Avoiding water ingress
How the SATRA STM 184H test equipment can be used to assess a footwear design’s resistance to the entry of water.
by Peter Allen
Image © www.georgeclerk.com
Water ingress into footwear can result in a number of performance failures. The most obvious of these is water penetrating to the wearer’s foot, although other effects can also be encountered. Water penetrating into insulation layers can lead to a significant reduction in thermal efficiency as water, which is highly conductive of heat, replaces the efficient insulation previously provided by trapped air in the dry footwear. Water ingress can lead to rotting of internal components, a reduction in material strength (for example, contributing to sole bond failures), bacterial growth and associated bad odours, and significant weight increase due to retained water.
The degree of water resistance
which footwear needs to provide will depend significantly on the product’s intended use. Contrast, for instance, the water resistance requirements for a wellington boot and a woman’s court shoe. Some footwear requires defined levels of water resistance, such as a hiking or military boot which needs to maintain water resistance in a significant puddle depth, or a golf shoe which may be worn for significant periods of time on wet grass. In these cases, a number of important issues need to be considered in the selection of materials, which should be both water-resistant and non-wicking.
However, a correct specification and use of materials is only part of the requirement. Two other key considerations are necessary to achieve water resistance: firstly, how to combine these materials within footwear through attention to detail in the design (for example, providing water-resistant
seams and ensuring there are no wicking paths to the shoe lining from the outer construction) and, secondly, that manufacturing methods and quality of production maintain the integrity of the footwear overall.
The inclusion of water-resistant membranes, which is one technique used to provide water resistance, does not, in itself, remove the requirement for care in the selection of materials, detail design or manufacturing controls. For instance, the use of a bootie-type membrane construction within poorly designed or poorly manufactured footwear can result in water becoming trapped between the upper and the membrane. While this does not result in water penetrating to the wearer’s foot, it can still lead to the performance failures listed at the start of this article.
Using the SATRA whole shoe flex tester
SATRA has developed a number of test methods and test machines which allow an assessment to be made of the effects of water on the materials used in footwear construction and, hence, verify the water resistance of the selected materials. In addition, SATRA has developed test methods and a family of test machines which can be used to evaluate the water resistance of whole footwear, such as the SATRA STM 184H high-level whole shoe flex tester.
If footwear is required to offer resistance to water ingress, this not only needs to be achieved when the product is new, but should also be maintained during the anticipated life of the footwear in use. The STM 184H whole shoe flex tester allows a number of tests to be conducted within a range of water immersion levels, up to 120mm in depth, and with the footwear subjected to repeated flexing cycles representative of the flexing produced during walking. This versatile test machine can be used to conduct two water resistance tests: SATRA TM77 (flexing machine water penetration test) and SATRA TM375 (whole boot flex and water resistance tests for wellington boots), in addition to a dry flex test to SATRA TM92 (resistance of footwear to flexing). This particular test machine, with its deep-water tank, is suitable to test wellingtons or other longer leg boots.
As an example, to conduct a test to SATRA TM375 (whole boot flex and water resistance tests for wellington boots) using the STM 184H machine, the footwear is clamped by a bar across the toe via a spacer (figure 1) and an internal clamp plate against a datum surface. Then the heel is attached to a flexing frame. The test method gives instructions regarding how to determine the flexing angle to which the footwear is flexed. It also includes a process to identify the flexing line for footwear of different sizes and how to position the internal clamp plate. The test machine comes with a range of clamp plates and toe spacers to suit a range of footwear sizes, along with an adjustable setting gauge to aid correct positioning of the clamp plate to ensure the flexing line is aligned to the pivot axis of the flexing frame.
Steel ball bearings are used to fill out the volume of the boot under test, filling the foot and lower half of the leg. The upper section of the boot is secured to the upper arm of the flexing mechanism and the top of the boot is sealed against water ingress from water splashes. Water is introduced into the tank up to a maximum depth of 120mm. The test machine’s polycarbonate cover is placed over the tank and the test is started, during which the machine flexes the footwear at a rate of 140 cycles per minute. After a set number of cycles, the interior of the boot can be inspected for water ingress or damage to the upper or the foxing as a result of the flexing.
Tests conducted to SATRA TM77 (flexing machine – water penetration test) are used to assess a wider range of footwear types. The water level is set to 5mm above the featherline, up to the top of the sole, or with a wet surface to the bottom of the sole – depending on the type of footwear under evaluation.
SATRA produces other test machines which can be used for water resistance testing, such as the STM 184 (a variant used for testing to SATRA TM92 for dry flex testing, but which can also be used for SATRA TM77, which is a wet flex test). Additional relevant pieces of SATRA equipment are the STM 505 (dynamic footwear water resistance tester), which flexes the footwear with an internal pneumatic action, used to conduct tests to SATRA TM230. The new STM 640 ‘SATRAFuge’, allows tests to be carried out in line with the SATRA TM444 method. Full details of the SATRAFuge test machine can be found in the article ‘Footwear in a spin’.
How can we help?
Please email email@example.com for advice on the selection of the appropriate machine for your application, or contact firstname.lastname@example.org for assistance with producing water-resistant footwear.
This article was originally published on page 8 of the May 2019 issue of SATRA Bulletin.