404
Ind. Eng. Chem. Prod. Res. Dev. 1985, 2 4 , 404-412
Mechanism of Core/Shell Emulsion Polymerization Deborah R. Stutman,+ Andrew Klein,' Mohamed S. El-Aasser, and John W. Vanderhoff Emulsion Polymers Institute and Departments of Chemical Engineering and Chemistry, Lehigh University, Bethlehem, Pennsylvania 180 75
The influence of 12 process variables on the ultimate particle morphology of poly(buty1 acryiate)/poiy(styrene) "polymerizatknwas studled. A Plackett-Burman experimental design was employed [(PBA)/(PS)]core/she#e for the initial screening for variable main effects, and propertles of later polymerization recipes were analyzed by using linear regression techniques. It is proposed that aqueous-phase polymerization leads to secondary PS particle formation and that the predominant potymerkation mechanism is a balance between capture of the newly nucleated particles and polymerization in a monomer-rich PBA seed particle surface layer.
Introduction Modern technology has developed polymers that were designed to perform in a wide range of functions, from adhesives and protective and/or decorative coatings to structural materials. It is known that polymer properties can be tailored by homopolymer blending, plasticization, or copolymerization. For thermoplastic materials the most desirable attributes are high impact strength and resistance to failure under stress/fatigue. Core/shell polymer morphology, the rubber core and glassy shell type, has been known to yield improved impact strength. In the past two decades numerous patents have been claimed on the polymerization of particles with a "core" composition different from that of the "shell". Representative patents are those by Ferry et al. (1976) and Purvis and Grant (1976). Usually a semicontinuous process is used. Various other process strategies in buffering, water surfactant content, and monomer feed rate are also used to produce the specific latex. In the recent literature Matsumoto et al. (1976),Yamazaki (1977))and Stabenow and Haaf (1973) described several morphologies of core/ shell polymers. Some of the differences in morphologies could be inferred to be due to process change. The goal of this research was to determine which process parameters influence the morphological features of the seeded polymerization and from this information propose a mechanism of the graft/coating process. The poly(buty1 acrylate) seed/poly(styrene) shell system was chosen for analysis because of the known anomalous behavior of this system. Twelve operator-controllable parameters were chosen for investigation, and a Plackett-Burman experimental design (unsaturated factorial) was used for early screening of major variable effects. Several of these variables were then investigated in more depth, and a polymerization mechanism was proposed. Latex particle morphology was primarily characterized by using TEM, SEM, TLC, IR, soap titration, and solvent extraction. Materials and Methods All solvents used for the study were reagent or certified grade, used as received. Two stocks of sodium lauryl sulfate (SLS) were used: (1)Maprofix 563 from Onyx Co., primary and secondary isomeric mixture, commercial grade, without purification; (2) Stepanol WA-100 from Stepan Chemical Co., primary isomer only, purified by recrystallization from boiling 100% ethanol followed by Present address: Owens-CorningFiberglass Corp., Technical Center, Granville, OH 43023.
soxhlet extraction from diethyl ether for 48 h. The purified SLS was stored in a desiccator. nButyl acrylate (BA) was from Rohm and Haas Co. and Badisch (BASF) Corp. Styrene was from Fisher Scientific Co. Inhibitors were removed by alkaline washing according to Collins et al. (1973). The monomers were then distilled under vaacuum. BA was distilled at 30-35 torr, 56-61 "C. Styrene was distilled at 20-25 torr, 48-53 "C. The receiving flasks of all distillations were kept in an ice bath. Dowex ion-exchange resin was purified according to Vanderhoff (1977). Cleaning of dilute latexes was also done according to methods described therein. Soap titration with purified SLS was done as described by Stutman (1984). Weight percent solids were found by placing 3-7 g of latex in an aluminum dish and drying to constant weight at 105 "C. Dried Latex Films. Drawn filmswere made from some of the design series latexes and a 1/1 (PS/PBA) copolymer for comparison. The films were air-dried for 4 h and then placed in an oven at 55-60 "C for 14 h. Scanning electron micrographs (SEM) of the films were taken and examined. Samples of air-dried films of the design series latexes were dissolved in THF filtered with a nucleopore syringe filter, and the molecular weight distributions were found by gel permeation chromatography. A Waters Associates Model 126 was used. Some of the films dissolved easily while some contained considerable amounts of gel even after 3 weeks of slow agitation. Analysis for Percent Graft. Thinchron Sm TLC rods were spotted with 2 MLof a 2 g/L polymer solution and then developed by using a CC4:EtAc 6011.5 binary solvent mixture. This solvent mixture will cause migration of PS and PS-g-PBA copolymer as was shown by Min et al. (1983). Four developments are required with the solvent mixture to ensure complete migration. The solvent front is allowed to advance to 5.0-5.5 cm, at which time the rods are removed from the solvent and dried for 4 min at 60 "C. The rods are cooled to room temperature between developments. Four further developments to 10 cm by toluene, to cause migration of only homo-PS, followed. The Iatroscan TH-10 TLC-FID was used to scan the rods. The samples prepared for use with the Iatroscan-FID were analyzed by using a Perkin-Elmer IR spectrometer. Solution was spotted (110-180 pL) on NaCl cells and scanned fro 3500 to 400 cm for 24 min. Using PS and PBA calibration solutions, we calculated the copolymer compositions of the soluble fractions. Transmission Electron Microscopy. A new technique using phosphotungstic acid (PTA) for observation of soft latex particles by transmission electron microscopy @ 1985 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 3, 1985
(TEM) was employed. The acid serves as a support and a negative stain for the soft particles. The procedure is described by Shaffer (1983). A later technique involves dilution of the latex with double-distilled deionized water (DDI). Plugs placed over the cooling-chamber openings permit a colder sample tip. The procedure may be used for final samples, but for the PBA seed and aliquots the PTA was still necessary. General Seed Polymerization Procedure. The master batches of seed latex were polymerized by using a recipe adapted from Dickie (1973): 110-130 g of BA (for D, = 110 nm), 280 mL of DDI, 0.8 g of K2S208,and 0.35 g of purified primary isomer SLS. This recipe and procedure resulted in a monodisperse particle latex, with a particle diameter close to the 100 nm desired, and the only ingredients were water, SLS, monomer, and K,S2Os. Once made, the shell-stage nonionic character of the surfactant and the overall ionic strength could be easily controlled. Batches MS-1, MS-7, and MS-8 were prepared with unpurified SLS (Maprofix 563). The latexes MS-9 through MS-16 were prepared with purified SLS (Stepanol WA100). Because of the higher effectiveness per gram of purified primary isomer vs. unpurified isomeric mixture, as shown by Stutman (1984),the amount of surfactant in these recipes was decreased. The DDI, minus 35-40 mL, the SLS, and 20% of the monomer were placed in a resin kettle. The reactor mixture was brought to 70 “C in a thermostated bath and agitated at 18&190 rpm for 15 min. The initiator was mixed in 15 mL of DDI and added to the emulsion. After 45 min the remaining monomer was added at a rate of 0.25% per min. To keep the weight percent solids of the latex low and to limit coagulation, 10 mL of DDI was added to the emulsion at about t = 255 and 360 min. After all monomer was administered, the reaction was continued for an additional 45 min. The seed recipes were unbuffered, and the final pH was 3 or below. Before use as a seed, the latex was steam stripped. Copolymer latexes used for the A- (the molecular adsorption area) determination study were prepared in the same semicontinuous manner, using a premixed monomer mixture. General Procedure for Shell-Stage Polymerizations. Half of the DDI, minus 37-42 mL, was placed in a 500-mL resin kettle. The phosphate buffer, a mixture of 0.01 M KzHPOl and 0.04 M NaH2P04,kept in a stock solution, any additional surfactant required at the start of this stage, and any sodium hydroxide were added to the water. The volume of seed latex was measured in a graduated cylinder and placed in the resin kettle. The rest of the DDI and methanol, if used, was used to rinse the graduated cylinder and added to the kettle. The last ingredient added was the K2S04solution, as this could cause coagulation of the latex if added earlier. A glass stirrer with a Teflon blade, or an equivalently sized stainless steel stirrer, was put in place and the kettle lid secured. The latex mixture was stirred at 180-190 rpm in a thermostated water bath at the desired temperature, 60, 65, or 70 “C depending upon the experiment, for about 10-20 min, while the rest of the equipment and ingredients were readied. As for the seed recipes, nitrogen blanketed the system. The required “slow addition” soap was emulsified by using ultrasonification and added in 12 mL of DDI. A small addition funnel was fixed in place, the surfactant emulsion pipetted into it, and then sealed with a glass or Teflon stopper. During this period nitrogen was bubbled through styrene monomer. A constant-rate syringe pump was set in place and a Teflon 3/16 in. tube fixed into the kettle lid with an
405
adapter. Forty to fifty milliliters of styrene was drawn into a 50-mL syringe, ensuring the tip was constantly immersed in monomer. The end of the Teflon tube was immediately placed over the needle fixture of the syringe, the syringe secured on the pump, and the monomer pumped until just above the exit to the kettle. The initiator was mixed with 15-20 mL of DDI and added to the seed latex mixture. The monomer feed was begun at the correct rate, the “slow addition” surfactant feed started at 0.2 mL/min, the stirring rate was adjusted to the correct level. The monomer feed was halted by shutting off the pump and pulling back on the syringe plunger to draw the monomer remaining in the Teflon tube back into the syringe. The polymerization was continued 30-40 min longer, when the reactor was removed from the bath and about 1.5 mL of a 3 70solution of hydroquinone (HQ) in isopropyl alcohol was added to the kettle and agitated to mix. Aliquots were withdrawn with a glass tube and the nitrogen flow increased during sampling. All samples were short-stopped by adding an alcohol solution of HQ, with the exception of samples for surface tension measurement. These samples were short-stopped by using HQ crystals. Aliquots for TEM purposes were usually 2-3 mL. For surface tension monitor samples, 40-mL samples were withdrawn and weighed as described above. Recipe Schemes. A special arrangement of a saturated fractional factorial design, a Plackett-Burman design discussed by Isaacson (1970), has been set up for the purpose of screening out only the main effects on a particular product property caused by a specific variable. The design matrix used for this research was for 16 observations, 12 actual factors, and 3 dummies; each factor is at the high level in 8 experiments and at the low level in 8 experiments. The actual design matrix is shown in Table IV, and the associated variable levels with run numbers are shown in Table VI. The design matrix can be rearranged into a standard fractional factorial, with the precise blocking as listed in the NBS Applied Mathematics Series, for a ‘/I6 replicate of a 212 design. This is shown in Table V. The following 12 independent variables were chosen for the study: (1) (a) pH; (2) (b) initiator concentration; (3) (c) surfactant concentration; (4) (d) water solubility of styrene; (5) (e) temperature; (6) (f) weight percent solids; (7) (8) agitation, rpm; (8) (h) mode of addition of additional surfactant; (9) (j) monomer addition rate; (10) (k) ionic vs. nonionic surfactant character; (11)(1) ionic strength; (12) (m) seed size. When required by the design, the aqueous-phase solubility of styrene monomer was increased by using a mixture of 10/90 methanol/DDI. The nonionic character of the surfactant was adjusted by adding amounts of Triton N-101 (nonphenylpoly(ethoxy)ethanol, 10-11ethoxy units) such that it comprised 64-70 mol % of the final total surfactant at the high level. The slow surfactant addition comprised 0 or 35-40 mol % of the final total surfactant in the system. The ionic strength was raised to the desired levels by addition of 0.144 M K2S04solution. Calculation of ionic species contributions from the seed latex was based upon the weight percent solids and the weight percent salts/polymer in the seed recipe, minus coagulum recovered and allowing for monomer and water removed by steam-stripping. Three series of (PBA/PS) core/shell latexes were prepared by using PBA seed from master batches MS-13 (IS-X) and MS-15 (ICS-X, KFS-X). For the first, IS-X, values of the 12 screened process variables were chosen that were either average or favored core/shell morphology:
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I d . Eng. Chem. Rod. Res. Dev.. Vol. 24, No. 3. 1985
Table 1. PBA/F'S Latex Particle Characteristics: ICs-X and KFS-X Study area/mL, sample ionic str, g-equiv/L cmzX lo4 wn 4,nm M R I.I.
ICS-1A ICS-3A ICS-1D ICS4B ICS-3B KFS-1C KFS-2 KFS-3 MS-15 ST-ICSdB
0.040 0.033 0.017 0.078 0.077 0.033 0.033 0.033
3.987 6.799 3.979 5.612 6.894 4.m 5.613 6.894
210 210 210 210 210 300 300 300
0.077
6.9
210
D., nm
I.l. f. i
1n7
127 128 122 127 122 120 128 132 116
120 125 119 123 119 116 125 128 107
see gen D, nm 45 16-20 39 23-48 35-105
acorns 4,nm 109 95 83 96 96 92 90 93 (?)
Figure 1. TEM micrograph of MS-16. PBA need latex.
X I Figure 3. TEM micrographs of core/shell latexes of ionic strength study.
I
Figure 2. TEM micrographs of eore/shell latexea of design aeries.
pH was buffered near 5, the nonionic character of the final surfactant content set at 50 mol %, a seed diameter chosen at 110-115 nm, etc. About 70 mol % of the total surfactant concentration was dissolved in about 6% of the water and added slowly over the first 60 min of polymerization. The final total surfactant molar concentration based on water phase was kept constant throughout the recipes. The monomer feed rate was a constant 0.15 mL/min for every recipe, regardless of solids content. The ratio of final
polymer mass:initial polymer mass was kept constant at 21. The basic recipe used previously was repeated in the ICs-X and KFS-X series to study the effect of ionic strength, initiator level, and stirring speed. For the IS-X series latexes, only the ionic strength and solids content were varied; for latexes of these series, the initiator level was reduced to one-fourth of the previous level, to about based on aqueous phase. Total sur0.00195 M K2S208. factant concentration, based on aqueous phase, was 0.008 M, 50 mol 70of each surfactant type. The pertinent recipe conditions and particle characteristics for the ICSX and KFS-Xseries are given in Table I. Figure 1is a micrograph of MS-16, a typical PBA seed latex, showing good moncdispersity and clarity. Figure 2 shows four micrographs of latexes in the design series. Both E and F were on different parts of the same grid of DS-2A, showing evidence of secondary particles. K shows the appearance of the unusual lumps, called Yacornsn,for sample DS-5B. and SS is sample DS-16B, a latex that appeared to be evenly coated but film-formingat room temperature. Figure 3 shows micrographs of two latexes of the IS-X series, IS-1B (b) and IS-5 (a, c). IS-5 was evenly mated by the PS with very few secondary particles, but IS-lB, while appearing to be evenly coated, did not result in the predicted growth on the seed; i.e., much PS ended up elsewhere. TEM micrographs of the aliquots
Id. Eng. aUrm. Rod. Rm. M.. Vol. 24. No. 3. 1985 407
.-
Figure 4. TEM micrograph of core/sheU latex ICs-ID, no F T A
d. revealed a large number of tiny PS particles of a size below 30 nm, some as low as 10 nm. Latex DS-2B also had a strong acorn feature that was not as severe in the final sample. Figure 4 is a micrograph of I C S l d taken by using no PTA. I t can easily be seen that there is an acorn present, large enough to have limited shell thickness to 5-8 nm instead of the predicted 12-16 nm. Figure 5 shows micrographs of aliquots from the surface tension monitor experiment. Surface Tension Monitor. To fmd the location of the surfactant during the polymerications and to ascertain whether mixed micelles were a likely locus of the "aqueous-phase" polymerization, a scaled-up recipe of sample ICS2B was prepared, labeled ST-ICS2B. Mquots were tested for surface tension by using a bubble tensiometer and dried for weight percent solids, and the particles were examined by using TEM. A jacketed cell was used to keep the sample temperature near the reaction temperature. TEM micrographs of aliquots 4 (at t = 52 min) and 9 (at t = 233 min) are shown in Figure 5. Results and Discussion The amounts of recoverable coegulum varied among the recipes, as did the amount of seed particle growth. For some latexes with very little size growth, such as DS-llC, an increase in weight percent solids indicates that the majority of the styrene monomer polymerized elsewhere rather than on/in the seed particle. Some latexes had particles that were homogeneous and reached the size predicted from monomer feed, solids content, and seed size data. For some latexes, such as DS-2A, both grown seed and the second generation PS particles could be seen. Other latexes showed peculiar seams fded with either FTA or PS. The acorns are similar to the 'warta" obasrved by Stabenow and Haaf (1973). The particles appeared to be p(BA), a bit larger than the seed, with dark circular centers. These centers are sometimes eccentrically located or a t the edge. A t the edge they can be seen as hemispherical protrusions, and the particle looks like an acom. In areas of the grid without the PTA the latex particles collapsed, with most of the particle now indistinguishable from the background and unevenly shaped remnants remainii. These remnants are often of a size similar to that of the acorns seen elsewhere for that sample. Second generation particles are usually polydisperse. Acorns, when present, are more uniform and are often observed on almost all particles. It is proposed that, for some latexes, the addition of PS to the surface is initially localized on the surface into regions; later an even, overall shell covers the seed and acorn, preserving the acorn structure. By the end of the polymerization, most of the
Figure 5. TEM micrograph of ST-ICSdB aliquots.
small particles had been swept up by the larger particles. Table I1 lists those variables that significantly affect several features of intereat in the core/shell latexes. Only some of the factors of the initial 12 can be deemed crucial to the formation of "true" core/shell particles. The amount of recovered styrene, the first category in the table, refers to styrene actually polymerized, calculated from weight percent solids and coegulum collected. The pH, monomer feed rate, and stirring speed did not seem to he important. Rather, the parameters usually thought to influence the existence of micelles, such as seed size, weight percent solids, surfactant, and initiator concentrations, were important. The second category represents particle capture behavior. The characteristics of design series latexes were coded to analysis by using the statistical package SPSS, Statistical Package for Social Sciences. Factors were set a t levels 1 and 2, corresponding to the conditions present during polymerization in the statistical design matrix. A series of linear regressions were run to weed out insignificant factom and investigate certain In (x) factors and interaction terms. The coefficients obtained from the analysis are not quantitatively accurate due to the qualitative nature of some of the factors. The following equation relates the parameter (Ni/N,), which is the ratio of seed particle number Ni to the final particle number Nf, to the process variables, along with the F value and the confidence level for the equation. The sign of each coefficient appears below it. In (Ni/Nf)= bo
bdb
+
X C)
+ b,a + b,& + + b,z(m)z -
X
b) +
+ bn(f X g) + b d b / n + b d d X 13 +
+
+
+
b,(f x m)
+
F = 14.02, sign. = 99.99% S i m i i equations were found correlating such features as evidence of acoms, growth of seed particle during the
408
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 3, 1985
Table 11. Variables Affecting Several "Des pH init conc effect 1 2 recovery coagulum conversions see gen -acorns size = pred. __ swell? film: dissolve (THF) ++ smooth cloudy M,, higher M high tail M , low tail -SEM of film: cracks, holes lumps, individ. particles ~~
of MorDholoau" wt % solids stirring speed 6 7
monomer rate
ionic strength
seed size
8
9
11
12
~
-
+ + +
+
++
+
soap mode
++
-
__
++ ++
++ ++
+ + ++
+
means that variable increased the amount of feature significant a t the 80% level. 90% or above level. For soap mode, means slow addition augments the property.
+
shell stage, etc., with the process variables and various interaction effects. The equations were all of high statisical significance. The parameter Ni/Nfrepresents the balance between the rates of new particle formation, radical capture by the particle, and particle-particle agglomeration. The rate of the particle capture process seems to be an exponential function, as the In (Ni/Nf)term is more desired (higher F) than a linear relationship. The (m)2area term agrees with the supposition of a capture mechanism. The equation also contains an (f X m) term. Hansen and Ugelstad (1979) find the term (N X r ) important to Nf/Ni, and the term ( f / m ) would directly correspond to it. Perhaps inverting the Ni/Nf factor would correspondingly alter ( f / m )to (f X m). The term ( b X c ) may be due to the ultimate surface charge density and the "surface active" character of water-phase PS formation. An interaction between pH and initiator concentration may be due to (1) extension of surface groups and hydrodynamic size and (2) acid-catalyzed reaction of initiator. The term ( b / f )could be interpreted as a nominal radical flux to the surface. The removal of interaction terms and replacement with powers of the main variables failed to produce an acceptable regression equation. The common appearance of the factor (f X g) in several equations points directly toward a coagulation/capture mechanism. The effect of increased ionic strength seems to be important to the destabilization and capture of PS particles and was found to be directly involved with an increase of the acorn nature. An increase in particle size (1)enlarged and permitted more acorn formation, (2) thus caused less even growth (size), (3) permitted more water-phase surfactant due to a lower surface area at constant percent solids, and (4) therefore increased stability against flocculation ( % coagulum). Similar arguments can be used to support the claim that the major polymerization mechanism, at least during the first part of the reaction, takes place in the water phase. Consequences of this theory could explain the coefficients and other terms in the regression equations. It has been assumed that if one begins with a seed latex with insufficient emulsifier to form micelles, after the addition of monomer and initiator the batch system will behave as if one has passed the nucleation step and is proceeding to polymerize in or on the seed particles. Semicontinuous monomer addition for the same seeded system should also yield the same result: no new particles.
--
+ ++
_-
__
++ ++ ++
-
__
_-
-
__ __
__
++ means the property is raised, significant at the
Many systems do not behave in this manner. Seeded polymerizations, both batch and semicontinuous, often result in a significant crop of new secondary particles, even with moderately low amounts of surfactant. This was shown for polymerization of styrene onto p(S) by Tseng et al. (1980),for styrene-acrylonitrile onto poly(butadiene) by Hagiopol et al. (1981), for styrene onto poly(ethy1 acrylate) by Matsumoto (1974), and for p(BA) by Yamazaki et al. (1976); for various (meth)acrylates onto p(S) Chainey et al. (1982) have demonstrated this behavior. In all cases the amount of emulsifier was insufficient to form micelles. Polymerization with extremely low amounts of emulsifier, including emulsifier-free, has been performed with monomers having a wide variety of water solubility by Snupmek (1979,1980),Fitch et al. (1971),and Hansen and Ugelstad (1979). The polymerization rate depends both on the specific propagation rate constant and also on the water solubility. In such cases it has been proposed that water-soluble oligomeric radicals grow and precipitate from solution after reaching a critical size. These new particles then serve as sites to adsorb surfactant, imbibe unreacted monomer, or capture other newly formed particles. They may also be captured themselves by previously existing seed particles. Smith-Ewart-type model kinetics for an emulsion polymerization assume low solubility of monomer in water and a negligible amount of aqueous-phase polymerization. Gershberg (1965) studied vario& monomers and cataloged literature data for several others regarding the dependence of polymerization rate (thus perhaps particle formation) on surfactant concentration for a batch system. Gershberg (1965) finds significant deviation of behavior from the standard model when the monomer solubility in the aqueous phase exceeded 0.04 wt % . Gershberg (1965) presumed that a possible explanation for the deviation is significant water-phase polymerization and/or increased radical desorption from the particles. The possibility of aqueous-phase polymerization of styrene in this system is strong. Three aqueous solutions of various surfactant concentrations were prepared with approximately the same buffer strength as the ionic strength study latex series reaction serum (IS-X). Room temperature estimates of the solubility of styrene in the solutions used in this work were made by using turbidity measurements. They indicated an increase of up to 250% over that in pure water. Fitch and Tsai (1971) developed an equation describing the rate of capture of homogeneously nucleated particles
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 3, 1985 409 INITIAL SURFACE AREA (cm*/ml latex, x
possibility that the balance between polymerization in/on the seed particle and in the water phase may change. Changing the solubility of PS oligomers in the water phase may affect the oligomer molecular weight at which critical solubility is reached, as may the balance of nonionic surfactant. The monomer feed rate may influence the partitioning between the water phase and diffusion into the seed particles. In studies of an emulsifier-free PBA/PS core/shell system, Min (1983) discovered a relationship between the amount of grafted copolymer and the degree of phase separation. Polymer particles of 6% graft polymer exhibit a large amount of phase separation while particles with about 12% graft do not separate as much, and particles with more than 30% graft show very little phase separation. Gasperowicz et al. (1982) studied the effect of various process variables on the amount of graft polymer formed in a S/PBA batch system initiated with K2S208.He finds that a small range of initiator concentration will maximize graft polymer, with little or no grafting occurring outside this range. The recipes in the DS-X and IS-X series called for an initiator concentration far too high on the cited scale to expect much grafting of styrene onto the seed PBA material. The recipes for the ICS-X and KFS-X series were prepared with one-fourth the previous initiator concentration to investigate the grafting reaction. The acorn particle morphology still occurred. As expected, due to the greatly reduced ionic strength and the same initial surface area range, very little coagulation of primary particles was found. Most of the latexes showed evidence of secondary PS particle generation at some time during the polymerization. Less initiator should raise the molecular weight of polymer formed on/in the seed particles and in the water phase and should decrease early termination in the water phase, encouraging formation of new, higher molecular weight PS. Surface Tension Monitor. The surface tension of the latex decreased during the first hour of the run as expected, due to the feed of additional surfactant. The surface tension began to rise, reached a maximum, and then decreased again. After the monomer feed was halted, the surface tension continued to decrease for 15 min and then leveled off. The TEM pictures indicated the formation of secondary particles, mostly after sample number 2, and these particles are swept up by the larger seed particles before the end of the reaction. Tiny, solid-looking, round particles were assumed to be PS particles. An identical “serum” sample was prepared and tested for surface tension behavior. The same buffer strength, emulsifier mixture and concentrations, and pH levels were used. A concentrated SLS solution was added until the surface tension reached the initial conditions, and then a solution of “added surfactant” mixture was added until the lowest surfactant tension measured during the run was reached. The initial aqueous concentration corresponds to about 0.21 mM SLS in the aqueous phase. The seed latex is known to contain about 2.4 mM SLS (after dilution to the initial conditions); this test indicates that only 8-970 of the surfactant is present in the aqueous phase. The lowest surface tension experienced during the polymerization would correspond to approximately 0.078 mM Triton N-101 and 0.24 mM SLS. This implies that only 2% of the nonionic surfactant and 6% of the anionic surfactant remain in the aqueous phase. The rate of conversion calculated for latex ST-ICS-2B revealed a very low interval conversion for the first 20 min of the reaction. Thereafter,
) 1.5
1
0.0
0. 0
/
I
2. 0
I
I
4.0
I
I
I
I
6. 0
8. 0
Figure 6. Particle number ratio vs. initial seed surface area.
by particles already present in the system. A plot of the ratio of the number of initial particles, Ni, to the number of final particles, Nf, should give an idea of the relative rates of particle nucleation and capture by primary seed particles. A ratio less than 1.0 indicates formation of secondary particles. A ratio of greater than 1.0 implies flocculation of seed material. Neither case excludes the other phenomenon from occurring to a lesser degree. A plot of Ni/Nf vs. a total initial surface area parameter, 4 7 ~ N ~ rfor - ~ ,the design series latexes was consistent with the nucleation/capture mechanism, indicating a homogeneous nucleation model for styrene polymerization in the present system. Several recipes produced a ratio of unity for Ni/Nf, indicating likely polymerization in or on the seed particles. Experiments DS-SA, DS-12A, DS-16A, and DS-4A indicated that some combination of low surfactant concentration in the initial stages of shell formation was necessary, and a low level of aqueous-phase styrene initiation was important. This would imply a low feed rate, lower aqueous solubility of styrene, and/or a low level of initiator concentration (favoring (1)diffusion to seed particles and (2) formation of higher molecular weight oligomers, hence less capacity to stabilize the system). Figure 6 is a plot of Ni/Nfvs. initial surface area constructed by using the seven core/shell latexes of the IS-X series. As can be seen in Figure 6, the core shell latexes run at higher ionic strength fall on a curve consistent with less second generation of PS particles and/or more coagulation of particles. The general trends indicate that an increase in available seed surface area causes an increase of Ni/Nf, i.e., fewer remaining new PS particles and/or increased agglomeration of the primary seed particles. At all times the aqueous surfactant concentration was well below the cmc for both surfactants. Mixed micelles are unlikely due to the high surface tensions occurring (45-56 dyn/cm) during the polymerization reaction. An increase in solids content serves two direct purposes: the increase in particle surface area absorbs more soap and thus less is available in the water phase to stabilize new particles and also to furnish more surface area for particle capture. If electrostatic stabilization is important, the increase in ionic strength would decrease the effectiveness of this stabilization and allow for more capture by the seeds. If the rate of nucleation of these particles is changed, such as by initiator concentration, there is a good
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 3, 1985
Table 111. Percent Graft Efficiencies, SS-XSeries graft % of graft % of latex surfactant soluble PBA soluble PS SS-1 all SLS 25.1 11.5 SS-2 nonionic 8.2 11.0 Triton N-101 SS-3 nonionic 40-45 16-17 Igepal CO-730
the incremental conversions were close to 100%. A monodisperse seed latex of P(BA), MS-16, was prepared similar to MS-1 through MS-15. Three core/shell recipes using the MS-16 seed latex were prepared to study the effect of the shell-stage surfactant on the final particle morphology. The recipes for these latexes, SS-1, SS-2,and SS-3, are similar to the other core/shell latex schemes in this work. The various surfactant amounts were adjusted to preserve similar effectiveness concentrations (based upon cmc values): about 0.0077 M added for nonionics and about 0.0095 M for SLS, surfactant (based on total final DDI). Igepal CO-730(nonylphenylpoly(ethoxy)ethanol, 20 ethoxy units) was chosen to investigate the effect of the surfactant cloud point. The latex SS-22 was made by using the seed latex MS-16 that had been cleaned with ion-exchange resin described earlier and reconcentrated. Surfactant and “initiator fragments” (approximated by K8O4) were added to replace all species expected to have been removed by the ion-exchange procedure. Final samples and aliquots were examined by using TEM. Soluble fractions of these SS-X series latexes and the MS-16 seed latexes were collected. IR analysis and TLC were done on the soluble fractions to find percent graft. The ion-exchange process removed a considerable amount of PBA seed material. This could only have been accomplished by removal of low molecular weight polymer with a high level of ionic end groups per gram of polymer. This material is either in the water phase itself or is weakly
adsorbed to the seed particle surface. To minimize gross error, 2-7 rods for each soluble fraction were developed, scanned by FID, and averaged together. Each set of eight rods contained a random selection of samples. A bad scan, usually identifiable by huge water peaks, necessitated a complete repetition of the combinations. Calculations of the grams of each species along the rods were done, assuming that the “top” spot was pure PS and the “center” was grafted copolymer. The amount of grafting of the soluble polymer fractions decreased markedly for SS-2, the sample with ion-exchanged seed. It is interesting to note the amount of polymer of each species that is grafted. Table I11 contains this information. The analysis clearly indicates that a major locus of grafting is in the water phase. Among the three samples the overall efficiency of grafting PS is similar, with little change between SS-1and SS-2. This may indicate a fairly constant monomer flux through the water. The efficiency rose for the heavier nonionic, possibly due to induced chain transfer and enhanced power over the Triton N-101 to solubilize oligomeric radicals and seed material (leaving surfactant-like PBA free to react or adsorb). There is a near absence of grafted soluble PBA in the SS-2 latex. It is evident that the ion-exchange process has removed many of the potential graft sites, i.e., the water-phase of surface-adsorbed polymer. If there is surface-adsorbed polymer, the presence of nonionic rather than anionic surfactant enhances grafting by reducing repulsion from the incoming charged primary or oligomeric radicals.
Conclusions In the semicontinuous polymerization of a PBA/PS core/shell emulsion, variation of 12 process parameters produced a large change in the morphology of the resulting particles. Some of the changes measured were (i) pro-
Table IV. Statistical Matrices for Design Experiments Independent Variables 1 a PH initiator concentration 2 b surfactant concentration 3 C 4 d water solubility of styrene 5 e temperature 6 f wt % solids agitation, rpm 7 g mode of addition of additional surfactant 8 h monomer addition rate 9 j 10 k ionic vs. nonionic surfactant character ionic strength 11 1 12 m seed size 13-14 n-p dummy variables Plackett-Burman design independent variable labela exptno. a b c d e f g h j k 1 m n o p 1 t 2 3
4 5 6 7 8 9
+
+ + + + + +
10 11 12
13 14 15 16
+ -
+
+ + + + + +
+
-
+
+ + + + +
+ +-
+
+ + +
+ + + + -
+ + + +
+ + + + -
+
+ + + + + + + -
and no entries represent the low level of a variable.
+ +
+ + + + + + -
+ +
+ + + + + + -
+
+ + + + +
+ +
-
+
+ + + + + + +-
+
+ + + + + + + -
+ + + + + + + -
+ + + +
+ + + +
-
+
+ + + + + + + -
+ + +
+ + + + + -
no. of +‘s
8 7
6 5 .5
6 6 7 6 6 6 7 7 7 7 0
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 3, 1985
Table V. Statistical Matrices for Design Experiments NBS 212Factorial Design expt no. 212 independent variable label factor (NBS)” P-Bno. A B C D E F G H 1 1 2 9 + + + 3 7 + + + 4 2 5 13 + + + 6 15 + + + 7 12 8 11 9 6 10 3 + + + 11 10 + 12 4 13 5 14 14 + + 15 8 16 16 -
+
+
+ +
+ +
+ +
factor (P-B)* NBS,
+ + +
+ + + +
+
+
+ + + +
+
+ +
d
c
a
b
+
+
+
+
+
+
+ +
+
+ + +
+ +
h
j
+ +
+
f
+ + + +
J
K
+ + + +
+ +
+ +
+
+
+ +
+ +
L
M
+ + + + + + + +
+ + + +
+ +
411
no. of +’s 8 6 6 7 7 7 7 6 6 6 6 5 5 7 7 0
+ + +
+
m
replicate of a 212 fractional factorial design. bP-B, Pluckett-Burman design.
Table VI. Conditions for Screening Latexes run 1
a 9
b 0.056
0.0083
d 1.137
2B
5.5
0.150
0.0095
1.208
3B
5.75
0.065
0.0093
0.821
4B
6
0.061
0.0053
1.130
5B
4
0.058
0.0110
0.644
‘6
6
0.166
0.0064
1.261
7
4
0.140
0.0084
1.055
8
6
0.138
0.0092
0.639
9c
5.55
0.150
0.0054
0.608
10B
4
0.053
0.0065
1.288
11D
3.5
0.150
0.0095
0.569
12A
6.5
0.045
0.0046
0.480
12B
6
0.005
0.006
0.551
13
4
0.126
0.005
0.562
14B
3.75
0.150
0.005
1.127
15D
4
0.061
0.0054
0.628
16B
4.25
0.061
0.0054
0.628
C
variable conditionsavb f g 60 180190 0 70 235240 10 70 235240 70 1800 10 11.89 70 180190 11.53 60 235240 0 60 180190 0 70 180240 60 18010 190 0 60 235240 0 60 235240 0 70 235240 0 70 245240 8.42 70 180190 70 23510 240 0 60 180190 0 60 180190 e 8.75
h 0.16
i 155.4
0
0
1 184
0.078
228
0
0
111
0.078
139
0.304
0.294
111
0.22
136
0.40
0
111
0.078
191.5
0
0
184
0.13
197.5
0.287
0
184
0.078
155
0.394
0.294
184
0.22
235
0.379
0.849
184
0.22
228
0
0.547
111
0.22
194
0
0.878
184
0.16
136
0
0
111
0.078
159
0
0.718
184
0.078
135
0
0.72
184
0.078
136
0
0.636
184
0.22
228
0.40
0.772
111
0.13
228
0.40
0
111
0.13
228
0.40
0
111
j
k
a, pH; b, ionic strength; m equiv/liter; c, starting initiator conc-molar; d, final soap concentration-mmolar; e, mL of MeOH; f, temperature; g, agitation, rpm; h, average feed rate mL/min; i, total liquid volume; j, fraction of total soap moles “slow add”; k, mole frac. triton N-101 in total final soap; 1, seed size D,, nm. Ionic strength averages 0.146, 0.059; initiator concentration averages 0.00094 M, 0.0056 M; surfactant concentration averages 0.0118 M, 0.00059 M.
duction of a second generation of particles, (ii) production of acorn PS domains, (iii) change in the amount of grafting, (iv) change in average thickness and coherence of the ultimate PS shell, (v) film drying properties, and (vi) coagulation of the primary PBA seed particles. A statistical experimental design was used to screen out variable main effects. The following variables were shown to influence several types of morphological change: (i) pH, (ii) initiator concentration, (iii) weight percent solids, (iv)
stirring rate, (v) rate of addition of shell-stage surfactant, (vi) rate of addition of monomer, (vii) ionic strength, and (viii) seed size. Both the Plackett-Burman design and linear regression analysis indicate that the most important variables influencing the aqueous nucleation with evenly distributed capture are (i) ionic strength, (ii) pH, (iii) initial seed particle surface area/mL, (iv) stirrer speed, and (v) mode of shell-stage surfactant addition. Nucleation increased
Ind. Eng. Chem. Prod. Res. Dev. 1985, 2 4 , 412-417
412
Registry No. SLS, 151-21-3; BA, 141-32-2; (PS/PBA) co-
with increasing initiator concentration, stirring speed, and seed size. Linear regression done on the same latexes and morphological properties showed that (i) The ratio In (Ni/Nf) was related to the pH, initiator concentration, stirring speed, percent solids, ionic strength, and seed size. This ratio increased with increasing pH and decreased with seed size. The rest of the parameters entered as second-order interactions. (ii) The production of the acorn morphology was dependent upon the following variables: pH, initiator concentration, monomer feed rate, ionic strength, stirring rate, and seed size. The tendency to form acorns decreased with pH and increased with the other significant parameters. A significant amount of seed material was removable by treatment with ion-exchange resin. Polymerization onto this cleaned seed latex resulted in a lower amount of grafted PS and significantly less grafted PBA than did comparable polymerization onto an uncleaned seed latex. It is proposed that the major locus of the styrene polymerization is in the aqueous phase. The styrene material nucleates new particles and interacts both with surfactant and any water-soluble seed polymer. These particles are then captured by the seed. The capture of particles by the seed, coupled with polymerization in these seed particles, does not always result in an even shell. The PS acorn domains form early on, with an even shell growth taking precedence as the domains grow larger than 60-90 nm in diameter. The domains form due to (i) incompatibility during the polymerization of the monomer near/on the seed surface and (ii) an overall minimization of interfacial energy resulting from the different polymer/polymer and polymer/water interfacial tensions. Surfactant addition rate and concentration play a large role in the tendency to form acorns.
p o l y m e r , 25767-47-9; styrene, 100-42-5.
Triton N - l O l / I g e p a l CO-730, 9016-45-9;
Literature Cited Chainey, M.; Hearn, J.; Wilkinson, M. C. Br. Polym. J. 1981, 73(3), 132. Chainey, M.; Hearn, J.; Wiikinson, M. C. Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 171. Collins, E. A.; Bares, J.; Biilmeyer, F. W. "Experiments in Polymer Science"; Wiley: New York, 1973;p 334. Dickie, R. A.; Cheung, M. F.; Newman, S. J. Appl. Polym. Sci. 1973, 17,
65. Ferry, W. J.; Jones, D. R.; Graham, R. K. US. Patent 3965703, 1976. Fitch, R. M.; Tsai, C. H. "Polymer Colloids, ACS Symposium on Polymer Colloids"; American Chemical Society: Washington, DC, 1971;p 73. Gasperowicz, A.f Kolendowicz, M.; Skowronski, T. Polym. 1982, 23, 839. Gershberg, D. AIChE-I. Chem. E . Symp. Ser. No. 3 1985, 4. Hagiopoi, C.; Dimonie, V.; Georgescu, M.; Deaconescu, I.; Deleanu, T.; Marinescu, M. Acta Polym. 1981, 32(7)390. Hansen, F. K.; Ugelstad, J. J. Polym. Sci.. Polym. Chem. Ed. 1979, 17,
3033. Isaacson, W. Chem. Eng. 1970 (June), 69. Matsumoto, T.; Okubo, M.; Imai, T. Kobunshi Ronbunshu 1974, 37,5767. Matsumoto, T.; Okubo, M.; Onoe, S.Kobunshi Ronbunshu 1978, 5 , 771. Min, T. I.; Klein, A.; El-Aasser, M. S.; Vanderhoff, J. W. J. Polym. Sci. Polym. Chem. Ed. 1983, 21, 2645. Purvis, M. T.; Grant, R. P. U.S. Patent 3983296, 1976. Shaffer, 0.L.; El-Aasser, M. S.; Vanderhoff, J. W. "Proceedings of the 41th Annual Meeting of the Electron Mlcroscopy Society of America"; Electron Microscopy Society of America: Phoenix, AZ, 1983;p 19. Snuparek, J., Jr. J. Appl. Polym. Sci. 1979, 24, 909. Snuparek, J., Jr. Angew. Makromol. Chem. 1980, 88, 61. Snuparek, J., Jr.; Tutalkova, A. J. Appl. Polym. Sci. 1979, 2 4 , 915. Stabenow, V. J.; Haaf, F. Angew. Makromol. Chem. 1973 29/30(359),1. Stutman, D. Ph.D. Dissertation, Lehigh University, Bethlehem, PA, 1964. Tseng, C. M. Ph.D. Dissertation, Lehigh University, Bethlehem, PA, 1983. Vanderhoff, J. W. "Characterization of Latexes by Ion Exchange and Conductometric Titration"; Academic Press: New York, 1977;p 77. Yamazaki, S. Kobunshl Ronbunshu 1976, 1 1 , 1. Yamazaki, S. Shikizai Kyokaishi 1977 5 0 , 267. Yamazaki, S.;Hattori, S.:Hamashima, M. Kobunshi Ronbunshu 1978. 5 ,
662.
Received for reuiew D e c e m b e r 26, 1984 Accepted April 22, 1985
Latex Paint Rheology and Performance Properties Cheng-Fa Lu Hercules Incorporated, Wilmington, Delaware 19894
Viscoelastic measurements reveal a distinct difference in rheology between high and low PVC latex paints. For low PVC (pigment volume concentration) paints, the translational-rotational Brownian motion dominant regime in which G"(o) > G'(o) occurs at frequency w lower than 0.1 (or 0.01) rad/s. However, G"(o) > G'(o) for high PVC paints can appear at a much higher frequency ( > l o rad/s). The correlation between G"(w) at low o and paint leveling was found to be excellent. Murphy's equation, which describes the leveling mechanism, was tested. Lab results confirm its accuracy. Extensional viscosities of paints were obtained by using convergent flow analysis. I t was found that paints showing increasing extensional viscosity with applied stress have very poor spattering property.
its better protective properties (e.g., cracking and corrosion resistance (e.g, Patton, 1979; Lin, 1975). Quach (1973)gave a detailed review of many aspects of leveling. Most theorties consider that surface tension is the major driving force. Gravity is just a minor cause. In many cases, paints are supplied on a substrate by using a roller coater. During the application, numerous paint filaments are pulled out from the interstices among fibers on the coater, and this may result in paint spattering. In addition to shear deformation, uniaxial extension of the
Introduction The application of a paint to a substrate by brushing involves two rheological extremes. During the brushing process, the encountered shear rate is believed to be within the range from 5 000 to 20 000 s-l (Patton, 1979). After brushing, the leveling of brushmarks is brought about by the driving of surface tension at a corresponding shear rate less than 1.0 (Patton, 1979). A level film is important not only for its physical appearance (e.g., smoothness, gloss, and color) but also for 01 96-43211851 1224-0412$01.50/0
0
1985 American Chemical Society
