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Water Vapour Transfer through Pigment Finished Leather testing method

2019-05-17 15:54
Physical and mechanical properties of leather can be varied depending on the applications (footwear, gloves,clothing, purses, furniture upholstery, saddles, etc.). It is required that leather to be very soft, thin and extensible for gloves and clothing, more rigid for footwear, hard and stiff for soles, etc.
After to be dyed leather is subjected to finishing to improve wear properties of leather in general and to protect it from wetting and soiling, to level out patches and grain faults, to apply an artificial grain layer to splits or corrected grain leathers, to modify the surface properties (shade,lustre, handle, etc.) [1, 2]. For this purpose various finishing materials (casein, nitrocellulose, polyurethane,acrylic, other resin and polymer compositions) and techniques (glaze, plate, embossed, spray and curtain coating finish) can be used.
In the case of pigment coating tanned leather is first coloured with a penetrating dye. The dye penetrates the surface of the leather giving it colour but not covering over natural markings. Then leather can be finished in one or more coating operations, with clear or pigmented finishes that do not penetrate the surface [3]. Pigment coat imparts the desired appearance of the leather and levels out the surface. Generally, the more finish leather has, the stiffer it becomes. Usually coating thickness does not exceed 0.15 mm.
It is been recognized that the moisture transport under a humidity transient is a key property influencing the dynamic comfort of the apparel: boots, clothes, and gloves. In the footwear water vapour transfer through leather, that is, perspiration, directly affects the thermal comfort of the foot [4, 5]. In great deal water vapour transfer is a result of the special properties of leather surface coating. The polymer coating must act as a barrier to liquid water entry from the environmental, but to be sufficient permeable to water vapour to allow significant amount of sweat to evaporate through leather.
The goal of this investigation is to study influence of leather finish coating characteristics on moisture transport properties in the exothermic conditions in order to predict comfortability of leather goods. 
Testing Method:
Nappa leather – soft and thin leather, which can be applied for manufacturing of footwear, gloves and clothes– was used for the investigation of water vapour transmission. This leather surface was coated with opaque pigmented acrylic emulsion by spraying with compressed air.
To determine water vapour transfer versus coating thickness the pigment coat layers number was varied. In this way several types of coated leather were obtained.
Nappa 0 is aniline leather – full grain leather that has been soaked in aniline dye, but does not have any subsequent pigmented finish coats applied. This dye penetrates the hide with colour allowing the natural grain to show through (Fig. 1, a). Aniline leather is the most natural looking leather with unique surface characteristics.
Nappa 1 is semi-aniline leather, also attributed to the full grain leather, but more durable than aniline whilst still retaining a natural appearance and character imperfections(Fig. 1, b). The increased durability is provided by the application of the light surface coating, which contains a small amount of pigment. 
Pigmented leather is obtained by a polymer surface coating that contains pigments. This surface imparts greater wear resistance, water resistance and protection against the staining. Depending on the hide quality and requirements, it may be obtained full grain pigmented leather (nappa 2) and corrected grain pigmented leather(nappa 3) with the solid top layer. Full grain pigmented leather is leather whose natural grain has not been removed before applying a surface coating containing pigment(Fig. 1, c). Fig. 1, d presents corrected grain pigmented leather, which is obtained by sanding off a thin layer of the grain surface before the pigment coating is applied.
Because this process removes surface blemishes, a lower grade of hide can be used. Because it also removes the natural grain pattern, an artificial grain is embossed onto the surface coating, although full grain pigmented leather is often embossed too. 
Finally, after pigment coating in all cases the topcoat with the acrylic lacquer layer was used. The topcoat determines the final appearance, the handle of the leather surface, and has decisive influence on the fastness properties of the finish.
Before the test all specimens were conditioned for at least 48 h at a standard atmosphere 20/65 (temperature T = 20 ±2 °C, humidity RH = 65 ±5 %) in accordance with LST EN 12222:1997. The water vapour permeability was measured according to EN 13515:2001 at constant temperature and relative humidity. A sample of leather was placed over a jar, which contained a solid desiccant –calcium chloride (Fig. 2). The whole set-up was placed upright in a conditioned chamber at 20 °C and relative humidity near to RH = 100 %. 
In order to determine the influence of leather coating on kinetics of water vapour sorption, test was continued up to 176 h until water droplets and humidity spots appeared on the leather surface. The changes in leather surface topography due to the water vapour sorption were examined by a standard image processing system,converting surface unevenness into soft lines. 


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