Cellulose-polyhydroxylated fatty acid ester-based bioplastics with tuning properties: Acylation via a mixed anhydride system

Highlights

• New cellulose esters were synthesized by condensation of cellulose and aleuritic acid.

• The mechanical behavior was changed from rigid to ductile, keeping the same toughness.

• These materials show similar properties to commercial cellulose derivatives.

Abstract

The synthesis of microcrystalline cellulose (MCC) and 9,10,16-hydroxyhexadecanoic (aleuritic) acid ester-based bioplastics was investigated through acylation in a mixed anhydride (trifluoroacetic acid (TFA)/trifluoroacetic acid anhydride (TFAA)), chloroform co-solvent system. The effects of chemical interactions and the molar ratio of aleuritic acid to the anhydroglucose unit (AGU) of cellulose were investigated. The degree of substitution (DS) of new polymers were characterized by two-dimensional solution-state NMR and ranged from 0.51 to 2.60. The chemical analysis by attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) confirmed the presence of aleuritate groups in the structure induces the formation of new H-bond networks. The tensile analysis and the contact angle measurement confirmed the ductile behavior and the hydrophobicity of the prepared bioplastics. By increasing the aleuritate amounts, the glass transition temperature decreased and the solubility of bioplastic films in most common solvents was improved. Furthermore, this new polymer exhibits similar properties compared to commercial cellulose derivatives.

Introduction

Petroleum-based synthetic polymers are versatile and multifunctional materials for the large-scale production of commodities for the growing population of our planet. They present very valuable features such as an attractive cost/performance ratio, a wide range of physical and chemical properties, low weight, and facile processability (Heredia-Guerrero and Athanassiou, 2016, Mülhaupt, 2013). In fact, the global production of plastics in 2015 was ∼322 million t, highlighting the demand for common polymers like polypropylene, polyethylene, polyvinyl chloride, polyurethanes, polyethylene terephthalate and polystyrene (PlasticsEurope, 2016). However, the accumulation of such non-biodegradable materials is causing important problems in the environment. This has stimulated the development of polymers with a high degree of degradability (Tokiwa, Calabia, Ugwu, & Aiba, 2009).

Cellulose is the most common renewable and biodegradable polymer with an estimated annual biomass production between 10¹¹ and 10¹² t (Klemm, Philipp, Heinze, Heinze, & Wagenknecht, 2004). It is mainly produced by plants and forms part of their cell walls as a structural component (Cosgrove, 2005). Cellulose is a polydisperse, linear and syndiotactic polysaccharide whose basic repeating unit is anhydroglucose (AHG) (Varshney & Naithani, 2011). It is used in different forms such as a construction material, natural fibers for textiles, paper or cardboard (Varshney & Naithani, 2011). Cellulose is inherently a hydrophilic and highly water-absorbing material, which is a serious hint for becoming an alternative for traditional plastics. It also displays a low barrier capacity against water and gases and it cannot easily be processed since it is infusible and insoluble in most solvents. Usually, this is overcome by the chemical modification of cellulose. Indeed, there is a vast number of chemical functionalizations of cellulose such as esterification, etherification, silylation, amination, etc. (Klemm, Heublein, Fink, & Bohn, 2005; Meng & Edgar, 2016). In particular, cellulose esters are commercially exploited in several applications (Edgar et al., 2001). For instance, they are used in waterborne coatings, radiation curable coatings, controlled release systems, plastics, resins, composites, optical films, and membranes for separation processes (Edgar et al., 2001). Specifically, long-chain cellulose esters have been studied in order to improve the thermal processability of these derivatives since the temperatures of melting and thermal decomposition are very close for short-chain cellulose esters (e.g., cellulose acetate, cellulose acetate propionate and cellulose acetate butyrate). In general, such long-chain cellulose esters are produced by a variety of chemical syntheses both in solution or heterogeneous mixture (Edgar et al., 2001, Heinze and Liebert, 2001, Heinze et al., 2003; Meng, Matson, & Edgar, 2014a, 2014b; Meng, Roy Choudhury, & Edgar, 2016; Meng, York, Liu, & Edgar, 2015). Among the diverse possible acylations in solution, the mixed anhydride system (traditionally known as “impeller method”) consists of the formation of reactive intermediates (the above-mentioned mixed anhydrides), which can subsequently react with the hydroxyl groups of cellulose and produce the corresponding cellulose esters (Tristram, Mason, Williams, & Hinkley, 2015). When trifluoroacetic acid anhydride (TFAA) is used as a promoter of the reaction, this methodology shows several advantages such as short reaction times, mild reaction conditions, reagents saving and easy isolation of final products (Huang, 2012). The reactions involved in the mixed anhydride system using TFAA are schematized in Fig. S1A. Typically, trifluoroacetic acid (TFA) and chloroform are used as solvents when TFAA is employed as a promoter (Huang, 2012). TFA can solubilize cellulose by the trifluoroacetylation of its hydroxyl groups. When TFA is removed, the trifluoroacetyl groups are readily hydrolyzed by the humidity in the environment (Bayer et al., 2014, Liebert et al., 1994). The ability of cellulose to be solubilized by TFA has been employed to produce different blends with natural or synthetic polymers (Guzman-Puyol et al., 2016, Tran et al., 2016).

Most of the molecules used for the synthesis of long-chain cellulose esters are saturated fatty acids ranged between C₆ and C₂₀ (Edgar et al., 2001). There are relatively few studies about the use of fatty acids with other functionalities, with little investigation into the use of unsaturated (Nakagami, Amimoto, & Yokota, 1974; Uschanov, Johansson, Maunu, & Laine, 2011) and ω-carboxyesters (Liu, Ilevbare, Cherniawski, Ritchie, Taylor, & Edgar, 2014) fatty acids. However, the presence of other additional chemical groups, such as hydroxyl groups, could directly affect the final properties of cellulose esters (Meng and Edgar, 2015, Meng et al., 2015). In fact, saturated hydroxy fatty acids have a higher reactivity, solubility, and viscosity compared to saturated fatty acids and are used in pharmaceutical, chemical, food, and cosmetic industries (Kim & Oh, 2013). 9,10,16-trihydroxyhexadecanoic or aleuritic acid is a polyhydroxylated fatty acid employed as a substitute for α-hydroxy acids (e.g., glycolic and lactic acids) in cosmetic formulations (Kokate, Purohit, & Gokhale, 2008), food additive (Sagis, 2015), and coating material for tablets and capsules (Perale & Hilborn, 2017). It is a renewable material isolated from shellac, a natural lac resin (Benitez et al., 2016). Currently, its annual commercial production is ∼150 t per year (Pal, Jaiswal, Bhattacharya, & Yogi, 2009).

In this work, we have synthesized novel cellulose esters by esterification of aleuritic acid with microcrystalline cellulose at different molar ratios. The reaction was based on the mixed anhydride synthetic route using a mixture of TFAA and TFA. Free-standing films were obtained after the evaporation of the solvents. The effect of the degree of substitution on the thermal and mechanical properties as well as on the solubility, wettability and water uptake of films was studied. Furthermore, the hydrophobicity and thermal and mechanical properties of these materials were compared with those of other important cellulose derivatives.