The ability to measure and control the viscosity of coating materials is essential to ensure consistent quality across different application methods. Viscosity plays an important role in the paint and coatings industry. Whether in the DIY or industrial sector, the right viscosity has a decisive influence on the processability of a coating and then ultimately on the surface properties of the dried coating film.
If coatings will be applied with a brush or roller, they need to have a higher viscosity and show a slight thixotropy, which is important for good processability. If the coating is painted on a wall, it has to be thin enough to be spreadable without running down the wall. The thixotropic effect prevents this by quickly making the coating more viscous so it won’t run.
If coatings will be applied by spray or dip coated, they need to have a lower viscosity. In the case of spray coatings, too much viscosity can clog the nozzle of the spray gun, which inhibits the ability to produce a fine spray mist. Instead, drops will be shot onto the substrate without forming an even coating film. For dip coatings, a very low viscosity coating is also important to ensure all parts of the material can be reached and evenly coated.
The right viscosity plays an important role in making an additive or polymer easier to process. The higher the viscosity, the more difficult it is to dose an additive or a polymer. Depending on the paint formulator’s application needs, the viscosity can be set within a certain range, which varies from customer to customer. The dynamic viscosity η (Greek letter eta) is generally given in millipascal seconds (mPas) and is usually determined with the aid of a rotational viscometer. In the past, viscosity was also given in poise (or centipoise with 1 cP = 1 mPas).
The choice of container can also impact the processability of an additive or polymer. For example, an IBC or bunghole drum would not be appropriate for a highly viscous product, which would come out poorly or sometimes not completely. In this case, a clamping lid barrel might work best.
Viscosity Determination Methods
At Lubrizol, we employ multiple methods to determine viscosity, including run-out-time and rotational viscosity, which improves our ability to supply consistent quality.
Run-out time testing is determined according to ISO standards and the German industry norm known as DIN (which is short for Deutsche Industrie Norm). The test is conducted by filling a test cup with the additive/polymer being tested to determine the viscosity and stop time it takes for the substance to flow out of the cup. The run-down time is the time that elapses while the substance to be tested begins to flow out of the nozzle of the filled test cup until the substance has completely run out of the test cup.
Rotational viscosity is determined using the Rheolab QC and the Brookfield viscometer. These rotational viscometers, also known as viscosimeters, use a spindle (cylinder or disk) that is dipped into the test sample. It measures the force that the spindle requires to overcome the liquid inertia. The angle between the spring and spindle is recorded electronically and converted into a torque value. The torque value depends on the rotation speed of the spindle and the spindle geometry. From this value, the viscosity of the sample can be directly given in mPa s/cP (dPa/P).
Whether very low-viscosity additives are needed, such as those used in spray coatings for example, or paste-like additives for the printing ink sector, Lubrizol uses a range of equipment to accurately determine viscosity to be able to precisely control it.
With many years of experience and the ability to use a variety of viscosity determination methods, we are able to ascertain the optimal measurement method for each product while offering each paint formulator the measurement method they prefer. This allows us to optimally monitor the viscosity of our products and deliver consistent quality.
Contact us to learn more about our viscosity testing and control capabilities and how this can bring value to your formulations.