Sustainable Leather Prospective: realistic objectives and future opportunities
SUSTAINABLE LEATHER MANUFACTURE: REALISTIC OBJECTIVES AND FURTHER OPPORTUNITIES Dr.-Ing. Heinz-Peter Germann N-Zyme BioTec GmbH, Innovation Center Leather & Collagen, Reutlingen / GermanySustainable development is a pattern of resource use that aims to meet humanneeds while preserving the environment so that these needs can be met not onlyin the present, but also for future generations.The term was used by the Brundtland Commission – formally the World Commis-sion on Environment and Development (chaired by the former Norwegian PrimeMinister Gro Harlem Brundtland) that was convened by the United Nations in 1983– which coined what has become the most often-quoted definition of sustainabledevelopment as “Development that meets the needs of the present without compromising the ability of future generations to meet their own needs”.By IUCN, “The World Conservation Union”, it has been shown that the field ofsustainable development can be conceptually broken into three constituent parts(see figure 1): • environmental sustainability (objective: preserving nature and environment for future generations) • economic sustainability (i.e. a manner of economy enabling sound income and prosperity) • socio-political sustainability (i.e. development of the society as a way of enabling participation for all members of a community) Fig. 1: Scheme of sustainable development – at the confluence of three constituent partsSustainability can be related to different levels of consideration, i.e. it can beconsidered as a local, regional, national or global issue. While it is increasingly
2considered as a global issue from an ecological point of view, the focus ofeconomic and socio-political sustainability is often a national one.Practical approaches to realizing the idea of sustainable development in manu-facturing companies are followed up by cleaner production which includes − reduction of energy use, − use of renewable resources, − minimization of water consumption a n d − reduction of waste generation.However, the ‘destination’ of sustainability is not a fixed place in the normal sensethat we understand destination. Instead, it is a set of wishful characteristics of afuture system as pointed out by A.M. Hasna in 2007.“The origin of leather manufacture dates far back in the prehistoric ages, and wasprobably one of the earliest arts practised by mankind” as already stated by HenryRichardson Procter who has been called “the father of leather chemistry”.Having a look some hundred thousand years back when stabilized animal skin bychewing (principle of chamois tannage) and smoke tannage (following to thediscovery of fire) were discovered by mankind, this type of leather making andusing resources could certainly be considered a sustainable process, at that time.And also the “Iceman”, a human being from Copper age, found in the EuropeanAlps in the frontier area between Italy and Austria who has been reported aswearing fur clothes “tanned” by a special fatliquor procedure that has to be seenas an “impregnation” rather than a tannage, could still be seen as acting in thesame line.But in those early times the (main) interest of mankind had been the stabilisationof hides and skins – available from animals hunted for the need of nourishment –just for their effective use in personal protection. From today’s scientificperspectives, this could possibly be considered as the first attempt for pollutionprevention and control by avoiding uncontrolled rotting of hunting/slaughteringwaste in the environment.And, indeed, leather manufacturing is in itself recycling – or as recently cited:“Leather is a sustainable environmental solution to the very real disposal problemof a large volume of hides and skins that originate from the meat industry …”.On the other hand, leather production, like many other industrial processes, doesindeed involve the generation of polluted waste water and solid by-products orwastes, so that an appropriate effluent treatment and waste management are thesubject of a must, nowadays. This also includes the reuse of valuable by-productslike e.g. splits or split off-cuts that can be used in the production of gelatine andsausage casings. Fleshings e.g. can be used in biogas production or directly toreplace the fuel for energy generation after separation of fat, as recently
3demonstrated by Suedleder GmbH & Co. KG, a leading German contract tannery.In Suedleder tannery this has resulted in 50% CO2-reduction (savings of 4.500 t/aCO2).Of course, some thousand years ago, by-products such as connective tissueshould not have created any problems since they were most probably consumedas food.However, factors like fashion, commercial interests and globalisation had notexisted yet. Even the rule of unhairing was not invented that time, a process whichlater became a crucial step in leather manufacturing for generations of tanners.Also preservation in today’s sense should not have played a major role; skins wereeither used freshly enough after separation from the animal body or certain defectsresulting from putrefaction were probably considered as normal. Today, in thepreservation of hides and skins common salt is still the most extensively usedagent, although the 30-40% sodium chloride which is needed for this process iscausing serious problems, especially, in countries with warm and dry climate,where fresh water resources are at a premium.Obviously the best solution from the environmental point of view is the use of fresh(green) uncured or chilled hides for processing wherever the proximity of thetannery to the slaughterhouse is given. In other words, this will always berestricted by the respective slaughtering and flaying conditions as well as theclimate in the place of processing. In those cases where preservation isunavoidable new ways of raw hide curing like e.g. the proposed use ofwasserglass will have to be further investigated.However, looking for a really sustainable leather manufacture, the sourcing ofhides and skins becomes of equal importance to controlling the manufacturingprocess. This will always call for processing of raw hides at the place of origin.The beamhouse processes have always been considered the most importantoperations in leather production as far as quality and texture of the resultingleather are concerned. On the other hand it is well known that the unhairingprocess of hides and skins, including the removal of hair and epidermis as well asthe opening-up of the fibre-structure, ranks among the most polluting operations inthe tannery. Although there have been significant developments andimprovements, including e.g. the reduction of processing time and effluent load,made in the past decades, we are still looking for a more desirable, cleanerunhairing technology.Development of the unhairing process: • sweating process (hair-loosening by a “natural” bacterial process; difficult to control) • traditional liming (risk of uncontrolled bacterial growth and enzyme action becomes unmanageable under the conditions of industrial production)
4 • rapid hair-destroying by sharpening agents (became most widely used standard procedure as a consequence of economic considerations including leather quality improvement)Attention was then turned to the development of new liming systems to protect theenvironment. These developments included reduced water consumption (e.g. byreplacing rinsing by washing processes, decreasing the float ratio and trials on therecycling of lime and liming wash liquors), the reduction of sulphide and reductionin COD loading of the effluent.For environmental reasons new unhairing technologies have been developed: • hair-saving liming systems (based on the principle of a partial immunization of the hair) • low sulphide liming systems (usually based on combination of certain organic (thio-)compounds with inorganic sulphides) …and also: • oxidative unhairing process (applying hydrogen peroxide in alkaline media – which has not been commercially adopted, for certain limitations) At present, from a technological and ecological point of view low-sulphide systems using either organic thio-compounds or amines, sometimes in conjunction with enzymes, have to be considered as the best technologies existing; and it is merely depending on regional and individual conditions concerning possibilities of hair utilization and on the technical equipment available, whether these systems should be used in hair-destroying or hair- saving mode. A number of leather scientists have proposed that hair-saving • enzymatic unhairing will someday replace sulphide unhairing as an ecological sound alternative, so that the wheel would come full circle – the oldest method of hair-loosening, after some improvements, would become the latest one. As early as 1910, Otto Roehm had patented the unhairing with pancreas tryptases aiming at the replacement of the almost uncontrollable sweating method by a much more controllable process. Unfortunately, some of the disadvantages already observed by O. Roehm still could not be fully overcome (incomplete removal of short hair under practical conditions). For this reason the use of some sulphide in enzymatic unhairing processes is indispensable, till the present time.Regarding the tanning process itself, in the pre-industrial period lasting nearly tillthe end of the 19th century, vegetable tannage or tanning with Aluminium (tawing)have been the tanning techniques employed depending upon the intended use ofthe resulting leather – i.e. based on the leather characteristics required.Following the discovery of the tanning effect of chromium salts by the GermanFriedrich Knapp in the 1850’s and further development of the process byM. Dennis (1893) who brought it into a practical system, chrome tannage becamethe worldwide accepted standard method of tanning, during the last century.
5It should be noted that in these times there were no significant environmentalconcerns. The main interests were based on rationalisation of the tanning process,the benefits in cost and speed of the reaction – hence faster throughput time – anda highly versatile basis for leather manufacture. All of these factors representedmajor steps towards industrialisation.It has just been within the last few decades that the sensitivity to the ecology andhealth concerns by the majority of people, starting from the highly developedcountries, has been steadily increased. In the tanning sector, this has resulted indeveloping different approaches for an improved chrome management (seefigure 2). Fig. 2 Within the last 50 years: Different approaches for an improved chrome management • chrome recycling techniques • better understanding on the influence of process parameters: – mechanical action – temperature – pH – concentration – processing time – Cr offer • development of high exhausting chrome tanning systems … based on the general principles of: low chrome offer, optimised process parameters and application of special auxiliariesThis has involved chrome-recycling techniques as well as a better understandingof the individual and combined influences of process parameters.This also formed the basis of the development of the high exhausting chrometanning systems, based on the general principles of low chrome offer, optimisedprocess parameters and application of special auxiliaries (“crosslinking maskingagents”).Today‘s “greener” way of thinking brings about new requirements on process-chemicals and -technologies in leather manufacturing. Moreover, this way ofthinking has been much intensified by an increasing number of press reports andtelevision programmes raising questions concerning the environmental burden andconsumer health. In addition, there are the increasingly stringent requirements ofgoverning tannery effluents as regards to their chrome content and also certainissues involving the disposal or utilization of by-products containing chrome forwhich chromium tannage in particular has come under criticism.Before considering any possible alternatives to chrome tannage, over and aboveany basic technical limitations in terms of the tanning effect, the question of actualecological benefits gained from using a different tanning method has to beconsidered.In this event it is interesting to consider for instance the amount of tanning agentrequired for converting a bovine hide into leather (see figure 3):
6 Fig. 3: Amount of tanning agent required for a bovine hide (40 kg pelt weight)Considering the worldwide consumption of 350.000-400.000 tons/annum chrometanning agents used for the manufacture of ≥ 80% of the world’s leatherproduction and the consumption of vegetable tanning agents of approximately200.000-250.000 tons/annum, it becomes more than evident that a (complete)replacement of chromium by vegetable tannins is not realistic. Increasing theproduction of vegetable tannin extracts by the necessary factor, in the case of wildgrowing trees like Quebracho, would immediately decimate their existence and inthe case of trees (e.g. mimosa) grown in plantation, the required cultivation areawould not be available; or in other words: extending the plantation area of tanninproducing plants to the debit of agricultural food production may not in the least beconsidered as sustainable – in a world where lots of people are still suffering fromhunger.Experimental studies and an eco-efficiency analysis, which were carried out tocompare different tanning methods from an ecological viewpoint, showed that, inpractice, it is safe to assume that chrome tanned leather produced in accordancewith the ‘best available technique’ principle – including all environmental measures– can be classified as environmentally compatible. Basically, the indication of thetanning method in itself cannot be regarded as an ecological quality criterionirrespective of the leather article.Considering, however, the importance of the tanning methods today, it has to bementioned that although 80-85% of the worldwide leather production is based onchromium tannage, there is an increasing share of chrome-free tannages. This ismainly due to the growing demand for so-called free-of-chrome leathers (“FOC-leathers”) in the automotive industry where the current share of FOC-leather isabout 25-30%.The principle of FOC-leather production is based upon a wet-white pre-tannage bye.g. glutaraldehyde tanning agents (partially modified), THPS or other reactiveorganic-synthetic crosslinkers (all originating from fossil fuels), which is followed bya retanning using polyphenols (vegetable and syntan tanning agents) and otherorganic-synthetic retanning agents.Compared to chrome leathers their advantages include
7 − the absence of “heavy metal”, − less problems in solid waste disposal (including sludge) and − improved performance in dry-shrinking behaviour.On the other hand, well-known disadvantages include − higher COD in the effluent, − reduced fixation of dyestuffs and fatliquoring agents, − extremely difficult hydrophobing, − more difficult “handling” in production and processing, − lower stability and mould resistance of semi-finished products (wet-white).In principal, the course of tanning in modern processing can be divided in 2 steps(see figure 4): Fig. 4 The course of tanning in modern processing Pre-tanning/Main Tanning Retanning/Filling stabilisation for adjustment of the mechanical treatment required leather (sammying, shaving) characteristics production of a storable (e.g. fullness, shape- and transportable semi- retention ability, grain finished leather product firmness, embossability, (wet-blue, wet-white, …) buffability)1. The pre-tanning or main tanning stage for the stabilisation in order to permitmechanical treatment and the production of a storable and transportable semi-finished leather product and2. The retanning/filling process for adjustment of the required leathercharacteristics.The retanning step includes mainly the application of vegetable tannins andsynthetic tanning/auxiliary tanning agents like syntans, resins and polymers; butalso mineral tanning agents and aldehydes are applied.At this stage of leather manufacturing, which is also connected to the processes offatliquoring and dyeing, the most important technical issues concerning ecologyand consumer health are – avoiding chromium VI – avoiding formaldehyde release – avoiding carcinogenic amines (from azo dyes)All of these can be achieved best by the right selection of process chemicals.However, neither wet-blue nor wet-white production can be considered assustainable processes in the proper sense, since both chromium salts andglutaraldehyde (or any other reactive organic-synthetic tanning agent) are non-renewable raw materials.
8More recent research and developments concerning ecologically beneficialmethods of pre-tanning or stabilising the skin (collagen matrix) include − wasserglass stabilisation, − enzymatic crosslinking and − application of natural plant crosslinkers (especially, from olive waste)There are three main objectives of these studies: – To establish an innovative, sustainable and environmentally friendly tanning system – as an appropriate complement to the existing tanning methods – The stabilisation/pre-tanning of the skin material – under the avoidance of conventional chemical tanning agents – so that a satisfactory mechanical processing (sammying/splitting and shaving) can be achieved – To obtain by-products/waste (shavings) enabling practically unlimited usageSkin stabilisation by wasserglass results in a white, stable and shaveable materialthat proved to be storable for months. However, there is no significant increase inthe shrinkage temperature, and therefore reluctance for technical application.The enzyme transglutaminase produces irreversible crosslinking with denaturatedcollagen (gelatine). However, sufficient stabilisation (significant increase in theshrinkage temperature) – as required for leather manufacture – could not beachieved.Application of an activated extract from olive solid waste produced technicallypromising results. In the meantime, by serial trials at LGR, a stable leatherintermediate – called “wet-green®” – with good shaveability and a shrinkagetemperature of ≥70°C could be obtained. This new leather intermediate forms anexcellent basis for the production of crust leather (see figure 5). Fig. 5 Natural plant crosslinkers ☺ Application of an activated extract from olive solid waste produced technically Olea europea promising results: a stable leather intermediate (“wet-green®”) with good shaveability and TS ≥70°C forming an excellent basis for crust leather production
9Olive solid waste material includes leaves – originating from olive harvesting (seefigure 6) – as well as the effluent and residues from olive oil- and table oliveproduction.Manual olive harvestingMechanical olive harvestingFig. 6 Generation of leaves and twigs during manual and especially, mechanical oliveharvestingIn Europe (>90% of world cultivation area for olive trees) approximately 30 milliontons of olive-solid waste materials are produced, annually.The wet-green® technology uses a new innovative tanning agent based on anaqueous olive leaf extract. The extract is produced from olive leaves thataccumulate in huge amounts as waste during the olive harvest and the trimming oftrees without any valorisation today. In the meantime, the new tanning agent canbe produced in industrial scale. Integration ability of the wet-green® tanningprocess in leather production has been demonstrated within the scope of aresearch project carried out by N-Zyme BioTec GmbH and LederinstitutGerberschule Reutlingen.Another issue to be dealt with in future is the undesirable problem of mould occur-ring in the wet leather intermediate stage. A delay of further processing in the wet-blue, wet-white or wet-green® stage can lead to mould growth and hence apotential loss in quality and value. Avoiding such losses includes the use ofeffective preservatives in the (pre-)tanning system. Although our formerinvestigations showed that the initial fungicide content is reduced significantly bypost-tanning operations and finishing, there is an interest in avoiding the presenceof those chemical agents mainly on behalf of consumer protection.Therefore, further investigations included the development of an universal inter-mediate stage (pre-/tanned) that requires no inclusion of preservatives / fungi-cides. I.e., in addition, this product should allow drying, and subsequently wetting-back without any problem.
10The aims of this approach are to avoid losses from mould attack, and avoid theapplication of undesirable chemical agents.This requires an appropriate stabilisation/crosslinking and sufficient increase in theshrinkage temperature to enable sammying and shaving as well as appropriate“fibre separation” (e.g. by the filling effect of certain tanning agents or fatliquors) toenable drying without sticking within the fibrous collagen structure.Although the new technology suggested will still need some optimisation, resultsare extremely promising and giving inspiration about a further increase in thesustainability of leather manufacture.Cleaner production in finishing includes: • avoidance / reduction of organic solvents increasing application of water based systems • avoidance / reduction of “overspray” improvements in application techniques, e.g. optimized spraying techniques (HVLP) innovative roller coating technique • ecological improvements in cross-linking in-line cross-linking substitution of harmful highly reactive crosslinkers radiation curing (e.g. by UV) • new innovative finishing technologies…?Sustainable leather manufacturing: wishful thinking or realistic objective?The ‘destination’ of sustainability is not a fixed place …Instead, it is a set of wishful characteristics of a future system!So, what are the future challenges for sustainable leather manufacture?− In principle, leather manufacturing is in itself ‘recycling’ – i.e. it is a sustainable solution to the disposal problem of a by-product that originates from the meat industry.− The concept of ‘globalization’ in leather production has to be updated / adjusted by taking more into account additional factors like e.g. raw material sourcing that is also relevant to the subject of sustainability.− Sustainability of leather manufacture can be further increased by using resources (i.e. water, fossil fuels and other natural resources) sparingly, which includes controlling the production processes and improving the systematic re- use of by-products whenever possible, and giving priority to the use of renewable resources.References see:- H.-P. Germann, B.M. Das Memorial Lecture 2010: Sustainable leather manufacture – Realistic objective or wishful thinking?, Leather 212, 4/2010, 28-30.- H.-P. Germann et al., Heidemann Lecture: A natural plant crosslinker from olive waste for leather tanning, Proceedings XXXI. IULTCS Congress, Valencia, Spain, September 2011.