**1. Introduction**

Currently, a global plastics production of 368 million tons was recorded in 2019, an increment of 2.5% from 2018. Conventional polymers such a polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), poly(ethylene terephthalate) (PET), polystyrene (PS), and polyamide (PA), represent approximately 70% of plastic demand in Europe. With regard to the packing industry, which represents around 40% of total demand, these polymers are leading the plastic demand [1]. The majority are non-biodegradables as well as manufactured by petrochemical industries (non-renewable resources) [2]. The food packaging industry generates a large volume of waste due to it short-lifespan and its recycling is often limited to those not contaminated with food products. According to Plastics Europe 2020, about 39.5% of post-consumer waste was recycled, while 18.5% (3.2 million tons) ended up in landfills [1]. These non-recycled plastics need to be managed to avoid the presence in seas, lakes, and rivers which threatens the environment [3,4].

Concerning biodegradable polymers, their presence is increasing in the food packing industry. Several biodegradable polymers such as poly(lactic acid) (PLA), thermoplastic polyurethane (TPU), and polyhydroxyalkanoates (PHAs) have been applied as new alternatives [5–8]. The most employed polymer is PLA (about 10.9%), which is obtained by fermentation of polysaccharides or sugar extracted from potato, sugarcane, corn, etc., thus obtained by renewable resources [9]. PLA is currently manufactured for common applications such as salad cups, lamination films, drinking cups, containers, etc. [10].

**Citation:** Dominguez-Candela, I.; Ferri, J.M.; Cardona, S.C.; Lora, J.; Fombuena, V. Dual Plasticizer/Thermal Stabilizer Effect of Epoxidized Chia Seed Oil (*Salvia hispanica* L.) to Improve Ductility and Thermal Properties of Poly(Lactic Acid). *Polymers* **2021**, *13*, 1283. https://doi.org/10.3390/ polym13081283

Academic Editor: Beom Soo Kim

Received: 20 March 2021 Accepted: 12 April 2021 Published: 14 April 2021

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**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/).

Biodegradation of PLA is produced by hydrolysis, resulting in harmless and non-toxic substances [11,12]. Nowadays, PLA is considered economically competitive and its properties such as good processability, high transparency, water solubility resistance, biodegradability, recyclability, etc., make it suitable for good packaging [13,14]. Besides, the energy saved in production is around 22–55% with regard to petroleum-based polymers, thus contributing to a decrease in environmental impact [15]. PLA is characterized by its high tensile modulus, although some drawbacks such as brittle nature with elongation at break of less than 9%, a narrow processing window, poor thermal stability, etc., are detected [16,17]. Several methodologies to improve ductile properties have been carried out successfully using copolymers, blends, or plasticizers in a PLA matrix [18–20].

The plasticizer market is increasing its annual demand in the polymeric industries. Phthalates (PTs) are a common plasticizer and additive to provide transparency, flexibility, and durability properties to a polymer matrix, commonly found in food packaging, medical equipment, building materials, toys, etc. [21,22]. The annual world production of PTs as plasticizer is approximately 80% [21]. However, studies show a migration phenomenon from polymer matrix to element in contact causing health and environment impact. It is well known that exposure of PTs to human lives produces endocrine damage, and reproduction and carcinogenic effects [23]. According to the European Union and other organizations, specific PTs such as diisobutyl phthalate (DIBP) or diethylhexyl phthalate (DEHP) among others, are banned for contents above 0.1 wt.% [24]. Among other alternatives, poly(ethylene glycol) (PEG), polyethylene oxide (PEO), and adipates are widely studied in a PLA matrix, obtaining an excellent improvement of ductile properties [25–27]. However, petrochemical-based plasticizers are questioned for toxicity and therefore there is a continuous attempt to obtain bio-based plasticizers [28].

Vegetable oils (VOs) are an interesting route to achieve renewable plasticizers for three main reasons: they are widely available, have a low toxicity, and are biodegradable. Two reactive sites are identified in fatty acids of vegetable oils to bring compatibility with a polymer matrix: double bonds and ester groups [29]. To increase this compatibility, vegetable oil can be epoxidized, which consists of introducing epoxy groups (oxirane ring) in double bonds. Several studies reported the use of epoxidized vegetable oils in PLA matrix, thus obtaining a bio-based polymer with high performance as a plasticizer. Some epoxidized oils such as epoxidized linseed oil (ELO) and epoxidized soybean oil (ESBO) are available commercially at competitive prices. Several studies using epoxidized oil with a non-elevated number of oxirane groups have been reported. The study performed by Qiong Xu et al. reported an improvement of elongation at break from 3.98 to 6.5% using 9 wt.% of ESBO [30]. Further percentage led to a decrease in ductile properties due to plasticizer saturation. Garcia-Garcia et al. found an increment of elongation at break from 7.8 to 15% with 5 wt.% of epoxidized Karanja oil [31]. In respect to impact energy, an evident improvement of 32% was obtained, confirming an effective plasticization. More interesting results were found by Balart et al., who reported an increment of 450% of elongation at break with respect to neat PLA employing ELO with 8% of epoxy groups [32].

Chia seed oil (CO) is a promising VO characterized by its high iodine value (above 190 g I2/100 g oil) [33], becoming suitable to be epoxidized. Epoxidized chia seed oil has not been previously applied in a polymer matrix and could present an elevated epoxy content, improving the compatibility between PLA.

As different authors have reported that the interaction between PLA and epoxidized chia seed oil (ECO) could occur between the carbonyl group, from ester linkage, present in the PLA main chain and the epoxy group of ECO. This reaction mechanism was proposed by Al-Mulla et al. [34]. Although the interaction mechanism is not very strong, the terminal location of the hydroxyl groups in PLA gives them a high availability to react with the epoxy groups [30]. Thus, based on these previous studies, Figure 1 shows the chemical interaction between PLA and ECO. This new bio-based plasticizer could be an alternative to ELO and ESBO plasticizers. For this reason, the aim of this work is studying the potential of epoxidized chia seed oil as a new bio-based plasticizer for PLA to be used in the packaging

sector. Mechanical and thermal properties were tested to evaluate the effect of different percentage of ECO in a PLA matrix. A migration test was evaluated as an important property in the packaging sector. Finally, a disintegration test was carried out to evaluate the effect of ECO in polymer degradation. effect of different percentage of ECO in a PLA matrix. A migration test was evaluated as an important property in the packaging sector. Finally, a disintegration test was carried out to evaluate the effect of ECO in polymer degradation.

*Polymers* **2021**, *13*, 1283 3 of 17

alternative to ELO and ESBO plasticizers. For this reason, the aim of this work is studying the potential of epoxidized chia seed oil as a new bio-based plasticizer for PLA to be used in the packaging sector. Mechanical and thermal properties were tested to evaluate the

**Figure 1.** Schematic representation of chemical interactions between PLA and ECO. **Figure 1.** Schematic representation of chemical interactions between PLA and ECO.
