MANUAL FOR OXAZOLIDINE TANNED LEATHER

Transcription

1 () MANUAL FOR OXAZOLIDINE TANNED LEATHER LIFE+ Proyect Enviromentally Friendly Oxazolidine-Tanned Leather (OXATAN) PROJECT COORDINATOR CENTER FOR TECHNOLOGY AND INNOVATION PROJECT PARTNERS

2 CONTENTS 1. INTRODUCTION THE MANUFACTURE OF LEATHER The origins of tanning The industrial processing of the hides and skins Beamhouse operations Tanning Post-tanning Finishing THE OXAZOLIDINE LEATHER TANNING TECHNOLOGY Background Starting materials: hides and skins, equipment and chemicals Technical procedure Technical validation of oxazolidine-tanning technology Basic safety and health guidelines for the oxazolidine-tanning process ANNEX I. SAFETY DATA SHEETS OF (GENERIC) CHEMICALS ANNEX II. LEATHER QUALITY STANDARDS (PHYSICAL PARAMETERS) ANNEX III. BIBLIOGRAPHY

3 1. INTRODUCTION. The traditional tanning process, used in more than 90% of leather tanned worldwide, consists in the application of trivalent chromium salts, which interact with the skin s collagen, acting this way as a tanning agent. This process confers leather excellent physical properties and a high stability to manufacturing processes and the passage of time. However, in some cases, it can cause allergies to chromium and even on some occasions the chemical characteristics of chromium can change and be harmful to the environment. Given the growing environmental pressure to which tanning industries are subjected and the tendency to increase the environmental requirements for leather, recently INESCOP, Centre for Innovation and Technology, has been carrying out different R&D actions to develop new tanning techniques, alternative to the traditional chrome tanning, that allow the environmental performance of tanneries to be improved. As a result of these actions, the project titled Environmentally-friendly Oxazolidine-tanned Leather (OXATAN) was launched, co-funded by the European Commission through the LIFE+ Environment Programme. This project proved that tanning with oxazolidine, combined with other vegetable or synthetic tanning agents, makes it possible to obtain high performance leather while avoiding the presence of metals (chromium) both in liquid and solid waste; this way, the environmental impact generated during the whole tanning process can be dramatically reduced. This Manual describes the methodology for oxazolidine leather tanning developed in the framework of the project Environmentally-friendly Oxazolidine-tanned Leather (OXATAN)

4 2. THE MANUFACTURE OF LEATHER. Tanning is one of the oldest trades of humanity, with references on the use of leather in cave paintings and archaeological sites worldwide that prove this. Tanning had an accidental nature in its prehistoric origins, slowing evolving until turning in the Middle Ages into a craft. At the end of the nineteenth century, the first scientific studies were started which, with the industrial revolution and the development of the relevant machinery, derived into the current leather production technology The origins of tanning. Primitive man hunted wild animals for food; he removed the hides and skins from the dead animal carcass and used them as crude tents, clothing and footwear. The earliest record of the use of leather dates from the Palaeolithic period, cave paintings discovered in caves near Lerida in Spain depict the use of leather clothing. Excavation of Palaeolithic sites has yielded bone tools used for scraping hides and skins to remove hair. The main problem for the prehistoric man was to avoid the rapid putrefaction of the skins, by first drying and then rubbing them with fat to give them a greater softness. It is assumed that primitive man also discovered accidentally that the smoke from wood fires could preserve the hides and skins, as did softening them with tannin-containing barks, leaves, twigs and fruits of certain trees and plants. With the passage of time, they discovered ways to remove the hair using wood ash and burnt limestone and, much later, the capacity of certain minerals (alum) to stabilise the skins obtaining whiter and softer leather thanks to their content in aluminium

5 Figure 1. Cave paintings of La Valltorta in Castellón-Spain and a cowhide shoe found in Armenia, BC. The first civilisations (Mesopotamia, Sumer, Assyria, Persia, etc.) used tanned leather for footwear manufacturing and other articles as shown in different icons drawn on ceramic pots, jars and engravings found in burial tombs. The Egyptian civilisation is the one that has brought the most information on leather tanning, in the form of engraving in papyruses or in wall paintings and objects in Egyptian tombs; sandals, clothes, gloves, buckets, bottles, shrouds for burying the dead and military equipment, due to their belief that the dead should be buried with all of their possessions to enjoy them in the next life. Figure 2. Reproduction of an Egyptian papyrus showing leather tanning

6 The ancient Greeks and Romans also made extensive use of leather. The Romans used leather on a wide scale for footwear, clothes, and military equipment including shields, saddles and harnesses. Figure 3. Roman leather footwear. Oiasso, Irún, Basque Country. The tanning process evolved with time until turning, during the Middle Ages, into a craft. Most towns and villages had a tannery, situated on the local stream or river, which they used as a source of water for processing and as a source of power for their machines. In many towns there is still evidence of this in street names. Also, at that time religious communities had a prominent role in the progress of this activity, since their monks were expert at making leather, especially vellum and parchment for writing purposes and bookbinding. During the Middle Ages, rawhides and skins were treated by first immersing them in a fermenting solution of organic matter in which bacteria grew and attacked the hides or skins, resulting in a loosening of the hair or wool and some dissolving out of skin protein

7 The hair or wool was then scraped off with primitive scrapers and fat or meat still adhering to the flesh side was removed in a similar manner. Tanning was done by dusting the rawstock with ground up bark or other organic matter and placing them in shallow pits or vats of tannin solution. The skins remained immersed for a long time in these tannin solutions given that it was a static process, finalizing it when the tannin solution had penetrated right through the skin structure. The majority of the leather was tanned with oak bark but softer and whiter clothing, gloving and footwear leathers were tanned with alum, oil, and combinations of these two materials. The leather was then hung up for several days in open sheds. The dressing of the leather involved paring or shaving it to a level thickness, colouring and treatment with oils and greases to produce attractive surface finishes. Finally the grain surface was treated with waxes, proteins such as blood and egg albumins, to produce attractive surface finishes. Figure 4. Medieval tanneries from the poor area of San Segundo in Avila (Spain)

8 At that time, leather was used for all kinds of purposes such as: footwear, clothes, leather bags, cases and trunks, leather bottles, saddlery and harness, for the upholstery of chairs, and couches, bookbinding and military uses. It was also used to decorate coaches, sedan chairs and walls. From the Middle Ages till the end of the seventeenth century, there were not many changes in the skin transformation processes, until Colbert, minister of King Louis XIV of France, gave a strong impetus to scientific analysis of tanning methods. This way, at his request, Des Billettes wrote in 1708 a work called La tannerie et la préparation des cuirs, which marked the end of the oral tradition in this technical field. At the end of the eighteenth century, the growth of industrialisation created a demand for many new kinds of heavier-duty leathers, e.g. belting leathers to drive the machines being introduced into industry, special leathers for use in looms in the textile industry, leathers for use as diaphragms and washers, leathers for use in transport and for furniture upholstery, etc. as well as other softer, suppler, colourful leather for clothing, footwear and glove making that the traditional vegetable tanning could not provide. Similarly, the progress in chemistry at the end of the nineteenth century was critical for the development of the tanning industry, with the discovery and introduction of basic chemicals like lime and sulphuric acid, the range of coal tar dyes, etc. Finally, the discovery of new tanning products, mainly chromium and aluminium, marked the beginning of industrial scale tanning

9 In 1853, the French Cavalin discovered the use of trivalent chromium salts that reacted with the collagen fibres of the skin, thus obtaining strongly tanned leathers with never before obtained characteristics, like the boiling and tear resistance, as well as a good elasticity and water vapour permeability. In 1858, Knapp patented tanning with two baths and finally, between 1887 and 1892, A. Schultz achieved the first tanning with only one bath. Since then, trivalent chromium salts have been used as a tanning agent, and presently 90% of leather is tanned this way worldwide. Likewise, various artificial substitutes of the natural substances always used for tanning were discovered. These synthetic tannins marked an important field of application of chemistry to the leather industry and since then they have continued to be studied. This change in what refers to tanning substances was accompanied by something similar with regard to processes applied to tanning, which were mechanized in an extremely rapid way. The tanneries of the beginning of the twentieth century were big factories where the treatment of hides and skins, with all its complexity, was slowly carried out, manually and with very few mechanical means. In recent years, the work conditions have been radically transformed with the incorporation of the instrumentation that engineering, electronics and computing provide, so that a spectacular optimization could be achieved both from the output of the process and the final quality of the leathers, trying also, to minimize the impact of this activity on the environment by developing clean and innovative technologies

10 Figure 5. Paddles and drums in tanneries in Igualada, Barcelona (Spain) at the end of the nineteenth century and appearance of a current tannery. In the current industrial process, leather tanning is carried out dynamically in drums and paddles, in which the hides and skins interact with different chemical agents, mainly trivalent chromium salts, which react with the collagen s fibres to obtain stable and durable leather. After tanning, the skins are drained and shaved, obtaining an intermediate product named wet-blue due to the blue-greenish colour of chromium sulphate. If other tanning products are used, this intermediate product is known as wet-white. Then, different operations are conducted to improve its aspect and feel and confer it different textures. All the process is carried out using a sequence of well-known chemical processes and mechanical operations that are described below

11 2.2. The industrial processing of the hides and skins. The tanning process consists in turning a putrescible organic product, fresh animal skin, into a resistant, durable and very nice-looking material that can be used for the manufacture of footwear, leathergoods, upholstery, garments, etc. The hides and skins used are mainly those of cattle, sheep, goat, pig and, in a lesser amount, reptiles, birds and fish. Leather processing can start little after slaughtering the animal, but in many cases the hides and skins are stored for a long time, a preservation treatment being necessary to prevent the growing of microorganisms and the associated putrefaction. The curing process is carried out in collection centres, inside or outside abattoirs, to avoid the putrefaction and loss of quality of the raw material, which requires the time between collection and preparation to be as brief as possible. For this, when the material cannot be processed immediately, it must be cured by one of the available methods: refrigeration (chilling) for short periods, drying (by air or in chamber), salting, dry salting and use of biocides or products to prevent bacterial attack. Hides and skins are usually cured in abattoirs and sometimes the gatherers/ wholesalers represerve them, putting hides and skins in pallets, leaving them in cold areas of the warehouse or refrigerated. The degrees of curing of the skin vary, although currently all the skins are prepared to be able to be transported to any place in the world, which involves undergoing a curing treatment that prevents them from deteriorating over long periods of time and enduring very aggressive transport conditions

12 Once in the tannery, if the skins are to be stored for long time, they are kept in refrigerated to avoid deterioration and quality loss. In the industrial process numerous chemical and mechanical operations are carried out, using different technologies and reactives according to the type of skin that needs to be processed and the use of the final product, each tannery adapting basic processes to their needs. The process to obtain finished leather from fresh hides or skins can be divided into multiple steps, which in turn can include four stages: Beamhouse operations, Tanning, Post-tanning Finishing. The Beamhouse operations for a conventional process are the following: soaking, dehairing, liming, fleshing and splitting. The objective of this stage of the process is to clean the skin, remove the adipose tissue and the hair and adapt the thickness of the skin to the desired value. The Tanning steps for a conventional process are the following: deliming, bating, pickling and tanning

13 The objective of this stage of the process is to partially degrade the structure of the skin to facilitate the penetration and the subsequent fixing of chemicals, to adjust the ph to the adequate value for tanning, and to stabilise the structure of the collagen by adding tanning agents (the most common ones are chromium salts or vegetable extracts). Also, for sheepskin there is usually a degreasing step after pickling. After the tanning step the skins are now stable and in this state they are called wet-blue, if tanned with chromium, or wet-white, if another tanning agent was used. The Post-tanning steps for a conventional process are the following: shaving, neutralisation, retanning, dyeing, fatliquoring, sammying and drying. The objective of this stage of the process is to adjust the desired thickness for the skin, achieving the characteristics of fullness and colour, and to bring the skin to a suitable moisture content. In this stage the skin is called crust. The Finishing stage for a conventional process consists in diverse mechanical operations and/or the application of various products on the surface to give the leather the final texture and appearance desired. According to the type of starting hide or skin or the final product to be obtained these stages can be carried out in a different way. Consequently, many variations to the conventional process can be found

14 The stages of the tanning process are listed in Table 1: BEAMHOUSE TANNING POST-TANNING FINISHING RAW HIDES OR SKINS Sorting - Trimming Soaking - Liming Fleshing Pelt splitting Deliming - Bating Degreasing Pickling Tanning Sammying Splitting Shaving Retanning Neutralisation Dyeing Fatliquoring Sammying Drying Staking Finishing Mechanical operations Sorting Packing - Dispatching FINISHED LEATHER Table1. Stages of the tanning process

15 Beamhouse operations. In this stage the initial steps for the processing of skins are carried out from its arrival to the factory until the tanning is carried out. These are chemical processes and mechanical operations to remove all unwanted components from the skin, which are not adequate to obtain the leather, as well as to prepare the structure of collagen for the tanning stage. Most processes are carried out by immersing the skins in water and adding different chemicals in adequate containers, which can be: - Pits: These are used when the skins must remain static in the presence of liquids. In these vessels there is no considerable mechanical action, although in some cases the skins have a slight rocking motion or the liquid is re-circulated using a pump. Normally they are used in soaking and in vegetable tanning although at present they are obsolete. - Paddle vats: The skins are immersed in a bath inside a vat fitted with a rotating paddle wheel. In this case the skins are bent and there is a soft mechanical action that facilitates the penetration of products. - Drums: In these the mechanical action is much greater than in pits and paddle vats. It is internally fitted with shelves or pegs to tumble the skins. As the drum rotates, the skins are hit, bent and subjected to strong tensions and they finally fall, all of which favours the penetration of chemicals. These are cylindrical vessels that can be made of wood or stainless steel and are fitted with an airtight door to load and unload the skins

16 Figure 6. Diagram of paddle vats, drums and mixers used in different stages of the tanning process. In addition to the traditional drums, there are those called cement mixers or mixers, Y - shape drums, etc. The sequence of steps in the beamhouse stage is described below. Soaking. The aim is to remove all foreign matter from the skins and to return the skins to the hydration state they were in when fresh. This operation can be carried out both in drums, and in paddle vats. Fresh skins do not require soaking, just a wash to remove blood, dirt and dung. Cowhides present some problems in soaking, as they are thicker and therefore salt is more adhered to the fibres, which makes it more difficult for water to penetrate. The water that is used for soaking must be free from organic matter, at a temperature of around ºC and free from ferric salts that can produce stains on the skin. The products added to the soaking bath are: - Alkaline products: when the hair is not relevant given that these products break the hydrogen bridges that exist in collagen molecules and subsequently favour rehydration

17 - Surfactants: the function of which is to decrease the surface tension of water and thus facilitate water penetration in the skin. They are also able to emulsify the natural fats of the skin. - Prepared enzyme products: the function of which is to accelerate the soaking of the skin in a controlled way. - Bactericides: to avoid bacterial growth problems during soaking. The wastewater of this operation shows high organic matter content and high salinity, which can cause putrefaction problems with bad odours. Liming. The hydrated, cleaned skin, some of the proteins of which having been removed in the soaking stage, now passes to the liming stage. This operation aims to remove the epidermis and hair and to loosen the fibrous structure of collagen. To achieve this, it is necessary to use lime. However, in most hides the desired effect is not achieved, so the bath has to be reinforced with sulphides. The skins must not be left for an excessive time in this bath, since otherwise the hair would be attacked by the alkaline bath, which would dissolve hair and would not be able to be separated in the filter. The ph of the skins after leaving the bath is of about After draining the bath the skins are subjected again to some washes. At present, the hair removed in this process is filtered out from wastewater and the lime liquor is re-circulated. This hair can be used as a fertiliser

18 Figure 7. Facility for hair filtering from the liming bath. Trimming and Fleshing. The dehaired skins are brought to the fleshing area. The aim of fleshing is to clean the skin by removing the adhering fat and tissues on the underside of the skin to facilitate the penetration of chemicals applied in subsequent stages. This operation is carried out using a fleshing machine. With this process we obtain the skin ready to be split and a by-product called tallow that can be marketed. Figure 8. Cowhide fleshing stage

19 The operation ends with a manual retouch to removes what the machine hasn t removed properly. Splitting. This operation consists in placing the skin on two cylinders and using a knife, separating horizontally the skin in two layers, one called grain, which will be used do produce leather, and another one called split, which is the flesh part that can be tanned in the same factory, to obtain another external layer or can be applied to other uses Tanning. Deliming. After the splitting process, the grain layer of the skin and the suitable split undergo a deliming process. Deliming is the process that removes lime and alkaline products from the inside of the skin. This way the swelling of the limed skin is achieved. The lime dissolved in interfibrillary liquids and the one deposited on the fibres can be easily removed using some washes prior to liming. The rest of the lime is removed by adding buffered solutions of ammonium or organic salts, or carbon dioxide. These agents, combined with the alkaline products of the limed skin, provide readily water-soluble products that can be removed by a simple wash. When adding theses acids, the ph has to be equal to or greater than 7.5, otherwise the skin would undergo undesired acidic swelling

20 Bating. The objective of bating is to achieve, using proteolytic enzymes, the de-swelling and relaxation of the skin, while cleaning up any remaining epidermis, globulins, elastins, hair and fat, as a secondary effect, making the grain finer and softer grain. Traditionally, animal dung, mainly dog and chicken dung would have been used as the source of the enzymes. The digestive tracts of these animals are a rich source of the relevant enzymes. In more recent times the enzyme was extracted from pancreas of cattle. This gland produces a series of active substances able to decompose the proteins and fats. Bating with such extracts, being unhygienic and hard to control, has been displaced by artificial bates that simplify the exact dosage and correct control of the operation. Some interesting bates are: - Pancreatic enzymes-based bates. - Fungal and bacteria proteases-based bates, their activity being lower than that of the pancreatic trypsin. Degreasing. At this stage the natural fat of the skin is removed to facilitate the penetration of reagents and to avoid undesired reactions and stains on the skin. This operation is carried out by direct emulsion of fat in an aqueous medium using surfactants or solvents. When degreasing using surfactants, the process can be improved using lipolytic enzymes that degrade the skin s fat, which facilitates fat emulsion and solubility to reduce its molecule size

21 Pickling. This stage can be considered as a complement of deliming and definitive interruption of the enzymatic effect of the bating process. Also, after pickling the skin is prepared for the tanning operation. The pickling operation is more important with respect to the subsequent tanning operation, since, if unpickled, the ph of the skin would be high and the salts of the mineral tanning agent would gain higher basicity. In these ph conditions, over-tanning would affect the outer layers, which would make it difficult for the tanning agent to diffuse into the internal layers, thus shrinking the grain layer and precipitating the hydrolyzed mineral agent on it. In the pickling operation, the skin is treated with acid products that add an important quantity of acids to the skin and at the same time manage to lower its ph to 3-3.5, removing totally the skin s alkali. The acids that are used are: sulphuric acid, formic acid and acetic acid. Neutral salts are also added to the pickling bath before adding acid so as to be able to prevent the acidic swelling of the collagen. The salt that is normally used is sodium chloride, which is practically not combined with the skin, so its concentration remains almost unchanged in the waste bath. In addition to sodium chloride, it is also possible to use sodium sulphate, sodium formiate and polyphosphates

22 Tanning. The skin, once adequately prepared in previous processes, is subjected to the tanning process in which it turns into leather, namely, the skin becomes rot-proof, stabilizing its protein structure by crosslinking collagen chains with the tanning agent by means of chemical bonds. Due to the wide variety of hides and skins, it is easy to assume that there are many different types of tannages. All these tannage types can be grouped into: Tanning with inorganic products or mineral tanning, using chromium salts, aluminium, iron, titanium, etc. Vegetable tanning, using natural vegetable extracts. Tanning with organic products like syntans, aldehydes and quinones, sulphochlorinated paraffins and multiple resins. Figure 9. Unloading cowhides from tanning drums

23 Sammying. Chromium-tanned leather contains between % of water, but in order for the leather to be adequately split and shaved, it must not contain more than 50-55%. Leather sammying is easier if the skin has been previously split. This operation is carried out making the leather go through two rollers covered with filter nets. The pressure of the cylinders is transmitted to the leather s fibres and forces them to squeeze the water out, while the filters absorb the water and drain it to the outside Post-tanning. Shaving. The leather is passed through a cutting machine subjecting it to a knife that adjusts its final thickness, generating waste called shavings. Re-tanning. In the re-tanning stage, one or various tanning products are added to provide the leather with certain qualities that are not easily obtained using only one tanning agent. There are many types of re-tanning agents that can be grouped as follows: Cationic products such as metallic salts like chromium, aluminium, zirconium, organo-chromium and organo-aluminium salts. Anionic products such as vegetable extracts like mimosa, quebracho, chestnut, tara, sumac, synthetic substitution products, neutral or acidic auxiliary synthetic products, auxiliary-substitution and vegetable extract-substitution products or mixed blends

24 Resins: anionic, cationic, amphoteric, pre-polymerized, polymerized, monomeric, urea-based, melamine, dicyandiamide and acrylic resins. Various re-tanning agents, like silicate, aldehydes, polyphosphates, tanning oils and fillers of different types. Neutralisation. The main purposes of neutralisation are to remove the remains of shavings adhered to the skin, to remove unfixed tanning agents and to remove part of the initial acidity of the skin by adding alkaline salts to facilitate the penetration of the dyestuff and fatliquoring products. Dyeing. Leather dyeing includes a group of operations aiming to confer a certain coloration to the tanned skin, be it superficial, partial or total. From a chemical point of view, dyes are classified as natural and synthetic; the same way there are vegetable and synthetic tanning agents. The commercial series of dyes gather dyes of very different chemical composition, but of similar dyeing behaviour with regard to fastness, penetration power, matching capability, degree of opacity, method of use, etc. According to these characteristics dyes are classified as: acid dyes, direct dyes, basic dyes, metal-complex dyes and reactive dyes. Fatliquoring. The fibres of the wet leather move easily since it is a very flexible material; but when the leather dries it can become hard due to the fact that the fibres have dehydrated and have grouped forming a compact substance

25 The fatliquoring operation is carried out with the aim to obtain flexible and soft-feeling leather, which is achieved by the incorporation of water soluble or insoluble fats. These maintain the fibres separated and lubricate them so that they can glide off each other. The greater or lesser degree of flexibility of leather depends on the quantity and type of fat used, which conditions the product that needs to be obtained so that by varying the fat percentages and the combinations of fatliquoring agents different products are obtained. Drying. Through this operation, the moisture content of the skin is reduced to be able to carry out the finishing operations. Drying can be achieved through different systems: Drying chamber: is the oldest and most economical system and consists in natural air drying but, to avoid problems that ambient humidity can produce, it is carried out inside a chamber equipped with fans and heating batteries until the leather dries. The air can go in and out completely or be recirculated in a controlled way. Drying tunnel: consist of a tunnel through which the leathers pass slowly, hanging from a conveyor, while the hot air circulates perpendicularly to the leathers path. The drying tunnel can be divided into various sections; each one has its own temperature and air recirculation. This drying method is mainly used for leather that must have a very soft touch. If used for vegetable-tanned leather, low temperatures must be used to avoid a darkening of the leather s colour. Paste drying: consists in sticking the leather s grain side onto a glass plate. The glass plates covered with leather circulate slowly and vertically inside a drying tunnel. At the end of the process the leathers have dried, and they are removed from the glass, the glasses are washed and wet leather is stuck once again

26 Using this drying system very flat leathers are obtained with high surface yield, since the leather cannot shrink during drying because it is stuck to the glass. The grain is very fine. Secoterm drying: the leather is stuck, on its grain side, on a metallic plate inside which hot liquid circulates. The leather moisture evaporates on the grain side and must go through the whole leather thickness to be able to come out so the leather is less compact than after paste drying. It is usually used for suede and industrial leathers, being not recommended for vegetable-tanned leather because the operating temperature is ºC. Vacuum drying: the leather is spread out on a heated horizontal plate, the machine s airtight hood is placed over the leather and a strong vacuum is applied to quickly remove moisture from the leather. The drying duration depends on the thickness of the leather and will take several minutes. This drying method is normally used as previous drying to obtain a finer grain, the still humid leathers needing to be hanged for complete drying. Currently, this is the most commonly used method. Figure 10. Vacumm drying

27 Finishing The finishing operations include a series of processes aimed to improve the leather s surface appearance, protect it against chemical and mechanical effects, even out colour and shine and improve the feel of the leather. The finish provides resistance to rain, blows, rubbing and any type of external mechanical stress, and at the same time gives the desired appearance to the leather. Depending on the appearance of the leather s surface and the desired result, the finish application will be different, so if the leather s characteristics are to be highlighted, finer coats will be applied that give it shine and texture but if the leather s imperfections need to be corrected it is necessary to apply thicker coats. Also, pigments are generally added to the finish to even out and adjust the colour achieved in the previous stages. Prior to the application of the finishing products it is necessary to carry out a series of mechanical operations, some of them optional: Moisture conditioning: during drying, the leather was left with a very low moisture content, so it is necessary to carry out a conditioning with the aim to achieve a relative humidity of 20-22%. Staking: this operation intends to obtain more flexible leather. This is achieved by applying a bending and/or stretching mechanical action to the leather which separates the fibres from each other, which were joined before to confer stiffness. The softening can be carried out manually or with staking machines. Usually vibrating pivot machines are used to soften leather for footwear and leather goods, and roller machines or blades are used for garment leather

28 Final drying: the softened leather still contains certain moisture that must be removed before continuing the finishing operations. This can be achieved by nailing the skin on a board and leaving it to air dry, using a vacuum dryer, or a peg dryer. In this latter, the skins are stretched out using pegs that are placed at the edges of the leather and then fixed on perforated plates. With this final drying, the leather moisture is reduced to 12-14%. Trimming and ironing: in these operations, the creased parts or defects such as peg marks, holes, etc. are removed to give a better presentation and obtain a completely flat leather surface so that the application of finish products can be as uniform as possible. Buffing: the leather surface must be sanded with emery paper. When it is done on the grain side, it can serve to obtain nubuck or to smooth surface defects. When it is done on the flesh side, it serves to remove flesh and improve the final appearance, and if it is on split, velvety skins are obtained. If done thoroughly, suede can be obtained. The buffing machine has a metallic cylinder on which an emery paper is placed. It is applied with two simultaneous movements, one circular on its axis and the other to and fro. Dedusting: after buffing, dedusting is necessary to remove the dust from the surface of the leather. For this, air blast or brushing machines are used. The brushing machines are of a very simple construction and have two cylinders provided with brushes which rotate in an opposite direction and between which the leathers are fed. Air blast machines remove the dust using a powerful jet of air projected perpendicularly to the leather surface using some special blowers. Finishing products can be classified in different groups:

29 - Dyestuffs: are substances that are used to dye undyed or dyed leather in which it is intended to level, correct or make the colour brighter, without hiding the support (they are transparent) - Pigments: are coloured, insoluble substances that are used in aqueous or organic dispersion. Unlike the colourants, pigments hide the support s appearance so they are used to conceal defects in the leather (scratches, insect damage, etc.) - Binders: are polymers able to form films and retain the rest of the finishing products in them. They can be of various types: Protein binders (albumin, casein), which are soluble in water, insensitive to heat, form hard and discontinuous films and need rubbing or glazing to provide a glossy look; Cellulosic binders, which are insoluble in water, giving a lot of shine and are heat sensitive; and Thermoplastic binders. - Finishing auxiliaries: are substances that change the characteristics of the binder used. Some of these products are: wax, plasticizers, matting agents, fillers, thickeners, etc. - Solvents: are the ones that contain the dissolved finish products. Once applied they evaporate. These can be organic solvents or water. With regard to the type of finish, there are multiple possible combinations between the different components. Depending on the type of binder used, the most usual finishes are:

30 - Glazable finishes: they have a smooth pore, are transparent and have an excellent gloss because protein binders (casein) are used and the leather is subjected to a glazing operation. - Nitrocellulose finishes: use nitrocellulose as a binder. - Thermoplastic finishes: consist in the application of a thermoplastic emulsion (acrylic resin or polyurethane) on the leather which results in a very homogeneous coat able to cover all imperfections. Furthermore, depending on the amount of pigment contained in the finish, the following types of finish can be found: - Full aniline finish, which is completely transparent without any type of pigment, - Semi-aniline finish, which is a finish with a certain covering power that is achieved by moderately adding pigments and dyestuffs, - Pigmented finish, which has a great covering effect by adding high quantities of covering pigments. Regarding the methods of application, we can distinguish: - Spray guns: it is the most commonly used method and with it, it is possible to achieve a very thin, uniform coat that enhances the leather quality. The system is basically comprised of a conveyor belt, a spray booth and a drying tunnel. The spray booth contains the spray guns from which air and pigment are atomised (< 5 g/foot 2 in one row) in a fine mist that is not completely deposited on the leather and the excess finish must be removed using fans at the exit. The drying tunnel is made up of different sections, in each of which the air temperature can be regulated

31 Figure 11. Spray gun for the application of finishing products. - Roller coating machine: in this case, the finish is transferred to the leather by passing between two rollers. The quantity applied in just one coat ranges between 5-50 g/sq.foot. This system has a disadvantage of being very sensitive to the differences in leather feel and thickness. The machine has a metallic roller and a blade. The space between the roller and the blade is filled with the finish preparation. By rotating the engraved roller, it ends up covered with a finish layer that is later deposited on the leather while being able to impart special effects. There are other machines that do not apply finishing products but are used in the finishing section as an intermediate step in the application of the different finish coats, like the ironing press, the plating press, the glazing machine, the polishing wheel, etc. In short, after this laborious production process comprising numerous chemical processes and mechanical operations, tanned hides and skins are obtained, which are suitable for use in the manufacture of different articles. Also, as in every industrial activity, the process has a significant impact on the environment, which is summarised in Figure 12:

32 INPUTS OUTPUTS Salted raw hide kg Leather kg Water m2 Water m2 QOD BOD 5 SS Chrome (III) Sulphides kg 100 kg 150 kg 5 6 kg 10 kg Chemicals kg Solid waste kg Untanned trimmings 20 kg Untanned fleshings kg Untanned scraps 225 kg Finishing dust 2 kg Finishing trimmings 30 kg Wastewater sludge 500 kg Energy 9 42 GJ Air 40 kg Organic solvents Figure 12. Mass balance of the tanning process. However, the integrated pollution prevention and control policies and the new clean production technologies, which are increasingly known and implemented, reduce this environmental impact at source, in both the consumption of natural resources and in the generation of contaminated effluents and waste

33 3. THE OXAZOLIDINE LEATHER TANNING TECHNOLOGY. This section describes the developed methodology, based on the use of oxazolidine as a main tanning agent combined with vegetable or synthetic tanning agents and the final conditioning treatment to remove the free formaldehyde from leather. This technology is applied in those cases where it is required to tan hides and skins without using mineral tanning agents, obtaining metal-free leather but with similar appearance, quality, properties and technical applications Background. As described in previous sections, the transformation of the animal skin into leather involves a series of chemical processes and mechanical operations, where a putrescible material, constituted mainly by proteins, is transformed into a resistant material, suitable for use in the manufacture of footwear, leathergoods, upholstery, garments, etc. In the traditional tanning process, used in more than 90% of the leather tanned worldwide, the stabilization of the protein structure is carried out using trivalent chromium salts that interact through chemical bonds with the carboxyl groups of the collagen present in the skin, providing the leather with its stability and resistance properties (see Figure 13). COLLAGEN-CHROMIUM CROSSLINKING Figure 13. Collagen-chromium crosslinking

34 This process gives the leather excellent physical, mechanical and chemical properties and a high stability to manufacturing processes and the passage of time, with shrinkage temperatures over 100 ºC. However, in some cases, chromium allergies may arise or even, under certain conditions, trivalent chromium can oxidize to hexavalent chromium, which is a carcinogenic compound that can be present in tannery wastewater and solid waste with a considerable impact on the environment and human health. There is also recent environmental pressure on tanning industries and a tendency to stricter environmental requirements for leather, which has led to the implementation of improvements in the tanning processes to reduce pollution, and the search for innovative tanning technologies alternative to chromium, thus avoiding at source the problems derived from its use. In this context, among the existing alternatives, the use of mineral tanning agents combined with other metallic ions, like aluminium (III), zirconium (IV) or titanium (IV) has been proposed. However, similar restrictions could be faced with when the market demands metal-free leather. As a result, organic tanning is an alternative technology that has been widely studied in recent years. Organic tanning products, including vegetable tannins, glutaraldehyde, oxazolidine, phosphonium salts, melamine and methacrylic resins, show different properties and collagen reaction capacity. Depending on the type of organic radical, the leathers obtained can reach shrinkage temperatures of up to 80-85ºC which gives them a thermal stability adequate for the manufacture of footwear, leathergoods, garments, etc. However, in some cases the leathers can show an unnatural appearance, little fullness and flexibility, as seen in the case of phosphonium salts and melamine and methacrylic resins, or cause a greater degree of contamination of the process wastewater, as it happens with vegetable tannins

35 In the case of glutaraldehyde, the leather obtained shows adequate appearance and physical properties, but has the drawback of being a substance of high risk in its handling and use for the operators. Also, the leather shows little fastness to light and yellowing problems. Therefore, there is a need to develop a new tanning technology that provides, using processes with less environmental impact, quality leathers that comply with the market s requirements in terms of quality and content in restricted substances. In this sense, previous research studies carried out by INESCOP have proved that the use of oxazolidine as a tanning agent, combined with other agents (vegetable or synthetic) allow quality leathers to be obtained, which can be employed by footwear, upholstery and leathergoods industries. The main advantage of oxazolidine tanning is that it allows high performance leather to be obtained, while managing to avoid the presence of metals both in liquid and solid waste derived from the tanning process, given that to date there is no record of problems derived from the use of oxazolidine. This way, it is possible to considerably reduce the environmental impact generated during the tanning process and also at the end of the leather lifecycle, either in the form of leather trimmings when different goods are manufactured or when they are disposed of after use. Oxazolidines are saturated heterocyclic compounds prepared by reacting primary amino alcohols with formaldehyde. Monocyclic or bicyclic oxazolidine ring structures are formed depending on the choice of starting chemicals. It is therefore possible to synthesize a variety of oxazolidines from different amino alcohols

36 Oxazolidines are highly useful chemicals for a wide variety of applications: corrosion inhibitors, emulsifiers, diluents or tanning agents, etc. The oxazolidines marketed for use as tanning agents are water soluble compounds, compatible with most chemicals commonly used in tanning operations. Table 2 shows the main types of oxazolidines used as tanning agents: Name Type Oxazolidine A Oxazolidine E Oxazolidine T 4,4-Dimethyl-1-oxa-3- aza -cyclopentane 5-Ethyl-1-aza- 3,7-dioxabyciclo [3,3,0] octane 5-Hydroxymethyl-1- aza-3,7-dioxabyciclo [3,3,0] octane Molecular structure CAS number Molecular weight (g/mol) ph Appearance Yellowish liquid Yellowish liquid White powder Table 2. Properties of the oxazolidines used as tanning agents The capacity of oxazolidine as a tanning agent is based on the formation of a reaction intermediate due to two possible mechanisms: - the protonation of oxygen of each ring in acid medium, which weakens the C-O bond, or; - the opening up of oxazolidine rings, by hydrolysis in acid medium, to provide an intermediate with two N-(hydroxymethyl) groups

37 and the subsequent nucleophilic attack of this intermediate species to the amino groups of collagen (lysine, hydroxylysine, tyrosine and methionine) by means of stable covalent bonds. (Figure 14). COLLAGEN-OXAZOLIDINE CROSSLINKING COLLAGEN-OXAZOLIDINE CROSSLINKING Figure 14. Collagen-oxazolidine cross-linking Leather tanned with chromium salts has high stability, determined by a shrinkage temperature (Tg) over 100 C, while leather tanned with oxazolidine alone reaches a shrinkage temperature below 75º C. It is therefore necessary to carry out oxazolidine tanning in combination with synthetic or vegetable tanning agents to achieve higher shrinkage temperatures and obtain leather of comparable quality to mineral tanned leather. Synthetic or vegetable re-tanning agents react directly through their hydroxyl groups (-OH) with the collagen s amino acids by means of hydrogen bridges and also indirectly with oxazolidine s reactive groups by means of covalent bonds (see Figure 15)

38 Figure 15. Collagen-oxazolidine- retanning agent bond. This way, the tanning structure stability is improved and an increase between 5-10 ºC in the shrinkage temperature is achieved. Furthermore, aldehyde tanning, including oxazolidine tanning, tends to give positive results in the free formaldehyde content of the leather, so it is necessary to carry out a final conditioning treatment of the leather to comply with increasingly strict limitations. The initial approach to control formaldehyde in oxazolidine-tanned hides is based on the intensification of the skin washing (closed door to avoid great water consumption) and the prevention of formaldehyde using formaldehyde-free syntans and natural vegetable tanning agents, that is, free from dispersants or other products that could contain formaldehyde

39 Given the strict limitations existing on the market, a conditioning procedure was optimized to remove free formaldehyde by adding a reducing substance with the capacity to react with the leather s free formaldehyde, converting it into a soluble product which is removed by washing without affecting the leather s properties. In the reaction an oxime is formed, which is a soluble compound that is removed from leather by washing (see Figure 16): Formaldehyde Hydroxylamine Oxime Figure 16. Formaldehyde reacting with hydroxylamine sulphate to form a soluble oxime. It was determined that adding 2% of hydroxylamine sulphate in the final wash of the leather reduces formaldehyde content to less than 50 ppm. Moreover, the use of a greater percentage of this product reduces the formaldehyde content even more, obtaining values of less than 15 ppm with 4% of hydroxylamine sulphate, although this involves an increase in the production costs that must be assessed by the user. Likewise, it was found that adding this product did not cause the un-tanning of the leather, given that the shrinkage temperature and the physical properties of the leather were maintained. Under these operating conditions, it was possible to obtain white, odourless leather with good physical resistance, with a thin grain and with an adequate smoothness, softness, fullness and flexibility, showing no significant differences between both combinations (synthetic or vegetable re-tanning)

40 Regarding the oxazolidine ratio used (3 or 5%), no differences were found in the leathers, neither in their appearance nor in their physical properties, so adding 3% was considered enough, reaching a Tg of 80 ºC which also saves costs in the process. With regard to the selection of a synthetic or vegetable re-tanning agent, in each case it will depend on the specific use of the leather, choosing synthetic products for lighter colours and with a high fastness to light, and vegetable re-tanning agents for darker colours and with no light fastness requirements. Likewise, the leather obtained meets the quality standards recommended for the manufacture of different leather articles as well as the criteria established for the European Ecolabel for Footwear (Commission Decision 2002/231/EC). With regard to the manufacture of leather for different uses (footwear, leathergoods, garments, upholstery, etc.) the process to be carried out is quite similar, the most important being to make an adequate choice of the re-tanning and fatliquoring products for the requirements for each use, as well as of the intermediate mechanical operations to be performed (milling, staking, etc.) and the final finish applied to the leathers that gives them their final appearance. Moreover, the quality of the leathers tanned with oxazolidine was checked by manufacturing different leather articles. In all cases, the manufacturing process was carried out as usual and no differences were observed in the processes or in the final appearance of the produced articles with regard to those obtained from mineral tanned leather

41 With regard to the environmental impact of this technology, both the effluents and the waste derived from oxazolidine tanning are metal-free and proved to be more biodegradable than those derived from chrome tanning. In short, oxazolidine tanning implies a significant benefit given that it is possible to dramatically reduce the environmental impact produced during the tanning process and at the end of the lifecycle of articles made of this type of leather Starting materials: hides and skins, equipment and chemicals. The tanning methodology was developed using pickled cowhide, sheepskin and pigskin as starting materials, obtained from raw skins subjected to a standard beamhouse process made up of the following stages: soaking, dehairing/liming, splitting, deliming, bating and pickling. These operations were performed as usual, without making any change in the processes and reagents used. With regard to the necessary equipment, the chemical operations were conducted in tanning drums of suitable size for the weight of the hides and skins to be processed and equipped with the control systems usually found in tanneries: water dosage and temperature control, reagents dosage, drum rotation speed, and manual or automatic mode operation

42 The mechanical operations for shaving, staking, milling, etc. were carried out as usual, using normal tannery equipment. In the case of the shaving machine, if the machine is to be used in both processes (tanning with or without metals), the skin shavings were adequately separated (with or without metals) to be suitably managed. Likewise, in trimming operations, both types of tanned leather trimmings were separated for their separate management. Furthermore, given the limit on the metal content of metal-free leather (<0.1%), in the sammying operation precaution should be taken to carry out an adequate wash of the sammying machine s felt mat to avoid the contamination from leathers with metals, if the machine is used in both processes (metal and metal-free tannage). Finally, it is very important to control and adjust the operating temperature in the drying chambers and tunnels and, above all, in the vacuum drying, due to the lower shrinkage temperature of the oxazolidine-tanned leather. With regard to chemicals, in the different stages generic products were used, like sodium formiate and bicarbonate, formic acid, etc., together with commercial products selected for their chemical composition for this specific application. It is important to adequately select the substitutive synthetic re-tanning products with a very low content in formaldehyde to minimize its content in processed leathers. Likewise, in the dyestuff selection, those which contain azo groups (-N=N-) must be avoided since they may form aromatic aryl amines of demonstrated carcinogenic potential, as well as metal complex dyes to avoid the metal contamination from leather waste (shavings and trimmings) in wastewater and treatment sludge

43 Table 3 shows the products used in the process stages: TANNING STAGE NEUTRALISATION Sodium chloride CHEMICALS Pre-fatliquoring: combination of phosphoric esters and synthetic fats Oxazolidine: type E Substitutive synthetic re-tanning agent: condensation product of sulphone and aromatic sulfphonic acid, with low formaldehyde content. Vegetable reagent: mix of tara, quebracho and mimosa. Sodium formiate Sodium bicarbonate DYEING / FATLIQUORING Dyestuffs Fatliquoring: sulphonated triolein Fatliquoring: waterproofing agent Reagent with low formaldehyde content Formic acid CONDITIONING AUXILIARIES Sequestering agent Indicators: bromcresol green Table 3. Main chemicals used in the oxazolidine tanning process The safety data sheets of the chemicals used with their most relevant data are included in Annex I

44 3.3. Technical procedure. The tanning procedure developed allows metal-free leathers to be obtained with similar appearance, qualities, properties and technical application possibilities. This technology is based on a group of chemical and mechanical operations intended to achieve stabilization of the protein structure of the skin by means of the reaction of collagen proteins with oxazolidine and the subsequent treatments to improve their physical and chemical properties. This procedure comprises the following stages: - tanning with oxazolidine - re-tanning using synthetic or vegetable products - neutralisation using sodium salts - dyeing and fatliquoring - leather conditioning to remove free formaldehyde The oxazolidine tanning process, which is described in detail below, is carried out without substantial modifications to the conventional processes used in traditional tanning with trivalent chromium salts. Moreover, the described process is adequate for pickled cowhide, sheepskin, and pigskin, no important differences being found in tanning these three types of hides and skins with oxazolidine

45 (1) Oxazolidine tanning. The oxazolidine tanning procedure starts by soaking the skins in a saline pickling bath. 1 This bath is made of 70% water at 25 ºC and 7% sodium chloride (by pelt weight), letting the drum rotate at slow speed (4-6 rpm) for 10 min in order for salt to dissolve. Next, the salinity of the bath is checked (6-7º Baumé) using a densimeter. If this value is not achieved, the parameters of the bath are readjusted by adding salt (successive doses of 0.2%), the drum is left to rotate at a slow speed (4-6 rpm) for 10 min and the salinity is checked again until reaching the values indicated. Once the pickled bath s salinity is adjusted, the skins are loaded in the drum and they are left to rotate for 15 min for them to get re-moisturized, checking the ph of the bath that must have a value of 3. Otherwise, the parameters of the bath are readjusted by adding formic acid (successive doses of 0.2%) and the drum is left to rotate at a slow speed (4-6 rpm) for 10 min, checking again the ph until reaching the indicated value. Then, a pre-fatliquoring is done to lubricate the fibres, facilitate the penetration of the products and improve the feel of the skins. 1 This stage can be avoided if the process is carried out after beamhouse operations and the skins are already in the pickling bath

46 This process is carried out adding 5% (by pelt weight) of a commercial product combination of phosphoric esters and synthetic fats, leaving them to rotate in the drum for 30 min at a slow speed (4-6 rpm). After the pre-fatliquoring, a tanning agent is added, in this case 3% (by pelt weight) of oxazolidine, and the skins are left to rotate for 90 min at a speed of 6-8 rpm and for the whole night with the drum working automatically (5 minutes in motion, with the previous speed and 55 minutes stopped). Once the reaction of oxazolidine with the skin collagen has finished, the ph of the leather is checked, which must have a value of , and the penetration of oxazolidine is also checked by cutting a small sample and applying the indicator (bromcresol green) to the cross section. If the oxazolidine has penetrated through the whole cross section of the skin, the cross section will show a homogeneous greenish coloration, while if it has not penetrated the whole cross section there will be a lighter shade at the centre of the cross section. In this case, the rotation of the drum will be extended for 60 min and the penetration will be checked again, repeating this operation as much as needed 2. Table 4 shows the formula used: 2 If the tanning has been carried out during the whole night with the drum turning automatically, the penetration should be complete and it would then not necessary to extend the tanning stage

47 PROCESS/PRODUCTS % pelt weight Tª (ºC) Time (min) ph Remarks TANNING Soaking/pickling bath Water NaCl 7 10 Check 8º Be 3 Pre-fatliquoring Pre-fatliquoring agent 5 30 Tanning Oxazolidine (100%) 3 90 Add the skins 15 Check ph<3 4 Automatic over night Check ph ( ) and cross section Table 4. Formulation of oxazolidine tanning stage. (2) Retanning. At this stage, according to the type of re-tannage selected (synthetic or vegetable), the following products are added to the same oxazolidine tanning bath: 3 If it not reached, successive doses of 0.20% sodium chloride are added (although it is not usually necessary) 4 If the ph>3, successive doses of 0.20% formic acid are added (although it is not usually necessary)

48 - 15% (by pelt weight) of substitution synthetic retanning agent: sulphone condensation product and aromatic sulphonic acid or - 15% (by pelt weight) of vegetable retanning agent: mix of tara, quebracho and mimosa. The addition is carried out in three successive doses of 5% (by pelt weight), separated by a 60 min interval, with the drum rotating at a speed of 8-10 rpm. Once the reaction of synthetic and vegetable retanning agents with oxazolidine and with the skin collagen has finished, the ph of the leather is checked, which must have a value in the range of and the penetration of the retanning products is also checked by cutting a small leather sample and applying the indicator (bromcresol green) to the cross section. If the retanning agents have penetrated through the whole cross section of the leather, the cross section will show a homogeneous green-bluish coloration, while if it has not penetrated the whole cross section, the centre of the cross section will have a lighter shade. In this case, the rotation of the drum will be extended for 60 min and the penetration will be checked again, repeating this operation as much as needed. 5 Finally, the bath is drained and the drum unloaded, leaving the leathers to rest on a horse for 12 hours to stabilize the tanning bonds formed. 5 In general, the products used, both synthetic and vegetable, have a good penetration in the skin so it is not necessary to extend the retanning stage

49 The mechanical operations required for the article to be processed are then carried out, which mainly consist of sammying and shaving the leathers to give them the desired thickness. Table 5 shows the formula used: PROCESS/PRODUCTS % pelt weight Tª (ºC) Time (min) ph Remarks RETANNING Synthetic/vegetable tanning 5 60 Synthetic/vegetable tanning 5 60 Synthetic/vegetable tanning 5 60 Check ph 6 Check ph ( ) and cross section Drain and remove leathers Leathers are left to rest for 12 hours Sammying and shaving Table 5. Retanning formulation. (3) Neutralisation. The leathers shaved to the desired thickness are loaded in the drum for neutralisation. The alkaline nature of oxazolidine simplifies the neutralisation stage, reaching the final tanning ph with a lower addition of basifying agent. In these conditions, the bath is formulated with 200% water at 30 ºC, 1% sodium formiate and 0.5% sodium bicarbonate (by shaved leather weight), and the drum is rotated at 10 rpm. 6 If the ph>3, successive doses of 0.20% formic acid are added (although it is not usually necessary)

50 After 40 min, the ph of the leather is checked, which must have a value in the range of and the penetration of the neutralising products is also checked by cutting a small leather sample and applying the indicator (bromcresol green) to the cross section. If the neutralizing agents have penetrated through the whole cross section of the leather, the cross section will show a homogeneous bluish coloration, while if it has not penetrated the whole cross section; the centre of the cross section will have a lighter shade. In this case, successive additions of 0.2% sodium chloride will be made to reach this ph value, extending drum rotation for 30 min and repeating this operation as much as needed. 7 Once the desired ph has been reached, the leathers are gently washed to remove the excess neutralising reagents and the bath is drained. Table 6 shows the formula used: PROCESS/PRODUCTS % shaved weight Tª (ºC) Time (min.) NEUTRALISATION Water Sodium formiate 1.0 Sodium bicarbonate Check ph 8 ph Check ph ( ) and cross section Drain bath and wash leathers Remarks Table 6. Neutralisation stage formulation. 7 In general, leather neutralisation is achieved with an initial dose of formiate and bicarbonate so it is not necessary to extend the neutralization stage. 8 If the ph >3, successive doses of 0.20% formic acid are added (although it is usually not necessary)

51 (4) Dyeing/fatliquoring. Once the leathers have been neutralized, washed and the bath has been drained, the dyeing operation is carried out, if desired. For this, the desired dyestuff is added on the wet leathers (dry dyeing, without water, to improve the dyestuff penetration crossing through the whole section of the leather) on a ratio (by shaved leather weight) of 3% for sheep and pigskin and 5% for cowhide (for its greater thickness), and the drum is rotated for 30 min at 10 rpm so that the colour can penetrate. Next, the colour penetration is checked by cutting a small leather sample. If the dyestuff has penetrated through the whole cross section of the leather, the cross section will show a homogeneous coloration, while if it has not penetrated the whole cross section; the centre of the cross section will show a lighter shade. In this case, successive additions of 0.5% dyestuff are made in order for the dyestuff to penetrate the leather, extending the rotation of the drum for 20 min and repeating this operation as much as needed. Once the colour has penetrated the leather, the fatliquoring is carried out; in this case, it is done in two successive doses. The fatliquoring bath is formulated (by shaved leather weight) with 100% hot water (40 ºC), 2% sulphonated triolein and 4% waterproofing fatliquor, and the leathers are tumbled at rpm for 30 min. Next, a similar bath is prepared; it is added to the drum and left to rotate at rpm for 30 min

52 After this time, in which the fatliquor has penetrated the leather lubricating the fibres, 5% re-tanning agent (by shaved leather weight) is added to the bath to improve the appearance and properties of the leather, such as its fullness, feel, compactness, physical resistance, improvement on the retention capacity of the print, of the buffing, etc. The type of final re-tanning agent used will depend on the appearance and intended use for which each leather is processed, but it must have a low formaldehyde content. The retanning agent is left to act for 30 min with the drum rotating at rpm. Finally, 2% formic acid (dilution 1:10) is added (by shaved leather weight and on two consecutive doses separated by 15 minutes), which causes a decrease in the leather ph (<3) and a change in the polarity of the leather that favours the fixation of the dyestuff and fatliquoring products. The formic acid is left to act for min, with the drum rotating at rpm. Once the dyestuff and fatliquor are fixed, the drum is drained and the leathers are washed to remove the excess unfixed reagents, using 200% water (by shaved leather weight), in a closed-door drum for 5 min and then the bath is drained. Table 7 shows the formula used:

53 PROCESS/PRODUCTS % shaved weight Tª (ºC) Time (min.) ph Remarks DYEING/FATLIQUORING Dyeing Dyestuff Check cross section Fatliquoring Water Fatliquor (sulphonated triolein) 2 Waterproofing fatliquor 4 30 Water Fatliquor (sulphonated triolein) 2 Waterproofing fatliquor 4 30 Synthetic/vegetable tanning agent 5 30 Formic acid (1:10 v/v) 1 15 Formic acid (1:10 v/v) 1 15 <3 Check ph and Tg Check ph <3 Drain bath and wash leathers Table 7. Dyeing/fatliquoring stage formulation

54 (5) Final conditioning. The conditioning treatment of the drained and washed leathers is carried out to remove the free formaldehyde. For this, the formaldehyde sequestering bath is added, which is formulated with 100% water at 35 ºC and 2% hydroxylamine sulphate (by shaved leather weight), leaving it to act for 60 min. Finally the bath is drained and the leathers are unloaded, leaving them to rest for their subsequent sammying, drying, mechanical operations and finishing in accordance with their desired final appearance. Table 8 shows the formula used: PROCESS/PRODUCTS % shaved weight Tª (ºC) Time (min) FINAL CONDITIONING Water Sequestering agent 2 60 Check Tg Drain and remove leathers Rest Sammying, drying and mechanical operations Finishing operations ph Remarks Table 8. Final conditioning stage formulation. Table 9 shows the overall formulation used for the complete monitoring of the process:

55 PROCESS/PRODUCTS % pelt weight Tª (ºC) Time (min.) ph Remarks TANNING Soaking/pickling bath Water NaCl 7 10 Check 8º Be 9 Pre-fatliquoring Pre-fatliquoring agent 5 30 Tanning Oxazolidine(100%) 3 90 RETANNING Add the skins 15 Check ph<3 10 Automatic over night Check ph ( ) and cross section Retanning Synthetic/vegetable tanning agent Synthetic/vegetable tanning agent Synthetic/vegetable tanning agent Check ph 11 Check ph ( ) and cross section Drain and remove leathers Leathers are left to rest for 12 hours Sammying and shaving 9 If this salinity is not reached, successive doses of 0.20% sodium chloride are added (although it is usually not necessary) 10 If the ph>3, successive doses of 0.20% formic acid are added (although it is usually not necessary) 11 If the ph>3, successive doses of 0.20% formic acid are added (although it is usually not necessary)

56 PROCESS/PRODUCTS NEUTRALISATION Neutralisation % shaved weight Sodium formiate 1,0 Tª (ºC) Time (min.) Sodium bicarbonate 0,5 40 Check ph 12 DYEING/FATLIQUORING Dyeing ph Check ph ( ) and cross section Drain bath and wash leathers Dyestuff Check cross section Remarks Fatliquoring Water Fatliquor(sulphonated triolein) 2 Waterproofing fatliquor 4 30 Water Fatliquor (sulphonated triolein) 2 Waterproofing fatliquor 4 30 Synthetic/vegetable tanning agent 5 30 Formic acid (1:10 v/v) 1 15 Formic acid (1:10 v/v) 1 15 <3 Check ph and Tg Check ph <3 Drain bath and wash leathers 12 If the ph>3, successive doses of 0.20% formic acid are added (although it is usually not necessary)

57 PROCESS/PRODUCTS FINAL CONDITIONING Final conditioning % shaved weight Tª (ºC) Time (min.) Water Sequestering agent 2 60 Check Tg Drain and remove leathers Rest Sammying, drying and mechanical operations Finishing operations ph Remarks Table 9. Overall formulation of the oxazolidine-tanning process. In these operating conditions, white, odourless leathers are obtained, with a thin grain and with similar properties to leathers tanned with chromium salts. However, these leathers are metal-free and have a lower impact on the environment due to their higher degree of biodegradability Technical validation of oxazolidine-tanning technology. The results derived from the development of oxazolidine tanning technology, in combination with synthetic or vegetable tanning agents, showed that the obtained leather had good physical strength and adequate appearance and feel for the manufacture of different articles. Moreover, no significant differences between the two tanning agent combinations were detected

58 Therefore, the selection of a synthetic or vegetable tanning agent will depend on each individual case, choosing syntans for lighter colours and high light-fastness, while vegetable tanning agents can be used for darker colours with no light-fastness requirement, as they are cheaper. Regarding the ratio of oxazolidine used, it was considered that 3% was the optimum dose, which favours the financial viability of this technology. The tanning degree of leather was assessed through the determination of the Shrinkage Temperature (Tg), giving values between 75 and 82 ºC, which were acceptable for the production of most products (footwear, handbags, jackets, etc.). The technical validation of oxazolidine tanning technology was first assessed according to standardised quality control tests to check the compliance with recommended values and the suitability of leather for footwear manufacturing. Table 10 shows the results obtained in the different tests carried out, compared with recommended values. PARAMETERS Thickness (mm) Tear strength (N) Tensile strength (N/mm2) Elongation at break (%) Grain burst (mm) Shrinkage temperature (ºC) STANDARD CATTLE HIDES SHEEP SKINS RECOMMENDED VALUES ISO 2589: > 1.1 ISO : > 50 ISO 3376: > 15 ISO 3376: > 40 ISO 3379: > 8 ISO 3380: > 70 Table 10. Physical characterisation of oxazolidine-tanned cattle hides and sheepskins

59 Likewise, some footwear styles (even incorporating vulcanised soles) and upholstery, leathergoods and apparel products were manufactured and no differences were observed in leather processing or in the final appearance of the articles produced, as shown in Figures 17, 18 and 19. Figure 17. Children s footwear (DECHICS), men s penny loafers (MOSEIPE) and occupational footwear - clogs (DIAN) made from oxazolidine tanned leather. Figure 18. Women s footwear (TPSP), casual and vulcanised footwear (CALZADOS CANÓS GARCÍA, S.L.) made from oxazolidine tanned leather. Figure 19. Upholstery, leathergoods and apparel products made from oxazolidine tanned leather

60 Furthermore, oxazolidine-tanned leather meets the requirements relative to the limit content in hazardous substances according to the criteria of the European Ecolabel for footwear set forth in the Commission Decision 2002/231/EC: PARAMETERS STANDARD LIMITED VALUES (*) Chromium (VI) ISO 17075: ppm Formaldehyde ISO : ppm Pentachlorophenol ISO 17070:2006 Not detected (*) Lead (Pb) ISO :2011 Not detected (*) Cadmiun (Cd) ISO :2011 Not detected (*) Arsenic (As) ISO :2011 Not detected (*) Aromatic amines (derived from azo-colorants) ISO :2010 Not detected (*) Not detected (*): below the detection limit Table 11. Chemical characterisation of oxazolidine-tanned leather Regarding the environmental impact of this technology, the characterisation of the effluents from oxazolidine tanning processes showed similar values to those obtained in chrome tanning. However, oxazolidine tanning effluents are chrome-free and consequently the oxidation of trivalent chromium to its hexavalent state is avoided and the metal-free sludge derived from wastewater treatment is more likely to be reused, e.g. for agriculture. Furthermore, the respirometry tests conducted on activated sludge proved that oxazolidine tanning effluents are more biodegradable, which reduces the environmental impact of the process and a priori implies a higher feasibility of the biological treatment of wastewater

61 Finally, biodegradability tests were performed on leather, which allowed the different tanning technologies to be compared with regard to the environmental impact of their resulting waste. Since there is currently no test method or specific standard for the determination of the biodegradability of leather, we employed a method jointly designed and optimised by INESCOP and the University Miguel Hernandez (Elche-Spain). In the tests carried out, the biodegradation of chrome-tanned leather and oxazolidine E- tanned leather was compared with a pure collagen standard. As expected, pure collagen, used as a test standard, showed a biodegradation rate of 85% after 700 hours, while chrome-tanned leather showed 12% and oxazolidine-tanned leather 55%. This implies an increase by 43% (see Figure 20). 85% 55% 12% Figure 20. Graph comparing the biodegradability rate of collagen, oxazolidine-tanned leather and chrome-tanned leather