With the increasing demand for low-VOC or VOC-free waterborne coating systems, there has been a call for a new class of thickeners -- known as modern rheological additives. These additives allow specific modification of the viscosity of the coating system at various shear rates.2-5 As a result, such key coating properties as vertical flow, leveling, gloss, film thickness, covering power, spattering tendency, brush and roll resistance, sedimentation tendency and pigment stabilization are significantly affected. In most cases, only small quantities of between 0.1 and 4%, calculated on the formulation, are enough to obtain ideal flow properties for a particular application. In addition, such additives also have an influence on the properties of the coating after its application, for example on adhesion and elasticity and on the resistance to soiling, abrasion, water, and corrosion.

Consequently, the right choice of rheological additives for a specific task is becoming increasingly important,3,6-7 especially as optimum flow properties play a major role in the success of waterborne coating systems. The large number of different, commercially available products does not make this choice very easy. The aim of this article is to first give a brief summary of the different rheological additives on the market, and to then make recommendations on the choice of suitable systems with the aid of a few selected examples.

Coarse Classification of Rheological Additives

Rheological additives can be roughly divided according to their chemical nature into inorganic and organic thickeners, with a subsequent distinction being made between thickeners for solventborne systems and thickeners for waterborne systems (see Figure 1).

A more useful classification, particularly with the variety of types suitable for aqueous systems, which will be dealt with in more detail later, can be made on the basis of their rheological profiles, e.g., the viscosity as a function of the shear rate used.8 Unlike Newtonian liquids, in which the viscosity is not dependent on the shear rate applied, surface coatings are complex multi-component systems that usually have pseudoplastic properties. This means that their viscosity falls rapidly as soon as significant shear forces act on the system.

If we take this distinguishing characteristic as a basis, thickeners can be broadly divided into two different groups.9

One group is made up of the high-molecular-weight, non-associative thickeners such as cellulose ethers and polyacrylic esters. Complementary to these are the associative thickeners such as HEUR polyurethane thickeners (Hydrophobically modified Ethylene oxide Urethane Rheology modifiers), HASE (Hydrophobically modified Alkali-Swellable Emulsions) and HMHEC (Hydrophobically Modified Hydroxy Ethyl Cellulose) (see Figure 2).

Dissolved, non-associative rheological additives exert a thickening effect by forming a three-dimensional structure through interaction with the aqueous phase. The interactions with other components of the coating system can be ignored. They are highly effective especially at low shear rates (from approx. 0.001 to 1 sec-1) and medium shear rates (from approx. 1 to 1,000 sec-1), but have esentially no effect at high shear rates. Important advantages and disadvantages of the two most common non-associative thickeners are shown in Figure 3.

Unlike the previously mentioned group, the associative thickeners interact only weakly with the aqueous phase. They have an ambiphilic structure from a hydrophilic base polymer, which is linked with defined quantities of hydrophobic segments. In the case of polyurethane thickeners, for example, these are polyether polyols reacted by way of a diisocyanate with a long-chain alcohol. The other associative thickeners, produced by way of different synthetic pathways, have similar structures. Their thickening effect is attributed to the fact that the relevant hydrophilic segments react weakly with the aqueous phase, whereas the hydrophobic side chains form relatively stable associates with the hydrophobic ingredients of the coating. In this way, the raw materials become three-dimensionally, reversibly and physically crosslinked, so that corresponding formulations retain virtually constant viscosity even at medium and especially at high shear rates up to approx. 10,000 sec-1. Figure 4 shows their main strengths and weaknesses.

Because of the different thickening effect, it is difficult to compare the rheological profiles of associative thickeners with those of non-associative types. The rough comparison in Figure 5 illustrates the strengths and weakness of the various thickener types.

The given shear ranges are of differing importance for the surface coating. While in the lower shear range, properties such as pigment stabilization, syneresis, anti-settling tendency and leveling are negatively affected, the medium shear range is of importance for properties such as coating consistency, miscibility and pumpability. High viscosities, even under high shear, are responsible for film buildup, covering power, spattering tendency, brush drag and rolling resistance.1

Important Factors for Choosing a Suitable Rheological Additive

When selecting one or more suitable rheological additives for a formulation, various factors must be considered. To begin with, it is the kind of coating system and method of application that play the most important role. Decorative paints for doors and window frames and emulsion paints for brushing or rolling make different demands on the rheological characteristics of a coating system than industrial coatings for application by spraying, roll coating, curtain coating or dipping. Since rheological additives can also influence film properties such as gloss, water resistance, weather stability, wash resistance and corrosion resistance, the intended application -- and consequently the specifications for the surface coating -- are of particular importance when selecting a rheological additive.

Type of Coating System

Which rheological additive is suitable for a given system depends on the coating formulation and, in particular, on the type of binder being used. The widest choice is with formulations based on polymer dispersions that can be combined very well with both non-associative and associative (apart from a few exceptions) thickeners. By nature, formulations with a high binder content react far better to the addition of associative thickeners than formulations with a low binder content (see Figure 6). The same can be said of fine dispersions, which respond far more intensively to the use of associative thickeners than coarse dispersions.

If, however, a thickener is needed for an emulsion (e.g., an epoxy or alkyd resin emulsion), the choice is somewhat more limited. Here, associative additives are generally ruled out because they thicken the emulsion merely through intrinsic association, which is not particularly effective. In such cases, alternatives would be high-polymer additives such as cellulose ethers or acrylate thickeners.

Application

The intended method of application is another factor influencing the choice of a suitable rheological additive. If the paint is to be sprayed, good leveling (results in good gloss and covering power) and a low sagging tendency (prevents the paint from running down on vertical surfaces, also called curtaining) are needed. Ideally, use would be made here of a thickener with pronounced pseudoplastic properties. High shear forces, introduced as the coating passes through the spray nozzle, lower the viscosity enough to ensure good leveling of the spray droplets. Subsequently, the higher initial viscosity returns, and sagging is avoided. The high level of effectiveness of a pseudoplastic thickener under low shear forces can, in appropriate formulations, also prevent sedimentation of the fillers and pigments.

The properties of a paint can not directly be related to its rheological profile, but the direct comparison of rheological profiles allows predictions of the paint's behavior as illustrated in Figure 7. Here, a water-reducible two-pack polyurethane top coat formulation (white) based on Bayhydrol VP LS 2235/1 combined with Bayhydur VP LS 2319 was modified with various associative thickeners to generate an optimized rheology for spray application. Without any rheology modifier (graph a) the coating shows insufficient flow and sagging. The choice of a less pseudoplastic thickener (graph c), results in an even worse flowing paint which is not affected by different thickener concentrations. Finally, the use of the right PU-thickener (graph b) results in a paint that convinces due to its excellent flow and reduced sagging.

For dip application, an even more extreme pseudoplastic profile is required. On the one hand, the thickener must prevent the pigments and fillers from settling in the low shear range, while on the other, the viscosity at low shear forces (as are generated on dipping a substrate) must be reduced to such an extent that the substrate is completely wetted by the coating. As it is pulled out of the dipping bath, the viscosity of the applied coating must increase again considerably to prevent sagging.

With curtain coating, the result is dependent on the stability of the curtain coating film. Any tearing of the film leads to coating defects. In such cases, Newtonian (high shear) thickeners have proved particularly useful because they keep the coating viscosity virtually constant, even in zones of different shear.

Coatings, including emulsion paints, for brushing or rolling make similar demands on the rheological profile and necessitate a thickener that is highly effective even at high shear rates.1 Good coating results can only be achieved by brushing or rolling if the viscosity of the relevant coatings remains high even under high shear rate (brush drag). If the brush drag is inadequate, a considerable proportion of the coating applied to the surface is pushed along in front of the brush or roller without sticking to the substrate. This results in poor coverage, uneven application and poor leveling (see Figure 8).

In addition, a relatively high viscosity at high shear also prevents the coating from being carried away by centrifugal forces too easily from the brush or roller. In this way, a suitable high-shear thickener can have a significant effect on the spattering tendency of a paint (see Figure 9).

Use of the Coating System

Apart from the kind of rheological profile created by a rheological additive, there are other properties that play a key role in whether a certain additive is suitable for a specific application. Cellulose ethers, for example, have properties that sometimes make them irreplaceable, especially for emulsion paints. Through their intensive association with water, they delay the drying of an emulsion paint (water retention) and prevent the water from "disappearing" into highly absorbent substrates (see Figure 10). Thus they help to create a uniform coating even on surfaces with varying absorption. The higher the quantity of cellulose ether (normally approx. 0.5%) in the formulation, the better the effect.

Formulations in which the price is the dominant factor generally contain a relatively small quantity of a high-molecular-weight cellulose ether -- enough to allow the desired low shear viscosity to be attained easily and inexpensively. The poorer water retention is normally tolerated. However, if the formulation must also satisfy the demands of professional painters, it is normal to use combinations of low-molecular-weight cellulose ethers and higher molecular weight grades. In this way, larger quantities of cellulose ether can be incorporated without raising the low shear viscosity to such an extent that the coating becomes "pudding-like."

For extremely cheap emulsion paints, the cellulose ether can also be replaced with an acrylate thickener.10 It must, however, be taken into account that coatings formulated in this way are sensitive to pH fluctuations, have inadequate water retention and also have certain deficits in their water resistance, which means that they can only be used indoors.

If, in addition, the brush drag and anti-spattering tendency of the paint are to be optimized, this is done with the aid of suitable high-shear thickeners. New polyurethane thickeners such as Borchi Gel VP 0013 and the virtually twice as effective Borchi Gel VP 0024 have been developed specifically for this purpose (see Figure 11).

Although good water retention of cellulose ethers may be important for an emulsion paint, this property also restricts its application. High quantities of cellulose ether have a negative influence on the water resistance of the surface coatings. When developing the formulation, a compromise therefore often has to be found between water retention, acceptable low shear viscosity, and good wash and abrasion resistance. In facade paints for outdoor application, for example, the use of cellulose ethers is usually limited for these reasons.

In paints exposed to weather, it is therefore more common to use only polyurethane thickeners. Although they are often not as effective as acrylate thickeners or cellulose ethers in the low shear range, they do have very good weather stability and water resistance. Thanks to the variety of commercially available products that are optimized to the different shear ranges, it is possible to create almost any desired rheological profile (see Figure 12).

When optimizing a coating with associative thickeners, it must however be kept in mind that the effectiveness of the respective products can be influenced very considerably by the other raw materials in the coating formulation. In particular, the type and quantity of the binder and its fineness play a major role (see Figure 13).

Polyvinyl acetate dispersions, for example, are very difficult to thicken, whereas styrene acrylates and straight acrylates generally thicken relatively easily. Differing polarity or morphology of an acrylate dispersion also exerts a considerable influence on the effectiveness of an associative thickener, as is shown by the comparison between various commercially available acrylate dispersions (see Figure 13).

However, other components in the coating such as solvents and emulsifiers also have a considerable influence on the rheology. Nonpolar solvents, for example, intensify the thickening effect of associative thickeners, whereas polar solvents (e.g., glycol ethers) appreciably reduce in particular the low shear thickening.3,11

Conclusion

In an article such as this, it is only possible to give a general picture of the variety of decision criteria for or against a particular kind of rheological additive. A true optimization of the rheological properties to the requirements of a coating system can only be achieved through practical work with the relevant systems. However, keep in mind that expert advice can save a lot of time and money.

This article was originally presented at the 6th N?rnberg Congress, April 2001, in N?rnberg, Germany.

For more information on rheological additives, contact Peter Manshausen, Borchers GmbH, Alfred-Nobel-Stra?e 50, Bldg. 6610, D-40765 Monheim, Germany; phone +49 (0) 2173 38 2678; fax +49 (0) 2173 38 2696; e-mail peter.manshausen. pm@borchers.de; or Circle Number 139.

References

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