Effects of shear thinning on forward roll coating

This paper described the forward mode roll coating process of generalised non-Newtonian fluids characterised by the Ellis model. The fluid in the coating bead and the free surface formation are described by the lubrication approximation and the stability is also considered using a perturbation analysis of the downstream meniscus. Results highlight the complex behaviour of this coating process and volume flow rate and film thickness results are obtained for a range of operating conditions. The stability of the downstream meniscus is observed to improve with increasing level of shear thinning when τ_1/2 (the shear stress at which the viscosity is half that of the zero shear stress viscosity) is small, however as τ_1/2 increases the meniscus stability decreases with increasing levels of shear thinning.     

High speed wire coating by withdrawal from a bath of viscoelastic liquid

Data are presented for the thickness H_∞ of liquid coating entrained by continuous withdrawal at speed U of a wire of radius R from the free surface of a large bath. For Newtonian fluids of viscosity μ, density ρ and surface tension σ, the data are carried out to coating speeds beyond the applicability of current theories, to Capillary numbers of nearly one hundred. In the high speed range the data, which cover several orders of magnitude in viscosity, can be well represented by the equation for U_μ/σ = N_ca > 3. All data presented are at an essentially constant Goucher number of 0.08, where N_Go ≡ R(ρg/2σ)^1/2. Data for viscoelastic fluids show phenomena quite distinct, qualitatively and quantitatively, from Newtonian observations. In particular, strongly elastic fluids show a markedly reduced ability to be entrained onto the wire. Further, the coating thickness appears to become independent of Capillary number at high speed.     

Thinnest uniform liquid films formed at the highest speeds with reverse roll coating

Reverse roll coating is probably the most widely used coating operation, much less investigated than its counterpart and inherently unstable forward roll coating. A new data to complement earlier work which was limited to large gaps and thus “thick” films is presented. The intention is to assess the feasibility of reverse roll coating to yield very thin films (<10 μm) at high speeds (>1 m/s) for application in the newer technologies, such as the production of solar cells and plastic electronics. The data obtained demonstrate this is possible but at the lowest permissible gap (25–50 μm) with low‐viscosity fluids (～7 mPa s). The study also developed a new understanding of how instabilities are controlled. It was seen that the size of the inertia forces generated by the applicator roller in relation to surface tension, as expressed by the Weber number and not the applicator Capillary number (viscous forces/surface tension) which is the critical parameter. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3083–3091, 2013     

Air entrainment in a roll coating system

Experiments are performed to determine the critical speed at which air is entrained by a roll, rotating partially submerged about an axis parallel to t free surface. Data for Newtonian fluids correlate in terms of a critical Capillary number, Ca* = U* μ/σ, which is found to depend on a parameter γ = σ(?/μ⁴g)^{$\text{1}\text{3}$} over four decades in γ. A model based on simple force balances at the dynamic meniscus reflects these observations.Data obtained with viscoelastic polymer solutions indicate that elasticity is a strong stabilizing factor.     

A boat-casting technique for the preparation of very thin plastic films suitable for optical studies

A meniscus coating technique utilizing a Teflon “boat” for dispensing a viscous solution of a polymer in a volatile solvent upon a suitable substrate provides very thin plastic films (>ca. 0.5 μm) which are usually of acceptable optical quality for spectrophotometric measurements.     

Experimental study of film thickness and free surface velocity around a rotating roll for non-newtonian fluids

Measurements are performed on film thickness and free surface velocity around a rotating roll for Newtonian, non-Newtonian inelastic, and viscoelastic fluids by using noncontact methods of a capacitance probe and a laser Doppler velocimeter probe. The film thickness decreases with increasing inspection angle for Newtonian fluids. For non-Newtonian fluids, it retains an approximately constant value, owing to shear-thinning of viscosity, except for a meniscus region development, which is dependent on fluid elasticity. Comparison of current results with the film thickness from a previous work is also made. With the increment of inspection angle, the free surface velocity increases rapidly in the meniscus region and maintains a constant value almost equal to a roll speed in the other region for viscoelastic fluids, while it increases linearly for Newtonian fluids. The shear rate at a roll surface is presented assuming that the velocity distribution in the liquid film is a polynomial equation. It is found that viscoelastic fluids exhibit different behavior from that of Newtonian and non-Newtonian inelastic fluids. Evaluation of the force acting on the liquid film for Newtonian fluids implies that the velocity gradient at a roll surface in an ascending region may be steeper than the parabolic form assumed in this study.     

The effect of shear‐thickening on the stability of slot‐die coating

Slot‐die coating is an economical roll‐to‐roll processing technique with potential to revolutionize the fabrication of nano‐patterned thin films at high throughput. In this study, the impact of shear‐thickening of the coating fluid on the stability of slot‐die coating was investigated. For the coating fluid, a model system fumed silica nanoparticles dispersed in polypropylene glycol was chosen. These dispersions exhibit shear and extensional thickening characterized through steady shear and capillary break‐up measurements. The critical web velocity for the onset of coating defect for different flow rates was measured, while the type of coating defect was visualized using a high speed camera. For the shear thickening particle dispersions, the coating failed through the onset of a ribbing instability. The critical web velocity for the onset of coating defect was found to decrease with increasing particle concentration and increasing fluid viscosity. The minimum wet thickness was studied as a function of capillary number for the particle dispersions and compared with a series of Newtonian fluids with similar viscosities. In all cases, shear‐thickening behavior was found to stabilize coating by reducing the minimum wet coating thickness when compared against a Newtonian fluid with similar viscosity at the same capillary number. Conversely, the shear‐thinning fluids tested destabilized the coating by increasing the minimum wet thickness when compared against a Newtonian at the same capillary number. The impact of shear‐thickening on slot‐die coating was further studied by quantifying the evolution of the ribbing instability with increasing web speed and by conducting tests over a wide range of coating gaps. © 2016 American Institute of Chemical Engineers AIChE J, 62: 4536–4547, 2016     

Low-speed tack measurements of fluids and inks

Low-speed tack measurements (≈ 0.1–6.0 rad/s or ≈ 1–57 RPM) have been determined for polymeric-based solution inks and oil-based dispersion inks, tripropylene glycol (TPG), Igepal (I-530), and N-350 (viscosity calibration standard) fluids using a metal roller/incline method. The inks and fluids were tested under “unaged” or reference conditions at 25°C. The inks were “aged” at 70°C and, subsequently, tack measurements were made at 25°C. The tack (T_i) and angular speed (W_i) for the inks were empirically fitted as functions of incline angle (α) and “aged” time, t_cd. A correlation was also made for tack and percent weight change, ΔW_i. Other factors, such as viscosity, surface tension, humidity, and “aging” temperature, T_cd, were also found to affect the magnitude and variation of tack. Additionally, the results suggest that low-speed tack measurements are quite useful for selecting solution inks containing polymeric substances that possess the desirable spreading, mixing, and pressing properties in high-speed/high-volume printing and the component compatibility necessary for long-term performance. © 1995 John Wiley & Sons, Inc.     

Capillary pressure and viscous pressure drop set bounds on coating bead operability

In premetered slot or extrusion coating and related sheet coating a “bead” of liquid is held between the coating die and the moving sheet by capillary forces, which depend on gap clearances, surface tension, contact line attachment, and dynamic contact angle; by viscous forces, which depend on clearances, viscosity, meniscus, location, and coating thickness; and by the externally applied pressure difference, which must fall within bounds for the bead to be operable.New bounds are derived for quasi-static beads with variable meniscus location, extending Ruschak's [1] analysis. Viscous effects are modeled by Couette and Poiseuille contributions that account for nonuniform clearances and are important except in limiting cases like Ruschak's. Operating bounds are derived for viscous coating beads.     

ROLL COATING IN THE PRESENCE OF A FIXED CONSTRAINING BOUNDARY

When a cylindrical roll rotates about its axis while partially submerged in a liquid, as suggested in Figure 1, a layer of the liquid is entrained and withdrawn from the bath. In the absence of a constraining boundary the coating thickness H is found to exhibit a strong dependence on roll speed, as well as on the fluid viscosity and interfacial tension. In a previous publication (Middleman, 1978) from this laboratory we have presented experimental results and a successful data correlation for the case of a roll operating under such conditions that the boundaries of the fluid reservoir exert no influence on the coating dynamics. In this paper the effect of an adjacent constraining boundary is considered, a mathematical model based on lubrication theory is presented, and the data are found to exhibit the general features predicted by the model. Under some conditions it is observed that a uniform coating is not achieved. Instead, a regular set of lines or “ribs” is developed, in the direction of motion. Figure 2 shows this situation. This instability is anticipated and described in the work of others, and a theory of Savage (1977) is evaluated with respect to our own data on critical conditions for onset of “rib” formation. The general features predicted by that theory are consistent with our observations.     

Polysulfobetaines and corresponding cationic polymers. VI. Synthesis and aqueous solution properties of cationic poly(methyl iodide quaternized acrylamide–N,N-dimethylaminopropylmaleimide copolymer) [poly(MIQADMAPM)]

The copolymer prepared by copolymerizing with acrylamide and maleic anhydride was imidized with N,N-dimethylaminopropylamine. The obtained acrylamide–N,N-dimethylaminopropylmaleimide (ADMAPM) copolymer was then reacted with methyl iodide to yield a poly(methyl iodide quaternized acrylamide–N,N-dimethylaminopropylmaleimide) copolymer [poly(MIQADMAPM)]. Its aqueous solution properties were studied by measurements of reduced viscosity, intrinsic viscosity, and flocculation test in this study. The reduced viscosity and intrinsic viscosity of this cationic polyelectrolyte were related to the types and concentration of the added salt. “Soft” salt anions were more easily bound to the quaternary ammonium cation (R₄N⁺) of poly(MIQADMAPM) than were “hard” salt anions. Halide anions are hard anions; consequently, hard cations were more easily attracted to halide anions and reduced the binding degree of halide anion on the quaternary ammonium group (R₄N⁺). The intrinsic viscosity behavior for cationic polyelectrolyte resulting from the electrostatic repulsive force of the polymer chain was contrasted with polyampholyte. The effect of various flocculants on flocculation in different pH values was accessed in this study. © 1996 John Wiley & Sons, Inc.     

Organic polymeric coatings for silica fibers. I. UV-curing epoxy acrylate (V1F)

Coated optical fibers (～100 μm) were prepared exhibiting fracture strengths in excess of 750 ksi (5.3 GN/m²) for lengths up to 100 meters. To achieve this we have developed a novel coating and coating application process. The coating is an epoxy acrylate, cured by UV irradiation, which may contain a multifunctional silane coupling agent. The coating liquid was applied to the fiber prior to solidification by an applicator, which facilitates centering of the fiber in the coating. Our coating system offers advantages in that there is no solvent to remove, thereby avoiding possible surface “corrosion” of the glass fiber. The reasonably low viscosity of the coating formulation allows for the extensive wetting of the fiber surface irregularities prior to solidification.     

A kinetic model for pyrolysis of cellulose

It has been shown that the pyrolysis of cellulose at low pressure (1.5 Torr) can be described by a three reaction model. In this model, it is assumed that an “initiation reaction” leads to formation of an “active cellulose” which subsequently decomposes by two competitive first-order reactions, one yielding volatiles and the other char and a gaseous fraction. Over the temperature range of 259–341°C, the rate constants of these reactions, k_i (for cellulose → “active cellulose”), k_v (for “active cellulose” → “volatiles”), and k_c (for “active cellulose” → char + the gaseous fraction) are given by k_i = 1.7 × 10²¹e^{? (58,000/RT)} min ^?1, k_v = 1.9 × 10¹⁶e^{? (47,300/RT)} min^?1, and k_c = 7.9 × 10¹¹e^{? (36,600/RT)} min^?1, respectively.     

Polysulfobetaines and corresponding cationic polymers. IV. Synthesis and aqueous solution properties of cationic poly(MIQSDMAPM)

A cationic poly(methyl iodide quaternized styrene-dimethylaminopropylmaleimide) copolymer [poly(MIQSDMAPM)] was synthesized by imidizing styrene-maleic anhydride (SMA) copolymer. Its aqueous solution properties were studied by measurements of reduced viscosity, intrinsic viscosity, and a flocculation test. The reduced viscosity and intrinsic viscosity of this cationic polyelectrolyte were related to the types and concentration of the added salt. “Soft” salt anions were more easily bound to the quaternary ammonium cation (R₄N⁺) of poly(MIQSDMAPM) than “hard” salt anions. Halide anions are hard anions; consequently, hard cations were more easily atracted to halide anions, and reduced binding degree of halide anion on the quaternary ammonium group (R₄N⁺). Some salt ions were observed to strongly attract the quaternary ammonium group of the cationic polymeric side chain and resulted in agglomeration of the polymers. A comparison of various flocculants as to the effect of flocculation was made in this study. © 1996 John Wiley & Sons, Inc.     

Fluid absorption during forward roll coating of porous webs

Forward roll coating is a common process to deposit thin liquid films onto a continuous web. When the web is porous, some amount of the fluid is forced into the web in the nip. This removal of fluid, along with the deformation of the backing material, influences transfer in the nip as well as operational issues such as misting and coating defects. While much has been reported on forward roll coating for non-porous webs, little has been done when the web is porous.A laboratory roll coating device is used to characterize the pressure profile, the rubber deformation, and the film thickness as the fluid is in contact with a porous web. A pressure transducer is used to record the pressure profile in the nip. The film thickness on the steel roll surface and the gap between the rolls are measured with capacitance probes. Silicone oils, with three different viscosities, are used as test fluids. Three different papers are used in these tests. A model is proposed to describe the pressure profile, rubber deformation, and absorption in a forward roll coating device. The differential equations are solved to describe the nip behavior. A simplified model is also proposed, using an average nip pressure and Darcy's law, to predict penetration in the nip. The proposed models compare well with the experimental results and predict the dependence on viscosity, nip load, and paper permeability. The experimental results with low viscosity fluids show some speed dependence that is not captured by the model, but the high viscosity fluid behavior agrees well with model predictions.     

Aqueous solution properties of poly(trimethyl acrylamido propyl ammonium iodide) [poly(TMAAI)]

The aqueous solution properties of a cationic poly(trimethyl acrylamido propyl ammonium iodide) [poly(TMAAI)] were studied by measurements of reduced viscosity, intrinsic viscosity, and flocculation test. The reduced viscosity and intrinsic viscosity of this cationic polyelectrolyte were related to the types and concentration of added salt. “Soft” salt anions were more easily bound on the quaternary ammonium (R₄N⁺) of poly(TMAAI) than those of “hard” salt anions. Halide anions are hard anions; consequently, hard cations were more easily attracted to halide anions for reducing the binding degree of halide anion on the quaternary ammonium group (R₄N⁺). Some salt ions were observed to strongly attract the quaternary ammonium of the cationic polymeric side chain for coagulation of the polymers. This effect would make the polymeric aqueous solution become turbid. The intrinsic viscosity behavior for cationic polyelectrolyte resulting from the electrostatic repulsive force of the polymer chain is contrasted with polyampholyte. A comparison of various flocculants as to the effect of flocculation was also studied. © 1994 John Wiley & Sons, Inc.     

Flow through porous media of a shear-thinning liquid with yield stress

Darcy's law for the laminar flow of Newtonian fluids through porous media has been modified to a more general form which will describe the flow through porous media of fluids whose flow behavior can be characterized by the Herschel-Bulkley model. The model covers the flow of homogeneous fluids with a yield value and a power law flow behavior. Experiments in packed beds of sand were carried out with solutions of paraffin wax in two oils and with a crude oil from the Peace River area of Canada. The model fitted the data well. A sensitivity analysis of the fitting parameters showed that the model fit was very sensitive to errors in the flow behavior index, n , of the Herschel-Bulkley model. A comparison of the “n” values calculated from viscometer measurements and from flow measurements agreed well. A more general Reynolds number for flow through porous media, which includes a fluid yield value, was developed. The data were fitted to a Kozeny-Carman type equation using this Reynolds number. The constant in the Kozeny-Carman equation was determined for the two packed beds studied using Newtonian oils. The data could all be represented, within the experimental error, by the relationship f^* = 150/Re^*. Since the mean volume to surface diameter of the packing was determined by the measurement of its permeability to a Newtonian oil, assuming C' = 150, the new definition of the Reynolds number allows the direct use of the Kozeny-Carman equation with Herschel-Bulkley type fluids.     

Molecular weight and solution viscosity characterization of PVC

As a means of developing the most accurate possible Mark-Houwink relationship possible for PVC, the literature Mark-Houwink coefficients for the weight average molecular weight (M_W) of PVC in THF are plotted and the derived “grand average” Mark-Houwink relationship (K = 15.56 × 10^?3 ml/g, a = 0.7690) is shown. High pressure-size exclusion chromatography (HP-SEC) data from two independent laboratories was used along with the “grand average” Mark-Houwink coefficients to calculate absolute number of average molecular weight (M_N) and weight average molecular weight values for PVC. An easy-to-use table has been developed to detail the relationships between M_N, M_W, K value, and inherent viscosity (I. V.).     

The coating of Newtonian liquids onto a roll rotating at low speeds

A theoretical treatment is presented for the prediction of the rate at which a liquid is entrained by a roll which is rotating partially submerged in a Newtonian liquid, and also for the prediction of the variation in the thickness of the film around the roll. The method involves the integration of the momentum equation after neglecting inertia forces in the dynamic meniscus region and the matching of the surface curvature with that derived for the static meniscus region near the liquid surface. The resulting equation allows the prediction of the relationship between a dimensionless film thickness parameter, T, and the capillary number, Ca, as a function of the radius of the roll, the withdrawal angle and the position of the roll periphery. The liquid flux on the roll can also be predicted.     

Toughening mechanism of rubber reinforced epoxy composites by thermal and microwave curing

The carboxyl terminated polybutadiene (CTBN) is utilized to improve the toughness of diglycidylether of bisphenol A epoxy cured by heat and microwave. The change of viscosity, chemical groups, and the glass transition temperature (T_g) of system are analyzed. The impact performance is characterized to evaluate the fracture toughness, and tensile properties also are investigated. The fracture morphologies are observed by the scanning electron microscopy for exploring toughening mechanism. The viscosity results indicate that viscosity of system increases with increasing of CTBN, demonstrating the formation of precrosslinking and interpenetrating network structure of two phases. The Fourier transform infrared spectrometer results show that effective chemical bonds are formed between CTBN and epoxy resins. The T_g decreases with introducing CTBN, indicating the decline of crosslinking density, which further suggests inherent three‐dimensional structure have been changed. The impact strength and energy increase with increasing of CTBN, and reach a maximum value of 5.92 kJ/m² and 0.13 kJ at 15% for thermal curing, respectively, increased by 36.8% and 23.1% relative to microwave curing system, while tensile strength and modulus reach the optimum at 5%. Scanning electron microscopy observation finds that “plastic tensile” and “microvoid” deriving from “sea‐island” structure exist, presenting the ductile fracture features. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45767.