* * *

The present invention relates to fireproof leather and to a method for manufacturing it.

The decay processes of tissues, including skin, begin immediately after an animal is slaughtered. Even before reaching a state of tissue putrefaction, processes are triggered which can result in more or less severe damage to the skin and which quickly deteriorate quality and value in tanning terms.

It is therefore necessary to preserve the skinned hides quickly by brining and drying techniques.

The tanning process is rather long and complex. It is substantially a chemical process consisting of several successive steps separated by mechanical operations. The entire process can be divided into 3 macro-steps:

• tanning

· re-tanning

• finishing.

Each macro-step is then divided into steps with specific purposes. A peculiarity of the tanning process is that some operations, both mechanical and chemical, may be performed in different points of the process, according to the features of the leather to be obtained or, sometimes, to organizational requirements of companies.

All chemical operations, up to the post-tanning treatments, are performed using water in a drum, the latter being the typical tannery machine. Substantially, the drum consists of a cylinder rotating about its axis in which water, skins and chemical reagents are introduced. Initially, it was a simple revolving wooden cylinder, but today it is a complex computer- controlled machine, offering the possibility of adjusting water input, speed and sense of rotation, door opening and closing, introduction of reagents, temperature adjustment, checking the weight (if mounted on load cells). Therefore, a processing "recipe" can be performed automatically, whereby avoiding many possibilities of manual errors. Most drums are still made of wood, but drums may also be made of steel or resin.

There are also alternative machines to drums, such as for example mixers or immersion drums (similar to household washing machines), but the most widespread reactors are still drums.

After having been brined or dried for preservation, the skin is prepared for tanning through several mechanical processes. Tanning is the chemical operation which transforms the putrescible skin into imputrescible material, i.e. leather.

The tanned leather cannot yet be used to produce articles. This is a wet material which, even if it were dried, would give rise to a rather stiff, "cardboard-like" product, which is not very flexible and which has the typical color of the tanning by which it was obtained. In order to transform it into a marketable product, useful for making products, it must be subjected to further chemical and mechanical treatments, referred to as post-tanning treatments.

Finally, the leather is subjected to a step referred to as finishing, which is the most complex step of the tanning process and includes all the operations performed on dry leather to modifying its features in terms of appearance, function or both.

A very important aspect relates to the leather used for upholstery on motor vehicles, aircraft, ships or other vehicle for transporting passengers and/or animals. Indeed, said leather must comply with the regulations in force in the respective countries; for example, said leather must have a given fire resistance or must emit a very low amount of smoke in the event of a fire or must be completely fireproof. Obtaining the latter features is of fundamental importance for the leather used for upholstering vehicles and is often difficult to obtain. For this reason, manufacturers often replace natural leather with synthetic fabrics.

Various methods for tanning, re-tanning and finishing animal skins to obtain fire-resistant leather are described in literature.

WO 2008/105020A1 describes a method for obtaining fire-resistant leather suitable for vehicles. Said method provides stabilizing the leather with sodium bicarbonate to reach pH 6/6,5, adding natural and synthetic tannins at 3 % for 90 minutes. The leather is then left to stand for 10/12 hours. It is then left to dry and is then sprayed with a coating of fire-resistant products. Said coating comprises aliphatic polyurethanes, acrylic resins and aromatic polyurethanes.

CN 104450990 describes a method for making fireproof leather. Said method provides, among other steps, one of adding acrylic resins, a fireproof treatment and the use of oxalic acid.

Disadvantageously, the methods described allow fire-resistant leather to be obtained by spraying or applying a product capable of preventing the burning of the leather.

Another disadvantage is that said leather is not completely fireproof but offers fire and smoke release resistance only for a given time.

Yet a further disadvantage is that the application of fire-retardant products is often associated with a loss of appearance features of the leather, such as for example softness or pleasantness to the touch.

It is the object of the present invention to provide a method for finishing animal skins which allows a fireproof leather having excellent appearance to be obtained.

It is another object to provide a method for making fireproof leather which may be used for upholstering the interiors of vehicles for transporting passengers and/or animals, such as for example cars, airplanes, boats.

In accordance with the invention, such objects are achieved by a method described in claim 1.

It is a second object of the present invention to obtain leather suitable for use on passenger transport vehicles.

According to the invention, such an object is achieved by the leather as described in claim 15.

These and other features of the present invention will become more apparent from the following detailed description of a practical embodiment thereof.

After several experiments, the applicant obtained a method for tanning skin of animal origin which allows a fireproof leather to be obtained.

Said method allows a fireproof leather to be obtained from any type of skin of animal origin, e.g. the hide of cattle, the skin of sheep, goats, crocodiles, ostriches, pigs, horses, fish, kangaroos, deer, ostriches, or of any other non-human animal origin.

Preferably, quality skins of animal origin will be used, e.g. crocodile skin, such as the skins of Nile crocodiles (Crocodylus niloticus), American alligators (alligator mississippiensis), Siamese crocodiles {Crocodylus siamensis) or of any other species of crocodile.

Said skins can be used for any type of destination, e.g. for leather goods, or furniture, upholstering motor vehicles, airplanes, boats, or to any other destination.

Usually, immediately after slaughtering the animal, the skins are tanned, i.e. treated so as to prevent decay by putrefaction or contamination by bacteria or microorganisms. The skins are then received in brine for re- tanning and finishing.

Skins of animal origin comprise an outer side of the animal skin, which we will refer to as grain side hereinafter, and an inner side, i.e. the side which is in contact with the internal tissues of the animal, which is referred to as "crust" in the jargon.

The skins are then processed according to the various steps, the main processing steps are:

re-tanning,

dyeing,

- and finishing of the leather. Said steps are carried out in the order listed above.

Re-tanning is a treatment with tanning agents, often different from those used for the main tanning process, which are used to modify the features imparted by the main tanning process as desired.

Leather re-tanning according to the present invention provides that the leather is stabilized by a buffer solution in order to obtain pH homeostasis; it is very important to take the leather to the correct pH for the subsequent steps to act correctly on the tissue. Having reached the desired pH, the leather is then treated with tannins, e.g. synthetic tannins, to make it soft and fireproof. This is followed by a series of washes with water and oxalic acid to remove any residues of previous treatments on the leather.

The leather is then treated with an aqueous dispersion of anionic, acrylic and auxiliary polyurethane base, and a crosslinker based on a polyfunctional nitrogen group to confer fire -resistance properties.

The leather can then be treated with a solution of tannins, fatliquors and resins in order to confer tear resistance to the leather and optimal performance in fireproof terms.

The leather is then swelled to oxygenate the grain side by virtue of a mixture of anionic fatliquors derived from fatty acids, anionic fatliquors based on sulphited fish oils and a modified polymer with a filler and is finally washed to prepare it for the step of dyeing. A further bath is then performed by immersing the leather in a solution of water and oxalic acid.

The leathers can be then pass directly to the step of finishing or can be dyed with the addition of dyes, preferably metal-free aqueous base dyes; the color is then fixed with formic acid. After dyeing, the leather is washed in cold water to remove dye residues. The leather is then dried in an oven at controlled temperature and humidity.

The leather finishing step provides treatments of stretching and fixing with acrylic based products and resins, e.g. polyurethane or acrylic - polyurethane. During the step of re-tanning, the leather is then treated with an aqueous dispersion of anionic, acrylic and auxiliary polyurethane base, and a crosslinker based on a polyfunctional nitrogen group to confer fire- resistance properties.

Finally, feel modifiers are sprayed to confer softness and silkiness to the leather, e.g. a solution is used comprising at least one polysiloxane in aqueous emulsion, an organic poly-condensate and a crosslinker based on a nitrogen group.

More in detail, in the method described above, the pH of the leather is stabilized with a solution of bicarbonate of calcium (Ca(HC03)2) in water in a drum at a temperature comprised between 25 °C and 50°C, preferably between 30°C and 48°C, still more preferably between 35°C and 45°C.

Preferably, an amount of water ten times greater than the weight of the leather, i.e. 1000%, is added. Said solution of bicarbonate dissolved in water comprises calcium bicarbonate in a final concentration by weight comprised between 0,5% and 8%, preferably between 0,8% and 6%, still more preferably between 1% and 3%.

Having reached the desired pH, i.e. comprised between pH 7 and pH

8, the method proceeds with the treatment using a solution of synthetic tannins. Said solution comprises synthetic tannins in a final concentration by weight comprised between 2% and 25%, preferably between 5% and 20%, still more preferably between 8% and 18%.

Said treatment with synthetic tannins lasts for a time between lh and

3h, preferably between lh and 2h.

The synthetic tannin used may be, for example, the product referred to as Tergotan PMB made by Clariant, containing a special copolymer based on synthetic and natural materials completely formaldehyde-free, or the product referred to as Tanicor WZ made by Clariant.

The solution of synthetic tannins may be prepared with a concentration of Tergotan PMB comprised between 5% and 20%, preferably between 10% and 15% final by weight and Tanicor WZ between the 5% and the 15% final by weight, preferably between 8% and 12%.

Said treatment with synthetic tannins has the main purpose of increasing the softness of the leather and making it fireproof.

After the treatment with synthetic tannins, the method proceeds with at least one leather wash with water and oxalic acid in a final percentage comprised between 0,2% and 1%, preferably between 0,4% and 0,8% final v/v, in order to remove any residues of tannins used on the leather grain.

In order to confer tear resistance and optimal performance in fireproof terms, the leather is then immersed in a bath of water with the addition of tannins, fatliquors and resins. Preferably, an amount of water ten times greater than the weight of the leather obtained from the previous step, i.e. 1000% water, is added in the drum. The tannins are added in a final percentage comprised between 5% and 25% by weight, preferably between 10% and 20% by weight. The fatliquors are added in a final percentage by weight comprised between 5% and 20%, preferably between 8% and 15% and the resins are added in a final percentage by weight comprised between 2% and 12%, preferably between 4% and 8%. Said treatment lasts for a time comprised between 30 minutes and 2 hours, preferably between 45 minutes and 90 minutes, even more preferably between 55 minutes and 70 minutes.

The leather is then swelled to oxygenate the grain and to stabilize the fireproof effect. The swelling occurs by virtue of a mixture of anionic fatliquors derived from fatty acids, anionic fatliquors based on sulphited fish oils and a modified polymer with filler. Said polymer modified with filler can impart a selective swelling of leather and provide a slightly "swelling" effect.

For example, the mixture consists of anionic fatliquors derived from fatty acids in a final concentration v/v comprised between 2% and 10%, preferably between 3% and 6%, anionic fatliquors based on fish oils sulphited in a final concentration v/v comprised between 2% and 10%, preferably between 3% and 6%, and a modified polymer with filler in a final concentration v/v comprised between 2% and 10%, preferably between 3% and 6%. The products used for the composition of the swelling mixture may be, for example DERMINOL WWL made by Clariant Products (Italy) S.p.A., DERMINOL 051 made by Clariant Products (Italy) S.p.A. and MAGNOPAL IPF made by TFL Ledertechnik GmbH.

Finally, a bath is performed by immersing the leather in a solution of water and oxalic acid. Preferably, an amount of water ten times greater than the weight of the leather obtained from the previous step, i.e. 1000% water, is added and oxalic acid in a final percentage v/v comprised between the 2% and 10%, preferably between 4% and 8%, is added.

After re-tanning, the leather is prepared to be dyed during the step of dyeing.

The leather is immersed in a water bath at a temperature comprised between 20°C and 80°C, preferably between 30°C and 70°C, still more preferably between 40°C and 60°C. Metal-free aqueous base dyes are added to said bath and finally the process is fixed with formic acid. In particular, an amount of water ten times greater than the weight of the leather obtained from the previous step is added, the dyes are added to a final concentration by weight comprised between 0,2 and 5%, preferably between 0,5% and 3%; the formic acid is added in a final concentration v/v comprised between 15% and 25%, preferably between 10% and 20%.

The duration of the process may vary according to the desired color to be obtained.

Having reached the desired color, the method proceeds by washing the leather in cold water, at a temperature comprised between 10°C and 30°C, preferably between 15°C and 25°C, to eliminate possible dye residues from the leather grain.

The method then proceeds with drying in oven at controlled temperature and humidity, preferably at a temperature comprised between 15°C and 45°C, still more preferably between 25°C and 30°C, with a relative humidity comprised between 50% and 80%, preferably between

60% and 70%. The drying time of the leather may vary between 30 minutes and 3 hours, preferably between lh and 2h.

The method then proceeds with the step of finishing of the leather through the treatments of stretching and fixing and final sizing to obtain the required appearance and feel.

For acrylic based mixtures and resins are used stretching and fixing, e.g. polyurethane or acrylic-polyurethane in final concentration v/v comprised between 2% and 10%, preferably between 3% and 6%.

For example, mixtures can be used contained in the commercial products referred to as AKRAL PU-MF made by ALPA S.p.A. in a final concentration by weight comprised between 1% and 10%, preferably between 3% and 8%, and ASTACIN UH, made by BSF IF in a final concentration by weight comprised between 0,2% and 4%, preferably between 0,5% and 2%, in order to confer a given degree of resistance to rubbing, to sunlight and to aging to the leather.

Polyurethane products are used in order to confer fireproof properties to the leather, e.g. an aqueous dispersion of anionic, acrylic and auxiliary polyurethane base, and a crosslinker based on a polyfunctional nitrogen group (e.g. polyaziridine).

Preferably, the solution comprises an aqueous dispersion of anionic polyurethane base in a final concentration v/v comprised between 2% and

10%, preferably between 3% and 6%, an acrylic and auxiliary water-based polyurethane composition in a final concentration v/v between 2% and 10%, preferably between 3% and 6%, and a crosslinker based on a polyfunctional nitrogen group in a final concentration v/v between 0,5% and 5%, preferably between 0,8% and 2%.

For example, R.P.U. 030 made by Leather Chemicals, RODA TECH B02 made by Rodatech and Aqualen AKU made by Leather Chemicals, which are fixed and catalyzed in the measure of 10% by the weight of the previously prepared base, can be used.

Finally, feel modifiers are sprayed to confer softness and silkiness to the leather. The solution containing feel modifiers is applied in the percentage between 1-2% by weight of the fixing and stretching solution through a solution comprising at least one organic poly-condensate soluble in water in a final concentration v/v between 0,5% and 5%, preferably between 0,8% and 2%, a polysiloxane in aqueous emulsion in a final concentration v/v between 0,5% and 5%, preferably between 0,8% and 2%, and a crosslinker based on a nitrogen group in a final concentration v/v between 0,5% and 5%, preferably between 0,8% and 2%.

For example, solutions containing feel modifiers of the following commercial products, i.e. SFT 2009, SFT 160 made by ALPA S.p.A. and Aqualen AKU made by Leather Chemicals, can be used.

In addition, various passages can be added, such as for example washing and/or drying known to those skilled in art of tanning leather of animal origin.

A piece of leather obtained with the method described above was subjected to various tests to assess the actual fire resistance.

In particular, said leather which was the object of the tests was obtained with the following steps in sequence:

pH stabilizing with a solution of calcium bicarbonate in water in a drum at a temperature of 38°C, in which the final concentration by weight of bicarbonate dissolved in water was 2%;

- after pH stabilizing, treating with a solution of synthetic tannins (a product referred to as Terotan PMB) in a final concentration by weight of synthetic tannins of 12%, for a duration of lh and half;

leather washing with water and oxalic acid in a final percentage of 0,6% v/v;

- in order to confer tear resistance, adding water into the drum, with tannins in final percentage by weight of 12%, fatliquors in a final percentage by weight of 12% and resins in a final percentage by weight of 6%; this step lasted for 60 minutes;

leather swelling with a mixture comprising anionic fatliquors derived from fatty acids in a final concentration v/v of 5%, anionic fatliquors based on sulphited fish oils in a final concentration v/v of 4%, and of the modified polymer with filler in a final concentration v/v of 4%; immersing the leather in a solution of water and oxalic acid, the latter in a percentage v/v of 5%;

- immersing the leather in a water bath at a temperature of 45°C, with addition of dyes with a final concentration by weight of 2% and formic acid with a final concentration v/v of 18%;

having reached the desired color, washing in cold water at a temperature of 22°C;

- drying in oven at 30°C with a humidity of 70% for lh and 45 minutes; stretching and fixing with polyurethane resin in a final percentage v/v of 4%;

conferring fireproof properties with a dispersion of dispersion of anionic polyurethane base in a final concentration v/v of 4%, with an acrylic and auxiliary water-based polyurethane composition in a final concentration v/v of 5%, and a crosslinker based on a polyfunctional nitrogen group in a final concentration v/v of 1,5%;

spraying of feel modifiers in percentage by weight of 1,5% in a solution comprising organic polycarbonate in a final concentration v/v of 1%, a polysiloxane in a final concentration v/v of 1%, and a crosslinker in final concentration v/v of 1%.

The test first assessed resistance to a flame applied directly on the leather; the leather was subjected for about 60 seconds to a flame, at a temperature higher than 800°C and lower than 900°C. The result was that the leather was only minimally damaged by the flame but did not catch fire. Table 1 shows the parameters used for the tests.

Table 1

The test results are shown in table 2.

Table 2

As shown in Table 2, the leather resists fire to the extent that it does not ignite when directly exposed to the flame and has a very small burn length. The leather thus complies with the main standards for use on passenger transport vehicles, such as aircraft or boats or any other vehicles.

Indeed, said regulations require the average extinguishing time to be less than 15 seconds and the average drip extinguishing time to be is less than 3 seconds. The extinguishing time of the leather which is the object of the present invention cannot be evaluated because it does not ignite and is therefore lower than said parameters.

Table 3 shows the data measured as a result of direct exposure of the leather to fire.

The flaming test is performed by positioning the leather vertically, with a flame positioned near the free, non-constrained portion of the leather. The flame is applied for a time of at least 20 seconds and 60 seconds and is then extinguished. The flame reaches a temperature between 800°C and 900°C. The test is performed inside a sealed enclosure to evaluate the release of gases and toxic substances.

Table 3 Test mode: FLAMING

Concentrations in ppm

From the tests performed and shown in Table 3, it can be inferred that the leather, obtained using the method described above in detail has a very low release of the main toxic compounds which cause of death during a fire.

For example, the release of hydrofluoric acid, hydrochloric acid, hydrocyanic acid, sulfur dioxide, sulfuric acid and nitric oxide and nitrogen dioxide, which are very toxic and, in some cases, odorless compounds, is less than 1 ppm (parts per million).

Concurrently, the concentration of carbon monoxide is between 543 and 577 ppm, on average 560 ppm.

Advantageously, said leather is fireproof and may be used for upholstering passenger transport vehicles.

A second advantage is that the leather, in addition to being completely fireproof, releases a very low amount of toxic substances during combustion.

Another advantage is that the obtained leather has excellent appearance.

A yet another advantage is that the leather treatment method can be applied to any type of skin of animal origin.