Reinforcement of polycaprolactone with microfibrillated lignocellulose

Highlights

• We produced microfibrillated cellulose (MFLC) with high residual lignin content.

• We compared the reinforcement potential of MFLC with MFC from bleached pulp.

• We show that MFLC shows considerable advantages compared to MFC in the reinforcement of polycaprolactone films.

• We demonstrate that MFLC has some potential to serve as a replacement for chemically modified MFC in certain applications.

Abstract

Cellulose-reinforced polycaprolactone (PCL) nanocomposite films were produced by casting from organic solvent. Microfibrillated cellulose derived from bleached pulp (MFC) and from pulp with high residual lignin content, termed microfibrillated lignocellulose (MFLC) were used as filler without any kind of chemical surface modification. Visual inspection revealed better dispersion of MFLC in organic solvent compared to MFC, resulting in a more homogeneous distribution of MFLC in PCL natrices compared to MFC. Tensile tests revealed an improvement of nanocomposite mechanical properties at 1% and diminishing performance at higher filler content. Overall, the performance of MFLC filler resulted in more favorable nanocomposite tensile performance than MFC. This was particularly true when MFLC was not continuously processed in solvent, but converted to a powder by drying from non-polar solvent prior to compounding with the polymer solution. Thus a novel dry nanocellulosic filler option for hydrophobic polymers obtained in a simple process is presented.

Introduction

Microfibrillated cellulose (MFC) is a fibrous material with typical diameter of individual fibrils in the order of 10–50 nm and length in the μm range, which is usually obtained by mechanical disintegration of wood pulp (Dufresne, 2012). Owing to the fascinating potential of this relatively new bio-based material, abundant literature exists, among them some excellent reviews, such as by Eichhorn et al., (2010), Klemm et al., (2011), Moon et al., (2011), or Charreau et al., (2013), to which the reader is referred for more in-depth information. Consisting mainly of cellulose and certain amounts of hemicellulose, which may vary depending on pulp source and processing conditions, MFC is an essentially hydrophilic material. This hydrophilicity manifests itself in a high capacity to bind water, as is obvious for aqueous MFC slurry, which is gel-like at an MFC content >2%. The hydrophilicity of MFC poses a severe challenge in view of its potential use for the reinforcement of polymers. While MFC disposes of excellent mechanical properties and a high specific surface area, it collapses irreversibly when dried conventionally, and shows poor surface-chemical compatibility with many important technical polymers such as polypropylene, polyethylene, or polystyrene.

Different drying methods other than conventional drying at ambient pressure and elevated temperature have been studied for MFC (Peng et al., 2012a, Peng et al., 2012b). Drying in supercritical carbon dioxide leads to excellent results in terms of avoiding collapse and preserving the nano-scale morphology of MFC. Consequently, this method is widely and successfully used in studying cellulosic aerogels at the laboratory scale (Liebner et al., 2015), but it is doubtful whether this technically sophisticated and comparably expensive method is suitable for up-scaling to industrial dimensions. Freeze-drying is a potential alternative, as it also preserves nanofibrillar morphology to some extent, and only entails limited collapse due to the formation of ice crystals. However, the heavily networked structure of freeze-dried MFC may hinder dispersion during compounding with polymers (Peng et al., 2012a). Finally, spray-drying was also evaluated as a potential route to drying MFC (Peng et al., 2012b). This method delivers mostly particulate and only partly fibrous structures with sizes of few microns. Spray-dried MFC particles disperse well in polymers presumably due to hornification effects, but their reinforcement efficiency is limited by small aspect ratios, which originate from the partial collapse of fibrous structures during droplet formation and evaporation in the spray-drying process. Spray drying is recommended as an industrially viable route to drying MFC, however, the problem of loss of nano-scale MFC dimensions due to agglomeration to micron-scale particles is potentially prohibitive with regard to efficient polymer reinforcement.

Thus the challenge of drying MFC is not fully overcome. By contrast, the problem of lack of surface-chemical compatibility of hydrophilic MFC with non-polar polymeric matrices can be very efficiently tackled by means of chemical surface modification. As summarised in a recent extensive review (Habibi, 2014), numerous approaches to surface modification enable tailoring of MFC surface polarity within a very wide range spanning from hydrophilic to superhydrophobic. However, surface-chemical hydrophobisation involves more or less laborious wet-chemical methods, necessitating the use of different organic solvents and sometimes expensive reagents. In this context, the role of lignin as a potential natural compatibiliser is of interest. It was reported recently that MFC nanopapers containing substantial amounts of residual lignin tend to be less hydrophilic than nanopapers derived from bleached pulp (Rojo et al., 2015). More specifically, dispersive intermolecular forces gain in significance at high lignin content, which entails a higher fraction of non-polar moieties compared to pure cellulose (Notley and Norgren, 2010). In agreement with this finding relying on contact angle measurements, variability in surface chemistry of lignins was also demonstrated by hydrophobic interaction chromatography. Here, the presence of lignin fractions with significantly varying surface chemistry from polar to non-polar was demonstrated (Ekeberg et al., 2006). In good agreement with these surface-chemical properties of lignin-rich cellulose, a compatibilising effect of lignin in MFC with high residual lignin content, termed microfibrillated lignocellulose (MFLC) was recently proposed (Gindl-Altmutter et al., 2015). Using the example of polycaprolactone and polystyrene, it was shown that MFLC can be dispersed more homogeneously compared to MFC. Also, significantly better mechanical reinforcing effects were achieved with MFLC compared to MFC.

In the present paper, the mechanical properties of MFLC-reinforced polycaprolactone are studied in more detail. In particular, it is shown that MFLC can be dried from organic solvents without loss of fibrillar morphology, resulting in dry MFLC powder, which can be successfully compounded with polymers.