TITLE

TANNING PROCESS FOR PRODUCING LEATHER WITH HIGH ELASTIC PROPERTIES AND

LEATHER OBTAINED

TECHNICAL FIELD

The present invention concerns a chemical process included in the tanning process and suitable to obtain leather having peculiar aesthetic and functional qualities.

The invention also concerns leather having peculiar aesthetic and functional qualities obtained by certain tanning processes.

STATE OF THE ART

The tanning process is long and complex and it is essentially a chemical process consisting of successive phases that alternates with mechanical operations, in which animal hides are made not only rot-proof but also commercially attractive by giving them certain features of colour, softness, texture, smell, tenacity, resistance and other features.

The hides used in the tanning process can be derived from any species of land, marine, amphibian or bird animal, whose use is of course permitted or regulated by law.

The whole process can be divided into three general stages: tanning, post/re-tanning and finishing treatments. To each of these macro-phases belong a plurality of individual phases with specific purposes.

The tanning consists of all the operations preparatory for tanning, also known as the beam house operations, and the tanning itself. The preparatory operations include: soaking, fleshing, dehairing, liming, splitting, deliming, bating, degreasing.

Soaking has the aim to restore the hides hydration that had lost in the preservation process, remove dirt, blood, dung and the salt in the case of salted skins or hides. It is carried out in drums with the use of water, auxiliary substances and surfactants.

Dehairing is used to remove hair and epidermis, and it is then always performed unless you have to produce a leather with hair or fur. Liming is needed to loosen the fibrous weave of the leather. Since dehairing and liming are carried out with the same chemicals, actually the two phases are carried out simultaneously and therefore are usually referred to as dehairing-liming phase or, more simply, liming phase. Dehairing and liming take place conventionally in a wet environment, preferably in a drum with a suitable amount of water and with the addition of reducing agents, in particular sodium sulphide (Na2S), sodium hydrogen sulphate (NAHS) and lime (Ca (OH) ₂ , calcium hydroxide).

Deliming, soaking and degreasing are other important chemical operations. With the deliming much of the lime used in the process of dehairing is removed from the leather. As deliming agents can be used strong acids (sulfuric, hydrochloric), weak acids (formic, acetic, boric acid etc.), or salts in acid dissociation, such as ammonium sulfate or chloride. The bating completes the action of the liming phase causing a further relaxation of the collagen structure. At this aim proteolytic enzymes are used. Degreasing is used to eliminate or, at least, to reduce the original fat content of the hide that could lead to difficulties in the absorption and fixation of chemicals and to various types of defects on the finished leather. It is carried out with the use of ionic or non-ionic surfactants. Deliming, soaking and degreasing take place in a wet environment, preferably in a drum in the presence, in addition to the above substances, of an appropriate amount of water.

Fleshing is a mechanical operation by which are eliminated residual of flesh and adipose tissue in the subcutaneous layer.

After the preparatory operations, however, the hide is still putrescible and it must be submitted to tanning, usually after pickling. The most common and popular types of tanning processes are chrome tanning and vegetable tanning while other types of tanning processes may use other reagents such as synthetic tannins, phosphonium, aluminum, or zirconium salts, aldehydes, oils or mixtures of them. The chrome tanned leathers are called Wet Blue because at the end of the operation have a typical bluish / blue colour. The tanning takes place in a wet environment.

The tanned leather is still only a rigid semi-finished product, similar to cardboard and with the colour characteristic of the type of tanning with which it was obtained. To become a marketable finished product it must be subjected to the post tanning and finishing treatments. The post tanning treatments include: sammying, pressing, shaving, splitting, re-tanning, dyeing, fattening, drying, and others.

Sammying, pressing, shaving, splitting and drying are mechanical operations.

Retanning, dyeing and fattening are chemical fundamental processes for the determination of product characteristics desired in the leather production. In particular, re-tanning is an optional treatment made with auxiliary agents and tanning agents, often different from those used for the main tanning, which serves to modify the characteristics imparted to the leather by the main tanning. For example, if you want to produce a leather less flexible than a pure chrome tanned leather, you have to perform a re-tanning with vegetable tannins. Conversely, if you want to produce a more flexible and soft leather from a pure vegetable tanned leather, you have to perform a chrome retanning. Moreover, often the tanning agents based on aluminum, zirconium or titanium are used as retanning agents to obtain a leather more reactive towards the dyestuffs and therefore to obtain more intense and brilliant colours.

Dyeing is an operation which gives the leather the desired colour. Dyeing is usually done in the drum, at a predefined temperature, with dyes, acid dyes usually, but also organometallic, cationic, reactive, sulfur, etc. dyeing agents.

The fattening (or fatliquoring) is a chemical operation which is used to introduce between the fibers of the leather a lubricant that keeps separate the fibers and allow them to slide over each other. In this way the leather can assume flexibility, softness and other qualities commercially appreciated. For fattening are used oils or fats of animal, vegetable, mineral or synthetic origin. Of course, the oils or fats as such, in order to be usable as lubricants for leather, are chemically modified or added with emulsifiers to make them water-soluble and especially to give them a certain capacity to bond with the leather. Usually, to make the lubricants soluble anionic hydrophilic groups are introduced (sulfonated, sulfated, phosphate, phosphonate, etc.), cationic (quaternary ammonium, etc.) or non-ionic. In the frequent case of fattening with anionic products, the chemism of fattening is quite similar to that of the dye for which, at times, dyeing and fattening are performed simultaneously. When the post-tanning treatments are completed, and in particular the drying, the leather is ready to be subjected to the delicate and complex operations, both mechanical and chemical, of finishing, useful to confer the desired aesthetic and functional characteristics.

Chemical finishing consists in covering the surface of the leather with a thin film of synthetic material (acrylic resins, butadiene, polyurethane) or natural material (casein, albumin, cellulose-based modified film-forming protein) which may contain pigments, colorants, opacifiers, polishing, and various auxiliary agents. The film can be made to form on the surface of the leather starting from monomers or polymers of different nature or it can be preformed and made to adhere to the leather surface with the aid of adhesives. In the first case the mixture of film-forming binder and auxiliaries is deposited on the leather surface by spraying, with rollers (roll-coater), with the curtain coater (production of "patent leather"). In the second case, the film is most frequently polyurethane based, with fancy designs, with variable thickness, on different media from which it is transferred onto the leather. For this reason, this type of finishing is often referred to generically as "transfer finishing."

In actual practice, the search for innovative products or other production needs often lead to reverse or modify the sequence of operations, coming to perform finishing at first and then re-wet the leather, retan, dye and grease, by combining and exchanging all the normal stages of the tanning process.

However, a fundamental difference between the chemical operations of finishing and chemical operations of the stages of tanning and post-tanning consists in the fact that while the latter are carried out with high absolute humidity of the leather, the former are carried out substantially dry, i.e. with absolute humidity of the leathers, referred to their dry weight, less than 25% (average finishing operations are performed with absolute humidity less than 15% of the leather, while the operations of tanning and post tanning treatments are performed with absolute humidity of the leather usually higher than 50% and even up to 2000% and more).

It is known since time the use of elastomers in the operations of finishing. For example in US Patent 4,581 ,034 it is described a process of finishing according to which the leather surface is applied a synthetic carboxylate elastomer obtained by emulsion through the polymerization of unsaturated carboxylic acids, butadiene or isoprene, styrene and acrylonitrile. The coating with elastomer based substances gives the leather properties of impermeability, elasticity, mechanical resistance, however, these characteristics depends on the integrity and uniformity of the coating layer.

Treatments are also known that are part of the tanning process in which elastomers are added to the treatment bath in a wet phase of the process, preferably during the phases of the post-tanning or tanning. In the following such a process will be called "rubberizing" and it is part of the tanning process, and "rubberized leather" will be the leather obtained from a tanning process containing a rubberizing treatment. For example, the US patent 2,949,335 describes a treatment of rubberizing, performed in the presence of at least one tanning agent, which provides for the addition in the treatment bath of an aqueous emulsion of a polymer or copolymer or mixture of polymers or elastomeric copolymers. The document cites the possibility of using mixtures of polymers and copolymers of the most varied types to get tanned leathers that have peculiar properties which are a function, among other things, of the type of elastomers added. In particular, it is mentioned the use of an emulsion containing a copolymer obtained from butadiene, acrylonitrile and styrene to obtain a leather with characteristics of high water-proofness, high abrasion resistance and high stitch-tear resistance.

Although the process defined in the cited patent US 2,949,335 can actually bring improvements in the mentioned characteristics of the leather obtained with it, it is necessary to search for solutions capable of allowing the practical application of what summarily suggested and able to optimize some specific characteristics of the process of rubberizing and to the related rubberized leathers.

Moreover, the insertion of the treatment of rubberizing in the tanning process determines the need to search peculiar mode of execution of the subsequent drying processes in order to obtain the best final result and avoid problems inherent in the characteristics of elasticity mainly acquired by a leather treated by adding elastomers during a wet phase of tanning.

Conventionally, the most often used drying process is vacuum drying that, in its broader meaning, comprises the steps of sammying, pressing (if needed) and drying. The leather, coming from the stages of the tanning process that occur in a wet environment, and therefore containing on average a quantity of water equal to approximately 400% of its dry weight, is drained by hanging or lying it by means of grippers or in another way, after which it is passed into a machine in which the rollers relaxes it and removes about 70% - 80% of moisture contained in it. The subsequent vacuum drying is done by inserting the leather between two plates heated at a temperature between 30° and 65°. Once resting the leather with grain facing down on the lower plate, the upper plate is made to descend and a suction device creates a certain degree of vacuum between the two plates. Proper drying of the leather up to a residual humidity less than 20% of the dry weight is obtained by setting the temperature, the degree of vacuum and the drying time.

When drying has to be performed on a rubberized leather it is necessary to prevent in a particularly accurate way wrinkling which would be particularly difficult to remove by sammying and drying or even by using other means specifically designed for the purpose. Conventionally, damages such as wrinkles or folds, or excessive shrinkage of the leather, are avoided by keeping the leather stretched, between the output from the last wet phase and the sammying or vacuum drying, through the use of mechanical organs, in particular pincers, acting close to the perimetral areas of the leather itself. This mechanical action causes however a considerable lack of homogeneity of physical and mechanical characteristics of the leather between the pinched areas and the areas most distant from them, namely the central areas of the piece of leather.

Conventionally, after drying, finishing steps are performed which include pressing. The pressing conventionally performed by means of presses equipped with punches, requires temperatures higher than the drying temperature, typically between 90° and 130° C and pressures typically between 250 and 300 Bar. Another important variable of the pressing process is the duration of the pressure that is on average between 5 seconds to 20 seconds. It is known that pressing can be performed simultaneously with vacuum drying, using the vacuum drying machines equipped with three- dimensional plates provided with the pattern to be imprinted in the leather. This type of vacuum drying with pressing is very advantageous as it allows savings in time and costs for machinery, however, does not give optimal results when used on not rubberized leather or on conventional rubberized leather. SUMMARY OF THE INVENTION

Object of the present invention is to propose a tanning process comprising a treatment of "rubberizing", i.e. a treatment in which substances containing elastomeric polymers are added during a wet phase of the tanning process, which allows producing leather with peculiar aesthetic and functional characteristics.

Another object of the present invention is to propose a tanning process comprising a process of "rubberizing" which allows making leather that have peculiar and optimized pressability characteristics with various pressing systems used in the field, and / or stitch-tear resistance and / or break grain resistance.

Another object of the present invention is to propose a tanning process comprising a process of "rubberizing" which allows making rubberized leather with chemical-physical characteristics substantially constant throughout the thickness of the leather.

Another object of the present invention is to propose a tanning process comprising a process of "rubberizing" which allows avoiding the problems typically associated with the drying of rubberized leather.

Another object of the present invention is to propose a tanning process comprising a process of "rubberizing" which allows optimizing the processes of pressing and drying of the leather obtained.

Further object of the present invention is to propose rubberized leather that have better characteristics, compared to conventional rubberized leather, from the point of view of the stitch-tear resistance, the break grain resistance, the homogeneity of the physical and mechanical characteristics in the entire thickness of the leather, the printability, the shape memory.

According to an aspect of the present invention the above objects and others are achieved by means of a method according to what set forth and characterized in the independent claim 1 .

The claims dependent on claim 1 describe other characteristics of the present invention or variants to the main inventive idea.

Typically, the tanning process comprises a plurality of chemical operations in the wet phase, with absolute humidity of the leather, referred to their dry weight, more than 25%. A conventional rubberizing process provides, during at least a wet phase of the tanning process, the adduction into the processing environment of a given quantity of elastomers.

According to the present invention, the aforesaid elastomers are constituted by a polymer or a mixture of polymers belonging to the family of carboxylated thermoplastic elastomers.

Advantageously, the polymers used are carboxylated polymers belonging to the group R according to DIN ISO 1629, said polymers or mixture of polymers being added in an amount (referred to the dry weight of the product) of at least 0,1 % of the weight of the fresh salted leather, or 0.5% of the weight of the wet shaved leather, or 1 % of the dry weight of said leather.

The rubberizing treatment performed by adding to the treatment bath a polymer as defined above generates between the leather and the polymer itself carbo-amide bonds which are very strong stable and planar covalent bonds, substantially not obtainable with the addition of elastomers of different type, which are mainly responsible for the high qualities of stitch-tear resistance and break grain resistance of the leather rubberized with the process of the invention.

Alternatively, the above polymer is a polymer selected from carboxylated chloroprene rubber (neoprene)(XCR), carboxylated isoprene rubber (XIR), carboxylated nitrile rubber (XNBR), carboxylated styrene butadiene (XSBR), or a mixture of these and possibly of the said carboxylated styrene butadiene copolymer.

Advantageously, when the elastomer is to be added, the treatment environment (bath) is anionic or cationic with alkaline pH referred to the isoelectric point of the leather, conditions in which the penetration of the elastomer in the dermis of the leather is promoted, and subsequently to a determined treatment time, the treatment bath is made acidic with pH advantageously comprised between 2 and 4.

Still advantageously, the rubberizing treatment according to the invention is followed by at least one treatment phase at controlled temperature and / or pressure.

More precisely, a tanning process according to the invention provides, after the rubberizing treatment, the preservation of the leather in a wet environment such that, between the last wet phase of said tanning process and a subsequent drying step of said tanning process, the humidity of the leather is never lower than 250% of the dry weight of said leather.

Advantageously, the wet preservation is done by keeping the leathers in containers for liquids, immersed in water or other water based liquid.

Advantageously the drying step comprises a step of sammying and a step of vacuum drying.

Preferably, after the extraction of the leather from the aforementioned wet preservation environment the leather is subjected to a drying process in such a way that between the extraction of the leather from the wet preservation environment and the moment in time in which the residual humidity of the leather is not more than 40% of the dry weight of the leather a period elapses not exceeding two hours.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the process of the invention will become more easily understood from the following description of specific embodiments, non-limiting, provided with the aid of the attached drawings in which:

Figure 1 shows enlarged photographs taken at microscope of a section of leather tanned without rubberizing process: Fig. 1 a shows a magnification of 500x, 100 urn, Fig. 1 b shows a magnification 500x, 20μιτι;

Figure 2 shows enlarged photographs taken at microscope of a section of the leather tanned with a leather tanning process comprising a rubberizing process according to the invention: Fig. 2a shows a magnification of 500x, 100 urn, Fig. 2b shows a magnification of 500x, 20μιτι;

Figure 3 shows the results of a FTIR - ATR analysis performed on various layers of a sample of leather tanned with rubberizing process according to the invention: Fig. 3a shows the surface of the grain, Fig. 3b shows a second layer from the grain side, fig. 3c shows a third layer from the grain side, fig. 3d shows a fourth layer from the grain side; Figure 4 shows a diagram of comparison of the spectra obtained with ATR analysis on the surface of the grain side of a rubberized leather according to the invention and on the surface of the grain side of a leather not rubberized;

Figure 5 shows the spectra obtained with FTIR- ATR analysis on a sample of leather treated with elastomers in the finishing process (non-wet tanning phase): Fig. 5a shows the surface of the grain, Fig. 5b shows a second layer from the grain side, fig. 5c shows a third layer from the grain side.

DESCRIPTION OF PREFERRED EMBODIMENTS

Example 1

A load of hides intended for the production of leather suitable to be finished by painting processes, consisting of 100 kg of the shaved weight of half calf full grain, of a thickness of about 1 mm, which were tanned with chrome tanning and retanned with synthetic tannins, acrylic resins and synthetic greases, are subjected in a drum to the further post-tanning treatments specified below.

It is performed a first operation of washing by adducting in the drum a chemical bath consisting of a mixture of 600 kg of water at 30° C and 0.5 kg of ammonia in aqueous solution at 30%, and keeping the leathers in such environment for about 30 minutes, after which the leathers are drained, a wash cycle is run with 200 kg of water and the leather are then drained again for about 5 minutes.

It is then performed a fatliquoring operation by adding 100 kg of water at 30° C and 4 kg of a chemical product for synthetic polymer fatliquoring. After 30 minutes of treatment are added a further 12 kg of chemical products for fatliquoring, both synthetic and natural, comprising inter alia natural sulfitated, for a treatment period of 90 additional minutes. Finally, 1 kg of acetic acid is added with which the treatment continues for further 30 minutes, after which the leathers are drained and washed with 300 kg of water.

Then, it is performed a dying operation by adding acid dyes to the chemical bath chemical and maintaining for about 30 minutes.

Then, several washing cycles are performed lasting 5 minutes each with 300 kg of water at 30° C to bring the leathers to a pH above 4.5. It is then performed a retanning lasting 10 minutes with a chemical bath consisting of 50 kg of water at 30° C and 4 kg of a retanning aldehyde.

Subsequently, in the same environment of treatment, which is anionic at alkaline pH, the rubberizing operation is performed by adding 24 kg of AMBRASAN DAS ^® (commercial name of a mixture of carboxylated thermoplastic polymers) and keeping the environment of treatment for 20 minutes, after which are added 100 kg of water at 50° C and 3 kg of formic acid, which makes the treatment bath with acidic pH equal to about 3.5, for a further duration of 5 minutes, at the end of which the leathers are drained.

It Is then carried out a further dyeing operation by inputting 2 kg of an acid dye and maintaining for 10 minutes and then entering 2 kg of formic acid and keeping for a further 30 minutes, after which the leathers are drained and washed.

Finally, a further fatliquoring operation is performed by entering 200 kg of water at 50° C and 4 kg of a natural sulphited product for fatliquoring, and keeping the leathers in such an environment treatment for about 60 minutes, after which they are drained. A further 100 kg of water at 50° C and 1 kg of formic acid are then adducted, for a duration of 60 minutes.

At the end, the leathers are drained and washed with 300 kg of water. Then, the leathers are removed from the treatment environment (drum) and immediately placed in a tube full of water in which they are kept immersed until they are subjected to the drying phases. Drying can be done in the same production facility in which tanning took place or in a different one. In this second case it is obviously essential that the tube or other container in which the leathers are stored can be sealed to prevent leakage of water during transport.

The drying operation is then carried out which takes place in an environment with pressure below atmospheric pressure while keeping the leathers at a temperature between 40° C and 45° C up to obtain leathers with absolute humidity compared to their dry weight not exceeding 30%.

More precisely, the leathers are extracted from the wet preservation bath. At this point, the leathers, which have a humidity degree more than 300% of their dry weight, are subjected to sammying in a machine comprising several opposed rollers that squeeze the leather and by at least one roller provided with blades oriented so as to comb and lay the leather.

Immediately after sammying, the leathers are subjected to vacuum drying, preferably in a multilayer leather vacuum dryer. The degree of vacuum, the temperature of the plates and the drying time (4 - 5 minutes) are adjusted to obtain leathers with a residual humidity content not exceeding 40% of their dry weight.

Subsequently, the drying is completed at room temperature up to a value of absolute humidity equal to 12% (measured with a hygrometer).

The leathers obtained can then be subjected to the desired finishing operations, with final painting.

The advantages derived by the insertion of the rubberizing procedure of the invention in the tanning process as described above, are immediately apparent in the following Tables 1 .1 and 1 .2 in which there is a comparison between certain technical characteristics of a tanned leather ready for the next finishing process (preferably by painting) produced according to a conventional tanning process and a leather produced for the same purpose but which has undergone the process of rubberizing of the invention according to the example above.

Table 1 .1

In particular, Table 1 .1 refers to the experimental values for "Distension and tensile strength of the grain - Ball burst test method (UNI 1 1308)", while Table 1 .2 refers to the experimental values of "Determination of the stitch-tear strength- single-edge tear (UNI EN ISO 3377-1 ) and double-edged tear (UNI EN ISO 3377-2). "

From the table it is clear immediately an average increase of about 30% of the above values.

The above-mentioned improvement of the performance of the leather, in particular the stitch-tear resistance and tensile strength of the grain are obtained thanks to the peculiar rubberizing process described above.

The polymer used which is contained in the product AMBRASAN DAS ^® has excellent solubility and elasticity. Compared with the same polymer not carboxylated from which it comes, it is a polar copolymer obtained by subjecting it to a further process of carboxylation (addition of a carboxyl group- COOH on the diene bonds) thus acquiring polar characteristics with carboxyl groups arranged on the molecular surface.

The polarity property of the carboxylated polymer molecule is of primary importance in the rubberizing process as it makes the polymer more soluble in polar solvents (the tanning bath is in fact a polar bath since it is aqueous)^ and capable of forming covalent carbo-amide bonds with the protein compounds of the hides. In fact, the hides is chemically constituted for the most part by collagen whose structural unit is constituted by the tropocollagen.

The tropocollagen is nothing more than a repetition of Glycine-hydroxyproline (or proline)-X (where X is any amino acid). These repetitions of sequences are held together by hydrogen bonds to form a kind of triple helix.

The various fibers of tropocollagen are then linked by cross-links between two allysine or between a lysine and an allysine. The carboxylated polymer added to the treatment bath will link to the lysine / allysine, resulting in the formation of the carbo-amide bond.

The carbo-amide bonds formed are planar stable covalent bonds, where all components lie on the same plane: in fact, there is no rotation in the bond C-N that turns out to be shorter than a C-C simple bond This bond is appreciable by SEM analysis conducted on samples of bovine hides tanned in drum using a conventional receipt for the obtaining of a conventional chrome tanning (see Fig. 1 and Fig. 1 A) and the same hides tanned in drum by using the rubberizing procedure with AMBRASAN DAS® (see Fig. 2 and Fig. 2A).

From the comparison of the morphologies obtained with the SEM of the two hide samples (in particular of Fig.1 and Fig. 2) it can be noted that, in the case of rubberized leather (Fig. 2), the arrangement of the fibers of the leather does not follow any more a random and chaotic order but they are neatly arranged in parallel one over the other: the carboxylated polymer fills the usually empty interstices of the dermis and immobilizes them. As supposed, in fact, the rubberized leather appears much more organized along its entire cross section (therefore also more in depth).

To support this argument, namely the coupling / impregnation of the polymer through its carboxyl within the dermis, in addition to SEM analysis, IR analysis were also conducted. Namely, FTIR-ATR analysis were carried out, directly on the leather with the aid of Chemical Imaging technique.

In particular, the leather was "split" into many thin slices that were subjected to FTIR-ATR analysis on the surface of the grain side in order to control the compliance of the characteristic peaks of the polymer within the layers of the dermis.

The results showed that the carboxylated polymer is penetrated in the totality of the section of the hide, as shown by the spectra of Fig. 3.

To appreciate the differences between a non-rubberized leather and a rubberized leather according to the process of the invention, the spectra were drawn up on the surface of the grain, of the not rubberized sample and of the sample rubberized with AMBRASAN DAS ^® (Fig. 4).

Furthermore, from the spectra of Fig. 5 it is also possible to deduce the significant differences between a rubberized leather according to the process of the invention and a leather treated only in the finishing stage with rubberizing products, albeit with the aid of penetrating agents.

As it can be seen in Fig. 5, the characteristic peaks of the polymer disappear by removing the surface coating applied by the finishing process. The carboxylation may also affect the possible formation of hydrogen bonds that may form on the surface and in the section of the dermis for producing a more stable rubberized leather.

The ability of the carboxylated polymer to form carbo-amide bonds in the entire thickness of the treated leather is influenced by the treatment environment. In fact, the preparation of a treatment bath initially anionic and alkaline prevents the precipitation of the product containing the elastomeric polymer which remains therefore available for penetration into the leather and the formation of bonds with the latter. Moreover, the penetration of the polymer (polar) in the leather is strongly promoted by the anionic nature of the treatment bath as the basic pH of the treatment bath gives a positive charge to the leather and promotes the penetration of the polymer that is negatively polarized.

Subsequently, the treatment bath is made acidic, thus creating the best conditions for the formation of carbo-amide bonds between the leather and the elastomeric polymer.

The "rubberized" leather obtained by the rubberizing process according to the invention has mechanical properties very similar to those of the elastomer contained in it. However, the chemical bonds that give the leather such features are consolidated only with the loss of humidity by the leather, while as long as the leather remains moist it can be worked and its shape can be changed. As the leather loses moisture it contains, the internal chemical bonds are consolidated and the leather acquires the shape memory typical of the elastomer contained in it. For this reason, it is essential that, not only the actual drying stage, but also any loss of moisture of the leather subsequent to the rubberizing treatment occur in a controlled manner. According to the present invention, between the end of the tanning processes in a wet environment and the beginning of the drying phase, the leather is kept immersed in water or other suitable aqueous liquid so that the chemical bonds that give the rubberized leather its peculiar properties keep being "relaxed" until the drying process begins, during which the leather is stretched and kept lying. In this way, the formation of wrinkles or other malformations is effectively avoided and the rubberized leather is microscopically and macroscopically homogeneous.

Example 2 A load of crust ex wet-blue leathers consists of 100 kg dry weight of half calf French nubuck, which have been tanned with chrome tanning and retanned with synthetic and vegetable tannins and fatliquored with natural fattening agents, are subjected in the drum to the further post-tanning treatments specified below.

It is performed a first operation of wetting back by entering in the drum a chemical bath consisting of a mixture of 600 kg of water at 30° C and 0.5 kg of ammonia in aqueous solution at 30%, and keeping the hides in that environment for about 30 minutes, after which the hides are drained, a wash cycle is run with 200 kg of water and the hides are drained again for about 5 minutes.

It is then performed a fatliquoring operation by adducting 100 kg of water at 30° C and 6 kg of a mixture of polymeric synthetic fatliquors. After 30 minutes of treatment are added a further 25 kg of chemical products for fattening, both synthetic and natural, comprising inter alia amphoteric natural fatliquors and sulfated natural fatliquors, for a treatment period of 90 additional minutes. Finally, it is added 1 kg of acetic acid with which the treatment continues for further 30 minutes, after which the hides are drained and washed with 300 kg of water at 30° C.

It is then performed a conventional dyeing operation.

Several washing cycles are then performed lasting 5 minutes each with 300 kg of water at 30° C to bring the hides to a pH above 4.5.

It is then performed a retanning lasting 10 minutes with a chemical bath consisting of 50 kg of water at 30° C and 4 kg of a retanning aldehyde.

Subsequently, in the same treatment environment, the rubberizing operation is performed by adding 20 kg of a thermoplastic carboxylated elastomeric copolymer, and keeping the treatment for 30 minutes, after which are added 100 kg of water at 50° C and 3 kg of formic acid for a further duration of 10 minutes, after which the leathers are drained.

Finally, one last chemical operation is performed by entering 100 kg of water at 50° C and 1 kg of formic acid, for a duration of 60 minutes.

At the end, the leathers are drained and washed with 300 kg of water. The leathers are then extracted from the drum and preserved as described in the previous example. It is then performed the drying operation. In this case, the vacuum drying occurs in a vacuum drying machine in which the plates between which the leather is laid are suitably three-dimensionally shaped to constitute respective moulds and counter moulds. In this way, a pressing phase of the leather takes place simultaneously with the step of vacuum drying with obvious advantages from the point of view of economy of the overall process. In fact, out of the vacuum drying machine you get a rubberized leather that is three-dimensionally shaped and / or pressed with patterns in relief, logos and other graphic symbols.

The above process of vacuum drying and simultaneous pressing is conventionally known, but it is particularly advantageous thanks to the use of a rubberized leather according to the present invention. In fact, it is possible to perform the pressing with parameters of temperature and pressure much lower than what is the case of the press in the finishing process, thanks to the fact that the leather still has a high degree of humidity that significantly lowers the plasticization temperature/pressure values of the polymer contained in it. The rubberized leather may then be pressed during the drying phase deforming it with a high degree of precision and definition and, after drying, the high shape memory due to the homogeneous presence of the elastomer throughout the thickness of the leather ensures that the shape and patterns achieved are kept. In addition, the execution of pressing at low temperatures (average 30-65° C instead of 90-130° C) prevents thermal stress to the leather surface causing hardening, burning or other damage.

Finally, the rubberized leather obtained may possibly be subject to the desired finishing operations.

The advantages derived by the insertion of the rubberizing procedure of the invention in a tanning process as described above, are immediately apparent in the following Tables 2.1 and 2.2 in which is performed a comparison between the values of certain technical properties of a tanned leather from semi-finished wet-blue produced according to a conventional tanning process and a leather produced for the same purpose but which has undergone the rubberizing process of the invention according to the example above.

Table 2.1

I Specimen | Not Rubberized leather | Rubberized leather Elongation at break Elongation at break

N° mm mm

1 6.89 9.89

2 7.25 10.01

3 7.89 10.18

Average 7.34 10.02

Table 2.2

In particular, Table 2.1 refers to the experimental values for "Distension and tensile strength of the grain - Ball burst test method (UNI 1 1308)", while Table 2.2 refers to the experimental values of "Determination of the tear strength- single-edge tear (UNI EN ISO 3377-1 ) and double-edged tear (UNI EN ISO 3377-2)."

From the table it is clear immediately an average increase of about 30% of the above values.

Moreover, even the press in the finishing process is particularly advantageous with a leather produced with the rubberizing process of the invention as it is possible to obtain a pressed pattern with very high definition thanks to the physical and mechanical properties of the elastomeric polymer contained in the leather which are acquired substantially by the leather itself, with possible advantages in preventing counterfeiting.

Obviously, the processes described above are only exemplary and not limiting and a method of rubberizing according to the present invention, as well as the tanning process of which it is part, can also be significantly different.

Indeed, hides that, during the tanning process, can be subjected to the rubberizing procedure described above may arise from any species of terrestrial, marine, amphibious or volatile animal and the tanning process is optimized as a function of the type of hides to be treated and aesthetic and functional properties of the leather to be obtained which, in turn, may be suitable to realize large and small leather goods, footwear, uppers and soles for shoes, clothing, furniture, furs, bookbinding, technical articles, upholstery, and more yet. The rubberizing procedure is advantageously implemented by means of the drums inside of which the proper chemical treatment environment can be created and contextually they are able to exert a mechanical action on the leather. However, the rubberizing procedure could also alternatively be implemented in other machines for wet tanning phases such as stationary tanks, reels, washing machines, blenders or other suitable containers.

The elastomers used are preferably carboxylated polymers selected from carboxylated chloroprene (neoprene) (XCR), carboxylated isoprene (XIR), carboxylated nitrile rubber (XNBR), carboxylated styrene butadiene copolymers (in various percentages) (XSBR), but other different carboxylated thermoplastic elastomers can be used.

The above listed elastomers, soluble or solubilized in water, are used individually or in admixture between them or with other types of products.

Preferably the rubberizing method occurs simultaneously or in succession to operations of retanning, dyeing or fatliquoring. However, the process of the invention may also be carried out simultaneously or in succession to one or more of the preparatory operations, or even simultaneously or in succession to the operation of tanning, de-acidifying or fixation.

In any case, it is advantageous that, at the time of the addition of the elastomer, the treatment environment is anionic with alkaline pH referred to the isoelectric point of the leather and that, subsequently to a certain treatment time, preferably of a minimum of 15 minutes, the pH is made acidic (still with reference to the isoelectric point of the leather), for example by the addition of acids such as formic acid, acetic acid, or other organic acids preferably with short chain. In such conditions it is initially promoted the penetration of the elastomer in the dermis of the leather and subsequently its fixation and exhaustion.

The minimum amounts of elastomers (referred to dry weight) used in the rubberized process of the invention, referred to the weight of the hides are equal to 0.1 % compared to the weight of the fresh salted hide, or 0.5% compared to the weight of the wet shaved hide, or 1 % compared to the dry weight. Obviously amounts of elastomers can be used also much higher than those indicated depending on the properties of the leather that are desired. For example, particularly appreciable properties are obtained with amounts of elastomers equal to 1 % by weight of the fresh salted hide, or to 5% compared to the weight of the wet shaved hide, or 10% compared to the dry weight.

The leathers are held in a chemical bath containing elastomers for a period of time that is a function of the leather article to be produced and preferably of at least 15 minutes. This allows a sufficient penetration of the elastomers in the inner layers of the leather. However, the concentration of elastomer in the chemical bath and the duration of treatment are variable depending on the type and properties of the leather article required and / or desired.

The rubberizing procedure can be performed, still in the wet phase, even subsequently to the finishing operations. Especially in this case it is particularly advantageous that the rubberizing procedure is followed by drying operations that occur at controlled temperature and / or pressure. In particular, the drying may advantageously be made by using vacuum dryers or drying ovens.

The leathers which have undergone the rubberizing procedure are consistent, elastic, rubbery, have excellent printability, high-definition, have excellent stitch tear resistance and tensile strength of the grain and improved mechanical strength in general, better resistance to high temperatures and pressures, better resistance to water absorption and at bleeding. Leather articles for smooth items, appear to have adherent and not blown grain; leathers with drummed grain are characterized by acquiring a very full and natural appearance. In addition, flesh side leathers (or splits) presents a particular compactness.

The advantages of a process according to the invention remain unaffected even in the presence of further variants or changes, made still remaining within the scope of protection defined by the following claims.