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Waterborne latex coatings of color: II. Surfactant influences on color development and viscosity.

A matrix of coating variables, nonassociative versus associative thickeners, different latex median particle sizes, individual surfactants and colorants [carbon black (CB), red, and yellow pigments], was examined for their influence on variances in coatings rheology and color development. Within the different coating groups, the variable of interest in this study was the surfactant added to the colorant formulation. In all three colorant formulations, sodium dodecyl sulfate (an anionic surfactant) provided poorer color development (CD) than in applied formulations containing an equivalent nonylphenol oxyethylene (EO) surfactant. In CB formulations, nonionic surfactants with higher EO content provide improved color development at low (2 mM) concentrations, but near equality in CD is achieved with low EO surfactants at higher concentrations. In contrast to CB formulations, red and yellow colorants exhibit good color development with high EO content nonionic surfactants only at low nonionic surfactants concentrations. This variance appears to be related to the interactions of surfactants with inorganic pigments (talc and laponite) in the colorant formulation.

The coating's rheology is related to latex, thickeners, and surfactant components of the paint, as has been noted in previous studies, but not to the nature of the color pigment. The viscosity of the hydroxyethyl cellulose (nonassociative type) and HEUR (associative type) thickened paint decreased with colorant addition due to dilution effects. There were no unusual deviations with the NP(EO)[.sub.x] surfactants, except when a large hydrophobe nonionic surfactant [e.g., [C.sub.18][H.sub.37](EO)[.sub.100]] is added. In HEC thickened coatings, the viscosity decreases when [C.sub.18][H.sub.37]-(EO)[.sub.100] is in the colorant due to that surfactant inhibiting depletion flocculation. In the [C.sub.18][H.sub.37](EO)[.sub.100] coatings containing the HEUR thickener, significant increases in viscosity were observed, above the dilution values observed with the colorant addition. This is related to the viscosity maximum in the low concentration of HEUR with the [C.sub.18][H.sub.37](EO)[.sub.100] surfactant. Color development is independent of the viscosity profile of the coating.

Keywords: Colorant, color retention, surfactant, latex, carbon black, thickener, talc, and laponite

INTRODUCTION

Dispersion, Flocculation, and Stability

Proper dispersion of pigments is critical for color development. In order to obtain a stable dispersion, three processes must occur. Disruption of the aggregate structure (1) is accomplished by dispersing equipment that supplies the external energy to break up the agglomerates into smaller, primary particles, which require stabilization. Wetting, the displacement of air from the surface of the pigment, is followed by the formation of a solvent layer on the pigment and by adsorption of stabilizing components. (2) Stabilization through the adsorption of a surfactant or dispersant on the colorant occurs by steric or electrostatic repulsive forces between particles. Flocculation is unwanted and results in a loss of optical and color properties because it changes the pigment size distribution in the medium. This study starts with carbon black (CB) because of the extensive adsorption studies of surfactants on CB (described later) and the surface similarities with Ti[O.sub.2] that we have examined over the past decade. (3-5)

Properties of Carbon Black

Carbon black is manufactured from hydrocarbon feed-stocks and is formed by partial combustion in the gas phase at high temperatures. There are many varieties of carbon black, which are classified by their production method. The reaction that forms carbon black is not well understood. (6) It is suggested that the process occurs in a number of stages. (7) Hydrocarbons are decomposed, C-C bonds are cleaved, and hydrogen is lost. The smaller units are polymerized to form small intermediates of carbon clusters that condense from the vapor phase. (8) During the decomposition of the feedstock, the addition of a free radical to ethyne (H-C[equivalent to]C-H) takes place. The radical containing ethyne then reacts with other gases, such as butadiene, to form cyclic intermediates containing free radical sites, and these cyclic intermediates containing radicals are able to propagate polymerization and dehydrogenate saturated compounds that are still present in the intermediate.

[FIGURE 1 OMITTED]

Carbon black is considered to be spherical in shape, and the particle size can vary from 5 to 500 nm. The surface area of CB ranges from 10 [m.sup.2]/g to 1,000 [m.sup.2]/g, and the density is between 1.8 and 2.1 g/ml. The structure of the interior of carbon black is considered to be an arrangement of a loose polycrystalline matrix. (9) Hundreds of these crystallites form a particle.

The surface of carbon black is important to its properties and is dependent on the diameter of the particle and on internal porosity. Porosity is considered to be a major factor in high adsorption isotherms. Oxidization increases the porosity, and the surface area can be six to eight times higher than the calculated surface area. Carbon black is generally oxidized after its synthesis. Oxygen-containing functional groups on the surface of carbon black greatly influence the properties of dispersions. The most common after treatment is heating to 350-700[degrees]C in the presence of air. The oxidization treatment can result in CB with up to 15% oxygen, resulting in a strongly hydrophilic pigment. Another common type of oxidization is carried out during pelletization, which is the type of carbon black used in this study. This type of oxidization occurs by the addition of nitric acid as the beading agent. The oxidization occurs while the pellet is dried at elevated temperatures. The type of carbon black used in this study was not treated but is still slightly oxidized (ca. 10%). (10)

Adsorption onto Carbon Black

Adsorptions of surfactants on CB studies (11) are numerous and some of the studies have evaluated their influence on the stability and viscosity of the dispersions. Octyl (OP) and nonyl phenol (NP) surfactants with oxyethylene segments of 10 to 100 repeating units decrease in adsorption with increasing EO content. The results are similar to previous adsorption studies on a 100 nm acrylic latex. (12) The larger hydrophobe (NP) adsorbed on CB in greater amounts than the smaller OP unit. Increasing the EO content led to an increase in the limiting surface area of the molecule with decreasing adsorption, but an increased adsorbed layer thickness, and stability of carbon black. With a larger hydrophobe, (13) [C.sub.16][H.sub.37](EO)[.sub.20], adsorption isotherms are similar to the lower ethoxylated NP(EO)[.sub.12], but [C.sub.16][H.sub.37](EO)[.sub.20] exhibited greater adsorption than NP(EO)[.sub.40] and NP(EO)[.sub.100]. Ethoxylated acetylenic diol-based surfactants adsorbed on CB and on phthalocyanine blue exhibit similar results. (14,15) The limiting surface area of the acetylenic diol increased with decreasing adsorption as the ethylene oxide content was increased, but the area per molecule of adsorbed acetylenic diols with -(EO)[.sub.10] and -(EO)[.sub.20] was two to three times larger than OPE[O.sub.10] and NPE[O.sub.9], on both CB and on phthalocyanine blue.

In competitive adsorption studies, the adsorption of NP(EO)[.sub.12] decreased in the presence of sodium dodecyl sulfate (SDS). (11) The amount of NP(EO)[.sub.n] displacement by SDS increased with increasing EO content. In competitive studies of ethoxylated octyl phenol (16) (EO = 8, 10, and 12), the octyl phenol surfactants adsorbed stronger on carbon black than the SDS surfactant. Electronic repulsion of the negatively charged head groups with those on the surface of CB results in lower SDS adsorption. SDS adsorption is improved by the addition of salt.

Organic Pigments

Organic pigments are intensely colored organic solids that are essentially insoluble. (17) They are richer in color and more expensive than inorganic colored pigments. There are six major categories of organic pigments: monoazo, diazo, acid and basic dye pigments, phthalocyanine, quinacridone, and other polycyclic pigments. Azo pigments dominate the yellow, orange, and red shade area in terms of commercial organic pigments and have an azo function (-N=N-) between two [sp.sup.2]-hybridized carbon atoms. (18) Two types of untreated azo pigments were used in this study: a monoazo yellow pigment 3 and a naphthal AS pigment 188. Adsorption data on monoazo pigments were not found.

MATERIALS AND METHODS

Preparation of Paint Bases

Paint bases were formulated with an acrylic latex binder (0.32 volume fraction (VF) with a 108 or 600 nm size) and Ti[O.sub.2] (R900, Dupont) as the hiding pigment (at 0.07 VF). Paints with the different latices were formulated with either a high Mv HEC (100 MH) (950,000 molecular weight, Dow) or a step-growth telechelic HEUR associative thickener where the active structure is [C.sub.12][H.sub.25]-[[H.sub.12]MDI-(EtO)[.sub.182]][.sub.3]-[H.sub.12]MDI-[C.sub.12][H.sub.25] in a broad mixture of other components generated in a step-growth synthesis.

The pigment grind was first prepared using a Cowles blade, and the formulation is shown in Table 1. Immediately after dispersing, the Hegman grind was measured (pass 7 NS) according to the ASTM 1210 testing procedure. The pigment grind was divided and added to the latex, coalescing aid, and thickener. The paint bases containing HEC were formulated to 105 KU, and the paint bases containing [C.sub.12][H.sub.25]-[[H.sub.12]MDI-(EtO)[.sub.182]][.sub.3]-[H.sub.12]MDI-[C.sub.12][H.sub.25] were formulated to 120 KU. The paint base formulation and amount of thickener to reach a certain Kreb unit are given in Tables 2 and 3.

Preparation of Concentrated Colorants

Colorants were formulated to 0.062 volume fraction (VF) colorant and 0.12 VF talc. SDS, three ethoxylated nonyl phenol (Igepal) surfactants, and a [C.sub.18][H.sub.37](EO)[.sub.100] (Brij 700) were added individually to the colorant as the dispersing agent. The colorant formulations also contained AMP 95 (2-amino-2-methyl-propanol) (ANGUS Chemical) as a neutralizing agent, PEG [poly(ethylene glycol)], Drew Plus L475 (Ashland Chemical) as the defoamer, and laponite as the antisettling agent.

Colorants were prepared by the following sequence. Laponite was first dispersed in deionized water containing AMP 95. The surfactant PEG dispersant and defoamer were added to the dispersed laponite, and the solution was agitated until the surfactant was dissolved. The colorant and talc were added, and the pigments were ball milled for 20 hr. The colorant formulation is illustrated in Table 4.

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Rheology Measurements

Tinted paint bases were allowed to sit after shaking for 16 hr. Shear profiles were obtained using a Carri-Med CSL-100 controlled stress rheometer equipped with a cone and plate (4 cm with a 2[degrees] cone angle). Shear profiles were run with a three-minute increasing and three-minute decreasing ramp cycle.

Color Measurements

Colorants added to the paint base, at 15.4 wt% to obtain deeper color tones, were agitated on a Red Devil shaker for three min. The tinted paint bases were cast (3 mils, wet) on black and white Leneta cards and allowed to dry for two days. The color properties of the paint bases and tinted paints were measured (i.e., the coated area of the white part was measured, followed by a measurement of the coated area of the black part of the Leneta card) using a Macbeth Color-Eye 7000 spectrophotometer with the [D.sub.65] light source and an observer angle of 10.

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The color data were analyzed using the Propalette 5.0 program. The CIELAB color coordinate system was used to examine the difference in color and used L* (lightness), a* (green to red), and b* (blue to yellow) as parameters for evaluating color. Additional discussion of CIELAB values from XYZ tristimulus values determination can be found in reference 19.

Color Differences in the CIELAB System

The color difference between any two colors in CIE space is the distance between the color locations. The distance of the total color difference can be expressed as [DELTA]E*:

[DELTA]E* = ([DELTA][L*.sup.2] + [DELTA][a*.sup.2] + [DELTA][b*.sup.2])[.sup.1/2], (1)

where [DELTA]L* is the lightness difference, [DELTA]a* is the red/green difference, and [DELTA]b* is the yellow/blue difference (Figure 1).

[FIGURE 4 OMITTED]

Color differences can also be determined from the L*C*h* color space, using the equation:

[DELTA]E* = ([DELTA][L*.sup.2] + [DELTA][C*.sup.2] + [DELTA][H*.sup.2])[.sup.1/2], (2)

where [DELTA]L* is the lightness coordinate, [DELTA]C* is the chroma difference (e.g., the attribute of color used to indicate the degree of departure of the color from a gray of the same lightness), and [DELTA]H* is the hue difference (e.g., the attribute of color by which a color is perceived to be red, yellow, green, blue, purple, etc. Pure white, black, and grays possess no hue). Chroma and hue differences will not be discussed in this study.

Color Development Studies

Color values for this study were chosen based on the color variables in Table 5. A large color difference in the negative dL* occurs with carbon black (CB) addition to the paint, and -dL* is used as the color parameter to define color development in the black colorant. There is a negative db* component in the black because this pigment particle size has a blue tint undertone, as mentioned in the product literature. Typically, fine blacks give a brown undertone while coarse carbon black pigments give a blue undertone. There are changes in all of the parameters in the red colorant, the dominate one being da*, this value reflecting the green-red parameter. The reason for the slight db* component in the red 188 pigment is related to a yellowish, red shade. (18,20) The paint containing the yellow colorant is mainly dominated by the db* parameter, which is blue-yellow. There is also a negative da* value; as yellow 3 is considered to be a greenish, yellow pigment. (18)

[FIGURE 5 OMITTED]

RESULTS AND DISCUSSION

In the previous study, a mix of different surfactants was added to carbon black to determine the effects colorant formulations have on the viscosity of a commercial paint with unknown components. Reducing the total amount of surfactant or eliminating the two most surface-active components, which were anionic surfactants, inhibited the viscosity decrease in a commercial paint. In this study, colorants were formulated with individual surfactants and added to paints with known ingredients (i.e., median particle size of the latex, thickener, Ti[O.sub.2] stabilizer, etc.). The colorant formulations were added to the paint at higher concentrations (15.4 wt%) compared to the 3 wt% and 12 wt% concentrations used in the previous article. Due to the different pigment levels used in this study, the increase in weight percent of colorant resulted in similar volume fractions of pigment in the final tinted paint formulation. A comparison of the amounts is given in Tables 6 and 7.

[FIGURE 6 OMITTED]

The following discussion of color development and coatings rheology will include a comparison of an anionic (sodium dodecylsulfate, SDS) with a conventional nonionic surfactant (nonylphenol with an average 12 mole ethoxylate of NP(EO)[.sub.12]). This nonionic surfactant is slightly higher in molecular weight than the anionic SDS. To have achieved comparable molecular weights, a lower ethoxylate level would have been needed that would have limited the nonionic's aqueous solution solubility. In the second part of this study, the role of structure among nonionic surfactants will be examined, first by using only 12.5 mM of different nonionic surfactants based on a dispersant demand analysis. In the final comparison, the concentration of different nonionic surfactants will be varied.

Comparison of SDS and NP(EO)[.sub.12]

COATINGS RHEOLOGY: Colorants were formulated with SDS, an anionic surfactant, and with NP(EO)[.sub.12], a nonionic surfactant, and were added to the paints containing a step-growth HEUR thickener and a 108-nm latex, which contained 0.45 wt% of the synthesis surfactant, NP(EO)[.sub.15]. In the previous study (21) of nontinted coatings, the synthesis surfactant was removed by dialysis. The colorants are very low in volume fraction (0.03 VF, Table 7) and should not contribute to the rheology. Differences in rheology of the paint with the different colorant pigments (Figure 2) were not observed. The viscosity of the coatings initially decreased with addition of the colorant due to a reduction in the volume fraction of components from dilution effects (Table 8). The viscosity did not continue to decrease with increasing surfactant with the NP(EO)[.sub.12] colorant due to limited desorption of associative thickener from disperse phases and synergy in aqueous phase associations (Figure 2, A1-C1). Unlike the colorants with NP(EO)[.sub.12], the paints containing the colorants with SDS exhibited large decreases in viscosity with increasing amounts of SDS, decreasing from roughly 5.5 Pa.s for the paint without colorant to about 2 Pa.s from the paint containing colorant with 45 mM SDS in the colorant formulation (6.9 mM, 0.2 wt%, SDS in paint). When the surfactants brought in with the latex and added to the pigment grind are included, a total of roughly 17 mM surfactant is present in the paint and the viscosity decreases dramatically (Figure 2, A2-C2). The additional SDS added into the aqueous phase displaced the associative thickener from the disperse latex phase (22) and also disrupted the HEUR/SDS network in the aqueous phase. These rheological trends agree with previous studies from our group, and differ only in the removal of the latex synthesis surfactant by dialysis. (23)

The viscosity decrease due to colorant addition was even larger with the 600 nm latex paint (Figure 3). The influence of surfactant on the rheology of coatings with different size lattices and associative thickener was delineated in our most recent publication. (21) The difference in adsorb layer thicknesses, interparticle distance, and bridging of disperse phases by adsorbed HEUR associates led to a greater viscosity in smaller particle latex coatings. The addition of the excess surfactant with the larger latex removed all of the HEUR from the surface of the latex. Less surfactant was needed to displace the HEUR into the aqueous phase, and the greater amount in the aqueous phase will disrupt thickener networks. There were also dilution effects occurring, noted in Table 8. The viscosity did not decrease with increasing NP(EO)[.sub.12] concentration (Figure 3) in the colorant. The nonionic did not displace all of the HEUR from the disperse phases and there is an interaction synergy in the aqueous phase between the nonionic surfactant and the HEUR.

[FIGURE 7 OMITTED]

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[FIGURE 10 OMITTED]

COLOR DEVELOPMENT: SDS VS NP(EO)[.sub.12]: Color properties of the paints containing SDS and NP(EO)[.sub.12] were examined in both the 108 and 600 nm latex paint with the HEUR thickener. In both paints, and with all three colorants, NP(EO)[.sub.12] promoted much better color properties than SDS (Figure 4). NP(EO)[.sub.12] has a stronger adsorption (16) and steric stabilization of the colorants, which is not sensitive to the salinity of the colorant or paint. In basic media, SDS adsorbs weakly onto carbon black due to electronic repulsion of the negatively charged head groups with the negatively charged carbon black. This result suggests that surfactants with ethoxylated groups aid in color development in concentrated colorants.

Structural Influences of Nonionic Surfactants At 12.5 mM

In the second part of this study, the structure of the nonionic surfactant on color development will be examined primarily with nonyl phenol (NP), with increasing oxyethylene (EO) content. With increasing EO content, greater stabilization (at a constant adsorption level) will be realized; but with an increasing EO content at a constant hydrophobe size, there will be less adsorbed. To counter this in an [EO.sub.100] surfactant, one with a larger hydrophobe (i.e., [C.sub.18][H.sub.37]-) was also studied. In the first phase of this study, the concentration of surfactant was held constant at 12.5 mM, based on the standard dispersant demand curve (Figure 5). The properties of these nonionic surfactants are listed in Table 9. The critical micelle concentration (CMC) increased with increasing ethylene oxide due to their greater hydrophilicity.

[FIGURE 11 OMITTED]

The [C.sub.18][H.sub.37]E[O.sub.100] surfactant has a lower CMC due to the large hydrophobe. It is important to note that the addition of 0.01 VF colorant to the paint resulted in a surfactant concentration of 1.9 mM addition to the paint. With increasing ethylene oxide, more surfactant on a weight percent basis needs to be added to obtain 12.5 mM in the colorant formulation, due to the varying molecular weights of the surfactants (Table 9). The only other surfactant used in our coating formulation was that added with the Ti[O.sub.2] [([C.sub.6][H.sub.13])[.sub.2]CH(OC[H.sub.2]C[H.sub.2])[.sub.9]OH)] and one with a similar structure used in the synthesis of the latex. The two latex (i.e., 108 and 600 nm) formulations were thickened by both HEC and a telechelic HEUR.

[FIGURE 12 OMITTED]

INFLUENCE OF NONIONIC SURFACTANTS AT 12.5 MM ON COATINGS RHEOLOGY: The HEC-thickened coatings were very shear thinning (Figure 6) due to flocculation of Ti[O.sub.2] and latex disperse phases by a depletion mechanism. (24) The insoluble talc and colorant pigments in the colorant formulation also would be expected to flocculate in an HEC-thickened coating if not properly stabilized. HEC-thickened coatings are insensitive to surfactant changes. In the other series of latex coatings, the thickener was a telechelic HEUR ([C.sub.12][H.sub.25]-[[H.sub.12]MDI-(EtO)[.sub.182]][.sub.3]-[H.sub.12]MDI-[C.sub.12][H.sub.25]) that can be very sensitive to surfactant changes. The inflection in shear thinning profile at ca. 100 [s.sup.-1] is associated with the disruption in hydrophobic associations of associative thickeners. The associative thickener was added to obtain a higher 120 KU formulation at 75 [s.sup.-1] than the HEC formulation (105 KU) in order to achieve a higher viscosity in the absence of the depletion flocculation. Poly(methacrylic acid), void of hydrophobes in its structure, was used as the Ti[O.sub.2] pigment dispersant.

[FIGURE 13 OMITTED]

Addition of the colorant/surfactant formulations at a 15.4 wt% level led to dilution of the coating (Table 8: 0.39 VF decreases to 0.33 VF with the additional water; with the talc and colorant present, the VF decreases to 0.36). This led to a drop in the coatings' viscosity (Figure 6). There was little variation in the rheology of the paint containing colorants with different surfactants, although a trend to lower viscosities was observed with the surfactants capable of providing stability in the depletion flocculation media of a HEC-thickened paint. This could reflect a greater stabilization of the disperse phases.

In the HEUR-thickened coating with the [C.sub.18][H.sub.37](EO)[.sub.100] surfactant, there was a reversal and the low shear rate viscosities were significantly higher, particularly in the 600 nm latex formulation. With this surfactant, there is a greater tendency to form networks in the aqueous phase (25) (1.93 mM added to paint, roughly 100 times its CMC, a multiple noted for occurrence of the solution viscosity maximum with this surfactant). With the [C.sub.18][H.sub.37](EO)[.sub.100] large hydrophobe, an additional viscosity synergy is possible, as observed with larger hydrophobic HEUR thickeners. (21) The rheology of the HEUR formulations containing CB is reproduced in Figure 7 for comparison with formulations containing the red and yellow colorants, also at 1 VF in the coatings. There was no influence of the colorant at 1 VF on coatings rheology.

[FIGURE 14 OMITTED]

COLOR DEVELOPMENT: 12.5 MM NONIONIC SURFACTANTS: In the previous section, the NP hydrophobe containing an average of 12 EO units provided greater color development (CD) than an equivalent SDS anionic surfactant. From the data in this section, more than 12 EO units provided greater CD. With the CB colorant, the 600 nm latex had slightly better color development than the 108 nm latex paint (Figure 8 for the HEC and Figure 9 for the HEUR-thickened coatings). In the HEUR-thickened coatings there appears to be support for the adsorption and stabilization concepts by balancing the hydrophobe size with the number of EO units.

Comparison of the changes in CD with variation in nonionic surfactant structure is clearly more complex, for the concepts do not describe what was observed when the colorant formulation contained the red or yellow colorant (Figure 9). The complexity was investigated below by changing the nonionic surfactant concentration.

VARYING NONIONIC SURFACTANT CONCENTRATIONS: The 600 nm latex coating is emphasized in this part of the study: these coatings had more interesting rheology compared to the 108 nm latex paint and slightly better color development.

Varying Concentrations of [C.sub.18][H.sub.37](EO)[.sub.100] Surfactant -- The three colorants containing [C.sub.18][H.sub.37](EO)[.sub.100] surfactant in the 600 nm latex paint thickened with HEC resulted in similar shear profiles (Figure 10, A1-C1). At low concentrations of surfactant, there was only a slight decrease in viscosity due to the dilution effect. As the concentration of surfactant in the colorant increased, the viscosity of the paint decreased at low shear rates due to inhibition of depletion flocculation. With the change in colorant, there was little deviation in the rheology, but there were significant variations in CD with the [C.sub.18][H.sub.37](EO)[.sub.100] surfactant concentration (Figure 10, A2-C2). Unlike the CB colorant that plateaued in -dL* with increasing surfactant, the da* of the red colorant and db* of the yellow colorant showed an initial increase at low concentrations of [C.sub.18][H.sub.37](EO)[.sub.100], but both da* and db* decreased sharply at higher surfactant concentrations. In addition, the yellow colorant had an additional discontinuity in the db* parameter with increasing concentration dependence. The latter appeared real, but we have no explanation for this dependence. This discontinuity CD in the yellow colorant will not occur when EO/PO triblock polymers are used as dispersants.

[FIGURE 15 OMITTED]

The three colorants containing the [C.sub.18][H.sub.37](EO)[.sub.100] surfactant were also examined in the HEUR-thickened 600-nm paint. There were minor differences in rheology between carbon black and the organic pigments (Figure 11, A1-C1). As described earlier, the increase in low shear viscosity may have been due to the surfactant level being near the viscosity maximum, even at the highest concentration of surfactant (25 mM surfactant in colorant equates to 3.8 mM surfactant in paint), and a possible synergy of large hydrophobe surfactants and HEUR thickeners. (21) The color values of all three colorants are similar to the color values of the paint containing HEC, indicating that there was little effect of the thickener on large scale CD changes (Figure 11, A2-C2).

CD Comparisons with Nonionic Surfactants of Lower EO Content -- All of the surfactants were examined in the CB tinted coatings. In the paint thickened with HEC, the -dL* of the colorants increased as each nonionic surfactant increased in concentration, and then plateaued, much like a Langmuir isotherm curve (Figure 12A). The R(EO)[.sub.100] surfactants need the least amount of surfactant to reach the plateau, suggesting that the larger ethoxylated surfactants were adsorbing strongly and stabilizing the CB formulations. The CD (-dL) plateaued at a higher concentration in NP(EO)[.sub.40] formulations; an even higher concentration of NP(EO)[.sub.12] was required to attain the plateau. The color values of the paint containing the HEUR thickener were similar to the HEC paint in the amount of surfactant needed to reach a plateau; however, the color differences among the different surfactants were more differentiated (Figure 12B). The smallest ethoxylated surfactant, NP(EO)[.sub.12], had the poorest CD even though it should have adsorbed strongly onto carbon black. (11) Previous adsorption studies of NP(EO)[.sub.x] surfactants onto lattices indicated that adsorption of these types of surfactants increases with the addition of unionized acid groups onto the surface of the latex, suggesting that the acid groups on a latex surface interact with the ethylene oxide segments and promote higher adsorption of the surfactants. (12) The carbon black in this study was untreated but still has oxidation sites on its surface. This mechanism may be happening on the surface of carbon black, leading to improved color development.

The differences in CD of the red colorant (in da* units) and the yellow colorant (in db* units) were only slightly sensitive to the median size of the latex (Figure 9). The color trends were similar in the HEC and HEUR paint, but there were very distinct differences among the surfactants (Figures 13-15). Two surfactant extremes, NP(EO)[.sub.12] or NP(EO)[.sub.40], and [C.sub.18][H.sub.37](EO)[.sub.100], were chosen for the red and yellow colorant studies with varying surfactant concentrations. At low concentrations, the [C.sub.18][H.sub.37](EO)[.sub.100] had a higher da* than the NP(EO)[.sub.12] formulation (Figure 13). As the surfactant concentration increased, the da* drastically decreased in the colorant containing [C.sub.18][H.sub.37](EO)[.sub.100], but continued to increase with increasing concentration in the NP(EO)[.sub.12] surfactant formulations.

A similar result, even though the curve was irregular, was observed in the yellow colorant comparison with [C.sub.18][H.sub.37](EO)[.sub.100] and NP(EO)[.sub.40] (Figure 14). Clearly, at higher concentrations of surfactant, additional ethylene oxide decreases the color development in the red and yellow colorant. This effect did not occur with the black colorant and the mechanisms used in explaining the variations in CD noted in the CB colorant formulations are not valid for the red and yellow pigments (Figures 10, 11 A2-A3, and 12 A2-C3). To address this issue we reviewed the pigment characteristics in Table 10 and emphasized the presence of talc (Table 10) in the formulations. CB was the smallest pigment with the largest surface area. Naphthol Red 188 was larger, and Hansa yellow 3 had the largest pigment particle size with the smallest surface area. Talc was the largest disperse phase (in an unpeptized state) and had the smallest oil adsorption. The data in Table 10 were collected from a number of different sources, with different measurement precision and day-to-day variances in the commercial products.

Talc was present at 0.125 VF in the colorant formulation; laponite was present at 0.0125 VF. In the final coating, they represent 0.02 (twice that of the colorant) and 0.005 volume fractions, respectively. Previous studies have indicated that ethylene oxide units have the ability to adsorb on the surface of talc, (26) laponite, (27) and similar types of clays (28) in proportion to the number of EO units. This could lead to flocculation due to interparticle bridging with increasing EO content. In addition, the structures of the red and yellow colorants contained many different functional groups that could also lead to interbridging (and viscosity increases). The color properties drastically improved when talc (Figure 15) and laponite were removed from the red and yellow colorants with high levels of surfactant. The CB formulation remained unaffected by the changes in pigments. The high surface area and high porosity of carbon black (Table 11) would result in both adsorption and absorption of the surfactant. There would be less surfactant available for interaction with talc and laponite, and certainly less of an excess of surfactant to promote interbridging of the clays.

CONCLUSION

The colorant formulation contained two primary disperse phases: the colorant at volume fraction (0.0625 VF) and talc (added to improve colorant dispersion) at 0.125 VF. After being added to the waterborne coating (W-BC) that contained latex at 0.32 VF and the hiding pigment (Ti[O.sub.2]) at 0.07 VF, the colorant and talc were 0.01 and 0.02 VF, respectively. The surfactant types and amounts (1 to 45 mM) in the colorant were reduced to a concentration range of 0.2 to 6.9 mM in the coating formulation where the surfactant concentration from the latex synthesis and Ti[O.sub.2] addition were in the 0.75 wt% (10 mM) range. Thus, the surfactant changes in the colorant could affect the coating's rheology and color development in the applied films. There were dramatic decreases on the coating's viscosity upon addition of the colorants to the coating due to dilution effects, but there were no surfactant influences noted that have not been observed in prior nontinted W-BC studies. A large hydrophobe surfactant with a significant number of ethylene oxide (EO) units, [C.sub.18][H.sub.37](EO)[.sub.100] (not previously studied), produced notable viscosity responses with increasing concentration. In HEC-thickened formulations, [C.sub.18][H.sub.37](EO)[.sub.100] led to decreasing viscosities, relatable to the inhibition of depletion flocculation of the disperse phases. In the associative thickener (HEUR) coating formulations, [C.sub.18][H.sub.37](EO)[.sub.100] surfactant promoted higher viscosity due primarily to the interaction synergy in building network structure in the aqueous phase of the larger hydrophobe with the HEUR hydrophobes. The colorant pigments did not affect the rheology of the paint, and there was no influence of coating viscosities on color development (CD).

With the three colorants, CB, red, and yellow, nonionic surfactants promoted better CD, due to the stronger adsorption and stabilization, than the anionic surfactant, SDS. Nonionic surfactants with larger ethylene oxide units improved the color development of the CB colorant. Increasing the ethylene oxide length of the surfactant led to poor color development in the red and yellow tinted coatings. With the larger ethoxylated surfactants, there was an initial increase in color at very low surfactant concentration, but a sharp decrease occured at higher concentrations of surfactant. Lower EO surfactants promoted a plateau behavior in CD with the red and yellow colorants, as was noted in CB-tinted formulations.

Although at a low VF in the coating, the amount of talc present was twice that of the colorant. The talc and laponite need to be stabilized by surfactant adsorption, but clays are complex systems that interact strongly with EO units. The decrease in color development with increasing concentration and EO content was due to adsorption of the ethylene oxide units onto the surface of talc and laponite, resulting in interparticle bridging. There was less opportunity for this to occur with CB due to its greater surface area and decreasing surfactant concentration in the aqueous phase, by both adsorption and absorption processes.

References

(1) Parfitt, G.D., J. Oil & Colour Chem. Assoc., 22, 822 (1967).

(2) Kaluza, U., Prog. Org. Coat., 10, 289 (1982).

(3) Lundberg, D.J. and Glass, J.E., "Pigment Stabilization Through Mixed Associative Thickener Interactions," JOURNAL OF COATINGS TECHNOLOGY, 64, No. 807, 53 (1992).

(4) Kaczmarski, J.P., Tarng, M.-R., Glass, J.E., and Buchacek, R.J., Prog. Org. Coat., 30, 15 (1997).

(5) Tarng, M.-R., Chen, M., Glass, J.E., and Dickinson, J.G., "Unifying Model for Associative Thickeners Influences on Water-Borne Coatings: II. Competitive Adsorption of Nonionic Surfactants and HEUR Thickeners on Titanium Dioxide Pretreated with Inorganic Stabilizers and Organic Oligomeric Dispersants," JOURNAL OF COATINGS TECHNOLOGY, 74, No. 935, 45 (2002).

(6) Christie, R.M., Pigments: Structures and Synthetic Procedures, Oil & Colour Chemists' Association, Wembley, U.K., pp. 9-10, 1993.

(7) Buxbaum, G., Industrial Inorganic Pigments, VCH, Weiheim, Germany, Chapt. 4, 1993.

(8) Abrahamson, J., Nature, 266, 323 (1997).

(9) Stoy, W.S. and Garret, M.D. in Pigments: Part I, Myers, R.R. and Long, J.S. (Eds.), Marcel Dekker, Inc., New York, Chapt. 5, 1975.

(10) Private conversation with employee of Cabot Corp.

(11) Ma, C. and Xia, Y., Colloids Surf., 66, 215 (1992).

(12) Ma, Z., Chen, M., and Glass, J.E., Colloids Surf., 112(2/3) (1996).

(13) Bossoletti, L., Ricceri, R., and Garielli, G., J. Dispersion Sci. Technol., 16(3 & 4), 205-220 (1995).

(14) Musselman, S.W. and Chandler, S., J. Colloid Interface Sci., 256, 1 (2002).

(15) Musselman, S.W. and Chandler, S., Colloids Surf. A. 206, 497 (2002).

(16) Schwuger, M.J. and Smolka, H.G., Colloid Polym. Sci., 255, 589-594 (1997).

(17) Smith, H., in Pigment Handbook, Patton, T. (Ed.), Wiley-Interscience, New York, Chapt. I-D-a-1, 1973.

(18) Herbst, W. and Hunger, K., in Industrial Organic Pigments, VCH, Weiheim, Germany, 1997.

(19) Burns, R.S., Billmeyer and Saltzman's Principles of Color Technology, Wiley-Interscience, New York, 2000.

(20) Schunck, R.P. and Hunger, C., in Pigment Handbook, Patton, T., (Ed.), Wiley-Interscience, New York, Chapt. 14., 1973.

(21) Mahli, D.M., Steffenhagen, M.J., Xing, L.L., and Glass, J.E., "Surfactant Behavior and Its Influence on the Viscosity of Associative Thickeners Solutions, Thickened Latex Dispersions, and Waterborne Latex Coatings," JOURNAL OF COATINGS TECHNOLOGY, 75, No. 938, 39 (2003).

(22) Thibeault, J.C., Sperry, P.R., and Schaller, E.J., in Water-Soluble Polymers: Beauty with Performance, Glass, J.E. (Ed.), Advances in Chemistry Series 213, American Chemical Society, Washington, D.C., Chapt. 20, 1986.

(23) Chen, M., Wetzel, W.H., Ma, Z., Glass, J.E., Buchacek, R.J., and Dickinson, J.G., "Unifying Model for Understanding HEUR Associative Thickener Influences on Water-Borne Coatings: I. HEUR Interactions with a Small Particle Latex," JOURNAL OF COATINGS TECHNOLOGY, 69, No. 867, 73 (1997).

(24) Goodwin, J.W. and Hughes, R.W., in Technology for Waterborne Coatings, Glass, J.E. (Ed.), Advances in Chemistry 663, American Chemical Society, Washington, D.C., Chapt. 6, 1997.

(25) Lundberg, D.J., Brown, R.G., Glass, J.E., and Eley, R.R., Langmuir, 10(9) 3027-3034 (1994).

(26) Pugh, R.J. and Tjus, K., Colloids Surf., 47, 179-194 (1990).

(27) Zebrowski, J., Prasad, V., Zhang, W., Walker, L.M., and Weitz, D.A., Colloids Surf. A, 213 (2-3), 189-197 (2003).

(28) Glass, J.E., Ahmed, H., and Karunasena, A., Colloids Surf., 21, 335-346 (1986).

David M. Mahli, Jon M. Wegner, and J. Edward Glass** -- North Dakota State University*

Daniel G. Phillips -- Degussa Corp. ([dagger])

Presented in part at the 81st Annual Meeting of the Federation of Societies for Coatings Technology, November 13-14, 2003 in Philadelphia, PA.

* Dept. of Polymers and Coatings, 1735 NDSU Research Park Dr., Fargo, ND 58105.

([dagger]) 2 Turner Place, Piscataway, NJ 08854.

** Author to whom correspondence should be addressed: University of Lake Wobegon, Coatings Plus Dept., 1751 S. 23rd St., Fargo, ND 58103, email: eglass@fargocity.com.
Table 1 -- Pigment Grind Formulation

Pigment Grind Amount Added in g

Tamol 850 (30%) 9.3
Tergitol 15-S-9 5.0
Defoamer L 475 6.4
Ethylene glycol 26.7
DDI 77.8
Biocide 1.0
Ti[O.sub.2] 373.8
Total 500 g

Table 2 -- Paint Base Formulations Used in This Study

Ingredients Amount

Latex 0.32 VF

Dispersion 0.07 VF
Texanol 2.5 g
HEUR or HEC Amount to obtain 120 KU (HEUR) or 105 KU (HEC)
Water Amount needed to total 150 ml
Total amount 150 ml

Table 3 -- Amount of Thickener Added to Paint (0.32 VF Latex and 0.07 VF
Ti[O.sub.2]) to Achieve the Correct Kreb Unit (KU) Viscosity

 wt% HEC to Obtain 105 KU wt% HEUR to Obtain 120 KU

108 nm Latex paint 0.27 0.17
600 nm Latex paint 0.30 0.31

Table 4 -- Colorant Formulation

 Amount of Ingredients in
 Paint After Dilution
 Amount of Ingredients in Colorant (15.4 wt% Colorant Added
 Formulatio n to Paint Base)

Water Varied Varied
Laponite RD 1 wt% 0.0019 VF
AMP 95 0.1 wt% 0.019 wt%
Surfactant Varied Varied
Defoamer 0.15 wt% 0.029 wt%
PEG 5 wt% 0.96 wt%
Colorant 0.063 VF 0.0096 VF
Talc 0.125 VF 0.0192 VF
Total 0.195 VF 0.0307 VF

Table 5 -- Color Values of Paint (Ti[O.sub.2]/600 nm Latex/HEUR) with
0.01 VF (15.4 wt%) Black, Red, and Yellow Colorant Containing 12.5 mM
NP(EO)[.sub.40] in the Colorant Formulation

 L* a* b* dL* da* db* dE*

No colorant 97.378 -0.733 0.919 -- -- -- --
Black 46.231 -1.108 -5.241 -51.146 -0.396 -6.277 51.532
Red 70.764 41.614 13.224 -26.821 42.334 12.337 51.612
Yellow 94.771 -10.783 43.601 -2.85 -10.05 42.683 43.942

Table 6 -- Comparison of Surfactant Amounts Used in Previous Study with
the Current Study

 wt% Surfactant wt% Surfactant from
 Added to Colorant Colorant in Paint

 3 wt% Colorant 12 wt% Colorant
Previous study in Paint Base in Paint Base

S-5 15.6 0.47 1.87
S-3 9.1 0.27 1.09
(S-5)[.sub.max] 5 0.15 0.60

This study (at 12.5
mM) 15.4 wt% Colorant in Paint Base

[C.sub.18][H.sub.37] 4.6 0.72
E[O.sub.100]
NPE[O.sub.100] 4.6 0.72
NPE[O.sub.40] 2.0 0.31
NPE[O.sub.12] 0.75 0.12
SDS 0.29 0.045

Table 7 -- Comparison of Colorant and Extender Pigment Amounts in the
Colorant and in the Final Tinted Paint Used in Previous Study and in
This Study

 This Study Colorants in Previous Study

Amounts in colorant formulation
VF colorant pigments 0.063 0.055-0.15
VF pigments including talc 0.19 0.16-0.20

Amounts in final tinted paint
wt% colorant formulation 15.4 3 12
VF colorant pigment 0.01 0.005 0.01
VF pigments including talc
(excluding pigment from paint) 0.03 0.01 0.03

Table 8 -- Latex Paint (0.32 VF Latex and 0.07 VF Ti[O.sub.2]) After
Dilution of 15.4 wt% Colorant (21)

 V[F.sub.eff]
 Volume Fraction w/10 nm at Relative Viscosity
 (VF) of Paint Adsorbed Layer Viscosity (Pa.s)

108 nm latex
No colorant 0.39 0.61 179.6 0.45
15.4 wt% colorant 0.33 0.51 14.2 0.04
excluding VF of
pigment
15.4 wt% colorant 0.36 0.56 31.8 0.08
w/VF of pigment

600 nm latex
No colorant 0.39 0.43 6.1 0.02
15.4 wt% colorant 0.33 0.36 3.9 0.01
excluding VF of
pigment
15.4 wt% colorant 0.36 0.40 4.8 0.01
w/VF of pigment

Table 9 -- Properties of Nonionic Surfactants Used in This Study

 wt% of 12.5 mM
 Molecular Surfactant
 Moles of EO Weight CMC (Mm) in Colorant

NPE[O.sub.12] 12 749 0.075 0.8
NPE[O.sub.40] 40 1949 0.11 2.0
NPE[O.sub.100] 100 4626 0.30 4.6
[C.sub.18][H.sub.37] 100 4670 0.02 4.7
E[O.sub.100]

Table 10 -- Properties of Pigments Used in This Study

 Cabot Clariant Clariant
 Black Pearls Novoperm Red Hansa Yellow
Pigment Talc 120 HF3S 10G
Class of Magnesium Carbon Black Naphthol AS Monoazo Yellow
Pigment Silicate Pigment Pigment
 Red 188 Yellow 03
Structure [Mg.sub.3]
 [Si.sub.4]
 [O.sub.10]
 (OH)[.sub.2]
SA ([m.sup.2]/ N/A 25 16 11
g)
Particle size 2,200 nm 75 nm 145 nm 405 nm
Oil ad,
([m.sup.2]/g) 42 72 74 59

Table 11 -- Total Surface Area of the Three Colorants

 [M.sup.2] of Surface in 0.0625 VF Colorant

Carbon Black 225
Red 126
Yellow 87
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Author:Phillips, Daniel G.
Publication:JCT Research
Geographic Code:1USA
Date:Oct 1, 2005
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