Electrohydrodynamic Processing of PVP-Doped Kraft Lignin Micro- and Nano-Structures and Application of Electrospun Nanofiber Templates to Produce Oleogels

Abstract

The present work focuses on the development of lignin micro- and nano-structures obtained by means of electrohydrodynamic techniques aimed to be potentially applicable as thickening or structuring agents in vegetable oils. The micro- and nano-structures used were mainly composed of eucalyptus kraft lignin (EKL), which were doped to some extent with polyvinylpyrrolidone (PVP). EKL/PVP solutions were prepared at different concentrations (10–40 wt.%) and EKL:PVP ratios (95:5–100:0) in N, N-dimethylformamide (DMF) and further physico-chemically and rheologically characterized. Electrosprayed micro-sized particles were obtained from solutions with low EKL/PVP concentrations (10 and 20 wt.%) and/or high EKL:PVP ratios, whereas beaded nanofiber mats were produced by increasing the solution concentration and/or decreasing EKL:PVP ratio, as a consequence of improved extensional viscoelastic properties. EKL/PVP electrospun nanofibers were able to form oleogels by simply dispersing them into castor oil at nanofiber concentrations higher than 15 wt.%. The rheological properties of these oleogels were assessed by means of small-amplitude oscillatory shear (SAOS) and viscous flow tests. The values of SAOS functions and viscosity depended on both the nanofiber concentration and the morphology of nanofiber templates and resemble those exhibited by commercial lubricating greases made from traditional metallic soaps and mineral oils.

Effect of Solution Properties in the Development of Cellulose Derivative Nanostructures Processed via Electrospinning

“…As was previously mentioned, the electrospinning process depends on many variables and physicochemical properties of solutions, such as viscosity, surface tension, and electric conductivity. In particular, the viscosity of the solution can be adapted by modifying the polymer concentration [29][30][31] and in fact, some correlations were applied in order to find the minimal concentration that leads to adequate viscosity values to perform the electrospinning process. Table 1 displays the values of shear viscosity of different solutions of AC and EC, as well as other physicochemical parameters, namely, extensional viscosity, density, surface tension, and conductivity.…”

“…On the one hand, the main factor is due to the molecular weight, since EC has a molecular weight of 45,000 g.mol −1 , while CA has a molecular weight of less than 30,000 g.mol −1 . As is well known, systems made with a polymer with a higher molecular weight have higher viscosities, in addition to causing the displacement towards lower concentrations of the critical concentration (Ce) from which these systems are in the semi-dilute entangled regime and can generate micro and nanofibers during the electrospinning process [31,33].…”

Impact of the Morphology of Electrospun Lignin/Ethylcellulose Nanostructures on Their Capacity to Thicken Castor Oil

“…This fact could be related to the polar character of lignin, due to the presence of phenolic and aliphatic hydroxyl and carboxyl groups as well as substructures based on β-O-4′alkyl ethers in its chemical structure [ 19 ]. In addition, the increase in conductivity with the LSL concentration suggests that these solutions are below the concentration at which the macromolecules start to overlap (overlap concentration), delimiting the transition between the dilute and semidilute regime, from which the conductivity decreases due to the reduced mobility of the entangled polymeric molecules [ 43 ]. Moreover, the higher the lignin proportion in the blend, the lower the surface tension of the LSL:EC solutions (see Table 1 ).…”

Lignocellulosic Materials for the Production of Biofuels, Biochemicals and Biomaterials and Applications of Lignocellulose-Based Polyurethanes: A Review

“…The production of nanofibres from lignin by using the electrospinning technique has likewise permitted the use of these materials as oil structuring [ 189 ]. In addition, not only liquid lubricants but also semi-solid ones, i.e., lubricating greases, have been developed by using lignin [ 190 , 191 ].…”