Innovative system for conductive safety footwear
Risk of ignition or explosion due to electrostatic discharge is very common in several working situations. An innovative system to avoid this hazard has been developed by the Footwear Technology Centre of La Rioja (CTCR) and is reported in this article.
Conductive gels, contained in canals which cross the sole and confined by conductive polymeric stoppers, create an electrical connection between foot and floor, allowing reduction of the electrostatic charge produced in the worker. This system can be used to reach either conductive or anti-static values, regardless of the kind of rubber used in the soles, and can be assembled without modifications in the usual productive process.
Electrostatic discharge risks
Conductive safety footwear is a type of PPE (Personal Protective Equipment) which seeks to reduce, by grounding, the electrostatic charge generated in a worker. This is an unbalanced electrical charge at rest created by the friction caused when the worker comes into contact with different materials or surfaces. One surface gains electrons while the other one loses them.
The effects of charge exchange are usually only noticed when the worker wears insulating footwear, because he/she has a high resistance to electrical flow. These charges then remain on the body until they either bleed off to ground or are quickly neutralised by an electrostatic discharge (ESD).
The accumulation of electric charge may therefore involve risk of ignition and explosion in situations where highly flammable materials, including gas, fuel vapour and coal dust are present. In general, it can be said that this risk would exist if all the following conditions are present
• Potentially explosive atmosphere • Generation of electric charges • Accumulation of electric charges • Electrostatic discharge • Sufficient discharge energy
As a consequence, the risk of ignition would disappear if we are able to avoid at least one of these conditions.
The main situations in which these risks could exist and conductive safety footwear is required include: power lines, refineries, gas stations, explosives or weapons industries, computer rooms, chemical laboratories, places with machinery that generates friction, rooms with magnetic fields, non-electrical maintenance works in carpeted buildings or electrical centres – exhibitions, or hotels – and electrical works in places with plastic or vinyl floors.
Many accidents caused by ESD in recent years have given rise to growing interest in this field, and protective clothes and footwear have been developed to avoid ESD hazards.
Electrical resistance standards
International Standardization Organization’s (ISO) standards use electric resistance test methods contained within the ISO 20344:2011 standard, relating to personal protective equipment. In this test, samples of whole footwear are assessed with an instrument able to measure electrical resistance between an external electrode, consisting of a copper sheet, and an internal electrode, composed of stainless steel balls (conforming to the requirements of ISO 3290-1 standard) connected to the measure instrument by a copper wire.
The test piece is filled with these balls to a total mass of 4kg and placed on the copper plate. A voltage of (100±2) V DC between both electrodes is then applied for one minute and electric resistance is calculated. Tests are carried out under wet (20±2º C and 30±5 % of Relative Humidity for seven days) and dry conditions (20±2º C and 85±5 % RH for seven days). ISO 20345:2011 standard establishes three categories of safety footwear according to its electrical resistance, listed in Table 1.
Nowadays, it is possible to achieve ‘C’ category for some kinds of rubber, such as Nitrile Butadiene Rubber (NBR), by just adding fillers like carbon black, a form of paracrystalline carbon which is a very good conductor of electricity.
This type of copolymer, however, has a higher wear than other rubbers, such as Thermoplastic PolyUrethane (TPU), which cannot reach conductive levels with additives. In addition, carbon black has a number of disadvantages, like possible release of fine volatile particles which could generate dust clouds affecting both employees and machinery. What is more, carbon black will transfer black colour to the final mixture, so when used in soles only that colour can be used, which could be an aesthetic problem for a footwear manufacturer. As mentioned before, obtaining conductive soles made of TPU or PolyUrethane (PU) by adding fillers is not possible. Nevertheless, an anti-static category can be reached by mixing commercial additives, such as ortegol or deuteron. Moreover, some recent studies also describe the use of other components, such as graphene, graphite, carbon nanotubes or 1-ethyl-3-methylimidazolium ethyl sulfate, to increase electrical conductivity in PU or TPU rubbers until a certain value, which is impossible to exceed even though the amount of additive is increased. Other solutions have been recently performed to improve the conductivity of PU/TPU soles and achieve the ‘C’ (conductive) category, like using conductive threads which are sewn through the rubber in order to create a low impedance path for electrical charges, regardless of the material used in the sole. This system loses its effectiveness over time, however, because of thread wear, given it is placed in a very abrasive area.
Metallic wires could be used as well, instead of threads, crossing the sole and connecting both sides, but this system would present several disadvantages. Being a rigid material inside a flexing rubber, metallic wires could open cracks in the sole and finally break it. In addition, friction between metal and floor could cause the generation of sparks, which could produce an explosion in the presence of flammable materials.
Overcoming the limits
In this context we propose an innovative system based in conductive gels which grounds by connecting both foot and floor, through canals perforated in the sole.
A gel is a colloidal system – an intimate mixture of two substances: a dispersed phase or colloid uniformly distributed in a finely divided state through the dispersion medium. Here, a solid colloid is dispersed in a liquid phase. A gel has a density similar to liquids but its structure resembles a solid, remaining fluid while stirring and solid when stationary.
Depending on the content of the dispersed phase, gels may become electrically conductive, so they can replace the normal conductors formed by liquid electrolytes and, by easy confinement, leakages can be prevented and they can be used in systems which require higher operation temperatures.
Their mechanical properties make them easily molded, manageable and enforceable, which is an advantage over other conductive materials. Moreover, conductive gels are commonly used in medical ultrasound tests or electrocardiograms, facilitating the conductivity of the skin in this context, avoiding skin burns and increasing the collected signal.
These gels are generally generated in aqueous solvents and can incorporate in their structure small portions of alcohol mixtures, thickeners or ionic salts, such as chlorides, sulphates or metal nitrates. These materials can be produced by using a gelling agent, such as agarose, which acts as a structural gel and different electrolytes can be incorporated. The aqueous mixture should have a non-neutral pH, otherwise it would not present a high conductivity.
As mentioned earlier, in the proposed system shown in Scheme 1, conductive gels are placed into small canals that cross the sole and confined inside of them by using conductive polymeric stoppers with a specific shape to avoid leaks. The gel’s viscosity allows a perfect adaptation to movement and deformation of the sole when it is used.
The material which stoppers are made of has been selected because of its high conductivity and less wear than rubber, so permanent contact between both foot and floor is always guaranteed. Metal tops have been avoided since they could generate sparks due to friction against floor.
The system includes two canals: one in the front part of the sole and one in the heel, to guarantee a permanent contact when walking.
Gel is formed from an ionic salt, a crosslinking agent and a gelling agent dissolved in an organic solvent with a high boiling point. This was changed from normal aqueous solvent-based gels so it could remain unmodified even when subjected to tests in dry atmospheric conditions.
This system provides some additional advantages to those described above. It can be assembled at the end of the production process, so changes are not necessary. When boots are finished, canals are made in the soles with a drill and top stoppers are placed from outside of the boot thanks to their original shape, avoiding its assembly from inside, which is more uncomfortable.
Stoppers of the canals are visible inside the red circles. See Figure 1.
Subsequently, these canals are filled with the conductive gel by injection and bottom stoppers are placed, creating an electrical connection between floor and inside of the boot. Gels with several conductivity values could be used to reach the ‘C’ or ‘A’ (anti-static) category with the same process. This system works regardless of the material used in the sole – PU, TPU, NBR, or Styrene Butadiene Rubber (SBR), for example.
Several prototypes with this system have been assembled (shown in Figure 1) and tested following the section 5.10 of the ISO 20344:2011 standard, relating to electric resistance. All of them obtained the ‘C’ category for safety footwear, achieving an electric resistance of 0.04MΩ, lower than 0.1MΩ, the maximum value set by the ISO standard.
These prototypes have been also tested according to section 5.11 (slip resistance), section 8.3 (outsole abrasion resistance), section 8.4 (flexing resistance of outsole) and section 5.14 (energy absorption of the seat region) of the ISO 20344:2011 standard, in order to guarantee that other properties required by the ISO standards have not been affected by the new system.
The Footwear Technology Centre of La Rioja (CTCR), a non profit organisation, first opened in 2007 with the clear aim of providing technology solutions and meeting the demands of businesses within the footwear sector.
While adapting and evolving in a constantly changing market, the CTCR demonstrates a clear commitment to promoting Research, Development and Innovation as a process for sustainable growth, and enhancing competitiveness among companies within the footwear sector.
During this period, the CTCR has managed to provide services to 97% of footwear companies in the region thanks to a successful collaboration among the different stakeholders in the footwear sector, a significant trajectory recognised and unconditionally supported by regional, state and international institutions.
The CTCR’s main objectives principally focus on:
• Boosting Research, Development and Innovation initiatives and projects
• Increasing the technology level of companies and the collaboration and cooperation among industry agents, regional, state and international organisations and other technology centres
• Providing assistance in quality control, company regulations, environmental care, manufacturing, physical-chemical tests and other areas within the footwear sector
• Promoting training and specialisation in human resources by providing conferences, seminars and courses, among other initiatives
• Executing agreements and/or contracts with sector agents, groups and public or private entities, as well as other institutions involved in the footwear sector
The nerve centre of the CTCR, in Arnedo, Spain, is also home for an important cluster of footwear industries which gives the town the name of ‘Shoe Town’.
In recent years the CTCR has developed several research projects directly related to safety footwear and based on nanotechnology and new materials. One of the success stories is the focus of this article, the development of a new concept of conductive footwear by using a new versatile method for all kinds of shoes, based on canals filled with conductive gels. Other developments include the design of totally adaptable boots for urban motorcyclists, able to dissipate the 90% of the energy of an impact coming from an accident; pellet-proof clothes for small game hunting based on nanofibres; nanometric additives for anti-slip soles; environmentally friendly fire proof rubber; bactericidal fabrics and rubber to avoid infections and bad odours.
H Kim, Y Miura, C W Macosko. Chem. Mater., 2010, 22, 3441. N Yousefi, M M Gudarzi, Q Zheng, S H Aboutalebi, F Sharif, J-K Kim. J Mater Chem., 2012, 22, 12709. A R Shafieizadegan-Esfahani, A A Katbab, A R Pakdaman, P Dehkhoda, M H Shams, A Ghorbani Polym Compos., 2012, 33, 397. A M F Lima, V G de Castro, R S Borges, G G Silva. Polimeros, 2012, 22, 117. F Prissok, F Schaefer, G Egbers, R Krech, C Guenther. US 20100221474 A1. 2011.
Published: 30th May 2014 in Health and Safety International