OPERATIONAL PROPERTIES OF ANTI-GRAFFITI COATING SYSTEMS FOR ROLLING STOCK

The article presents the comparative tests results of selected properties of anti-graffiti paint system for rolling stock industry. The measurements included an adhesion test, free surface energy, surface geometric structure, corrosion resistance and flame resistance test. The system consists of high solid corrosion protection primer, putty, filler, basecoat and anti-graffiti clearcoat.


INTRODUCTION
Paint systems for rolling stock must fulfill mechanical and qualitative properties to protective and decorative properties maintenance longer on the vehicle.These requirements include: adhesion, resistance to weather conditions (humidity, UV, corrosion) as well as hardness and specialized properties such as anti-graffiti.Coating systems intended for rolling stock in addition to the above-mentioned requirements, as well as ease of application and operation, must also have adequate fire performance [12].
The study of wettability and free surface energy is of interest to many disciplines [14]: physics, chemistry, materials engineering and biotechnology.Wettability of materials used in the industry by various liquids is of great practical importance in industrial processes such as bonding, sealing, painting, printing and application of various protective coatings.The paper examines the anti-adhesion properties of subsequent undesirable coatings onto the antigraffiti coating, determining the wettability of these varnishes.
At the next stage of the study, measurements of the surface geometric structure of the anti-graffiti coating system were performed.The surface geometric structure (SGS) substantially influences many processes that occur in the outer layer.A lot of publications deal with the measurement methods and the assessment of surface roughness and waviness [1,2,7,11,15].Analysis of properties of anti-graffiti coating systems requires many methods [8,9].
Another important issue is to protect paintings before end users and to ensure proper operation of the vehicle.Therefore, owner of the rolling stock should follow the manufacturer's instructions, how to handle with them and what may prolong the life of the product for next years.Application of various types of paints (graffiti) on the vehicle may lead to shortening durability of the coating surface.Spray paints should be treated as dangerous substances for coatings because they contain various solvents and other substances that might soften or migrate into protective coating and cause delamination of the coating system.Graffiti paints that are difficult to remove require the use of more aggres-  sive materials, which increase the possibility of mechanical or chemical damage of the coating system and consequently reduce the thickness of the protective coating or remove it completely.

OPERATIONAL PROPERTIES OF ANTI-GRAFFITI COATING SYSTEMS FOR ROLLING STOCK
In addition, aggressive chemicals removers are dangerous to the users and environment.The article presents the comparative tests results of selected properties of anti-graffiti paint system for rolling stock industry.

MATERIALS AND TREATMENT PARAMETERS
Coatings was applied with a SATA spray gun on S235 carbon steel, before the application the surface of steel was polished with 80-grit sandpaper.Coating system consisting of the following layers: anti-corrosion epoxy primer, repair filler, primer filler, basecoat and anti-graffiti clearcoats XPC60011, XPC60012, XPC60036, BO100-AGR.Each layer is applied and dried in accordance with the requirements of the technological cards.The prepared samples were conditioned at 23 °C and 50% humidity for minimum 7 days in order to perform tests on dry coating.

Operational tests
Cured coating was also tested for mechanical properties and corrosion resistance.After conditioning period had finished, thickness of dry coating was tested by magnetic induction method, adhesion tests were performed by pulloff test.Hardness of cured coating was measured by Koenig pendulum.Summary tests results are presented in Table 1.
Also measurements of "orange peel" was made, which is responsible for the final appearance of the coating.When measuring device's laser beam optically scans the coating surface undulation, like the eyes captures the dark-light pat-tern reflections.Results are shown in five wavelength ranges from 0.1 to 30 mm Table 2.
The measurements of gloss and color parameters was conducted using Byk micro-Tri-gloss and Minolta Spectrophotometer CM-600d on the samples after removing graffiti from coating surface.The measurements were performed both before application of graffiti after cleaning with water and additionally after cleaning using solvent nitro.The results are summarized in Table 3.After the test, the sample was dried, and the measurements of gloss and color parameters -L*, a*, b* for light D65.The results are shown together with diagrams of color change.The color change is expressed as ΔE, which is defined by the equation (1): where: ΔE -color difference expressed as the distance between two points in three-dimensional space, ΔL* -the difference in distance between two points in the dimension L -brightness, Δa* -the difference in distance between two points in dimension a -green and red color, Δb* -the difference in distance between two points in the dimension b -blue and yellow color.
The next test of corrosion resistance to aggressive environmental conditions were finished and assessed after 1000 hours of exposure.The corrosion resistance test was performed in the salt spray chamber according to PN-EN ISO 9227 at 35 o C using 5% saline solution.The result revealed no changes in the tested coatings (no blistering, cracking, corrosion or thread-like corrosion).Given time of 1000 hours is a minimum for the coating to be resistant to environmental effects (Table 4).Figure 1 shows an example a view of sample after corrosion resistance test for anti-graffiti BO100-AGR.However, this time could be extended when aluminum is used as a substrate instead of steel.

Measurement of contact angle and free surface energy
One of the most commonly used methods of determining the contact angle of a material [14] is a method based on the geometry of the droplet (Figure 2).The surface of the droplet is most often in the shape of a circular arc, and then the contact angle is calculated from the measurement of the height h and the radius of the contact surface of the drop r.The height of the circular arc is given by h = R (1 -cosQ) and the surface contact radius r = RsinQ.From these relationships we get a equation (2): where: h -height of circular arc, r -the radius of the contact surface of the drop.
The value of free surface energy of construction materials is determined indirectly by measuring the contact angles of selected measuring fluids.Distilled water and diododomethane (DIM) are used to measure the contact angle.The stereoscopic microscope with the camera and the MicroScan v 1.3 software were used for droplet observation and contact angle measurement.The following values of free surface energy constants of the measuring fluids and their polar and dispersion components were assumed: The measuring liquid was applied to the test surface with a 5 μl constant volume micropipette.
One of the most commonly used methods for determining free surface energy is the Owens-Wendt method [6,14,16] in which it is assumed that the free surface energy is the sum of two components, dispersion and polar: where: γ d S -dispersion component of free surface energy, γ p S -polar component of free surface energy.
The minimum number of contact angle measurement on each sample with another anti-graffiti coating system was 10 times, which allowed for averaging the test results.
Free surface energy (FSE) values were determined by measuring the contact angle.FSE estimation was done at least six times for each surface.Table 5 summarizes the averaged wetting angle and free surface energy measurements of the anti-graffiti coating systems.
The obtained results show that the lowest value of free surface energy was obtained by anti-graffiti BO100-AGR coating system.Also the highest values of measured wetting angles with distilled water and diododomethane were the BO100-AGR system.Analyzing the obtained results, we can observe high repeatability of the measurements, as evidenced by small values of standard deviations (Table 5).

Measurement of flammability properties
The fire properties for coatings intended for the vehicle's body of rolling stock in accordance with the European requirements of PN EN 45545-2 [10]   Measurements of maximum heat generated were made using a conical calorimeter using the oxygen consumption calorimetry principle: For most materials per kilogram of oxygen consumed by burning material, 13.1 MJ of heat is released to within ± 5% .The use of precision oxygen measurement equipment in gas extraction and gas flow measurement in the extract allows for a very precise determination of oxygen consumption and at the same time to accurately calculate the value of the released heat and its rate of release.
Samples with an anti-graffiti coating system were treated with a cone-shaped electric radiator with a radiation intensity of 50 kW/m 2 .The ignition was initiated by the spark and combustion was carried out in air atmosphere (0.024 m 3 /s).Classification according to EN 45545-2 [10] where: t -time, mostly t 1 = 0 q -the rate of heat release, mostly q 1 = 0 Figure 3 shows an example HRRs heat releasing curve for anti-graffiti BO100-AGR.Table 6 summarizes the results of the flammability tests of anti-graffiti coating systems.
From the analysis of Table 6, it is clear that the final values slightly differ from each other.The graphs and observations of the combustion processes also show their similarity and lack of significant impact on the course of the anti-graffiti layer used.This is due to the fact that in the tests carried out, the samples were subjected to radiation with a radiation intensity of 50 kW/m 2 , simulating the conditions of the fire [12,13].As a result, all layers of the coating system burned down.

Measurements of the surface geometric structure
Measurements of surface geometric structure were carried out at the Laboratory of Computer Measurements of Geometric Quantities of the Kielce University of Technology.Tests were performed using a Talysurf CCI optical profilometer using the coherent correlation interferometry method, enabling a resolution of 0.01 nm with a z axis resolution.The measurement result is recorded in a matrix of 1024x1024 measuring points using the x10 lens, giving a measured area of 1.65 mm x 1.65 mm and a horizontal resolution of 1.65 mm x 1.65 mm.Ten measurements were made on samples of anti-graffiti and S355 steel, allowing averaging of the results.The obtained images of surface stereometry and their analysis using the software TalyMap Platinium allowed to evaluate the geometrical structure of the examined surfaces.
Figure 4 shows a sample isometric roughness of the surface of the anti-graffiti XPC 60012 coating system, while Figure 5 shows the isometric image of the wavy surface of the coating system.Table 7 summarizes the most important SGS parameters of the tested anti-graffiti coating systems.
The tested anti-graffiti coating systems had averaged mean arithmetic surface roughness deviations from the average surface area Sa = 6.6÷26.9nm.Samples of S235 carbon steel sanded with P80 grain sandpaper on which coatings were applied had Sa = 1 234.5÷1863.2 nm.Parameter Sa is the basic amplitude parameter for quantifying the state of the surface being analyzed.A similar trend in the measurement of antigraffiti and S235 coating systems was observed for the quadratic surface roughness Sq, which has a strong correlation with the Sa parameter.As a result of coating application, the surface roughness was significantly reduced.

CONCLUSIONS
• The mechanical properties of each system are comparable and do not depend on the antigraffiti clearcoat used, whereas the BO100-AGR is characterized by the highest hardness, • Corrosion resistance for each system exceeds 1000 h in a salt chamber test using a 5% saline solution, • Tested rolling stock systems has similar fire test results, which means that anti-graffiti

Fig. 2 .
Fig. 2. Measurement of the contact angle of the drop geometry (a), view of the drop of a measuring liquid (b)

Fig. 4 .
Fig. 4. Isometric view of the surface roughness of the anti-graffiti XPC 60012 coating system

Fig. 5 .
Fig. 5. Isometric view of the waviness surface of the anti-graffiti XPC 60012 coating system

Table 1 .
Selected properties of dry coating applied on steel

Table 2 .
Orange peel measurement results

Table 3 .
Change of color and gloss parameter after cleaning for BO100-AGR

Table 4 .
Results after corrosion resistance test for BO100-AGR

Table 5 .
Results of contact angle measurements and free surface energy of anti-graffiti coating systems defines the MARHE parameter, which represents the maximum average rate of heat release (ARHE) over a 20 minute test.The ARHE value is calculated according to the following formula:

Table 6 .
Summary of fire test results