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Influence of Polarity in Water-Soluble Polymer Synthesis Thomas W. Beihoffer , David J. Lundberg, and J. Edward Glass 1
Polymers and Coatings Department, North Dakota State University, Fargo, N D 58105
A historical perspective on the development of hydrophobe-modified, water-soluble polymers is presented. The various synthetic procedures used to obtain different associative thickeners are discussed in terms of the complexities in ionogenic monomer polymerizations. This discussion serves two purposes. Thefirstis to present the peculiarities in anionic and cationic polymer synthesis in contiguity with previous work on water-soluble polymers that related only to their use. The second purpose is to draw parallels between the discontinuities in the classical chain-growth polymerization of nonionic with ionogenic monomers and those that should be expected to occur with hydrophobe-modified monomers, but for which there are insufficient data in associative thickener technology to define properly.
Historical Development of Hydrophobe-Modified, Water-Soluble Polymers The concept of hydrophobe modification of water-soluble polymers may have arisen from the hydrophobic half-esters of maleic anhydride-methyl vinyl ether copolymers, which were commercially available products in the 1960s (1). Replacement of the methyl vinyl ether with styrene produced more stable copolymers in general (2), but hydrophobes attached through the ester Current address: Amoco Production Company, Research and Development Center, Tulsa, OK 74102
0065-2393/89/0223-0151$06.00/0 © 1989 American Chemical Society
Glass; Polymers in Aqueous Media Advances in Chemistry; American Chemical Society: Washington, DC, 1989.
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linkage were easily hydrolyzed in alkaline media. Most applied formulations are alkaline. Alternative approaches to the hydrophobe-modified maleic acid polymers can be achieved by the copolymerization of maleic anhydride with α-olefins (3). Some of the unique features of hydrophobe-modified maleic acid copolymers are discussed in Chapters 16 and 17. The synthesis of styrene-maleic acid terpolymers (SMAT) was a more serious test of associative thickener technology. The termonomer was an adduct of the reaction of a nonylphenol ethoxylate anion with vinylbenzyl chloride. The key to its limited success was the separation of the surfactant hydrophobe from the main chain by 40 or more ethoxylate units. The thrust for the development of such a terpolymer (4) was the need for improved coatings rheology (5). There were a number of production and application problems with SMAT. For example, the ethoxylation of any hydrophobe results in a hydrophobe with variable ethoxylate chain lengths and the pro duction of free poly(oxyethylene glycol) with two active hydrogens. In the reaction of the ethoxylate anion with vinylbenzyl chloride, the glycol ether byproduct reacts to form a difunctional monomer (Scheme I). In the terpolymerization of the S M A T monomers, the difunctional monomer can lead to cross-linked product and insoluble material. This result and the rapid interaction in heat aging tests of the maleic acid units with pigments and fillers in a coating formulation limited the marketability of SMAT. During these developments, hydrophobically modified ethoxylated urethane ( H E U R ) thickeners were synthesized and patented (6, 7). Their ap plication in coatings was discussed, but the emphasis was on improved barrier properties (8), not on rheological influences. The H E U R polymers of the
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NaCI
Scheme I
Glass; Polymers in Aqueous Media Advances in Chemistry; American Chemical Society: Washington, DC, 1989.
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BEIHOFFER E T AL.
Water-Soluble Polymer Synthesis
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1960s and 1970s appear to have had their greatest success in the hydraulic fluids area. In this respect, it is interesting that a recent patent (9) claims the hydrophobe modification of acrylamide as an effective hydraulic fluid additive. The H E U R thickeners of the early patents were, in general, not complementary to water-borne latex systems popular in the United States. Patents related to thickeners effective with a wider variety of latex types were introduced in the United States in the early 1980s (10-12). With market acceptance of H E U R thickeners, which were more expensive than the cellulose ethers replaced, formulators made numerous efforts to maintain constant formulation costs. The blending of thickeners achieved this goal. Blending also permitted the separation of the 7 5 - s formulation viscosity (5) from the viscosity at high shear rates (10 s" ) and thereby allowed formulation of H E U R thickeners that were ineffective viscosifiers at formulation shear rates (75 s ) but effective viscosifiers at high (10 s ) shear-rate viscosities (HSV). These H E U R products (Chapter 27) probably bear a close structural resemblance to the H E U R products of the early 1960s. The introduction of hydrophobe-modified (hydroxyethyl)cellulose ( H M H E C ) eventually resulted in the definition of a linear relationship for HSVs in blends of H M H E C with "ineffective" H E U R s (Chapter 27). 1
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Petroleum recovery applications represent such large market potentials that even coatings companies organized subsidiaries to capture a share of the sales potential. Acrylamide, for reasons to be discussed, has been the synthetic monomer of choice. It is used as a copolymer, either by partial hydrolysis or by direct copolymerization with acrylic acid (hydrolyzed poly(acrylamide), H P A M ) . The neutralized acid enhances performance both by increasing the hydrodynamic volume (and viscosity) because of electrostatic repulsions and by decreasing the polymer's adsorption (Table I) on subterranean substrates. Such high molecular weight polymers have two inherent problems: poor mechanical stability (related to elongational behavior, discussed in Chapters 11 and 12), and divalent ion sensitivity. The chelating characteristics of carboxylate groups promote precipitation in brines high in divalent ion. This behavior has prompted the use of sulfonatecontaining monomers. Table I. Static Adsorption of Polyacrylamide on Silica Powder Polyacrylamide Unhydrolyzed 15% HPAM 25% HPAM 15% HPAM
Static Absorption (^g/g) 75 65 50
