28   Maggio   2013

Tiles with controlled slip surfaces

Research conducted as part of the InProCer Project (Development Objective 2).

Slip and surface texture: a new approach. Applications to ceramic tiles.

by Barbara Mazzanti and Patrizia Generali - Centro Ceramico Bologna

InProCer –Workshop for Ceramic Product and Process Innovation
Regione Emilia Romagna –Funding programme “From production districts to technological districts”

A number of players are involved in the safety of a floor covering, including designers and manufacturers of the surface covering materials, manufacturers of floor treatment and maintenance materials and safety sector operators.
When designing a floor with a given level of safety for users, it is essential to know the contribution to slip that comes from the surface covering materials, which must be chosen according to the intended use, the risk associated with specific conditions of use and applicable laws/legislation.
Amongst the various floor covering materials available, we will focus here on ceramic tiles, products that are widely used in areas where the above-mentioned safety requirements are particularly critical.
Because of its consequences for user safety, anti-slip resistance is one of the key characteristics prescribed by the harmonised standards for the purposes of CE marking of construction products, in this case ceramic tiles.
Measuring the coefficient of friction and other related variables is currently considered the primary method for determining the slipperiness of a floor covering. Various studies of walking mechanics have identified the factors that influence the friction of a surface and have determined the values of coefficient of friction of a surface that will cause users of that surface to slip.
Unfortunately, however, the problem of determining anti-slip resistance has been addressed independently and without any degree of coordination by different European and non-European countries, resulting in test methods that vary widely and are not directly comparable.
It should also be noted that all the test methods currently available involve testing a given surface subsequent to production. Only limited and largely qualitative information is available to producers wishing to design a ceramic surface with given characteristics of friction.
A new approach to research and development of a surface with anti-slip characteristics has now been developed based on a study of the product’s surface texture.
To meet the many performance requirements and at the same time the ever-changing aesthetic needs of the market, ceramic tiles come with an enormous range of surface texture characteristics that are difficult to group and classify according to a shared nomenclature. Given the difficulty of analysing these surfaces, it has become necessary to introduce categories of surfaces capable of distinguishing macroscopic from microscopic characteristics so as to enable them to be analysed separately. A product classification has therefore been created based on “families of surface texture”, where each family is defined by the combination of a “macro” characteristic (i.e. the surface effect that can be generated by the pressing punch: the “base”) and a “micro” characteristic (created by an application, generally of glaze or sand, on the prepared base: the “surface finish”).
The surface texture study required a method for quantifying the morphological characteristics of the tile surface in relation to the aspect under investigation, namely the product’s anti-slip resistance.
This can be done using a surface texture measuring instrument such as an optical profilometer. This latest-generation non-contact instrument has made it possible to proceed from scanning and analysing a single profile to scanning and analysing an area of the surface. This provides a much larger quantity of information that at the same time is easier to interpret than in the past and allows for a three-dimensional representation of the surfaces, thereby opening up new methods of inspection and analysis. Figure 1 and Figure 2 show examples of 3D images of scanned areas of sample ceramic tiles with different finishes.

One of the best known methods for slip testing is the ramp method with cleated boots as described by standard DIN 51130. So for this preliminary treatment, the slip ratings R adopted in the above-mentioned standard (from rating R9, corresponding to poor anti-slip resistance, to R13, high anti-slip resistance) were chosen to study the relationship between surface morphology and anti-slip resistance of the tested material.
However, an initial comparison between textural characteristics and non-slip behaviour reveal a situation that is still far from clear. Individual classes of surface texture may be associated with significantly different values of slip rating R introduced by DIN 51130. Conversely, a single slip rating R may contain products with different characteristics of both surface finish and base. For example, a high value of anti-slip resistance (R11 or higher) can be obtained from highly textured or rough tiles or from tiles that have a smooth base but a very rough or gritty finish. Likewise, class R9 may contain tiles with a smooth surface and uniform texture over the entire area, or natural stone imitation tiles that by their nature have structural irregularities.
However, by quantifying the morphological characteristics of the surfaces and identifying the significant parameters capable of describing and distinguishing between the various families of textures, it is now possible to verify the existence of a correlation between the surface texture of a product and its anti-slip resistance behaviour evaluated in accordance with the main standardised methods currently in force.
The correlation between texture and slipperiness was analysed separately considering a group of significant parameters for each macro category, “Textured base” and “Smooth base”.
Two parameters were chosen for the first group of samples:
− Rz : the sum of the highest peak of the profile and the depth of the deepest trough of the profile within the base length; this value provides an indication of the amplitude of the texture of the tile base
− Pk : depth of the roughness nucleus; this parameter represents the finest detail of surface texture that determines surface roughness.
For the class made up of samples with a smooth base, the substantial uniformity of the sample surface and more detailed observation of the surface have made it possible to analyse not just amplitude (Rz) but also profile characteristics such as:
− frequency of the peaks, using parameter Rsm: average amplitude of the profile elements within the base length;
− slope of the peaks, using parameter Rdq: mean square variance of the slope of the profile;
− shape and angularity of the profile, with the parameters Rsk: asymmetry of the profile, and Rku: kurtosis of the profile.
By analysing and comparing these surface texture parameters and assigning the specimen an R slip rating in accordance with DIN 51130, it is possible to formulate a classification model that will be capable of predicting slip characteristics based on the results of the surface texture measurements.

These models can provide effective support for:
− evaluating the contributions to slip made by floor covering materials,
− predicting surface anti-slip performance in specific target environments at the contractual stage,
− designing surfaces according to intended use,
− guaranteeing that the requirements of the various product batches are maintained during process control,
− assessing the long-term performance of floors in service.



References
- ISO 3274:1996 “Geometrical Product Specifications (GPS) -- Surface texture: Profile method -- Nominal characteristics of contact (stylus) instruments”
- EN ISO 4287:2009    GPS – Surface Texture: Profile Method – Terms, definitions and surface texture parameters
- UNI EN 623-4 2004: “Advanced Technical Ceramics - Monolithic Ceramics - General And Textural Properties  - Part 4: Determination Of Surface Roughness”
- W.R Chang, S. Matz, R. Gronqvist, M. Hirvonen “Linear regression model of floor surface parameter on friction between neolite and quarry tiles”, Applied Ergonomics (2010)
- “Development of Slip Resistant Standard Surfaces for a Safer Environment- SlipSTD” European Collective Research Project, VI PQ
- “Understanding and preventing falls”, R. Haslam, D. Stubbs, CRC Taylor&Francis
- DIN 51130 (06/2004): “Testing of floor coverings - Determination of the anti-slip property - Workrooms and fields of activities with slip danger, walking method - Ramp test”. Determination of the total angle of acceptance (average value)
- DIN 51097 (92): “Testing of floor coverings; determination of the anti-slip properties; wet-loaded barefoot areas; walking method; ramp test”. Determination of average slip angle.
- Test in accordance with BCRA (D.M. 236/89): determination of average coefficient of friction () in conditions of leather on a dry surface and rubber on a wet surface.
- ASTM C 1028-2007 E): “Standard Test Method for Determining the Static Coefficient of Friction of Ceramic Tile and Other Like Surfaces by the Horizontal Dynamometer Pull-Meter Method”. Determination of average coefficient of friction () in conditions of neolite on a dry surface - neolite on a wet surface.
- BS 7976-2:2002: “Pendulum Testers - Part 1: Specification”. PTV in dry and wet conditions.