EP0450539A2

EP0450539A2 - Process for cleaning hydrophilic porous matrices

Info

Publication number: EP0450539A2
Application number: EP91105096A
Authority: EP
Inventors: Enzo Ferroni; Gabrielli Gabrielli; Gabriella Caminati
Original assignee: Syremont SpA
Current assignee: Syremont SpA
Priority date: 1990-04-02
Filing date: 1991-03-28
Publication date: 1991-10-09
Also published as: IT9019912A0; EP0450539A3; IT1240734B; IT9019912A1

Abstract

Described is a process for cleaning hydrophilic porous matrices such as wall surfaces or works of art, which comprises treating the surface to be cleaned with dispersed heterogeneous systems stabilized by amphiphilic substances or with heterogeneous systems composed of molecular aggregates of said amphiphilic substances dispersed in an aqueous medium or in aqueous electrolyte solutions.

Description

The present invention relates to a process for cleaning hydrophilic porous matrices.
More in particular, the present invention relates to a process for removing hydrophobic impurities from wall surfaces or pictorial works of art etc., by means of circumscribed solubilization of said impurities.
The complex and numerous operations which are used for the restoration of a work of art and in particular of frescos comprise also "cleaning" operations, i.e. the removal of impurities - for the most part solid impurities - which may be present and usually are distributed at random, said operations often being of particular importance. Of particular interest is the removal of hydrophobic impurities, which are distributed in porous (and generally hydrophilic) matrices.
In these cases, the most obvious operation would be the treatment with hydrophobic organic solvents capable of solubilizing the impurities. Generally, however, said treatment only has the effect of distributing said impurities over a greater surface or to convey them into the porous hydrophilic material which is typical of the surfaces considered.
According to the present invention, the removal of hydrophobic impurities from generally hydrophilic porous matrices can be carried out, without the above drawbacks, by using dispersed heterogeneous systems composed of dispersions of a liquid in another liquid insoluble therein, stabilized by amphiphilic substances, or of molecular aggregates of said amphiphilic substances dispersed in an aqueous medium or in aqueous electrolyte solutions.
The term "amphiphilic substances", whenever used in the present specification and in the claims, denotes those substances whose molecular structure contains both a hydrophilic portion and a hydrophobic portion; therefore, said substances are stable at the interface of the heterogeneous system and act as surfactants.
The heterogeneous systems utilized in the process of the present invention are preferably in the form of:

(a) emulsions, apparently heterogeneous, optically non-transparent and only kinetically stable;
(b) micelles, i.e. aggregates of amphiphilic substances, surface-active in water;
(c) blisters and/or liposomes, i.e. molecular aggregates consisting of structures composed of a double layer of amphiphilic molecules having an approximately spheric shape; or
(d) microemulsions, apparently homogeneous, optically transparent and thermodynamically stable (in contrast to the above "real" emulsions).

The amphiphilic substances are employed to stabilize the heterogeneous system. They can be emulsifiers or surfactants.
Emulsions are composed of a continuous liquid phase and of a discontinuous liquid phase immiscible therewith, the latter being dispersed in the former by means of an emulsifier. Common emulsion types are oil-in-water and water-in-oil; the term "oil" being used to denote any water-in-soluble fluid, such as, e.g., an aliphatic or aromatic hydrocarbon or a halogen derivative thereof.
These systems are generally cloudy, lactescent and thermodynamically unstable. The emulsifier is distributed over the surface of the dispersed phase. Examples of suitable emulsifiers are cetyl alcohol and lauryl alcohol with sodium cetylsulphate or sodium laurylsulphate.
The term "micelles" denotes any molecular aggregate of about 50 to about 100 molecular units of an amphiphilic substance which spontaneously forms in aqueous media above a certain concentration which depends on the type of amphiphilic substance utilized. Said aggregates can exhibit different shapes such as spheric, cylindrical or disc-like.
The amphiphilic substance can be a known surfactant which is arranged with the hydrophilic portion turned towards the water phase and the hydrophobic portion turned towards the molecular aggregate inside. The surfactants capable of forming micelles can be cationic, anionic, non-ionic or zwitterionic surfactants.
Specific examples of suitable surfactants are sodium alkylsulphates of formula:

CH₃-(CH₂)_n - 0 - SO₃ ⁻ Na⁺

wherein n is an integer ranging from 6 to 30;
and the alkyl-ammonium halides of formula:

CH₃(CH₂)_n - N⁺ (R)₃ A⁻

wherein n is an integer of from 6 to 30, R is an alkyl radical containing from 1 to 3 carbon atoms and A⁻ is a halide ion such as bromide and chloride.
Suitable surfactants of the non-ionic type are, e.g., poly-(oxyethylene) alcohols or poly(oxyethylene) alkylphenols.
Preferred examples of surfactants are sodium dodecylsulphate and dodecyl-trimethyl-ammonium chloride.
If the amphiphilic substance exhibits a double alkyl chain, single-lamellar or multi-lamellar aggregation structures (which are called blisters or liposomes) are obtained.
Preferred double alkyl chain surfactants are didodecyl-methyl-ammonium chloride of formula

[ CH₃(CH₂)₁₁ ]₂ N⁺ (CH₃)₂ Cl⁻

and dipalmitoyl phosphatidyl choline of formula
Microemulsions are composed of two immiscible liquids such as water and oil and of a surfactant and a co-surfactant, both arranged at the interface of the two immiscible liquids.
Depending on whether the dispersed phase is oil or water, it is possible to obtain oil-in-water microemulsions or water-in-oil microemulsions. The microemulsions are stable, homogeneous and optically transparent and form spontaneously.
Among the above heterogeneous systems, micelles and oil-in-water microemulsions are preferred in the cleaning process of the present invention.
In particular, microemulsions have proved to be surprisingly effective. This system preferably consists of a microemulsion of an aliphatic hydrocarbon in water, stabilized with a surfactant of the non-ionic, anionic or cationic type and with a preferably alcoholic co-surfactant.
A specific example of such a microemulsion is a system composed of dodecane in water and containing sodium dodecylsulphate as surfactant and pentanol as co-surfactant.
By using the above heterogeneous systems it is possible to achieve a complete solubilization or to at least increase the solubility of hydrophobic compounds, as said systems contain compounds or molecular areas which are also hydrophobic and, therefore, capable of interacting with the hydrophobic substances to be solubilized. This is of particular importance when the continuous phase of the dispersion is water or an aqueous solution which by itself would be absolutely unsuited for solubilization. On the other hand, the hydrophilic phase of the dispersion is particularly suitable for the hydrophilic substrate onto which it has to act.
The above systems have two different types of solubilization areas; in particular, in the micelles and in the blisters, the solubilization of the hydrophobic impurities occurs in molecular areas which are also hydrophobic while in the emulsions and microemulsions the solubilization occurs in areas substantially consisting of the hydrophobic oil phase dispersed in the hydrophilic matrix.
All these systems act as solubilizers of the hydrophobic impurities, thus eliminating the drawbacks caused by the treatment of surfaces with organic solvents. Furthermore, all these systems are suitable for being applied onto hydrophilic substrates, such as those of wall surfaces and pictorial works in general, as they exhibit a continuous aqueous phase which secures the wetting of the hydrophilic matrices, and therefore their contact with the hydrophobic material, for a sufficiently long period of time.
The cleaning process usually comprises the following steps carried out in the order given:

onto the parts to be cleaned there are first applied compresses of cellulose or synthetic pulp saturated with water, which usually are kept in contact with said parts for about one to about two hours; during this step, the area of interest is saturated with humidity;
subsequently, there are applied onto the selected parts compresses of cellulose or synthetic pulp saturated with a heterogeneous system such as, for example, the above microemulsion;
the treated parts are sealed by means of duplex paper and/or a non-woven fabric in order to prevent both the evaporation of the volatile components of the heterogeneous system and the diffusion thereof beyond the selected area;
the cellulose or synthetic pulp compresses are allowed to remain in contact with the surface to be cleaned for about 2 to about 6 hours.

After removal of the compresses the hydrophobic impurities have been solubilized or at any rate detached from the substrate and are therefore readily and spontaneously removable.
The cellulose pulp can, for example, be of a type such as ARBOCEL® BC or mixtures thereof with ARBOCEL® BWW. The synthetic pulp can, for instance, be composed of polyethylene and/or polypropylene fibrils.
Naturally, instead of the cellulose or synthetic pulp, any other inert and absorbent substrate can be utilized such as, for example, fossil meal, micronized silicas etc.
The duplex paper usually consists of an inner plastic material sheet, such as polyethylene (which is brought into contact with the surface to be treated), and of an outer cellulose sheet.
An alternative method of application comprises causing the heterogeneous system to continuously flow, at a controlled flowrate, from a proper container (tank) to the area of interest - previously subjected to a humidification treatment - through a suitable micropiping, and removing the heterogeneous system downstream after solubilization of the hydrophobic impurities has occurred.
Said method permits a continuous renewal of the solubilizing agent.
The following non-limiting example is to further illustrate the present invention.

Example

A microemulsion was prepared by dispersing 0.08% by volume of dodecane in an aqueous solution of 0.5 moles of sodium dodecylsulphate and 1 mole of pentanol. Dodecane was gradually added under stirring. The system, opalescent at the beginning, became suddenly limpid after a few hours and remained in this state when maintained at a temperature ranging from 15 to 35°C.
With reference to the attached figure, the fresco surface to be cleaned, containing splashes of wax (H), was first treated with a compress of cellulose pulp (type ARBOCEL BC 200®) impregnated with deionized water, in order to saturate the plaster surrounding the wax-coated area with humidity.
After two hours, the cellulose pulp compress was removed and replaced by a compress (A) composed of 70% of cellulose type ARBOCEL® BC 200 and 30% of cellulose pulp type ARBOCEL® BWW 40, impregnated with the water/sodium dodecyl sulphate/pentanol/dodecane microemulsion prepared as described above.
Compress (A) was surrounded by a compress (B) of cellulose pulp type ARBOCEL® BC 300 impregnated with dionized water, and the whole was coated with Japanese paper and cellulose pulp impregnated with water (C).
The microemulsion was made to flow in contact with the surface to be cleaned from a bottle (L) through a pipe equipped with a flow regulator (I), the cannula (E) of said pipe being in contact with the part to be cleaned through a perforation in compresses (B) and (C). Compresses (B) and (C) were covered with a duplex paper consisting of a polyethylene film (D) and of a cellulose paper sheet (F) in order to prevent the components of the microemulsion from evaporating.
In the part below the surface to be cleaned, and beneath the polyethylene film (D), a duplex paper sheet (G) was arranged, which consisted of a polyethylene layer in contact with the fresco and of an outer layer of cellulose paper. In this manner the polyethylene layer isolated the picture while the paper absorbed the excess microemulsion, thereby preventing it from dropping onto the underlying picture surface.
After a treatment of about 4 hours it was observed that the treated part was completely clean and the splashes of wax had thoroughly been removed.

Claims

Process for cleaning hydrophilic porous matrices by means of solubilization of hydrophobic impurities, which comprises treating the surface to be cleaned with a dispersed heterogeneous system composed of dispersions of a liquid in another liquid, said dispersions being stabilized by amphiphilic substances, or of molecular aggregates of said amphiphilic substances dispersed in an aqueous medium or in aqueous electrolyte solutions.
Process according to claim 1, wherein the dispersed heterogeneous system is in the form of emulsions, micelles, blisters or microemulsions.
Process according to claim 2, wherein the emulsion is of the type oil-in-water or water-in-oil, said oil being a water-insoluble liquid, and the emulsion is stabilized with an emulsifier.
Process according to claim 2, wherein the micelles are molecular aggregates of about 50 to about 100 molecular units of amphiphilic substances in an aqueous medium.
Process according to claim 4, wherein the amphiphilic substances exhibit a double alkyl chain.
Process according to claim 2, wherein the microemulsion is composed of two immiscible liquids and is stabilized by a surfactant and a co-surfactant, preferably of the alcoholic type.
Process according to claim 6, wherein the microemulsion comprises dodecane in water and contains sodium dodecylsulphate as surfactant and pentanol as co-surfactant.
Process according to anyone of the preceding claims, comprising the following sequence of steps:
- applying onto the parts to be cleaned a compress of cellulose or synthetic pulp saturated with water and keeping said compress in contact with said parts for about one to about two hours;

- subsequently applying onto said parts a compress of cellulose or synthetic pulp saturated with the heterogeneous system;

- sealing the treated parts by means of duplex paper or a non-woven fabric;

- removing the compress after about 2 to 6 hours.
Process according to anyone of claims 1 to 7, wherein the heterogeneous system is caused to flow, at a controlled rate, from a container to the area of interest, which area had previously been subjected to a humidification treatment, through a micropiping, and removing the heterogeneous system downstream after the solubilization of the hydrophobic impurities has occurred.