In-production monitoring of water-resistant footwear
Making water-resistant footwear can be a challenge, however success can differentiate the product in the market to the benefit of the brand owner, retailer or manufacturer.
by Peter Allen
Particular care is required in the manufacture of water-resistant footwear. This includes careful selection and control of the materials used – for example, the upper and sole materials should be waterproof and non-wicking. Attention is also required to the design and production controls during manufacture of all joints (such as the water proofing of seams, and the upper-to-sole bond). If a breathable/waterproof membrane is used, any sub assembly and integration into the footwear needs both to be designed correctly and controlled during manufacture.
Avoidance of basic errors needs to be considered. These may include ensuring that a bellows tongue still performs its function of keeping water out of the footspace when the laces are tied, and ensuring that labels or decorative features are not sewn through to the linings, giving an immediate path for water to track into the footspace. SATRA has developed a range of test methods and associated test machines to allow the assessment of material properties used in the construction of water-resistant footwear. Examples of such tests include the water resistance of leather and the wicking of threads.
Avoiding development mistakes
Having selected appropriate materials, it is also very important to be able to assess the water resistance of the completed footwear at the development stage. This permits fundamental errors in design or construction to be identified. For instance, SATRA TM230: 2017 – ‘Dynamic footwear water penetration test’, can be conducted on the SATRA STM 505 dynamic footwear water resistance tester to assess the overall footwear design and construction against the ingress of water. The depth of water and number of cycles can be set to reflect the type and intended use of the footwear. During the test, the footwear sample is flexed by an internal mechanism at a flexing rate representative of that produced by walking. Longer tests also allow evaluation of the effects of wicking due to immersion.
When moving from development to production, difficulties are sometimes encountered in ensuring that all the footwear produced is water-resistant. This can be caused by non-conforming materials or errors in the production processes. The risk of this occurring can be reduced by good process and material controls. However, a quick, non-destructive test to verify that sample products from the end of the production line have achieved the water resistance required can make a significant contribution to quality control. This can be achieved by testing to SATRA TM444:2012 – ‘Water resistance of footwear – Centrifuge test method’ using the new SATRA STM 640 four-station SATRAfuge test machine. This is a simple ten-minute test which allows two pairs to be assessed at the same time. All conforming samples can be put back into the production batch. However, if any water resistance failures are detected, the causes of failure can be investigated early, thus preventing large quantities of nonconforming products from being produced and even being shipped to the customer.
The SATRA TM444 test
When carrying out a SATRA TM444 test using the STM 640 SATRAfuge, completed footwear samples taken from the production line are first weighed. Each sample is then clamped at the bottom of one of the test machine’s sample containers. Water is introduced into the container on the outside of the footwear and the selected water level is dependent on the footwear type and its intended use. One or two pairs of containers are inserted into the STM 640 machine. When the test is started, the machine quickly achieves the correct speed, allowing the sample containers to rotate about a vertical axis and swing out to a horizontal position. This effectively increases the pressure of the water against the outside of the sample for the duration of the evaluation.
After the test is finished, the outside of the footwear is dried by blotting with tissue paper and the sample is re-weighed. Any increase in weight is an indication of water ingress into the footwear construction. Leaks into the foot space can be detected with tissue. If the sample has not shown a definable increase in weight, and has not leaked, it can remain in the manufacturing batch and confidence will have been gained in the water resistance of the footwear. If, however, a leak or mass gain has been observed, an investigation can be instigated into the root cause of the failure before significant further production has been completed.
This test should not be confused with centrifuge tests used to test the water resistance of membrane bootees where the water is applied to the inside of the footwear bootee. These tests can verify that the bootee is leak-resistant but does not demonstrate that the footwear is water-resistant overall. While a membrane bootee can prevent water reaching the foot space, this does not in itself as described below make a water-resistant boot. Sometimes, water does not reach the foot space itself – for example, penetrating the upper – but is retained between the upper and a membrane lining or bootee. However, water ingress to any extent can be problematic. Footwear weight can increase significantly (we have found increases in weight of footwear of 35 per cent).
Water is a much better conductor of heat than is air, so water ingress can significantly reduce the thermal insulation of the footwear, resulting in discomfort in cold conditions. The water can remain trapped within the construction, take a long time to dry out and cause damage to the materials or construction (particularly the critical sole bond), while leading to the build-up of moulds and unpleasant odours. So even if footwear does not leak through to the footspace, trapped water in the footwear construction can affect the performance of the footwear, its durability and the wearer’s comfort. This is the advantage of the SATRA TM444 test, as it is an assessment of the footwear’s resistance to the ingress of water from the external environment. The test is equally applicable to water-resistant footwear made with or without a waterproof membrane.
The new SATRAfuge machine
The four-station STM 640 SATRAfuge test machine has been developed using the experience gained developing and using a two-station prototype in SATRA’s own laboratory. This has allowed for refinement of the design, along with tests to ensure that the machine is safe to use. One of the features of the design has been a focus on the ergonomics and ease of use, which is an important consideration when four containers may be needed to be installed and removed every 12 minutes.
When used in a development or testing laboratory rather than for production quality testing, we have found the SATRA TM444 test to be a good screening test for water resistance of footwear prior to the more demanding and lengthier SATRA TM230:2017 – ‘Dynamic footwear water penetration test’. If the footwear leaks or increases in mass after a SATRA TM444 test, this can save the time required for the SATRA TM230 test and is usually indicative of poor-quality manufacture or a fundamental design fault.
Other valuable assessments
Another consideration when designing water-resistant footwear is the durability of the water resistance after a period of use. This can readily be assessed by the SATRA TM444 test using either footwear samples from user trials or samples tested using the SATRA STM 528 Pedatron, which simulates a typical walking gait. Footwear tested to SATRA TM444 in which mass gain has been observed can also be subsequently tested on SATRA’s whole-shoe moisture and thermal management test machine (STM 567 Endofoot) to determine the effect of the trapped moisture on the thermal insulation properties of the footwear. The STM 567 machine can also be used to assess the breathability of completed footwear. Being able to verify the water resistance, breathability and durability of water-resistant footwear can offer a distinct market advantage to a brand owner, retailer or manufacturer.
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.
How can we help?
Please contact SATRA’s footwear testing team (firstname.lastname@example.org) for the assessment of water resistance in footwear.
This article was originally published on page 46 of the May 2020 issue of SATRA Bulletin.