Cationic finishing technology31 October 2002
Cationic finishing products were first available for tanning and finishing leather from the time modern ideas on finishing began to take a hold on the industry in the 1930s. New technologies, including acrylics, copolymers and polyurethanes were all anionic in form and only recently has there been a resurgence in the application of cationic technology in the leather industry. Products such as pigments and fillers with an anionic charge have little natural affinity for leather and, therefore, call for the use of resins and binders to provide the necessary levels of adhesion to the leather surface. Nevertheless they provide excellent physical properties. In contrast, cationic finishes are meeting requirements for a finish which may be best described as a 'skin cosmetic' rather than a 'paint' while performing to the same high aesthetic standards as anionic finishes. Stahl's range of cationic finishing products now includes ten principle pigment colours, casein binders, fine particle aliphatic polyurethane resins, a fine non-ionic acrylic resin, a soft oily emulsion, a filler and a number of other special effect products. They are being used to give a soft, natural, well-filled look to leather while providing excellent coverage to mask defects such as pin-hole, scars, mange and others. In so doing, they offer an excellent means of upgrading a wide variety of full grain softee shoe upper leather and sheep nappa clothing leathers. This early approach to the use of cationic products was to apply them as a sealer coat to regulate the absorption of subsequent finishing coats. Upgrading was achieved by sealing the parts of the grain surface where the enamel had been removed due to mechanical or natural causes. This resulted in relatively poor performance by today's standards, often with poor intercoat adhesion, wet rubs and other physical properties. Today, the chemistry is much better understood and results in a number of important advantages: * The cationic charge gives better bonding with both chrome and vegetable retans with less need to use resins and binders to promote adhesion * The pH of around 4.0 is close to the natural iso-electric point of leather, giving rise to gentle absorption of the finish by osmotic pressure without using wetting agents or solvents. This gives better penetration and adhesion of the finish * All cationic products in the Stahl range have a natural fine particle size and, with a Shore hardness of 10 or less, are softer than many of the anionic counterparts. This contributes again to improved penetration, good adhesion within the leather structure, a softer finish and often a tighter leather due to less wetting and, therefore, minimal swelling of the grain surface * Less film loading is formed on the surface of the leather * Cationic products can be used with less pigments and resin in the formulations. As pigments are generally dry and rough, they do not penetrate well into the leather and the ability to replace them with softer, fatty and waxy substances improves upgrading and aesthetics * Cationic finishes improve tensile strength, lasting properties, softness, and fill In addition to the general range of cationic products for use in finish formulations, Stahl have recently introduced a range of cationic pigments containing highly concentrated, high performance organic and inorganic colours. Other products in the range include a non-ionic acrylic resin to act as a bridge to subsequent anionic finishing products used in 'sandwich type' finishes. The company have also introduced a range of compact resins which offer opportunities for simplifying the finishing process, leaving the finisher free to concentrate on detailed adjustments to the precise formulation which will achieve the exact appearance, colour and performance required. There are two main disadvantages to using cationic finishes: the first is that wetrubfastness does not measure up to the performance of anionic counterparts. This is not necessarily a problem provided that it is considered when deciding on the finishing system to be used. The second is that, because of the softness of the finish film, print retention is usually poor. Where this is important to the continued good looks of the leather, the finish can be cross-linked to improve its performance. Crust leather should be treated in the retannage and fatliquor stage using any cationic topping products such as fixing agents or oils. The first coat of the cationic finish should then ensure natural and full wetting of the surface for optimal defect coverage. Application of the finish can be by roller coating, padding or spraying whichever gives the best results, followed by a recommended drying period of at least twelve hours prior to smooth plating and finishing. The next stage is referred to as the melting process. The leather is ironed or plated at low pressure and relatively high temperature, with sufficient dwell to fuse the products to the fibres. This causes the finish to flow into the damaged areas which receive greater product filling and sealing and surface uniformity, thus upgrading the leather. For the best results, plating temperature is normally 90-100°C, and pressure is between 100 and 175kg for 1 to 5 seconds on a hydraulic plating machine. Similar results are achieved using a slow speed rotary ironing machine at a temperature of 110-110°C and a pressure of 50 to 75 bar. If shorter times are used, satisfactory results may be achieved although the upgrading ability may be reduced. The leather is finally softened, preferably by milling overnight or by mechanical staking, which results in a uniform soft, well-filled leather with a very pleasant handle. Finishing requires the application of light coats and, therefore, the leather should be drum dyed to shade. If high quality results are to be achieved, all equipment throughout the whole process of cationic finishing must be kept clean to avoid precipitation which would occur should they become mixed with anionic materials. There are three choices of approach to final finishing of the leather: * A fully cationic finish is achieved by using only cationic products in the base coat and subsequent conventional top coats * A semi-cationic or multi-charge finish using a combination of cationic, non-ionic and even anionic products for upgrading using a more conventional type of finish * Sandwich finishes are finishes in which a fully cationic sealer coat is followed by anionic conventional finish coats to give maximum upgrading on very low quality leathers. For this type of finish, Butadiene resins must not be used in the base coat as this causes intercoat adhesion problems. The incorporation of non-ionic acrylic resins in the sealer coat is recommended for improving intercoat adhesion as this reduces the possibility of over-sealing and acts as a 'chemical bridge' to subsequent anionic coats. Using cationic technology, low selection crust material can be upgraded into a competitively priced, saleable article, natural looking and with acceptable physical properties. These products from Stahl help tanners to upgrade a wide variety of crust materials and produce a more natural looking leather than was previously possible. Acceptable physical properties are achievable. Looking ahead, cationic products may well become available that will extend their use into high performance leathers and early development work in this sector suggests that favourable results are possible, especially when applied as a pre-sealer coat prior to more conventional finish applications on both full and corrected grain leathers.