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QLK5-1999-00913
Copolymers of tannins and waste lignocellulosic materials with synthetic materials for non-toxic environment friendly chrome substitutes in leather tanning |
| Contract No: | QLK5-1999-00913 |
| Source: | Final Report Summary March 2003 |
| Second Annual Progress Report 2002 | |
| First Annual Progress Report 2001 |
The final technical report is divided into five chapters. The first chapter deals with the characterisation and figer-printing of all the raw materials which were used. The second chapter deals with the preparation of tanning agents by co-polymerisation of vegetable tannins with acrylics and with the application of these pre-polymerised tanning agents to prepare leather. The third chapter deals with the preparation and in situ (in the hide) co-polymerisation of the a novel, high performance low cost melamine-urea-formaldehyde (MUF) resin with natural vegetable tannins. It extensively details the application technology, the characteristics of the 130 experimental leathers obtained and the initial developments of very new, more scientific tests. The fourth chapter, explains in a detailed, yet concise way both the type of new test methods developed and the correlation between their interpretation of what occurs in leather tanning at the molecular level with the observed macroscopic effects, and the forecasting equations supporting such a correlation. It also presents, in detail, the overall technology of leather tanning with the new system that has been developed. Lastly, the fifth chapter deals with industrial bulk trials of the new tanning system, its advantages and (few) disadvantages.
The natural waste materials which have been used to co-polymerise with acrylic acid, acrylic resin and Melamine-Urea-Formaldehyde (MUF) resins have been analysed from several points of view and have been extensively finger-printed by using a variety of very different analytical techniques.
The following tannins were examined:
These were examined using two types of tannin/non-tannin analysis methods, namely hide powder and Divargan methods, for colour points, pH, sediments, insolubles and total dry matter. They were then analysed for formaldehyde and phenol content and for ash content by standard methods. They were extensively analysed for molecular mass distribution by Size Exclusion Chromatography (SEC), then by High Pressure Liquid Chromatography (HPLC), and finally by a very extensive liquid phase 13C NMR and Matrix assisted Laser Desorption/ionization (MALDI-TOF) mass spectrometry. This resulted in a series of quantitative analysis with complete chemical group identification. Also the more important synthetic raw materials that were used, namely acrylic acid and acrylic resins, were analysed by liquid phase 13C NMR quantitative analysis.
The results from 42 copolymers of a variety of tannins and waste lignocellulosic materials with acrylic acid are reported. Comprehensive chemical analysis by NMR, and Size Exclusion Chromatography (SEC) of each copolymer produced was carried out. In addition a number of experimental leathers were prepared in the laboratory in order to evaluate the co-polymers produced. Comprehensive testing of the leathers obtained was also carried out. All the leather samples tested had been obtained by tanning with co-polymers of different tannins with 5%, 15% and 50% acrylic acid. All the samples where the new products were used in re-tanning gave encouraging results. This was less so in the case of direct tanning.
The stability to light of the acrylic modified leathers was checked by ESR (electron spin resonance) techniques as well as being tested using a colorimeter and have shown some good, although not exceptional, results. Reaction kinetics as regards the extent of colour variation has been carried out by ESR as has comparison of ESR and colorimeter measures.
Improvement in resistance to shrinkage of the leather was not achieved as indicated by a shrinkage temperatures only slightly higher than that what was obtained for leathers obtained with control pure vegetable tannins.
Thermomechanical analysis (TMA) experiments directly on the leathers indicated that this method of analysis gave interesting insights on the process and its parameters at the molecular level, and on the improved flexibility of the leather obtained by tanning with the acrylic acid/tannin copolymers. Some improvement was obtained by use of copolymers of vegetable tannins with acrylic acid; these being an improved flexibility, some improvement in light fastness but practically no improvement in shrinkage temperature. All these problems were overcome, however with the second approach taken.
This report also outlines the technology developed for, and the research and results obtained in the manufacture of flexible soft leather by total and partial substitution of toxic chrome salts by the use of a vegetable tannin (several types) co-reacted in situ with a number of different melainine-urea-formaldehyde (MUF) polycondensation resins. The conclusions that were reached for the more than 130 leather treatment combinations tried are as follows:
Equations correlating the results of the traditional leather shrinkage test with the TMA test were obtained. Equations correlating the parameters for a proper colour evaluation system with the light fastness of the leather produced were also obtained.
Finally mixed leathers were manufactured in Italy based on the use of a mix of vegetable tannin + a syntan to which was coupled the same MUF resin; this also gave excellent results and particularly light coloured almost white leathers.
The results obtained were also applied at factory level in a series of bulk plant trials in the UK. The results obtained supported the results obtained in the laboratory especially as concerns shrinkage resistance to high temperatures as well as light stability. A low degree of "drawing" of the leather was observed in full scale industrial trials but this was tolerable and was minimised using appropriate industrial treating techniques.
The industrial trials indicated that the system developed is most apt for soft upholstery leather tanning and re-tanning.
The work-plan is divided into 4 main work-packages, and phases, which follow sequentially. In the initial two phases, the project was mainly of a chemical nature, and thus priority was given to chemical analysis, determination of structures. Greater preference was also given to aspects of co-polymerisation and reaction work, determination and optimisation of the parameters of manufacture, characterisation of the new materials formed and of their properties, and initial rapid evaluation of the potential of the new products in meeting the main objectives of the project. The work-packages that were addressed in the year under review are described below.
Testing of new tanning agents
Co-polymerisation with acrylic acid: The effectiveness at laboratory testing level and during limited plant trials of the different mixes of waste lignocellulosic residues as well as industrial tannins reacted with acrylic acid were checked in a pilot plant. The first type of co-polymerisation of all these material mixes, and possibly the more important to study, was that based on radical initiation. Of almost 100 reaction mixes that were investigated, more than 25% of them showed some promise. These were tested for leather production in the laboratory and one of them was tested in a limited plant trial both on pickled hides as well as in re-tanning on wet blue hides. This allowed evaluation of the applicability of the new materials to both primary tanning and re-tanning.
The following variables were investigated:
Although some encouraging results were obtained as regards the colour of the materials and of the leather produced with them, the leather produced remained rather dark and less stable than what hoped for. However, rather elastic and reasonably soft leathers were obtained, the shrinkage temperature of which was rather better than that of vegetable tannins alone but not quite up to the level of what was obtainable with chrome tanning. The conclusions were that the approach based on the co-polymerisation of vegetable tannins with acrylic acid gave some acceptable results, the main problem being however the smell of un-reacted acrylic acid. The level of this was small, but nonetheless sufficient to give a totally unacceptable smell to the leather produced with the copolymer. Furthermore the results as regards shrinkage temperature of the veg-tan/acrylic acids were not up to the required standard of chrome leather. While some minor improvement could be observed as regards leather shrinkage temperature this was of far too small a level to be significant. For this reasons this approach was discontinued.
Tanning agents produced by co-polymerisation of lignocellulosic raw materials with synthetic aminoplastic resins: The phenolic groups present in both hydrolysable (chestnut) and condensed flavonoid tannins, as well as those present in the degraded lignin in waste lignocellulosic residues are reactive with formaldehyde and with formaldehyde-based resins under both acid and alkaline reaction conditions. The reaction occurs quite readily, particularly for hydrolysable and condensed flavonoid tannins. All the natural products found suitable in previous work were reacted and co-polymerised with synthetic formaldehyde based resins already used as tannin agents - namely sulphonated melamineurea-formaldehyde resins in order to:
This new MUF resin gave really good results with vegetable tannins as regards leather softness, light colour of the leather and light fastnessof the leather produced Much better fullness of the leather was also achieved, both on standard untreated hides, pickled hides and re-tanned wet blue hides. The leather produced with 50%-80% vegetable tannin and 20%-50% new MUF resin was almost white, the colour hardly changed with 3 days UV irradiation and the leather produced without any fattening and finishing was softer than that obtainable with chrome.
These results were obtained in direct tanning and not just in re-tanning, that is on hides not containing any chrome. The tanning capability and astringency was also determined using standard hide powder or by using Divargan analysis methods and found to be good.
The new MUF resin, in combination with the vegetable tannin yielded exceptionally light-coloured leather and light-stable leather, when treated with a composition of 50% vegetable tannin (Chestnut, quebracho or mimosa) and 50% MUF co-reacted in situ during leather manufacture. A second extremely light-coloured composition showing considerable promise was based on a reaction in situ of light coloured chestnut tannin, sulphonated phenolic syntan (synthetic tannin) and the new, reformulated sulphonated MUF resin.
Many leather samples were produced using the new veg-tan/MUF resin system in order to check its ease of use, and even more important, the shrinkage temperature of the leather produced. The results obtained in pilot work were excellent, proving that the new technology can be transferred to industrial operations and does not loose perform less well when the size of the operation is increased.
The same leather shrinkage temperature (that of chrome leather) was obtained with veg-tan/MUF tanning. This was a very good result; one very exploitable commercially. Quite remarkable improvements were obtained in respect of temperature induced leather shrinkage, with some of the formulations showing shrinkage temperatures as high as 90 to 92oC. This compares with products based on vegetable tannin alone that have shrinkage temperatures in the mid 60s oC.
The stability to light of the leathers presenting good shrinkage resistance was checked along with other samples of leather using both a colorimeter and by UV exposure. The results obtained with these materials were good. Comparison of these colorimeter measures with those of others confirmed that the results obtained were good, consistent and comparable. On this basis it was concluded that the light-fastness problem of vegetable leathers appears to have been considerably improved.
Conclusions
Introduction
The leather tanning industry is one of the oldest still in operation. Although the technology of leather manufacture has evolved over centuries and even in recent years, the basic principles for the production of leather have remained the same. Hide proteins, mainly collagen, are rendered insoluble and dimensionally more stable by treatment with chemical products able to fix on them and render them more resistant to both mechanical wear and less susceptible to biological and other types of attack. The main products used today for leather tanning are as follows:
Each of the products mentioned above is more sited to a particular type of leather. The fact remains however, that the first two in the list account for more than 90% of all the leather manufactured today, and that the process based on trivalent Chrome salts accounts by itself for about 70% of the total. Chrome tanning is particularly suited for soft leather as it does not affect hides flexibility and renders the leather very stable both chemically and physically (imparts a high T.). The forestry derived vegetable tannins have instead a strong astringent effect (they fix very effectively on the collagen structure) and give considerable 'body', hardness and toughness to the leather produced with them. Conversely, synthetic resins, in particular the acrylates, give very soft, elastic, light-coloured leathers presenting high resistance to degradation induced by light and which are used for the preparation of clothing articles.
Tanning is carried out in a series of complex, discontinuous operations requiring a considerable amount of labour, and the treatment of waste waters represents one of the major problems in this industry, especially to-day that European norms impose ever more stringent effluent limits. The wastewaters are generally treated in biological plus chemical purification plants in particular to abate (never eliminate) Chrome and Aluminum salts residues. It has been amply demonstrated that natural vegetable tannin is biodegraded with considerable more ease than a synthetic resin, leading to improvements in the effectiveness of existing purification plants. Furthermore, well-defined quality standards as regards these aspects have also been introduced for finished products, for instance in clothing and interior car linings. In this respect two of the tougher requirement limits to comply with are the amount of both trivalent chrome, and of one of its tanning derivatives, namely the even more dangerous, highly toxic hexavalent chrome. Recent European Union directives limit severely the proportion of hexavalent chrome in leathers to be used in direct contact with human skin, such as watch straps, shoe uppers etc., as well as the amount of formaldehyde emission.
Objectives
The main objectives of the project are to obtain:
Activities
As regards the approaches of the proposed project these are centered on three main areas:
The workplan is divided into 4 main tasks that will be run in a phased manner. In the initial two phases, the work is mainly chemistry, with priority placed on chemical analysis, determination of structures, and even more important to copolymerisation and reaction work. This includes determination and optimisation of the parameters of manufacture, characterisation of the new materials formed and of their properties, and initial rapid evaluation of the potential of the new products as regards the meeting of the objectives.
Activities
Workpackages 1 and 2 were addressed during the first year. The activities carried out under these were as follows:
Workpackage 1: Physico-chemical characterisation of raw materials
The natural, waste raw materials to be used have quite a varied composition and had to be analysed and characterised in more detail, particularly in view of the subsequent modifications they were expected undergo. The materials included lignocellulosic waste products from wet-type high-density fibreboard industrial lines as well as the lignosulphonates waste liquors from paper pulp lines. The hydrolysable tannins (chestnut tannins for fibreboard), carbohydrates and lignin contents were determined. The main constituent groups of the materials of the mixes were separated by HPLC and analysed by FT-IR. Their fine chemical structure was determined by 13C NMR to establish the proportion of phenolic moieties present. These are capable of influencing both the capability of fixation on collagen of each material and the level of retardation imparted onto radical polymerisation reactions in the subsequent preparation of the tanning agents.
The capability of fixation onto hide proteins was also determined experimentally and correlated to their composition. Furthermore, the photooxidative capability of these materials, as well as their radical reaction capability was determined by electron spin resonance (ESR) based kinetics experiments and related to the structures found.
Sulphited procyanidin tannins, mainly derived from the bark of European and exotic tree species and the solid residues of the extraction of very valuable tannins for food consumption, such as the solid residues of the hydrolysable tannins extraction of Tara extract were also characterised exactly in the same manner as above.
The results were that several of the tannins, namely European hydrolysable standard chestnut tannin, European light-coloured chestnut tannin, quebracho tannin and mimosa tannin were found to be very suitable for applications such as copolymerisation with synthetic materials. Some of the other materials were also suitable but their use was not advisable due to difficulties in handling, solubility problems and relatively high operational costs compared to what was obtainable.
Workpackage 2: Preparation of new tanning agents
2. 1 Copolymerisation with acrylic acid: The various mixes of waste lignocellulosic residues as well as industrial tannins and the solubilised residues of speciality tannins extraction were reacted with acrylic acid. The first type of copolymerisation of all these material mixes, and possibly the more important to study, will be that based on radical initiation. Almost 100 reaction mixes were tried and copolymers of the four tannins and some of the waste lignocellulosic materials identified in Workpackage I were produced and tested. The following variables were tried:
This study was rather complex and hence was partially approached by factorial multivariate analysis as regards the design of the series of experiments that were conducted. The materials obtained were passed to partners 2 and 3 for leather making and for application assessment.
The variety of copolymerisation products which resulted were analysed and characterised by Partner I by both liquid phase and solid phase "C NMR to understand the nature and extent of the copolymerisation induced and the proportion of the different products formed in the various materials. Their tanning capability and astringency was determined by both Partners 2 and 3 by standard hide powder and Divargan methods. The photooxidative capability light colour and light-fastness of these materials, as well as their residual reaction capability was determined by electron spin resonance (ESR) based kinetics experiments and these related to the structures found.
Although some encouraging results were indeed obtained the colour of the materials and of the leather produced with them remained rather dark and less stable than what hoped for. However, rather elastic and reasonably soft leathers were obtained, the shrinkage temperature of which was rather better than that of vegetable tannins alone but not quit - up to the level of what obtainable with chrome tanning. The conclusions were that the approach based on the copolymerisation of vegetable tannins with acrylic acid gave some acceptable results, the main problem being however the smell of unreacted acrylic acid. The level of this was small, but nonetheless sufficient to give a totally unacceptable smell to the leather produced with the copolymer. Work is still in course to better this situation.
By far more interesting, far better and far more exploitable results are now being obtained in Workpackage 2.2.
2.2. Tanning agents by copolymerisation of lignocellulosic raw materials with synthetic aminoplastic resins: The phenolic groups present in both hydrolysable (chestnut) and condensed flavonoid tannins, as well as those present in the degraded lignin in waste lignocellulosic residues are reactive with formaldehyde and with formaldehyde-based resins under both acid and alkaline reaction conditions. The reaction occurs with relatively easily, particularly for both hydrolysable and condensed flavonoid tannins. All suitable natural products identified were or are to be reacted and copolymerised with synthetic formaldehyde based resins that are already used as tannin agents. These include sulphonated melamine-urea-formaldehyde resins. The purpose was to:
Considerable experience on these types of reactions already existed in the laboratories of two of the partners for application to a different field (namely wood adhesives), and thus optimisation of the best parameters of preparation and maximisation of yields has been undertaken with ease. The first result that is of interest is that all the commercial sulphonated MF and MUF resins used for this application are based on truly obsolete resin formulations. It is clear that in this field resin formulation has not advanced due to the very conservative nature of the industry itself. The first task in this field after trying the commercial formulations was to formulate a truly modem and effective sulphonated MUF resin. This was achieved, and with much lower melamine levels (hence a much cheaper resin than the commercial ones) than the commercial resins.
This newly formulated MUF resin gave exceptionally good results once copolymerised with all the four types of vegetable tannins as regards leather softness, light colour of the leather and light fastness of the leather produced. Much better fullness of the leather was also achieved. The leather produced with 50%-80% vegetable tannin and 20%-50% new MUF resin was almost white, the colour hardly changed with 3 days UV irradiation and the leather produced without any fattening and finishing was softer than what obtainable with chrome.
All this was obtained in direct tanning and not in re-tanning, hence on hides not containing any chrome. Their tanning capability and astringency was also determined by Partner 2 by standard hide powder and Divargan analysis methods. The photooxidative capability of these materials, as well as their residual reaction capability is in the process of being determined by ESR based kinetics as well as by controlled UV exposure and calorimeter measurements. Some of the UV exposure results that have been obtained are very positive (they are positively exciting!).Thermomechanical analysis (TMA) will be used in future to determine networking rates and kinetics, Formaldehyde content and emission tests according to existing European Norms will also be carried out.
In more detail, first of all reaction of the two materials was carried out before leather treatment, as well as during leather treatment by simultaneous treatment of the hide with a solution of the two materials as well as by sequential treatment with solution of the two materials. The temperature of treatment was also optimised at 50ºC after a series of experiments at different temperatures. The best results were obtained with simultaneous hide treatment and co-reaction of the two materials.
At the beginning two commercial sulphonated MUF resins both containing 80% of the expensive melamine were used, these resins being the ones traditionally used in the leather trade. Examination of these resins and formulations revealed that what was available in the trade at this stage is hopelessly obsolete and out of touch with modern MUF formulation practices. The MUF resin was then reformulated to prepare a much higher performance one, and a cheaper one due to its much lower melamine content (down to 47% of the total). This reformulated MUF, in combination with the vegetable tannin yielded exceptionally light-coloured leather and light-stable leather, when treated with a composition of 50% vegetable tannin (Chestnut, quebracho or mimosa) and 50% MUF coreacted in situ during leather manufacture. A second extremely light-coloured composition showing considerable promise was based on a reaction in situ of 1/3 light coloured chestnut tannin, 1/3 of a sulphonated phenolic syntan (synthetic tannin) and 1/3 the new, reformulated sulphonated MUF resin.
The stability to light of these leathers have been checked already by ESR (electron spin resonance) techniques as well as being tested by calorimeter and have shown truly excellent results. ESR testing needs to be continued however on several other samples of this series. Comparison of ESR and calorimeter measures will be carried shortly in the future to compare the two testing methods. It can be said that the light-fastness problem of vegetable leathers appears then to have been solved by the use of these formulations, and solved well. The leathers obtained were very light coloured (almost white), they were light- and colour-fast, they were extremely soft for a non-fattened vegetable-tanned hide and had a well-filled hand filled feeling and appearance. While still presenting some improvement the only property that showed instead only little improvement was the shrinkage characteristics of the leather so produced. Several suggestions in this regard will be implemented, namely the use of alternative non- polluting cross-linkers which will be tried in Workpackage 2.3 later this year, the indications being that even this property could be consistently and considerably improved by this latter approach. The conclusions were that the reformulation of the MUF resin to a level at present unknown to the leather and additives industry has provided the solution to two of the problems which had to be resolved in the project. That is colour and light stability and sufficient softeness to rival a chrome-treated leather before any fattening. This is already a considerable success.
A further result of considerable interest was achieved using a formulation based on 1/3 vegetable tannin (both light coloured chestnut and quebracho tannins were used), plus 1/3 of a synthetic oligomeric phenol sulphonate syntan + 1/3 of the reformulated sulphonated MUF resin. This gave slightly better results than the tannin+MUF system, this being the "whitest" leather of all those that has been achieved up to now.
As a consequence of all the above results it has been decided that the work on the coreaction of vegetable tannins with reformulated MUF resins had to be continued in earnest. It was decided for the following work to be carried out as soon as possible:
In this regard and with the three short-term aims expressed above taken into account:
Partner I will concentrate on the fundamental research aspects of the coreactions in leather of mimosa, quebracho and standard chestnut tannins with MUF resins ad on the further formulation of the MUF resins. They will also carry out fogging tests on automotive leathers to determine volatile components in the material.
Partner 2 (Silva, Italy) will concentrate on the fundamental research and the use of light colour chestnut tannin + MUF resin, light colour chestnut tannin + syntan + MUF resin and quebracho tannin + MUF resin. For all these combinations they will also study the parameters important for industrial application, namely pelt types, pH of treatment, type and quantity of fattening oils and tannin quantity.
Partner 3 (Holmes Halls, UK) will carry out tests of fattening and dyeing with the best samples which will be sent to them, as well as to start with some small scale semi- industrial trial and the consequent tests.
Conclusions
In the first year of operation of the project all the raw materials have been tested and fingerprinted. The copolymerisation of acrylic acid and tannins was achieved and the leather produced and tested yielding some improved results but not quite as good as hoped. However, the co-reaction of vegetable tannins with MUF aminoplastic resins has been a complete success yielding leathers of exceptionally light colour (almost white), light stable and very, very soft and this in complete absence of chrome.
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