Assessing resistance to water penetration

‘Water penetration resistance’ is a key performance requirement used to market many types of footwear.

There are many water resistance tests available for materials and whole finished products across different sectors, such as clothing, camping tents and gloves. With regard to footwear, there are some special considerations which relate to the nature of product use and specific requirements. For example, clothing does not fold and crease in the same manner as footwear does, nor is it necessarily immersed for considerable periods.

SATRA has developed various tests in a context specific to the footwear industry, as well as designing the test machinery on which these evaluations can be conducted. This article provides a brief overview of the SATRA test machines available for water penetration resistance tests.

SATRA STM 606D

The SATRA TM34:2019 – ‘Resistance to water penetration – Maeser test’ can be conducted using the SATRA STM 606D water resistance tester for upper leather (Maeser type), which is shown in figure 1. This machine is designed to ascertain the water resistance of upper leathers (and upper leather substitutes) under conditions of flexing similar to those experienced specifically by the forepart of a shoe during walking.

When using the SATRA STM 606D equipment, up to four rectangular test pieces are folded into vee shapes and held in watertight clamps (figure 2). A tank containing water is then raised so that the specimen is surrounded by water. One clamp is repeatedly moved up and down through a 32-degree angle by a driven crank linkage. This forms creases in the leather similar to those produced in the shoe vamp while the wearer is walking. A resettable cycle counter is fitted, and the number of flexes achieved before water penetration is observed and recorded as the test result.

Even if water penetration has not occurred, following careful sample conditioning, the mass gain of the sample can be measured. This can provide valuable information on how much water has been absorbed during the test.

The system incorporates an individual counter for each station. The detection of any water penetration is automatic, using individual visual and audible alarms to alert laboratory staff of any penetration. Detection level is adjustable, with a three-position switch to select one of three sensitivities (10 kΩ, 30 kΩ and 50 kΩ). The individual station counter stops recording when water penetration is first detected and the alarm is switched on while the remaining stations continue to perform the assessment.

SATRA STM 505

For whole shoe testing, factors in addition to the application of surface pre-treatments and water-resistant materials need to be considered in order to develop highly water-resistant footwear. These include membranes, seam sealing and the avoidance of wicking paths from the outside to the lining. For this reason, whole shoe water resistance evaluations should be conducted. This is done in accordance with SATRA TM230:2017 – ‘Dynamic footwear water penetration test’, for which the SATRA STM 505 dynamic water resistance test machine (figure 3) was developed. SATRA TM230 is also encapsulated in EN ISO 20344:2021 clause 5.19.2 for safety footwear to be used in Europe and also in a number of specifications for tenders – including those for service footwear in many countries around the world.

SATRA STM 505 is a two-station machine which allows two footwear samples to be assessed independently in line with SATRA TM230 or the previously-mentioned European standard. The machine comprises a foot form that is flexed by a pneumatically-activated mechanism positioned inside the footwear sample. The footwear is clamped in place during the test, and the toe is flexed through the relevant angle at a controlled flexing rate.

Each independently controlled station has its own water tank into which the sample and its associated flexing mechanism is lowered at the start of the evaluation. The method sets out the distance from the machine’s heel piece to the centre of the flexing pivot, and this depends on the footwear size. The water level in the tank is adjusted to meet a specific assessment or customer requirement for when the sample is lowered into the flexing position (figure 4).

A built-in counter enables the machine to be left unattended, and when the pre-determined count is reached (depending on the test), the footwear will rise out of the water. The footwear is then removed from the foot forms and is inspected for signs of internal leakage. If no obvious water penetration has occurred, absorbent paper is used to determine if and where water penetration has occurred. Areas of dampness are again ascertained in line with the demands for the particular product.

A modified version of the SATRA TM230 test for more extreme conditions has also been developed. This is SATRA TM446:2018 – ‘Resistance to waterborne abrasive particulates’, which evaluates the resistance of footwear by immersing and flexing it in water containing abrasive particles. The SATRA TM446 test replicates the effect that natural abrasive substances (such as sand, silt and mud) have on footwear that is submerged in water – for example, footwear for kayaking, wading, off-road running and military exercises.

This assessment uses an abrasive aluminium oxide grit, added to the water tank in which the footwear is submerged. This is diffused around the tank by a ‘baffle bed’ which produces a vigorous bubbling action through a supply of compressed air. The grit is thus prevented from settling at the bottom and so imitates running water, such as a running stream or surf waves.

During the course of the test, the aluminium oxide grit, which is hard and fine, penetrates into the footwear sample and may then become trapped between layers that are rubbing together. It may also lodge between the lining of the boot and the foot form, which can cause additional abrasion-related wear and tear. SATRA TM446 provides a realistic evaluation of the footwear’s water resistance properties, and quickly highlights any potential weaknesses in a product’s design or manufacture without having to resort to expensive and potentially unreliable field trials.

Both SATRA TM230 and SATRA TM446 require the mass gain to be determined. For SATRA TM230, this is the amount of water absorbed into the upper and construction. Even if there is no leak into the foot space, high mass gain indicates a potential on thermal properties and drying times. For SATRA TM446 – once reconditioned – the mass gain indicates the amount of ‘silt’ that has made its way into the construction. Often the location of the silt can be determined using SATRAs X-ray machine, the cause determined and the footwear re-engineered to make a better product.

Publishing Data

This article was originally published on page 12 of the May 2024 issue of SATRA Bulletin.

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