Reverse Gravure Research Articles

A Comparative Study on the Roll-to-Roll Processing of a Silicate-Polyvinyl Alcohol Composite Barrier Lacquer Using Slot-Die and Reverse Gravure Coating Techniques.

The integration of platelet-shaped montmorillonite particles to improve the oxygen barrier of polyvinyl-alcohol-based barrier layers is state-of-the-art, but research on roll-to-roll coatings of such composite barrier lacquers has not been widely published. In this study, two different coating techniques, slot-die and reverse gravure, were used on a roll-to-roll scale to apply barrier lacquers comprising polyvinyl alcohol and montmorillonite. The lacquers were analyzed regarding viscosity at certain shear rates and surface energy and the dried coating layers regarding oxygen barrier, surface morphology, and particle orientation. Low permeability coefficients delivering a high oxygen barrier of 0.14 and 0.12 cm3 (STP) 1 μmm2 d bar were achieved for the coating layers with slot-die and reverse gravure coating, respectively. It turned out that the properties of the barrier lacquer need to be adjusted to the coating technique to achieve high oxygen barrier performance. By tailoring the barrier lacquer formulation, the orientation of the platelet-shaped montmorillonite particles can be achieved using both techniques. A low solid content of down to 3 wt% is preferable for the premetered slot-die coating, because it results in low agglomeration quantity in the coating layer. A high solid content of up to 9 wt% is preferable for the self-metered reverse gravure coating to assure a homogeneously coated layer.

Low-cost, mass-producible nanostructured surface on flexible substrate with ultra-thin gold or silver film for SERS applications

Surface Enhanced Raman Spectroscopy has emerged as a powerful analytical technique for fingerprint recognition of molecular samples with high sensitivity. The Surface Enhanced Raman Scattering (SERS) effect has been extensively studied for the past few decades. However, only recently the commercialization of portable Raman spectrometers has taken SERS a step closer to real-world applications. Swift and convenient testing of analytes for point-of-care, environmental as well as food quality control and safety applications, is very lucrative. This can be realized with the use of low-cost, mass producible and environmentally friendly SERS active substrates in combination with portable Raman spectrometers. In this study, we demonstrate one approach to accomplish such a SERS-active substrate using nanostructured latex coated paperboard as a base substrate. The nanostructure is accomplished by applying a reverse gravure coater in combination with a short-wavelength infrared (IR) heater. The whole process is easily up-scalable. The SERS functionality is then obtained by physical vapor deposition of an ultra-thin layer of Au or Ag. The surface nanostructure was confirmed by atomic force microscopy, showing an additional nanoscale graininess after the deposition of Au or Ag. The successful metal deposition was confirmed by X-ray photoelectron spectroscopy and deposition homogeneity was also analyzed. To confirm the SERS effect, two model compounds; crystal violet and rhodamine 6G were tested in the concentration range of 1–1000 μM. The results confirmed that the nanostructured, flexible, paper-based substrate can perform as a SERS-active substrate with negligible background noise.

Cellulose nanofibrils and silver nanowires active coatings for the development of antibacterial packaging surfaces

An active ink composed of cellulose nanofibrils and silver nanowires was deposited on flexible and transparent polymer films using the bar coating process, achieving controlled thicknesses ranging from 200 nm up to 2 μm. For 350 nm thick coating on polyethylene terephthalate films, high transparency (75.6% transmittance) and strong reduction of bacterial growth equal to 89.3% and 100% was noted respectively against Gram-negative Escherichia Coli and Gram-positive Staphylococcus Aureus bacteria using AATCC contact active standard test. Retained antibacterial activity was found with films produced by reverse gravure roll-to-roll process, showing the promising capability of this antibacterial solution to be deployed industrially. Finally, the same ink was also deposited on polylactic acid substrate to investigate barrier properties: for 350 nm thick coating, a reduction of 49% of oxygen transmission rate (dry conditions) and 47% reduction of water vapor transmission rate was noted, proving the enhanced barrier properties of the coatings.

Roll-to-roll fabrication of cellulose nanocrystal-poly(vinyl alcohol) composite coatings with controlled anisotropy

Cellulose nanocrystal (CNC) composite coatings may impart many benefits in packaging, electronic, optical, etc. applications, however, large-scale cellulose coating production is a major engineering challenge. A versatile roll-to-roll reverse gravure process for the manufacture of cellulose nanocrystal-poly(vinyl alcohol) (CNC–PVA) coatings on a flexible polymer substrate was investigated in the present work. CNC content was varied from 0 to 100% to determine the effect of CNC concentration on coating anisotropy. Coatings were characterized by polarized light microscopy, UV–Vis spectrophotometry, cross-hatch adhesion testing and optical profilometry. This method produced uniform, highly transparent coatings with surface roughness less than 100 nm for all CNC–PVA weight ratios examined. The isotropic-anisotropic coating transformation was observed above 50% CNC, with a maximum anisotropy at 70% CNC along the shear direction. Anisotropic CNC–PVA coatings exhibited increased water vapor barrier performance due to the increased CNC packing and density.

Reverse gravure coating for roll-to-roll production of organic photovoltaics

Reverse gravure (RG) coating is reported here as an alternate film deposition method for potential large scale roll-to-roll production of organic photovoltaic devices (OPVs). The basic working principles of RG coating are shown and compared to the more well-known gravure printing. Gravure printing is similar to RG coating from a process point of view, but the films produced using each method are very different to each other. An optical thickness measurement system was developed and used to monitor film thickness variation of RG coated photo-active layers with various coating parameters in situ in the roll-to-roll process. Partially and fully printed OPV modules were fabricated using, primarily, the roll-to-roll RG coating process and devices showed 2.1% and 1.5% power conversion efficiencies, respectively.

Self-Aligned Metal Electrodes in Fully Roll-to-Roll Processed Organic Transistors

We demonstrate the production of organic bottom gate transistors with self-aligned electrodes, using only continuous roll-to-roll (R2R) techniques. The self-alignment allows accurate <5 µm layer-to-layer registration, which is usually a challenge in high-speed R2R environments as the standard registration methods are limited to the millimeter range—or, at best, to tens of µm if online cameras and automatic web control are utilized. The improved registration enables minimizing the overlap between the source/drain electrodes and the gate electrode, which is essential for minimizing the parasitic capacitance. The complete process is a combination of several techniques, including evaporation, reverse gravure, flexography, lift-off, UV exposure and development methods—all transferred to a continuous R2R pilot line. Altogether, approximately 80 meters of devices consisting of thousands of transistors were manufactured in a roll-to-roll fashion. Finally, a cost analysis is presented in order to ascertain the main costs and to predict whether the process would be feasible for the industrial production of organic transistors.

Rheological and Morphological Evaluation of Natural Rubber Latex-Based PSAs Containing Waterborne Aliphatic Hydrocarbon Tackifier Dispersions

The effect of three waterborne aliphatic hydrocarbon tackifier dispersions each with different softening points (70°C, 85°C, and 95°C) were evaluated with natural rubber latex at two addition levels (25% and 50%) for pressure-sensitive adhesive (PSA) applications. No other additives were incorporated into the PSA formulations so that rheological effects of waterborne aliphatic hydrocarbon tackifier resin dispersions in natural rubber-based PSAs could be clearly understood. Application of these waterborne PSAs was evaluated, in terms of rheology, since flow parameters have a very important influence on the convertibility (coating ability) of such adhesives. Morphological correlations with wet rheology for these waterborne PSA formulations and starting materials revealed that the interaction between the latex particle and tackifier dispersion particle has a major influence in determining the viscosity characteristics at low to medium shear rate, where stirring, pumping, and filtration processes occur. A shear-thinning effect was also predominant in formulations with lower tackifier dispersion levels. The extent of shear-thinning can be correlated well to morphology. Interestingly, all the PSA formulations tend to follow Newtonian behavior above a shear rate of 1000 s−1 and no shear-thinning or shear-thickening at higher shear rates was observed. The minimal change in viscosity at higher shear rates is a key parameter for high-speed coating techniques such as curtain coating and reverse gravure, since any change in viscosity can affect coating uniformity and the wetting of the substrate surface during coating.

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.

Top layer coatability on barrier coatings

In this study, the influence of different surface characteristics of dispersion-coated barrier layers on top layer coatability is examined. The barrier layer consisted of platy pigments, such as talc and kaolin, combined with different amounts and types of latexes. Coating of the top coat was carried out using the reverse gravure technique in which a dispersion consisting of mineral pigments and latex is applied under slight pressure onto the barrier-coated substrate. Wettability and coatability were measured as a function of surface energy and surface roughness of the barrier layer and the surface tension of the top coat dispersion. Plasma and corona surface treatments were used to increase the surface energy and wettability without affecting the surface topography. The most important aspect for predicting the coatability was the wettability, especially the water contact angle, because the top coat is an aqueous suspension. Decreasing the surface tension of the top coat dispersion also improved the coatability, whereas roughness had only a minor effect.

Roll-to-roll fabricated lab-on-a-chip devices

We present a high-volume fabrication technique for making polymer lab-on-a-chip devices. Microfluidic separation devices, relying on pinched flow fraction, are roll-to-roll fabricated in a cellulose acetate (CA) film at a volume of 360 devices h−1 for a cost of approximately 0.5 euro/device. The manufacturing process consists of two steps: (i) roll-to-roll thermal nanoimprint for patterning the microchannels into a CA film and (ii) roll-to-roll lamination for bonding another CA film onto the imprinted film to seal the microchannels. Reverse gravure coating is used to apply an adhesive polymer onto the CA lid film before roll-to-roll lamination in order to increase the bonding strength. The fabricated devices are compared with planar imprinted devices with regard to the cross-sectional profile of the imprinted channels and their separation functionality. The separation functionality is characterized using fluorescent polystyrene microspheres with diameters ranging from 0.5 to 5 µm.

All-printed low-voltage organic transistors

All-printed low-voltage organic transistors were manufactured on a plastic substrate and measured in normal room atmosphere. All electrodes were ink-jet printed, while the homogeneous semiconductor and insulator layers were subsequently applied by the reverse gravure coating technique. No destructive effects from the rough plastic substrate or the printed silver source and drain electrodes were observed. Changes in the transistor characteristics after 48 days of ageing in room atmosphere were mainly attributed to a reduced conductivity of the polymer gate electrode.

Roll-to-Roll Fabrication of Bulk Heterojunction Plastic Solar Cells using the Reverse Gravure Coating Technique

AbstractThe roll-to-roll reverse gravure (RG) coating technique was used to produce thin homogeneous films (∼100 nm) for organic bulk heterojunction solar cells. The conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and the active layer regioregular poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) were successfully subsequently RG coated on an ITO covered plastic substrate in ambient air. Working solar cells were achieved after annealing and thermal evaporation of the top contact. The AM1.5 power conversion efficiency (PCE) of the RG coated organic solar cells was determined to 0.74% (at 100 mW/cm2). This was very similar to the results of a reference device that was spin coated on a glass substrate in a nitrogen glove box.