2.2. Materials Characterization

The Fourier transform infrared (FTIR) spectra were obtained in a Spectrum 100 spectrophotometer from Perkin Elmer under the attenuated total reflectance (ATR) mode. A total of 64 scans were recorded per spectra, at a resolution of 4 cm⁻¹, in the range of 4000 to 600 cm⁻¹.

The melt flow index (MFI) was determined at 190 °C, with a load of 2.16 kg, according to ISO 1133 [34], using a 7053 device from Kayeness Inc (Dynisco Company, Franklin, MA, USA). The results are given as the average of 5 samples.

Water absorption was carried out according to ISO 62:2008 [35]. The specimens were immersed in deionized water and weighed periodically until the weight was constant. The water absorption was calculated using the following equation:

where W₀ is the initial weight of the sample, W_t is the weight of the sample at time t, and W(%) is the percentage increase in weight. Three replicas per sample were assessed.

The kinetics of water uptake can be obtained using Fick’s law:

where D is the diffusion coefficient (m²/s), h is the thickness of the original sample, W_m is the maximum moisture absorbed by the sample, and k is the initial slope of the curve of water uptake versus t^1/2, as described by Equation (3) [36]:

If the moisture uptake at each measurement is compared with the maximum water uptake of each sample, the parameters n, associated with the diffusion mode, and k, related to the interaction between the material and the water, can be calculated as follows [37]:

Differential scanning calorimetry (DSC) was performed on all the samples in a Perkin Elmer DSC 6 apparatus under a nitrogen atmosphere. Samples of approximately 5 mg were prepared in closed aluminum crucibles. The measurements were performed at 10 °C/min, from 30 to 200 °C, with two heating cycles. The melting temperature for both the heating cycles (T_m1 and T_m2, respectively) and the crystallization temperature (T_c) from the cooling step were determined, together with the melting and crystallization enthalpies (ΔHm₁, ΔHm₂, and ΔHc). The enthalpies were used for the crystallinity degree (χ) calculation using the following expression:

where ΔH₀ is the enthalpy for the 100% crystalline sample (93.7 J/g) [10]. Three tests were performed per material type, and the results are given as average values (less than 5% deviation for all the series).

The isothermal crystallization kinetics were determined using the same apparatus. Samples of approximately 5 mg were prepared in sealed aluminum crucibles under a nitrogen atmosphere and subjected to a first stage of heating from 30 to 200 °C at 30 °C/min, and then kept at 200 °C for 5 min to remove the thermal history of the material. Then, the sample was cooled down to the desired temperature at the same rate and kept at the temperature of the study for 30 min. Finally, the sample was heated again until it reached 200 °C for 5 °C/min to determine the values of crystallinity from the melting enthalpy.

The flow properties of the materials were assessed in an oscillatory rheometer AR G2, from TA instruments, with 25 mm diameter parallel plates and a 1.5 mm gap under a nitrogen atmosphere. The experiments were conducted at 210 °C. Preliminary assays were performed under the strain sweep mode in order to ensure that the later experiments were placed in the linear viscoelastic region (LVE). In these tests, the strain was varied between 0.1 and 5%. Frequency sweep tests were performed at 0.5% strain, in the LVE, in the 100 to 0.01 Hz range. Finally, flow tests were also performed at the same temperature, between 0.01 and 1 Hz shear rate.

The tensile properties of the injection-molded samples were determined following ISO 527–2:2012 [38], at a rate of 1 mm/min for the ultimate tensile strength and 0.25 mm/min for elastic modulus. The flexural properties were measured according to ISO 178:2019 [39] at 1 mm/min, with 22 mm between the cantilevers, determining the elastic modulus and flexural strength. The tensile and flexural tests were performed using an LS5 universal testing machine from Lloyd, with a cell load of 500 N for the modulus and 5 kN for strength, with 5 replicates per test. Charpy impact tests were performed on notched samples, following UNE-EN ISO 180:2019 [40], using a 7.5 J pendulum and an impact rate of 3.7 m/s in a Ceast Resil impactor P/N 6958.000, with 10 replicates. The results are given as average values and standard deviation (SD).

The thermomechanical properties of the different materials were evaluated by dynamic mechanical thermal analysis (DMTA) using a Tritec 2000 device, from Triton Technology, under the single cantilever bending method. A strain of 10 µm was applied at 1 Hz frequency between −50 and 120 °C, with a heating rate of 2 °C/min.

The color of the samples was assessed following the International Commission on Illumination (CIE) L* a* b* coordinates. In this system, L* is the color lightness, whereas a* and b* represent the coordinates of redness (green (−)/red (+)) and yellowness (blue (−)/yellow (+)). These parameters were determined by optical spectroscopy using an X-Rite SP64 portable spectrophotometer, measuring 5 samples per material. The total color difference parameter (ΔE*) was calculated as indicated in Equation (6) following ISO 7724 [41] standard:

The opacity was calculated using the same device by measuring the sample against a white and a black background. Finally, the yellowness index (YI) was used to quantify the degradation of the PLA due to its processing and was calculated as follows: