Test Methods for Decorative Wet Paints

Maintaining quality standards in paint production, meeting expectations at the highest level, ensuring unconditional customer satisfaction, adopting international quality systems and conducting quality control tests are possible through the implementation of proper procedures. The regular and careful execution of standard tests is important in that it enables timely intervention in problems that may arise during the production phase. Paint quality control tests are defined on both wet and dry paints. This article evaluates wet paint tests. Maintaining standards in quality control tests and obtaining test reliability requires all environmental conditions to be stable. All tests should be carried out on samples under the same conditions, and parameters such as paint thickness, environmental conditions, drying conditions and surface preparation should be defined. The main test methods performed on wet paints can be listed as follows: • Visual inspection, • Solid content, • Density, • Hiding power, • Flow and sag, • Viscosity, • Rheology.

Visual Inspection

In the visual inspection method, the paint surface is checked for the presence of a skin layer, gelation, phase separation, settling and foreign matter. Gelation is the part that becomes a non-flowing paste when the paint consistency is checked. Except for thixotropic paints, it is not possible to make a gelled paint fluid by stirring. Phase separation is the visible separation of substances in the paint from one another. Generally it is the case of an oil layer coming to the surface in water-based paints, or an alkyd phase in solvent-based paints. Paint with phase separation is stirred with the aid of a mixer until it becomes homogeneous and then allowed to settle. The absence of observed phase separation indicates that the paint is in usable condition. Settling is when solid materials in the paint form hard or soft sediment at the bottom. If the settled paint becomes homogeneous when stirred, it is soft settling and the paint is in usable condition. If it cannot be stirred and the settling is in mass form, the paint cannot be used.

Solid Content

Pigments and fillers that make up the paint have a large proportion in determining the solid content. Solid content can be formulated according to the manufacturer's cost calculations. Solid content determination is established using ASTM D4290/TS EN ISO 3251 standard methods. Approximately 1-2 grams of paint sample is added to the surface of a tared container. The painted lid is placed in a circulating-air oven at 105°C for 3 hours. After the time period, it is cooled in a desiccator and weighed again.

Density

It is a value that shows the relationship between weight and volume. Besides being a parameter evaluated for calculating paint consumption, paint density is important during the packaging of the product. Density measurement is performed using TS EN ISO 2811-1 standard methods. Density is typically measured with a pycnometer. After the density container is tared, it is filled with paint until it just overflows, the lid is closed and the overflowing paint is cleaned. It is weighed again and the difference between the two weights is the weight of 100 ml of paint, and the calculated value is divided by the internal volume of the container to find the density. When taking measurements, the paint temperature should be 20°C and the scale used should have an accuracy of at least 0.01 grams. Since the density value will vary depending on temperature, the temperature at which the measurement was made must be stated.

Hiding Power

Standardized by TS EN ISO 6504-3, the covering power of paints can be defined as the ability to completely cover the surface on which it is applied. This property results from the pigment it contains and its interaction in the formulation. This is tested using a simple zebra paper method. Zebra paper attached to a test plate is coated with 1, 2, 3, 4,... coats of paint to gradually increase the thickness value, obtaining a paint film. After the paint film is completely dry, the point where the black and white stripes of the zebra paper are no longer noticeable is marked and the film thickness on both sides of this point is measured. The average of the measured values is given as the dry film hiding power in microns. Images of the plates on which measurements were made are provided in Figure 1.   [caption id="attachment_130337" align="aligncenter"] Figure 1. Zebra paper method[/caption] Flow and Sag The flow and sag limit of all types of paint is determined using ASTM D4400/ ASTM D2801 standard methods. Paint is applied in 1 coat to the entire vertical test plate. Each time, starting 3-4 cm from the right, 2nd, 3rd, 4th, 5th... coats are applied. One minute after application is complete, the test plate is allowed to dry. After the test plate is dry, the film thicknesses on it are measured from thin film to thick film. The point at which the paint begins to flow or sag is determined in microns. The dry film thickness at the first point where sag is detected is determined as the paint's sag limit. Flow testing is shown in Figure 2. [caption id="attachment_130338" align="aligncenter"] Figure 2. Flow and sag method application[/caption]

Viscosity

Viscosity is one of the most important parameters that determine paint quality. It can be defined as the resistance of a liquid to flow. Viscosity measurement, as determined by ASTM D2196 / ASTM D1200 codes in standard methods, is one of the most widely used test methods in the industry. It provides a determining value that gives information about properties such as spreading, sag, flow, applicability with brush or spray. Viscosity ranges for different materials are provided in Figure 3. [caption id="attachment_130341" align="aligncenter"] Figure 3. Example materials in low and high viscosity ranges[/caption] If the ratio between the shear rate applied to a liquid and shear force is constant and does not change over time, such liquids are called Newtonian. Liquids such as water, solvent and oil are examples in this category. For Newtonian type liquids, viscosity measurement is performed using the DIN CUP4 method based on ASTM D1200 standard. For measurement, the viscosity measurement device is first cleaned, and the paint temperature is maintained at an average of 20°C in a water bath. The well-mixed paint, once free of air bubbles, is filled into the DIN CUP interior with the finger closing the hole until it is completely full. Care must be taken to prevent air from entering the container during this process. The moment the finger is withdrawn, the stopwatch is started and stopped when the paint stops flowing continuously and begins to drip. The value obtained from the measurement is given by specifying the temperature at which the measurement was made and the type of viscometer. For example; 45"/DIN CUP4 or FORD CUP 4/20°C. DIN CUP4 and FORD CUP4 measurement devices are shown in Figure 4. [caption id="attachment_130342" align="aligncenter"] Figure 4. DIN CUP4 and FORD CUP4[/caption] A significant portion of paints are classified as non-Newtonian liquids and in this case viscosity measurements are performed with a rotational viscometer. Shear force is applied to paint samples taken in a container with the aid of a rotating spindle and the liquid's resistance to shear force is measured. For measurement, rotational viscometers designed based on the Brookfield method are typically used. The viscometer device is shown in Figure 5. [caption id="attachment_130343" align="aligncenter"] Figure 5. Brookfield viscosity measurement device[/caption]

Rheology

Rheology is derived from the words rheos (flow) and logos (science) and describes the flow properties of liquids. Rheology helps determine properties that will affect the final product by applying shear force to liquid or solid material and evaluating the entire flow behavior of the liquid. Using a rheometer, parameters such as temperature, time and pressure or shear force that cannot be measured simultaneously when using a viscometer can be included in the evaluation via a flow curve. This allows the product to be shaped according to the application type, especially during the formulation development stage. In addition, the creep, relaxation, deformation and all deformation properties of viscoelastic materials widely used in industry can also be understood with the aid of rheology. Measuring flow behavior in paint types developed for different applications becomes extremely important. The factor that creates a difference between storage in warehouses, application with brush or spray use is the shear rate and shear force applied to the product. Through the measurement and evaluation of the flow curve, the packaged product is ensured to have the desired properties in brush or spray applications. Figure 6 shows viscosity changes with different shear rates in paint applications. [caption id="attachment_130344" align="aligncenter"] Figure 6. Rheology changes in paint applications[/caption] Rheology measurements are performed with a Rheometer device. Device selection should be made differently depending on the conditions under which the rheology study will be conducted and the properties of the product. The Rheometer device is presented in Figure 7. [caption id="attachment_130345" align="aligncenter"] Figure 7. Rheometer device[/caption] Rheology studies can be conducted over a wide range from liquids with low viscosity values to viscoelastic or solid materials in measurement capacities, and besides flow diagrams, properties of the product such as deformation, glass transition temperature and shelf life can also be measured.  

Duygu Topçu Kubat Sales Support Manager Reaksiyon Kimya