Langmuir 1996,11,4712-4718
4712
Water-in-Alkyd-ResinEmulsions: Droplet Size and Interfacial Tension P. Aurenty,t>SA. Schroder,?and A. Gandini*$t Laboratoire de m n i e des Prockdks Papetiers, URA 1100 CNRS, Ecole FranGaise de Papeterie et des Industries Graphiques (I.N.P.G.), B.P. 65, 38402 Saint Martin d'Hi?res, France, and Polychrome France, B.P. 116, 92164 Antony, France Received March 10, 1995. I n Final Form: August 30, 1995@ Ultrasound and dynamic shear measurements were used to investigate nonintrusively a water-inalkyd-resin emulsion in which the resin was a typical highly viscous component of an offset-ink vehicle. The aging of the emulsion was studied over 40 days and an increase in the size of the suspended water droplets was observed. The value of the mean radius R of these droplets was calculated from ultrasonic attenuation data at three stages of the emulsion aging, in the frequency range 600 kHz t o 2.5 MHz. At each stage, the value of the ratio between the interfacial tension y and the mean droplet radius R was obtained from dynamic shear measurements data in the frequency range 10-230 Hz. The values ofR and y were found to be in good agreement with the corresponding results obtained by conventional techniques. The merits of the concomitant application of the two nonintrusive techniques are emphasized in terms of the possibility of characterizingquantitatively the features of the highly viscous emulsions encountered in lithography.
Introduction The in situ characterization of the physicochemical properties of water-in-oil emulsions in which the continuous phase is a highly viscous liquid is of particular interest in the field of offset lithography. This printing process is based on the mutual repellency between a hydrophobic ink and an aqueous "fountain" solution. On the offset aluminum plate, the physicochemical interactions between the different interfaces promote a partition between the ink which wets the image area (a photopolymer layer of low surface energy), and the fountain solution which spreads onto the nonimage area (a thin film of aluminum oxide of high surface energy on an aluminum substrate) lying on the exposed surface. Both ink and fountain solution are submitted to high pressures and shear rates during the ink transfer through the roller nips. This process is responsible for the emulsification of a certain amount of water in the ink. It follows that in a steady state regime, the image areas of the plate become covered with a water-in-ink emulsion. The properties of this emulsion strongly influence the ink transfer on the press and also-theprint quality.' The radius of the aqueous droplets and the interfacial tension between the fountain solution and the ink are two parameters of primary importance.' Several techniques have been used for the determination of the size of the water droplets. Measurements using a time-size mapping system carried out on samples in which the viscous inkvehicle had been diluted with xylene2were limited to the determination of droplet diameters higher than 0.5 pm and gave size distribution curves centered around a few micrometers to several tens of micrometers depending on the mode of preparation of the emulsion. Optical microscopy can provide information about the droplet size without the need to dilute the vehicle and
* To whom correspondence should be sent, Alessandro.Gandini@ efpg.grenet.fr. Laboratoire de Genie des Procedds Papetiers. Polychrome France. Abstract published in Advance ACS Abstracts, November 15,
*
+
@
1995. (1)Fetzko, J. M. A literature survey,
Lehigh University, National Printing Ink Research Institute, Inc.: New York, 1986;part 1, 1988; part 2. (2) Chambers, D. Ph.D. Thesis, London University, 1994.
0743-7463/95/24l1-4712$09.00/0
values of a few micrometers have been determined,3s4but again of course this technique does not detect submicrometer droplets. The only reported way to characterize the latter type ofemulsion is scanning electron microscopy, which has been applied to cryofiactured system^.^ Likewise, various techniques have been used to determine the interfacial tension between an aqueous solution and a highly viscous ink. The most suitable method appears to be the determination of the dimension of a pendant water drop suspended at the end of a tube plunged into the vehicle.6 Other classical methods call upon the use of the Wilhelmy plate or the Du Nouy ring7-9 but require the dilution of the ink with a nonpolar organic solvent. The values of the interfacial tension are then obtained by extrapolation to zero dilution. This procedure leaves some doubts about the reliability of the figures obtained because of the quantitative aspect related to a correct extrapolation and the qualitative problem related to the real change in the polarity ofthe interface associated with the dilution. The same doubts apply to the results obtained by the spinning drop method which also requires a dilution procedure.1° A n alternative approach to the estimation of the interfacial tension consists of carrying out calculations based on the geometrical mean11J2or the harmonic mean method,I3 but the validity of the procedure related to the calculation of the nondispersive contribution to this parameter has been questioned.14J5 Moreover, these (3)Rosenberg, A. Proccedings of the 18th IARIGAI Conference; Banks: London, 1985;p 264. (4)Braun, F. Am. Ink Maker 1986,2,26. (5) Bassemir, R. W. TAGA Proc. 1992,365. (6)Andersen, P. W. Int. Bull. Printing Allied Trades 1968,6, 109. (7)Laraignou, R. Proccedings of the 7th IARIGAI Conference; Banks: London, 1963;p 411. ( 8 ) Bitter, J. H. Int. Bull. Printing Allied Trades 1966,1, 24. (9)Karttunen, S.;Lindquist, U. Graphic Arts Finland 1978,2,4. (10)Karttunen, S.;Virtanen, J.; Lindquist, U. Proccedings of the 18th IARIGAI Conference; Banks: London, 1985,p 241. (11)Owens, D. K.;Wendt, R. C. J.Appl. Polym. Sci. 1969,13,1741. (12)Kaelble, D. K.In Physical Chemistry ofAdhesion; Wiley: New York, 1971. (13)Wu, S. In Polymer Interface and Adhesion; Dekker: New York, 1982. (14)Fowkes,F. M.; Riddle,F. L.;Pastore, W. E.; Weber,A.A.Colloids Sui$ 1990,43,367. (15)Strom, G.; Vanderhoff, J. W. Proccedings of the 17th IARIGAI Conference;Banks: London, 1984;p 229.
0 1995 American Chemical Society
Water-in-ResinEmulsions
Langmuir, Vol. 11, No. 12, 1995 4713
Table 1. Physical Properties of Water and Alkyd Resin parameter waterz4 0.998 density p ( g ~ m - ~ ) 1497 sound velocity, c (mess1) 4.18 107 specific heat, C, (erg."C-'*g-l) 5.9 x 104 thermal conductivity, K (erg."C-l*cm-l*s-l) 2.4 x 10-4 thermal expansion coeff, /3 ("C-') 23 x 10-17fz (finHZ) sound attenuation, a (cm-') 0.98 10-3 shear viscosity around 1MHz, 7 (Paas) a
alkyd resin" 1.026 1511 1.91 107 1.7 x 104 7.5 10-4 3.2 x 10-9p.36(fin Hz) 170f0.64 (fin Hz)
Determined in our laboratory.
methods require the values of dispersive and nondispersive contributions to the surface free energy of the ink as obtained from diluted samples. Finally, the presence of surface-activemolecules in both inks and fountain solutions introduces an additional difficulty because if the amount of additive available is not sufficient to establish the equilibrium surface concentration when passing from a modest interfacial area (e.g. with static experiments) to a very high interfacial area in the actual emulsion, the interfacial tension will increase accordingly. These unsolved questions prompted us to seek a novel approach to determine the average droplet size and the interfacial tension in situ and without dilution. The combined use of ultrasonic attenuation and dynamic shear measurements has such potential. In face ultrasonic techniques can be excellent nondestructive tools for the characterization of materials and have been applied successfully to biphasic liquid media. With suspensions of spherical particles and more specificallywith emulsions, several theoretical and experimental investigations have shown that the ultrasonic attenuation is related to the radius R of the suspended spheres.16 Furthermore this technique, coupled with an adequate theoretical development dealing with the multiple scattering ofwaves,17does not require any dilution of the emulsion. Several investigations of water-in-oil e m u l s i o n ~ ~and ~ J ~polymerspheres-in-water suspensionsz0have shown that R values within the range ( ~ 5 nm, 0 %5pm) can be measured using the appropriate ultrasonic technique. In the present context, we measured the ultrasonic attenuation of water-in-alkyd-resin emulsions using a classical ultrasonic spike transmission technique and applied the theoretical formulation elaborated by Fukumotozlto calculate the mean radius R ofthe water droplets. Dynamic shear measurements have proved useful to determine the ratio ofthe interfacial tension y to the radius R of the inclusions in polymer-polymer emulsions.22 PalierneZ3has provided an equation applicable to spherical particles in a viscoelastic medium. This theoretical treatment relates the relaxation time of the dispersed droplets under shear to the ratio ylR. In the present study the shear modulus ofour emulsions was measured at different frequencies, and the ensuing values were introduced in Palierne's equation in order to calculate ylR. This ratio combined with the R values determined by ultrasound provided the corresponding values of the interfacial tension between the droplets and the matrix. To our knowledge this is the first time that the two above nonintrusive techniques have been used in conjunction to characterize an undiluted emulsion of water in a highly viscous medium. Materials and Methods The Emulsions. The alkyd resin, kindly provided by Sunchemical, was based on a typical composition resulting from the polycondensationof isophthalic acid, a polyol, and vegetable fatty acids. Its molar mass was determined by vapor pressure
osmometry as M , = 3470. This resin exhibited a Newtonian behavior a t low shear rates and its viscosity at 21 "C was 160 Pa s. Most physical properties needed for the equation of the ultrasound attenuation were taken from the literaturez4 or determined in our laboratory and are given in Table 1. Preparation of the Emulsions. A Dispermat CV homogenizer was used to disperse 20% (w/w) water in the alkyd resin. The stirrer's diameter was 40 mm and its speed was set at 6000 rpm. The alkyd resin was stirred for 30 s before adding the water dropwise during 15 s. The two liquids were homogenized for 3 min, manually for 1min more, and finally for 3 min more at 6000 rpm. During emulsification, the temperature rose to about 50 "C. Aflotation period of 5 days was necessary to rid the emulsion of air bubbles. We verified that no macroscopic phase separation occurred within this period and indeed for several weeks. The Ultrasonic Technique. Theory. There are a variety of theoretical formulations that describe ultrasonic propagation through suspensions of spherical p a r t i ~ l e s They . ~ ~differ ~ ~ ~ ~ ~ ~ ~ ~ from one another by the mathematical approach used, but the relevant point common to all is that the ultrasonic attenuation a of the suspension can be expressed as the sum of three terms, viz.
where 4 is the volume fraction of the dispersed phase, a1 and a2 are the intrinsic attenuation of the continuous and dispersed phases, respectively, and 6a is an "excess attenuation" in the emulsion corresponding to scattering of the ultrasonic wave a t the interface between the continuous matrix and the suspended droplets. If both matrix and droplets have similar density and sound velocity, as was indeed found in our system, the acoustic impedance is also similar in both media. It follows theoretically16 and experimentallyz0~z7 that the major factor influencing the excess attenuation 6a is a thermal dissipation effect of the ultrasonic wave. In this work we used the theoretical formulation derived by Fukumoto21which only takes into account the thermal dissipation of waves. This model expresses 6 a for an emulsion containing a volume fraction 4 of monodisperse droplets of radius R as
where f is the ultrasonic frequency, TO(K) is the temperature, po is the density of the continuous phase, and A is given by 21 (16)Harker, A.H.; Temple,J. A. G. J . Phys. D:Appl. Phys. 1988,21, 1576. (17)Lloyd, P.; Berry, M. V . Proc. Phys. SOC.1967,91,678. (18)McClements, D.J.;Povey, M. J. W. J.Phys. D:Appl. Phys. 1989, 22,38. (19)Urick,R. J.; Ament, W . S. J.Acoust. SOC.Am. 1949,21,115. (20)Allegra, J. R.; Hawley, S. A. J.Acoust. Soc. Am. 1972,51,1545. (21)Fukumoto, Y.;Yzuyama, T. Phys. Rev. A 1992,46,4905. (22)Graebling, D.;Muller, R.; Palieme, J. F. Macromolecules 1993, 26,320. (23)Palierne, J. F. Rheol. Acta 1990,29,204. (24)Handbook of Chemistry and Physics; CRC Press: Cleveland, OH,1973. (25)Isakovitch, M. A.Zh. Eksp. Teor. Fiz. 1948,18,907. (26)Epstein, P. S.;Carhart, R. R. J.Acoust. SOC.Am. 1963,25,553. (27)Schroder, A.;Raphael, E. Europhys. Lett. 1992,17,565.
Aurenty et al.
4714 Langmuir, Vol. 11, No. 12, 1995
~ ~ -{i)nfl. ( l - tanh[(l - i ) n f l l }
+
K~
tanh[(l - i ) n g I [ ( l- i)n,a + 11 (3)
no4,
I ,
-
&
I
&
1
I
L &
whee n, = ( W ~ ~ C , , J ~and K ~ subscripts ) ~ 1and 2 relate to matrix and inclusions. The other physical parameters of both media are given in Table 1. Equation 2 applies to dilute systems and is simpler than the corresponding expression for concentrated ones which take into account multiple scattering.21 Figure 1shows that in our context droplet-droplet interactions were negligible, whichjustifies the use of eq 2 to calculate 6a. One important point that is scarcelydiscussed in the literature is the effect of the polydispersity of the droplet size on the ultrasonic attenuation of the emulsion. We carried out a simulation by simply adding the contribution of each population size to a as shown in Figure 1. For this we used a Gaussian function for the droplet size distribution in our emulsion with a ratio between the average-volume and average-number radii, R,IR, = 1.4, as determined by optical microscopy (Figure 6 ) .In the frequency range of our measurements, the effect of polydispersity was limited to an overall increase of less than 1%in the values of the attenuation without any modification of the slope ofthe rectilinear plot. We thus decided to treat all the data presented in this paper using eq 1relative to a monodisperse distribution in a dilute medium. Experimental Section. All velocity and attenuation measurements were performed at 21 “C using an ultrasonic cell designed in our laboratory. This instrument was basically a typical broad band spike transmission cell whose principle is described elsewhere.28 A block diagram of the instrument is shown in Figure 2. Two identical circular piezoelectricceramic elements (center frequency of 2 MHz) were used as emitter and receiver, respectively. They were aligned coaxially and each was coupled with an “ultrasonic gel” to a brass buffer rod which constituted one wall of the cylindrical cell. Each buffer rod played the role of a delay line for the ultrasonic impulse and was thermostated to h0.05 “C using water circulation. A pulse generator designed in our laboratory gave 2 ps 5-V pulses (internal center frequency of 1MHz) every 100 ms and was connected to the emitter transducer. The ultrasonic impulses produced by the emitter undertook the following complex pathway: (i) traveled through the first buffer rod, (ii) were partially transmitted a t the brasdiquid interface, (iii)traveled through the liquid in the cell, (iv)were again partially transmitted a t the liquidbass interface, (v)traveled through the second rod, and (vi) were finally detected by the receiver. The signal thus received was sampled with a digital oscilloscope (Philips 3350) and stored in the memory of a PC 486 using a General Purpose Interface Bus (GP-IB). The cell geometry was designed so that the reception signal was well-defined in time (about Bps duration) thus allowing both time arrival measurements and fast Fourier transform (FFT) analysis. The measurement of the time offlight ofthe ultrasonic impulse was used to calculate the velocity of sound c in the liquid under study. For this purpose, the assembly was first calibrated with several homogeneous liquids which gave c value within 2% of the corresponding values reported in the literature?$ with a reproducibility of hO.l%. The FFT analysis of the reception signal was carried out by scanning the frequency range 600 lrHz to 2.5 MHz. The ultrasonic attenuation coefficient a of the emulsions was calculated by comparing their transmission spectrum with the reference spectrum obtained with only water in the cell (the energy losses due to partial reflections ofthe sound waves a t each liquidhrass interface were taken into account by calculating the impedance 2 = p*c of each liquid studied). Using various liquids, the technique was found to give an absolute experimental error of about hl%. Furthermore, once the cell geometry and the signal analysis parameters were fxed, the relative error corresponding to the reproducibility of measurements was found to be about (28)Methods of experimental Physics, Ultrasonics; Edmonds, P.D., Ed., Academic F’ress: New York, 1981; Vol. 19, Chapter 2. (29) Anson, L. W.; Chivers, R. C. Ultrasonics 1989,28,16.
x
monodisperse
+
multiscattering polydisperse
A
& & I I
*I I
I I
I ;Ir
Frequency (Hz) Figure 1. Calculated ultrasonic attenuation versus frequency using the parameters given in Table 1: (i) monodisperse distribution (R= 2pm) usingeqs 1and 2; (ii) same monodisperse distribution using Fukumoto’s equation dealing with multiple scattering effects;21(iii) polydispersedistribution with a volumeaverage radius R, = 2 pm and a ratio between the volumeaverage radius and the number-average radius RJR, = 1.4. Note that the three sets of data points are indistiguishible.
Figure 2. Schematic representation of the ultrasonic cell: (a) piezoelectric elements; (b) brass buffer rods; (c) measurement cell. All dimensions are given in cm. &O. 1%.The latter figure is highly relevant to the present context as discussed below. Dynamic Shear Measurements. Theory. Dynamic shear measurements have been carried out on different types of emulsion^.^^^^^ The general trends observed are an increase of the elastic modulus at the low frequencies and comparatively long relaxation times. These features have been rationalized theoreticallyZ3J1and related to the value of the ratio ylR of the emulsion. In a shear experiment the droplets are subjected to deformation of their spherical shape. The parameter ylR can be considered as a n elastic stress which acts on the deformed droplets to restore their original shape. Morphological observations of sheared emulsions have shown that the time required for such (30)Eshuis, A.; Mellema, J. Colloid Surf. Sci. 1984,262,159. (31)Oldroyd, J. C. Proc. R. SOC.London, Ser. A 1953,218, 122.
Langmuir, Vol. 11, No. 12, 1995 4715
Water-in-ResinEmulsions a return when the shear stress is stopped is of the same order of magnitude as the mechanical relaxation time obtained from shear dynamic Oldroyd's model31 describesthis phenomenon for dilute emulsions and, in a more general context, P a l i e ~ m eextended ~~ that mathematical treatment to a concentrated emulsion of two viscoelastic fluids, including the possible presence of an interfacial agent. The latter author took into account the fact that the interfacial tension depends on both the shear deformation and the variation of the interfacial area. The mechanical interactions between droplets were treated in the same way as the Lorentz spheres in electricity. In our case, since we did not use surfactant, the interfacial tension is obviously independent of the variation of the interfacial area and ofthe local shear.22Taking into account amonodisperse distribution of the droplet size, the resulting complex shear modulus of the emulsion is therefore obtained as a function of the morphology and dynamic properties of the biphasic system (volume fraction of the inclusions, interfacial tension, droplet size, complex shear modulus of each phase). Palierne's equation then takes the form
L
-1
100
10
Frequency (Hz) Figure 3. Elastic and viscous moduli versus frequency, of the pure alkyd (Gm) and G ( A ) ) and of the emulsion E l (experimental data, G(+) and G ( x ) ; best fits according to Palierne's equation: solid lines).
(4)
where G*(o)is the complex modulus of the emulsion, Gi*(o) is the complex modulus of the dispersed phase, G,*(w) is the complex modulus of the matrix, R is the radius of the droplets, 4 is the volume fraction of the droplets, and y is the interfacial tension. Very good correlations have been obtained between the theoretical predictions and the experimental results related to different emulsions oftwo polymers.22It was shown in particular that shear dynamic measurements could be used to estimate the interfacial tension in the emulsion if the size of the droplets was known. The effect of the polydispersity of the droplets' size was investigated and shown to be negligible22if the distributions with a given R, had a ratio between the volume-average radius and the number-average radius, RJR,,lower than 2. We chose therefore to fit our experimental data to Palierne's equation4 for a monodisperse distribution of the droplets' size and assumed that the interfacial tension was independent of the variation of the interfacial area during the deformation. The values of ylR could thus be determined. Experimental Section. The dynamicshear measurements were carried out at 21 "C on a Metravib viscoelasticimeter used in the frequency range 10-230 Hz. The bottomless annular pumping geometry was chosen in order to apply pure shear deformations. The amplitude ofthe oscillations was 50pm and the annular gap in which the sample was sheared was set a t 0.5 mm. We verified that these conditions still corresponded to a linear viscoelastic behavior. The elastic and viscous moduli of the samples G and G were recorded and were reproducible with 15%. The effect of the presence of water droplets in the continuous phase is shown in Figure 3. Because of a nonzero interfacial tension, the elasticity of the emulsion was greatly enhanced a t low frequency. In that domain, the G curve flattened progressively because of the relaxation of the water droplets discussed earlier. The corresponding relaxation time fell within the early portion of our domain of measurements, viz. about 20 Hz. After the fit of the experimental data and the calculation with eq 4, the values of ylR could be deduced. Other Measurements. Contact angles (It2O)were measured at room temperature with an image analysis system developed in our laboratory. Surface and interfacial tensions were determined with a classical Wilhelmy plate apparatus. A standard Olympus optical microscope ( x 100)equipped with a camera was (32) Scholtz, P.;Froelich, D.;Muller, R. J.Rheol. 1989,33, 481.
used for the observation of the water droplets. The FTIR and 'H-NMR spectra were run on routine high-quality instruments.
Results and Discussion Once the emulsions were prepared, a portion w a s used to fill the ultrasonic cell, while the remainder was put aside for furtehr shear and optical measurements. The observation of the evolution of the ultrasonic signal through the sample confirmed that all t h e air bubbles formed during the emulsification process had reached the surface within 5 days. However, a period of 13 days was allowed to pass before carrying out the first measurements. In fact, three series of measurement were performed respectively 13 (El), 26 (E2), and 35 (E31 days after emulsification. At each stage of aging, ultrasonic attenuation and velocity as well as shear measurements were performed and a set of optical microscope photographs of the emulsions w a s taken. Ultrasound. The first important observation was that the sound velocity did not vary with aging, i.e. remained the same for E l , E2, and E3. This implies that the volume fraction of inclusions stayed constant and therefore no creaming or sedimentation effect occurred during the whole period of our study. This does not m e a n however that these emulsions did not evolve. The attenuation data for samples E l , E2, and E3 a r e presented in Figure 4 which clearly shows that df2 decreased w i t h aging over t h e frequency range explored. For each series of measurements, the attenuation data were compared with calculated values determined from eq 1, using the parameters given in Table 1and assuming that the emulsified droplets were monodisperse. The best fit between experimental (data points) and calculated (solid lines) trends provided the corresponding values of t h e droplets' radius R , as shown in Figure 4 and given in Table 2. One c a n clearly see that aging is accompanied by an increase of t h e mean radius of the w a t e r droplets. This w a s confirmed by t h e observation of the emulsion samples u n d e r a microscope as shown in Figure 5. A fastidious visual classification of the droplet size w a s carried o u t and t h e ensuing distributions as a function of aging, shown in Figure 6, clearly confirmed the indirect conclusion d r a w n from ultrasound data. Note that for t h e two
Aurenty et al.
4716 Langmuir, Vol. 11, No. 12, 1995
lo6
3 lo6
Frequency (Hz) Figure 4. Ultrasound attenuation ofthe emulsion as a function of frequency. Effect of aging (experimental data, E l (x), E2 (+>, E3 (A);best fitsaccording to Fukumoto’se q ~ a t i o n s , l -solid ~ lines). Table 2. Mean Droplet Size and Interfacial Tension Determined by Ultrasonic and Shear Dynamic Measurements El E2 E3 1.9 (1.6) 2.3 2.6 (2.4) R h m ) (optical microscopy) 4390 3720 3230 ylR (N/m2) 8.3 8.6 8.4 y (mNlm) ~~
distributions the ratio between the volume-average and the number-average radii was less than 1.5. Moreover, a good quantitative agreement was obtained between the values of the volume-average radius R, measured from the microscopy pictures (Figure 6) and those calculated from eq 1 (Table 2). Whether the droplet size increases with time due to simple coalescenceor to diffusional pathways that deplete the emulsion of the smaller elements33cannot be ascertained at this stage, and indeed both phenomena might take place in our system. We did not dwell on this problem because it fell outside the main scope of the investigation given the very long time delays involved compared with the exposure times of emulsions on a printing press. Shear Dynamic Measurements. The shear experiments were performed on the emulsions a t the 3 aging times. On the one hand, an evolution of the elastic modulus of the emulsion was observed on the flattened part of the curve (10-30 Hz) corresponding to the shape relaxation of the droplets. This evolution has to be correlated with a variation of the ratio ylR because all other parameters remained constant during the aging period. The best fit of the three G curves using Palierne’s equation4 are shown in Figure 7 and gave the corresponding values reported in Table 2. On the other hand, a t higher frequency (above 100 Hz) the G curves tend to merge. The relaxation of the stress ylR does not occur because of the high frequency of the deformation, Le. the droplets lose their contribution to the viscoelastic behavior of the emulsion and should be visualized as “holes”in the matrix. Indeed, a t the higher frequenciesof our domain the G and G values were about 20%lower than those of the pure alkyd resin (Figure 3), (33) Tadros, T. F.;Vincent, B. Encyclopedia ofEmulsion Technology; Becher, P., Ed.; Dekker: New York, 1983; Vol. 1, Chapter 3.
Figure 5. Optical observations of the emulsions a t stage E l (a) and E3 (b).
5
n
25 20
El
1
/)
E3
15
10 5
0
0
0.5
1
1.5
2
2.5
3
3.5
4
Droplet radius (pm) Figure 6. Distribution of the water droplet radius in the emulsion at stage E 1and E3 as observed by optical microscopy.
consistent with the 20% volume fraction of water in the emulsion. The values of the interfacial tension y calculated from the ultrasound and dynamic shear measurements data are similar within experimental error, indicating that any specific orientation of the alkyd macromolecules at the interface and the mutual migration of component into the opposite phase (see below) must have been completed by the time the experiment were started.
Water-in-ResinEmulsions
Langmuir, Vol. 11, No. 12, 1995 4717 Laraignou7 for alkyd resins of different compositions, namely between 5 and 17 mN/m. The alternative approach proposed by several author~"-~ to~estimate interfacial tensions consists in calculating y from Owens-Wendt's equation which is an extension of Fowkes original work
using the dispersive contribution yd to the surface tension of the two pure liquids and their respective polar counterpart yp. The former is usually measured by contact angle experiments of the liquid deposited on a totally nonpolar surface. The latter can then be calculated as the difference between the measured total surface free energy and the dispersive contribution. In our case, the determination of the surface tension of the alkyd resin was conducted with a Wilhelmy plate between 80 and 120 10 100 "C in order to reduce the viscosity, and the values obtained Frequency (Hz) were extrapolated at room temperature. The equation obtained was y = 32.4 - 0.06T. This temperature Figure 7. Elastic modulus of the emulsions versus frequency. dependence is in total accordance with the data given in Effectof aging time (experimental data, E l (x), E2 (+), E3 (A); best fits according to Palierne's, eq~ation,~ solid lines). the literature for polymers in the melt.13 The surface tension ofthe alkyd resin at 20 "C thus obtained was 31.1 f 0.1 mN/m. In order to separate the dispersive and polar parts of this parameter, the classical contact angle method was used. A drop of the alkyd resin was deposited on a clean Teflon surface which was used as a purely dispersive E reference. Once the equilibrium was reached, i.e. typically within 7-8 min, the value of the contact angle was 60.9 E 20 f 1.7". The dispersive contribution to the surface tension c 0 was calculated as 25.4 f0.9 mN/m. The polar contribution 3 was then 5.7 f 1.0. The value of the interfacial tension between water and alkyd resin calculated using OwensWendt's equation5was 22.7 mN/m, which is considerably higher than both the values reported in the literature7 and those obtained with the two techniques used in this study (Table 2 and Figure 8). Given this apparent anomaly, we investigated the possible diffusion of a water-soluble fraction of the alkyd 5 80 100 resin into the aqueous phase. It was indeed reasonable to suppose that the alkyd resin used in this study might 0 20 40 60 have contained a fraction of monomers andor oligomers, % alkyd resin in toluene (wt/wt) e.g. the poly01 and its condensation products with isophthalic acid, which were soluble in water. It could be Figure 8. Interfacial tension between water and solutions of expected as a consequence that the diffusionofthis fraction the alkyd resin in toluene versus concentration. into the aqueous phase would induce a decrease in the surface tension of water and therefore a corresponding The Value of the Interfacial Tension. In order to decrease in the interfacial tension between the resulting assess the meaning of the y value determined in situ, we aqueous solution and the alkyd resin. carried out some experiments using two classical methods of characterization of the interfacial tension between water In order to test this possibility we dissolved a sample and highly viscous polymers. of alkyd resin into methylene chloride and shookvigorously The Wilhelmy plate method is one of the most used this solution with water in a separating funnel. After techniques for the determination of interfacial tensions decanting, we collected the aqueous fraction and vacuum between water and highly viscous l i q ~ i d s . ~The - ~ common evaporated the water. The residue was an oily liquid practice is to dilute the resin with a nonpolar solvent at which clearly showed the presence of glycerol and isovarious concentrations and to extrapolate the y values phthalic glycerides when analyzed by FTIR and lH-NMR spectroscopy. obtained to zero dilution. We determined the interfacial tension between water and alkyd resin using this approach A second test corroborated this positive evidence, namely the measurement of the surface tension of a water sample with toluene as diluent. The results are presented in Figure 8. As shown in the classical work of L a r a i g n ~ u , ~ (20%) separated afier 4 h of vigorous shaking with the the interfacial tension seems to reach a constant value alkyd resin (go%), which gave 55.0 mN/m. If this value above 30% alkyd in toluene. With a value 12.6 mN/m for reflects the real surface tension of the aqueous phase in a 57% solution, it seems reasonable to postulate that the the emulsion, then it is possible to calculate the resulting interfacial tension between the alkyd resin and water lies interfacial tension using Owens-Wendt's equation5 in which the dispersive contribution of the aqueous phase is between 10 and 12 mN/m. This value is in accordance kept at its nominal value for water. This calculation gave with the corresponding result obtained from ultrasound 11.6 mN/m, in excellent agreement with the values and shear dynamic measurements (Table 2). Moreover, determined experimentally. It can therefore be safely this result falls within the range of values obtained by
R
-
25L
.I
3
4718 Langmuir, Vol. 11, No. 12, 1995
concluded that water-soluble fractions of the alkyd resin can migrate into the emulsified water and that this can be an important phenomenon with respect to the ensuing properties of the interface. Such a behavior should be borne in mind in future studies on this type of system.
Conclusion The combination of ultrasonic and dynamic shear measurements provides a novel, noninvasive way to determine the interfacial tension and the particle size in water-in-oil emulsions. Aging effects and the diffusion of hydrophiliccomponents into the aqueous phase can readily be detected. Results are in reasonable agreement with those obtained by more cumbersome techniques such as optical microscopy coupled with droplet counting and classical tensiometry requiring multiple dilutions. The
Aurenty et al. next step in this investigation is the extension of this experimental approach to systems reflecting the actual lithographic process, namely using real ink vehicles, and fountain solutions as opposed to the simple model emulsion studied here. Such studies are in progress and should constitute a helpful complement to other ways34 of unraveling the complex phenomena encountered in the lithographic process.
Acknowledgment. We are grateful to Polychrome for generous financial support and to J. F. Le Nest and J. F. Palierne for stimulating discussions. LA950191X ~
(34)Morantz, D.Colloid Surf. 1994, A92, 221.
