WO1990001155A1

WO1990001155A1 - A test method for determination of moisture vapor transmission rate of absorbent materials

Info

Publication number: WO1990001155A1
Application number: PCT/US1989/003199
Authority: WO
Inventors: Philip Mark Crosby; Kathleen Ruth Driskill
Original assignee: W.L. Gore & Associates, Inc.
Priority date: 1988-07-27
Filing date: 1989-07-25
Publication date: 1990-02-08
Also published as: AU4036689A

Abstract

A test method is provided that comprises the testing for moisture vapor transmission of certain materials, such as leather, that absorb water in sufficient quantities to give erroneous moisture vapor transmission values by conventional methods. The test method incorporates an equilibration step for the sample in order to obtain steady state transmission data and not simultaneously absorption/transmission data. The test method finds utility in functional applications such as in the screening and ranking of leathers destined for end-use products such as footwear.

Description

TITLE

A TEST METHOD FOR DETERMINATION OF MOISTURE VAPOR TRANSMISSION RATE OF ABSORBENT MATERIALS

FIELD OF THE INVENTION

•5 The test method of this invention provides a way to easily screen water absorbent materials, such as leather, of varied abεorbency and thickness for moisture vapor transmission rate (MVTR) . The test method of this invention allows for accurate and reproducible MVTR values for materials, such as leather, that

'•0 absorb water in sufficient quantities to give erroneous moisture vapor transmission values by conventional methods. By utilizing the test method of this invention, such absorbent materials can be ranked by their moisture vapor transmission values.

BACKGROUND

15 It is useful to have a test method for the determination of moisture vapor transmission rate (MVTR) of such absorbent materials. Hereafter, these materials will be discussed by reference to leather, but it is understood that the test of the invention is not limited to leather. A major component of many

20 footwear articles is leather. Wear trials with footwear have shown a positive correlation of perceived comfort by the wearer with moisture vapor permeability of shoe upper material. The foot generates large amounts of sweat during the day, particularly during an activity such as walking or running. It is estimated that between about 4-17 g of sweat per foot per hour are generated for the average male. A laced shoe or boot allows only a small percentage of the generated sweat to escape by the pumping action of walking. Absorbent materials placed strategically near to the sweating foot will absorb water from the foot for a time, but these materials will gradually become overloaded in a non-permeable shoe. The wearer will then experience a sensation of hotness or wetness as the sweat vapor condenses and liquid water is felt by the skin of the foot. Therefore, it is desirable to have an accurate test method for the measurement of MVTR of leather so that appropriate leathers can be selected for use - particularly in footwear uppers. Leather is a variable material. Moisture vapor transmission through leather may depend upon the animal from which the leather was taken. Pigskin, for instance, generally is more moisture vapor permeable than cowhide of similar thickness. The tanning process also greatly affects the permeability of leather. A suede leather generally is more permeable than a similar leather with an embossed grain finish. Some patent leathers are non- permeable. After-finishes or sprays can render a leather basically impermeable, as can waxes or oils added at the tannery to render watertightness properties to leather. A test method is necessary that allows for, and accurately tests, all different types of leather. A number of test methods are known for the determination of moisture vapor transmission through materials. In one type test, upright water cup methods, a sample is cut to fit snugly in the mouth of a test cup that is partially filled with water. Air of known relative humidity is moved across the cup, drawing moisture vapor through the sample. The test is isothermal. The

AΞTM Method E96-80 and the SATRA Permeability/Absorption Test are examples of this type test.

Limitations of the upright cup test methods are listed below. 1) It is critical that the temperature and the relative humidity of the room, as well as the wind velocity be closely controlled, making the test expensive to operate. 2) The head space between the sample and the water inside the cup must be standardized, typically at 3/4-1 inch. Too small a head space makes cup handling difficult as any water accidentally splashed onto the sample may alter the MVTR value. However, as the head space increases, MVTR values decline, necessitating a long test time. A 24 hour test time is^' typical. 3) If the sample is an absorbent material, the MVTR obtained can be a non-steady state transmission value, i.e. a sample that is drier in humidity at the start of the test will pick up some water from the cup and hold it, giving a falsely low MVTR value. Likewise, a wetter sample at the start of the test will lose water, giving a falsely high MVTR value. Saryan, Sarkis S., in an article, "A Method of Determination of Water Vapor Absorption and Simultaneous Transmission through Shoe Upper Materials", presented at the annual meeting of ALCA in June, 1968, describes an upright water cup method. He utilizes gauze in the cup to mitigate water splashing onto the leather samples. However, other limitations of the upright cup method as outlined above are still present.

Seligsberger, Ludwig, in an article "A Versatile Method for Measuring the Water Vapor Absorption of Leather and Other Sheet Materials", presented at the Xlth conference of the IULCS on September 9, 1969, describes a upright water cup method which eliminates the head space completely by placing wicking felt into the cup of water, placing a microporous vapor-permeable, waterproof membrane upon the felt and placing the sample upon the membrane. However, the method is not isothermal and calls for a warmer temperature in the cup than in the air, adding an additional limitation to the test. And other upright water cup test limitations yet remain.

Another known method for moisture vapor transmission determination is called the modified desiccant method (MDM) . This is currently used with low-absorbing materials such as fabrics. In this method, a porous, non-hydrophilic, waterproof, vapor-permeable membrane is supported by a floating hoop or fixed bracket large enough to hold the sample and test cup assembly described below. The hoop with the membrane is floated upon a temperature controlled water bath. The sample sits upon the membrane. A desiccant cup is made by partially filling a cup with a solution of saturated potassium acetate salt. The mouth of the cup is sealed with similar membrane used in the hoop. The desiccant salt cup is weighed and placed upon the sample for a time, reweighed, and the MVTR is calculated from the water weight pickup. The MDM method eliminates many of the limitations of the upright cup type test. Specifically, there is no need for tight humidity control in the room as there is no air-sample interface. The problems associated with a head space of air between the sample and the water are eliminated with the use of the vapor- permeable waterproof membrane. However, the method still does not yield steady state transmission values for thick absorbent materials, such as shoe upper leather. A method which yields reproducible, accurate moisture vapor transmission rates for leather and allows various permeability leathers to be ranked accordingly would be desirable. The present invention provides a more satisfactory test method of measuring the moisture vapor transmission rate of leather.

SUMMARY OF THE INVENTION

A discovery has now been made which comprises a test method for the determination of moisture vapor transmission rate (MVTR) of water absorbent materials such as leather. A number of leathers have been tested by the method of this invention. I has been determined that leathers can be ranked by the MVT values obtained by this test. The test gives reproducible results that are not easily influenced by operator error.

Referring to Figure 1, which is an expanded view of the test assembly of Figure 2, the test method of this invention comprises, in sequence: a. equilibrating a sample(1) to steady state transmission of moisture vapor by i) placing the sample onto a waterproof, moisture vapor permeable membrane(2) in a support (3) on a controlled temperature water bath(4) held at a temperature that is substantially the same as the test environment, ii) inverting a test cup(5), which contains a saturated salt solution(6) and is sealed with a waterproof, moisture vapor permeable membrane(7) , onto the sample until steady state transmission values can be obtained, b. testing the sample by removing the salt cup used in step a, and inverting a weighed salt cup, and placing the cup on the sample, c. after a predetermined time, weighing the cup and recording the water pickup over the weight of the cup prior to step b.

To translate the weight pickup into a means for comparin rates between leathers, this can be done by calculating th moisture vapor transmission rate (MVTR) according to th following formulation:

MVTR (g/[m² x 24 hr.]) = (Weight (g) water pickup in cup / [Area (m²) of cup mouth x Time (days) of test]) DETAILED DESCRIPTION OF THE INVENTION

'Steady state transmission' will be achieved for the purposes of the test method of this invention when it can be shown that two successive one hour MVTR tests (by the test method of this inventi n) yield values which are within 10%, and preferably, within 5%. The time taken to achieve steady state transmission varies - dependent upon several factors - for instance: the thickness, density, grain or oil content of the sample, the moisture vapor gradient used in the test method, and the moisture content that an absorbent sample contains at the test start. Obviously, thicker samples will require more time to achieve a steady state transmission than thinner samples. Of prime importance to the time required to reach steady state for an absorbent sample, is the moisture contained in the sample at the start of the test. It is wise to condition very thick and absorbent samples in a relative humidity environment which is not too distant from the relative humidity (RH) conditions of the test. For instance, if the relative humidity differential for the isothermal test is to be from 100% RH to 76% RH, then storing leather samples at 20% RH is not recommended.

Preferred membranes are microporous, moisture vapor permeable membranes of expanded poly(tetrafluoroethylene) , (ePTFE), such as are described in USP 3,953,566. Other types of membranes include those made from breathable polyurethanes. It is sufficient that the two membranes used in the test method of this invention be moisture vapor permeable. However, preferably, the combined moisture vapor permeability of the two membranes should be high enough to yield an MVTR of at least about ten times that of the sample.

A preferred salt is sodium chloride. Saturated salt solutions of certain salts maintain definite relative humidities at constant temperature. These solutions should have a surplus of the salts to ensure saturation at all times. Other useful salts include potassium acetate, potassium carbonate, magnesium chloride and the salts described as suitable for this purpose in ASTM Method E 104-51 (1971).

An experiment was performed on leathers of various animal origin, thickness and moisture vapor transmission rate. The experiment utilized the test method of this invention, with the exception that two one hour MVTR tests were performed in succession after a 17 hour equilibration time. The results of the two MVTR tests were compared for closeness of value. Table 1 shows that for the sheep, pig, and cowhide leathers tested, that the two successive MVTR values varied less than 5%, indicating that steady state transmission was obtained by the test method of this invention. Additionally, the water absorption of the samples during the equilibration step and testing was carefully noted. The samples were conditioned prior to the testing at 50% RH and 23 C. A definite measurable amount of water absorption into the samples was noted. TABLE 1

I. SUCCESSIVE MOISTURE VAPOR TRANSMISSION RATE fMVTR^ TESTS PERFORMED ON LEATHER AFTER A 17 HOUR EQUILIBRATION STEP

II. WATER ABSORPTION OF LEATHER SAMPLES DURING ENTIRE TEST PROCEDURE

LEATHER-THICKNESS MVTR (g/CirT X 24 hr.]) WATER ABSORPTION

-1st -2nd -change -gm -increase Hour Hour Value Value

Cowhide-0.20 cm 1145 1117 ^■2.5= 0.77 ιe^~

Sheepskin-0.13 cm 2795 2747 -1.7^s, 0.17 11%

Pigskin-0.11 cm 2912 2899 -0.4% 0.31 16-i

The test method of the invention is novel and useful because the required equilibration step in the method ensures that only steady state transmission values are yielded. Yet the test is easy to set up and to perform, there is no need to control the relative humidity of the test room, and there is a lower error risk as: possible liquid water contact with the sample is eliminated, careful measurements of the water height level as required in the upright cup method are eliminated, and humidity differences in leather pre-conditioning which might have affected the test results are corrected by the equilibration step required by the method of this invention. MVTR values reflect the permeance of the specimen only, without any associated layers of air (head space) . The importance of the equilibration step as taught by the test method of this invention is shown as follows. A permeable sample of cowhide leather (0.060 inches thick) was tested. Also tested was an impermeable sample that was created by backing the permeable leather with a piece of impermeable plastic. The modified desiccant method (MDM) was used. The water bath and room were held at 23C. Sodium chloride desiccant was used, giving a relative humidity environment of 76% within the salt test cup at 23C. The leather samples were stored for two weeks at 23C and 50% relative humidity prior to testing. Table 2 results show that, without equilibration, falsely low, even negative, MVTR values are obtained for both the permeable and impermeable leathers. This is due to the fact that in the first hour, the leather samples, which are at 50% RH will absorb some water from both the salt cup, which is at 76% RH, and the water bath, which is at 100% RH.

The same leathers were re-conditioned to 50%, and rerun by the test method of this invention. Table 2 results show that the MVTR value for the permeable leather as obtained by the test method of this invention was 84% higher than the MVTR obtained by testing without the equilibration step, indicating that the equilibration step is important, as MVTR does increase with test time for an absorbent material that is dryer at the start of a test than at test condition. The MVTR value obtained by the test method of this invention for the plastic-backed leather was 0 g/[m² x 24 hr.], indicating that a true MVTR value was gained only after steady state transmission had been reached with the equilibration step.

Table 2

EFFECT OF EQUILIBRATION STEP ON MVTR VALUES OBTAINED FOR PERMEABLE AND IMPERMEABLE LEATHER SAMPLES

Permeable Leather Sample MVTR (g/[πr x 24 hr. ])

Without Equilibration 1060

After Equilibration 1950

Impermeable Leather Sample MVTR (g/[ιrr x 24 hr.])

Without Equilibration -900

After Equilibration

It is believed that even if the leather samples discussed in the experiment above had been conditioned to 88% RH (midpoint of 100% RH and 76% RH) prior to the MVTR test, that some further degree of absorbance would still have taken place upon the test apparatus until a steady state transmission at test condition is reached. Leather is not a uniform material—having a skin side and a flesh side. We believe that equilibration of the sample in an active test environment that ranges from 100% RH to 76% RH is better than, and much different than, passive pre-conditioning of the sample at 88% RH.

Example 1

Example 1 demonstrates that various leathers can be tested and ranked for moisture vapor transmission rate (MVTR) by the test method of this invention. Eight leathers of various thicknesses, animal origins, and finishes were tested for MVTR. The leathers were die-cut to circles of 7.25 cm. diameter. A constant temperature water bath (23C) was set up in a temperature controlled room (23C) . 4.5 fluid ounce polypropylene cups were charged with 35 gm. sodium chloride and 15 gm. distilled water, making a saturated salt solution. An expanded^' poly (tetra luoroethylene) membrane, available from W. L. Gore and Associates, Incorporated, was heat- sealed to the 6.5 cm. diameter mouth of the cups. A similar expanded pol (tetrafluoroethylene) membrane was mounted taut in a 5 inch embroidery hoop and floated upon the water bath. The sample for MVTR testing was placed upon the membrane in the hoop, and a test salt cup was inverted and placed upon the sample. After 17 hours, the salt cup was removed, weighed and replaced upon the sample. After one hour, the cup was removed and weighed. The MVTR was calculated from the following formulation:

MVTR (g/[m² x 24 hr.]) = (Weight (g) water pickup in cup / [Area (m²) of cup mouth x Time (days) of test])

Table 3 lists the description, thickness and MVTR of the leather samples tested by the test method of this invention.

TABLE 3

LEATHER DESCRIPTION THICKNESS MVTR

(cm.- 23C) (g/[m x 24 hr.])

Sheepskin-glove-untreated 0.084 2930

Pigskin-shoe liner 0.122 2230

Cowhide-glove-untreated 0.079 2000

Cowhide-shoe upper 0.145 1900

Cowhide-glove-"sprayed" 0.076 980

Cowhide-Boot upper 0.249 950

Cowhide-Boot upper-oiled 0.262 420

Cowhide-upper-'-waterproofed" 0.284 126

Cowhide-upper-glued 0.277 When a test was run with the test apparatus, including the two expanded pol (tetrafluoroethylene) membranes, but not including a test sample, the calculated MVTR was about 35,000 g/[m² x 24 hr.]. Thus, the combined moisture vapor permeability of the two expanded poly(tetrafluoroethylene) membranes had an MVTR of at least ten times that of the most permeable sample tested in this example.

Claims

What is claimed is:

1. A method for determining the degree of moisture vapor transmission of certain materials that absorb water in amounts that affect moisture vapor transmission values, which comprises,

5 in sequence: a. equilibrating a material sample by

(i) placing the sample onto a waterproof, moisture vapor permeable membrane in a support on a controlled temperature water bath held at a temperature that is substantially the same - as the test environment,

(ii) inverting a test cup which contains a saturated salt solution and is sealed with a waterproof, moisture vapo permeable membrane, onto the sample until steady stat transmission values can be obtained, 5 b. testing the sample by removing the salt cup used in step a, and inverting a weighed salt cup, and^' placing the cup on th sample, c. after a predetermined time, weighing the cup an recording the water pickup over the weight of the cup prior t 0 step b.

2. The method of claim 1 where the material is leather.