Polymers in Aqueous Media - American Chemical Society

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Optimizing Latex Paint Rheology with Associative Thickeners Downloaded by UNIV LAVAL on July 1, 2014 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1989-0223.ch027

A Practical A p p r o a c h Freidun M . Anwari and Fred G. Schwab Coatings Research Group, Inc., 2340 Hamilton Avenue, Cleveland, O H 44114

This chapter reviews the rheological properties of one-component thickener systems and presents a practical approach to blending different thickener types to optimize the rheological profiles of latex paints. Blend ratios are used to compare thickeners of different efficiencies and rheological properties for their overall contributions to paint viscosity in different shear-rate ranges. Paint rheology was modified by changing the blend ratio to adjust high-shear (10 s ) viscosity, changing thickener molecular weight tofine-tunelow-shear (sag and level) viscosity, and in some instances, by using a third thickener to prevent clear-liquid separation. The entire process was carried out at constant mid-shear (Stormer) viscosity. 4

T H E

-1

P E R F O R M A N C E O F A L A T E X C O A T I N G is evaluated with several stand-

ardized rheological tests, each of which is intended to simulate conditions in one part of the broad shear-rate range that occurs during actual use. These tests are Leneta anti-sag (I) and N . Y . P . C . (New York Production Club*) leveling (2) ratings, Stormer viscosity (3), and I C I (Imperial Chemical Industries) cone and plate (10 s" ) viscosity (4). These tests approximately correspond to the low-, mid-, and high-shear-rate regions of a viscosity profile (5), respectively. 4

1

Although these tests give some indication of actual performance, other *Now called New York Society for Coatings Technology. 0065-2393/89/0223-0527$06.00/0 © 1989 American Chemical Society

In Polymers in Aqueous Media; Glass, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

528

POLYMERS IN A Q U E O U S M E D I A

factors are involved. ICI viscosity is used to predict properties during application; as ICI viscosity increases, brushability (6) and spreading rate (7) decrease. However, substrate texture, porosity, wettability, drying rate, and thixotropy also influence these properties. During actual application, highshear rheology, substrate porosity, and other factors will also affect leveling and sag resistance (8) through changes in film-build. Greater substrate porosity increases viscosity through wicking, which reduces flow and leveling. Often, these interactions are not present when tests are performed that simulate actual conditions, and results may be misleading. The formulator should be aware of these interactions and ideally should attempt to change

Downloaded by UNIV LAVAL on July 1, 2014 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1989-0223.ch027

only one physical property of the coating at a time.

Paints Containing One Thickener Cellulosic Molecular Weight.

A direct relationship exists between

the molecular weight (MW) of a cellulosic component and the rheological profile it imparts to a paint. Several investigators (9-11) have shown that as cellulosic molecular weight increases,

Stormer thickening efficiency and

roller spatter increase, and ICI viscosity decreases. This observation is valid only if paints of nearly equal Stormer viscosities are compared. Nonrheological paint properties are also affected by cellulosic molecular weight. In particular, overall enzymatic resistance or stability increases, and scrub resistance decreases as cellulosic molecular weight decreases (11).

H E U R Effective Molecular Weight. Several factors that influence the rheological behavior of hydrophobically modified ethoxylated urethane (HEUR)-type thickeners are discussed in the preceding chapter. Unlike cellulosic thickeners, which are well-characterized by the manufacturer (12-14), the specific structural features of the commercial H E U R thickeners are generally unknown to the formulator. In this chapter, H E U R thickeners will be described by their "effective" molecular weights on the basis of their efficiencies in achieving a Stormer viscosity (about a 50-s" shear rate). A l though the relative ordering of H E U R s may change from formulation to formulation, their effectiveness using the Stormer reference point is generally fairly constant and is designated as low (HEUR-1514), middle ( H E U R 825), or high (HEUR-275) molecular weight. H E U R - 8 2 5 is equivalent to 1

H E U R - 7 0 8 and H E U R - 2 7 5 is equivalent to H E U R - 2 7 0 . These relationships were discussed in the preceding chapters. The commercial numbers given to the products in this chapter refer to variations in nonvolatile content and cosolvents. The latex used in these studies is the large-particle-size (>500 nm) vinyl acetate/butyl acrylate binder (Ucar 367) used previously in associative thickener comparisons (15). The latex is sterically stabilized by (hydroxyethyl)cellulose grafts (16).


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Table I compares typical test results for low-, mid-, and high-effective molecular weight thickeners in a vinyl-acrylic latex paint. Many of the same trends are present with cellulosic and H E U R thickeners when the effective molecular weight designation for H E U R thickeners is used. As the effective molecular weight of an H E U R increases,

Stormer thickening efficiency,

roller spatter, and water resistance increase, whereas ICI viscosity decreases. The major difference between these two thickener types is in their low-shear behavior. As effective molecular weight increases, cellulosic paints improve in leveling and decrease in sag resistance. H E U R paints decrease in leveling and

improve in sag resistance. Other property differences between these


two thickener types are highlighted in Table II.

Modifying Latex Paint Rheology Through Blending Generally, the rheological properties that are desirable in a practical paint system are somewhere between those imparted by an H E U R and a cellulosic. Flow and leveling should be sufficient without excessive sagging, and the ICI viscosity should be high enough to ensure adequate coverage during application without causing excessive brush drag. This compromise is frequently achieved by incorporating two thickeners with different rheological profiles into one formulation. In this approach, a thickener with fairly Newtonian behavior is blended with one with a very shear-thinning rheology. Typical combinations include HEUR/(hydroxyethyl)cellulose ( H E C ) (see Chapter 26), H E U R / hydrophobically modified H E C (discussed in Chapters 18 and 19),

HEUR/hydrophobically modified alkali-swellable latex, and

HEUR/organoclay. Blend Ratio. The overall thickener level can be specified as pounds per 100 gallons or, in the case of thickener solutions, as dry pounds per 100 gallons. When two or more thickeners are combined, the thickener ratio becomes very important for predicting rheological properties. Often, the thickener ratio is expressed on a dry-pound-per-dry-pound basis. A better method is to use a blend ratio (i.e., the weight ratio of base paints blended to produce a particular thickener system) in which each base paint is made at equal Stormer viscosity and contains one of the thickeners in the system. The base paints have the same ingredients except for the thickener (often, they are made from a common pigment dispersion and letdown). For example, a 60/40 H E U R / H E C - t h i c k e n e d paint can be made by blending 300 g of an HEUR-thickened paint with 200 g of an HEC-thickened paint, both at equal Stormer viscosity. In effect, 60% of the Stormer viscosity of the final paint blend was developed by the H E U R and 40% by the H E C . By specifying thickener combinations as blend ratios, rather than as dryweight ratios, thickeners with different Stormer efficiencies can be compared more easily. Also, by fixing the Stormer viscosity at the desired value,



13.0 7.5 6.5 6.5 8.8 16.0

Thickener Level (dry lb/100 gal)

fo

"Leneta anti-sag, 12.0 = best. N.Y.P.C. leveling, 10.0 = best. Susceptibility of dry film to blistering. ^Spatter during roller application.

Low-MW H E C Mid-MW H E C High-MW H E C Low-MW H E U R Mid-MW H E U R High-MW H E U R

Thickener System (P) 1.6 1.1 1.0 3.8 2.6 1.9

ICI Viscosity 12.0 9.0 8.0 4.0 4.0 5.5

Anti-sag" 0.0 4.0 4.0 7.0 6.5 6.0

h

Level

good very good very good poor very poor very poor

0

Water Resistance

fair poor very poor good very good very good

Spatter Resistance

Table I. Rheological Properties of Latex Paint Thickeners in a 60% PVC-30% N W Interior Vinyl-Acrylic Flat at 100 K U


d

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ANWARI & SCHWAB

Latex Paint Rheology 6- Associative Thickeners

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Table II. Properties of Cellulosic- and HEUR-Thickened Paints


Property

C ellulosic-Thickened

Flow and leveling Sag resistance Spreading rates Brush drag Roller spatter Resistance properties (scrub, adhesion, and ammonia sensitivity) Gloss Cost Sensitivity to other paint ingredients (e.g., colorant, surfactant, cosolvent, dispersant, latex) Pigment settling Color-float problems Susceptibility to bacterial attack

HEUR-Thickened

Poor Good High Low High

Good Poor Low High Low

Good Low Low

Poor High High

Insensitive Little, if any Few Susceptible

Sensitive Tendency toward More Resistant

blending will affect only the low- and high-shear-rate regions of the rheological profile (see Problems with Blending for exceptions). HEUR/HEC. H E U R / H E C is probably the most commonly used thickener combination for the reasons discussed earlier. Once the Stormer viscosity has been selected, ICI viscosity can be adjusted by changing the blend ratio. Figure 1 is a plot of ICI viscosity as a function of blend ratio for several H E U R / H E C combinations. Blends that are 0% and 100% H E U R are the H E C - and HEUR-thickened base paints, respectively. These data show that the ICI viscosity of a blend is between the ICI viscosities of the base paints used to make the blend, and that the relationship is not linear. For a specific ICI viscosity, decreasing the molecular weight of the H E C will shift the required blend ratio to be richer in H E C and leaner in H E U R . Increasing the effective molecular weight of the H E U R has a similar effect. Through construction of graphs of this type for a particular formulation, the correct blend ratio to achieve targeted ICI and Stormer viscosities can be predicted. Leveling and sag resistance can be fine-tuned by deciding which H E C molecular weight to use. In blends, sagging is decreased as H E C molecular weight decreases. This behavior results from the intrinsic decrease in these properties that occurs with lower molecular weight H E C s , and because lowering the H E C molecular weight will shift the blend ratio required to obtain a targeted ICI viscosity more to the H E C side. Because H E C s impart less sagging than H E U R s , the shift in the blend ratio will further decrease this property. This shift in the blend ratio has a much greater influence on leveling and sagging than the actual thickener molecular weight because the difference in these low-shear-rate properties is greater between the two types



Figure 1. ICI viscosity as a function of thickener blend ratio in a 60% PVC/30% NW vinyl-acrylic flat at 100 KU.


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ANWARI & SCHWAB

Latex Paint Rheology à- Associative Thickeners

537


in developing Stormer viscosity that an unreasonably large quantity would be required to obtain the desired Stormer viscosity. Other thickeners may produce base paints that are so thixotropic that only an approximate Stormer viscosity can be determined. Once the rheology of a paint has been optimized through blending, the final composite formulation should be remade without blending as a final check for unforeseen interactions. The order of ingredient addition is very important when using associative thickeners, and varying the order can result in large Stormer and ICI viscosity changes (18). Maintaining the proper order of addition is not always possible when making base paints for blending. For example, when making an unblended HEUR/cellulosic paint, the cellulosic is often added during the pigment dispersion phase and the H E U R is added afterwards. When making an H E U R base paint, the liquid in the grind phase must be reduced to achieve the proper consistency for pigment dispersion. This liquid is then added during the letdown to maintain percent nonvolatile by volume ( N W ) , and changes the order of addition. Differences between the viscosity profiles of a production-made paint and the laboratory-made blend may occur. Again, the order of ingredient addition may change. Shear rate, temperature, and dispersion time often differ. Also, production-made paints are manufactured with continually changing lots of raw materials that have different properties, whereas developmental work is often carried out with only one lot. Whenever possible, laboratory conditions should simulate production conditions as closely as possible.

Other Ways To Modify Paint Rheology N W , Pigment Volume Content, and Latex. Pigment volume content (PVC) and latex type and grade are important for optimizing dry film properties. Percent N W is usually governed by the desired cost of the final product. The formulator usually does not use these variables to optimize paint rheology but should be aware of how they affect thickener efficiency. Generally, decreasing the percent N W will decrease the efficiency of thickeners and require more thickener to obtain a specific Stormer viscosity. Decreasing P V C at equal solids contents will increase the efficiency of H E U R thickeners because of their greater association with latex. This effect is much more pronounced in higher N W formulations (23, 24). Very few, if any, H E U R s work properly in l o w - N W , high-PVC formulations. Increasing P V C content will increase the efficiency of hydrophobically modified H E C because H E C associates more with pigment. Also, hydrophobically modified H E C associates more, strongly with calcined clay than with calcium carbonate or titanium dioxide (17). For H E U R s , thickening efficiency is increased as latex particle size is decreased (20, 25, 26). Because cellulosic thickeners do


538


not associate with latex or pigments, P V C or latex choice does not affect cellulosic thickening efficiency significantly.


Components with More Formulating Latitude.

For hydrophob-

ically modified H E C , hydrophobically modified alkali-swellable latex, and H E U R s , increasing the level or hydrophile-lipophile balance (HLB) of nonionic surfactants decreases their Stormer thickening efficiency, as does increasing the level of glycols or water-miscible coalescing aids (19, 20, 22, 23, 27, 28). ICI viscosity is only slightly affected by these additives. Waterinsoluble solvents have little effect on these thickeners. Colorants, with their high glycol and surfactant content, often will affect the viscosity of paints with hydrophobically modified H E C , hydrophobically modified alkali-swellable latex, or H E U R s in the thickener system. The ingredients just mentioned are frequently used to modify paint rheology (20, 22, 23). In certain formulations, H E U R s and hydrophobically modified alkali-swellable latexes have such a high degree of association that the Stormer viscosity is too high at the desired ICI viscosity. If this is the case, the level of water-soluble cosolvents or surfactants can be increased to depress the Stormer viscosity of the paint. This procedure, in effect, decreases the effective molecular weight of the thickener by decreasing the degree of association. Although this approach will work, often other paint properties are affected. Higher cosolvent levels can prolong dry time and reduce block resistance; lower levels can reduce freeze-thaw stability, coldtemperature coalescence, and open time. Surfactant type and level can affect color acceptance, defoaming, and scrub-resistance properties. Great care must be taken by the formulator to investigate all possible consequences of making these types of formula changes.

Char-Liquid Separation Clear-liquid separation is the appearance of a clear supernatant layer above the bulk of opaque paint after shelf-aging. This separation can occur within 1 day and be several inches thick in a 1-gal can. In tinted paints, colorant may float up into this layer and produce a very displeasing in-can appearance. Reincorporation of the supernatant layer by hand stirring or mechanical agitation usually solves the problem only temporarily because the clearliquid layer returns after several days. H E U R / H E C - and H E U R / h y d r o phobically modified HEC-thickened paints are particularly prone to this separation.

Blend Ratio, H E U R Effective Molecular Weight, and H E C M o lecular Weight. Table III presents data that show the effect of these variables on the clear-liquid separation that occurs in a vinyl-acrylic flat after 1 month of shelf-aging. Generally, clear-liquid separation was greatest


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Latex Paint Rheology à- Associative Thickeners 539


near a 40/60 H E U R / H E C blend ratio, fairly close to the blend that had optimum rheological properties. Although this ratio is specific to this formulation, similar results in other formulations may occur. Thickener molecular weight also had an effect on clear-liquid separation. Lower molecular weight H E C s and higher effective molecular weight H E U R s produced more separation. Surfactants. Table IV presents data that show how surfactant H L B affects clear-liquid separation in a 50/50 H E U R / H E C - t h i c k e n e d v i n y l acrylic flat. Generally, higher H L B surfactants decrease the amount of separation; the reduction is greater in low effective molecular weight H E U R systems. Small amounts of separation can be reduced or eliminated by adjusting the surfactant system. However, h i g h - H L B surfactants must be used judiciously because of their adverse effect on the Stormer thickening efficiency of H E U R s . Also, high surfactant levels may affect other paint properties such as color acceptance, water resistance, and scrub resistance. Use of a T h i r d Thickener.

The use of a third thickener is another

method for eliminating clear-liquid separation. The third thickener must be very shear-thinning to build sufficient structure at rest to prevent separation, yet break down at high shear to permit proper application. It must also be very thixotropic so that the structure will not rebuild quickly and adversely affect leveling. Once paint rheology has been optimized, a base paint containing the third thickener can be blended incrementally into the formulation until clear-liquid separation is no longer present. Only low levels should be used because the optimized rheology may change. Typical thickeners for this purpose are polysaccharides, attapulgite clays, smectite clays, and hydrophobically modified alkali-swellable latexes. Because of the low blend ratios involved with these materials, the Stormer viscosity of the third thickTable HI. Clear-Liquid Separation Occurring in a Half-Pint Can (60 mm Full) After 1-Month Shelf-Aging Thickener System Associative Low-MW H E U R Low-MW H E U R Low-MW H E U R Mid-MW H E U R Mid-MW H E U R Mid-MW H E U R High-MW H E U R High-MW H E U R High-MW H E U R

Blend Ratio (Associative/Cellulosic) 70/30

Cellulosic

30/70

40/60

50/50

60/40

Low-MW H E C Mid-MW H E C High-MW H E C Low-MW H E C Mid-MW H E C High-MW H E C Low-MW H E C Mid-MW H E C High-MW H E C

3

4

4

3

3

4 4

3

3

2

3

3

3

2

1

4

4

3

2

3

4 4

2

2

3

3

3

2

3

1 0

1 1 1 0

1 0 0 0

1 0 0 0 0

1

KEY: 0, no clear-liquid separation; 1, up to 8.3% of total depth; 2, up to 16.7% of total depth; 3, up to 25.0% of total depth; and 4, over 25.0% of total depth.


540


Table IV. Effect of Surfactant H L B on Clear-Liquid Separation After 1-Month Shelf-Aging Nonylphenol-Based Surfactant (HLB) Thickener System

4.0

12.9

17.1

Low-MW H E U R Mid-MW H E U R High-MW H E U R

3 3 3

3 2 1

3 2 0

0


N O T E : Surfactant level of 4.0 active lb/100 gal. K E Y : 0, no clear-liquid separation; 1, up to 8.3% of total depth; 2, up to 16.7% of total depth; 3, up to 25.0% of total depth; and 4, over 25.0% of total depth. "Associative thickener blended with mid-MW H E C cellulosic thickener (50/50).

ener base paint has little effect on the Stormer viscosity of the rheologically optimized blend. The results of such a study are presented in Table V.

Summary The basic formulating procedure to optimize latex paint rheology is as follows. 1. Base paints should be made, each with a different thickener, at the desired Stormer viscosity. 2. Two base paints should be blended, one with a high ICI viscosity and one with a low ICI viscosity, to obtain the desired ICI viscosity. During this process, the Stormer viscosity should remain constant. Typical thickener combinations used are H E U R / c e l l u l o s i c , H E U R / h y d r o p h o b i c a l l y modified H E C , and HEUR/hydrophobically modified alkali-swellable latex. 3. Low-shear properties can be fine-tuned in HEUR/cellulosic systems by changing thickener molecular weight in the base paints and reblending to obtain the desired ICI viscosity. Higher molecular weight thickeners may be used to improve flow and leveling; lower molecular weight thickeners are used to improve sag resistance. 4. If clear-liquid separation occurs, a third highly thixotropic, shear-thinning thickener can be incrementally blended in to determine the minimum amount required to stop the separation. Only small quantities of the third thickener should be used or the optimized paint rheology may change. 5. Paint rheology may also be changed by varying other formulation ingredients; however, possible changes in other paint properties may result.


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Latex Paint Rheology i? Associative Thickeners

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Table V. Effect of a Third Thickener on Clear-Liquid Separation After 1-Month Shelf-Aging 15.0%* 10.0% Third Thickener 5.0% 0 1 1 Polysaccharide Attapulgite clay 1 0 0 Smectite clay 3 2 1 1 2 HASE-615 2 NOTES: Third thickener blended into a 50/50 mid-MW HEUR/ mid-MW HEC-based paint that had a clear-liquid separation rating of 3. KEY: 0, no clear-liquid separation; 1, up to 8.3% of total depth; 2, up to 16.7% of total depth; 3, up to 25.0% of total depth; and 4, over 25.0% of total depth. "Level (%) of third thickener in the blend. ^Hydrophobically modified alkali-swellable latex thickener. e

a


fe

6. Remake the final formulation without blending to check for possible interactions that were not present during the blending process. This systematic approach to rheology optimization should save the formulator considerable time in developing thickener systems for good application properties. Although this method was developed mainly for use in vinyl-acrylic formulations, it should be applicable to a wide variety of latex systems. This procedure enables the formulator to modify independently the rheological profile of latex paints in different shear-rate regions with only a minimal effect on nonrheological properties. The more commonly suggested approach of changing Stormer thickening efficiency with cosolvents or surfactants and adjusting the level of thickener to produce the desired ICI viscosity often does not accomplish this goal.

References 1. Sag Resistance of Paints Using a Multinotch Applicator, Method A—Horizontal Stupes; ASTM D 4400-84. 2. Leveling Characteristics of Paints by Drawdown Method; ASTM D 2801-69. 3. Consistency of Paints Using the Stormer Viscometer; ASTM D 562-81. 4. Determination of Viscosity of Paints and Varnishes at a High Rate of Shear by the ICI Cone/Plate Viscometer; ASTM D 4287-83. 5. Patton, T. C. J. Paint Technol. 1968, 40(522), 301. 6. Patton, T. C. Off. Dig., Fed. Soc. Paint Technol. 1964, 36(474), 745. 7. Beeferman, J. L.; Bergren, D. A. J. Paint Technol. 1966, 38(492), 9. 8. Smith, N . D. P.; Orchard, S. E . ; Rhind-Tutt, A. J. J. Oil Colour Chem. Assoc. 1961, 44, 618. 9. Glass, J. E . J. Coatings Technol. 1978, 50(640), 61. 10. Blake, D. M . J. Coatings Technol. 1983, 55(701), 33. 11. Croll, S. G.; Kleinlein, R. L. In Water-Soluble Polymers; Glass, J. E . , Ed.; Advances in Chemistry 213; American Chemical Society: Washington, DC, 1986; pp 333-350. In Polymers in Aqueous Media; Glass, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

542 12. 13. 14. 15. 16.


17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28.

POLYMERS IN AQUEOUS M E D I A

Methocel methylcellulose, product literature, Dow Chemical Company. Natrosol hydroxyethylcellulose product literature, Aqualon Company. Cellosize hydroxyethyleellulose product literature, Union Carbide Corporation. Hall, J. E . ; Hodgson, P.; Krivanek, L . ; Malizia, P. J. Coatings Technol. 1986, 58(738), 65. Craig, D. H . In Water-Soluble Polymers; Glass, J. E . , Ed.; Advances in Chem istry 213; American Chemical Society: Washington, D C , 1986; pp 351-367. Shaw, K. G . ; Leipold, D. P. J. Coatings Technol. 1985, 57(727), 63. Schwab, F. G. In Water-Soluble Polymers; Glass, J. E . , Ed.; Advances in Chem istry 213; American Chemical Society: Washington, D C , 1986; pp 369-373. Glancy, C. W. J. Coatings Technol., Am. Paint &Coatings J., Aug 6, 1984. Rheolate 255 and Rheolate 278 product literature, N L Chemicals. DSX-1514, DSX-1550, and DSX-1600 product literature, Henkel. Acrysol RM-825 and Acrysol RM-1020, Rohm & Haas Company. Ucar thickener SCT-275 product literature, Union Carbide Corporation. Fernando, R. H . ; Glass, J. E . J. Oil Colour Chem. Assoc. 1986, 69, 263. Glancy, C. W.; Bassett, D. R. Proceedings of the ACS Division of Polymeric Materials: Science and Engineering 1984, 51, 348. Murakami, T.; Fernando, R. H.; Glass, J. E . Abstracts of Papers, 190th National Meeting of the American Chemical Society, Chicago, IL; American Chemical Society: Washington, D C 1985; PMSE 113. Glass, J. E. In Water-Soluble Polymers; Glass, J. E . , Ed.; Advances in Chemistry 213; American Chemical Society: Washington, D C , 1986; pp 391-416. Thibeault, J. C.; Sperry, P. R.; Schaller, E . J. In Water-Soluble Polymers; Glass, J. Ε., Ed.; Advances in Chemistry 213; American Chemical Society: Washington, DC, 1986; pp 375-389.

RECEIVED for review April 14, 1988. ACCEPTED revised manuscript October 14, 1988.


Polymers in Aqueous Media - American Chemical Society

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