**1. Introduction**

Polylactic acid (PLA) is one of the most widely consumed biodegradable and compostable polymers. In the field of medicine, it is used for its excellent compatibility with the human body [1]. However, its low toughness makes it necessary to modify it by incorporating additives to obtain a more ductile material, with better barrier properties, higher hydrophobicity, and higher stability to temperature and external agents (UV, humidity, etc.). Its modification allows greater applicability, there being several industrial sectors

**Citation:** Aldas, M.; Ferri, J.M.; Motoc, D.L.; Peponi, L.; Arrieta, M.P.; López-Martínez, J. Gum Rosin as a Size Control Agent of Poly(Butylene Adipate-Co-Terephthalate) (PBAT) Domains to Increase the Toughness of Packaging Formulations Based on Polylactic Acid (PLA). *Polymers* **2021**, *13*, 1913. https://doi.org/10.3390/ polym13121913

Academic Editor: Łukasz Klapiszewski

Received: 11 May 2021 Accepted: 7 June 2021 Published: 8 June 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/).

in which it can be used, among them agriculture [2], packaging [3,4], medical fields [5], 3D printing [6], textile fibers [7], and composites [8–10].

The PLA processing method with the more favorable industrial viability is its physical mixing or blending [11,12]. The modification of biodegradable polymers through physical blending with another biodegradable polymer shows many advantages since it offers the opportunity to create a new material with desired properties. Moreover, it is relatively simple and cost-effective to blend polymeric materials in the melt state, based on available processing technologies commonly used at the industrial level (i.e., extrusion, injection molding, film-forming, etc.) [13]. Many biodegradable polymer formulations have been obtained by blending polymeric matrices to modulate their mechanical, thermal, rheological, and morphological behavior. The literature refers to blends of PLA with other polymers or copolymers such as poly(ε-caprolactone) (PCL) [14], thermoplastic starch (TPS) [15], poly(butylene succinate) (PBS) [16], poly(butylene succinateco-adipate) (PBSA) [17], poly(glycolic acid) (PGA) [18], poly(hydroxybutyrate) (PHB) [12,19], poly(hydroxybutyrate-co-valerate) (PHBV) [20], and poly(butylene adipate-co-terephthalate) (PBAT) [21], among others. Among these, PBAT has gained interest in the development of PLA/PBAT blends intended for film manufacturing due to its high flexibility [22] and its inherent biodegradable character [23]. However, in most of the scientific works reported up to now, poor miscibility or total immiscibility between components of the blend was observed and the expected synergism to improve the overall properties was not achieved.

Currently, additives can be used to increase the miscibility between polymers through various modification mechanisms that seek to increase either the interaction between the different polymeric phases through compatibilization, the plasticization of one of the components to increase the free volume, which facilitates the miscibility of a second component, or interaction through free radicals (reactive mixing). The components of the blend, together with the reactive agent, undergo a reaction and molecular chemical change that influence the mechanical and thermal properties. Wang et al. formulated PLAbased blends with different PBAT contents, additivated with 0.75% of Joncryl ADR 4368. As a result, the compatibility of both polymers was increased. Specifically, the elongation at break was increased by 18% and the impact absorption energy went from 4.85 kJ/m<sup>2</sup> for the PLA/20PBAT to 5.21 kJ/m<sup>2</sup> for the formulation 80PLA-20PBAT\_0.75Joncryl [24]. Arruda et al. also used Joncryl ADR-4368 on PLA-PBAT blends and demonstrated its compatibilizing effect. The formulation 60PLA-40PBAT-0.6Joncryl increased the elongation at break by 1200% (compared to the percentage of elongation of neat PLA). Such a compatibilizer acts as a crosslinking and/or branching agent for both polymers, providing higher strength and Young's modulus to each of the polymers separately, although it increases the ductility of all the studied formulations [25]. Wu et al. added 0.3, 0.5, and 0.75 wt.% of 2,5-bis (tert-butyl peroxy) -2,5-dimethyl hexane (Luperox 101), observing a crosslinking reaction and some interaction between the components (PBAT and PBS) of the ternary PLA-based blend. By adding 0.3 phr of Luperox 101 to the PLA/20PBAT/20PBS formulation, it was possible to increase by up to 10 times the impact energy absorbed, with respect to the polymeric matrix without additives, considerably reducing the glass transition temperature, Tg, of PLA and increasing the elongation at break [26]. On the other hand, some natural plasticizers such as vegetable oils (VO), obtained from seeds, are presented as an effective alternative, sometimes acting as compatibilizers [27,28]. Their use in the packaging sector is of interest due to the high resistance of plasticizers to migration in food contact conditions [29,30], being able to increase the solubility of the blend components and therefore, act as a compatibilizer in polymeric blends. These can be chemically modified allowing greater interaction with the polymeric chains thanks to the added functional groups, reacting with polar groups such as hydroxyl, carboxylic, etc. [31,32]. According to Bocque et al., an efficient plasticizer has to be able to increase the molecular free volume and be endowed with ester groups (reactive functional groups that provide cohesion) and aromatic groups that increase its compatibilizing effect [33]. In this sense, Carbonell-Verdú et al. compatibilized PLA blended with 20 wt.% of PBAT by using cottonseed oil-based

derivatives [22]. They observed that the low miscibility of PLA/PBAT could be improved by compatibilization with epoxidized cottonseed oil (ECSO) and maleinized cottonseed oil (MCSO). Moreover, both additives were able to considerably increase the elongation at break of the PLA/PBAT blend without compromising mechanical strength.

On the other hand, colophony or gum rosin (GR) and its derivatives have gained interest in the field of polymeric materials as highly versatile and multifunctional natural additives, both with synthetic plastic matrices and with biodegradable matrices [34–36]. For example, Arrieta et al. used a gum rosin ester as a natural viscosity increasing agent in a blend based on polyvinyl chloride (PVC) plasticized with epoxidized linseed oil (ELO). In that study, the gum rosin derivative showed good compatibility with the PVC synthetic matrix. Furthermore, it was verified that a composition between 40 and 50 phr of rosin ester present in PVC can increase the viscosity of the blends up to 10 times [37]. On the other hand, in previous work, the effect of GR and two gum rosin esters on the properties of a commercial blend of TPS, PBAT, and PCL was studied, and the versatility of the resin and its derivatives were verified. Furthermore, the GR acted as a plasticizer, and on the other hand, the gum rosin esters provided a solubilizing and compatibilizing effect of the biodegradable blends. This behavior influenced the properties of each of the studied formulations, especially in those with 15 wt.% of GR where the processing temperature was reduced by up to 50 ◦C and the toughness increased up to 500%, compared to the neat polymeric matrix [38]. Finally, when the interaction between gum rosin and gum rosin derivatives with Mater-Bi type bioplastic was studied through microscopic techniques, the improvement of the miscibility of the components and the solubility effect conferred to the PBAT phase thanks to the compatibilizing effect of the GR and its derivatives were confirmed [39,40].

The main objective of this work is to compatibilize PLA/PBAT-based blends with GR and to study the plasticizing effect conferred by the GR on the PLA/PBAT binary blends, focusing on the use of a natural resin as the main novelty of the present work. The processability aspects, as well as the main mechanical properties of the formulations, the thermal stability, and the barrier properties were assessed. In addition, the microstructure was studied by field emission scanning electron microscopy (FESEM) which allowed to see the interface of both polymers and to evaluate the plasticizing effect of GR on the toughness of the PLA/PBAT blend.
