Evaluating waterborne abrasive resistance

An explanation of the science behind the SATRA TM446 abrasive water durability test and its application.

Image © Doug Berry | iStockphoto.com

SATRA has a long history of developing innovative test methods and equipment to help members to develop their products and provide them with a competitive edge. Some of the advanced methods are used to assess finished footwear and are designed to model real-world scenarios. One example is SATRA TM446:2018 – ‘Resistance to waterborne abrasive particulates’ – a method designed to assess the durability and resistance of whole footwear.

SATRA is always looking for ways to expand its testing capabilities and to ensure that members are offered the test conditions that are appropriate for their products and as representative of intended end use as possible. With the rise in popularity of extreme sports and a corresponding demand for tough footwear that can withstand long periods of use in harsh and wet conditions, SATRA had for some time considered incorporating grit and silt in water resistance tests to assess footwear likely to be used in such conditions.

SATRA’s preferred method for whole footwear water resistance is SATRA TM230:2017 – ‘Dynamic footwear water penetration test’. Accepted globally as the ‘go-to’ test, the method actually uses ‘potable’ water (see the box ‘The purity of water used in testing’). In the real world, of course, water in the environment often contains small particulates which can migrate into materials and constructions. These particles can be highly abrasive, and once embedded in the construction they tend to remain there for the life of the footwear, often to the detriment of longevity and durability.

The purity of water used in testing

In some articles in this magazine and SATRA test methods, the term ‘potable water’ is used. Water can be at different levels of purity – the purest and most expensive to produce being ‘distilled’ water. To distil water, it is heated to a vapour and then condensed, using special containers that keep the water pure. Distillation removes all the dissolved ions, particles and organic matter, such as bacteria and viruses. ‘Deionised’ water has the dissolved ions removed, but it can still contain other impurities. It is very important to have pure water for some tests, as the dissolved ions can act as additional reagents – especially if the test is actually looking for certain trace elements. However, it should be noted that distilled and deionised water must be handled carefully, as contact with any material (including CO₂ in the air) will add impurities and dissolved ions, as well as affect the pH levels.

SATRA has commercial laboratories and takes the purity of its water very seriously. In SATRA’s facilities, extremely pure water is available ‘on demand’. Of course, for many laboratories the availability of distilled or deionised water is limited due to cost implications. However, there are test methods that do not need this level of purity. For example, water resistance tests involve the immersion of whole footwear or upper material specimens into water, which instantly renders the water impure. This is especially true of methods such as SATRA TM446, where additional impurities (abrasive particulates) are added. In addition, SATRA TM230 is a whole footwear water resistance test which typically uses ten litres of water, and there is no scientific justifiable reason that this should be distilled or deionised.

SATRA regularly reviews test methods and they are updated if necessary, with major revisions – based on sound research – occasionally taking place. During this process, the purity of the water required is also considered, and the term ‘potable’ will often be used. Potable is simply the scientific term to describe water that is sufficiently pure to be drinkable.

Many types of footwear will ultimately get wet and, in some cases, water will even get into the foot space. Activities such as wading through rivers and trail running often mean that the footwear can become fully submerged. Even if immersion is avoided, the fine particles can find their way into upper textiles and seams, thus leading to abrasion of the materials and even failure of the seams if the thread is not resistant. These types of footwear are often exposed to long periods of immersion in muddy water, and subject to the ingress of sand and grit particles, as well as water.

This attack by sand and grit can introduce abrasive elements into the footwear, thus greatly accelerating wear and tear. SATRA’s research was intended to develop a test that recreated the effect of immersed flexing in muddy or silty water. This was not so much to establish a footwear product’s resistance to water ingress, but more to establish its durability under conditions that could greatly accelerate wear due to abrasion. Non-lace closure systems can also be affected by undue wear and the clogging of mechanical parts.

ChristopherBernard | iStockphoto.com

The SATRA TM446 test is as applicable to children’s beach sandals as it is to military footwear

The SATRA TM446 test method is designed to reveal weaknesses in both materials and constructions to this type of environment. While the test is often seen as only suitable for high-performance extreme footwear, this is not always the case. Footwear to be used on a beach is not only subjected to salt water (brine) and frequent wetting and drying cycles, but is also exposed to sand – especially if it is used in the surf zone, where there is an almost constant suspension of sand in the water. As the brine dries, it will leave behind salt crystals which are also sharp and abrasive. The method is as applicable to children’s beach sandals as it is to military footwear.

Safety footwear is another example of a situation where sediments can make their way into footwear. The slurry from animals is very fine, and often can be found embedded deep within footwear. This can actually become a danger if, for example, the slurry builds up and compacts on the underside of the toe cap, reducing the toe volume and hence the safety margin.

The test

SATRA TM446 is conducted on a specially modified SATRA STM 505 machine. In the water baths are specially-designed baffles that provide a constant agitation of the water to ensure that the silt used is kept in constant circulation. The footwear being tested can be fully submerged or the test can be done at a specific depth, depending on whether or not the footwear is designed to be fully submerged in use. Jungle footwear, sandals and footwear for wading are good examples, where they will have some mechanism to drain excess water and will become wet on the inside.

The silt used is actually calibrated and very consistent in nature, which ensures that the method is highly repeatable. Thus, if a problem is detected, the footwear can be re-engineered and exactly the same protocols followed to ensure the weakness has been addressed – perhaps as simple as the choice of stitching thread or the redesign of seams to ensure no ingress of silt.

Un-tested and tested footwear, showing the discoloration that has taken place, as well as the general fraying of textiles (the footwear that was tested had its insock removed before testing)

The footwear is weighed before the test commences and then again after it is removed. This gives total mass gain in water and particulates. The footwear is given a simple wash as would be the case in actual use and then allowed to dry. Once dry, the footwear can be weighed again to measure the mass of the silt within the construction. SATRA research has shown that once the silt makes its way into the construction, it remains there and is almost impossible to remove. These particulates can continue to abrade components, such as threads and membranes.

SATRA also has an X-ray machine which is used in the evaluation of footwear. With some shoes and boots, there are no apparent signs that the silt has ingressed, other than from the mass increase. Of course, the footwear can be deconstructed to discover its location. However, before this is done, it can be X-rayed to clearly pinpoint where the silt is actually located – for instance, around a particular seam.

An X-ray of a boot containing abradant from the SATRA TM446 test

The SATRA TM446 method is proving to be of particular value to companies producing products that are expected to stand up to attack by both water and the abrasive effects of waterborne particulates.

A full SATRA test report covers such aspects as whole shoe aesthetics, the continued functionality of any closure system and the durability of stitching. Optional additional tests can be included such as sole adhesion and perhaps the ease of cleaning after use.

It should be reiterated that this test does not assess the footwear’s leak resistance, as the water is not static but is ‘frothing’ and ‘bubbling’ – often at a pre-determined level above the featherline or vamp. The water will inevitably work its way up and into the footwear unless it is very high-legged, which would not ordinarily happen during standard immersed flexing tests.

Being able to assess an item of footwear’s ability to withstand abrasion, and other associated damage caused by immersed flexing and the action of the grit circulating within the turbid water is a most useful inclusion within SATRA’s catalogue of whole shoe tests.

SATRA test methods now available online

SATRA test methods can now be purchased online. Please visit www.satra.com/test_methods to download the current catalogue and to order these documents.

How can we help?

Please email footwear@satra.com for assistance with the TM446 test.

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