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Factors Affecting the Water-Vapor Permeability of Leather testing Method

2019-05-17 15:39
The basic principle in all water vapor .permeability tests developed for leather is the use of the specimen as a diaphragm with a region of high relative humidity on one side and one of low relative humidity on the other. The gain in weight per unit area on the low relative humidity side in unit time is expressed as the water vapor permeability. One of the earliest and simplest tests was developed by Wilson and Lines.
They put sulfuric acid in a bottle, over which a leather specimen was placed by fastening it in the cap, which was then screwed down on the top. The bottle and contents were weighed from time to time to determine the gain in weight.
Worle by Mitton [3] and by the Institute of Paper Chemistry [6] has shown that a static air space between the specimen and the desiccant influences the result since the diffusion of water vapor is retarded by air. Thus, for highly pCl'meable leathers the result may be the permeability of the underlying air space rather than that for the specimen. The atmospheres of high and low relative humidities should, therefore, make immediate contact with the opposite faces of the speClmen.
This condition is readily achieved on the outer face by rapid ventilation. On the low relative humidity side, circulation involves complicated apparatus, but has been accomplished by the rapid passing of dry air. The n ecessary condi tions can be more conveniently met in a cell (dish) by placing the desiccant in contact with the leather.
The former has been used successfully by Edwards and by MaeseI' [5], whereas the latter has been used successfully by Carson [7] and by the InstitutE' of Paper Chemistry [6] and is applied in the present ALCA test. 
Description of Method Used:
The method used in this work is a modification of the ALCA test. This procedure was adopted because of its speed, convenience, case of application, and case of duplication of conditions. The water vapor permeability cell devised is shown in figure 1, A. I t consis ts of a circular aluminum cup with a flange. The total diameter is about 4 in. The flange is bent in such a way that a raised rim is formed at the edge of the cup upon which the leath er fits tightly. In preparing the cells, the cup is filled with the desiccan t, and the leath er specimen having a larger diameter than the cup is placed over the rim. A circular copper template (fig. 1, D) having one surface machined to have the same diameter as the cup is then fitted as neal'ly as possible directly over the cup. Molten microcrystalline wax is poured around the groove formed by the template and the flange. This seals the edge of the leather and fastens it to the celL After the wax has ha rdened,the copper template is removed, leaving an area of 25 cm2 of the leather exposed. Cooling the template in a refrigerator prior to usc assists in preventing adhesion of the wax.
In figure 1, C the position of the desiccant with respect to the specimen is shown, and figure 1, E shows the side next to the desiccant of a leather specimen on which tests have been completed.
The completed cell is hung in a cabinet of the type described by Carson and Worthington at the desired temperature and relative humidily.
The cells are weighed at variolls intervals of time in place wi thou t removal from the cabinet to determine the rate at, which the leather transmiLs moisture.
Unless otherwise stated, the tempera ture at which the measurements are made is 1000 F. This temperatlll'e is used because it closely approximates body temp er ature. Less time is also required to reach a steady state at this temperature than at a lower temperature. Conditions of relative humidiLy in the range 40 to 65 percent were found satisfactory. The time required for a measurement by this method is 4 to 5 hI'S compared to 4 or 5 days required in the present ALCA test. 
The cells may be prepared in about 5 percent of the time required by the ALCA test.. The period over which the rate of gain of the cell was most nearly constant was taken as the value for the water vapor permeability. This was usually over the time in terval of 2 to 4 hI'S. The ra tes of gain during this period were averaged and expressed as gain per 100 min.
The success of the desiccant method depends upon the capacity for water vapor of the particular salt used.

The salt used as the desiccant should maintain a .high capacity for water over a great enough time period for completion of a test. This time period must permit the specimen to come to equilibrium and then continue the adsorption of water at a constant rate. The rate of adsorption per unit time may be taken at any point after a constant rate of gain has been reached.

In figure 2 the behavior of anhydrous calcium chloride, anhy?lrous magnesium perchlorate, and Drierite (anhydrous calcium sulfate) when used in the cell is shown. The results were obtained with chrome-tanned calfskin at 44-percent r('lative humidity on the high vapor pressure side. They show tbat calcium chloride is highly superior to Dri('rite and better than magnesium perchlorate with r espect to the maintenance of a high capacity for water vapor over an extended period of time.
The curve foJ' calcium chloride is a straight line,whereas the other two curves tend to level off.All of the following determinations, unless otherwise stated, were made at 100° Ii' with a relative humidity of 53 percent on the high-pressure side and calcium chloride in contact with the specimen on the low-pressure side. 
Some Typica l Results With the Method:
In table 1 3 arc shown water vapor permeability values obtained with the above-described method for a number of different types of leather. The thickness and grease contents of the leathers are also given. It may be observed that the water vapor transmission is dependent upon both grease content and thickness.
The correlation between the ALCA method and the new method used in making the tests in this invesigation is shown in figure 3. In the ALCA method, a cell is prepared by cementing the leather specimen on the top of a tannin dish .
This i a circular dish, about 3 in. in diameter and 1.5 in. deep. Drierite is placed in the dish prior to the sealing of the specimen. The prepared cell is placed on a rack in a room at a constant temperature and relative humidity. The cell is turned specimen side down, so that the desiccan t is in contact with the specimen. Weighings are made from time to time to determine the rate at which water vapor passes through the leather. The results obtained with this method shown in figure 3 were obtained at 50-percent relative humidity and 73 ° F. The correlation is good except that it is curvilinear. The nature of the correlation is undoubtedly caused by the fact that Drierite is' used in the ALCA test. Its capacity for holding water is low, and therefore the rate of pickup will decrease rapidly with time. There will th en be a tendency for the curve showing increasing water vapor permeabilities to become asymptotic to some maximum value that would approach the sa turation point of the Drierite used in the cup. 


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