**1. Introduction**

Since the end of the 20th century, environmental concerns and the development of sustainable strategies have been on the rise [1]. This concern carries over to the present day, where plastic represents a large part of human life, with up to 368 million tonnes of plastic produced globally in 2019 [2]. The problem arises mainly because of the negative impact on the ecosystems generated by the amount of plastic waste, since only 173 million tonnes are collected for recycling or landfill [3], but also by the exploitation of non-renewable natural resources, such as petroleum. The problem is magnified in the packaging industry, as it is the largest consumer of plastics, accounting for 39.6% of total plastic consumption in Europe [2]. In view of this situation, research is mainly focused on obtaining polymeric materials with formulations based on renewable resources or even with the property of being biodegradable. A wide variety of biopolymers or bio-based materials have now been obtained that contribute to this sustainable development.

Bio-based polymers are polymers that are made from biological substances, i.e., nonfossil materials [4]. These materials may or may not be biodegradable depending on their ability to be broken down by microorganisms [4]. Currently, some of the most promising biopolymers are extracted from biomass (starch, cellulose, protein, chitin, etc.), such as thermoplastic starches (TPSs), given the abundance of polysaccharides in the biomass. Another promising alternative are also those obtained by microbial production, such

#### **Citation:** Perez-Nakai, A.;

Lerma-Canto, A.; Domingez-Candela, I.; Garcia-Garcia, D.; Ferri, J.M.; Fombuena, V. Comparative Study of the Properties of Plasticized Polylactic Acid with Maleinized Hemp Seed Oil and a Novel Maleinized Brazil Nut Seed Oil. *Polymers* **2021**, *13*, 2376. https://doi.org/10.3390/ polym13142376

Academic Editors: Andreia F. Sousa, José Miguel Ferri, Vicent Fombuena Borràs and Miguel Fernando Aldás Carrasco

Received: 30 June 2021 Accepted: 16 July 2021 Published: 20 July 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/).

as polyhydroxyalkanoates (PHAs) and polyhydroxybutyrate (PHB) or polylactic acid (PLA) [5–7].

In 2020, bioplastics accounted for only 1.2 million tonnes, less than 1% of all plastics produced [3]. However, the market for these polymers has been growing for years and the trend is expected to increase, with production estimated to increase by 11.28% from 2019 to the end of 2025 [8]. Specifically, PLA is one of the biopolymers with the greatest potential for industrial use as a substitute for petroleum-based polymers [9,10]. This is due to its great potential due to the combination of its mechanical properties, easy processability and its price compared to other biopolymers [11,12]. PLA has a suitable thermal stability and resistance to being industrially processed by injection molding, welding, thermoforming or extrusion [13]. Moreover, as it is biodegradable, one of its most interesting applications is mainly in the packaging sector [14]. In terms of mechanical properties, this material is comparable to non-degradable polymers in the "commodity" range [14,15]. However, there is a major limitation of PLA, its brittleness, which is a major drawback in the packaging industry [16]. To alleviate this disadvantage, there are several proposals. One solution to improve the flexibility of this polymer is its blending with other polymers such as polyethylene glycol (PEG), thermoplastic starch (TPS) or polybutylene succinate-co-adipate (PBSA), among others. However, the lack of miscibility between the components makes it difficult to achieve this improvement in toughness [5,11,17,18]. For this reason, some authors proposed modified vegetable oils (MVOs) as compatibilizing agents or even as plasticizing additives [12,19]. The reason for this is that they are a renewable and sustainable substitute for synthetic modifiers [20,21], which are also respectful of human health due to their non-toxicity, as they do not generate the migration of substances such as Bisphenol A (BPA), as is the case with conventional epoxy resins used in consumer products [22].

Several techniques can be used for chemical modification, such as epoxidation [23,24], maleinization [5,10,25], acrylation or hydroxylation. The MVOs available on the market today are mainly epoxidized soybean oil (ESBO) [25] and epoxidized linseed oil (ELO) [26]. Authors such as Garcia-Garcia et al. or Chieng et al. [26,27] have reported the improvement of PLA stiffness properties with these oils. Additionally, other authors investigated other modified oils for PLA formulations such as epoxidized cottonseed oil (ECSO) [24] or epoxidized palm oil (EPO) [27]. Another commercial option is maleinized linseed oil, which has provided excellent properties to PLA, as reported Ferri et al. [28].

In this work, the chemical modification of process of oils carried out is the maleinization. Maleinization is a chemical process, usually carried out in a single step, which consists of incorporating maleic anhydride molecules into the triglycerides that make up vegetable oils when conjugated carbon–carbon double bonds are present. For this purpose, there are several methods that can be used, such as the so-called "ene" reaction, Diels-Alder addition and free radical copolymerization, the first being the most favorable and the one used in this work [5,29]. This procedure requires a temperature of about 200 ◦C to result in the addition of the anhydric groups at the allylic positions of the fatty acid, as shown in Figure 1.

Both Brazil nut (*Bertholletia excelsa*) and hemp seed (*Cannabis sativa* L.) are interesting as MVOs, since both have an interesting lipid profile for functionalization. The Brazil nut is a brown fruit generally cultivated in the Amazon [30]. Its oil is an interesting object of study, since it has between 60.8% and 72.5% lipids [30,31]. Among them, it has 75.6% unsaturated fatty acids (UFA) [30], and both monounsaturated (MUFA) and polyunsaturated (PUFA), which means a significant amount of double bonds that allow its functionalization by chemical processes such as maleinization. On the other hand, hemp seed oil is also attractive for this purpose, as it is high in linoleic acid (55.3%) and linolenic acid (20.3%) [32]. Both allow their oil to be extracted by cold pressing with a good yield and also have good oxidative stability [31,33]. Thereupon, this work evaluates the potential of MBNO and MHO as a bio-based plasticizer to improve the ductile properties of PLA

related to its brittleness and the comparison of these with commercial maleinized linseed oil (MLO). MBNO and MHO as a bio-based plasticizer to improve the ductile properties of PLA related to its brittleness and the comparison of these with commercial maleinized linseed oil (MLO).

also have good oxidative stability [31,33]. Thereupon, this work evaluates the potential of

*Polymers* **2021**, *13*, x FOR PEER REVIEW 3 of 18

**Figure 1.** Schematic representation of the maleinization process of the triglyceride presents in vegetable oil. **Figure 1.** Schematic representation of the maleinization process of the triglyceride presents in vegetable oil.

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

LLC (Minnetonka, MN, USA) in pellet form.
