Footwear in a spin

Describing SATRA’s new centrifuge test machine used to conduct the SATRA TM444 test.

by Peter Allen

The effects of water from the external environment and moisture from internally generated perspiration can have a significant impact on the performance of footwear, its durability and the wearer’s comfort. SATRA has developed a number of test methods and associated test machines to allow these effects to be assessed. This article describes one of these tests – SATRA TM444:2012 – ‘Water resistance of footwear – centrifuge test method’. SATRA TM444 permits the assessment of water ingress from the external environment, and can be conducted on SATRA’s new four-station STM 640 ‘SATRAfuge’ test machine.

Care is required when designing and manufacturing footwear which is intended to prevent water ingress from the external environment. This includes the overall design, the selection of materials, the effectiveness of any membranes if used, and the construction methods. Water can find its way into the foot space in a number of ways – for example, via i) the use of materials which are permeable to water, ii) poor design or manufacture of seams, iii) wicking effects, iv) the poor application of membranes, and v) overall design errors – such as bellows which channel water into the footwear rather than keeping it out.

Sometimes water does not reach the footspace itself, perhaps penetrating the upper but being retained between the upper and a membrane lining or bootie. However, water ingress to any extent can be problematic. The weight of footwear can increase significantly to the detriment of wearer perception in a number of aspects. In addition, water is a much better conductor of heat than air. Therefore, water ingress can significantly reduce the thermal insulation of the footwear, resulting in discomfort in cold conditions. Water can remained trapped within the construction, taking a long time to dry out and causing damage to the materials or construction (particularly the critical sole bond), and leading to the build-up of moulds and unpleasant odours. Hence, even if footwear does not allow water to leak through to the footspace, trapped water in the footwear construction can affect footwear performance, durability and wearer comfort.

Having selected appropriate materials (that exhibit resistance to water ingress and wicking), and taking care of the concept and design details, it is also important to ensure that the water resistance is not undermined by poor quality footwear manufacture. The SATRA TM444 test method, conducted using the SATRA STM 640 test machine, allows for a simple and rapid assessment of footwear against the ingress of water. As the test causes no damage, marking or degradation of the test shoes, it is ideal for use as a non-destructive test of completed footwear samples from a production line. The assessment, following a test, investigates leaks into the footspace, but also looks for any mass gain in the sample. This is a good indicator of water ingress, even if it has not penetrated through to the footspace. The test takes just over ten minutes to perform, and allows two pairs of shoes to be tested at the same time. It does not replace other tests (such as the more demanding SATRA TM230:2017 – ‘Dynamic footwear water penetration test’, which must always be used to test the longer-term resistance of prototype and sample footwear to water penetration during wear), but sits alongside them as a much quicker assessment of water resistance for production footwear.

When conducting the SATRA TM444 test, the footwear sample is first weighed. The test footwear is clamped at the bottom of a container and water is carefully introduced into the container on the outside of the footwear. The water level selected depends on the footwear type and its application, and the containers (in pairs) are inserted into the STM 640 centrifuge test machine. When the test is started, the machine quickly reaches the test speed, rotating the sample containers about a vertical axis and allowing them to swing out to a horizontal position. This effectively increases the pressure of the water against the outside of the sample. The test has a duration of ten minutes, after which the outside of the footwear is dried by carefully blotting with tissue and the sample is re-weighed. Any increase in weight is an indication of water ingress into the footwear. If the sample has not leaked or increased in mass, it can remain in the manufacturing loop and confidence will have been gained in the water resistance of the footwear. However, if a leak or mass gain has been observed, an investigation can be made into the root cause of the failure before significant further production has been completed.

SATRA’s four-station STM 640 centrifuge test machine has been developed with the experience of developing and using a two-station prototype in SATRA’s own laboratory. This has allowed for the refinement of the design, along with the use of safety tests to ensure that the machine is safe to operate. One of the features of the design is its lightweight and easy to install testing containers, in which the footwear samples are held. This feature – along with a good access door – improves the loading and unloading of the samples, which is an important consideration when four containers may need to be removed and reinstalled every ten minutes.

When used in a development or testing laboratory rather than for production quality testing, the SATRA TM444 test is good for screening the water resistance of footwear prior to the more demanding and lengthy SATRA TM230 test. SATRA TM230 includes flexing of the footwear and a longer immersion time, during which wicking can occur. Any leaks or increases in mass after a SATRA TM444 test is usually indicative of poor quality manufacture or a fundamental design fault. Therefore the time required for the SATRA TM230 test can be saved. If mass gain is observed in footwear tested to SATRA TM444, the product can also be tested on SATRA’s STM 567 Endofoot, which is a whole-shoe moisture and thermal management test machine used to determine the effect of the trapped moisture on the comfort properties of the footwear.

In summary

As mentioned, SATRA TM444 is not an alternative to other SATRA test methods used for water resistance of footwear, such as SATRA TM230, SATRA TM77:2017 – ‘Flexing machine – water penetration test’ and SATRA TM375:2010 – ‘Whole boot flex and water resistance tests for wellington boots’. However, it is complementary to these methods as it allows a rapid assessment of the quality of footwear in production.

The SATRA TM444 test should not be confused with other centrifuge tests, where water is introduced to the inside of footwear or the inside of a membrane bootie. Such tests are used to determine the integrity of the lining or bootie construction, and the use of water inside footwear requires lengthy and potentially harmful drying procedures before the test samples can be returned to production. The SATRA TM444 test allows a simple, rapid and non-destructive assessment of the water resistance of whole footwear during production, thereby enabling rapid quality control of this critical performance property with minimal disruption to production schedules.

SATRA test equipment is designed and manufactured in the UK and supplied to a global market. The range of over 300 test machines and devices draws on the experience gained over its 99 years’ history, from fundamental research, development of over 400 test methods and the extensive use of SATRA machines in our own commercial laboratories.

Publishing Data

This article was originally published on page 16 of the October 2018 issue of SATRA Bulletin.

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