Resistance to microorganisms

Determining the resistance of footwear components to attack by microorganisms.

by John Hubbard

When footwear is worn outdoors, the outer materials are exposed to both water from snow and rain and the ubiquitous microorganisms in the environment. The interior components of the footwear are also exposed to microorganisms which are present on human skin and thrive in warm, moist conditions. It is important for the life expectancy of footwear that the materials employed in its construction are not adversely affected by the action of these microorganisms.

The Soil Burial Test

In order to assess the resistance of footwear components to attack by commonly occurring microorganisms, a Soil Burial Deterioration Test (SATRA TM309) was developed. The principle of this test is that the material under assessment is buried in a standard soil with a specified moisture content (to give the desired level of microbiological activity), and incubated for a set period of time. The duration of the tests will vary, depending on the test material. The method is commonly applied to textiles and fibreboard materials, but can be carried out on other materials such as polyurethane solings, which can also be subject to microbiological deterioration.

As the soil used can be variable, it is important to ensure a level of activity which will actually affect susceptible materials. This is done by adjusting the moisture content – to optimise the conditions for the growth of the microorganisms – to within the range of 20-30 per cent, either by the addition of water or by allowing the soil to lose excess moisture by evaporation before the start of the test. In addition, a control cloth of known susceptibility is used to measure the activity level. This control cloth is buried alongside the samples and then, after one week’s exposure to the soil, both the exposed and unused samples of the control cloth have their breaking strengths measured according to SATRA TM29. If the strength of this control cloth has not fallen by 80 per cent, the soil will be considered not to have sufficient activity, and the test should be restarted with fresh soil.

The test duration selected will reflect the types of materials used and the intended final application. Typical durations are four weeks for fabrics and threads, between two and four weeks for insole boards, and several months for soling materials.

Evalution of samples

At the end of the exposure period, the samples are removed and sterilised. Initially, they can be assessed on simple parameters – such as a visual assessment and measuring the gain or loss of weight of the sample. However, it is more common for the change in a particular physical property to be measured. An unexposed sample is always tested alongside the buried sample, so that the change in property can easily be measured.

Normally, the assessments are typical strength tests – such as breaking strength for fabric and threads, or tensile strength for insole boards or soling materials. Sometimes, the tests can be for resistance to a type of damage (for example, scuff resistance for insole boards or abrasion loss for solings), or a chemical assessment of degradation such as a measure of the low molecular weight matter in polyurethanes.

Where more information is required about the rate of degradation, a number of samples can be exposed and removed periodically, when the loss in strength or other parameter can be plotted against time. This can be extrapolated into potential implications for how footwear ages. In addition, variants of the test using specific contamination can be carried out for materials which one might expect to be left with external contamination for long periods. This may include such footwear as hiking boots or boots for farm, stable or abattoir workers that may be exposed to animal faeces and urine.

The test could also be used to demonstrate the biodegradability of materials.

Publishing Data

This article was originally published on page 32 of the March 2012 issue of SATRA Bulletin.

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