It is extremely important to supply footwear with adequately slip-resistant soles. While it is impossible to make footwear resistant to slip under all conditions that may be encountered in wear, the risk of slip should be minimised if guideline friction coefficients are achieved. Hannah Boughey and Mike George, footwear technologists at SATRA Technology give an overview of this safetycritical property.
Any slip accident – in or out of the workplace – can lead to injury. Therefore, slip resistance is a very important property to consider and is a big part of risk assessments. Slip accidents happen for a number of reasons – often when the person is walking normally with a confident gait, and something unexpected or unnoticed is encountered. Factors influencing slip can be split into various categories, such as location and environment, human factors, flooring and footwear. The cause of a slip accident may just be one of these factors, or it may be a combination. In this article we mainly focus on the factors relating to footwear.
The majority of slip accidents occur on wet or contaminated surfaces. General housekeeping is therefore a major factor, as well as the type and surface roughness of the flooring. However, it is still possible for some footwear to be slippery on dry, smooth surfaces. It is impossible to make any footwear resistant to slip under all conditions which may be encountered in wear but, with careful selection of footwear and achieving minimum friction guideline levels, the risk of slip accidents can be minimised.
A soling tread pattern may not be necessary on exclusively clean and dry underfoot surfaces. Soft soling materials nearly always offer the best dry friction, probably because they make more intimate contact with the floor at a microscopic level, conforming with tiny asperities rather than bridging them. However, if they are too soft, they can almost stick to the floor which can inhibit normal gait, or even induce a tripping hazard. It is also important to remember that a good performance in the dry does not necessarily mean a good performance in the wet.
“the need for testing is driven by an increasing obligation within worldwide markets to supply a demonstrably safe product”
On contaminated surfaces (such as where lubricants are present), an effective tread pattern is needed to sweep aside the lubricant, the same way as does, for example, the tread on a car tyre. The foundations of a good tread pattern design include soft, flexible constructions that maximise contact with the floor, as well as smooth, flat wearing surfaces and leading edges in all directions, and good unimpeded drainage to the sides. The tread pattern will only be effective while it lasts, so depth of tread and material durability must also be adequate.
Testing of slip resistance
The need for testing is driven by an increasing obligation within worldwide markets to supply a demonstrably safe product which is fit for its intended purpose.
For a standard walking movement, there must be friction between the sole and underfoot surface in order to start and sustain the movement, as well as change both the speed and the direction of that movement. If no friction was present, the smallest force would cause a slip between two surfaces. Even standing upright and motionless would be impossible without any friction.
When assessing slip resistance of footwear, the coefficient of friction is measured between the sole of the footwear and the underfoot surface, and with the most realistic possible dynamics and forces. The coefficient of friction is a proportionality constant that is calculated by dividing the force required to make the two surfaces move against friction (the horizontal resisting force), by the load which is pressing them together (the vertical force). The higher the coefficient of friction between the sole and underfoot surface, the greater the resistance to slip. However, if it becomes too high, it may create a stumbling risk – often referred to as ‘sticktion’.
Over the years, SATRA has conducted considerable research into the testing of slip resistance and the development of test methods, as well as the SATRA STM 603 slip resistance tester. The SATRA TM144:2011 – ‘Friction (slip resistance) of footwear and floorings’ test method is now a well-established assessment that has been adopted worldwide and is applicable to all types of footwear, outsole units, heel top-pieces in addition to sheet materials. Slip resistance testing is, of course, best carried out on the finished footwear, but it can also be useful for evaluation during pre-production stages. The key principles of the test are shared with the methods specified in the safety footwear standards discussed in the next section of this article.
The footwear is mounted onto a footwear making last and then attached to the slip-resistance tester. The relevant underfoot surface, which includes the flooring and any contaminants, are prepared and positioned under the footwear. While many of the test methods or standards relating to slip resistance specify a standard underfoot surface, we also encourage extending the testing to include a range of surfaces relevant to the intended use – especially for special-purpose footwear.
There are three test mode options that determine the set-up and positioning of the footwear, all of which are aimed at replicating the phases of a typical walking step when slip is most likely to occur. A normal walking step begins with heel strike and ends as the toe is lifted from the ground. Biomechanical studies have shown that slip is most likely to occur just after heel strike and then just before toe lift, when only the forepart of the sole is in contact with the underfoot surface. These two stages are replicated by the ‘forward heel’ and ‘backward forepart’ test modes. In the ‘forward heel’ mode, the test simulates the heel trying to slide forward with a shallow contact angle of seven degrees between the heel and the underfoot surface. In the ‘backward forepart’ mode, the centre forepart is trying to slide backwards, with the heel raised clear of the underfoot surface. The third ‘forward flat’ mode is where the whole footwear sole makes contact with the underfoot surface, perhaps to stop a rapid forward motion, which has a higher friction demand than for normal walking.
Once set up in the correct position, a constant vertical force is applied to bring the footwear in contact with the underfoot surface. This force is specified in the test method based on the size of the footwear, but is intended to approximately represent half-body weight that is being applied during those most critical phases of the walking step. After a split second of static contact, the footwear and flooring surface are moved horizontally to one another (with the STM 603 slip resistance tester, the flooring moves under the footwear). The horizontal force acting on the footwear during this movement is measured, and this allows the coefficient of friction to be calculated. Again, the time at which these values are determined is specified in the selected test method.
Slip resistance in safety footwear
A number of standards are used around the world for safety footwear, which may have different demands. The requirement for slip resistance is not included in every safety footwear standard. Major standards include EN ISO 20345:2011 – ‘Personal protective equipment – Safety footwear’, ASTM F2413-17 – ‘Standard specification for performance requirements for protective (safety) toe cap footwear’ and CSA Z195-14 – ‘Protective footwear’.
The EN ISO 20345 specification is used in Europe, and has also been published by the International Organisation for Standardisation (ISO). For the test method to be used, this standard refers to EN ISO 13287:2019 – ‘Personal protective equipment – Footwear – Test method for slip resistance’. The current standard has three possible slip ratings, coded ‘SRA’, ‘SRB’ and ‘SRC’, which relate to the underfoot surface conditions. For all of these, the test is carried out in both ‘forward heel’ and ‘forward flat’ test modes. The underfoot surface for SRA is a ceramic flooring tile, known as ‘Eurotile 2’, coated with soapy water (0.5 per cent sodium lauryl sulphate (NaLS) solution). For SRB, the underfoot surface is stainless steel plate coated with a 90 per cent glycerol solution. Both of these surfaces are certainly very slippery conditions which offer a severe examination of footwear designed to encounter many challenging situations in daily use. The SRC marking includes both SRA and SRB surface conditions to be tested.
“footwear should be selected according to the type of underfoot conditions most likely to be encountered”
All of these conditions, along with the minimum coefficient of friction requirements, are presented in Table 1.
It is a common misconception that the SRC rating is better than either SRA or SRB. Footwear should be selected according to the type of underfoot conditions most likely to be encountered – for example, mainly wet (to which SRA would best apply) or mainly greasy (to which SRB would best apply). It is better to achieve a strong performance in the most relevant conditions (SRA or SRB) than a mediocre pass in all conditions (SRC). Footwear which scores very well in SRB conditions may not be best designed for rough and/or soft outdoor surfaces, for which larger, deeper cleated tread patterns are best advised, and those might only be capable of passing ‘SRA’.
The CSA Z195-14 standard also refers to the EN ISO 13287 (undated) test method for slip resistance testing. There are no minimum coefficient of friction values stated, although the manufacturer must state the values achieved on either a label affixed to the footwear, on the packaging or on the product information sheet.
The ASTM F2413-17 standard is used in the USA, but does not include requirements for slip resistance. However, the test method ASTM F2913-19 – ‘Standard test method for measuring the coefficient of friction for evaluation of slip performance of footwear and test surfaces/flooring using a whole shoe tester’ is available. This method is very close to SATRA TM144:2011 and uses ‘forward heel’ and ‘backward forepart’ as the standard test modes.
Proposed developments affecting safety footwear
At the time of writing, there are several developments which are likely to have some impact, or be further decided upon, in early 2021.
SATRA’s own TM144 test method is completing a revision process, and the testing and certification of PPE footwear in Europe and the UK, postBrexit, is entering a period of flux.
The EN ISO 20345/6/7 specifications, referred to above, are currently under review with the formal enquiry stage now complete. However, the complexity of the comments received, coupled with the currently challenging working environment caused by the pandemic, has led to some delay in taking this further. It is probably worth noting the proposals within this draft specification, with the caveat that they are not yet (as of December 2020) guaranteed to take effect.
It is proposed that slip resistance of PPE will become a basic requirement and, as such, the codings SRA, SRB and SRC will disappear from the footwear label. The basic requirement test is proposed to use the same ceramic floor tile and NaLS lubricant as described above. The heel test mode (condition A in table 1) will be retained, but the forward flat test mode (condition B) will be replaced by the backward forepart test mode (new condition E) with a two-year transition period during which either conditions B or E may be used alongside condition A.
The required friction value for condition A will be effectively unchanged although building in the correction factor that was previously applied to condition A results. Thus, the new value is 0.31 (minimum). Likewise, the condition B requirement will be effectively unchanged but stated as 0.39. The new requirement for condition E is 0.36. All of these values are as directly measured, without correction (at present, the condition A and B requirements must be achieved after subtracting 0.03 and 0.07 respectively from the measured results. This ‘corrects’ the results to what would have been expected from a previous version of the tile – ‘Eurotile 1’ – in use when the specification was written but no longer commercially available).
Under the proposal, if users wish to achieve a higher level of performance than this basic one, an additional floor condition may be tested. This will be the ceramic tile (‘Eurotile 2’ again) with glycerol in 90 per cent solution as the lubricant. The test modes will be forward heel slip (condition F requiring friction at least 0.19) and backward forepart slip (condition G requiring friction at least 0.22). Achieving these values (in addition to the basic requirements with the tile plus NaLS lubricant) will permit the product coding ‘SR’ to be used. Alternatively, during a two-year transition period from the date of publication of the new standard, the present conditions C and D may be employed (steel/ glycerol, heel and flat), permitting the present coding SRB to be used (note – not SRC – even though the present SRA conditions may also have been fulfilled).
All of this is for ‘conventionally soled’ footwear, and there is an exclusion made for ‘special purpose footwear containing spikes, metal studs or similar and used in very special workplaces (where) this test is not applicable’. This type of footwear is to be marked with symbol ‘N’ (for ‘no slip resistance’), which warns the user that not even the basic requirement test has been carried out. Conventional soled footwear cannot be designated ‘N’.
All of the above proposals are completely dependent upon the new draft standard being ratified and published in due course.
Table 2 summarises the proposal requirements for slip resistance which may apply from sometime in 2023, after the end of the aforementioned two-year transition period.
Entirely separate, but parallel to this specification development, is the situation that has applied in the UK since 1st January 2021 when Brexit took full effect. The UK has adopted its own legislation with regard to placing PPE goods onto the GB market (that is, England Scotland and Wales). The European PPE Regulation (EU 2016/425) has been brought into UK law to become the UK PPE Regulations. For safety footwear, third-party certification is required and the PPE needs to be marked with a new ‘UKCA’ mark. The process is very similar to that previously used for CE marking.
Over time, there may be a gradual divergence of UK PPE legislation and standards away from that of the EU. Although it is part of the UK, products being sold into Northern Ireland will require either CE marking or UKNI marking and not the new UKCA mark. Footwear currently requiring CE marking for sale in the EU will continue to need a CE mark, because the UKCA mark will not be recognised in Europe.
There are some transition periods as follows:
During all of 2021, footwear newly placed on the GB market may carry either the UKCA mark awarded by a UK Approved Body (such as SATRA UK) or the CE mark – but only if that Notified Body is based in an EU27 member state. This could be, for example, SATRA Technology Europe Ltd based in Dublin, Republic of Ireland. During this time, the UKCA mark can be fixed to the packaging instead of the footwear. Applicable rules are those at the time of placing individual products on the market which, under current guidance, is when manufactured and offered for sale. So, any product that was manufactured by the end of 2020 is considered to be already placed on the market under the old (2020) legislative regime.
During 2022, it will only be possible to place on the GB market footwear carrying the UKCA mark, and not footwear which only carry a CE mark. The UKCA mark may be placed on the packaging instead of the footwear.
From 1st January 2023, it will only be possible to place on the GB market footwear carrying the UKCA mark, displayed on the footwear.
Therefore, to summarise all of this in the context of slip resistance requirements for PPE footwear sold in the UK, it appears highly likely that, in the short to medium term at least, the same performance will be required under the UKCA mark as under the CE mark, even as those EN specifications continue to evolve.