Getting to the Soul of the Sole

Shoe soles can be made of different materials, depending on the type of footwear and its use. Whether you require resistance to slips, compression or abrasion, or perhaps lightfastness or anti-fatigue characteristics, you need to be able to trust that your footwear will perform. In this article, Leticia Sanz details the varying materials used and the required standards and testing associated with different types of footwear depending on its application.

Normally the materials for high demand footwear are rubber, PU and TPU. The footwear categorised as ‘high demand’ includes safety footwear, as well as sports shoes and children’s footwear. For medium-demanding footwear, such as for leisure, and women’s and men’s fashion footwear, rubber, PU, TPU, TR, EVA, PVC, PS or ABS are used. The footwear of low exigency footwear, or in other words domestic footwear, is made with rubber sheets and low intensity EVA. The classification of the materials and their use as a sole material in one type of footwear or another depends on the technical requirements that the final product needs, i.e. a trekking boot or boot for professional use must have good sliding behaviour, good resistance to tearing, abrasion and/or elongation at breakage to avoid excessive wear or falls when using the footwear. However, domestic footwear must offer very good thermal comfort and energy absorption.

Each material used for the manufacture of soles has a series of associated characteristics that we must take advantage of to guarantee the good use of each type of footwear that we are going to make.

“each material used for the manufacture of soles has a series of associated characteristics that we must take advantage of”

Table 1. Materials used in footwear floors

Footwear standard

The footwear models must comply with a series of technical requirements for their marketing that are specified in both national and international regulations that stipulate the limit values and methods for carrying out the tests.

They are as follows:

• UNE 59910 – Women’s shoes
• UNE 59900 – Men’s footwear
• UNE 59930 – Free time footwear
• UNE 59920 – Footwear for children and schoolchildren
• EN ISO 20345 – Protective equipment – safety footwear
• EN ISO 20347 – Personal protective equipment – working footwear

Quality assurance

The quality control of footwear components carried out in laboratories is of vital importance for detecting those faults that are not visible to the naked eye or have not been detected during production. These controls are carried out on both raw materials and finished products, and guarantee the final quality and durability of the models. Investing small resources to ensure the materials used for the manufacture of footwear will guarantee the good use of it, as well as ensuring the image and prestige of the brand and company.

Physical tests

The physical tests carried out on footwear are aimed at determining parameters such as the hardness and density of the material, as well as the resistance they present to different tests such as, among others, abrasion, traction and tearing.

It is necessary, for example, to pay attention to the design of the sole, where factors such as the internal relief, the geometry, the thickness of the walls or of the ribs and soles, the dimensions of the cavities or even the presence of reinforcements are very important so as not to find deficiencies later on when carrying out tests.

Image 1. Sole designs

Another test that is carried out on floor materials is the resistance to abrasion or wear of the sole, according to the UNE-EN 12770 standard, which represents the loss produced by friction with another surface and the result of the test determines the volume loss of the material in mm3.

The requirements for good sole performance are summarised below, depending on the material used and its density.

Image 2. Abrasimeter

The tensile strength, tearing strength or elongation at break tests, according to the UNE-EN 12803 standard, are carried out on a dynamometer and consist of measuring the force (N) until the failure of the tested material in the specimen.

In the tensile strength test, the value is expressed in N/mm2 and the most commonly used test piece is the halterium 2. With this same standard, the elongation is determined, which is expressed in %, and indicates the capacity of a material to stretch until it breaks, an important property for the behaviour of soles in bending: the greater the elongation, the better the behaviour against breakage by bending.

The tear resistance test determines the breakage of a material once a cut or incision has been made in it; trouser-type, angled or crescent-shaped specimens are used.

Image 3. Tear resistance test

Below are tables of values with the requirements to be taken into account according to the regulations and which the different sole materials must meet for the above-mentioned physical tests, taking into account their density.

Image 4. Dynamometer

Table 2. Test requirements for tensile strength, tearing strength and elongation at break

Slip resistance (EN ISO 13287) is another physical test carried out on complete footwear, but it is very common to previously check the behavior of the soles. During the test, the sample is placed on the equipment and a certain vertical and horizontal load is exerted on it depending on the size of the footwear. The sole can be tested on two surfaces: steel with glycerine or tile with soap and water. After a movement of the sole under these forces and on these surfaces, the dynamic coefficient of friction (CoF) is calculated for each of these surfaces. This test provides information on the behaviour of footwear on wet surfaces with different contaminants, a situation that may commonly occur in working situations and which should not give rise to accidents for users

“after a movement of the sole under these forces and on these surfaces, the dynamic coefficient of friction (CoF) is calculated for each of these surfaces”

Table 3. Slip test requirements

The purpose of this type of test is to analyse the resistance characteristics of materials when they are subjected to variable stresses with a certain frequency. An example of this is the Bending Test (UNE-EN ISO 17707, EN-ISO 20344). For very rigid footwear, the flexion test is not done. Prior to the test, a 2mm long incision is made in the sole, the sole is subjected to 30,000 bending cycles and the incision must not increase above the requirement set out in the above-mentioned standard (≤ 10mm/30kcycles for street footwear and ≤ 4mm/30kcycles for footwear for professional use).

Another fatigue test is resistance to compression (UNE 59536). In the latter, materials are subjected to compression for 100,000 cycles at 400N of force with a certain frequency. No more than 10% of the physical properties analysed should be lost.

Image 5. Slip resistance test

Image 6. Slip resistance test

Image 7. Floor flexometer

Ageing tests

Lightfastness (UNE-EN ISO 105-B02) is a test performed in the Suntest light chamber (Xenon arc lamp) on white or light-coloured sole materials. For this purpose, the specimen is exposed to artificial light under controlled conditions together with a blue reference scale for controlling the test time. The requirement for the solidity index of a sole should be Hydrolysis ageing, according to the UNE EN 12749 standard for street footwear, establishes the conditions under which the sample is subjected to modified atmosphere conditions with 85-90% RH (relative humidity) and 70ºC temperature for seven days. Once the ageing period has passed, the material tested must not lose 80% of its properties, whether it be abrasion, traction and/or tearing, among others. On the other hand, EN 20344 develops a specific test method for safety footwear made of polyurethane, in which the resistance to hydrolysis of the sole is determined. In this test, soles are exposed to the same ageing conditions mentioned above and after hydrolysis is completed, they are subjected to Ross Bending (UNE 59532) for 1500,000 cycles. The initial incision made is 2mm and the requirement specifies that it should not exceed 6m.

Image 8 and 9. Above and centre) specimens after light fastness test. Below) evaluation booth with illuminant D65

Chemical tests

The resistance to hydrocarbons (EN ISO 20344) of materials for professional footwear soles is measured as the change in volume experienced by a test tube when immersed in Isooctane, for 22 hours at 23ºC and protected from light. The requirement specifies a maximum of 12% volume variation after the test.

Other tests for professional footwear requiring resistance to acids and bases are based on UNE EN 13832-2 for footwear protecting against chemical products.

Image 10. Ross flexometer equipment

 Electrical tests

The electrical resistance tests are carried out in the laboratory according to the EN 20344 standard, for footwear for professional use. The electrical resistance is determined by passing a direct current of defined voltage through the shoe or boot. The measurements are made in Volts and the electrical resistance of the sample is calculated (MΩ). The footwear should be tested after being subjected to a dry and wet atmosphere for one week.

The requirements are:

• Conductive footwear – resistance to current passage at MΩ <0.1
• Antistatic footwear – resistance to the passage of current in MΩ between 1 and 1000

Image 11. Electrical resistance equipment