Ind. Eng. Chem. Prod. Res. Dev. 1984, 2 3 , 569-572
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GENERAL ARTICLES Surface Sealing of Microvoid Latex Paints by Exclusion of Small Diameter Latex Particles Daniel P. Durbln,+Yohamed S. EI-Aasser,’ and John W. Vanderhoff Emulsion Pokmers Institute and Departments of Chemical Englneerlng and Chemistry, Lehlgh University, Bethlehem, Pennsylvania 180 15
Improved scrub resistance has been obtained in commercially formulated microvoid coatings utilizing a particle exclusion mechanism investigated by the authors in “model” latex films. It was possible to remove more than 50% of the TiO, pigment from a typical indoor flat white latex paint formulation without loss of hiding power and stlll retain adequate scrub resistance. Pigment removal from commercial paints is not only cost effective but reduces the shlpping weight by nearly 10% .
Introduction Pigments have been a continual source of problems for the paint industry. For example, they react undesirably with other paint constituents, fade, and chalk, and if the particle size is not closely controlled, the particles may settle or float. In spite of these and other technological problems, pigments continue to be the most widely used means of opacifying a paint film. The problems which have forced the paint industry into a search for new opacifying techniques have been economic in nature. The most significant of these problems are the rising cost of high quality pigments such as TiOz and the increased shipping cost of paints (which are sold by volume instead of weight) containing high density pigments dispersed within them. The most promising new opacifying method to be developed recently has been the microvoid coating concept. Microvoid coatings derive their opacity from the scattering of light by microscopic or submicroscopic air bubbles dispersed in the paint film replacing the traditional pigment particles. Following the rules of any opacification mechanism, the void size, void volume fraction, and the relative refractive index ratio of air to the binder matrix are the most important factors. A review of the world literature on incorporating microvoids into polymeric coatings has been presented by Seiner (1978). Two significant problems which have kept the microvoid coating concept from commerialization have been the nonuniformity of void size and dispersion as well as the substantial reduction in the physical properties of the coatings because of increased porosity. One method in which microvoids of a regular size can be uniformly dispersed in a model microvoid film is that described by El-Aasser et al. (1976). In this approach, monodisperse polystyrene latex spheres (high Tgpolymer) are coated with a soft sticky-film forming polymer by a second stage emulsion polymerization technique (Bradford et al., 1956). Shell Development Co., Houston, TX. 0196-432118411223-0569$01.50/0
The thickness of this layer is sufficient to cause coalescence of the latex spheres at the points of contact but insufficient to fill the interstices between the particles when packed in a rhombohedral arrangement during the drying process. The film-forming polymer coating on the latex particles acts as the binder in the system supplying film integrity, while the interstices provide the sites for light scattering. Incorporation of this concept into a formulated latex paint, using it to replace a portion of the binder while reducing pigment content, causes a substantial reduction in scrub resistance as well as increases in film porosity. Recent studies (Vanderhoff and Bradford, 1973; Durbin, 1980) have been performed on the exclusion of small diameter latex particles to the air/polymer film interface in model latexes comprised of mixed monodisperse polystyrene latexes. It has been found that the packing order of a single monodisperse polystyrene latex below its Tgis dependent upon the colloidal stability of the system throughout the drying process. Particles which are stable throughout the drying process pack rhombohedrally, but particles which undergo slow coagulation during drying pack in a less ordered fashion. This is presumably because doublets and triplets of particles cannot pack as ordered as single particles driven by capillary forces. By raising the anionic surfactant concentration of the latexes to levels near the cmc (critical micelle concentration), or adjusting the electrolyte concentration at low anionic surfactant levels, it was possible to achieve colloidal stability in the early drying stages. This is followed by instability (slow coagulation) late in the drying process, presumably because the concentration of surfactant is becoming so great that it is beginning to act as an electrolyte and compress the diffuse electrical double layer surrounding the individual latex particles and inducing instability. The result is a rhombohedrally packed array of particles at the vapor/polymer film interface with a gradual transition to a less ordered packing arrangement at the polymer film/solid interface. When mixed monodisperse latexes consisting of largediameter particles and small diameter particles are dried 0 1984 American Chemical Society
570
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 4, 1984
below the Tgof the polymer, it is found that the smalldiameter particles are excluded to the air/polymer film and polymer film/solid interfaces if their total volume cannot be accommodated by the void volume of the large-diameter particles packed in an array. The smalldiameter particles must be small enough to fit within and move through the interstices of the larger-diameter particle array. If the volumetric ratio of small to large particles is greater than what can be accommodated by the rhombohedral packing arrangement of the larger particles, yet small enough to be accommodated by a less ordered packing arrangement, the small particles will appear in profusion at the vapor/polymer film interface of the mixed monodisperse latexes. The void volume within the film gradually increases from the minimum value (corresponding to the rhombohedral packing arrangement) at the vapor/polymer film interface to the maximum at the solid substrate (less ordered packing arrangement) where all the particles may be accommodated in the voids. The purpose of this study was to test the particle exclusion mechanism developed with the mixed monodisperse polystyrene latexes as a method of sealing commercial microvoid coatings. In these coatings over 50% of the TiOz pigment was removed from a typical indoor flat white paint formulation and replaced with an equivalent solids content of hard core-soft shell latex as developed by El-Aasser et al. (1976). A small-diameter film forming latex was added in theoretically sufficient quantities to be excluded to the vapor/film interface and seal it by adjusting the surfactant concentration. A t the same time approximately equivalent quantities of vinyl-acrylic binder were removed resulting in a net reduction in the density of the paint.
Experimental Section To approach the optimum microvoid size for maximum opacity determined by El-Aasser et al. (1976),a microvoid latex (46.4% solids) was prepared using an 800-nm diameter monodisperse polystyrene seed latex and polymerizing a 67:33 vinyl acetateethyl acrylate copolymer shell (25:75, shel1:core) onto the core in a second-stage emulsion polymerization. During this process careful control of the surfactant concentration was necessary to ensure colloidal stability, yet avoid the appearance of a “second generation” of new small particles. Due to the low surfactant concentration needed to achieve this, the latex was post stabilized with 3.25% (wt based on aqueous phase) anionic emulsifier (Aerosol-MA, American Cyanamid Co.). This high surfactant concentration was desired to achieve the packing density variations in the final formulated paint films as observed while studying the model colloids. The small diameter latex was a 70-nm diameter 67:33 vinyl acetate:ethyl acrylate copolymer. These latexes were formulated into modified versions of a typical flat white indoor paint, as given in Table I. The full control contained the usual 22.76 kg/ 100 L of titanium dioxide pigment, and the half control contained 10.78 kg/ 100 L. The seven modifications that were tested are as follows: (1)the basic microvoid formulation is given in Table I. In this formulation more than half of the TiOl pigment has been removed and the UCAR 366 binder (37.138 kg/100 L) has been replaced with a mixture of UCAR 366 (9.824 kg/100 L) and microvoid latex (25.817 kg/100 L). This results in a net reduction of polymer binder on a solids basis with 11.98 kg/100 L of microvoid latex solids replacing 14.98 kg/100 L of vinyl acrylic solids. Water is withheld in the formulation to balance the additional water added with the lower solid content micro-
Table I. Modifications to a Typical Latex Paint Formulation wt, kg/100 L full half pigment pigment material control control microvoid water 40.229 40.229 35.197 titanium dioxide pigment 22.762 10.782 10.782 37.138 9.824 UCAR 366 vinyl acrylic latex” 37.138 (55% solids) microvoid latex (46.4% solids) 25.817 totals 100.129 88.149 81.620 Union Carbide Corporation.
void and small-diameter latexes added; (2) the basic microvoid formulation given above in (l), with sufficient UCAR 366 latex added to bring the total polymer solid content up from 11.98 kg/100 L to 14.98 kg/100 L; (3) the basic microvoid formulation plus sufficient 70-nm diameter particles to give a 2501 small/large particle number ratio; (4) the basic microvoid formulation plus sufficient 70-nm diameter particles to give a 500:l small/large particle number ratio; (5) the same as paint 4 plus 0.120 kg/100 L of tributoxyethyl phosphate (TBP) plasticizer (FMC Corp.); (6) the same as paint 4 plus 0.240 kg/100 L of tributoxyethyl phosphate plasticizer; (7) the same as paint 4 plus 0.360 kg/100 L of tributoxyethyl phosphate plasticizer. The Full Control is an example of a typical high quality flat paint with the usual quantity of titanium dioxide, and the Half Control shows the effect of decreasing the titanium dioxide concentration by 11.98 kg/100 L. Paint 1 shows the effect of substituting microvoid latex for the film-forming UCAR 366 latex at the lower level of titanium dioxide. Paint 2 shows the effect of increasing the amount of UCAR 366 latex to the same polymer solids level used in the control paints. Paints 3 and 4 show the effect of 250:l and 5001 particle small/large particle number ratios, respectively, at the same microvoid solids level as paint 2. Paints 5, 6, and 7 show the effect of increasing tributoxyethyl phosphate plasticizer concentration in paint 4. The aqueous phase of all the paints had a surface tension of approximately 29 mN/m. The scrub resistance of the paint films were measured by using a Gardner Washability Machine, Model WG-2000 (Gardner Laboratory, Inc., Bethesda, MD) and following ASTM-D2486 guidelines. For hiding power measurements ASTM-D2805 guidelines were followed by measuring the spectral reflectance of the samples every 10 nm wavelength using a Kollmorgen KCS-40 spectrophotometer (Kollmorgen Color Systems, Attleboro, MA) equipped with a DEC PDP-8L minicomputer (Digital Equipment Corp., Maynard, MA). Discussion Figures 1 and 2 are plots of the spectral reflectance curves for the various paints applied to black-and-white smooth surface opacity charts (Leneta Co., Ho-Ho-Kus, NJ) in 2 mil (5.1 X cm) wet film thicknesses. Figure 1 shows how the opacity of the control paint is severely hindered by removal of half of the titanium dioxide pigment during formulation. A t 560 nm, the percent reflectance for the half control paint decreased by 12% over the black substrate, and decreased by 2% over the white substrate. Figure 2 shows the effect of increasing the number of small particles in the microvoid containing paint system on the percent reflectance in the visible region. Paint 1,
Ind. Eng. Ct". Rod. Res. Dev.. VOI. 23. No. 4. 1984 571
Table 11. Physical Properties of Microvoid Paint Films paint Gardner scrub cycles hiding power, m2/L full control 357 9.12 half control 7.85 1 25 10.63 2 87 R~9R 3 129 9.51 3. 129 9.57 4 188 6.92 5 183 1.76 6 174 1.73 7 171 1.22 ~
'Dried at 50 O C ; all others dried at room temperature.
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14.98 kg/lOO L of UCAR 366 solids replaced with 11.98 kg/lOO L of microvoid latex solids, shows equivalent refleetance over black and white substrates as the Full Control. At 560 nm, the percent reflectance for paint 1 is within 1% of the full control over the black substrate.
Figure 3. Scanning electron micrographs of the surfaces of paints 3 (left) and 4 (right), dried at room temperature.
and 2.5% over the white substrate. Comparing paint 1 with paints 3 and 4 (250:l and 5Oo:l small particles added, respectively), the effect of adding an increasing number of small particles on the percent reflectance of the paint can be observed. Although the reflectance over the white substrate is not appreciably changed, substantial reductions are observed over the black substrate. Table I1 summarizes the results for the scrub resistance and hiding power tests performed on the various paint films. Although paint 1shows hiding properties superior to those of the full control, and this was achieved in spite of 50% TiOp pigment removal, the scrub resistance has decreased substantially due to the increased porosity of the system. By introducing the small-diameter (ca. 70 nm) film forming particles into the system (paint 3) and utilizing the exclusion mechanism developed with the "model" latexes, the scrub resistance may he increased from 25 to 129 cycles, while at the same time retaining a hiding power equivalent to the full control. The hiding power as well as the scrub resistance is lower than nearly an equivalent amount of UCAR-366 (ca. 300-nm diameter) is replaed in the microvoid formulation (paint 2), presumably because the UCAR-366 particles are not small enough to fit between the interstices of the dispersed particles in the system during the drying process, and the interstices are filled, destroying the light scattering sites. The scrub resistance is poor because the air/paint film interface is still porous. If the ratio of small to large diameter particles is increased to 5004 (paint 4) a dramatic decrease in hiding power is related because of the loss of microvoid scattering sites. The scrub resistance increased due to lower film porosity. Scanning electron microscopy (SEM) studies given in Figure 3 showed paint 4 to possess better sealing at the air/polymer film interface than paint 3. Paints 4 to I show the effect of increasing plasticizing concentration on microvoid paints with a 500:l ratio of small to large particles. The scrub resistance decreased only slightly with the addition of tributoxyethyl phosphate plasticizer. Although the experimentalerror using scrub tests of this type
Ind. EW. Chem. RM. Res. Dev. 1904, 23, 572-581
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Figure 4. Scanning electron micrographs of the surfaces of paints 4 (left) and 7 (right), dried at rmm temperature.
is generally sasumed to be gmater than the observed trend, SEM studies given in Figure 4 showed a slight reduction in surface sealing with increasing phtizicer concentration. This reduction may be attributed to either reductions in the tensile strength of the polymer binder due to increased plasticization or the premature coalescence of the small film forming particles in the channels needed for particles to reach the film surface. Since reductions in surface sealing were observed, premature coalescence of the particles is believed to play the major role in the decreased scrub resistance. Conclusions The present experimental study testing the exclusion mechanism investigated hy the authors in "model" latex films in formulated microvoid paints demonstrates that improved physical properties may be accomplished involing this mechanism. The efficiency of the exclusion of small particles, however, is limited by nonuniformly sized voids and channels reducing the packing efficiency of larger
particles as well as blocking passage of the smaller particles. Another factor limiting the exclusion mechanism is the premature coalescence of the small film-forming particles in the channels needed for particles to reach the film surface. The coalescence effect was further enhanced by the addition of external plasticizing agents that lower the temperature needed for film-forming particles (large and small) to coalesce in the channels at the beginning of the second falling rate drying period. This prohibits particles in the interior of the film from pushing to the air/paint film interface. Increasing the small particle/large particle number ratio provided increased scrub resistance at the expense of opacity by filling greater numbers of voids within the film. Optimization of the process would involve balancing the number of small particles in the system needed for adequate scrub resistance with opacity properties provided by the microvoids. Acknowledgment
We gratefully acknowledge the contributions of Dr. Daniel F. Herman and the support of NL Industries and The Emulsion Polymer Institute, Lehigh University. Registry No. Ti02, 1346867-7;polystyrene (homopolymer), W3-53-6;(vinylacehte).(ethyl acrylate)(coplymer),25190-97-0. Literature Cited Bradfad, E. 8.; VandertoH. J. W.; Alhey. T. J. C&WScI. 1956. 11. 135. owbin. 0.P. m.0. o b w a ~Lehgh . university. bthbk" PA. 1980. El-Aasser, M. S.: Iqbai. S.; VacdehH. J. W. " M I O M acd Inlerfece S c ! e n ~ " .VCi. V Academic Press: New Ywk. 1976: p 381. Seiner. J. A. I&. Eng. me" W .Res. &v. 1978'. 17.302. VandwhoH, J. W.; Bradtmd. E. B. P a p k 1973. 27. 52.
Receiued for reuiew March 14, 1984 Accepted July 26, 1984
Improving Adhesion between a Segmented Poly(ether-urethane) and a Fluorocarbon Copolymer Coatingt D. Mark Hofhnan; Comb M. Walkup. and Ing L. Chlu l
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NawOnal Laboratory. UvSnroe. CaMomia 84550
A moisture barrier coating of KeCF 800, developed at U N L to reduce uranium corrosion. had to be bonded to a porous ceramic. The adhesive could not bond too slrongiy or react with the coating and jeopardize its barrier properties. We studied methods of improving adhesion to the KeCF coating. Silane and titanate coupling agents
and a fluwocarbon surfactant were somewhat effective at increasing adhesion dapending on the application procedure. X-ray photoelectron spectroscopy (XPS) was used to demonstrate the presence of fluorosurfactant at tha fracture intMace. Postcuring at elevated temperatures (85 OC) also significantly improved adhesive strength to the fluorocarbon coating. This was anributed to thermal acceleration of interfacial diffusion of the urethane adhesive into the fluoropolymer surface.
Introduction Recently the polymer group at Lawrence Livermore National Laboratory (LLNL) developed a fluoropolymer barrier coating besed on the copolymer Kel-F 800 Walkup, 1983) produced by 3M Corporation, to reduce uranium 'Work performed under the auapicea of the US.Department of Energy by Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48.
corrosion due to moistwe. Epoxy (Hammon and Althouse, 1976) and poly(urethane) (Hammon et al., 1980. Hoffman, 1981,1983) adhesives, developed at LLNL. were used to bond Kel-F coated uranium parts. Two engineering requirements were placed on the adhesive/coating interface: (1) the fluoropolymer's moisture barrier properties must be retained to protect the uranium and (2) failure must not m r at the Kel-F/uranium interface since this defeats the purpose of the barrier coating. Further requirements of the adhesive were that the ultimate bond strength be
