Forming and dewatering of a microfibrillated cellulose composite paper

Abstract

An approach is demonstrated for the manufacturing of a microfibrillated cellulose (MFC) composite paper. A key element in the manufacturing paradigm is the use of high consistency suspensions to improve retention and minimize the need for water removal after forming. The rheological characterization of the composite furnish, which contained 70% structured pigment, 20% MFC, and 10% pulp fibers, revealed a gel-like shear thinning behavior of the suspension, which differs greatly from traditional fiber-based papermaking furnishes. The results from laboratory and pilot scale studies show that the headbox consistency range from 5 to 10% offers the best combination of processing, forming characteristics, retention, and dewatering. While the furnish dewatering in laboratory scale was very problematic, under suitable dynamic conditions the wire section dewatering was excellent. The results of this study suggest that the MFC composite can be manufactured on a modified paper machine and that the final product will have an attractive cost structure.

Full Article

Forming and Dewatering of a Microfibrillated Cellulose Composite Paper

Juuso Rantanen,*^a Katarina Dimic-Misic,^a Jukka Pirttiniemi,^b Petri Kuosmanen,^b and Thad C. Maloney ^a

An approach is demonstrated for the manufacturing of a microfibrillated cellulose (MFC) composite paper. A key element in the manufacturing paradigm is the use of high consistency suspensions to improve retention and minimize the need for water removal after forming. The rheological characterization of the composite furnish, which contained 70% structured pigment, 20% MFC, and 10% pulp fibers, revealed a gel-like shear thinning behavior of the suspension, which differs greatly from traditional fiber-based papermaking furnishes. The results from laboratory and pilot scale studies show that the headbox consistency range from 5 to 10% offers the best combination of processing, forming characteristics, retention, and dewatering. While the furnish dewatering in laboratory scale was very problematic, under suitable dynamic conditions the wire section dewatering was excellent. The results of this study suggest that the MFC composite can be manufactured on a modified paper machine and that the final product will have an attractive cost structure.

Keywords: Composite; Dewatering; Forming; MFC; Nanocellulose; Papermaking; Rheology

Contact information: a: Department of Forest Products Technology, Aalto University School of Chemical Technology, P. O. Box 16300, 00076 Aalto, Finland; b: Department of Engineering Design and Production, P. O. Box 14400, 00076 Aalto, Finland; *Corresponding author: juuso.rantanen@aalto.fi

INTRODUCTION

The vast majority of paper products use macroscopic pulp fibers as the primary structural component. Most pulp fibers have a length in the millimeter range, contain cellulose as the main load-bearing component, are capable of forming inter-fiber hydrogen bonds from a water suspension, and have suitable surface area, swelling, and permeability that allow efficient dewatering. It is largely these factors that dictate the range of paper and paper board products that have been developed over the previous decades. Likewise, it is the characteristics of pulp fibers that largely define the design and operation of the modern paper machine.

The advent of nano and microfibrillated celluloses (NFC and MFC) opens up great possibilities to reengineer paper, expand the property space, and develop a range of new products. This is largely because the dimensions of various MFC/NFC grades are several orders of magnitude less than those of fibers, and this allows a higher degree of freedom in engineering the final product. For example, a 60 g/m² sheet composed of softwood kraft pulp (SWBK) may contain 10 to 12 layers of fibers. This leads to a fairly high mass variation, surface roughness and z-directional defects. However, the same sheet composed of 10 to 20 nm wide MFC/NFC will contain thousands of layers of fibril strands. Thus, in principle, a more homogenous material with less defects and higher performance can be produced. While laboratory studies (Eriksen et al. 2008; González et al. 2012; Henriksson et al. 2008; Rantanen et al. 2013; Subramanian et al. 2008) have shown the potential of MFC/NFC-based papers, information on the potential manufacturing strategies is still lacking.

Many of the current paper grades have matured and are approaching the end of their lifecycle. One consequence of this is that it has become increasingly difficult to innovate highly differentiated products at a reasonable cost structure. While the modern large-scale paper machine is highly efficient, its operating window is fairly narrow. This means that it is not easy to use furnishes that have very different characteristics, such as MFC/NFC-based compositions. If MFC/NFC composite papers are to be produced at large scale, then a new manufacturing platform will need to be developed, and this should be at least as efficient as the current manufacturing solution. Some of the most important requirements for the production of these papers are:

1. The MFC/NFC must be available in stable, defined quality at a reasonable cost. For high volume manufactures this probably means an onsite preparation.
2. The rheology of the furnish must be such that components can be mixed, cleaned and pumped to the paper machine under steady-state conditions, with a minimum of disturbances.
3. The furnish rheology and web forming technology must allow for the jet and MD (machine direction) and CD (cross- direction) basis weight control typical of high volume machines.
4. An efficient water removal strategy must be devised for the MFC/NFC-based furnish that takes into account the high water binding, low permeability and high surface area typical for this kind of material.
5. The web consolidation and shrinkage must be controlled in such a way that desirable product properties are obtained.

It has been shown in many studies that aqueous suspensions of MFC/NFC are highly shear thinning and show gel-like properties, especially at increased concentrations (Herrick et al. 1983; Pääkko et al. 2007; Agoda-Tandjawa et al. 2010; Rezayati Charani et al. 2013; Saarikoski et al. 2015; Žepic et al. 2014). The rheology of a high consistency furnish that contains large amount of MFC/NFC together with fillers and cellulose fibers was also observed to be governed by the swelling and gel-like properties of the MFC/NFC, as described by Dimic-Misic et al (2013). Since the behavior of MFC/NFC furnishes is so different than traditional furnishes, it follows that the manufacturing strategy will also be unique. Furthermore, it is difficult to study the manufacturing solution in the laboratory, because many of the relevant phenomena in reel-to-reel manufacture are dynamic and must be observed in a continuous operation. Therefore, highly flexible and non-standard pilot scale studies are required. The development of MFC composite papers is further complicated by the fact that the raw material base, the product properties, and the manufacturing strategy must be approached simultaneously.

In this study, some first steps in demonstrating the forming and dewatering strategy in pilot scale for a MFC composite paper are taken. Of the above listed points, the focus was mainly on items 2 to 4. Relevant laboratory measurements were included to help demonstrate the unique characteristics of the furnish, to suggest the outlines of the large-scale manufacturing paradigm, and to hint at possible products.

The development of a web manufacturing solution requires that the furnish be defined. This has been done in an earlier study (Rantanen et al. 2013) by mapping the property space for various combinations of macroscopic fiber, pigment, and MFC. Based on sheet quality, cost and potential processing characteristics, a furnish containing 10% fiber to impart tear strength, 70% coarse S-PCC to give smoothness, light scattering, permeability, and favorable dewatering, and 20% MFC, which acts as the main bonding component, was chosen.

It was clear from the outset of this work, that dewatering of the MFC composite furnish would be a major challenge, so that high-consistency forming solutions would be of interest. For example, if one can form at 10% solids instead of the usual 1% solids, then the amount of water that must be removed from the web is reduced by about 99%, the recirculation systems are simplified, and the corresponding production energy is greatly reduced.

In traditional fiber-based high-consistency furnishes it is important that the fiber suspension reaches a “fluidized” state for processability. Fluidization refers to a deflocculated state that a fiber suspension reaches when enough shear is applied (Bennington et al. 1991; Chen and Chen 1997; Cichoracki et al.2001). In a fluidized state, the furnish viscosity is greatly reduced and Newtonian behavior is observed. In most modern low-consistency headboxes, energy in the stock flow is sufficient to break up flocs by means of suitably arranged static elements. Grundström et al. (1973) developed a headbox capable of forming decent quality webs from 2.5 to 3.5% solids content fibrous suspensions. When furnish consistencies become sufficiently high, an external source of energy is needed to break up the flocs. In the present work, a rotating element inside the headbox has been used. Since the reflocculation rate is directly related to the consistency (Kerekes 2006), successful high consistency forming demands that the web is formed directly from the fluidized suspension.

A prototype headbox, based on forming from a high consistency fiber suspension which was fluidized with applied shear was developed by Gullichsen and coworkers (Gullichsen and Härkönen 1980; Hietaniemi and Gullichsen 1996; Cichoracki et al. 2001). In pilot scale work, webs with reasonable formation were produced at up to 10% furnish consistency. The headbox design that Gullichsen and coworkers developed was taken as a starting point for this project. The present study is the first time that this technology has been applied to MFC-based furnishes.

EXPERIMENTAL

Raw Materials

Softwood bleached kraft pulp (SBKP) fibers from a Finnish pulp mill were delivered in once-dried form. The length-weighted average fiber length of the pulp was 2.24 mm. The pulp was refined to SR=18, which is a measure of drainability and is defined by a method that is based on the standard ISO 5267-1 (1999). Microfibrillar cellulose (MFC) was a commercial grade Daicel Celish KY-100G delivered at 10 wt-% aqueous suspension. Precipitated calcium carbonate (PCC, grade FS240) of scalenohedral shape and weight average particle size of 3.97 μm, measured with Malvern Mastersizer, was delivered by Omya AG as a water dispersion of 35 wt-%. The MFC composite furnish was a mixture of 70% PCC, 20% MFC, and 10% SBKP fibers. The components were mixed with a high shear laboratory mixer for the rheological and web coherence measurements. In the pilot runs the components were mixed in the storage tank which was equipped with a high consistency mixer. The consistencies of the furnishes varied between 4 and 20% in the experiments. Previously dried birch kraft pulp, refined to SR 18, was used in the web coherence experiments as a reference.

Rheological Measurements

Rheological measurements of the furnish were made at 4 and 7% solids to evaluate behavior in high-consistency processes. The viscoelastic rheological investigations were performed at 23 °C using an Anton Paar Physica MCR-300 rheometer (Anton Paar Germany GmbH, Germany) with a plate-plate geometry. The rheometer was equipped with roughened top (PP20) and bottom (P-PTD200) plates. The bottom plate was also equipped with temperature control. The gap was initialized to 2.3 mm. Prior to measurement, the sample was pre-sheared at 10 (rad)s^-1 and strain deformation of 0.1 % for 10 min, followed by a rest stationary state time of 10 min. Different types of rheological measurements were performed on the same rheometer preserving the geometry: viscoelastic measurements, structure recovery tests, and steady state flow with low to high shear rate. To prevent evaporation of the water medium, a layer of silicone oil was spread over the surface of the sample in contact with the air. The rheological measurements were repeated five times including pre-shear protocol, and for the calculation of rheological parameters average values of five measurements were used.

Viscoelastic properties and steady state flow

Dynamic viscoelastic moduli, storage modulus (G´) and loss modulus (G´´), were measured as a function of angular frequency (ω = 0.1 to 100 s^-1) using oscillatory tests. To perform the frequency sweep test, the linear viscoelastic range of the sample (LVE) was obtained from an amplitude sweep using constant angular frequency (ω = 1 s^-1) with varying strain amplitude between 0.01 and 500%. The influence of shear rate (r) on the dynamic viscosity (η) was measured at a steady state flow by increasing the shear rate from 0.01 to 1000 s^-1.

3ITT (3 interval thixotropy test) recovery measurements)

The purpose of the 3ITT measurement was to determine how the furnish consistency affects the recovery of the dynamic elastic network structure after removal of high shear. It is desirable that the initially high dynamic viscosity of the composite furnish can stay in a disrupted state at the head box slice and during forming on a fabric screen (“wire”), in order to be in a flowing state. Initially, samples were subjected to low shear rate (0.1 s^-1), then subsequently high shear rate (500 s^-1), and finally once again low shear rate, the recovery stage (0.1 s^-1). Structure recovery was traced in respect to recovery of dynamic transient viscosity (η⁺) in the third interval, expressed as percentage (%) of ratio η⁺/ η₀after 300 s of measurements, where η₀ is the low shear viscosity at the beginning of first interval (Dimic-Misic et al. 2013a).

Data processing

Some of the rheological data contained mechanical noise, which, unless stated otherwise, was subsequently smoothed by Tikhonov regularization (Yeow et al. 2007; Dimic-Misic et al. 2014). Solutions with smaller curvatures were preferred by setting the forward operator to the identity matrix and the regularization operator to a discretized form of the second derivative.

Web Coherence

The flow behavior of the furnish was studied with a laboratory headbox based on a device used by Gullichsen and Härkönen (1981) in their medium consistency research. Similar construction with a vaned rotor inside a chamber has been also used by other researchers studying the rheology of high solids content fiber furnishes (Bennington et al. 1991; Chen and Chen 1997; Andersson et al. 1999; Pettersson and Rasmuson 2004).

The rheometer consisted of a rotor installed inside a chamber of 4 · 10^-3 m³ in volume. The rotor had a rhombic patterned surface to allow energy transmission from the rotor to the suspension. The chamber wall, acting as a stator, had a grooved surface for the same reason. The speed of the rotor could be adjusted in the range 0 to 3000 RPM with sufficient power to “fluidize” (deflocculate) a pulp suspension up to about 10% solids for ordinary pulps. The rotational speed (based on inverter frequency) and torque (based on current) were measured. The rheometer was equipped with a slice which could be opened while the furnish was fluidized, thus forming a web under quasi-static state conditions. Different slice geometries could be fitted to the rheometer, and based on initial flow tests a converging slice geometry with 2 mm opening was chosen for the furnish comparison experiments. A hydraulic piston with adjustable pressure control was used to pressurize the chamber. A high speed video of the jet was analyzed to determine under which conditions the best jet coherence was achieved. A schematic illustration of the headbox rheometer is presented in Fig. 1.