Experiences with Scaling-Up Production of TEMPO-Grade Cellulose

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Chapter 12

Experiences with Scaling-Up Production of TEMPO-Grade Cellulose Nanofibrils Richard S. Reiner* and Alan W. Rudie USDA Forest Products Laboratory, Madison, Wisconsin 53726, United States *E-mail: [email protected].

The USDA Forest Products Laboratory (FPL) has worked toward production of cellulose nanofibrils (CNF) to a level suitable for supplying for many preliminary research and development projects. In particular, this chapter discusses the process steps involved in scaling the production of CNF from grams in the laboratory to pilot batch sizes up to 4 kg, namely the reaction, washing, dispersion, separation and freeze drying. The carboxylation of the pulp nanofibril surface was carried out using the catalyst TEMPO with two reaction protocols utilizing hypochlorite at pH 10 and sodium chlorite at pH 7. Various methods for mechanical dispersion, of oxidized pulp fibers into individual nanofibrils are also compared. Currently, aqueous suspension and freeze-dried TEMPO-grade cellulose nanofibrils produced using hypochlorite are available through the University of Maine’s Process Development Center (along with other cellulose nanomaterials). However, end-use applications will ultimately dictate the process optimization of chemistry and dispersion that are engineered into producing CNF with particular properties.

Not subject to U.S. Copyright. Published 2017 by American Chemical Society


Introduction The act of modifying the bonding surface area of cellulosic fibers to change their papermaking properties has existed since the ancient Egyptians began hammering overlaid papyrus fibers into sheets or the 2nd century Chinese began pounding rags, wood and grasses into individual fibers that were collected onto mats and dried into sheets. In an over simplification, the cellulose fibers used to make paper are individual plant cells. In the growing plant, these cells are bound together with other natural polymers in a composite structure to form the structure of the plant: trunk, stems, branches, etc. The cellulosic structure of the plant cell wall might be compared that of rope, namely small individual threads of a material are twisted and braided together to form larger and larger fiber-like structures. Plant cell walls are composed of layers of microfibers, which in turn are composed of bundles of nanofibers, which are composed of bundles of cellulose chains that were produced by the plant during its growth. The process of pulping is, through a combination of mechanical and chemical means, to separate these individual fibers so as to be able recast them into thin sheets to make paper. The process of beating and refining pulp fibers prior to sheet formation extends a certain amount of microfibrils from the fiber surface and increases the cellulose surface area available for bonding. Further refining and beating, begins to not only break down the fiber structure, but also begins to extend nanofibrils from the surface of the microfibrils. This acts to further alter the sheet strength, uniformity, density, opacity and porosity. Taken to an extreme, such refining was used to produce glassine, a strong, thin, glazed, semitransparent form of paper. However, recently, more aggressive mechanical methods have been used to completely disrupt the plant fiber cell walls and produce a very high surface area fibrillar product. The first report is not recent, 1980, using multiple passes through a homogenizer to get microfibrillated cellulose (MFC) as a gel-like cellulose suspension (1). Addition of enzymes (2) and acids (3) have helped to reduce processing energy and further extend the structural disruption toward nanofibrils. The most complete nanofibrillar dispersions are achieved by chemical treatments which act to add ionic functional groups to the surfaces of the nanofibril: examples, TEMPO oxidation which oxidizes the C-6 carbon of cellulose to a carboxylic acid (4–6), chloroacetic acid, which substitutes acetate groups for any of the three hydroxyl groups on cellulose (7) or oxidation with periodate which oxidizes C-2 and C-3 of cellulose to carboxylic acids (8). When suspended in water, these ionic groups act to repel nanofibrils from one another and with the aid of mechanical action can produce a colloidal suspension of highly dispersed cellulose nanofibrils. Recently, cellulose filaments (CF), a long, thin ribbon of cellulose that is mechanically peeled from from pulp fibers has also started being produced (9). Table I lists cellulose nanofibril production or announced production facilities around the world. By definition, nanocellulose must have at least one dimension less than 100 nm in length. However, some of the materials listed may not fit this definition but are included here as they are being developed for many of the same types applications as nano-scale CNFs.

228

Table I. Cellulose nanofibril production facilities worldwide


Company

Country

Product notesa

Productivity kg/day

Ref.

Forest Products Laboratory

USA

4

TEMPO CNF

Paperlogic

USA

2000

CNF

(10, 11)

Univ. of Maine

USA

1000

CNF

(10)

American Process

USA

500

Hydrophobic CNF

(10)

Kruger

Canada

5000

CF

(1, 12)

Performance Biofilaments

Canada

Pre-Commercial

CF

(1)

Oji Paper

Japan

100

Phosphate CNF

(13)

Nippon Paper

Japan

1400

TEMPO CNFs

(14)

Daio Paper

Japan

275

CNF

(15)

Borregarrd

Norway

1000

MFC

(10, 16)

Betulium

Finland

Pilot

CNF

(17)

InTechFibers

France

100

CNF

(10)

Innventia

Sweden

100

CNF

(18)

Stora Enso

Finland

Commercial

MFC

(19)

UPM

Finland

Pre-Commercial

CNF

(10)

Norske Skog

Finland

Pilot

CNF

(10, 20)

VTT/Aaalto

Finland

Pilot

CNF

(10, 21)

Luleå Univ. of Technology

Sweden

Lab

CNF

(10)

Imerys

UK

1500

MFC with minerals

(10)

Cellucomp

Scotland

1000

CNF with resins

(22)

SAPPI

Netherlands

20

CNF

(23)

a

Cellulose nanofibrils (CNF), microfibrillated cellulose (MFC), and cellulose filaments (CF).

The pioneering work for the production of chemically-modified cellulose nanofibrils uses a catalyst, (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO) radical, to oxidize accessible primary alcohols of the cellulose chains to carboxy groups, which when combined with mechanical or ultrasonic energy, produced an extremely dispersed grade of cellulose nanofibrils (4–6). A pilot plant at 229

the United States Department of Agriculture-Forest Service Forest-Products Laboratory (USDA-FS-FPL) in Madison, Wisconsin has been working to scale the production of TEMPO-grade cellulose nanofibrils (CNFs) in order to make the material available for Research and Development purposes. This chapter will summarize the scale-up efforts from gram-scale to the kilogram-scale production in the pilot plant.


TEMPO Reaction with Cellulose The TEMPO system oxidizes the C6 primary alcohol of cellulose chains. The reaction appears to be physically restricted to nano-scale fibril surfaces by the composition and organization of the cell wall leaving an unoxidized core of the cellulose chains that make up the nanofibril. The limitation of the TEMPO reaction to only the primary alcohols on the nanofibril surface keeps the fundamental structural unit of the nanofibril intact, with the modified surface chemistry. There are two sets of reaction conditions, both using catalytic levels of TEMPO to functionalize cellulose with carboxylate groups. The first uses hypochlorite as the terminal oxidant along with sodium bromide. This reaction is maintained at pH 10 and is carried out over the course of several hours at room temperature (5) A second reaction system uses sodium chlorite as the terminal oxidant. This reaction is maintained at pH 7 and is carried out over the course of several days at 70°C (6). Generally speaking, the TEMPO/hypochlorite system at pH 10 can achieve higher levels of carboxylate on the nanofibril surface, up to 1.5mmol -COONa/g dry pulp, but also results in lower degree of polymerization (DP) of the cellulose. The TEMPO/sodium chlorite system at pH 7 will result in lower levels of carboxylate on the nanofibril surface, up to 0.8 mmol -COONa/g dry pulp, while the cellulose retains a higher DP. Furthermore, the higher carboxylate of the TEMPO/hypochlorite system at pH 10 is likely to produce nanofibrillated suspensions with a higher level of light transparency and films cast will also likely have a higher level of light transparency than those produced by the TEMPO/sodium chlorite system at pH 7. The trade-off is with the DP of the cellulose and is effect on the strength of cast films. The TEMPO/sodium chlorite system at pH 7 provides stronger films than those produced from the TEMPO/hypochlorite system at pH 10 (6) Another consequence of the oxidation level and DP is that aqueous suspensions of the nanofibrils produced from TEMPO/sodium chlorite at pH 7 has a higher viscosity than an equal concentration of cellulose nanofibrils produced from the TEMPO/hypochlorite system. There are two distinct stages to the mechanism of the TEMPO reaction. The TEMPO catalyst, in its cationic oxidized state, first oxidizes the primary alcohol on the cellulose surface to the aldehyde. Then the terminal oxidant, as well as its various oxidation states, oxidizes the TEMPO catalyst back to the active state, and oxidizes the aldehyde to a carboxylic acid (carboxylate). The reaction produces one mole of acid (the carboxylic acid) on completion of the oxidation cycle, resulting in a drop in pH. Maintaining the reaction rate requires 230


controlling the pH within a fairly tight window, pH 9-11. This is accomplished with addition of alkali during the reaction, or use of a buffer. The reaction using chlorite as primary oxidant has a particular problem in that TEMPO is most active in the pH 9-11 range, but chlorite is inactive at that high of a pH. Chlorite is generally unreactive, but under acid conditions reacts with hypochlorous acid to make chlorine dioxide which is quite reactive. Chlorine dioxide returns to chlorite when it oxidizes TEMPO. Cycling though the process multiple times is required to utilize the majority of the 4-electron oxidizing capability of chlorite. This cycle is more rapid at low pH. The result is a compromise with operation at a pH around 7 and a much slower reaction. If the pH drops too far below 7, the TEMPO oxidation stops. If the pH rises too much above 7, the reaction producing chlorine dioxide stops (6). Furthermore, the TEMPO/hypochlorite system at pH 10 is facilitated with the presence of sodium bromide (5), while the TEMPO/sodium chlorite system is dramatically improved if the reaction is kick-started with a small dose of sodium hypochlorite (6). Since the cellulose modification proceeds through the aldehyde as the alcohol is oxidized to carboxylic acid, residual aldehydes may be present on the pulp. This residual aldehyde level may be affected by the ratio of pulp to sodium hypochlorite or sodium chlorite, the reaction time, the initial pulp (i.e., species, previous dry state, hemicelluloses, etc.). Residual aldehydes may be oxidized to carboxylic acid using a treatment with sodium chlorite and acetic acid (4). In practice, the USDA Forest Products Laboratory has chiefly produce cellulose nanofibrils using the TEMPO/hypochlorite reaction, but has explored the reaction with TEMPO/sodium chlorite. The TEMPO/hypochlorite method offered several advantages. First and foremost, the higher light transparency of the suspension was a desirable characteristic for many of the researchers interested in the materials. This is a unique characteristic relative to many other nanomaterials, such as, carbon nanotubes and nanoclays. Other factors that favored use of the sodium hypochlorite procedure included ambient reaction temperatures, reaction times of hours instead of days, and maximizing yield (at least with regard to maximizing light transparency by removing less well-dispersed residue as discussed below). There is an expectation that as CNFs are developed into potential products, it will be possible to optimize the properties of the CNFs for the application either by changing the primary oxidant, or controlling final carboxylic acid levels. Bleached kraft Eucalyptus machine dried pulp has been used as the starting material for the TEMPO reaction. Four kilograms of drylap sheets were repulped using a Voith 50L hydropulper (Appleton, WI) at about 8% solids. Repulping was continued for about 30 min to assure the fibers were well dispersed and help swell the fiber wall. The TEMPO reaction proceeds similarly with dried and never dried pulps, but there are some differences in water retention values after TEMPO treatment (5). The pulp was pretreated by soaking in water containing 0.2% sodium chlorite on pulp and adjusted to pH 2 with sulfuric acid. This was stirred slowly overnight, then washed well with reverse osmosis (RO) water. The pretreatment oxidizes any residual lignin and quinones that would otherwise consume oxidant during the TEMPO reaction. The low pH removes trace metals such as calcium, magnesium and iron. Once the pulp is carboxylated, multivalent 231


cations are tightly bound to the pulp, and can add color to the pulp and inhibit dispersion of CNFs. A bicarbonate/carbonate buffer is used to maintain the TEMPO/hypochlorite reaction pH close to 10. Sodium carbonate, 1000 g is dissolved in 150 L of RO water in a stirred, glass-lined reactor equipped with a baffle. Wet, pretreated pulp, equivalent to 2 kg oven-dry, is added and stirred for 30 min. Then 200 g sodium bicarbonate is added along with 225 g sodium bromide. Thirty-two grams of TEMPO is added as a dry powder and mixture stirred for 30 to 60 min. While TEMPO is slightly soluble in water, its dissolution is relatively slow. It was helpful to minimize TEMPO crystal size by melting an appropriate amount, pouring it into a ziplock bag, then cooling in ice. The bag could then be gently kneaded to produce a relatively fine TEMPO powder for sprinkling into the reactor. The reaction is diluted to 400 liters and sufficient sodium hypochlorite solution added to provide 5 mol NaHClO per kilogram pulp; this was typically about 7L of 12% solution. (The concentration of stock sodium hypochlorite needs to be determined through an appropriate analytical method.) The reaction was allowed to proceed overnight. The carbonate/bicarbonate buffer provides an initial pH near 10.5 and the final pH near 9.5. The contents of the reactor were collected by filtration on a 40-inch diameter Nutsche filter, and washed in place with RO water. Typical, carboxylate levels are 1.4 mmol -COONa/g with a TEMPO treated pulp yield between 90-95%. Recently, FPL has assembled an auto titrating system that adds NaOH solution as needed to control the reaction pH at 10 during the reaction. This method provides a way for clearly determining the reaction endpoint as the pH stabilizes and the control system no longer requires additional NaOH solution. A small amount of sodium carbonate/bicarbonate is still used (50/10 grams respectively in 400L) in order to improve pH control and prevent localized spikes that could be detrimental to both the reaction and complicate the pH control. With good control at pH 10, the reaction is complete after about 3 h. Using this method, carboxylate levels on the pulp increased to about 1.5 mmol -COONa/g. This increase was sufficient to further swell the treated pulp making it harder to wash and necessitating some procedural changes discussed below. Pulp yields have been about 88 to 92%.

Chemical Analysis of the Carboxylated Pulp To analyze the carboxylate content of TEMPO reacted pulps, FPL has used two methods. The first is a modification of TAPPI method T-237, “Carboxyl Content of Pulp,” in which the reagent concentrations have been increased due to the significantly higher carboxylate contents of these pulps. This method only works using the reacted pulp before dispersion as nanofibrils. The pulp is converted to the acid form and washed well with high quality deionized water. A known test quantity is reacted with a standardized solution of 0.1N NaHCO3 and 0.25M NaCl, which bubbles off CO2 generated by the acid groups on the pulp. The pulp is removed and an aliquot of the reagent solution is titrated for NaHCO3 consumption using 0.1N HCl to a methy red endpoint. Due to the buffering of 232


H2CO3/NaHCO3, the endpoint is transition is broad so the titration must be boiled to drive off CO2 near the endpoint in order to sharpen the methyl red color change. Another method used to determine the carboxylate content is through a conductometric titration. The conductivity of the solution is monitored while standardized acid or base added to a known quantity of CNF. The endpoints are determined by changes in the slope of the titration data as the titrant is added. This test can be performed on reacted pulp or dispersed CNF suspensions. Care must be taken that the system has equilibrated with each addition of titrant. Residual aldehydes may be oxidized to carboxylic acid by a follow-up treatment with sodium chlorite and acetic acid (4). One can than measure the difference in carboxylate levels before and after this post-treatment to determine the residual aldehyde levels of the TEMPO reaction. The carbohydrate analysis was performed using HPLC with an amperometric detector (24). However, quantification of uronic acids is limited by lack of standardization, which is reflected in the significantly lower total carbohydrates detected.

Washing the Reagents from the Carboxylated Pulp Due to the addition of carboxylic acid groups to the pulp, washing and handling provides additional challenges during scale-up. At low ionic strength, the pulp swells significantly in water slowing filtration. Further, the pulp is also susceptible to fibrillating, even with just modest shear environments. Care is taken to minimize handling until washing is completed. The 400 L reactor gravity drains to the Nutsche filter eliminating the need for a pump to transfer pulp for washing. A Nutsche filter is an agitated horizontal plate filter enclosed in a vertical tank. The pulp mat is sensitive to compacting which significantly slows the filtration process. Appling pressure or pulling a vacuum creates an initial surge in filtration flow rate, but this quickly subsided to a much slower rate. Increasing the pressure differential across the mat again increases flow, but only for a short time until additional fiber packing slows filtration even further. To prevent slow drainage of the pulp, the lower chamber of the Nutsche filter is flooded to the filter prior to transferring the TEMPO/hypochlorite reaction. The bottom valve is opened slowly to begin draining and limit the, differential pressure to a small head. There is a significant enough difference in ionic strength between the reaction solution and the RO water used for washing such that the carboxylated cellulose fibers swell during washing. This increases the sensitivity to collapse as washing progresses. Once the bulk of the reaction has been drained above the fiber mat but before air is drawn through the mat, RO water is sprayed over the top of the fiber mat and allowed to percolate through. This is repeated several times with a total of about 200L of wash water until conductivity exiting the fiber mat is sufficiently low. After the final wash water has been drained through the mat, modest air pressure (~1 bar) is applied to dewater the mat. In this way, two kilograms of carboxylated pulp can be washed with a 40-inch diameter Nutsche filter in about an hour. 233


This displacement washing method worked well in early reactions when carboxylate buffer was used to control the reaction pH. However, using the autotitrator, during the TEMPO hypochlorite reaction to hold the pH at 10 has resulted in a small increase in the final carboxylate level of the pulp. This has resulted in an even higher level of swelling of the pulp mat as RO water displaced the reagents. In fact, the swelling was significant enough to cause rupturing of the pulp mat and creating a fractal pattern of cracks that allowing wash water to channel rather than flowing evenly through the pulp mat; to avoid this, a small change to washing procedure was required. The completed TEMPO/hypochlorite reaction was transferred to the Nutsche filter and drained as before, but the spent reagents were then drained from the lower chamber of the Nutsche filter. Then, while under vacuum, wash water is introduced from the bottom. This bottom flow releases the pulp mat more cleanly from the filter cloth and refills the lower chamber of the Nutsche filter once again. Once the water level was above the Nutsche filter mixer, the pulp was mixed gently for a short period to swell the pulp while suspended in the initial wash water. The water was drained and, the pulp mat washed further using the displacement procedure. Dewatering was complete with air pressure as described above. A small amount of yield is lost through the filter cloth before the pulp fibers form a mat thick and tight enough to fully retain cellulose particles. Even with this procedure, the TEMPO treated pulp requires longer wash times. This washing approach is not suitable for larger scale processing and another approach beyond increasing filtration area per kg of pulp is needed. A likely approach might be washing with a series of decanters; one will need to take some care with balancing the g-force affecting the settling rate of the swollen, oxidized pulp with the compaction of the solids and the intensity of mixing that is required between stages so as to avoid significantly increasing the water retention value of the pulp or yield losses due to dispersion of CNFs.

Mechanical Dispersion of the Carboxylated Pulp Once the carboxylation of the pulp has been achieved, a certain amount of shear energy must be added in order to disintegrate and disperse the material as nanofibrils in water. Various methods have been explored, including stirring, pumping, blending, ultrasound, refiners, homogenizers, etc., with success to various degrees. At FPL, the goal is to produce a material that can be used for research and development over the broadest array of potential applications balanced with issues such as capacity, yield and process simplicity. This goal has, channeled FPL CNF production toward the TEMPO/hypochlorite method and maximizing the carboxylate content of the pulp in order to produce a dispersed material with high levels of suspension light transparency. As applications are developed, one can return to the variables of CNF production in order to optimize a material with the appropriate balance of carboxylate content, cellulose DP, suspension transparency with chemistry, energy, and other process considerations. The initial scale-up fibrillation of CNF was performed by circulating a tank of carboxylated pulp (1.3 mmol -COONa/g) at 0.1 % solids using a 234


multistage centrifugal pump (Grundfos CR2) for five days. Periodic aliquots of the suspension were centrifuged in the laboratory at 4500G for 10 min and the supernatant analyzed for solids content. A centrifuge was used to separate smaller CNF particles that remain suspended at high gravity-force from larger particles. This suspension was passed twice through a Sharples cylindrical-bowl-type centrifuge operating at 12,500G with about a one minute residence time for each pass. The supernatant was concentrated to nearly 1 wt% solids by circulating through a tubular ultrafiltration membrane (4-ft B1 module with polyvinylidene fluoride (PVDF) 200,000 molecular weight cut off (MWCO) tubes; Membrane Solutions). This suspension was clarified by passing once through a homogenizer (Microfluidics M-110EH-30) equipped with an 87 micron orifice. Solids analysis showed about 57% of the oxidized pulp was converted to a clear, colloidal suspension of CNF product in this first fibrillation cycle. The solid residue from the centrifuge was suspended in 200L of water and circulated with a multistage centrifugal pump for another five days. This yielded an additional 18% of the oxidized pulp as a colloidal suspension. While, this initial procedure was able to generate nanofibrils and confirmed that laboratory methods work at larger scale; it was an impractical solution for pilot production. These results are summarized in Table II. In order to increase the rate at which pulp fibrillation could be processed, FPL also evaluated use of a disk refiner. A Sprout-Walden stainless-steel disk refiner was outfitted with plate pattern D2B503. A variable speed Moyno® pump was used to continuously recirculate the CNF suspension through the disk refiner. An inline heat exchanger was added to the circulation line in order to prevent the CNF suspension from overheating during processing (the system typically reached a steady state temperature near 50°C while processing CNF suspension). Warming of the suspension and disk refiner during the processing caused the refiner gap to decrease, so the gap was adjusted on-the-fly such that the processing power was maintained at 20 kW; the gap was estimated to be 150-200 microns. A valve for an air bleed was installed near the intake of the disk refiner to prevent the disk refiner from flooding, which causes a pumping suction at the inlet and a surge in power consumption. Disk refiner tests were done using 60L of 0.1 wt% carboxylated pulp (0.76 mmol -COONa/g), which had been produced using the TEMPO/sodium chlorite method. This suspension was processed in the disk refiner for 45 min while circulating at a rate of 60 L/min. As before, samples of the suspension were centrifuged at 4500G for 10 min to remove material that was not fully suspended. Just 13% of the CNF successfully suspended. The process was repeated using a 2 wt% suspension. The suspension became quite viscous and it was necessary to manually stir the tank to prevent channeling of processed CNF through the tank. Periodic samples were collected, diluted to 0.1 wt% and centrifuged as before. At the higher percent solids, 45 % of the CNF was colloidally suspended. At pilot scale, the bulk suspension was diluted to 0.1 wt% passed twice through the Sharples centrifuge, concentrated to about 0.5 wt% solids using tubular ultrafiltration, and clarified by passing once through the Microfluidizer homogenizer. 235

Increasing the disk refiner processing time to 2 h increased the colloidal yield marginally to 50%. The Sharples centrifuge, operates as a batch process with respect to retained solids. It needs to be stopped periodically to empty the solids from the bowl. The run time could be increased with a crude initial separation of the largest particles using a side-hill screen prior to centrifugation.


Table II. Percent of CNF dispersed using various dispersion techniques Disk refiner TEMPO/sodium chlorite pH 7 0.76 mmol COONa/g pulp, 0.1 wt%

Multistage centrifugal pump TEMPO/hypochlorite pH 10 1.3 mmol COONa/g pulp, 0.1 wt% 0.125 days

6 %

5 min

4.4 %

1 day

21 %

10

7.8 %

2 days

31 %

15

8.6 %

5 days

57 %

20

9.2 %

30

10.7 %

45

13.0 %

Continued processing of rejects 5 days

18 %

Disk refiner TEMPO/sodium chlorite pH 7 0.76 mmol COONa/g pulp, 2 wt%

Disk refiner TEMPO/hypochlorite pH 10 1.4 mmol COONa/g pulp, 2 wt% 0 min

5.8 %

0 min

3.7 %

5

24.7 %

5

9.5 %

10

47.9 %

10

16.4 %

15

65.1 %

15

21.3 %

20

76.3 %

20

23.9 %

30

89.2 %

30

31.7 %

45

91.3 %

45

45.2 %

The refined solids were diluted from 2 to 0.35 % and circulated across a 100mesh, stainless-steel screen tilted at a 45 degree angle. Additional water was added to the circulating rejects until the filtrate passing the screen ran relatively clear, Dilution was intentionally limited to not exceed 20 times the refining volume. Removing the coarsest material allowed the Sharples centrifuge to be operated with limited stoppages for emptying collected solids. Testing the refiner method on TEMPO oxidized pulp using the hypochlorite method reduced the amount of time needed recirculating in the refiner while also increasing the yield to 90%. Whereas the chlorite method CNF could only be concentrated to 0.5% in the membrane system, the combination of higher carboxylate levels, up to 1.5 mmol -COONa versus 0.8 mmol -COONa, combined with the lower degree of polymerization of the cellulose, allowed the tubular UF 236


system to increase the concentration of hypochlorite oxidized CNF to about 1 wt% solids before the viscosity became too high to maintain flow in the membrane tubes. The comparison of CNF dispersion using a disk refiner is shown in Table II with sugar analysis of treated pulp as well as refined CNF and rejects in Table III for the disk refiner at 2% solids produced using the TEMPO/sodium chlorite method.

Table III. Sugar analysis of carboxylated eucalyptus pulp and refined (2% solids) CNF produced using the TEMPO/sodium chlorite method buffered at pH 7 with phosphate followed by disk refining. (Arabinose, galactose and mannose were not detected.) Glucose

Xylose

Total Carbohydrates

Eucalyptus

79.7 %

15.1 %

94.9 %

TEMPO reaction, 0.76 mmol/g

61.9 %

12.5 %

74.4 %

Colloidal CNF

60.4 %

11.1 %

71.6 %

CNF rejects

65.2 %

7.8 %

73.0 %

Sample

Figure 1 shows rheology data for samples of TEMPO-CNF prepared with the different oxidants using a Bohlin Rheometer operating with a C25 cup and a 2000g-cm torsion element. The viscosity of a 0.38% solids CNF suspension using the TEMPO/sodium chlorite method is comparable to the 0.95% solids CNF suspension using the TEMPO/hypochlorite method. Additionally, Figure 1 shows both CNF suspensions are thixotropic, meaning it is a non-Newtonian fluid, with a viscosity changing with shear rate, and with shear history. Thixotropy is a common characteristic of many gels. Having determined that other equipment was not doing an adequate job of dispersing the TEMPO treated CNF to a clear suspension, FPL switched to using homogenizers for the complete dispersion process rather than just the clarification step. Using the TEMPO/hypochlorite reaction provides a near 100 % yield during the dispersion without the need for separation or concentration steps. The Forest Products Laboratory has had access to three different homogenizers over the years for dispersing CNFs: a Microfluidizer Model M-110EH (MicroFluidics, Inc., Westwood, Massachusetts), a mini-DeBee (Bee International, South Easton, Massachusetts) and a GEA Niro Soavi 3015EH (GEA North America, Columbia, Maryland). All three use one or multiple plungers or pistons, to develop high fluid pressures. The fluid is then dropped rapidly across a reaction zone to create massive amounts of turbulence that act to disrupt the fiber structure into individual nanofibrils. The Microfluidizer combines a small orifice with impinging flow: a “Y-cell” which divides the high-pressure stream into two which are each passed through small orifices that are directed at each other to increase turbulence further. The Mini DeBee also passes the high-pressure fluid through a small orifice followed by a secondary turbulent zone. There is some 237


flexibility as to how this secondary zone is set-up; FPL only used a series of varying diameter orifices that created numerous zones of shear and turbulence in series after the primary homogenizing orifice. The GEA Niro Soavi system uses an opposing valve where the high-pressure fluid is passed through an annular gap formed by a static “passage head” and free-floating “impact head.” The size of the gap is controlled by the pressure applied to the “impact head;” the gap size is also influenced by fluid properties and flow rates.

Figure 1. Viscosity characteristics of CNF suspensions as shear rates are increased then decreased. The three systems have comparable performance. Operating procedures generally consisted of three passes, each with increasing operating pressure or decreasing orifice diameter or gap size. With the microfluidizer, the initial pass was used a 200-micron Y-cell operating at about 600 bar followed by two passes using an 87-micron Y-cell operating at 1400 bar. The Mini DeBee also used a 200-micron orifice operating at about 600 bar for the first pass. This initial pass was conducted at near 5% solids followed by dilution to 1% solids. Then the CNF suspension was then passed through a 125-micron orifice operating at about 1000 bar followed by a pass through a 100-micron orifice operating at about 1400 bar. Both the microfluidics and Mini DeBee suffer plugging of the orifice with dirt and debris that presumably came in with the pulp. This problem was reduced by pumping the suspension through a 150 mesh screen after the first pass through the homogenizer using the 200 micron orifice Both the Microfluidizer and Mini DeBee were limited by capacity, operating at rates near 200-250 mL/min. For the FPL pilot plant producing TEMPO treated 238


pulp in 2 kg batches, five full working days were required to process 2 kg of CNF. FPL has recently purchased a GEA Niro Soavi 3015EH. With a similar methodogy, CNF is dispersed by an initial pass at 600 bar at 1.5 to 2 wt % solids. After dilution to 1 wt% solids, a second pass is performed at 900 bar followed by a third at 1200 bar. This system is capable of operating at up to 240 L/hour so processing 2 kg of CNF suspension (200L at 1 wt% solids) over three passes can be produced in about 3 h. This system has not had problems with debris plugging the opposing valve. The limited amount of debris likely accumulates at the valve until the end of a pass, when the valve is opened. However, with this opposing valve system, great care must be taken with plumbing design and operation to ensure that no significantly sized air bubbles are fed into the system as this causes catastrophic failures of the valves. Maintenance appears to be a normal process for machines operating at such high pressures. All three systems have been hampered by routine maintenance and repair needs. The use of a homogenizer, or any aggressive processing method for that matter, for the dispersion of CNF seems to be a balance between improving the dispersion of CNF as seen through increasing light transparency versus degradation of the CNF as seen through decreasing viscosity and mechanical properties. Figure 2 shows that the first passes of CNF through the homogenizer have a significant effect on CNF suspension light transparency, subsequent passes show diminishing improvements with each additional pass. Furthermore, the initial passes results in a significant increase in suspension viscosity, but beyond the first a couple passes at the most extreme homogenizer conditions, the suspension viscosity is observed to plateau then decline. This drop in viscosity with extensive processing probably indicates damage to the CNF and will likely result in diminished mechanical strengths of films or other applications.

Figure 2. Light transmittance of CNF suspension with increased levels of processing using a Microfluidizer. 239


Freeze Drying Cellulose Nanomaterials Cellulose, is hydrophilic and when the surface has been modified with carboxylate groups, as with cellulose nanofibrils (CNFs), or sulfate groups, as with cellulose nanocrystals (CNCs), they are even more hydrophilic. There are obvious economic issues associated transporting bulk CNFs around the country, or the world, as 1 wt % suspensions. Furthermore, many of research efforts are aimed at incorporating cellulose nanomaterials into hydrophobic matrixes and require a dry starting material. The critical need is for methods that provide a dry cellulose nanomaterial that can be redispersed easily at the point of use. Ideally, the method doesn’t have additives which need to be removed before use or which are detrimental to certain applications. For some applications, a material that is highly porous such that it can be impregnated with other matrixes is also of value. Attempts to redisperse air-dried cellulose nanomaterials demonstrated this was inadequate for many potential applications. FPL focused on freeze drying as a means to provide a dry form of cellulose nanomaterials that best fit most application needs. It should be noted that these techniques were developed at FPL using cellulose nanocrystals but were generally assumed to be applicable to cellulose nanofibrils as well.

Figure 3. Cellulose nanocrystals freeze dried and resuspended (6 wt%) with only stirring after different freezing methods compared to the initial (ini) CNC suspension (6 wt%). Sample A: simple, slow freezing of dish placed in freezer. Sample B: Rapid freezing of dish placed in liquid nitrogen. Sample C: Frozen layer that developed in aqueous suspension in small ice cream maker. Sample D: Frozen in small ice cream maker with 10 vol% t-butanol added. 240


When a suspension of aqueous cellulose nanomaterials is frozen and the water removed under high vacuum, e.g., freeze drying, the resulting dry material is a crunchy, crumbly mass containing large flakes and gaping pores. When mixed with water, the dried material produces a hazy suspension with some particles large enough to be seen with the naked eye (Figure 3A). Some of the ice crystals that formed during freezing, grew very large, and glacially concentrated the cellulose nanocrystals to such an extent that CNC films were literally cast around the ice crystals. The laboratory solution was to freeze the CNC and CNF suspensions as rapidly as possible by adding the suspension dropwise into liquid nitrogen (LN2). While the ability to redisperse is improved (Figure 3B), the method is not practicable at larger scale. Among applications where water needs to be frozen and ice crystal growth minimized includes ice cream and some types of frozen drinks. FPL purchased a small one-pint kitchen ice cream maker. The initial test with CNC suspension did not go well as a thick layer of ice quickly formed on the bowl of the mixer because the blade was unable to scrape the ice from the sides of the bowl. Freeze drying of this frozen layer, however, produced a CNC powder that resuspended easily and looked similar to material dried after flash freezing with LN2 (Figure 3C). In ice cream, the dissolved sugar and suspended fats help control how the ice cream suspension freezes. What was needed was an additive that would provide a freezing point depression like the sugar in ice cream. More specifically a freezing point depression results in freezing over a larger temperature range than is obtained with pure compounds. In addition, the additive needed to have a number of other properties to make it easier to work with and protect the freeze drying equipment: a freezing point similar to water so it would be captured in the freeze drier condenser and not damage the vacuum pump, and a vapor pressure higher than water so it would sublimate from the frozen CNC suspension and not contaminate the product. Tertiary-butyl alcohol, was selected. It has very high water solubility, a freezing point of 25°C and a boiling point of 83°C. Ten percent by volume was added to a rapidly stirred CNC suspension and in which was then transferred to the mini ice cream maker. Twenty minutes later a small bowl of CNC similar to a soft serve ice cream was transferred to a freezer to complete the freezing. After freeze drying the starting dry mass of dry CNCs was attained, confirming that the t-butanol had been removed and much of the t-butanol was captured in the condenser of the freeze drier. Upon resuspending in water with minimal mixing, the CNCs looked similar to the original, never-dried CNC suspension (Figure 3D). Optimizing the t-butanol addition, a 5% concentration did not prevent the freezing problems with the mixer seizing, and addition levels above 10% did not appear to provide additional benefits. The 10% addition level was selected to provide a margin of error for scale-up. FPL was able to obtain an unused convenience store frozen drink machine to produce the 150L of cellulose nanomaterial as a frozen slush needed to load a Vertis GPFD-XL35 freeze drier. With this equipment, a 150 L batch requires about three weeks to complete the drying process. The freeze drier has limited condenser capacity and needs to be operated at reduced vacuums to avoid collecting water in the vacuum pump oil. The t-butanol is recovered from the condenser and distilled to the 87 vol% t-butanol azeotrope for reuse. 241

The CNCs are typically freeze dried from a 10 wt% suspension in water and t-butanol and collected as dry sheets that crumble to a fine powder. TEMPO CNFs are freeze dried from a 1 wt% suspension in water and 10% t-butanol. The resulting sheets are similar to a Styrofoam board.


Conclusion FPL has been working toward producing cellulose nanomaterials at a scale suitable for preliminary research and product development. A simple process schematic that FPL is currently using for the conversion of bleached Kraft pulp to CNFs is shown in Figure 4. Cellulose nanocrystals have been distributed through the University of Maine’s Process Development Center since 2012 while cellulose nanofibrils produced using the TEMPO/hypochlorite method at pH 10 have been available since late 2015. Both are available as never-dried aqueous suspension and freeze-dried solids. FPL has scaled established laboratory methods for production of TEMPO-grade cellulose nanofibrils to several kilograms per batch. First, commercial bleached eucalyptus drylap is repulped and pretreated with sulfuric acid and with sodium chlorite at pH 2 to remove non-process elements and oxidize any residual lignin or chromophores that might consume reagents during the TEMPO treatment. The pulp is oxidized using TEMPO and sodium hypochlorite at pH 10 over the course of several hours under ambient conditions resulting in carboxylation of the surface of the elementary nanofibrils within the pulp fiber structure to levels near 1.5 mmol/g pulp. The washed, carboxylated pulp fiber, now swollen but intact, is subjected to a high shear environment in order to disrupt the fiber structure and disperse the carboxylated nanofibrils as a clear, viscous, colloidal suspension of approximately 1 wt% solids for distribution. FPL has also developed a method producing freeze-dried CNF. The aqueous suspension is mixed with 10% by volume t-butanol and partially frozen with a commercial soft-serve machine. This is spread into trays, frozen solid overnight and freeze-dried over the next several weeks. (The FPL freeze dryer is limited by its condenser capacity.) There are multiple aspects of the CNF chemistry and processing that must be balanced with each other to optimize once potentially successful uses begin to emerge. Does the application utilize the carboxylate surface chemistry of the TEMPO-grade CNF? If so, what level is required? Questions of surface chemistry must also be balanced with their effect on physical properties. Generally speaking, the higher the carboxylate level of the CNF, the more easily the CNF can be dispersed into a colloidal suspension. High levels of suspension and film transparency can ultimately be achieved with higher carboxylate levels on the CNF. However, higher levels of carboxylation generally results in a lower degree of polymerization of the cellulose chains within the nanofibril. This can potentially limit ultimate strength of the nanofibril and films or matrices into which it is incorporated.

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Figure 4. Process schematic for producing CNF from commercial bleached Kraft pulp. CNF is produced as aqueous suspension following mechanical dispersion of pulp carboxylated using TEMPO and sodium hypochlorite. Freeze drying is carried out as needed to produce dry CNF. Cellulose nanofibrils, and cellulose nanomaterials more generally, are a novel form of cellulose where the potential applications are just beginning to be explored. As with any new material, there are likely to be opportunities where cellulose nanomaterials provide just the right combination of chemical and physical properties to create new and unique applications that couldn’t be achieved, or even conceived, prior to their availability. But just as likely, cellulose nanomaterials will also be quietly incorporated into many products we are already using. Since cellulose nanomaterials are derived from renewable, bio-based feedstock, any application utilizing them will be considered to be “greener” than it was before, either from the standpoint of the ease of recyclability or from limited environmental impact caused by disposal. We are even likely to find some products, after use, might be disposed of by tossing it in the backyard compost pile. But “greener” also includes improving durability, increasing longevity and reducing material consumption, so perhaps we’ll also have things in front of us for decades with cellulose nanomaterials inside.

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