[0001] The present invention relates to a process for cleaning hydrophilic porous matrices.
[0002] 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.
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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).
[0008] The amphiphilic substances are employed to stabilize the heterogeneous system. They
can be emulsifiers or surfactants.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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.
[0014] Suitable surfactants of the non-ionic type are, e.g., poly-(oxyethylene) alcohols
or poly(oxyethylene) alkylphenols.
[0015] Preferred examples of surfactants are sodium dodecylsulphate and dodecyl-trimethyl-ammonium
chloride.
[0016] If the amphiphilic substance exhibits a double alkyl chain, single-lamellar or multi-lamellar
aggregation structures (which are called blisters or liposomes) are obtained.
[0017] 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
![](https://data.epo.org/publication-server/image?imagePath=1991/41/DOC/EPNWA2/EP91105096NWA2/imgb0001)
[0018] 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.
[0019] 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.
[0020] Among the above heterogeneous systems, micelles and oil-in-water microemulsions are
preferred in the cleaning process of the present invention.
[0021] 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.
[0022] 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.
[0023] 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.
[0024] 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.
[0025] 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.
[0026] 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.
[0027] 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.
[0028] 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.
[0029] 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.
[0030] 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.
[0031] 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.
[0032] Said method permits a continuous renewal of the solubilizing agent.
[0033] The following non-limiting example is to further illustrate the present invention.
Example
[0034] 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.
[0035] 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.
[0036] 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).
[0037] 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.
[0038] 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.
[0039] 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.
1. 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.
2. Process according to claim 1, wherein the dispersed heterogeneous system is in the
form of emulsions, micelles, blisters or microemulsions.
3. 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.
4. 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.
5. Process according to claim 4, wherein the amphiphilic substances exhibit a double
alkyl chain.
6. 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.
7. Process according to claim 6, wherein the microemulsion comprises dodecane in water
and contains sodium dodecylsulphate as surfactant and pentanol as co-surfactant.
8. 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.
9. 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.