Dispersants associative thickeners

Syneresis. This chapter began with consideration of the depletion layer effect. This phenomenon can be seen in coatings that contain large latices (>300 nm) not highly stabilized by surface-attached (hydroxy-ethyl)cellulose fragments (16), and is in part the problem observed in the last sections of Chapter 27. The phenomenon is not necessarily restricted to HEC-thickened formulations and depletion flocculation. In our studies, syneresis is observed in thickened aqueous solutions and in dispersed systems containing the model trimer associative thickener (Scheme II) it can be overcome by addition of conventional surfactants. Syneresis in HMHEC-thickened solutions is discussed in Chapter 19 in the absence of a dispersed phase. Syneresis is discussed in the following chapter where additives that substantially enhance low shear viscosities are added to inhibit syneresis. 

Depletion layer effects occur in associative thickener formulations when the latex is larger in size ( 500 nm) and not highly stabilized with surface (hydroxyethyl)cellulose fragments. Syneresis is also observed in simple aqueous solutions and in latex dispersions when the hydrophobicity of the associative thickener is high. 

The presence of surfactants, besides altering the latex particle surface, can also interact with the water-soluble polymer. For instance, poly(ethylene oxide) homopolymer and block copolymers interact with sodium dodecyl sulfate surfactant [109], and hence alter the latex viscosity behaviour [110]. Other water-soluble polymers are also capable of interacting with specifle surfactants [111]. When pigmented latex dispersions are thickened with associative thickeners one must consider the interactions with some of the pigment stabilizers [112] and other additives, like coalescing aids [113]. 

This exciting field of biodegradable poly(aspartic acids) has generated many new opportunities for anionic polymers. Some examples of an ever-expanding list include dispersants for oil production [133], tobacco filters [134], cosmetics [135], hydrophobic associating thickeners [136], adhesives [137], and CTOSslinked superabsorbents [138-140]. A recent review on polyaspartic acids up to 1997 includes a large body of information [141]. 

Latexes are typically low in viscosity since they are water-based dispersions. However, in some cases, it may be necessary to increase the viscosity of the final latex product. For example, the viscosity of latex paints should be such that they flow evenly on a substrate, but do not run off. Latex viscosities may be controlled by the addition of a viscosity modifier (thickener) to the aqueous phase (371). Polyethylene oxide (PEO), hydroxyethyl cellulose (HEC), and various associative thickeners (such as HEUR (91, 281)) or HASE (130, 282) thickeners) are often used as viscosity modifiers. 

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