Evaluating odours in footwear

Investigating the causes of unacceptable odours in new footwear and how this problem can be included in specifications.

The perception of odour by an individual is how they respond to the presence of certain volatile compounds which react with the olfactory nerves in the nose. Therefore, for a product to possess an odour, it must have some volatile components. The rate at which these volatile compounds are emitted is dependent on the ambient temperature. Because the substance is emitted from the product, this would continually diminish the amount of substance remaining in the product. Hence, the odour associated with a product will reduce over time and, while at the point of sale it may be very strong, after the product has been worn for a period, this may reduce to little more than a background level. However, the nose is a very sensitive detector, so very strong odours may rapidly overwhelm a person’s sense of smell.

When writing a specification for the acceptance of completed footwear from a supplier, many organisations have included a requirement for odour. This parameter needs to be carefully defined in terms of both strength and type, as the assessment of odour is always subjective and may lead to the rejection of acceptable products.

Many materials used in footwear production have an inherent odour. These may not necessarily be classed as ‘unpleasant’ and, in the case of leather, it is often closely associated with the customer's expectation of the product. It is therefore important that any specifications stipulate uncharacteristic and/or unpleasant odours, and not just try to determine if there is an odour present. This can introduce the previously-mentioned element of subjectivity into the assessment of the product, as it is not certain that individuals would always agree on how to characterise a specific odour.

The assessment of odours

The identity and relative quantities of volatile organic compounds (VOCs) can be determined instrumentally by the use of analytical equipment such as Gas Chromatography Mass Spectrometry (GC-MS) headspace. A small specimen of material is heated in a sealed vial and the air above the specimen is analysed using the GC-MS. This can identify key volatile components, but does not automatically confirm whether they are the specific substance responsible for the odour. Nevertheless, this information is vital when attempting to investigate why a product has a particular odour.

Determining whether products have an uncharacteristic or unacceptable odour is normally carried out by odour assessment panels. These are similar to panels used to assess odour-absorbing materials under SATRA TM435:2010 – ‘Qualitative odour absorption test’. However, rather than describing the change in odour, the panellists would in this case characterise the odour and make a judgement on whether or not it would be acceptable to an end consumer.

An odour assessment panel should consist of at least four individuals who are not smokers and are in good health. Screening with containers containing dilute odours to determine whether the panellist’s smell threshold is acceptable is often carried out before constituting the panel. In some applications, a larger panel may be appropriate, as used in automotive odour assessment panels. The sense of smell will vary with age and gender, so this needs to be considered when selecting panellists.

There are also health risks associated with some contaminants, which can often arise from the build-up of volatile material during the transportation of shoes. In some cases, it may be necessary to allow transport containers to air before they are unpacked at their destination.

Analysing the problem

If products are identified as having an unacceptable odour, the analytical determination of the key components may help in pinpointing the origin of the odour, as well as identifying whether it is inherent or caused by contamination, degradation or an interaction between different materials.

The description of the odour from the panellists may also help in suggesting possible origins. Damp and ‘musty’ odours may be the result of moisture contamination and subsequent mould growth, while acrid odours may result from chemical contamination such as by some ethylene vinyl acetate (EVA) or polyurethane (PU) degradation compounds. Solvents used during manufacture in adhesives, paints, polishes or cleaners will usually have characteristic smells. In most cases where there is a residual solvent odour, it is because footwear has been packed too soon after production and these more volatile compounds were not given the opportunity to be released at the manufacturing stage.

National standards bodies have created specification standards for footwear which include odour as a requirement. If such a requirement is to be put into a product specification, it should be clear that it applies only to unacceptable odours and not necessarily inherent odours. Investigation of unacceptable odours can be undertaken by using a combination of trained assessors working as an odour panel and instrumental analysis.

Publishing Data

This article was originally published on page 28 of the November 2021 issue of SATRA Bulletin.

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