Assessing water vapour permeability and absorption

Highlighting three pieces of SATRA equipment that have been designed to test these key footwear characteristics.

Understanding an upper construction’s water vapour permeability and absorption is often vital when selecting a material for this application. SATRA offers a number of items of test equipment that are used for specific evaluations to enable these characteristics to be identified.

SATRA STM 473

The SATRA STM 473 water vapour permeability apparatus determines the amount of water vapour a material will transmit through its structure in a specified time. The test is mainly applicable to leathers and textiles used in footwear uppers and clothing, where it gives a measure of the ability to remove perspiration from the surface of the wearer’s skin.

The SATRA STM 473 water vapour permeability apparatus

This equipment incorporates six separate test stations on a rotary table, each having a specimen holder. Silica gel desiccant is placed in the holder, after which the test material is placed over the neck of the holder and this is secured by a screw top. After weighing each holder, complete with silica gel and specimen material, the holders are placed and locked into the rotary mechanism.

Placing silica gel desiccant into an STM 473 specimen holder

After seven to eight hours, the complete holder is weighed again and the gain is used to determine the degree of permeability. It is essential to ensure that the entire operation is carried out in a conditioned atmosphere.

The SATRA STM 473 unit features a built-in timer that is used to record the number of hours during which testing has been conducted.

SATRA STD 478

The STD 478 water vapour absorption test apparatus measures a material’s affinity to absorb water vapour.

A test specimen is clamped between the open end of a test pot (containing a specified volume of water) and an impermeable seal. The pot is then stored at a specified temperature for a set time, after which the change in mass of the specimen is determined.

SATRA STD 478 is used to measure a material’s affinity to absorb water vapour

The STD 478 test apparatus features six test pots, each having an internal diameter of 35 mm and a volume of 100 cm³, in addition to a removable lid (which can be securely clamped over the open end of the pot) and an impermeable seal held over the end of the pot by the lid. In addition to the STD 478 apparatus, a laboratory balance and a stopwatch are necessary in order to determine water vapour absorption.

SATRA STM 175

The SATRA STM 175 permeability and absorption test machine

The SATRA STM 175 permeability and absorption test machine is used to perform a preferred SATRA test (SATRA TM47:2019 – ‘Water vapour permeability and absorption’) which, unlike many other methods, attempts to simulate the environment within a shoe – 100 per cent relative humidity at foot temperature.

As well as measuring the amount of moisture transmitted through the material, the STM 175 machine can be used to determine the quantity of moisture absorbed by the upper

As well as measuring the amount of moisture transmitted through the material, the quantity of moisture absorbed by the upper can also be determined. This is important because, although some materials may be impermeable, the ability to absorb moisture means that the foot will feel dry to the wearer. The moisture can desorb from the material by evaporation once the shoe is off the foot – for example, overnight.

An assembly of discs of outer material, the lining (if used in the footwear) and a standard hose (sock) fabric is clamped and sealed across the top of a plastic pot containing distilled water. The water inside the pot is maintained at a temperature of 32°C by resting the pot in a temperature-controlled water bath. The atmosphere outside the pot is maintained by carrying out the test in a conditioned room and maintaining a flow of air across the top of the pot at a speed equivalent to a brisk walking pace. If the outer is permeable, water vapour passes through the cotton hose and the test material in the same way that perspiration would pass through a shoe.

The permeability of the material is determined by measuring the weight loss of the assembly at intervals throughout a six-hour period. The absorption of the upper material, lining and standard cotton hose is measured by weighing each disc at the start, during and after the test.

How can we help?

These items of test equipment form only a small part of the comprehensive range available from SATRA. For further information, please visit www.satra.com or email test.equipment@satra.com

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