10
Downloaded by UNIV OF LIVERPOOL on October 17, 2015 | http://pubs.acs.org Publication Date: January 15, 1996 | doi: 10.1021/ba-1996-0248.ch010
Associative Thickeners An Overview with an Emphasis on Synthetic Procedures Wylie H . Wetzel, Mao Chen, and J. Edward Class Department of Polymers and Coatings, North Dakota State University, Fargo, ND 58105
The lack of concern for and understanding of the structure of surfactant-modified, water-soluble polymers is widespread. This lack is surprising, because structural variations can promote significant differences in solution behavior and application properties. In this chapter, we consider the structure of surfactant-modified, water-soluble polymers and evaluate comparative differences in the placement of hydrophobes in chemically different families.
H YDROPHOBÉ MODIFICATION of three chemically different families is considered in this chapter. 1. Cellulose ethers. In this family, a few large hydrophobes or many small ethyl groups are added to hydroxyethylcellulose (HEC) to form hydrophobe-modified H E C ( H M H E C ) or ethyl H E C ( E H E C ) ; 2. Chain growth terpolymers. Three monomers are commonly modified: acrylamide, methacrylate, and styrene. All three hydrophobe-modified monomers are included in small percentages. Methacrylate and styrene contain large amounts of ionogen monomers and have at least 0065-2393/96/0248-0163$ 12.00/0 © 1996 American Chemical Society
In Hydrophilic Polymers; Glass, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.
Downloaded by UNIV OF LIVERPOOL on October 17, 2015 | http://pubs.acs.org Publication Date: January 15, 1996 | doi: 10.1021/ba-1996-0248.ch010
164
HYDROPHILIC POLYMERS 20 oxyethylene units between the hydrophobes and the methacrylate or styrene group. These terpolymers (hydrophobe-modified, alkali-swellable emulsions, or HASE) also contain small amounts of difunctional monomers that form cross-linked networks. 3. Step-growth hydrophobe-modified, ethoxylated urethane (HEUR). This series is the only one in which hydrophobe placements can be quantified, but the polymers have a broad molecular weight distribution and, depending on the molecular weight, a significant amount of unreacted poly(oxyethylene) (POE).
Cellulose Derivatives H M H E C . The reaction of alkali cellulose with ethylene oxide (discussed in Chapter 8) produces pendant nonionic oxyethylene units that promote solubility in water, in part by disruption of hydrogen bonding among the glucopyranosyl (GP) repeating units of cellulose and in part by a hydrophilic contribution arising from a unique interaction of water with the oxyethylene group (1). The distributions of oxyethylene chains on the segmentally rigid cellulose chain are given in Table I. H E C has been the primary thickener of latex coating formulations for 25 years. The limitations of formulations thickened with H E C are viscosities that are too high at low shear rates (< 1 s" ) (2), viscosities that are too low at high shear rates (10 s " *) (2), spatter and misting during roll applications (3), high water sensitivity (4), and poor gloss in applied films (5). These limitations are addressed in the references just cited. The first report on the synthesis and solution behavior of H M H E C was published in 1982 (6). This and subsequent publications from the Aqualon laboratories (7, 8) discuss hydrophobic associations and viscosity maxima with increasing anionic surfactant concentrations. On average, only three hydrophobes (ranging from C12H25- to C16H33- in size) are placed on a cellulose chain (9). The original H M H E C studies contained only 2.5 mol of oxyethylene units (molar substitution [M.S.]) per molar G P unit. This product has limited solubility in aqueous solutions because of the size of the hydrophobes, and a second H M H E C derivative, with an M.S. of 3.3, was introduced for use in coatings formulations. The structure H M H E C is illustrated in Figure 1. In the commercial or laboratory synthesis of H E C (10,11), all of the hydroxyl groups are available, and the placement of oxyethylene units becomes a statistical matter depending on the relative reactivities of the oxyanions generated (Chapter 8). The structure of H E C has been defined by 1
4
In Hydrophilic Polymers; Glass, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.
10.
WETZEL ET AL.
Associative
Thickeners
165
Table I. Characteristics of Monosubstituent Cellulose Ethers Unsubstituted Units in Trains ο/
Cellulose Ether M.S.'
Downloaded by UNIV OF LIVERPOOL on October 17, 2015 | http://pubs.acs.org Publication Date: January 15, 1996 | doi: 10.1021/ba-1996-0248.ch010
HEC
HPC
% Unsubstituted Length η Glycopyranose Units η %
1.0
37.0 ± 1.4
2.0
17.0 ± 1.1
4.3
2.1 ± 0.6
1.0
35.0 ± 0.8
2.0
11.0 ± 0.3
4.0
0.8 ± 0.3
2 3 4 5 2 3 4 5 2 3
26.4 14.9 9.1 4.9 21.9 3.6 1.8
± ± ± ± ± ± ± 0.0 11.1 ± 0.0
2.9 2.4 0.7 2.0 7.6 3.5 3.1
2 3 4 5 2 3 4 2
29.1 12.0 12.2 2.9 16.6 2.8
2.7 3.4 1.5 1.5 6.9 2.8
± ± ± ± ± ± 0.0 0.0
6.4
Adduct Units η
%
2 3 4 5 2 3 4 5 2 3 4 5 2 3 4
32.1 9.5 2.5 0.5 34.6 18.2 8.8 3.4 31.3 26.2 14.0 3.5 23.6 1.8 0.3 0.2 36.2 11.7 3.1 32.9 25.3 13.6 5.1
5
2 3 4 2 3 4
5
a b c
Degree of Order in Dry Powder b
c
10-20% crystalline
