Elastomer Friction –Fundamental and Footwear Research

Slipping, tripping and falling is with 18.9% in Denmark the second most frequent cause of serious occupational accidents. In fact, general fall accidents in 2016 were estimated to result in annual additional costs of 3.9 billion DKK due to lost production and further 4.8 billion DKK for treatment and care.

The cause of fall/slip accidents is complex and multi-factorial, but is most often a result of insufficient friction between footwear and surface. However, varying work environments where different surfaces and contaminants (oil, dirt and food residues) are present, makes it difficult to maintain sufficient friction. Friction occurs as a result of contact between shoe sole and surface, but is difficult to determine realistically, as the friction between footwear and surface is strongly influenced by the test methodology. It is therefore necessary to replicate realistic slipping accidents, to gain the best possible insight into the slipping mechanisms.

Shoe soles are often made from elastomers, which are viscoelastic materials in nature. This means that elastomers have both viscous (behaves like liquids) and elastic (behaves like solids) characteristics and the friction properties are therefore extremely complicated. This is due to a complex combination of material properties such as low modulus of elasticity, adhesive and hysteresis components, real contact area, as well as dependence on sliding velocity, temperature normal load and frequency of normal load.

The purpose of this PhD thesis is to investigate the shoe/surface slipping phenomena from a basic research perspective, using elastomer blocks and from an application-oriented perspective, using actual footwear. The thesis is thus divided into research that deals with elastomer friction of block specimens and research that is applied to a system, which, in this case, is shoe slip resistance.

Study I has dealt with the application-oriented research and presented and evaluated a test system to quantify the slip resistance of footwear. The system can operate in accordance with "ISO 13287 Personal protective equipment – Footwear – Test method for slip resistance" and at the same time can be adjusted to accommodate testing parameters replicating realistic slip accidents. Studies II and IV have used this test system to determine the slip resistance properties of footwear models using an application-oriented research approach. In study II, three shoes constructed from the same model, but with different outsole materials, have been studied. The three materials are referred to as PU, TPU and RU. In study IV, five commercially available footwear models have been studied, all of which are certified as slip resistant according to ISO 13287.

Studies III and V have been characterized as more fundamental research of elastomer friction, since they use elastomer blocks and do not include whole shoes. The complexity of the tribosystem is thereby reduced and the results are to a certain extent more generic and can therefore be used in other applications.

In Summary, the studies (Studies II, III and V) have showed that material selection have a large impact on the friction properties and is context dependent. The RU material has exhibited the highest dynamic friction on a cold (-10 °C) ice surface, whereas the PU has exhibited the highest dynamic and static friction on a warm (0 °C) ice surface. PU has also demonstrated the highest dynamic friction on tile and steel surfaces contaminated with glycerin and canola oil. TPU has exhibited the highest dynamic friction on dry steel surface at high sliding velocity (up to 2.4 m/s). Thereby, all three materials have showed the highest friction under different conditions. This concludes that material performance/friction properties are highly dependent on testing conditions and environmental circumstances. Furthermore, study IV has showed that the test conditions and footwear design have influenced slip resistance considerably. Future research should therefore focus on optimizing slip resistant footwear to meet area- and industry-specific environments, where different surfaces and contaminants are present. At the same time, the focus should be on reducing the gap that exists between the academic research in slip resistance and the industrial stakeholders in the footwear industry, who produce slip resistant footwear and surfaces, as well as the industrial buyers/users of non-slip footwear.