The transformation from cathode ray tubes to LCD displays has been rapid since the early 2000s. We now have thinner, lighter and bigger screens available with affordable prices. You have probably also seen pictures of flexible displays and read stories about flexible mobile phones and foldable screens. I'm sure many of you have also thought if we really need those and would it in the end be practical to have a foldable display in your pocket. Probably not, but flexible displays allow new product opportunities for many industries such as car industry and consumer products. However, one of the biggest drivers for the flexible displays is actually related to the manufacturing of the displays.
Flat panel displays
Flat panel displays (FPD) are traditionally manufactured by batch method. The functional materials of the displays are deposited on a glass plate which is then divided to different screen sizes. Due to practical limitations (size and weight of panels), there is a growing interest to produce displays in a continuous roll to roll process which would allow mass production of screens at low cost. In order to achieve the required flexibility for the process, plastics are used instead of glass, as substrate. However, the limiting factors for the change to roll to roll process are related to the high requirements set for the substrate materials. Plastic substrate should have high processing temperature, low thermal expansion, low gas permeability, high chemical resistance, high flexibility and good optical transparency. Typical plastics used for FPD include polycarbonate (PC), polyester (PET), polyethersulfone (PES), and polyimide (PI). The major challenge with these plastics is that they either have low processing temperature, poor printability or have relatively large coefficient of thermal expansion which may create strain and cracks during the manufacturing process of the display.
LG Display introduced in 2014 a flexible OLED (organic light emitting display) display to the market and has since continued the development of flexible and transparent displays for multiple end use purposes.
MFC in flexible displays
The ideal substrate for active matrix LCDs and OLEDs would combine the barrier, thermal and scratch resistant properties of glass with the flexibility, toughness and processability of plastic. We have previously discussed about the barrier and film forming properties of microfibrillated cellulose (MFC).
Hu et al. reported in 2013 that they had produced a transparent and flexible OLED device on transparent nanocellulose paper, made of TEMPO oxidized nanocellulose. The nanopaper that they produced by filtration had tensile strength of 287 MPa, Young`s modulus of 9 GPa, coefficient of thermal expansion 2.7 ppm/K with bending radius of 1 mm. In addition to mechanical properties, the nanocellulose paper omitted anti-glare effect which makes the display more pleasant for the eyes and comfortable to read.
Keeping in mind that MFC also has strong adhesion to glass, one could imagine achieving stronger glass films by adding a thin layer of MFC on to the glass. Hu et al. coated flexible glass with TEMPO oxidized nanocellulose resulting in a material with high optical transparency and increased optical haze from <1% to ~56%.
When we think about the multifunctionality of MFC, we can clearly see that the potential of MFC is not only limited to displays. In fact, there are multiple opportunities for the electronic industry to utilize the mechanical and physical properties of MFC to produce bio based components and products. MFC will enable the industry to produce stronger, better performing and greener products for future demand.