## 1 Introduction  
Polymers have become essential materials in our lives mainly due to their unique properties featuring lightness and durability and, as a consequence, their consumption has increased during the last decades [1]. However, the mass production of plastics and the limitation of non-renewable sources have led to problems with their final disposal and in end-of-life options [1; 2]. This has promoted the search for alternatives to the use of fossil-based polymers with materials that present better environmental performance, particularly for short term applications (i.e., food packaging, disposable cutlery, agricultural applications) [2; 3]. In recent years, biopolymers have risen as an alternative to traditional plastics, mainly for short term applications. Many research and industrial efforts have been focused on the development of sustainable polymers, mostly for single use disposal applications such as bottles, cold drink cups, thermoformed trays and container bids, blister packages, overwrap as well as flexible films, which are currently commercialized [2; 3; 4]. However, many commercially available biopolymers possess a low glass transition temperature, above which the polymeric matrix loses its rigidity leading to plastic deformation, which make them unsuitable for hot beverages and/or hot food applications. Biopolymers are materials whose synthesis involves micro-organisms and that have a reduced lifespan after final disposition [5,6]. Biopolymers can be produced naturally or obtained from biomass. They can be produced from: (i) wood derivatives such as cellulose [7,8], lignin [9] or terpen [10], (ii) lipids as fatty acids [11] and (iii) polysaccharides as sugar [12], chitin, chitosan [12] or starch [3,13]. Polysaccharides account for 75% of the world's annual biomass production, with 170 trillion metric tons. Starch is the second most abundant and available polysaccharide in nature [14]. It is composed of two D-glucose polysaccharides: amylose, a linear polymer and amylopectin, a branched polymer [3]. Starch can be obtained from various sources including cassava, wheat, rice, potato, pea, and corn. Corn is the largest source of starch in the world [3,14]. Starch is widely used in the production of biopolymers given its abundance and low cost [3,15]. Starch is used as a main polymeric matrix as well as minor component or filler in polymer blends [16]. Moreover, some bioplastics in the market are produced from starch, such as poly(lactic acid) (PLA) [2,4]. Starch in its native state does not exhibit a thermoplastic behaviour. Hence, to use starch in the plastic industry it is necessary to break down the structure of granular starch [17]. The molecular order of the starch granules is destroyed by converting its molecular structure to thermoplastic through the gelatinization process, changing its crystalline structure to an amorphous structure [14]. To produce plasticization in the starch structure these features are required: (1) temperatures greater than 70 \({}^{\circ}\)C, (2) presence of plasticizers (water or polyols) and, (3) shear stress [17,18]. These conditions can be achieved in the extrusion process, which will allow an amorphous paste, known as thermoplastic starch (TPS) to be obtained, where most of the inter-macromolecular hydrogen bonds of starch are destroyed [3]. TPS can melt and flow allowing it to be processed by conventional moulding and extrusion processing techniques used for most thermoplastic polymers [18]. Glycerol is the most widely used plasticizer for the production of TPS [3]. From an environmental point of view, glycerol is a by-product of biodiesel production and the re-volarization of glycerol increases its added value from a low-grade by-product to a useful biopolymer plasticizer [19]. TPS has gained industrial attention, compared with other biopolymers, due to its high availability and low cost, being one of the cheapest biopolymers in the market [5,15,16].