Scratch-proofing - European Coatings Scratch-proofing Towards a standard scratch resistance test for

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Transcript of Scratch-proofing - European Coatings Scratch-proofing Towards a standard scratch resistance test for

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    European Coatings Journal

    01-02/2005

    36

    Scratch-proofing

    Towards a standard scratch resistance test for parquet coatings. Rico Emmler, Rolf Nothhelfer-Richter. Scratch resistance is one of the key performance parameters on wood coatings systems, particularly in parquet coatings. However, as yet no convincing standard method exists that at the same time allows to differentiate between different coatings systems, correlates well with scratch resistance performance in real life, is easily applicable and not too costly. The study presented here aims at developing such a test. In order to reduce emissions from solvents, many environmentally friendly surface coatings on wooden flooring have been established. On multi-layer parquet, systems of 100% UV lacquers are dominant. Another common approach is to apply oiled or waxed surfaces. With sealing systems being applied on site, water-based lacquer systems have come to be widely adopted. These systems, however, require more care during application, and the place of application has a decisive influence on the selection of the appropriate system in order to prevent subsequent complaints. The useability of wooden floor lacquers is determined by complex parameters and should not be judged by single properties, such as abrasion resistance, alone. Thus, further to basic properties, such as adhesive strength and surface appearance, wooden floor coatings must also be resistant towards staining caused by domestic chemicals, rock particles and dirt. They must also show sufficient elasticity to the highest degree possible to follow dimensional changes of the hygroscopic wood substrates, or deformations caused by impact, e.g. by high-heeled shoes. In this context, the Institute for Wood Research Dresden (ihd) has developed a requirement profile for wooden floor coatings which aims to provide a complex assessment of hardness and elasticity properties [1]. The use of environmentally friendly systems, particularly of water-based coatings as well as oils and waxes, has been delayed partly due to negative experiences of users, because considerably differing qualities are available on the market. For instance, when products were chosen that are not compatible with the place of application, this often resulted in negative experiences, e.g. with respect to scratch resistance performance. Partial aspects of the usability evaluation, namely the scratch resistance, have been addressed in a joint project of the ihd and the Research Institute for Pigments and Coatings Stuttgart (FPL), with an enphasis on the methodology of testing the scratch resistance. This paper reports intermediate results of these investigations, focusing on multiple scratching test methods.

    Single scratch tests - too unpredictive or too costly Generally, test methods for scratch resistance of coatings can be divided into single and multiple scratch tests. The most frequently applied single scratch test of floor coatings is DIN 68861 p.4/EN 438 p.2, producing scratches by applying a constant force. However, although an orientating statement for the scratch resistance is possible with this test, the prognoses with the EN 438 parameters only rarely coincide with assessments of floor surfaces in practical use [2]. Scratch procedures using an increasing nominal force (nano-scratch procedures) permit plastic deformation processes and brittle fracture events to be registered more

    simply and can be correlated with viscoelastic phenomena [3]. In this procedure, however, the extensive effort in constructing application apparatuses and the requirement of a surface to be even on a micrometer scale are regarded to be disadvantageous.

    Comparing multiple scratch tests For multiple scratch tests the Martindale procedure, the linear abrasion procedure [4] and the Rota Hub Scratch Tester [3] can be considered the most important test methods. Felts, Scotch Brite pads, special papers and sanding papers are used as scratching media in these procedures. The principles of the methods are explained in the following:

    The Martindale Procedure Various defined abrasive materials are applied on an area of 80 mm diameter. The applied force is variable. The scratch material is pressed evenly onto the substrate and a so-called Lissajous pattern combining linear, circular and elliptical motions in all directions is generated. The test device has its origin in the textile industry (Figure 1).

    Linear Scratch Procedure In this scrub procedure, which is standardised for interior wall paint according to DIN EN ISO 11998, a scratch body is moved linearly in lift and push intervals across the coated surface. Variable scratch materials that have an impact on the surface area can be applied with defined weights.

    Rota Hub Scratch Test In this procedure, a rotating disk with variable scratch materials attached to it is brought onto the surface, applying defined weights. The test surface is placed on a table, across which the disk can be moved in the x and y axes [3]. For the evaluation gloss measuring and optical assessment are used. Additionally, simple test procedures accompanying product development in the lacquer industry are in use, e.g. 800 g hammers covered in Scotch Brite pads and moved linearly fifty times across a surface area. In this case, too, the loss in gloss is used for evaluation.

    Coatings and scrub materials The types of coatings used in this study so far, and some of their key performance data, are listed in Table 1. Table 2 provides details of the scrub materials used. The following procedure was applied in the research project, aiming at an improved standard for the evaluation of scratch resistance of parquet coatings: 1. Analysis of the surface structure of parquet surfaces used in practice 2. Selection of representative coating systems and surfaces 3. Laying of various types of floor covering and their systematic monitoring 4. Analysis of scrub materials 5. Development of test methods for determining the scratch resistance 6. Testing of various evaluation methods 7. Draft of coordinated test standards

    Testing methods and parameters A linear scrub tester, a Martindale device and a Rota Hub tester were used as test devices. Apart from the normal requirements on test procedures, such as differentiability, reproducibility and repeatability, the following fringe

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    European Coatings Journal

    01-02/2005

    36

    conditions were agreed upon with the project-accompanying committee: - Practice-relevant scratch patterns shall be produced - Scratches deeper than 5 µm had to be simulated (up to this depth, preserving agents can compensate scratch traces) The following influential parameters had to be investigated as varying test parameters: - type of scrub agent - applied pressure - velocity of movement - number of cycles required The determination of gloss and mass, as well as roughness measurements and optical assessments were used to judge the applicability of the evaluation procedures.

    Surface structures on used parquet coatings An example of a parquet surface that was in use for five years is shown in Figure 2. The scratch pattern is completely haphazard and inhomogeneous. An underlying system cannot be recognised. Size and shape of scratches vary to a high degree. Scratch traces are partly linear, but also irregularly curved. There are frequent changes in direction and crossovers. Ragged, splintered marginal areas of scratches are visible, as are clear-cut and sharp-edged fringes.

    Martindale scrub agents differ in effectiveness When applying the Martindale procedure, the following conclusions were reached regarding the application of the scrub agent (Table 2). - The fabric of the abrasive agent produces no utilisable scratches but only features of wear. - The steel chips could not be utilised, since they rendered impossible to produce a homogeneously sanded surface. - Of the four scrub fleeces investigated only fleeces SB 7440 (medium) and SB 7447 (very fine) proved to be suitable to produce scratch traces of the desired depth and intensity. Scrub agent SB 7448 (ultra fine) did not result in any visual change of the test surface. Product SB 7446 (medium) attacked the surfaces too much.

    Applied force needs to be calibrated to the scrub material In its standard version, the abrasion plate exerts a force of 4 N, distributed on the scrub material dispersed across 80 mm in diameter, onto the sample. The force was varied up to 8 N by adding mass pieces. With the very fine scrub material SB 7447, at 6 N load, the effect of making scratches very clearly visible occurred and differentiability improved. Therefore, this level of load exertion with this scrub material was to be selected. With somewhat rougher scrub material an increase in force to 6 N resulted in undesired abrasive effects. For that reason a force of 4 N was maintained.

    Scrub velocity does not matter - the number of cycles does Velocity was varied within the range of device parameters (half, normal and double velocity factor). Significant dependence of changes in gloss and scratch patterns on velocity could not be detected. The number of cycles applied varied between 16 (= 1 Lissajous movement (LM)) up to 1120 (= 70 LM). With the selected scrub materials SB 7440 and SB 7447, best differentiability was established in the range of 1 LM to 5 LM (Figure 3). As a preliminary result, the number of cycles has therefore been determinded at 5 LM.

    Gloss measurement distinguishes most clearly The registration of change in mass proved to be of too little confidence, also in terms of