Following on from the November edition of Health and Safety International, Simon Courtney brings us the second of our two part series on safety footwear. Having previously looked in detail at testing, impact and compression resistance, this final instalment delves further into the subject, covering slip resistance, nail penetration resistance, metatarsal protection and electrical resistance properties of footwear.
Slip resistance is a complex property with many factors involved; however, well designed footwear can help to reduce the risk of slipping accidents and injury. On a clean and dry surface a tread pattern is not always necessary, but on contaminated surfaces such as where lubricants are present, an effective tread pattern is required to sweep aside any liquid or contaminants. This works in a similar way to a tyre tread pattern, returning the sole to almost dry conditions. Elements of good tread pattern design include soft, flexible constructions which maximise contact with the floor, smooth flat wearing surfaces and leading edges in all directions.
Both the European version of the International Standardization Organization’s standards (EN ISO) and the Canadian Standards Association’s (CSA) standards use slip resistance test methods contained within the EN ISO 13287 standard. In this test, samples of whole footwear are assessed against sections of test surfaces wetted with an appropriate lubricant. The CSA standard also contains a dry test.
The test mimics the first contact of the footwear with flooring, and the speed of movement is chosen to represent the early stages of the interaction between the flooring and the footwear. If the friction is high then slip will not occur, but if the friction is low then slipping could take place. The footwear is tested with the sole in flat contact or with the heel angled at seven degrees to the test surface.
The EN ISO standard contains a choice of test surfaces and lubricants which allow use of different codes on the final footwear which passes these tests. The three conditions are listed in Table 1.
The lubricants and surfaces used simulate possible working conditions the footwear is likely to encounter during normal use. The sodium lauryl sulphate solution is a mild detergent solution and simulates a freshly cleaned floor with a degree of slippery soap solution remaining on the flooring; glycerol simulates an oily contaminant.
In order to meet user demands, the CSA standard has provisions for test surfaces and slipping agents other than the primary test conditions, as described in Table 1. Other slipping agents suggested include oils, greases and detergents. The ASTM International standards do not include a slip resistance test; however, following the publication of ASTM F2913-11: Test Method for Measuring the Coefficient of Friction for Evaluation of Slip Performance of Footwear and Test Surfaces/Flooring Using a Whole Shoe Tester, slip resistance testing is now available to manufacturers.
This test is derived from SATRA’s TM 144 slip resistance tests and differs slightly from the EN ISO standards, including the mode of testing carried out with forepart contact rather than flat contact.
The EN ISO standards contain performance requirements for each test condition, whereas the Canadian standard only requires that the test data is supplied to the end user of the footwear which is usually provided on a printed sheet included in the shoe box.
Nail penetration resistance
Additional components may be incorporated into footwear to produce soles with nail penetration resistance, also referred to as puncture resistance. This is achieved by the inclusion of a metallic or textile insert usually placed between the outsole and the insole materials. Traditionally, metallic inserts have been widely used; however, more recently tightly woven multiple layered textiles have become more common.
The EN ISO standard requires the whole sole unit containing the penetration resistant insert to be assessed for puncture resistance, and the ASTM and CSA standards require that the penetration resistant device is tested prior to being put into the footwear. Both tests are similar, with a truncated steel nail being forced through the sole and inserted from the underside through to the foot side, simulating a wearer accidentally standing on a nail during wear.
For metal inserts, testing is ceased once the nail has penetrated through the footwear and the load to penetrate the footwear is measured. The EN ISO standard has a different end point for testing non-metal inserts.
In this test a fixed load of 1100N is applied to the nail until visible signs of the first emergence of the nail point through the sole. The EN ISO standard has a requirement of 1100N and the ASTM and CSA standards have a slightly higher requirement of 1200N.
The EN ISO and CSA standards also contain dimensional requirements for metal inserts to ensure that the insert provides suitable coverage for the whole foot. If the insert is too small or not seated centrally the degree of protection for the wearer is reduced, as unprotected regions exist at the edges of the footwear where there is a risk of nails penetrating through to the foot. The width of the unprotected region is measured at several points on the footwear.
The EN ISO standard requires that the unprotected region is not greater than 6.5mm in width (17mm at the heel) and the CSA test requires that the width is not greater than 8mm (13mm at the heel). The ASTM standard does not contain any dimensional requirements.
Further testing of these inserts is also required to ensure that the protection they offer to the wearer is maintained throughout the life of the boot. As the foot flexes during normal use, inserts are subjected to testing to ensure that they can resist flexing across the forepart of the footwear. Cracking of the insert during use would result in loss of protection to the wearer, as penetration resistance would be lost at the site of any insert cracking, making it possible for nails to penetrate through these cracks.
The inserts are removed from the footwear, or are tested prior to manufacturing, and are subjected to one million flexes in the EN ISO test and 1.5 million in the ASTM and CSA standards’ tests.
To protect the complete dorsum, or top side of the foot, metatarsal protection may also be provided to the wearer by additional integrated protective components. Metatarsals are the five long bones that connect the toes to the bony mass forming the rear of the foot and they are at risk of injury from an accidental blow to the foot. Additional protection for the wearer will extend behind the toecap and further up the footwear.
Manufacturers may achieve this by inclusion of an externally attached shaped plate, which is permanently fixed to the footwear close to the toecap and fits over the instep of the foot. This plate may be a metallic or composite material and is usually covered with the footwear’s upper material for aesthetic purposes. In addition, it may also be folded forward when the boot is secured onto the foot and folded back into place to provide protection.
Increasingly, metatarsal protection is provided by the incorporation of internally attached foam material which is intended to absorb energy during impact.
This method of manufacture allows production of more aesthetically pleasing boots, as although the footwear needs to be fuller at the instep region to allow incorporation of these materials, normal upper designs can be used.
The EN ISO, ASTM and CSA standards all contain tests and performance requirements to assess the effectiveness of these materials by use of an impact resistance test. This is a similar test to the impact resistance test for toecaps and uses the same equipment with some modification.
The impact positions and strikers of the methods do differ slightly so direct comparison between the standards is difficult. The impact energies of the methods are very similar, with the EN ISO test requiring 100J and the ASTM and CSA methods requiring 101.7J of energy delivered to the footwear on impact.
To assess the clearance provided by the protective devices a foot form is placed within the boot. In the EN ISO and ASTM methods the form is moulded from wax which is shaped from either the appropriate making last (one size smaller than the footwear to be tested) or by setting wax into the forepart of the footwear to be tested.
In the CSA method, a plastic foot form with a cut channel at the site of impact is used and this channel is packed with soft modelling clay. The wax and clay set into the channel deform on impact, allowing the clearance to be measured.
Electrical resistance properties
The EN ISO, ASTM and CSA standards all contain tests and requirements for the electrical resistance properties of footwear in three categories of protection. The test methods, requirements and category descriptions differ between the standards and these are included in Table 2.
The purpose of conductive footwear is to minimise electrostatic charges in the shortest possible time and to prevent an electrical discharge that might ignite volatile, flammable materials in close proximity to the wearer, such as when handling explosives.
The purpose of antistatic footwear is to minimise electrostatic build-up by dissipating electrostatic charges, thus avoiding the risk of electric shock from any electrical apparatus or live parts that have not been completely eliminated, along with eluding the risk of spark ignition from, for example, flammable substances and vapours. The ASTM and CSA standards describe this property as static dissipative.
The EN ISO standards also contain requirements for electrically insulating footwear which is used to protect the wearer when there is a possibility of a large potential difference in voltage between the wearer’s hand or body, and the ground on which they are standing.
In the ASTM standards the category of footwear that offers the highest level of electrical resistance is described as electrical hazard resistant. In the CSA standards this level is described as electrical shock resistant, although both use the same test method and requirements.
The EN ISO standards are more comprehensive than the ASTM and CSA standards, which incorporate whole shoe and material component tests and requirements that are not key safety features of footwear.
The standards contain, for example, requirements for tear strength, abrasion resistance, water vapour permeability and absorption testing of lining materials. This applies to all materials used as lining materials within the footwear and these tests allow the general quality of components to be assessed.
There is a similar range of tests for upper outer materials, soling materials and insole or insock materials and these tests apply to all relevant materials within the footwear. For complex designs of upper patterns where more than one upper or lining material has been used, multiple testing is required to verify compliance with the standard.
Some of the more complex trainer style footwear designs may incorporate three or four upper materials and three different lining materials, which obviously greatly increases the amount of testing that is required. In these more complex designs some of the materials may feature as only very small pieces which may be too small to take the necessary test samples from. For this reason, it is common practise for manufacturers to supply sheet materials for testing.
The most appropriate series of standards for a manufacturer to test and be in accordance with will depend on where in the world the footwear is intended to be sold. The range of requirements the footwear must meet will depend on the claims they make.
Published: 18th Jan 2013 in Health and Safety International