Testing for resistance to water penetration
Considering two of SATRA’s test machines used to evaluate this important footwear characteristic.
Water penetration resistance is a key performance requirement used to market many types of footwear. SATRA has produced a number of relevant tests by which to ascertain this characteristic, in addition to the equipment required on which to perform these assessments. This article provides a brief overview of the SATRA machines available on which to conduct the appropriate evaluations.
SATRA STM 606D
The SATRA TM34:2019 – ‘Resistance to water penetration – Maeser test’ may 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 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.
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 K, 30 K and 50 K ohms). The individual station counter stops recording when water penetration is first detected and the alarm is switched on while the remaining stations continue the assessment.
SATRA STM 505
In order to obtain highly water-resistant footwear, factors in addition to the application of surface pre-treatments and water-resistant materials need to be considered. 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, such as 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 20344:2011 clause 5.15.2 for safety footwear to be used in Europe.
SATRA STM 505 is a two-station machine which allows two footwear samples to be assessed independently in line with SATRA TM230 or the above 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 an angle of 22 degrees at a rate of 60 flexes per minute.
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.
How can we help?
SATRA STM 606D and STM 505 are just two of the many test machines developed and built by SATRA. Please contact firstname.lastname@example.org for further information on these items or visit www.satra.com/test_equipment for an overview of the wide variety of equipment and test methods available.
This article was originally published on page 38 of the July/August 2021 issue of SATRA Bulletin.