In the rapidly evolving field of flexible electronics, the demand for efficient, sustainable, and precise fabrication methods is paramount. Traditional gravure printing techniques have long relied on chromium (Cr) coatings for their durability and print quality. However, environmental and health concerns associated with chromium have spurred research into alternative materials. One promising candidate is diamond-like carbon (DLC), known for its exceptional hardness, chemical inertness, and environmental friendliness. This article delves into the comparative study of DLC and chromium-coated gravure surfaces, focusing on their efficacy in printing conductive fine-line electrodes for flexible electronics.

Gravure printing stands as a cornerstone in the production of both graphic inks and conductive materials for electronic applications. Its ability to deliver high-resolution patterns at rapid speeds makes it indispensable in manufacturing processes. Traditionally, gravure cylinders are coated with chromium due to its hardness and wear resistance. However, the electroplating process used for chromium deposition generates toxic waste, posing environmental and health risks. Consequently, there is a pressing need to identify sustainable alternatives that do not compromise on performance.

Diamond-like carbon (DLC) emerges as a compelling substitute. DLC coatings exhibit remarkable surface hardness and resistance to abrasion, properties that are advantageous in the context of gravure printing. Moreover, the fabrication of DLC does not produce toxic waste, aligning with environmental sustainability goals. This study aims to evaluate the performance of DLC-coated gravure surfaces in printing conductive fine-line electrodes, comparing them against traditional chromium-coated surfaces under various printing conditions.

Materials and Methods

The study involved designing fine-line electrode patterns, engraving these designs onto gravure surfaces, and assessing the print quality under different conditions. Key variables included the type of coating (DLC vs. chromium), ink formulations, substrates, and doctor blade materials.

Design and Fabrication of Gravure Surfaces

Digital artwork of diamond-shaped square grids with line widths of 20 μm and 30 μm was created using Adobe Illustrator. These designs were laser-engraved onto soft copper surfaces, which were subsequently coated with either chromium or standard DLC. The engraving depth was maintained at 10 μm to ensure consistency across samples.

Characterization of Coated Surfaces

Surface properties such as free energy, contact angle, and roughness were measured to understand their influence on ink transfer and print quality. A KRÜSS Mobile Surface Analyzer was employed to assess surface free energy and contact angles using water and diiodomethane as test liquids. Surface roughness parameters were evaluated using a white light interferometer.

Ink and Substrate Selection

Two types of conductive inks were used: LOCTITE ECI 7007 E&C and Versa HR Black 60. The substrates selected were Melinex® ST339 (white PET) and Melinex® ST506 (clear PET), both known for their relevance in flexible electronic applications. The interaction between these inks and substrates was analyzed by measuring viscosity, surface tension, and contact angles.

Printing Trials

Eight printing trials, as shown in the table below, were conducted using an RK Print Coat Instruments K Printing Proofer. Variables such as doctor blade material (metal vs. plastic), ink type, and substrate type were systematically altered to assess their impact on print quality. Each trial was performed on both DLC and chromium-coated surfaces to facilitate direct comparison.

Results and Discussion

Surface Characterization

The DLC-coated surfaces exhibited higher surface free energy compared to chromium-coated surfaces, particularly in the polar component. This suggests better wettability, which is advantageous for ink transfer during printing. Surface roughness measurements indicated that DLC surfaces had lower roughness parameters, potentially contributing to more uniform ink distribution and reduced defects in printed lines.

Print Quality Assessment

Line width measurements revealed that prints from DLC-coated surfaces consistently exhibited increased line widths for both 20 μm and 30 μm lines compared to those from chromium-coated surfaces. This increase in line width, or line gain, is indicative of better ink release and transfer efficiency. Electrical resistance measurements showed that DLC-printed samples had lower resistance values, correlating with the observed increase in line width and suggesting more effective deposition of conductive material. The lines printed using chrome and DLC surfaces in Trial 1 and Trial 3 are shown in the figure below. For more images of lines from other trial experiments can be found in the article Materials Advances, 5(16), 6535–6553 and article access link is given in the references section.

Influence of Printing Conditions

The type of doctor blade used significantly impacted print outcomes. Metal blades, being harder, provided higher shear during doctoring, leading to more effective ink filling in the engraved lines. Plastic blades, being softer, resulted in less effective ink transfer. The choice of substrate also played a role, with substrates exhibiting higher surface free energy facilitating better ink adhesion and transfer.

Statistical Analysis

Statistical analysis using two-sample t-tests confirmed that the differences in line widths between DLC and chromium prints were significant, with p-values less than 0.005 in most cases. This underscores the superior performance of DLC-coated surfaces in terms of ink transfer and print quality.

Conclusion

The findings of this study suggest that DLC-coated gravure surfaces offer a viable and sustainable alternative to traditional chromium coatings for printing conductive fine-line electrodes in flexible electronics. The superior ink transfer efficiency, coupled with the environmental benefits of DLC fabrication, positions it as a promising material for future applications in printed electronics. For more details on the complete experiment, results and conclusions, see the article published in Materials Advances, 5(16), 6535–6553 and its access link is given in the reference section below.

Future Work

While this study provides compelling evidence of the advantages of DLC-coated gravure surfaces, further research is warranted to explore their performance in large-scale roll-to-roll printing processes. Additionally, investigating the interaction of DLC surfaces with a broader range of conductive inks and substrates will provide deeper insights into their applicability across various flexible electronic applications.

Reference:

1. Seetharamiahsrinivasaraju, C., Shetty, R., Cohen, D. K., Sharma, P., & Springstead, J. R. (2024). DLC-engineered flat gravure surface: Enabling sustainable fabrication to replace chrome for printing conductive line electrodes in flexible electronics. Materials Advances, 5(16), 6535–6553. https://doi.org/10.1039/D4MA00562G

Dr. Chandramohan Seetharamiah Srinivasaraju is an expert in the printing, packaging and paper industries. His research has been published in leading scientific journals, and his work on developing new technology cylinders with environmentally friendly, durable surfaces for graphic printing earned him GAA Technical Paper award and the prestigious Technical Association of Graphic Arts (TAGA) Individual Award. Dr. Chandramohan holds a Master’s degree in Print Media Technology and a PhD in Paper & Printing Science. With over 15 years of experience in the printing industry, he has worked extensively in areas such as software development, color management, printed electronics, inkjet and conventional printing, as well as the manufacturing and qualification of printing methods and substrates. Currently, Dr. Chandramohan Seetharamiah Srinivasaraju is pursuing advanced research focused on sustainable materials for use in printing and packaging. He can be reached at [email protected].