New Associative Thickeners and Their Use In Waterborne and High-Solids Coatings
Thickeners are included in coating formulations to bring about certain specific required rheological properties. A coating’s rheology influences the properties of the coating during manufacture, storage and application. Traditionally thickeners such as the cellulosics have been used successfully in waterborne and bentonite as well as hydrogenated castor oils in solventborne coatings. However, approximately two decades ago a new class of thickeners, known as the associative thickeners, was proposed for use in coatings and, in the meantime, found a variety of applications. These thickeners offer properties including improved flow and leveling. So far, associative thickeners have only been known for application in waterborne coatings, notably dispersion paint.
The trend to reduce the emission of organic solvents led to an increased interest in high-solids paint. These paints are usually formulated with resins characterized by a relatively low molecular weight. The low molecular weight of the resin explains the stronger sensitiveness to sagging during the application and storage of high-solids paint. To avoid sagging, additives such as bentonites, hydrogenated castor oils, silicates and colloidal clays are used. The incorporation of these solid materials in the formulation is rather difficult and requires strict conditions. Moreover, these additives may show a detrimental effect on the appearance — notably the gloss — of the paint film or lose their effect during the stoving procedures or lead to poor leveling.
Liquid rheology-control agents have been used as well in solventborne coatings. These include over-based sulphonate gels, polyamide waxes and sulphonated castor oils. The strengths and weaknesses of traditional thickeners are summarized in Table 1.1
In this article, the mechanisms, properties and applications of polyurethane associative thickeners in waterborne coatings will be addressed. Furthermore, a new class of associative thickeners, identified as solventborne associative thickeners (SBAT), will be introduced for use in organic solventborne coatings.
Associative Thickeners in Waterborne Coatings - Generic Properties of Associative Thickeners for Waterborne CoatingsOne of the most important developments in the area of additives for waterborne coatings is the use of associative thickeners, and their use has so far been restricted to waterborne coatings only. The associative thickeners may be identified chemically as styrene-maleic anhydride terpolymers (SMAT), hydrophobically modified alkali-swellable emulsions (HASE) and hydrophobically modified ethoxylated urethanes (HEUR).
The most popular associative thickeners in waterborne coatings are the HEUR thickeners, also known as PUR (polyurethane) associative thickeners. These thickeners offer substantial benefits to the coating formulator in comparison to the traditionally used thickeners.2 PUR associative thickeners are a group of synthetic thickeners characterized by a relative low molecular mass (about 10,000–50,000). They permit the formulation of waterborne coatings, with rheological properties virtually identical to those of alkyd resin coatings (see Table 2). The properties of these PUR associative thickeners (compared with cellulose thickeners) are summarized in Table 2.
Figure 1 presents a generic but characteristic viscosity profile of a dispersion paint, thickened with a polyurethane associative thickener vs. a similar paint thickened with one of the conventional thickeners.
Chemical Composition of PUR Associative ThickenersThe PUR associative thickeners usually contain nonionic hydrophobic polymers, which are available either in liquid form (e.g., as a 50% solution in water and/or organic solvents) or in powder form. The PUR polymers are obtained by reacting diisocyanates with diols and hydrophobic blocking components. An example is the chemical structure shown in Figure 2. In this structure, R and R’ are each a hydrophobic, aliphatic or aromatic group.
In the molecule, a distinction can therefore be made between the following three segments.
- Hydrophobic terminal segments
- Several hydrophilic segments
- Urethane groups
The polymer is end-capped with hydrophobic segments. Possible hydrophobic segments are, for example, oleyl, stearyl, dodecylphenyl and nonylphenol. To give optimal association interactions, more complicated hydrophobic segments (e.g., more than one separated hydrophobic group in each segment) are preferred for practical applications. The decisive factor for the viscosity increase effect is that each molecule contains at least two terminal hydrophobic segments. The hydrophilic segments used are polyethers or polyesters. Examples are polyesters of maleic acid and ethylene glycol and polyethers, such as polyethylene glycol or polyethylene glycol derivatives. However, polyethers are the preferred hydrophilic segments, as these polymers offer the best chemical resistance and hence the best viscosity stability during storage of the paint. The polymer chain is extended by polyisocyanates. Possible isocyanates include IPDI, TDI and HMDI. The reaction conditions are carefully controlled in order to obtain narrow molecular weight distributions and optimal reproducibility. The product properties of these PUR thickeners are determined not only by these base components but also by the ratio of hydrophobic to hydrophilic segments.3
Thickening MechanismThe presence of hydrophobic and hydrophilic groups in the same PUR thickener molecule indicates a certain surface activity. On solution in water, formation of micelles does in fact occur above a characteristic concentration. In contrast to monomeric surfactants, the same PUR associative thickener molecule may be present in more than one micelle. This results in the formation of structures that reduce the mobility of the water molecules and an increase of the viscosity, which may result in the formation of a gel structure (see Figure 3). The viscosity increase achieved with a PUR associative thickener is the sum of various physical phenomena, notably:
1. Increase in the viscosity of the solution by dissolution of the PUR polymer
2. Micelle formation and/or formation of links between PUR micelles
3. Association with emulsion polymer particles
It was found empirically that, when used in dispersion paint, the contribution to the viscosity increases in the order of 1 < 2 < 3.3 Mid shear thickening (Brookfield viscosity or shear rates of 1 up to approx. 100 sec-1) is assumed to be largely influenced by the micelle formation: relatively weak interactions that are broken spontaneously under shear conditions. The association of the hydrophobic groups with the surfaces of the emulsion particles is more important for the viscosity increase during use in emulsion systems. Because each PUR associative thickener molecule contains at least two hydrophobic segments, it is possible for two emulsion polymer particles to be linked to one another via the PUR molecule and to form a sort of “skeleton.” The polymer particle is bound to the micelles of PUR associative thickener molecules in exactly the same way (see Figure 3).
The extent to which association takes place with the polymer particle depends on the properties of the hydrophobic group and on emulsion particle surface properties. The strength of the structure build-up between the PUR associative thickener and the dispersion particle obviously depends on the strength of the association of the PUR thickener to the dispersion particle surface.
In practice, the strength depends on the composition of the association bonds and the number of the bonds per associated molecule. The quality of the bonds, as far as the thickener molecule is concerned, is influenced by the physical characteristics of the hydrophobic segment. Increasing the hydrophobicity tends to increase the adsorption and as a result increases the viscosity.2 Regarding the quantity aspect, it has been proven that the number of bonds per molecule is proportional to the thickening effect under high shear conditions.4
It may be assumed that the interactions between the thickener molecules present in one micelle are unstable and may be already disrupted under low shear conditions. This indicates that the association with the dispersion particle largely determines the high shear thickening. Increasing the number of bonds per associated molecule makes a detraction of this molecule from the surface thermodynamically less attractive. It has been found that associative thickeners with multiple hydrophobic functionality are very effective high-shear thickeners. The mechanism for high-shear thickening is demonstrated in more detail in Figure 4.
Application PropertiesAs presented earlier, PUR associative thickeners function only through interactions with other components of the coating system. This indicates that the final thickening performance not only depends on the type, quality and dosage of the PUR thickener, but also on the other components of the coating system. This is certainly relevant to the properties of the polymeric dispersion particles as well as the composition of the liquid phase. From practical results, it is obvious that there is a relationship between the thickening effect and the total surface area of the dispersion particles; stronger thickening is observed if the total surface area is increased. Also, water-soluble organic solvents and anionic surfactants decrease viscosity, mainly in the low-shear rate range. Contrarily, water-insoluble coalescents increase the high shear thickening. Optimal performance is reached if the coating formulation is adjusted to suit all ingredients and conditions involved in the formulation, storage and application. The structure built up between the PUR associative thickener and the emulsion particles is relatively resistant to mechanical effects, leading to a virtually Newtonian flow behavior. Figure 5 demonstrates the rheology profile of a PVC–dispersion paint, thickened with a PUR associative thickener (SER-AD FX 1010) and an HEC thickener.
Further optimization of the extreme high-shear viscosity is reached by using a combination of a standard or a mid-shear thickener, and a high-shear associative thickener as described in the Thickening Mechanism section. The advantage of a further increase of the high-shear viscosity is mainly seen in low-PVC, high-gloss paint. However, the high-shear thickener may also be applied in conjunction with typical low-shear thickeners, to increase brush-drag. The effect of the addition of a typical high-shear associative thickener on the rheology profile of a low-PVC, gloss paint is demonstrated in Figure 6.
Properties such as flow and leveling of a paint layer are greatly determined by the viscosity in the low and mid shear range, typically in the range between 0.01 and 1 sec -1. The thickening properties of polyurethane thickeners in this range are much weaker than, for instance, the thickening effect of cellulosics. This explains the improved flow and leveling properties of paint based on associative thickeners opposed to paints thickened with cellulosics. The strong thickening effect in the region of high shear velocity explains the improved brush-drag — which results in higher film build during application — compared with, for example, cellulose thickeners. Related to this high viscosity under high shear condition is the increased film-build, layer thickness and consequently the improved opacity of the paint layer: a one-coat coverage can be achieved (see Table 3). Associative thickeners prevent spraying during roller coating. This is due to the relatively low molecular weight of the PUR molecule. Precisely because of the low tendency to spray, PUR associative thickeners are used in dispersion paints with a medium to high pigment content and are generally combined with a cellulose thickener.
Associative Thickeners in Solventborne Coatings - A New Class of Liquid Thickeners for SB CoatingsConventionally formulated solventborne paints usually show an almost Newtonian flow behavior and differ considerably in this respect from dispersion paint systems. Thickeners are added to adjust the viscosity, for instance, to avoid sagging during the film formation or to reduce pigment sedimentation. Contrary to waterborne coatings, the solventborne coatings are characterized by dissolved resin molecules, rather than dispersed polymeric particles. So any similar kind of association thickening as seen for associative thickeners with binder particles in waterborne coatings seems to be unlikely in solventborne coatings. However, dispersed pigment particles in solventborne coatings offer adsorptive sites for association interactions. This is exactly what is used for the newest class of associative thickeners: associative thickeners for use in solventborne coatings (abbreviated as SBAT).
Characteristics of SB Associative ThickenersThe SBAT thickeners designed for use in SB coatings belong to the polyurethane class of polymers (see Figure 7), and are illustrated by a schematic model. The chemistry of these SBAT compounds is quite complex and different from their counterparts being used in aqueous coatings. In contrast to PUR associative thickeners discussed for use in WB paint, SBAT has associating groups distributed more or less randomly down the polymer chain. Nevertheless, a main similarity is in the low molecular weight of the polymers, which are characterized by multifunctional groups capable of hydrogen bonding and their nonsolubility in water and solubility in most organic solvents. The association is obtained by way of the adsorptive functional groups in the PUR molecule. The associative SB thickener may be formulated as a low viscosity liquid. The adsorptive groups are based on ester-, amide-, and/or urea-functionalities.
A main advantage of these SBAT thickeners is that they are liquid and show good stability to higher temperatures (see Table 4).
Thickening MechanismIt is known that polymers with randomly distributed associating groups down the chain when placed in polar media will form either gels that fracture in steady shear or strongly shear-thinning solutions or that undergo shear induced gelation. The particular behavior observed depends upon concentration and molecular mass, on the density of associating groups and on the strength of the association.5,6 It is supposed that comparative interactions, based on hydrogen bonding — instead of associations through hydrophobic segments as with aqueous solutions — play a main role in the thickening mechanism of SBAT. The thickening is based on association interactions of the SBAT molecule and the pigment particles, and the solvents as well as intermolecular bonding between the thickener molecules. As a result of all these physical interactions, a pseudo-polymeric network is built. However, the actual kinetics of the physical interaction involved in the formation of the gel structure are very complex and not all fully understood at present.
- Intermolecular Associations. The intermolecular association of SBAT molecules is explained by amide-amide hydrogen bonding and amide-ester bonding (see Figure 8–9).
- Association onto the Pigment. Hydrogen bonding is assumed as being involved with the interaction between the SBAT molecules and the pigment surface, for instance through amide-hydrogen bonding (see Figure 10), leading to an increase of the apparent pigment volume and consequently to volume restriction. Moreover, these adsorbed SBAT molecules contribute to the formation of an extensive network with the dissolved SBAT thickener molecules.
- Association with the Solvent. Hydrogen bonding is also involved in the intermolecular association between SBAT molecules and solvent molecules. This contributes to an increase of the molecular volume of the SBAT molecule and chain entanglement.
The overall result of these associations is the formation of a structure that contributes to an increase in the viscosity of the coating system (see Figure 11).
Notably, the interaction with the pigment seem to be very essential for the thickening efficiency: experimentally it has been shown that the thickening effect of the SBAT thickener in nonpigmented clear varnishes is very limited (see Table 5). It is assumed that the contribution to the formation of a gel structure diminishes in the following order.
1. The association of the thickener to the pigment.
2. Intermolecular association between thickener molecules.
3. Association with the solvent.
The indicated interactions under 2 and 3 are of second order, but they do support and strengthen the contribution to the primary thickening effects as indicated under 1. Given the low molecular weight and consequently the low cumulative functionality of each SBAT thickener molecule, all these structures are considered as being loosely associated. The attractive forces are easily disrupted under application stress conditions such as involved during paint mixing, spraying or brushing. The network is already broken under such relatively mild shear conditions, explaining the strong shear thinning of coatings formulated with these SBAT thickeners. After paint application and removal of the shear field, the network is quickly rebuilt leading to improved sag resistance.
The effect of pigmentation on the thickening efficiency of SBAT can be seen in Table 6.
Application PropertiesThe addition of the SBAT thickener improves the rheology properties of SB coatings by increasing the low-shear viscosity providing the coating with a pseudo-plastic or thixotropic flow behavior (see Figure 12). This results in improved anti-sagging and anti-settling properties in which a good leveling is maintained (see Table 7). The influence on the mid- and high-shear rheology is very limited. A main advantage is the easy incorporation, which is related to the liquid aspect of the thickener. Operating temperature during incorporation is not relevant. The molecular mass of the SBAT thickener is rather low and consequently the number of bonds per SBAT molecule the hydrogen bonds are easily separated under mechanical shear conditions, such as involved during the paint application. Consequently, the paint formulated with SBAT thickener is easy to apply. SBAT is crosslinked during the curing process with typical hardeners such as aminoplasts and isocyanates and becomes part of the polymer matrix. This explains the excellent mechanical and film exposure properties, with respect to the low molecular mass of the SBAT (see Table 8). Solvents showing a high polarity (e.g. Butanol) reduce the thickening effect of SBAT and their use in the coating formulation should be limited as much as possible in order to reach optimal benefit of the SBAT.
ConclusionAssociative thickeners offer interesting properties to waterborne and solventborne coatings. In most typical coating systems these associative thickeners can be stirred in under mild agitation at any common temperature. Post addition also is applicable. Benefits obtained in waterborne coatings include optimal flow and leveling properties, reduced roller spattering, improved opacity and water resistance. A new class of a liquid associative thickener is used in solventborne coatings and offers improved sag resistance, also during stoving conditions.
This paper was presented at the International Waterborne, High-Solids and Powder-Coatings Symposium Feb. 10-12, 1999, in New Orleans.
For more information on thickeners, contact Richard Meijer, CONDEA Servo LLC, P.O. Box 365, 2 Turner Place, Piscataway, NJ 08855; call 732/981.5478; fax 732/981.5497.