Associative cellulosic 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). 

A few years ago, Landoll (2-4) reported that grafting a small amount of long-chain alkyl hydrophobes onto a nonionic water-soluble polymer leads to associative thickening behavior (i.e., enhanced viscosity, surface activity, and unusual rheological properties). This chapter deals with the general methods of preparation and solution properties of hydrophobically modified nonionic WSPs. Particularly described are the solution properties of hydrophobically modified (hydroxyethyl)cellulose (HMHEC) in aqueous and surfactant systems. 

HMHEC polymers are improved rheological modifiers for latex paints. They retain the benefits of traditional cellulosic ethers (compatibility with a wide range of coatings ingredients, storage stability, better sag resistance, etc.) but overcome many deficiencies of synthetic associative thickeners (sensitivity to latex systems and pH of paint formulations, poor sag resistance, etc.). The combined advantages offered by HMHEC are best suited for interior and exterior flat paints (5). 

The HMHECs used in these experiments provided good examples of an associative thickener, in that clear evidence of a critical aggregation concentration was seen from the dilute-solution behavior. Although some limited molecular aggregation may occur below this concentration, once this concentration is achieved, a three-dimensional network starts to form. This network formation leads to a significant enhancement of adsorption onto polymer latex particles from which the surfactant has been removed. The adsorption density is high for a cellulosic polymer of the equivalent molecular weight. 

Hydrophobically Associating Copolymers. Hydrophobically modified cellulose derivatives (28) and N-alkylacrylamido copolymers (24, 25, 27) were among the first nonionic associative thickeners reported in the patent literature. The concentration of hydrophobic units allowed for dissolution in aqueous solution is usually less than 1-2 mol %. Like conventional polymers, apparent viscosity is proportional to molecular weight and concentration. However, with associative copolymers, a very dramatic increase in apparent viscosity occurs at a critical concentration, C, which clearly is related to a phenomenon other than simple entanglement. Viscosity dependence on hydrophobe concentration, size, and distribution suggests mi-croheterogeneous phase formation. Surfactants enhance viscosity behavior in some instances (24), yet clearly reduce viscosity in others (i). 

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. 

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. 

Associative thickener blended with mid-M W HEC cellulosic thickener (50/50). 

The introduction of these new rheology modifiers provides paints manufacturers with a very attractive alternative to cellulosic thickeners. These rheology modifiers are the first acrylic associative thickeners that offer a similar rheology, in-can feeling, appearance, and application properties as cellulosic thickeners. They also show some key advantages compared to cellulosic thickeners. Supplied as liquids, they are easier to handle and because of their polymeric chemistry, they are less sensitive to microbial attack In application trials, professional painters found that new rheology modifier based paint performs similarly to cellulose thickened paint but with a better spattering resistance. 

Thickeners, mainly cellulose derivatives (e.g., methyl cellulose, ethylhydroxy-propyl cellulose) or polyacrylates, are generally used in emulsion paints. Recently polyurethane thickeners (associative thickeners) with more favorable leveling properties are also increasingly used. 

Because the HEUR thickeners are relatively low in molar mass (30000-100000 g mol ) compared to the alkali soluble or cellulosic associative thickeners, and the poly(ethylene oxide) backbone is so flexible, almost all of the thickening power comes from associations [106,107]. Molar mass must be high enough to provide efficient network formation, but veiy high molar mass simply dilutes the effectiveness of the hydrophobic clusters [108-110]. While the placement of the hydrophobes may have subtle effects on surface-active behaviour, the size and number of hydrophobes appear to be fee major determinants of performance [110,111]. 

Once an associative thickener is completely desorbed from the latex, it behaves like a non-adsorbing conventional thickener and can flocculate the latex by the depletion mechanism (see Section 13.3.1.1). Flocculation phase diagrams can be constructed showing the state of flocculation or deflocculation of the latex at a specified solids content, over a range of thickener and surfactant or cosolvent concentrations [97]. Because of their lower molar mass and less volumefilling backbones, the threshold concentration for desorbed HEUR thickeners to cause depletion flocculation is much higher than that for high molar mass cellulose ethers. 

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. 

Chem. Descrip. Carboxylated styrene-butadiene copolymer latex Uses Associative thickener for adhesives, suitable for coated and uncoated paper. Mylar, cellulose acetate, metalized polyester, aluminum, nonskid coating, and mastic substrates Features Very efficient... 

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. 

As will be shown in the next section, the rate of sedimentation decreases with increasing volume fraction of the disperse phase, and ultimately approaches zero at a critical volume fraction rji (the maximum packing fraction). However, at p, the viscosity of the system approaches oo. Thus, for most practical emulsions, the system is prepared at high molecular weight polymers (such as xanthan gum, hydroxyethyl cellulose or associative thickeners), finely divided inert solids (such as silica or swelling clays) or a combination of the two. 

Hydrophobically modified polymer chains, referred to as associative thickeners, can produce gels at low concentrations, e.g. hydrophobically modified hydroxyethyl cellulose (Natrosol Plus, Hercules) or hydrophobically modified poly(ethylene oxide) (Rhom and Haas). 

Associative thickeners increase the viscosity of the paint by interacting with other paint ingredients through bridging of particles (surface to surface) or interaction of adsorbed layers. The interactions are assumed to be through hydrophobic forces. The operative mechanism is illustrated in Figure 5.9. Important groups in this class are associative acrylic thickeners, associative cellulose ethers and hydrophobically modified ethoxylated urethanes (HEUR). 

Figure 5.9 Operative mechanism of associative thickeners 5.3.3.2 Cellulosic thickeners...

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