In recent years there has been brought to attention the demand for bio-derived resources because of overriding environmental contamination and diminishing fossil reserves worldwide (Willberg-Keyriläinen, Orelma, & Ropponen, 2018). At present, polyethylene (PE) has extensive application for packaging and low price which makes it the most broadly used thermoplastic. It is mostly obtained from petrochemistry. Polylactic acid (PLA), a biopolymer that has the potential to replace fossil-based materials, is becoming increasingly popular. However, its drawbacks are brittleness, low thermal stability, and a narrow processing window (Willberg-Keyriläinen et al., 2018).
Biopolymers like cellulose, starch, and chitin produce over 170 × 109 tons of biomass, of which cellulose is 35–50%. This concludes that cellulose is the most abundant biopolymer. It is possible to chemically modify it and the synthesis of cellulose allows for obtaining needed strength and biodegradability. However, its difficult structure and insolubility in traditional solvents are restricting its full potential for usage. (Gustavsson, Adolfsson, & Hakkarainen, 2020; Onwukamike, Grelier, Grau, Cramail, & Meier, 2018; Shibata, 2011; Willberg-Keyriläinen et al., 2018)
Cellulose esters are the most common derivatives. Many esters are commercialized e.g. cellulose nitrate (CN), cellulose acetate (CA), cellulose acetate propionate (CAP), and cellulose acetate butyrate (CAB). These materials have a shorter chain length of < C6. Cellulose acetate is used in a variety of industries, including medicine, textiles, and cigarette filters. To improve thermoplastic properties, long-chain esterification can be used. They are synthesized from longer side-chain fatty acids (≥ C6). (Onwukamike, Grelier, Grau, Cramail, & Meier, 2019; Shibata, 2011; Willberg-Keyriläinen et al., 2018). Longer fatty acid esters include a side-chain length from C6 to C18. Because of their high price and complicated production, they are not as widely used commercially. But there is a big potential for its use as bioplastics. They have higher strength, good water barrier properties and a wider temperature processing window before it goes into the thermal decomposition territory. (Willberg-Keyriläinen, Vartiainen, Harlin, & Ropponen, 2017).
Previous researches have shown the ductile behaviour as the side chain length and $ values increase (Crépy, Chaveriat, Banoub, Martin, & Joly, 2009; Onwukamike et al., 2018; Wen, Wang, Wei, Wang, & Liu, 2017). Generally, the ranges are as follows: elastic modulus (E), 150 to 350 MPa; tensile strength (σ), 8 to 20 MPa; tensile strain (ε), 5 to 65% (Crépy et al., 2009). The elastic modulus E values are lowered with a longer side chain, thus, is more ductile. Given result is coherent since longer chains restrict materials' movement. Additionally, the degree of substitution (DS) increases tensile strength and strain level until a certain point. However, newer source reports no major correlation between DS and its mechanical properties in fatty acid cellulose esters (FACEs) (Duchatel-Crépy et al., 2020).
For the packaging material, the hydrophobic properties are important and cellulose esters contact angle is over 89°, therefore, having the desired hydrophobic properties. Additionally, the values are reported to be similar to the commercially used polyethylene (i.e. LDPE, 91°) (Crépy et al., 2009). However, it is dependent on the ester’s composition. A higher degree of substitution and chain length increases the contact angle. (Crépy et al., 2009; Duchatel-Crépy et al., 2020; Willberg-Keyriläinen, 2020) When discussing the matter of the influence of the degree of substitution even further, the reason for the increase of contact angle value is the influence of aliphatic chains. Since the hydroxyl groups in cellulose are hydrophilic, when substituted with a hydrophobic aliphatic chain, it inhibits water absorption. (Wen et al., 2017) Especially if the hydrophilic groups are completely substituted.
Previous studies have been focusing on the different solvent systems to dissolve cellulose. However, the mechanical and surface properties (wettability and roughness) of the esters obtained by homogeneous transesterification with mTBNOAC ionic liquid is not thoroughly studied. In the present study, we aim to investigate the tensile and surface hydrophobicity properties of the novel synthesized cellulose derivatives films. The cellulose derivatives obtained were characterized by means of FTIR spectroscopy, contact angle and tensile test.