*2.3. Coating of PLA and Paper*

The coating strategy of the nanoparticle dispersions onto polymer substrates was empirically developed for CNC dispersions. Qualitative parameters were considered regarding the application

of a nanoparticle dispersion onto a flat substrate and the subsequent drying process. PLA was corona-treated (Corona Station, Softal, Hamburg, Germany) at 400 W and 5 m min−<sup>1</sup> to increase the surface energy with the aim to achieve good spreadability of the aqueous coatings on the substrate surface [41]. For paper this pretreatment was not necessary. The effect of viscosity of the coating medium was investigated for CNC dispersions with concentrations of 3 ≤ *c* ≤ 8 wt %. It was found that with increasing concentration and hence increasing viscosity, contraction of the wet film could be fully avoided. The dynamic viscosity of a dispersion with 6 wt % of CNC was >2000 Pa s at a shear rate of 0.01 s−<sup>1</sup> and decreased exponentially to 0.05 Pa s at 1000 s<sup>−</sup>1.

The concentration of the nanoparticle dispersions was adjusted in a rotary evaporator (Rotavapor R-100, Büchi, Flawil, Switzerland) at 40 ◦C and 70 mbar to ~63 g kg−<sup>1</sup> and ~73 g kg−<sup>1</sup> of hydrolyzed product for cellulose and starch, respectively. The dispersions were applied onto the substrates with a semi-automatic coating unit (CUF5, Sumet Systems, Denklingen, Germany).

The interplay of blade velocities *<sup>v</sup>* of 5 ≤ *<sup>v</sup>* ≤ 60 mm s−<sup>1</sup> and wet film thicknesses *<sup>d</sup>* of 10 ≤ *<sup>d</sup>* ≤ <sup>100</sup> <sup>μ</sup>m was tested. At *<sup>v</sup>* = 10 mm s−<sup>1</sup> and *<sup>d</sup>* ≈ <sup>51</sup> <sup>μ</sup>m under a normal load of 40 N, the coatings showed no macroscopic cracks or delamination on both PLA and paper. The same parameters led to conformable coatings using SNP dispersions on both substrates.

Drying was tested for temperatures of 40 ≤ *T* ≤ 60 ◦C for PLA and 40 ≤ *T* ≤ 120 ◦C for paper. The drying time was 1 ≤ *t* ≤ 10 min. Drying at high temperatures and within narrow time spans led to contraction of the coating medium and eventually delamination of the coating layer on PLA. Furthermore, high temperature for extended time spans bears the risk of denaturation of plastic substrates and desulfation of the nanoparticles [42]. Eventually, a temperature of 40 ◦C for 10 min was chosen for PLA. Paper was dried at 70 ◦C for 3 min to avoid wrinkling [43]. To avoid possible pinholes in the coating layer on the paper substrate due to its surface roughness it was coated and dried two times with the same parameters.
