THE IMPACT OF HEEL MATERIAL, DETERGENT AND ACID ETCH ON THE RISK OF SLIPPING AND SLIDING ON WET FLOORINGS

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1 THE IMPACT OF HEEL MATERIAL, DETERGENT AND ACID ETCH ON THE RISK OF SLIPPING AND SLIDING ON WET FLOORINGS François Quirion, and Patrice Poirier QI Recherche et Développement Technologique inc. 165 Boulevard Lionel-Boulet Varennes, Québec Canada, J3X 1S2 Abstract : Since 1997, the Institut de recherche Robert-Sauvé en santé et en sécurité du travail and our group have addressed the problem of slipping on greasy floors through the optimisation of floor cleaning. But clean floors can also be slippery, in particular when they are. In this paper, the slipperiness was evaluated by measuring the sliding distance of a falling plate on floorings and the aquaplaning threshold was defined as the critical thickness of water that leads to aquaplaning for a given combination of flooring and heel material. Glazed ceramic, vinyl composite and quarry tiles were tested with a slider made of smooth stainless steel and a slider made of smooth Neolite. The impact of adding a detergent to water and the impact of acid etch of the glazed ceramic and quarry tile on the aquaplaning threshold were also investigated. 1. AQUAPLANING ON WET FLOORS It is believed that most slip and fall accidents occur on floors contaminated with grease, oil or aqueous solutions (Leclercq, 1999). Since 1997, our work has concentrated on the elimination of small amounts of fats and oils by the use of an adequate floor cleaner and cleaning method (Quirion, 24a, 24c). Field tests confirmed that the slipperiness of floorings can be reduced significantly with optimised cleaning methods (Massicotte and Quirion, 22, Quirion, 24b). But clean floor can also be slippery, in particular when they become worn and fouled or when they are. The risk will depend on the ability of the flooring and heel to eliminate the liquid layer trapped at the time of heel strike. If the amount of liquid is small, its elimination is rapid and there will be solid contacts between the heel material and the flooring. If the amount of liquid is too high, the heel will slide on the liquid layer with little or no contact with the flooring, a phenomena very similar to aquaplaning. The objective of this investigation is to determine the aquaplaning threshold, i.e. the critical water thickness that a flooring-heel combination can withstand with limited risk of aquaplaning, and to evaluate the impact of heel material, detergent and acid etch on the risk of aquaplaning. In a recent paper (Quirion, 25), the falling plate method was presented as a very simple approach to evaluate the aquaplaning threshold of a smooth stainless steel plate sliding on a flooring. In this paper, the approach is extended to a smooth Neolite plate, a material often used to represent heel or sole materials (Chang, 1998), sliding on floorings having an average roughness in the range,9 m < Ra < 2 m with no definite patterns. The first part compares the aquaplaning threshold of stainless steel and Neolite on different floorings. The second part deals with the impact of detergent on the aquaplaning threshold of the Neolite plate on different floorings. In other words, is a detergent solution more likely to induce aquaplaning than pure water? Finally, the third part compares the risk of aquaplaning of smooth or worn tiles before after they are treated with strong acids such as hydrofluoric acid solutions. 2. THE FALLING PLATE METHOD The falling plate method is based on the hypothesis that the sliding distance of a plane object over a surface can be related to the apparent friction of the object with the surface. This approach was used for many years to determine the initial velocity of a car from the length of the skid marks and the known friction coefficient between the tyres and the pavement (Adamson, 1976). In our case, the sliding distance, d, is used to determine the apparent dynamic friction assuming that the plate always has the same initial horizontal kinetic energy, E K,H, corresponding to a fraction, Φ, of its potential energy, E P. The sliding energy is then dissipated by a combination of forces acting against the forward motion of the plate and these forces are combined into an apparent dynamic friction coefficient, K,app. L EK, H = Φ EP = Φ m g = K, app m g d (1) 2 Φ L K, app = (2) 2 d

2 To keep the method as simple as possible, it is assumed that Φ = Φ and the apparent friction of the flooring is expressed as the ratio of the apparent friction of the plate on the and flooring. R R = = K, app, K, app, F + ( ) exp i Φ = Φ F d d d d a ( t ) (4) t * (3) Experimentally (see Figure 1), the base of a plate of length L is leaning on a fix holder and standing at an angle θ with the flooring. When the plate falls, it acquires kinetic energy which is partly used to slide over a distance, d. The sliding distance is measured as a function of the apparent thickness of water on the flooring, t (m) = 1 6 (V water (m3)) /(A flooring (m2)) to generate R vs t data sets (equation 3). Figure 1: Illustration of the falling plate method. In this study, the drop angle θ is 9 so that the plate initially stands normal to the test sample (A). As it strikes the liquid surface (B), it slides over a given distance and then stops (C). The R vs t data set for a plate sliding on a given flooring is then analysed with an empirical model (equation 4) to determine the critical apparent thickness of water that leads to aquaplaning, t*. The initial friction ratio, R,i, is always unity and the final friction ratio, R,F, is strongly dependent on the apparent friction ratio at high apparent water thickness. This leaves mainly two fitting parameters, a, an exponent that describes the rate of friction drop and t* the critical apparent thickness of water, also referred to as the aquaplaning threshold. Figure 2 shows two typical examples for the sliding distance and friction ratio of a Neolite plate on a smooth vinyl composition tile (VCT) covered with water and a slip resistant composition tile covered with an aqueous solution of sodium laurylsulfate (SLS=.15%). A) 6 B) 1. Sliding distance, d (mm) Friction ratio, R Water thickness, t (m) Water thickness, t (m) Figure 2 : A) Sliding distance and B) friction ratio of the Neolite plate (width=41 mm, Length=64 mm, mass=78 g) on, a smooth vinyl composition tile (VCT, Ra ~ 2.7m) covered with water and, a slip resistant vinyl composition tile (VSR, Ra ~ 17m) covered with a detergent solution (SLS =.15% in water). The solid lines are fits deduced from the empirical approach (equation 4) for VSR ( R,F =.14, a = 1.6, t* = 53 m) and VCT ( R,F =.21, a = 1.3, t* = 18 m).

3 3. FLOORINGS AND SLIDING PLATES TESTED The sliding distance will depend on the texture of the plate and the texture of the floorings. Table 1 summarises the physical characteristics of the floorings tested. In this study, both the plate and the floorings had no definite patterns. Table 1: Physical characteristics of the floorings tested Symbol Description Ra (m) 1 2 K, d (mm) Φ QTM Quarry tile, Monogres QTF QTM worn and fouled QTFHF QTF treated with a HF solution CER Glazed ceramic Cecrisa CERHF CER treated with a HF solution VCT Vinyl tile, Azrock VS34, stripped VCTA VCT + acrylic finish Sureshine VSR Vinyl tile, Azrock SR Determined with a DekTak 33 analyser. Average of 5 runs. 2 Determined with the Neolite plate (78 g) pulled at 25 mm/s. Average of 5 runs. For this investigation, the sliding distance was determined using a 41 mm x 64 mm plate have a sliding surface made of smooth Neolite ( Ra ~,6 m ) and weighing 78 g. The normal load and sliding pressure are thus very small compared to other devices commonly used for the determination of friction (Chang et al, 21). The initial sliding velocity is determined by the length of the plate and the absolute value of parameter Φ with v H 2 = Φ g L. Estimates based on the sliding distance and the dynamic friction coefficient on surfaces suggests values of Φ around.12 ±.2 for our Neolite plate on the floorings tested (without QTFHF and CERHF). This results in an average initial horizontal velocity around v H ~.27 m/s, in the range usually accepted for friction measurements. 4. AQUAPLANING THRESHOLD AND WET FRICTION RESULTS This section discusses the aquaplaning threshold, t*, determined through the empirical analysis of the R vs t data sets obtained with a Neolite plate sliding on different floorings under different conditions. The fitting parameters of the empirical model, combined with the dynamic coefficient of friction on the floorings (refer to Table 1), can be used to evaluate the apparent dynamic coefficient of friction at any liquid thickness. We chose to evaluate the friction at a liquid thickness of 1 m (1 ml/m 2 ), K,app,1, because it corresponds to experimental conditions suggested for the determination of the friction with other devices (Cholet et al, 2). The results are summarised in Table 2 and discussed in the following sub-sections. Table 2: Summary of the aquaplaning threshold, t*, and friction, K,app,1, for the sliding of the Neolite plate on different floorings covered with either water or an aqueous solution of SLS at.15%. Symbol Description Water SLS (.15% t* (m) K,app,1 t* (m) K,app,1 QTM Quarry tile Monogres QTF QTM worn and fouled QTFHF QTF treated with a HF solution CER Glazed ceramic Cecrisa CERHF CER treated with a HF solution VCT Vinyl tile, Azrock VS34, stripped VCTA VCT + acrylic finish Sureshine VSR Vinyl tile, Azrock SR The heel material : Stainless steel vs Neolite Figure 3 compares the aquaplaning threshold, t*, obtained from the empirical analysis of the R vs t data sets for the Neolite plate with those obtained previously (Quirion, 25) with a smooth stainless steel plate having essentially the same dimensions and mass.

4 8 SS Neo 6 t* (m) 4 2 QTF CER VCTA VCT QTM VSR Figure 3: Comparison of the aquaplaning threshold, t*, obtained with a stainless steel plate ( SS, Ra ~.15 m ) and a Neolite plate ( Neo, Ra ~,6 m ) on various floorings covered with water. Refer to Table 1 for the definition of the acronyms. The first observation is that the aquaplaning threshold is essentially the same for the smooth stainless steel and the smooth Neolite plates, suggesting that the nature of the material has little effect on the aquaplaning threshold. This could be the consequence of the small sliding pressure of both plates. Under such conditions, both stainless steel and Neolite experience little deformation so that they both behave as hard materials. In other words, at low sliding pressure, the draping effect of the shoe material over the roughness of the flooring does not contribute much. 4.2 Adding a detergent : Water vs SLS solution It is often believed that a detergent solution is more slippery than water. In a typical floor cleaner formulation (L Homme and Quirion, 1999), the detergent concentration will be around.15%, a concentration high enough to reduce the surface tension of water from ~72 to ~3 mn/m but low enough to have little impact on the viscosity and density. The effect of adding.15% of sodium laurylsulfate (SLS) to water was tested for the sliding of the Neolite plate on the floorings and the results are presented in Figure 4. A) 8 B).25 6 SLS.15% Water.2 SLS.15% Water t* (m) 4 K,app, QTF CER VCTA VCT QTM VSR. QTF CER VCTA VCT QTM VSR Figure 4: Comparison of A) the aquaplaning threshold and B) the apparent dynamic friction coefficient of the Neolite plate sliding on the floorings covered either with water (solid) or an aqueous solution of SLS at.15% (dashed). Refer to table 1 for the definition of the acronyms.

5 As shown in Figure 4a, the impact of the detergent on the aquaplaning threshold depends on the type of flooring tested. It decreases for CER, QTM and VSR, while it increases for VCT, and has little effect on QTF and VCTA. However, the relative changes are always small ( < 3 % ) so that it is not possible at this time to discriminate between a real effect and the experimental uncertainty. Figure 4b shows that in all (except CER where K,app,1 is unchanged) cases, the friction at an apparent liquid thickness of 1 m is significantly lower in the presence of SLS. Once again, more data are needed to ascertain that these changes are significant within experimental uncertainty. Nevertheless, these results suggest that the onset of aquaplaning may increase or decrease slightly upon the addition of SLS to water but that the sliding distance will be higher when sliding on a detergent solution. VSR and QTM, the rougher floorings tested in this investigation, both show a decrease of the sliding friction and of the aquaplaning threshold, suggesting that they would become more slippery in the presence of detergent. 4.3 Acid etch of tiles The average roughness of worn and fouled quarry tiles (QTF) is much lower than that of the original new quarry tile (QTM) and these smoother tiles also have a lower aquaplaning threshold (Figure 4a) and friction (Figure 4b). Based on these observations, a surface treatment that regenerates the roughness of worn tiles, or provides roughness to smooth tiles, should theoretically increase their aquaplaning threshold and friction. Acid etch is one of the treatments that claims to generate surface roughness and to improve floor friction of smooth tiles. New quarry tiles were worn and fouled according to a procedure developed in our laboratory (Massicotte and Quirion, 22 ). These tiles and the glazed ceramic tiles, were treated with a hydrofluoric acid (HF) solution according to the manufacturer s instructions. The results obtained with water and the SLS solution were very similar so that the average values of the aquaplaning threshold and friction are presented in Figure 5. A) t* (m) B).15 No treatment HF etch No treatment HF etch.1 K,app,1.5 QTF CER. QTF CER Figure 5: Impact of an acid etch with hydrofluoric acid (HF etch) of worn quarry tiles (QTF) and glazed ceramic tiles (CER) on A) the aquaplaning threshold, t*, and B) the friction at 1 m of liquid, K,app,1. The average values obtained for the sliding of the Neolite plate on water and on the detergent solution of SLS are presented. The treated tiles (dashed) corresponds to QTFHF and CERHF. The treated tiles (QTFHF and CERHF) had a lower shine (light reflection), a higher friction and almost the same average roughness than the untreated tiles (QTF and CER). However, under conditions, the acid etch treatment resulted in a slight decrease of the aquaplaning threshold. The friction of the treated ceramic remained unchanged while that of the treated worn quarry tile was higher. These preliminary results suggest that the acid etch of glazed ceramic and worn tiles has little effect on their ability to prevent aquaplaning. It was also observed (Massicotte et al, 21) that the surface of HF treated tiles were more sensitive to the cleaning action than untreated tiles so that the impact of such treatments may disappear rapidly with time. For instance, the mass loss of glazed ceramic tiles treated with HF was about 2 times higher after 6 months of daily mopping or machine scrubbing with respect to untreated glazed ceramic tiles. A systematic investigation would be required to conclude on the impact of such treatments.

6 5. CONCLUSIONS The aquaplaning threshold determined from the analysis of the sliding distance of a flat plate can be used to evaluate the ability of a flooring to prevent aquaplaning. The higher the aquaplaning threshold, the better the flooring. Not surprisingly, rougher floorings are better in the prevention of aquaplaning. The lack of sensitivity of the method to the shoe material (Stainless steel or Neolite) can be explained by the low sliding pressure involved in the experiment, thus limited contribution of the draping effect generally attributed to the deformation of the shoe material. The values obtained with water and a solution of sodium laurylsulfate ( SLS =.15% ) suggests that the presence of the detergent has little impact on the threshold of aquaplaning but that it decreases the sliding friction of the flooring, increasing the sliding distance. Finally, the treatment of fouled quarry tiles and glazed ceramic tiles with an aqueous solution of hydrofluoric acid (HF) seems to have little impact on the average roughness and the aquaplaning threshold of the tiles. The falling plate method is simple to use and can be adapted easily for field studies. The aquaplaning threshold and friction extracted from the empirical analysis of the sliding friction show great potential for the investigation of the ability of floorings to prevent slipping and sliding under conditions. 6. ACKNOWLEDGEMENTS This study received financial support from the Institut de recherche Robert-Sauvé en santé et en sécurité du travail (IRSST) under grant No The authors wish to thank Louis Bousquet (IRSST) for his assistance during the course of this study. Plain vinyl composition tiles (VCT) and slip resistant vinyl composition tiles (VSR) were provided by Tarkett. 7. REFERENCES 1. Adamson, A., W., 1976, Physical Chemistry of Surfaces, John Wiley & Sons, 3 rd edition. 2. Chang, W.-R., 1998, The effect of surface roughness on dynamic friction between neolite and quarry tile, Safety Science, 29, Chang, W.-R., Grönqvist, R., Leclercq, S., Myung, R., Makkonen, L., Strandberg, L., Brungraber, R. J., Mattke, U., Thorpe, S. C., 21, The role of friction in the measurement of slipperiness, Part I: Friction mechanisms and definition of test conditions, Ergonomics, 44, Cholet, C., Salimbeni, E., Vetter, F., 2, Glissance des revêtements de sol : Étude expérimentale., Cahier du CSTB, No 3234, Livraison 411, juillet-août Leclercq, S., 1999, The prevention of slipping accidents : a review and discussion of work related to the methodology of measuring slip resistance. Safety Science, 31, L Homme, P., Quirion, F., 1999, Répertoire des nettoyants à planchers: vol 1, IRSST, Institut de recherche Robert Sauvé en santé et en sécurité du travail, Technical report R-23, 11 p. 7. Massicotte, A., Boudrias, S., Quirion, F., 21, Impact de l entretien sur la résistance au glissement des planchers, Institut de recherche Robert Sauvé en santé et en sécurité du travail, Technical report R-283, 66 p. 8. Massicotte, A., Quirion., F., 22, Étude préliminaire de la friction des planchers recouverts de matière grasse, Institut de recherche Robert Sauvé en santé et en sécurité du travail, Technical report R-294, 27 p. 9. Quirion, F., 24a, Optimal cleaning for safer floors, Ergonomics 24, CRC Press, Paul MacCabe Editor, Quirion, F., 24b, Improving slip resistance with optimal floor cleaning : A preliminary field study, Ergonomics 24, CRC Press, Paul MacCabe Editor, Quirion, F., 24c, Floor cleaning as a preventive measure against slip and fall accidents, IRSST, Technical guide RF-366, 12 p. 12. Quirion, F., Poirier, P., 25, Experimental determination of the aquaplaning threshold on floorings, Safety Science, submitted for publication on March 17 th, 25. P.S. : IRSST Technical reports can be downloaded free by typing the report number ( example RF-366 ) on the search engine of the IRSST web site