Focus on architectural: Correct use of viscometers and side tests to characterise the rheology of latex paints

Artur Palasz, Spektrochem, demonstrates the appropriate use of equipment using standardised test methods to ensure the correct rheological properties of a paint 

We encounter the rheological properties of latex paints when we open the paint bucket. We see whether the paint has a water layer separation (syneresis), then when we start mixing it, we encounter resistance, in-can feel and sometimes also the result of settling on the bottom (sedimentation). When we pour paint into a tray, we measure its pourability and flow and when we reach for painting tools such as a brush or roller, by taking paint from the tray we can assess its brush or roller loading and in the case of spray painting, its sprayability.

This is not the end, because the paint applied to the wall, while still wet, may drip due to gravity, especially when applied in a thicker layer. Also, during the drying process, we expect that the paint will spread well enough that there will be no visible brush marks or roller marks and that it will have adequate leveling.

All these are rheological properties that both professional and amateur painters encounter. The components of the formulation are responsible for this, especially rheological additives and their interaction with other raw materials. However, in order to properly adjust rheological additives to ensure optimal rheological properties, it is necessary to learn procedures that allow for reliable and repeatable testing of rheological properties using appropriate equipment using standardised test methods and not just subjective feelings during the application of paint on the wall.

Rheological properties

As I mentioned in the introduction, rheological additives called thickeners are responsible for the rheological properties. There are several basic groups of them, characterised by a diverse chemical base, ranging from cellulose thickeners, through acrylic, polyurethane and inorganic, mainly phyllosilicates. Latex paints are formulated to meet all the needs to ensure rheological properties, starting from the paint production process, ensuring its stability in the packaging and application properties. Rheological properties are usually related to the paint viscosity, which may be characterised by advanced flow relationships, depending on the rheological additive used and the paint formulation containing the remaining components. It should be mentioned right away that most typical latex paints do not use only one rheological additive, but two or more to ensure boosting of the formulation in the area responsible for specific paint flow. Figure 1 shows diagrams of Newtonian, pseudoplastic and thixotropic flow, well known from theoretical technical materials and books.

Figure 1. Flow as a dependence of viscosity on shear rate to be correlated in formulations

Depending on the desired method of applying the paint, the surface on which it will be applied (vertical or horizontal), the thickness of the layer in which it will be applied or the interaction with the thickeners among themselves, the rheological properties are set at a level that ensures the correlation of all of them in order to provide the market with a paint with good application properties, good leveling, does not leave brush marks, does not drip in a thick layer or shows no syneresis even after several months in the package.

Each property of the paint in relation to rheology finds its characteristics as a function of the shear rate and viscosity. These relationships in the paint industry are divided into low, mid and high shear rate areas. In each of these areas there are properties that characterize the rheological properties of the paint, ranging from viscosity stability in the low-shear forces area, through in-can feel and sagging in the mid-shear area or workability when painting with a brush, roller slip or sprayability characterized in the high-shear rates area (Figure 2).

Figure 2. Characterisation of the dependence of rheological properties on shear rate

To characterise all these rheological properties, it is necessary to use appropriate test methods that allow for the measurement and interpretation of the properties responsible for them. There is often an idea to characterise these properties only using the dependence of viscosity on shear rate or shear stress, using advanced rheometers allowing multi-point viscosity characterization in the full shear rate area. However, this is not a good idea because some rheological properties cannot be interpreted this way. This includes, among others: about sagging, leveling, storage stability, workability, etc. Rheometers of this type are not applicable here, and their use can be limited only to viscosity, however, in the latex paint industry, commonly understood methods are used to measure viscosity using various simple viscometers, which are used both for research at the R&D and QC stages and all tests are performed in accordance to specific ASTM or ISO standards.

To characterise the rheology of latex paints, the rest of the article discusses how to use various measurement methods, how to properly use instruments and how to interpret the results. The measurements are divided into two categories: viscosity measurements, where viscosity is measured using various viscometers, and side (secondary) rheological properties, where other rheological properties are assessed using instruments other than viscometers.

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Viscosity measurement

Viscosity measurements are probably the most frequently performed tests for latex paints. Not only because viscosity is an important parameter, but primarily because viscosity is an indicator of many changes that occur in the paint. Viscosity drift over time indicates that the formulation should be modified because there is some incompatibility between the ingredients, viscosity is an indicator showing whether the tinting base is well adapted to accept large amounts of pigment concentrate, and finally, by interpreting the viscosity it is possible to conclude how the paint will behave when painted.

The global paint industry, and especially latex paint producers, use standardised viscosity measurements to characterize those parameters in which viscosity is an indicator of the rheological behaviour of paints. These are typically measurements made using various viscometers (Figure 4) in which the viscosity is measured representative of a given area of shear rates, as discussed earlier. There are specific standards and requirements for the viscosity that latex paints should meet in order to have specific rheological properties.

Figure 3. Station for basic viscosity measurements in the Spektrochem lab

These measurements are divided into viscosity designations using:

• Brookfield viscometers, representing low-shear area
• Stormer (KU) viscometers, representing mid-shear area
• ICI-viscometers, representing high-shear area

Each viscometer device uses the principle of measuring the resistance exerted on a rotating spindle, which, depending on the measurement method and device, has a different shape and dimensions. Figure 4 shows examples of rotors used for viscosity determinations.

Figure 4. Rotors (spindles) used in viscosity measurements (examples)

In Figure 4, the photos from the left are two examples of spindles for determining low-shear viscosity, for samples with low viscosity (L), another is a disk for determining Brookfield viscosity also in the low-shear forces range, but for standard viscosity range (HA), next to the paddle-rotor of the Stormer viscometer for determining KU-viscosity, and the first on the right is the conical rotor for determining ICI-viscosity in the high-shear area using a cone-plate viscometer.

The geometry of individual rotors, the principle of operation of viscometers and the procedures for conducting viscosity measurements are specified in standards containing guidelines ensuring repeatability and reproducibility of measurement methods. This is the first point to ensure the reliability and usefulness of such measurements. The second point is to ensure the quality of measurements by using high-class instruments and secondary materials, regularly calibrated viscometers by certified reference materials (e.g. viscosity standards) or calibrated thermometers for measuring the temperature of samples, etc.

Low-shear viscosity

Determination of viscosity in the low-shear forces area, in which the standard determination of apparent viscosity is the Brookfield method, is performed in accordance with ASTM D2196 method A. When measuring the viscosity of latex paints, it is particularly important that the test sample is repeatable each time. It is not about the composition of the paint, but about its degree of shear thinning. After all, most latex paints have pseudoplastic flow, some have thixotropic flow, so in both cases the way they are mixed before the test is crucial for the repeatability and reproducibility of the tests, which in turn translates into compliance with the specifications or lack of compliance with respect to viscosity.

Therefore, before viscosity measurements, the ASTM D2196 standard specifies, in addition to the standard bringing of paint samples to the standard temperature or other agreed-upon temperature prior to test, but also mixing the paints by shaking and setting aside for a precisely defined period of time, during which, after shaking, viscosity measurements are to take place. This approach to viscosity measurements provides an appropriate level of repeatability and reproducibility, which is missing in the ISO 2555 standard.

Viscosity measurement is performed in a standard Griffin beaker, which is also defined for repeatability of measurements. The obvious thing is to calibrate the viscometer using certified reference materials, such as calibration oils of known viscosity. In the case of known samples, where it is known that the type of measuring container used does not affect the viscosity, other types of beakers and test containers can be used. Often, such measurements at the stage of development of raw materials in formulations are performed in smaller beakers, as is also the case in the Spektrochem lab. However, for arbitral and independent tests, use of the Griffin beaker specified in ASTM D2196 is required.

Figure 5. Brookfield (HA) viscometer for standard low-shear viscosity measurements

Disks and cylinders (see Figure 4) are used as rotary elements to attach to the viscometer and their size determines the viscosity range of the samples and the viscometer that is dedicated to their use. The ASTM D2196 method A standard uses viscometers with rotating disk elements (Figure 5). This type of viscometers is used for standard measurements of the viscosity of materials such as latex paints.

However, it is also possible to measure low viscosities using the same method, but using L-type viscometers (Figure 6) with rotating cylindrical elements, which are used to measure the viscosity of some raw materials, e.g. polymer latex dispersions, thickener solutions, etc.

The measurement of apparent viscosity according to ASTM D2196 method A takes place after stabilising the reading on the viscometer in the torque range between 10% and 100%, but in practice it is best to read in the range of 50% to 95%, and the result is expressed in m · Pas (millipascal seconds). The standard viscosity measurement is used not only to determine the viscosity as a rheological component of paints, but also in various tests as an indicator of viscosity changes, e.g. informing about disturbances in storage stability.

Figure 6. IKA ROTAVISC lo-vi for low-viscosity measurements

The ASTM D2196 standard also specifies measurements using the B method, which involve determining viscosity under changing speed, degree of shear thinning and thixotropy. All these measurements are made with viscometers working according to method A, usually to the standard viscosity range in the low-shear area.

Figure 7. Degree of shear thinning of latex paint with pseudoplastic flow

The ASTM D2196 method B test is fundamentally different from the measurements often found in the paint industry using advanced rheometers determining flow curves, however, it is very useful for interpreting how the paint behaves in terms of recovery of viscosity in the low-shear forces area at two measurement points and a third additional reference point after very intensive mixing of the sample. In this way, information is obtained about the reduction of viscosity after high shear and the recovery of viscosity after readings within a set time. The test results are graphs such as Figure 7 showing a typical pseudoplastic rheological profile or Figure 8 showing a thixotropic flow profile.

Figure 8. Degree of shear thinning of latex paint with thixotropic flow

Mid-shear viscosity

The next area on the shear force diagram where standard viscosity measurements are made is the mid-shear area. In this area, the rheology of latex paints is responsible for the in-can feel, i.e. how we organoleptically feel viscosity, brush or roller loading, as well as sagging after application on vertical surfaces, or the rheology of pouring and pumpability of the paint begins in this area.

The standard measurement representing this area is viscosity, expressed in KU (Krebs Units) and measured with a Stormer viscometer (Figure 9). This viscometer measures at a steady speed of 200 rpm via a paddle-type spindle (Figure 4, second from right), and the test is performed in accordance with ASTM D562. This standard distinguishes between method A, using older types of meters requiring the use of special weights to drive the device, or method B, using a digital viscometer.

Figure 9. Stormer viscometer BYK byko-visc for measuring KU-viscosity (mid-shear)

Not every latex paint can be measured using this viscometer and the ASTM D562 method due to the limited viscosity range to a maximum of 141 KU. This depends on the market habits, where paints are prepared in such a way that their rheological properties do not involve increasing the viscosity in the low-shear forces to such a level that it is impossible to measure the viscosity in the mid-shear forces. The typical range of KU viscosity for latex paints is 90-110 KU and is characteristic of formulation habits in the USA, Canada, but also Asia and the Pacific area, as well as some EU countries. In Europe, however, the very high low-shear viscosity prevails, which makes measurements with a Stormer viscometer impossible. It is worth emphasising here that latex paints, apparently “thinner in consistency”, having a viscosity of 90-110 KU do not mean that they are too thin to paint or tend to splash. Such paints usually have quite low PVC and the thickeners used in them allow for obtaining ideal rheological properties but with apparently lower viscosity, but this is an issue for a separate article about thickeners and formulation habits.

High-shear viscosity

High shear is the ICI-area responsible for influencing the paint when applied with a roller, brush or spray. It is in this range that the appropriate viscosity will allow for easy application, roller slippage, limited brush drag as well as for film building. Measurements in this shear rate range are made with a cone-plate viscometer (Figure 10), which uses an extremely small gap in which the test sample is placed and is subjected to the shear of a rotating cone relative to a stationary plate, , which maintains the set test temperature (Peltier plate). The test is performed in accordance with ASTM D4287 at one speed, e.g. 12,000 s^-1, but it is also possible to create a curve of viscosity changes in the high-shear forces area using software coupled with a viscosimeter.

Figure 10. Brookfield CAP 2000+ viscometer for ICI-viscosity measurements (high-shear)

The result of the measurements is the viscosity in P (poise), but also graphs of the dependence of viscosity on shear rate, shear stress, etc. This viscometer is extremely useful when designing rheological properties using advanced high-shear rheology modifiers and ICI-viscosity builders.

With regard to this type of viscometers or more advanced cone & plate rheometers, there are many myths that talk about the possibility of measuring the viscosity curve in the full range of shear forces and interpreting properties such as settling, storage stability, sagging, leveling or spattering on its basis. Unfortunately, this is not true and although this is a topic for a separate article debunking these myths, later in the article I will try to explain with examples why other rheological properties, referred to as secondary rheological properties, cannot be interpreted based on viscosity measurements with a rheometer or viscometer.

Secondary rheological properties

The term secondary rheological properties or side rheological properties covers those properties of liquid paints that are also responsible for thickeners and are related to their flow, i.e. rheology, but cannot be characterised using viscometers/rheometers. We are talking about sagging resistance, leveling, spattering and storage stability. In the case of the latter parameter, a number of other raw materials are also responsible for storage stability, and thickeners are typically identified as responsible for syneresis, i.e. the appearance of a water layer after storage in the packaging. These properties, their importance in practice, and their correct use in characterizing both thickeners and latex paint formulations are discussed below.

And I will emphasise this again: These properties cannot in any way be interpreted using curves determined even by the most advanced rheometers, because they do not determine the key features for their characterisation.

Sagging

Sagging resistance is a test indicating at what gap of the applicator used for drawdown the wet coating does not drip from the vertical surface. It is very important to understand that sagging resistance expressed in mils or µm does not mean the thickness of the wet paint layer, but the gap of the applicator with which it was applied (the thickness of the wet paint layer is not equal to the gap used for drawdown using the gap applicator). Sagging resistance test is performed according to ASTM D4400 and the result is the Anti-Sag Index. This is a result derived from the largest gap at which the paint does not show sagging and a mathematical expression of the relationship between the next gap which shows sagging but describes how it merges with subsequent strips of paint (complete, half, just touching, etc.)

The test procedure for aquaeous paints is to drawdown at high speed using a multinotch applicator (Anti-Sag Meter) of the paint given before pre-shearing using a syringe and needle. This procedure is intended to reduce the viscosity caused by application in real conditions, as well as by applying a set of paint stripes of increasing thickness at one time (Figure 11).

Figure 11. Right after automatic drawdown with Anti-Sag Meter

Immediately after drawdown, the cards with the ink stripes are placed vertically and, after drying, the Anti-Sag Index is read in a manner strictly defined in the ASTM D4400 standard (Figure 12).

Figure 12. Wet paint strips of varying thickness showing sagging at a specific gap number

Anti-Sag Index assays are necessary for testing thickeners, their dosages and combinations and provide answers about differences in sagging from vertical surfaces by first complementing secondary rheological tests with any viscosity measurements. As you can see, these results cannot in any way be obtained by measuring the viscosity alone, because gap thickness and merging of the strips cannot be concluded on this basis.

Leveling

Leveling is another test about rheological properties, this time about the ability of the paint to flow and leveling marks on the surface after painting with a brush. The test is performed in accordance with ASTM D4062, which uses a special spiral-grooved LTB (Leveling Test Blade), which draws down on plain charts at high application speed. After drawing down the paint using a leveling test blade, a coating is obtained which shows all the grooves which are expected to be reduced during drying (Figure 13). After drying, the coating is compared to leveling standards. The test result is leveling on a scale from 0 to 10, where 0 is poor leveling and 10 is perfect leveling.

Figure 13. Comparison of leveling on wet paint on the right poor leveling, on the left perfect leveling

Leveling test is another assessment of rheological properties that cannot be determined using a viscometer or rheometer. This test is used not only for the development of formulations in which this parameter is optimised using thickeners, but also for many surfactants whose task is to improve leveling.

Spattering

The spattering test shows how the paint behaves when applied with a roller. There would be nothing new about it if it weren’t for the fact that it is not applied with a roller used for painting, but with a special standardised notched spool roller. It is a steel roller with grooves and indentations that provides an assessment of the paint’s tendency to spatter, excluding the influence of the type of paint roller. The test is performed according to ASTM D4400 on a wet layer of paint that has just been applied by drawdown, after which a specified number of repetitions of the roller movements are performed in reverse at a tempo measured with a metronome to ensure uniform testing (Figure 14).

Figure 14. ASTM spattering test during notched spool roller movements over a wet coating

Paint drops are caught on catch paper and assessed using standard photographic standards. The results of the tests are numerically expressed ratings, from 0 (poor spattering resistance) to 10 (no spattering), and additionally, photos from the tests are also presented (Figure 15).

Figure 15. Comparison of different spattering ratings for paints with different thickeners

Spattering resistance is another in a set of tests that are impossible to interpret after determining the viscosity curve, even with the most advanced rheometer. It cannot also be interpreted based on the viscosity results measured by Brookfield or Stormer, because this property depends on many factors related to, e.g., the association of the binder with the thickener, the characteristics of the thickener itself, etc.

Storage stability

Storage stability is an extremely important parameter, which is ensured not only by rheological properties. Waterborne paints must be suitable for storage for up to five years from the date of production and retain all their properties both as a liquid and after a formed coating. One of such parameters is viscosity stability and homogeneity during storage. These parameters include not only dependence on thickeners, but also other raw materials, but thickeners are also largely responsible for them. This is about the stability of viscosity over time, the lack of a drastic increase in viscosity resulting in deterioration of application properties, or a decrease that usually prevents workability. Another parameter is syneresis, i.e. the appearance of a water layer on the paint surface, which is permitted to a small extent. Ultimately, it is about reducing settling, but dispersants are mainly responsible for this. Storage stability tests cover, to some extent, the issues of secondary rheological properties in terms of syneresis or viscosity changes. Tests that are typically performed are accelerated stability in accordance with ASTM D1849, in which samples of liquid paint in packaging are exposed to a temperature of 125 °F (52°C) for a period of 1 month, which is also often shortened to 14 days during the development stage to verify after two weeks of initial stability.

Figure 16. Various degrees of syneresis in paints after storage stability tests

After this time, it is checked whether syneresis has occurred (Figure 16), and then, after mixing the paint, whether viscosity drift has occurred (Figure 17) using the previously described measurement methods. Syneresis assessments are made by assessing the appearance of a water layer, where at Spektrochem we have developed our scale from 1 to 5, where 1 is a very thick water layer and 5 is no syneresis.

Figure 17. Changes in the viscosity of paints with different thickeners after storage stability tests

These assessments, which are part of storage stability tests, are the last of the set of parameters discussed in this article that cannot be interpreted using only viscosity measurements, especially initial viscosity. Many technical materials indicate that a tendency to syneresis can be inferred from a specific behaviour of the viscosity curve, but this is only a theory and has no practical basis.

Summary

In summary, in this article I have extensively presented each of the test methods used primarily during the raw material development stage for its performance characterization in formulations. These methods allow for a complete characterisation of the rheological properties of latex paints and waterborne paints for many applications, mainly addressed to decorative wall paints.

These methods are clearly understood in the paint industry and test results using them can be found in many of our and other technical materials for characterising thickeners and building formulations to optimize rheological properties. You cannot focus only on viscosity, assessed using single-point viscosity measured using a Brookfield or Stomer viscometer but the rheological properties must be expanded with side rheological properties, also called secondary ones. They provide extremely practical knowledge that is impossible to obtain even with the most advanced rheometers, which are often incorrectly used to interpret all rheological properties.

 Artur Palasz, Ph.D., SPEKTROCHEM – Technical Center of Raw Materials for Architectural Paints, Poland

E-mail: [email protected]