*3.3. Characterization of the Multilayer PET Films*

After optimization of the PLA deposition, achieving superhydrophobicity was attempted by depositing SiO2 as a second layer on the PET/PLA bilayer structure. For this, different flow-rates were tested during electrospraying, that is, 20 mL/h, 30 mL/h, 40 mL/h, and 50 mL/h and the water contact angle was assessed prior to annealing. Figure 6 shows the values of the contact angle as a function of the flow-rate. One can observe that, after the SiO2 deposition, superhydrophobicity (>150◦ on water contact angle) was achieved in all cases, regardless of the flow-rate used.

Since the all tested flow-rates resulted in superhydrophobicity of the multilayer, the lowest flow-rate, that is, 20 mL/h, was chosen for further applying the annealing. It provides higher reproducibility of the application of the SiO2 microparticles and the optical properties would not be so impaired. Furthermore, superhydrophobicity was already achieved at this flow-rate so there would not be a need to use higher flow-rates, which would have implied a higher production cost in a potential upscaling.

In order to promote adhesion between layers, three different annealing temperatures were chosen, that is, 150 ◦C, 160 ◦C, and 170 ◦C, for the thermal post-treatment in view of the above results on the PET/PLA films. Top and cross-section views of the SEM images of the PET/PLA/SiO2 films after applying these temperatures are gathered in Figure 7. All the annealing temperatures provoked strong changes in the surface of the multilayer films rendering a rougher topography. The PLA fibers were seen to adhere strongly to the substrate while the SiO2 microparticles remained well-spread on the PET/PLA bedding substrate.

**Figure 6.** Apparent water contact angle of polyethylene terephthalate (PET)/polylactide (PLA)/silica (SiO2) films at the electrospraying flow-rates of 20 mL/h, 30 mL/h, 40 mL/h, and 50 mL/h.

**Figure 7.** Field emission scanning electron microscope (FESEM) micrographs of the cross-section (left and middle columns) and top views (right column) of polyethylene terephthalate (PET)/polylactide (PLA)/silica (SiO2) films: (**A**) Without annealing; (**B**) Annealed at 150 ◦C; (**C**) Annealed at 160 ◦C; (**D**) Annealed at 170 ◦C. The annealing time was 15 s in all samples and scale bars are 50 μm (left column) and 100 μm (middle and right columns).

As one can observe in Figure 8, the thermal treatments applied on the multilayers also resulted in an improvement of the contact transparency when compared with the untreated PET/PLA/SiO2 film. The transparency values, which take into account the thickness differences among the samples, were, however, similar across the treatments. T values of 6.71, for the treatment at 150 ◦C, 6.50 for

the treatment at 160 ◦C, and 5.65 for the highest temperature, that is, of 170 ◦C, were observed. The T value for the non-treated PET/PLA/SiO2 was 7.35, while for the uncoated PET it was 1.32 and for the PET/PLA without annealing it was 4.42. In the case of the PET/PLA annealed at 150 ◦C and 160 ◦C, the T values were 2.15 and 2.50, respectively. A higher transparency was presented in the PET/PLA samples when comparing the T values with the transparency values of the PET/PLA/SiO2 films. Likewise, the transmittance light values were lower as compared with the PET/PLA films (see Figure 4), due to the extra layer of sprayed SiO2 microparticles that diffract the light.

**Figure 8.** (**A**) Ultraviolet-visible (UV-Vis) transparency measurements of the polyethylene terephthalate (PET)/polylactide (PLA)/silica (SiO2) films at different annealing temperatures. (**B**) Contact transparency of the PET/PLA/SiO2 films: A—Without annealing; B—Annealed at 150 ◦C; C—Annealed at 160 ◦C; D—Annealed at 170 ◦C. The typical sample size of the films in the pictures is of ca. 2 <sup>×</sup> 1.5 cm2.

Lastly, Figure 9 includes the results of the contact angle measurements of the PET/PLA/SiO2 films. From this figure, it can be observed that all multilayer films rendered a superhydrophobic behavior with contact angle values in the 155◦–170◦ range. The contact angle of the untreated multilayer films was 156.75◦ ± 1.16◦. Su et al. [7] reported water contact angles around 165◦ for superhydrophobic structures fabricated via simultaneous electrospraying of SiO2/dimethylacetamide (DMAc) colloids and electrospinning of polyvinylidene fluoride (PVDF)/DMAc solutions.

In this context, it was observed that the thermal post-treatment affected the surface superhydrophobicity of the multilayer systems. The film sample annealed at 150 ◦C showed a low reproducibility in the values of contact angles. It was probably due to the fact that at 150 ◦C, a partial loss of the fibrilar morphology occurred, provoking a non-homogeneous adherence between layers. When the film samples were treated with the highest temperature, that is, 170 ◦C, the PLA fibers were completely turned into a continuous film and therefore the contact angle was decreased up to 158.00◦ ± 4.37◦. The film sample post-treated at 160 ◦C showed an increase in the contact angle values of around 8%, compared with the untreated multilayer film, obtaining a value of 170.63◦ ± 1.49◦. Based on these results, it can be stated that thermal post-processing at 160 ◦C not only improved the adherence between layers but also provided the highest superhydrophobicity. The entrapped air and the roughness structures throughout the depth of the membrane was thought to lead to a continuous water-air-solid interface, resulting in a more hydrophobic surface [7]. In addition, this multilayer material promoted the desired hierarchical nanoparticle-agglomerates-fiber structure (see Figure 7) to increase the tortuosity of the water-air-solid interfaces, thus resulting in an easy sliding, with a sliding angle of 6◦.

**Figure 9.** (**A**) Shape of the water droplets on the polyethylene terephthalate (PET)/polylactide (PLA)/silica (SiO2) films; (**B**) Quantitative measurements of the apparent water contact angle of PET/PLA/SiO2 films without annealing and annealed at 150 ◦C, 160 ◦C, and 170 ◦C.

#### **4. Conclusions**

A one-step co-continuous process that can increase the water repellency to PET substrates was herein developed. To this end, first, PLA ultrathin electrospun fibers were deposited onto the PET films by electrospinning and, thereafter, the coated PET/PLA films were post-treated at different annealing temperatures in the 90–170 ◦C range for 15 s. The control of the solution and electrospinning parameters allowed the creation of a homogeneous deposition of PLA fibers onto the PET films. The presence of the here-developed PLA coatings led to an increase of the surface hydrophobicity of the PET films but did not lead to superhydrophobic properties. For this reason, the co-deposition of SiO2 microparticles by electrospraying was added to the process. Different electrospraying flow-rates of the SiO2 microparticles, from 20 to 50 mL/h, generated multilayer films presenting superhydrophobicity. It was observed that the superhydrophobic behavior of the PET/PLA/SiO2 multilayer films was somewhat improved when thermal post-treatments were applied up to 160 ◦C. In the optimal formulation, an apparent contact angle of 170◦ and a sliding angle of 6◦ were obtained.

Unfortunately, the very high contact and background transparency of PET could not be retained in the double coated samples but after annealing, the optical properties of the coated films with superhydrophobicity were optimal and showed good contact transparency. The generated PET films coated with PLA and SiO2 reported in this work could be of potential use in easy emptying transparent packaging applications. Furthermore, the reported use of a scaled-up setup with a co-continuous process should facilitate its industrial implementation.

**Author Contributions:** Conceptualization was devised by J.M.L.; Methodology, Validation and Formal Analysis was carried out by M.P.-F., S.T.-G., A.L.-C. and J.M.L. Investigation, Resources, Data Curation, Writing Original Draft Preparation and Writing-Review & Editing was performed by M.P.-F., A.L.-C., S.T.-G. and J.M.L.; Supervision J.M.L.; Project Administration, J.M.L.

**Funding:** This study was funded by the EU H2020 OPTINANOPRO project (No. 686116).

**Acknowledgments:** S.T.-G. would like to thank the Ministry of Science, Innovation, and Universities (MICIU) for his Juan de la Cierva contract (IJCI-2016-29675).

**Conflicts of Interest:** The authors declare no conflict of interest.
