Energy curable technologies and printed electronics

Paul Boughton

Printed electronics is the broad term for a relatively new technology that defines the printing of electronics on common substrates such as paper, plastic, and textile using standard printing processes.

The technology, according to Rahn and Energy Curing, is believed to be one of the fastest growing in the world with an estimated USD$300 billion dollar market potential.

Common press equipment used in the graphics arts industry, such as screen printing, flexography, gravure, ink-jet and offset lithography are employed. Instead of printing graphic arts inks, families of electrically functional electronic inks are used to print devices, such as thin film transistors.

Printed electronics is forecast to facilitate the development of very low-cost electronics useful for applications not typically associated with conventional (ie, silicon-based) electronics, for example: Thin/Flexible Batteries; Conformal/Flexible Displays; E-readers; Photovoltaics; Transistors & Logic; Roll-to-Roll Manufacture Challenges/Opportunities; Stretchable Electronics for Clothing; Conductive Materials; Organic & Inorganic Semiconductors; Graphene Carbon Nanotubes; Sensors & Actuators;  Lighting; RFID; Smart Substrates & Stretchable Electronics.
 
Energy curing is the broad term used to describe the technology used to 'cure' or rapidly dry specially formulated inks, coatings and adhesives. The technology is widely deployed in conventional graphic arts printing bringing the following motivations versus other technologies like solvent based and water based solutions.
 
Energy Curing is normally broken down to two curing techniques.

* UV curing, this means formulations that are cured by exposure to light in the 200-400nm sector of the electromagnetic spectrum.
* EB curing, this means formulations that are cured by exposure to high energy electrons.
Components of energy curable formulations are:

Oligomers – the overall properties of any coating, ink, adhesive or binder crosslinked by radiant energy are determined primarily by the oligomers used in the formulation. Oligomers are moderately low molecular weight polymers, most of which are based on the acrylation of different structures.
The acrylation imparts the unsaturation or the 'C=C' group to the ends of the oligomer.

Monomers – Monomers are primarily used as diluents to lower the viscosity of the uncured material to facilitate application. They can be monofunctional, containing only one reactive group or unsaturation site, or multifunctional. This unsaturation allows them to react and become incorporated into the cured or finished material, rather than volatilizing into the atmosphere as is common with conventional coatings. Multifunctional monomers, because they contain two or more reactive sites, form links between oligomer molecules and other monomers in the formulation.

Photoinitiators - This ingredient absorbs light and is responsible for the production of free radicals. Free radicals are high energy species that induce crosslinking between the unsaturation sites of monomers, oligomers and polymers. Photoinitiators are not needed for electron beam cured systems because the electrons are able to initiate crosslinking.

Additives - The most common are stabilisers, which prevent gelation in storage and premature curing due to low levels of light exposure. Color pigments, dyes, defoamers, adhesion promoters, flatting agents, wetting agents and slip aids are examples of other additives. There will be many emerging applications in printed electronics that might benefit from utilising energy curing as the technology to cure or dry the printed functional pattern.

Energy Curing could also be employed to protect such printed patterns or for other parts of the fabrication process.
 
Some possible benefits that can be imparted to energy curing formulations for PE applications by Rahn energy curable raw materials include: Adhesion to Difficult Substrates; Low Shrinkage; Anti-reflective; Heat Restistance; Moisture and Oxygen Barriers; High Flexibility; Hydrophobic Properties; Thin and Thick Film Curing.

For more information, visit www.rahn-group.com