Comparison of vertical and horizontal slot die coatings

Comparison of vertical and horizontal slot coatings was carried out experimentally using aqueous glycerin solutions with varying viscosity on a laboratory‐scale slot coater. The maximum coating speeds achieved in vertical slot coating were found to be higher than those of horizontal slot coating for low‐viscosity solutions. The coating defects observed below the minimum wet thickness changed from ribbing to air entrainment at a critical solution viscosity of around μ = 75 mPa s. Above this critical viscosity, a lower minimum wet thickness was obtained for the horizontal coating but not for vertical coating. The difference between vertical and horizontal coating was mainly the length of the coating bead. The dimensionless minimum wet thickness was correlated as a function of Reynolds number. There existed three regions of minimum wet thickness depending on Reynolds number. Initially, the dimensionless wet thickness increased in the low Reynolds number region (Region I), then reached a plateau in the intermediate Re region (Region II), finally dropped off sharply in the high Re region (Region III). The boundaries of these three regions were identified by two critical Reynolds numbers, i.e., Re = 1 and 20. Only Regions I and II of the wet thickness could be found for horizontal coating, while mainly Regions II and III were obtained for vertical coating, depending on the viscosity of the coating solution. POLYM. ENG. SCI., 47:1927–1936, 2007. © 2007 Society of Plastics Engineers     

Effects of the molecular weight and concentration of poly(vinyl alcohol) on slot die coating

An experimental study was carried out to investigate the effects of the molecular weight and concentration of aqueous poly(vinyl alcohol) (PVA) solutions on the stable operating window of slot die coating. Various coating defects were observed outside the operating window. The window was found to expand with the PVA concentration and molecular weight increasing at low concentrations and to reach a maximum size at the gel point concentration, which corresponded to a critical Deborah number. Beyond this point, the effect of fluid elasticity became dominant, and the window began to shrink. This phenomenon was in contrast to that observed for low‐molecular‐weight glycerol solutions, in which the operating window was found to contract with the concentration or viscosity increasing. This anomalous behavior was attributed to the stretching or extension of long‐chain PVA molecules, which effectively stabilized the fluid motion in the coating bead region. The maximum coating speed could be correlated with the PVA concentration and molecular weight in terms of the capillary number as a function of the dimensionless concentration. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Minimum wet thickness for double‐layer slide‐slot coating of poly(vinyl‐alcohol) solutions

A combined slide‐slot coating die, with the slide coating on top, was designed and built to investigate the double‐layer coating of poly(vinyl‐alcohol) solutions. The operating coating windows were examined as a function of flow rates and viscosities of the two coating layers. The top coating layer could be made much thinner as compared to the double‐layer coating so long as a stable thin film could be formed on the slide. A minimum wet thickness of the top layer was found to be as thin as 5 μm or less. A large viscosity ratio of the two layer solutions appears to be helpful in expanding the coating windows. Addition of a small quantity of polymer, such as carboxymethyl cellulose, can further enhance the coating speed and reduce the top layer thickness. A flow visualization technique was employed to observe the coating bead region. It was found to be easier to change the flow direction in the slide‐slot coating die than the double‐layer slot die, resulting in a more stable coating flow and much thinner top layer. POLYM. ENG. SCI., 45:1590–1599, 2005. © 2005 Society of Plastics Engineers.     

Coating weight reduction by a carrier layer for tensioned‐web slot coating

The feasibility of using a dilute bottom carrier layer to reduce the thickness of a viscous top layer for tensioned‐web slot coating was examined experimentally. Aqueous poly(vinyl) alcohol solutions were prepared for both the carrier and the top layers. The solutions were coated on poly(ethylene terephthalate) films for observation. The thickness ratio of the two layers has to be in a certain range for stable coating, otherwise different types of coating defects would appear. The effects of viscosity, wrapping angle, surface tension, and web tension were studied. It was found that the viscosity ratio of the top layer to the carrier layer is the most critical factor; a large ratio is more helpful in reducing the thickness of the top layer. Increasing the wrapping angle, web tension, and surface tension of the top layer can also decrease the coating thickness of the top layer. The drying load is reduced substantially owing to the significant reduction of the top layer thickness. It was estimated that 50–90% of the energy required for drying could be saved by introducing a carrier layer in this study. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Spinning and drawing properties of ultrahigh‐molecular‐weight polyethylene fibers prepared at varying concentrations and temperatures

The concentrations and temperatures of ultrahigh‐molecular‐weight polyethylene (UHMWPE) gel solutions exhibited a significant influence on their rheological and spinning properties. The shear viscosities of UHMWPE solutions increased consistently with increasing concentrations at a constant temperature above 80°C. Tremendously high shear viscosities of UHMWPE gel solutions were found as the temperatures reached 120–140°C, at which their shear viscosity values approached the maximum. The spinnable solutions are those gel solutions with optimum shear viscosities and relatively good homogeneity in nature. Moreover, the gel solution concentrations and spinning temperatures exhibited a significant influence on the drawability and microstructure of the as‐spun fibers. At each spinning temperature, the achievable draw ratios obtained for as‐spun fibers prepared near the optimum concentration are significantly higher than those of as‐spun fibers prepared at other concentrations. The critical draw ratio of the as‐spun fiber prepared at the optimum concentration approached a maximum value, as the spinning temperature reached the optimum value of 150°C. Further investigations indicated that the best orientation of the precursors of shish‐kebab‐like entities, birefringence, crystallinity, thermal and tensile properties were always accompanied with the as‐spun fiber prepared at the optimum concentration and temperature. Similar to those found for the as‐spun fibers, the birefringence and tensile properties of the draw fibers prepared at the optimum condition were always higher than those of drawn fibers prepared at other conditions but stretched to the same draw ratio. Possible mechanisms accounting for these interesting phenomena are proposed.     

Drying air-induced disturbances in multi-layer coating systems

A range of new experimental techniques is developed to quantify drying-air induced disturbances on low viscosity single and multi-layer coating systems. Experiments on prototype slide-bead coating systems show that the surface disturbances take the form of a wavelike pattern and quantify precisely how its amplitude increases rapidly with wet thickness and decreases with viscosity. Heat transfer measurements show that the redistribution of water to form an additional lower viscosity carrier layer while increasing the solids concentration of the upper layer or layers enables the maximum drying rate, for which drying-air induced surface disturbances are acceptably small, to be increased with significant commercial benefits.     

Properties of hydroxypropyl methylcellulose–polyvinyl alcohol water systems,dispersants in vinyl chloride suspension polymerization

Aqueous solutions of partly hydrolyzed polyvinyl acetate (polyvinyl alcohol, PVA) and hydroxypropyl methylcellulose (HPMC) are used together as dispersants in vinyl chloride suspension polymerization. Surface tension, viscosity, and cloud point (CP) of diluted PVA and HPMC solutions at room temperature, viscosity of concentrated solutions as a function of temperature, and incipient gelation temperatures (IGT) were determined. Viscosity measurements show synergism of binary polymer mixtures. The synergetic effect diminishes with rise of temperature. IGT of PVA–HPMC solutions do not differ remarkably from those of solutions containing only HPMC and are much higher than the corresponding CP. At definite relative concentrations of HPMC and PVA, separation of the mixtures into two immiscible phases occurs. Each phase contains both polymers, but in the denser and more concentrated phase (lower layer), PVA is the prevailing component. The mutual influence of the two polymers causes changes in solute–solvent interaction, that is, dehydration of the polymers' molecules. It is assumed that in the processes occurring in the system, preferential molecules of definite structure take part. The merits of a system using both polymers as dispersants in the vinyl chloride polymerization are discussed in the light of ascertained data. © 1993 John Wiley & Sons, Inc.     

Viscometric study of aqueous poly(vinyl alcohol) (PVA) solutions as a binder in adhesive formulations

Commercial grade poly(vinyl alcohol) (PVA) is widely used in adhesive and textile industries. The density and viscosity of an aqueous PVA solution are important properties in the various processing stages in these industries. In this work, the density and viscosity of aqueous PVA solutions are measured and the average molecular weight of PVA is determined. The measured densities are expressed by a simple linear equation in terms of PVA weight fractions. The coefficients of the density equation are calculated and presented. The measured viscosities are fitted to the modified Eyring viscosity model developed by the authors and the parameters of the model are evaluated. The viscosity model with known parameters for aqueous PVA solutions has the added advantages that it can be used in practical applications in various industries for predicting viscosity of aqueous PVA solutions without recourse to expensive and time consuming experimental measurements.     

Synthesis and rheological properties of hydrophobically modified poly(vinyl alcohol)

A novel series of water-soluble hydrophobically modified poly(vinyl alcohol) (HMPVA) with various hydrophobe contents was prepared by grafting poly(vinyl alcohol) (PVA) using 1-dodecanol and toluene-2,4-diisocyanate as hydrophobic monomer and coupling agent, respectively. The chemical structure of HMPVA was analyzed by Fourier Transform Infrared Spectrometer (FTIR) and ¹H NMR. Rheological properties of the aqueous solutions also confirmed the incorporation of hydrophobic groups into PVA. In dilute concentration regime, HMPVAs exhibited lower intrinsic viscosity than PVA, suggesting that HMPVA molecules were more shrunken. While the aqueous solution viscosity was enhanced due to hydrophobic modification at a high concentration, and HMPVAs with higher hydrophobe contents exhibited lager values of apparent viscosities. Over a frequency range of 1 to10² rad/s, the dynamic storage modulus of PVA solution was smaller than the dynamic loss modulus whereas the dynamic storage modulus of HMPVAs solutions was greater than the dynamic loss modulus, indicating the evolution of viscoelastic solid properties in HMPVAs solutions. The yield stress of PVA was nearly zero whereas that of HMPVAs represented positive values, implying that networks were present in HMPVAs solutions.     

Solution properties of ionic hydrophobically associating polyacrylamide with an arylalkyl group

Copolymers of acrylamide, 2‐acrylamide‐2‐methylpropanesulfate (AMPS), and hydrophobic monomer N‐arylalkylacrylamide (BAAM) were synthesized by free‐radical micellar copolymerization. The effects of the copolymer, BAAM, AMPS, and NaCl concentrations and the pH value on the apparent viscosity of the copolymers were studied. The solution viscosities increased sharply when the copolymer concentration was higher than the critical associating concentration. The apparent viscosities of aqueous solutions of poly(N‐arylalkylacrylamide‐co‐acrylamide‐co‐2‐acrylamide‐2‐methylpropanesulfate) (PBAMS) increased with increasing BAAM and AMPS concentrations. PBAMS exhibited good salt resistance. With increasing pH, the apparent viscosities first increased and then decreased. Dilute PBAMS solutions exhibited Newtonian behavior, whereas semidilute aqueous and salt solutions exhibited shear‐thickening behavior at a lower shear rate and pseudoplastic behavior at a higher rate. Upon the removal of shear, the aqueous solution viscosities recovered and became even greater than the original viscosity, but the salt solution viscosities could not recover instantaneously. The elastic properties of PBAMS solutions were more dominant than the viscous properties, and this suggested a significant buildup of a network structure. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 316–321, 2005     

Single‐Point Determination of Intrinsic Viscosity of Semirigid Thermotropic Copolyesters in Phenol/1,1,2,2‐Tetrachloroethane

A series of semirigid thermotropic copolyesters with different compositions were prepared from p‐hydroxybenzoic acid (HBA), hydroquinone (HQ), terephthalic acid (TPA) and poly(ethylene terephthalate) (PET) by acidolysis reaction and following polycondensation. Fourteen procedures of calculation of the intrinsic viscosities from a single viscosity measurement for polymer solutions, including three proposed ones, have been applied for the copolyesters in phenol/1,1,2,2‐tetrachloroethane (60/40, v/v) at 30°C. It is found that various forms of the Huggins and Kraemer equations, used singly or in a combined form, yield intrinsic viscosities in good agreement with those extrapolated values obtained in the usual manner from multipoint viscosity measurements over a wide range of concentrations.     

The effect of substrate roughness on air entrainment in dip coating

Dynamic wetting failure was observed in the simple dip coating flow with a series of substrates, which had a rough side and a comparatively smoother side. When we compared the air entrainment speeds on both sides, we found a switch in behaviour at a critical viscosity. At viscosity lower than a critical value, the rough side entrained air at lower speeds than the smooth side. Above the critical viscosity the reverse was observed, the smooth side entraining air at lower speed than the rough side. Only substrates with significant roughness showed this behaviour. Below a critical roughness, the rough side always entrained air at lower speeds than the smooth side. These results have both fundamental and practical merits. They support the hydrodynamic theory of dynamic wetting failure and imply that one can coat viscous fluids at higher speeds than normal by roughening substrates. A mechanism and a model are presented to explain dynamic wetting failure on rough surfaces.     

High‐pressure viscosity of polystyrene solutions in toluene + carbon dioxide binary mixtures

High‐pressure viscosity of polystyrene (M_w = 50,000, M_w/M_n ≤ 1.06) solutions in toluene and in mixtures of toluene + carbon dioxide was measured using a falling cylinder‐type viscometer at 320, 340, and 360 K, and up to 35 MPa. Solutions with polystyrene concentrations of 3, 5, and 7 wt % were investigated. Carbon dioxide levels in the range from 0 to 14.7 wt % were evaluated. Viscosity was observed to vary linearly with pressure at the temperatures and polymer concentrations investigated. Viscosity of the polymer solutions decreased as the concentration of carbon dioxide in the mixture was increased. The largest viscosity reduction was observed at the lowest temperature and at the highest concentration of polymer. The viscosity of the solutions was correlated with the solution density for different compositions. It was found that solutions of the same density have different viscosities, depending upon the carbon dioxide concentration in the mixture. The solutions with the higher carbon dioxide content display the lower viscosities at a given density. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 306–315, 2000     

Continuous polymerization of ω‐lauryl lactam in an intermeshing corotating twin‐screw extruder

This article describes the synthesis of poly(ω‐lauryl lactam) by a reactive extrusion process. Anionic ring‐opening polymerization was performed in an intermeshing corotating twin‐screw extruder. We investigated the evolution of conversion of ω‐lauryl lactam as a function of reaction time, screw speeds, different feed rates, and different screw configurations along the screw axis in a twin‐screw extruder. For comparison with continuous polymerization in a twin‐screw extruder, we studied polymerization in an internal mixer, which was considered a batch reactor. We found the final conversion of ω‐lauryl lactam made in a twin‐screw extruder was higher than in an internal mixer. Higher molecular weights are found at lower screw speeds and feed rates. Melt viscosities and mechanical properties of the polymers were measured. Residence time, molecular weights, and shear mixing have the main effect on the mechanical properties of products. The twin‐screw extruder performance was interpreted in terms of commercial software. It was found that twin‐screw extruder reaction rate was higher than those in the batch reactor and increased locally with screw speed and feed rate. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1605–1620, 2005     

Syndiotacticity‐rich ultrahigh molecular‐weight poly(vinyl alcohol) film. I. Determination of optimum polymer concentration by zone‐drawing method in film preparation

A new method using a simple zone‐drawing technique has been suggested for determining the optimum initial concentration of a polymer solution that has suitable macromolecular entanglements. This method was developed to replace the incorrect inherent viscosity‐measuring method for syndiotacticity‐rich (syndiotactic diad content of 63.4%) ultrahigh molecular‐weight (number‐average degree of polymerization of 12,300) (UHMW) poly(vinyl alcohol) (PVA) solution. Syndiotacticity‐rich UHMW PVA films were prepared from dimethyl sulfoxide (DMSO) solutions with different initial concentrations: of 0.1, 0.2, 0.3, 0.4, and 0.5 g/dL. In order to investigate the drawing behavior of the syndiotacticity‐rich UHMW PVA films with different solution concentrations, the films were drawn under various zone‐drawing conditions. Through a series of experiments, it was discovered that the initial concentration of PVA solution in DMSO caused significant changes in the draw ratio of the syndiotacticity‐rich UHMW PVA film. That is, the one‐step and maximum zone draw ratios of the film at an initial concentration of 0.3 g/dL exhibited its maximum values and gradually decreased at higher or lower concentrations. Thus, it was disclosed that the initial concentration of 0.3 g/dL is the optimum polymer concentration to produce a maximum draw ratio in this work. Based on the above results, it may be concluded that the optimum concentration of the initial PVA solution can be determined directly by measuring the zone draw ratio. The draw ratio, birefringence, crystallinity, degree of crystal orientation, tensile strength, and tensile modulus of the maximum drawn PVA film were 32.9, 0.0449, 0.61, 0.991, 1.91, and 46.2 GPa, respectively. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 123–134, 2000     

Molecular weight effect on theta-gel formation in poly(vinyl alcohol)–poly(ethylene glycol) mixtures

Injectable hydrogel formulations that undergo in situ gelation at body temperature are promising for minimally invasive tissue repair. This work focuses on the investigation of injectable poly(vinyl alcohol) (PVA) and poly(ethylene glycol) (PEG) mixtures. The injectable PVA–PEG aqueous solutions form a hydrogel as temperature is reduced to near body temperature, while filling a defect in the injection site. Gamma sterilization of these solutions compromises injectability presumably due to crosslinking of PVA. We hypothesized that by modifying the PEG molecular weight and its concentration, injectability of radiation sterilized PVA–PEG hydrogels can be optimized without compromising the mechanical properties of the resulting gel. The use of a bimodal mixture of higher and lower molecular weight PEG (600 and 200 g/mol) resulted in lower PVA/PEG solution viscosity, better injectability, and higher gel mechanical strength. The PVA/bimodal-PEG had a lower viscosity at 2733 ± 149 cP versus a viscosity of 5560 ± 278 cP for PVA/unimodal-PEG (400 g/mol). The gel formed with the bimodal PEG mixture had higher creep resistance (61% total creep strain under 0.5 MPa) than that formed with unimodal PEG (84%). These hydrogel formulations are promising candidates for minimally invasive tissue repair. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

N-methyl-2-pyrrolidone as a solvent for poly(vinyl alcohol)

NMP (N-methyl-2-pyrrolidone) and its mixtures with water have been investigated as solvents for PVA [poly(vinyl alcohol)]. Judging from the higher exponent in the Mark-Houwink equation, NMP appears to be a better solvent than water. The equation found is for solutions in NMP at 30°C. Although the viscosity of NMP–water mixtures goes through a maximum when the molar ratio of NMP to water is 1:2, the intrinsic viscosity for PVA exhibits neither a maximum nor a minimum in mixtures ranging from pure water to pure NMP. As in aqueous solutions, the intrinsic viscosity in NMP solutions decreases slightly with increasing temperatures from 20 to 50°C.     

Effect of extensional properties of polymer solutions on the droplet formation via ultrasonic atomization

Ultrasonic atomization was conducted using 2.4 MHz ultrasound generator to produce polymeric particles from the solutions of water‐soluble polymers like poly (ethylene oxide) (PEO), poly(vinyl alcohol) (PVA), and poly(vinyl pyrrolidone) (PVP). In the series of solutions containing these polymers of various molecular weights, the shear viscosity and surface tension were varied; such differences, however, could not account for the observed trends in the atomization behavior. It was found that the availability of droplet formation and size of the droplets depend strongly on the extensional characteristics of polymer solutions along with the apparent extensional viscosities and the longest relaxation times. The extensional properties of polymer solutions were effectively measured by capillary breakup extensional rheometry (CaBER) and provided a rational basis to understand the essential role of fluid viscoelasticity in ultrasonic atomization. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

Adhesion of ultrahigh molecular weight polyethylene plates photografted with hydrophilic monomers and evaluation of failure location by X‐ray photoelectron spectroscopy

In this study, methacrylic acid (MAA) and acrylic acid (AA) were photografted onto the ultrahigh molecular weight polyethylene (UHMWPE) plates at different monomer concentrations and temperatures, and the grafted UHMWPE plates were bonded with aqueous polyvinyl alcohol (PVA) solutions. The tensile shear adhesive strength of both grafted UHMWPE plates was also discussed in relation to wettability and water absorptivity. The location of failure was also estimated by X‐ray photoelectron spectroscopy (XPS). Wettability of the MAA‐grafted UHMWPE plates became constant, when the UHMWPE surface was fully covered with grafted PMAA chains. Conversely, wettability of the AA‐grafted UHMWPE plates passed through the maximum value and then gradually decreased against the grafted amount probably due to aggregation of grafted PAA chains. Water absorptivity of the grafted layers formed at lower monomer concentrations or temperatures sharply increased at lower grafted amounts. The adhesive strength increased with the grafted amount and substrate breaking was observed at higher grafted amounts, indicating that a main factor to increase the adhesive strength is the formation of a grafted layer by shorter grafted polymer chains and/or the restriction of the location of photografting to the outer surface region. In addition, surface analysis by XPS showed that failure occurred in the boundary between the layer composed of grafted polymer chains and PVA chains and the ungrafted layer at a low adhesive strength, and the location of failure was shifted to the grafted layer containing PVA chains at the grafted amount increased. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40133.     

Effect of emulsion polymerization conditions of vinyl acetate on the viscosity fluctuation and gelation behavior of aqueous poly(vinyl alcohol) solution

Poly(vinyl alcohol) (PVA) was prepared by emulsion polymerization of vinyl acetate (VAc) at 40, 50, and 60°C. PVAs of various molecular parameters were dissolved in water at concentrations of 3, 5, and 7% (g/dL) and aged at 30 and 60°C. The effects of the molecular weight and polymerization temperature on the viscosity fluctuation and gelation behavior of aqueous PVA solutions were investigated. Viscosity was increased with increasing molecular weight when other parameters were held the same. PVA of superior molecular regularity due to the lower polymerization temperature of VAc had higher relative viscosity. Viscosity and its fluctuation of PVAs with aging time varied with the polymerization temperature of the precursors, concentration of the aqueous solutions, and aging time. For PVA with a number‐average degree of polymerization of 2800 prepared from poly(vinyl acetate) polymerized at 40°C, a sharp increase in viscosity was observed after 500 min of aging at a concentration of 7%. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 1897–1902, 2001