Alkali-swellable thickeners

The subject of surfactant-modified, water-soluble polymers, briefly discussed in Water-Soluble Polymers, is addressed in the last three sections (Chapters 16-28) of this book. These associative thickeners are covered in detail, ranging from the maleic acid copolymers of variable compositions introduced in various commercial markets in the early 1960s to the most recent entries (that is, in the open literature), hydrophobe-modified poly (acrylamide). Chapter 23 is complementary to the spectroscopic studies in Chapters 13-15 it explores new approaches to understanding associations in aqueous media. The three hydrophobe-modified polymers that have gained commercial acceptance in the 1980s, (hydroxyethyl)cellulose, eth-oxylate urethanes, and alkali-swellable emulsions, are discussed in detail. In particular, hydrophobe-modified (hydroxyethyl)cellulose, which is... 

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). 

The industrial problems limiting earlier industrial thickeners in the coatings area were overcome with HEUR thickeners. With increasing sales, production limitations were addressed and the batch processes were replaced with semicontinuous techniques. Hydrophobe modification of industry standards (hydroxyethylcellulose and alkali-swellable emulsions) produced by proven industrial processes entered the market and broadened the technology. 

Alkali Swellable and Alkali-Soluble Thickener Technology... 

Alkali-swellable and alkali-soluble thickeners (asts), as defined herein, are carboxyl functional copolymers produced by the free-radical polymerization of ethylenically unsaturated monomers. The copolymers are substantially insoluble in water at low pH, but exhibit thickening on swelling or dissolution in aqueous media at higher degrees of ionization. In their ionic form (partially or fully neutralized), ASTs generally belong to the broad... 

The AST process receiving the greatest industrial and academic attention by far has been that of emulsion polymerization, and because of the relative importance of the emulsion thickeners, these polymers will be the object of considerable discussion in this review. When conventional ASTs are specifically prepared by this process, alkali-swellable or alkali-soluble emulsions (ASE) are obtained. For conventional ASTs produced by processes other than emulsion polymerization (nonemulsion), the acronym ASNE has been adopted herein. The associative ASTs have analogous designations. HASE is the common acronym for hydrophobically modified, alkali-swellable or alkali-soluble emulsion, and HASNE is the adopted acronym for the associative nonemulsion thickeners. The family of AST polymers reflecting this classification scheme is shown in Figure 2. 

Figure 2. Classification scheme for the family of alkali-swellable and alkali-soluble thickeners (AST). All acronyms are defined in the text and in the list of Symbols and Abbreviations.

Shay Alkali-Swellable and Alkali-Soluble Thickener Technology and Improvement Claims in AST Patents... 

The thickener is an encapsulated alkali-swellable polymer (EASP) dispersion. 

X HE RELATIVE COST OF MANUFACTURING hydrophobically modified, water-soluble polymers is the primary determinant in their market acceptance. The lowest-cost associative thickeners used in coating formulations are hydrophobically modified, alkali-swellable emulsions (HASE). The extensive history leading to the development of this technology is reviewed in the preceding chapter their utilization and limitations are discussed in Chapters 27 and 28. 

HASE (hydrophobically modified alkali-swellable emulsion, discussed in Chapters 25 and 28) and HEUR thickeners are readily displaced from acrylic latex surfaces (32) by sodium dodecyl sulfate (SDS). A surface-active cellulose ether was also reported (33) to desorb from monodispersed poly(styrene) latices with SDS addition. In these studies, the relative critical micelle concentrations of the anionic surfactant and thickener appear to be more important than buffering of surface charges. 

Hydrophobically modified alkali-swellable latex thickeners (Chapter 25) can also be used as cothickeners for HEURs, and, like hydrophobically modified HEC, do not suppress the ICI viscosity of HEURs. Therefore, blend ratios of HEUR/hydrophobically modified alkali-swellable latex systems will have less HEUR thickener and be less expensive than HEUR/cellulosic systems. Some hydrophobically modified alkali-swellable latexes are even more cost-efficient than cellulosic thickeners, so a further savings may be realized. Another advantage to this approach is that the HEUR/hydrophobically modified alkali-swellable latex thickener systems are not biodegradable however. 

The reduction of scrub resistance is the main disadvantage of HEUR/ hydrophobically modified alkali-swellable latex thickener systems 18, 19). Hydrophobically modified alkali-swellable latex-thickened paints often show increased water and alkali sensitivity, reduced wet and dry adhesion, and lower scrub resistance than similar cellulosic-thickened paints. 

Two base paints should be blended, one with a high ICI viscosity and one with a low ICI viscosity, to obtain the desired ICI viscosity. During this process, the Stormer viscosity should remain constant. Typical thickener combinations used are HEUR/cellulosic, HEUR/hydrophobically modified HEC, and HEUR/hydrophobically modified alkali-swellable latex. 

Hydrophobically modified alkali-swellable latex thickener. 

Rheology results are shown in Figure 5 for combination of dispersants with a commercially available HASE (hydrophobically modified alkali swellable acrylic emulsion) thickener. 

Chem. Descrip. Acrylic alkali-swellable associative thickener Uses Thickener in interior flat paints, textiles Featiaes Provides low cost/high performance alternative to traditional cellulosic thickeners provides outstanding resist, to roller spattering, better film build and leveling alkali-sol. 

Uses Thickener for latex systems, textile coatings, adhesives Features High efficiency alkali swellable highest efficiency between pH 7-8 produces short and buttery rheology Properties Vise. 10 cps 30% NV by wt. 

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