SLABS OF STONE MATERIAL, RESISTANT TO WEAR ₅ TO CORROSION CAUSED BY ACIDS AND TO THE STAINING ACTION OF OILY SUBSTANCES. Field of the invention: The present invention relates to the field of processes aimed at protecting stone surfaces from mechanical wear, from corrosion and from oily stains. Prior art:

It is known, particularly in the building sector, that stone materials, such as marbles and granites, are being increasingly utilized for functional uses such as kitchen countertops, finishes of sanitary fixtures, important parts of interior furnishings and for the exterior part of a building. It is also known that the use of these materials, of great prestige and aesthetic effect, is substantially limited by the wear and corrosion to which they are subject in time. In fact, stone materials deteriorate in time due to various causes, such as:

^■ wear, in the case of floors or pavements for example. The action of abrasive powders, of walking and/or of rubbing causes consumption of stone surfaces; ■ corrosion caused by atmospheric conditions and/or by contact with corrosive acid substances: for example acid rain, or lemon juice, the latter being a typical substance with which the stone material can come into contact if used for kitchen countertops. ■ the staining action of oily substances: as these materials are extremely porous, they irreversibly absorb any oily liquid substances (i.e. olive oil) with which they come into contact.

Moreover, it is known that the surface of stone materials is subjected to mechanical finishing treatments such as, for example, smoothing/polishing/flaming and the like and/or to the application of layers of synthetic material on the finished surface, in order to prolong their useful life. However, these techniques have always provided

somewhat unsatisfactory results, particularly with regard to preservation in time of the appearance and of the function characteristics of the finished product.

Plasma deposition of thin films, generally indicated with the acronym PECVD (Plasma-Enhanced Chemical Vapour Deposition) and substantially consisting in the use of a cold plasma to deposit, in high vacuum conditions, a thin film on the surface of the substrate to be coated, and plasmochemicai treatment (grafting of functional groups, crosslinking and/or ablation), have proved to be extremely versatile technologies to change the surface characteristics of conventional materials, providing them with particular performances and properties. The patent application MI2005A1672 describes flat stone slabs protected by a transparent film with chemical composition of SiO _x deposited through a plasmochemicai process, using hexamethyldisiloxane (HMDSO) mixed with argon as precursor.

The stone material, obtained with the aforesaid method, shows high resistance to mechanical wear and to corrosive acids, but poor duration in time: in fact, one month after plasmochemicai treatment, crumbling of the slabs of stone material exposed to the action of atmospheric conditions (rain, wind, etc.) was observed. Therefore, the marble slabs are once again vulnerable to attack by the corrosive agents against which it was wished to protect them. This phenomenon is probably due to the crumbling action that water has on marble, given the carbonate and porous nature of the stone material considered. In fact, water penetrates the pores of the marble and essentially transforms it into soluble calcium bicarbonate. A protective coating of the surface of the substrate in question must be therefore able to cover these pores. It is known that films with composition of SiO _x deposited through the plasma process, grow on the surface of materials on which they are deposited, following their morphology and consequently do not block any pores. Therefore, water penetrates the film and crumbles the substrate.

In the light of the above, therefore, there is an evident need to produce a compact protective covering of the surface of stone materials, in order to eliminate the porosity of the substrate, whose effectiveness remains unchanged in time as long as possible. Brief description of the figures:

Fig. 1 schematically shows a plasmochemical reactor for treatment of stone surfaces according to the invention.

Fig. 2 schematically shows the section of a slab (in this case a flat slab) of stone material protected according to the present invention. Detailed description of the invention:

The present invention allows long-term protection of the surface of stone materials against mechanical wear, against the corrosive action caused by acid substances and against stains caused by contact with oily substances. These results are obtained through the deposition of a hard and compact film, with chemical composition of SiO _x , on the surface of marble slabs, through the use of cold plasmas fed by vapours of organosilane substances (tetraethoxysilane, TEOS; hexamethyldisiloxane, HMDSO; vinyltrimethylsilane, VTMS; etc.) mixed with oxygen and argon, the surface of which is subsequently crosslinked, compacted and hardened in argon plasma. Moreover, the aforesaid film is perfectly transparent and therefore does not alter the aesthetic properties of the marble on which it is deposited. With reference to the accompanying figures, to simplify the description a stone element will be identified below with the generic term "slab", without specific reference to the use made of this term in the technical sector of reference and irrespective of whether its surface is flat or curved. According to the present invention, the term stone material is intended as a material made of stone normally used in the building sector, such as marble, covering materials in general for building use, such as limestones, travertines, ceramics, concrete materials, etc..

The coating material, deposited with the plasmochemical process, is essentially composed of a film with chemical composition of SiO _x , where x can vary from 1.5 to 2.05.

Preferably, said film has a thickness ranging from 1 to 5 μm, in order to obtain high resistance to acid attack, to oily stains and to mechanical wear.

The application substrate normally has a thickness ranging from 0.5 to 3 cm.

As shown in figure 1 , the plasmochemical reactor (100) used for plasmochemical treatment of slabs (10) is substantially composed of a single reaction chamber (110), connected to feed and control equipment, such as:

^■ vacuum forming devices (120);

^■ system for reading the pressure inside the reaction chamber (130);

^» radiofrequency generator (RF, 13.56 MHz) (140);

■ matching network (150);

^■ devices to feed reagents in the gaseous or vapour state (160); » a flat electrode 170, connected to the radiofrequency generator, disposed inside the reaction chamber 110;

^■ a flat electrode (ground) (180), disposed inside the reaction chamber (110), on which the element in stone material (10) to be coated is placed. According to the invention, the process for the preparation of stone material with surface protection involves the following steps:

■ providing a slab of stone material (10) with finished, cleaned, washed and degreased surface;

■ placing the slab (10) in the reaction chamber (110); ■ forming a process vacuum in the reaction chamber (110);

■ feeding gas into the reaction chamber (110);

■ performing, inside the reaction chamber (110), a pre- treatment (etching) on the surface of the slab (10) to be coated;

^■ stopping gas feed to the reaction chamber (110); ■ forming a process vacuum in the reaction chamber (110);

■ simultaneously feeding reagents to the reaction chamber (11)

■ plasmochemical deposition of the protective film (20) for a programmed time interval;

^■ stopping reagent feed to the reaction chamber (110); ■ forming a process vacuum in the reaction chamber (110);

^■ feeding gases into the reaction chamber (110);

■ argon plasma treatment (crosslinking) of the protective film deposited (20);

^■ stopping gas feed to the reaction chamber (110); ■ switching off the pumping system (120) and allowing air to enter the reaction chamber (110);

■ removing the finished slab (10).

If wished or preferred, instead of a system composed of a single reaction chamber, operating discontinuously as describe above, a system with three distinct reaction chambers can be used, in which the process can therefore be implemented continuously. Therefore, said system will be provided with:

^■ a chamber for pre-treating the marble slabs;

^■ a chamber for depositing the protective coating; « a chamber for the crosslinking treatment of the surface of the protective coating.

If necessary, two additional chambers for loading and unloading the marble slab can be added to these three chambers.

The relative auxiliary devices will, in this case, be adapted to the enlarged configuration of the system, while the ranges relative to the values of the work parameters indicated below will remain the same.

In fact, according to preferred embodiments of the process:

1. the step to pre-treat the surface of the slab (10) is performed in the following experimental conditions:

■ pressure ranging from 1 to 10 ^"3 torr;

■ power (13.56 MHz) ranging from 0.06 to 1.00 W/cm ² ; ■ argon flow ranging from 0.5 to 2.0 seem/ cm ² ;

■ treatment time ranging from 2 to 20 minutes.

2. the step to deposit the film with chemical composition of SiO _x (20), on the surface of the slab (10), is performed in the following experimental conditions: ■ pressure ranging from 1 to 10 ^~3 torr;

■ power (13.56 MHz) ranging from 5 to 40 W/cm ² ;

^■ plasma feed gas flows; vapour flow of the organosilane precursor ranging from 0.15 to 1.00 seem/ cm ² , oxygen ranging from 0 to 30 sccm/cm ² , argon ranging from 0 to 30 seem;

^■ the plasmochemical deposition time varies in relation to the characteristics to be obtained for the finished product and, therefore, to the thickness of the protective coating (20) to be deposited. In particular, a time ranging from 3 to 60 minutes is estimated for the deposit of up to 5 μm of film with chemical composition of SiO _x;

3. the plasmochemical crosslinking treatment of the film with chemical composition of SiO _x (20) is performed in the following experimental conditions:

■ pressure ranging from 1 to 10 ^"3 torr;

■ power (13.56 MHz) ranging from 0.03 to 1.90 W/cm ² ;

■ argon flow ranging from 1 to 20 seem/ cm ² ,

^■ treatment time ranging from 5 to 10 minutes. Experimental tests described below confirmed that the surface of the stone materials, coated with the protective film described, offers considerable resistance to mechanical wear, to corrosion caused by

acids and to oily stains that lasts for long periods of time. Moreover, the coatings in question are completely transparent and therefore do not alter the ornamental characteristics of the stone material treated.

EXAMPLE 1 : LONG-TERM RESISTANCE TEST: CRUMBLING TESTS IN WATER

Without doubt, water can be considered the main cause of crumbling of stone materials. In fact, the porous structure of these substrates facilitates water penetration, which transforms the marble into soluble carbonate. Part of the surface of sample of slab of white stone material was protected with special adhesive tape. The surface of the sample thus obtained was modified, in the plasmochemical reactor, in the following experimental conditions:

Pre-treatment Pressure = 10 ^"2 torr

Power = 0.8 W/cm ²

Ar = 0.8 sccm/cm ²

Treatment time = 5 minutes

Plasmochemical deposition Pressure: 10 ^'2 torr

Power = 10W/cm ²

Ar = 1.8 sccm/cm ²

O ₂ = 3.6 sccm/cm ²

HMDSO = 0.16 sccm/cm ² Deposition time = 30 minutes (thickness of the film deposited = 3 μm)

Treatment (Crosslinking)

Pressure: 10 ^"2 torr

Power = 1.2 W sccm/cm ²

Ar = 2 sccm/cm ² , Treatment time = 7 minutes

At the end of the process, after having removed the protective tape, the stone sample was immersed in water for 3 hours. After this period of

time, it was seen that water wets the untreated surface of the sample

(the strip previously covered by the adhesive tape appears "shinier") to a much greater extent than the part of surface modified by the plasmochemical process. Subsequently, resistance in time of the protective coating deposited in the aforesaid experimental conditions was assessed, in real conditions.

Specifically, the samples of stone material, obtained following the experimental procedure described above, were exposed to atmospheric conditions (rain, wind, etc.) for 3 months. After this period of time, the surface of the stone samples was observed under the electronic microscope, excluding the presence of crumbling areas of the substrate, and of cracks and/or areas of detachment of the film from the surface of the stone material examined, just as any other form of alteration of the protective coating. EXAMPLE 2: ASSESSMENT OF IRIDESCENCE AND OF

TRANSPARENCY

The surface of a slab of white stone material was modified in the following experimental conditions:

Pre-treatment Pressure = 10 ^"2 torr

Power = 0.8 W/cm ²

Ar = 0.8 sccm/cm ²

Treatment time = 5 minutes

Plasmochemical deposition Pressure: 10 ^"2 torr

Power = 10W/cm ²

Ar = 1.8 sccm/cm ²

O ₂ = 3.6 sccm/cm ²

HMDSO = 0.16 sccm/cm ² Deposition time = 30 minutes (thickness of the film deposited = 3 μm)

Treatment (Crosslinking)

Pressure: 10 ^"2 torr

Power = 1.2 W sccm/cm ²

Ar = 2 sccm/cm ²

Treatment time = 7 minutes

After the plasmochemical treatment, the film deposited on the surface of the stone sample met the need for absence of iridescence and for transparency, required for the expected uses.

EXAMPLE 3: CORROSION TEST: ASSESSMENT OF RESiSTANCE

TO LEMON JUICE

As it is known, nothing is more corrosive than lemon juice for a stone kitchen countertop. Therefore, this test is without doubt discriminating in judging the resistance to acids of the surface of the material in question.

A slab of stone material was modified through the plasmochemical process in the following experimental conditions:

Pre-treatment Pressure = 10 ^"2 torr

Power = 0.8 W/cm ²

Ar = 0.8 sccm/cm ²

Treatment time = 5 minutes

Plasmochemical deposition Pressure: 10 ^'2 torr

Power = 10W/cm ²

Ar = 1.8 sccm/cm ²

O ₂ = 3.6 sccm/cm ²

HMDSO = 0.16 sccm/cm ² Deposition time = 30 minutes (thickness of the film deposited = 3 μm)

Treatment (Crosslinkinq)

Pressure: 10 ^"2 torr

Power = 1.2 W sccm/cm ²

Ar = 2 sccm/cm ² Treatment time = 7 minutes

A drop of lemon juice was deposited on the surface of the aforesaid sample. After 30 minutes of contact, the surface of the sample in

question was rinsed with water and dried. At the end of these operations there were no signs of corrosion on the surface of the substrate in question. On the other hand, the stone material as is was irreparably damaged by the lemon juice after two seconds of contact. On the basis of the experimental results described above, it was possible to conclude that the protective film, deposited in the experimental conditions described above, is capable of guaranteeing sufficient resistance to acidity (i.e. for a kitchen countertop, on which the operation can be performed in a short time) of up to 30 minutes. Finally, films of greater thickness will presumably be able to protect the surface of marble from corrosion caused by lemon juice for longer times.

EXAMPLE 4: TEST FOR RESISTANCE TO OILY STAINS:

ASSESSMENT OF RESISTANCE TO OLIVE OIL

A slab of stone material was modified through the plasmochemical process in the following experimental conditions:

Pre-treatment

Pressure = 10 ^'2 torr

Power = 0.8 W/cm ²

Ar = 0.8 sccm/cm ² Treatment time = 5 minutes

Plasmochemical deposition

Pressure: 10 ^"2 torr

Power = 10W/cm ²

Ar = 1.8 sccm/cm ² O ₂ = 3.6 sccm/cm ²

HMDSO = 0.16 sccm/cm ²

Deposition time = 30 minutes (thickness of the film deposited = 3 μm)

Treatment (Crosslinkinq)

Pressure: 10 ^"2 torr Power = 1.2 W sccm/cm ²

Ar = 2 sccm/cm ²

Treatment time = 7 minutes

A drop of olive oil was deposited on the surface of the aforesaid sample. After 30 minutes of contact, the surface of the sample in question was rinsed with water and dried. At the end of these operations there were no stains on the surface of the substrate in question. On the other hand, the stone material as is was irreparably stained by the olive oil after two minutes of contact. On the basis of the experimental results described above, it was possible to conclude that the protective film, deposited in the experimental conditions described above, is capable of guaranteeing sufficient resistance to oily stains (i.e. for a kitchen countertop, on which the operation can be performed in a short time) of up to 30 minutes. Finally, films of greater thickness will presumably be able to protect the surface of marble from the staining action caused by olive oil for longer times.

EXAMPLE 5: MECHANICAL WEAR TEST: TRIBOLOGICAL ANALYSIS

Some samples of slab in stone material were modified through the plasmochemical process in the following experimental conditions:

Pre-treatment

Pressure = 10 ^"2 torr Power = 0.8 W/cm ²

Ar = 0.8 sccm/cm ²

Treatment time = 5 minutes

Plasmochemical deposition

Pressure: 10 ^~2 torr Power = 10W/cm ²

Ar = 1.8 sccm/cm ²

O ₂ = 3.6 sccm/cm ²

HMDSO = 0.16 sccm/cm ²

Deposition time = 30 minutes (thickness of the film deposited = 3 μm) Treatment (Crosslinkinq)

Pressure: 10 ^"2 torr

Power = 1.2 W sccm/cm ²

Ar = 2 sccm/cm ² Treatment time = 7 minutes

The samples thus obtained were subsequently subjected to tribological tests aimed at determining the friction coefficient, assumed as magnitude representing the surface hardness of the substrate. The tests performed showed that a slab covered with the coating, deposited in the experimental conditions described, has a greater relative surface hardness than that of the surface of the stone material as is. Therefore, the process described allows elements of stone material (marbles and/or granites) to be produced in a precise, safe and reliable manner with a surface coated in a long-lasting protective layer, which improves the characteristics of surface resistance to acids, to oily substances and to mechanical wear, without altering its ornamental properties. The stone material, obtained following the aforesaid process, is therefore potentially usable for various functional purposes. Moreover, the process in question, allows all sides and edges of a flat element exposed to the plasma to be covered.

It can be noted that with the treatment described, even delicate stone material, such as particularly prestigious white marble, the use of which has been abandoned in recent times due to its extreme delicateness, may once again become commercially interesting. In fact, with the treatment described, the life of any stone material, regardless of its resistance, will be the same as, or even greater than, that of granite, and therefore these materials can be used for exterior coverings, interior furnishings for bars, kitchens and bathrooms.

Moreover, the treatment takes on a particular significance for use on complex stone structures, such as works of art exhibited outdoors which, as it is known, suffer the effects of particularly aggressive pollutants, such as acid rain, smog, etc..