The present invention relates generally to the field of rheology measurement for paints, inks, and coatings, and in particular to a new and useful device and method for measuring the transition or force to achieve a shear rate to more accurately determine the rheology of a given coating and use this force to forecast the results of the various application processes, to make better applicator designs, and to make improved formulations.
Coatings and surface treatments are commonly applied in industrial processes for reasons including merely changing the outward appearance of a product, protecting product surfaces from corrosion, weather or other environmental conditions, making the surface receptive to ink, creating liquid or gas barriers, creating a non-stick surface, and making a surface more or less adhesive, to name some. Several application processes and devices are known for applying these coatings, including sprayers, brushes, fan coaters, flow coaters, curtain coaters, roll coaters, Meyer rod coaters, Gravure coaters, blade coaters, air knife coaters, and meniscus coaters, among others.
It is well understood in the art that for any of these coaters to work properly, the rheology of the applied coating must be within a particular range, depending on the device and process conditions such as pick-up and speed. Accurate determination of coating fluid rheology is important to ensuring good and consistent quality coatings. For example, consistency is desirable for products coated with different batches of coating fluid on an assembly line. If the coating fluid does not have the proper rheology for the application, a poor quality coating will result.
Many times, the expense of the product and coating materials make errors in the rheology of the coating fluid very costly. Often, the only way to find the rheology of a coating is to run many test coatings to find one which works well for a particular application. But, the use of coating fluid samples and substrates in trial-and-error analysis becomes costly very quickly, due to time and materials wasted.
And, the inability to accurately predict the rheology of coating fluids is so deficient that coating dies cannot be made to run multiple coatings. That is, a single die is typically designed to run a specific coating composition and it is very difficult to make a different composition run well through the same die. The distribution pressure losses compared to the die lip exiting flow losses vary enough between formulations that operations resort to using a number of dies to make different grades for the same coating.
Difficulties in accurately predicting process results from rheology measurements for a coating fluid arise from the fact that substantially all paints, sealers, protective coatings, etc., exhibit non-Newtonian behavior. Thus, in order to accurately predict the process a particular application, the measurement must mimic the application process conditions as much as possible. In particular, it is necessary to duplicate the intensity and duration of shear rate, shape of the flow field, and time for accurate viscosity or fluid friction measurements to model the process. All coating processes have a region with an extensional flow field of a short duration, so that a measurement intended to predict the fluid performance in a process must do the same.
Transition or entrance effects can be generally defined as the effect on a fluid as it passes between two regions of differing areas or different shear rates. For example, when a fluid enters a tube or channel, or at a point where the same tube or channel tapers wider or narrower. Viscosity measurements generally discard or effectively ignore transition effects of a fluid entering the test flow field. Reynold's work on laminar flows has demonstrated that between 10 and 20 pipe diameters displacement are needed after entering a pipe for laminar flow to develop.
Similarly, U.S. Pat. No. 3,071,001 states that it is established a non-linear pressure drop occurs at the pipe transition which is related to the density times the square of velocity, or the velocity head of the fluid. For non-Newtonian fluids, however, we have a different finding.
Many viscometers elect to minimize the transition effects. They either wait for the transition effect to pass before making any measurements, or they subtract a value related to the velocity head. For example, U.S. Pat. No. 6,470,736 teaches that the transition flow effect and Reynold's number should be minimized so as to allow interpretation of flow rate by the Poiseuille equation. This value is simply the pressure drop through a test pipe under steady state flow.
Viscometers which minimize or eliminate the transition effects are useful and effective for measuring the resistance of an equilibrium process like the flow through a pipe. And, they are helpful and correlate to some extent with process conditions within a narrow range of chemistries. But, generally, they are ineffective at predicting process results over a broad range of fluid chemistries using a single predictive model.
Some capillary viscometers presently pass over the transition effect by taking data only at a point after the transition effect has occurred, so that it is small (near zero) compared to the equilibrium viscous force. Other capillary viscometer measurements subtract out the force needed to reach the measured shear rate. They measure this force with a standard fluid—typically water—and use this value for all fluids tested. But, simply subtracting the transition or entrance force value of one fluid type from all fluid measurements will result in error because the actual force varies with each fluid, especially for viscous non-Newtonian fluids. So existing capillary viscometers either ignore or improperly consider the actual transition shear force.
While most viscometers minimize the transition effect in the reported reading, some do not. Such viscometers are taught in several patents, including U.S. Pat. No. 4,449,394, which has a short capillary tube at the bottom of a cup for receiving the fluid under test. The height of the fluid provides the pressure source. However, this viscometer does not match the process conditions and does not forecast process results over a broad range of fluid types. It operates at a declining shear rate. As the fluid level in the cup drops, the shear reduces to near zero, until the fluid flow stops. This viscometer cannot be used to measure the resistance to flow at specific shear rates, and only works well at low shear rates since it is not pressurized beyond the inherent fluid pressure. This viscometer has the effect of averaging out the shear force versus time as well, so that it measures an average viscosity over an average shear rate. And, process rates of 300,000/second or higher are not obtainable.
Another capillary viscometer is taught by U.S. Pat. No. 4,793,174 in which fluid flow through a capillary tube is started at a low pressure and then the pressure is suddenly increased. The resulting flow increase is recorded as a function of time. The capillary tube is significantly longer than 20 pipe diameters, so that the transition effect is essentially eliminated at the measurement point. The transition effect is lost in the average with the equilibrium viscosity through the tube. The length of the tube dilutes the initial shear force, while the overall flow does not have a constant shear rate.
The viscometer of U.S. Pat. No. 3,952,577 includes a plurality of pressure sensors along the flow path. The flow path is a rectangular channel of decreasing cross-sectional area. The decreasing size of the channel necessarily prevents the flow from having a constant shear rate. The viscometer is provided for use with laminar flows only.
Another tool which is useful but still fails to account for transition effects is an oscillating viscometer. This viscometer begins motion in one direction and then reverses to measure the elasticity of a fluid. The viscometer is experiencing the force to initiate flow, but measurement is not taken until after several oscillations, and the force to destroy initial gel structure of the fluid is not lost.
Concentric cylinder viscometers measure the force after the flow establishes a fully developed velocity gradient even when the velocity is steadily increased. This measurement is commonly referred to as a rheogram. One tool which can plot shear force versus time after a change in shear rate to measure the energy to “beat out” a coating is known as the Haake rheometer. But, this device fails to duplicate all coating application processes as it has a longer than realistic process duration, and it is not an extensional flow field.
None of the existing viscometers measures the transition effects accurately at the process conditions that correspond to the various coating and paint application processes.
It has been discovered that the transition force can vary greatly between fluids, and therefore, this force needs to be measured in each case. Thus, a rheology measurement device and method for accurately determining and accounting for transition effects of fluids is still needed.