2.3. Characterization

The residual water content for all polymers (despite BioHDPE) was determined by Karl Fischer titration, which was performed at 140 °C on an “899 Coulometer” and an “885 Compact Oven SC” (both: Deutsche METROHM GmbH & Co. KG, Filderstadt, Germany). The resulting water content was <150 ppm.

A Ray Ran Melt Flow Indexer Model 3A (Industrial Physics, United Kingdom, Warwickshire) was used to measure the melt flow rate (MFR), following the standard ISO 1133-A and using 2.16 kg weight and 2 mm capillary at 190 and 210 °C. However, the melt flow rate is a weak parameter to estimate the processability of polymers, at least when the process temperature lies far above the standard’s test definition and (thermal) degradation is an issue, as is common for various biopolymers. Consequently, (shear-) rheological characterization was executed additionally to gain a deeper insight into and estimation of the materials’ flow behavior.

Shear rheological experiments in the temperature and time-sweep modes were performed on a “Physica MCR 501” rheometer (Anton Paar Group AG, Graz, Austria) in plate–plate geometry at different temperatures. Polymer granules were placed on the lower plate (25 mm in diameter), and the gap was adjusted to 1.0 mm. Afterward, excess material was removed, and the test was performed under a nitrogen atmosphere (strain: 10%; angular frequency: 10 rad·s^–1). Temperature ramps were performed under adjustment of the gap to maintain a constant normal force over the measurement. The strain amplitude was proven to be in the linear viscoelastic regime by strain sweep tests at a constant angular frequency of 10 rad·s^–1. Time-sweeps were executed at selected temperatures (strain: 10%; angular frequency: 10 rad·s^–1) to validate the thermal stability of the materials.

Differential scanning calorimetry (DSC) (Erich NETZSCH GmbH & CO, Holding KG, Selb, Germany) was used to analyze the thermal behavior and crystallinity of different polymers. Two heating scans from 25 to 210 °C at the heating rate of 10 °C/min were performed. The cooling rate between heating scans was 10 °C·min^–1. The crystallinity (X_c) was measured as per the DIN EN ISO 11357-1 standard, and the degree of crystallinity, X_c, was calculated as

Here, ΔH_m is the measured melting enthalpy of the sample, ΔH_cc is the enthalpy of cold crystallization (J·g^–1), and ΔH⁰_100% is a theoretical literature value for the melting enthalpy of the 100% crystalline polymer. The PBS literature value for the 100% crystalline polymer is 110 J·g^–1,37. PLA has a ΔH⁰_100% of 93.6 J·g^–138, and for PE, the ΔH⁰_100% value is 293 J·g^–139.³⁹

Thermogravimetric analysis (TGA) is a method where the mass of the sample is measured over time while the temperature increases. TGA equipment has a highly sensitive scale to accurately determine changes in sample mass. TGA equipment STA 499 F1 Jupiter (Erich NETZSCH GmbH & CO, Holding KG, Selb, Germany) was used for analyzing the thermal degradation of selected polymers.

The fiber diameter distribution was determined based on scanning electron microscopy (SEM). Therefore, a round sample was punched out of the nonwoven and placed on the SEM carrier, which was sputtered in argon plasma (40 s under a vacuum of 0.1 mbar, with a distance of 35 mm, a current of 33 mA, and a voltage of 280 V) with a gold–palladium layer of 10–15 nm. Three SEM micrographs per sample were taken with a magnification of ×1000 using a “TM-1000 tabletop electron microscope” from Hitachi High-Tech Corporation (Tokyo, Japan) with an accelerating voltage of 15 kV in the “charge-up reduction mode”. The magnification was chosen to evaluate roughly 40 single fibers per image (see Figure S2). Contrast and brightness were adjusted to gain an image of straight monochromic fibers in front of a dark monochrome background. To analyze the images with regard to automated fiber diameter distribution, the β software “MAVIfiber2d”, developed by Fraunhofer ITWM (Kaiserslautern, Germany), was used.⁴⁰ First, the images are smoothed and binarized by the software, before a statistical analysis is performed over each fiber pixel without segmentation into individual fibers.^41,42 After merging the output of the three images, the mean and median fiber diameters as well as the standard deviation and interquartile range were determined.

The area base weight of nonwovens was determined referring to DIN EN ISO29073-11, adjusted by cutting out and weighing square sections of 10 × 10 cm (100 cm²). To include homogeneity scattering along the cross direction (CD) of the nonwovens, three samples 10 × 10 cm were taken in CD and averaged. Generally, the sample size of 250 × 200 mm is not applicable on the meltblow line of 500 mm width, and we chose the smaller sampling size of 100 × 100 mm to increase the “resolution” of our measurement taking three samples along CD and three series of this in MD.

The thickness of the nonwoven fabrics was measured on the samples of the base-weight measurements using a test head of 1 cm² and a test force of 0.2 cN·cm^–2. Five measurements were executed along one sample, determining a median value for the thickness (δ).

In accordance with the base-weight sampling, the air permeability was measured on three 10 × 10 cm sections in accordance with EN ISO 9237:1995-12 with a testing area of 20 cm² and a differential pressure of 200 Pa.

The volume weight of the samples was determined using the median base weight (BW) and the median thickness (δ) of the nonwoven sample as follows in eq 2

Using the volume weight of the nonwoven fabric, its porosity can be determined using the polymer’s density (ρ), as shown in eq 3.

Here, P represents porosity, ρ_w represents the density of the nonwoven fabric, ρ_f represents the density of the polymer (i.e., PLA, 1.33 g/cm³; PBS, ∼1.25 g/cm³; BioHDPE, 0.955 g/cm³),⁴³ BW represents the basis weight of the nonwoven fabric, and δ represents the thickness of the nonwoven fabric.

Tensile tests of the nonwovens were carried out on an “Instron UPM 4301” instrument from Instron GmbH (Darmstadt, Germany) with a 100 N measuring head. The sampling size was 20 × 150 mm according to ISO 9073-3, and the test speed of 100 mm/min. Three samples were cut out in MD and in CD each and tested. The tenacity was calculated over the sample dimensions, fabric thickness, and measured peak force. Additionally, the elongation at maximal force and elastic modulus were measured. The median and standard deviation of all properties of the measurements are used to compare the nonwoven characteristics.