Introduction
The growing concerns on environmental issues motivate the development of biobased and biodegradable materials such as biocomposites. One of the sub-classes of biocomposites is all-cellulose composites (ACCs) (Nishino et al.
2004). The principle of ACC fabrication is based on the concept of all-polymer composites in which the matrix and the reinforcing fibers are made from the same matter (Capiati and Porter
1975). The advantage of this concept is that the adhesion between the matrix and the fibers is perfect, and there is no need of compatibilisers. The difference between all-polymer and all-cellulose composites is that cellulose is not melting, and thus ACC preparation requires a dissolution step followed by washing out the solvent and drying.
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) two-step 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 man-made fibers, aligned or isotropic (dispersed short fibers, a fabric, or a filter paper). Nanocellulose based composites will not be considered here as it is out of the scope of this work.
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 al.
2016; 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. For example, when aligned native ramie fibers were immersed in LiCl/DMAc for 2 h, exceptional longitudinal tensile properties of 460 MPa tensile strength and 28 GPa Young’s modulus were obtained (Soykeabkaew et al.
2008). One of the strongest man-made fibers, Bocell, was also used for making ACCs with volume fractions of up to 90% of fibers, resulting in an average tensile strength of 910 MPa and a Young’s modulus of 23 GPa with 8% elongation at break (Soykeabkaew et al.
2009). The above-mentioned mechanical properties far exceed the values reported for the traditional unidirectional natural fiber reinforced polymer composites. Nevertheless, the majority of the tensile properties of ACCs lies in the interval of 1–20 GPa for Young’s modulus and 50–200 MPa for tensile strength (Baghaei and Skrifvars
2020). The results depend on numerous parameters such as the origin of the reinforcing fibers, their concentration and alignment, and processing conditions (type of solvent, dissolution or impregnation time and temperature, drying mode, etc.) (Baghaei and Skrifvars
2020). It should be noted that the values obtained with tensile testing also depend on sample geometry and the method used to obtain the specimen elongation (directly from the tensile machine, from extensometer in which gauges are “clipped” on the material, using video extensometers or digital image correlation (DIC)). We will not consider here “contact” extensometer as it may induce damage or additional stress on the sample. Mechanical analysis will be performed using DIC technique; for example, it is commonly used for the analysis of the deformation mechanisms of polymers (Hild and Roux
2006). This method is accurate and provides crucial information on the strain field at macroscale level. It also allows measuring non-uniform deformation and 3D effects. However, DIC is rarely used for natural fiber reinforced polymer composites (Xu et al.
2019; Ramakrishnan et al.
2020) and, to the best of our knowledge, was never considered for the evaluation of the tensile properties of all-cellulose composites.
There are different ways to assess the tensile properties of a material. Below we recall the background equations determining nominal stress (σ
n) and strain (ε
n):
$$\sigma_{n} = \frac{F}{{A_{0} }}$$
(1)
$$\varepsilon_{n} = { }\frac{d}{{L_{0} }} = { }\frac{{L - { }L_{0} }}{{L_{0} }}$$
(2)
where F is force, A
0 is cross-section area of the sample, d is displacement, and L
0 and L are the initial length of the sample and the length at a given applied force, respectively. The displacement d can be monitored in different ways. One is using data obtained from the machine displacement sensor; in this work such method will be named “machine approach”. The other uses local measurement of sample displacement, i.e. either with video extensometer or DIC techniques; it will be named “local approach”.
Irrespective of the approach used, stress–strain dependences are built to characterize the material with Young’s modulus in the linear region, maximal stress (often called “tensile strength” or “ultimate stress”), elongation at break and elongation at maximal stress. The as-obtained results will include different levels of errors depending on the approach used. For instance, the measurement of displacement made with the machine approach includes not only the elongation of the sample in the gauge length region, but also the displacement induced by the stiffness of the tensile machine, the last one inducing erroneous mechanical properties (G’Sell et al.
1992). To exclude this problem which is common for all tensile testing machines, non-contact measurements of the displacement were developed. For example, video extensometer was introduced in the pioneering work of G’Sell et al. (G’Sell et al.
1992) to study highly deforming materials showing “necking”, the latter resulting in the alteration of the cross-section during experiment. However, the displacement of the specimen may not be homogeneous along the specimen length. The displacement measured using video extensometer may depend on the location of the marks on the sample and also on the length L which can be either sample length (Eq.
2) or the distance between selected two points. Finally, the most precise (but the most time-consuming) way of having a complete information on the deforming material is digital image correlation that allows obtaining a map of local strains on the specimen surface due to video cameras in stereovision which monitor the displacements of the markers (or speckles) randomly placed on the sample surface (Sutton et al.
2009; Candau et al.
2016).
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). In all examples mentioned above nominal stress–strain dependences were used to calculate Young’s modulus, tensile strength and elongation at break. The influence of the method on the stress–strain curves and on the values of Young’s modulus, tensile strength and elongation at break of all-cellulose composites has never been studied before.
The goal of this work was to analyse the tensile properties of all-cellulose composites applying local and machine approaches and understand the influence of each method on the values of the main mechanical characteristics of the material. Filter paper-based ACCs were produced using the ionic liquid 1-ethyl-3-methylimidazolium acetate ([EMIM][OAc]) as cellulose solvent, and the influence of the dissolution time on composite morphology, density, crystallinity and optical properties was investigated. Then, the tensile properties of ACCs were evaluated using data obtained directly from the machine and with a local technique, the latter using a two-cameras system and digital image correlation. Stress–strain data corresponding to various approaches were obtained and discussed together with the main tensile characteristics of the composite materials.
Conclusions
All-cellulose composites were made via controlled impregnation with ionic liquid, [EMIM][OAc]. First, the properties of the composites (density, porosity, crystallinity, cellulose II fraction, transmittance, haze and morphology) were investigated as a function of impregnation time. The results showed that cellulose was dissolved during the first 5 min, and further evolution of properties was due to the distribution of dissolved phase within the pores.
Tensile testing was conducted using different methods to obtain the specimen elongation: i) data directly taken from machine sensors and ii) using digital image correlation technique and local approach. The latter excludes the errors related to the stiffness of the tensile testing machine itself. Although the nominal tensile strength values from DIC and machine are similar, the nominal Young’s modulus, strain at maximal stress and, as a consequence, toughness, are very different. This must be taken into account when comparing results from different literature sources. ACC fabrication strategy using in this work resulted in the increase of tensile strength, Young’s modulus and toughness by almost 10, 5 and 25 times, respectively, as compared to the initial filter paper.
Publisher's Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.