## 1 Introduction  
Starch is one of the most abundant biopolymers and is used in many areas of industry [1]. It can be used as a biodegradable ecofriendly alternative to the conventional oil-based polymers, applied for example, in food packaging, agriculture and pharmacy. Due to the high glass temperature (T\({}_{\text{g}}\)) of starch close to its degradation, caused by strong H-bonding, its thermal processing is impossible. This polysaccharide, which consists of two polymer fractions, amylose and amylopectin, needs some additives that facilitate their processing. Small-molecule polar additives that decrease T\({}_{\text{g}}\), called plasticizers, disrupt H bonds between OH groups of polysaccharide chains. Newly formed H bonds between small plasticizers' molecules and OH groups of the polysaccharide lead to the production of thermoplastic starch (TPS). This process can be performed via starch gelatinization in water or via thermal-pressure processes, such as extrusion or thermocompression. Starch plasticizers, i.e., polyols (the most common being glycerol), sugars, amides or amines, ionic liquids or deep eutectic solvents (DESs) [2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16] can be applied. Urea (U) is amide compound that can be used for starch plasticization. However, when used as a single component as a starch plasticizer, U tends to recrystallize in the polymer matrix [17, 18, 19]; thus, it is mixed with other co-plasticizers [5, 13, 20, 21, 22, 23, 24, 25, 26]. It can form deep eutectic solvents with, for example, choline chloride (CC) [27, 21], betaine (B) [28] and polyalcohols [22] and also melts with sugars [29]. Deep eutectic solvents are mixtures with much lower phase-transition temperatures than their individual components [27]. Due to the application of urea as a fertilizer (nitrogen source), TPS plasticized with U can be used as agricultural bioplastics, facilitating fertilization [30; 31; 32; 33]. [PERSON] et al. prepared TPS films (from cassava), plasticized with U and glycerol with bagasse as a filler, via the casting method [31]. They studied, for example, the influence of the U content on the biodegradation rate, revealing that higher U content was correlated with higher biodegradability. The films with 50 wt% urea showed a total weight loss of 57% and a 95% release of the active compound after ca. 2 weeks. [PERSON] et al. prepared TPS/U via extrusion [32]. In their work, the influence of the TPS/U materials on plant growth (including the determination of such parameters as the fresh and dry matter of plants and their visual evaluation) was examined. The study revealed that the materials exhibited a stimulating effect on the growth and development of the cultivated model plants.  
Most of the urea-based DESs used for starch plasticization have a low molar ratio with the second component and there are no studies related to TPS with mixtures containing excessive U. Additionally, B (anhydrous and monohydrate) was chosen because there are few works related to B-based eutectic mixtures for polysaccharide processing [5; 34; 35; 36]. In this work, two types of DES, based on CC and B, with molar ratios CC or B to U up to 1:5, were prepared, thermally characterized (DSC, TGA) and used for potato-starch plasticization. The influence of the U content in the DESs on the TPS's mechanical properties (tensile test), morphology (XRD and FTIR) and sorption/dissolution behavior were investigated. Moreover, a dynamic thermal-mechanical analysis (DMTA) was performed on the TPS films. The DESs with the highest U content were selected for the preliminary test of extrusion. The TPS extrudates were characterized via mechanical tests, XRD and sorption behavior. The aim of these tests was to investigate whether U-rich DESs are able to plasticize starch via a more industrially acceptable continuous method of functional TPS-material production that could be used for agriculture or horticulture.