Vacuum Verbiage: Today's paper: smoother, clearer, and fit for flexible electronics

By Dr. Charles A. Bishop

Cellulose in its various forms is a growth substrate. Of the opaque products, paper is attracting plenty of interest at present as sustainability rises in importance, not least of which marketing importance. There is a large market for paper in replacing polymers in packaging applications. Switching from flexible polymers to paper has a variety of challenges. Paper tends to be opaque, the surface rough, and there is little barrier performance in comparison to polymers. To improve these deficiencies the paper can be calendered, polished, coated [1] or laminated, which coupled with sustainability credentials, enables paper to challenge the dominance of polymers.

Part of the challenge is to not impair the recycling opportunity for the final product when improving the properties of the paper. It is easy to laminate a polymer to the paper to provide better barrier performance, but this defeats the objective as it still uses a polymer, that may not be from a sustainable source, as well as preventing the final product being recycled because of the mixed materials. Coatings and laminates may be from polymers that are compostable and so allow the final product to be compostable, but this is less desirable than being able to repulp the paper to produce new products.

A clearer way to go

Historically there have been several transparent or translucent cellulose products from the opaque papers that were impregnated with oils, waxes, gums, resins or varnishes that impregnated the woven fibers, making the paper anywhere from translucent to transparent depending on the materials and thickness. The high-clarity, transparent, cellulose materials were the cellophane, cellulose acetate or cellulose triacetate group of films [2] that start as regenerated cellulose. This is where the wood pulp is ground up and dissolved into a solution known as viscose that is chemically treated to regenerate the cellulose in a form that can be extruded to make films. Glassine is a special form of paper as it has been supercalendered to densify, smooth and more align the fibres so that the end product has smoother and more shiny surfaces as well as being translucent. This has been used for pouches for stamps, protective sheets for photograph albums as well as separator sheets for foods.

In the area of flexible electronics, the idea of having recyclable circuits and devices also is becoming more appealing, and paper is included in the toolbox of solutions. This is not to be confused with e-paper, which is a display device and is so named because it was aimed at replacing books, magazines or newspapers with a compact digital device. For flexible electronics, the challenges for paper are greater than for packaging. It also is preferable for many of the electronic applications the substrate should be transparent.

Making paper work for flexible electronics

The traditional paper roughness is a problem when trying to deposit fine-line conducting circuits without breaks in the circuits, particularly when flexing the paper. To address this problem, the whole papermaking process was reviewed, and changes were made to the starting material. Cellulose from wood can still be used, but if the fibers or microfibrils that make up ~40% of the wood (~25% random amorphous hemicellulose & ~30% lignin, the glue that holds everything together) are further broken down by unbundling the component nanofibrils. This means that from the original microfibril of ~20- to 30-micron-diameter nanofibrils a thousand times smaller, at ~20 to -30-nm diameter and an aspect ratio >50, are released which are then used to make the paper with a surface roughness considerably reduced. If this cellulose material is also oxidized, such as by using TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl), which converts the hydroxyl group to a carboxyl group, improved dispersion and stability of the nanofibrils improves the uniformity of the paper [3].

This nanofibril cellulose (NFC) can have a transparency of >80% with a <20% haze. This NFC paper-manufacturing process has been combined with silver nanowires (AgNW), carbon fibers or carbon nanotubes (CNT) to produce transparent, conducting paper [4]. Similarly, nanoparticles have been incorporated into the paper to make biosensors or antimicrobial paper. This technology has demonstrated the use of this transparent paper for OLEDs, displays, photovoltaics, sensors, batteries, RFID tags and demonstrated them to be highly flexible.

Flexible…AND sustainable

However, these are demonstration devices and not full-scale production applications yet. This sustainable substrate is not yet widely available for roll-to-roll (R2R) production and at a competitive cost to allow products to switch to paper substrates. But…it does give an insight of where flexible electronics might change in the future.

These flexible electronic applications may or may not need vacuum-deposited coatings. Currently many of the layers are solution, wet-coated or printed onto the paper. This does not exclude vacuum coating as the paper has the mechanical performance to withstand winding and has a lower water content than many papers that are metallized, and so if there is an advantage to be gained by using vacuum-deposited coatings, I am sure this will be done.

References

1. R. Bollström and M. Toivakka. “Paper substrate for printed functionality,” Advances in Pulp and Paper Research, Cambridge 2013, Trans. of the XVth Fund. Res. Symp. Cambridge, 2013, (S.J. I’Anson, ed.), pp 945–966

2. Paunonen, S. “Strength and barrier enhancements of cellophane and cellulose derivative films: A Review,” BioRes (2013) 8(2), 3098-3121

3. Hongli Zhu, Zhiqiang Fang, Colin Preston, Yuanyuan Li, and Liangbing Hu. “Transparent paper: fabrications, properties, and device applications,” Energy Environ. Sci., 2014, 7, 269-287

4. Ren’ai Li, Kaili Zhang and Guangxue Chen “Highly Transparent, Flexible and Conductive CNF/AgNW Paper for Paper Electronics,” Materials 2019, 12, 322; doi:10.3390/ma12020322