Slip testing for icy surfaces

Highlighting SATRA’s ice surface slip resistance testing capabilities for footwear likely to be worn in such challenging conditions.

Every year, each country that experiences sub-zero winters will almost certainly experience pedestrian slip accidents on snow and ice. In northern regions such as Scandinavia and Canada this is a major annual problem, with significant human injury and economic costs. Thus, where possible, there is a need to improve the slip resistance performance of winter footwear for children, adults and the elderly for everyday, leisure and workplace use. A particular area of concern is footwear for delivery people such as mail and parcel delivery services.

Of course, this concern is not restricted to outdoor applications. It also applies to footwear and overshoes worn in cold stores, walk-in refrigerators and similar cold environments indoors. Then, there are various sports where participants and maintenance staff have particular requirements – for example, curling and skating (on ice rinks), bob-sleighing, and indoor and outdoor skiing.

To help manufacturers and suppliers develop and measure the slip resistance properties of footwear (including overshoes), SATRA designed and constructed a refrigerated ice tray (shown in figure 1).

This fits into a SATRA slip test machine – SATRA STM 603 (figure 2) – which is housed in a standard laboratory atmosphere. Testing can then be carried out according to the SATRA TM144:2021 – ‘Friction (slip resistance) of footwear and floorings’ method.

A range of ice surfaces

This equipment enables different types of ice surfaces to be formed in the laboratory. These represent different winter situations, from ‘wet’ ice near zero degrees Centigrade to ‘dry’ ice at -10ºC. The surface may be used in a naturally uneven condition (a consequence of the expansion of water as it freezes) or be smoothed flat before the evaluation using a heated dressing tool.

From a test control point of view, smooth ice has the advantage of being reasonably easy to form and to reproduce. The disadvantage is that it is not really a commonly-encountered surface, except on ice rinks, for example, and it tends to promote extremes of performance, depending on whether it is wet or dry.

Smooth, wet ice is one of the most dangerous surfaces that can be encountered underfoot. With regard to conventional footwear, it is of limited value as a test surface because virtually all test results are likely to be very low, probably below 0.05 coefficient of friction (CoF). This means that wet ice provides very little differentiation between footwear products and, more importantly, confirms that no conventional shoe soles are likely to provide any useful slip resistance on such an inherently slippery surface. It can, however, be a useful surface for assessing the effectiveness of various footwear add-on devices, such as metal chains or spikes.

Hard, aggressive ‘cleats’ penetrate into the ice surface, to interlock and gain traction with the ice. Because the SATRA TM144 test method forces slip to occur, testing such devices on ice will immediately damage the surface which will then need re-preparing before further testing. At the other extreme, quite ordinary footwear can achieve high levels of CoF on very cold (for example -10ºC) smooth, dry ice, but these conditions are found only in the most extreme climates.

A good compromise is a smooth ice surface at about -7ºC. At this temperature and in a standard atmosphere laboratory, the surface of the refrigerated ice tray will be nominally dry. However, it will have the potential for a film of water to form under the cleats of a shoe sole, depending on the loading conditions and the temperature of the footwear. These conditions generally give results in the range 0.10 to 0.25 CoF and give reasonable differentiation between different footwear solings. It is worth noting that the science of ice in relation to the nature of its surface under different temperature and pressure conditions is exceedingly complex and still not fully understood. It is no longer universally accepted, for instance, that it is solely pressure that causes ice to melt under an ice skate blade – other factors may come into play.

Frosted ice is a particularly interesting surface. This is where the water has been allowed to freeze naturally to form the ice and the ice tray is left in the humid laboratory atmosphere at 23ºC, 50 per cent relative humidity (RH) for around two hours to allow frost to grow on the surface to a measured depth. Tread patterns can bite into the frost and achieve a degree of traction. While this is a realistic surface, it does have a degree of variability associated with it. Smoothing the ice with the heated dressing tool prior to frosting improves the reproducibility of the surface, but this action gives slightly lower results.

Pre-conditioning chilling of the soling

The ice tray system comes complete with a cooling tray. This is also refrigerated, but is filled with an ethanol-water solution that does not freeze at low working temperatures. An item of footwear can be placed in the cooling tray and left for a defined time to lower the temperature of the soling compound before testing. This is done in order to replicate the cold hardening effects which are associated with many rubbers, plastics and other polymers, and which will tend to reduce their frictional properties. Alternatively, the footwear may be stored in a refrigerator or freezer prior to testing. Either way, it is important to dab the sole dry immediately prior to the test action to prevent liquid transfer or condensation interfering with the test performance.

Footwear types

In addition to conventional solings and traditional add-on overshoes, ice chains and spikes, there are many innovative potential solutions currently under development. There are solings with temperature sensitive rubbers, with integral retractable spikes, with very hard polymeric spikes integrated into the main tread pattern, or with textile and other fibres blended into the soling itself. All such concepts can benefit from being tested in the laboratory under realistic conditions before field trials.

International standards and requirements

We have found no national or international requirements for the slip resistance of footwear on ice or cold surfaces. It can certainly be argued, however, that there is an onus on suppliers who make safety related claims for their products, such as ‘prevents slipping on ice’, to support and justify their claims. This is certainly the case for the European market.

The European PPE Regulation (EU) 2016/425 identifies the need to protect the user against specific risks such as slipping and, indeed, goes on to say that ‘the outsoles of protective footwear intended to prevent slipping must be designed and manufactured or equipped with additional means so as to ensure adequate grip, having regard to the nature or state of the surface’. To demonstrate conformity with the EU PPE Regulation, the usual course of action is to test the product using the appropriate European harmonised standard or, in the absence of such a standard, by the most appropriate other means identified by the Notified Body.

The European slip resistance test standard EN ISO 13287:2019 does not currently include testing on ice, although the European technical committee is considering extending the method. In lieu of this, SATRA would advise members to use SATRA TM144 to test products on ice, to verify claims and give confidence to the end user.

Publishing Data

This article was originally published on page 30 of the November 2023 issue of SATRA Bulletin.

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