Through the fermentation of low molecular weight sugars, some bacteria, such as Komagateibacter are able to produce a highly crystalline and ultrapure form of cellulose. This bacterial cellulose is grown in the form of a three-dimensional intermeshing network which results in a tough membrane known as pellicle. The toughness of the membrane is translated into the dried BC network, where the fracture toughness of BC nanopaper has been measured to be as high as 6.6 kJ m^1/2. Despite the high fracture toughness, BC is a brittle material, as well as inherently opaque. To increase its value as a reinforcing material in transparent impact applications, a mercerisation process is employed here to increase both ductility and transparency of the BC nanopaper.

The nanopapers were prepared by immersing the pellicle into a sodium hydroxide solution (0, 5, 10, 15, 20 and 30 wt.% NaOH) for 30 min while clamped, to avoid lateral shrinking, followed by a washing process with DI water still in the same setup. The obtained nanopapers were dried at room temperature in between filter papers. Upon drying the nanopapers were immersed in a liquid, UV-curable resin overnight at reduced pressure. The impregnated nanopapers were then UV-cured directly or sandwiched in between two layers of impact-modified (IM) acrylic followed by UV-curing to create laminated composites. XRD measurements of the nanopapers confirm a structural change of the cellulose after the treatment with the solutions containing 15 wt.% or more of sodium hydroxide. This translates into an increase in total light transmittance from 40 % to 80 % at 550 nm after this treatment, compared to the samples treated with no, or lower amounts of sodium hydroxide. Furthermore, the fracture toughness of the nanopapers is increased by up to 70 %, and the strain at failure increases from 3 % to 13 %. These trends are then translated into both the thin plies and the laminated composites, which all reach a total transmittance of 75 – 80 % at 550 nm. Moreover, the impact strength, measured by Charpy impact testing, was increased from 11.6 kJ m^-2 for the IM-acrylic to 23.8 kJ m^-2 for the laminated composite reinforced with a nanopaper treated with the 30 wt.% NaOH-solution.

Daniela Wloch

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