Leather finishing by means of electromagnetic radiation29 June 2001
Current leather finishing techniques have led to the development of completely solvent-free systems which avoid chemical reactants that are potentially toxic to man and the environment.The past few years have seen the substitution of solvent-based products with waterborne systems. In Fenice's case, this culminated with the research and development of the Aquagrade system. However, a critical point has to be highlighted - in reality, all systems that use noVOCs need to use chemically active crosslinkers that are potentially dangerous for man and the environment. These types of products are polyaziridines and polyisocyanates. Fenice's study is aimed at the development of leather finishing technology based on crosslinkable polymers by means of electromagnetic radiation (ultraviolet rays and electronic beams). The final aim is not only to achieve leather finishing with VOC-free products, but also finishes that do not require external crosslinkers to reach the desired mechanical properties. The ambitious objective is to obtain finishes with high attractiveness, excellent touch and good mechanical characteristics through the use of simple installations and systems. Leather finishes can nowadays be obtained in two ways: turning to conventional solvent-based products (NC lacquers or polyurethanes in organic solvent) or using polyurethanes in water dispersion that are integrated and formulated and then during the application phase are reacted with a chemical crosslinker. Solvent lacquers are generally characterised by a dry content of 10 - 20% and a VOC content of 80-90%. Also, they are then further diluted 1:2 or 1:3 with organic solvents. The new generation waterborne fixing agents, such as Aquagrade, have a dry content between 18 and 32%, according to the type, with a VOC content below 5%. In the application phase, they are again diluted with water and a 5-10%, by weight, of a chemical crosslinker is added. The crosslinkers used are generally 50% solutions of polyaziridine or polyisocyanates in a water-based solvent. Keeping this in mind, it can be deduced that the VOC content in the application mix of a water-based fixing agent is generally around 4-8%. The new idea is to finish leather by means of crosslinking with products, which can be crosslinked using electromagnetic radiation. The aim is to eliminate the use of solvents as well as the chemical crosslinking, thus eliminating the VOC content in the emissions. From a technical point of view, there are two ways of crosslinking to be hypothesised: * In the first case, the use of fixing products with 100% active substance is foreseen, made up of crosslinkable polymers, diluted with reactant monomers and then irradiated. Chemically speaking, this involves urethanes, epoxides or polyesters modified with polyfunctional acrylates. After application on the surface an appropriate photoinitiator is added and exposure to radiation completes the polymerisation to produce a contiguous film, which incorporates both the polymers as well as the crosslinkable monomers. The use of the latter is necessary so as to regulate the viscosity and rheological characteristics of the polymers in order to meet the needs of the techniques with which the products are applied (by spray or roller application). In this way, 100% of the finish applied goes to make up the film, eliminating emissions * The second possibility deals with the use of crosslinkable polymers by means of electromagnetic radiation. The product is applied conventionally in the form of a water-based emulsion/ dispersion. It involves modified urethanes with polyfunctional acrylates. In this case, after application on the leather but before the crosslinking via polymerisation, the water must be removed. Thus, there are emissions, but they are limited to water. The introduction of the new technology, whether it is carried out through crosslinkable systems at a 100% of active substances or crosslinkable systems in water dispersion, in any case, is the same, the elimination of VOCs. Current research Current research involves: * characterisation of existing polymers in the market that could be used as raw materials for crosslinkable finishing products using electromagnetic radiation * characterisation of suitable photoinitiators to be used as promoters * the formulation of a complete range of finishing products based on these, which are able to meet the modern needs of the finishing sector * an analytical investigation of prototypes for the purpose of characterising them in a complete and appropriate way. This is in relation to the chemical-physical characteristics of the products and the film formed by them * characterisation of the suitable technological equipment for carrying out the crosslinking of the new products after their application on the leather * application trials which are studied in more detail and repeated with the aim of: * verifying the actual possibility of crosslinking through radiation of the formulated prototypes * optimising and improving the products as well as the operative conditions with the purpose of making the system simple and efficient to use * comparing the results obtained with those of conventional finishing technology * a study of the risks associated with the use of new technology and the precautions needed * in case the polymers that are available today are not deemed suitable or satisfactory, research is needed for the realisation and development of a new system. There needs to be the formulation of polymers aimed at producing suitable products, at first on a laboratory scale and then on a pre-industrial scale, using a polymerisation pilot plant. The environmental impact of leather finishing processes is essentially due to the products applied in the last stage of finishing, the so-called fixing agents or topcoats. On the other hand, some important parameters such as the aspect and touch of the finished leathers are related to this phase of the whole process, as well as the fastness properties. These parameters are largely related to the final destination of the leather. The current products that carry out the fixing can be these two types: Solvent-based lacquer This deals with lacquers that are normally made up of nitrocellulose or polyurethanes dissolved in organic solvents, with the eventual addition of plasticizers, matting additives, flowout promoters etc. These lacquers as supplied have a dry content of 10-20% and, consequently, a VOC content from 80-90%. Next to the solvent lacquers are the nitroemulsions. In this case, it involves solutions of plastified nitrocellulose or CAB (cellulose acetate butyrate) in solvent, emulsified in water with the help of surfactants. The dry content as supplied generally varies from 8 to 16%, with a VOC content of 40-60%. Figures 1 to 3 show illustrative formulations. On application the solvent lacquers are normally diluted 1:2 or 1:3 with organic solvents (the nitroemulsions 1:2 or 1:3 with water). Water-based fixing agents In this case, the fixing product is based on polyurethane aqueous dispersions, having a dry content that varies from 18% and 32% depending on whether they are the ready-to-use type or diluted with water. The organic solvent content, ie residual solvents from polyurethanes or added solvents like coalescents, is generally less than 3-5%. Example formulations are shown in figures 4 and 5. In this case, the additives are made up of flow promoters, possible matting agents, feel modifiers etc. Analysing the formulations shows a large reduction in pollution that results from using waterbased fixing agents. Above all, it is vital to consider that the use of waterborne fixing agents involves the combined use of chemical crosslinkers so as to reach the final required fastnesses. These latter fixing agents are generally supplied as 50% active components dissolved in water-based solvents. Thus, the problem of VOCs is not completely resolved, but is strongly reduced. Furthermore, the health situation of the working environment and security for the workers is still a problem due to the danger and reactivity of chemical crosslinkers. Crosslinking with electromagnetic radiation Chemical products that crosslink by means of electromagnetic radiation could resolve the problem of the impact of chemical crosslinkers on the environment, in as much as it would create the possibility of working without organic solvents and without the crosslinkers. The crosslinking of finishing products with electromagnetic radiation is possible by using polymers that have functional groups in their structure susceptible to further polymerisation under certain conditions. From a practical point of view radical polymerisation, induced by the presence of photoinitiators, is of interest. Crosslinking can occur when molecules containing double bonds are exposed to particular electromagnetic radiation, principally UV rays or electron beams. This reaction proceeds in three fundamental stages: * the initiation, in which the radicals are formed * the propagation, during which the polymerisation takes place with the formation of polymeric macromolecules. In this phase, the chain reaction produces free radicals, which promote the extraction of hydrogen atoms from saturated molecules, thus increasing the number of active centres for polymerisation * the termination, in which two radicals combine and bring about an end to the process A substantial difference between photo-polymerisation induced by UV rays and that promoted by electron beams is that in the first case the use of a photoinitiator is necessary for the production of free radicals to trigger the process. In the second case, the formation of free radicals for starting the reaction is initiated by bombarding the polymer with electrons. The polymerisation induced from the ultraviolet radiation seems to be the more interesting to investigate given the likely lower costs. The source of radiation consists of a UV lamp while in the case of photo-polymerisation with electron rays, an electron accelerator is required. The treatment presented, which follows, is therefore specifically orientated to crosslinking by means of UV radiation. Two possibilities exist, as a rule, for realising products for innovative fixing: * products crosslinkable through radiation, by applying 100% active substance * or products crosslinkable through radiation, in an aqueous emulsion/dispersion The first one is made up of low molecular weight oligomers and crosslinkable monomers through exposure to radiation, in the presence of photoinitiators. In such a case, the monomers carry out the function of the solvent and are necessary for conferring on the product a viscosity suitable for its application (that can be done by spray or roller). The polymer and the monomer have reactive groups, both of which take part in the crosslinking reaction. This is an integral part of the finishing film. In this way, the problem of emissions is eliminated by applying a 100% active substance product. Aqueous emulsion / solution products have the advantage of water as a solvent and, therefore, need a drying stage before the crosslinking is induced by electromagnetic radiation. In the second case, the system will have a reduced content of active substance, but the emissions generated will consist of water vapour only. The double acrylic bond seems to be the most suitable, given its reactivity, for this type of application. The polymers to investigate for creating crosslinkable products by means of radiation will be those with two or more acrylics groups. Among these are included urethane-acrylates, polyester-acrylates, epoxy-acrylates and polyacrylates. Among the mono and bifunctional reactive monomers Fenice propose the following: acid acrylic, hydroxyethylacrylate, glycidyl acrylate, octyl acrylate, 1-6 hexandiol-diacrylate and propyleneglycol diacrylate. Chemically speaking the following reactions are thought to be involved a) urethane-acrylates. The reaction is between a urethane pre-polymer with a free isocyanate group and a hydroxyethylacrylate b) polyesters-acrylates. The esterification is between a polyester of a hydroxyfunctional moiety and acrylic monomers. The esterification between the hydroxylgroup of hydroxyacrylates and acrylic acid opens a way for the creation of bifunctional reactive monomers. The final products of interest will probably have compositions similar to those set out in table 1. As regards the dispersion in water of the reactive polymers this can be achieved according to the following ways: * aqueous emulsions of reactive polymers through use of emulsifiers (for this purpose, the literature indicates the epoxy-acrylates as being the most easily emulsionable). * synthesis of a) water dispersible polyurethanes following the incorporation in their structure of solubilising functional groups, eg carboxylic acids, during the polymerisation or water soluble molecules, eg polypropylene glycol chains In both ways it is possible to obtain water dispersible urethane pre-polymers without the addition of external emulsifiers, by taking advantage of molecular groups inserted correctly into the polymer chain. It is also necessary to have molecules which are able to form free radicals when exposed to UV radiation. The suitable photoinitiators available on the market belong to the following groups: * alkyl-aryl-ketones * benzoin or benzoin-ethers * -hydroxyalkylphenones * benzophenone derivatives Table 2 relates the difference between the two crosslinking technologies. Analysis of the points mentioned in the table 2 suggests concentrating on u/v crosslinking. Objectives Looking at the research as described in the preceding paragraph, the following objectives were identified: * Formulation of the basic polymer * Formulation of the crosslinkable prototypes * Application on the leather and verification of the results Polyurethane synthesis in aqueous dispersion uses the following steps: a) Reaction between diisocyanates and polyols (polyesters, polyethers and polycarbonates) to a urethane prepolymer. This reaction normally takes place without solvents or with the use of limited quantities of N-methyl-2-pyrrolidone (5 - 10%) so as to regulate the viscosity within acceptable levels for the work in hand b) Reaction between the isocyanic pre-polymer and bifunctional molecules such as diols and / or diamines that are able to react with the isocyanic functions of the pre-polymer. It is in this phase that the molecular weight of the polymer increases more rapidly, as the poly-addition of pre-polymer blocks creates larger chains. It is still in this phase that the final polymer with the solubilising functions can be introduced into the macromolecules which are responsible for the dispersibilty in water of the polymer. An example of a substance used for this purpose is the dimethylolpropionic acid that allows the introduction of carboxylic functions in the polymeric chains. c) After the lengthening of the chain and insertion of the solubilising groups the pre-polymer can be dispersed in water, by means of a basic action, eg triethylamine) that solubilisise the carboxylic functions present in the polymeric chains. The general plan is as follows: * Polyol + isocyanate = polyisocyanate adduct * Adduct + dimethylolpropionic acid = ionomeric polymer * solubilisation with amine produces the salt * Alkaline water and ionomeric salt produces the final dispersion With the polyurethane illustrated here there are multiple ways of modifying the structure to influence the physical-mechanical properties of the final polymer. These include the choice of the isocyanate, of the polyols, the choice of the chain extenders and the operating conditions. These are the parameters that Fenice propose to modify so as to obtain suitable polymers for the finishing of the leather. In addition to what has been stated above, into the polymeric backbone some acrylic functional groups need to be inserted. These will be responsible for photo-polymerisation when exposed to radiation in presence of a photoinitiator. To achieve this, the isocyanate pre-polymer is reacted with an hydroxyacrylate. The next step with regards to the formulation of the experimental products is to meet the needs of finishing different types of leather. The diversity of the fixing products used on leathers is designed for final use, with parameters related to the tangible characteristics of the finish, eg slippery, fatty, waxy; optical characteristics such as bright, matt, satin film and the chemical-physical-mechanical characteristics of the finishing film The addition of additives normally takes place during the formulation of the fixing agent, eg when adding matting agents, feel modifiers such as silicon or waxy emulsions. The characteristics of chemical resistance or the use of aqueous agents and the mechanical fastness properties such as flexibility, hardness and rub resistance are directly related to the choice of isocyanates and/or polyols used in the polymer synthesis. It is necessary to have a series of basic polymers with different chemical-physical-mechanical characteristics, so as to be able to achieve suitable fixing agents for various types of leather, each which must meet determined needs regarding fastness. Summary The finishing processes are carried out with the aim of enhancing the leathers and to give ideal general optical characteristics as well as good resistance properties. It is likewise important to enhance the aesthetic aspect of the leather. However, Fenice feel that the most critical factor is reaching the standards of fastnesses that are acceptable for finished leather according to its final use. At the end of the research, Fenice hope that an innovative procedure will be available for leather finishing, based on new technology to this sector, that allows the elimination of chemical crosslinkers as well as organic solvents. Thus, processes with just a minor impact on the environment will be achieved.