J. Phys. Chem. 1980, 84, 1615-1620
1615
Redispersible StyreneIButadiene Latexes. 1. Preparation and Properties B. W. Greene,* A. R. Nelson, and W. H. Keskey The Dow Chemical Company, Designed Latexes and Resins Laboratory, Midland, Michigan 48640 (Received October 12, 1979) Publicatlon costs assisted by The Dow Chemical Company
Redispersible styrene/butadienelatexes, i.e., those which yielded films which re-formed into a latex in the presence of water, were prepared by emulsion polymerization with various monoalkyl maleates as functional modifiers. It was found that both the colloidal properties of the latexes and the redispersibilityof their films were strongly pH dependent, and that redispersibility was primarily due to the presence of high concentrations of hydrated carboxyl groups on the surface of the latex particles.
Introduction Synthetic latexes prepared by emulsion polymerization are colloidal dispersions of essentially spherical polymer particles in water. They are of varied polymer composition and usually clonsist of particles with diameters in the range 0.02-0.5 pm. They are often referred to collectively as emulsion polymers or polymer colloids. Synthetic latexes are used widely as coatings, binders, and adhesives in the paper, textile, automobile, and building industries. In these latter applications, the latexes are required to be film forming. 'This means that the latexes must, consist of polymer particles with a glass transition temperature (T,) sufficiently below the temperature of application in order to yield films which are continuous, ntrong, and water insensitive. The ultimate strength and integrity of the films will, of course, depend on the extent of coalescence of the polymer particles which are brought into close proximity upon removal of the water by evaporation during film formation. Conventioinal styrene/butadiene (S/B) copolymer latexes, e.g., those containing 20-60570 styrene by weight, generally have Tgvalues considerably below or near 0 "C. They, therefore, yield films with good strength and integrity at all application temperatures > 5 "C. The S/B latexes described in this paper were film forming like conventional S / B latexes. However, they differed from the latter in that they yielded films of poor strength and integrity, i.e., those which were weak, discontinuous, and so water sensitive that they re-formed into a latex upon addition of water. Because of the latter film behavior which is indicative of limited particle coalescence, these S / B latexes are referred to as redisperisble latexes. (Powders of several redispersible latexes are being used commercially in dry tape joint, paint, and cement compounds.) There h a w been several reports in the past on redispersible latexes consisting of polymers or copolymers with Tgin the regiion 30-100 "C.la However, the literature is almost devoiid of information on redispersible latexes consisting of polymers or copolymers with Tglower than the above! The primary objective of this and other papers in this series is to contribute information which will fill some of the void in this area. The present paper deals specifically with (i) the preparation and properties of redispersible SJB latexes and (ii) the properties of films of redispersible latexes before and after addition of water. The reader should bear in mind that the latexes described here are complex systems typical of those used commercially, and that the redispersible phenomenon exhibited by them would not be observed in rigorously purified model latexes which were prepared by using low 0022-365418012084- 16 15$01,0010
TABLE I: Polymerization Recipe
ingredient
parts b y weight
monoalkyl maleate water bromoform sodium salt of alkylarylsulfonate sodium persulfate
45-55 40-50 5.0-7.5 100 1.0 0.6 0.5 0.2 0.02
styrene
butadiene
sodium hydroxide
tetrasodium pyrophosphate
levels of emulsifier or functional modifier.
Experimental Section Materials. The styrene and butadiene monomers as well as the surfactant used here were technical grade and used without further purification. The methyl, ethyl, propyl, and butyl half-ester of maleic acid which were used as functional modifiers were prepared by allowing equimolar quantities of maleic anhydride and the desired alcohol to react at 25 "C: 0 H-C-C\
I/
1)
H-C-C'
H-C-C-OH
0 t ROH
II
il
II
H-C-C-OR
0
11
0
R = methyl, ethyl, . . . etc.
The purity of the half-esters was determined by NMR to by 95-98%. The main impurities in these products were maleic acid and the corresponding diester. Later Preparation. The redispersible latexes were prepared at 90 "C by 'using conventional emulsion polymerization techniques, The mixed monomer and aqueous feeds were added to the initial charge in the reactor over a period of 5.5 h, and polymerization was allowed to proceed 1h thereafter. After polymerization, the latexes were filtered through a 100,mesh screen to remove any coagulum formed and subjected to steam distillation to remove unreacted monomers. The basic latex polymerization recipe used here is given in Table I. Latex Characterization. Determination of Particle Size. The weight-average diameters of the latexes were determined on diluted latex samples by light scattering with the Brice-Phoenix photometer at a wavelength of 4380 A and by hydrodynamic chromatography (HDC). The procedure used for the latter method has been previously described.' The number-average diameters and particle size distributions of several of the latexes were determined by measuring the diameters of 800-1000 particles in 0 1980 American Chemical Society
1010
The Journal of Physkal Chemistty, Vol. 84, No. 12, 1980
electron micrographs with a Zeiss particle counter. Determination of Surface Carboxyl Groups. The concentration of carboxyl groups bound to the surface of the latex particles and in the serum phase was determined from potentiometric titrations carried out on the whole latexes as well as the isolated polymer and serum phases. The separation and titration procedures used here were similar to those previously d e s ~ r i b e d .From ~ ~ ~ titration results on samples containing known weights of the polymer, the concentration of surface carboxyl groups was calculated and expressed as milliequivalents of carboxyl groups per gram of polymer (mequiv/g). Determination of Hydrolysis of Half-Esters. Dilute solutions of the half-esters of maleic acid used here as well as sera isolated from the latexes were maintained at temperatures between 68 and 100 " C for various periods of time prior to analysis by NMR to determine the extent of half-ester hydrolysis. The NMR analyses were carried out by J. P. Heeschen at DOW'SAnalytical Laboratory. Determination of Electrolyte Stability. The electrolyte stability of the latexes was determined turbidimetrically by following the course of the early stages of coagulation of the diluted latexes (0.005-0.01% by weight) in the presence of various concentrations of various electrolytes. The change in optical density, OD, of a given latex with time was measured continuously, commencing a few seconds after electrolyte addition, with the instrumentation and procedure described previously.1° Stability curves of the latexes, log-log plots of dt/d(OD) against the molar concentration of added electrolyte, were also derived as previously described.1° Determination of Electrophoretic Mobility. The mobility of the latexes was measured as a function of latex pH and electrolyte concentration at the same latex concentration as used for electrolyte stability measurements. The measurements were carried out on the Rank Brothers particle microelectrophoresis apparatus, using the cylindrical cell in a bath thermostated at 25 f 1 "C. Parabolic curves of mobility against cell depth were obtained on several latex samples in order to locate the stationary level (level of zero liquid flow). Measurements made on a given latex at the stationary level were reproducible within f2%. Measurement of Surface Tension. Surface tension measurements were carried out on the latexes as a function of pH before and after dialysis of the latexes in cellulosic tubing against deionized water for 1week. All measurements were made by using the du Nouy tensiometer at 25 f 1 "C. Film Preparation. Films 20 mil in thickness were cast from latexes at various pH onto clean, smooth glass plates with a drawing bar, The films were allowed to dry in air at room temperature for a period of 24 h or longer prior to being subjected to tests described below. Film Characterization. Determination of the Type of Redispersibility. Film redispersibility was assessed semiquantitatively as follows: Several drops of deionized water were placed near the center of the air-dried film; and the film was left to stand - 5 min before extracting a sample of the redispersed portion for particle size measurement by light scattering. The film redispersibility was then classified as type I, 11, or I11 according to the following definitions: Type I. The film spontaneously redisperses into a latex with essentially the same average particle size as the original latex upon addition of water after more than 24 h of aging. Type II, The film spontaneously redisperses into a latex with an average particle size somewhat larger than that
Greene, Nelson, and Keskey
of the original latex upon addition of water after 24 h of aging. Thereafter, light rubbing (shear) is required for redispersion. Type III. The film redisperses into a latex upon addition of water after 24 h of aging only when shear is applied. A more quantitative measure of film redispersibility was obtained in several cases from the comparison of the particle size distribution curves of reconstituted latexes (those derived from films) with those of original redispersible latexes. Determination of the Mechanical Properties. The tensile strength and percent elongation at break of airdried and cured films were measured on an Instron tester. Determination of Water Content. The water released from air-dried films in the temperature range -25-100 "C was determined by combined thermogravimetric-mass spectrometric analyses. Analyses were carried out on 2560-mg samples of films of redispersible latexes which had been neutralized to various pH with different monovalent bases with a DuPont 950 thermogravimetric analyzer. Scanning Electron Microscopy. Changes in morphology of the latex particles in films were assessed from micrographs of air-dried films which had aged for various periods of time. Preparation of Reconstituted Latexes. Reconstituted latexes were prepared by mixing the desired weights of air-dried film and deionized water together for 2 min in a Waring blender set at high speed.
Results and Discussion General Properties of the Latexes. In order to demonstrate that the redispersible S/B latexes were unique, behaving more like hydrophilic than hydrophobic colloids, it was necessary to characterize three systems. These were the original latexes prepared by emulsion polymerization, films of the original latexes, and reconstituted latexes, those prepared by redispersing films of the original latexes in water. Pertinent results on these systems are discussed below. Some of the physicochemical properties of the original latexes which were prepared by using half-esters of maleic acid are given in Table 11. The solids of the latexes which were charged initially to be 50% varied from 45 to 48%. The latex pH and particle size increased systematically with increasing size of the alkyl group (chain length) of the half-ester., whereas the concentration of carboxyl groups on the latex surface decreased systematically with increasing chain length of the half-ester. The latter trends actually reflected differences in the water solubility and hydrolytic stability of the different half-esters. The NMR analyses mentioned earlier showed that the methyl halfester was essentially completely hydrolyzed to maleic acid under the conditions of the latex polymerization (low pH, high temperature), whereas the degree of hydrolysis of the ethyl, propyl, and butyl half-esters were -82,45, and 9%, respectively. Because of this, the concentration of surface carboxyl groups would be expected to be greater for the latex prepared with the methyl half-ester than for that prepared with the propyl or butyl half-ester. Besides this, the titration results indicated that a considerable portion (18-27 %) of these latter less water-soluble comonomers was polymerized in the bulk of the latex particles, whereas essentially all of the methyl and ethyl half-esters used in the polymerization were found to be polymerized on the surface of the particles (67-75% of total used) and in the serum phase (2533%). It was a consequence of the greater hydrolytic instability of the methyl and ethyl half-esters during polymerization that much higher con-
The Journal of Physical Chemistry, Vol. 84, No. 12, 1980
Redispersible 8yrene/Butadiene Latexes
1617
TABLE 11: P'roperties of S/B Latexes Prepared by Using Half-Esters of Maleic Acid distribution of carboxyl groups
parts by weight
run no,
oster deriv used
styrene
124-1 124-2 124-3 124-4 124-5 124-6
Me Me Me Et Pr BU
45 55 45 45 45 45
butadiene half-ester 5 50 5 40 7.5 47.5 5 50 5 50 5 50
% solids
latex pH 2.8 2.3 2.6 3.1 3.8 4.0
48.2 47.4 47.5 47.8 45.0 47.7
surface carboxyls, mequiv/g 0.502 0.518 0.511 0.392 0.219 0.047
av particle diam, A 2050 2240 2250 2360 2480 2550
surface/ totala 0.72 0.71 0.73 0.70 0.68 0.71
a Ratio of the concentration of surface carboxyl groups in mequiv/g of polymer to the concentration of total carboxyl groups in mequiv/g 0f polymer found in the latex by potentiometric titration. Log Initial Coagulation Rate;' [dt/d(O.D.)I
Log Initial Coagulation Rate;' [dt/d(O.D.)1
1
Electrolytes Used: Chlorides. Na, Ca, La Sulfates . Mg, AI, Na
-4
-3
-2
-1
0
i
1
1
Log Molar Concentration of Electrolyte
Figure 1. Effect of various electrolytes on the initial coagulation rate of the redispersible latex (pH 4.0).
centrations of carboxyl groups were found to be associated with the surlface of the latex particles than is normally found by using conventional vinyl comonomers (e.g., methyl methacrylate). In addition, it was due primarily to the much higher concentration of surface carboxyl groups that !films of several of these latexes were redispersible. This will become evident from results to follow. Colloidal Properties of the Latexes. Electrolyte Stability. The electrolyte stability of the redispersible latexes was determined as a function of the concentration of added mono-, di-, arid trivalent electrolytes. The results obtained on one of these latexes, no. 124-1, at low and high pH are given in Figures 1 and 2, respectively. They are similar to those obtained on most of the latexes. They show that the stability of the latex was essentially unaffected by the addition of mlolar quantities of 1:l monovalent electrolyte, decreased some in the presence of 1:2 monovalent electrolyte, and decreased markedly in the presence of di- and trivalent electrolytes. The quantities of the latter required to destabilize the redispersible latex were much greater, however, than those usually required for conventional S/B latexes. The shape of the stability curves of the redispersible latexes in the presence of cli- and trivalent electrolytes was adso different from that usually obtained with these latter. I t was indicative of' specific binding of multivalent counterions to carboxyl groups on the latex surface. Thiei was confirmed later when it was found that several of the latexes at higher concentrations (10-15%) could be gelled in the presence of Ca2+or Mg2+ions. p H Dependence of the Stability, Mobility, and Surface Tension. The stability, mobility, and surface tension which were obtained on the redispersible latex prepared with the methyl half-ester are summarized in Figure 3. They are typical of those obtained on several of the latexes. They show, first, that the stability of the latex in the presence
O
-4
l
3
1
.3
.
1
L
I .2
2
.
Electrolytes NaCl CaCI, 1 LaCI,
Used: 1-1
2.1 3-1
1
I
.l
0
1
Log Molar Concentration of Electrolyte
Figure 2. Effect of various electrolytes on the initial coagulation rate
of the redispersible latex (pH 9.0).
of CaClz decreased with pH. This behavior was due to the specific binding of Ca2+ions and is generally not found with conventional S/B latexes.1° They also show that the mobility and surface tension of the latex were essentially independent of pH up to pH 6.5-7.0, and, thereafter, increased with increasing pH up to pH -9. This behavior is also atypical for conventional S/B latexes. For the latter, the mobility is usually found to increase with pH in the region pH 3-9, whereas the surface tension at a given concentration is found to be practically independent of pH. In an attempt to explain the unusual behavior of the redispersible latex, the mobility and surface tension were redetermined after the latex had been subjected to purification to remove low molecular weight constituents in the serum phase. Both dialysis and a centrifugation-serum replacement techniquell were used to purify the latex. Both led to somewhat similar results. The results obtained before and after dialysis are given in Figure 4 They show that the pH dependence of the latex surface tension and mobility was much less after dialysis than before, and, in fact, more as expected for a conventional S/B latex. From these experiments which also showed that there was a partial loss in film redispersibility after latex purification, it was concluded that low molecular weight water soluble surface-activeconstituents in the serum phase contributed to the colloidal behavior of the redispersible latexes. This result will be discussed again later in the section on the mechanism of latex film redispersibility. Properties of Films of the Latexes. Observations Made on F i l m . It was while making a routine check of the water sensitivity of films of these latexes that it was found that most of the films were dispersible in water. It was also observed that the type of film redispersibility depended on several factors: the chain length of the half-ester used
1618
Greene, Nelson, and Keskey
The Journal of Physical Chemistry, Vol. 84, No. 12, 1980 a:
moles
C.C.C.,liter CaCI,
-c:
dynes cm surface tension, -
70
66
0.010
Flgure 3. pH
-
0
2
4
6
8
1
0
-1 0
62
58
54 2
4
6
S
10
I
0
2
I
4
I
6
I
5
I
1
I
0
Latex pH
dependence of the stability, mobility, and surface tension of redispersible latex no. 124-1 before dialysis. TABLE 111: Effect of the Type Base and the Half-Ester Chain Length on Film Redispersibility
y , d e 70 cm
65
60
1
55
I
I
1
1
1
"I--& -4
run no.
halfester
base used to neutralize latex
124-1 124-1 124-1 124-1 124-2 124-2 124-3 124-3 124-3 124-4 124-4 124-4 124-5 124-6
Me Me Me Me Me Me Me Me Me Et Et Et Pr Bu
none CH,CH(NH,)OH ",OH NaOH none NaOH none ",OH NaOH none ",OH NaOH NaOH NaOH
type relatex disperspH ibility 2.8 9.0 9.0 9.0 2.3 9.0 2.6 9.0 9.0 3.1 9.0 9.0 9.0 9.0
none I11 I1 I none I I11 I1 I none I11 I1 I11 none
TABLE IV: Water Contents of Uncured Films of a Redispersible Latex, No. 124-2, Neutralized with Various Monovalent Bases mol of H,O/mol of COOH -1
I
~
0
2
4
6
8
10
12
Latex p H
pH dependence of the surface tension and mobility of redispersible latex no. 124-1 before (A)and after ( 0 )dialysis. Flgure 4.
to make the latex; the pH of the latex used to cast the film; the type base used to neutralize the latex; the copolymer composition (S/B ratio) of the latex; and the age (or time and temperature of curing) of the film. The effects which some of these factors had on redispersibility are discussed below. p H Dependence of the Redispersibility of Films. The data given in Table I11 illustrate the dependence of film redispersibility on latex pH, chain length of the half-ester, and type base used to neutralize the latex. It can be seen from these data that, when latexes prepared with the methyl half-ester were used at low pH to cast films, film redispersibility was either not observed or poor (type 111), whereas when these latexes were adjusted to high pH with NaOH, good film redispersibility (type I) was observed. It can also be seen from these data that film redispersibility was poorer when NH40H and CH3CH(NH2)0Hwere used
base LiOH NaOH KOH ",OH
pH4.0a
pH 5.9
pH 10.0
3.1 1.5 1.6 1.5
5.1 2.9 2.0 1.8
10.1 5.4 2.6 2.4
a pH is that of latex used to make film. All films were air dried for 3 days at room temperature prior t o analysis.
to neutralize the latexes than when NaOH was used. This latter behavior was found irrespective of the type half-ester used to prepare the latex. It was presumed to be due to the difference in hydration characteristics of the counterions of the bases used for neutralization. This was also indicated by the water content data given in Table IV which were obtained on films by TGA-mass spectra analysis. If the results obtained on films of latexes of the same composition which were made with different half-esters (no. 124-1, -4, -5, -6) and adjusted to pH 9 with NaOH are compared, it can be seen that film redispersibility increased with decreasing chain length of the half-ester, i.e., with increasing concentration of acid on the surface of the
Redispersible StyrenelButadieneLatexes
The Journal of phvsical Chemistry, Vol. 84. No. 12, 1980 1819
TABLE V: Mechanical Prowrties of Latex Films latex designatn
S/B ratio
type relatex disperspH ibility
tensile
elon-
psi
%
strength, gation,
45/50 2.8 none 479 725 45/50 9.0 I (253)' (278) 45/50 9.0 I 304 311 55/40 2.3 none 718 532 55/40 9.0 I (342) (249) 275 417 55/40 9.0 I control -47/50 8.7 none 560 835 Values in parentheses are for uncured films; all other films were cured for 5 min at 135 'C prior to measurements. Tgdetermined with cured films by DTA was -23 to -18 "C. Control is a Dow latex used in textile applica. 124-1 124-1 124-1 124-2 124-2 124-2
a. Original Latex 0
tions.
+ ; . 0
I 0
0
I . .
A
0
b. Reconstituted Latex
F
Figwe 8. Electron micrographs of ths &ginal redispersible latex no. 124-1and ths reumstnuiedlatex dertved hom n (magnification17 600 X)
F
Flgure 5. Scanning electron micrographs of airdried films of ths dispersible latex no. 124-1 and a conventional SIB latex.
particles (see Table 11). Finally, it should be mentioned that the pH dependence of film redispersihility and latex colloidal properties (Figure 3) was similar. Type I film redispersihility was never observed unless the latex used to cast the film was at pH 27. Mechanical Properties of Films. The tensile strength and percent elongation of cured and uncured (air-dried) f h of two of the redispersible latexes and a conventional S/B latex (control) are compared in Table V. It can be seen from the latter that cured films derived from the latexes at low pH where redispersibility was not observed had moderately high tensile strength and elongation similar to the control, whereas cured and uncured films derived from the latexes at high pH where redispersibility occurred had much lower tensile strength and elongation than the control. These results showed definitely that the redispersible latexes were characterized by films of poor strength and integrity. It was hypothesized that this was due to limited coalescence of the particles in the films. In order to determine if this were so, the scanning electron micrographs, given in Figure 5 were obtained on air-dried films of the redispersible latex (no. 124-1 a t pH 9) after various periods of aging. They show that after 1-day aging the particles in the film existed as discrete (noncoalesced), somewhat deformed units, and that after 30-day aging there was still little change in the particle morphology. However, after 60-day aging there were some definite indications of particle coalescence. In contrast to the latter, the results in Figure 5 show that the particles in the film of the conventional S/B latex lost their discreteness after 1-day aging. Usually particles in films which initially exhibited type I redispersihility lost their discreteness after
4
'"I Ill 80
! I
Part#deDmamner. A
Flgure 7. latexes.
Patticla size distributionsof the wiglnal and reconstnuled
being aged 3 month or after being cured 30 min at 135 "C.
Properties of Reconstituted Latexes The electron micrographs obtained on one of the original redispersible latexes and the corresponding reconstituted latex are compared in Figure 6 They show that both the average particle size and dispenity of the two latexes were similar. The results given in Figure 7 show that the particle size distribution curves of both latexes were bimodal but that the average particle size of the reconstituted latex was somewhat larger than that of the original latex. The smaller particles present in these distribution curves were not detected when the particle size of the latexes was determined by the light scattering or HDC technique used here. However, as can be seen from Figure 8, they were evident in electron micrographs made a t higher magnifications. The small particle fraction was later found to arise due to the fact that the concentration of surfactant which
1820
The Jocnvrl of PhVJica/ Chemktry, Vd. 84, No. 12, 1980
& m e . Nelson, and Keskey
-
Marker
b. Rseonnitpd Latex D=2630A h=8.28
FILNO 8. E 61 500 X).
m miaosaphOr re65perswe latex 124-2 (magWcaIjm
was continuously added during latex polymerization had been sufficient to cause particle nucleation throughout the polymerization. When the level of surfactant was reduced, the smaller particle fraction was eliminated without altering other latex properties. The average particle size of original and reconstituted latexes was also determined by HDC. The results on the latex designated as no. 124-1are given in Figure 9. They are similar to those given in Figure 7 in that they show that the average diameters as well as the peak heights and widths of the HDC curves of the two latexes were practically the same. However, the average diameter of the latexes as determined by HDC was always larger than that determined by electron microscopy. The stability, mobility, and surface tension of the reconstituted latexes derived from films of the latexes designated as 124-1and -4were measured at high pH and found to be similar to those of the original latexes (Figures 2 and 3). Tentative Mechanism of Film Redispersibility It was shown here previously that water soluble constituents in the latex serum phase contributed to the latex colloidal properties and film redispersibility (Figure 4). It was later determined by NMR analyses and surface tension measurements that the main constituents of the sera were maleic acid, the hydrolysis product of the half-ester, and 1,2,3,6-tetrahydrophtbalicacid (THP), the Diels-Alder reaction product of butadiene and maleic acid, and that THP was surface active and could be post-added to conventional S/B latexes to render their films redispersible. From other data in the literature which indicate that the mechanism of film redispersibility may be different for different polymer type~,1A.~ it is realized that our understanding of this phenomenon is not complete. However, on the basis of the data given here, it is believed that the redispersibility of films of these S/B latexes was due (1) primarily to the unusually high concentration of carboxyl groups plus hydration shell on the surface of the latex particles and (2) secondarily to low molecular weight soluble constituents in the latex serum phase. (These served as physical barriers preventing the close approach of particles on evaporation of the water). Such a mecha-
Flaw 0.
Tim Hoc results on redfspaslble latex no. 124-1.
nism would satisfactorily explain the strong pH dependence of film redispersibility found here and by others8 (if it is assumed that a certain degree of neutralization of the carboxyl groups was needed to render the polymer particles hydrophilic) and the partial losa in redispersibility after dialysis of the latexes. The results of studies carried out to confirm this mechanism will be given in another communication. A t present, there is no theory which deals with the redispersibility or repeptization of hydrophobic colloids. However, in a recent assessment of this problem by Overbeek,'* it was stated that redispersibility would be favored by a low Hamaker constant, a finite distance of separation between particles in the flocculated state, and a high surface charge density (or potential) as was found here. Acknowkdgment. We are grateful to the Dow Chemical Company for permiasion to publish this paper, to Dr. R. A. Lauzon, a former colleague, for many helpful suggestions, and to Drs. J. F. Vitkuske and D. S. Gibbs whose earlier work on redispersible latexes at Dow contributed much to our understanding of this system. References and Notes (1) V. D. FlorisandR. A. Mak. Paper Rsaantedat lhe 129m Waethg of Uw American Chemlcal Soclsty. Dabas, Tex.. Apll. 1958. (2) G. 0.Marlson. U.S. Patern 2800463 (1057). (3) L F. (w8k and W. N. Wadey. J. &p/. Pc+m. Sd..7,2249(1983). (4) W. J. Bray. U S . Patent 3 104234 (1983). (5) J. C. h a . U.S. Patent 3409578 (1988). (8) C. P. Sung. U.S. Patent 3904587 (1975). (7) H. Small. J. CWkM Interface Sci.. 48, 147 (1974). ( 8 ) E. W. O m . J . CdM Interfaca Sci.. 43, 449 (1973). (9) D. A. Kangas. paper Resemed at me 1 7 M Mssanpotme Amrfarn Chemlcal Saclely. San Franslsca Caln.. Aug. 1976. s as,393 (10) E. W. cleen,and F. L. S a m , J. G&ij I ~ c Sd.. (1970). (11) Y. L. Wang. J. CoWInferleca Sd.. 32. 633 (1989). (12) J. Th. G. overbsek. J . &MU Imerface Sci.. 50, 408 (1977).
