of the painting. Over the last decade, precisely these cases were put into question through several meetings and publications about the benefits and drawbacks of this practice. However, in many cases the lining intervention remains the only and necessary operation to solve and remedy the excessive and unrecoverable brittleness of the original canvas and it is fundamental to act in the best possible manner. A lining (or relining) intervention is carried out when the original canvas support of a painting has lost its support strength (whether by deterioration of the cellulose, large-size lacerations or rips, etc.). The operation consists in applying to the back of the original canvas one or more new canvases. The new or lining canvas is temporarily stretched onto a stretcher frame that is larger than the canvas (temporary stretcher). In order to obtain these results, the lining treatment usually requires pressure and heat which liquefies the adhesive allowing it to penetrate; during the operation hand irons or other types of heat sources are used. If these tools are not used with sufficient caution, it is possible to damage the painting. Lining methods are generally characterized by the type of adhesive that is used: a glue-paste lining (with the use of a glue-paste adhesive), a wax-resin lining (in general out of use), a lining using synthetic resins (in use since the second half of the twentieth century). These latter lining types are identified by the commercial name of their adhesive. In the field of restoration, painting lining using adhesives based on aqueous mixtures, such as animal glue, flour, resin and Venice turpentine, became common in the Eighteenth century. In the Nineteenth century, mixtures of bee wax and resin began to be used as lining adhesives along with ironing to join the two canvasses. In the course of the years, many improvements were made to said hot-melt coating, including the introduction of the hot table in 1946 and the following use of vacuum pressure in addition to heat pressure in 1955. In this way, the combined application of heat and moisture has a plastifying effect on the varnish, as well as on the ground, and pressure makes it possible to smooth distortions, whereas it is reckoned that the restrictive effect of a stiff, strongly adhering support keeps the reappearance thereof under control. The first real starting point of the debate dates back to the early Seventies of the twentieth century with the Greenwich Lining Conference of 1974, during which new methods and materials for lining were introduced, for the purpose of replacing these old products which had been causing plenty of problems several times. Indeed, in many cases the wax-resin adhesive entailed a considerable irreparable darkening of the supports and preparation, owing to the impossibility to limit the penetration of the adhesive into the tissue. Another problem was the risk of tissue shrinking on account of the humidity inherent in the process. Moreover, these glue/paste adhesives were hard and brittle, favoured the growth of moulds and became difficult to be removed as a result of the increase of the irreversibility caused by aging. Through these ancient traditional lining techniques, the painting surface is also vulnerable to deformation on account of the heat and pressure require to establish a bond. This is particularly true with regard to strong -impasto paintings or to those contemporary paintings wherein the paint film is far softer than that of a traditional oil painting. Furthermore, hot-table processes expose paintings to longer heating times compared with manual coating and professionals have been using increasingly high pressures which involved new kinds of structural change. Since the late Sixties, new techniques have developed and during the Greenwich Conference the most important and innovative breakthroughs were introduced: heat-seal coating with BEVA 371 and cold coating with Plextol B500. BEVA 371 is certainly the most used for lining and for strip coating, even more than forty years after the publication of its recipes in 1972. It is an adhesive consisting of a complex thermoplastic mixture of copolymers (ethylene vinyl acetate), paraffin wax (namely a semicrystalline mixture of long- chain hydrocarbons), and two plasticizing resins, that is Laropall® K80, a ketonic resin, and Cellolyn™ 21 , a phthalate ester of hydroabiethyl alcohol. The original formulation was a solution with about 40% solid mass in a mixture of toluene solvent and naphtha, in order to ease application through brushing or sterilization, sometimes diluted by restores with naphtha or xylene. After application, the solvents evaporate and the adhesive is reactivated with heat, at a temperature comprised from 40 to 70 °C, depending on the solvent concentration: dilution actually lowers the reactivation temperature. In addition, a slight pressure is required for a proper formation of the bond. After BEVA official trademark by Berger in the mid-Eighties, a similar product was introduced by the Lascaux company, which had initially distributed Berger’s synthetic resin until 1989, under the name of Lascaux® heat-seal adhesive 375. BEVA 371 original formulation and Lascaux 375 were compared and studied by a lot of researchers in the course of the years: the two products have similar chemical-physical properties and the same rheological, optical and mechanical features. In 2010 the original formulation was forced to change, since the production of the Laropal® K80 ketonic resin was dropped by manufacturers, and the new formulation, named BEVA 371 b, currently contains an alternative ketonic resin giving the product a yellow hue, although the mechanical performances remained unaltered. Plextol B500 is an acrylic adhesive, manufactured based on an aqueous dispersion formulation manufactured by Vishwa Mehra at the end of the Sixties. It is formed by methylmethacryilate and by ethylacrylate copolymer, which has the advantage that it can be applied without using heat, with the aid of an airflow table favouring the process of drying and removing water. Despite its ability to form a moderately rigid film and a uniform and moderately weak bond ensuring the reversibility of the coating, its spread was at first limited by restorers’ caution regarding water-based treatments. However, Mehra’s studies and approach to structural conservation became enormously influential. The conservation of the painting appearance and the positive aspects of aging (namely the cracks formed on the surface) were stressed. Moreover, pre-treatment before lining was strongly emphasized, as were flattening and consolidation: this gradual approach allowed the whole treatment to be wholly monitored, in contrast with the traditional view that lining was essential to support the successful result achieved during pre-treatment. Later developments of products for lining followed the minimalistic method introduced by Mehra, both in terms of minimum intervention and compliance with the correct aesthetic appearance of objects, and in terms of method of application with the least possible handling and treatment (namely heat activation, pressure and heat application and so on). Ketnath introduced in 1977 an acrylic coating with low-heat activation with Plextol D36, an acrylic dispersion applied as dried, heat-sealed film at a temperature of less than 50 °C. Later on, in 1984, Fieux proposed, quite unsuccessfully, a cold lining with pressure-sensitive adhesives such as Fabrisil, which is a glass fibre fabric impregnated with teflon coated with a vulcanized silicone adhesive. Also, a series of further searches were devoted to lining by activating the solvent and pressure-sensitive adhesives, trying to minimise the bond strength, improve reversibility and reduce humidity and heat to the point of removing them from lining processes. However, over the last decades few enhancements were brought to the lining technology and the methods which became widespread and commonplace with restorers mostly envisage the use of BEVA 371 and acrylic dispersions. A recent poll showed that BEVA 371 is used by 79% of restorers. Accordingly, as described above, even the most recent technologies which have been available so far for the purpose of safeguarding/conserving/restoring works of art, particularly paintings on canvas, do not warrant either the minimum technical requirements needed for restoring the work of art, such as:

• Adhesion: strong enough to withstand the characteristic stresses artistic materials are subjected to;

• No chemical reactivity: any possible reactions should occur during the application and not damage the object in the future;

• No structural interaction: minimum shrinking and expanding; a certain plasticity and elasticity to absorb long-term stresses;

• Compatibility: no dyeing; similarity to the object in pattern and appearance;

• Durability: chemical and physical stability;

• Applicability: the application techniques should not jeopardise the object;

• Reversibility: possibility of safe removal; or the new requirements of:

- lower toxicity on account of the lack of dangerous and harmful solvents (for example toluene or other organic solvents), replaced with as biocompatible a solvent as possible, namely water, the lack of manufacturing wastes and the removal of heat activation, which simplifies the process of safeguarding, conserving and restoring;

- obtaining an easy, effective, rapid and cost-effective coating process by monitoring the water vertical diffusion,

- removing dangerous solvents and heat activation, enhancing transparency, high stability to UV rays and avoidance of yellowing in the course of time, as well as enhancing flexibility,

- being able to perform the operation of restoring/safeguarding/conserving the work of art also on site, namely where the work of art is situated/conserved, avoiding relocation to a specialized laboratory.

A strong need therefore existed to implement new products, systems and methods within the framework of the technologies for safeguarding/conserving/restoring works of art, particularly paintings, overcoming the drawbacks of currently available technologies.

Summary of the invention

Continuing research in the present technical field, the Applicant surprisingly and unexpectedly implemented as objects of the present invention:

- a liquid foam of ethylene vinyl acetate (EVA) resin/s, preferably an aqueous or water-based liquid foam of ethylene vinyl acetate (EVA) resin/s, preferably EVA resin/s wherein the weight relative percentages of the fractions of ethylene and vinyl acetate present as blocks in the EVA resin/s are ethylene from 43 to 63 and vinyl acetate from 37 to 57, preferably ethylene from 43 to 57 and vinyl acetate from 43 to 57, more preferably ethylene from 43 to 45 and vinyl acetate from 55 to 57, the most preferred being ethylene at 43 and vinyl acetate at 57, said liquid foam, preferably aqueous or water-based liquid foam, having:

- a viscosity not less than 50 Pa s, and/or

- density p comprised in the range from 0.1 to 0.5, preferably from 0.15 to 0.4, more preferably from 0.35 to 0.2, the most preferred being 0.278 g/cm ³ , and/or

- a gas fraction - <t> comprised in the range from 0.52 to 0.96, preferably from 0.54 to 0.88, more preferably from 0.60 to 0.80, the most preferred being 0.69, and/or

- a foam quality - F [%] comprised in the range from 52% to 96%, preferably from 54% to 88%, more preferably from 60% to 80%, the most preferred being 69%;

- a process of preparing a liquid foam of EVA resin/s, preferably an aqueous or water-based liquid foam of EVA resin/s according to the present invention, said process comprising the steps of:

A) providing a liquid dispersion of ethylene vinyl acetate (EVA) resin/s, preferably an aqueous or water-based liquid dispersion of EVA resin/s, said EVA resin/s being present between 40% and 60%, preferably between 42% and 56%, more preferably between 48% and 53%, the most preferred being 50% wt. of the dispersion,

B) subjecting to a foaming step the aqueous dispersion of the step A) by blowing thereinto a foaming agent in the form of an inert gas, such as for example nitrous oxide, CO2, nitrogen, and the like, in order to obtain the liquid foam of EVA resin/s, preferably the aqueous or water-based liquid foam of EVA resin/s, according to the present invention; preferably in said process of preparing a liquid foam of EVA resin/s, preferably an aqueous or water-based liquid foam of EVA resin/s according to the present invention, the weight relative percentages of the ethylene and vinyl acetate fractions present as blocks in the EVA resin/s are ethylene from 43 to 63 and vinyl acetate from 37 to 57, preferably ethylene from 43 to 57 and vinyl acetate from 43 to 57, more preferably ethylene from 43 to 45 and vinyl acetate from 55 to 57, the most preferred being ethylene at 43 and vinyl acetate at 57;

- a method of safeguard! ng/protecting/conserving/restoring works of art, particularly paintings on canvas, named “onsite lining”, said method comprising providing a liquid foam of ethylene vinyl acetate (EVA) resin/s, preferably an aqueous or water-based liquid foam of EVA resin/s according to the present invention, and applying as such, preferably at room temperature and/or without heat sources, preferably with a spatula, said liquid foam, preferably aqueous or waterbased liquid foam of EVA resin/s, as an adhesive in direct contact with the painted canvas, as a lining or sheath protecting the painted canvas itself; preferably in said method of safeguarding/protecting/conserving/restoring works of art, particularly paintings on canvas, named “onsite lining”, the weight relative percentages of the ethylene and vinyl acetate fractions present as blocks in the EVA resin/s are ethylene from 43 to 63 and vinyl acetate from 37 to 57, preferably ethylene from 43 to 57 and vinyl acetate from 43 to 57, more preferably ethylene from 43 to 45 and vinyl acetate from 55 to 57, the most preferred being ethylene at 43 and vinyl acetate at 57;

- kit or set for safeguarding/protecting/conserving works of art, particularly paintings on canvas, said kit or set comprising: i. a device for manufacturing a liquid foam, preferably aqueous or water-based liquid foam, of EVA resin/s according to the present invention, said device comprising: a container (a) comprising a pressurized inert gas, such as for example nitrous oxide N2O, carbon dioxide CO2, nitrogen N2, and the like, suitably connected, for example by means of an inlet check valve (a1 ), to a container (b) adapted to comprise, or which comprises, a liquid dispersion of EVA resin/s, preferably an aqueous dispersion or a water-based liquid dispersion of EVA resin/s, and adapted to blow into said dispersion as a foaming agent the pressurized inert gas coming from the container (a), forming outside said container (b), through an outlet valve (b1 ), the liquid foam, preferably aqueous or water-based liquid foam, of EVA resin/s according to the present invention; ii. a device, preferably a spatula, adapted to apply the liquid foam, preferably aqueous or water-based liquid foam, of EVA resin/s according to the present invention, on the surface of a painted canvas/fabric and/or a lining canvas/fabric; preferably in said kit or set said EVA resin/s is/are present from 40% to 60%, preferably from 42% to 56%, more preferably from 48% to 53%, the most preferred being 50% wt. of the dispersion, and/or the weight relative percentages of the ethylene and vinyl acetate fractions present as blocks in the EVA resin/s are ethylene from 43 to 63 and vinyl acetate from 37 to 57, preferably ethylene from 43 to 57 and vinyl acetate from 43 to 57, more preferably ethylene from 43 to 45 and vinyl acetate from 55 to 57, the most preferred being ethylene at 43 and vinyl acetate at 57;

- a painted canvas/fabric and/or a lining canvas/fabric which is coated by the liquid foam of EVA resin/s, preferably aqueous or water-based liquid foam of EVA resin/s, as described herein according to the present invention, or on whose surface the same is applied.

Aqueous liquid dispersions of EVA resin/s available on the market are currently employed in the restoration field as liquids, mostly as consolidants or fixatives (as referred to in the relevant technical data sheets), not as adhesives in lining processes aimed at restoring painted canvasses. Indeed, the important constraint related with the use of these water-based liquid dispersions of EVA resin/s as adhesives in the lining process for restoring paintings on canvas is closely linked to their low viscosity, obviously due to their aqueous nature, which amplifies vertical diffusion, above all in porous materials. Water can penetrate along the stratigraphic structure of the painting, which in most cases consists of different hydrophilic materials, and bring irreversible damages in terms of chemical interactions and structural resistance. In order to overcome such constraint, but in the double perspective of exploiting the adhesive properties of EVA resins on one side and developing a restoration system for paintings on canvas with the utmost environmental compatibility, the Applicant has developed, as invention of the present application, a process which modifies the viscosity of said aqueous liquid dispersions of EVA resin/s, so as to make it possible to control their wetting power and their penetrability into the canvas, in order to be able to use said EVA resins as adhesive in the lining process for restoring paintings on canvas.

The invention developed by the Applicant hence entailed the implementation of new adhesives in the form of liquid foam of EVA resin/s, adapted to be used in the lining process for the restoration of paintings on canvas, starting from liquid dispersions of EVA resin/s which are radically modified to this end in their chemical-physical properties, such as rheological properties.

According to the present invention: the abbreviation DA/DL EVA is to be construed as an aqueous liquid dispersion of EVA resin/s, as product to be found on the market, such as the one available under the name of Eva Art; the abbreviation SLA EVA is to be construed as an aqueous liquid foam of EVA resin/s according to the present invention, obtained through the process/proceeding comprising the step of foaming, according to the present invention, the aqueous liquid dispersion of EVA resin/s (DA/DL EVA)

Description of the figures

Figure 1 shows the aqueous dispersion of EVA resin/s a) in liquid form, as manufactured and available on the market under the name of Eva Art, and b) in the form of aqueous liquid foam of EVA resin/s (SLA EVA), according to the present invention, obtained after/through a process/proceeding comprising the step of foaming, according to the present invention, the aqueous dispersion of EVA resin/s in liquid form (DA/DL EVA), commercially named Eva Art.

Figure 2 a) and b) show: figure 2 a) shows the flow curve of the SLA EVA in the shear stress [Pa] vs shear rate [s’ ¹ ] diagram; figure 2 b) shows (1 ) the viscosity curve of the SLA EVA in foam form and (2) the viscosity range specified in the technical data sheet of the DA/DL EVA product between the temperatures of 25°C [ I ^-25 ] and 15°C [ I" ¹⁵ ] in the viscosity [Pa s] vs shear rate [s’ ¹ ] diagram.

In Figure 3, the diagram shows the average trend of the drying speed, namely the percentage weight loss as a function of time, expressed in hours, at a temperature T 20°C, of the aqueous dispersion of EVA resin/s in liquid form DA/DL EVA (Liquid) and of the dispersion of the aqueous liquid foam of EVA resin/s SLA EVA obtained by foaming, through the process according to the present invention, the same aqueous dispersion of EVA resin/s (Foam). The gravimetric tests were performed by applying the systems: liquid DA/DL EVA and foam SLA EVA as a thick film on a Petri plate, monitoring them for 48 hours and repeating 5 times for the Liquid (DA/DL EVA) and the Foam (SLA EVA).

Figure 4 shows the method of execution of the peel test on a sample.

Figure 5 represents in a diagram the results of the peel test as the average of five repetitions and relevant standard deviation. The used naming designates as A or M the kind of specimen: ancient of modern, respectively; as O or I the kind of lining canvas: Origam® or Ispra®, respectively; as F or L the form in which the EVA resin/s is applied between the specimen and the lining canvas: the foam of EVA resin/s according to the present invention SLA EVA (F) and the aqueous dispersion of EVA resin/s DA/DL EVA, therefore the liquid form (L); as a or b the quantity of used product: the ideal quantity of foam of EVA resin/s (a) and the ideal quantity of aqueous dispersion of EVA resin/s (b), respectively. The threshold of 280 N/m indicates the minimum acceptable value for an adhesive to be used to this end, namely for the purpose of restoration through the lining method (Phenix A, Hedley G., 1984).

Figure 6 shows the method of execution of the lap shear test on a sample. The ends are fastened to the machine for tensile testing by two clamps and a force is applied until the coupling breaks (Figure 6-a). As in the peel tests, the machine for tensile testing was provided with a screw clamp with lateral action and rectangular sides having a width of 5 cm and a height of 2.5 cm, with rubberized surface (Figure 6-b).

Figure 7 represents through a diagram the lap shear resistance values [in MPa] for a series of samples comprising as adhesive the aqueous liquid foam of EVA resin/s SLA EVA (F) according to the present invention or the aqueous liquid dispersion of EVA resin/s DA/DL EVA (L), for both samples as such and aged samples.

Figure 8 shows the IR spectra (% transmittance vs wavenumber cm ¹ ) obtained on the stratigraphic section of the specimens to which the aqueous liquid foam of EVA resin/s SLA EVA according to the present invention (AIF-a) and the aqueous dispersion of EVA resin/s DA/DL EVA as such (AIL-b) are applied as adhesive on the layer of the pictorial preparation, compared with the IR spectrum obtained from the stratigraphic section of a specimen without application of adhesive (A-0), either in the form of aqueous liquid foam of EVA resin/s or in the form of aqueous dispersion of EVA resin/s, and the standard I R spectrum of the aqueous dispersion of EVA resin/s DA/DL EVA.

Figure 9 shows the different degrees of penetration and migration of the adhesive in the specimens depending on whether it is applied as aqueous liquid foam of EVA resin/s SLA EVA (Foam), according to the present invention, or as aqueous dispersion of EVA resin/s DA/DL EVA (Liquid). In both cases the adhesive, that is both the aqueous liquid foam of EVA resin/s and the aqueous dispersion of EVA resin/s, was applied in a mixture with a red dye (Ponceau S) so that penetration into the tissue could be visible at the optical microscope. The penetration and spread level inside the images - two in the right column (relating to the application of the aqueous liquid foam of EVA resin/s SLA EVA to the specimens) and two in the left column (relating to the application of the aqueous dispersion of EVA resin/s DA/DL EVA to the specimens) - is highlighted by the length of the arrows which were purposely added in said images.

Figure 10 shows the images obtained through videomicroscopy (200x magnification) of the specimen with and without application of the lining canvas through the aqueous liquid foam of EVA resin/s SLA EVA, according to the present invention, before and after artificial aging of both the front and the back, comparing the front of a specimen without application of the lining canvas after artificial aging (A-0).

Figure 1 1 shows a specimen with lining canvas applied through the aqueous liquid foam of EVA resin/s SLA EVA, according to the present invention, through the photos of the front in visible light before (a) and after aging (e), front in UV light before (b) and after aging (f), back in visible light before (c) and after aging (g), back in UV light before (d) and after aging (h).

Detailed description of the invention and embodiments thereof

It is therefore an object of the present invention:

- a liquid foam of ethylene vinyl acetate (EVA) resin/s, preferably an aqueous or water-based liquid foam of ethylene vinyl acetate (EVA) resin/s, preferably EVA resin/s wherein the weight relative percentages of the fractions of ethylene and vinyl acetate present as blocks in the EVA resin/s are ethylene from 43 to 63 and vinyl acetate from 37 to 57, preferably ethylene from 43 to 57 and vinyl acetate from 43 to 57, more preferably ethylene from 43 to 45 and vinyl acetate from 55 to 57, the most preferred being ethylene at 43 and vinyl acetate at 57, said liquid foam, preferably aqueous or water-based liquid foam, having:

- a viscosity not less than 50 Pa s, and/or

- density p comprised in the range from 0.1 to 0.5, preferably from 0.15 to 0.4, more preferably from 0.35 to 0.2, the most preferred being 0.278 g/cm ³ , and/or

- a gas fraction - <t> comprised in the range from 0.52 to 0.96, preferably from 0.54 to 0.88, more preferably from 0.60 to 0.80, the most preferred being 0.69, and/or

- a foam quality - F [%] comprised in the range from 52% to 96%, preferably from 54% to 88%, more preferably from 60% to 80%, the most preferred being 69%.

A further preferred form of embodiment of the liquid foam of EVA resin/s, preferably aqueous or water-based liquid foam of EVA resin/s, according to the present invention, is a liquid foam, preferably aqueous or water-based liquid foam, having:

- a viscosity not less than 50 Pa s, or

- density p comprised in the range from 0.1 to 0.5, preferably from 0.15 to 0.4, more preferably from 0.35 to 0.2, the most preferred being 0.278 g/cm ³ , or

- a gas fraction - <t> comprised in the range from 0.52 to 0.96, preferably from 0.54 to 0.88, more preferably from 0.60 to 0.80, the most preferred being 0.69, or

- a foam quality - F [%] comprised in the range from 52% to 96%, preferably from 54% to 88%, more preferably from 60% to 80%, the most preferred being 69%.

A further preferred form of embodiment of the liquid foam of EVA resin/s, preferably aqueous or water-based liquid foam of EVA resin/s, according to the present invention, is a liquid foam, preferably aqueous or water-based liquid foam of ethylene vinyl acetate (EVA) resin/s, having:

- a viscosity not less than 50 Pa s, and

- density p comprised in the range from 0.1 to 0.5, preferably from 0.15 to 0.4, more preferably from 0.35 to 0.2, the most preferred being 0.278 g/cm ³ .

A further preferred form of embodiment of the liquid foam of EVA resin/s, preferably aqueous or water-based liquid foam of EVA resin/s, according to the present invention, is a liquid foam, preferably aqueous or water-based liquid foam of ethylene vinyl acetate (EVA) resin/s, having:

- a viscosity not less than 50 Pa s, and

- a gas fraction - <t> comprised in the range from 0.52 to 0.96, preferably from 0.54 to 0.88, more preferably from 0.60 to 0.80, the most preferred being 0.69.

A further preferred form of embodiment of the liquid foam of EVA resin/s, preferably aqueous or water-based liquid foam of EVA resin/s, according to the present invention, is a liquid foam, preferably aqueous or water-based liquid foam of ethylene vinyl acetate (EVA) resin/s, having:

- a viscosity not less than 50 Pa s, and

- a foam quality - F [%] comprised in the range from 52% to 96%, preferably from 54% to 88%, more preferably from 60% to 80%, the most preferred being 69%.

A further preferred form of embodiment of the liquid foam of EVA resin/s, preferably aqueous or water-based liquid foam of EVA resin/s, according to the present invention, is a liquid foam, preferably aqueous or water-based liquid foam of ethylene vinyl acetate (EVA) resin/s, having:

- density p comprised in the range from 0.1 to 0.5, preferably from 0.15 to 0.4, more preferably from 0.35 to 0.2, the most preferred being 0.278 g/cm ³ , and

- a gas fraction - <t> comprised in the range from 0.52 to 0.96, preferably from 0.54 to 0.88, more preferably from 0.60 to 0.80, the most preferred being 0.69.

A further preferred form of embodiment of the liquid foam of EVA resin/s, preferably aqueous or water-based liquid foam of EVA resin/s, according to the present invention, is a liquid foam, preferably aqueous or water-based liquid foam of ethylene vinyl acetate (EVA) resin/s, having:

- density p comprised in the range from 0.1 to 0.5, preferably from 0.15 to 0.4, more preferably from 0.35 to 0.2, the most preferred being 0.278 g/cm ³ , and

- a foam quality - F [%] comprised in the range from 52% to 96%, preferably from 54% to 88%, more preferably from 60% to 80%, the most preferred being 69%.

A further preferred form of embodiment of the liquid foam of EVA resin/s, preferably aqueous or water-based liquid foam of EVA resin/s, according to the present invention, is a liquid foam, preferably aqueous or water-based liquid foam of ethylene vinyl acetate (EVA) resin/s, having: - a gas fraction - <t> comprised in the range from 0.52 to 0.96, preferably from 0.54 to 0.88, more preferably from 0.60 to 0.80, the most preferred being 0.69, and

- a foam quality - F [%] comprised in the range from 52% to 96%, preferably from 54% to 88%, more preferably from 60% to 80%, the most preferred being 69%.

A further preferred form of embodiment of the liquid foam of EVA resin/s, preferably aqueous or water-based liquid foam of EVA resin/s, according to the present invention, is a liquid foam, preferably aqueous or water-based liquid foam of ethylene vinyl acetate (EVA) resin/s, having:

- a viscosity not less than 50 Pa s, and

- density p comprised in the range from 0.1 to 0.5, preferably from 0.15 to 0.4, more preferably from 0.35 to 0.2, the most preferred being 0.278 g/cm ³ , and

- a gas fraction - <t> comprised in the range from 0.52 to 0.96, preferably from 0.54 to 0.88, more preferably from 0.60 to 0.80, the most preferred being 0.69.

A further preferred form of embodiment of the liquid foam of EVA resin/s, preferably aqueous or water-based liquid foam of EVA resin/s, according to the present invention, is a liquid foam, preferably aqueous or water-based liquid foam of ethylene vinyl acetate (EVA) resin/s, having:

- density p comprised in the range from 0.1 to 0.5, preferably from 0.15 to 0.4, more preferably from 0.35 to 0.2, the most preferred being 0.278 g/cm ³ , and

- a gas fraction - <t> comprised in the range from 0.52 to 0.96, preferably from 0.54 to 0.88, more preferably from 0.60 to 0.80, the most preferred being 0.69, and

- a foam quality - F [%] comprised in the range from 52% to 96%, preferably from 54% to 88%, more preferably from 60% to 80%, the most preferred being 69%.

A further preferred form of embodiment of the liquid foam of EVA resin/s, preferably aqueous or water-based liquid foam of EVA resin/s, according to the present invention, is a liquid foam, preferably aqueous or water-based liquid foam of ethylene vinyl acetate (EVA) resin/s, having:

- a viscosity not less than 50 Pa s, and

- a gas fraction - <t> comprised in the range from 0.52 to 0.96, preferably from 0.54 to 0.88, more preferably from 0.60 to 0.80, the most preferred being 0.69, and

- a foam quality - F [%] comprised in the range from 52% to 96%, preferably from 54% to 88%, more preferably from 60% to 80%, the most preferred being 69%.

A further preferred form of embodiment of the liquid foam of EVA resin/s, preferably aqueous or water-based liquid foam of EVA resin/s, according to the present invention, is a liquid foam, preferably aqueous or water-based liquid foam of ethylene vinyl acetate (EVA) resin/s, having:

- a viscosity not less than 50 Pa s, and

- density p comprised in the range from 0.1 to 0.5, preferably from 0.15 to 0.4, more preferably from 0.35 to 0.2, the most preferred being 0.278 g/cm ³ , and

- a foam quality - F [%] comprised in the range from 52% to 96%, preferably from 54% to 88%, more preferably from 60% to 80%, the most preferred being 69%. A further preferred form of embodiment of the liquid foam of EVA resin/s, preferably aqueous or water-based liquid foam of EVA resin/s, according to the present invention, is a liquid foam, preferably aqueous or water-based liquid foam of ethylene vinyl acetate (EVA) resin/s, having:

- a viscosity not less than 50 Pa s, and

- density p comprised in the range from 0.1 to 0.5, preferably from 0.15 to 0.4, more preferably from 0.35 to 0.2, the most preferred being 0.278 g/cm ³ , and

- a gas fraction - <t> comprised in the range from 0.52 to 0.96, preferably from 0.54 to 0.88, more preferably from 0.60 to 0.80, the most preferred being 0.69, and

- a foam quality - F [%] comprised in the range from 52% to 96%, preferably from 54% to 88%, more preferably from 60% to 80%, the most preferred being 69%.

“Liquid foam of EVA resin/s” or “aqueous or water-based liquid foam of EVA resin/s” according to the present invention is to be construed in particular as a liquid foam as a heterogeneous system consisting of a liquid dispersing phase and a gaseous dispersed phase, that is a heterogeneous mixture wherein a gaseous phase, such as an inert gas, is dispersed in a liquid dispersing phase or liquid means, such as a liquid dispersion of EVA resin/s or an aqueous or water-based dispersion of EVA resin/s.

The liquid foam or aqueous or water-based liquid foam which is the object of the present invention is a liquid foam of ethylene vinyl acetate (EVA) resin/s, preferably an aqueous or water-based liquid foam of ethylene vinyl acetate (EVA) resin/s, having adhesive properties, which on account of its optimal chemical-physical properties of viscosity, density and gaseous fraction to liquid fraction ratio, when used in restoration through the technique of lining of paintings on canvas as adhesive layer between the support canvas and the painting canvas, provides the painting canvas with the required properties of mechanical resistance, avoiding the drawback of altering the structure of the canvas to be restored from a mechanical as well as chemical viewpoint.

This is because the liquid foam of EVA resin/s, preferably aqueous or water-based liquid foam, according to the present invention, is such as to minimise the quantity of EVA resin/s remaining applied per unit of surface of the painting canvas to be restored, although sufficient to enable the lining canvas to properly support the original canvas of the painting to be restored, as well as the quantity of water coming into contact with the canvasses, thus limiting spread and dispersion therein and hence reducing the risks of possible chemical-physical alterations of the painting itself.

As hinted above, the liquid foam of EVA resin/s or aqueous or water-based liquid foam of EVA resin/s, according to the present invention, is obtained through a physical foaming mechanism, named liquid-gas phase transition, in a simple and rapid manner, making an inert gas, such as nitrous oxide N2O, CO2, nitrogen, and the like, preferably pressurized, namely at a pressure higher than atmospheric pressure, bubble, as foaming agent, for example through a siphon, such as a kitchen siphon, in a liquid dispersion of EVA resin/s, preferably aqueous or water-based dispersion of EVA resin/s. It is therefore a further object of the present invention a process of preparing a liquid foam of EVA resin/s, preferably an aqueous or water-based liquid foam of EVA resin/s according to the present invention, said process comprising the steps of:

A) providing a liquid dispersion of ethylene vinyl acetate (EVA) resin/s, preferably an aqueous or water-based liquid dispersion of EVA resin/s, said EVA resin/s being present between 40% and 60%, preferably between 42% and 56%, more preferably between 48% and 53%, the most preferred being 50% wt. of the dispersion,

B) subjecting to a foaming step the aqueous dispersion of the step A) by blowing thereinto a foaming agent in the form of an inert gas, such as for example nitrous oxide, CO2, nitrogen, and the like, in order to obtain the liquid foam of EVA resin/s, preferably the aqueous or water-based liquid foam of EVA resin/s, according to the present invention; preferably in said process of preparing a liquid foam of EVA resin/s, preferably an aqueous or water-based liquid foam of EVA resin/s according to the present invention, the weight relative percentages of the ethylene and vinyl acetate fractions present as blocks in the EVA resin/s are ethylene from 43 to 63 and vinyl acetate from 37 to 57, preferably ethylene from 43 to 57 and vinyl acetate from 43 to 57, more preferably ethylene from 43 to 45 and vinyl acetate from 55 to 57, the most preferred being ethylene at 43 and vinyl acetate at 57.

“Liquid dispersion of EVA resin/s” according to the present invention is to be construed as a dispersion of EVA resin/s in a dispersing agent in the liquid state, said agent comprising one or more liquids, either miscible, partially miscible or immiscible with one another; “aqueous dispersion of EVA resin/s” according to the present invention is to be construed as a dispersion of EVA resin/s in water; “water-based liquid dispersion of EVA resin/s” according to the present invention is to be construed as a dispersion of EVA resin/s in a liquid dispersing agent comprising several liquids, wherein water amounts to at least 50% by weight of the dispersing agent.

The liquid foam of EVA resin/s or aqueous or water-based liquid foam of EVA resin/s, according to the present invention, can be manufactured in an effective manner using foaming devices characterized by a container (a) comprising a pressurized inert gas, suitably connected, for example by means of an inlet check valve (a1 ), to a container (b) adapted to comprise, or which comprises, the aqueous dispersion of EVA resin/s, and adapted to blow into said dispersion as a foaming agent the pressurized inert gas coming from the container (a), forming outside said container (b), through an outlet valve (b1 ), the liquid foam of EVA resin/s according to the present invention.

These are common foaming devices available on the market, such as for example:

- siphons, for example kitchen siphons or whipping siphons, wherein the aqueous dispersion of EVA resin/s, loaded into the siphon bottle, is foamed by bubbling the pressurized inert gas, coming from a special cartridge which can be replaced once it is exhausted, and loaded into the siphon through a suitable gas-in valve situated on the siphon closing cap; the foaming generates, at the outlet of the siphon cap through a gas-out extrusion valve, the liquid foam of EVA resin/s, according to the present invention, or

- purpose-made devices characterized by a tight-sealed bottle filled with the aqueous or water-based liquid dispersion of EVA resin/s and surrounded by an outer jacket vessel suitable to contain a pressurized inert gas, so that the gas is bubbled in the dispersion, foaming it and producing, outside the device itself, through an appropriate system such as a valve operated by finger pressure, the liquid foam of EVA resin/s or aqueous or water-based liquid foam of EVA resin/s, according to the present invention. Examples of said devices include spray cans loaded with the aqueous dispersion of EVA resin/s available in the inner tight-sealed bottle and with a pressurized inert gas available in the outer jacket vessel, which is also tight-sealed.

Precisely on account of its peculiar chemical-physical properties as described and claimed herein, the liquid foam of ethylene vinyl acetate (EVA) resin/resins, preferably aqueous or water-based liquid foam of ethylene vinyl acetate (EVA) resin/s, according to the present invention, proved to be an excellent means or tool usable for the purpose of safeguarding/protecting/conserving/restoring works of art, particularly for lining paintings on canvas, highlighting the achievement of those benefits and objectives which were strongly sought for and not yet obtained with the methods and materials described in prior art, such as:

• Adhesion: strong enough to withstand the characteristic stresses artistic materials are subjected to;

• No chemical reactivity;

• No structural interaction: minimum shrinking and expanding; a certain plasticity and elasticity to absorb long-term stresses;

• Compatibility: no dyeing; similarity to the restored object in pattern and appearance;

• Durability: chemical and physical stability;

• Applicability: the application techniques of the liquid foam according to the present invention do not jeopardise the object to be restored;

• Reversibility: possibility of safe removal; as well as:

- lower toxicity on account of the lack of dangerous and harmful solvents (for example toluene or other organic solvents), replacing them with as biocompatible a solvent as possible, namely water, the lack of manufacturing wastes and the removal of heat activation, which simplifies the process of safeguarding, conserving and restoring;

- obtaining an easy, effective, rapid and cost-effective coating process by monitoring the vertical diffusion of any solvent which may be present, such as water;

- removing dangerous solvents and heat activation, enhancing transparency, high stability to UV rays and avoidance of yellowing in the course of time, as well as enhancing flexibility;

- being able to perform the operation of restoring/safeguarding/conserving the work of art also on site, namely where the work of art is situated/conserved, avoiding relocation to a specialized laboratory.

It is therefore a further object of the present invention a method of safeguarding/protecting/conserving/restoring works of art, particularly paintings on canvas, named “onsite lining”, said method comprising providing a liquid foam of ethylene vinyl acetate (EVA) resin/s, preferably an aqueous or water-based liquid foam of EVA resin/s according to the present invention, and applying as such, preferably at room temperature and/or without heat sources, preferably with a spatula, said liquid foam, preferably aqueous or water-based liquid foam of EVA resin/s, as an adhesive in direct contact with the painted canvas, as a lining or sheath protecting the painted canvas itself; preferably in said method of safeguarding/protecting/conserving/restoring works of art, particularly paintings on canvas, named “onsite lining”, the weight relative percentages of the ethylene and vinyl acetate fractions present as blocks in the EVA resin/s are ethylene from 43 to 63 and vinyl acetate from 37 to 57, preferably ethylene from 43 to 57 and vinyl acetate from 43 to 57, more preferably ethylene from 43 to 45 and vinyl acetate from 55 to 57, the most preferred being ethylene at 43 and vinyl acetate at 57.

In the step of applying as such the liquid foam of ethylene vinyl acetate (EVA) resin/s, preferably aqueous or waterbased liquid foam of ethylene vinyl acetate (EVA) resin/s, according to the present invention, the spreading of the foam on the surface of the canvas to be restored is favoured by an appropriate device, preferably a spatula, thus favouring spreading with an even thickness, as well as the collapse of part of the cells of the liquid foam of ethylene vinyl acetate (EVA) resin/s, preferably aqueous or water-based liquid foam of ethylene vinyl acetate (EVA) resin/s, according to the present invention.

In order to implement the method of safeguarding/protecting/conserving/restoring works of art, particularly for lining paintings on canvas, according to the present invention, a kit or set for safeguarding/protecting/conserving works of art was implemented and is a further object of the present invention, said kit or set comprising:

I. a device for manufacturing a liquid foam, preferably aqueous or water-based liquid foam, of EVA resin/s according to the present invention, said device comprising: a container (a) comprising a pressurized inert gas, such as for example nitrous oxide N2O, CO2, nitrogen N2, and the like, suitably connected, for example by means of an inlet check valve (a1 ), to a container (b) adapted to comprise, or which comprises, a liquid dispersion of EVA resin/s, preferably an aqueous dispersion or a water-based liquid dispersion of EVA resin/s, and adapted to blow into said dispersion as a foaming agent the pressurized inert gas coming from the container (a), forming outside said container (b), through an outlet valve (b1 ), the liquid foam, preferably aqueous or water-based liquid foam, of EVA resin/s according to the present invention; ii. a device, preferably a spatula, adapted to apply the liquid foam, preferably aqueous or water-based liquid foam, of EVA resin/s according to the present invention, on the surface of a painted canvas/fabric and/or a lining canvas/fabric; preferably in said kit or set said EVA resin/s is/are present from 40% to 60%, preferably from 42% to 56%, more preferably from 48% to 53%, the most preferred being 50% wt. of the dispersion, and/or the weight relative percentages of the ethylene and vinyl acetate fractions present as blocks in the EVA resin/s are ethylene from 43 to 63 and vinyl acetate from 37 to 57, preferably ethylene from 43 to 57 and vinyl acetate from 43 to 57, more preferably ethylene from 43 to 45 and vinyl acetate from 55 to 57, the most preferred being ethylene at 43 and vinyl acetate at 57.

It is therefore a further object of the present invention a painted canvas/fabric and/or a lining canvas/fabric which is coated by the liquid foam of EVA resin/s, preferably aqueous or water-based liquid foam of EVA resin/s, as described herein according to the present invention, or on whose surface the same is applied, preferably wherein the weight relative percentages of the ethylene and vinyl acetate fractions present as blocks in the EVA resin/s are ethylene from 43 to 63 and vinyl acetate from 37 to 57, preferably ethylene from 43 to 57 and vinyl acetate from 43 to 57, more preferably ethylene from 43 to 45 and vinyl acetate from 55 to 57, the most preferred being ethylene at 43 and vinyl acetate at 57, said liquid foam, preferably aqueous or water-based liquid foam, having:

- a viscosity not less than 50 Pa s, and/or

- density p comprised in the range from 0.1 to 0.5, preferably from 0.15 to 0.4, more preferably from 0.35 to 0.2, the most preferred being 0.278 g/cm ³ , and/or

- a gas fraction - <t> comprised in the range from 0.52 to 0.96, preferably from 0.54 to 0.88, more preferably from 0.60 to 0.80, the most preferred being 0.69, and/or

- a foam quality - F [%] comprised in the range from 52% to 96%, preferably from 54% to 88%, more preferably from 60% to 80%, the most preferred being 69%.

The chemical-physical properties of the liquid foam of EVA resin/s, preferably aqueous or water-based foam of EVA resin/s, according to the present invention, such as:

- the rheological behaviour: viscosity

- gravimetric tests: weight loss and drying speed were studied and measured, comparing the relevant values with those obtained for the commercially available aqueous dispersion of EVA resin/s, from which the liquid foam of EVA resin, preferably aqueous or water-based foam of EVA resin/s, according to the present invention, is obtained by blowing the foaming/blowing agent.

Furthermore, the properties of:

- foam density

- foam quality of the liquid foam of EVA resin/s, preferably aqueous or water-based foam of EVA resin/s, according to the present invention, were analyzed and measured.

On the specimens of painted canvasses subjected to the lining process, either the liquid dispersion of EVA resin/s or the liquid foam of EVA resin/s, preferably aqueous or water-based foam of EVA resin/s, according to the present invention, was interposed between the painted canvas and the lining canvas, in order to perform side-by-side comparative tests, and the mechanical adhesion tests (peel and lap shear tests) were accomplished for the two canvasses.

Also analyzed, always on specimens of painted canvasses subjected to the above lining process, were colour and surface alterations (multispectral, spectrophotometry, microscopy), the penetration and migration of the adhesive (UVF marker test on cross section), the chemical interaction with painting materials and deterioration after the artificial aging process (micro-FTIR on cross section); further surveys and analyses were performed, such as: visible light photography, UV light photography, colorimetry and spectrophotometry, microscopy, stereomicroscopy, USB videomicroscopy, optical microscopy, Fourier Transform Infrared Spectroscopy, transmission mode, ATR mode, ATR- FTIR microscopy, both in the case of use of the liquid dispersion of EVA resin/s and in the case of use of the liquid foam of EVA resin/s, preferably aqueous or water-based foam of EVA resin/s, according to the present invention, obtained after foaming said liquid dispersions of EVA resin/s.

Rheological tests - Viscosity

The tests were performed on the foam adhesive in order to assess the viscosity variation compared with the liquid adhesive. An Anton Paar MGR 301 rotational rheometer was used. The measurement system chosen for the rheometer is a plate-dish configuration through a knurled surface, on which the upper plate has no smooth surface, but features a series of teeth increasing the contact surface between the plate and the analyzed fluid; it is useful for low-viscosity liquids. The diameter of the upper plate is 25 mm. The sample was applied directly on the lower fixed plate, which is provided with a heating system allowing a certain temperature, either stable or variable during the tests, to be selected. The temperature was fixed at 25 °C throughout the test: this is approximately the temperature of application of the adhesive for the implementation of the mock-ups or prototypes (that is about room temperature). The distance between the two plates was fixed at 1 mm, a logarithmic ramp of shear rate was set from 0.001 to 10 s ¹ , entailing a programmed rise of the flow rate of the material and measurement of the stress arising from the resistance to flow of the material (see for example Mezger, T., 201 1 , The Rheology Handbook. For users of rotational and oscillatory rheometers, Vincentz Network; Macoscko C. W., 1994, Rheology. Principles, measurements and applications, VCH Publishers, New York; Annesini M. C., 2009, Fenomeni di trasporto. Fondamenti e applicazioni).

Gravimetric tests: weight loss and drying speed

The method is based on water evaporation, in order to assess the amount of solid solute in a solution. The fundamental device for this kind of tests is the electronic analytical balance, which measures more than 0.1 mg of mass, accordingly detecting even the most imperceptible weight variations. The need to use these methods is due to the assessment of the following parameters. Firstly, the difference of the volatile portion (solvent) which is present in the adhesive applied as liquid and as form of foam is to be assessed and the mass of solid phase remaining after its complete drying is to be calculated. Afterwards, a second procedure was accomplished to assess the drying speed. The last observation was made to assess the volume of the foam in relation with its density and mass (see for example: The analyte is the component of a system to be analyzed. Nic M., Hovorka L., Jirat J., Kosata, B. and Znamenacek J., 2005, op. cit., page 83 ; Skoog D. A., West D. M., Holler F. J., Crouch S. R., 2014, Fundamentals of Analytical Chemistry, 9th ed., Cengage Learning, page 280 ; Skoog D. A., West D. M., Holler F. J., Crouch S. R., 2014, op. cit., page 19. Nic M., Hovorka L., Jirat J., Kosata, B. and Znamenacek J., 2005, op. cit., page 390).

Weight loss

In order to quantify exactly the weight loss after complete drying, a layer of the liquid product was applied on Petri plate and left to dry for 24 hours, the laboratory air having a temperature of about 20° C.

Drying speed

In order to analyze any difference in drying speed, the adhesive was applied on Petri plates, covering the overall surface and implementing a moderately thick layer, measuring approximately a few millimetres, and monitoring them for 48 hours with several measuring steps. The average percentages of weight loss for each step were plotted into a line chart in order to asses an average trend of the drying speed.

ABOUT THE PROCESS OF ADHESIVE FILM FORMATION

It is known that the drying process of an adhesive or a coating, the so-called process of film formation, depends on plenty of complex factors, depending on the type and the chemical composition of the adhesive itself. Two factors which may influence the process of film formation, in the case of an adhesive formed by polymer in a solution, are the vapour pressure of the solvent, which is the first to act on its evaporation, and the speed at which the solvent molecules spread on the film surface. In the case at issue, the solvent is the same for both applications (namely water, which has a vapour pressure of 2,339 kPa at 20 °C), and the vapour pressure cannot influence film formation; this means that the formation of the foam and its structure and internal morphology probably entail a higher spreading speed of the solvent molecules on the film surface, determining a more rapid formation of the latter.

Foam density

A beaker was filled with water and weighed, to obtain the exact volume of the liquid contained therein. The water density at 25 °C is about 0.997 g / cm ³ , accordingly about 1 g of water corresponds to 1 cm ³ of occupied volume. By filling the same beaker, that is the same volume, with the adhesive liquid foam and weighing the same, the foam density is obtained and can be compared with the density of the adhesive in liquid form, specified in the relevant technical data sheet.

ABOUT FOAM QUALITY

In general, the parameter to quantify the quality of a foam is Foam quality (F), expressed in %; for optimal foam quality, said parameter is comprised in the percentage range: 52%< l~<96%. The foam density is far lower than the liquid density on account of the presence of the gas fraction. As a matter of fact, the volume of the foam (Vf) is the sum of the volume of the liquid (VI) and the volume of the gas (Vg) composing it. In order to calculate the volume occupied by the gas which is present in the foam, the volume occupied by the liquid, or liquid component of the foam, must be obtained from the foam mass: by multiplying the weighed mass of the foam by the known density of the liquid from which the foam is obtained, in this case the density of the liquid dispersion of EVA resin, the density of the adhesive, what is obtained is the extent to which the liquid (the volume of the liquid portion of the foam) contributes to the volume of the foam, the remaining volume being ascribable to the gas, namely being the volume of the gas composing the foam. Knowing the gas volume, the gas fraction (<t>): (<t>) = Vg/Vf and the foam quality (F) : F = Vg/Vf x100 are calculated (see for example: Drenckhan W., Saint-Jaimes A., 2015, The Science of foaming, in “Advances in colloid and interface science », CCXXII, pages 228-259 ; Belyadi H, Fathi E., Belaydi F., 2017, Hydraulic Fracturing Fluid Systems, in Hydraulic Fracturing in Unconventional Reservoirs: Theories, Operations, and Economic Analysis, page 56).

Accordingly, as stated above, Vf is meant to designate the volume of the liquid foam of EVA resin obtained according to the present invention, VI is meant to designate the volume of the liquid portion/fraction/component of the liquid foam of EVA resin obtained according to the present invention and Vg is meant to designate the volume of the gas portion/fraction/component of the liquid foam of EVA resin obtained according to the present invention; also, in order to obtain the VI and Vg values, based on the measured Vf value of the liquid foam of EVA resin according to the present invention, the VI value is calculated by multiplying the density of the liquid, preferably aqueous, dispersion of EVA resin, such as for example EVA ART, by the weighed mass of the EVA liquid foam obtained according to the present invention, whereas Vg value, the volume of the gas component of the EVA liquid foam, is the difference between the measured Vf value and the VI value obtained as explained.

The gas fraction and the foam quality are closely related and are two of the plentiful factors that matter to understand the stability and the viscosity of the foam according to the present invention, obtained through the proceeding comprising the step of foaming the liquid EVA resin, according to the present invention. A foam having a quality of less than 52% shows gas bubbles which are not in contact with one another, with low viscosity, considering the high amount of free fluid in the system, and accordingly with a relative low stability thereof; at percentages between 52% and 96%, the bubbles are in contact with one another and viscosity and stability increase; at more than 96%, the foam degenerates and viscosity is lost (see for example Belyadi H, Fathi E., Belaydi F., 2017, op. cit. , page 57).

Peel test

In order to assess the resistance of the adhesive bond to peeling, the peel test is the most practical test to be performed. The purpose is to separate the two surfaces (namely the painting canvas and the coating canvas) joined by the adhesive, by applying a certain force to one of the surfaces, so as to assess the adhesion strength of the different systems, measuring the force required for peeling them apart. The last parameter calculated in the resistance to peeling (n), namely the average load per unit of width of junction line required to progressively separate a flexible element from a rigid element or from another flexible element, is measured as N/m. To this end, a machine for tensile testing, a device used to determine the response of a material to variable deformations, assessing the tensile properties thereof, was used (see for example: Ebnesajjad S., Landrock A., 2008, Testing of adhesive bonds, in Adhesive technology Handbook, pages 275-276; Davis J.R., 2004, Introduction to tensile testing, in Tensile Testing, ASM International, pages 1 -12; Berger G. A., 1972b, op.cit., page 181 ).

Lap shear test

The purpose is to assess the average shear stress (T), which is the applied load divided by the glued overlapped area (see for example Ebnesajjad S., Landrock A., 2008, op. cit., pages 274-275).

Visible light photography

VIS images allow an object to be documented in its real colour. The importance of photographs in visible light (wave length 400-700 nm) should not be underrated, since they are fundamental for several reasons. Firstly, because they allow documenting the interventions an object is subjected to, such as a restoration intervention, in all its steps, showing how the initial situation was and how it evolved after a more or less invasive treatment. Without a good-quality, high- definition photograph, it would be impossible to collect a documentation witnessing to the conservation history of an object having artistic, social, cultural and historic importance, which would be a significant gap in the field of cultural assets. A second reason is that a high-definition image in visible light warrants a macroscopic morphological analysis of the smallest details (namely brushstrokes in paintings) which would not otherwise be visible to the naked eye.

For these two reasons, it was necessary to take photographs of the mock-ups to document the initial situation before the application of the adhesive and the lining canvas, after the coating operation and after the aging process, in order to assess whether colour changes occur or other information need documenting. The photographs were taken with a light source positioned at 45° compared to the central axis of the specimens, in order to avoid the reflection of the light and obtain images with diffused light on the whole surface of the mock-ups. The photographs were taken on the front and on the back side, using a Nikon D800 (36 Mp) camera on which an AF MICRO Nikkor 60 mm 1 : 2.8 was mounted, with a Hama UV&IR CUT filter, shutter speed 1 / 50s, ISO 100 and f / 18 opening (see for example Stuart B., 2007, Analytical techniques in materials conservation, Wiley ed., page 72).

UV light photography

In the case at issue, UV fluorescence imaging was used to monitor any variations in the response of the mock-up surface, on the front and back side thereof, before the lining process, after said process and after the application of aging according to the protocol. The photographs were taken using a Nikon D800 (36 Mp) camera on which an AF MICRO Nikkor 60 mm 1 : 2.8 was mounted, with a Hama UV&IR CUT filter, shutter speed 30s, ISO 100 and f / 1 1 opening. Mada Tec ultraviolet sources were used (with 365 nm emissions) [see for example Pinna D., Galeotti M., Mazzeo R. (edited by), 2011 , Esame scientifico per I'indagine sui dipinti. Un manuale per i restauratori-restauratori, pages 204-205].

Colorimetry and spectrophotometry

Colorimetry is applied to describe, qualify and quantify human colour perception. A Minolta CM-2600d photoelectric spectrophotometer and colorimeter was used; the analyses were performed in a spectrum range from 400 to 700 nm, with three continuous scans and a small collimator (SAV) having a 3 mm diameter to increase resolution. Several measurements were made for each specimen, nine on the front and nine on the back side, to cover the possible larger surface and obtain a statistically valid value. Colorimetric measurements were performed before the lining process, then repeated after it and after the application of the aging protocol, over the same areas. To this end, a transparent mask was made, nine holes were created thereon and the mask was placed on the specimens, fixed in the same position and orientation. The obtained results were lastly compared with one another to assess whether colour changes occur, the entity and extent thereof, whether they are caused by the adhesive nature or by the natural degradation of materials, without interactions with the product, and whether at the end they fall within an acceptable tolerance range (see for example Pinna D., Galeotti M., Mazzeo R. (edited by), 201 1 , op. cit., pages 143-146).

Microscopy

A series of microscopic observations was performed, with different instruments and on different samples, in order to obtain the morphological description thereof. A first part of them was performed on pure materials, with particular attention to fibres and canvas tissues, used to classify and describe each single material used for the implementation of the mock-ups and eventually create brief technical data sheets summarizing their morphological features. A second part of microscopic observations was performed directly on the mock-ups, before the coating process, after the application of the adhesive and after the aging protocol, to monitor the morphological changes at each step. Hereinafter is a brief description of the microscopes in use, with their technical specifications.

Stereomicroscopy

An Olimpus BX 51 M stereomicroscope was used, with magnification from 7x to 45x, provided with an Infinity 1 Lumenera camera to display and transfer images directly onto the computer (see for example Pinna D., Galeotti M., Mazzeo R. (edited by), 2011 , op. cit., pages 206-207).

USB videomicroscopy A USB video or digital microscope is a variation of the traditional optical microscope, using optics and a digital camera to send the image of the samples directly to a monitor through a computer software. It is a compact and easy-to-use handheld device provided with its own incorporated LED light source; the image is focused on the digital circuit, hence lenses for the human eye are not present and the whole system is designed for the monitor image. It consists of an internal CCD camera, a light source which is often adjustable directly from software and a system of lenses to obtain different magnifications, in general from 10x to 250x, up to 400x even for the most expensive models. Resolution depends on the visual field of the objective used with the camera. The quality of the end image depends on the camera or on the megapixels available with the system, ranging from 1 .3 MP, 2 MP, 5 MP and more, as well as on the skills of the operator and the quality of lighting. The used device is a Dino-Lite USB handheld digital microscope, connected to a computer through the DinoCapture 2.0 software, to observe the surface of the mock-ups during the test steps, with 50x and 220x magnifications [see for example: Pinna D., Galeotti M., Mazzeo R. (edited by), 201 1 , op. cit. , pages 179- 183; Stuart B., 2007, Analytical techniques in materials conservation, Wiley ed., page 81 ].

Optical microscopy

Optical microscopy is an analytical technique allowing the morphological study of the surveyed samples, on account of the magnification thereof by several objectives. Both visible and ultraviolet light is used. The UV source enables to locate and identify more effectively organic materials, because they are excited by the UV light and show fluorescence effects which might be characteristic of specific materials. UV light and fluorescence make it possible to study the degree of penetration and migration of the adhesive through pictorial layers, using an unsharp fluorescent UV marker: some samples are taken from each mock-up before the lining process, after the application of adhesive canvasses and coating canvasses and after artificial aging; they are immersed in polyester resin and gradually polished with abrasive paper until cross-sections are achieved. The used instrumentation consists of an Olympus BX51 M microscope, with fixed 10x-magnification eyepieces and a set of objectives with 5x, 10x, 20x, 50x e 100x magnification; it is provided with a U25LBD Olympus filter for white light and an Olympus U-RFL-T UV source; for the acquisition of images, a PrimoPlus software connected to the microscope through an Olympus DP70 scanner was used (see for example Chelazzi D., Chevalier A., Pizzorusso, G., Giorgi R., Menu M., Baglioni P., 2014, Characterization and degradation of poly(vinyl-acetate)-based adhesives for canvas paintings, in “Polymer degradation and stability», CVII, pages 314- 320).

Fourier Transform Infrared Spectroscopy

Infrared spectroscopy allows the chemical characterization of both organic and inorganic materials. The concerned infrared region is comprised in the medium infrared fraction (MIR), ranging from 4,000 to 450 cm ¹ ; the analysis can be performed in reflection or transmission mode, depending on many factors such as the nature and the preparation of the samples.

Transmission mode

The used instrumentation is a Broker Tensor 27, with 4 cm' ¹ resolution and 64 scans. The powder samples (namely inorganic pigments) were dispersed and pressed in a KBr pellet and introduced into the sample holder; 1.0 mg of samples were used for 100 mg of KBr. The samples in liquid form (coating oil, EVA adhesive and rabbit skin) are applied as a thin film between two KBr pellets.

A TR mode

Attenuated total reflection (ATR) spectroscopy uses the phenomenon of total internal reflection. The used instrumentation is a Broker Tensor 27, with diamond crystal, 4 cm' ¹ resolution and 64 scans. This method was useful to characterize the samples taken from the canvasses used to implement the mock-ups and linen canvasses and to study the chemical composition thereof.

ATR-FTIR microscope

The combination of ATR infrared sprectroscopy and a microscope allows very small samples to be studied. In the field of cultural assets, it is applied to analyze the stratigraphic characterization of organic materials (namely medium or pool) of the cross section obtained from painted samples. Some samples were taken from every mock-up, before the coating process, after the application of adhesive and coating canvasses and after artificial aging. The samples were incorporated between two KBr pellets and the obtained pellets were in their turn embedded in polyester resin. After polishing with abrasive papers having different particle sizes, cross sections were implemented and analyzed to find out the degree of penetration of the adhesive into the specimen layers and the formation of by-products from the interaction between the adhesive and the materials composing the specimens which might be potentially dangerous for the paintings. The analysis was performed with a Niolet iNTMI OMX microscope coupled with a mercury-cadmium-tellurium (MCT) detector, cooled with liquid nitrogen to reduce the thermally originated electric noise. A tapered germanium crystal having a 300 pm diameter was used. Spectra were collected in a range from 4,000 to 675 cm ¹ , with a spectral resolution of 4 cm ¹ . For each sample, 3 to 5 line maps were acquired along the whole stratigraphic structure, with measurement steps of 20 pm (see for example Pinna D., Galeotti M., Mazzeo R. (edited by), 201 1 , op. cit., pages 151 -156; Stuart B., 2007, op. cit., pages 1 10-1 18; Derrick M. R., Stulik D. C., Landry J. M., 1999, Infrared Spectroscopy in Conservation Science, Los Angeles, pages 5- 15).

GENERAL CONSIDERATIONS

The liquid foam of EVA resin/s according to the present invention, obtained from the liquid dispersion of EVA resin/s, has several advantages comparted to the application of the latter. Firstly, it has an initial viscosity which is higher than that of the liquid dispersion of EVA resin/s, as shown by the rheological tests performed thereon; however, since this decreases as a result of the stress to which it is subjected during spreading, it provides good processability and excellent spreadability on the canvas. Said higher viscosity of the liquid foam of EVA resin/s according to the present invention, combined with the volume increase which allows a small amount of product to be used to obtain at any rate a uniform coating of the same surface, limits the penetration of the EVA resin with its aqueous content along the stratigraphy, as shown by the observations made on the “cross sections” of the specimens with the addition of dye before application. Also the FT-IR analysis on “cross stratigraphies” confirms this assumption, because the only samples in whose inner layers traces of EVA resin/s were found are those on which the application was performed with the liquid dispersion of EVA resin/s. In addition, the gravimetric tests allowed the stability and compactness of the liquid foam of EVA resin/s according to the present invention to be ascertained by determining the “foam stability” F [%] parameter, which shows values around 69% and which, falling within the 52%-96% range proposed by scientific literature as optimal, is reflected in a compact and stable liquid foam of EVA resin/s, which does not rapidly collapse, but preserves its structure and volume for the necessary time. A foam having a quality of less than 52% actually shows gas bubbles which are not in contact with one another, with low viscosity on account of the high amount of free fluid in the system, and accordingly with a relative low stability; at more than 96%, the foam degenerates and viscosity is lost. The optimal value of the quality of the liquid foam of EVA resin/s determines the stability of the liquid foam of EVA resin/s, which in the solute phase does not separate from the solvent and, as already hinted, has a high viscosity value. Accordingly, not only is the liquid foam of EVA resin/s stable, it also has a more rapid drying than the liquid dispersion of EVA resin/s, as proved by the drying tests. This higher drying speed, although not excessive compared to that of the liquid dispersion of EVA resin/s and to the processes of application which are of interest for the restoration of works of art, certainly influences the penetrability of the EVA resin/s in the form of liquid foam according to the present invention: since it dries more rapidly, it penetrates the substrate to a lesser extent, because the solute transport is stopped before the evaporation of the solvent. The tests performed on the specimens gave evidence that the properties of resistance and effectiveness as adhesive of the liquid foam of EVA resin/s according to the present invention are not negatively affected by the change of form from liquid dispersion of EVA resin/s to liquid foam of EVA resin/s.

However, considering the important reduction of the used amount, the results of the mechanical tests show that the application of the liquid foam of EVA resin/s achieves and exceeds the minimum acceptable value of 280 N/m for the peel strength, as defined in the literature referred to for the adhesives used in the coating operations, and also exceeds the minimum recommended value of 0.240 MPa for lap shear strength. Artificial aging does not cause loss of resistance to adhesion and does not impair the mechanical performance of the liquid foam of EVA resin/s according to the present invention as adhesive, since it always remains above said values.

As to colour alteration, in compliance with the performed colour tests, as shown in Table 4, both on the front and on the back of the specimens, it is shown that the front of the samples is not affected by the application of the liquid foam of EVA resin/s according to the present invention as adhesive, because the values of SCI AE*(ab) registered after artificial aging appear to be the same also for untreated samples.

Table 5 lists the SCI AE*(ab) values obtained from the colorimetric analyses by comparing the front of the specimens without adhesive and with adhesive (Tq-Ad), the front of the specimens with adhesive before and after artificial aging (Pre-post), the front of the specimens with adhesive after aging and the front of the specimens without adhesive after artificial aging (Ad(ag)-post) and the back of the specimens with adhesive before and after artificial aging (Pre-post). The abbreviation DA/DL EVA is meant to designate an aqueous dispersion of EVA resin/s, whereas the abbreviation SLA EVA is meant to designate an aqueous liquid foam of EVA resin/s according to the present invention and as described according to the present invention. A value below 3 indicates the absence of colour variation, a value between 3 and 6 indicates a perceptible but acceptable colour variation, a value above 6 indicates an alarming colour variation (see Hardeberg, 2001 ).

From the data of Table 5 it can be assumed that the adhesive layer does not directly cause the colour alteration of the used materials and is independent from the kind of used application as well as from the amount of applied product.

By contrast, the surface of the back side is subject to a more or less perceptible degree of variation and the amount of adhesive as well contributes to said alteration: the higher the amount of applied adhesive, the more perceptible the alteration.

The investigation about the degradation of the product through micro FT-IR analysis (see Figure 8) does not highlight the formation of particular by-products, since the acetic acid, which directly alters the EVA polymer, impairing the stability and technical features thereof, on account of vinyl deacetylation is not present in an appreciable amount according to the techniques in use.

ADVANTAGES

The liquid foam of ethylene vinyl acetate (EVA) resin/s, preferably according to the present invention, the relevant process of preparation and the aim of its use within the framework of safeguarding/protecting/conserving/restoring works of art, particularly for lining paintings on canvas, are characterized by several technical advantages, such as:

• the foaming step involves the formation of stable liquid foams having an optimal value of foam quality (68%), characteristic of high-viscosity foams;

• the increase of viscosity is sufficient to limit the penetration of the liquid foam into the substrate, but not so high as to impair the diffusion thereof;

• penetration is limited by the high viscosity and by the higher drying speed;

• the volume increase, due to the presence of gas which reduces the density of the liquid foam, allows a smaller amount of material up to about 90% to be used, preserving an optimal mechanical resistance;

• the liquid foam is not subjected to thermal aging and also the high amount of humidity combined with heating affects only moderately mechanical resistance.

Once the problem of vertical diffusion due to the low viscosity of these liquid foams, preferably water-based liquid foams, is solved, the main commercial benefit of proposing this coating method with high-viscosity liquid foams of EVA resin/s is related to:

• the green nature of these liquid foams, wherein attention for the environmental impact is combined with the protection of the operator’s health. The advantages of using an aqueous dispersion are actually connected to the lower toxicity on account of the lack of dangerous solvents and the lack of manufacturing wastes.

• operational simplification compared to other products on the market. The application does not need a step of heat activation which, in addition to being more complex, requires the use of specific devices (vacuum hot table, ironing and so on) and might expose the painting to thermal stresses.

• painting conservation. The use of these liquid foams warrants good transparency, resistance to UV rays and yellowing, high flexibility, according to the principle of minimum intervention respecting the work of art.

EXPERIMENTAL PART

As an example of the liquid foam of EVA resin/s according to the present invention, an aqueous liquid foam of EVA resin/s was prepared by applying the process of preparation of said aqueous liquid foam of EVA resin/s according to the present invention, starting from an aqueous dispersion of EVA resin/s found on the market under the name of Eva Art, studying and measuring the chemical-physical properties of the aqueous liquid foam of EVA resin/s: the rheological behaviour (viscosity), gravimetric tests (weight loss and drying speed), foam density and foam quality, comparing the values with the ones obtained for the aqueous dispersion of EVA resin/s found on the market, as well as performing comparison tests between said aqueous liquid foam of EVA resin/s and the aqueous dispersion of EVA resin/s found on the market, when used as adhesive layers in the method of safeguarding/protecting/conserving/restoring works of art through the technique of lining paintings on canvas, assessing mechanical properties and alteration of colour of the pictorial canvasses thus obtained.

PREPARATION

Aqueous liquid foam of EVA resin/s EVA: SLA EVA

The aqueous dispersion of EVA resin/s (DA/DL EVA) marketed under the name of Eva Art, having the appearance of a white milky liquid, with Density 1.1 Kg/dm ³ , pH: 6.0, Viscosity: 1 ,500 - 2,500 mPa s, wherein in the EVA resin the weight relative percentages of the fractions of ethylene and vinyl acetate present are 43 to 57, is loaded into a kitchen siphon and subjected to the foaming step according to the process according to the present invention, by blowing into said aqueous dispersion of EVA resin some nitrous oxide, operating under pressure (about 5 bar) with a consumption of 32 g of N2O per litre of aqueous dispersion of EVA resin, commercially named Eva Art.

Rheological tests - Viscosity

The tests were performed on the aqueous liquid foam of EVA resin/s: SLA EVA according to the present invention, as described in the paragraph PREPARATION, in order to assess the viscosity variation compared with that of the aqueous dispersion of EVA resin/s marketed under the name of Eva Art, as described in the paragraph PREPARATION, from which the aqueous liquid foam SLA EVA derives.

To this end, an Anton Paar MGR 301 rotational rheometer was used.

The measurement system chosen for the rheometer is a plate-dish configuration through a knurled surface, on which the upper plate has no smooth surface, but features a series of teeth increasing the contact surface between the plate and the analyzed fluid; it is useful for low-viscosity liquids. The diameter of the upper plate is 25 mm. The sample was applied directly on the lower fixed plate, which is provided with a heating system allowing a certain temperature, either stable or variable during the tests, to be selected.

The temperature was fixed at 25 °C throughout the test: this is approximately the temperature of application of the adhesive for the implementation of the mock-ups (that is about room temperature).

The distance between the two plates was fixed at 1 mm, a logarithmic ramp of shear rate was set from 0.001 to 10 s’ ¹ , entailing a programmed rise of the flow rate of the material and measurement of the stress arising from the resistance to flow of the material.

The collected data are exemplified in Figure 2 a) and b) wherein, figure 2 a) shows the flow curve of the SLA EVA in the shear stress [Pa] vs shear rate [s ¹ ] diagram; figure 2 b) shows (1 ) the viscosity curve of the SLA EVA in foam form and (2) the viscosity range specified in the technical data sheet of the DA/DL EVA product between the temperatures of 25°C [ I ^-25 ] and 15°C [ I" ¹⁵ ] in the viscosity [Pa s] vs shear rate [s ¹ ] diagram.

The rheological tests on the SLA EVA foam proved that this has a considerably higher viscosity than the DA/DL EVA liquid, when subjected to low stresses.

This trend of viscosity is due to the natural behaviour of a liquid foam: at first it is compact with a large amount of gas and a high viscosity on account of its inherent gel structure; then, when it is handled and stress rises, the gas bubbles start to break and collapse until they revert to resembling the initial liquid, while viscosity decreases.

Gravimetric tests: weight loss and drying speed of SLA EVA foam vs DA/DL EVA liquid system Weight loss

In order to exactly quantify the percentage of weight loss after complete drying, a layer of liquid product DA/DL EVA was applied onto the Petri plates and left to dry for 24 hours, the laboratory air being at a temperature of about 20 °C. The proceeding was repeated five times in order to ascertain the repeatability and reliability of the test.

The specimens were weighed by means of an electric analytical balance after 3 (t1 ), 20 (t2) and 24 (t3) hours and the percentage of weight loss at each measurement was calculated based on the average weight loss of the five measurements. In this manner, the solute content, namely water, and the mass of solid phase remaining left were obtained.

The same was done with the product after foaming, namely with the SLA EVA foam, in order to compare the weight loss in the two application methods, ascertaining whether the SLA EVA foam contains the same amount of water or whether a portion thereof evaporates with the formation of the liquid foam of EVA resin/s according to the present invention. The results are summarized in Table 1 .

Table 1 : Weight loss percentage in the case of the DA/DL EVA liquid and in the case of the SLA EVA foam measured after t1 (3h), t2 (20h) and t3 (24h) from the application on Petri plate and left to dry in the laboratory at about 20 °C.

As can be remarked, the system loses about the same weight, accordingly the same solvent amount, after complete drying both in the case of the DA/DL EVA liquid and in the case of the SLA EVA foam, with a difference of 1 % which is practically insignificant. Therefore, also the mass percentage of solid phase in both the compared systems is similar, amounting to 57% in the DA/DL EVA liquid and to 56% in the SLA EVA foam.

Drying speed

In order to analyze any difference in the drying speed of the two systems, the DA/DL EVA liquid and the SLA EVA foam, said systems were separately applied on Petri plates, covering the overall surface thereof and implementing a moderately thick layer, measuring approximately a few millimetres, and monitoring them for 48 hours with several measuring steps. The average percentages of weight loss for each step were plotted into a chart, shown in figure 3, in order to asses an average trend of the drying speed. The difference between the two systems is plain to see, particularly regarding the initial drying speed: after one hour from the application, the adhesive system of the SLA EVA foam loses about 38% of weight, whereas the system of the DA/DL EVA liquid loses 33% of weight; after two hours, the two systems start to have a similar loss of mass, amounting to 41% and 42%, respectively, for the SAL EVA foam and for the DA/DL EVA liquid, until complete drying was achieved with about 45% and 46% of loss, respectively, after 24 hours, remaining more or less close to these values.

Density of the SLA EVA foam

A beaker was filled with water and weighed, to obtain the exact volume of the liquid contained therein. The water density at 25 °C is about 0.997 g / cm ³ , accordingly about 1 g of water corresponds to 1 cm ³ of occupied volume. By filling the same beaker, that is the same volume, with the adhesive aqueous liquid foam of EVA resin/s (SLA EVA) and weighing the same, the SLA EVA foam density is obtained and can be compared with the density of the adhesive in liquid form DA/DL EVA, specified in the relevant technical data sheet. The proceeding was repeated five times in order to ascertain the repeatability and reliability of the test. The average results listed in Table 2 show that the density of the SLA EVA foam system drastically decreases compared with the density of the DA/DL EVA liquid system: from 1.1 g / cm ³ to 0.278 g / cm ³ .

Table 2: Density (p) calculated as mass (m) vs volume (V) ratio for the SLA EVA foam system and the DA/DL EVA liquid system. (*) The density of the DA/DL EVA liquid system is specified by the manufacturer in the technical data sheet (https://www.ctseurope.com/scheda-prodotto.php?id=3987).

The listed average results show that the density of the SLA EVA system, obtained after foaming the DA/DL EVA liquid system, drastically decreases.

Quality of the SLA EVA foam: Quality Foam - r [%] and Gas fraction - <D

The value of the parameters for assessing the quality of the SLA EVA foam according to the present invention are listed in Table 3.

Table 3: SLA EVA foam quality.

As can be seen, the SLA EVA foam obtained in the case at issue has a quality amounting to 69%, which means a compact and stable liquid foam which does not rapidly collapse, but preserves its structure and volume.

Application of the SLA EVA foam and of the DA/DL EVA liquid as adhesive layer, comparison of their characterization

A series of comparative tests were made between the aqueous liquid foam of EVA resin/s, according to the present invention, SLA EVA, and the aqueous liquid dispersion of EVA resin/s, when applied as adhesive layer between the painted canvas and the lining canvas, in the restoration and conservation proceeding called, in fact, lining, ascertaining the mechanical properties of adhesive force thereof, able to comply with the minimum acceptable value of 280 N/m for an adhesive to be used for restoration through the lining technique, also ascertaining, with regard to said adhesive layers, the alterations of the colour and of the surface of the treated canvasses, the penetration and migration of the adhesive, the chemical interaction with the painting materials and deterioration after the process of artificial aging.

The SLA EVA foam is applied by means of a spatula so as to be distributed evenly on all the contact surface between the canvasses to be overlapped; the same is done with the DA/DL EVA liquid system, by means of a brush, on as many pairs of canvasses. The amount of applied adhesive, both in the case of the SLA EVA foam and in the case of the DA/DL EVA liquid, also referred to as ideal quantity, is the amount of adhesive required and sufficient to coat the whole surface of the samples implemented by achieving the total adhesion of the lining canvas to the original painted canvas: that means the quantity, in mg, of adhesive per surface unit, in mm ² , of the canvasses, required and sufficient to have the coupled surfaces of said canvasses adhere completely to each other.

Mechanical adhesion tests

On specimens of painted canvasses subjected to the lining process, the mechanical adhesion tests (peel e lap shear tests) were made by interposing between the painted canvas and the lining canvas either the aqueous liquid dispersion of EVA resin/s DA/DL EVA or the aqueous liquid foam of EVA resin/s SLA EVA, according to the present invention, so as to perform side-by-side comparative tests.

Peel test

In order to assess the resistance of the adhesive bond to peeling, the peel test is the most practical test to be performed. The purpose is to separate the two surfaces (namely the painting canvas and the coating canvas) joined by the adhesive, by applying a certain force to one of the surfaces, so as to assess the adhesion strength of the different systems, measuring the force required for peeling them apart. The last parameter calculated in the resistance to peeling (n), namely the average load per unit of width of junction line required to progressively separate a flexible element from a rigid element or from another flexible element, is measured as N/m. To this end, a machine for tensile testing, a device used to determine the response of a material to variable deformations, assessing the tensile properties thereof, was used (figure 4). The specimens are formed by canvas stripes with a width of about 13 mm and a length of 10 cm, glued to the lining canvas for about 7.5 cm, leaving the end of both canvasses unglued. The free ends are fastened to the machine for tensile tests by means of two clamps, the upper clamp being movable both upwards and downwards, to perform the peel tests. In the case at issue, lateral screw handles were used. The used sides are rectangular and have a width of 5 cm and a height of 2.5 cm, with a rubberized surface, because the broad surface allows tissues to be grasped without risk of losing some fibres and the rubber coating limits the gliding of specimens (Figure 4). Measurements were made according to the ASTM D1876-08 and F88 I F88M-15 norms, at a speed of 256 mm/s, for a stroke of 75 mm. The specimens were unsupported during the test, as refereed to by the ASTM F88 / F88M-15 as one of the possible methods in use. The results of the peel test, expressed as peel strength, as the average of five repetitions and relevant standard deviation, are listed both in the diagrams of Figure 5 and in Table 4. The used naming designates as A or M the kind of specimen: ancient of modern, respectively; as O or I the kind of lining canvas: Origam® or Ispra®, respectively; as F or L the form in which the EVA resin/s is applied between the specimen and the lining canvas: the liquid foam of EVA resin/s according to the present invention SLA EVA, therefore the foam (F) and the aqueous dispersion of EVA resin/s DA/DL EVA, therefore the liquid form (L) , respectively; as a or b the quantity of used product: the ideal quantity of foam of EVA resin/s (a) and the ideal quantity of aqueous dispersion of EVA resin/s (b), respectively. The threshold of 280 N/m indicates the minimum acceptable value for an adhesive to be used to this end, namely for the purpose of restoration through the lining method (Phenix A, Hedley G., 1984).

Table 4: the results of the pee! test, expressed as peel strength, as the average of five repetitions and standard deviation, are listed Lap shear test

The purpose is to assess the average shear stress (T), which is the applied load divided by the glued overlapped area. The specimens are formed by the two more or less flexible elements, which are partially joined to each other by means of a square adhesion area.

They are formed by two stripes having a width of about 25 mm and a length of 12 cm. glued for about 25 mm by the adhesive layer.

The ends are fastened to the machine for tensile testing by two clamps and a force is applied until the coupling breaks (Figure 6-a).

As in the peel tests, the machine for tensile testing was provided with a screw clamp with lateral action and rectangular sides having a width of 5 cm and a height of 2.5 cm, with rubberized surface (Figure 6-b) .

The tests were performed in compliance with ASTM D1002-10 e D3163-01 , at a constant rate of 1 .3 mm I min, with a pre-load of 0.1 N.

For each kind of mock-up, five repetitions were performed on 5 different specimens.

Some of the collected lap shear test values are listed in the bar chart of Figure 7: the lap shear resistance values (in MPa) for a series of samples comprising as adhesive either the aqueous liquid foam of EVA resin/s SLA EVA (F) according to the present invention or the aqueous liquid dispersion of EVA resin/s DA/DL EVA (L) , both for samples as such and for samples subjected to aging.

FURTHER ANALYSES

Also analyzed, always on specimens of painted canvasses subjected to the above lining process, were: colour and surface alterations (multispectral, spectrophotometry, microscopy), the penetration and migration of the adhesive (UVF marker test on cross section), the chemical interaction with painting materials and deterioration after the artificial aging process (micro-FTIR on cross section); further surveys and analyses were performed, such as: visible light photography,

UV light photography, colorimetry and spectrophotometry, microscopy, stereomicroscopy, USB videomicroscopy, optical microscopy,

Fourier Transform Infrared Spectroscopy, transmission mode,

ATR mode, ATR-FTIR microscopy, both in the case of use of the liquid dispersion of EVA resin/s, DA/DL EVA, and in the case of use of the aqueous liquid foam of EVA resin/s, SLA EVA, according to the present invention, obtained after foaming said aqueous liquid dispersions of EVA resin/s.

As to colour alteration, in compliance with the performed colour tests, as shown in Table 5, both on the front and on the back of the specimens, it is shown that the front of the samples is not affected by the application of the liquid foam of EVA resin/s SLA EVA according to the present invention as adhesive, because the values of SCI AE*(ab) registered after artificial aging appear to be the same also for untreated samples.

The tests were performed through comparative tests, with the application also of the DA/DL EVA liquid system under the same conditions.

Table 5 lists the SCI AE*(ab) values obtained from the colorimetric analyses by comparing the front of the specimens without adhesive and with adhesive (Tq-Ad), the front of the specimens with adhesive before and after artificial aging (Pre-post), the front of the specimens with adhesive after aging and the front of the specimens without adhesive after artificial aging (Ad(ag)-post) and the back of the specimens with adhesive before and after artificial aging (Pre-post).

A value below 3 indicates the absence of colour variation, a value between 3 and 6 indicates a perceptible but acceptable colour variation, a value above 6 indicates an alarming colour variation (see Hardeberg, 2001 ). Table 5 From the data of Table 5 it can be assumed that the adhesive layer does not directly cause the colour alteration of the used materials and is independent from the kind of used application as well as from the amount of applied product.

By contrast, the surface of the back side is subject to a more or less perceptible degree of variation and the amount of adhesive as well contributes to said alteration: the higher the amount of applied adhesive, the more perceptible the alteration.

The investigation about the degradation of the product through micro FT-IR analysis (see Figure 8) does not highlight the formation of particular by-products, since the acetic acid, which directly alters the EVA polymer, impairing the stability and technical features thereof, on account of vinyl deacetylation is not present in an appreciable amount according to the techniques in use.

Figure 9 shows the different degrees of penetration and migration of the adhesive in the specimens depending on whether it is applied as aqueous liquid foam of EVA resin/s SLA EVA (Foam), according to the present invention, or as aqueous dispersion of EVA resin/s DA/DL EVA (Liquid). In both cases the adhesive, that is both the aqueous liquid foam of EVA resin/s and the aqueous dispersion of EVA resin/s, was applied in a mixture with a red dye (Ponceau S) so that penetration into the tissue could be visible at the optical microscope. The penetration and spread level inside the images - two in the right column (relating to the application of the aqueous liquid foam of EVA resin/s SLA EVA to the specimens) and two in the left column (relating to the application of the aqueous dispersion of EVA resin/s DA/DL EVA to the specimens) - is highlighted by the length of the arrows which were purposely added in said images.

The data of this experimental test show the reduced (and almost) inexistent vertical diffusion of the adhesive and of its aqueous component when applied as aqueous liquid foam of EVA resin/s SLA EVA, according to the present invention.