Quality standards in the paint production is an important goal to achieve. In order to satisfy the market expectations at the highest level and ensuring the unconditional satisfaction of the end user, is only possible with the adoption of international quality systems and applying quality control tests regularly.
It is important that standard tests are carried out regularly and meticulously in order to intervene in a timely manner to the problems that may occur during the production phase. Paint quality control tests are defined over wet and dry paints. In this article, wet paint tests are evaluated.
Ensuring standards in quality control tests and achieving test reliability require stable ambient conditions. All tests should be done under the same conditions, parameters such as paint thickness, ambient conditions, drying conditions, type of test panel, surface
preparation should be determined. The main test methods for wet paints can be listed as follow
• Visual inspection,
• Solid Content,
• Density,
• Hiding Power,
• Flow-Sagging,
• Viscosity,
• Rheology.
Visual Inspection
In the visual inspection method, it is checked whether there is a cream layer, gelation, phase separation, precipitation on the paint surface.
Cream surface on a dry layer might formed by the contact of the paint with air or the volatilization of the solvent. The thickness and hardness of this layer should be observed. The cream layer should be taken without scattering and should be used after the paint is filtered.
Gelation is the part that turns into a non-fluid gel when the paint consistency is controlled. It is not possible to convert a gelled paint into a fluid by mixing it, except for thixotropic paints.
Phase separation is the visible separation of the substances in the paint from each other. In general, it is the situation where the oil layer in water-based paints and the alkyd base in solvent-based paints come to the surface.
The paint with phase separation is mixed with the help of a mixer until it becomes homogeneous and rested. If no phase separation is observed, it indicates that the paint is usable.
Settling is the situation in which the solids in the paint form a hard or soft precipitate on the bottom. It is considered as soft slump and can be used, if the slump becomes homogeneous when the paint is mixed. The paint should not be used if it cannot be mixed and the settling is in mass.
Solid Content
Pigments and fillers have a large proportion in determining the amount of solid. The solid content can be formulated according to the manufacturer’s cost calculations. Determination of the amount of solid can be done according to ASTM D4290/TS EN ISO 3251 standard methods.
1-2 gr sample is added to the aluminum cap surface. The painted cover is kept in an oven with air circulation at 1050C for 3 hours. It is cooled in the desiccator and weighed again.
Density
Density shows the relationship between weight and volume. In order to calculate the paint consumption, the densities of both the raw materials and the finished paint are important during the packaging of the product. Density measurement is determined by TS EN ISO 2811-1 standard methods.
Density is usually measured with a pycnometer. After the density container is tared, it is filled with paint so that it overflows a little, and the overflowing paint is cleaned by closing the lid. It is weighed again and the difference between the two values is the weight of
100 ml of paint, and the density is found by dividing the calculated value by the inner volume of the container.
While the measurements are being made, the paint temperature should still be 20°C and the balance used should be at least 0,01 g sensitive. Since the density value will vary depending on the temperature, the temperature at which the measurement is made must
be specified.
Hiding Power
The hiding power of paints standardized with TS EN ISO 6504-3 can be defined as the potential to completely cover the applied surface. This feature is due to the pigment in it and its interaction in the formulation. It can be tested with a simple zebra paper method.
A paint film is obtained by increasing the thickness value by applying 1st, 2nd, 3rd, 4th,.. coats of paint to the zebra paper adhered to the test plate.
After the paint film has completely dried, the spot where the black and white stripes of the zebra paper are not noticeable is marked and the film thickness is measured on both sides of this point. The average of the measured value is given as the dry film coverage in microns. The plate images on which the measurements were made are given in Figure 1.
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Figure 1. Zebra paper method[/caption]
Flow and Sagging
It is the method used to determine the yield-sag limit of all kinds of paints. Determined with ASTM D4400/ ASTM D2801 standard methods.
Paint is applied to an entire vertical test plate. Condition to start 3-4 cm right each time with 2.,3.,4.,5... layers are applied. The test plate is left to dry 1 minute after the application is finished. After the test plate dries, the film thicknesses are measured from thin film to thick film.
The point where the paint starts to flow and sag is determined in microns. The dry film thickness at the first point where sagging is detected is considered as the sagging limit of the paint. Figure 2 shows the yield test.
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Figure 2. Flow-Sagging method application[/caption]
Viscosity
Viscosity is one of the most important parameters that determine the paint quality. It can be defined as the resistance created by the liquid against flowing. Viscosity measurement determined by ASTM D2196/ ASTM D1200 measuring Viscosity is widely used in the industry.
It provides a decisive value that gives information about paints properties such as spreading, sagging, flowing, applicability with a brush or spray. Viscosity ranges of different materials are given in Figure 3.
If the ratio between shear rate and shear force applied to the fluid is constant and does not change with time, such fluids are called Newtonian. Liquids such as water, solvent, oil can be given as examples of this group. Viscosity measurement for Newtonian type liquids is made by DIN CUP4 method based on ASTM D1200 standard.
The viscosity measuring device should be cleaned, and temperature in the water bath is ensured to be at an average of 20°C. When the paint is mixed well and there are no air bubbles in it, the inside of the DIN CUP is filled.
Air should not to be let into the container. As soon as the finger is removed, the
stopwatch is started and it is stopped when the paint stops flowing continuously and starts to drip.
The value obtained from the measurement should be define by specifying the temperature at which the measurement is made and the type of viscometer. E.g; 45”/DIN CUP4 or FORD CUP 4/20°C. DIN CUP4 and FORD CUP4 measuring device is given in Figure 4.
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Figure 4. DIN CUP4 and FORD CUP4[/caption]
A significant majority of paints are defined as non- Newtonian liquids a rotational viscometer used to measure viscosity. The shear force is applied to the paint samples by the rotating spindle and the resistance of the liquid to the shear force is measured. Rotational viscometers designed based on the Brookfield method are generally used, for the measurement. The viscometer device is shown in Figure 5.
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Figure 5. Brookfield viscometer[/caption]
Rheology
Rheology is derived from the words rheos (flow) and logos (science) and describes the flow properties of liquids. Rheological Analysis supports to determine the properties of the liquid that will affect the final product over its entire flow behavior by applying a shear force to the liquid or solid material.
Design parameters such as temperature, time and pressure or shear force, which cannot be measured simultaneously when a viscometer is used, can be included in the evaluation over the flow curve by using rheometer.
This enables the product to be shaped during the development of its formulation according to the type of application. In addition, the creep, relaxation, deformation and all other deformation properties of visco-elastic materials, which are widely used in the industry, can also be understood with the rheological analysis.
It is extremely important to measure the flow behavior of paint types developed for different applications. The factor that makes the difference between storage, applying with a brush or spraying is shear and shear rate which applied to the product. It is ensured that the final product has the desired properties stress by measuring and evaluating the flow
curve. In Figure 6, viscosity changes with different shear rates are seen in paint applications.
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Figure 6. Rheology changes in paint applications[/caption]
Rheology measurements are made with a Rheometer device. A device with a different capacity should be selected according to the conditions in which the rheology study will be carried out and the characteristics of the product. The rheometer device is presented in Figure 7.
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Figure 7. Rheometer device[/caption]
Rheology studies can be performed in a wide range materials from low viscosity liquids to visco elastic or solid materials, additionally. Other properties of the product such as deformation, glass transition temperature, shelf life. Can be measured as well as flow
diagrams.
Duygu Topçu Kubat
Sales Support Executive
Reaksiyon Kimya
