Testing breathability

Investigating how this important footwear characteristic can be assessed.

by Maria Agueda Sanchez

'Breathability' is one of a number of key factors that influence comfort in footwear, as it is important that the upper materials are able to effectively manage moisture generated through perspiration. In order to keep the foot dry and consequently maintain comfort properties in the long term, materials need to draw moisture – in liquid and water vapour form – away from the foot. There are several tests available to assess moisture management of materials, with SATRA TM47:2002 – ‘Water vapour permeability and absorption’ being the preferred method by SATRA. This is explained in the article 'Results of extreme conditions testing'.

SATRA TM47 is used to assess the permeability and absorption properties of materials or materials combinations over a period of time. The method mimics what happens inside the shoe, where the humidity and temperature are higher than in the outside environment. To reproduce these conditions, the test method uses a pot that is filled with water. This is then partially immersed in a water bath as a heat source, the open top of the pot is sealed with the specimen (see figure 1).

The water vapour generated during the test either will permeate through the sample or be absorbed by it, and it is these properties that are measured. This provides a measure of the ability of the material to remove perspiration from the skin of the foot and move it towards the outside environment. This test is carried out in standard laboratory conditions – 23°C/50 per cent relative humidity (RH) or 20°C/65 per cent RH) – which allows results to be replicated anywhere in the world. However, the standard environment conditions might not be representative of real life, especially for those regions with extreme temperature and/or humidity conditions. For example, the relative humidity in the rainforest will in general be above 80 per cent, whereas in arid areas such as the Sahara Desert, it sits at around 25-30 per cent. These extreme conditions fall at both sides of the spectrum and trigger different physiological responses in human body.

Testing in context

In order to maintain the comfort properties of footwear in the long term, it is paramount that the materials and construction are tested in context with the expected conditions of use and that they are able to perform in such conditions. For a number of years now, SATRA has been investigating the effect of ambient temperature and humidity on the moisture management of materials and whole footwear. The latest study (known as the ‘Multi Climate project’) focuses on the permeability or breathability and absorption properties of selections of materials, constructions and whole footwear across a wide range of environmental conditions. Testing is carried out in a climatic chamber that ranges from -40°C to +50°C, in temperature and from 10-100 per cent RH – and are thus able to reproduce any climate on earth.

For the testing in the climatic chamber at such extreme conditions, a modified version of the equipment used for the SATRA TM47 test (STM175 – see figure 2) was developed. This features a system able to both heat and cool the water in the water bath, which is vital when the temperature of the water bath needs to be kept lower than the ambient temperature. One of the main advantages of SATRA TM47 is its versatility. It allows not only the testing of upper materials, but also the combination of different layers of materials as they are to be assembled in the finished shoe – including linings or membranes, seams and seam sealing systems. For the purpose of the project, 20 different material and combinations were tested, including linings and membranes, as well as innovative new materials – so-called ‘smart’ materials. Specimens were tested across the different environmental conditions and the results were compared to those from standard testing. Testing of the whole shoe is also carried out alongside SATRA TM47 testing. For this, 12 different footwear styles were chosen and tested in accordance with SATRA TM376:2009 – Advanced moisture management test’, commonly known as the ‘AMMT’.

The main conclusion drawn from this Multi Climate project is the lack of predictability of the performance of materials/combinations/footwear in extreme environments when just looking at their performances at standard laboratory conditions. Materials that have very similar performance in standard conditions can perform entirely differently when in more challenging ambient conditions. Therefore, for those products intended for climates sufficiently different from standard laboratory conditions, testing in the environments to which they will be exposed becomes vital. This is in order to confidently select materials and constructions able to manage moisture effectively and, ultimately, provide a long-term comfort. Such bespoke testing is applicable to any type of footwear.

The value of standard testing

Testing in context with the application of the finished shoe represents the ideal scenario when looking at developing products able to meet customers’ expectations. However, standard testing can also be a useful tool to be used in combination with tailored testing. Having the products tested to standard conditions can both provide a base-level performance for a given material/combination/shoe, and also allow for a comparison with other products. The selection of the type of testing will vary depending on the application of the product and the stage in the development process. SATRA can guide members to select the most appropriate breathability tests.

Publishing Data

This article was originally published on page 8 of the April 2016 issue of SATRA Bulletin.

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