Unidirectional All-Cellulose Composites from Flax via Controlled Impregnation with Ionic Liquid

Abstract

Mechanically strong all-cellulose composites are very attractive in the terms of fully bio-based and bio-degradable materials. Unidirectional flax-based all-cellulose composites are prepared via facile room-temperature impregnation with an ionic liquid, 1-ethyl-3-methyl imidazolium acetate. To determine the optimal processing conditions, the kinetics of flax dissolution in this solvent is first studied using optical microscopy. Composite morphology, crystallinity, density, the volume fraction of cellulose II and tensile properties are investigated, indicating that flax dissolution should be within certain limits. On the one hand, the amount of cellulose II formed through dissolution and coagulation should be high enough to “fuse” flax fibers, resulting in a density increase. On the other hand, only the surface layer of the fibers should be dissolved to maintain the strength provided by the inner secondary layer and avoid a detrimental decrease in crystallinity. The highest Young’s modulus and strength, 10.1 GPa and 151.3 MPa, respectively, are obtained with a crystallinity of 43% and 20 vol% of cellulose II.

Exploring digital image correlation technique for the analysis of the tensile properties of all-cellulose composites

“…Finally, the specimen was dried by hot pressing (around 50 bar, 80°C for 2.5 h and then at 60°C for 0.5 h). A temperature gradient during drying was shown to prevent the warping and/or heterogeneous contraction of the sample (Chen et al 2020a). ACCs produced with 5 min, 10 min, 30 min and 60 min impregnation are named P5, P10, P30 and P60, respectively.…”

“…We also plotted Young's modulus as a function of crystallinity (Figure S3); modulus decrease with crystallinity increase shows that the crystallinity does not play an important role here as it is the presence of pores and heterogeneities that control mechanical properties. It has been demonstrated in the previous studies that density plays the essential role in the mechanical performance of ACCs (Korhonen et al 2019;Chen et al 2020a); an adequate comparison with literature can thus be made only for the materials either of the same density or for the specific values of mechanical characteristics. Unfortunately, the density of ACC is not always reported.…”

“…There are two main routes for making all-cellulose composites: (1) one-step method via partial dissolution of fibers in cellulose solvent with the dissolved cellulose forming the composite matrix (Soykeabkaew et al 2008;Huber et al 2012;Piltonen et al 2016;Khakalo et al 2019;Chen et al 2020a) and (2) twostep method which involves the preparation of a cellulose solution used for the impregnation of the reinforcing fibers (Nishino et al 2004;Spörl et al 2017;Labidi et al 2019;Korhonen et al 2019). In both cases the reinforcing phase can be natural or manmade fibers, aligned or isotropic (dispersed short fibers, a fabric, or a filter paper).…”

“…Various cellulose solvents have been used to make ACCs: LiCl/dimethylacetamide (DMAc) (Nishino et al 2004;Soykeabkaew et al 2008), NaOH-water based solutions (Piltonen et al2016;Korhonen et al 2019) and, recently, ionic liquids (Spörl et al 2017;Khakalo et al 2019;Chen et al 2020a). For application reasons, the tensile properties of ACCs were always in the focus of most of the publications.…”

“…As far as all-cellulose composites are concerned, the majority of tensile properties were obtained using a machine method (see, for example, ref. Soykeabkaew et al 2008;Soykeabkaew et al 2009;Piltonen et al 2016;Sirviö et al 2017;Korhonen et al 2019;Wei et al 2020;Chen et al 2020a). Few works report the results obtained with a video extensometer (Kröling et al 2018;Mat Salleh et al 2017;Duchemin et al 2009).…”