Formaldehyde in leathergoods: remedies and solutions

19 December 2005

Evolution of the market, application and cognitive technologies have in recent years significantly modified the requirements that must be met in the production of consumer goods. The constantly growing interest in 'health' at a worldwide level and, therefore, with regard to environmental problems, has led the industrial system to an increased attention to 'environment-friendly quality' articles. For this reason, Ecolabel trademarks and design labels have been established, such as: * Oeko - Tex standard * SG trademark * ICT ECO - tox guidelines No.1/96 * Draft EU Ecolabel for footwear and copious technical drafts for specific sectors including car upholstery or footwear and involving numerous, more or less restrictive control parameters. Of all the parameters, special attention must be paid to the aspect of free formaldehyde, its presence in leathergoods being considered strictly linked with the kind of chemical products used in the manufacturing process. In articles, free formaldehyde is permitted only within very strict limits of 150/50 ppm max for the purposes of certain trademarks (SG) and usage; in some cases even lower limits are applied, eg in the car and footwear industries. In order to handle this important subject, it is crucial to know and systematically assess all aspects, to enable us to prepare all the corrective and preventive measures to produce articles that meet the necessary requirements. Formaldehyde in leather Formaldehyde, the simplest of aliphatic aldehydes, is of considerable importance in the wider chemical industry, in synthesis, and has known industrial scale production for more than a century. It is used in the production of foam plastics, plastic products for engineering, regenerated wood, paints, dyes, vitamins and in preparations used in the paper industry. In the tanning industry, it is used as a tanning agent, as a casein fixing agent in finishing and is used in the production of pre-tanning, tanning and re-tanning agents based on synthesis and condensation, such as tannins and resins. Products most commonly used in tanning, involving synthesis based on polymerisation and condensation of various substances, including formaldehyde, are as follows: * Synthetic tannins * Melamine and dicyandiamide resins * Urea - formaldehyde etc As is known, synthetic tannins contain more phenolic functions, obtained by synthesis and used as principal or auxiliary agents (to aid solubilisation and penetration of vegetable tannins and dyes) in tanning. They were studied and synthesised mainly to produce substitutes for natural tannins, commencing from a base molecule such as phenol which, after preliminary sulphonation treatments, underwent condensation with a loss of one water molecule. Figures 1 and 2 show two reaction diagrams. In the case of the tanning industry, synthetic resins are achieved by polymerisation and/or condensation from a monomer, then the required reactions (suitably varying temperature and pH) are performed in situ (in the leather) in order to obtain polymers, possibly straight chain. From the various diagrams, it is clear that formaldehyde used as a condensing agent will certainly be present as a non-reacted element in products and will also be encountered, not only in tannins or resins, but also in all other products synthesised by this process. However, it must be emphasised that the use of products in which formaldehyde is present as a residue of reactions, will offer a modest contribution with respect to its direct use. In fact, the quantity of formaldehyde may be optimised, ie so that its presence is the minimum possible, if at factory level the production process respects the stoichiometric ratio and takes account of certain important parameters such as: temperature, rotation times, agitation speeds etc. As a matter of fact, the situation is completely different when using compounds such as vegetable tannins, polymer tanning agents and glutaraldehyde, which offer no contribution to the formaldehyde in the leather. Naphthalenic and sulphonic tanning agents are also comparable with vegetable tanning agents provided that the above parameters are respected. In tanning and re-tanning, formaldehyde can react with collagen-free base groups to form methylene compounds via an alcohol condensation reaction (a stable C-C methylene bond is formed which remains intact under analysis conditions) as shown in Figure 5. In this kind of tanning, the bond is covalent and, in particular, the pH interval of 6-8 used in practice is ideal for achieving the set maximum quantity. When tanning application conditions are such that they guarantee this kind of bond, the problem of free formaldehyde would seem to be inexistent, in the sense that drastic hydrolysis conditions would be needed to free it. It is no coincidence that the main analytical method to determine the bonded formaldehyde involves hydrolysis of the leather using sulfuric acid and subsequent distillations. The use of formaldehyde as such, and of products that release it by decomposition, will become increasingly less diffused as formaldehyde limits are lowered, with the aim of achieving environment-friendly articles. Formaldehyde: characteristics and assessment analysis methods Formaldehyde is not easy to handle: it has a penetrating odour, irritates the eyes and mucous membranes, and skin exposure to formaldehyde in those subject to allergies may induce allergic reactions. According to current regulations on the subject of labelling and hazardous preparations, formaldehyde in a maximum 50% solution is classified with the toxic T symbol, with the following hazard warnings: R 23/24/25 Toxic by inhalation, in contact with skin and if swallowed R 34 Causes burns R 40 Possible carcinogenic effects R 43 May cause sensitisation by skin contact Besides its allergenic properties, formaldehyde was indicated as a potential carcinogenic agent during experiments conducted in 1980 by the Chemical Industry Institute of Toxicology in North Carolina (USA). Nevertheless, formaldehyde was classified by the European Union as a suspected carcinogen under Category 3, and this has certainly accelerated efforts to lower limits in leather and articles in general. Formaldehyde may be present in articles in free form, reversibly or irreversibly bonded. Among the numerous methods developed to identify it, the more significant are those operating in the gaseous phase (AUDI/VW) PV 3925 and in aqueous media DIN 53315. Free formaldehyde, which is not bonded to the leather itself or to other substances applied to the leather, may be measured by the gaseous phase method. Reversibly bonded formaldehyde is measured by the water extraction method (results obtained via the extraction method represent the total of reversibly bonded and free formaldehyde); irreversibly bonded formaldehyde, ie the total formaldehyde content, may be assessed for example by treating the leather with sulfuric acid and subsequent distillation. Water extraction and gaseous phase analysis methods are compared in Table 2. Skins containing formaldehyde within required limits: how are they produced? The tanning industry needs to aim for clean products and processes. Therefore, innovative solutions are required to achieve this objective. Clearly, the greater undertaking falls to those producing and marketing chemical products for manufacturing processes but the technical and applicational contribution to the product is equally fundamental, with a mental approach that aims to optimise the application by exploiting all acquired know-how. To achieve the objective, steps are necessary on various fronts: * to produce products such as tannins, high-quality resin products, by developing advanced chemistry and technical systems that reduce formaldehyde content to a minimum * to produce skins by methods that take into account the characteristics of applied products * to introduce the use of new concept substitute products that overcome the problem and achieve the same product results * to study new products and application techniques, for cases in which the use of condensation products requiring formaldehyde is inevitable, in order to reduce its quantity In this respect, FGL International, aware of this need and with the intention of remaining close to customers by meeting user requirements, have produced a range of low formaldehyde content products which, on application, respect the limits set by law and/or technical drafts. * Research and production optimisation has led to the Lecosin range of synthetic tannins and Lecoren melamine and dicyandiamide resins with a reduced free formaldehyde content which, if used in appropriate quantities and type, allows the production of articles that respect legal requirements. * Re-tanning agents have also been researched as an alternative to classic phenol, melamine and dicyandiamide condensation products, by producing synthetic polymers with strong crosslinking power and re-tanning ability in the Permasol and Idrosin range. * The research and constant efforts underlying FGL International business activities has also produced Permasol TFR, a product able to intervene when the use of products containing formaldehyde that increase values beyond set limits is inevitable. Permasol: characteristics and applicational advantages: Reacts irreversibly with free formaldehyde and forms stable bonds, reduces the formaldehyde level to bring values well below set limits. The use of Permasol TFR is a valuable aid when the use of classic condensation products containing formaldehyde is inevitable in order to respect the technical requirements of particular articles. By application in various kinds of skin recipes involving the use of phenolic, dihydroxyphenyl sulfone and melamine resin synthetic tannins and repeating the same process on the other half of the skin using Permasol TFR, free formaldehyde values proved to be halved. The resulting average values are given in Table 3. Conclusions The continuous development of market and legal requirements force industry, including the tanning industry and related sectors, into facing constant challenges and adaptation. With the awareness that all of this is necessary to improve workplace and environmental quality, the aim for all parties involved must be that of undertaking the research and development of production and application techniques to achieve products that meet requirements.

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