Residual VOCs in footwear

It is not only worn footwear that can contain unpleasant odours. Volatile organic compounds used in manufacturing can also be present in new products.

by Martin Heels

Some of the materials used in the construction of footwear can have inherent odours. These odours are not necessarily undesirable and, indeed, some may be perceived positively by consumers. For example, the characteristic smell of leather is usually associated with higher-value products, especially footwear and furnishings. Some polymers, such as vulcanised rubber and ethylene vinyl acetate (EVA) foams could be considered to have more of an unpleasant chemical odour.

Residues from solvent-based adhesives or cleaning solutions can remain in footwear materials and become trapped when the footwear is packed. Once the shoe box is opened, those trapped solvent vapours will be released, potentially resulting in a risk to health and strong odours. If many boxes have been transported for several weeks inside the same container, the amount of solvents inside the container might exceed legislative exposure levels for those who are unloading the boxes once the door is opened.

Odours from unworn footwear

Rubbers, leathers, and polyvinyl chloride (PVC) all contain internal lubricants which can contribute to the odour of finished footwear. Leathers contain fatliquors in order to soften the leather and increase its flexibility. PVC formulations contain plasticisers to increase flexibility, whereas rubbers contain lubricants to aid mixing when the polymers are at the formulation stage and to make the final material more malleable. EVA might contain residues of dicumyl peroxide – a cross-linking agent that is used in production. This can react to form acetophenone and 2-phenyl propanol, which have characteristic odours. It is normal for these specific materials to contain slight odours from the presence of these volatile organic compounds (VOCs).

Why could other VOCs be present?

There are two particular processes during footwear manufacture that may involve significant amounts of VOCs. Firstly, when components are bonded together using adhesives (cemented), these adhesives may contain high amounts of solvents such as toluene, methyl ethyl ketone (MEK), ethyl acetate and hexane. After the adhesive is applied, these solvents evaporate, leaving the binder bound to the cemented components. However, solvents can sometimes be absorbed into materials and, depending on the temperature of storage and other factors such as air circulation around the material, some residual solvent can reside in foams and other footwear materials until they reach the consumer.

The other manufacturing stage where VOCs could be used is at the end of the process, when the footwear is cleaned, polished or dressed with a finish immediately prior to packing. If solvent-based solutions have been used and the time between their application and the product being packed is insufficient, the solvents may not have completely evaporated and dissipated before packing. Materials that are inherently absorbent (such as foams, laces and textile linings) can retain solvent vapours. Harder plastic materials – including nylon and vulcanised rubber – are less easily penetrated by solvents, which are more likely to remain on the surface and evaporate more readily. Further information about the benefits of changing to water-based adhesives were discussed in the article ‘Cemented constructions and sustainability'.

The effects of VOCs

Retailers, manufacturers and suppliers of consumer products are duty-bound to ensure that their products are safe for consumers and do not present significant hazards. Although there are no legislative levels for VOCs in consumer products, the levels of solvent vapour that workers are exposed to in the workplace are generally restricted. For example, there are legal maximum exposure levels that workers can be exposed to in the USA, and the UK has ‘workplace exposure levels prescribed in the document EH40 (updated most recently in January 2020).

EH40 sets an eight-hour exposure limit of 500 parts per million (ppm) for acetone, 200ppm for ethyl acetate and MEK, 50ppm for toluene and 20ppm for hexane. Prolonged or repeated exposure to these solvents may cause damage to internal organs, and short-term exposure may cause drowsiness or dizziness. In line with the earlier-mentioned scenario, there have been instances when containers transporting footwear have been held at a European port because the solvent vapours inside exceeded the maximum exposure levels for those workers who would be unloading the container. VOCs can also be a serious atmospheric pollutant as, in combination with oxides of nitrogen, they can form a photochemical smog which can irritate the eyes, worsen existing respiratory problems and have a negative effect on the wider environment.

Controlling odours

Improving existing production methods and restricting solvents used in the manufacturing process will diminish the potential for harmful odours arising from the presence of VOCs. Simple tasks can be utilised to improve existing practices. Increasing the drying time between cleaning and packing, using adhesives or cleaning solutions with low solvent contents or switching to water-based adhesives or cleaners are changes that will reduce the potential for VOCs to be present in footwear. However, the effects of alternative preparations must be assessed to ensure these do not detrimentally affect key properties of the final product, such as a reduction in the strength of the bonds.

Testing at SATRA

The chemical testing laboratory at SATRA can identify VOCs that are present in unworn footwear using headspace Gas Chromatography with Mass Spectrometry (GC-MS). A specimen of the material to be tested is heated in a sealed vial and a sample of the air from the vial is injected into the GC-MS. This separates the mixture of solvent vapours into their component parts, so that they can be identified and quantified if required by analysing alongside standards prepared with exact concentrations of the individual solvents.

Publishing Data

This article was originally published on page 26 of the April 2020 issue of SATRA Bulletin.

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