Dispersion of Solid Particles in Organic Media - Advances in

24 Dispersion of Solid Particles in Organic

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 10, 2013 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0099.ch024

Media G. E . MOLAU and E. H. RICHARDSON The Dow Chemical Co., Polymer Science, Physical Research Laboratory, Midland, Mich. 48640

Block copolymers are dispersing agents which stabilize dispersions of solid particles in organic media. Dispersions of titanium dioxide in toluene stabilized by partially carboxylated styrene-butadiene block copolymers are studied as model systems. The block copolymer is modified by adding thioglycolic acid to some of the butadiene units. Through these carboxylated sites, the butadiene ends of the block copolymer molecules are adsorbed selectively at the substrate surface, while the styrene ends are dissolved in the dispersion medium. We believe that a "polymeric double layer" is formed, and that the unusually high stability of the dispersions against settling is a direct consequence of the interaction of the double layers of different particles.

TTVispersions of solid particles i n organic media are of considerable technological importance, particularly i n oil-based paints and printing inks, but their stabilization has not been studied as extensively as the stabilization of dispersions i n aqueous systems. Romo (16) studied the stability of dispersions of titanium dioxide i n pure solvents and i n solutions containing organic resins, and C r o w l and Malati (2) stabilized dispersions of titanium dioxide and iron oxide in benzene with polyesters of adipic acid and neopentyl glycol. M c G o w n and Parfitt (9, 10) dispersed titanium dioxide i n solutions of oil-soluble surfactants i n xylene and treated the stabilization mechanism on the basis of the DerjaguinLandau-Verwey-Overbeek theory. The influence of water on dispersions of titanium dioxide in organic solvents has been investigated b y M c G o w n and Parfitt (9) and by Zettlemoyer, Micale, and L u i (19). W e have shown (12, 14) that block and graft copolymers ( B G copolymers have emulsifying properties i n "polymeric oil-in-oil emulsions." 379 In Multicomponent Polymer Systems; Platzer, N.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.


380

MULTICOMPONENT POLYMER SYSTEMS

These emulsions are l i q u i d - l i q u i d systems comprising immiscible polymer solutions i n nonpolar solvents and B G copolymer emulsifiers. The emulsifying power of B G copolymers has been attributed (12) to coalescence barriers formed b y accumulation of the B G copolymers i n the emulsion interface. This interface apparently has the structure of a double layer consisting of the different subchains of the B G copolymers which are solvated by the organic solvent. The chemically different sequences i n B G copolymers are separated i n different layers i n the interface because polymer chains of different chemical structures are usually incompatible (3, 4), particularly i n nonpolar solvents. It appeared attractive to extend the work on emulsification of l i q u i d liquid systems b y B G copolymers to solid-liquid systems. As a first approach a model system was studied which comprises titanium dioxide dispersed i n toluene with modified styrene-butadiene block copolymers as dispersants. These studies are reported here. The work was planned on the basis of a model of a dispersed solid particle onto which one type of sequences of a B G copolymer is adsorbed selectively while the other type sequence is dissolved i n the dispersion medium. A sketch of this model is shown i n Figure 1. The model is the result of applying the same arguments which had been advanced (12) in discussing the mechanism of stabilization of polymeric oil-in-oil emulsions b y B G copolymers to the problem of stabilization of dispersions of solid particles i n organic media. Previously, essentially the same arguments had led to the demonstration of micelle formation of styrenebutadiene block copolymers i n organic media under certain conditions (15). In the model a solid particle is coated with a "polymeric double layer" formed by a number of B G copolymer molecules consisting of monomer units A and monomer units B. The sequences of A units are

ADSORBED LAYER OF A-SEQUENCES SOLVATED LAYER OF B-SEQUENCES

Figure 1. Model of a solid particle in an organic medium. The particle is coated with a BG copolymer consisting of A sequences (adsorbed) and B sequences (dissolved).

In Multicomponent Polymer Systems; Platzer, N.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.


24.

Dispersion of Solid Particles

MOLAU AND RICHARDSON

381

absorbed onto the surface of the solid substrate, while the sequences of B units are dissolved completely in the surrounding organic medium. N o electrical charges are involved i n the formation of coalescence barriers according to this model. The stabilizing principles are based entirely on the interactions of the polymer chains with each other and with the surrounding organic medium. The principles underlying this model have been discussed i n detail (12), and the model itself and experimental data showing the dispersant activity of block copolymers i n dispersions of solid particles in organic media have been presented (13). Recently, a patent (1) appeared which also describes the use of block or graft copolymers for coating pigment particles and dispersing the coated particles in organic liquids. Experimental The components of the model system titanium dioxide, toluene, and styrene-butadiene block copolymers were selected for the following reasons: titanium dioxide is available in various particle sizes because it is a widely used pigment; toluene is a good solvent for the styrene-butadiene; block copolymers and has a low viscosity, so that the stabilizing effect of the block copolymers can be observed without overlapping stabilization effects resulting from a high viscosity of the dispersion medium; styrene-butadiene block copolymers were chosen because samples were available which had structures designed to give good performance as emulsifiers in polymeric oil-in-oil emulsions. Materials. Titanium dioxide was pigment-grade material ( d u Pont Ti-Pure R-900 brand) with spherical particles i n the range of 0.2-0.5 microns. In most experiments, the material was used without further treatment. In one experiment (specified in the text), the material was dried for 18 hours at 145 °C. The block copolymers were A B type styrene-butadiene block copolymers prepared by anionic polymerization. Unless specified differently in the text, a block copolymer of number average molecular weight 110,000 containing 70 wt % styrene and 30 wt % butadiene (S30B) was used. Samples of different composition used in one series of experiments had approximately the same molecular weight. The block copolymers were modified by introducing carboxylic groups into the butadiene sequences to provide sites for selective adsorption of one end onto the TiOo surface. The modification was accomplished by adding thioglycolic acid, making use of the well-known addition of mercaptans to carbon-carbon double bonds. ~CH —CH = CH—CH ~ + HS-CH —COOH 2

2

2

~CH —CH—CH—OH ~ I I H S I CH C()OH 2

2

2



382


Addition of Thioglycolic Acid (TGA) to Styrene-Butadiene Block Copolymers. The addition of thioglycolic acid to homopolymers of butadiene has been described by Marvel and co-workers (8). Starting with conditions described in their paper, various procedures were tested for achieving various degrees of carboxylation of the block copolymers. In a typical recipe, 20 grams of block copolymer (S30B) were dissolved in 280 grams of 1,4-dioxane, and 0.6 gram of thioglycolic acid (Evans Chemetics, Inc.) was added. The solution was stirred for 24 hours in a nitrogen atmosphere. The carboxylated block copolymer was recovered by precipitation with either methanol or n-hexane followed by washing with nonsolvent. The degree of carboxylation, expressed as mole % conversion of butadiene units to adduct units, was determined from the oxygen content determined by neutron activation analysis. To assure that the oxygen analysis was not falsified by methanol entrapped in the polymer, the last reprecipitation of the samples was always carried out with n-hexane. The degree of carboxylation ( D C ) obtained with 0.6 gram thioglycolic acid was 1.1% conversion of butadiene units; for other degrees of carboxylation, the amount of thioglycolic acid in the same basic recipe was varied. The carboxylated block copolymers were redissolved in the solvent in which they were to be used (usually in toluene) immediately after reprecipitation because they crosslink when they are stored in the solid state. Adsorption of Carboxylated Block Copolymer in o-Dichlorobenzene. In a three-necked flask equipped with stirrer, reflux condenser, and N inlet, 200 grams of a 1 % solution of carboxylated styrene-butadiene block copolymer in o-dichlorobenzene and 2 grams TiO? were stirred at various temperatures for 3 hours. (Typical temperatures in this treatment were 110°, 150°, and 178°C.) After the heat treatment the T i 0 was isolated from the block copolymer solution by centrifuging for 1-2 hours at 2,000-2,200 rpm. After decanting the supernatant solution, the solid particles were washed with solvent and dried in vacuo for about 16 hours at 50°C. Adsorption of Carboxylated Block Copolymer in Toluene. The later adsorption experiments were carried out in a W a r i n g Blendor which had been wrapped with heating tape. Solutions could be conveniently heated to and maintained at 100 °C with this arrangement. 50 Grams of T i 0 and 500 grams of a 1 % solution of the carboxylated block copolymer were agitated for half an hour. W h e n heat treatment was applied, the block copolymer solution was heated to the desired temperature before adding TiOo. Typical temperatures were 100°C and room temperature. The resulting dispersion was used directly for the settling test without isolation and washing of the coated substrate by centrifugation. In all recipes blank runs were made to test the effect of the heat treatment on pigment dispersibility in the absence of block copolymer. Determination of Adsorbed Block Copolymer. The titanium dioxideblock copolymer composite was isolated from the dispersion medium by centrifugation. After decanting the supernatant, the centrifugate was redispersed twice in toluene and collected again by centrifugation to ascertain removal of block copolymer not adsorbed. The final centrifugate was washed with methanol, dried in vacuo, and submitted for carbon2

2

2



24.

MOLAU AND RICHARDSON

383

Dispersion of Solid Particles

hydrogen analysis. The amount of adsorbed block copolymer, which was the only portion of the polymer-substrate composite containing carbon, was calculated from the carbon content of the total composite. Settling Test for Measuring Dispersion Stability. A quantity of 20 m l of a freshly prepared dispersion was placed in a graduate cylinder equipped with a stopper and allowed to stand motionless at room temperature. As the particles settled slowly to the bottom of the graduate, fractionation occurred because the fraction of larger particles settled at a faster rate than the fraction of smaller particles. Two zones became discernible. The boundary between the zones could be seen more easily by shining an oblique beam of light into the dispersion. The upper zone consisted of a very stable dispersion of fine particles, while the lower zone consisted of a more concentrated dispersion containing coarse as well as fine particles. The finer particles remained dispersed throughout the test and for long periods after the coarser particles had settled out. In control experiments, in which titanium dioxide without adsorbed block copolymer was dispersed under otherwise the same conditions, a l l particles including the fine particles settled out within a few seconds, and the supernatant was entirely clear. The rate of settling was measured by recording the motion of the pseudo-boundary between the two zones as a function of time. Plots of the volume fraction of the lower zone as a function of settling time were essentially linear. When most of the coarser particles had settled out, the volume fraction of the lower zone became constant—i.e., the slopes of aJl of the straight lines changed abruptly, and the oblique lines combined into one horizontal line. The times at the breaking points i n the lines were recorded as "total settling time" ( T S T ) , usually expressed i n days, and were taken as a measure of dispersion stability. Since the finer particles i n the dispersion remained dispersed for long times, and the settling rate of only the coarser particles was measured by the settling test, the recorded total settling times represent the minimum of attainable stability of a given dispersion because the test measures the worst performers in the system. Results In the presentation and discussion of the data, the compositions of the block copolymer samples are indicated by symbols such as S30B • T G A (block copolymer of 70 wt % styrene and 30 wt % butadiene modified by adding thioglycolic acid ( T G A ) onto part of the butadiene units). The degree of carboxylation ( D C ) of the butadiene chains is expressed as the percentage of butadiene units modified by a thioglycolic acid unit. When the dispersion is prepared, s grams of substrate (TiOo) are added to a solution of p grams of block copolymer. The substrate/polymer ratio is denoted by the symbol S/P. The amount of block copolymer actually adsorbed onto the substrate, which is determined b y carbon analysis of the block copolymer/Ti0 composite, is denoted by the symbol L ("Langmuir"), where L is the number of m g of block copolymer adsorbed onto 1 gram of TiOo. The "total settling 2



384


time," which has been defined i n the experimental section is denoted by T S T . As a first approach to the adsorption of carboxylated styrene-butadiene block copolymers onto titanium dioxide particles, experimental procedures and conditions were used which had been developed by Schechter (18) i n his studies on the adsorption of fatty acids onto titanium dioxide and on the effect of the fatty acid coating on the dispersibility of the particles i n organic solvents. Schechter had treated titanium dioxide with solutions of fatty acids i n o-dichlorobenzene at 150°-200°C for varying periods of time. After this heat treatment, he isolated the coated titanium dioxide particles by filtration, washing with ether, and drying in vacuo. Redispersion of the coated particles i n n-heptane and measuring the rate of settling as described i n the experimental section gave total settling times of 20-60 minutes, depending on the chain length of the fatty acid. Following the guidelines established by Schechter's work, we dispersed titanium dioxide particles in 1 % solutions of carboxylated styrenebutadiene block copolymers and stirred the dispersions at elevated temperatures i n a nitrogen atmosphere. Typical data are shown i n Table I. The dispersions (primary dispersions) i n o-dichlorobenzene were quite stable. T h e titanium dioxide particles were isolated from these primary dispersions b y centrifugation and were washed with toluene and finally with methanol. After drying in vacuo, samples of the block copolymertitanium dioxide composites were submitted for carbon analysis. The Table I.

Adsorption of S3 OB • T G A onto T i 0

Exp. No.

2

in o-Dichlorobenzene*

T , °C

S/P

DC,%

RA6 RA9 RA10

110 150 178

0.8 0.8 0.8

11.6 11.6 11.6

26 40 34

RD42 RD41 RD43 RD44

150 150 150 150

1.0 1.0 1.0 1.0

11.6 12.6 15.9 17.4

12 12 15 14

RD46 RD48

150 150

1.0 1.0

25.8 25.8

29 35

RD47*

150

1.0

1.4

17

RD36

Dispersion of Solid Particles in Organic Media - Advances in

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