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Test Method for Improving Water Vapor Permeability of Leathe
Leather is an unique material that has the ability to absorb water vapor, transmit through its cross section and permeate into the atmosphere.This unique characteristic of leather offers comfort to the wearer of shoes in hot and humid conditions. In addition upper leathers should possess comfort properties, strength properties,functional properties and aesthetic properties.
Under normal conditions, 5 gms/hour sweat is produced by a human when the temperature condition is between 30 and 35°C. Under industrial working condition, the sweat produced by a human foot is around 10 gms/hour. Leather footwear has the ability to absorb the sweat produced and transmit to the upper part of the leather through wicking process which is known as water vapour permeability or water vapour transmission [1]. Water vapour permeability is the ability of leathers allowing water vapour to transfer through from higher humid condition to lower humid condition [2]. This characteristic mainly depends on the porosity characteristic of leather. The ability to transmit water vapour is one of the important properties of leather that makes it most desirable for shoe making.
There are plenty of capillaries among collagen fibres in leathers as well as a lot of hydrophilic groups on the collagen chains. They may endow leathers with good water vapour permeability, compared with other synthetic materials [3]. Water vapour permeability is the volume of permeating water vapour in unit time and unit area, its unit is mg/10cm2h.
The property depends upon a number of factors including porosity, thickness of leather,grease content and the relative humidity and temperature of the atmosphere [4]. It is greatly reduced by the presence of the natural glyceride greases. The solution is to remove natural fat during degreasing process with chemicals which do not affect the water absorbency. The finish on the grain layer also has an influence on water vapour transmission. Although leathers have a certain degree of water vapour permeability,improved water vapour permeability is required for special purposes such as application in sports shoes. Hence a study is undertaken to improve water vapour permeability without affecting those strength properties required for upper leather.
MATERIALS AND METHODS Materials：
Fresh wet salted goat skins (24”-28”) without any hair slip were chosen for experiments. Commercial grade protease and lipase (with activity in the range of 80 – 100TU), urea and acetic acid were chosen for experiments.
Processing Methods：
Two pronged approach has been attempted to achieve enhanced water vapour permeability of skins viz., extensive enzymatic treatment during pre-tanning and treatment with lyotropic agents after chrome tanning.
Liming：
The purpose of liming is to remove hair,epidermis, natural fats and greases, interfibrillary proteins, to swell and split up the fibre bundles,to soften the collagen fiber lattice and make the final leather soft and pliable. Conventional sulphide unhairing methods often do not yield clean pelt leaving residual scud which hinders the permeation. But enzymatic unhairing using proteolytic enzymes helps in complete removal of hair from its follicle yielding clean pores so that permeability of water vapor is enhanced.
Enzymes are biocatalyst, hence they are specific in their action and they act specifically at the hair follicle and remove hair in shorter duration.
Bating：
The objective of bating the delimed pelts with proteolytic enzymes is to remove non leather making substance and to make the grain surface clean, smooth and fine. The purpose is to remove the hair roots, break down of the non-structural proteins and to get clean pelt. The efficiency of bating depends on temperature, concentration,pH and duration. The experiment is carried out with varying concentrations of enzyme.The physical tests were performed at crust and finished leather stages.
Degreasing：
Degreasing is the process of removing natural fat present in between the collagen fibers. If this fat is not removed completely,it affects the water vapour transmission in the final leather apart from forming fat spew.
Conventionally huge amounts of solvents and surfactants are used for this challenging process.During the present study, lipase was used which acts specifically on lipids and frees the pores from blockage which enhances the permeation of water vapor.
Treatment of Chrome Tanned Leathers：
When chrome tanned leathers are treated with lyotropic agents like urea (CH4N2O), and acetic acid (CH3COOH) hydrogen bonds are broken and cementing substances held by the fiber bundles are removed. The quantities of the chemicals were optimized after several experiments.
Determination of Water Vapour Permeability：
Leather sample test specimens were taken from the official sampling position (ISO 2418) and were conditioned for 48 hours at 20°C ± 2°C and 65 ± 2% RH (ISO 2419). The leather sample under test is sealed on the mouth of a desiccant(dry silica gel) filled jar that provides a less humid gradient across the test sample and outside. The test jars sealed with dry silica gel held in a vertical wheel which rotates during the test. A large fan is also incorporated within the equipment, and this rotates in the opposite direction to the sample wheel. This creates air turbulence over the samples sealed in the mouth of the jar, thus removing the layer of still air. The sealed jars are weighed before and after test, and the increase in weight of desiccant is used to calculate the WVP of the sample, usually expressed as milligrams of water vapor per square centimeter per hour.The water vapour permeability tests were done for dyed crust leathers and for finished leathers.
Determination of Tensile Strength and Elongation at Break ：
The tensile strength, % elongation, tear strength and grain crack strength were measured as per official procedures (SLTC Methods, 1965).
The test specimens were conditioned for 48 hours at 20°C ± 2°C and 65 ± 2% RH. Strength characteristics of the leathers were tested for tensile strength and tongue tear strength tests in a Universal Instron testing machine (Instron 4501, England). A crosshead speed of 100 ± 20 mm/min was used and the distance between the supports was 40 mm. A load was applied to the center of the samples until fracture occurred and the fracture load was recorded.
The crust leathers were also tested for grain crack and grain burst using a lastometer (SATRA 1992). The test specimen was tightly clamped between the circular rings facing grain side upwards and the machine started by forcing the plunger at the rate of 0.2 ± 0.05 mm/s.The surface of the specimen was continuously observed at the center for initial crack on the grain and the maximum distance and force were recorded.
Capillary Flow Porometry Analysis：
PMI capillary flow porometer was used in this study to analyze the pore size and its distribution. The experimental and conventionally processed leather samples from different locations of crust leathers were cut into 20 mm diameter and the thickness was noted. Calwick with a defined surface tension of 15.9 dynes cm-1 was used as wetting liquid for porometry measurements. In this technique,at first a non-reacting gas was sent through a dry sample. Second, the same sample was wetted with liquid of known surface tension, through which the above mentioned gas was sent. The changes in flow rate were measured as a function of pressure for both dry and wet processes. The samples which had better water vapour permeability are chosen for wet and dry profiles (pressure vs. gas flow rate) which are measured for porosity.
Scanning Electron Microscope：
The fibre structure of the control and experimental leathers have been studied using SEM analysis. Leather samples were coated with gold using an Edwards E 306 Sputter coater and analyzed by a Cambridge stereoscan S 150 scanning electron microscope.
RESULTS AND DISCUSSIONS：
Water vapour permeability of control and experimental crust and finished leathers are shown in Table 1 and Table 2. According to the results, the leathers undergone enzymatic dehairing and bating processes have improved water vapour permeability compared to conventionally processed leathers. The better opening up of fibre structure by proteases may be the reason for more water vapour transmission without affecting strength properties. The best degreasing effect was obtained with the use of lipase alone. The effective saponification of natural greases increased the water vapor permeability compared to conventional surfactant based degreasing. When chrome tanned leathers were treated with unionized acetic acid, it attaches as a molecule to the collagen chains breaking the hydrogen bonds present in the collagen and thus resulting in fibre splitting. The treatment of skin matrix by urea has been proved to affect mainly the non ionic links like peptide group and breaks the hydrogen bond. Urea and acetic treatments improved water vapor permeability considerably. The water vapour permeability of experimental crust and finished leathers are better when compared with conventionally processed leathers. Even though surface finish coatings affect the water permeability, the effect was found to be negligible. The porosity parameters described in Figure 1 indicate the leathers which have undergone integrated treatment with acetic acid and urea have better porosity compared to conventional leathers.
CONCLUSIONS
The objective of improving water vapor permeability through enzymatic and chemical methods has been achieved without affecting other mechanical properties of leathers.
When compared to acetic acid treatment, urea treatment has resulted in increased water vapor permeability. There was not much variation in strength properties between control and experiments with chemical and enzymatic treatments. Differences in water vapour permeability between crust and finished leathers were minimal. The approach provides an opportunity to improve the needed functional properties of leathers.