Swelling and Its Suppression in the Cleaning of Polymer Fouling Layers

4846

Ind. Eng. Chem. Res. 2007, 46, 4846-4855

Swelling and Its Suppression in the Cleaning of Polymer Fouling Layers Phanida Saikhwan, John Y. M. Chew, William R. Paterson, and D. Ian Wilson* Department of Chemical Engineering, UniVersity of Cambridge, New Museums Site, Pembroke Street, Cambridge, CB2 3RA U.K.

The swelling and cleaning behavior of layers of a non-cross-linked acrylate-styrene copolymer, simulating fouling layers found in emulsion polymerization reactors, in aqueous sodium hydroxide (NaOH), was studied using the novel measurement technique of fluid dynamic gauging. The polymer exhibited swelling, dissolution, and deswelling, depending on temperature and pH in the range 7-13.4. The mechanisms were elucidated using FT-IR spectroscopy and AFM. The swelling profiles exhibited case II diffusion behavior, the rate of swelling being proportional to the change in free volume on swelling. The effect of thickness was also studied using single and double polymer layers; the swelling rate and extent of swelling were independent of the initial thickness only when the films were made of a single layer. 1. Introduction 1.1. Context: Fouling and Cleaning. Unwanted polymer coatings are frequently encountered in emulsion polymerization processes, where a fouling film may form by the deposition of coagulum, the result of (i) colloidal instability or (ii) polymerization by alternative mechanisms that give polymer in forms other than the desirable latex.1 These coagula may deposit on the walls and roof of the polymerization reactor and serve as nuclei for further coagulation, leading to the formation of fouling films which reduce heat transfer efficiency.2 Moreover, they can cause cross-contamination between batches. In some membrane-based waste treatment systems, unwanted fouling films may be generated from polymers present in the waste stream or deliberately added to promote the removal of heavy metal ions from wastewater.3,4 The removal of such coatings is often achieved by aggressive solvents, the environmental impact of which has prompted investigations of alternative formulations, which are usually water-based. 1.2. Swelling. During cleaning, swelling of the polymer film is often the first step; a polymer that does not swell can rarely be cleaned without considerable effort. Swelling and subsequent dissolution must occur over a time scale useful for the cleaning of items of process equipment. Understanding swelling behavior is therefore of great importance in elucidating the behavior and kinetics of the cleaning. Polymer swelling and dissolution are also important in materials recycling and drug delivery (e.g., ref 5). A detailed review of polymer dissolution is given by MillerChou and Koenig.6 When a dry polymer film is exposed to solvent, the solvent molecules penetrate into the initially glassy polymer matrix, occupying any free volume and promoting morphological and structural changes so that the glassy region becomes a rubbery gel.7 This phase transition can result in swelling or cracking depending on the volume phase transition, the stresses generated by the transformation, and the elasticity of the polymer matrix. The work reported here considers noncross-linked polymers; cracking is not observed. The glassy and swollen gel regions are sometimes seen to be separated by a sharp penetrant front. Dissolution occurs when the polymer chains disentangle from the gel matrix and diffuse into the bulk solvent. Most dissolution studies have focused on disentangle* To whom correspondence should be addressed. E-mail: diw11@ cam.ac.uk. Phone: +44 1223 334 791. Fax: +44 1223 334 796.

ment, which is not necessarily essential for cleaning: the softer swollen film can often be removed by shear (e.g., from highpressure lancing). Alternatively, detachment of the film from the equipment surface can be promoted by surfactants which diffuse through the gel and break down the adhesive interactions at the surface, as reported by Chew et al.8 when comparing methylethylketone (MEK, a good solvent for the polystyrenebased films studied, causing swelling and disentanglement) and a commercial alkaline cleaning formulation. Our particular interest is in the rates of swelling of copolymer films exposed to aqueous alkaline solutions, which are commercially attractive as they are more environmentally acceptable. The thermodynamics of polymer swelling, i.e., of transitions between the glassy and swollen equilibrium states, has been investigated for some time, and quantitative models of swelling of charged polymers in aqueous solution exist (e.g., ref 9). For the polyester copolymers studied here, the hydroxyl ions present in alkaline solutions drive saponification, and the resulting -COO- groups create on the polymer chain a fixed charge, which is compensated by entry into the gel of a cation from the bulk solution. The concentration of cations within the gel thus increases, and so swelling is driven both by water entering the gel in response to osmotic pressure, and by mixing effects. The rate of swelling is usually discussed in terms of either Fickian diffusion (case I) behavior, where the rate decreases with time due to the thickness of the gel restricting transport of solvent or solutes to the gel/glass interface, or case II behavior, characterized by the advance of a swelling front at uniform speed (a velocity termed the case II relaxation constant by Ugˇur and Pekcan10). We observe case II behavior in this work. Ugˇur and Pekcan describe the rate of swelling as being determined by the relaxation constant, k (essentially a polymer kinetic term), and the volume phase transition, ∆V, i.e.

R ) k∆V

(1)

The volume phase transition is determined by thermodynamics, but the factors determining the relaxation constant are not well established. A working hypothesis is that the velocity of the penetration front is related to the mobility of the swollen chains and therefore to the viscosity of the matrix, although this is likely to be a non-Newtonian material exhibiting viscoelasticity. The Williams-Lander-Ferry (WLF) relation for viscosity and temperature is based on the Doolittle equation11 in which the

10.1021/ie0615943 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/07/2007

Ind. Eng. Chem. Res., Vol. 46, No. 14, 2007 4847 Table 1. PZC Latex Propertiesa mean molecular mass mean particle size acid number Hansen solubility parameter Tg (onset) Tg (midpoint) a

30 kDa 77 nm 46 mg KOH/g polymer 18.5 MPa0.5 -12 °C 1 °C

Supplied by NeoResins.

viscosity, η, of a polymer melt is related to the free volume (the volume that is free to be used for molecular motion), f, via

η ) A exp

(Bf)

(3)

where A and B are constants. The WLF result is obtained by introducing a linear relationship for f

f ) R(T - T∞)

Figure 1. Schematic diagram of FDG apparatus.

(4)

where T∞ is the Vogel temperature at which the free volume is zero, typically around 50 K below the glass transition temperature.11 In this work, we investigate the effect of temperature, pH, and ionic strength on the swelling rate of a commercial polyester copolymer representative of commercial coating lacquers. 1.3. Fluid Dynamic Gauging. Experimental techniques employed to study polymer swelling kinetics include weighing,12,13 optical microscopy,14 nuclear magnetic resonance (NMR15), and Fourier transform infrared (FT-IR) spectroscopy.16 In a previous paper,8 we reported the use of a novel noncontact fluid technique, fluid dynamic gauging (FDG), to monitor the swelling/cleaning characteristics in aqueous sodium hydroxide of polystyrene-based polymeric fouling films mimicking those generated in emulsion polymerization reactors. A brief description of the technique is given in appendix I: a detailed description is given in the earlier paper. We observed linear, i.e., case II, swelling behaviors for those non-cross-linked polymer films over a range of temperatures and found that the temperature dependency could not be described by timetemperature superposition approaches such as Williams-LanderFerry (WLF) descriptions. We report here a more detailed study of one such polymeric coating, augmented by other techniques, which illustrates that the swelling and dissolution of charged copolymers is a phenomenologically rich topic. 2. Experimental Section 2.1. Formation of Films. Polymer films were generated on circular (50 mm diameter) 316 stainless steel coupons using 80 or 200 µm wet film rollers to apply 40 wt % latex suspensions in water (pH ∼ 9.2). The films were left to settle at room temperature for approximately 4 h, and then dried in a vacuum oven (Gallenkamp, U.K.) for 12 h at 50 °C, yielding dried film thicknesses of 26 and 65 µm, respectively (measured by micrometer). Further layers were sometimes applied to the dried film to build up the film thickness. Most experiments were performed with an acrylate-styrene copolymer, labeled PZC (18 wt % styrene, balance 2-ethylhexyl acrylate/butyl methacrylate) provided by NeoResins b.v. (Waalwijk, NL) and representative of commercial coatings. Table 1 summarizes some characteristic properties of this material, which was manufactured by emulsion polymerization in aqueous

solution. PZC resembled the polymers studied in the previous paper, all being examples of non-cross-linked polystyrene copolymers. FT-IR spectra of films were obtained from a Nicolet 510M reflectance spectrometer using the clean, dry crystal as background. Before each scan, the polymer film was immersed in NaOH or water and then rinsed with reverse osmosis (RO) water; excess surface liquid was removed using filter paper. Topographic images of films were obtained using a Veeco Dimension 3100 AFM operating in tapping mode. Before each scan, the film was rinsed with RO water and the surface dried, as described above. 2.2. Swelling Studies and FDG. Aqueous NaOH solutions were prepared by dissolving NaOH pellets (Analytical grade, Fisher Scienctific) in RO water. Solutions with different ions or ionic strengths were prepared from LiCl (Analytical grade, Fisher Scientific), NaCl (pure dried vacuum, Weston Point, U.K.), and KCl (99+%, Sigma-Aldrich). Solution pH was measured using a pH electrode (Hannah). The thickness of the polymer films was monitored using the apparatus shown in Figure 1. A detailed description of the apparatus and its operation is given in ref 8. The temperature was controlled by circulating water through the external jacket, and measured using a K-type thermocouple. The solvent was charged through the supply port and its level maintained constant (to give fixed hydrostatic pressure) either by changing the feed rate, or by draining through the discharge valve. The flow rate through the gauge nozzle was measured using a gravimetric balance ((0.005 g) connected to a computer. FDG measurements were performed with a 1 mm i.d. nozzle. Once the polymer sample was fixed in position, thereby setting the start time, and a steady flow established, the clearance was adjusted to give a flow rate in the band corresponding to values of clearance, h, of 150-200 µm. This initial adjustment period lasted up to 3 min, so initial thickness data are not available. The gauging flow was checked throughout the experiment to keep h approximately constant and thereby minimize the influence of gauging flows on the swelling of the film. Separate testing of this protocol confirmed that gauging flows did not affect swelling. The measurements were stopped at least 15 min after the plateau stage was reached or, when no swelling was observed, after 1 h. The latter time scale was chosen as this represents the limit of inter-

4848

Ind. Eng. Chem. Res., Vol. 46, No. 14, 2007

Figure 2. Thickness-time profile of a PZC film (dry thickness 65 µm) in NaOH (pH ) 12.5, T ) 20 °C). The construction lines show how the various parameters were extracted.

Figure 3. AFM images (height scale of 100 nm) of PZC films before and after swelling in NaOH (pH 12.5, T ) 20 °C): (a) t ) 0 min, (b) t ) 10 min, (c) t ) 20 min, and (d) t ) 30 min.

est for cleaning studies. Experiments were performed at least twice to confirm reproducibility. The accuracy of thickness measurement was (10 µm. The observed swelling behavior suggested that swelling involved the movement of a penetrant front. An attempt was conducted to track the movement of this front using a dial

micrometer ((5 µm, Mitutoyo, Japan), acting under conditions of approximately constant force, to try to locate the boundary of the rubbery and glassy layer. In these tests, the sample was immersed in NaOH in a glass beaker, and readings were taken at regular intervals. These proved inconclusive: the probe did record a smaller thickness as swelling proceeded but did not

Ind. Eng. Chem. Res., Vol. 46, No. 14, 2007 4849

Figure 4. FTIR spectra of PZC film immersed in aqueous NaOH solution (pH 12.5, T ) 20 °C); transmittance scale for all the profiles was from 45% to 100%.

reach a steady value near zero when the film had swollen completely, indicating that the swollen layer maintained significant elasticity that opposed further penetration by the probe. 3. Results and Discussion 3.1. Swelling Behavior. Figure 2 shows a typical thicknesstime profile observed for a PZC film exposed to NaOH at a bulk pH of 12.5, at 20 °C. The film starts to swell almost immediately, increasing in thickness in a linear manner; a plateau stage follows. This pattern of behavior was reported in the previous paper for related styrene copolymers. The figure shows the constructions used to extract the characteristic parameters, namely the swelling rate, R, and final thickness, δmax. The latter can be used to evaluate the maximum extent of swelling, defined as

max )

δmax δ0

(5)

where δ0 is the initial thickness of the dry film. The free volume of the swollen layer, f, can be estimated from

f)

δmax - δ0 ) max - 1 δ0

(6)

The linearity in the thickness-time profile indicates that swelling is not controlled by Fickian transport of solvent through the swollen layer (case I diffusion behavior), but is consistent with the advance of a penetrant front (case II diffusion). The evolution of a swollen film under these conditions is apparent from the AFM images in Figure 3 which show the development of granularity at the length scale associated with the latex particles (77 nm) over this time scale from an initially uniform film (30 µm × 30 µm). The swollen matrix (Figure 3d) contains holes, but there is no evidence of removal or detachment. Figure 4 shows the molecular transformation associated with swelling as tracked by reflectance FT-IR. The peak at 1728 cm-1

4850


Figure 5. Effect of temperature on swelling for films exhibiting measurable linear swelling. Solid symbols, PZC; pH as shown. PA and PX films at pH 13.4 (after ref 8). Initial wet layer thickness 100 µm.

evident in the sample before exposure to NaOH (t ) 0) corresponds to the stretching of the carbonyl groups within 2-ethylhexyl acrylate and butyl methacrylate. The ester linkage is clearly hydrolyzed to acid and hydroxyl groups on exposure to alkali, with evident increases of peaks 1558 (asymmetric stretching of -COO-), 1650 (water), 3320, and 3400 cm-1 (water and intermolecular H-bonds). The spectra obtained after the plateau region was reached (t > 30 min) do not show any further change, which is consistent with the gauging measurements. PZC films were studied over the temperature range 2060 °C and pH values 9-13.4. Various swelling behaviors were observed, which are discussed further in the next section; we here analyze the rates for those cases in which linear (case II) swelling profiles similar to those of Figure 2 were observed. Figure 5 shows that for those (pH, T) conditions under which linear swelling behavior was observed, the measured free volume followed the linear dependency in eq 4. The linear regression trend lines show that the value of R was strongly affected by pH and exhibited a maximum between pH 12 and 13: it was not straightforward to obtain R values in this region as the film detached from the surface readily at temperatures above 20 °C. The linear dependency was also observed in films of other polymers, as shown by the data in the figure for two styrene/methacrylate copolymers PX and PA (mol wt 30 and 125 kDa, respectively) in 2 wt % NaOH (pH ∼ 13.4), taken from the previous paper. The R value in these cases was similar, at 0.02 K-1, but smaller than those recorded for PZC which were in a range 0.03-0.05 K-1. The pH and temperature ranges were not investigated in the previous study. The PZC data sets in Figure 5 suggest a common value of T∞, lying between 10 and 15 °C. The associated glass transition

Figure 6. Effect of pH on extent of swelling of PZC films in NaOH. Initial wet thickness 100 µm. The solid line is a trend line linking data at 20 °C: the dash-and-dot lines show the behavior expected at higher temperatures. The dashed line is the locus of conditions at which the swelling film is disrupted by the stresses imposed by the gauging flow. A data point at pH 7 (not shown) has the same value of f as that at pH 9.

temperature (Tg) for the film is therefore around 60 °C, which is clearly very different from the values in Table 1. This is not surprising as these values are for polymers in latex form, at pH ∼ 9 and when they are plasticized by the solvent. The Tg


Figure 7. FTIR of PZC (a) before swelling experiment. FTIR of PZC after immersion for 1 h at T ) 20 °C in (b) RO water pH 7, (c) NaOH solution pH 11, and (d) NaOH solution pH 13.4. The transmittance scale is from 40% to 100%.

of the dry PZC film was measured separately by DSC (Perkin-Elmer Pyris-1 at 20 °C/min), and a value of 46 °C was obtained, which is much closer to the 60 °C estimate based on T∞. The effect of pH on the swelling of PZC is plotted in Figure 6. The 20 °C data set shows little discernible swelling at pH values less than 11 and greater than 13.4: we refer to these values as swelling thresholds. Dissolution was not evident in any of these cases; there is a sharp increase in swelling with pH above 11 followed by suppression above 12.5. The FT-IR spectra in Figure 7 show that no hydrolysis occurred within 1 h at pH 11 whereas hydrolysis did occur at pH 13.4, but to a smaller extent that at 12.5 (Figure 4), with the peaks at 2960, 2935, and 2875 cm-1 (-CH3 and -CH2 stretching) being noticeably diminished. In Figure 8, AFM images of the films exposed to these conditions exhibit correspondingly different topographies, confirming that there is a threshold for hydrolysis and swelling at pH 11 and a physical effect limiting swelling at higher pH despite the polymer having been hydrolyzed, with the result that swelling is completely suppressed over the time scale of interest. The topography images taken after exposure to solutions appear rougher. That attempted after exposure to pH 11, Figure 8b, could not be completed as the change in the surface (to softer or more sticky) made it difficult for the AFM tip to scan without damaging the film. This effect indicates that hydrolysis, and hence swelling, occurred, albeit at a slow rate under these conditions.

It is noteworthy that the onset of swelling did not correspond to the pKa of any of the polymer components (6 for 2-etylhexyl acrylate and 5.4 for methacrylic acid) or the formation conditions (for PZC, ∼9.2). This could be because the rate of hydrolysis at pH < 11 was so slow that swelling was not observable within the time scale of these observations. It was estimated that the rates of hydrolysis of 2-ethylhexyl acrylate and butyl methacrylate were almost zero at neutral condition17 (Freidig et al., 1999); at pH 8.8, their half-life values were reported as 33 and 17 days, respectively. Few data are plotted in Figure 6 in the pH range 11-13.2 at temperatures greater than 30 °C because in these cases the polymer films swelled rapidly and were too soft to measure reliably using the FDG technique (normal and shear stresses imposed by the gauge are ∼170-200 and 4-6 Pa, respectively): inspection of the figure suggests that this behavior is associated with f ∼ 1. At higher temperatures, this limit was reached at pH values nearer the thresholds. The effect of pH and temperature on the swelling equilibrium for charged polymers in aqueous solvents has been investigated by several workers and modeled in terms of elastic, entropic, and osmotic components by workers such as described in ref 9. The dependence of equilibrium swelling ratio on pH, showing the same trend as in Figure 6, has been reported and modeled by Mercade´-Prieto et al.,18 who modeled the swelling and dissolution characteristics of an analogous system, namely heat-induced whey protein gels, in terms of charge

4852


Figure 8. AFM images (30 µm × 30 µm) of PZC before and after immersion in solution at T ) 20 °C: (a) pH 7, (b) pH 11, and (c) pH 13.4.

and osmotic pressure effects. Their context was the cleaning and dissolution of biopolymers, viz., the whey proteins found in milk, which denature during thermal processing and foul dairy equipment. Milk fouling deposits are usually cleaned using sodium-hydroxide-based solutions, which cause the aggregated proteins to swell and then dissolve. The rate of swelling and rate of cleaning exhibit a concentration optimum, first identified by Bird and Fryer;19 Figure 6 indicates that this phenomenon also occurs with synthetic polymers. The forces imposed on the film surface by the fluid dynamic gauging technique can be comparable to those generated by the flow of cleaning solutions in industrial equipment (see ref 8). The figure shows that the swollen polymer film is not weak enough to be removed by fluid shear across the whole pH range at 20 °C, but at higher temperatures the deposit can be removed at certain pH values. The rate of removal is also expected to be a function of pH and temperature. One may define the particular combinations of pH and temperature at which the deposit is removable as a “cleaning window”. The application of these numerical models (refs 9 and 18) to the films considered here has not been attempted and represents an avenue for further work. Our primary interest lies in understanding dissolution, and we proceed with experimentally measured values of f; we investigate the effects of parameters

Figure 9. Effect of ion screening on swelling of PZC films. Film thickness 65 µm, T ) 20 °C, bulk pH 12.5.

known from this body of literature to influence f and demonstrate that they have a parallel effect on swelling.


Figure 10. Dependence of rate of swelling on free volume. Data are at various temperatures except the data set at pH 12.5, which are at 20 °C but with different salt concentrations.

3.2. Factors Influencing Swelling. The deswelling behavior at high pH in our system is attributed to ion screening, where the higher ionic strength of the solution at higher NaOH concentration results in screening of the charges on the polymer chains and reduction in the difference in osmotic pressure between the bulk solution and that in the matrix. Ion screening was studied by adding chloride salts to NaOH solutions at pH 12.5 and 20 °C. Figure 9 shows that both the initial rate of swelling and maximum extent of swelling were reduced by increasing the ionic strength, as expected. This effect varied slightly with the electronegativity of the added cation, following the order of electronegativity, EN: Li, EN ) 0.98, f ) 0.3; Na, EN ) 0.93, f ) 0.35; K, EN ) 0.82, f ) 0.45. For instance, K has a lower electronegativity than Na, resulting in a higher concentration of mobile cation and thereby higher osmotic pressure. More water will be drawn into the gel, increasing f, and the osmotic pressure may control the swelling rate. Similar effects of ionic strength and cation electronegativity have been reported for whey protein gels.18 3.3. Analysis of Swelling Rates. Whereas the effect of variables such as pH and temperature on equilibrium swelling has been studied for some time, their effect on swelling rates, particularly for fast cases, has not been studied to the same extent, partly due to the difficulty in measuring the rates reliably. The corresponding plot (not shown) of linear swelling rate (R in Figure 2) against pH for PZC showed the same features as Figure 6, namely very small values outside the pH range 1113.4 and a distinct optimum around pH 12.5. The rate of swelling is therefore related to f, with swelling expected to be retarded by a small free volume. The dependence can be explained qualitatively using either (i) a transport approach

(hindered diffusion through the swollen matrix), (ii) a fluid model (osmotic pressure dragging water through a swollen matrix whose permeability is an increasing function of f ), or (iii) mechanical arguments (high viscosity or slow reordering being related to freedom of chains to move). Data taken from experiments where R and f could both be measured using FDG, plus data from Figure 9 measured at different ionic strengths, are plotted in Figure 10: they show a very strong proportional relationship between R and f where R (in m s-1) ) 1.27 × 10-4 f. This relationship is consistent with eq 1, implying that the relaxation constant k is insensitive to pH or ionic strength, in contrast to the sensitivity of f to those variables. The lack of stable swelling plateaus at higher temperatures meant that the temperature dependency of k could not be determined. The data could also be fitted to eq 3 by assuming R ∝ 1/η ) (1/A)exp(f/B), which yielded parameters A ) 0.04 s µm-1 and B ) 0.6. An interpretation of these values is not proposed here, as the fit (not shown) was substantially poorer. 3.4. Effect of Thickness and Construction Method of the Film. The effect of initial film thickness was investigated at fixed solvent temperature and pH with initial film thicknesses of 26, 65, and 130 µm, the latter film consisting of two 65 µm layers. The mean rate of swelling proved to be insensitive to the initial thickness of the film (data not shown), the gradients of the film-time profile during swelling being similar. The maximum extents of swelling of the 26 and 65 µm films were similar (f ) 0.84 ( 0.10 and 0.73 ( 0.14, respectively). The film consisting of two layers, however, showed different behavior. Figure 11 shows that the two profiles (for single and double layers of 65 µm initial thickness) can be superimposed

4854


Figure 12. Schematic showing operation of dynamic gauging technique.

Figure 11. Effect of deposit thickness: (a) single layers of 26 µm and 65 µm in NaOH, pH 12.2, T ) 20 °C, and (b) single layer of 65 µm and double layers of 65 µm films in NaOH, pH 11.8, T ) 20 °C.

on each other by shifting down the profile of the latter by only 55 µm, indicating that the outer layer prevents complete penetration of the solvent into the inner layer. In addition, leaving the composite film in NaOH for longer times resulted in two swollen layers that could be peeled off individually. The behavior of double layers, therefore, could not be predicted in a simple way from the properties of single layers, in sharp contrast to the predictability of the behavior of double layers of polystyrene copolymers reported in studies by Chew et al.20 This new behavior would be problematic for cleaning of polymerization reactors, as deposits are likely to form in layers after each batch of production. Conclusions The FDG technique recently introduced into polymer studies was successfully applied, in situ and in real time, to monitor the swelling of films of an acrylate-styrene copolymer, labeled

PZC, in aqueous caustic soda solutions. The results from this new technique proved entirely consistent with those measured by other physical and chemical techniques, namely AFM and FT-IR. The existence of a linear swelling period, followed by a plateau period, is consistent with the observations of Chew et al.8 on two related polystyrene copolymers. The dependence of equilibrium swelling ratio on pH is similar to that observed for a biopolymer, namely a heat-induced whey protein gel, by Mercade´-Prieto et al.18 In particular, a “cleaning window” was observed; at 20 °C, neither discernible swelling nor dissolution occurred outside the range 11 < pH < 13.4. The identification of a pH optimum for rate of swelling and cleaning (at 20 °C, at around pH 12.5) is analogous to that discovered for the cleaning of whey protein foulant deposits by Bird and Fryer.19 The discovery, however, that the behavior of films consisting of double layers, each of dry thickness 65 µm, could not be predicted in a simple way from the properties of single layers contrasts sharply with the corresponding predictability discovered by Chew et al.20 Ion screening effects were studied at 20 °C by dosing the caustic soda solutions, pH 12.5, with chloride salts of Li, Na, or K: both the initial rate of swelling and the maximum extent of swelling were reduced by increase in the ionic strength. Perhaps the most striking observation is the proportional dependence of the rate of swelling on the free volume of the polymer film (Figure 10), obtained over a range of pH (913.4), temperature (20-60 °C), and added cations (Li, Na, K). Fluid dynamic gauging was thus demonstrated to be a powerful addition to the tool kit available to investigators of polymer swelling kinetics and polymer swelling equilibria. Appendix I. Fluid Dynamic Gauging Figure 12 illustrates the principle of this technique. The coated surface being studied is immersed in a Newtonian liquid, and a suction is applied to the end of a siphon tube fitted with a convergent nozzle so that liquid is drawn into the nozzle at A. For a fixed hydrostatic pressure H, the flow rate, m, is intimately linked to the distance between the end of the nozzle and the film surface, labeled h. This information can be used to locate the surface and thus measure the film thickness, δ, from δ ) h0 - h. The device used in this work affords accurate ((10 µm) in situ measurement of film thickness in real time. Acknowledgment Funding for P.S. from the Royal Thai government is gratefully acknowledged. Assistance with the FT-IR work from Dr. Mark Eccleston in the Cambridge Unit for Responsive Biopolymers


is also gratefully acknowledged, as is the provision of polymer by NeoResins and helpful discussions with Dr. Alan Gould. Nomenclature Roman A, B ) constants in eq 1 ds ) diameter of siphon, mm dt ) diameter of nozzle throat, mm f ) free volume h, h0 ) clearance between nozzle tip and film surface, substrate, mm H ) hydrostatic head, mm k ) relaxation constant, µm-2 s-1 m ) mass flow rate, kg s-1 w ) width of nozzle rim, m R ) rate of swelling, µm s-1 t ) time, min T ) temperature, °C T∞ ) Vogel temperature, °C Greek R ) divergence angle of nozzle δ, δ0, δmax ) thickness of film, µm , max ) extent of swelling ∆V ) volume phase transition, µm3 AbbreViations AFM ) atomic force microscopy EN ) electronegativity FDG ) fluid dynamic gauging FT-IR ) Fourier transform infrared NMR ) nuclear magnetic resonance PS ) polystyrene RO ) reverse osmosis SS ) stainless steel TPU ) TPU cleaning mix WLF ) Williams-Landel-Ferry Literature Cited (1) Vanderhoff, J. W. The Formulation of Coagulum in Emulsion Polymerization. In Emulsion Polymers and Emulsion Polymerization 165; Basset, D. R., Hamielec, A. E., Eds.; American Chemical Society: New York, 1981; pp 199-208. (2) van de Ven, Theo, G. M. The Capture of Colloidal Particles on Surfaces and in Porous Material: Basic Principles. Colloids Surf., A 1998, 138, 207.

(3) Garcia-Molina, V.; Lyko, S.; Esplugas, S. Ultrafiltration of Aqueous Solutions Containing Organic Polymers. Desalination 2006, 189 (1-3), 110. (4) Canizares, P.; Pe´rez, A.; Camarillo, R. Recovery of Heavy Metals by Means of Ultrafiltration with Water-Soluble Polymers: Calculation of Design Parameters. Desalination 2002, 144, 279. (5) Narasimhan, B.; Peppas, N. A. The Physics of Polymer Dissolution. AdV. Polym. Sci. 1997, 128, 157. (6) Miller-Chou, B. A.; Koenig, J. L. A Review of Polymer Dissolution. Prog. Polym. Sci. 2003, 28, 1223. (7) Astarita, G.; Paulaitis, M. E.; Missinger, R. G. Thermodynamics of the Glass Transition. J. Polym. Sci., Part A: Polym. Chem. 1989, 27, 2105. (8) Chew, J. Y. M.; Tonneijk, S. J.; Paterson, W. R.; Wilson, D. I. Mechanisms in the Solvent Cleaning of Emulsion Polymerization Reactor Surfaces. Ind. Eng. Chem. Res. 2005, 44 (13), 4605. (9) Brannon-Peppas, L.; Peppas, N. A. Equilibrium Swelling Behavior of pH-sensitive Hydrogels. Chem. Eng. Sci. 1991, 46, 715. (10) Ugˇur, S.; Pekcan, O. Determination of Relaxation and Diffusion Activation Energies During Dissolution of Latex Film Using in situ Fluorescence Technique. Polymer 1997, 38 (22), 5579. (11) Rubinstein, M.; Colby, R. H. Polymer Physics; Oxford University Press: Oxford, U.K, 2003. (12) Blackadder, D. A.; Le Poidevin, G. J. Dissolution of Poly-propylene in Organic Solvents: 2. The Steady State Dissolution Process. Polymer 1976, 17, 768. (13) Blackadder, D. A.; Le Poidevin, G. J. Dissolution of Poly-propylene in Organic Solvents: 4. Nature of the Solvent. Polymer 1978, 19, 483. (14) Ueberreiter, K.; Asmussen, F. Velocity of Dissolution of Polymers Part I. J. Polym. Sci. 1962, 57, 187. (15) Hyde, T. M.; Gladden, L. F.; Mackley, M. R.; Gao, P. Quantitative Nuclear Magnetic Resonance Imaging of Liquids in Swelling Polymers. J. Polym. Sci., Part A: Polym. Chem. 1995, 33 (11), 1795. (16) Koenig, J. FTIR Imaging of Polymer Dissolution. AdV. Mater. 2002, 14 (6), 457. (17) Freidig, A. P.; Verhaar, H. J. M.; Hermens, J. L. M. Quantitative Structure-Property Relationships for the Chemical Reactivity of Acrylates and Methacrylates. EnViron. Toxicol. Chem. 1999, 18 (6), 1133. (18) Mercade´-Prieto, R.; Falconer, R.; Paterson, W. R.; Wilson, D. I. Swelling and Dissolution of β-Lactoglobulin Gels in Alkali. Biomacromolecules 2007, 8, 469. (19) Bird, M. R.; Fryer, P. J. An Experimental Study of the Cleaning of Surfaces Fouled by Whey Proteins. Trans. Inst. Chem. Eng. 1991, 69 (C), 13. (20) Chew, J. Y. M.; Tonneijk, S. J.; Paterson, W. R.; Wilson, D. I. Solvent-Based Cleaning of Emulsion Polymerization Reactors. Chem. Eng. J. 2006, 117, 61.

ReceiVed for reView December 11, 2006 ReVised manuscript receiVed April 17, 2007 Accepted April 25, 2007 IE0615943

Swelling and Its Suppression in the Cleaning of Polymer Fouling Layers

Recommend Documents