Flooding and floating in latex paint.

Flooding and floating are problems in many paint applications. If pigment concentration is uniform on the surface but not through the thickness of the film, one refers to 'flooding' (horizontal separation). If, however, concentration differences are visible across the surface of the paint film, one refers to 'floating' (vertical separation). In this article, the influence of pigment, filler, additives, and processing conditions on the flooding and floating of colored latex paint were investigated. It was discovered that too broad a distribution of pigment and filler particle size can lead to flooding and floating. Different levels of pigment (Ti[O.sub.2]) or filler (kaolin) loading cause diverse degrees of flooding and floating. Waterborne coatings that do not exhibit flooding or floating may show these conditions when diluted. Using dispersants or thickeners with hydrophobic constituents, increasing viscosity, reducing surface tension, etc., all help to prevent or reduce flooding and floating. Comparison tests revealed little influence of processing conditions on flooding or floating.

Keywords: Flooding, floating, latex paint, exterior wall coating

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When pigment and emulsion dispersions in waterborne paint are not stable, asymmetric separations can take place. They are often accompanied by flocculation. If there is sufficient dissociation and flocculation, stripe or grid patterns can be seen on films. This defect is called color floating. In other cases, the separations are rather regular, pigments concentrate on the surface, causing a uniform color difference from the normal paint. This is called flooding. Floating may be looked upon as a vertical separation of pigments, and flooding as a horizontal separation. Figures 1 and 2 show floating in a latex paint and its conversion to flooding with the addition of silicon oil. Flooding and floating occur during the application of colored latex paint. (1) They complicate color matching, waste color paste or pigment, and can hurt appearance, flow and leveling, hiding power, tint strength, gloss, and the resistance of the paint film to water and alkali. (2-4)

It is widely accepted that there are many components and factors that influence flooding and floating. Among these factors are:

(1) Stability of pigment and emulsion dispersion -- Inorganic pigments in aqueous coatings have been investigated using atomic force microscopy and microprobe analyzers. (5) Dispersability of organic pigments aggregation degree has been determined, (6) and rheological, electrokinetic properties and surface chemistry of waterborne dispersions have also been studied. (7-8) When excessive flocculation and precipitation occur, flooding and floating happen. So absorbing suitable dispersants on pigments and forming an optimum absorption layer will exert a beneficial influence on flooding and floating resistance.

(2) Flow currents within the film (9-10) -- In the wet film, as water volatilizes, the temperature, surface, and interfacial tension will decline, more hydrophilic pigments will be carried with water to the surface, and Benard cells are formed. (11) Benard cells in a wet film are illustrated in Figure 3. Benard cells will persist until the coating is too viscous for the particles to move. In many cases, flooding and floating are more likely to occur in humid circumstances than in dry air. Increasing the viscosity and reducing the surface tension of the system can alleviate flooding and floating. (12)

(3) The emulsion used -- Binder, like pigment, requires surfactants for dispersion and stabilization. If the emulsion and color paste are not compatible, or if the emulsion or color paste is deprived of surfactants, the stability of the dispersion will be reduced, and flooding and floating may appear. (13) So testing compatibility between emulsion and color paste before production is essential. Methods for assessing pigment dispersion have been compared by Van et al. (14)

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

(4) Application conditions -- Humidity, temperature, and processing are also influential. (15)

A variety of approaches have been used to alleviate flooding and floating, such as forming coflocculates, (16-17) using leveling agents, (18) or adding shear thickeners. (19) However, how the essential components in latex paint influence the defects of flooding and floating has seldom been reported.

In this article, the influence of pigment, filler, additives, and processing conditions on flooding and floating is studied. Correlative measures to prevent or alleviate flooding and floating are also proposed.

[FIGURE 3 OMITTED]

EXPERIMENTAL

Materials

Primal AC-261, from Rohm and Haas, was used as emulsion. CPS Monicolor universal color pastes were used for color. Dupont Ti[O.sub.2], kaolin from Jinyang in ShanXi, China, and talc from Longguang in GuangXi, China were used as pigment and fillers. Henkel and BYK additives were used as dispersants and defoamers, etc.

Instruments

An MP200A electronic scale from Shanghai, China and a GFJ-0.4 high speed dispersing plant in Shanghai, China were used to produce the paint. A 480KU viscometer from Sheen Instruments Ltd., U.S. and a Brookfield DV-II viscometer from Brookfield Engineering Laboratories, U.S., ICI cone and plate viscometer from Research Equipment Ltd., a QXD-25 to QXD-150 fineness of grind gauge from Tianjin, China, a tensionmeter 70535 surface tension apparatus from CSC-Dunouy, and a WGG-B three-angle digital glossmeter from Fujian, China were used to evaluate and survey the experiments.

Experimental Design

The acrylate emulsion and color paste were tested for compatibility. First, the emulsion and color paste were blended at a 50:1 ratio. After storage at 50[degrees]C for 30 days, the fineness was measured. If the fineness was below 30 [micro]m, the emulsion and color paste were considered compatible. If the fineness was above 50 [micro]m, they were considered incompatible. If it was between 30 and 50 [micro]m, they were considered partially compatible. Other ingredients were let-down and the latex paint was produced. Paint was applied on the substrate (asbestine plank) to form films and flooding and floating of the wet films were evaluated.

To study how pigment and filler influence flooding and floating, we designed the following experiments:

(1) Different amounts of Ti[O.sub.2] (4, 10, 23 wt%) and various amounts of monoazo red (0.1, 0.5, 1, 2, 5, and 10 wt%), were added to the basic paint.

[FIGURE 4 OMITTED]

(2) Using a formulation with 10 wt% Ti[O.sub.2] in the basic paint, colored by 0.1 wt% monoazo red, we added 5, 10, and 15 wt% water to dilute the paint and stored it naturally for seven days.

(3) With no change to the other ingredients in the paint, we used differing kaolin contents; the kaolin contents used were: 8, 11, and 14, and 17 wt%, colored by monoazo red and carbon black both at 0.1 wt%.

(4) We produced three groups of paint with kaolin and talc of various particle sizes, Ti[O.sub.2], and other ingredients as above:

(a) kaolin (38 [micro]m) + talc (38 [micro]m)

(b) kaolin (38 [micro]m) + talc (10 [micro]m)

(c) kaolin (10 [micro]m) + talc (10 [micro]m)

(d) kaolin (2 [micro]m) + talc (10 [micro]m)

All were colored with monoazo red and carbon black, both at 0.1 wt%.

(5) Several groups of surfactants were investigated to reveal how the surfactants influence flooding and floating:

(a) 0.8 wt% SN-Dispersant 5040

(b) 0.8 wt% SN-Dispersant 5027

(c) 0.2 wt% SN-Dispersant 5040 + 0.6 wt% SN-Dispersant 5027

Other ingredients were unchanged; the formulations were colored by monoazo red and carbon black, both at 0.1 wt%, and then brushed. After the films dried, we measured the brightness and saturation to compare dispersion stability.

(6) Three types of thickeners were used to increase viscosity: cellulose QP-4400, acrylic C[R.sub.2], and polyurethane SN-612. We adjusted the viscosity between 85-95 KU with the three thickeners; QP-4400 was added before other ingredients, while C[R.sub.2] and SN-612 were added in the last phase of production. They were all colored by 0.1 wt% monoazo red and carbon black.

(7) To see whether processing affects flooding and floating of coating, we added 0.1 wt% monoazo red in the coating in four different ways: (a) Dispersed with titanium under high speed agitating; (b) Added after high speed agitation dispersion of titanium and filler, and before emulsion addition; (c) Added after the basic paint is produced, that is, the last in order; and (d) Color paste premixed with emulsion, then added into the paint in the usual order. Flooding and floating were evaluated for each after natural and accelerated storage at 50[degrees] for 30 days.

[FIGURE 5 OMITTED]

RESULTS AND DISCUSSION

Influence of Emulsion

The results of the compatibility experiment of acrylate emulsion and color paste are shown in Table 1.

It can be seen that most color pastes are compatible with the emulsion, except that carbon black paste is partially compatible. However, in the paint produced with the same emulsion and different paste, flooding and floating appear in varying degrees, in which monoazo red and carbon black paint are most severe. That is why we chose monoazo red and carbon black paste for the following experiments. In the case of quinacridone violet and delphine violet, white color floats on the surface. It can be concluded that, in most cases, the emulsion is not the main cause of flooding and floating.

Influence of Pigments and Fillers

The influence of Ti[O.sub.2] content on flooding and floating is shown in Table 2 and Figure 4. It can be seen that paints with different Ti[O.sub.2] content differ in flooding and floating. This is because pigment particles vary in hydrophilicity. While a volatile component like propylene glycol which is used as a cold-resistant agent evaporates, it carries rather more lipophilic particles to the surface, so the scission of color emerges. Usually, 20-30% Ti[O.sub.2] content latex is used for the preparation of light tint paint, 8-13% Ti[O.sub.2] content latex is used for the preparation of a medium shade coating, and <4% Ti[O.sub.2] content latex is used in deep color production. To summarize, high Ti[O.sub.2] makes flooding and floating worse, especially with high paste content. The 10 wt% level may not be the optimum, but it certainly is better than the others at low paste content.

The effect of dilution with water on flooding and floating is shown in Table 3.

It is observed that, when a nonfloating paint is diluted with water, flooding and floating may occur, or their severity may increase. When more water is added to the paint, the dispersants are diluted and there is comparatively less dispersant available to stabilize the pigment. The paint becomes more hydrophilic, so lipophilic components are more likely to phase separate. Last, but by no means least, the decrease of viscosity makes the movement of particles easier. The addition of water also tends to raise the surface tension, inducing more severe Benard cell flows.

The influence of kaolin content on flooding and floating is shown in Table 4, and the floating results of carbon black are plotted in Figure 5.

The main component of kaolin is [Al.sub.2][O.sub.3]*2Si[O.sub.2]*2[H.sub.2]O. It has some other names, such as hydrated aluminum silicate, China clay, white bole, etc. It can engender thixotropic structure, form a kind of spatial network structure, and prevent aggregation of pigment particles, thus alleviating flooding and floating.

It is found that there is an optimum content of kaolin. Increasing concentration favors dispersion stability, but makes dispersion difficult. Application properties and gloss are impaired at high concentrations. Based on Table 4, 14 wt% is a comparatively effective dosage. In practice, several fillers are often used in combination to meet performance requirements.

Small particle size tends to alleviate flooding and floating. According to Stokes' law, the velocity of spherical particles sinking in a liquid is given by:

V = D([d.sub.pi]-[d.sub.bi])[r.sup.2]/[eta]

where V: velocity; D: proportional constant; [eta]: viscosity; [d.sub.pi]: density of pigment; [d.sub.bi]: density of binder; r: pigment radius. This equation indicates that sinking velocity decreases when the particle size decreases, or the difference between pigment and binder density is reduced, or binder viscosity increases. The effect of particle size on flooding and floating is shown in Table 5.

Influence of Additive Agents

The influence of dispersants on flooding and floating is shown in Table 6. SN-Dispersant 5040 is a special polysodium carboxylate dispersant for latex paint. SN-Dispersant 5027 is a polyammonium carboxylate with higher molecular weight than 5040. 5027 is more lipophilic and has lower surface tension. For the higher steric hindrance and lower surface tension, 5027 is good at alleviating flooding and floating; but 5040 has higher dispersing efficiency when used alone, so the use of both shows the best effect.

Sedimentation can be adjusted by increasing viscosity. C[R.sub.2] is an acrylic thickening agent; a schematic of its structure is shown in Figure 6. Its hydrophilic "segment" coalesces with water to thicken, and its lipophilic segment's hydrophobic function keeps the particles detached, and forms a steric network, thus controlling flooding and floating. It can be seen from Table 7 that steric hindrance plays an important role in the control of flooding and floating.

QP4400 is a kind of hydroxyethylcellulose from Cellosize, and SN-612 is another hydroxyethylcellulose from Henkel Ltd. Hydroxyethylcellulose is widely used in paint. However in this case, it is less effective in reducing floating, for C[R.sub.2] is more similar in structure to and compatible with the dispersants. So, the compatibility between thickener and dispersant is very important.

The influence of different viscosities (adjusted using C[R.sub.2], since it is the most effective thickener among the three) on flooding and floating is elucidated in this experiment; the results can be seen in Table 8. It is found that higher viscosity reduces flooding and floating.

[FIGURE 6 OMITTED]

Influence of Processing

It can be seen from Table 9 that processing has little influence on flooding and floating. In practice, color paste is added as process number 3 according to the principles of convenience, speediness, and economy.

CONCLUSIONS

In this article, flooding and floating in latex paint were studied and the causes were summarized from the aspect of components. Ways to alleviate flooding and floating in waterborne coatings were explored. The following conclusions can be drawn:

(1) Generally, color pastes are compatible or partially compatible with the emulsion. However, in the paint produced with the same emulsion and different paste, flooding and floating appear in various degrees. So, emulsion is not the main cause of flooding and floating.

(2) Paints with different pigment or filler contents differ in flooding and floating conditions. It is because pigment particles are different in hydrophilic property and lipophilic nature. Usually, 20-30% Ti[O.sub.2] content latex is used for preparing light tint paint; 8-13% Ti[O.sub.2] content latex is used for preparing a medium shade coating; and < 4% Ti[O.sub.2] content latex is used in deep colored paint production. Reducing the surface tension, increasing the viscosity, and using finer particles help to prevent flooding and floating.

(3) Steric hindrance in both dispersants and thickeners plays an important role in the control of flooding and floating.

(4) Processing conditions have little influence on flooding and floating.

To sum up, reducing the surface tension, forming spatial network structure, decreasing the particle size, minimizing the difference between pigment and binder density, and increasing viscosity are helpful in preventing flooding and floating.

Table 1 -- Compatibility of Acrylate Emulsion and Color Paste

                                Emulsion and Color Paste Only
                      Bin stability             Fineness
Color Paste           (50[degrees]C, 30 days)  ([micro]m)  Compatibility

Anthraquinone red     Uniform                     28       Compatible
Monoazo red           Uniform                     21       Compatible
Indian red            Uniform                     30       Compatible
Quinoline yellow      Uniform                     26       Compatible
Monoazo yellow        Uniform                     22       Compatible
Disazo yellow         Uniform                     25       Compatible
Ferrite yellow        Uniform                     29       Compatible
Brown iron oxide      Uniform                     28       Compatible
Phthalocyanine green  Uniform                     28       Compatible
Phthalocyanine blue   Uniform                     26       Compatible
Quinacridone violet   Uniform                     23       Compatible
Delphine violet       Uniform                     25       Compatible
Carbon black          Flooding (2)                40       Partially
                                                           compatible

                      Coatings with Other Ingredients
Color Paste           Inside   Surface (a)

Anthraquinone red     Uniform  Red floating at the brim (2),
                               flooding (2)
Monoazo red           Uniform  Floating (4), flooding (3)
Indian red            Uniform  Floating (1), flooding (1)
Quinoline yellow      Uniform  Floating (1), flooding (1)
Monoazo yellow        Uniform  Floating (1), flooding (1)
Disazo yellow         Uniform  Floating (2), flooding (1)
Ferrite yellow        Uniform  Floating (2), flooding (1)
Brown iron oxide      Uniform  Floating (1), flooding (1)
Phthalocyanine green  Uniform  Floating (4), flooding (3)
Phthalocyanine blue   Uniform  Blue floating at the brim (2),
                               flooding (2)
Quinacridone violet   Uniform  White floating (1), flooding (1)
Delphine violet       Uniform  White floating (1), flooding (1)
Carbon black          Uniform  Floating (4), flooding (3)

(a) Note 1, 2, 3, 4 indicate the degree of floating and flooding: 1 =
slightest, 4 = most severe; the same coding scheme is used in all
tables.

Table 2 -- Influence of Titanium Content on Flooding and Floating

                           Flooding and Floating Condition
Ti[O.sub.2] Content        4 wt%          10 wt%         23 wt%

Color paste           0.1  Floating (2),  Uniform        Floating (2),
                           flooding (2)                  flooding (2)
 content (wt%)        0.5  Floating (2),  Uniform        Floating (2),
                           flooding (2)                  flooding (2)
                      1    Floating (1),  Floating (1),  Floating (2),
                           flooding (1)   flooding (1)   flooding (2)
                      2    Floating (1),  Floating (1),  Floating (3),
                           flooding (1)   flooding (1)   flooding (3)
                      5    None           Floating (2),  Floating (3),
                                          flooding (2)   flooding (3)
                     10    None           Floating (2),  Floating (4),
                                          flooding (2)   flooding (4)

Table 3 -- Flooding and Floating Difference of Latex Paint Diluted with
Water

                          Flooding and Floating Condition

Dilution ratio            5 wt%          10 wt%         15 wt%
0.1 wt% paste + 10 wt%    Flooding (1),  Flooding (2),  Flooding (3),
Ti[O.sub.2]               floating (1)   floating (3)   floating (4)

Table 4 -- Influence of Kaolin Content on Flooding and Floating

              Kaolin Content                 8 wt%         11 wt%

Carbon black  Film status after natural      Floating (1)  Floating (1)
              setting for one week           flooding (2)  flooding (1)
              Film status after accelerated  Floating (3)  Floating (2)
              storage for one month          flooding (3)  flooding (2)
Monoazo red   Film status after natural      Floating (2)  Floating (1)
              Setting for one week           flooding (2)  flooding (1)
              Film status after heat         Floating (3)  Floating (2)
              accelerating storage for one   flooding (3)  flooding (2)
              month

              Kaolin Content                 14 wt%        17 wt%

Carbon black  Film status after natural      Floating (1)  Floating (2)
              setting for one week           no flooding   floating (2)
              Film status after accelerated  Floating (1)  Blocking
              storage for one month          no flooding
Monoazo red   Film status after natural      Flooding (1)  Floating (2)
              Setting for one week           no flooding   floating (2)
              Film status after heat         Floating (1)  Blocking
              accelerating storage for one   no flooding
              month

Table 5 -- Influence of Particle Size on Flooding and Floating

Test Item               Kaolin (38 [micro]m)        Kaolin (38 [micro]m)
                        +talc (38 [micro]m)         +talc (10 [micro]m)
Monoazo red (0.1 wt%)   Floating (3), flooding (2)  Floating (3),
                                                    flooding (1)
Carbon black (0.1 wt%)  Floating (3), flooding (3)  Floating (2),
                                                    flooding (1)

Test Item               Kaolin (10 [micro]m)       Kaolin (2 [micro]m)
                        +talc (10 [micro]m)        +talc (10 [micro]m)
Monoazo red (0.1 wt%)   Floating (1), no flooding  None
Carbon black (0.1 wt%)  Floating (1), no flooding  None

Table 6 -- Influence of Dispersants on Flooding and Floating

                                                            Surface
                                                            Tension
              Dispersants       Flooding and Floating       (dyn/cm)

Carbon black  5027 (0.8 wt%)    Floating (1), flooding (1)  34.5
              5040 (0.8 wt%)    Floating (2), flooding (2)  41.5
              5027 (0.6 wt%) +  None                        37.1
              5040 (0.2 wt%)
Monoazo red   5027 (0.8 wt%)    Floating (1), flooding (1)  34.3
              5040 (0.8 wt%)    Floating (1), flooding (2)  41.8
              5027 (0.6 wt%) +  None                        37.0
              5040 (0.2 wt%)

Table 7 -- Influence of Different Thickeners on Flooding and Floating

              Thickener
              QP4400                      C[R.sub.2]     SN-612

Content        1.5 wt%                     1.2 wt%        1 wt%
Viscosity     92.1                        92.3           91.8
Monoazo red   Floating (3), flooding (3)  Floating (1),  Floating (2),
                                          flooding (1)   flooding (2)
Carbon black  Floating (3), flooding (3)  Floating (1),  Floating (3),
                                          flooding (1)   flooding (3)

Table 8 -- Influence of Viscosity on Flooding and Floating

C[R.sub.2]   0.8 wt%        0.85 wt%        0.9 wt%        0.95 wt%
content
Viscosity   89             94             103            112
(KU)
Monoazo     Floating (3),  Floating (2),  Floating (1),  Floating (1),
red         flooding (3)   flooding (2)   flooding (1)   flooding (1)
Carbon      Floating (3),  Floating (3),  Floating (1),  Floating (1),
black       flooding (3)   flooding (3)   flooding (1)   flooding (1)

Table 9 -- Influence of Processing on Flooding and Floating

             Processing                 1              2

Monoazo red  After natural              None           Almost none
             setting (3 days)
             After natural              Almost none    Almost none
             setting (7 days)
             After accelerated storage  Floating (2),  Floating (2),
             for 1 month                flooding (2)   flooding (2)

             Processing                 3              4

Monoazo red  After natural              Floating (1),  Floating (1),
             setting (3 days)           flooding (1)   flooding (1)
             After natural              Floating (1),  Floating (1),
             setting (7 days)           flooding (1)   flooding (1)
             After accelerated storage  Floating (2),  Floating (2),
             for 1 month                flooding (2)   flooding (2)

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Huang Ying, ([dagger]) Cheng Jiang, Wen Xiufang, and Yang Zhuoru--South China University of Technology*

* Research Institute of Chemical Engineering, Guangzhou, 510640, China.

[dagger] Author to whom correspondence should be addressed. Voice/fax: [phone]; [e-mail].