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.     

Effect of type and amount of comonomer on the properties of four copolymers of ethylene

The plastics industry is now coming to realize that copolymers are unique thermoplastics which have broad areas of application. It is believed that ethylene-methyl acrylate copolymers of 2 melt index may be extrusion coated to paper by a cap-coat process or by use of a chill roll with an adhesive coating. Copolymers of ethylene and vinyl monomers can be made into film by the same methods used for polyethylene. Methyl acrylate and methyl methacrylate copolymers may be processed on a wide range of molding equipment. Table VII in the article gives a list of applications for each polymer, along with expected physical properties.     

Study of the stability of the film casting process

A theoretical study of the film casting process has been carried out. In this industrial process, a molten polymer is extruded through a flat die, then stretched in air and cooled on a chill roll. This involves mainly an extensional flow. Between the die and the chill roll, thick edges are formed and a neck-in phenomenon is observed. Above a critical take-up speed, a drawing instability, known as draw resonance, may occur. In this paper, a one-dimensional model adapted from the classical model of the fiber spinning is developed (for a Newtonian or a Maxwell fluid). The influence of the processing parameters (draw ratio, Deborah number, and aspect ratio) on geometry of the lateral free surface (the so-called neck-in phenomenon) is studied. An unattainable zone very similar to the one encountered in fiber spinning is predicted, which only slightly depends on the stretching geometry (initial film width and stretching distance). The onset of draw resonance is studied through the linear stability method. A stability zone, depending on the geometry of the process, the elasticity of the polymer, and the draw ratio, has been obtained. This instability is observed with simultaneous width and thickness film variations. It is proved that the aspect ratio (stretching distance divided by die width) has a strong influence on the onset of the draw resonance instability.     

Visualization study of liquid surface stability for full reverse 3-roll coater with rigid gravure roll

Roll coating systems are widely used to coat a thin liquid layer on a moving substrate, and many publications discussing forward roll coating systems and reverse gap control coating systems are available. Recently, thinner and more uniform layers are required for use with the high-performance solutions involved. Moreover, high line speed coating is also required in order to reduce manufacturing costs. A reverse gravure roll system can coat a thin layer film at high line speed more easily than a flat roll coating system because only the liquid that enters the gravure cell is transferred to the next roll or substrate. However, the existence of the gravure cell complicates the flow between the rolls. To our knowledge, no systematic study in the literature has explored this condition in depth. In the present study, the flow between a reverse deformable roll and a solid stainless steel gravure roll is visualized in order to determine how the uniformity of coating in the high roll speed region is affected by operating parameters, that is, the speed ratio between the rolls and the properties of the coating liquid. The range for coating uniformity is compared with the case of a gap control system. The results show that the stable region can be extended by using the reverse gravure roll system.     

Experimental studies on the development of a cast film

The cast film process is of significant industrial interest. This work presents quantitative experimental results for the characterization of a film casting operation in the region between the die exit and the chill roll. The polymer melt employed is a commercial PET sample that has been characterized in both shear and extension‐dominated flows. Pointwise measurements of both the film temperature and the local velocity are made over the entire region between the die and the chill roll. The velocity increases in the gap, although the strain rate decreases as the polymer moves from the die to the chill roll. For a particular axial position, the velocity decreases significantly near the edges of the film. There are modest (～20 °C) decreases in the temperature between the die and the chill roll, with the greatest variations again occurring near the edges of the film. The experimental results are explained by the coupling between the temperature, velocity fields, and geometry of the experimental apparatus. POLYM. ENG. SCI. 45:443–450, 2005. © 2005 Society of Plastics Engineers.     

Real‐time wide‐angle X‐ray diffraction during polyethylene blown film extrusion

Real‐time wide‐angle X‐ray diffraction (WAXD) measurements during blown film extrusion of low‐density polyethylene are reported in this study. WAXD patterns were obtained at different axial positions in the blown film line starting from a location near the die and extending up to the nip‐roller. The X‐ray diffraction patterns from the bubble were analyzed for crystalline growth along the bubble. From the evolution of (110) and (200) peaks, it is evident that the crystallization process starts near the frost‐line height (FLH), shows a steep growth immediately past the FLH, and then plateaus at higher axial distances near the nip‐rolls. The real‐time crystallinity profiles obtained from WAXD were consistent with those measured using real‐time Raman spectroscopy. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Estimation of roll press design parameters based on the assessment of a particular nip region

Granular solids are compacted in roll presses to obtain dust free products from fine powders. This mainly takes place through the increase of the bulk density due to the pressure applied by the press. The well known theory by Johanson [J.R. Johanson, A rolling theory for granular solids, Transactions of the ASME, Journal of Applied Mechanics, December 1965, 842-848.] was based on defining the nip angle which identifies a nip region where the material is trapped and pressed to higher bulk density. The nip region was introduced in terms of sets of parameters which needed a lot of experimental work to be obtained. In this study it is shown that these parameters are not needed to be determined. The output of the machine as described by the ratio of bulk density can give the design parameters of the roll press; mainly described by the roll dimensions and roll force. This is mainly obtained using one single test, where a particular nip region can be defined, which accounts implicitly for the parameters which represent the flow properties and the material-machine interaction parameters.     

管式法挤出涂层专用LDPE的性能及质量改进

针对管式工艺生产的挤出涂层专用低密度聚乙烯(LDPE)产品存在涂层与基材的黏结强度低、淋膜缩幅大等问题,通过与同类釜式产品的性能和结构的对比研究发现,管式产品的分子支化程度过低是造成其性能不佳的主要原因。分析了影响产品长链支化程度的工艺因素,并提出了增加产品长支链含量的方法。初步试生产表明:适当提高反应温度、降低反应压力并改变分子量调节剂的注入速度,可以有效地改善挤出涂层产品的性能。     

Measurement and modelling of the film casting process: 2. Temperature distribution along draw direction

In the case film process a polymer melt is extruded through a slit die, stretched in air and cooled on a chill roll. During the path in air the melt cools and a reduction of both thickness and width takes place; obviously, temperature distribution, thickness and width reductions are function of draw ratio and stretching distance.Temperature distribution along the draw direction was measured as function of flow rate during film casting experiments performed with an iPP resin. A non-contacting method of measurement, based on a narrow-band IR pyrometer, was adopted.A good qualitative agreement is shown between experimental temperature data and predictions of a model accounting of radiation emissivity dependence upon film thickness. Differences are consistent with discrepancies of film thickness evolution along draw direction, indeed the model slightly over predicts both film thickness reduction and, parallel, temperature decrease along the draw direction.     

铸片系统对BOPP薄膜的影响研究

介绍了双向拉伸聚丙烯（BOPP）薄膜铸片系统的设备结构和工作原理，详细地论述了铸片系统中模头、气刀、压边嘴、激冷辊以及水槽对BOPP薄膜的生产过程和性能的影响，并提出了一些注意事项和解决方法，对生产实践有相当的参考价值。     

INFLUENCE OF EXTRUSION COATING CONDITIONS ON STRUCTURE AND TENSILE PROPERTIES OF POLYETHYLENES

It is well known that the properties of a final polymer product are determined not only by the polymer characteristics but also, via process-related structural changes, by the processing technique and by processing conditions such as cooling rate and machine speed. In this study, the influence of the extrusion coating process on the structure and properties of LDPE-, LLDPE-, and m-LLDPE-grades was evaluated. Process variables such as the melt temperature, the draw ratio, the length of the air gap between the die exit, and the laminating nip and the machine speed were studied. The primary interest was in the influence of the process parameters on the degree of orientation and its corresponding impact on the tensile and barrier properties (water vapor transmission rate) of the polymer films. The degree of orientation of the films, assessed by shrinkage measurements, was found to be directly related to the elongational strain rate imposed on the films during the drawing stage of the coating process. Furthermore, the strain at break of the films in the machine direction correlated well with the shrinkage of the films, that is, the strain at break was strongly related to the imposed elongational strain rate. The relation is naturally also dependent on the polymer grade used and on how its rheological properties are affected by the process temperature. The melt temperature had a distinct influence on the water vapor transmission rate through the polymer films. This is discussed in terms of the orientation of the specimens.     

A method to model web trajectory and release in forward roll coating

The trajectory of the web at the exit of a roll coating operation can influence the quality of the final coating. While methods to model the web trajectory have been given in the literature, these methods are limited in various ways. A method is proposed to describe the web trajectory and the pressure distribution in the fluid at the exit of a forward roll coater. The Reynolds lubrication equations for the fluid are coupled with the web by a force balance on web node points. The fluid pressure in the coating layer generates forces on the web. These forces deflect the web. Integration in time gives the web dynamics. The angle that the web is pulled from the nip and the tension are found to influence the pressure pulse in the divergent section of the nip to a large extent. Low tensions lead to a second pressure pulse followed by a sub-ambient or tack pressure. Pulling the web at various angles from the nip can cause the tack pressure to increase or decrease. Pressure pulses are predicted that are comparable to measurements by a laboratory device.     

An engineering study of the film embossing process

A mathematical model of the dynamics and heat transfer of the film embossing process has been developed. The thermal analysis around the preheat roll is determined from an unsteady, two-dimensional heat conduction equation along with appropriate boundary conditions by neglecting the curvature of the preheat roll and choosing a Lagrangian reference frame. The heat transfer occurring between the preheat roll and the embossing rolls is based on a one-dimensional analysis, including both convective and radiative effects. The deformation occurring in the nip region is analyzed for two different situations. For the case where the surface features are small in comparison with the film thickness, a modified one-dimennsional calendering analysis is given, accounting for the irregular geometry of the embossing roll surface. For the case where the polymer does not make complete contact with the surface of the engraved channel, the local deformation is determined by means of a simple one-dimensional cavity filling model. The required pressure distribution is determined by means of a simple one-dimensional cavity filling model, The required pressure distribution is determined by means of a conventional calendering analysis. The analysis for the case of a Newtonian and power-law model is presented in detail. The model yields qualitatively correct results and is computationally simple.     

Increasing the efficiency of the extrusion process

The article presents constructional methods of increasing the efficiency of the polymer extrusion process. Increasing the efficiency of the polymer extrusion process consists in the increase of the flow rate of the processed polymer and the energy efficiency of polymer processing extruders and the decrease of the specific energy consumption and the pulsation of the flow rate and polymer pressure, as well as quality improvement of the received extrudate as a result of better homogenization of mechanical and thermal properties and polymer structure. Therefore, the improvement of the constructional solution of the screw and barrel of the extruder plasticating system was discussed. The improvement of the construction of the screw lies in using the active and passive grooved barrel section. Original constructional solutions of the active grooved section with both longitudinal and helical grooves were shown. The devices increasing the efficiency of the extrusion were also shown, which included a screen changer, a melt pump, and a static mixer. Appropriate conclusions were drawn. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Process limits in two-layer reverse roll transfer

Reverse roll coating in which a thin single layer of liquid is applied onto a substrate has been used in industry for decades and has been extensively analyzed in the literature. Modern coatings, however, are often composed of more than one layer to improve the product performance and to reduce the manufacturing cost. Premetered methods such as slot, slide, and curtain coatings are typically used to produce such multilayer coatings. If the caliper of the substrate to be coated is not constant, then the coating gap and consequently the final film thickness deposited on the web will also be nonuniform. In this study, we focused on the use of reverse roll technique with slot die liquid delivery system to produce a uniform thin two-layer coating. The use of this coating technique to produce such a coating has not been previously explored. The liquid film surface as it is transferred from a rigid steel roll to a deformable urethane-covered roll was visualized to find out how the uniformity of the two-layer coating is affected by the speed ratio between two rolls, layers’ wet thicknesses, and liquid viscosities. The effect of these parameters on the ribbing frequency and amplitude was also investigated. The results show that in the two-layer coating, as in the single layer reverse transfer, there is a critical web speed above which ribbing occurs. The critical speed is determined by the bottom layer viscosity.     

Nanomechanical study of polymer‐polymer thin film interface under applied service conditions

Single layer and multilayer polymer thin film coating on polymer substrate are gaining significant importance in different industries. The quantitative and qualitative estimation of interface response for thin film coating under different service conditions is significantly important from the perspective of modeling and designing novel materials. However, to characterize an interface between the soft polymer layer and soft polymer substrate is challenging because of the confinement effect, surface roughness, the viscoelastic nature of the polymers involved, and most importantly, the comparable mechanical properties of soft polymeric film and polymer substrate. Nanoindentation technique was applied in this work to find out the mechanical response of thin film PMMA (100–200 nm) and Epoxy interfaces of different interfacial strengths. Interfaces of different strengths were obtained by exposing the film‐substrate system to different service conditions. It has been observed from this study that pile‐up plays a major role in finding out the mechanical response of the interfaces of different strengths. The hardness was observed to increase as the interfacial strength reduces. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43532.     

Plasma treatment of automotive steel for corrosion protection – a dry energetic process for coatings

In the thrust of pursuing new environmentally friendly technology for automotive application, a new corrosion protection coating system for automotive steel has been developed through interface engineering affected by an energetic plasma process. The plasma treated coating system outperformed the current phosphated galvanized steel system in scab corrosion tests. In the plasma process, the steel substrate was subjected to plasma cleaning and in situ plasma polymerization deposition. Plasma of a mixture of argon and hydrogen was created to remove the surface contaminants and the inherent oxide layer. A very thin film (50–100 nm) was then deposited by a plasma generated from alkylsilanes (e.g. trimethylsilane (TMS)). The interface can be so designed that strong corrosion-resistant interfacial bonds such as Fe–Si, Fe–C, and Si–C can be obtained. The interfacial chemistry involved in the plasma process and corrosion reaction are characterized by reflection absorption infrared spectroscopy (RAIR), X-ray photoelectron spectroscopy (XPS), and sputter neutral mass spectroscopy (SNMS).     

Analytical prediction of roll coating with counter-rotating deformable rolls

Roll coating is a common technique for applying thin coating films on continuous substrates, e.g., papers and foils. Key advantages are the comparatively simple technology and the possibility of coating thin films using highly viscous fluids. Since roll coating is a self-metered process, the prediction of film thicknesses is of fundamental interest for industrial process control. In the present work, a new analytical approach for the prediction of the film thickness in roll coating with deformable rolls and negative gaps is developed. This method is based on the fluid dynamic theory of lubrication approximation. The film thickness is calculated depending on the geometry of the rolls (including the elasticity of the rubber), the fluid properties of the applied film and the roll velocities. This is gained by using boundary conditions for pressure and—in contrast to former literature—for force. The quality of the predicted results is validated with experimental data from literature. The comparison shows good agreement and thus the derived analytical model offers new possibilities for predicting film thickness and understanding of the associated influence parameters.     

Comparison of large scale coating techniques for organic and hybrid films in polymer based solar cells

Polymer based solar cells (PSC) can be manufactured in a continuous roll to roll process as a low cost regenerative energy source. Coating ink properties and film thicknesses of 30–200 nm are challenging with respect to the manufacturing process, which itself has an important impact on film properties and cell efficiencies. In this paper we compare the large area coating methods: knife coating, slot-die coating, and spray coating with laboratory spin coating. Properties of coating inks and a viscosity model for commercial PEDOT:PSS types are discussed. The significantly smaller viscosity to surface tension ratio, of typical coating inks for PSC compared to conventional coating inks, causes a different behavior during the coating process. Wet film thickness, homogeneity, and process stability and their dependence on process parameters are addressed for each coating method. Hole-conductive and photoactive layers, consisting of polymer-fullerene and polymer-nanoparticle blends, are then compared with respect to homogeneity, AFM topography and absorption spectra. First results indicate that the coating method itself has an impact on polymer-fullerene film morphology and opto-electric properties.