Marcus Chen
Traffic paint is a type of paint used to mark roads, and it is important that it has the correct viscosity to ensure a uniform finish and to prevent problems during application. Viscosity is a measure of a liquid's resistance to flow, and there are several methods and tools available to test it. One of the most popular methods is the Krebs Stormer viscometer, which uses a paddle rotating at 200 rpm in a standardised container to measure the resistance created by the paint. Other methods include the efflux method, which measures the time taken for a given quantity of material to flow through a given size of orifice, and the cone and plate viscometer, which was developed by a paint manufacturer specifically for measuring the viscosity of paint.
| Characteristics | Values |
|---|---|
| Method | Efflux method, cone and plate viscometer, Krebs Stormer viscometer, ICI viscometer, flow cups |
| Sample preparation | Mix, strain, submerge in material |
| Measurement | Time taken for material to flow through orifice, resistance created by paint, drag on rotation, reflectance |
| Units | Krebs units (KU), centipoises (cps), centistokes (cS or cSt), stokes, poise, grams (gm) |
| Factors affecting viscosity | Stirring, pumping, transferring, sagging, application, solvents, thinners, temperature changes, storage conditions, drying, sealing |
| Coverage capacity | Depends on pigment type, percentage, thickness of coat, light absorption, light diffusion |
| Gloss | Depends on type of resin used to prepare paint |
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What You'll Learn
Using a Krebs Stormer viscometer
The Krebs Stormer viscometer is a popular and versatile instrument used for viscosity testing. It is particularly useful for paints that will be spread with a brush or roller, which are typically applied at a medium shear rate from 10 to 100 s-1. This viscometer is also commonly used in industries such as paint manufacturing, coatings, adhesives, pharmaceuticals, and food applications.
The Krebs Stormer viscometer features a paddle that rotates through the paint at 200 rpm in a standardized container. The resistance created by the paint is then measured and expressed in Krebs units (KU) on the instrument's LED digital display. The higher the KU number, the more viscous the paint. Modern Krebs Stormer viscometers can also display viscosity in centipoise (cP) and grams (gm). This method of viscosity testing is simple, easy to use, and does not require calculations, making it accessible to those with limited experience in viscosity testing.
To use the Krebs Stormer viscometer, follow these general steps:
- Prepare the paint sample: Ensure the paint is well-mixed and homogeneous. The sample size should be sufficient to fill the standardized container of the viscometer.
- Fill the container: Pour the paint sample into the standardized container of the Krebs Stormer viscometer. Fill it to the appropriate level as specified in the instrument's instructions.
- Insert the paddle: Carefully insert the paddle into the paint-filled container. The paddle should be positioned correctly according to the instrument's guidelines.
- Rotate the paddle: Start the rotation of the paddle at a constant speed of 200 rpm. This can usually be set on the viscometer's controls.
- Measure the resistance: As the paddle rotates, it will experience resistance from the paint. This resistance is measured and recorded in Krebs units.
- Interpret the results: The KU value indicates the viscosity of the paint. Higher KU values represent higher viscosity, while lower KU values indicate lower viscosity.
It is important to refer to the specific instructions provided with your Krebs Stormer viscometer, as there may be slight variations in the setup and operation between different models. Additionally, proper calibration and maintenance of the instrument are crucial to ensure accurate and reliable results.
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Testing with a cone and plate viscometer
Cone and plate viscometers are a standard test for dynamic viscosity measurements, adhering to ASTM standards. They are a practical tool for any lab requiring quick and easy testing of materials, regardless of application. They are suitable for Newtonian or non-Newtonian materials with viscosities up to 15,000 poise and shear rates from as low as 20s-1 up to 13,000 s-1.
To conduct a viscosity test for traffic paint using a cone and plate viscometer, follow these steps:
- Prepare the paint sample: Ensure you have a sufficient amount of paint to be tested. The paint should be well-mixed and homogeneous.
- Mount the sample: Place a small amount of the paint on the plate of the viscometer. The plate should be clean and dry to ensure accurate results.
- Lower the cone: Carefully lower the shallow cone onto the paint sample. The cone should be clean and free of any residue.
- Rotate the cone: Revolving the cone at a specified speed will induce shear stress on the paint. The rotation speed can vary depending on the specific cone and plate viscometer being used and the desired shear rate. It can range from as low as 20s-1 up to high shear rates of 12,000 s-1 or even 13,000 s-1.
- Measure the drag: As the cone rotates, the paint will create drag or resistance on the rotation. This resistance is measured and recorded. The higher the drag, the more viscous the paint is.
- Clean the equipment: After testing, clean the cone and plate to remove any paint residue. This ensures that the equipment is ready for the next test and prevents contamination.
It is important to note that cone and plate viscometers can be used for a variety of substances beyond paint, including resins, inks, varnishes, cosmetics, oils, adhesives, and even heavy-duty substances like bitumen. Additionally, recent innovations include modifications to the cone/plate viscometer, such as variable-speed drive and integrated temperature control, enhancing its versatility and accuracy.
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Measuring viscosity in Krebs units
Modern Krebs Stormer viscometers, such as the BYK byko-visc DS, simultaneously display viscosity in Krebs units and centipoise (cP) and grams (gm). The BYK unit is usable with viscosities in the 40-141 KU range, which is equivalent to 27-5274 centipoise (cP) per ASTM D562. These viscometers are simple, easy to use, and yield operator-independent results with no calculations needed.
Krebs units can be converted to centipoise using ASTM D 562. One Krebs unit (KU) is the weight in grams that will turn a paddle-type rotor, submerged in the sample, 100 revolutions in 30 seconds. This is typically measured using a Krebs Stormer-type viscometer with a paddle spindle rotating at 200 rpm.
Other methods for testing the viscosity of paints include the use of cone and plate viscometers, which are suitable for testing at high shear rates, and flow cups, which are a low-cost and quick way to check viscosity. However, flow cups should only be used for Newtonian liquids as they are error-prone for the measurement of thixotropic paints.
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Using a digital viscometer
Paint viscosity can be measured using several types of viscometers. One of the most famous paint manufacturers, Imperial Chemical Industries (ICI), developed a viscometer that uses a cone and plate arrangement to measure the viscosity of paint samples. A small amount of paint is mounted on a plate, and a shallow cone is lowered onto the sample and revolved. The paint creates drag on the rotation that can be measured – the more viscous the paint, the greater the drag. This type of viscometer is now available in a digital version, known as the REL Digital, and can be purchased online.
Digital viscometers are widely used to measure the viscosity of various liquids, including paint. They are highly adaptable, with gearless drive and an LCD for information display and timer function selection. They can be used to measure the viscosity of liquids such as grease, foodstuff, dope, and paper-making, as well as paint.
When using a digital viscometer to test the viscosity of traffic paint, it is important to follow the correct procedure. First, gather the necessary equipment, including the digital viscometer, a standard spindle, a protective frame for the spindle, a seat and levelling feet, an ascending and falling rack, and a power adapter. Ensure that the viscometer is calibrated correctly and set up the equipment according to the manufacturer's instructions.
Next, prepare the paint sample by following any specific instructions provided by the viscometer manufacturer. This may include stirring or thinning the paint to achieve the desired consistency. Once the paint sample is ready, carefully load it into the viscometer, being careful not to introduce any air bubbles.
Start the testing procedure by activating the viscometer. Follow the instructions provided with the equipment to set the appropriate parameters, such as speed, spindle speed, volume of the sample, and time. Monitor the display on the viscometer, which will show the viscosity measurement in units such as mPa.s or cps.
Finally, record the viscosity measurement and compare it to the desired range for traffic paint. The viscosity of approximately 100 cps (1 P, 0.1 Pa•s) is considered acceptable for spraying, brushing, or roll coating. However, this may vary depending on the specific type of paint and its delivery method.
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Testing paint coverage with an optical spectrometer and reflectometer or glossmeter (considering the specific weight of paint)
Testing paint coverage with an optical spectrometer involves spreading the paint on test charts with black-and-white colours and measuring reflectance. The relationship between the two types of reflectance obtained is called the "contrast ratio", indicating an approximation of full coverage. The optical spectrometer can also be used with a database to determine the amount of colourant that must be added to the paint to match a model colour.
A reflectometer or glossmeter can also be used to measure the amount of light reflected by a surface. Gloss or specular reflection is the capacity of a paint coat to reflect the light it receives. If the capacity is high, the paint will be glossy; if it is low, the paint will be satin; and if it is zero, the paint will be matte. The smoother the surface, the glossier the paint.
The glossmeter is widely used in many industries, including paper mills, automotive, aerospace, and consumer electronics. It is used at each stage of the manufacturing process, from goods receipt through to final inspection. The measurement scale, gloss units (GU), of a glossmeter is a scaling based on a highly polished reference black glass standard with a defined refractive index. This standard is used to establish an upper point calibration of 100, with the lower end point established at 0 on a perfectly matte surface. This scaling is suitable for most non-metallic coatings and materials (paints and plastics) as they generally fall within this range.
To carry out an accurate gloss test, you need a glossmeter, following ISO standards. You can test the specular gloss of paint films using a reflectometer geometry of 20°, 60°, or 85°. A 60° geometry is applicable to most paint films. When dealing with very high levels of gloss and near-matte films, the 20° or 85° options may be more suitable.
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Frequently asked questions
The viscosity of a coating material is an indication of its ability to resist flow. The flow characteristics of fluids are directly related to the degree of internal friction.
There are several methods to test the viscosity of traffic paint. One of the most common methods is the efflux method, which involves using a Zahn or Ford cup with a given quantity of material and measuring the time it takes for the material to flow through an orifice at the bottom of the cup. Another popular method is using a Krebs Stormer viscometer, which measures the resistance created by the paint as a paddle rotates through it at 200 rpm. The viscosity is expressed in Krebs units (KU) or centipoise (cP). Cone and plate viscometers are also widely used and allow for control over the paint's viscosity during application.
ASTM D4287 and BS 3900-A7 are widely accepted standard test methods for determining the high-shear viscosity of paints and related products. ISO 2884 is another technique that measures the viscosity of paints at a high rate of shear using flow cups.
The units of viscosity include Krebs units (KU), centipoise (cP), and grams (gm).
Yes, a simple trick to measure paint viscosity is to dip a stir stick into the paint and hold it at a 45-degree angle over the paint. When the paint stops free-flowing, it should drip consistently once every second for at least 5 seconds, indicating the correct viscosity for a cup gun.
