TRIBOELECTRIFICATION OF SHOE COVER AND FLOOR IN HOSPITALS

TRIBOELECTRIFICATION OF SHOE COVER AND FLOOR IN HOSPITALS

El-Sherbiny Y. M.¹, Ali A. S.² and Ali W. Y.³

¹Dept. of Civil and Architectural Engineering, National Research Center, Dokki, Giza, EGYPT,

 ²Petrojet Company, Cairo, EGYPT,

³Faculty of Engineering, Minia University, P. N. 61111, El-Minia, EGYPT.

ABSTRACT

Electric static charges generated from friction of engineering materials have a negative effect on their applications. The increased use of polymeric materials raised the importance of studying that effect. Electric static charges building up on human skin and or clothes in direct contact with human body are very harmful and can create serious health problems. The present study investigates the electric static charge generated from the dry and water wet sliding of shoe cover against floor for people who are working in hospitals.

It was found that, at dry testing condition, friction coefficient displayed by sliding of polyethylene shoe cover against epoxy floor increased with increasing normal load. It is necessary that friction coefficient should have reasonable values so that the foot slip should be avoided to prevent accidents, where the values satisfied the European standards for safe use at dry sliding condition. Electric static charge, of relatively higher values, generated on the shoe cover from contact and separation against dry floor was similar to that generated by sliding. As the load increased the charge decreased. This observation can confirm the necessity to develop new materials to be applied as shoe cover of low electric static charge. At water wet testing condition, friction coefficient displayed relatively lower values. Voltage generated on the shoe cover from its contact and separation as well sliding against water wet epoxy floor showed lower values than that observed for dry condition due to the ability of water to conduct the charge from the contact surfaces.

KEYWORDS

Electric static charge, contact and separation, dry, water wet, sliding, shoe cover, floor, hospitals.

Introduction

Triboelectric charging is the transfer of electrons which occurs when two materials are in contact and are then separated.  One material gains an excess of negative ions and the other an excess of positive ions.  The charge generated can be more than 25,000 volts. It is well known that when two different materials contact each other, they may get charged. This tribocharging phenomenon is also known as triboelectrification when materials rub against each other, [1 - 3]. The mechanism of charge transfer in tribocharging can be explained by three mechanisms: electron transfer, ion transfer, and material transfer, [4 – 6]. The metal to metal contact electrification successfully explained by electron transfer mechanism. When two different materials come to contact, electrons transfer happens until their Fermi level equals. Difference in work functions between them is the main driving force, [7]. As for insulators, the electron transfers only happen on the surfaces of insulators, where electrons move from the filled surface of one insulator to the empty surface of the other insulator, [8 – 10]. Few researchers have drawn up triboelectric series to predict the polarity of the charge that is transferred from one surface to another, [11]. When two kinds of materials contact each other, the upper one in the triboelectric series will get positively charged and the other one will be negatively charged. It is becoming increasingly evident that more than one of these mechanisms may occur simultaneously, [12].

The electrostatic charging of unstrained and strained latex rubber sheets contacted with a series of materials such as polytetrafluoroethylene (PTFE), polyurethane (PU) and stainless steel (SS) was studied, [13]. For SS, strain reduces the frequency of electrical discharges occurring. It was found that material strain can strongly influence triboelectric charging. Besides, straining a material can produce ions, electrons, and radicals that can react to form charged species. Silicon carbide is electrically semiconducting. The friction and wear behaviour of silicon carbide based materials may be influenced by electric potentials applied to the tribological system, [14 – 17]. Also, it was found that the surface state of SiC ceramics can be influenced by electric potentials.

Triboelectrification and triboluminescence were measured from the sliding or rolling frictional contacts between polymers of PA66, POM, ABS, PET, PP, PVC, PE, and PTFE in various humidity conditions, [18]. Triboluminescence intensity was higher in sliding friction. The saturation charges of all the sliding couples showed their maxima at the humidity from 10 to 30%. It was found that the humidity enhanced charge transfer which resulted in the increase or decrease of electrification, [19]. The contact and separation process leads to the charge transfer between dissimilar materials. When charges are accumulated, they are measured as triboelectrification.

Charge and discharge associated with the rubbing between shoes and carpet are less experienced in summer rather than in winter. It indicates that the charge is suppressed in higher humidity. Experimental data have exemplified this tendency [20 - 22]. However, other data show that water molecules on the surfaces convey charges in the form of ions to enhance charge separation between two surfaces [23, 24]. These contradictory results require precise measurement of the effect of humidity on charge generation.

It was found that voltage generated by the contact and separation of the tested upholstery materials of car seat covers against the materials of clothes showed great variance according to the type of the materials, [25]. The materials tested showed different trend with increasing load. The contact and separation of the tested against polyamide textiles generated negative voltage, where voltage increased down to minimum then decreased with increasing load. The behaviour can be interpreted on the fact that as the load increased the two rubbed surfaces, charged by free electrons, easily exchanged the electrons of dissimilar charges where the resultant became relatively lower voltage. High density polyethylene displayed relatively lower voltage than cotton and polyamide textiles, while polypropylene textiles displayed relatively higher voltage than that shown for high density polyethylene. The variance of the voltage with load was much pronounced. Remarkable voltage increase was observed for contacting synthetic rubber. This observation can limit the application of synthetic rubber in tailoring clothes. Materials of high static electricity can be avoided and new materials of low static electricity can be recommended.

The wide use of polymer fibers in textiles necessitates to study their electrification when they rubbing other surfaces. The electric static charge generated from the friction of different polymeric textiles sliding against cotton textiles, which used as a reference material, was discussed, [26]. Experiments were carried out to measure the electric static charge generated from the friction of different polymeric textiles sliding against cotton under varying sliding distance and velocity as well the load.  It was found that increase of cotton content decreased the generated voltage. Generally, increasing velocity increased the voltage. The voltage increase with increasing velocity may be attributed to the increase of the mobility of the free electrons to one of the rubbed surfaces. The fineness of the fibers much influences the movement of the free electrons. The electrostatic charge generated from the friction of polytetrafluoroethylene (PTFE) textiles was tested to propose developed textile materials with low or neutral electrostatic charge which can be used for industrial application especially as textile materials, [27]. Research on electrostatic discharge (ESD) ignition hazards of textiles is important for the safety of astronauts. The likelihood of ESD ignitions depends on the environment and different models used to simulate ESD events, [28]. Materials can be assessed for risks from static electricity by measurement of charge decay and by measurement of capacitance loading, [29].

Less attention was considered for the triboelectrification of the textiles. Friction coefficient and electrostatic charge generated from the friction of hair and head scarf of different textiles materials were measured, [30]. Test specimens of head scarf of common textile fibres such as cotton, nylon and polyester were tested by sliding under different loads against African and Asian hair. Electric static charge measured in voltage represented relatively lower values. This behaviour may be attributed to the ranking of the rubbing materials in the triboelectric series where the gap between human hair and nylon is smaller than the gap between hair and cotton as well as hair and polyester.

Little attention has been devoted so far to the electrostatic properties of hair although these properties are very sensitive to the friction between hair and head scarf textiles. Hair has a tendency to develop static charge when rubbed with dissimilar materials like human skin, plastic and textiles. Human hair is a good insulator with an extremely high electrical resistance. Due to this high resistance, charge on hair is not easily dissipated, especially in dry environments. Many macroscale studies have looked at the static charging of human hair, [31, 32]. Most of these studies include rubbing hair bundles with various materials like plastic combs, teflon, latex balloons, nylon, and metals like gold, stainless steel and aluminum. Hair in these cases is charged by a macroscale triboelectric interaction between the surface and the rubbing element.

The present study investigate the friction coefficient and electric static charge generated from the contact and separation as well as sliding of shoe cover against epoxy floor for people who are working in hospitals at dry and water wet working conditions.

EXPERIMENTAL

The present work investigates the measurement of electric static charge generated by the contact and separation as well as dry and water wet sliding of polyethylene shoe cover against floor (epoxy). The experiments simulate the walking of people working in hospitals. The electrostatic fields (voltage) measuring device (Ultra Stable Surface DC Voltmeter) was used to measure the electrostatic charge (electrostatic field) for test specimens, Fig. 1. It measures down to 1/10 volt on a surface, and up to 20 000 volts (20 kV). Readings are normally done with the sensor 25 mm apart from the surface being tested. The tested textiles were adhered to the wooden block of 50 × 50 mm². Tests were carried out at room temperature under varying normal loads. The epoxy floor tiles of 400 × 200 × 8 mm³ were placed in a base supported by two load cells, the first can measure the horizontal force (friction force) and the second can measure the vertical force (normal load), Fig. 2. The shoe cover was pressed to epoxy tile by foot. Friction coefficient was determined by the ratio between the friction force and the normal load.

Fig. 1 Electrostatic field measuring device.

During test running, horizontal and vertical load cells connected to two digital monitors read normal and friction load respectively. Friction coefficient is the ratio between friction load and normal load. Each run was replicated five times, and the mean value of the friction coefficient was considered. Friction tests were carried out at different forces (loads) and ranged from 0 - 800 N. The tested shoe cover and floor are shown in Fig. 3.

Fig. 2 Arrangement of the test rig.

Fig. 3 Shoe cover against floor.

RESULTS AND DISCUSSION

Friction coefficient and electric static charge measured in volts and generated at the dry contact and separation as well as sliding of shoe cover against floor, are shown in Figs. 4 – 8. Friction coefficient displayed by sliding of polyethylene shoe cover against epoxy floor at dry condition, Fig. 4, increased with increasing normal load. It is necessary that friction coefficient should have reasonable values so that the foot slip should be avoided to prevent slip accidents. The lowest and highest friction values were 0.37 and 0.45 at 250 and 720 N load respectively. The friction values at light loads satisfied the European standards for safe use at dry sliding condition, where the safety increased with increasing applied load.

Fig. 4 Friction coefficient displayed by sliding of shoe cover against dry floor.

Voltage generated at the shoe cover from contact and separation with dry floor is shown in Fig. 5. The values were ranged between -2600 and -1000 volts. As the load increased the voltage decreased. This behaviour might be attributed to the increase of the contact area with increasing load. The voltage generated at  the surface of epoxy floor, Fig. 6, showed positive values ranging from 120 to 200 volts. As the load increased, voltage slightly decreased due to the increased interference between the shoe and floor, where the charge transfer became easier. Due to the nature of the voltage, the scatter in the values measured during experiments was relatively high.

Dry sliding of shoe cover against floor generated much higher voltage measured on the shoe, Fig. 8. The highest voltage reached -2400 volts, while the lowest was -1050 volts. This observation can confirm the necessity to develop new materials to be applied as shoe cover of low electric static charge. As the load increased, the negative voltage remarkably increased. Voltage generated on the floor surface recorded very low voltage value of 220 volts at 720 N, Fig. 9. As the load increased the positive voltage increased. It seems that friction coefficient critically depended on the value of the generated voltage. This behaviour can be explained on the basis that, generation of equal voltages on the sliding surfaces of different signs would increase the attractive force between the two surfaces and consequently the adhesion increased leading to friction increase.

Fig. 5 Voltage of shoe cover generated from its contact and separation against dry floor.

Fig. 6 Voltage of floor generated from its contact and separation against dry shoe cover.

It was observed that, at sliding, the charge value was higher than that recorded for contact and and separation. When two materials contact each other, the upper one in the triboelectric series will get positively charged and the other one will be negatively charged. As the difference in the rank of the two materials increases the generated voltage increases. It is known that shoe cover (polyethylene) is ranked as negative charged material, while epoxy is above propethylene so it is positive charged, Fig. 7. It is therefore necessary to select the materials based on their triboelectric charging.

Fig. 7 Generation of voltage on the sliding surfaces.

Fig. 8 Voltage of shoe cover generated from its sliding against dry floor.

Fig. 9 Voltage of floor generated from its sliding against dry shoe cover.

The results of experiments measuring friction coefficient and voltage at water wet contact are illustrated in Figs. 10 – 14.Friction coefficient is considered as the main factor in evaluation the safety of walking against floor materials. The measure of the safety is the friction coefficient displayed between shoe cover and floor. As the friction coefficient increased, the safety of walking increased. Friction coefficient displayed by sliding of shoe cover against epoxy floor at water wet condition is shown in Fig. 10, where friction coefficient slightly decreased with increasing the load. The lowest friction value was 0.23, while the maximum value was 0.34. The values of friction were lower than that observed for dry contact.

Fig. 10 Friction coefficient displayed by sliding of shoe cover against water wet floor.

Voltage generated on the shoe cover from its contact and separation against water wet epoxy floor is shown in Fig. 11. Voltage values were -59 and -13 volts at 360 and 580 N load respectively. This observation confirmed that, the intensity of voltage depended on the load. The contact and separation, Fig. 12, displayed positive voltage for the floor reached 96 volts. The values of voltage were approaching that shown for the opposite side (shoe). It seems that, the low values of charge were from the ability of water to conduct the charge from the contact surfaces. It is recommended to measure the charge simultaneously on the two opposing surfaces.

Fig. 11 Voltage of shoe cover generated from its contact and separation against water wet floor.

Fig. 12 Voltage of floor generated from its contact and separation against water wet shoe cover.

Fig. 13 Voltage of shoe cover generated from its sliding against water wet floor.

Fig. 14 Voltage of floor generated from its sliding against water wet shoe cover.

Voltage generated on shoe cover from its sliding against floor is shown in Fig. 13. The maximum voltage value was -105 volts at 620 N. The voltage generated on the floor from its sliding against cover is shown in Fig. 14. The voltage observed was in positive sign with relatively low values.

CONCLUSIONS

1. At dry testing condition, friction coefficient displayed by sliding of polyethylene shoe cover against epoxy floor increased with increasing normal load. It is necessary that friction coefficient should have reasonable values so that the foot slip could be avoided to prevent accidents. The friction values at light loads satisfied the European standards for safe use at dry sliding condition, where the safety increased with increasing applied load. Voltage generated on the shoe cover from contact and separation against dry floor was ranged between -2600 and -1000 volts distributed on the shoe cover. As the load increased the charge decreased. Sliding of shoe cover against floor generated values of voltage approaching that observed for contact and separation. This observation can confirm the necessity to develop new materials to be applied as shoe cover of low electric static charge.

2. At water wet testing condition, friction coefficient displayed by sliding of shoe cover against epoxy floor slightly decreased with increasing the load. The lowest friction value was 0.23, while the maximum value was 0.34. The values of friction were lower than that observed for dry contact. Voltage generated on the shoe cover from its contact and separation as well sliding against water wet epoxy floor showed lower values than that observed for dry condition due to the ability of water to conduct the charge from the contact surfaces.

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