Panasonic's "stretchable resin film"
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Panasonic Corp developed a "stretchable resin film" (stretchable insulating material) as well as a transparent electrode and a conductive paste for interconnection (wiring) for use with the resin film.

In a sense, it is an advanced version of flexible substrate, enabling extension and restoration to the original shape, which cannot be done with existing flexible substrates. Panasonic plans to exhibit the new product at the 17th Printed Wiring Boards Expo, which will take place from Jan 13 to 15, 2016, at Tokyo Big Sight.

Featuring repeatable stretchability, the insulating material has a tensile elongation ratio of 150% or more and a restoration ratio (elongation speed: 25mm/min, restoration speed: 0.1mm/min) of 98% or more. There are some urethan- and rubber-based materials that have such a stretchability, but they have problems concerning adhesiveness, heat resistance and embrittlement.

This time, Panasonic employed a thermosetting resin for the stretchable resin film. The heat resistance of thermosetting resins lowers when an elastomer, etc are added to improve flexibility.

Therefore, the company used its own resin design technology to make use of the three-dimensional bridge structure of the thermosetting resin and reduce the internal stress applied at the time of stretching the resin so that the resin returns to the original shape. The resin formed into a film is used as an insulating material.

CNT used as material for transparent electrode

Panasonic also developed a transparent electrode and a conductive paste for interconnection for use with the insulating material. It is possible to use ITO (indium tin oxide) for transparent electrodes, but ITO-based electrodes can be easily cracked when they are folded or stretched.

This time, the company used the stretchable resin as a base material and formed a thin conductive layer by using carbon nanotubes (CNTs) with a large aspect ratio. As a result, it ensured both a flexibility and conductivity and realized a transparent electrode that can keep its conductivity even after being repeatedly elongated and contracted.

The conductive paste for interconnection is made by using the stretchable resin as a binder and combining it with a silver filler. The paste can also keep its conductivity even after being repeatedly stretched and contracted.