Feasibility of using powder coating22 August 2001
For the first time the feasibility of using powder coating technology, a process involving dry solid particulates, has been investigated for leather finishing. Results showed that powders can be applied to leather successfully using conventional electrostatic spraying facilities, provided that the leather is properly wetted with water. Powders can also be successfully applied to leather using non-electrostatic application methods, after pre-treating the leather surface with a tackifying agent, such as a soft resin, that makes the leather surface tacky and, therefore, receptive to powders. Finishing trials on leather using currently available commercial powder coatings have shown that a surface coating with an appearance no different from a conventional pigmented finished leather can be achieved without loss of area or softness, provided that stoving conditions are controlled (160°C for 10 minutes). Although the coating has excellent wet and dry rub fastness, sufficient dry adhesion and adequate wet adhesion properties, the major problems are the occurrence of surface cracking and poor flexing endurance. This was because the powder coating used had not been specifically formulated for leather finishing. Powder coating1,2 is a complete solid coating technology where coating material in the form of a fine powder is applied to the substrate surface and then fused to form a continuous film at elevated temperatures. In so far as possible, each particle of the powder should contain all ingredients in the formulation, well mixed with minor components such as colorants, flow agents and crosslinkers etc, in a matrix of the major binder components. There are two broad categories of powder coatings, thermosetting and thermoplastic; by far the major portion of the market is for thermosetting powders. When environmental and economic factors are taken into consideration, the distinct advantages of powder over liquid systems can be listed as follows1,3: * With no emission of volatile organic compounds (VOCs) it will satisfy the most stringent legislation on VOC emission * Reclaiming powder is possible, leading to almost 100% utilisation of the material and, therefore, cuts in material cost and effluent * An even coating is achieved without the need for multiple coats and drying or extensive and overlapping spraying, thus save time and energy Powder coating is, therefore, an ecological, economic and energy-efficient technology; cost per unit area is actually cheaper than conventional methods. Although powder coating offers an interesting alternative, it has not yet been regarded as feasible for leather finishing. This is primarily because (1) the stoving temperature of current powder coatings, typically ~ 200°C, is too high for leather, and (2) common powder application methods (electrostatic spraying, fluidised bed etc) are not applicable to leather which is neither an electric conductor for the electrostatic spraying method, nor a good heat dissipater for the fluidised bed method. However, powder coatings requiring stoving temperatures as low as 120°C are now in the early stages of commercialisation; and there is extensive research effort on lowering the temperature to 80°C or below. Therefore, we believe that it is timely to investigate the feasibility of powder coatings for leather finishing. Our primary concerns are about developing appropriate methods for applying powders onto the leather surface, and stoving conditions that will have no adverse effect on leather. For these purposes we have chosen a commercial thermosetting powder requiring a relatively low stoving temperature (160°C) for our preliminary. Experimental Materials A thermosetting epoxy/polyester hybrid powder, Interpon 700, was supplied by Courtaulds Coatings. The stoving condition for this powder recommended by the manufacturer is 160°C for 20 minutes. The leather used was a grain split (2 - 2.2mm) of chrome-tanned bovine crust leather prepared in our tannery. Application of the Powder to Leather Surface (a) Electrostatic spraying A Ransburg-GEMA electrostatic spraying system with a GEMA Volstatic PGC1 control unit was employed. A chrome-tanned crust leather sample (grain split) was wetted with tap water and then hung vertically in the spraying booth and earthed. Electrostatic spraying was performed, at 70 kV and with a carrier gas flow rate of 2 m³/h, until the leather surface (grain side) was visually fully covered with powder. (b) Applying the powder to a resin-treated leather surface An aqueous soft acrylic resin, made by mixing one part of RU9611 (Stahl Ltd) and one part of water, was sprayed onto the surface of the crust leather using a plastic hand spraying bottle. After drying, the surface of leather became tacky. In a fume-cupboard, the tacky surface of the leather was pressed onto a bed of powder. Excess powder was removed by gently shaking the leather whilst it was held vertically or with the powder-coated surface facing downward. Alternatively, the leather was laid horizontally and flat, with the tacky surface upwards. The powder was spread over the surface by vibrating a laboratory test sieve which contained the powder, and excess powder was removed in the same way as described before. (c) Applying the powder to a wetted leather surface Leather was immersed in water for a few minutes. The powder was then applied to the wet surface and excess powder removed using the same techniques as described above for the resin-treated leather surface. Stoving Stoving was carried out in a conventional convection oven. Typically, leather samples (15x15cm) coated with a thin layer of the powder, were placed in the oven at different nominal temperatures (130, 140, 150 and 160°C) for different lengths of time. Physical Testing Softness The softness of leather samples, before and after the application of powder coating, was measured using a BLC softness gauge after conditioning at 20°C, 65% humidity for 24 hours. Colour fastness to dry and wet rubbing Dry and wet rub fastness of the powder coating finish was determined by the Official Method (SLF5)4 using a rub fastness tester (STM 461, SATRA Footwear Technology Centre). Colour changes of leather and felt pad were visually assessed according to the Grey Scale. Finish adhesion Wet and dry adhesion properties of the finish were measured according to the Official Method (SLF11)4. PVC strips were firmly adhered to the finish surface of the leather sample using a polyurethane adhesive (Chelsea C/70, Caswell & Company Ltd), in a direction either parallel or perpendicular to the backbone. The force required to peel the leather substrate from the finish starting at one end of the strip was determined with a Leather Finish Adhesion Tester. The force was expressed in grams weight. Results and discussion Powder Application Techniques for Leather Among various trials5 that were conducted for applying powders to the leather surface, the three methods described in the experimental section have achieved various degrees of success. Results from our initial trials have shown that powders can be applied successfully and most satisfactorily to a leather surface by conventional electrostatic spraying, provided that the leather has been wetted with water. Figure 1 shows the results of electrostatic spraying on dry and wetted leather. Clearly electrostatic spraying has not worked for dry leather but worked well for wetted leather. Reverse pick-up of powders by the back side of the wet leather was also observed (see Figure 2), which clearly demonstrates, beyond any doubt, that the powders were picked up by the leather with electrostatic force. Therefore, the leather has been made electrically conductive simply by wetting with water. The powders can be coated evenly onto wetted leather with good control, just like a piece of metal, and the powders adhere well to the leather surface. Pre-treating the leather surface with a tackifying agent so that the powders will adhere to the leather surface is also a successful technique, although the results are less satisfactory than electrostatic spraying in terms of the evenness and coverage of the powder coats. This less satisfactory result is partly, if not entirely, due to the unsophisticated manual operations of applying powders in our trials (spreading the powder with a laboratory test sieve or pressing the tacky surface to a powder bed). The use of appropriate machinery or a spraying device should improve the evenness and coverage. Although in our experiments a soft acrylic resin was used to make the leather surface tacky, other appropriate adhesives should be applicable as well. This method of tackifying the leather surface may be more versatile than electrostatic spraying of wetted leather since the latter method may be problematic for water-repellent leather. It was found that wetting of the leather surface with water also provided a receptive surface for powders by the same spreading or pressing methods used for the tacky surface. However, the evenness and coverage of the powder was poorer than when using the soft resin. The differences between the above three methods are best illustrated by the appearance of the finish after stoving, as shown in Figure 3. The brighter spots or patches which can be seen on the surface of the finish are due to varying light reflection caused by the uneven surface, rather than an incomplete covering of the powder coating. The continuity and smoothness of the film formed under the same stoving conditions (see samples b, c and d shown in Figure 3) clearly depend on the powder application technique, and are in the following order: - electrostatic spraying of wet leather > pre-treating the surface with tacky resin > tackifying the surface with water Stoving Condition and Area Loss Figure 4 shows the dependence of area loss of powder-coated leather on the time and temperature of stoving, after conditioning at 20°C and 65% relative humidity for 24 hours. Stoving for 25 minutes at 130°C, 15 minutes at 140°C, 10 minutes at 150°C or 160°C resulted in no area loss. Thus the use of a stoving temperature as high as 160°C should only last for up to 10 minutes, if the area yield of the leather is to be maintained. Stoving and the Softness Results for softness before and after powder coating finishing under various stoving conditions are given in Table 1. If stoving is carried out under the conditions which result in no area loss, there is also no loss of softness. In most cases, the softness is little affected by stoving but slightly decreased for prolonged stoving. In fact, as far as the important quality of softness is concerned, leather samples can endure the stoving condition required by this powder, ie 160°C for 20 minutes, with no detrimental effects. Note that for the samples after coating and stoving, the softness was measured with the coating on. Thus it appears that the leather softness is unaffected by this particular powder coating. Stoving and Appearance The appearance of powder coating finished leather samples are shown in Figure 3. The surface characteristics, in terms of smoothness, are clearly dependent on the powder application methods employed, and more critically on the temperature of stoving. Low temperature stoving, even for a prolonged period, fails to produce a smooth surface, due to poor flow at low temperatures. Higher temperatures improve the flow of the polymer and consequently improve the smoothness of surface, as is apparent when samples a) and b) in Figure 3 are compared. It should be pointed out here that the finished leather had not been subjected to the hot compression procedure normally used in finishing. The smoothness of the surface would certainly be improved if such hot compression were used. Generally, if the leather samples are either electrostatically sprayed or pre-treated with soft resin followed by a non-electrostatic application method, stoving at the required 160°C will result in an appearance which is no different to a conventionally coated leather with a pigmented finish. Stoving at 160°C for 10 minutes (producing no area loss) is sufficient and prolonged stoving does not appear to improve the appearance any further. Therefore, it may be assumed that the 20 minutes stoving at 160°C, as recommended by the manufacturer, is to ensure a sufficient curing to optimise the mechanical performance of the coating. The major problem for this powder coating finish was the occurrence of surface cracks that can only be observed under microscope. The problem tends to become more serious at higher stoving temperatures (eg 160°C). When the finish was bent through 180° and then pressed, cracks were visible to the naked eye. Cracks developed after re-conditioning. One sample that was kept dry has no occurrence of surface cracks. Therefore, the dimensional change of leather during stoving and after conditioning appears to be one of the crucial factors which cause surface cracking. During stoving at high temperature the surface area of leather contracts due to loss of moisture. After conditioning the surface area expands and pulls the coating so cracks occur, as the film is not fully cured and has low tensile strength. Thus, leather may be able to endure 160°C for 10 minutes, but its dimensional change at high stoving temperature makes it incompatible with powder coatings. This suggests that low temperature powder coatings, preferably below 100°C, would be essential for leather finishing. Dry and Wet Rub Fastness Table 2 shows the results for rub fastness under the stoving conditions that do not lead to any area loss. It can be seen that dry and wet rub fastness increase as either the stoving temperature or the time increase. Even though the finish is not sufficiently cured under these zero area loss conditions, the rub fastness of these powder coating finished leathers is excellent, particularly the wet rub fastness which reaches 1,024 revolutions without problem. This should be contrasted to the situation with the widely used water-borne finishes where wet rub fastness has been a major concern.6-8 Finish Adhesion Satra recommends dry adhesion should be between 200g weight and 700g weight and it is clear that dry adhesion is adequate (see Table 3) after curing at 130°C for 25 min or 160°C for 10 min, conditions giving no area loss. However, the procedure of soft resin treatment followed by powder coating leads to poor adhesion properties. The acrylic resin used appears to have impaired the adhesion between the powder coating and leather. This may result from a poor adhesion between leather and the acrylic resin, or more likely a poor adhesion between the resin and the finish, or perhaps both. When stoving at 160°C for 10 minutes, wet adhesion is great enough (minimum 100g weight as recommended by Satra). Conclusions Powders can be applied successfully and most satisfactorily with good control onto a leather surface by using conventional electrostatic spraying facilities, provided that the leather is made electrically conductive simply by wetting with water. Powders can also be successfully applied to dry leather using non-electrostatic application methods, after pre-treating the leather surface with a tackifying agent, such as a soft resin, that makes the leather surface tacky and, therefore, receptive to powders. Wetting of the leather surface can also make it receptive to powders applied using non-electrostatic methods, but evenness and coverage is less satisfactory than that obtained from treating with a soft resin. Provided that stoving is carried out at 160°C for 10 minutes (but no longer), leather can be finished with currently available commercial powder coatings without a loss of area yield or softness. The surface coating so formed has an appearance no different to a conventional pigmented finished leather. It also has excellent wet and dry rub fastness, sufficient dry adhesion and adequate wet adhesion properties, even though not sufficiently cured. However, the coating is not satisfactory due to surface cracking and poor flexing endurance. The surface cracks are partly due to the dimensional change of leather resulting from high stoving temperature exceeding the limit of film formed by powder coatings. Therefore, our results strongly suggest that the application of powder coatings to leather finishing is feasible, but further development of low temperature powder coating formulation suitable for leather finishing is needed.