José María Baydal Bertomeu; Néstor Arroyo Gómez; Sergio Puircerver Palau; Clara Solves Camallonga; Juan Carlos González García; Sara Gil Mora

Instituto de Biomecánica (IBV)
Universitat Politècnica de València
Camino de Vera s/n. Edificio 9C
46022 Valencia, Spain

The science of nanomaterials has opened the door to new solutions, given that nanostructured surfaces have been shown to have specific properties as far as wear, lubrication and friction are concerned. These surfaces can be modulated depending on the nanoparticles that are used in their production, and which may include, for example, hydrophobic or lipophobic characteristics. The NANOFRICTION project combines the use of nanomaterials with biomechanically designed soles in order to improve the friction of the footwear and to reduce the risk of falls in industrial environments."

INTRODUCTION

 Statistically, falls from slipping are the most common accidents in work environments and, accordingly, are responsible for a high percentage of injuries. The risk of falls from slipping involves several factors, the most important of which is the friction between the floor and the sole of the shoe.

 Most accidents occur when there is an unexpected change in surface friction conditions (for example, when moving from a dry surface to a wet area) and the user is unable to adapt his or her pattern of activity to the new conditions. In industrial environments, this change in the friction coefficient (defined as the ratio between the horizontal and the vertical force) is normally due to the presence of pollutants such as water, oils, soaps, etc.

 In this sense, conventional materials used in the manufacture of professional footwear (PUs, TPUs or PVCs) and in the designs of soles, present problems when it comes to complying with the quality standards for anti-slip safety, as they are unable to reduce the risk of slips and falls at work. Advances in the science of nanomaterials has opened the door to new solutions, given that nanostructured surfaces have been shown to have very specific triblological properties as far as wear, lubrication and friction are concerned. These surfaces can be modified depending on the nanoparticles that are used in their production, which may include, for example, water-repellent (hydrophobic) or grease and oil-repellent (lipophobic) characteristics. The NANOFRICTION project combines the use of these nanomaterials with biomechanical criteria in order to substantially improve the friction properties of soles.

 The NANOFRICTION project has been coordinated by AVANZARE, a company based in La Rioja that specializes in nanotechnology. Other participants include ANALCO, a company based in Elche that manufactures components for footwear and BASEPRO, the Italian manufacturer of safety footwear. The Instituto de Biomecánica (IBV) has also collaborated in the project, bringing to the table its expertise in such areas as biomechanics, friction analysis and the design of non-slip soles.

DEVELOPMENT AND RESULTS

IBV has participated actively in all six phases of the project. The development and the main results obtained in each one of the phases are described below.

1.     Characterization of the industrial working environment.       
IBV has participated in carrying out research into a wide sample of companies that represent the major industrial sectors: air transportation, packaging, automobiles, distribution and logistics, chemicals, catering, construction, food and ceramics. We studied this sample of firms in order to determine the most common combinations of pollutants and soil types in industrial environments, that together increase the risk of causing falls from slipping. In this sense, the results indicate that the combinations that imply greater risk for workers in terms of danger and frequency, are: terrazzo-water and steel-glycerin. Figure 1 shows a number of pollutants that are frequently found in industrial environments.

In addition, we also analyzed the activities that cause the greatest number of falls from slipping. These were: walking, running, 90º and 180º turns, walking down stairs and walking down slopes.

Figure 1. Contaminantes detectados en ambiente industrial.Pollutants detected in industrial environments.

2.     Biomechanical characterization of the process of slipping.  
Friction between the footwear and the floor is essential if there is to be the necessary mechanical momentum to produce the displacement of the body mass. Thus, if the Load Factor (LF), which represents the minimum force of friction necessary to perform a specific activity, is greater than the friction provided by the floor-footwear combination, slipping may occur which may in turn precipitate a fall. Table 1 shows the results obtained on the minimum LF values so that falls do not occur depending on the type of activity undertaken.

On the other hand, the areas on which forces are transferred between the person and the floor occurs are also different depending on the activity carried out. In order to optimize the location of the nanoparticles, it is very important to know how the areas of the sole that remain in contact with the floor are distributed.

IBV has participated in identifying these areas of support in those activities that involve the greatest risk of falls in industrial environments. To do so, we studied 10 male subjects between 20 and 40 years of age. The instrumentation used was the Biofoot/IBV templates system and the Footscan® System pressure platform. In order to superimpose the different footsteps of a given subject with the footsteps of the other subjects, we designed a mathematical algorithm based on Principal Components Analysis. Figure 2 shows the areas on which there is greater support, depending on the activity analyzed.

Figure 2. Zonas de contacto entre calzado y el suelo en función del tipo de actividadContact areas between the footwear and the floor depending on the type of activity

3. Selection and mechanical characterization of nanocomposites. 
In this phase we carried out a selection of the most appropriate nanoparticles in terms of their integration on a chemical level with the materials used for soles that we selected for the project (rubber and TPU). We also selected the nanoparticles according to their capacity to repel water or oils. Once we had manufactured the nanocomposites with the nanoparticles we had selected (Figure 3), we carried out research to determine their friction coefficient. To do so, we used a friction test machine developed by IBV that makes it possible to simulate the intervening forces and the position adopted by the foot in different stages of the gait cycle (Figure 4). In this way it was possible to determine the friction coefficient of the nanocomposites.

Figure 3. Compounds with rubber-based nanoparticles.

Figure 4. Friction testing machine.

Following on from the friction study, we selected the rubber and TPU nanocomposites that demonstrated the best friction characteristics in the following conditions: dry terrazzo, terrazzo with water and steel with glycerin. Table 2 shows the friction results of the rubber nanocomposites. Here we can appreciate improvements in friction of up to 626% with respect to materials without nanoparticles in the “Steel with Glycerin” condition. In the “Wet Terrazzo with Water” condition, the results showed friction improvements of 193%. In the “Dry Terrazzo” condition, the results are similar to the reference material without nanoparticles.

Table 2. Nanocomposites developed on a rubber base improve friction behavior.  

4.     Development of criteria for the design of soles.        
During this phase of the project, IBV conducted a comparative study of the level of friction of different sole designs. This study allowed us to obtain design recommendations and criteria for anti-slip footwear soles. These criteria focus on:

♦ General recommendations for soles.

♦ Recommendations on how to improve impact cushioning.

♦ Recommendations on how to improve flexibility.

♦ Recommendations on how to improve the friction between the floor and the sole.

5.       Geometric design of an anti-slip sole. 

Based on the design criteria generated in the previous phase and taking into account the choice of nanoparticles for each material (rubber and TPU), we prepared a geometric design for an anti-slip sole (see figure 5). This design is valid both for rubber soles and for TPU soles. 

Figure 5. Anti-slip sole design.

CONCLUSIONS

 The development of the NANOFRICTION project has made it possible to obtain combinations of nanocomposites on a rubber base and on a TPU base that can significantly improve the friction of current conventional materials, especially in the presence of pollutants such as water and oil. This project has also made it possible to increase our awareness of which activities carried out in industrial environments entail a greatest risk of falling, and of the biomechanical interaction between the worker, his or her footwear, the sole and the floor surface. On the basis of the information obtained as to the areas of the sole that are subject to greater friction requirements for various activities, we have put forward a new design for a sole that incorporates those nanoparticles that have been shown to possess improved anti-slip behavior and that presents great potential for reducing the risk of falls in the presence of pollutants in industrial environments.

ACKNOWLEDGEMENTS

This project has been funded by CDTI within the action framework of the EUROSTARS 

International Intercompany Subprogram. It has been supported by the Ministry of Economy and Competitiveness (MINECO) with Community co-financing. (Reference E7926).