Printed electronics (PE) is the basis of technologies for logic, memory and light-emitting functions and what lies behind developments in e-paper, smart systems for communicative labels and low-cost RFID tags as well as intelligent and interactive packaging, displays and signage.
Recent years have seen corporate researchers and academia collaborating in European-funded projects to develop large area and low cost manufacturing technologies for printing on new structures, with a view to investing substrates, especially paper-based materials, with electric and electronic features that were previously too expensive to be built into disposable consumer goods.
Kimberly-Clark Corporation has been exploring diverse 'out of domain' applications in its research and development of inexpensive, easy-to-manufacture conductive papers, tissues and nonwovens.
The global health and hygiene company's progress in PE has shown that disposable / semi-durable keypads, self-heating packaging, heated cleaning wipes and disposable / semi-durable thermal therapy pads are within the realms of commercial possibility.
The technology opens new areas for resistively heated and disposable electronic consumer goods by using non-metallic materials, principally cellulose and carbon fibre.
"The material removes metals to create the conductive and electronic functionality. Although preliminary, the removal of metals takes the material in a more sustainable direction to avoid harm to the environment and disruption in the recycling infrastructure," said Tom Ales, a Kimberly-Clark research scientist specialising in technology based on pulp and paper chemistry applied to absorbent products.
Most recently, Ales was working on novel material for passive and active sensing that offers the proposition of add-on benefits in disposable forms such as baby garments. Since 2004, when he started on the project, the scope has widened to incorporate simple components that are now becoming a reality in the world of printed electronics such as transistors, resistors, and memory.
Kimberly-Clark's conductive nonwovens (cNonwovens) comprise raw materials such as paper, carbon fibre and synthetics. They can be sheeted, cut, folded and converted in conventional ways. According to Ales, these materials can be semi-durable or reusable, but they are also low enough in cost to form disposable goods making them functional raw materials. The materials were investigated by Kimberly-Clark and pursued with attention to the balance between the benefit and value. He adds that the carbon fibre content is high enough to enable the materials to do simple, discrete signalling with low electronic signal integrity, and remain disposable at a low cost.
The materials hold promise as a very flexible converting and end-use raw material with a variety of applications. Ales goes on to say that Kimberly-Clark, having reached the stage of a "very precursory analysis of uses and processing", is looking for partners and licensees to develop applications and products that are outside of the traditional consumer market where Kimberly-Clark operates.
For example, when the resistive characteristic is integrated into packaging material it produces a self-heating container that will keep food warm. The material is capable of a wide range of heating temperatures but is limited to the flash point of the given substrate, Ales explains. While cellulose fibre will heat to 300°C (420°F), the addition or use of synthetic fibres can extend this heating range.
The next step
How fast can PE developers move to exploit its seemingly great potential? When introducing new technology "you start simple then go complex," says Ales. It is important to "draw consumers to the new concept" and not to "throw the consumer through a loop", he warns.
In creating a heat source, one of the challenges is to achieve a temperature that is uniform and reproducible; another is to make heat 'customisable': similar to an electric blanket, it must be possible for the heat to be turned on and off, and regulated up and down.
Carbon fibres are cost efficient, and are available with water soluble sizings to allow for uniform formation of the material during manufacture. Consequently, the technology lends itself to use in active RFID at pallet or case level, and in such applications it will bring the cost down markedly. Kimberly-Clark speculates a 30% cost reduction compared with conventional systems. About 50% of the cost of RFID tags is in the converting - in the inlay, carrier sheet processing and conductive ink.
In conductive papers and nonwovens, a conductive substrate results when between 5%-30% (by weight) carbon fibre is mixed with natural or synthetic fibres. The conductive and resistive nature of the paper and nonwoven can be varied by changing the ratios of the constituent materials.
Compared with using wires embedded in wovens or nonwovens, the result is less bulky and more flexible, offers lower resistivity (10% carbon fibre/cellulose mixture demonstrates 40Ohms/sq) and can demonstrate higher resistivity to provide anti-static properties.
Kimberly-Clark's large-scale test produced 60t of sample paper material. It has two published patent applications and six unpublished patent applications.