Machines for EU safety footwear testing – part 5

Highlighting test equipment used to assess outsoles.

This series of articles considers the SATRA test machines that can be used to conduct footwear tests to the requirements of European safety footwear standard EN ISO 20345:2011 – 'Personal protective equipment – safety footwear' and the associated test methods found in EN ISO 20344:2011 – 'Personal protective equipment – test methods for footwear'. The focus of this article is on basic requirements for outsoles (clause 5.8 of ISO 20345:2011).

Within EN ISO 20345:2011, footwear is classified as either 'Class I' or 'Class II'. Class I footwear is made from leather and other materials, excluding all-rubber or all-polymeric footwear. Class II footwear is all-rubber (that is, entirely vulcanised) or all polymeric (entirely moulded). The particular requirements for 'outsoles' are applicable to all Class I footwear and Class II footwear. Interlayer bond strength is applicable only to multi-layered soles.

Basic requirements for outsoles

Sub clauses within clause 5.8 of EN ISO 20345:2011 cover a number of characteristics: i) design (clause 5.8.1), ii) tear strength (clause 5.8.2), iii) abrasion resistance (clause 5.8.3), iv) flexing resistance (clause 5.8.4), v) hydrolysis (clause 5.8.5) and vi) interlayer bond strength (clause 5.8.6). In this article, we will examine the test equipment required for each of these test categories.

Design

Clause 5.8.1 of EN ISO 20345 sets out the requirements for outsole thickness, taking account of whether the sole is cleated or non-cleated. A differentiation is also given for the thickness requirement, dependent on whether the footwear is Class I or Class II. Where the sole is cleated, the standard sets out some additional requirements for the cleat dimensions and their location within the sole area. The thickness of the outsole is assessed by means of a graduated eyepiece used to examine sections cut across the footwear. SATRA supplies a suitable instrument for making this assessment – SATRA STD 405M.

Tear strength

Within EN ISO 20345, clause 5.8.2 sets out the requirements for outsole tear strength. This clause references ISO 20344:2011 clause 8.2 which, in turn, references method 'A' of ISO 34-1:2010. This is a trouser tear test conducted using a tensile tester. SATRA’s STM 566 tensile tester fitted with appropriate jaws (such as SATRA STM 566F jaws) can be used to conduct this test. Care and suitable equipment is required to prepare the test specimen (100 x 15 x 2mm) which must be cut from the sole.

Abrasion resistance

Clause 5.8.3 of EN ISO 20345 specifies the requirements for the abrasion resistance of outsoles, and references ISO 20344:2011 clause 8.3 which, in turn, references for non-leather outsoles ISO 4649:2010 method A. The test can be carried out using SATRA’s STM 602 soling materials abrasion machine (see figures 1a and 1b). This is a three-head machine which allows three specimens to be tested at the same time. During the test, a specimen of defined dimensions cut from the outsole is weighed and then installed in a holder on the machine. A 10N load is applied between the soling specimen and a drum covered with a specified abrasive paper. The drum is rotated under the specimen at a fixed speed, while the material under test moves along the drum for a specified 40m distance.

Figure 1a: SATRA’s STM 602 sole abrasion tester

Figure 1b: A specimen of soling under test

The specimen is then removed and reweighed, allowing the mass of soling material lost to abrasion to be established. The abrasive resistance of the specimen material can then be determined by relating the test results to those of a control rubber compound defined in ISO 4649. SATRA also supplies the certified control rubber (STM 469CR).

Flexing resistance

Clause 5.8.4 of EN ISO 20345 specifies the requirements for the flexing resistance of the outsole, and references ISO 20344:2011 clause 8.4. Before carrying out the flexing resistance test, an assessment of the rigidity of the whole footwear is required as set out in clause 8.4.1 of ISO 20344. The rigidity test is carried out on one complete item of footwear, which should be the middle size of the footwear model size range.

The SATRA STM 465SG test gauge allows for the footwear rigidity, as specified in the standard, to be determined. The footwear sample is clamped at the toe and positioned in the gauge such that the flexing line (as defined within the method) is positioned at the pivot point of a hinging rear section of the gauge. By raising the rear section of the gauge, the footwear is forced to flex. As this flexing takes place, a load cell within the STM 465SG gauge allows the flexing force to be determined.

When a load of 30N is applied, the angle of flex is recorded. If the 30N load is achieved at an angle of flex less than 45°, the footwear is considered too stiff to carry out the flexing test specified in clause 8.4.2 of ISO 20344:2011.

The requirements for the flexing test are set out in clause 8.4.2 of ISO 20344:2011. This test is conducted on footwear after establishing (via testing to clause 8.4.1 of ISO 20344) that it is flexible enough to allow the flex test to be carried out. When conducting the repeated flexing test, the test specimen is cut from the forepart of the sole (and associated insole) and separated from the upper. The specimens are clamped over mandrels in the test machine and during the test are repeatedly flexed through 90°. A single cut is produced in the sole, at a specified position, using a cutting tool defined in figure C2 of ISO 5423:1992.

Figure 2: The SATRA STM 465 whole sole flexing machine

After 30,000 flexes, a visual inspection is carried out to see if the crack produced by the cut has expanded. The acceptance criteria against cracking are set out in clause 5.8.4 of ISO 20345:2011. This flexing test can be carried out using the SATRA STM 465 whole sole flexing machine (figure 2). This test machine allows three specimens to be tested at the same time. SATRA also supplies a chisel and jig (STM 465J) to allow the specified cut to be made in the sole prior to the test.

Hydrolysis

Within EN ISO 20345, clause 5.8.5 specifies the requirements to assess whether polyurethane (PU) outsoles are resistant to the effects of hydrolysis, and references clause 8.5 of ISO 20344:2011 – 'Determination of resistance to hydrolysis of outsole'. In turn, this specifies ISO 5423:1992 Annex C as the method to be used to determine the outsole hydrolysis behaviour. The requirements for preparing and hydrolysing the specimens are set out in Annex E of this same standard, for which a humidity chamber is required. A flexing test is required on the hydrolysed specimens, which is carried out using a Ross flexing machine operating at a temperature of -5°C. This can be carried out using SATRA’s STM 141F low temperature Ross flexer. A cut of defined geometry is made in the test specimens before testing. A suitable jig and chisel (STD 405S) are available from SATRA to allow this to be carried out. Clause 5.8.5 of ISO 20345 specifies the maximum allowable crack growth after 150,000 flexing cycles using the low temperature Ross flexing machine.

Interlayer bond strength

Clause 5.8.6 of EN ISO 20345 sets out the requirements for the interlayer bond strength for outsoles. It is applicable where soles are made up with distinct layers – for example, to assess the bond strength between an outer cleated layer and a midsole. This clause references ISO 20344:2011 clause 5.2, which covers both the upper to outsole bond strength and the sole interlayer bond strength. The upper to outsole strength requirement is referenced from clause 5.3.1.2 of ISO 20345, and was covered in part 2 of this series of articles.

The interlayer bond strength test can be conducted with SATRA's STM 566 tensile tester fitted with the appropriate jaws (for instance, STM 566SB and STM 566F). The preparation of test specimens is set out in clause 5.2.3.3 of ISO 20344:2011, and indicates the orientation for cutting specimens from footwear. A hot knife should be used when separating sole layers at an adhesive surface. This sole bond strength test is not applicable when the bond has been made using pins, screws or stitching.

SATRA test equipment is designed and manufactured in the UK and supplied to a global market, including to brand owners, retailers, manufacturers, suppliers of components and materials, test houses, government institutions, universities and research institutes. SATRA test equipment draws on the expertise gained over its 97 years of history, from fundamental research, development of over 400 SATRA test methods and the extensive use of SATRA machines in our own laboratories.

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Publishing Data

This article was originally published on page 46 of the January 2017 issue of SATRA Bulletin.