Use of novel polyetheralkanolamine comb polymers as pigment dispersants for aqueous coating systems.
Keywords Dispersants, Rheology, Stabilization, Pigments, Waterborne, Color measurement, Water-based surface chemistry
The dispersion of a pigment governs the properties of the final film formed by an ink, paint, or any other pigmented coating. Stable, concentrated dispersions with small particles and narrow particle size distribution potentially can lead to higher gloss and color strength per unit mass of pigment. The push towards water-based formulations and increasing pigment loading underlines the need for good dispersion and stabilization properties. There are two stabilization mechanisms for pigment dispersions: electrostatic stabilization and steric stabilization. As the particles come close together, the Van der Waals attractive force, which exhibits power law distance dependence, will dominate, causing flocculation. Systems with high pigment loadings are often stabilized by a steric repulsion mechanism which arises from the interactions between adsorbed polymer chains as the particles come together. Unlike electrostatic stabilization, the total energy interaction in sterically stabilized systems is repulsive also at very short distances and as a result flucculation is prevented. Steric stabilizers are often comb polymers, with an anchoring group adsorbing strongly to the pigment surface to be stabilized and with a flexible side chain extending into the solvent. The adsorption polymer layer thickness affects the dispersion stability (1) and is of importance in the understanding of the mechanisms of steric stabilization such as the conformation of adsorbed molecules on particle surfaces. Napper (2) and Killman and Eisenaluer (3) reported that, depending on the amount of polymer added to a dispersion, a polymer could stabilize or flocculate a dispersion by forming a steric barrier or interparticle bridges between particles having different thicknesses of the adsorbed polymer layer. There are various methods used to determine the adsorption polymer layer thickness. These include photon correlation spectrometry, (4) microelectrophoresis, (5) determination of sedimentation coefficient, (6) and viscometry. (7) Siffert and Li (8) used these four techniques to determine the thickness of a polyethylene glycol layer at the solid/liquid interface. They reported that photon correlation spectroscopy produced the most accurate results, followed by microelectrophoresis. Viscometry and sedimentation coefficient methods are not recommended unless the dispersions are stable during the measurements and the difference in the viscosity between dispersions with and without polymer is high enough to overcome the error caused by the measurements. In this paper we employed the "brush" (9) to estimate the adsorbed layer thickness for the comb polymers.
The preparation and stabilization of concentrated suspensions of solids in liquids are of prime importance in different fields of applications from both a theoretical and technological point of view and has been reviewed by Sato. (10) Several investigators have studied the adsorption of polymers at the solid/liquid interface extensively. Zhou et al. (11) investigated the effect of ammonium salt of styrene maleate copolymer (SMA-[NH.sub.4] on the rheology of quinacridone red pigment dispersion and revealed that the rheology of the treated dispersion is a nearly Newtonian behavior while the untreated one shows a pseudoplastic character. The rheological properties and stability of aqueous quinacridone pigment dispersion with SMA-[NH.sub.4] were strongly affected by both pH and ionic strength. The dispersion became unstable when the pH is below 5 or the ionic strength is above 0.001 mol/d[m.sup.3]. Silber et al. (12) showed that the copolymers of vinyl ethers with maleic anhydride is effective at dispersing inorganic pigments such as [TiO.sub.2] and iron oxide in water. The authors did not indicate if these copolymers would be effective for organic pigments and carbon black. a poly (acrylic acid), a lignosulphonate, and a comb polymer (consisting of a polycarboxylate backbone modified with sulphonate groups and grafted polyoxyethylene chains) as dispersants for highly concentrated aqueous alumina suspensions have been investigated by Palmyqvist et al. (13) At pH of 10, a poly (acrylic acid) was the most effective dispersant, providing both electrostatic and steric stabilizations. Schwartz et al. (14) examined an acrylic/polyether comb-branched copolymer as a dispersant for inkjet inks.
Adsorption at the solid/liquid interface involving the determination of adsorption isotherms, zeta potential, particle wettability, and heats of adsorption has been studied. (15) These results gave an insight into the mechanisms of surfactant adsorption, particularly the formation of the adsorbed layer. Fluorescence techniques are commonly used to elucidate specific structural details of surfactant aggregates such as surfactant aggregation number, polarity, and microviscosity. (16)
The main goal of the present study was to evaluate the efficiency of polyetheralkanolamine comb polymers as dispersants for a variety of pigments in waterborne coating systems. A series of polyetheralkanolamine comb polymers was prepared by fixing the anchoring group while varying the molecular weight and the ethylene oxide/propylene oxide ratio of the side chain. The adsorption of these comb polymers has been investigated at the carbon black/water interface by measuring adsorption isotherm, contact angle, wetting rate, zeta potential, and particle size distribution. Once the optimum molecule was achieved, its performance in pigment concentrates with and without a resin was compared to other conventional dispersants such as a styrent acrylate copolymr, an acetylenic diol surfactant, and a styrene maleic anhydride comb polymer modified with a polyether.
Materials and methods
Carbon black was received from Cabot Corporation. It has a particle size of 24 nm and a surface are of 112 [m.sup.2]/g, which was provided by the supplier. Pigment blue 15:3, pigment yellow 14, and pigment red 57:1 were received from Magruder Color Company. Styrene acrylic resin was from Johnson Polymer LLC, now BASF.
Decorative commercial white paints were from Sherwin Williams Company. Conventional dispersants including an acetylenic diol surfactant, a styrene acrylate polymer, and a styrene maleic anhydride modified with a polyether comb polymer were from different dispersant suppliers.
Polyetheralkanolamine comb polymers consist of a hydrophobic aromatic backbone and hydrophilic side chains. The backbone, being hydrophobic and aromatic, has strong affinity for the pigment surface, thereby anchoring the dispersant molecule to the pigment particles. The side chains are based on polyether (mostly polyethylene oxide) and dissolve in the aqueous media, creating a stabilizing shell around the pigment particle. Some pigments also contain acidic groups, and as a result there is also an acid/base interaction between the amine group (base) of the dispersant and the carboxylic group of the pigment. The chemical structure of the polyetheralkanolamine comb polymers used in the present study is given below: are given in Table 1. The number after the letter P designates the moles of ethylene oxide.
Table 1: Polyetheralkanolamine comb polymers Product Moles of Propylene oxide Moles of ethylene oxide P7E 3 7 P19E 3 19 P41E 3 41 P58E 8 58
Adsorption isotherm measurements
Carbon black suspensions (C = 0.40% w/w) were prepared at room temperature in the presence of polyetheralkanolamine solutions of different concentrations. The dispersion was obtained by treatment in an ultrasonic bath for 30 min and followed by rotating the samples end-over-end for 18 h to reach equilibrium. Each suspension all the solids, leaving a clear supernatant at the top. The concentration of the remaining polyetheralkanolamine in the solution was carried out by UV absorption and the amount adsorbed was estimated from the difference between the amount of polyetheralkanolamine added and the amount remaining in the supernatant.
Surface and interfacial tension measurements
Surface tension measurements were determined with a K12 Tensiometer (Kruss GmbH) using a Wilhelmy plate method. Dynamic interfacial tension measurements were made using a DVT-10 drop volume Tensiometer (Kruss GmbH). The instrument measures dynamic interfacial tension as a function of surface age.
Contact angle measurements
Contact angle measurements of water on a clean slide coated with a carbon black dispersion containing a dispersant of desired concentration were made using a K12 Tensiometer. The coated glass slide was air-dried for 36 h before use.
Pigment adsorption studies
K12 Tensiometer was employed with the powder holder FL12. A circle of filter paper was placed in the bottom of a perforated cell to prevent powder from leaking out the bottom of the cell. A second piece of filter paper was placed on top of the powder that was placed in the cell to prevent powder from rising through holes in the piston. About 1.02 g of carbon black was placed inside the cell and the piston was screwed completely down to insure a constant pigment volume for each experiment. The cell was then brought in contact with the liquid contained in a beaker and the weight gain due to absorption of the liquid as a function of time was recorded.
Zeta potential measurements
Zeta potential measurements were done with a DT-1200 Acoustic Spectrometer (Dispersion Technology, Inc.) by means of Colloid Vibration Current (CVI). Details about the technique can be found elsewhere. (17) Basically, an ultrasonic wave is introduced and disturbs the double layer. The displacement of the ionic cloud creates a dipole moment. The sum of these dipole moments over many particles creates an electrical field which causes the CVI to flow. The instrument measures the CVI from which the zeta potential is determined from an empirical equation. Samples were prepared by milling of 5 and 30 wt% of carbon black in water prior to zeta potential measurements. One of the main advantages of electroacoustics over traditional main advantages of electrokinetic methods is the elimination of dilution as the instrument can measure up to 50% volume of solids.
Particle size measurements
DT-1200 Acoustic Spectrometer was employed to measure the particle size. Details about the technique can be found elsewhere. (17) Sound waves are transmitted through a sample. The attenuation of these pulses is measured over a wide range of ultrasonic frequencies. The particle size is then deduced from the measured spectra using an empirical equation. Using this technique, the particle size (5 nm to 1000 [micro]m) of a concentrated dispersion (up to 50% by volume) can be measured. Samples were prepared by milling of 5 and 30 wt% of carbon black in water prior to measurements.
Pigment dispersion viscosities were determined with a Bohlin CVO 120 Rheometer (Malvern Instruments Ltd.) which used a truncated cone with a gap setting of 0.15 mm and a viscometer (Brookfield Engineering Laboratories, Inc).
Initially a premix was made by weighing water, binder (optional), defoamer, and dispersant into a blender container and mixing for 20 s. The pigment was then added slowly in two equal parts, followed by mixing after each addition. After incorporating all the pigment, the premix was blended for 10 min to deflocculate the pigments. The formulation was then introduced to a Mini-100 horizontal media mill (Eiger Machinery, Inc.). The mill contained about 70 mL of 1.2 mm glass media and was grinded at a speed of 5000 rpm for 10 min. All dispersants were tested on actives.
These tests were performed over a regular bond paper (The Leneta Company, Inc.) with a #40 wire wound rod, received from the Paul N. Gardner Company, Inc. with wet film thickness = 102.5 [micro]m. The rub-up test consists of rubbing with an index finger over a time span of 5 s. For rub-up tests, a color mill base was added to a commercial white paint (tint base) to obtain a 2.5% color pigment in the finished formulation. It was mixed using a SpeedMixer[R] (FlackTek. Inc.) for 10 min at 3000 rpm. A Spectro-guide 45/0 Gloss Meter (BYK Chemie USA, Inc.) was used to measure the L*a*b* values from which the [DELTA]E was calculated.
Water resistance tests
Drawdowns were made on a glass substrate using a #40 wire wound rod. The film was dried at ambient and water droplets were placed on the dried coating and observed for any blisters.
Color (tint) strengths
Color strengths of all dispersions were determined by mixing a commercial coating containing [TiO.sub.2] and a pigment concentrate using a SpeedMixer [R] (FlackTek, Inc.). A # 40 wire wound rod was used to draw down on a regular bond paper (The Leneta Company, Inc.). The purpose of evaluating color strength is to ascertain the amount of color generated during milling. Color strength is calculated by designating one formulation as the standard (i.e., 100%). The percent deviation in color strength of other samples is calculated based on this standard. Optical density (OD) of the samples was measured using an X-RITE[R] Spectrodensitometer (X-rite Incorporated) and converted to reflectance values (R) by:
R = [10.sup.-O.D] (1)
Using the Kubelka-Munk equation, the reflectance values for samples ([R.sub.u]) are compared against the standard ([R.sub.s])
% Strength vs. Standard = [(l - [R.sub.u]).sup.2]/2[R.sub.u]/[(l - [R.sub.s].sup.2]/2[R.sub.s] x 100 (2)
Results and discussions
Optimization of polyetheralkanolumines
The Washburn equation for liquid penetration into a column of porous particles is given by:
[W.sup.2] = ([[rho].sup.2][gamma]c cos[theta]/[eta]) t, (3)
Where W = weight of liquid gained (g) within time (s):
[rho] = density of the liquid (g/[cm.sup.3]): () = contact angle;
[eta] = viscosity (g/cm. s); c = solid material constant: and
[gamma] = surface tension (dyne/cm).
The solid material constant c is determined by performing an absorption measurement within n-hexane, assuming that contact angle is zero (perfect wetting). It follows from equation (3) that a plot of [W.sup.2] vs. t should be linear. However, the slope of the above plot is not a constant in the whole range for all the polytheralkanolamines investigated (Fig. 1 shows a plot of [W.sub.2] vs t for P19E as an example where after about 120 s deviations from linearity was observed). Various authors also observed such deviation from linearity. (18-20) For example, Varadaraj et al. (19) investigated the wettability of sodium dodecyl sulfate on resin-coated sand and their result is shown in Fig.2. As can be seen, there is no single relationship for the whole range of experimental points. They attribute the contact changes during capillary rise and gravity could account for observed deviations from linearity. Labajos-Broncano et al. (21) use parabolic forms of Washburn's equation to analyze the results of the capillary-penetration wetting technique instead of its linear expression. They define the total weight gained being the sum of the gained mass due to the imbibition expressed by equation (3) and the increased of weight caused by the force of the liquid surface tension exerts over the porous body due to the contact between solid and liquid.
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Since the slope of the [W.sup.2] vs. time curves is not constant, the Washburn equation could not be applied directly to analyze the data. As a result we adopted the procedure from Varadaraj et al. (19) who defined two parameters to express wettability: (1) wetting effectiveness and (2) wetting rate. Wetting effectiveness is the maximum amount of liquid gained into the pigment column. Wetting rate is the weight of liquid gained at half saturation divided by the time required for half saturation. Figures 3 and 4 show the effect of ethylene oxide (EO) on wetting effectiveness and wetting rate for 4% polymer solutions on carbon black, respectively. Wetting effectiveness increases with EO content and levels off when the EO is above 41 moles. Wetting rate, however, appears to reach a maximum value around 41 moles EO and then starts to decrease slightly. Figure 5 shows the effect of EO on surface tension. As expected, surface tension increases with increasing EO content as the polymer becomes more hydrophilic. A similar result was observed for interfacial tension measurements using mineral oil as a dispersed phase (Fig. 6). The effect of EO on carbon black dispersion viscosity is illustrated in Fig. 7 with 37% pigment loading and 15% dispersant on carbon black. A minimum in viscosity was observed at 41 models EO, corresponding to a highest wetting rate as was seen in Fig. 4. An increase in dispersion viscosity of P58E, despite the possibility for steric stabilization due to the long EO chains, may be explained by a larger effective particle diameter. The adsorbed layer thickness of a comb polymer can be added to the volume fraction of the solid particles, [phi], (calculated solid loadings), to give an effective volume fraction, [[phi].sub.eff], which can be considerably larger than [phi].
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[[phi].sub.eff] = [phi](1 + [delta]/R).sup.3].(4)
where [delta] is the adsorption layer thickness and R is the radius of the particle. (22) In a good solvent, [delta] can be estimated by the following. (9)
[delta] = n[l.sup.5/3/[s.sup.2/3]. (5)
where n is the number of EO units, l is the EO length (0.369 nm), and s is the average distance between grafting points (1.6 nm), i.e., the average distance from one nitrogen to another nitrogen of a comb polymer. Using equations (4) and (5) with R = 24 nm, Table 2 lists the values for steric thickness ([delta]) and effective volume fraction ([[phi].sub.eff] along with the calculated volume fraction of solid particles (density of carbon black = 1.8 g/cc). It can be seen that the layer thickness increases with increasing EO units, suggesting that the polymer was adsorbed by anchoring the hydrophobic backbone onto the surface of carbon black with the EO chain extending into the water and providing steric stabilization to the carbon black particles. Other authors also found a similar trend using different methods. For example, Ma and Xi[u.sup.23] employed microelectrophoresis and indicated that the adsorption layer thickness of ethoxylated nonylphenols onto carbon black particles increases with increasing EO contents. They also concluded that the thickness of the adsorption layer, not the adsorption amount, governs the stabilization of the carbon black dispersion. Using photon correlation spectroscopy, the adsorbed polymer layer thickness was found to increase with the molecular weight of polyethylene glycol at [TiO.sub.2]/water interface. (8) Recently, using equation (5) above, Palmqvist et al. (13) calculated the layer thickness of 33.5 nm for a comb polymer consisting of a polycarboxylate backbone with PEO chains. For [phi] = 0.24, the [[phi].sub.eff] for P58E is about 0.64 which is close to the maximum packing limit of 0.74 for hexagonal close packing. As a result particle/particle interactions can occur and this is why P58E gave a higher carbon black dispersion viscosity than P41E. Similarly, Palmqvist et al. (13) attributed a high dispersion viscosity observed for a grafted polymer in alumina to a high effective volume fraction. The adsorption layer thickness for P41E is about 6.11 nm, compared to a theroetical value of 14.35 nm (length of each EO unit is about 0.35 nm). It is concluded that the EO chain may loosely coil at the solid/liquid interface rather extend straight into the water. It has been shown that an adsorbed layer thickness of 5-10 nm is usually considered sufficient to screen the attractive Van der Waals forces when the particles are less than a few microns in diameter. (1) As a result, only P41E and P58E can provide steric stabilization. Since P41E gave the lowest dispersion viscosity, and also provided steric stability, we will focus on the physicochemical and performance properties of P41E only.
Table 2: Calculation of steric thickness and effective volume fraction EO content [delta] (nm) [[phi].sub.eff] [phi] 7 (P7E) 1.39 0.29 0.24 19 (P19E) 3.05 0.35 0.24 41 (P41E) 6.11 0.48 0.24 58 (P58E) 9.16 0.64 0.24 NP [40.sup.a] 2.3 Note: Length of each EO unit is about 0.35 nm. For 41 EOs, the length is 14.35 nm, compared to 6.11 nm. It is concluded that the EO chain may loosely coil at the solid/liquid interface rather than extend straight into the water
(a) Ma and Xia (23)
The surface tension curve for P41E in water is shown in Fig. 8. The polymer is slightly surface active, reducing the surface tension of water by 20 dyne/cm at 10 ppm. It also exhibits a distinct break similar to the critical micelle concentration, called critical aggregation concentration (cac) at about 20 ppm. Above the cac, the surface tension is essentially constant and is about 50 dyne/cm. Using the Gibbs adsorption equation, the amount of P41E adsorbed at the air/water interface is 1.86 X [10.sup.-10] mole/[cm.sup.2] and the area per molecule at the air/water interface is 89 [[angstrom].sup.2]. These values are close to dodecyl alcohol ethoxylates (30 moles of EO) (1.6 X [10.sup. -10] mole/[cm.sup.2] and 105 [[angstrom].sup.2]). (24)
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In Fig. 9, the zeta potential curves for carbon black with various amounts of P41E together with a references curve for untreated carbon black are shown. For the untreated carbon black, the isoelectric point (IEP) is around 10.2. This value is close to the p[K.sub.a] of the phenol which is normally present on the surface of the carbon black. It is clear that adsorption of P41E has taken place since the IEP is lowered to around 6.9 in the presence of P41E. An increase in the absolute values of the zeta potential. At pH 2.0, the zeta potential for 15% P41E is about 8 and -4 mV at pH 11. These values are not sufficient to provide electrostatic stability since it has been reported that at least -30 or +30 mV is considered for electrostatic stabilization. (25) It is not surprising that P41E imparts much less charge since it is a nonionic polymer. The low zeta potential indicates that the grafted EO chains extend into the water phase and shift the shear plane further away from the carbon black surface, which is expected for a nonionic polymer. (26)
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The following equation was used to calculate the specific energy of interaction between the carbon black surface and the dispersant P41E. (27)
[[DELTA]p[H.sub.iep] = 1.0396[C.sub.0] exp( - [DELTA][G.sub.SP.sup.0]/RT) (6)
[DELTA][G.sub.SP.sup.0] = - RT(ln [DELTA]p[H.sub.iep] - ln [C.sub.0] - ln 1.0396), (7)
where [delta] p[H.sub.iep] is the shift in the IEP at the additive concentration [C.sub.0] and [delta][G.sub.SP.sup.0] represents the specific free energy of interaction between the surface carbon black and the dispersant. R and T are the standard gas constant and the temperature in Kelvin, respectively. The calculated data of interaction energy ([delta] [G.sub.SP. sup.0] are shown in Table 3. The value of -7.3 RT was obtained for the interaction energy, indicating that the P41E molecules are strongly adsorbed onto carbon black surface. This in turn suggests a strong interaction of carbon black with P41E dispersant and shows suitability of P41E as dispersant for carbon black dispersions.
The adsorption isotherm of P41E at water/carbon black interface is shown in Fig. 10. The isotherm is of the Langmuir (L) type. (28) The values of free energy of adsorption can be estimated from the following equation:
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[DELTA][G.sub.ads] = - RT ln (IS x 55.5/plateau value), (8)
where IS is the initial slope of moles adsorbed per g adsorbate vs. molar equilibrium concentration curve and plateau value is the maximum adsorption in moles per g adsorbate. The limiting adsorption or the plateau values, the values of free energy of adsorption, and the limiting area per molecule are reported in Table 4. For comparison, we also include the values for nonylphenol ethoxylates from the literature (NP40 = nonylphenol with 40 EO; NP100 = nonylphenol with 100 EO; NP20 = nonylphenol with 20 EO). The adsorption energy for P41E is slightly more negative than those for nonylphenol ethoxylates, indicating that P41E molecules absorbed more strongly onto carbon black surface than nonphenol ethoxylates. The area per molecule for P41E, calculated from maximum adsorption at water carbon black interface (1380 [Angstrom].sub.2] is much larger than that obtained from adsorption at water/air interface (89 [[Angstrom].sub.2]). This can be attributed to the porosity of carbon black which has not been taken into accounts for these experiments. Figure 11 shows the initial concentrations vs. the concentration absorbed onto the carbon black for P41E which is the difference between the initial concentration and the amount in the supernatant after centrifugation. It can be seen that a fairly linear relationship was observed with [r.sup.2] = 0.998. The slope represents the partition coefficient which is defined as K = [C.sup.a]/[C.sup.I]. where [C.sup.a] is the concentration of adsorbed P41E at the carbon black/water interface and [C.sup.I] is the total added concentration of P41E. Since K is large, most of the added P41E adsorbed onto the carbon black and only a small amount stayed in the aqueous phase. This is very important as the dispersion stability is governed by the amount the dispersant adsorbed onto the pigment and the foamability is affected by the amount in the bulk phase and the amount adsorbed at the water/air interface.
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Table 3: Computed value of specific free energy of interaction for carbon black and P41E System Concentration p[H.sub.iep] (ppm) No dispersant - 10.2 Carbon black/ 7500 6.9 P41E System [DELTA] -[DELTA] p[H.sub.iep] [G.sub.SP.sup.0] (RT units) No dispersant - - Carbon black/ 3.3 7.27 P41E Table 4: Adsorption data Sample Limiting adsorption [DELTA] Limiting values Gads area per ([mu]mol/[m.sup.2]) (kJ/mol) molecule (A2) P41E 0.12 -40.6 1380 NP [40.sup.29] 0.87 -31.8 191.7 NP [100.sup.29] 0.71 -30.1 234.2 NP [20.sup.30] 0.41 - 407 NP [40.sup.30] 0.17 - 967
Pigment particle size and size distribution affect the color properties of the pigment. Smaller particles provide higher color strength, higher gloss, and better transparency. Small particles and uniform particle size distribution are especially critical for inkjet inks. A large particle size distribution is more prone to floeculation. Figures 12, 13, and 14 show the particle size distributions for the 5% untreated carbon black. 5% carbon treated with P41E, and 30% carbon black treated with P41 E, respectively. The distribution of the untreated carbon black is clearly quite bimodal. The smaller mode, corresponding to the primary carbon black particles, was on average 0.523 [micro]n. The larger mode which represents large amounts, indicating some degree of aggregation of these carbon black particles, had a median size of about 16.9 [micro]m. When treated with 20% P41E on carbon black, both 5% and 30% carbon black dispersions showed uniform distribution (unimodal) and had very small median sizes of about 0.0924 and 0.1032 [micro]m, respectively. Such small particle size and uniform distribution make P41E a suitable dispersant for inkjet ink applications.
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One of the three steps involved in the pigment dispersion process is the pigment wetting which is the interaction of liquid with pigment surface. Contact angle can provide such information. A low contact angle is indicative of good wetting. water contact angles of carbon black treated with various dispersants are shown in Fig. 15. The carbon black is quite hydrophobic as reflected by a high contact angle of 87[degrees]. When treated with P41E, it is lowered to 26[degrees], indicating that P41E adsorbed onto the carbon black and modified the surface to hydrophilic. For comparison, the modified styrene maleic anhydride (MSMA) reduced the contact angle to 56[degrees], less effective than P41E.
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Comparison of P41E with other conventional dispersants
It has been shown that a polyetheralkanolamine with 41 moles EO gave the best performance among its homologues and hence we will compare its efficiency with conventional dispersants including a styrene acrylate copolymer, an acetylenic diol surfactant, and a MSMA. The results will be divided into two sections: resin-containing pigment concentrates and resin-free pigment concentrates.
Resin-containing pigment concentrates
Figures 16-19 compare the dispersion viscosities of carbon black, pigment yellow (PY 14), pigment blue (PB 15:3), and lithol rubine (PR 57:1), respectively, containing P41E, MSMA, and acetylenic diol surfactants. The pigment to resin (binder) ratio is 4.7:1. As can be seen. P41E exhibits a much lower dispersion viscosity than other dispersants. A low viscosity may suggest a stable system because the aggregation of pigment particles is low for a well-dispersed system, the spindle experiences only a small hindrance from aggregates at a given shear rate and therefore detects a smaller shear stress and a lower viscostiy. In contrast, a higher shear stress and thus a higher viscosity is observed for a poorly dispersed system because the agglomerates form networks that hinder the spindle. (31) Unlike dispersions with P41E which shows relatively Newtonian behavior, those with other dispersants show pseudoplastic character. Steric immobilization, in which solvent is trapped in the interstices of clusters of the particles, is caused by particle/particle interactions. As these clusters are broken upon shearing, the apparent viscostiy is decreased, resulting in pseudo-plasticity. (32) Newtonian behavior indicates pigment deflocculation and is desirable during the grinding stage. This behavior may be attribute to the strong hydrophobicity of the P41E comb polymer backbone, which results in strong adsorption on the pigment particles, and the hydrophilic and flexible character of The polyether groups extended into the water phase. This configuration stabilizes the dispersion. The benzene ring was introduced as the hydrophobic portion of the P41E comb polymer and is similar to the pigment molecules based on aromatic structure. It has been reported that viscometry can be used to assess the degree of pigment flocculation. (11).(14).(33) Newtonian behavior is an indication of a lack of particle/particle interaction, and the degree of deviation from Newtonian is a measure of flocculation (i.e., particle/particle interaction). Linear plots are obtained by plotting log viscosity against the reciprocal square root of the shear rate. The larger the slope, the greater the degree of flocculation, as shown in Figs. 20 and 21. P41E had a smaller slope than the MSMA and the acetylenic diol surfactant. In Figs. 17 and 18, the polyethermonoamine is the side chain grafted onto the P41E. Comparison of the polyethermonoamine itself with P41E shows that combining a hydrophobic anchoring group with a polyether side chain to make P41E reduced the yellow and blue dispersion viscosities significantly. This an be attributed to a stronger adsorption of P41E onto the pigment surface. After a three-week aging period, P41E produced a much better viscosity stability in the yellow dispersion than an acetylenic diol surfactant (Fig. 22).
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Tint strength comparison of P41E with other dispersants are shown in Figs. 23-25. P41E developed a higher color strength (4-11% more strength) in carbon black, PB 15:3, and PR 57:1 than others. Gloss at 60[degrees] angle is also higher with P41E (Fig. 26).
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Resin-free pigment concentrates
Basically, resin-free pigment concentrates were made and let down with white paints having a variety of resins (polyesters, alkyds, acrylics, etc.). The advantage of having resin-free pigment concentrates is that they can be blended with various resins or other components to make a finished product. As a result, by mixing a few pigment concentrates with white-based paints, a range of paints in different colors can be produced, making the paint manufacturing process cost-efficient and less time consuming.
A dispersant used for making resin-free pigment concentrates should meet the following criteria:
(1) wide compatibility with different resins
(2) good solubility with various solvents
(3) high pigment loading
(4) low dispersion viscosity
(5) stable dispersions/paints
The compatibility of P41E has been tested in a number of resins by mixing it with a resin at 1:1 ratio and observing the appearance (clear, hazy, precipitation). Table 5 summarizes the results. It can be seen that P41E was compatible with a variety of resins. Figure 27 shows that P41E was much more effective at reducing the blue pigment concentrate viscosity at high pigment loading (45%) than a styrene acrylate copolymer. A similar result was also observed for the lithol rubine concentrate at 45% pigment loading (Fig. 28). Figure 29 compares the viscosity stability of P41E and styrene acrylate dispersants in 30% carbon black resin-free concentrates. After one week at 50[degrees]C, the viscosity of P41E was essentially constant while the viscosity of styrene acrylate increased significantly.
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Table 5: Compatibility of P41E in different binder systems Binder system Compatibility 2 Pack Epoxy Good Polyester resins Good Alkyd resins Good PU dispersions Good Acrylic dispersions Good Melamine resins Good
Resin-free lithol rubine, phthalo blue and carbon black dispersion containing MSMA, styrene acrylate and P41E with 36.5%, 43%, and 30% pigment loadings were prepared, respectively. To test the quality of dispersions, these pigment concentrates were tested with 2.5% by weight into decorative commercial white paints such as latex wall paint flat, latex paint semigloss, latex paint high gloss, and alkyd emulsion semigloss. Drawdowns were made with #40 wire would rod on uncoated and coated papers (The Leneta Company, Inc.). The gloss, color strength, and color compatibility were then measured. The rub-up test is commonly used to assess the degree of color separation. The color difference between rubbed and unrubbed areas ([DELTA]E) is a measure of the degree of color separation. If [DELTA]E > 1, color separation has occurred. For a black high-glossy latex paint and a black semigloss latex paint, the color strength for P41E was about 10% and 16% higher than those for other dispersants, respectively (Figs. 30 and 31). The color strength was much more pronounced for a black semiglossy alkyd resin emulsion paint in which P41E showed an improvement of 28% over a styrene acrylate (Fig. 32). P41E also exhibited a much higher gloss than others (Fig. 33). Water resistance for all three dispersants was acceptable. Table 6 lists the [delta]E values for carbon black in a semigloss latex paint. Color compatibility improved with an increase in the dosage of P41E. For example, when the dosage was increased to 50% on carbon black, a minimum in color separation was observed ([DELTA]E < 1). Tables 7-10 summarize the results of a blue in various white paints. For a blue high-glossy latex paint, all three dispersants gave a comparable color strength. P41E produced a slightly higher color strength than others in a blue flat latex paint but the difference was insignificant. Highest color strength, however, was observed for P41E in the blue alkyd resin paint. With the exception of alkyd resin paint, color compatibility was acceptable for flat, semigloss, and high-gloss latex paints ([delta]E < 1). P41E gave the highest gloss in alkyd resin paint. Water resistance was good for all three dispersants except for a blue flat latex paint containing a styrene acrylate.
[FIGURE 30 OMITTED]
[FIGURE 31 OMITTED]
[FIGURE 32 OMITTED]
[FIGURE 33 OMITTED]
Table 6: Rub-up [delta]E values for carbon black in semi-glossy paint Dispersant Dosage on black (%) [delta]E P41E 30 4.68 P41E 40 2.83 P41E 50 0.81 MSMA 30 3.94 Styrene acrylate 40 3.12 Table 7: Properties of phthalo blue containing various dispersants in flat latex paint Dispersant Dosage on blue [delta]E Color Water pigment (%) strength resistance P41E 20 0.09 104.90 Good Styrene acrylate 20 0.47 100 Blisters MSMA 20 0.16 102.9 Good Table 8: Properties of phthalo blue containing various dispersants in semigloss latex paint Dispersant Dosage on blue [delta]E Color Water pigment (%) strength resistance P41E 20 0.59 117.2 Good Styrene acrylate 20 0.25 100 Good MSMA 20 0.69 107.1 Good Table 9: Properties of phthalol blue containing various dispersants in high-gloss latex paint Dispersant Dosage on blue [delta]E Color Water pigment (%) strength resistance P41E 20 0.39 101.50 Good Styrene acrylate 20 0.14 100 Good MSMA 20 0.41 102.01 Good
Tables 11-14 list the properties of lithol rubine containing various dispersants in white paints. It is noteworthy that P41E was tested at half the dosage of other dispersants and still proved effective. The most striking result in the color development of P41E in an alkyd resin paint in which P41E was 100% and nearly 50% more efficient than a MSMA and a styrene acrylate, respectively. Unlike a MSMA, P41E produced the[delta]E values of rub-up les than 1 in various white paints, indicating that color compatibility is acceptable. Water resistance is good for all three dispersants.
Table 10: Properties of phthalo blue containing various dispersants in alkyd resin semigloss paint Dispersant Dosage on [delta]E Gloss Color Water blue (%) strength resistance P41E 20 3.24 43.2 114.2 Excellent styrene acrylate 20 3.51 15.3 110.05 Excellent MSMA 20 3.39 33 100 Excellent Table 11: Properties of lithol rubine containing various dispersants in flat latex paint Dispersant Dosage on lithol [delta]E Color Water rubine (%) strength resistance P41E 10 0.17 104.70 Good MSMA 20 0.20 103.3 Good Styrene acrylate 20 0.16 100 Good Table 12: Properties of lithol rubine containing various dispersants in semigloss latex paint Dispersant Dosage on lithol [delta]E Color Water rubine (%) strength resistance P41E 10 0.49 111.09 Good A 20 2.20 110.59 Good Styrene acrylate 20 0.67 100 Good Table 13: Properties of lithol rubine containing various dispersants in high-gloss paint Dispersant Dosage on [delta]E Gloss Color Water blue (%) strength resistance P41E 10 0.15 33.83 100.55 Good MSMA 20 0.27 30.2 101.48 Good acrylate 10 0.20 30.03 100 Good Table 14: Properties of lithol rubine containing various dispersants in alkyd resin semigloss paint Dispersant Dosage on [delta]E Gloss Color Water blue (%) strength resistance P41E 10 1.00 28.93 263.98 Excellent MSMA 20 5.07 26.2 100.00 Excellent Styrene acrylate 20 0.87 10.6 180.01 Excellent
In this work, a series of polyetheralkanolamines was prepared by fixing the anchoring group while varying the molecular weight and the EO/propylene oxide (PO) ratio of the side chain. The surface tension and interfacial tension increase with increasing EO as the polymer becomes more hydrophilic. Zeta potential measurements and adsorption layer thickness results suggest that polyetheralkanolamines with high EO contents provide only steric stabilization and they absorb at the interface with the ethylene oxide chains in a coil conformation. The area per molecule for the polyetheralkanolamine calculated from maximum adsorption at water/carbon black interface is orders of magnitude larger than that obtained from adsorption at water/air interface, perhaps due to the porosity of carbon black not being taken into account in this study. A polyetheralkanolamine with 41 moles of EO and 3 moles of PO provides the best performance among its derivatives, showing a lowest carbon black dispersion viscosity and highest pigment-wetting rate. This structure also provides small, uniform, and narrow particle size distribution of carbon black dispersion. When compared with a styrene acrylate copolymer, an acetylenic diol surfactant, and a MSMA copolymer, the polyetheralkanolamine produces lower viscosity with nearly Newtonian flow behavior water-based resin-free and resin-containing pigment concentrates. Better results such as gloss, high pigment loading, color strength, water resistance, and stability were also observed with organic pigments and carbon black. The pigment concentrates containing the polyetheralk-anolamine exhibit excellent color compatibility in various decorative commercial white paints containing a wide range of resins.
Acknowledgments The author would like to extend his gratitude to Ms. Judy Perez for skillful laboratory assistance and to Lisa Fine for valuable discussion. All information contained herein is provided "as is" without any warranties, express or implied, and under no circumstances shall the authors or Huntsman be liable for any damages of any nature whatsoever resulting from the use or reliance upon such information. Nothing contained in this publication should be construed as a license under any intellectual property right of any entity, or as a suggestion, recommendation, or authorization to take any action that would infringe any patent. The term "Huntsman" is used herein for convenience only, and refers to Huntsman Corporation, its direct and indirect affiliates, and their employees, officers, and directors.
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Presented at the 2006 FutureCoat! Conference, sponsored by the Federation of Societies for Coatings Technology, in New Orleans, LA. on November 1-3, 2006.
Huntsman Corporation. TheWoodlands, TX, USA
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