Langmuir 1994,10, 2619-2628
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Role of Water in Particle Deformation and Compaction in Latex Film Formation P. R. Sperry,*B. S. Snyder, M. L. O’Dowd, and P. M. Lesko Rohm and Haas Company, 727 Norristown Road, Spring House, Pennsylvania 19477 Received January 3, 1994. In Final Form: May 19, 1994@ The particle deformation and compaction stage of latex polymer film formation was investigated via minimum film formation temperature (MFT) measurements; variables included polymer composition, particle size, time, and, especially, the water content of the deposited film and in the drying environment. The water content of the system ranged from very low (latex film predried well below the MFT before imposition of the temperature gradient, with low relative humidity maintained throughout) to very high (wet latex cast on the gradient bar and high humidity maintained during drying). A film predried well below the MFT-a turbid deposit owing to interparticle voids-exhibits a “dryMFT”transition, from turbid to clear film, as it is heated. With hydrophobic polymer compositions, the dry MFT is virtually identical to that from a wet casting, indicating that so-called capillary forces associated with the presence of liquid water have little if any role. With hydrophilic compositions the “wet MFT” is lower than the dry MFT by as much as 10 “C or more; this is ascribed simply to plasticization by water. Dry MFT values decrease with log time similar to the WLF glass temperature-time shift, consistent with viscoelastic relaxation driven by interparticle van der Waals attractive forces/polymer-air surface tension such as described by the Johnson-Kendall-Roberts model of particle adhesion and deformation. For a given polymer composition, the dry MFT data correlate with a simple model of surface tension driven collapse of the interstitial voids.
Introduction The particle compaction or deformation step in latex polymer film formation during drying (Figure 1) has received continuous theoretical and experimental attention since the seminal work of Bradford and co-workers in the early 1 9 5 0 ~ that ~ 1 ~ modeled film formation as a Frenkel viscous flow of contacting polymer spheres under polymer-air and/or polymer-aqueous-phase interfacial tension. Shortly thereafter Brown3 made compelling arguments that the role of liquid water was not only contributory but central to the deformation process. The principal force was proposed to be capillary compression of the particle assemblage as water evaporated, controlled by the latex serum-air surface tension, against the deformation resistance of the polymer characterized by its (visc0)elasticity. Virtually all work since has comprised attempts to variously refine, extend, verify, or refute Brown’s theory and premises and to propose alternatives, but forces arising from surface energies involving aqueous latex serum persist in the m ~ d e l s . ~ - It l ~ is rather surprising that extremely little attention has been given to comparative examinations of the compaction step in the absence as well as the presence of water. We quote some of Brown’s arguments for immediate reference because they are so historically connected to Abstract published in Advance A C S Abstracts, July 15,1994. (1)Dillon, R. E.;Matheson, L. A.; Bradford, E. B. J. Colloid Sci. 1951,6,108. (2)Henson, W.A.;Tabor, D. A.; Bradford, E. B. Ind. Eng. Chem. 1953,45,735. ( 3 )Brown, G. L. J. Polym. Sci. 1956,22,423. (4) Brodnyan, J. G.; Konen, T. J. Appl. Polym. Sci. 1964,18,687. (5)Sheetz, D. P. J. Appl. Polym. Sci. 1965,9,3759. ( 6 )Bertha, S. L.; Ikeda, R. M. J. Appl. Polym. Sci. 1971,15,105. (7)Vanderhoff, J. W.;Bradford, E. B.; Carrington, W. K. J. Polym. Sci.: Symp. No. 4 1973,155. ( 8 )Mason, G. Br. Polym. J . 1973,5,101. (9)Lamprecht, J. Colloid Polym. Sci. 1980,258,960. (10)Kendall, K.;Padget, J. C. Int. J . Adhes. Adhes. 1982,2,149. (11)Eckersley, S.T.;Rudin, A. J. Coat. Technol. 1990,62(780),89. (12)Dobler, F.;Pith, T.; Lambla, M.; Holl, Y. J.Colloid Interface Sci. 1992,152, 1. (13)Dobler, F.;Pith, T.; Lambla, M.; Holl, Y.J.Colloid Interface Sci. 1992,152,12. (14)Kan, C . S.TAPPI Notes, Adu. Coat. Fundam. 1993,101.
Figure 1. Particle deformatiodcompactionstep in film formation.
@
mechanism development and comprise the motivation for our own studies. (1)“Itis observed that film formation in many polymer emulsion systems occurs concurrently with the evaporation of water and is complete when water evaporation is complete. In this case, the surface tension of the polymer could not supply the driving force. Rather, this would be replaced by the polymer-water interfacial tension, which in the presence of the emulsifier would in general be lower in value than the surface tension [counteringBradford et By analogy with oil-water systems which can be measured, it may be in the range of 0-10 dynes/cm,rather than about 30 dynes/cm as postulated by these workers. In addition, it would be remarkable indeed if the times required for water evaporation and particle coalescence ~
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2620 Langmuir, Vol. 10, No. 8, 1994
were so comparable for a wide variety of polymers unless the processes were interdependent.’’ (2) “It may demonstrated experimentally that the rate of water removal may determine the coalescence of polymers of borderline “filmability”. This rate may be altered by changes in substrate capillarity or in the relative humidity of the surrounding atmosphere.” (3)“Porous,incompletely coalescedfilms may be formed from many polymers simply by maintaining, during water evaporation, a temperature lower than a certain critical value. It is observed that for certain polymers a higher temperature exists which is insufficient for coalescenceof the porous structure previously formed a t a lower temperature, but is adequate for complete coalescence if applied during the entire course of water evaporation. In addition to the plasticization of polymers by the water, the water exerts a strong force responsible for coalescence. The role of water in the process is of extreme importance.” (4) “Lightly crosslinked emulsion polymers may form films which are continuous, though lacking in strength. Purely viscous flow is not possible in such a system, and presumably the particles have deformed and the surface cohered, but interpenetration by self-diffusion has not occurred. Thus, film formation is possible in the absence of purely viscous flow.)) Argument 3, in particular, is very strongly stated. Brown offers no experimental verification for debate, although many practitioners in latex technology might be inclined to agree with the observation, albeit without real proof that the phenomenon described is in fact not simply a hydroplastic effect. de la Court16has called attention to the potentially significant role of hydroplastic effects on polymer moduludviscosity reduction in film formation, and the relation of these effects to drying conditions such as humidity and air velocity. He also made the important suggestion that efforts to address fundamental film formation mechanisms should be done with waterinsensitive or hydrophobic polymer compositions to avoid these very ambiguities of interpretation. Recently, Dobler et al.13 did compare the results of “coalescing”a styrenehutyl acrylate-based latex, of 40 “C glass temperature, a t 36 “C under three conditions: (a) under water (presumably by forces of polymer-water interfacial tension), (b) “standard conditions” of drying in air at 94% relative humidity (RH), and (c) a t 94% RH a “packing” of particles from which water had been preremoved at 23 “C/O%RH. The film formed under condition a was still opaque after 2 h while that formed under condition b was clear. The predried deposit under condition c was also opaque after 2 h, but became slightly transparent after 24 h. The authors concluded that coalescence under standard conditions is due neither to particle-air interfacial tension nor to particle-serum interfacial tension. “Under standard conditions, deformation ofparticles results from forces specifically connected to evaporation ofwater. This confirms the early suggestion of Brown.” l3 We would not be so firm in conviction as Dobler et al.,noting that the predried deposit does in fact tend toward transparency given adequate time. Just a few degrees increase in temperature might well have accelerated the coalescence toward the time scale observed for their so-called standard conditions because of the extremely high temperature sensitivity of polymer viscoelastic compliancein the temperature region where latex polymers do form films. We think also that there is a substantial possibility that the desiccated particle mass may not have readily reequilibrated to the same internal water content, and therefore to the same degree of (15) de la Court, F. H. FATZPEC Congr. 1970,10,293.
Sperry et al. hydroplastic effect that would lower the modulus, as when it was deposited from water under standard conditions. Kan14has recently criticized much of the earlier work that had attempted to verify the criteria for film formation of Brown and others on the basis that the values of the polymer modulus used were obtained on dry films and not in the water-saturated environment of the film formation measurement, and moreover failed to use the proper time-dependent viscoelastic modulus, Le., that corresponding to the film formation time. Kan’s experiments on several styrenehutadiene latices, taking those effects into account, led him to conclude that his results better fit the Johnson-Kendall-Roberts theory16 of contact adhesion forces, operating under polymer-aqueous serum interfacial tension, than those of Brown and others. Our experiments consist mainly of minimum film temperature (MFT)17measurements of latices using a conventional temperature gradient bar apparatus, but in which a principal variation includes observing, like Dobler et al.,13 the coalescing behavior of packed but nondeformed particle deposits obtained by casting and drylng latex films well below the MFT. Latices of particle diameter on the order of 200 nm and greater will dry below their MFT to yield deposits that are visually turbid due to light scattering from the interparticle voids. van Tent et al.1s-20 have recently described the quantitative application of light scattering to characterize several of the elements of the film formation process, including the deformation/ compaction step.
Experimental Section k Latex Polymers. Latices (Table 1)were prepared using persulfate initiator at 85 “C and 0.2% sodium lauryl sulfate surfactant based on the weight of monomers and contained 1w t % acrylicacid (AA)in the monomer chargeto aid colloidalstability. A semicontinuous process was used to favor random copolymerization. Polymer solid contents were approximately 48% by weight. A small amount of ammonium hydroxide was added to adjust the pH to about 6, and the latices were used as such. We have seen no significant differencethus far in results from those furtherneutralized to about pH 9. Particle sizeswere determined by photon correlation spectroscopy using a Brookhaven BI-90 instrument. The percent sol was determined by solvent extraction with tetrahydrofuran (THF). Molecular weights were determined by gel permeation chromatography, using reagent grade THF as the solvent, a Polymer Laboratories Type A (20pm) column, and poly(methy1methacrylate) calibration standards. Glass transition temperatures were obtained by differential scanning calorimetry (DSC) using a DuPont Model 912 instrument and a 20 “C/min heating rate. The half-height midpoint in the heat flow vs temperature curve was selected as the Tg. B. MFT measurements. Measurements were performed on a Sheen Instruments, Ltd., “MFFTBar 90”. The instrument is designed to permit MFT measurement according to ASTM D-2354, in which wet latex is cast on the bar or plate with a preimposed and equilibrated temperature gradient and (supposedly) dry air flow as detailed in Table 2. The ASTM test defines MFT as the higher of the temperatures of the transition from turbid or cracked film to clear and coherent film. Where possible, we have identified both temperatures, but our main interest is in the turbid-to-clear transition as the indicator of polymer deformation and compaction. The turbid-to-clear tran(16)Johnson, K. L.; Kendall, K.; Roberts, A. D. Proc. R. SOC.London, A 1971,324,301. (17)Though there is increasing tendency to use the abbreviation MFFT, we use MFT for minimum film temperature in accordance with the originators ofthe measurement technique [Protzman, T. F.; Brown, G. L. J.Appl. Polym. Sci. 1960,4,811,and ASTM D-2354. (18) van Tent, A. Thesis, Technical University of Delft, 1992. (19) van Tent, A.; te Nijenhuis, K. J.ColloidInterfuceSci. 1992,150, 97. (20) van Tent, A.; te Nijenhuis, K. Prog. Org. Coat. 1992,20, 459.
Role of Water in Particle Deformation and Compaction
Langmuir, Vol. 10, No. 8, 1994 2621
Table 1. Latex Characterization0 sample composition (wt %) diam (nm) % sol (THF’) MJMn (GPC, x 1000) Tg(DSC,midpoint)(“C) EMS 340 47 EM52 W1 AA 340 100 55454 50 BNS 530 35 BM64 SI1 AA 530 100 241/44 48 43 EM56 M W 1AA 360 90 nm 47 EA/MMA 360 -. BA/MMA430 37 BM62 M W 1AA 430 60 nm 44 84 B W 1 5 M W 1AA BAhfMMA 540 540 100 404155 50 BMA 135 99 B W 1 AA 135 37 99 B W 1 AA BMA 350 350 35 99 B W 1 AA 466 BMA 466 35 99 B W 1AA BMA 565 100 480l109 565 36 99 B W 1 AA BMA 700 700 36 43 EN56 SI1AA ENS 533 533 54 VAc 310 VAc 310 42 a Abbreviations: AA = acrylic acid,EA = ethyl acrylate, BA = n-butylacrylate, BMA = n-butylmethacrylate,MMA = methyl methacrylate, S = styrene, VAc = vinyl acetate, GPC = gel permeation chromatography, nm = not measureable. Table 2. Experimental Conditions for MFT Measurements Conventional (ASTMD-2354-86) ”In this test method the minimum film formation temperature (MFT) is determined by visual observation o crackin or whitec;f in films that have dried over a sukE€Fii&ng a con ro ed temperature gradient.” latex filmed on gradient, chamber is covered, dry air purge (4 Um) Special Conditions (WhiteningTransition Observed) casting method wet onto preequilibrated gradient bar (W) wet onto cold bar, dried, gradient applied (D) test environment high flow of dry air into large air volume (D) open to room (65%humidity at 25 “C) (H) combinations of casting method and test environment WH (similarto conventional) WD (exposure only to bulk water) DH (exposure only to water vapor) DD (no water effects)
sitions were either equal to or a few degrees lower than the crack transitionsin the studiesreported here. In additionto the ASTM conditions, we operate the bar under the “special conditions” noted in Table 2 to permit measurement on predried as well as wet film castings at both very low as well as moderate humidity (ca. 65% RH) over the film. Temperature gradients are on the order of 2 Win. With operator consistency in selection of the appearance ofthe transitionpoint,the precision of determination of a transition is estimated to be at least within f l “C. Reproducibility from run to run, Le., complete startup with new samples, is estimated t o be on the order of f 2 “C. Wet latex was applied with a 250-pm (10-mil)gate casting block on the gradient bar parallel to a stripe of the predried but similarly applied latex, approximately 15 min after powering the bar. Thermal equilibriumwas substantiallyachievedin this time, to within 1“C of final values. The wet film thickness was approximately 125 pm (5 mils), yielding a dry film thickness of about 60 pm (2.4 mils) and, by difference, water corresponding to a film of about 65 pm (2.6 mils). Thus, data were obtained as pairs, viz., DD with WD and DH with WH with reference to Table 2. Accordingly,the comparativeprecision is likely greater within pairs than between runs. MFT data collection began, in most of our work, 1h after casting the wet film. ASTM D-2354 recognizes the need to provide for removal of moisture of evaporation to prevent water condensation in the apparatus that would interferewith film formation,and calls for a flow of 4 Umin of dry air through the gradient bar chamber (formedfrom a transparent flat cover)from the low-temperature to the high-temperature end. However, our commercial apparatus, operated under these conditions, failed to prevent condensation,and in many instancescompletedrying of the film to enable an MFT reading was very slow,up to 30-60 min at the lower values of MFT. Our “D” test environment in Table 2 was achieved by providing a much larger chamber formed by placing a plastic bag over the apparatus, and using dry air flows on the order of 50 Umin. Wet castings at room temperature under the D environment on even the unenergized MFT bar lost virtually
all water in less than 2 min. Thus, the ASTM condition in our hands is a much more humid environment for a substantially longer period of time during film formation than even our “WIT condition. Results Figure 2 is a photograph of film castings of a 565-nm poly (n-butyl methacrylate) (poly(BMA))latex that demonstrates our principal experiment. The upper casting was made with the MFT bar cold and unenergized, and was allowed to dry thoroughly. In addition to having coarse cracks, presumably arising from shrinkage stresses resulting from particle array gelation at less than close packing, the film was highly turbid owing to light scattering from the interstices. The bar was then energized and allowed to attain thermal gradient equilibrium. The lower casting of wet latex was then made, Le., with the gradient imposed. We do in fact observe a rather distinct transition temperature for the predried as well as wet cast films (Figure 2), above which the films are substantially clear, implying that particle deformation and compaction has occurred at least to the degree to render the size of any remaining voids invisible. In other words we are able to measure a “dry M F T . The upper predried film does show some residual turbidity, even well above the transition temperature, and this appears to arise simply from microfissures between fine grains of particle deposit throughout its entire length that cannot be thermally healed. These fissures are also evident at the low-temperature end of the wet casting, but they are largely absent at the hot end, suggesting that the overall particle packing dynamics are better when drying occurs above the MFT. Our dry MFT is then the deformation and compaction process of the particle deposit within the grains. The principal series of latices examined are in the approximate size range of 350-550 nm, a range selected for high turbidity of the noncompacted dry film for ease of observation of the compaction process. Initial work was done with poly(BMA) latices, but for the most part work was done with compositions of somewhat higher glass temperature for convenience of preparing the noncompacted film deposits by drying the latex at room temperature (25 “C);the larger diameter poly(BMA) latices yielded initially noncompacted deposits at 25 “C,but much like those of Dobler et al., these tended to compact and clarify with time. That result led us not only to select latices of high Tgfor experimental convenience, but also to recognize that time of measurement was a n important variable. The various copolymer compositions were selected to explore anticipated hydroplastic effects; we would anticipate on grounds ofpolarity that ethyl acrylate/ methyl methacrylate (EA/MMA) compositions would imbibe the most water, styrene (S) containing compositions
2622 Langmuir, Vol. 10, No. 8, 1994
Sperry et al.
Figure 2. Photographs of latex films on the MFT bar: (upper)cast and dried before energizingthe bar; (lower)cast after the bar was energized and equilibrated.
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Figure 3. MI?"-time traces for various casting and drying conditions: ENS 340. the least. Brodnyan and Konen4did observe such effects with latices having related monomeric compositions. k Time Dependence of the MFT. Traces of MFT values with time measured under the casting and drying conditions described in Table 2 are shown in Figures 3-8. The sets of MFT data for each latex composition are plotted in both linear and logarithmic time, measured from the time of film contact with the thermal gradient. The same temperature and time scales are used in all of the figures for ease of visual comparison among data sets. The logarithmic time plots result in quite linear MFT dependence in essentially all cases. MFT values obtained under standard ASTM conditions are also indicated as a single point at 1h, the time suggested for assuring equilibration. The ASTM MFTs of many of our latices are lower than
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Figure 4. =-time traces for various casting and drying conditions: BNS 530. the values obtained under our special conditions and are quite time independent up to at least two days. The E N S and the BNS (BA = n-butyl acrylate) latex data (Figures 3 and 4)are perhaps the most remarkable in that the MFT-time curves are nearly coincidental, regardless of wet or dry conditions of origin and environment. The ASTM MET is only about 2 "C lower. As described, the DD and W D curves were obtained simultaneously in one run, and the DH and WH in another; the ca. 2-3 "C separation of the sets of curves in Figure 4 may not be significant. The implication of these results would appear to be that there is little, if any, role of w a t e r e i t h e r "special" or even hydroplastic. For the EA/MMA and BA/MMA latices, Figures 5 and 6,the DD and DH MFT dependence on time is like that
Role of Water in Particle Deformation and Compaction
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Langmuir, Vol. 10, No. 8, 1994 2623
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Figure 5. MFT-time traces for various casting and drying conditions: ENMMA 360.
Figure 6. MFT-time traces for various casting and drying conditions: BNMMA 430.
of the styrene copolymers. However, the MFT of the wet latex castings is significantly lower, and is essentially time invariant a t least for times greater than 1h; the ASTM value is about 2 “C below that of WH. While this might be the behavior expected if “special forces” due to water evaporation were operative, it is plausible that the cause is simply hydroplasticization. It is generally accepted among practitioners in coatings that such acrylate copolymers, especially EA/MMA, are quite water sensitive. Indeed, Brodynan and Konen4 reported modulus-temperature curves for a 50 wt % EM50 wt % MMA latex copolymerfilm in both dry and water-swollen states, with a n approximately 10-15“C shift of the curve toward lower temperature for the latter. This is very similar to the shifts of wet vs dry MFT curves we see a t shorter times in Figure 6. Results for the BMlYMMA copolymer (Figure 7) indicate behavior between those ofthe styrenic and of the acrylate1 MMA copolymers, in that the WD and WH curves at short times fall somewhat below the DD and DH curves. Our last case, that of poly(BMA1, is particularly interesting. Our experience related to the general water resistance properties of coatings led us to expect that this polymer would behave quite “hydrophobically”,Le., little difference between wet and dryresults, a t least no greater than for the B W M A copolymer. However, the MFT traces of Figure 8 more closely resemble those of the acrylate/MMA copolymers. There is a complication, though, related to the nature of the experiment, in
particular maintaining controlled local humidity along the gradient bar. While the prevailing environmental RH is about 65%, the local RH along the bar that dictates equilibrium moisture content is a function of the local bar temperature. The BMA polymer has the lowest glass transition temperature within this series of polymers and would be subjected to higher RH a t this point than the copolymers of higher Tg. Figure 9 reproduces the semilogarithmic plot of Figure 4 for ENS 340 latex under DD and WD conditions but where we show data obtained for times of less than 1 h of exposure of the film on the gradient bar. This experiment is complicated by the presence of the time lag in achieving a steady-state gradient after turning on the apparatus, although the Sheen instrument is remarkably rapid. We estimate equilibration to within about 1“C of final values along the bar after 15min or less of activation. Time zero is taken a t this time, upon application of the wet latex film, parallel to the predried film. The DD and WD results in Figure 9 appear separated by a t most 1“C a t short times, and we do not attach much significance to the differences. B. Water Evaporation. We also show in Figure 9 curves labeled water evaporation. This is a n estimate of the time a t a specified temperature required to evaporate a film of water alone, of 65-pm (2.6-mil) thickness, equivalent to the interstitial water in our films as noted in the Experimental Section. The curve on the right is temperature vs the reciprocal of water vapor pressure as
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2624 Langmuir, Vol. 10,No. 8,1994
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a measure of time to evaporate, keyed to actual data for the evaporation rate a t 25 "C, viz., 2.1 x (g/cm2)/s, measured by us and very near those values reported by others in quiescent The left-hand curve is the same as the right, shifted by a factor of 2.5. This is the ratio of room temperature evaporation rates found by C r o l P for a n air velocity of 4 mph, selected to simulate air currents over drying paint films, to quiescent air (5.34 x 10-4/2.16 x lo-* in the same units). We also made a n attempt to track the evaporation time of water on the energized MFT bar itself simply by monitoring the progression of drying of castings of water. Some of these data points are shown in Figure 9, both as cast a t 125 pm (5 mils), and corrected to estimate the time had they been 65 pm (2.6 mils). These data agree rather well with the estimated curve for the 4 mph air velocity, which is not unreasonable as the chamber was exposed to circulating laboratory air during the measurement. In any event, these various results and estimations would appear to provide a reasonable bound on the water evaporation componentof the film formation process. As they obviously should, our MFT data even a t the shortest accessible measurement times fall to the right of the water evaporation curve. (21)McEwan, I. H.J.Paint Technol. 1973,45 (583),33. (22) Sullivan, D.A. J . Paint Technol. 1978, 47 (610), 60. 123) Croll, S. G. J. Coat. Technol. 1986, 58 (734),41.
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C. Comparison of Tg and the Various MFT Transitions. Table 3 summarizes selected numerical values of observed MFT transitions in comparison with measured dry polymer glass temperatures, including differences in transition temperatures for various conditions. Crack MFTs average only about 2 "C above the turbid-clear transition, dry MFT values center about Tgas measured by DSC, and the hydroplastic effects discussed in the preceding section are clearly revealed. The last column
Langmuir, Vol. 10, No. 8, 1994 2625
Role of Water in Particle Deformation and Compaction Table 3. Summaw of Key MET Values
E N S 340 B N S 530 EAMMA 360 BAlMMA 430 B W M A 540 BMA 135 BMA 350 BMA 466 BMA 565 BMA 700 ENS 533 VAc 310
50 48 47 44 50 37 35 35 36 36 54 42
42 44 29 29 45 clear 25 29 28 29 52 clear
46 46 30 30 50 27 27 30 29 29 56 17
47 51 43 43 55 clear 33 36 43 42 51 clear
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Figure 10. MFT(DD)-time traces vs poly(BMA) particle diameter.
In contrast a film from the 350-nm latex is quite strongly scattering. However, the 135-nm latex does exhibit a transition from clear and continuous to cracked or microfissured when cast wet on the gradient bar, i.e., a conventional crack MFT value.
in Table 3 contains the slopes of the MFT(DD)vs log time plots of Figures 3-8. Multiple entries reflect repeat determinations for the same latex samples. The bottom two entries of Table 3, E N S 533 and VAc 310, have not been previously mentioned; the former is simply a slightly higher S content analog of ENS 340, and the results thoroughly verify the highly hydrophobic character of these S-based polymers. Indeed the drywet MFT difference is the least of any of the polymers. Like the lower acrylates, VAc polymers have a reputation for high water sensitivity in coatings and related applications; the results of Table 3 certainly are consistent with this, exhibiting by far the highest difference between Tgand ASTM MFT. D. Latex Particle Diameter. To date we have examined only the poly(BMA) compositionfor particle size effects, although in view of the results obtained relating to hydrophiliclhydrophobic effects, it would be very desirable to obtain such data for the hydrophobic styrenebased compositions. With poly(BMA)there is insignificant dependence of MFT on particle size, under the ASTM condition or under our WH condition, over the range of 135-700 nm. MFT(DD) does appear to increase significantly with an increase in particle diameter; the time traces of MFT(DD) are shown in Figure 10 and are approximately parallel. Films of poly(BMA) of 135-nm diameter dried a t room temperature show essentially no visual turbidity due to interstitial void scattering, and thus the MFT(DD) cannot be measured. (Though not reported here, the same lack of significant turbidity was observed with similarly small particle size counterparts that were made of several of the other latex compositions.)
Discussion That we and Dobler et al.13 observe a time-dependent cloudy-clear transition for a predried latex particle deposit, and that the very existence of a n MFT transition indicates that, below the MFT, water evaporates without deformation occurring, suggests that we can construct a time-temperature bound for the deformatiodcompaction process by decomposing the process into separate water evaporation and film compaction events. This decomposition is depicted in Figure 11in which a wet latex, at the point of particle contact, is separated into equivalent films of water alone and a n as yet undeformed dry film deposit of the particles. Each component is heated instantaneously to a selected temperature and observed for the time required to complete the event-the time required for the water to evaporate, and the time required for the dry particle array to compact. The time for water evaporation is a function of vapor pressure, humidity, and air velocity, and is measurable as shown in Figure 9. The time for (dry) particle compaction is also measurable, as we have shown also, and will be dependent on the viscoelastic compliance of the polymer, perhaps on the particle size, and certainly on whatever interfacial and colloidal forces act between the particles in the array including air-polymer surface tension and/or van der Waals attraction. We qualitatively depict this model in Figure 12. At temperatures sufficiently below the MFT, the time for water evaporation clearly is (by definition) less than that for polymer particle compaction. The existence of an MFT, above which films compact in the presence or
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2626 Langmuir, Vol. 10,No. 8, 1994
Table 4. Equilibrium Water Content of Polymers (20-25
t
“C) wt% wt% vol % (calcd)“ (measd) (measdlextraD)b
VAc
MA EA MMA EMA BA BMA I
’.
Wafer Evaporation
TIME FOR EVENT -+ Figure 12. Model for rate-limiting steps in film formation.
absence of water regardless of temperature, implies that the curves (dashed in Figure 12) for particle compaction and water evaporation cross. The temperature-time bound for the overall film compaction event would be dictated by the slowest process (rate-limiting step) a t any temperature and would tend along the bold line(s) in Figure 12. This is nothing more than a graphical statement of Brown’s observation that “It is observed that film formation in many polymer emulsion systems occurs concurrent with the evaporation of water and is complete when water evaporation is complete.” In contrast to Brown, we don’t believe that it is particularly remarkable that the times for water evaporation and film formation are so similar; it is simply that water evaporation is the rate-controlling process for temperatures above the MFT. We do not mean to imply, a t least as yet, that the MFT is simply the point of intersection of the individual process rate curves; one can certainly envision a “special role” of water, such as the generation of osmotic or capillary forces in the drying particle deposit. This kind of circumstance would result in a time-temperature path indicated by the arrow marked with a n asterisk in Figure 12. As temperature is increased toward this position, a special force or pressure due to water evaporation could conceivably result in compaction a t a temperature lower than that required for dry compaction. Our measurements on the more hydrophobic BNS and E N S per Figures 3 and 4 (and Table 3 which includes the wet and dry MFT results with E N S 533 as substantiation of the more extensively studied E N S 340) suggest the absence of water effects of any kind. An apparent inconsistency in our experiments lies in the observation that the cloudy-clear transition measured under the ASTM condition is up to about 5 “C lower than our dynamic MFTs in the otherwise moisture-insensitve E N S and BAJS systems. Our interpretation of this result is that even these polymers still have a degree of water sensitivity and in the extremely wet environment of the ASTM condition, relative even to our WH condition described in the Experimental Section, are subject to hydroplasticization early on in the experiment. Resolution of this would be aided by actual measurements of water absorption and/or glass temperature measurement as a function ofwater content. Scatena et aLZ4have performed such measurements on poly(viny1 acetate) via DSC,and their data indicate a plasticization effect of about 6 “C/wt % water dissolved in polymer over the range of 0 to about 4 wt % water. Table 4 contains a variety of data to give some, albeit quantitatively incomplete, perspective on the expected degree of equilibrium water absorption of polymers used here. If we consider the estimates, based 124) Scatena, M.; Sanmartin, P.; Zilio-Grandi, F. FATZPEC Congr. 1968,(Section 3, Brussels), 68.
2-EHA S 57 w t % M W 4 3 wt % EA 63 wt % M W 3 7 wt % BA 85 wt 9% B W 1 5 w t 9% MMA
BMA 52 wt % SI48 wt % EA 64 wt % 5/36 wt% BA a
4.2 4.2 3.6 3.6 3.2 2.8
-4c 3.0 2.2d
2.5 1.6
2.5 1.9 0.09 3.6 3.3 2.7 2.5 1.8 1.1
Reference 25. Reference 26. Reference 24. Reference 27.
on the van Krevelen group contribution along with some supporting data we have f ~ u n d ,of~ the ~,~~ equilibrium water content of the ENS and BNS polymers along with Scatena et al.’s estimates of glass temperature sensitivity, the hydroplastic effects predicted are certainly of the correct magnitude. It is also likely that our latex systems are more water sensitive than the pure copolymers would be in that they contain copolymerized carboxylate and have not been cleaned of surfactant and initiator residues. The remaining latex systems show even larger hydroplasticization effects, consistent again with the higher expected equilibrium water contents suggested in Table 4. The present experiments may not unambiguously deny the role of special coalescence forces arising from the presence of liquid water in film formation from latex. We do believe, a t least for some of our systems-the more hydrophobic latices-that the need to invoke liquid water driven forces is minimal. Structure compaction to eliminate at least the visible voids appears to proceed as readily in the absence of water as in its presence. The case for lack of special forces is nevertheless compelling enough to warrant more sophisticated examination of the film formation event with careful control of the environment during the critical moments of film formation at very short times. We also believe that a t a minimum our results highlight the simple interrelation of water loss rate and hydroplastic effects with the rate of the compaction process, and lack of necessity to invoke a liquid water driven model. We have made no effort to evaluate for the forces that do cause compactionupon water loss; polymerair interfacial tension and van der Waals attractive forces between particles in close contact per the JKR model16 proposed by Kendall and Padgetlo are a reasonable alternative. We embellish the scheme of Figure 12 to include speculation on the role of variables that would affect the evaporation rate of water, e.g., humidity and air velocity, and the effect that polymer solubilized moisture might play in the particle compaction process, other than any possible role in the generation of special forces such as capillary pressures. Figure 13 reproduces Figure 12 but with additional lines; one of these depicts slower evapora(25) van Krevelen, D. W. Properties o f Polrmers, 3rd ed.; Elsevier: Amsterdam, 1990; p 572. (26) Brown, G. L. In Water in Polymers; Rowland, S. P., Ed.; ACS Symposium Series 127; America1 Chemical Societv: Washinpton, - DC. 1980; p 441. (27) Moore, R. S.; Flick, J. R. In Water in Polymers; Rowland, S. P., Ed.; ACS Symposium Series 127; American Chemical Society: Washington, DC, 1980; p 555.
Role of Water in Particle Deformation and Compaction
\IFT
g
~~
~~
TIME FOR EVENT -+ Figure 13. Effect of drying conditions on the film formation
model.
tion of water obtained, for instance, by increasing the relative humidity over the drying film, or by a relative decrease in air velocity. The effect is to lower the film formation temperature from point A to point B, a n effect well known to the practitioner. We view the effect as one of simply allowing more time for the forces of compaction to operate before the water leaves. Likewise,ifthe polymer particles can have a finite equilibrium moisture content, the polymer will be plasticized by water and the timetemperature curve for particle compactionwill be lowered, as depicted by the curve labeled plasticized in Figure 13. The MFT would be lowered from point A to point C. If both effects are operative, e.g., high humidity which would both slow water evaporation and plasticize the particles, the MFT would be lower still, at point D. Overall, it is believed that essentially all of the arguments and observations of Brown3 as quoted in the Introduction are adequately accounted for in our relatively uncomplicated model. Certainly a major role of water in the film formation process is to provide a stabilizing and suspending medium for the particles that allowsfor thermal motion and particle mobilityto permit approach to close packing ofthe particles a t the point of contact or gelation of the array. Also, there may well be a role of air-water surface tension driven capillary forces in establishing good polymer-polymer interaction at the instant of particle contact, i.e., rupture of the hydrophilic stabilizing layers around the particles as described by Chavalier and Joanicot et al.28329This effect would have been operative in our predried systems as well as those deposited directly from water on the energized gradient bar. The linear dependence of turbidity MFT on log time, particularly for the DD condition, is consistent with WLF (Williams-Landel-Ferry) type polymer relaxation proc e ~ s e s ~ Ooperating ,~l under the stress of the interparticle attractive forces such as described by Kendall and Padget and recently by Kan,14 although the overall context of Kan’s application of WLF principles, to adjust modulus data, is rather different from that meant here. Here we mean to draw a parallel between the well-known application of the WLF equation to correlate Tgdata obtained a t different time scales of measurement, and our MFTtime results with the concept that the MFT is in fact a measure of the glass transition under the particular conditions of the measurement. The slopes of MFT(DD) (28) Chevalier, Y.; Pichot, C.; Graillat, C.; Joanicot, M.; Wong, K.; Maquet, J.; Lindner, P.; Cabane, B. Colloid Polym. Sci. 1992,270,806. (29) Joanicot, M.; Wong, K.; Cabane, B. Proc. TAPPZ Coat. Conf. 1993, 175. (30)Elias, H.-G.Macromolecules,2nded.;Plenum Press: New York, 1984; Vol. 1, p 411. (31) Tanner, R.I. EngineeringRheology,revd. ed.;OxfordUniversity Press: New York, 1988; p 353.
Langmuir, Vol. 10, No. 8, 1994 2627 vs logtime obtained by least-squares fits ofthe data shown in Figures 4-9 range from about 3-8 and are given in the last column of Table 3. The initial slope of Tgvs log time in the generalized form of the WLF equation is approximately 3,30although it may be significantly higher according to polymer composition and is dependent on the choice of reference temperatures. A crucial mechanistic question is what the environment is, Le., liquid and sorbed water, at the rate-limiting locus offilm formation. We propose that this is a n environment in which liquid water is absent. This would likely be the uppermost layer or layers of particles in the drying film where the surface and/or van der Waals forces acting toward pore closure are at a maximum, unmediated by liquid water. Certainly the surface region is the first to lose water via transport to the vapor state. This opinion would contrast with that of Kan whose mechanistic views we believe are otherwise consistent with our own; i.e., Kan considers that the polymer-aqueous serum interfacial tension, specifically the work of adhesion between the particles in the serum environment, is controlling. In many systems the dry and wet works of adhesion may differ by no more than a factor on the order of 2, and this may not be distinguishable within the error of available measurements and uncertainties in present JKR theory. We call attention to the papers of C ~ O ~ (and P somewhat , ~ ~ less explicitly Sheetz6) who does indeed discuss the possibility of an “...outermost “dry”layer...”, as well as the concept of film formation control by whatever is the ratelimiting step-water evaporation or intrinsic coalescence kinetics. Croll furthermore believes his observations of latex film formation, comprised mainly of drying rate experiments through and past the particle contact and compaction stage, are also more consistent with the Kendall and Padget model than with the capillary force model. To be clear on the usage ofthe phrase “...outermost ‘‘dry” layer...)),we mean dry with respect only to local liquid water; equilibrium absorbed water resulting in hydroplastic effects is a secondary yet allowed effect in these views. Critical experiments that define the particle environment a t the onset of deformation a t the surface of the particle layer, when forming films from the dispersion, are certainly called for. As noted we have not yet extensively examined particle size effects on MFT, with the exception of the series of poly(BMA) latices. MFT(DD) was found to be most sensitive to this variable (Table 3)) and does appear to increase with increasing particle diameter; this is anticipated by most proposed film forming mechanisms, including the Kendall and Padget model. We further find that the MFT vs log time curves are parallel, with values of d MFT(DD)/d log time given in Table 3. While it is conceivable that the particle size dependence of the MFT has its origins in film formation mechanism, there is clearly a geometric component which must be considered. In a close-packed particle array, there are two types of interstice, with either octahedral or tetrahedral geometries. The former are larger, with a n equivalent sphere size equal to 41.4% of that of the particle, and likely account for the majority of scattering observed in moderately size latices (250-500 nm). The latter, with an equivalent sphere size of 22.5% of that of the particle, become significant contributors to the scattering as the particle size exceeds approximately 500 nm. As the viscoelastic relaxation ofthe polymer plays a key role in the eradication of these voids, it is reasonable to expect that a n increase in the distance over which the polymer must flow would (32) Croll, S. G. J . Coat. Technol. 1987, 59 (5711, 81.
Sperry et al.
2628 Langmuir, Vol.10,No.8,1994
show no significant particle size dependence. We are inclined not to attempt to mechanistically rationalize the earlier results vis-&-visours in view of the fact that many of the latices studied were quite hydrophilic acrylate compositionsand that the film forming environments were not as well controlled as we believe they need to be. It is also probably extremely important to ensure, when testing for film formation mechanism from the wet latex, that the test latices are compositionally uniform, without surface layers enriched with hydrophilic material that might vary with particle diameter in synthesis and confound the MFT results. We certainly accept the caution of de la that film formation mechanistic studies should be performed with latex polymer compositions of minimum intrinsic water sensitivity. .1
1
10
100
1000
NORMALIZED HOURS = HOURS [(500 310)/(d 310)]
-
-
Figure 14. Application of time-diameter superposition model to poly(BMA) particle size results. require a concomitant increase in the time necessary for closure at a given temperature. To test the preceding concept in a simple model, we imagine the interstitial space as equivalent sphericalvoids and search for the time-dependent shrinkage of the void in a viscous medium under the interfacial tension. With the principal simplifying assumption that inertial effects are negigible, it can be shown that the void radius decreases linearly with time.33 Thus, the time for void closure to the point of visual transparency is proportional to the difference between the initial void radius and that a t transparency. Since void radius scales as particle diameter, the time for void closure should be proportional to the difference between the latex particle diameter and the diameter of a latex particle whose uncompacted film is on the verge of transparency. In the case of the poly(BMA) latices, we know from preceding discussion that this critical diameter should be between 350 nm (shows dry opacity) and 135 nm (largely clear). We have so treated the time coordinate of the data of Figure 10 by dividing the times byd(partic1e) - &critical), and also normalized the result to that expected for a reference latex, e.g., 500 nm by multiplying by the factor 500 - &critical). d(critica1)was established by trial and error to find avalue that gave a n approximate minimum standard deviation in a linear fit of the normalized results; a value of &critical) of about 310nm was obtained, and the final result is shown in Figure 14. The model despite its crudity and assumptions certainly correlates the data rather well, although the value of d(critical) probably should not be taken too seriously. Nevertheless, we plan to establish how well this value relates to the uncoalesced visual transparency of a series of specially prepared poly(BMA)latices covering the particle size range in question. Literature reports of the particle size dependence of the (conventional) MFT range from, e.g., nil4 to a slight increase,ll to a somewhat stronger increase34than our results for MFT(DD). Our ASTM MFT results, as noted, (33) Gebhard, M. S. (Rohm and Haas Co.). Private communication, 1993. In work conducted with L. E. Scriven at the University of Minnesota it was demonstrated that the differential equation for the limiting interfacial tension driven collapse of a spherical bubble in a viscous melt, not limited by gas diffusion, reduces to a simple linear decrease of the bubble radius with time, viz. dRldt = -yI2q were R = bubble radius, t = time, y = liquid-gas interfacial tension, and rj = liquid viscosity. For background: see Scriven, L. E. Chem. Eng. Sci. 1962, 17,55. (34) Jensen, D. P.; Morgan, L. W. J.Appl. Polym. Sci. 1991,42,2845.
Conclusions The present results demonstrate the existence of a latex transition to a compacted state in the absence of water. That this dry MFT occurs in the temperature range of the glass transition, and decreases linearly in logarithmic time, similar to the classical dependence of glass temperature itself on measurement time expressed through the WLF formulation, provides compelling evidence for the lack of a special role for water in this process. These features are strictly adhered to in the case of adequately hydrophobicpolymers,with MFT values virtually identical in both wet and desiccated latex states. Deviation from this behavior, observed only for materials deemed to be hydrophilic, can be entirely rationalized on the basis of hydroplastic effects; this results in MFT values in the presence of water considerably below those obtained in the dry state, as well as below the glass transition temperature. These effects have been found to coincide with earlier reports on the changes in polymer modulus and Tgas a function of water content. Limited study of particle size effects revealed a significant particle size dependence of the (turbidity) MFT only for the predried films of latex; ASTM MFT values for the poly(BMA) latices were the same within experimental error over the range from 135 to 700 nm. The increase of time-dependent dry MFT with increasing particle size correlates with a simple viscous flow model expressing the longer time required for the larger interstitial voids to shrink to a size that results in visual transparency of the film. For the case of the hydrophobic polymers we support the application of JKR contact viscoelastic deformation theory to adequately describe the forces which serve to induce particle deformation in a drying film. While it is highly likely that a similar conclusion can be drawn for the case of the more hydrophilic systems, the present measurements do not exclude the possibility of special forces arising from bulk water acting on these particles during film formation. Finally, we have modeled the observation of Brown on the coincidence of water evaporation with film formation as owing to the rate-limiting nature of the former process above the MFT. Our model also accounts for environmental effects on film formation observed in practice including changes in the water removal rate and the aforementioned hydroplastic effects.
Acknowledgment. The authors are pleased to acknowledge Ms. Angie Le for latex preparation, Mr. David Larson for advice on apparatus modifications, and members of the Rohm and Haas Film Formation Forum for valuable discussion. We would also like to acknowledge the Rohm and Haas Co. for support of this effort.
