2.5.2. Dry Mass, Ion Chromatography and Yield

The mass of nanoparticles *m*np in the product was determined gravimetrically in combination with ion chromatography and calculated according to Equation (4):

$$m\_{\rm np} = m\_{\rm dry} - m\_{\rm wet} \sum\_{i} \frac{\gamma\_{\rm IC,i} V\_{\rm IC}}{m\_{\rm IC}}.\tag{4}$$

About 2 g of wet product *m*wet were freeze-dried (2–4 LSCplus, Christ, Osterode am Harz, Germany) and the dried product was weighed again to obtain *m*dry. Ion chromatography (820 IC Separation Center, Metrohm, Herisau, Switzerland) was performed on aliquots with a mass *m*IC of about 0.1 g, diluted in H2O with a volume *V*IC of 50 mL. The mass concentration γIC,*<sup>i</sup>* of cations (Metrosep C 4—150/4.0 column) and anions (Metrosep A Supp 4—250/4.0 column) was obtained. The yield *Y* is the quotient of the initially provided mass of the feed stock *m*raw and *m*np.

#### 2.5.3. Optical Properties of Dispersions, Coatings, and Cast Films

Optical microscopy was performed on a polarized light microscope (BX51-P, Olympus, Hamburg, Germany) equipped with a camera (XC50, Olympus, Hamburg, Germany) for the qualitative analysis of the coatings and the cast films.

Scanning electron microscopy (SEM) was carried out on a JEOL JSM-IT100 (Akishima, Japan) with a secondary electron detector and an acceleration voltage of 3–5 kV. The nanoparticle dispersions were freeze-dried beforehand and mounted on conductive carbon tape.

The optical absorbance *A* of dispersions and transparent films was measured using a Specord 50 Plus Spectrophotometer (Analytik Jena, Jena, Germany). The absorbance at a wavelength λ is proportional to the extinction coefficient ε(λ), the concentration *c* and the optical path length *d*, according to the Beer-Lambert law (Equation (5)).

$$A = \varepsilon(\lambda)cd\tag{5}$$

#### 2.5.4. Physical Properties

#### Water Content

The water content of the raw materials was determined by volumetric Karl Fischer titration (TitroLine KF, Xylem Analytics, Weilheim, Germany). The iodine titrant was determined with the water standard. Methanol and formamide were used as solvent in a ratio of 1.5:1. About 0.3 g of the sample was added to the solvent. The titration was started when the sample was completely dissolved. The solvent was exchanged after each measurement. The water content of the raw materials was considered regarding the calculation of the mass ratios for the preparation of nanoparticles and cast films.

#### Surface Tension

The surface tension of untreated and corona-treated PLA was evaluated with test ink pens (Arcotest, Mönsheim, Germany). The pens are filled with an ink of defined surface tension. The ink is applied with the pens to the substrate surface. If the line of ink does not separate into drops after at least 2 s, the surface energy of the substrate is the same or higher than the surface tension of the fluid. Then, the pen with a higher surface tension is applied until a separation of the line of fluid into drops is observed.

#### Pinhole Testing

The grease resistance of the coated films was measured using an internal method from Fraunhofer IVV [44]. A test area of 25 cm2 of the film surface was covered with a fleece—for constant and sufficient covering—and saturated with a solution of colored peanut oil. No further weight was applied. After 24 h at 23 ◦C and 50% r.h., the fleece and oil residues were removed, and the stained area was characterized by digital image evaluation. At least four specimens were characterized. No pinholes are present, when no fatty spots on the back of the sample are detected.

#### Thickness

The film thickness was measured mechanically (Precision Thickness Gauge FT3, Rhopoint Instruments, East Sussex, UK) on 5 evenly distributed measuring points. The coating thickness *d*laminate was calculated from the thickness of a coated film and the same substrate without a coating *d*substrate.
