Application of microcalorimetry to the study of interactions in coating formulations.
Abstract Water-based coating formulations contain many components, such as latex binder, pigment, dispersant, thickener, and surfactant. Complex interactions between these components affect properties of the coating in both the wet and dried states. For example, rheology of the paint is dependent on the interactions of components in the formulation. Pigment interaction can affect the degree of dispersion and, therefore, have ramifications on end use properties such as hiding and tint strength of the paint film. Isothermal titration calorimetry has been used to determine the enthalpy of interaction of the coating components. Representative examples will be given to demonstrate structure--property relationships that correlate the interactions of latex, pigment, surfactant, dispersant, and thickener with paint viscosity as well as end use performance parameters such as adhesion and tint strength.
Keywords Microcalorimetry, Latex pigment interaction, Coating
In many water-based coating applications, latex binders are used in formulations in conjunction with other components, e.g., pigments, surfactants, thickeners, dispersants, etc. Complex interactions between these components are manifested in many properties of the formulations such as rheology and coating structure affecting mechanical and optical properties. To control the properties of the formulated products, it is important to understand the interactions between the various components.
Interactions between components used in coating formulations have been studied. For example, Husband (1) characterized the interactions between latex, starch, and kaolin clay by studying the adsorption and rheological behavior. Stenius and coworkers (2), (3) studied the adsorption of surfactants and dispersants, such as sodium dodecyl sulfate (SDS), polyethylene oxide nonylphenol ethers (NPEO), and sodium hexametaphosphate (SHMP), on kaolin clay and polystyrene latex. They found that the addition of NPEO will cause SHMP to desorb from kaolin clay and SDS to desorb from polystyrene latex. Sandas and Salminen (4) studied the interactions between English clay and polyvinyl alcohol (PVOH), carboxymethylcellulose (CMC), starch, and synthetic thickeners. They found that the CMC and the two synthetic thickeners they studied show high interactions with clay and form a strong three-dimensional structure. PVOH also shows a high interaction but does not lead to formation of a three-dimensional structure. Young and Fu (5) studied the adsorption of thickeners on kaolin clay and styrene/butadiene (S/B) latex. They found anionic CMCs are adsorbed less on kaolin clay than nonionic hydroxyethylcellulose and hydrophobically modified cellulosic thickeners (HMCT). Adsorption on S/B latex was observed only with HMCT. Interactions between surfactant and water-soluble polymers were also studied. For example, Qiao and Easteal (6) studied the interaction between nonionic surfactant and poly (ethylene glycol), Fishman and Eirich (7) and Murata and Arai (8) studied the interaction between SDS and poly (vinyl pyrrolidone). Baxter et al. (9) studied the interactions between latex and [TiO.sub.2] with different water-soluble polymers by comparing their phase partition behavior.
Isothermal titration calorimetry (10) (ITC) has been used to study adsorption and interaction processes. The interactions between surfactants and water-soluble polymers have been studied. Wyn-Jones and coworkers (11-15) used ITC to study the interactions between SDS/poly (N-vinylpyrrolidone), SDS/nonionic dendrimers, and cationic surfactants/nonionic polymers. Kevelam et al. (16) studied the interactions between hydrophobically modified water-soluble polymers and surfactants. Interactions between SDS, cationic surfactants/poly(ethylene oxide) and ethyl(hydroxyethyl)cellulose ethers, (17) anionic and cationic surfactants/poly (N-vinylpyrrolidone), (18) sodium alkylsulfates/poly(ethylene oxide), (19) and surfactants/thickeners (20-22) have also been studied by other researchers. The micellization of surfactants has also been studied extensively using microcalorimetry. (23-35) Winnik et al. (36) used microcalorimetry to study the interaction between nonionic surfactants and poly (butyl methacrylate) latex. El Attar Sofi and Foissy (37) used microcalorimetry to study the adsorption of poly(acrylic acid) and polyacrylamide on [TiO.sub.2]. Pettersson and Rosenholm (38) studied the adsorption of octylamine on [TiO.sub.2]. Microcalorimetry has been used by Sanders and Roth (39) to study the interactions between pigments, binders and water-soluble polymers. Wernett and coworkers (40) used microcalorimetry to determine the interaction between S/B latex, CMC, and pigments such as clay and calcium carbonate.
In this paper, the interactions between components of interest in coating applications, such as pigments, latexes, thickeners, and dispersants, were studied using microcalorimetry.
The heats of interaction between components were determined using the Omega isothermal titration calorimeter (ITC) from MicroCal, Inc. Figure 1 shows a schematic of the calorimeter.
[FIGURE 1 OMITTED]
The ITC directly measures heat evolved or absorbed in liquid samples as a result of injecting precise amounts of reactants. A spinning syringe is utilized for injecting and subsequent mixing. In this study, a 250-[micro]L syringe was used at a stirring rate of 400 rpm. A pair of identical coin shaped cells of 1.35 mL volume are enclosed in an adiabatic jacket. The reference cell was filled with water. The sample cell contained the liquid of interest, either the latex or pigment slurry. During the experiment, the reference cell was heated by a very small constant power, the reference offset. The temperature difference between the two cells was constantly measured and a proportional power was increased or reduced to the sample cell by the cell feedback system to keep the temperature difference very close to zero. A signal proportional to the cell feedback is called dCp, and with the instrument temperature and time, constitutes the instrument's relevant raw data. The dCp is calibrated in [mu]Cal/s. An injection which results in an exothermic reaction within the sample cell causes a negative change in the dCp power since the heat evolved provides heat that the cell feedback is no longer required to provide. Alternatively, an endothermic reaction would result in a positive change in dCp. Since the dCp has units of power, the time integral of the peak yields a measurement of thermal energy, [DELTA]H. The detector sensitivity is 0.2 [micro]cal with a noise level of 0.005 [micro\]cal/s. For reactions in aqueous media, the thermal response half-time is 8 s.
The latex and pigment slurries were usually diluted to 5-10 wt% solids, degassed, and stored with agitation to prevent the particles from settling. The concentration of surfactants and dispersants is 1% based on solid content unless indicated otherwise.
The titration experiments were carried out at 22[degrees]C. For example, in the determination of the heat of interaction between pigment and latex, the degassed 10% pigment slurry was loaded into the sample cell using a 2.5-mL glass syringe. The syringe was carefully inserted into the cell until it touched the bottom of the cell. The cell was filled with the pigment slurry from the bottom up in order to exclude air bubbles.
The injection syringe was loaded, in this case with 10 wt% of latex. It is very important to ensure complete absence of bubbles. The injection syringe was carefully inserted into the entry port of the sample cell. The syringe was inserted until the upper bearing was seated flush with the top of the cell port entry hole. The stirrer motor-injector stepper motor stand was then rotated into place so that the injector shaft aligned with the syringe plunger with the stirrer belt engaged. The injector shaft, which controls the movement of the plunger, was then lowered into place to abut the syringe plunger using computer control. A coupler was used to link the movement of the injector shaft with that of the plunger. The stirrer was then turned onto a speed of 400 rpm. The cell was allowed to equilibrate until a stable base line was obtained.
The heat of interaction between the 10% pigment slurry in the sample cell and the latex in the syringe was determined by injecting ten 10 [micro]L injections into the cell with a wait of 300 s between injections. The duration of each injection was 6.29 s.
Latex pigment interaction
Interactions between components in the coating formulation are important and affect many coating properties and the end use performance. For example, Mahli et al. (41) studied the effect of surfactant on color development. They found SDS gave poorer color development compared to nonylphenol oxyethylene surfactant and attributed the difference to interactions of the surfactant to pigments in the formulation. Ziani et al. (42) used inverse gas chromatography to study the interaction of pigments dispersed in coating formulation. Design of latex products capable of interacting with titanium dioxide ([TiO.sub.2]) has been reported in the literature. (43)
In this study of latex pigment interaction using microcalorimetry, the titration cell was filled with 10 wt% of the pigment slurry. It was titrated with a 10 wt% latex. The volume of each injection was 10 [micro]L. Ten injections were carried out. Figure 2 shows a plot of a typical titration. It can be seen that, in this case, the addition of ten 10 [micro]L aquilots of 10% latex resulted in an endothermic interaction. This endothermic interaction is the sum of the four pair-wise interactions; interaction of the pigment surface with the latex surface, the interaction of the pigment supernatant with the latex surface, the interaction of the pigment surface with the latex aqueous phase, and the interaction between the two aqueous phases. The following experiments are carried out to determine these four enthalpies. In addition to determining the enthalpy of interaction between 10% pigment slurry and 10% latex, the enthalpies of interaction between 10% pigment slurry and latex serum, pigment supernatant and 10% latex, and pigment supernatant and latex serum were also determined. The latex sera were isolated using a column eluate concentrator with 25 [micro]m microporous monolayer polypropylene membrane. The supernatant of pigment slurry was obtained by subjecting the slurry to ultra-centrifugation at 15,000 rpm for 1 h. Using these four experimentally determined enthalpies, interaction of the pigment surface with the latex surface, the interaction of the pigment supernatant with the latex surface, the interaction of the pigment surface with the latex aqueous phase can be calculated.
[FIGURE 2 OMITTED]
Results of the experimentally determined and calculated interactions between 10% calcined clay and 10% latexes are listed in Table 1. The interaction of latex with calcined clay indicates polyvinylacetate (PVAc) latexes interact with calcined clay exothermically. Both the aqueous phase interactions and the surface interaction are exothermic. The overall interactions of S/B latexes with calcined clay studied here are endothermic. The surface interactions of S/B latexes with calcined clay are endothermic, whereas the net aqueous phase interactions are slightly exothermic.
Table 1: Interaction of latex and calcined clay Latex Delta H Delta H (10% Delta H (10% clay/ clay/serum) (supernatant/10% 10% latex) [mu]cal latex) [mu]cal [mu]cal S/B latex 1 722 140 -344 S/B latex 2 526 172 -371 PVac latex 1 -1949 -1109 -647 PVac latex 2 -1086 -728 -330 Latex Delta H Delta H (clay Delta H (supernatant/serum) surface/serum) (superantant/latex [mu]cal [mu]cal surface) [mu]cal S/B latex 1 -187 309 -175 S/B latex 2 -159 315 -228 PVac latex 1 -394 -755 -293 PVac latex 2 -177 -570 -172 Latex Delta H (clay surface/latex surface) [mu]cal S/B latex 1 757 S/B latex 2 582 PVac latex 1 -547 PVac latex 2 -186
Effect of dispersant on the interaction of precipitated calcium carbonate with S/B latex
The effect of added polyacrylate dispersant on the interaction between pigment and latex was studied using a precipitated calcium carbonate pigment, OptiCal Print [TM] from Imerys (PCC 1) and S/B latex 3, varying the dispersant level from 0% to 0.75%. Results are shown in Table 2, and plotted in Fig. 3.
Table 2: Interaction of PCC 1 with 10% latex: effect of dispersant % Dispersant Delta H Delta H (10% DeltaH (10% PCC/serum) (supernatant/10% PCC/10% [mu]cal latex) [mu]cal latex) [mu]cal 0 -7807 115 -4424 0.1 -8384 -228 -3241 0.2 -8566 -213 -4091 0.275 -7953 111 -5098 0.3 -8114 84 -4156 0.5 -6621 421 -5928 0.75 -6576 575 -6310 % Dispersant Delta H Delta H (PCC Delta H (supernatant/serum) surface/serum) (supernatant/latex [mu]cal [mu]cal surface) [mu]cal 0 419 -262 -4802 0.1 21 -247 -3260 0.2 449 -617 -4494 0.275 341 -195 -5405 0.3 553 -582 -4654 0.5 570 -92 -6441 0.75 451 170 -6716 % Dispersant Delta H (PCC surface/latex surface) [mu]cal 0 -3118 0.1 -4895 0.2 -3859 0.275 -2659 0.3 -3376 0.5 -601 0.75 -436
[FIGURE 3 OMITTED]
The overall interaction between PCC 1 and S/B latex 3 is exothermic, ranging from -6576 to -8566 [micro]cal depending on the amount of dispersant added. The pigment surface/latex surface interaction is most exothermic with the PCC 1 containing 0.1% dispersant and decreases to-436 [micro]cal for PCC 1 with increased level of dispersant. Correspondingly, the pigment supernatant/latex surface interaction is at a minimum with the PCC 1 containing 0.1% dispersant and increases with PCC 1 containing higher amount of dispersant. In comparison, the interactions of latex serum with pigment surface or pigment supernatant is relatively small. The results can be interpreted as a result of competitive interactions between pigment surface and pigment supernatant with the latex surface. With increasing amount of added dispersant, the pigment supernatant interaction becomes increasingly exothermic at the expense of the interaction between the pigment surface and latex surface.
A similar trend of the effect of dispersant on the interactions between PCC 1 and S/B latex 3 was also observed using pigment that has been washed to remove adsorbed species. Table 3 shows the effect of washing the pigment on the interactions with S/B latex 3. A PCC surface with dispersant removed interacts to a greater extent with both the latex surface and latex serum.
Table 3: Interaction of PCC 1 with 10% latex: effect of washing the pigment Delta H Delta H Delta H Delta H (10% PCC/ (10% (supernatant/10% (supernatant/serum) 10% latex) PCC/serum) latex) [mu]cal [mu]cal [mu]cal [mu]cal Washed -8408 152 -4890 692 pigment PCC -7296 365 -5595 393 Delta H (PCC Delta H Delta H (PCC surface/serum) (supernatant/latex surface/latex [mu]cal surface) [mu]cal surface) [mu]cal Washed -471 -5513 -3047 pigment PCC 12 -5949 -1712
Effect of latex serum replacement on the interaction of PCC with S/B latex
Since adsorbed dispersant on pigment surface and dispersant in the pigment supernatant affects the interaction of pigment and latex, the effect of surfactant in the latex is also studied by comparing the interaction of latex with serum phase material removed to the latex prior to serum phase replacement. Albafil [R] M (PCC 2) and Albaglos [R] S (PCC 3) from Special Minerals Inc. were used. PCC 2 and PCC 3 are identical except PCC 2 is dispersant free. Results are shown in Table 4.
Table 4: Interaction of PCC with 10% latex: effect of latex serum replacement Delta H (PCC/10% Delta H Delta H latex) [mu]cal (PCC/serum) (supernatant/10% [mu]cal latex) [mu]cal 10% S/B latex 4 with 5% PCC 2, pH = 8.7 S/B latex 4 1640 -150 2098 pH = 8.5 S/B latex 4 49 -91 857 ASS pH = 8.5 10% S/B latex 4 with 5% PCC 3 pH = 10.0 S/B latex 4 -1532 -1011 -937 pH = 8.5 S/B latex 4 -1423 -141 -766 ASS pH = 8.5 Delta H Delta H (PCC Delta H (supernatant/serum) surface/serum) (supernatant/latex [mu]cal [mu]cal surface) [mu]cal 10% S/B latex 4 with 5% PCC 2, pH = 8.7 S/B latex 4 65 -212 2040 pH = 8.5 S/B latex 4 -80 -15 929 ASS pH = 8.5 10% S/B latex 4 with 5% PCC 3 pH = 10.0 S/B latex 4 -555 -484 -437 pH = 8.5 S/B latex 4 -73 -72 -700 ASS pH = 8.5 Delta H (PCC surface/latex surface) [mu]cal 10% S/B latex 4 with 5% PCC 2, pH = 8.7 S/B latex 4 -249 pH = 8.5 S/B latex 4 -793 ASS pH = 8.5 10% S/B latex 4 with 5% PCC 3 pH = 10.0 S/B latex 4 -84 pH = 8.5 S/B latex 4 -585 ASS pH = 8.5
The presence of surfactant and water-soluble material in the latex serum affects the interaction with the PCC pigment. Removing the serum phase material resulted in increased interaction of the latex surface with both the pigment surface and pigment supernatant. For example, for PCC 3, the pigment surface interaction increased form -84 to -585 [micro]cal when serum phase materials were removed. Concurrently, the pigment supernatant interaction also increased from -437 to -700 [micro]cal. As expected, the presence of serum phase material in the latex increased the interactions that involves the latex serum. The interaction between S/B latex 4 serum and the pigment surface was -484 [micro]cal and decreased to -72 [micro]cal when the serum phase materials were removed. The pigment supernatant/latex serum interaction also decreased from -555 to -73 [micro]cal. Similar trend was observed with PCC 2, with the exception of pigment supernatant/latex serum interaction where no trend was observed within the error of the measurements.
The interactions between the different components, especially in the aqueous continuous phase, can affect coating rheology. The effect on rheology is manifested in various ways, such as viscosity, viscoelastic behavior, and structure formation. It has been observed that the viscosity of some ground calcium carbonate (GCC) containing coating formulations increases when aged at elevated temperature. The role of the interaction of aqueous phase components on the heat aged viscosity was studied. The Brookfield viscosity of coatings containing 100 parts GCC, 16 parts latex at a solid of 67.5% and pH of 8.5 aged at 43[degrees]C was measured as a function of time, using a #4 spindle at 20 rpm. Results are listed in Table 5.
Table 5: Interaction between GCC and latex and coating viscosity S/B latex 4 S/B latex 5 [DELTA]H (5% GCC/20% latex), [mu]cal 893 1,653 [DELTA]H (5% GCC/Serum of 20% latex), -593 -366 [mu]cal [DELTA]H (Supernatent of 5% GCC/20% latex), -870 -2,267 [mu]cal [DELTA]H (Supernatent of 5% GCC/ serum of -377 -285 20% latex), [mu]cal [DELTA]H (surface of 5% GCC/surface of 20% 1,999 4,015 latex), [mu]cal [DELTA]H Aqueous phase interactions, -1,106 -2,363 [mu]cal Initial viscosity, cps 850 1,080 24 h viscosity, cps 1,220 >14,000 48 h viscosity, cps 1,400 >14,000
The viscosity of coating containing S/B latex 4 remained relatively constant upon heat aging, whereas the viscosity of the coating containing S/B latex 5 increased significantly. It was observed from the ITC results that the aqueous phase interactions in the coating containing S/B latex 5 was higher than the corresponding coating with S/B latex 4, -2363 vs -1106 [micro]cal. The interaction between the GCC supernatant and S/B latex 5, -2267 [micro]cal, is predominately responsible for the interaction observed. Therefore, in this case, the nature of the latex surface has significant impact on the continuous phase properties, such as viscosity.
Titanium dioxide is a very effective white opacifying pigment because the large difference in the refractive index between [TiO.sub.2] and the paint binder results in high scattering efficiency. It has been known that the scattering efficiency of pigments such as [TiO.sub.2] decreases with increasing PVC. (44), (45) This is explained by the "crowding" or dependent scattering. (46) Therefore, it has been proposed that fine particle size extenders can act as "spacers" between the [TiO.sub.2] particles and increase the scattering efficiency resulting in an increase in opacity and tint strength. The effectiveness of the tint strength improvement depends on the interaction of the polymeric pigment with the [TiO.sub.2] particles, as shown by ITC.
The effect of the incorporation of polymeric pigments * on the tint strength of an acrylate based 23 PVC semi-gloss paints was determined (3% per 100 gal paint). The formulation of the paint is shown in Table 6.
Table 6: Formulation of 23 PVC semi-gloss paint Ingredient Weight (gm) Water 72.08 Nuosept 95 1.94 Tamol 731 6.79 KTPP 0.10 Ti-Pure R-940 222.67 DrewPlus L-475 0.48 Grind at high speed Acrylate latex 281.90 Water 219.79 Propylene glycol 25.95 AMP-95 2.00 Triton CF10 4.00 Acrysol RM-8 34.91 Acrysol RM 1020 13.55 Texanol 7.91 DrewPlus L-475 1.00 Phthalo green 888-5511 5.00 Polymeric pigment 60.47 Water 33.36
The improvement in the tint strength, [DELTA]TS, is well correlated to the enthalpy of interaction between [TiO.sub.2] and the polymeric pigments (Fig. 4). The linear regression line of the data is % Delta tint strength = -0.23 Delta H -5.56, and [R.sup.2] is 0.90.
[FIGURE 4 OMITTED]
The efficiency of light scattering of certain wavelength depends on the mean particle size of the scattering center and is maximized when the particle size is half the wavelength of the incident light. For example, [TiO.sub.2] pigments are designed to have a mean particle size of 240 nm in order to achieve maximum light scattering in the center of the visible region of light, i.e., 550 nm. When [TiO.sub.2] particles are crowded or flocculated, the effective mean particle size increases and longer wavelength light would be scattered more efficiently. Balfour and Hird (47) have shown that when [TiO.sub.2] particles flocculate in a paint film, light scattering in the infrared region (wavelength = 2500 nm) increases. The better spacing of the [TiO.sub.2] particles in the presence of the interacting polymeric pigment is studied using Near Infrared Total Reflectance Spectroscopy (NIR). The total reflectance of light in the wavelength region from 2000 to 2600 nm from the paint films were determined using a Shimadzu UV 3101PC spectrometer with a total reflectance attachment. The spectrum was determined at medium scan speed with a slit width of 2 nm and sampling interval of 1 nm. Figure 5 shows the NIR of paint films comparing the effect of interacting and noninteracting pigment.
[FIGURE 5 OMITTED]
It can be seen that the noninteracting pigment has little effect on the % Reflectance when compared to the base paint, whereas the % Reflectance decreased in the paints containing interacting pigment indicating that [TiO.sub.2] was better dispersed in this paint.
Pigment surfactant interaction
Surfactants are frequently present in coating formulations. The adsorption and desorption of surfactants on latex and pigment surfaces can affect the colloidal stability and rheology of the formulation. The enthalpy of interactions between 5% pigment and 1% surfactant solutions was determined. Results are shown in Table 7. Surfactants used in this study are a nonylphenol ethoxylate based phosphate ester, Rhodafac [R] RE610 (Surfactant 1), a sodium dodecyl diphenyloxide disulfonate, Dowfax [TM] 2A1 (Surfactant 2), a nonylphenol ethoxylate, Dowfax [TM] 9N40 (Surfactant 3), and a sodium diisobutyl sulfosuccinate, Aerosol [R] IB45 (Surfactant 4). For [TiO.sub.2] pigment, surfactant 1 was found to interact more strongly than surfactant 2. The interactions of surfactants 2, 3, and 4 with Hydrafine clay are also exothermic. However, for surfactants 2 and 3, the interactions were primarily due to the interaction of the surfactant with the clay supernatant. The net heat of interaction was calculated to be negligible. The heat of interaction between surfactant 4 and kaolin clay was found to be much more exothermic than either surfactants 2 and 3,--6994 [micro]cal vs--162 and--1469 [micro]cal. A net interaction of--5498 [micro]cal between kaolin clay and surfactant 4 was calculated after accounting for the aqueous phase interactions. This suggests that surfactant 4 should adsorb more on kaolin clay surface than either surfactants 2 and 3.
Table 7: Interaction of 5% pigment with 1% surfactant Surfactant Delta H Delta H (5% Delta H (5% (5%Ti-Pure Hyrafine 90 Hydrocarb 90/1% R940/1% clay/1% surfactant) surfactant) surfactant) [mu]cal [mu]cal [mu]cal/ Dowfax 2A1 -49 -162 -146 Rhodafac RE 610 -1,086 n.d. n.d. Dowfax 9N40 n.d. -1,469 -140 Aerosol IB45 n.d. -6,994 -28,213 n.d., Not determined
The interactions of surfactants 2, 3, and 4 with Hydrocarb 90 calcium carbonate were also determined. It can be seen that the interactions of surfactants 2 and 3 with Hydrocarb 90 calcium carbonate are similar to Hydrafine 90 clay. The enthalpies of interaction are relatively small. However, the interaction between Hydrocarb 90 calcium carbonate and surfactant 4 is exothermic, -28,213 [micro]cal. The net interaction after accounting for the aqueous phase interaction is--16,744 [micro]cal. Therefore, surfactant 4 interacts more with Hydrocarb 90 calcium carbonate than with Hydrofine 90 clay.
The adsorptions of surfactants 2 and 4 on Hydrafine 90 clay and Hydrocarb 90 calcium carbonate have been determined. Various amounts, from 0.25 to 5 g, of a 1% surfactant solution were added to 50 g of 5% pigment dispersion. The dispersions were stirred for at least 8 h. The supernatants were then collected. The concentration of surfactant in the supernatants was then analyzed using liquid chromatography. Results are summarized in Table 8 and plotted in Fig. 6.
Table 8: Adsorption of surfactant on pigment Pigment Surfactant Amount of Amount of Amount of surfactant added surfactant in surfactant ([mu]g) supernatant adsorbed ([mu]g) ([mu]g) Kaolin Dowfax 2A1 2,730 2,866 - clay 5,120 5,281 - 10,090 10,187 - 20,070 22,278 - 50,170 54,600 - Aerosol 2,480 1,910 570 IB45 5,220 4,178 1,042 9,980 8,342 1,638 20,000 17,325 2,675 30,190 28,291 1,899 50,000 47,250 2,750 GCC Aerosol 2,480 907 1,003 IB45 5,000 2,160 2,840 9,960 7,323 2,637 30,100 27,781 2,320
[FIGURE 6 OMITTED]
It can be seen from the data that surfactant 2 did not adsorb onto Hydrafine 90 clay, as predicted from the enthalpy of interaction determined by microcalorimetry. Surfactant 4 was found to adsorb on both Hydrafine 90 clay and Hydrocarb 90 calcium carbonate. It was found that at low surfactant concentration surfactant 4 adsorbed on Hydrocarb 90 to a larger extent that on Hydrafine 90 Clay. The amount adsorbed reaches approximately the same value when an increasing amount of surfactant was added.
In coating formulations where both pigment and latex are present, the distribution of surfactant will also depend on the interaction between the surfactant and latex surface. Due to the complexity of latex surfactant interaction, this subject will not be discussed in the present paper. Recently, Winnik et al. (36) studied the interaction between nonionic surfactants and poly(butyl methacrylate) latex and found the interaction to be endothermic. They concluded that the endothermic interaction is indicative of entropic driven process. It is important to study the interaction between the surfactant and latex used in coating formulations to develop a complete picture of the complex interactions between pigment, latex, and surfactant.
Stain resistance of coatings is a performance attribute especially important for interior application. The staining and washability are related to the interaction of the stain with the coating. Mustard stain has been found to be difficult to prevent and remove. The mustard stain resistance of a 20 PVC coating based on Latex 6 was found to be better than the corresponding coating based on Latex 7. Delta E of the stained coatings were measured and shown as Fig. 7. The staining of the nonpigmented coatings is also shown. It is clear that coatings based on Latex 6 have better mustard stain resistance than coatings based on Latex 7. The heats of interaction of 0.1% turmeric with 10% Latexes 6 and 7 were determined to be 34 [micro]cal/injection and -97 [micro]cal/injection, respectively, for a 10 [micro]L injection. The endothermic interaction of turmeric with Latex 6 indicates the interaction is thermo-dynamically less favorable and therefore mustard should not stain Latex 6 coating significantly. Delta E of Latex 6 was found to be 1.4 and is not visually apparent. This is to be contrasted with the significant staining of Latex 7, with delta E determined to be 15.2. This is to be expected since turmeric interacts exothermically with Latex 7, indicating the interaction is thermodynamically more favorable. The 20 PVC coating based on Latex 6 stained more than Latex 6 alone indicating there are components in the pigment grind that also interact with turmeric and contribute toward the staining.
[FIGURE 7 OMITTED]
The complex interactions between components in coating formulation affect properties of the coating in both the wet and dry states. Understanding and controlling these interactions are important to the design of the coating for optimal performance. This paper demonstrates the application of ITC to study pigment latex interaction and the role of surfactant and dispersant on the interaction affecting the rheology of the coating, as well as end use performance, such as tint strength and stain resistance.
Acknowledgments The authors wish to thank Imerys and Specialty Minerals Inc. for pigments used in this study. Management support is appreciated.
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* Composition of the polymeric pigments are proprietary information of The Dow Chemical Company.
C. Kan ([??]), M. Keefe, K. Olesen, P. Saucier
Dow Coating Materials, The Dow Chemical Company, Midland, MI, USA
Charles Kan, Melinda Keefe, Keith Olesen, Peter Saucier
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|Author:||Kan, Charles; Keefe, Melinda; Olesen, Keith; Saucier, Peter|
|Date:||Jan 1, 2011|
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