# Structural Features and Properties' Characterization of
## 1 Introduction  
Polymers have firmly entered our lives as constructional and special materials, successfully competing with and replacing traditional materials, such as glass and metals. They are especially well-suited for the production of packaging materials due to their availability, relevant properties, and manufacturability. However, certain operational properties of synthetic polymers turn from being advantages to drawbacks when the polymers are recycled or scrapped. Unlike organic natural materials, polymers do not degrade for a very long time (80-100 years) and remain almost in their \"initial\", stable state without decomposing, due to their resistance to chemical, atmospheric, and biological elements. Packaging materials and plastic wrappers have become quite a problem because of the huge manufacturing volumes of such products and their rather short operation life. One solution to the problem of polymer waste can and should be the manufacture of biodegradable plastics that degrade under natural conditions. Presently, one of the most common bioplastics is polylactide (PLA), a synthetic polymer of lactic acid. However, the unmodified PLA is brittle, with low impact elasticity and break elongation. Its properties are similar to those of another relatively brittle polymer--polystyrene [1, 2, 3]. These properties significantly limit the application of PLA in the polymer packaging manufacture. This flaw in the mechanical properties is eliminated by combining the PLA with other polymers [4] and elastomers [5, 6, 7, 8], primarily natural rubber (NR), which is a natural biopolymer [9]. The PLA-NR based composites have been extensively studied; however, all the tests havepointed out the problem of insufficient compatibility among these polymers. A partial solution to this problem is to introduce epoxidized natural rubber [10; 11; 12; 13] and its modifications [14; 15; 16], use functional modifiers in the process of reaction mixing [17; 18], and pre-treat NR in order to increase its content of oxygen-containing groups [18; 19]. However, all these methods require rather expensive materials.  
Commercial PLA and its blends have been modified by the wide spectrum of plasticizers and compatibilizers where the synthetic chemicals dominate. However, to preserve the environment, the non-toxic and eco-friendly modifiers should be used, especially in such a delicate area of implementation as food packaging. Replacing petroleum-based plasticizers and compatibilizers with bio-based ones obtained from natural renewable resources, such as palm oil [20], epoxidized cottonseed oil [21], and limonene from the main component of citrus oil [22], could be an attractive alternative not only for environmental sustainability but also for circular economy trends [23].  
This paper first covers our research of the use of epoxidized soybean oil (ESO) as a simultaneous compatibilizer and plasticizer for the PLA-unmodified NR-based compound. Epoxidized soybean oil provides excellent performance to elastomers and plastics. ESO is a non-toxic ingredient; therefore, it is used in the production of medical devices and food packaging. ESO is also a product made from renewable plant materials. In polymer compositions, ESO is characterized by low volatility and high migration resistance. The presence of a large number of oxygen-containing groups should ensure compatibility of the PLA-NR blend as the potential candidate for innovative packaging.