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Ind. Eng. Chem. Res. 1996, 35, 434-449
Surface Elastic Effects in Premetered Coating Techniques Jose E. Valentini,*,† John T. Chandler,† Qiang Jiang,‡ Yee C. Chiew,‡ and Leslie J. Fina‡ Medical Products, Research & Development, E. I. du Pont de Nemours, Brevard, North Carolina 28712, and Chemical and Biochemical Engineering, Rutgers University, Piscataway, New Jersey 08855
The effect of the dilational surface elasticity on coating was investigated in two-layer slide coatings and single-layer slot and curtain coating experiments, respectively, up to 2 m/s. In the slide experiments both the surfactant concentration and were varied in the lower layer. While the position of the static contact line (SCL) is a function of the elasticity modulus , the dynamic contact line (DCL) position is insensitive to this variable. Also, the vacuum range increases with increasing the modulus . Best correlations were obtained when was measured at 1-2 Hz with a longitudinal wave generator. In curtain coating, the curtain edges can be stabilized and the wet thickness decreased by increasing the elasticity . Also, in slot coating experiments the minimum wet thickness correlates with as well. The modulus that best characterizes these coating techniques was determined at 0.1-0.2 Hz by means of a Langmuir trough. 1. Introduction The selection of surfactants and optimization of their concentrations for the manufacture of photographic films is proprietary, nonsystematic, and in some cases controversial. Some of the reasons are the complexity of the numerous and different coating processes, the multiple and not well understood mechanisms by which surfactants work in different coating techniques, the fact that within a given coating technique there are zones (bead, curtain, slide, edges, etc.; for examples, see Figures 3 and 7) which can be affected differently by the same surface-active materials, and finally, and most important, the limited understanding about the relationship between the physico/chemical characteristics of surfactants in solutions and their effect on coatings. A review of the literature suggests a renewed interest to pursue research in this area. Ruschak (1986) in a lubrication type of analysis investigated the effects of surfactants on the waves induced by air currents. He concluded that surfactants at the liquid/air interface will induce surface tension gradients that prevent the onset of waves and therefore coating defects. In an earlier study, Cerro and Whitaker (1971) used a slide similar to the one shown in Figure 3 to study the effect of surfactants on the hydrodynamics of thin films. They found the velocity profile to be affected by the surface elasticity. Schunk (1989) used a computer model to predict the diffusion of sodium decyl sulfonate in the slide coater and explored how the curvature of the upper meniscus (see Figure 3) is modified as a function of the liquid/air concentration. Using flow visualization techniques, we have also found that highly diffusible surfactants in the upper layer of a two-layer package promote wettability of the layers on the coater slide, increase the curvature of the upper meniscus, and widen the operating window of the slide coater (Valentini et al 1990, 1991). “Coating window”, “coatability window”, or “operating window”, which are used interchangeably throughout this work, refer to a locus of operating conditions (equipment setup, coating solution properties, and coating speed) that ensure both coating quality and a film free of defects. By extension, the expressions * To whom correspondence should be addressed. † E.I. du Pont de Nemours. ‡ Rutgers University.
0888-5885/96/2635-0434$12.00/0
“wide coating window” or “enhanced coating latitude” mean that the process conditions can be varied from the optimum ones without significantly altering the coating performance or the quality of the product. For multilayer coatings, there is not sufficient theoretical and experimental information to ascertain the role that surfactants play on the onset of coating defects. A recent patent application by Ishiwata et al. (1990) proposes that defect-free multilayer coatings can be achieved by adjusting the dynamic surface tension (DST) of each layer. According to the inventors, this ensures even and defect-free coatings. The importance of the dynamic surface tension on the quality of the flow down the slide was also recently pointed out by Valentini et al. (1990) and Tsuchiya et al. (1994). Theoretical results by Schunk (1989) show that surfactants in the lower layer induce a downward movement of the static contact line (SCL) on the die face (see Figure 3). He concludes that the final position of the SCL is determined by the equilibrium properties of the surfactant rather than the dynamic ones. Wu et al. (1985) proposed that secondary flows in the vicinity of the static contact line promote convection of surface-active materials; thereby, surface tension gradients are unlikely. Gutoff (1992) sustains that, in a multilayer package, surfactants are only required to promote spreading of the solutions on the web. The purpose of this work is to study how surfactants affect coating quality particularly when they are used in the lowermost layer of a two-layer arrangement or in single-layer slot/curtain coating experiments. The impetus of this research is to understand the impact of the dilational surface elasticity on the operating window of the slide, slot, and curtain coaters. 1.1. Dilational Surface Elasticity. High rate surface deformations are typical of all coating operations. Dilational surface elasticity, unlike the Gibbs elasticity, is a dynamic property that quantifies the resistance of the interface to undergo deformation. In this work, this property is characterized by the elasticity () defined as the surface tension variation dσ that occurs following a fractional area change (dA/A) due to either dilation or compression of the interface. The elasticity is represented by
)
dσ dσ ) dA/A d ln A
© 1996 American Chemical Society
(1)
Ind. Eng. Chem. Res., Vol. 35, No. 2, 1996 435
When the interface of a soluble surfactant solution expands, there is a local change of surface tension followed by an immediate adsorption of surface-active agents from the bulk onto the surface layer. This has the following two consequences: (a) the surface tension deviation dσ that occurs, for instance, after an expansion, is partially offset and (b) the change in surface tension, dσ, may not be instantaneous but lags behind the fractional change in area d ln A. The latter occurs when the rate of adsorption is slower than the rate of surface expansion/compression. The surface viscoelasticity may be written as a complex quantity, i.e.,
) ||eiθ ) ′ + i′′
(2)
where ′ represents the interfacial dilational elasticity, while ′′ denotes the interfacial viscosity. They are also expressed as the dilational modulus || ) {′2 + ′′2}1/2 and the loss angle θ ) tan-1 (′′/′). The quantity ′ gives the elastic energy stored in the surface film, while ′′ accounts for the dissipative effects due to matter interchange between the solution and the interface. When the adsorption process is slow, the frequency ω at which the interface is deformed is important and usually measurements of the surface elasticity are carried out in the frequency domain. Theoretical models predicting (ω) based on the assumption of diffusion-controlled adsorption have been developed by Lucassen et al. (1972, 1975), van den Temple and Lucassen-Reynders (1983), and, more recently, Jiang et al. (1995b). These models have been successfully used to interpret experimental data, to identify the adsorption mechanism, and to quantify the characteristic diffusion frequency ωDiff for a number of low molecular weight surfactants. For these systems, the dilational modulus || and loss angle θ can be shown to obey the following equations according to Lucassen and Hansen (1966, 1967):
|| ) 0/(1 + 2ξ + 2ξ2)1/2
(3a)
tan θ ) ξ/(1 + ξ)
(3b)
and
Here, 0 ) (ω)∞) represents the high-frequency limit of the viscoelasticity and may be identified with the Gibbs elasticity, i.e., 0 ) -(dσ/d ln Γ)Γ. The parameter ξ is given by
ξ ) xωDiff/ω
and
ωDiff ) D/2(dC/dΓ)2
(4)
where Γ is the surface excess, C is the bulk concentration, and D represents the diffusion coefficient of surfactant in the bulk solution. The loss angle θ for diffusion-controlled adsorption varies between 0 and 45°. Also at high frequencies where ωDiff < ω the measured modulus is equal to its high-frequency limit 0. 2. Experimental Section 2.1. Surface Elasticity. We measured the dilational viscoelasticity in coating solutions over a wide range of frequencies (from 0.001 to 1000 Hz) by using three separate experimental devices. At low frequencies, in the cycle per minute range, both ′′ and ′ were determined in a modified Langmuir trough in which the interface is expanded and compressed by means of two tight fit Teflon barriers. In this device, which has been previously described by Lucassen and Giles (1975) and
Figure 1. Schematic diagram of the apparatus for the generation and detection of capillary waves. The blade is the capillary wave generator.
Jiang et al. (1990,1995a), the surface tension variations ∆σ is generated from harmonic area variations ∆A induced by the moving barriers. These surface tension variations are measured by means of a platinum plate which is connected to a Cahn 2000 electromicrobalance. The apparent surface dilational modulus || is obtained from
|| )
dσ |dA|/A
(5)
The loss angle θ is given by the phase difference between the surface tension and the area variation curves. Surface aging experiments were also performed with the modified Langmuir trough. In this approach both the surface tension and surface elasticity modulus were measured as a function of time (t) on an initially clean surface. Results are usually described as vs t. Measurements of the in the high frequency range (50-1000 Hz) were accomplished by determining the damping of electrocapillary waves in an experimental setup developed in our laboratory (Jiang et al., 1992, 1993a). This equipment, which is similar to that used by Sohl et al. (1978), is shown in Figure 1. In this technique the waves are excited by applying a sinusoidal and dc off-set voltage between a thin metal blade (8 cm long) and the solution under study. The capillary waves are detected by specular reflection of a focused laser beam and measured with a position-sensitive detector (PSD) from which both the wavelength λ and the damping coefficient β can be determined. The surface elasticity is determined from the experimental values of wavelength, damping coefficient, frequency, and liquid bulk properties through the dispersion equation described by Lucassen (1969) and Hansen et al. (1971) as follows:
[Fω - Fgk - σk3 + 2iηωk2][iηω(m2 + k2) - mk2] ) [-i(Fgk + σk3) - 2ηωmk][2ηωk2 + ik3] (6) Here, g, F, η, and σ are the gravity, the liquid density, the viscosity, and the static surface tension, respectively.
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ing coefficient βL. The wave number kL is obtained by linear fitting the plot of the phase angle vs scanned distance. Values of surface elasticity can be calculated from the pairs βL and kL through the dispersion equation for a shallow trough previously described by Hansen and Ahmad (1971) as follows:
[Fω2 - (γk3 + Fgk) tan{h(kh)}](ρω2 - mk2) k3(γk3 + γgk) + 4iFµω3k2 + 4µ2ω2k3(m tan{h(kh)} - k) ) 0 (11) where most of the variables are the same as those used in eq 6, k ) kL + iβL, ω is the capillary wave frequency, and h is the depth of the trough (h ) 1 cm). The surface viscoelasticity (complex) can be calculated by solving the following equation:
) d - iωηd Fω2[Fω2 - (γk3 + Fgk) tan h(kh)] + 4iFµω3k2 + 4µ2ω2k3(m tan h(kh) - k) )
Figure 2. Schematic diagram of the longitudinal wave generator and detection system used in the experiment. Here, LG and CG represent the longitudinal and capillary wave generators, respectively.
The parameters k and m are given by the following equations:
k ) (2π/λ) + iβ
(7)
m2 ) k2 - (iωF/η)
(8)
Two different procedures as described by Jiang et al. (1993b) have been used to solve eq 6 from which the modulus || can be extracted. Measurements of in the middle range frequency, that is, 1-8 Hz, were carried out by using a longitudinal wave generator (Figure 2) which was also developed in our laboratory and described by Jiang et al. (1993b). In this case, the longitudinal waves are generated by oscillating a paraffin-coated, stainless steel barrier at the air/liquid interface. This barrier is connected, via a 15 cm long tube, to the axis of a small dc motor which generates a very small amplitude oscillation to ensure that the motion is parallel to the surface. The propagating longitudinal wave is detected by using capillary waves as described by Miyano et al. (1983). In our case we have used a new “double lock-in” technique that measures both φ and ∆φ. Both parameters are capillary wave phase angles in the absence and presence of longitudinal waves, respectively, at the detection line. They are related to each other by the following equations:
φ ) φ0 + ∆φ
(9)
∆φ ) ∆φ0e-βLx+iωLt-ikLx
(10)
where ∆φ0 is the longitudinal wave amplitude at x ) 0, βL is the damping coefficient, and ωL and kL are the wave angular frequency and the wavenumber, respectively. The longitudinal wave can be scanned over the length of the capillary wave generator along the detection line. The longitudinal wave amplitude and phase angle (i.e., kLx in eq 10) are recorded in the computer as a function of the scanned distance by using a second lock-in amplifier. The slope of the logarithmic plot of the amplitude vs scanned distance provides the damp-
mk2[Fω2 - (γk3 + Fgk) tan h(kh)] + k3(γk3 + Fgk)
(12)
Here d and ηd are the dilational elasticity and dilational viscosity, respectively. Throughout this work, we refer to the elasticity values measured at different frequencies. as “” or “elasticity modulus ”. 2.2. Solution Properties. Dynamic surface tensions (DST) were measured with a Sensadyne 6000 surface tensiometer, which is based on the maximum bubble pressure method (Adamson, 1982). The bubble generation rate was kept at 1 bubble/s. Both the calibration and surface tension readings were performed at both 40 and 25 °C as required. The calibration range of surface tensions was 33.8-70 mN/m. The accuracy of the measurement is (0.5 mN/m. Static surface tensions (SST) were determined with a Fisher Surface Tensiomat Model 21, which operates on the Du Nouy ring method (Adamson, 1982). Calibrations and readings were performed at both 40 and 25 °C, respectively. The accuracy of the measurements is (2 dyn/cm Since surfactants modify the bulk rheological properties of coating solutions as well, the type and concentrations of surfactants were chosen in such a way that the bulk elasticity (G′) and viscosity (G′′) remained essentially the same for all the solutions used within an experimental protocol. These properties were measured in a dynamic mode with a Fluids spectrometer RFSII. A Brookfield viscometer was used to measure the steady-state viscosities at 60 rpm (Spindle 18). Static contact angles were measured with a Rame-Hart NRL (100-00-15) goniometer. 2.3. Coating Experiments. 2.3.1. Slide Coater. The equipment to carry out slide coating was first described by Mercier et al. (1956), and a diagram is shown in Figure 3. In this technique, one or more solutions are dispensed through channels, onto an inclined plane, then across a gap, and onto a moving web or support. The bead, where most of the stretching of the surface occurs, is established between the moving web and the coater die face. The bead is bounded by both the upper meniscus and the lower one which is maintained at subatmospheric pressures, i.e., P < Pa, where P is the pressure in the vacuum chamber and Pa is the atmospheric pressure. In coating technology, this is referred as using a “bead vacuum” or “vacuum”. However, under certain conditions, it is possible to coat
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Figure 5. Projection of “scratch shadow” allowing one to detect the dynamic (DCL) and static (SCL) contact line positions, respectively. Here, H is the distance from the tip of the coater up to the SCL position.
Figure 3. Cross-cut diagram of the slide coater. Coating solutions flowing down the slide, across the gap, and onto the moving web. The arrows at the lower meniscus show how the liquid accelerates as it moves from the SCL to the DCL.
Figure 4. Combination of a laser line projector and B&L Monozoom 7 that allows the “Monitor View” shown in Figure 5.
when P ∼ Pa. Both menisci extend across the gap, which is the distance between the die face and the moving web. The web, which is a 13.75-cm-wide polyester base, has a previously applied gelatin subcoating to ensure proper wettability. The static contact angle of the coating solutions on the polyester base were in the range 1831°. All the coatings were performed at either 40 or 25 °C, and the slide was also kept at the same temperature to ensure a constant delivery temperature up to the bead. The solutions were filtered prior to coating and fed into the slots by use of gear pumps (Zenith Co.). A flow visualization technique was used to determine the position of both the static and dynamic contact lines. The static contact line (SCL), where the film formation process begins, is the locus on the die face where the liquid takes off and forms the bead. Similarly, the dynamic contact line (DCL) is the locus where the coating solutions intercept the moving web. The apparent contact angle at the DCL is known as the apparent dynamic contact angle. Detection of both wetting lines has been carried out by looking at the lower meniscus through a 0.25-in.thick optical-quality glass skid plate mounted on a stationary “coating roll” as shown in Figure 4. The coating roll, which supports the web, ensures that the latter one remains parallel to the die face at the application point. A mirror is required to redirect the bar tip and the image from the radial to axial direction
into a scope. The scope (Bausch and Lomb monozoom 7), which is used with a 0.75× objective, 3× amplifier at 3× zoom, is attached to the Sony XC-77 camera. As shown in Figure 4, the scope is slid into the open end of the coating roll so the axes of both the coating roll and the scope lens are aligned. As shown in Figure 5, profiling of the shape is done by aiming a sharply focused line of light under the bead at a grazing angle. A permanent mark or “scratch” on the glass window induces a sharp shadow which is bent at both the liquid/solid interface (SCL) and the point where the bead intercepts the moving web (DCL). Also, the addition of a contrast medium (silver halides) to the solutions further helps to define the position of the liquid/solid interface at the static contact line. The SCL position is characterized by H, which is the distance from the coater tip to the SCL. H can be determined from the monitor. The stability of the flow was characterized by measuring the pressures in the vacuum chamber at the onset of “ribbing” as later defined. At a certain coating speed, subatmospheric pressures are required under the lower meniscus to maintain the bead as explained above. This pressure, above which one or both of the edges usually breaks, is known as the minimum vacuum, Pm. Edges are the locus of flow instabilites and coating flow detachment. Once the coating has been stabilized, it is possible to reduce the pressure (or to increase the vacuum) to a new level, Pr, in which parallel fine lines or ribbing can be observed on the surface of the coating in the machine direction, provided that the coating speeds are higher than 0.3-0.5 m/s; otherwise, “chatter” (lines transversal to the coating direction) is most likely to occur. Ribbing is the primary manifestation of a flow instability or a three-dimensional flow at high speed. The “vacuum range at the onset of ribbing” or “vacuum range” is the difference Pr - Pm. In this study, the vacuum range, Pm, and Pr are reported in inches of water (in. H2O). 2.3.2. Slot Coater. A simple diagram of the coating head is shown in Figure 6, but additional details of this technique have been well described by Gutoff (1992). The coating solution, which wets the coater die face above and below the slot exit, flows from the slot and then across the gap and onto the moving web. Like in the case of the slide, pinning of the bead is accomplished by reducing the pressure P in the vacuum chamber. The experiment consisted of determining the minimum flow or wet thickness t, required to maintain both a stable and a ribbing-free coating. Since slot coating is also a premetered technique and all the fluid fed to
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Figure 6. Schematic diagram of the single-layer slot coater. The coating solution flows from the slot, throughout the gap, G, and onto the moving web. Here, t is the wet coating thickness. Vacuum is required to hold the lower meniscus in position.
Figure 7. Schematic diagram of the curtain coater. The solution, which is delivered from the “delivery slot”, forms a vertical curtain pinned at the edges by the solid edge guides. The solution touches the web at the “dynamic contact point”.
the coating head is applied onto the web, the minimum flow correlates with the minimum thickness. The slot opening and the coating width, which were maintained constant for all experiments, were 100 µm and 15.24 cm, respectively. The distance between the coater head and the web, G, was maintained constant at 100 µm. The initial step is to establish a coating which is both stable and free of ribbing. Subsequently, the solution flow is slowly decreased until the edges become unstable and the thin bead breaks. This flow is considered to be minimum if after repeating the same sequence the coating breaks at similar minimum flows. The experiments were carried out at a vacuum setting that ensured ribbing free coatings (less than 2 in. H2O). 2.3.3. Curtain Coater. In this coating technique previously described by Hughes (1970) in U.S. Patent 3,508,947, solutions are delivered from the slots onto the slide to form a multilayered structure. Then, these solutions form a free-falling vertical curtain that both impinges on the moving web and leads to the wet coated product. The equipment used in our experiments, as shown in Figure 7, consists of a slide, similar to the one previously described. On each side of the slide, two vertical stainless steel rods are used to hold the edges of the curtain in place, thereby avoiding receding of the edges or “necking in”. Both edge guides extend from the slide down to 0.2-0.3 cm above the moving web. The web movement is horizontal with respect to the floor and perpendicular to the edge guides. The distance between the slide and the web is kept constant and equal to 10 cm in all experiments. The quality of the coating is largely determined by the properties of the liquid curtain, the geometry of the slide (Joos et al., 1995), and the flow characteristics (Conroy et al., 1994).
Surfactants affect the stability of the film at the edges. Edge stability is sensitive not only to the geometry of the edge guides but also to the type and flow of liquid used as lubricating/wetting agent. In this study, we investigated the effect of the surface elasticity of the coating solutions on the stability of the curtain. Here, stability is defined as the minimum flow required to establish and to maintain a stable curtain. To define the minimum flow, two failure modes were used: edge breaking and/or the appearance of uncoated areas on the web due to curtain disturbances at the dynamic contact line. After generating the curtain, the film is disturbed by piercing it either at the dynamic contact line in the proximity of the web or at the “quasi “ static contact line where the liquid film touches the edge guide. The curtain is defined as stable if, following the induced perturbation, the falling film either remains stable or breaks at the edges (“edge breaking”) and reforms immediately with minimum operator assistance. In this case, the flow is further reduced and the procedure is repeated. On the contrary, if the curtain is not stable, it suddenly breaks. In this case, the only alternative to reform the film is by increasing the flow to a higher setting than before. If the new setting permits one to obtain a stable curtain, this flow is considered to be the minimum flow. The second failure mode (“bead breaking”) occurs when the curtain breaks in the proximity of the dynamic contact line and regenerates by itself. In this case, small uncoated patches appear on the web. This defect is eliminated by increasing the flow as well. The lowest flow, that ensures both stable edges and uninterrupted coatings are considered the minimum flow, is the number reported in this work. 2.4. Solutions. Gelatin solutions in the range 5-10% (w/w) were prepared with Kind and Knox deionized gelatin and deionized water. The solid gelatin is added into a fast-stirred container with water, and the suspension is kept at 18-20 °C for 5 min. After the slurry is soaked, it is heated up to 60 °C during 20 min until complete dissolution of the gelatin particles. All gelatin solutions were delivered and coated at 40 °C. Glycerin solutions were prepared by dilution of Fisher Scientific glycerin G33-20 with deionized water. Poly(acrylamide) solutions were supplied by Allied Colloid (MW ∼ 120 000) and diluted with deionized water. All solutions were filtered as required prior to coating. Low molecular weight surfactants used in these experiments include Dowfax (Dow Chemical), SS2 (Union Carbide), C12E6 (Nikkol), and Triton X-100 (Union Carbide). These surfactants, that were used as received, are described by formulas S1, S2, S3, S4, and S5, respectively. C12H25 O –O
(S1)
3S
SO3–
C12H25 –O
C12H25 (S2)
O 3S
SO3–
CH3 H3C
Si CH3
CH3 O
Si
CH3 O
Si 1.5
CH3
(S3)
CH3
C3H60(–C2H4O)7.5CH3 C12H25O(–CH2CH2O)6H C8H17
O(–C2H2O)x = 9, 10H
(S4) (S5)
Ind. Eng. Chem. Res., Vol. 35, No. 2, 1996 439
Figure 9. Axis system used in the optical model to determine the chain orientation angle χ at the liquid/air interface (XY plane). The arrow represents the hydrophobic moiety of the surfactant molecule with respect to the interface normal (Z). Figure 8. Optical diagram of an oriented monolayer on an isotropic liquid substrate.
In addition, experiments were carried out using oxazoline polymeric surfactants as shown in the formula S6. (–CH2CH2N)x CO
(–CH2CH2N–)y CO
(S6)
CH2CH2C8F17 CH3
X =1
Y = 6–23
This is a copolymer that results from the copolymerization of 2-ethyl-2-oxazoline and 2-(fluorooctyl)-2-oxazoline in the presence of the initiator 3-[(perfluorooctyl)ethyl]-2-oxazolinium triflate. The details of this reaction have been described by Valentini et al. (1992, 1993). Polymers with y/x ratios between 6 and 23 were used for the elasticity characterization program, although most of the coatings were carried out with oxazolines that had y/x ratios of 13 or higher. 2.5. Fourier Transform Infrared Measurements at the Liquid Surface. We carried out experiments to determine the orientation of the surfactant molecules at the liquid/air interface, using both a Fourier transform infrared (FTIR) technique and an optical model previously described by Fina et al. (1991). Orientation in this context is the relative position of the hydrophobic chain of the surfactant with respect to the liquid/ interface normal. Two structural variables can be extracted from this vibrational spectroscopy technique: molecular conformation and orientation. The conformation state is reflected in the frequency of a peak, while the orientation is reflected in the intensity. As shown in Figure 8, the three phases involved are (1) ambient atmosphere, where the infrared source and detector are located, (2) the surfactant monolayer, and (3) the aqueous subphase. The relationship between the peak intensities and the chain orientation is established through the optical constants. The constants, n and k, are shown in Figure 8 for each layer. For the monolayer, the thickness (d) and the anisotropy in the optical constants are indicated. Also shown are the vectors Es and Ep which represent the fact that the infrared radiation can be polarized in two orthogonal directions. The anisotropy in the optical constants must be known in order to find the surfactant chain orientation. The anisotropy is related to the molecular chain through the vibrational dipole moment (M) and its position in the laboratory frame of reference (Figure 9). The angles
Figure 10. Experimental diagram showing the infrared radiation path from the source to the sample and detector.
which define the average chain in the laboratory coordinate system are R, β, and χ, where χ is the angle between the surface normal and chain axis. This angle (χ) is reported as the “chain orientation angle”. Φ defines the angle between the chain axis and M. The optical model has been developed using the parameters defined in Figures 8 and 9. In addition, three assumptions are made which serve to simplify the mathematics and to approximate the experimental surfactant systems. They are (1) uniaxial chain orientation (i.e., R ) β in Figure 9), (2) no preferred orientation about the chain axis (i.e., M exists at all angles with Φ constant), and (3) the average molecular conformation of the system is constant in the range of parameters studied. A diagram of the experimental setup is shown in Figure 10. The source of light is interferometrically modulated, broad band infrared radiation. The optical components are such that the light reflected from the surface layer is sent to the detector and subsequently processed with the computer associated with the spectrometer. Measurements were carried out on the Dowfax/oxazoline mixture solutions. 3. Results and Discussion 3.1. Slide Coater. 3.1.1. DowfaxsCoatability. The effect of Dowfax on the onset of ribbing was investigated while coating a two-layer package at 2 m/s. In this experiment, the upper layer consisted of a 10% gelatin solution and Triton X-100 surfactant. The lower layer (or underlayer), which is the one under investigation, was a 5% gelatin solution with increasing concentrations of Dowfax surfactant from 10-4% up to 10-1%
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Table 1 solution
surfactant (% w/w)
viscositya (cP)
SSTa (dyn/cm)
DSTa (dyn/cm)
Pr - Pmb (in. H2O)
underlayer A underlayer B underlayer C underlayer D upper layer
(10-4)
5.1 5.2 5.2 5.4 15
52.2 52.2 48.3 43.1 34
66.5 65.5 63.3 59.1 43
0.39 0.85 1.18 0.75
Dowfax Dowfax (10-3) Dowfax (10-2) Dowfax (10-1) Triton-X100 7 × 10-2
a Measurements carried out in the gelatin solutions at 40 °C. DST measured at 1 bubble/s. b Vacuum range figures at steady state (plateau).
Figure 12. Vacuum range, Pr - Pm, at the onset of ribbing as a function of both time after coating start and Dowfax surfactant concentration in the underlayer. Coating speed: 2 m/s. Temp: 40 °C. Two-layer coating.
Figure 11. Surface elasticity modulus of aqueous Dowfax solutions as a function of frequency (25 °C). Data obtained with both the Langmuir trough (0.003-0.17 Hz) and longitudinal wave generator (2 Hz).
(w/w). Viscosities and static (SST) and dynamic (DST) surface tensions are reported in Table 1. The surface elasticity modulus of Dowfax aqueous solutions vs concentration is plotted in Figure 11. In general, as the concentration increases, so does the modulus due to the surface excess. At a certain concentration, increases up to a maximum, signaling that diffusional processes become relevant, and then it decreases monotonically. Relaxation of the surface by diffusion is important when the amount of surfactant in the subsurface is significant compared to that on the surface. Since the subsurface thickness is ∼(D/ω)1/2, where D is the diffusion coefficient and ω is the testing frequency, it is expected that as ω increases and so does the bulk concentration at which reaches a maximum. While at ω ) 0.2 cpm, reaches the maximum (∼40 dyn/cm) at log c (%) ∼ -3.2; at a higher frequency than that, for instance, ω ) 2 Hz, the peak (∼150 dynes/cm) occurs at log c (%) ∼ -2.0. Immediately after establishing the coating bead, several determinations of the vacuum range (Pr - Pm) were performed as a function of time for each solution. For all runs the minimum vacuum Pm was ∼0.08 in. H2O. As shown in Figure 12, as the coating progresses, it is required to apply higher vacuum (lower pressure) to induce ribbing. This means that the operating window becomes wider with time. This trend is distinctive up to 2.5-3.0 min, beyond which the values seem to reach a plateau except in the 10-4% solution. Also, from Figure 12, at any given time after the start of coating, the vacuum range increases as a function of the surfactant concentration up to 10-2% and then it decreases. In other words, it seems that there is an optimum concentration that minimizes the ribbing instability. Higher or lower concentrations than that can be equally detrimental. This is shown in Figure 13 in which both the vacuum range values at t ∼ 3 min and the modulus measured at 2 Hz are plotted vs the
Figure 13. Vacuum range, Pr - Pm, at the onset of ribbing vs Dowfax concentration in the underlayer. Coating speed: 2 m/s. Temp: 40 °C.
solution Dowfax concentrations. The matching of these two curves indicates that, if all other bulk properties remain constant, ribbing seems to be more sensitive to variations of elasticity modulus rather than to changes of either static (SST) or dynamic (DST) surface tensions. Similar results were found while coating a two-layer coating package with Triton X-100 in the underlayer (Valentini et al., 1991). We speculate that the increase of the vacuum range vs time shown in Figure 12 is related as to how the surface elasticity of the lower meniscus varies as a function of time once the bead has been established. To further investigate this assumption, adsorption kinetic measurements of the modulus (t) were carried out on fresh liquid/air interfaces of Dowfax solutions in a Langmuir trough as shown in Figure 14. The idea is to simulate the surfactant transfer that takes place at the bead in an instrument that allows one to characterize the air/liquid interface as a function of time. The curves, which, in general, have a sigmoidal shape, show that is relatively small soon after the interface is established (lag time) and then it sharply raises up to a plateau value. Both the length of the lag time and the magnitude of the plateau value are functions of the surfactant concentration. It is interesting to notice that at times shorter than 5 min, which is the period within
Ind. Eng. Chem. Res., Vol. 35, No. 2, 1996 441
Figure 14. Elasticity modulus and surface tensions of 10-410-1% aqueous Dowfax solutions vs time after a fresh interface was formed in the Langmuir trough.
Figure 15. Model of the liquid/air interface at the lower meniscus in slide coating. The length of the arrows is proportional to the liquid /web velocity. The black circle is a schematic representation of the surfactant molecules at the interface. Additional details shown in Figure 3.
which the vacuum range measurements were carried out (∼3 min), the 10-2% Dowfax solution exhibits the largest modulus, ∼30 dyn/cm. This is concomitant with this solution having the largest vacuum range. The 10-1% solution reaches plateau in a short time as well, but its magnitude (7 dyn/cm) is not as high as that for the previous solution. The largest modulus, 70 dyn/cm, was measured on the 10-3% solution, but becomes significantly large only 10 min after the interface is formed, and this time is beyond the range at which the coating measurements were performed; therefore, the coating window is not the widest. Finally, the modulus of the 10-4% solution was essentially flat throughout the testing range and that resulted in the smallest vacuum range (0.4 in. H2O). 3.1.2. DowfaxsFlow Visualization. Flow visualization studies carried out while coating 5% gelatin/ silver solutions with different Dowfax concentrations in the underlayer clearly show that, immediately after the coating start, the SCL moves from the coater tip down into the vacuum chamber. Certainly during this transient time (∼0.5 min), Dowfax migrates to both the liquid/air and liquid/coater interfaces as they generate. Once all involved forces are balanced, the position and geometry of the lower meniscus reaches a quasi static position. Inspection of the monitor reveals erratic and low-amplitude movements of the SCL around a certain position. Figure 15, which is a hypothetical depiction of the lower meniscus, shows a high concentration of hydrophobic heads (black circles) at the SCL as Dowfax
is able to diffuse and adsorb onto the liquid/air interface. Close to the dynamic contact line (DCL), the deformation rates are extremely high (10-4 s), and convective processes are unable to replenish the interface. Therefore, it becomes depleted. At a molecular level, the final arrangement of the surface is a function of molecular interactions. Hydrophobic interactions promote attraction between the adjacent alkyl groups and tend to pack the molecules together on the air side. Meanwhile, on the liquid side, the hydrophilic heads have a strong affinity for water; therefore, they tend to space out to allow as much water as possible to solvate the head groups. In the case of the Dowfax surfactant (S1, S2) repulsion is exacerbated because the groups are charged. The balance between attractive and repulsive forces will lead to a surface packing that not only decreases the free energy of the system (Israelachvilli, 1987) but also will affect how the interface relaxes after deformation; for instance, as it occurs after the vacuum is increased. The region of the interface which contains a high surfactant concentration is practically stagnant and it behaves as if the liquid were covered by a rigid thin plate or “membrane” very similar to that found in bubble lamellae. The presence of this “rigid” surface has significant implications: (a) The membrane modifies the flow field of the adjacent liquid since the tangential stress at the air/ liquid interface should be balanced by the surface tension gradient. The boundary condition along the interface becomes η(dνs/dy) ) dσ/ds, and this imposes a “retarding “ effect on the flow. This boundary condition should be included in modeling coating flows. (b) Because of surface elasticity, this rigid region is expected to counteract potential surface disturbances. For instance, following a very small vacuum increase, the lower meniscus is forced into the gap with a concomitant increase of its area. Although, in principle, this deformation may threaten the integrity of the bead, it also leads to the sudden appearance of surface tension gradients along the stretched area. To balance these gradients, low tension areas of the surface flow toward the high tension ones (Marangoni, 1878). In doing so, the moving membrane tends to carry with it the adjoining bulk liquid, that not only helps to restore the original shape of the bead but also delays the onset of ribbing. The correlation from Figure 13 certainly supports this view. (c) Finally, at steady state, the presence of a rigid membrane could conceivably affect the geometry of the lower meniscus. This hypothesis was investigated by optical inspection of both the SCL and DCL positions as follows. Underlayers with Dowfax surfactant at different concentrations in the range 10-4-10-1% were coated as part of a two layer coating structure at 2 m/s. The underlayer consisted of a 5% gelatin solution containing 0.4% silver halide which was added as an optical contrast medium. The upper layer was the same as that reported in Table 1. The vacuum under the bead was kept constant at 0.7 in. H2O. The SCL positions (H) vs surfactant concentrations are shown in Figure 16. Because the coater die is made of stainless steel, results of static contact angle measurements carried out with the coating solutions on this material were included. The elasticity modulus at 2 Hz was also included in this figure.
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Figure 16. SCL position (H) determined on a two-layer coating at a coating speed of 2 m/s. Elasticity moduli at 0.16 and 2 Hz were determined in aqueous solutions at 25 °C. The contact angles of the underlayer solutions were measured on stainless steel at 40 °C. All variables were measured as a function of the Dowfax concentrations.
As the Dowfax concentration increases up to 10-2%, the wetting characteristics of the underlayer improve and the SCL moves downward (H increases). The contact angle values decrease. Above that Dowfax concentration, the SCL begins to recede back toward the coater tip. Conversely, the static contact angle values of the same solutions continue decreasing monotonically due to lower surface tensions. The maximum of the H curve and those of the vacuum range at the onset of ribbing and surface elasticity occur approximately at the same surfactant concentration range (10-3-10-2%) (see Figure 13). The tape used to film the flow visualization experiments was edited to be able to show the SCL positions of all four solutions in the same frame. These positions as well as the position of the coater tip are indicated by an arrow in Plate 1 (top). For instance, it shows the relative position of the SCL on solution D (white/blue color interface) as well as those that had been obtained while testing solutions A-C. The proposed mechanism to explain this outcome is that as the Dowfax concentration increases, the wetting properties of the solution improve; therefore, H increases. At larger concentrations than 10-2%, the surface elasticity of the rigid membrane becomes significant and the interface is less likely to withstand stretching. As a consequence, the contact line recedes back toward the coater tip. First of all, these results indicate that the region close to the SCL is dynamic and it is sensitive to any surfactant action. Second, provided that all variables remain constant, the SCL position seems to be the result of a balance between two competing forces: one of capillary origin and the other one due to surface gradients, respectively. While the former ones promote wetting, the latter ones determine the extent of the deformation and the final geometry of the meniscus. Overall, the SCL position H correlates with the underlayer surface elasticity which is essentially a dynamic property. In the proximity of the web or DCL (Figure 15), the surfactant concentration at the interface is low; thereby η(dνs/dy) f 0 and the liquid velocity profile should adjust to the new boundary condition. In this region the membrane should be both diffuse and “mobile” because of a different surfactant packing at the interface. Using the same tape editing technique, we confirm that speculation. The DCL remains in the same position as the Dowfax concentration increases from 10-4 up to 10-1%. As shown in Plate 1 (middle), all four arrows overlap in the same place. This region is less sensitive to any surfactant action than the SCL.
Plate 1. (top) Arrows show the SCL positions obtained while testing solutions A (10-4%), B (10-3%), C (10-2%), and D (10-1%). The coater tip position is indicated by “TIP”. This picture shows superimposed images from all four coating experiments. (middle) The DCL positions for all four solutions A, B, C, and D are the same, and they are shown by the upper single arrow. The letters D and C beneath indicate where the static contact line positions of these two solutions are located with respect to the DCL. This picture shows superimposed images of the DCL and SCL flow visualization experiments. (bottom) Once the injection of the oxazoline solution begins (INJECTOR ON), the SCL position moves downward into the gap. It can be recognized by the difference in colors (white/black) Coating speed: 2 m/s.
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A significant conclusion for the coating practitioner is that, according to Figures 11 and 13, there is a frequency range at which the modulus can be measured and maximized to prevent ribbing. In general, that is at low frequencies, for all practical purposes in the range 0.2-2 Hz. Considering the potential high rate of extensions that may be conceivable at the bead, these values seem to be low. We speculate that this is a consequence of the multiple deformation rates and different concentrations that are possible along the interface. The lower meniscus can be imagined as a sequence of elements i which have both variable areas Ai and different surface tensions σi, according to their position. Elements close to the SCL will deform at a slower rate than those near the DCL. Therefore, the surface elasticity of the ith element can be described as
i(x) )
dσi(Γi,x) d ln Ai
(13)
Figure 17. Elasticity modulus and surface tensions of 10-410-1% Dowfax solutions without and with oxazoline polymer (10-2%) vs time after a fresh interface was formed in the Langmuir trough.
where Γi and σi are the surface concentration and surface tension, respectively, and x is the position of that element. At the location of the ith element, i.e., x, there is a charcteristic surface deformation frequency ω which will be increasingly higher as the element gets closer to the DCL. Therefore, i from equation 13 can be converted into an expression i(ω) in the frequency domain. Because the “effective” elasticity should account all elements present at the lower meniscus, it is necessary to define a weighed average expression
j )
∫0∞i(ω) ψ(Γ,ω) dω
(14)
that includes the modulus value i in the range ω and ω + dω as calculated from eq 13 as well as ψ(Γ,ω) which takes into account the fraction of elements that have that elasticity. This leads to a spectrum function which predicts not only the modulus of the lower meniscus but also the critical frequencies at which the effective elasticity j becomes significant. As discussed, diffusional processes are unable to short-circuit surface tension variations at high extension rates as found at the DCL; thereby, the expected proportion of elements that both undergo high-frequency deformations and have i(ω) > 0 is probably very small. Most likely, j will be influenced by those elements that deform and relax very slowly. 3.1.3. Dowfax-Oxazoline MixturesCoatability. The addition of oxazoline surfactant (10-2%) to the aqueous Dowfax solutions (10-4-10-1%) has a significant impact on the surface properties, as shown in Figure 17. For instance, (t) at short times, at least within the first 5 min, is higher than the same parameter for the pure Dowfax solution, particularly in the case of the10-4, 10-3, and 10-1%. At the 10-1% Dowfax concentration, the oxazoline addition does not significantly impact the surface tension as much as the surface elasticity modulus which is on average ∼13 dyn/cm higher than the that of pure Dowfax. Equally important is the fact that, in the case of a Dowfax/oxazoline mixture, the polymer enhances the surface modulus at low frequencies, as shown in Figure 18. Gelatin solutions, containing the same Dowfax concentrations in the range 10-4-10-1% and oxazoline (10-2%), were coated in the underlayer. To rule out upsets of the bulk rheology due to the presence of oxazoline, both the elastic and viscous moduli G′ and
Figure 18. Elasticity modulus vs frequency of Dowfax/gelatin (5%) solutions and Dowfax/oxazoline (y/x ) 14)/gelatin (5%). All measurements were carried out at 40 °C.
Figure 19. Vacuum range at the onset of ribbing vs Dowfax concentration without and with oxazoline polymer (10-2%) in the underlayer. Measurements carried at coating speeds of 2 m/s.
G′′ were measured. The addition of oxazolines, at least in this concentration range, does not significantly affect these bulk rheological properties. The vacuum range was determined according to the same protocol used in the prior study; although, in this case the measurements were performed at constant time (3-4 min after coating start). The overcoat layer and the coating conditions were reproduced as indicated above. As shown in Figure 19, the addition of the polymeric surfactant has a significant and positive impact on the vacuum range at low and high Dowfax concentrations. For instance, at 10-4% Dowfax concentration, both the reduced static surface tension and increased surface elasticity enhance the vacuum range window from 0.35 to 0.9 in. H2O. A similar effect, although not as intense, was observed for the 10-1% solution. In this case, the vacuum range increased from 0.7 to 1.15 in. H2O. In general, the vacuum range coating window vs concentration becomes flat and less responsive to surfactant variations. For instance, the addition of oxazoline to the underlayer
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Figure 22. Vacuum at the onset of ribbing Pr and vacuum at edge break Pm for a two-layer coating at coating speeds of 1.52 m/s vs oxazoline monomer ratios y/x in the underlayer. All underlayers have Dowfax surfactant (0.004%). The SST range of all solutions is 42-44 dyn/cm.
Figure 20. Chain orientation angle χ (see Figure 9), vs oxazoline monomer ratio y/x. Small χ values imply a vertical hydrophobic chain with respect to the liquid/air interface normal. Parenthetical values indicate surface tensions.
Figure 21. Elasticity moduli of oxazoline surfactant/(5%) gelatin solutions vs y/x oxazoline monomer ratios at 0.033 and 0.082 Hz at 40 °C.
allows one to reduce the Dowfax concentration from 10-3 to 10-4% without a significant upset of the coating latitude. The use of surfactant mixtures in photographic solutions is customary. Polymeric surfactants are very attractive to the coating scientist because their structures can be tailored to meet properties that are different according to the final use of the surfactant (Schmidt et al., 1990). Similarly, oxazolines can be modified to be able to vary the surface elasticity of aqueous solutions as desired. For instance, in our experiments, the monomer ratio (y/ x) as defined in S6 was varied from 6 to 23. As y/x increases, the fluorocarbon concentration at the liquid/ air interface decreases, leading to more spacing between chains. As shown in Figure 20, small orientation angle values (χ) at high fluorocarbon concentrations (y/x f 6) suggest that the chains adopt an almost perpendicular position with respect to the surface of the liquid. The interface behaves similarly to that of a solution containing a high concentration of a low molecular weight fluorocarbon surfactant. As y/x increases, the hydrophobic moieties tilt toward the air/liquid interface and the surface elasticity of the solution increases up to an asymptotic value (Figure 21). A coating study, which included underlayers with oxazoline surfactants of different y/x monomer ratios, was performed. The underlayer consisted of a 5% gelatin solution which contained both oxazoline and Dowfax surfactants at constant concentrations, 0.01 and 0.004%, respectively. The monomer ratio (y/x) of the oxazoline surfactant was varied from 6 to 23. The overcoat had the same composition as the one described
in Table 1. Both Pr, the maximum vacuum at ribbing, and Pm, the minimum vacuum to hold the edges, are shown in Figure 22. While Pm remains essentially constant or increases slightly, Pr seems to match the surface elasticity curve. It is as if the polymeric oxazoline modulates the behavior of the interface and the onset of ribbing. The polymer configuration on the liquid surface plays an important role on both the adsorption kinetics and the dynamic properties of the liquid/air interface (Glass (1968) and Serrien et al. (1992)). The dynamic behavior of surfaces which contain surface-active polymers in the presence of low molecular weight surfactant (i.e., Dowfax/oxazoline) is difficult to predict. It is expected that, following a surface expansion, both surfactants will compete to access the interface based on their relative diffusivities. The orientation of the polymer on the surface will depend not only upon the y/x ratio but also on the relative surface concentrations of the other surfactant and the surface interaction between the two chemicals. Synergism, that is, enhanced adsorption of one species due to the presence of a relatively insoluble polymer at the interface, should not be ruled out as well (Jiang et al., 1995b). 3.1.4. Dowfax-Oxazoline Flow Visualization. The SCL position of the mixture Dowfax-Oxazoline was investigated by injecting an oxazoline into the underlayer. Both the overcoat and underlayer were prepared as described in Table 1. After injection and mixing, the final Dowfax and oxazoline concentrations in the underlayer were 10-1% and 10-2% (w/w), respectively. The experiment was started without injection and the SCL position of the underlayer containing Dowfax determined. This position is shown by the arrow “INJECTOR ON” in Plate 1 (bottom). At that time the injection flow was turned on up to 0.1% of the total flow, and very slowly the SCL began moving downward. After 2-3 min, the SCL reached a final position as shown in the same Plate. The surfactant concentrations of the mixture correspond to case D in Figure 17. The underlayer static surface tension of the Dowfax/oxazoline mixture should be essentially the same as that of the Dowfax only underlayer. However, after the injection process started, a significant enlargement of the liquid/air interface was observed, presumably as a consequence of the change in surface elasticity resulting from the oxazoline adsorption/interaction at the liquid/air interface. An increased modulus promotes a change of the geometry of the meniscus: essentially an increase of its area. Variations of shear surface viscous forces due to the oxazoline surfactant cannot be totally ruled out, but there is
Ind. Eng. Chem. Res., Vol. 35, No. 2, 1996 445 Table 2. 11% Gelatin Solutionsa
Table 4 b
Triton viscosity SST (σ) DST X-100 (%) (µ)a (cP) (dyn/cm) (dyn/cm) (dyn/cm) 0.0001 0.005 0.03
27.2 26.8 26.0
53.8 44.6 32.8
66.8 61.1 50.8
15 22 8
Cac
t (µm)
Triton X-100 (%)
viscosity (µ)a (cP)
SST (σ)a (dyn/cm)
DSTa (dyn/cm)
0.25 0.3 0.4
43 25 51
0.0001 0.001 0.003 0.007 0.01 0.05 0.0001 0.001 0.003 0.007 0.01 0.05
14.2 14.1 14.1 14.0 13.7 13.5 5.3 5.2 5.0 5.1 5.2 5.3
64.2 57.8 53.1 49.2 43.9 33.9 57.6 52.7 47.0 39.0 36.7 31.2
70.6 70.0 68.2 68.0 64.9 55.3 69.0 69.6 67.8 65.7 64.1 55.5
a All properties were determined at 40 °C. b modulus measured at 0.17 Hz. c Capillary number (µ/σ)U.
Table 3. 6.7% Poly(acrylamide) Solutionsa Triton viscosity SST (σ) DST b X-100 (%) (µ)a (cP) (dyn/cm) (dyn/cm) (dyn/cm) 0.0001 0.005 0.03 a
29.0 29.0 28.6
60.6 43.3 37.1
70.0 64.0 53.0
15 17 0
Cac
t (µm)
0.24 0.34 0.4
32 26 105
a
All measurements carried out at 40 °C.
b
All properties were determined at 25 °C. modulus measured at 0.17 Hz. c Capillary Number (µ/σ)*U.
Figure 23. Minimum wet thickness “t” vs in a single-layer slot coating experiment at a coating speed of 0.5 m/s, (Tables 2 and 3). Binders are 6.7% poly(acrylamide) (white circles) and 11% gelatin (black circles). Triton X-100 was used as the surfactant.
uncertainty regarding the determination of these forces. They can be associated with enhanced thin film stability and changes in geometry of a liquid/air interface as well (Clunie et al., 1971). 3.2. Slot Coater. All experiments were carried out at constant coating speed and vacuum: 0.5 m/s and 2.5 in. H2O, respectively, using the hardware previously described. Two solutions consisting of gelatin (11%) and poly(acrylamide) (6.7%) were used in two different runs. The surface elasticity modulus of each coating solution was adjusted by adding Triton X-100 surfactant. Although the delivery and the coating temperatures were the same for all runs, the gelatin and poly(acrylamide) experiments were carried out at 40 and 25 °C, respectively. All measured solution properties are shown in Tables 2 and 3. The coated layer wet thickness t was calculated for each run from a mass balance and it is shown in Tables 2 and 3, respectively. Also, in the case of the gelatin coatings, the t values were obtained from gravimetric gelatin coating weight determinations on dried film samples and compared to the theoretical values (Tables 2 and 3). There was good agreement between the two numbers. As shown in Figure 23, t correlates with the elasticity modulus . Lee et al. (1992) had reported minimum wet thickness values obtained from slot coating experiments, and their results have been discussed by Gutoff (1992). According to their charts, we should have expected t/G ratios ∼0.7; hence, t ∼ 70 µm which is significantly higher than our results. At low capillary numbers and low coating weights, surface forces become relevant and (rather
Figure 24. Minimum flow and vs Triton X-100 concentrations for a single-layer curtain coating experiment using glycerin as the binder. Coating speed: 1 m/s. Flow decreases in the positive y direction.
than σ) seems to determine both the resistance of the film to rupture and thereby the minimum wet thickness. In both experiments the Triton X-100 concentrations that maximize the surface elasticity in both binder solutions, gelatin and poly(acrylamide), are similar. Surface forces at maximum are usually determined by the surfactant. 3.3. Curtain Coater. 3.3.1. Curtain CoatersOne Layer. Solutions were delivered throughout a single slot and the minimum flows determined as previously explained in the experimental protocol. The coating speed was kept at 1 m/s, and a combination flow visualization/tape recording technique was used to study the stability of the curtain. The first series of experiments was carried out with Triton X-100 surfactant dispersed in glycerin solutions with viscosities ∼5 and ∼ 14 cP, respectively. Properties are shown in Table 4. Values of minimum flow determined during the coating experiment as well as elasticity moduli obtained on the low viscosity solution are shown in Figure 24. At both viscosities, the curtain is most stable at an optimum surfactant concentration (cm) approximately equal to 0.003%. The surface elasticity modulus measured in the range 0.15-0.2 Hz reaches a maximum at that concentration as well. This frequency is smaller than the 1-2 Hz found to be the best to study coatability
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Figure 25. Curtain breaks at the dynamic contact point and forms a Λ-shaped defect. The curtain remains pinned at the edges. The coated film shows patches.
of the slide coater. Presumably, this is due to the different residence times of the liquid in both the curtain and the slide coater bead surfactant types and binders. A review of the experimental observations on the tape shows that the onset of the first failure mode, that is, breaking at the edges, occurs at the edge guides, close to the web. This mode is prevalent at surfactant concentrations lower than cm or when d/dc > 0 where c is the surfactant concentration. That is on the left side of the vs concentration curve. Occasionally, the curtain breaks in the middle close to the web as well. This leads to the formation of a Λ-shaped disturbance in the center of the vertical film, while the edges remain intact (Figure 25). This event is normally followed by an abrupt rupture that leads to aimless streamers falling from the hopper. This decreased stability of a film at low surfactant concentrations (c < cm) or at the left-hand side of the maximum elasticity has been previously pointed out by Lucassen (1981). Normally extension of these films invariably leads to areas that both have low elasticity and are prone to rupture. The second failure mode, that is, the appearance of uncoated patches on the web, occurs at surfactant concentrations significantly higher than cm. In this surfactant concentration range, the bead is susceptible to disturbances that lead to both breaking of the coating at the dynamic contact point rather than in the curtain and the formation of uncoated patches on the web. Overall, in this experiment the stability of the curtain is largely controlled by the edges as previously pointed out by Christodoulou and Do (1994). The edges can be stabilized by maximizing the elasticity modulus of the coating solutions. Edge guide wetting additives are commonly used in industry, and the benefit of using them might be related to their surface elastic properties as well. A potential dilution of the coating solution might be encountered if the concentration of the edge guide wetting agent is not properly optimized. As a consequence, an unstable curtain due to a less than optimum surfactant concentration may become totally unstable because of dilution at the edge guides. The opposite effect, an improvement of the vertical film stability, may occur if the original coating solution concentration is significantly larger than cm. In this case, dilution may help to stabilize the edges. This is illustrated in Figure 26, which was obtained while coating glycerin solutions containing variable amounts of Triton X-100. The experiment was carried out both ways: with and without wetting of the edges. To illustrate this effect, distilled water at a low flow was used in both edge
Figure 26. Minimum flow and vs Triton X-100 surfactant concentration while coating a single-layer experiment at a coating speed of 1 m/s with and without wetting edges. Wetting agent: water. Flow decreases in the positive y direction.
Figure 27. Surface elasticity modulus vs SS2 surfactant concentrations without (white triangles) and with C12E6 (10-5%) (black circles). Measurements were carried out in a 60% glycerin solution.
guides, during the “WET EDGE GUIDE” studies. For instance, while coating a solution with high surfactant concentration (log c (%) ∼ -1.5) and low elasticity modulus , the flows can be reduced from ∼120 down to ∼55 cm3/min‚cm by using water at the edges. On the contrary, at c , cm, the opposite effect is possible. In this case (log c (%) ∼ -4), a dilution of the edges weakens the curtain and the flow must be increased from 85 up to 110 cm3/min, in order to be able to sustain the vertical film. Surfactant mixtures were studied as part of this protocol as well. The goal was to define the stability of the curtain as a function of the relative concentration of each species in the mixture. Synergistic effects have been reported before on the stability of thin films containing binary surfactant systems. An increased stability of sodium lauryl sulfate foams was observed because of the presence of a small amount of dodecanol (Prins and Van den Temple, 1964; Clunie et al., 1971). In this study, C12E6 and SS2 described by formulas S4 and S3, respectively, were chosen because the elasticity modulus of this mixture is discernibly different from that of the SS2 (Figure 27). The coating experiments were carried out to study the stability of the glycerin film in the presence of both the pure SS2 surfactant and that of the mixture containing SS2 and C12E6. The SS2 concentrations in the mixture were varied from 10-6 up to 10-1%, while the C12E6 one was kept constant at
Ind. Eng. Chem. Res., Vol. 35, No. 2, 1996 447 Table 5 Triton X-100 (%) 0.0001 0.001 0.003 0.007 0.01 0.025 overcoat 0.05 a
viscosity (µ)a (cP)
SST (σ)a (dyn/cm)
DSTa (dyn/cm)
8.7 8.6 8.5 8.5 8.5 8.5 22.0
45.8 39.4 42.1 37.8 34.3 32.2 34.0
67.5 67.3 66.2 62.8 60.8 47.2 51.3
All solutions contained 0.005% methanol.
Figure 28. Minimum flow vs SS2 surfactant concentration in a curtain coating experiment at a coating speed of 1 m/s using a glycerin binder without (black circles) and with C12E6 (10-5%) (white triangles). Flow decreases in the positive y direction.
10-5%. At low SS2 concentrations, the modulus of the mixture is determined by the concentration of C12E6. As the SS2 concentration increases, it becomes the dominant species at the interface and determines the dynamics of the interface. Figure 28 shows that the curtain is most stable at the same SS2 concentration at which reaches the maximum, that is, in the range 0.001-0.01%. For instance, at a concentration close to 10-6%, the quality of the curtain operating at 70 cm3/min‚cm is poor or severely compromised. However, the addition of C12E6 to the glycerin solutions stabilizes the edges at that low flow. Τhe implication is that, by using such a mixture, it is feasible to reduce the SS2 concentration, i.e., down to 10-5%, without a significant decrease of . Modeling the surface elasticity of a surfactant mixture has been discussed by several authors (LucassenReynders, 1973; Garret and Joos, 1976; Jiang et al., 1995b) and the interpretation of experimental data is complicated. For instance, in the case of two soluble surfactants, i.e., i ) 1, 2, whose adsorption/desorption is controlled by diffusion, the resulting surface dilational elasticity is given by (Jiang et al., 1995b)
)-
( )
1 ∂σ b ∂T1
[n1a1Γ1 + n2a2Γ2 + n1n2(a1a4 -
Γ2
a2a3)Γ1] - -
( )
1 ∂σ b ∂Γ2
[n2a4Γ2 + n1a3Γ1 + n1n2(a1a4 -
Γ1
a2a3)Γ2] (15) where the surface tension of the mixture and the bulk surfactant concentrations are σ and ci, respectively. Γi and Di are the surface concentration and the diffusion coefficient for the i species, respectively. The slopes of the adsorption isotherms, ai, are as follows:
a1 )
( )
( )
( )
( )
∂Γ1 ∂Γ1 ∂Γ2 ∂Γ2 ; a2 ) ; a3 ) ; a4 ) ∂c1 c2 ∂c2 c1 ∂c1 c2 ∂c2 c1 (16)
and
b ) 1 + n1a1 + n2a4 + n1n2(a1a4 - a2a4)
(17)
ni ) (iω/Di)1/2
(18)
where
Figure 29. Minimum flow and vs underlayer Triton X-100 concentration in a two-layer curtain coating experiment at a coating speed of 1 m/s. Flow decreases in the positive y direction
3.3.2. CurtainsCoatersTwo Layer. All experiments were carried out according to the same protocol as used in the single-layer coatings (3.3.1). The underlayer consisted of a glycerin solution with variable amounts of Triton X-100 surfactant. As before, the minimum flow of the bottom layer was used to characterize the curtain stability. Triton X-100 was used in the overcoat as well, although its concentration was kept constant at 0.05%. The flow of the overcoat solution was maintained at 29 cm3/min‚cm throughout all experimental runs. The solution properties at 25 °C are shown in Table 5. All coating results are shown in Figure 29 which indicates that the minimum flow to ensure a stable curtain correlates with the surfactant concentration of the underlayer. This has been previously reported by Franke et al. (1994); however, it becomes clear that the surface elasticity modulus of the lowermost layer is the critical variable as in slide coating experiments. The lower layer with the highest Triton X-100 concentration (0.025%) was very unstable on the slide, and the curtain was never properly established. Presumably this is due to the low underlayer dynamic surface tension with respect to that of the overcoat. Flow instabilities on the slide due to an unusual ratio of dynamic surface tensions have been reported before (Valentini, 1991). 4. Conclusions Slide, curtain, and slot coater operations involve the use of additives, surfactants, and polymers, that impact the quality of both the coating process and that of the final product. Under dynamic conditions, for instance, following extensions, these species lead to surface tension gradients and surface forces. The magnitude of the latter ones can be predicted by the elasticity
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modulus . As such, this parameter is an important variable in coating processes. With everything else the same, the surface elasticity of the layers determine the onset of ribbing and the minimum wet thickness that can be achieved before the bead breaks. This is particularly important in the case of the lowermost layer of a multilayer package. For most solutions, the surface elasticity modulus vs surfactant concentration increases, reaches a maximum, and then decreases as a consequence of the mass transfer in the diffusional sublayer. In this study, surface elasticities were successfully measured at low, intermediate, and high frequencies by using the Langmuir trough, the longitudinal wave generator, and the capillary wave generator, respectively. In the specific case of the slide coater, the surface elasticity of the lowermost layer seems to be a key parameter that affects the SCL position as well as the vacuum range at the onset of ribbing. Flow visualization studies show that the SCL position seems to be the result of a balance between capillary forces that promote wetting of the solution on the solid surface and surface forces due to gradients, that control the deformation of the liquid/air interface. As the elasticity of the lower layer increases, the SCL position moves further down into the coating gap and increased vacuum levels are necessary to induce ribbing. The modulus should be measured at 1-2 Hz to optimize the surfactant concentration of this layer. A conclusion that requires further research is that, following a deformation of the meniscus, the area close to the SCL might be intimately involved in the onset of ribbing. We postulate that the same mechanisms that explain the thinning behavior of thin soap films (Langevin, 1994) might be involved in coating, especially during transient deformations of the liquid/air interface. On the contrary, the dynamic contact line (DCL) position seems to be less sensitive to the lowermost layer surfactant concentration. Surface elastic forces are equally important in curtain coating operations. They seem to control the stability of the vertical film both at the edge guides and at the dynamic contact line, preventing abrupt ruptures or poor coating quality. Increased surface elasticities allow one to decrease the wet thickness of the final coating in both single and double-layer structures. In the last case, proper surfactant optimization of the lower layer is equally important to achieve a stable vertical film at the vertical edge guides. The elasticity modulus , which should be measured at 0.1-0.3 Hz, is a better predictor of curtain stability than the static (SST) or the dynamic (DST) surface tensions. Experiments performed with the slot coater show that, at both low coating weight and low viscosities, the minimum wet thickness can be reduced by increasing the surface elasticity modulus of the coating solutions. Surfactant mixtures containing low molecular weight species or polymer/low molecular weight surfactants enhance the operating windows of both the slide and curtain coaters. Although this effect seems to be additive, the possibility of synergism at the liquid/air interface is under investigation. Although diffusional processes are the key to explaining most surface elasticity phenomena, the chemical structure of the surfactant at the liquid/air interface plays a significant role, especially in a homologous series as well as surfactant mixtures. FTIR techniques proved to be useful in characterizing the orientation of the hydrophobic chains on the liquid surface.
Acknowledgment The authors thank Drs. E. B. Gutoff and J. Lucassen for their helpful comments and suggestions and W. R. Thomas for his work on flow visualization techniques. Also the authors thank Mr. L. E. Hall and Mr. C. T. Wilson for performing the slot coater experiments. Literature Cited Adamson, A. W. Physical Chemistry of Surfaces; Wiley-Interscience: New York, 1982; pp 2-23. Cerro, R. L.; Whitaker, S. J. The effect of surfactants on the hydrodynamic development of thin films. Colloid Interface Sci. 1971, 37 (1), 33-51. Christodoulou, K.; Do, D.-V. On the stability of Curtain Coater. Presented at the National Spring Meeting of AIChE, Atlanta, GA, April 17-21, 1994. Clunie, J. S.; Goodman, J. F.; Ingram, B. T. Surface and Colloid Sciences; Matijevic, E., Ed.; Wiley: New York, 1971; Vol. 3, p 167. Conroy, E. J.; Ruschak, K. J. Curtain Coating Method and Apparatus. U.S. Patent 5,358,569, 1994. Fina, L. J.; Tung, Y. S. Molecular orientation of monolayers on liquid substrates: Optical model and FT-IR methods. Appl. Spectrosc. 1991, 45, 986. Franke, M.; Hirshburg, R. I. Personal communication, 1994. Garret, P. R.; Joos, P. Dynamic dilational surface properties of submicellar multicomponent surfactant solutions. J. Chem. Soc., Faraday Trans. 1 1976, 10, 2161. Glass, D. Adsorption characteristics of water-soluble polymers Poly(vinyl alcohol) and Poly(vinylpyrrolidone) at the aqueousair interface. J. Phys. Chem. 1968, 72 (13), 4450. Gutoff, E. B. Premetered Coating. In Modern Coating and Drying Technology; Cohen, E., Gutoff, E., Eds.; VCH Publishers: New York, 1992 Hansen, R.; Ahmad, J. Waves at interfaces. J. Prog. Surf. Membr. Sci. 1971, 4, 1. Red from 49 Ishiwata, M.; et al. Process of producing photographic materials. EP 0383347 A2, publication number 0383347, 1990. Israelachvili, J. Physical principles of surfactant self-association into micelles, vesicles and microemulsion droplets. In Surfactants in Solutions; Mittal, K. L.; Bothorel, P., Eds.; Plenum: New York, 1987; Vol. 4, p 3. Hughes, D. J. Method for simultaneously applying a plurality of coated layers by forming a stable multilayer free falling vertical curtain. U.S. Patent 3,508,945, 1970. Jiang, T. S.; Lee, O. H.; Yen, S. C.; Valentini, J. E.; Thomas, W. R.; Sevenhuysen, P. The essence of surface dilational modulus in thin film coating. Presented at the Spring National Meeting of the AIChE Session IV, Orlando, FL, March 18-22, 1990. Jiang, Q.; Chiew, Y. C.; Valentini, J. E. Damping of cylindrical propagating capillary waves on monolayer-covered surfaces. Langmuir 1992, 8, 2747. Jiang, Q.; Chiew, Y. C.; Valentini, J. E. The study of surface dilational properties of nonionic surfactant solutions by propagation of electrocapillary waves. J. Colloid Interface Sci. 1993a, 155, 8. Jiang, Q.; Chiew, Y. C.; Valentini, J. E. An apparatus for the study of surface longitudinal waves at the air/water interface. J. Colloid Interface Sci. 1993b, 159, 477. Jiang, Q.; Chiew, Y. C.; O’Lenick, C.; Valentini, J. E. Dynamic penetration of surfactant into an insoluble monolayer. Langmuir 1995a, 11, 1138. Jiang, Q.; Chiew, Y. C.; Valentini, J. E. Theoretical model for the dynamic dilational surface properties of binary surfactant mixtures. J. Colloid Interface Sci. 1995b, in press. Joos, F. M.; Ruschak, K. J. Curtain Coater Slide Hopper with Improved Transition Profile and Method. U.S. Patent 5,399,385, 1995. Langevin, D.; Sonin, A. A Thinning of soap films. Adv. Colloid Interface Sci. 1994, 51, 1-27. Lee, K.-Y.; Liu, L.-D.; Liu, T.-J. Minimum wet thickness in extrusion slot coating. Chem. Eng. Sci. 1992, 47, 1703-1713. Lucassen, E. H. Properties of Capillary waves. J. Adv. Colloid Interface Sci. 1969, 2, 347. Lucassen, J.; Dynamic Properties of Free Liquid Films and Foams. In Anionic Surfactants; Lucassen-Reynders, E. H., Ed.; Dekker: New York, 1981.
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Received for review June 29, 1995 Accepted October 23, 1995X IE950398L
X Abstract published in Advance ACS Abstracts, December 15, 1995.
