Hydrophobically associating polymer HEUR

HEUR hydrophobically associating polymer (hydrophobically modified, ethoxy-lated urethane resin)... 

Surfactant concentration (varied after polymerization) greatly affects the viscosity of associating polymer systems. Iliopoulos et al. studied the interactions between sodium dodecyl sulfate (SDS) and hydrophobically modified polyfsodium acrylate) with 1 or 3 mole percent of octadecyl side groups [85]. A viscosity maximum occurred at a surfactant concentration close to or lower than the critical micelle concentration (CMC). Viscosity increases of up to 5 orders of magnitude were observed. Glass et al. observed similar behavior with hydrophobically modified HEC polymers. [100] The low-shear viscosity of hydrophobically modified HEC showed a maximum at the CMC of sodium oleate. HEUR thickeners showed the same type of behavior with both anionic (SDS) and nonionic surfactants. At the critical micelle concentration, the micelles can effectively cross-link the associating polymer if more than one hydrophobe from different polymer chains is incorporated into a micelle. Above the CMC, the number of micelles per polymer-bound hydrophobe increases, and the micelles can no longer effectively cross-link the polymer. As a result, viscosity diminishes. 

HydrophobicaHy Modified, Ethoxylated Urethane. HEUR associative thickeners are in effect poly(oxyethylene) polymers that contain terminal hydrophobe units (66). They can be synthesized via esterification with monoacids, tosylation reactions, or direct reaction with monoisocyanates. There are problems associated with aH of the methods of synthesis. The general commercial procedure for their synthesis is by a step-growth addition of... 

HEUR associative thickeners are in effect poly(oxyethylene) polymers that contain terminal hydrophobe units. They can be synthesized via esterification with monoacids. tosylation reactions, or direct reaction with monoisocyanates. 

The two associative thickeners examined in the remainder of this text whose synthesis has not been discussed are hydrophobe-modified alkali-swellable emulsions (HASE) discussed in Chapters 25, 27, and 28, and hydrophobe-modified (hydroxyethyl)cellulose (HMHEC, discussed in Chapters 17, 18, and 27). HASE thickeners, by far the lowest cost hydrophobe-modified thickeners produced, should have achieved the largest market share on the basis of cost of production, but this situation does not appear to be the case (discussed in Chapter 28) in large part because of the poor properties observed with the lowest cost latex, vinyl acetate, used to form the continuous film. The applied-film properties 46) of vinyl acetate can be substantially improved through the use of HEUR polymers. HMHEC, synthesized by a matured (30-year-old) commercial slurry process (47) has achieved commercial acceptance, in large part because of linear high shear rate viscosities achieved in blends with HEUR thickeners (Chapter 27). 

Hydrophobically modified, ethoxylated urethanes (HEURs) are the thickeners of choice in exterior coatings and in corrosion-resistant, high-gloss industrial latex finishes. This chapter presents an overview of our research efforts and discusses approaches to the synthesis of HEUR polymers with different molecular architectures, geometrical influences on solution properties, and viable mechanisms by which this class of associative thickener effects desirable coatings rheology and applied film properties. 

Viscosity Maxima. The low-shear-rate viscosities of both commercial and model associative thickeners below their c /, values will increase with the addition of conventional low molecular weight surfactants or coalescing aid (22). With HEUR polymers, solution viscosities are observed to increase, achieve a maximum value, and then decrease with continued increase in surfactant concentration (23). This type of behavior is illustrated (Figure 5) for four commercial HEURs with a nonionic surfactant (typical of that used in coating formulations). A similar behavior has been observed (24) with a classical anionic surfactant and hydrophobically modified (hydroxy-ethyl)cellulose (HMHEC) and is reviewed in Chapter 18. Intermicellar networks, formed by the participation of one or more hydrophobes from different polymers in the micelles of conventional surfactants, were again recently suggested (25) to affect viscous solutions. 

Non-ionic associative thickeners are usually poly(ethylene oxide) polymers whose molar mass has been extended 1 some linking group and in which hydrophobic blocks have been incorporated, usually by the same linking group. Urethane linking groups are most commonly used, and this type is now commonly called a HEUR (hydrophobe-modified ethylene oxide urethane) thickener. Because they can provide well-characterized model systems, academic research has ccxicentrated on HEUR thickoiers with tominal hydrophobes, but commercial materials may contain either or both tominal and internal hydrophobes. 

Associative thickeners are low-molecular-mass water-soluble polymers with at least two hydrophobes such as hydrophobically modified ethylene oxide-urethane block copolymers (HEUR) or hydrophobically modified hydroxyethylcellulose (HMHEC). The hydrophobes can associate with themselves or with hydrophobes on surfactant, cosolvent, latex, and pigment. This sets up a loose network that is sensitive to mechanical disturbance but re-forms quickly. The result is that pigment settling and film sag are reduced because the network structure increases the low shear viscosity, but the formulations show easy... 

Finally, by way of illustration, we mention a polymer type currently receiving much attention. The polymer is referred to as hydrophobically modified ethoxylated urethane (HEUR), the reaction product of a PEG and a diisocyanate, end-capped with a long chain alcohol or amine. HEURs are already recognized as having much potential as associative thickeners in coating formulations. Their structure suggests they will show pronounced interaction with surfactants in solution and this is indeed the case. Here we refer to two recent studies. The first by Hulden (95) included surface tension studies of the polymer... 

In addition, hydrophobically modified ethoxylated urethanes (HEUR) are prepared by hydrophobe capping of urethane prepolymers. Comb, block and star HEUR s have been synthesized. (Schemes 1.2-1.4) [27]. Such polymers are very effective associative thickeners for urethane paints and coatings. 

In the second study Wang and Winnik [6] reported pyrene probe fluorescence experiments on PEO polymers end-capped with n-hexadecyl groups through urethane linkages. These polymers are examples of hydrophobically-modified ethoxylated urethane (HEUR) associative thickeners (Fig. 2.7). Two... 

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