The detected increase of 6.85% on average in the biodegradation rate for glycerolcontaining PVA may be due to the changes in the hydrophilic characteristics of the glycerol that reduce internal hydrogen bonds in the polymer chain and decrease the residual mass as described by Abdullah and Dong (2019) [49]. They observed an increase of 23.33% in biodegradation rates when adding glycerol. Moreover, raw glycerol has biodegradation on natural water between 68 and 78% [50].

It should be noted that due to the lack of research about biodegradability on microparticles from bioplastics, this study can be only compared to studies using larger fragments (films, pellets, etc.). There is a gap of information in this area that needs to be covered, given that the final faith of microplastics and their degradation products is not well known [51].

Some labels have been developed by industry to distinguish plastics that can biodegrade in the environment. For instance, the label created by Vinçotte OK Biodegradable WATER applies the standard BS EN ISO 14851:2019 [38,52]. Nevertheless, as Harrison et al. pointed out, there is no agreement on which standards to use for plastic biodegradation [53], and a more realistic point of view unifying all the criteria required for these specific materials is needed. It is also important to bear in mind that biodegradability is an intrinsic property of a material, but performed biodegradation will necessarily depend on environmental conditions, and thus no single standard will be able to be representative of the multiple end-of-life scenarios present in the aquatic environments.

#### *3.2. Toxicity Test*

Toxicity of the materials using the standard sea urchin model with a concentration of 10 g/L is reflected in Table 3. According to the current EU classification of chemicals, based on their short-term aquatic toxicity [54], all samples are classified as harmless (EC50 > 100 mg/L). Following the TU classification (see Table 2), the polymer composed of

plain PVA showed a total absence of toxicity with a TU < 1. This result is supported by toxicity bioassays in seven other aquatic species including marine water individuals *Hyalella azteca* and *Leptocheirus plumulosus* that showed a low toxicity for the PVA material [55]. The materials PVA.029 and PVA.030 presented a certain degree of aquatic toxicity (2.3 and 2.2 TU, respectively) with a larval growth lower than the PVA.031 sample (Figure 2). Consequently, PVA.029 and PVA.030 could be classified as slightly toxic.

**Table 3.** Toxicity of different materials expressed as no observed adverse effects concentration (NOEC) and the lowest observed adverse effect concentration (LOEC); toxic units (TU); 50% decrease in larval growth with respect to the control (EC50).

**Figure 2.** *Paracentrotus lividus* larvae size increase ratio compared to control treatment (ΔLc) for the different sample dilutions. \* *p* < 0.05, \*\* *p* < 0.01, \*\*\* *p* < 0.001.

ECHA [56] shows a CE50 for glycerol on freshwater invertebrate organisms at 48 h of 1995 (1851 to 2068) mg/L, corresponding to a TU < 1. According to studies carried out on three different aquatic bioindicators, glycerol ethers can be classified as harmless for the environment in acute exposure [57]. Glycerol has been presented as a sustainable solvent for its good biodegradability and low toxicity on certain organisms [58], but its effects when blended with polymers and other materials need further research in order to confirm that these compounded materials do not pose a threat for the environment. In fact, we have found a slight but significant increase in toxicity for glycerol-PVA blends, compared to plain PVA.

Similarly, it is necessary to include the assessment of the potential toxicity of degradation products in the scheme of evaluation of biopolymers before they can be presented as more environmentally friendly alternatives compared to conventional plastics.

#### **4. Conclusions**

This study characterized the biodegradation and ecotoxicity of PVA polymers in marine environments. The results obtained, though limited, support the lack of biodegradability of PVA materials under conditions representative of a natural marine environment. The slight biodegradability of the blended materials was attributable to the glycerol component of the blend. Since plastics of different nature are increasingly found in marine environments, standards with a view to assess biodegradation under realistic marine end-of-life scenarios are urgently needed.

On the other hand, none of the tested polymers pose a relevant risk to the model marine organism used, the sensitive Sea urchin Embryo Test (SET), but slight toxicity arises for glycerol-containing blends. These tests should be conducted with a broader group of aquatic species. These standards should take into account not only biodegradability in terms of mineralization to CO2, but also mechanical degradation, potential release of microplastics and lack of toxicity of additives and biodegradation products.

**Author Contributions:** Conceptualization, O.A.-L., S.L.-I. and R.B.; data curation, S.L.-I.; formal analysis, O.A.-L.; funding acquisition, R.B.; investigation, O.A.-L., S.L.-I. and R.B.; methodology, O.A.-L., S.L.-I. and R.B.; project administration, R.B.; resources, O.A.-L., S.L.-I. and R.B.; supervision, R.B.; writing—original draft, O.A.-L.; writing—review and editing, O.A.-L., S.L.-I. and R.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by the RisBioPlas Project, financed by the Ministry of Science and Innovation—Spanish Agency of Research (AEI: 10.13039/501100011033); by LABPLAS Project (LAnd-Based Solutions for PLAStics in the Sea, Grant Agreement Nr.: 101003954, financed by the EU H2020 program); by the RESPONSE Project (toward a risk-based assessment of microplastic pollution in marine ecosystems, financed by the Joint Programming Initiative, Healthy and Productive Seas and Oceans, JPI Oceans-Spanish National Research funding Agency: PCI2020-112110). S.L.I. was supported by the Ministry of Universities of Spain (FPU grant reference FPU19/02280).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors thank the Plastic Technology Centre AIMPLAS (Valencia, Spain) for supplying the different PVA materials. Funding for open access charge: Universidade de Vigo/CISUG.

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

#### **References**

## *Article* **Effect of Microcapsules with Waterborne Coating as Core Material on Properties of Coating for Tilia Europaea and Comparison with Other Microcapsules**

**Xiaoxing Yan 1,2,\*, Yu Tao 2 and Xingyu Qian 2**

**Abstract:** Urea formaldehyde was used as wall material and waterborne coatings as a core material to prepare microcapsules. So as to explore the influence of mass ratio of core to shell, reaction temperature and standing time on the performance of microcapsules, the orthogonal test of three factors and two levels was put into effect. The orthogonal experimental results showed the mass ratio of core to shell was the most important factor. With the increase of the mass ratio of core to shell, the output and clad ratio of microcapsules increased first and then decreased. The microcapsule with the mass ratio of core to shell of 0.67:1 had better appearance, output, and encapsulation performance. The optical properties of waterborne wood coating with the microcapsules of waterborne coating as core materials did not decrease significantly, while the hardness, impact resistance, and toughness were improved. At the same time, the microcapsules have a certain self-repairing effect on coating micro-cracks. Compared with the properties of waterborne coatings with other microcapsules, the coating with waterborne coating as core material has better comprehensive performance. The results provide a new research idea for the performance enhancement and self-healing of wood waterborne coating.

**Keywords:** waterborne coating; microcapsule; self-healing; core material; clad ratio

#### **1. Introduction**

In the process of using the coating, owing to the changes of environmental factors (such as light, temperature, humidity), the low toughness, the poor adaptability, and the complex process, the micro-cracks are generated and expanded inside the coating, and even destroy the internal structure of the coating, thus influencing the operation life of the coating and wood [1–3]. Because of the unique wet swelling and drying shrinkage and dimensional instability of wood [4], the microcapsule self-healing technology has a broad prospect in wood materials [5,6]. When the coating [7] is cracked by the external force, the microcapsule is cracked and the core material healing agent is released [8], which can enhance the performance of wood and prolong its service life. Waterborne acrylic coating is a commonly used waterborne coating [9,10]. It uses water as the dispersion medium, and does not contain formaldehyde and heavy metals [11,12]. It is non-toxic and has no pungent smell, and its harm to human health and environmental pollution is relatively small, so it can meet the increasingly stringent environmental protection requirements [13–15].

At present, the microcapsules have been widely used in wood materials [16,17]. In the combination of microcapsules and wood materials, Jeong et al. [18] used microencapsulated phase change materials to increase its heat storage, and introduced them into wood-based panels by physical mixing of microcapsules and adhesives, in order to produce high thermal efficiency wood flooring. Pinkl et al. [19] further proposed that if the self-healing

**Citation:** Yan, X.; Tao, Y.; Qian, X. Effect of Microcapsules with Waterborne Coating as Core Material on Properties of Coating for Tilia Europaea and Comparison with Other Microcapsules. *Polymers* **2021**, *13*, 3167. https://doi.org/10.3390/ polym13183167

Academic Editors: Florian J. Stadler and Alberto García-Peñas

Received: 13 July 2021 Accepted: 12 September 2021 Published: 18 September 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

material is added to repair the damaged adhesive joint, the adhesive closer to the bonding line may be better. He et al. [20] used microwave expansion and other pretreatment technologies to increase the gap of wood to achieve the purpose of introducing larger diameter microcapsules. For the application of microcapsules in wood functional materials [21], it is mainly explored how to introduce microcapsules into wood or wood materials. However, this combination process is more complex. According to the requirements of wood outputs for single or composite functional characteristics, microcapsule are evenly introduced into wood and wood materials, or attached to the surface of wood and wood materials by surface modification technology [22–24]. In the application of microcapsules to self-healing coatings [25], Siva et al. [26] used linseed oil and mercapto benzothiazole as core materials, and compounded urea formaldehyde microcapsules by emulsion polymerization. The self-healing ability of the anticorrosive coating with microcapsules was discussed by SVET. Lang et al. [27] encapsulated linseed oil in polyurea formaldehyde (PUF) shell and prepared a self-healing coating on account of microcapsules successfully. Meanwhile, the influence of the amount of microcapsules on the repair property of the paint-coat was explored. Yang et al. [28] explored a new self-healing system based on microcapsule organogels, where there would be no damage in the healing area. The self-healing coating was prepared by microencapsulation of urea formaldehyde polymer into polymer coating. As for the application of microcapsule in self-healing coating, scholars mainly explored its role in metal surface anti-corrosion coating, and often used linseed oil and other dry oil as core material repair agent, while there are few studies on the application of microcapsules in coatings of wood surface [29]. In order to simplify the combination of wood and self-healing microcapsule, the self-healing microcapsule with waterborne coating as the core material is applied to the coating on the surface of wood in this paper. It is of great significance to apply self-healing microcapsules to prepare self-healing coatings for wood products. The core material of self-healing microcapsules has an important influence on the properties of microcapsules. If the core material repair agent and coating are two different substances, after the coating has micro cracks, when the core material repair agent is curing and healing micro cracks, the combination of the repair agent and coating may produce interfacial stress, which affects the repair effect of micro cracks. Therefore, the choice of microcapsule core material will not only affect the performance of the microcapsule itself, but also have a great impact on the self-healing performance of the coating. As the wall material of microcapsules, urea formaldehyde resin has the advantages of low cost, mature process, and good coating of core materials. However, free formaldehyde gas may be released in the preparation process, causing harm to the human body. Therefore, it is necessary to control the content of formaldehyde and reduce the release of formaldehyde.

The performance of waterborne coating is better than that of single resin because it contains resin, additive, and other compounds. Using waterborne acrylic coating as the core material, the repairing agent can be cured at room temperature to form a coating after micro cracks [30,31]. Because the composition of the core material is consistent with that of the waterborne coating itself, the interface compatibility will be better when the crack is healed, and the performance of the coating can be further improved. In order to further explore the effect of the core material, this paper used waterborne coating itself as the core material repair agent to prepare microcapsules, and its process parameters were optimized. Then, the prepared microcapsules were added to waterborne coating to explore the effect of the self-healing microcapsules on the coating properties. On this basis, the coating properties of microcapsules were compared with those of other microcapsules with different core materials reported in the literature [32–34]. The main objective of the research is to explore the influence of different core materials on the coating and obtain better selfhealing effect, so as to provide the basis for the application of self-healing microcapsules in wood materials in the future.

#### **2. Materials and Methods**

#### *2.1. Materials*

The chemical reagent information required for the experiment is shown in Table 1. Nippon waterborne acrylic coatings, which are mainly composed of waterborne acrylic copolymer, additives, and water, was provided by Nippon (Changzhou) Co., Ltd., Changzhou, China. Tilia europaea with size of 100 × 50 × 4 and uniform color were provided by Guangdong Yihua life science and Technology Co., Ltd., Shantou, China.

**Table 1.** Chemical reagent information in experiment.

#### *2.2. Preparation of Microcapsules*

Firstly, according to the related literature of microcapsules [35], three vital factors affecting the preparation of microcapsules were determined. As shown in Table 2, the orthogonal test of 3 factors and 2 levels was conducted. Two levels of mass ratio of core to shell, reaction temperature, and standing time were chosen to prepare urea formaldehydecoated waterborne coating microcapsules.

**Table 2.** Influencing factors and levels.

Samples 1–4# microcapsules of the orthogonal test were prepared, and the details of each substance dosage were listed in Table 3. Taking sample 1# of orthogonal experiment as an example, the preparation process of microcapsules was explained. The formation process of microcapsules is shown in Figure 1. The first step was the synthesis of urea formaldehyde (UF) prepolymer: after 20.0 g urea was poured into the beaker, 27.0 g of 37.0% formaldehyde solution was added into it and fully stirred. Then, the triethanolamine was put in slowly to regulate the pH to 8.5–9.0, and the mixture was continuously stirred at the rate of 100 rpm in a stationary temperature water bath at 70 ◦C for 30 min to obtain a faintly thick and transparent UF prepolymer solution. The second step was the emulsification of waterborne coatings for core material: 0.975 g of SDBS was put into distilled water and stirred thoroughly to obtain a mixture with concentration of 1.0% as emulsifier. Next the 12.5 g waterborne coating was mixed with 97.0 mL emulsifier solution. The mixture was emulsified for 60 min at the rate of 1200 rpm and 50 ◦C reaction temperature to gain steady emulsion. The last step was microencapsulation: the wall material prepolymer was slowly dropped into the core material emulsion obtained in the second step at the speed of 300 rpm, then citric acid solution was dropped gradually, and the mixture was continuously stirred until the citric acid crystal was completely dissolved. After adjusting the pH value to 3.0–3.5, the temperature was tardily increased to 50 ◦C and kept for 2.5 h. After 1 d of standing time, the production was filtered and distilled water was used at the same time to wash off the surplus emulsifier. Then, the production was laid into the drying oven and heated at 45 ◦C for 24 h. The powder gained was the required sample 1# in the orthogonal experiment. The preparation process of other microcapsules samples is consistent with the above.

**Table 3.** Detailed list of various substances used in orthogonal and single factor tests.

**Figure 1.** The formation process of microcapsules.

Through the above orthogonal experiment, the most influential factor of the three factors on the performance of microcapsules and the better preparation scheme of microcapsules were determined, and next the independent test was developed for the most influential element. The experimental materials, equipment, and preparation method of microcapsules remained unchanged. The reaction temperature was regulated to 70 ◦C, and the standing time was set at 5 d. As an independent variable, the mass ratio of core to shell was 0.42:1, 0.50:1, 0.58:1, 0.67:1, 0.75:1, 0.83:1, and 0.92:1 (samples 5–11#), respectively. The consumption details of each substance are shown in Table 3.

#### *2.3. Preparation of Coating with Microcapsules*

According to the relevant literature, when the mass fraction of epoxy resin microcapsule, shellac microcapsule, and waterborne acrylic acid microcapsule in waterborne coating is 10.0%, the coating performance is better [32–34]. Therefore, in order to carry out the subsequent comparative analysis, the optimized mass ratio of core to shell of 0.67:1 waterborne coating microcapsules were added to the waterborne coating with a mass fraction of 0 and 10.0%, respectively, and stirred evenly. The prepared coating was applied on Tilia europaea substrate to form a uniform coating on its surface. After the coating was completely dry, 800 mesh sandpaper [36] was used to grind the paint coating to make it smooth and the surface was mopped up with dry cloth. The above procedure was performed three times in the same way to obtain a coating thickness of about 60 μm.

#### *2.4. Testing and Characterization*

In the aspect of micro morphology of microcapsules, a Zeiss Axio scope A1 optical microscope (OM) and a FEI Quanta−200 scanning electron microscope (SEM), produced by FEI Company, Hillsboro, OR, USA, were employed for characterization. In terms of chemical composition, A VERTEX 80 V Fourier infrared spectrum analyzer (FTIR), produced by Bruker (Beijing) Scientific Technology Co., Ltd., Beijing, China, was applied

for analysis. For the microcapsule clad ratio test, the mass of microcapsule (*m*1) was fully ground and soaked in ethyl acetate for 72 h. The solution should be changed every 24 h. Finally, the filtered product was washed with deionized water, and the mass of the residue (*m*2) after drying was weighed. The encapsulation rate (c) of microcapsules was figured out by Formula (1).

$$\mathbf{c} = \frac{m\_1 - m\_2}{m\_1} \times 100\% \tag{1}$$

In the aspect of coating optical performance, the chromaticity value of waterborne coating on Tilia europaea surface was measured by RM200-PT01 color detector (Pantone Inc., Carlstadt, NJ, USA). The HG268 trigonometric glossmeter (Shenzhen Forbes Instrument Co., Ltd., Shenzhen, China) was applied to test the gloss of waterborne coating film, and the gloss of coating at 60◦ incidence angle was recorded. As for the mechanical performance of the coating, the pencil with known hardness label was applied to characterize the hardness of the coating. In the process, the hardness of the pencil was gradually increased until the surface of the coating appeared defects such as plastic surface or cohesive damage. The pencil hardness of the coating was expressed by the hardest hardness that does not cause a scratch of 3 mm or more. The QFH-HG600 film scribbler (Guangzhou Yuanxiao Marine Equipment Co., Ltd., Guangzhou, China) was operated to characterize the adhesion of the coating, and the resistance of the coating falling off from the substrate was evaluated when the coating penetrated the substrate with a right angle grid pattern. According to the surface appearance of the cross cutting area, it can be divided into six grades: 0, 1, 2, 3, 4, and 5, and the first three grades were satisfactory. The QCJ film impact test instrument (Tianjin World Expo Weiye chemical Glass Instrument Co., Ltd., Tianjin, China) was applied to test the coating impact resistance. A 1.0 kg heavy hammer was fixed at different heights that were less than 50 cm, and next the heavy hammer was allowed to fall naturally to impact the test piece to observe whether cracks and spalling phenomena appeared on the surface of coating. Finally, when there was no crack in the coating, the maximum height of the hammer was the impact strength of the coating. The tensile strength of the coating was tested by AG-IC100KN universal testing machine (Guangzhou Kehui Instrument Co., Ltd., Guangzhou, China). Then, the elongation at break of the coating was counted by the tensile strength. All the tests were repeated 4 times, and the error was less than 5.0%.

#### **3. Results and Discussion**

#### *3.1. Analysis of Orthogonal Test Results*

#### 3.1.1. Analysis of Micro Characteristics of Microcapsules

The macroscopic images of microcapsule powder are shown in Figure 2. The SEM appearance of the orthogonal samples 1–4# in Table 3 was displayed in Figure 3. The morphology of sample 4# microcapsule (Figure 3D) is relatively good, followed by sample 1# (Figure 3A). Sample 1# has a general coating condition, with some agglomeration, but the particle size is basically uniform, about 5–8 μm. Sample 2# (Figure 3B) has a poor coating condition, with more urea formaldehyde precipitation and agglomeration, and the particle size is about 8–10 μm. Sample 3# (Figure 3C) has a general coating, with less precipitation and agglomeration, and its particle size is about 5–8 μm. Sample 4# microcapsule is coated well with a little agglomeration, and the particle size is about 8 μm. It can be found that the UF resin coated waterborne coating microcapsules whose particle size was approximately 8 μm have been successfully prepared.

**Figure 2.** The macroscopic images of microcapsule powder: (**A**) sample 1#, (**B**) sample 2#, (**C**) sample 3#, (**D**) sample 4#.

**Figure 3.** The SEM morphologies of microcapsules under different orthogonal experimental parameters: (**A**) sample 1#, (**B**) sample 2#, (**C**) sample 3#, (**D**) sample 4#.

#### 3.1.2. Infrared Analysis of Components in Microcapsules

Figure 4 shows the FTIR spectrogram of wall material, core material and orthogonal test sample 4#. The special absorption peaks at 3350 and 1556 cm−1 are assigned to N-H and C-N functional groups, which belong to the functional groups of UF resin. The absorption at 1641 cm−1 is consistent with the stretching vibration of C=O functional group in UF resin. The absorptions at 2961 and 1447 cm−1 are the special absorption peaks of C-H. The special absorption at 1726 cm−1 belongs to the characteristic absorption of C=O functional group in the waterborne acrylic, and the corresponding peak also appears in the infrared spectrum of sample 4# in the orthogonal test. It is confirmed that the corresponding UF resin and waterborne acrylic exist in the prepared microcapsule, and the composition is not damaged. It proves the successful preparation of UF resin cladding waterborne coating microcapsules.

**Figure 4.** The FTIR spectrogram of wall material, core material, and sample 4# microcapsule.

#### 3.1.3. Output Analysis of Microcapsules

The microcapsule samples 1–4# made by orthogonal experiment were weighed, and the range (R) and variance results of output are shown in Tables 4 and 5. When the mass ratio of core to wall and the standing time increased, the output of the microcapsule increased, and when the reaction temperature increased, the output of the microcapsule decreased. It can be found from the Table 4 that sample 3# microcapsule has the largest output at 33.32 g. From the range results, it can be seen that in the three factors of the mass ratio of core to shell, reaction temperature, and standing time, the mass ratio of core to shell makes the greatest influence on the output of urea formaldehyde-coated waterborne coating microcapsules, followed by the standing time. Only from the results of output, the better preparation parameters of the microcapsule are mass ratio of core to shell of 0.67:1, the reaction temperature of 50 ◦C and the standing time of 5 d. In Table 5, F ratio was obtained by comparing the square sum of the average deviation of each factor with the square sum of the average deviation of the error. This ratio reflected the influence of each factor on the test results. If F ratio was greater than the F critical value, it showed that this factor had a significant influence on the test results. It can be seen from Table 5 that the F ratio of the mass ratio of core to shell is greater than the F critical value, indicating that the mass ratio of core to shell has a significant impact on the output of microcapsules.

**Table 4.** Range results of microcapsule output by the orthogonal test.

**Table 5.** Variance results of microcapsule output by the orthogonal test.

1 F ratio is obtained by comparing the square sum of the average deviation of each factor with the square sum of the average deviation of the error. 2 F critical value is the critical value of given significant level.

#### 3.1.4. Clad Ratio Analysis of Microcapsules

The range and variance of clad ratio of UF resin cladding waterborne coating microcapsules made by orthogonal test are displayed in Tables 6 and 7. When the mass ratio of core to wall, the reaction temperature, and the standing time increased, the clad ratio of the microcapsule increased. Sample 3# of the microcapsule has the highest clad ratio of 43.0%. On the basis of the variance results of the clad ratio, it is obvious that the mass ratio of core to shell has the greatest influence on the clad ratio of the microcapsules, followed by the standing time. Based on the results of clad ratio, it can be found that the mass ratio of core to shell of 0.67:1, the reaction temperature of 70 ◦C and the standing time of 5 d are the optimum preparation process of the microcapsules.

**Table 6.** Range results of microcapsule clad ratio by the orthogonal test.

**Table 7.** Variance results of microcapsule clad ratio by the orthogonal test.

The orthogonal test results for this kind of microcapsule were comprehensively analyzed. Among the three elements, the mass ratio of core to shell is the most important element affecting the output and clad ratio results of microcapsule. In order to further optimize the appearance and optical and mechanical properties of microcapsules, a single factor test was carried out for the most significant influencing factor (mass ratio of core to shell) according to the above orthogonal experimental results. Based on the analysis of output results, it can be determined that the best levels of the two other factors are reaction temperature of 50 ◦C and standing time of 5 d, respectively, and the best levels of the analysis of clad ratio results are 70 ◦C and 5 d. Compared with the output, the clad ratio is a more important factor in judging the performance of microcapsules. Because the micro morphology of sample 4# microcapsule was the best, the reaction temperature was regulated to 70 ◦C in the process of preparing sample 4#, finally the reaction temperature was adjusted to 70 ◦C and the standing time was set at 5 d in the single factor experiment.

#### *3.2. Analysis of Single Factor Test Results*

3.2.1. Analysis of Micro Characteristics of Microcapsules

The OM and SEM of microcapsules optimized by single factor experiment with mass ratio of core to shell are displayed in Figures 5 and 6. Figure 5H is the magnification image of Figure 5D. It can be seen from Figure 5H that the outside of the microcapsule sphere is the dark part, which is the wall material, and the bright spot of the microcapsule is the core material. The figure shows that the microcapsules with small mass ratio of core to shell of 0.42:1, 0.50:1, 0.58:1, and 0.67:1 (Figure 6A–D) have great morphology, consistent particle size, and almost no precipitation. However, when the mass ratio of core to shell increases, the content of core material increases, and the microcapsules with mass ratio of core to shells of 0.75:1, 0.83:1, and 0.92:1 (Figure 6E–G) can be found to have serious agglomeration and precipitation. The microcapsule with mass ratio of core to shell of 0.67:1 (Figures 5D and 6D) has the best micro morphology, and the particle size is 5 μm.

**Figure 5.** The OM appearance of microcapsules by single factor experiment: mass ratio of core to shell (**A**) 0.42:1, (**B**) 0.50:1, (**C**) 0.58:1, (**D**) 0.67:1, (**E**) 0.75:1, (**F**) 0.83:1, (**G**) 0.92:1, (**H**) 0.67:1.

**Figure 6.** The SEM appearance of microcapsules by single factor experiment: mass ratio of core to shell (**A**) 0.42:1, (**B**) 0.50:1, (**C**) 0.58:1, (**D**) 0.67:1, (**E**) 0.75:1, (**F**) 0.83:1, (**G**) 0.92:1.

#### 3.2.2. Infrared Analysis of Components in Microcapsules

Figure 7 shows the FTIR of seven kinds of microcapsules prepared by single factor experiment. The special absorption peaks of N-H and C-N functional groups are at 3350 and 1556 cm−1. The absorption at 1639 cm−1 is the stretching vibration of C=O functional group in UF resin. The absorption at 2966 cm−1 is the special absorption peak of C-H functional group, and the peak at 1726 cm−1 represents the special absorption of C=O functional group in waterborne acrylic acid coatings. It can be found from the picture that the special absorption peaks of microcapsules by different ratios of core to wall are similar, which can prove that the chemical component part of microcapsules by different ratios of core to wall is still present, and the microcapsules are successfully prepared.

**Figure 7.** FTIR spectrum of microcapsules by single factor experiment.

3.2.3. Output Analysis of Microcapsules

The output of microcapsules with seven ratios of core to shell of waterborne coatings as core material is displayed in Table 8. The output results displayed that the microcapsule with mass ratio of core to shell of 0.67:1 had the highest output. As can be seen from the table, the overall output of microcapsules basically increased first and then decreased, with the increasing of mass ratio of core to shell. When the mass ratio of core to shell increased from 0.42:1 to 0.67:1, the output of microcapsules increased from 28.93 g to 37.80 g. The reason may be that the increase of the weight of the core material can increase the weight of the wall material to cover the repair agent, thus increasing the output of the microcapsule. However, as the mass ratio of core to shell continued to increase, the output decreased. The reason may be that the increase of core material led to the increase of agglomeration and precipitation. As a result, the excess core material cannot be completely covered and the mass loss occurred during the filtration process.

**Table 8.** Output results of microcapsules by the single factor test.

#### 3.2.4. Clad Ratio Analysis of Microcapsules

The results of clad ratio of microcapsules with seven different mass ratios of core to shell in single factor experiment are shown in Table 9. From the results in the Table 9, when the mass ratio of core to shell was 0.67:1, the clad ratio of microcapsules was the highest. With the enlargement in the mass ratio of core to shell (the raise of the mass of core material), the clad ratio of waterborne coating microcapsules basically rose first then fell. When the mass ratio of core to shell increased from 0.42:1 to 0.67:1, the clad ratio of microcapsules rose from 37.0 to 49.0%, but when the mass ratio of core to shell rose to 0.92:1, the clad ratio of microcapsules fell to 32.0%. Owing to the raise of the core material mass, the content of the core material cladded in the microcapsule wall increased, which enhanced the clad ratio to a certain degree. Nevertheless, when the mass ratio of core to shell rose, the microcapsule would produce too much precipitation and agglomeration, which affected the performance of the microcapsule, resulting in the decrease of the clad ratio.

**Table 9.** Clad ratio results of microcapsules by the single factor test.

Comprehensive analysis of the results in Figure 6, Tables 8 and 9 showed that when the mass ratio of core to shell was 0.67:1, the microcapsule had better morphology, the highest output, and the highest clad ratio, and the preparation was more successful.

#### *3.3. Effect of Standing Time on the Morphology of Microcapsules*

In order to understand the formation mechanism of microcapsules and explore the effect of standing time on the morphology of microcapsules, the microcapsules with mass ratio of core to shell of 0.67:1 were aged for 0, 1, 2, 3, 4, and5drespectively. The OM of the microcapsules prepared under different standing times is shown in Figure 8. With the increase of standing time, the observed spherical particles gradually increased, and with the extension of time, the particles formed a partial agglomeration phenomenon. In order to explore whether the agglomeration phenomenon of microcapsules would seriously increase with the infinite extension of time, the prepared microcapsules were aged for two months. The SEM under different multiples is shown in Figure 9. The microcapsules had good morphology and a small amount of agglomeration and bonding. The result showed that although a small amount of agglomeration occurred with the increase of standing time, it was not serious due to the long standing time. It also showed that the standing time had little effect, and the core wall ratio was a more important factor affecting the morphology of microcapsules.

**Figure 8.** OM morphologies of microcapsules prepared under different depositing time: (**A**) 0, (**B**)1d, (**C**) 2 d, (**D**) 3 d, (**E**) 4 d, (**F**) 5 d.

**Figure 9.** SEM morphology of microcapsule prepared under two months of deposition: (**A**) low magnification, (**B**) middle magnification, (**C**) large magnification.

#### *3.4. Influence of Microcapsules on the Performances of Waterborne Coatings*

The performance of the coating is shown in Table 10. Properties of coating with 10.0% mass fraction microcapsules were compared with that of the coating without microcapsule. In terms of optical properties, the color difference of the coating increased from 0.6 to 2.0 after adding 10.0% waterborne coating microcapsules, and the gloss of the coating fell from 29.4 to 11.9% at 60◦ incident angle. It may be due to the uneven surface of the coating and the enhancement of diffuse reflection after adding microcapsule particles, and the influence of the color of the microcapsule itself. In terms of mechanical properties, the hardness of the coating increased from HB to 3H, the adhesion remained unchanged, and the impact resistance raised significantly from 6.0 to 13.0 kg·cm. After comprehensive analysis, it can be concluded that the performance of the waterborne coating added with 10.0% waterborne coating microcapsules did not decrease significantly, but its hardness and impact resistance were enhanced. At the same time, the elongation at break of the coating had risen from 9.94 to 16.18%, which was a significant increase. It is obvious that the toughness of the coating with microcapsule concentration of 10.0% is improved, and the existence of microcapsule can prevent the appearance of micro-cracks to a certain extent.

**Table 10.** Performance comparison of waterborne coatings with different microcapsules.

1 Shellac is a kind of purple natural resin secreted by shellac insect after absorbing the sap of host tree.

In terms of self-healing performance, the blade was used to mark the cracks on the paint coating on the surface of Tilia europaea, and the OM was used to record the cracks. After an interval of 5 d, the OM was used to observe the self-healing effect of the paint coating again. According to Figure 10, the scratch width of the coating without microcapsules changed from 22.85 to 20.09 μm, and there was no obvious change. However, it can be seen from Figure 11 that the scratch width of the coating with 10.0% microcapsules decreased significantly from 28.72 to 20.71 μm after 5 d. It can be seen that the self-healing effect of the coating with microcapsule is better than that without microcapsule. Therefore, 10.0% microcapsules have a certain self-repairing effect on the micro cracks on the surface coating of wood. The self-healing mechanism of Tilia europaea surface coating containing microcapsules is shown in Figure 12. When the coating cracks, the microcapsules in the coating will break, and the waterborne coating core repair agent in the microcapsules will flow out. The waterborne paint repair agent can be cured into a film at room temperature. The curing of waterborne coating mainly has three levels of mechanisms. The first is the volatilization of water and other film-forming aids to fully reflect the basic properties of the thermoplastic resin itself, the second is the aggregation and fusion of the emulsion particles, and the third is mutual diffusion of particles. When the temperature is higher than the glass transition temperature, the final film is formed. The micro-cracks on the coating surface can achieve a certain self-repairing effect through the curing of the core material repairing agent.

**Figure 10.** The OM diagram of paint coating without microcapsule: (**A**) 0 d, (**B**) 5 d.

**Figure 11.** The OM diagram of paint coating with 10.0% microcapsule: (**A**) 0 d, (**B**) 5 d.

**Figure 12.** The self-repairing mechanism of waterborne coating microcapsules.

The performances of the waterborne coatings with the same content, the same wall material and different core materials reported in the literature were compared and analyzed [32–34]. In Table 10, the optimal mass ratio of core to shell with waterborne coating microcapsules was 0.67:1, that of waterborne acrylic acid microcapsules was 0.58:1, that of epoxy resin microcapsules was 0.83:1, and that of shellac microcapsules was 0.75:1. The results showed that the optimal core-wall ratio may be different for different kinds of core materials. From Table 10, it can be found that the optical and mechanical properties of waterborne coatings were significantly different after adding microcapsules with different core materials. Among them, the coating with waterborne coating microcapsules had the highest gloss, the coating with epoxy resin microcapsules had a higher elongation at break, and the coating with shellac microcapsule had the lowest hardness. It may be that epoxy resin is more flexible than waterborne coatings and shellac, so the elongation at break is higher. But matched with epoxy resin, the waterborne coating and shellac can be solidified at normal atmospheric temperature without heat, which is more practical for surface coating of wood furniture and wood products. However, the shellac is insoluble in water and soluble in organic solvents, which has certain difficulties in the preparation process. The coating with waterborne coating as the core material has better comprehensive properties.

#### **4. Conclusions**

Through orthogonal test of three factors and two levels, UF resin cladding waterborne coating microcapsules were prepared. The best reaction temperature is 70 ◦C and the best standing time is 5 d. The microcapsule with a mass ratio of core to shell of 0.67:1 has better comprehensive properties such as morphology, output, and clad ratio. Under the optimum preparation conditions, the clad ratio was 49.0%. Compared with the coating without microcapsules, the hardness of the coating with 10.0% mass fraction microcapsules increased to 3H, the impact strength increased to 13 kg·cm, and the elongation at break increased to 16.2%. Moreover, the microcapsules have a certain self-healing effect on micro

cracks in wood surface coating. At the same time, compared with the coating with other different core materials, the coating prepared by adding microcapsules with waterborne coating as core material has better comprehensive properties. This may lay a technical foundation for the industrial application of waterborne coating microcapsules on wood surface. The future research direction is to prepare microcapsules with higher coverage rate and better repair effect for waterborne coatings on wood surface, at the same time, the wall materials will be environmental protection materials without formaldehyde emission.

**Author Contributions:** Conceptualization, methodology, validation, resources, data management, supervision, X.Y.; Formal analysis, investigation, writing-review and editing, Y.T.; Investigation and writing-review, X.Q. All authors have read and agreed to the published version of the manuscript.

**Funding:** This project was partly supported by the Natural Science Foundation of Jiangsu Province (BK20201386) and Youth Science and Technology Innovation Fund of Nanjing Forestry University (CX2016018).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

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

#### **References**

## *Article* **Preparations of Poly(lactic acid) Dispersions in Water for Coating Applications**

**Giada Belletti 1,2, Sara Buoso 2, Lucia Ricci 3, Alejandro Guillem-Ortiz 4, Alejandro Aragón-Gutiérrez 4, Olga Bortolini 1 and Monica Bertoldo 1,2,\***

**Abstract:** A green, effective methodology for the preparation of water-based dispersions of poly(lactic acid) (PLA) for coating purposes is herein presented. The procedure consists of two steps: in the first one, an oil-in-water emulsion is obtained by mixing a solution of PLA in ethyl acetate with a water phase containing surfactant and stabilizer. Different homogenization methods as well as oil/water phase ratio, surfactant and stabilizer combinations were screened. In the second step, the quantitative evaporation of the organic provides water dispersions of PLA that are stable, at least, over several weeks at room temperature or at 4 ◦C. Particle size was in the 200–500 nm range, depending on the preparation conditions, as confirmed by scanning electron microscope (SEM) analysis. PLA was found not to suffer significant molecular weight degradation by gel permeation chromatography (GPC) analysis. Furthermore, two selected formulations with glass transition temperature (Tg) of 51 ◦C and 34 ◦C were tested for the preparation of PLA films by drying in PTFE capsules. In both cases, continuous films that are homogeneous by Fourier-transform infrared spectroscopy (FT-IR) and SEM observation were obtained only when drying was performed above 60 ◦C. The formulation with lower Tg results in films which are more flexible and transparent.

**Keywords:** poly(lactic acid); water emulsions; water dispersions; film formation

#### **1. Introduction**

In recent years, in an effort to produce more eco-friendly products, renewable and biodegradable polymers have been investigated as emerging coating materials. One of the most promising polymers for such a purpose is poly(lactic acid) (PLA) due to its biodegradability, biocompatibility and production on an industrial scale at relatively low cost [1–3]. PLA is already used for a wide range of applications, particularly in the packaging sector [4], for instance for making food and beverage containers, cups, overwrap, blister packages, as well as coating on paper and board [5–7]. The most frequently used conversion method for the preparation of PLA items is melt processing, such as cast extrusion, extrusion coating, lamination and blown extrusion [8,9]. An alternative technology for the preparation of coatings that does not involve treatment at high temperature is solvent casting [10,11]. The method requires the solubilization of the polymer in a suitable solvent, followed by casting of the solution onto the substrate. Despite the possibility to modulate the film thickness by varying the concentration of the polymer solution, the release of toxic and harmful organic solvents during the process gave rise to health and environmental issues. For this reason, the latter method is not typically used to produce biodegradable

**Citation:** Belletti, G.; Buoso, S.; Ricci, L.; Guillem-Ortiz, A.; Aragón-Gutiérrez, A.; Bortolini, O.; Bertoldo, M. Preparations of Poly(lactic acid) Dispersions in Water for Coating Applications. *Polymers* **2021**, *13*, 2767. https://doi.org/ 10.3390/polym13162767

Academic Editor: George Z. Kyzas

Received: 30 June 2021 Accepted: 12 August 2021 Published: 18 August 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

films. Indeed, in the last decades, many efforts have been devoted to the replacement of solvent-borne polymer coatings with their waterborne counterparts, even for those applications where they were traditionally used such as adhesives and inks [12,13]. Water, indeed, is considered as a cheap, safe, non-toxic and environmentally benign solvent [14]; however, polymers are rarely soluble in water. A typical method to use water as a vehicle, in spite of the absence of solubility, is the preparation of polymer latexes consisting of polyacrylate, polymethacrylate and more recently, polyurethane as well [15–22]. Water dispersions are gaining land in many application fields such as paints [23], adhesives [24] and inks [25,26], particularly for the food packaging sector [27,28], where the restrictions on the possible substances used forced the producers to substitute old technologies with safer ones.

When water dispersions (or latexes) are used to prepare coatings, the preparation is cast onto the chosen substrate and, as water evaporates, the polymer particles merge and form a homogenous film on the coated surface [29]. The ability of a polymer latex to form a continuous film depends on the minimum film formation temperature (MFFT) of the polymer. When the casting takes place above the MFFT, the particles undergo deformation and cohesion, thus creating a homogeneous film. On the other hand, when the film is cast below its MFFT, a brittle discontinuous film or compact powder may be obtained [30]. Typically, the MFFT tends to be close to the glass transition temperature (Tg) of the polymer or above it [31]. The most commonly accepted mechanism of film formation from waterborne dispersions of preformed polymers is composed of three stages: drying, deformation and coalescence [32,33]. During drying, the water evaporates from the dispersion surface at a constant rate until the polymer has reached 60–70% of volume fraction. As the water evaporation rate slows down, deformation occurs and the particles come into irreversible contact. In the final step, polymer chain interdiffusion takes place and the particles coalesce into a continuous film.

The most common methods to produce water-based coatings are emulsion polymerization [34] and emulsification of preformed polymers. Polymer emulsions of acrylate and methacrylate have been produced for many years with emulsion polymerization [34–38]. More recently, water dispersion of low molecular weight resins that are converted into high weight mechanically performing coatings by UV-curing have also entered into the market [39,40]. Unluckily, emulsion polymerization is applicable only to free radical polymerizable monomers and lactide and lactic acid are not included among them. Indeed, in these cases the production of water-based preparations is achieved by dispersing the preformed polymer [41]. For instance, in the acetone process for the preparation of polyurethane-polyurea water dispersions, the hydrophilic isocyanate prepolymer is firstly chain extended in acetone to give the desired molecular weight. Subsequently, by the addition of water and removal of the solvent, a purely waterborne dispersion is formed [42,43].

To the best of our knowledge the preparation of aqueous emulsions and dispersions of PLA has been reported only for applications in the biomedical field, especially for the obtainment of PLA nanoparticles for drug delivery [44,45]. Some of the methods used in these cases involve the use of non-harmful solvents, such as acetone and ethyl acetate. Very diluted dispersions of PLA particles have been obtained by nanoprecipitation [46–49], salting-out [41,50–52] and emulsification-diffusion methods [46,53,54]. Nevertheless, these works aim at the isolation through centrifugation or lyophilization of the PLA nanoparticles. As a consequence, the dispersion stability over time is usually not investigated and besides the final PLA content is usually lower than 5 wt.%. On the other hand, with the use of chlorinated solvents by emulsification-evaporation methods, PLA dispersions with higher polymer contents are accessible (maximum 20 wt.%) [27,55–57]. Generally, the simplest emulsion-based procedures require the solubilization of PLA in a water-immiscible organic solvent, followed by subsequent dispersion of this organic phase in fine droplets into the aqueous medium. After that, the organic solvent evaporates leading to the formation of PLA nanoparticles dispersed in water [44]. This technique was also exploited to prepare blends of PLA with hydrophilic polymers such as chitosan [58,59], lignin [60] and

nanocellulose [61,62]. However, in the latter cases, the studies focus on the preparation of thermoplastic composites and consequently, the stability of the dispersion was not an issue and thus it was not reported. Besides, the toxicity associated with the chlorinated solvents used in these processes limits the biocompatibility and the use of the dispersions thus prepared for food contact applications. Additionally, the preparation of waterborne PLA dispersions for coating applications has been the subject of some recent patents [63,64]. In these cases, preparations with high dry content (25–70 wt.%) are afforded in the absence of organic solvent by continuously and simultaneously introducing melted PLA and aqueous phase into the blender of an emulsifying unit. Nevertheless, with these techniques the polymer is always processed at high temperatures and usually, high amounts of viscosity reducing agents, plasticizers or additives are needed. The PLA particles thus obtained are in the micrometric size range.

Aim of the present work was the design and optimization of a process to obtain stable dispersions of PLA in water suitable for coating film formation. The process had to be accomplished without chlorinated solvents to allow the use of the produced coatings in food packaging application. To this aim, an amorphous PLA soluble in non-chlorinated solvents and a solvent approved for food contact application, such as ethyl acetate, were selected. Different homogenization methods were tested for the mixing of the organic phase, in which PLA was dissolved, and the water-phase containing emulsifier and stabilizer. With the purpose of ensuring the stability of the preparation during storage and during evaporation of the organic solvent, the use of several emulsifiers and different polysaccharides as stabilizers were examined. In fact, to prepare kinetically stable emulsions, an emulsifier is needed to protect the newly formed droplets against the different destabilization mechanisms. In particular, the emulsifier forms a protective interfacial layer when it adsorbs on the surface of the droplets, preventing the latter from merging together [65]. Emulsifiers can be produced using either petrochemical or bio-based feedstocks and they are commonly classified according to the polarity of their head group: anionic, cationic, non-ionic and amphoteric [66]. In recent years, due to the continuous consumption and depletion of fossil fuels, renewable substrates have been of increasing interest for the production of bio-based surfactants [67]. On the other hand, non-surface-active polysaccharides, like gelatinized starch, may contribute to interfacial stabilization of an oil-in-water emulsion by interaction with the surfactant layer already located at the interface [65,68]. Additionally, polysaccharides are able to enhance the stability of emulsions by increasing the viscosity of the aqueous phase [69]. In this work, various chemicals and polysaccharides approved for food contact application were chosen as promising surfactants to be tested. Furthermore, several emulsifiers were selected on the basis of the relative strength of their hydrophilic and lipophilic moieties (known as hydrophilic–lipophilic balance or HLB) [70]. Surfactants with both high and low HLB values were examined. Regarding the choice of the stabilizers, the attention was focused on bio-based polysaccharides, which are largely employed in the food industry such as starch and xanthan gum (XG) [71,72] or promising emulsion stabilizers such as microfibrillated cellulose (MFC) [73]. Additionally, the film forming ability of two selected PLA dispersion formulations was studied at different temperatures in the 25–110 ◦C range. The morphology of the obtained layers was investigated through scanning electron microscopy (SEM) to identify the MFFT of the innovative biodegradable dispersions in water.

#### **2. Materials and Methods**

#### *2.1. Materials*

PLA Ingeo Biopolymer 4060D (PLA) was supplied by NatureWorks LLC (Blair, NE, USA.). Sodium dodecyl sulfate (SDS), ethyl acetate, sodium alginate (SA), sodium carboxymethylcellulose (NaCMC), Tween 80, Tween 20, stearoyl-2-lactylate (SSL) and Span80 were purchased from Sigma-Aldrich (Milan, Italy). Starch C\*Icoat 07,525 (starch) and xanthan gum (XG) were generously supplied by Cargill (Krefeld, Germany,). A solution of starch at 3% in ultrapure water was prepared by heating under reflux 6 g in 200 mL

for nearly 1 h. Metolad 368 (Met. 368), Metolad 388 (Met. 388), Leukonol LB-2 (Leu.) and Tafigel AP 15 (Taf.) were kindly provided by Münzing Chemie GmbH (Abstatt, Germany). Synperonic PE/F68 (SYN) was generously supplied by Croda Chemicals (Mortara (PV), Italy). Microfibrillated cellulose MFC Exilva F 01-L (MFC) was kindly supplied by Borregaard (Sarpsborg, Norway).

#### *2.2. Preparation of PLA Dispersions with Sodium Dodecyl Sulfate as Surfactant and Starch as Stabilizer*

6.47 g of starch solution at 3% conc. was diluted with 28.5 mL of ultrapure water (starch conc. 0.5 wt.%) and 0.28 g of SDS were added (SDS conc. 0.8 wt.%). In the meantime, 5 or 7 g of PLA was solubilized in 45 or 87.5 mL of ethyl acetate (EtOAc) (11 or 8 wt./vol.%, respectively) under vigorous stirring for 4 h at room temperature. After that, proper amounts of the oil and water phases were mixed, in an ice bath, with the use of a homogenizer to obtain the emulsions. Different tests were carried out by changing the volume ratio between the two phases as well as the total phase volume (entries 1–13, Table 1). For homogenization, ULTRA-TURRAX (UT) or two different ultrasound probes (UVC and UH) were tested. Homogenization conditions were as follows: (a) UH for 30 s at amplitude 50% + 1 min 30 s at amplitude 80%; (b) UVC for 30 s at amplitude 50% + 2 min of overhead stirrer at 200 rpm + (30 s at amplitude 90% + 2 min of overhead stirrer at 200 rpm) three times; (c) UT for 1 min 30 s at 1100 rpm. Afterwards, the resulting emulsions were stirred for 20 h at room temperature and 200 rpm under the aspiration of a laboratory hood (suction speed: 0.5 m/s).

A similar procedure was adopted in tests of entry 11 and 12 in Table 1 except that the amount of the starch solution used was 0.38 g (starch conc. 1.1 wt.%) in test 11 and the amount of SDS employed in test 12 was 0.10 g (SDS conc. 0.3 wt.%).

1 Ratio between the weight of poly(lactic acid) and the volume of EtOAc. 2 Ratio between the weight of poly(lactic acid) and the volume of water. 3 UH = ultrasonication with Hielscher, UVC = ultrasonication with Vibracell, UT = Ultraturrax. 4 Assessed through visual criteria and DLS analysis both after the preparation and after 1 week. Stability as follows: coa. = coagulation, cl. = clot formation. 5 Determined through DLS analysis. 6 1.1 wt.% of starch in water was employed.

#### *2.3. Preparation of PLA Dispersions with SDS and Different Polysaccharides as Stabilizers*

Different amounts of starch, MFC or XG and 0.28 g of SDS were solubilized in 35 mL of ultrapure water (Table 2). In the case of MFC, the polysaccharide suspension was treated with UT at 10,000 rpm for 4 min before addition of SDS. In the meantime, 5 g of PLA was solubilized in 45 or 62.5 mL of EtOAc (11 or 8 wt./vol.%, respectively) under vigorous

stirring for 4 h at room temperature. After that, the oil and water phases were mixed together, in an ice bath, with the aid of the UVC homogenizer for 30 s at amplitude 50% + 2 min of overhead stirrer at 200 rpm + (30 s at amplitude 90% + 2 min of overhead stirrer at 200 rpm) three times. Afterwards, the resulting emulsion was stirred for 20 h at room temperature at 200 rpm under the aspiration of a laboratory hood (suction speed: 0.5 m/s).

**Table 2.** Screening of different stabilizers for the preparation of PLA dispersions in the presence of SDS as surfactant.

1 Ratio between the weight of poly(lactic acid) and the volume of EtOAc. 2 Assessed through visual criteria and DLS analysis both after the preparation and after 1 week. Stability as follows: coa. = coagulation, sed. = sedimentation.

#### *2.4. Preparation of PLA Dispersions with Different Surfactants Than Sodium Dodecyl Sulfate*

The proper amount of emulsifier was solubilized into 35 mL of ultrapure water. In the case of the presence of starch as stabilizer in the formulation, the 35 mL included 6.47 g of starch solution at 3% conc. (starch conc. 0.5 wt.%). The type of surfactant employed and its concentration in the water phase are listed in Table 3.

**Table 3.** Screening of different surfactants for the preparation of PLA dispersions.

1 Assessed through visual criteria and DLS analysis both after the preparation and after 1 week. Inverted = formation of water-in-oil emulsion instead of oil-in-water one. 2 Homogenization with UH, PLA/EtOAc ratio (wt./vol.%) = 8, EtOAc/H2O vol. ratio = 2.5, total volume = 70 mL.

Moreover, 5 g of PLA was dissolved in 45 mL of EtOAc under vigorous stirring for 4 h at room temperature. When Span 80 was employed as surfactant, the latter was solubilized in the organic phase together with PLA. After that, the oil and water phases were mixed together and homogenized while standing in an ice bath to obtain the emulsions. Homogenization was carried out with UH or UVC ultrasound homogenizer under conditions "a" or "b" mentioned in Section 2.2, respectively. Afterwards, the resulting emulsions were stirred overnight at 200 rpm under the aspiration of a laboratory hood (suction speed: 0.5 m/s).

#### *2.5. Preparations of PLA Films from Dispersions*

1.7–3.0 g of PLA dispersions prepared under the condition of entry 3, Table 1 (PLA\_SDS\_starch) or entry 4, Table 3 (PLA\_SYN), were transferred into a PTFE capsule (diameter 3.7 cm) and allowed to dry for 2–3 h at the selected temperature until solidification occurred. The drying temperature was room temperature and then the latter was gradually increased by 10 ◦C starting from 40 up to 110 ◦C.

#### *2.6. Instruments and Characterization Methods*

To weigh the starting materials Entris Sartorius and Sartorius BP61S laboratory balances (Sartorius AG, Goettingen, Germany) were used. To prepare the water and the organic phase of the dispersions, an IKA magnetic stirrer RCT basic (IKA, Staufen, Germany) was employed. Ultrapure water was produced with Millipore Direct-Q 3UV (Merck KGaA, Burlington, MA, USA).

For homogenization of oil and water phases, Ultrasound Hielscher Up200St (UH) equipped with a 14 mm diameter probe, Vibra-Cell Ultrasonic Liquid Processors VCX750 (UVC) (Sonics, Newtown, MA, USA) provided with a 13 mm diameter probe and IKA ULTRA-TURRAX T 25 basic (UT) equipped with an S 25 N-18 G dispersing tool (IKA, Staufen, Germany) were used.

Emulsions and dispersions were stirred with an ArgoLab AM20-D (ArgoLab group, Landshut, Germany) overhead stirrer equipped with PTFE blade.

The diameter of the particles was determined by dynamic light scattering (DLS) analysis at 25 ◦C using a NanoBrook Omni particle size analyzer (Brookhaven Instruments Corporation, Holtsville, NY, USA) equipped with a 35-mW red diode laser (nominal 640 nm wavelength), BI-SCP cell in backscattering (173◦) and software for data analysis. To give reliable results, dilute suspensions (0.15 vol./vol.%) were analyzed. For data elaboration, reflective indices of the dispersion medium and of the dispersed phase were assumed to be 1.330 and 1.596, respectively. Each measurement was repeated five times on the same sample and the reported data were the average over the five measurements. The standard deviation δ (Equation (1)) was assumed as data error, where N is the number of measurements, xi the value of the i-th measurement and μ the arithmetic mean. All the DLS data were analyzed by a multimodal function. Particles were assumed to be uniform spheres.

$$\delta = \sum\_{i=1}^{N} \frac{\sqrt{\left(\mathbf{x\_i} - \boldsymbol{\mu}\right)^2}}{\sqrt{N}} \tag{1}$$

The dispersions' stability was assessed through visual criteria and DLS analysis both after the preparation and after 1 week. Dispersions were assumed stable if no phase separation was observed and the particle size variation was lower than the standard deviation on the particle size value.

The dry residuum of the preparations was determined after heating the sample at 200 ◦C for 30 min in an oven.

The residual amount of EtOAc present in the emulsion was quantitatively determined by gas chromatography/mass spectroscopy analysis (GC/MS). A triple quadrupole GC/MS instrument from Agilent Technologies (Agilent Technologies, Inc., Santa Clara, CA, USA) equipped with a 20 × 0.18 mm, 0.18 μm column was employed, using hydrogen at 0.8 mL/min as carrier gas with a 10:1 split ratio. For the analysis, 100 μL of the emulsion

was added to a 20 mL chromatography vial and incubated at 85 ◦C for 30 min for headspace extraction. The GC oven was programmed for 40 ◦C for 3 min, followed by a stepped increase of 10 ◦C min−1 to 50 ◦C, where it was held for 5 min, and then the temperature was increased by 30 ◦C min−1 to 230 ◦C, where it was held for 3 min. The quantitative determination of EtOAc traces was conducted by using the selected ion monitoring (SIM) at the *m*/*z* values 88/73. Calibration was performed by injecting solutions of EtOAc in *N*,*N*-dimethylformamide at known concentrations into the GC/MS instrument.

Differential scanning calorimetric analyses were performed on a DSC 8000, PerkinElmer Inc. (Waltham, MA, USA) instrument equipped with IntraCooler II cooling device and Pyris software (Version 13.3, PerkinElmer Inc., Waltham, MA, USA) for instrument control, data acquisition and analysis. The instrument was calibrated for temperature and energy with high-purity indium and lead as standards. 3–10 mg of sample was analyzed in aluminum pans under dry nitrogen atmosphere (30 mL/min). Samples were at first heated up from 25 to 200 ◦C to erase the thermal history and to remove any trapped volatile substance such as residual solvents. Thus, samples were cooled down to −70 ◦C (cooling step), maintained at −70 ◦C for 5 min and finally heated up again to 200 ◦C (second heating step). Heating and cooling steps were all performed at 10 ◦C/min as the scanning rate.

Size-exclusion chromatography (SEC) analyses were performed with a Jasco (Jasco Europe srl, Cremella, Italy) instrument comprising a PU-2089 Plus quaternary pump and injector with a 20 mL loop, two in-series PLgel MIXED-D columns (Agilent Technologies Italia S.p.A., Cernusco sul Naviglio, Italy; linearity range 200 to 2,000,000 g/mol based on polystyrene equivalent) placed in a Jasco CO-2065 column oven set at 30 ◦C, a Jasco RI-2031 Plus refractive index detector, and a Jasco UV-2077 Plus multi-channel UV-Vis detector. The samples in the form of films or powders were dissolved in trichloromethane (HPLC grade Sigma-Aldrich, Milan, Italy) with the aid of sonication and filtered through a 0.2 mL pore size PTFE filter to remove the insoluble fraction before injection as 5 mg/mL solutions; elution was performed with trichloromethane at 1 mL/min flow rate. ChromNav Jasco software (Jasco Europe srl, Cremella, Italy) was used for data acquisition and analysis based on a calibration curve obtained by running a set of four monodisperse polystyrene standards (19,000, 50,000, 233,000, and 300,000 g/mol, respectively) and performing a 4th order fit.

Scanning electron microscopy analyses (SEM) were accomplished with a Zeiss EVO 40 microscopy (Carl Zeiss Microscopy Ltd., Cambridge, UK) equipped with a LaB6 source. Samples in the form of powder or film were gold sputtered before observation. Film sections were obtained by fracture in liquid nitrogen.

A Perkin Elmer Lambda 650 UV-Vis spectrophotometer (Waltham, MA, USA) was used to record the transmittance of free-standing films. Measurements were accomplished in transmission mode with air as reference. Film thickness was measured with an electronic outside micrometer that is able to perform measurement between 0.001 and 25 mm.

FT-IR spectra were recorded on an Agilent Cary 630 FTIR spectrophotometer (Agilent Technologies, Inc., Santa Clara, CA, USA) with ZnSe ATR element. Background and sample spectra were collected by accumulating 32 scans.

#### **3. Results and Discussion**

#### *3.1. Influence of the Preparation Conditions on the Stability of PLA Dispersions in Water*

Several tests to prepare ethyl acetate/water (EtOAc/H2O) emulsions of PLA with SDS as surfactant and starch as stabilizer were carried out (Table 1), by using Ultraturrax (UT) or Ultrasound processors (UH and UVC) as homogenizer. When Ultraturrax (UT) was used, the emulsion formed but it was unstable (entry 1, Table 1); during the subsequent evaporation of the organic solvent, the majority of the preparation coagulated. On the contrary, with the use of ultrasound processors, either Ultrasound Hielscher UP200St (UH) or Vibra-Cell Ultrasonic Liquid Processors VCX750–SONICS Materials (UVC), stable PLA emulsions in water were obtained (Figure 1a). As a matter of fact, by solvent evaporation, these were converted into stable dispersions with size of 200–215 nm. No significant effect of

the specific ultrasound processor used on the particle size was noticed (Table 1). Dry matter content was between 13 and 20 wt.%, which is much higher than any other reported result for stable PLA dispersions in water produced with emulsification-evaporation methods and non-chlorinated solvents [29,55].

Inspired by the good results obtained with ultrasound homogenizers, we attempted to further increase the PLA content in water by using a larger volume of organic phase, namely, 62.5 mL instead of 45 mL, to prepare the emulsion with the UVC homogenizer (entry 4, Table 1). However, even if the emulsion formed, the process was not effective since a cloth made of PLA formed on the stirrer during the subsequent EtOAc evaporation. When the process was replicated with the same oil and water phase volumes, but with 8% concentration of PLA instead of 11% in the organic phase, once again a homogeneous emulsion was obtained.

**Figure 1.** (**a**) Picture of the PLA dispersion of entry 5, Table 1; (**b**) comparison between DLS plots of the PLA dispersions prepared under the condition of entry 3, Table 1, after storing for increasing periods of time. Correlograms are normalized with respect to the intensity at time 2 μs. Insert refers to the particle size distribution as obtained by data fitting with a multimodal size distribution function.

In this case, the emulsion was stable enough to be converted into a stable dispersion by EtOAc evaporation. Particle size was smaller than the one obtained in the previous test with higher PLA concentration (entry 5, Table 1). Similar results were afforded when the total volume of the organic and water phases was reduced from 97 to 32 mL (entry 6, Table 1). The results suggest that the viscosity of the oil phase is the key parameter allowing the dispersion in the case of the higher oil/water phase ratio. Indeed, dispersion was achieved with PLA concentration of 11% and phase ratio of 1.3 or PLA concentration of 8% and phase ratio of 1.8. The decrease of the PLA concentration in EtOAc leads to a lower viscosity of the organic phase, thus reducing the energy input needed to disperse the organic phase into the aqueous one [74]; as a consequence, the treatment of a larger total volume becomes feasible. The further attempt to prepare a more concentrated PLA dispersion by increasing to 87.5 mL the volume of the 8% PLA solution (oil/water phase ratio of 2.5), once again gave an emulsion. However, this was not very stable and a clot formed on the stirrer during evaporation of the organic solvent, if the UVC homogenizer was used (entry 7, Table 1). On the contrary, with UH homogenizer, after solvent evaporation, the emulsion turned into a homogeneous dispersion with 20% dry matter content (entry 8, Table 1). Even when the preparation was performed in a smaller scale (with a lower total volume) but with the use of UVC homogenizer, a clot on the stirrer was formed during evaporation of

the EtOAc (entry 9, Table 1). This result clearly indicates the higher efficiency of UH in transferring the energy and in treating larger both total volume and EtOAc/H2O volume ratio than UVC does. Indeed, UH sonication was effective in treating EtOAc/H2O volume ratio up to 2.5 (entries 2, 8, 10 and 11, Table 1), whilst the ratio was only up to 1.8 for preparations performed with UVC (entries 3, 4, 5, 6, 7 and 9, Table 1). In any case, the maximum EtOAc/H2O volume ratio that could be treated is 2.5. As a matter of fact, no stable preparation could be obtained with a ratio of 5, even with the UH homogenizer, a relatively low total volume (60 mL) and a high concentration of stabilizer, 1.1% instead of 0.5% (entry 11, Table 1).

When the ratio between phases was lower than 2.5 for treatment with UH and not higher than 1.8 for UVC use, stable emulsions and then dispersions were obtained, even with SDS concentration lower than 0.8%. Indeed, good results were obtained with SDS 0.3% (entry 12, Table 1) plus 0.5% of starch.

#### *3.2. Influence of Ethyl Acetate Residua on the Emulsion Stability*

The homogeneous emulsions obtained in the experiments described in Table 1, if left standing without stirring, coagulated in a reasonably short time. However, it was clear that they turned into stable dispersions after complete evaporation of the organic solvent (Figure 1a). Dispersions can even be stored at room temperature or in the fridge at +4 ◦C for days and months without sedimentation and significant change of the particles' size and of their distribution (Figure 1b).

In order to better understand the emulsion preparation process, different samples were singled out at distinct times during the evaporation of the EtOAc and they were analyzed through DLS and head-space GC. Furthermore, the evolution of the dry matter content, while EtOAc evaporates, was followed. The experiment was carried out under the condition of entry 3 in Table 1. The evolution of the dry matter content shows an almost linear increase over time (Figure 2a). On the contrary, the EtOAc residue initially decreases slowly, and then almost exponentially. The difference among the two kinetics clearly indicates that during the evaporation stage not only the organic solvent but also a portion of water is removed from the preparation. This is due to the minimum temperature of the azeotrope between water and EtOAc.

**Figure 2.** (**a**) Kinetics of ethyl acetate residue (red line) and dry residue (blue line) evolution; (**b**) kinetics of particles size (blue line) and polydispersity (red line) evolution over time. Data are related to test 3 in Table 1. \* The highest data of ethyl acetate residua expressed in ppm are only indicative.

In the initial period (0–3 h) when the EtOAc residuum is very high (~40%) but also later on, when it is lower, e.g., 37% at 5 h, the preparation is unstable and coagulates by standing. On the contrary, when the EtOAc residuum reaches the value of 80 ppm, after 19 h, the preparation is almost stable; the DLS correlogram exhibits just a slight decrease of the intensity during the analysis time and the emulsion was visibly stable. Besides, when the EtOAc residuum is only 33 ppm (after 22 h), the dispersion is very stable, even after a month of storing in the fridge at 4 ◦C (Figure 1). Overall, the data collected show that, in order to ensure the stability of the dispersion, the organic solvent must be completely removed (residue of EtOAc lower than 40 ppm).

During EtOAc evaporation, particle size and polydispersity evolve, both passing through a maximum after 90–100 min and then decreasing to reach the minimum after 1 day (Figure 2b). The initial increase of size can be due to coalescence between distinct particles. Indeed, after preparation, the dispersed phase is made of drops of PLA solution in EtOAc. As the solvent evaporates, the particle volume decreases and then also its size. Furthermore, due to the evaporation of EtOAc, the concentration of PLA in the particle increases and therefore, the viscosity of the oil phase increases also. As a result, the rate of coalescence between distinct particles decreases and becomes irrelevant once particles are solid.

#### *3.3. Effect of Stabilizer on Dispersions Stability*

With the purpose of studying the role of starch on the stability of the emulsion/dispersion, different trials to prepare the emulsion with only SDS as surfactant or with other polysaccharides than starch were performed (Table 2). Two different tests were carried out with only SDS as surfactant. The water phase amount, composition and the total amount of PLA were the same. The only difference between the two tests was the concentration of the oil phase, which was 11% and 8%, (entries 1 and 2, Table 2, respectively), and as consequence the oil phase amount and the oil/water phase ratio also changed. In both cases, PLA dispersions unstable by DLS analysis were afforded. The comparison with the high stability of the dispersions prepared under comparable conditions with the additional presence of starch (entry 3 and 5 in Table 1, respectively) clearly indicates that the presence of such agent is of utmost importance for the stabilization of the resulting formulations. Usually, starch can act as an emulsifier only if hydrophobic modifications are introduced on its chain in order to afford substantial surface activity at the oil-water interface [75]. Gelatinized starch, as a non-surface-active polysaccharide, has been reported to act as stabilizer for the emulsion by forming a secondary steric stabilization layer through interaction with the pre-adsorber emulsifier [65,68].

After the result with starch, we decided to explore if similar results could be obtained with other polysaccharides. To this aim, microfibrillated cellulose and xanthan gum were tested. Microfibrillated cellulose (MFC) in water has been reported to form a threedimensional network capable of incorporating dispersed oil droplets, preventing their coagulation and sedimentation [76]. On the other hand, xanthan gum (XG) is widely used to increase the viscosity of the aqueous phase, hence preventing creaming of oil droplets [77]. However, in the case of the PLA, the attempt to employ MFC as stabilizer in a concentration of 0.5 wt.% in water, in the presence of SDS, gave a PLA dispersion that sedimented over time (entry 3, Table 2). On the other hand, any attempt to prepare PLA dispersions in the presence of higher MFC content was not successful because the mixtures could not be homogenized under ultrasound treatment due to the extremely high viscosity of the water phase.

Similar viscosity constraints were also observed with XG. Indeed, tests with XG as stabilizer provided stable dispersions only at XG concentration in water of 0.009 wt.% (entry 5, Table 2). The high viscosity of more concentrated XG solution, even 0.1 wt.%, prevented the effective emulsification of oil-and-water phases. Dispersion with 0.009 wt.% concentration of XG has nanoparticles with 192 nm diameter and dry residual of 14%. Both values are comparable to the ones afforded with starch as stabilizer, thus suggesting comparable mechanisms of stabilization for the two polysaccharides. Unluckily, when the preparation was performed at a larger scale (3 times higher) with XG, coagulation occurred (entry 6, Table 2). Overall, a PLA dispersion in water is formed in all the conditions listed in Table 2. However, only with starch as stabilizer is the formulation stable over time and it maintains its stability even when it is prepared at a higher scale, thus highlighting the key

role of this polysaccharide in efficiently stabilizing the preparations, without preventing the effective homogenization of phases during the emulsion preparation.

Furthermore, the good stability of dispersions with starch, in spite of the lower viscosity of starch solutions with respect to XG, suggests that the viscosity is not the parameter that controls the dispersion stability.

#### *3.4. Screening of Different Emulsifiers for the Formation and Stability of the Dispersion*

In order to study the role of the emulsion stabilizer used in the formation and stability of the preparations, several surfactants in different combinations and concentrations were tested (Table 3).

At first, SDS in combination with Span 80, SYN (a PEO-PPO block copolymer), alone and in combination with Span 80 and Tween 80 were tested in the absence of starch (from entry 2 to 9, Table 3). However, only SDS in combination with Span 80 (entry 2, Table 3) and SYN at 2.4% concentration (entry 4, Table 3) provided stable emulsions. In the case of SYN, an almost satisfactory emulsion was already obtained at 0.8% (entry 6, Table 3). However, PLA coagulated on the stirrer forming a clot during the subsequent solvent evaporation, when the concentration was other than 2.4% (entries 3, 5, 6 and 7, Table 3). The dispersion obtained with SYN possesses a particle size between 400 and 450 nm. The value is higher than the one obtained with SDS. On the contrary, the particle size observed with SDS and Span 80 was around 150 nm, which is comparable to the one with only SDS.

Met. 388, SA, NaCMC, SSL and Met. 368 (entries 12, 15, 16, 17 and 18, Table 3) tested in the presence of starch, all led to the formation of inverted emulsions (water in oil instead of oil in water). On the other hand, with the use of Tween 80 and Tween 20 (entries 9, 13 and 14, Table 3) all the preparations were unstable due to coagulation.

In order to rationalize the obtained results, they are visualized in Figure 3: the formation and stability of the PLA preparations are plotted as a function of the surfactants' hydrophilic–lipophilic balance (HLB) values and their concentration in the water phase. The data highlight that for relatively low values of HLB (between 5 and 11) and low concentration in water (from 0.5 to 0.1 wt.%), an inverted emulsion (water-in-oil) is formed (red symbols, Figure 3). When the hydrophilic–lipophilic balance (HLB) value is in the 15–20 range, without exceeding concentration values of 0.1 wt.% in water, the emulsion sometimes is still inverted and sometimes partially forms (respectively, red and orange symbols, Figure 3). However, in this HLB range it is sufficient to increase the concentration of surfactant from 0.8 to 1.3 wt.% to obtain a direct emulsion (oil-in-water) (light green symbols, Figure 4). However, emulsions are not stable over time. Indeed, for hydrophilic– lipophilic balance (HLB) values higher than 19 and a concentration of surfactant higher than 1 wt.%, a direct emulsion was obtained with all tested surfactants, even if it is stable only in a few cases. In summary, two stability zones can be highlighted from the chart (dark green symbols, Figure 3): (a) for a high HLB value (40), a relatively low concentrations of surfactant (0.8 wt.%) and starch as stabilizer; and (b) with modest HLB (29 and 34) and fairly high concentrations of emulsifier (2 and 3 wt.%). In the second case, the use of starch as stabilizer does not affect the stability of the preparations, suggesting that, in a certain range of HLB values and concentrations of emulsifier, no stabilizing agents are needed. As a drawback, the use of a higher amount of surfactant significantly affects the composition of the final coating, therefore most likely also the final properties of the layer.

**Figure 3.** Influence of hydrophilic–lipophilic balance (HLB) and concentration of surfactant in water on the formation and stability of the formulations.

**Figure 4.** DSC thermograms at 10 ◦C/min of neat PLA, dried PLA dispersion with SDS and starch as surfactant and stabilizer, respectively, (entry 3, Table 1) and dried PLA dispersion with SYN (entry 23, Table 3). Second heating step. Thermograms were arbitrarily vertically shifted for clarity.

#### *3.5. Characterization of the Dispersions*

The dried dispersions were characterized through gel permeation chromatography (GPC) analysis in order to verify if the PLA was subjected to degradation during the preparation of the formulation and the subsequent storage. The comparison between the molecular weight of pristine PLA and PLA of dispersions showed minor differences (3–5%) in the range of the experimental error (Table 4). Furthermore, no significant change in polymer dispersity was detected, thus indicating the absence of significant degradation of the polyester during the processing in water for the dispersion preparation under ultrasound treatment and moderate heating and the subsequent storage for a few months.

DSC analysis of neat PLA 4060D showed glass transition temperature at 54.1 ◦C and no melting or crystallization peaks (Figure 4), in agreement with the amorphous nature of the resin declared by the producer (NatureWork processing guide NWPG002\_020111; https://doi.org/10.1016/j.reactfunctpolym.2017.06.013, accessed on 1 September 2020). The analysis of dried dispersion with SDS and starch (entry 3, Table 1) showed only a modest shift of the Tg value towards a lower value (Tg = 51 ◦C) with respect to the pristine polyester, thus indicating a modest effect of the emulsion process even on the thermal properties of the resin. On the contrary, dispersion with SYN showed a significant decrease of ~20 ◦C in the glass transition temperature (Tg = 34 ◦C), indicating a plasticization of PLA by the PEO block of SYN [78]. This latter sample, in addition to the shift in the Tg's mean value, showed a broadening of the transition, thus reflecting the variety of thermal motions available [79]. This last effect can be due to a non-homogeneous distribution of SYN in the PLA particles or even to a non-homogeneous distribution of SYN among particles, resulting in particles richer in the block copolymer and others with comparable depletion.

**Table 4.** Molecular weight by GPC analysis of pristine PLA and PLA dispersions dried at different temperatures.

#### *3.6. Preparation of Films from PLA Dispersions*

PLA dispersions in water prepared with either SDS\_starch (entry 3, Table 1) or SYN (entry 29, Table 3) as surfactant and stabilizer were cast on PTFE capsules and left to dry. When drying was performed at room temperature, a layer of powder was obtained. SEM analysis revealed the presence of submicrometric particles (Figure 5a,b), whose dimension was comparable to the values by DLS data of the corresponding water dispersion (Section 3.2). This result indicates that particles did not merge, most likely because the processing temperature was lower than the glass transition value of both dispersions (Section 3.5). When drying was performed at 40 ◦C, particles started to merge, forming white continuous layers. However, these were brittle and fractured (Figure 6a,b). A further increase of the temperature to 50 ◦C had a partial positive effect only in the case of the SYN dispersion, which has a glass transition temperature of 34 ◦C. However, the formed film, even if partially transparent, was brittle and cracked. On the contrary, when films are dried at 60 ◦C, continuous free-standing films with thickness between 230 and 550 μm are afforded (Figure 6c,d) with both formulations.

The formulation with SYN gave even, almost transparent films, while in the presence of SDS and starch, opaque films are afforded. In both cases, individual particles cannot be distinguished by SEM observation of the film's surfaces (Figure 5c,e,g,h) and section (Figure 5d,f not shown), thus indicating effective film formation. Notice that 60 ◦C is higher than the glass transition temperature of both dispersions and of the pristine PLA. Processing at higher temperatures, up to 110 ◦C, provided films that look like the one obtained at 60 ◦C in the case of the SYN formulation. However, with the dispersions with SDS and starch, yellowing occurred at drying temperatures above 90 ◦C. Indeed, the SEM analysis of PLA dispersion in water prepared with SDS\_starch at 80 ◦C (Figure 5g) shows the presence of granules that are probably due to the degradation of the surfactant/stabilizer.

**Figure 5.** SEM pictures of the filming tests performed with water dispersions of PLA at room temperature (**a**,**b**), at 60 ◦C (**c**–**f**) and at 80 ◦C (**g**,**h**). Picture (**a**,**c**,**d**,**g**) refer to tests with the SDS\_starch formulation; (**b**,**e**,**f**,**h**), correspond to tests with the SYN formulation.

**Figure 6.** Pictures of PLA films obtained by solution casting at 40 ◦C (**a**,**b**) and 60 ◦C (**c**,**d**) of water dispersions with SDS and starch (**a**,**d**) or SYN (**b**,**c**).

Transmission measurements showed that PLA films with SDS and starch (thickness of 320 μm) possess a transmittance of 2.6% at 600 nm. The low transmittance is due to the whitish color of the film, probably given by the presence of starch. On the other hand, PLA film with SYN (thickness of 250 μm) showed a higher transparency (transmittance of 40% at 600 nm). However, the latter possesses a modest transparency considering that a pure PLA film prepared from EtOAc (thickness of 130 μm) has a transmittance of 72% at 600 nm.

ATR analysis of the upper and lower surfaces of the films showed comparable band intensity, hence indicating a similar composition. As a consequence, significant surfactant segregation or blooming effects can be excluded (Figure 7). However, films obtained at high drying temperatures (90 and 110 ◦C) showed a different composition between the upper and lower surface by ATR analysis (Figure 7). In particular, an enrichment in stabilizer/surfactant is observed on the lower surface of the films. Indeed, the IR spectra of the bottom surface of PLA\_SDS\_starch film prepared at 90 ◦C (Figure 7a) shows a broad band between 3000 and 3500 cm−1, which is typical of the starch's OH stretching. The enrichment of starch is further confirmed by the typical stretching band of sugar rings at around 1000 cm−1. Similarly, the film from the PLA\_SYN formulation obtained at 110 ◦C, in the bottom surface showed more intense signal than the top surface. These are signals at nearly 2800 cm−1 relative to CH2 stretching and the one at nearly 955 cm−1, which are both present in the ATR spectra of Synperonic.

The possible degradation of PLA during film formation was checked by GPC analysis (Table 4). A modest variation, even if detectable (10–15% decrement), was observed after drying at 60 ◦C. Higher processing temperature resulted in a more extended molecular weight decrement. No variation of the dispersity was detected as a result of a comparable decrement in both the *Mn* and *Mw* value, thus suggesting no preferential degradation of the high or low molecular weight fractions. In any case, no optimization of the drying time has been performed and it can be envisaged that better control over the molecular weight can be achieved by proper control of the heating time.

**Figure 7.** (**a**) FT-IR spectra of the top and bottom surfaces of PLA\_SDS\_starch films prepared at 60 and 90 ◦C; (**b**) FT-IR spectra of the top and bottom surfaces of PLA\_SYN film prepared at 60 and 110 ◦C.

#### **4. Conclusions**

In the present work, a procedure based on emulsification-evaporation method, which gives PLA dispersions in water with dry matter content up to 20 wt.%. has been developed. The procedure is based on the combination of an amorphous commercial PLA grade, ethyl acetate as solvent and surfactants all approved for food contact applications, so that the new preparation is also suitable for this application's purpose.

Surfactants with hydrophilic–lipophilic balance (HLB) in the 29–34 range at 2 and 3 wt.% concentration are needed to stabilize the formulations during the preparation stages. Surfactants with higher HLB (40) are also effective if starch is included as stabilizer. Emulsification needs high energy input by ultrasound sonication to achieve effective dispersion. Furthermore, the total treated volumes, the organic/water volume ratio and the viscosity of phases must be adjusted to the sonication capability to achieve the stability of the final dispersion. In addition, for long-term stability, the final dispersion formulation must contain negligible ethyl acetate residua (<40 ppm). Under this circumstance, dispersions do not sedimentate or cream and particle size does not appreciably change over several months at room temperature or in the fridge at 4 ◦C.

The obtained dispersions, consisting of submicrometric PLA particles, were successfully cast into films, with minimum formation temperature of 60 ◦C, which is a little above the glass transition temperature of the pristine PLA.

#### **5. Patents**

The method of preparing halogen-free aqueous dispersion of biodegradable polymers was submitted to the Italian Patent Office (Bertoldo, M.; Ricci, L.; Messina, T.; Belletti, G.; Gallur Blanca, M.; Guillem Ortiz, A. and Aragón Gutierrez, A. Italian Patent issue no.<PHONE_NUMBER>28640, 26 November 2020).

**Author Contributions:** Conceptualization, M.B.; methodology, M.B., G.B. and L.R.; validation, G.B. and S.B.; formal analysis, G.B. and S.B.; investigation, G.B. and S.B.; resources, M.B. and O.B.; data curation, M.B. and G.B.; writing—original draft preparation, M.B. and G.B.; writing—review and editing, M.B., G.B., A.G.-O. and A.A.-G.; visualization, M.B., G.B., A.G.-O. and A.A.-G.; supervision, M.B. and O.B.; project administration, M.B.; funding acquisition, M.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Bio Based Industries Joint Undertaking under the European Union's Horizon 2020 research and innovation program under grant agreement No 745718 (Sherpack).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** Professor Valter Castelvetro of the University of Pisa is kindly acknowledged for the GPC analysis. Franco Corticelli of Institute for Microelectronics and Microsystem (National Research Council) is kindly acknowledged for the SEM analysis. Cargill Spa is acknowledged for providing Starch C\*IcoatTM 07525 and xanthan gum, Borregaard for MFC Exilva F 01-L and Croda Chemicals for Synperonic PE/F68.

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

#### **References**

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