Viscosity measurements of latex paints in the area of high shear rates (ICI-viscosity)

Viscosity measurements with a Brookfield or Stormer viscometer are standard in R&D and QC of latex paints. However, they only cover a part of the shear area (low shear and mid shear) to which the paints are handled. The area responsible for the rheological behavior of the paint when painting with a brush, roller or spray is the high shear area where measurements are made using a cone-plate viscometer.

Cone and plate for ICI-viscosity measurements

Depending on the shear rate and thus the shear stress, the paint must retain its rheological behavior. Appropriate rheological additives are selected in order to optimize the flow in each area of the shear rate. Providing data on the effective operation of rheology modifiers and classifying them in terms of them is an extremely important element of technical markting, which must be based on data from research, case studies and ladder studies for thickeners tested in various doses, for various ranges of PVC, various latex binders and in compatibility with various coalescents, surfactants, etc.

This is the daily work of the Spektrochem laboratory, which we perform for manufacturers of additives and other raw materials for water-based paints. One of the stages of laboratory work on the development of guide formulations for rheological additives is the performance of measurements in the field of high shear forces, the so-called ICI-viscosity.

Thickening agents operating at the highest shear rates are the so-called ICI-viscosity builders. Brush painting takes place at shear rates from 102 to 104 s-1, and spraying from 103 to 106 s-1. Thickening agents effective in these areas of shear rate are responsible for the properties of latex paint during its application and constitute an extremely important group of additives.

Figure 2. A pictorial representation of the shear rate range when using high-shear area paints

High-shear thickeners

High-shear-effective thickeners (ICI-builders) are usually polyurethane thickeners (HEUR) and acrylic thickeners (ASE, HASE). Of course, also cellulose, mineral and other thickeners which impart viscosity to the paint in the area of low and medium shear rates also have their limited effectiveness in the area of high shear forces. However, ICI-builders require special measuring methods or a practical painting with a brush, roller or sprayer to assess their effectiveness. Usually, thickeners whose efficiency is strongly shifted into the area of building the paint structure at high shear rates will not be observable in building viscosity at lower shear rates. As an example, the results of the viscosity measurements are shown in Table 1.

Table 1. Comparison of the effect of the ICI thickener on the viscosity of the latex paint in the three shear regions

The viscosity of a paint using a high shear effective thickener does not change when measured with a Brookfield viscometer in the low-shear region (ASTM D2196 method A – apparent viscosity measurement using spindles). Determination of the mid-shear viscosity with a Stormer viscometer (KU viscosity, ASTM D562 method B – digital viscometer using a paddle as a spindle) shows a slight difference in viscosity and a slight increase of Δ = 6 KU for the sample with the ICI-builder thickener applied. The biggest difference is seen when measured with a cone & plate viscometer (ICI-viscometer according to ASTM D4287) at a high shear rate of 10,000 s-1. The increase in viscosity of the paint with the high-shear thickener was Δ = 1.11 P, which is an increase of 61% over the sample without the high-shear thickener.

That is why it is so important to assess the effectiveness of these thickeners to use viscometers working at high shear rates (10,000 – 12,000 s-1), because viscometers for KU viscosity or apparent viscosity measurements are not suitable due to too low velocity during measurements. High-shear thickeners are designed to increase the viscosity in the high shear area for better film building (providing a thicker coating when applied by roller, brush or spray application), elimination of spattering and maintaining this viscosity for a while after the shear rate drops so that the paint can pass smoothly for drying without sagging (in the mid-shear area).

The effectiveness of high-shear thickeners depends on many factors, such as the type of polymer dispersion (copolymer, surfactants, polymer particle size, surface tension), other thickeners and the strength of the association mechanism, the presence and polarity of coalescents, etc. The effectiveness of ICI thickeners also depends on the dose of active in formulation, stabilization of viscosity (initial, overnight, storage stability, etc.). Figure 3 shows the effect of the ICI-builder in question on viscosity stability immediately after preparation of the paint and after 7 days, in two doses of active substances in the paint formulation.

Figure 3. Viscosity graph with ICI thickener at two different doses and over time

Our Paint Technical Center deals with the assessment of the effectiveness of thickeners, also in the field of high-shear forces in latex paints. On the basis of the tests presented here, samples of paint formulations of various PVC are prepared, in which we test the doses of thickeners by ladder and check which of them is useful in particular ranges of shear rate. You can later find the effectiveness of these thickeners in technical materials and formulation guideliness when you receive samples from your suppliers. But how do we make the viscosity measurement itself using high shear rates?

ICI-viscosity measurement

Viscosity measurement at high shear rates is performed using a special viscometer. In the laboratory of the Spektrochem Paint Technical Center, we use the BYK CAP 2000+ L (Fig. 4) designed by AMETEK Brookfield. This model enables the viscosity of samples to be measured at temperatures from 5 °C to 75 °C. Viscosity measurement is carried out in accordance with the ASTM D4287 standard and is usually performed at 25 °C, however, it often happens that we determine the viscosity, e.g. at 10 °C for samples in which we test the effectiveness of coalescents and thickeners enabling painting at low temperatures, which allows us to conduct ICI-viscosity studies when painting at low temperatures and comparing e.g. brush drag and difficulties in application properties at different temperatures.

Figure 4. BYK CAP 2000+ L viscometer in the Spektrochem Paint Technical Center laboratory

A homogeneous sample of paint is sheared between the cone (rotating) and the plate (stationary). The viscometer plate is also the surface that thermostats the sample during the test. The swirling cone allows the paint to be sheared up to 13,300 s-1. Depending on the cone, the opening angle is 0.45° and for lower shear rates 1.8° – 3.0°. The volume of the sample to be tested is extremely small, starting from 24 µL. Fig. 5 shows a schematic view of the cone-plate configuration with blue marking where the paint is located during the measurement.

Figure 5. Schematic representation of the cone-plate interface for ICI-viscosity determination

The measurement is performed extremely quickly, because after 12-30 seconds of stabilization the result is displayed on the viscometer. The set speed is automatically converted into the shear rate for each cone, depending on its geometry. Along with the viscometer, we also use CAPCALC software, which is used for additional service in the event of the need to perform measurements in a series of changing programs of shear rate, rest periods, time readings, etc. The software also collects data on the calibration of cones with individual reference oils.

High-shear forces viscosity measurements are a supplement to the determination of the rheological properties of paints prepared with various thickeners. Assessment of rheological behavior in the full range of shear rates and supplementing these data with tests such as sagging, leveling, spattering, etc. they provide an image for creating a guide to raw materials, in this case various rheological additives for latex paints. This is especially important when introducing new and existing rheological additives to the market, because their effective operation allows professional painters to perform their work faster and more conveniently, and amateur painters to perform color changes in their homes without the need to hire a professional.

Finally, I leave a link to our video which shows in a nutshell the extent to which we test thickeners in relation to viscosity measurements in the area of high-shear forces.

PS. Don’t forget to subscribe to our YouTube channel.

Published by Artur Palasz

Scientist, paint formulator and testing expert.

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