Monitoring of cellulose depolymerization in 1-ethyl-3-methylimidazolium acetate by shear and elongational rheology

Highlights

• Highlight the efficiency of ionic liquids as direct cellulose solvent and their potential to produce new cellulose products from biomass.

• Characterize the depolymerization of cellulose in 1-ethyl-3-methylimidazolium acetate upon storage at high temperature via determination of the intrinsic viscosity and molar mass distribution.

• Analyze the kinetics of cellulose degradation with two models.

• Investigate the effect of cellulose degradation on visco-elastic properties of cellulose-IL solution by shear rheology.

• Elucidate the effect of cellulose depolymerization on the elongational visco-elastic properties of the cellulose solutions using a capillary break-up extensional rheometer (CaBER). The methodology to process CaBER data of solutions with a high cellulose concentration is explained in detail. To the best of our knowledge, no similar study is available.

Abstract

The thermal stability of cellulose in the ionic liquid (IL) 1-ethyl-3-methylimidazolium acetate, [emim]OAc was investigated. For this purpose, Eucalyptus urugrandis prehydrolysis kraft pulp was first dissolved in [emim]OAc by means of a vertical kneader and then stored at three different temperatures to study the time-depended behavior of the cellulose-[emim]OAc system. Cellulose depolymerization was assessed by characterizing the precipitated cellulose and the rheological behavior of the cellulose-[emim]OAc solutions. The results show decreases in the weight average molecular mass and in the shear viscosity at temperatures exceeding 60 °C, which can be related to progressing degradation of cellulose in the IL upon storage at elevated temperature. The changes in behavior of the solutions under extensional stresses also attest the gradual depolymerization of cellulose. The degradation has been analyzed using appropriate kinetic models. Propyl gallate appeared to be an efficient stabilizer of the cellulose-[emim]OAc system during the dissolution step even though the mechanism has not been fully understood yet.

Introduction

Over the last decades, the demand for consumption goods has significantly increased due to the world population growth and a rise in per-capita consumption, especially in developing countries. Concomitantly, growing concerns regarding the use of depleting crude oil feedstock as well as an increasing awareness towards sustainable resource management has raised the interest in the use of biodegradable and organic renewables. Cellulose being the most abundant natural polymer on earth offers opportunities as raw material for many applications.

The market for sustainable cellulose products such as films, foils, composite or man-made cellulosic (MMC) fibers, is expanding worldwide. In 2012, the total fiber consumption was 83.5 million tons (The Fiber Year, 2013) of which 33% were cellulosic fibers (cotton, Viscose/Rayon, Modal and Tencel® fibers). In 2012, cotton represented 82% of the total consumption of cellulosic fibers. However, owing to limited cotton production and thus increasing cotton costs, the cotton output decreased by 5.4% over the period 2011–2012. Moreover, the large amount of water required for its production and the extensive use of artificial fertilizers and pesticides can make cotton an unsustainable cellulosic fiber raw material (The Fiber Year, 2013). A study for the period 2010–2030 predicts an annual fiber production of 133.5 million tons in 2030 with an estimated share of 33 to 37% of cellulosic fibers, thus raising the demand of cellulosic fibers to 44–49 million tons in 2030. Assuming a maximum cotton production of 26 million tons, 18 to 23 million tons of MMC fibers will be needed to fill the so-called cellulose gap (Eichinger, 2012).

Nowadays, the market of man-made cellulosic fibers, cellulosic films and composites is still dominated by the viscose process that uses and forms highly toxic chemicals and gasses and, consequently, may pose a threat to the environment. The more environmentally acceptable lyocell process, commercialized in the early 1990s, using the non-derivatizing solvent N-methylmorpholine N-oxide (NMMO) has been so far the only viable alternative to the viscose process (Fink, Weigel, Purz, & Ganster, 2001). However, the thermal instability of NMMO demands important investments in safety technology to avoid thermal run-away reactions and cellulose degradation. Moreover, films manufactured with the viscose process show inferior barrier and mechanical properties compared to synthetic films. Therefore, new industrial technologies for the processing of cellulose in direct solvents are desired to manufacture cellulosic material with outstanding properties and to make their production cost- and eco-efficient (Hermanutz et al., 2008, Perepelkin, 2007). Ionic liquids that have been presented as a novel class of direct solvents for cellulose have been thoroughly investigated during the last few years. They appear to be a promising alternative to NMMO and offer new possibilities for cellulose processing (Cao et al., 2009, Feng and Chen, 2008, Kosan et al., 2008, Swatloski et al., 2002). Their low melting point, low vapor pressure, thermal stability and their ability to dissolve cellulose of high DP make ILs advantageous for industrial processes (Pinkert, Marsh, Pang, & Staiger, 2010). Furthermore, the possibility of preparing mixtures of cellulose with different substrates or activation of cellulose in ILs makes them attractive especially for the production of biocomposite (Simmons et al., 2011, Stefanescu et al., 2012, Wu et al., 2009).

The stability of dissolved cellulose in ionic liquids upon thermal impact is of special importance for the processing conditions of shaped cellulosic products such as films, composites or fibers. The high temperature used during the different preparation steps, from the dissolution until the regeneration of cellulose, may cause cellulose depolymerization (Haward et al., 2012, Wendler et al., 2009). The intrinsic viscosity measurement is a fast and convenient method to estimate the average DP of cellulose and thus the extent of degradation. However, the determination of DP provides only some information on the viscosity-average molar mass and no element regarding the molar mass distribution (MMD). A polymer cannot be fully characterized by a single molecular mass value; it is well defined by a molar mass distribution which is one of the key factors affecting the rheology of a visco-elastic cellulose–IL solution and, thus, its processability during coagulation (Collier et al., 2009, Dupont and Mortha, 2004). The assessment of cellulose degradation through the determination of the MMD requires the regeneration of the dissolved cellulose in ILs. This method is consequently time-consuming and necessitates being performed offline between the dissolution and shaping regeneration steps. Unlike the traditional methods to quantify the intrinsic properties of dissolved cellulose, rheological measurements offer the possibility of on-line monitoring the extent of cellulose depolymerization. Visco-elastic properties of cellulose–IL solution are dependent of the DP and MMD of dissolved cellulose. Thus, the change of the complex viscosity or the extensional viscosity of cellulose–IL can be used to determine whether the cellulose degrades during the process (Lu, Cheng, Song, & Liang, 2012). Moreover, shear and extensional deformations play a major role in cellulose processing operations which involve rapid change of shape such as fiber spinning or film blowing. The assessment of the rheological properties of cellulose–IL solution is, hence, a prerequisite for successful cellulose processing (Sammons et al., 2008a, Sammons et al., 2008b, Sammons et al., 2008a, Sammons et al., 2008b). Rheological characterization of cellulose–IL solutions is thus an important method for monitoring cellulose degradation and fundamental for the subsequent processing of polymer solutions.

In this study, we use Eucalyptus urugrandis prehydrolysis kraft pulp dissolved in 1-ethyl-3-methylimidazolium acetate to demonstrate how cellulose depolymerization upon storage in ILs at high temperatures can be monitored by shear and elongation rheology. Cellulose degradation is first assessed by traditional methods through the determination of the intrinsic viscosity and the MMD of the regenerated cellulose. The kinetics of cellulose depolymerization is calculated using two model types. In the second part, the shear and elongational properties of the cellulose–IL solution are evaluated in order to show how the intrinsic properties of the regenerated cellulose and the rheological properties of the cellulose–IL solution can be linked and, hence, how the rheological properties of cellulose–IL solution can be directly used to estimate the extent of cellulose depolymerization.