28 Hydrophobically Modified Alkali-Soluble Emulsions as Thickeners for Exterior Latex Paints S. LeSota, E. W . Lewandowski, and E. J. Schaller Rohm and Haas Research Laboratories, Spring House, P A 19477
Hydrophobically modified water-soluble polymers, used by latex paint manufacturers as associative thickeners, provide significant advantages in improving film build and leveling, and decreasing spatter from roller-applied paints. Nonionic hydrophobically modified ethoxylated urethane (HEUR) thickeners provide good performance in both interior and exterior paints, but are more costly than conventional thickeners. Low-cost hydrophobically modified alkali-soluble emulsion (HASE) thickeners, although particularly well-suited for use in interior wall paints, present a risk of early blistering over chalking repaint surfaces when used in exterior house paints. Their water sensitivity is increased until the volatile base used to solubilize the thickener has left the film. Ionic cross-linking of the HASE thickeners in the paint film by incorporating soluble zinc complexes or zinc oxide into the paint formulation can significantly reduce the risk of blistering early in the life of the film.
HYDROPHOBIC
A L K A L I - S O L U B L E E M U L S I O N (HASE) T H I C K E N E R S
impart
viscosity, flow, film b u i l d , gloss, a n d spatter resistance to i n t e r i o r latex paints (1—3); h o w e v e r , t h e y r e d u c e the w a t e r resistance o f e x t e r i o r latex paints to the extent that early b l i s t e r resistance o v e r some r e p a i n t surfaces is s e v e r e l y d o w n g r a d e d . W h e n state-of-the-art
h i g h - a d h e s i o n latex b i n d e r s are u s e d
w i t h H A S E t h i c k e n e r s , no b l i s t e r i n g p r o b l e m occurs o v e r n o n c h a l k i n g latex or o i l r e p a i n t substrates, o r c h a l k y o i l substrates. H o w e v e r , c h a l k i n g latex paints pose a severe risk of early b l i s t e r i n g to H A S E - t h i c k e n e d paints a p p l i e d
0065-2393/89/0223-0543$06.00/0 © 1989 American Chemical Society
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over them. Despite this blistering problem with H A S E thickeners, there is still a strong interest in using these inexpensive rheology modifiers in exterior paints. Reducing the amount of other water-sensitive paint ingredients helps, but not satisfactorily. This chapter describes ionic cross-linking of the H A S E thickener with zinc oxide or zinc ammonium complexes to improve early blister resistance. Zinc ammonium complexes have been used in ammonia-removable latex floor polishes for decades. Paint formulations containing zinc oxide have benefits but are prone to sporadic viscosity instability, which sometimes cannot be predicted by standard laboratory tests. However, many paint manufacturers have learned how to successfully stabilize these zinc oxidecontaining latex paints. Unlike conventional alkali-soluble thickeners, H A S E thickeners are in the class of anionic rheology modifiers also known as associative thickeners or polymeric surfactants. These thickeners are water-soluble polymers con taining hydrophobic groups capable of forming intermolecular associations and adsorbing onto the surfaces of dispersed particles; thus they provide thickening power much greater than unmodified polymers of similar mo lecular weight (1,2). The more widely used nonionic, hydrophobically mod ified ethoxylated urethane ( H E U R ) associative thickeners provide good performance in both interior and exterior paints, but are more costly than conventional and H A S E thickeners.
Experimental Details A typical exterior acrylic paint formulation (see Appendix A) was thickened with a commercial HASE thickener that contained 5.23 meq/g of carboxylic acid function ality (4). This thickener was modified with 1 to 25 lb (1 lb = 0.4536 kg) of zinc oxide (French process) in the mill base in one set of paints and with post-added zinc ammonium complex such as acetate, carbonate, bicarbonate, or glycinate in another set of paints. Two coats of these paints were applied 5 h apart over a heavily chalked latex paint on a white pine board. Because significant differences have been seen between individual chalky test substrates, common control paints containing unmodified HASE, (hydroxyethyl)cellulose (HEC), and H E U R thickeners were included in each test. Test paints were dried for 18 h at 25 BC and 50% relative humidity and then placed in a fog box for 5 h. (The fog box is a Plexiglas (poly(methyl methacrylate)) box with six spray nozzles on the inside top that continuously spray a fine mist of water over the painted panels placed about 15B off the vertical in racks about 10 in. (1 in. ζ 2.54 cm) below the nozzles.) We used two different fog boxes; one sprayed deionized water and one sprayed tap water. The paints were then rated for blistering by using American Society for Testing and Materials (ASTM) pictorial blister standards (5). An ASTM blister rating of 7 was chosen as the minimum acceptable level because experience has shown that these blisters will usually shrink after drying and leave no perceptible sign that blistering had taken place. Repeated rain or fog spray showed no further blistering. Presumably, some of the water-soluble material that caused blistering had been washed out of the film.
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Portions of these paints were oven aged for 10 days at 60 °C, as an indication of long-term stability. Freeze-thaw tests were also run that consisted of five cycles at -25 °C.
Results and Discussion Zinc Oxide.
We found that as little as 5 lb of zinc oxide per 100 gal
(1 gal = 3.785 L) of paint was sufficient to impart acceptable early blister resistance to the paint when tested in the rain or in the tap water fog box (Table I). Higher zinc oxide levels showed no further blister-resistance improvement in this paint. Blistering in the deionized water fog box was so severe that even the HEC-thickened control had a blister rating of 4 D . (The note to Table I explains the rating method used.) However, a noticeable effect occurred at zinc oxide levels of 5 lb /100 gal of paint and higher. The zinc oxide combined with the ammonia in the paint and formed a zinc ammonium complex that may ionically cross-link the carboxyl groups in the H A S E thickener and other paint constituents, and thereby reduce the overall water sensitivity of the paint. Zinc Ammonium Complexes. The zinc ammonium complexes were tested on a different chalky latex panel than the zinc oxide paints, and the entire series showed less blistering in the same deionized water fog box (Table II). The HEC-thickened control did not blister on this board. The H E U R control showed 7 M blisters on this board vs. 2 M D (Table I). Addition of the zinc ammonium complexes resulted in much-improved early blister resistance. Blister-resistance effectiveness differed depending on which complex was used. The four zinc ammonium complexes evaluated in the H A S E Table I. Blister Resistance of ZnO-Modified Paints with HASE Thickener: Comparison of Three Blister Tests HASE ASTM Blister Ratings Rheohgy Thickener, ZnO, Modifier or IbllOO gai lb/100 gai Deionized WaterTap Water Zn, Thickener of Paint of Paint eq/COOH Rain Fog Box Fog Box 0
HEC HEUR HASE HASE HASE HASE HASE HASE HASE HASE
— — 3.3
3.6 2.8 2.7 2.7 2.7 2.7 3.6
0 0 0 1 5 10 15 20 25
b
— — 0.5 4.0 8.6 13.6 17.9 22.1 0.5
4D 2MD ID ID 2D 2MD 2MD 2D 2D ID
8M to 10 105MD 5M 10 10 10 10 10 5M
10 7F 4M 5F 10 10 10 10 10 5F
"ASTM ratings: 10 is best, 0 is worst; the numbers refer to blister size and the letters refer to the population density of the blisters, as illustrated in ref. 5. Abbreviations: F, few; M , medium; M D , medium to dense; and D, dense. ^Zinc ammonium carbonate.
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Table Π. Blister Resistance of HASE-Thickened Paints Modified with Zinc Ammonium Complexes HASE Thickener, Zn Ammonium dry lb/100 gal Complex of Paint Glycinate
Acetate
Bicarbonate
Carbonate
None H E C control H E U R control a
3.00 2.94 3.08 2.93 2.94 2.60 2.36 2.79 2.34 2.19 2.74 2.48 2.33 3.30
— —
COOH eq in HASE Thickener Zn, eq 0.016 0.015 0.016 0.015 0.015 0.014 0.012 0.016 0.012 0.011 0.014 0.013 0.012
— — —
0.017 0.023 0.034 0.068 0.017 0.026 0.034 0.017 0.026 0.034 0.017 0.026 0.035
— — —
Zn,
eqICOOH 1.06 1.53 2.13 4.53 1.13 1.86 2.83 1.06 2.17 3.09 1.21 2.00 2.92
— — —
Deionized Water Fog Box
0
3MD 3MD 3M 10 3MD 6MD 10 3MD 5MD 10 2MD 5MD 10 ID 10 7M
A S T M blister ratings are described in Table I.
thickened acrylic paint in decreasing order of effectiveness in blister resist ance were acetate > carbonate > bicarbonate > > glycinate > > > no com plex. The stoichiometry at effective zinc ammonium complex levels suggests that zinc reacts with more carboxyl groups in the paint (latex, dispersants, etc.) than in the H A S E thickener. E v e n with the most efficient complex, zinc ammonium acetate, more zinc was required to obtain good blister re sistance than the stoichiometric equivalent of the carboxyl groups in the H A S E thickener (Table II).
Fog Boxes vs. Rain. Like many other accelerated tests, the fog box blister-resistance test shows trends rather than an exact correlation with the results observed in actual rainfall. When deionized water is used for the spray in the fog box, the test is most stringent, and blister differences can be observed in paints that do not blister when the much less stringent tap water fog is used. The tap water test correlates more closely to what is generally seen on the test fences when paint films no more than a few days old are exposed to rain. Early blistering due to rain depends on the rain's intensity, duration, and on how long the paints have dried before it rains. The p H of the water may also be an influential variable. The deionized water fog box had a p H of about 8 and the tap water fog box had a p H of 7, whereas the rain had a p H of about 4. The relatively more alkaline deionized water would be ex-
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pected to attack the alkali solubles in the paint more than the acidic rain would. Table II compares the zinc oxide paint results from all three sources of water. The more stringent deionized water fog box showed that even the HEC-thickened paint blisters badly, as does the H E U R paint, which shows no blistering on exterior exposures. The zinc cross-linked H A S Ε-thickened paints blistered much more in the deionized water fog box than they did in the tap water fog box or in the rain. No significant differences were noted between 5 and 25 lb of zinc oxide per 100 gal of paint. Paint Stability.
Table III shows that paints containing the higher
levels of zinc oxide had a greater viscosity drop after heat aging, and gelled after three freeze-thaw cycles. At 0.5 eq of zinc per C O O H unit, both the zinc oxide and zinc ammonium carbonate paints were stable. Table IV shows that paints containing the higher levels of zinc ammo nium complexes also generally showed the greatest viscosity drop on heat aging. Table IV also shows that these paints had poorer freeze-thaw stability. Close attention to formulation techniques, such as the judicious choice of dispersants and surfactants, can minimize or eliminate this problem in zinc oxide-containing latex paints.
Conclusions Ionic cross-linking of the water-sensitive carboxyl groups in the H A S E thick eners (and probably other carboxyl groups in the paint) with either low levels of zinc oxide in the mill base or post-added zinc ammonium complexes can be effective in improving the early blister resistance of H A S Ε-thickened latex paints over heavily chalked latex paint surfaces. O f the zinc ammonium complexes evaluated, the zinc ammonium carbonate and bicarbonate apTable HI. Viscosity Stability of Zinc Oxide-Modified Paints with HASE Thickener After Heat and Freeze-Thaw Testing ZnO, lb/100 gal of Paint
0.5 4.0 8.6 13.6 17.9 22.1 0.5
96 97 113 112 105 118 112 84
—
0 1 5 10 15 20 25 a a
Zn, eq/COOH
Equilibrated Viscosity (KU)
Viscosity Change After Heat Aging (KU) -3 -6 -9 -11 -12 -30 -26 -4
Zinc ammonium carbonate.
American Chemical Society Library 1155 16th St., N.W. Washington, O.C. 20036
Viscosity Change After Freeze-Thaw (KU) -2 -5
gel gel gel gel gel 1
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Table IV. Viscosity Stability of Zinc Ammonium Complex-Modified Paints After Heat and Freeze-Thaw Testing
Zn Ammonium Complex
Zn, eq/COOH
Equilibrated Viscosity (KU)
Viscosity Change After Heat Aging (KU)
1.60 1.53 2.13 4.53 1.13 1.86 2.83 1.06 2.17 3.09 1.21 2.00 2.92
91 91 94 94 92 90 94 90 88 90 89 89 93 96
-1 -1 -3 -4 -2 -3 -12 -3 -6 -11 -3 -5 -10 0
Glycinate
Acetate
Bicarbonate
Carbonate
—
None
Viscosity Change After Freeze-Thaw (KU) 9 7 12 41 gel gel gel 0 -5 gel -1 2 44 10
peared to have the combined advantage of providing acceptable blister re sistance and ability to maintain freeze-thaw stability in this particular paint formulation. However, the contribution of other water-sensitive species in the latex paints, varying requirements for the amount of H A S E thickeners to achieve required viscosity, and sensitivity to test conditions make it im portant that the performance be critically assessed under actual rainfall.
Acknowledgment We are indebted to M r . Charles W. Griffin for the preparation and testing of these paints. His exceptional cooperation is much appreciated. Appendix A. Acrylic Exterior Paint Formulation XW-64-7 Zn Material Water Tamol 960 (40%) Triton N-57 Potassium tripolyphosphate Colloid 643 defoamer Titanium dioxide (Ti-Pure R-902) Zinc oxide (AZO-66LP) or zinc ammonium complex Minex 4 Icecap Κ
Concentration (lb/100 gat) 100.0 7.1 4.0 1.5 1.0 225.0 0-25 148-160 50.0
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P r o c e d u r e : M i l l these ingredients o n a h i g h - s p e e d m i l l ( C o w l e s dissolver) for 20 m i n ; at l o w e r speed, a d d as follows:
Material Rhoplex A C - 6 4 (60.5%) Colloid 643 defoamer Texanol Super Ad-It Methylcarbitol Ammonia, 28%, to p H 9.5 H A S E thickener, H E U R , or H E C and water to 90 K U
Concentration (lb/100 gal) 306.0 3.0 9.3 9.0 59.0
— —
The formulation constants are as follows: pigment volume content, 45%; volume solids, 36%; Stormer viscosity, 90 K U ; and pH, 9.5.
References 1. Schaller, E. J. Surf. Coat. Austr. 1985, 22(10), 6. 2. Shay, G . D.; Rich, A . F. J. Coat. Technol. 1986, 58(732), 43. 3. Hall, J. E.; Hodgson, P.; Krivanek, L.; Malizia, P. J. Coat. Technol. 1986, 58(738), 65. 4. Acrysol TT-935; product bulletin 81A61, Rohm and Haas Company, Spring House, PA, October 1984. 5. Pictorial Standards of Coatings Defects; Federation of Societies for Coatings Technology: Philadelphia, P A , 1979. R E C E I V E D for review February 29, 1988. A C C E P T E D revised manuscript October 17, 1988.
