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Acknowledgements

I would like to express my gratitude to Ms.Nazia Noor madam, for her encouraging

guidance, exceptional advice in my research, and moral support through this dissertation

process.

I would also like to thank my honorable teacher, Dr. Muhammad Yusuf Miah and

Mohammad Saiful Alam, Md. Zakarul Islam for their advice and support in my project.

Sincere thanks also go to Md.Tanvir Hossain, Md. Touhidul Islam, Md.Zakarul Islam for

their suggestions, discussions, and friendships.

Finally, I give sincere thanks to my parents for supporting me throughout this process and for

their patience in waiting for me to finish this chapter of my life.

Most importantly, I think all of my loving friends, who has not only tolerated my

irritability and frustration during this process, but has encouraged me to be the best that I can

be.

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Summary

Nanocellulose (also known as microfibrillated cellulose) is a material made from wood

cellulose fibers, where the individual microfibrils have been detached from each other. It is

very thin (~20 nm) and long (70 nm to 1 μm) fiber. In low concentrations of a few per cent,

it is a gel-like, transparent material. Nanocellulose in composite materials with e.g.

polytrimethylene terephthalate, polyacrylate or polylactic acid has been studied.

A challenge to be solved is the economical, large scale production of nanocellulose. The

production of nanocellulose from wood is very energy consuming. Laboratory scale

production of nanocellulose is done by acid hydrolysis of microcrystalline cellulose.

In this review we describe various approaches to the preparation of nanocellulosic materials

from plant sources. The focus is on the extraction and investigation of microfibrillated

cellulose (MFC) in particular; however, to put this topic in context, cellulose whiskers and

bacterial cellulose are also briefly discussed in particular sections of the text and applications

of nanocellulosics in films or in other nanocomposite materials are included. Although it has

been suggested that cellulose nanofibers could be used as a rheology modifier in foods,

paints, cosmetics and pharmaceutical products, discussion of these applications is beyond the

scope of this paper.

A notable feature of cellulose nanofibers is their hydrophilicity, which makes them suitable

for combination with hydrophilic polymers but generally incompatible with hydrophobic

matri-ces. Various chemical modification methods have been explored to open up

possibilities for combining cellulose with hydrophobic polymers and these are discussed here.

Overall, this review aims to summarize current knowledge on the development and

application of Nanocellulose with a particular focus on manufacturing process of

nanocellulose.

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Table of Contents

SL No: Topics Page

1. Introduction 6-7

2. Introduction of the Report 8-9

3. Objective of the Report 09

4. Methodology 09

5. Limitations 10

6. Origin of Nano cellulose 11

7. Morphological Description 12-13

8. Source of Nano Cellulose 14-16

9. History 17-18

10. Literature Survey 19-26

11. Properties of Nano Cellulose 27-28

12. Uses of Nano Cellulose 29-32

13. Various Process of Nano Cellulose 33-36

14. Preparation & Characterization of Micro fibrillated Cellulose

37-42

15. Mechanical Treatment 44-46

16. Technology Selection of Nano Cellulose in Bangladesh

47-48

17. Preparation of Nano Composite Films

46

18. Major equipment in this process 49-51

19. Process Economics 52

20. Pollution, Safety Aspects 53

21. Conclusion 54

22. Reference 55-62

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Table of Figure

Sl No: (Figure No.)

Topics Page

1. Nanocellulose

06

2. Structure of cellulose micro fibril, showing hydrogen bonding with water

07

3. Crystalline and amorphous regions in cellulose microfibril

07

4. The Development of nanocellulosic fibers from cellulose

12

5. Length & width of Fibers & Fibrils. 12

6. structure and appearance of MFC by SEM: a) micro-scale; b) nano-scale

13

7. AFM images of MFC on mica after drying

13

8. Schematic diagram to show polymer nanofibers by electrospinning.

30

9. The main steps involved in the preparation of cellulose nanoparticles

40

10. The homogenzier 48

11. The microfluiizer 49

12. The micro-grinder 50

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Table of Data Sheet

Sl No. Topics Page

1. Nanocellulose Dimensions 11

2. Illustration of the annual number of scientific publications and patents since 1983, when the term microfibrillated cellulose was first introduced.

21

3. Examples of cellulose nanofiber

preparation procedures

41

4. Parameters used to procedure MFC from bleached and unbleached wood pulp fibers

42

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Introduction

Nanocellulose or microfibrillated cellulose (MFC) is a material composed

of nanosized cellulose fibrils with a high aspect ratio (length to width ratio). Typical

dimensions are 5–20 nanometers width and length up to 2000 nanometers. It is pseudo-

plastic. Moreover, nanocellulose exhibits the property of certain gelsor fluids that are thick

(viscous) under normal conditions, but flow (become thin, less viscous) over time when

shaken, agitated, or otherwise stressed. This property is known as thixotropy. When the

shearing forces are removed the gel regains much of its original state. The fibrils are isolated

from any cellulose containing source including wood-based fibers (pulp fibers) through high-

pressure, high temperature and high velocity impact homogenization (see manufacture

below). Nanocellulose can also be obtained from native fibers by an acid hydrolysis, giving

rise to highly crystalline and rigid nanoparticles (generally referred to as nanowhiskers)

which are shorter (100s to 1000 nanometers) than the nanofibrils obtained through the

homogenization route.

Nanocellulose or Microfibrillated Celluloses (MFCs) are generally considered to be fibrils

with diameters in the range of 10-100 nm liberated from larger plant based cellulose

fibers. MFCs have garnered much attention for use in composites, coatings, and films

because of high surface areas, renewability, and unique mechanical properties. Many of

the recent studies of MFC generated from wood pulp have focused on fully bleached

chemical pulps; however, these materials must be further modified to be incorporated in

hydrophobic matrices for composite reinforcement. The production of MFCs containing

less hydrophilic lignin may reduce the need for surface modifications. To investigate,

wood pulps of different chemical compositions were used to produce MFCs to

determine the effect of chemical composition on microfibril and film properties. It was

found, after homogenization, that the presence of lignin did not significantly decrease film

Figure 1: Nanocellulose

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mechanical properties, even with a significant decrease in density, contrary to the

physical properties of the produced hand-sheets. As expected, increasing film density

resulted in higher tensile indices. Samples containing lignin also had higher specific surface

areas and water retention values than samples without lignin.

Fig.2 Structure of cellulose microfibril, showing hydrogen bonding with water

Fig.3 Crystalline and amorphous regions in cellulose microfibril

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Three different types of mechanical processing equipment were used to generate MFC.

Results show that the micro fluidizer resulted in significantly tougher films than both micro-

grinding and homogenization and required less energy to obtain these properties,

offering great promise for producing MFC materials with lower energy input. The ability to

produce MFCs containing lignin could potentially provide new markets for MFC such as

composite reinforcements in hydrophobic matrices without surface modification, while

production with the micro fluidizer and the micro-grinder could provide a more

economically feasible production method as compared to the homogenizer. A

modification to the Congo red specific surface area measurement was developed to more

accurately compare surface areas between samples containing lignin and those without lignin.

One potential application for micro fibrillated cellulose is in the paper and packaging area.

Films of bleached hardwood produced by micro-grinding were modified using internal fillers

and surface coatings in an attempt to develop properties comparable to that of polyethylene.

It was determined that coating the MFC films with cooked starch, paraffin wax, and beeswax

resulted in water vapor transmission rates lower than low density polyethylene. The addition

of internal fillers such as kaolin clay and calcium carbonate decreased WVTR, hypothesized

to be due to an increase in path length because of decreased water vapor solubility

in the film. The determined pore radii of the MFC films by the Knudsen diffusion

model was in agreement with those calculated based on SEM imaging.

Introduction of the report

Each professional degree needs practical knowledge of the respective field of discipline to be

fruitful. Our Applied Chemistry & Chemical Engineering program also is similar, relating to

the exchange of theoretical knowledge into the real life practical situation. The report entitled

―Overall image & prospect of Nano Cellulose‖ originated from the partial fulfillment of the

Applied Chemistry & Chemical Engineering Course. The main purpose of the preparation of

the report is due to the partial fulfillment of the course of the Applied Chemistry & Chemical

Engineering Program.

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Objectives of the report

The General objective of the study is to provide an overview of Properties, Application &

Process of Nano Cellulose and fulfill the course requirement. Beside the general objective,

the report can be categorized into main objective and specific objectives. The objectives

behind this report are mentioned below:

Main Objective

The main objective of this study is to prepare a paper (which is a partial requirement of the

completion of this course) on the specified topic implementing the knowledge that has been

gathered over the semester at the Noakhali Science & Technology University.

Specific Objectives

The specific objectives of this report are as follows:

To understand the SWOT & 4P processes involved with the ROBI.

To identify the image of customer about ROB

Methodology

This report is a descriptive one, which was administered by collecting primary and secondary

data. Descriptive Research has an important objective: gives description of manufacturing

process, properties and uses of nano cellulose. The report tried to evaluate the manufacturing

process of Nano Cellulose. Before going in to the deep study, conceptual structure visualized

under which the whole study was conducted.

Preparing a report about the overall image & prospect of Nano Cellulose is a difficult and

complicated task and no single method is appropriate for preparing the report. For this reason,

a number of procedures have followed to prepare a meaningful report. The methodology of

the task can be depicted as follows:

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Sources of data

This study covered two types of data, which are:

• Primary data

• Secondary data

Primary Data:

Primary data will be collected through various websites.

Secondary Data:

Going through different documents and papers developed by the company personnel and by

others are the sources of secondary data.

Limitations

The study is not free from some practical limitations. Following limitations have faced during

the study and the time of working & data collection:

Time is the main limitation for my study. Due to unavailability of sufficient time, the

researcher will not be able to do survey among all of the sample size. That‘s why the

findings of the research will not be fully but partially true.

Due to lack of practical experience, some errors might be occurred during the study.

Therefore maximum efforts have given to avoid mistakes.

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Origin of Nanocellulose

MFC can be obtained from almost any cellulosic material such as beet pulp, potato

tuber cells, wheat straw, tunicin, crab shell chitin and bacterial cellulose [1]

. Among these

biomaterials, wood has been considered as an attractive source for production of MFC due to

its great abundance. [2]

To isolate MFC from wood, multi-stage process is required, involving chemical

delignification (pulping and bleaching), mechanical diminution and chemical diminution.

These processes are often used sequentially or in combination. [1]

The purpose of chemical delignification is to remove lignin, which connects microfibers

together. This process is considered as a promising initial step for the preparation of

nanocellulose materials [3]

. If colorless cellulosic nonmaterial‘s and a high degree of

crystalline are desired, bleaching treatments are required to allow any dissolved lignin or

carbohydrate byproducts to be incinerated, with the recovery of energy in the

delignification process[1]

. Mechanical diminution provides extensive separation of bleached

fibers into nano-sized fibrils. Conventional approach of refining is to pass the fibers between

rotating and stationary discs or cones having patterns of raised rectangular bars, separated by

groove spaces [1]

. Another approach of breaking fibers down to nano-sized fibrils is to

pass the material through a small nozzle at very high pressure. This approach is often used in

combination with other treatment [4]

Chemical diminution treatments are almost always combined with the mechanical

diminution treatments[1]

. Acid hydrolysis and enzymatic treatment are two major methods

for breaking down the amorphous cellulose and liberating cellulosic nano-sized crystals

into suspension.[5]

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.

Figure 4: The Development of nanocellulosic fibers from cellulose

molecules

Data Table 1: Nanocellulose Dimensions

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Morphological description

Techniques such as transmission electron microscopy (TEM), scanning electron

microscopy (SEM), atomic force microscopy (AFM) and wide-angle X-ray scattering

(WAXS) have been widely used to characterize MFC morphology. Although these

methods can provide information on MFC widths, it is hard to determine MFC lengths

due to entanglements. Both ends of individual MFC are very difficult to identify [6]

. Many

studies show that MFC are usually in the form of nanofiber aggregates due to the high

density of hydroxyl groups on the microfibril surface, which can strongly interact and lead

Figure 5: Length & width of Fibers & Fibrils.

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to aggregation [7]

. Examples of SEM and AFM images of MFC are showed in Figure.

Figure 7: AFM images of MFC on mica after drying

Figure 6: structure and appearance of MFC by SEM: a) micro-scale; b) nano-scale

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Source of Nanocellulose

Wood

The mechanical extraction of nanofibers from wood dates back to the 1980s when Herrick

and Turbak produced MFC from wood pulp using cyclic mechanical treatment in a high-

pressure homogenizer. The homogenization process resulted in disintegration of the wood

pulp and a material in which the fibers were opened into their sub-structural micro fibrils [8]

The resulting MFC gels consisted of strongly entangled and disordered networks of cellulose

nanofiber. Bleached Kraft pulp has often been used as the starting material for research on

MFC production. [9]

Agricultural crops and by-products

Although wood is certainly the most important industrial source of cellulosic fibers,

competition from different sectors such as the building products and furniture industries and

the pulp and paper industry, as well as the combustion of wood for energy, makes it

challenging to supply all users with the quantities of wood needed at reasonable cost. For this

reason fibers from crops such as flax, hemp, sisal, and others, especially from by-products of

these different plants, are likely to become of increasing interest. These non-wood plants

generally contain less lignin than wood and therefore bleaching processes are less

demanding. Other examples of agricultural by-products which might be used to derive

nanocellulose include those obtained from the cultivation of corn, wheat, rice, sorghum,

barley, sugar cane, pineapple, bananas and coconut crops.

Today, these agricultural by-products are either burned, used for low-value products such as

animal feed or used in biofuel production. Because of their renewability crop residues can be

valuable sources of natural nanofibers [10]

. In addition, when by-products, such as pulps after

juice extraction, are used as raw materials, fewer process-ing steps to obtain cellulose are

required [11]

. It should also be noted that in agricultural fibers the cellulose microfibrils are

less tightly wound in the primary cell wall than in the secondary wall in wood, thus

fibrillation to produce nanocellulose should be less energy demanding [12]

.

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Bacterial cellulose

In addition to its plant origins, cellulose fibers are also secreted extracellularly by certain

bacteria belonging to the genera Acetobacter, Agrobacterium, Alcaligenes, Pseudomonas,

Rhizobium,or Sarcina[13]

. The most efficient producer of bacterial cellulose (BC) is

Acetobacter xylinum (or Gluconacetobacter xylinus), a gram-negative strain of acetic-acid-

producing bacteria [14]

. There are important structural differences between BC and, for

example, wood cellulose. BC is secreted as a ribbon-shaped fibril, less than 100 nm wide,

which is composed of much finer 2–4 nm nanofibrils[15]

. In contrast to the existing methods

for obtaining nanocellulose through mechanical or chemo-mechanical processes, BC is

produced by bacteria through cellulose biosynthesis and the building up of bundles of

microfibrils [16]

. These microfibril bundles have excellent intrinsic properties due to their high

crystallinity (up to 84–89%; [17]

), including a reported elastic modulus of 78 GPa [18]

, which is

higher than that generally recorded for macro-scale natural fibers [19]

and is of the same order

as the elastic modulus of glass fibers (70 GPa;. Compared with cellulose from plants, BC also

possesses higher water holding capacity, higher degree of polymerization (up to 8,000), and a

finer web-like network [20]

. In addition, BC is produced as a highly hydrated and relatively

pure cellulose membrane and therefore no chemical treatments are needed to remove lignin

and hemicelluloses, as is the case for plant cellulose [21]

.

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History

The terminology micro fibrillated/nanocellulose or (MFC) was first used by Turbak, Snyder

and Sandberg in the late 1970s at the ITT Rayonier labs in Whippany, New Jersey, USA to

describe a product prepared as a gel type material by passing wood pulp through a Gaulin

type milk homogenizer at high temperatures and high pressures followed by ejection impact

against a hard surface.

The terminology (MFC) first appeared publicly in the early 1980s when a number of patents

and publications issued to ITT Rayonier on this totally new nanocellulose composition of

matter.[22]

In later work Herrick at Rayonier also published work on making a dry powder form of the

gel.[23]

Since Rayonier is one of the world's premier producer of purified pulps their business

interests have always been 1) to create new uses and new markets for pulps and 2) never to

compete with new or potentially new customers. Thus, as the patents issued,[24]

Rayonier

gave free license to whomever wanted to pursue this new use for cellulose.

Rayonier,as a company, never pursued scale-up. Rather, Turbak et al. pursued 1) finding new

uses for the MFC/nanocellulose. These included using MFC as a thickener and binder in

foods, cosmetics, paper formation, textiles, nonwovens, etc. and 2) evaluate swelling and

other techniques for lowering the energy requirements for MFC/Nanocellulose

production.[25]

ITT closed the Rayonier Whippany Labs in 1983–84 and further work on

making a dry powder form of MFC was done by Herric at the Rayonier labs in Shelton,

Washington, USA[23]

The field was later taken up in Japan in the mid 1990s by the group of Taniguchi and co-

workers and later by Yano and co-workers. and a host of major companies (see numerous

U.S. patents issued to P&G, J&J, 3M, McNeil, etc. using U.S. patent search under inventor

name Turbak search base). Today, there are still extensive research and development efforts

around the world in this field.

With the annual world consumption of plastic approaching 100 million tons, it is

imperative to develop bio-based, renewable alternatives (Clean Air Council 2006). In

addition to concerns related to land-filling, recent studies have shown that chemicals

commonly found in plastics can be a threat to human health. As an alternative to petroleum-

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based plastics, natural cellulose fibers are renewable, bio-degradable, do not contain

harmful chemicals, and can be used to develop materials with unique structural,

functional and optical properties. Currently, the amount of paper consumed globally is

around 400 million annual tons, an example of a hugely available renewable cellulose-based

material. Microfibrillated celluloses, MFCs, were first produced in 1983 by Turbak et

al. using wood pulp and a high pressure homogenizer, which promoted the

disintegration of cellulosic fibers into sub-structural fibrils and microfibrils having lengths in

the micron scale and widths ranging from 10 to a few hundred nanometers. This material was

found to form stable aqueous suspensions, providing an opportunity for multiple uses as

thickeners, emulsifiers or additives in food, paints and coatings, as well as cosmetics

and medical products.

Microfibrillated celluloses (MFCs) were first produced in 1983 by Turbak et al. using

purified cellulose from wood pulp and a high pressure homogenizer. Cellulosic fibers were

disintegrated into their sub-structural fibrils and microfibrils having lengths in the

micron scale and widths ranging from 10 to few hundred nanometers.

These materials were found to form stable aqueous suspensions providing an opportunity for

multiple uses as thickeners, emulsifiers or additives in food, paints and coatings, as

well as cosmetics and medical products. More recently, MFCs were produced by

combining mechanical shearing of wood pulp with enzymatic hydrolysis and TEMPO-

mediated oxidation.

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Literature Survey

This literature survey presents a brief but concise review of the current research efforts on

nanocellulose preparation, application and characterization. Review focuses on nanocellulose

based on all three types of multifunctional fillers: nanotube/fiber r einforced composites,

nanoplatelet-reinforced composites, nanoparticle-reinforced composites. Different processing

techniques for manufacturing the nanocellulose and their mechanical, thermal, electrical,

barrier and flammability properties are discussed in detail. Also, ordered mesoporous

nanosensors - photoluminescent and electroluminescent materials that are capable of

producing fast photon emissions with practical applications for military body armor, aircraft

and ship hull penetration detection is discussed.

Survey indicates that a new area of nanocellulose research has emerged in the last two

decades that utilizes nanoparticle fillers to alter the properties of polymers and other matrices.

Definitive results have not yet been achieved, but trends show that when processed properly,

small amounts (≤ 5 wt. %) of nanoparticle fillers can increase the modulus, strength,

toughness, resistance to chemical attack, gas impermeability, resistance to thermal

degradation, and dimensional stability of polymeric materials. A point worthy of note in this

study of nanoparticles and nanocomposites is the flammability resistance effect of

nanoparticles. The intumescent model indicates that the flame retardancy mechanism

involves a high-performance carbonaceous-silicate char; this char build-up insulates the

underlying material. Understanding this char build-up mechanism presents a challenge and

area of research interest in the effort to develop blast resistant materials and structures.

Nanocellulose extracted by a mechanical disintegration process from wood cell was

first obtained by Herrick et al. [26]

, and Tubark et al. [27]

, in 1983. This new type of

cellulosic material was named microfibrillated cellulose (MFC). MFC can be viewed as a

cellulosic material, composed of expanded high-volume cellulose, moderately degraded

and greatly expanded in surface area, obtained by a homogenization process [28]

.

Contrary to straight cellulose whiskers, cellulose microfibrils are long and flexible

nanoparticles. MFC is composed of more or less individualized cellulose microfibrils,

presenting lateral dimensions in the order of 10 to 100 nm, and length generally in the

micrometer scale [29]

, and consisting of alternating crystalline and amorphous domains.

Another noteworthy difference between these two kinds of nanoparticles is that MFC

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presents a web like structure [30]

. Very recently, researchers have managed to evaluate the

elastic modulus of single cellulosic micro fibrils using atomic force microscopy (AFM).

They proposed a value of 145.2 ± 31.3 Gap for microfibrils prepared by TEMPO-oxidation

and 150.7 ± 28.8 GPa for microfibrils produced by acid hydrolysis [31]

.

In the 1980s, different research groups reported MFC as a low-cost and totally new form of

cellulose. It has a large surface area as result of heat and mechanical action. In these

studies, the authors worked with a Gaulin homogenizer, model 100-KF3-8BS, using a

pressure of 8,000 psi. Cooling was used to maintain a product temperature in the range of

70–80 °C during the homogenization treatment. Initially, the wood pulp was precut to reduce

the fiber length to 0.6–0.7 mm. After repeated homogenization treatments, they obtained a

diluted dispersion of MFC, having a gel-like appearance.

Another piece of equipment appeared as an alternative to the use of the Gaulin homogenizer.

The microfluidizer from Microfluidics Inc., USA, is a piece of equipment that also allows the

defibrillation of cellulosic pulps. The fiber suspension is led through thin z-shaped

chambers under high pressure. Pressure can reach levels as high as 30,000 psi. When the

pressurized product enters in the interaction chamber and passes through geometrically fixed

microchannels, very high velocities are achieved. At that point, two primary forces act in the

product stream. One of the forces occurs as a result of product stream with the channel walls

at high velocity. The shear results in a deformation of the product stream. The other is

produced by the impact of the high velocity stream upon itself. At the end of the process a

heater exchanger returns the product stream to ambient temperature.

With either equipments—Gaulin homogenizer or microfluidizer—it is necessary to

repeat the procedure of homogenization several times, in order to increase the degree

of fibrillation [34]

.

Nakagaito et al. indirectly evaluated the degree of fibrillation of kraft pulp by water

retention, measured as moisture content after centrifuging a 2 wt% fiber suspension of

treated pulp slurry at 1,000 G for 15 min. The fiber slurry was passed through a high

pressure homogenizer 2, 6, 14, 22, and 30 times. They observed that the disintegration

could be improved by increasing the number of passes to 30 times. It is not difficult to

conclude that a higher number of passes results in an increased energy necessary for the

disintegration.

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Pretreatments have been developed by some researchers in order to solve the problem

of energy consumption during the process. This is one of the main drawbacks related

to the process of MFC production. A European project (SUNPAP, FP7) is aimed at scaling

up the MFC process for industrial applications. The target application is packaging.

Zimmerman et al. applied an acid hydrolysis step before pumping the sulfite pulp through the

homogenizer. In their experiments, 5 g of oven dried pulp were hydrolyzed by 200

mL of sulfuric acid (10 wt%) under stirring at 60 °C for 16 h. After centrifugation and

washing steps, the suspension was neutralized with sodium hydroxide (0.1 M). Finally, the

suspension was homogenized with a microfluidizer (M-100Y High Pressure Pneumatic

Microfluidic Processor, Newton, MA). The sulfuric acid treatment, combined with

mechanical dispersion, resulted in finer fibril structures than MFC obtained only by a

mechanical treatment. The former produced diameters below 50 nm, but their lengths were

still in the micrometer range.

Another treatment that has been used in combination with mechanical shearing is

enzymatic hydrolysis. Henriksson et al. [27]

treated cellulosic wood fiber pulps with pure C-

type endoglucanase in order to facilitate the disintegration of MFC. Hydrochloric acid was

also used as a pretreatment step.

In their work, these authors used an endoglucanase manufactured by Novozymes A/S

Denmark. They considered the enzymatic treatment as an environmentally friendly

process since it did not involve solvents or chemical reactants. The MFC obtained by

enzymatically pretreated pulps showed more favorable structures, with higher aspect

ratio than MFC resulting from acid hydrolysis treatment.

However, they demonstrated that a high concentration enzymatic treatment can increase the

extent of fine material and reduce the fiber length. An increasing fiber swelling in water was

observed due to the enzymatic treatment.

Similar studies were carried out by the group of Ankerfors et al. [28]

. First, sulfite pulp was

refined to increase the accessibility of the cell wall for subsequent enzymatic treatment

with endoglucanase (Novozym 476, Novozymes North America Inc., Franklinton, NC). The

enzymatic treatment was done at 50 °C for 2 h. The concentration was 0.17 μL of

monocomponent endoglucanases per gram fiber (5 ECU/μL). After stopping the enzymatic

treatment, the material was passed through the microfluidizer (Microfluidics M-110EH

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Microfluidizer Processor, Newton, MA). Additionally, the diameter of the interaction

chamber was varied by changing the interaction chamber. They first passed the slurry

through chambers of 400 and 200 μm three times, and then five times through a chamber

pair of 200 and 100 μm. The operation pressures were 105 and 170 MPa, respectively. They

highlighted the importance of milder hydrolysis provided by enzymatic treatment. Compared

to the more aggressive acid hydrolysis treatment, the enzymatic treatment yielded

longer and highly entangled nanoscale fibrils. They demonstrated that the enzymatic

hydrolysis step avoids blocking problems during the homogenization treatment. Finally,

the enzymatic step leads to reduced energy consumption allowing widespread use of

the material and an industrial pilot is being built in their laboratory since May 2010.

Saito et al. [29]

have proposed a new process to obtain MFC based on TEMPO

reaction and strong mixing. In their study [30]

, individualized MFC was obtained by TEMPO-

mediated oxidation at room temperature and stirring at 500 rpm. They determined that at pH

10, optimal conditions were reached, giving cellulose nanofibers with 3–4 nm in width and

a few microns in length.

Tubark et al. [27]

and Herrick et al. [26]

suggested a wide range of potential commercial uses

for MFC in the earliest 80s. They proposed some applications, e.g., in foods, cosmetics,

paints, paper and nonwoven textiles, oils field services, and medicine. Recently, because of

its properties such as high strength, flexibility and aspect ratio, many research groups have

focused their attention on the use of MFC as a reinforcing phase in nanocomposites.

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Properties of Nanocellulose

Due to their abundance, high strength and stiffness, low weight and biodegradability, nano-

scale cellulose fiber materials (e.g., microfibrillated cellu-lose and bacterial cellulose) serve

as promising candidates for bio-nanocomposite production. Such new high-value materials

are the subject of contin-uing research and are commercially interesting in terms of new

products from the pulp and paper industry and the agricultural sector. Cellulose nanof-ibers

can be extracted from various plant sources and, although the mechanical separation of plant

fibers into smaller elementary constituents has typically required high energy input, chemical

and/or enzy-matic fiber pre-treatments have been developed to overcome this problem. A

challenge associated with using nanocellulose in composites is the lack of compatibility with

hydrophobic polymers and various chemical modification methods have been explored in

order to address this hurdle. This review summa-rizes progress in nanocellulose preparation

with a particular focus on microfibrillated cellulose and also discusses recent developments in

bio-nanocomposite fabrication based on nanocellulose.

1. Nanocellulose dimensions and crystallinity

The ultra structure of cellulose derived from various sources has been extensively studied.

Techniques such as transmission electron microscopy (TEM),scanning electron

microscopy (SEM), atomic force microscopy (AFM), wide angle X-ray scattering (WAXS),

small incidence angle X-ray diffraction and solid state 13

C cross-polarization magic angle

Data Table 2: Illustration of the annual number of scientific publications and patents since 1983, when the term microfibrillated

cellulose was first introduced.

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spinning (CP/MAS) nuclear magnetic resonance (NMR) spectroscopy have been used to

characterize nanocellulose morphology. These methods have typically been applied for the

investigation of dried nanocellulose morphology.[35]

Although a combination of microscopic techniques with image analysis can provide

information on nanocellulose fibril widths, it is more difficult to determine nanocellulose

fibril lengths because of entanglements and difficulties in identifying both ends of individual

nanofibrils. It is often reported that nanocellulose suspensions are not homogeneous and that

they consist of cellulose nanofibers and nanofiber bundles.

Most methods have typically been applied to investigation of dried nanocellulose dimensions,

although a study was conducted where the size and size-distribution of enzymatically pre-

treated nanocellulose fibrils in a suspension was studied using cryo-TEM. The fibrils were

found to be rather mono-dispersed mostly with a diameter of ca. 5 nm although occasionally

thicker fibril bundles were present. It should be noted that, some newly published results

indicated that by combining ultrasonication with an "oxidation pretreatment", cellulose

microfibrils with a lateral dimension that belows 1 nm is observed by AFM. The lower end of

the thickness dimension is around 0.4 nm, which is believed to be the thickness of a cellulose

monolayer sheet.[36]

The aggregate widths can be determined by CP/MAS NMR developed by Innventia AB,

Sweden, which also has been demonstrated to work for nanocellulose (enzymatic pre-

treatment). An average width of 17 nm has been measured with the NMR-method, which

corresponds well with SEM and TEM. Using TEM, values of 15 nm have been reported for

nanocellulose from carboxymethylated pulp. However, also thinner fibrils can also be

detected. Wågberg et al. reported fibril widths of 5–15 nm for a nanocellulose with a charge

density of about 0.5 meq./g. The group of Isogai reported fibril widths of 3–5 nm for

TEMPO-oxidized cellulose having a charge density of 1.5 meq./g.[37]

The influence of cellulose pulp chemistry on the nanocellulose microstructure has been

investigated using AFM to compare the microstructure of two types of nanocellulose

prepared at Innventia AB (enzymatically pre-treated nanocellulose and carboxymethylated

nanocellulose). Due to the chemistry involved in producing carboxymethylated

nanocellulose, it differs significantly from the enzymatically pre-treated one. The number of

charged groups on the fibril surfaces is very different. The carboxymethylation pre-treatment

makes the fibrils highly charged and, hence, easier to liberate, which results in smaller and

more uniform fibril widths (5–15 nm) compared to the enzymatically pre-treated

nanocellulose, where the fibril widths were 10–30 nm.[38]

The degree of crystallinity and the

cellulose crystal structure of nanocellulose were also studied at the same time. The results

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clearly showed the nanocellulose exhibited cellulose crystal I organization and that the degree

of crystallinity was unchanged by the preparation of the nanocellulose. Typical values for the

degree of crystallinity were around 63%.[38]

2. Viscosity

The unique rheology of nanocellulose dispersions was recognized by the early

investigators.[39]

The high viscosity at low nanocellulose concentrations makes nanocellulose

very interesting as a non-calorie stabilizer and gellant in food applications, the major field

explored by the early investigators.

The dynamic rheological properties have been investigated in great detail and it has been

found that the storage and loss modulus were independent of the angular frequency at all

nanocellulose concentrations between 0.125% to 5.9%. The storage modulus values are

particularly high (104 Pa at 3% concentration) compared to results for cellulose nanowhiskers

(102 Pa at 3% concentration).[39]

There is also a particular strong concentration dependence

as the storage modulus increases 5 orders of magnitude if the concentration is increased from

0.125% to 5.9%.

Nanocellulose gels are also highly shear thinning (the viscosity is lost upon introduction of

the shear forces). The shear-thinning behaviour is, of course, particularly useful in a range of

different coating applications.

3. Mechanical properties

It has long been known that crystalline cellulose has interesting mechanical properties for use

in material applications. The stiffness of crystalline cellulose has been shown to be in the

order of 140–220 GPa, which is in the same size order as for instance Kevlar and is better

than, for example, glass fibers, both fibers are used commercially to reinforce plastics. Films

made from nanocellulose have been shown to have high strength (over 200 MPa), high

stiffness (around 20 GPa) and high strain (12 %).

4. Barrier properties

In semi-crystalline polymers, the crystalline regions are considered to be gas impermeable.

Due to relatively high crystallinity,[38]

in combination with the ability of the nanofibers to

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form a dense network held together by strong inter-fibrillar bonds (high cohesive energy

density), it has been suggested that nanocellulose might act as a barrier material.[39]

Although the number of reported oxygen permeability values is limited, reports attribute high

oxygen barrier properties to nanocellulose films. One study reported an oxygen permeability

of 0.0006 (cm³ µm)/(m² day kPa) for a ca. 5 µm thin nanocellulose film at 23 °C and 0 %

RH.[40]

In a related study, a more than 700-fold decrease in oxygen permeability of a

polylactide (PLA) film when an nanocellulose layer was added to the PLA surface was

reported.[37]

The influence of nanocellulose film density and porosity on film permeability remains

relatively unexplored. Some authors have reported significant porosity in nanocellulose

films,[40]

which seems to be in contradiction with high oxygen barrier properties, whereas

Aulin et al. measured a nanocellulose film density close to density of crystalline cellulose

(cellulose Iß crystal structure, 1.63 g/cm³)[41]

indicating a very dense film with a porosity

close to zero.

Changing the surface functionality of the cellulose nanoparticle can also affect the

permeability of nanocellulose films. Films constituted of negatively charged cellulose

nanowhiskers could effectively reduce permeation of negatively charged ions, while leaving

neutral ions virtually unaffected. Positively charged ions were found to accumulate in the

membrane.[42]

5. Foams

Nanocellulose can also be used to make aerogels/foams, either by itself or in composite

formulations. Nanocellulose-based foams are being studied for packaging applications in

order to replace polystyrene-based foams. Svagan et al. showed that nanocellulose has the

ability to reinforce starch foams by using a freeze-drying technique.[43]

The advantage of

using nanocellulose instead of wood-based pulp fibers is that the nanofibrills can reinforce

the thin cells in the starch foam. Moreover, it is possible to prepare pure nanocellulose

aerogels applying various freeze-drying and super critical CO2 drying techniques.

Aerogels and foams can be used as porous templates, potentially useful in various

nanoapplications.[44][45]

Tough ultra-high porosity foams prepared from cellulose I nanofibrill

suspensions were studied by Sehaquiet al. A wide range of mechanical properties including

compression was obtained by controlling density and nanofibrill interaction in the

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foams.[46]

Cellulose nanowhiskers could also be made to gel in water under low power

sonication giving rise to aerogels with the highest reported surface area (>600m2/g) and

lowest shrinkage during drying (6.5%) for cellulose aerogels.[47]

In another study by Aulin et al.,[48]

the formation of structured porous aerogels of

nanocellulose by freeze-drying was demonstrated. The density and surface texture of the

aerogels was be tuned by selecting the concentration of the nanocellulose dispersions before

freeze-drying. Chemical vapour deposition of a fluorinated silane was used to uniformly coat

the aerogel to tune their wetting properties towards non-polar liquids/oils. The authors

demonstrated that it is possible to switch the wettability behaviour of the cellulose surfaces

between super-wetting and super-repellent, using different scales of roughness and porosity

created by the freeze-drying technique and change of concentration of the nanocellulose

dispersion. Structured porous cellulose foams can however also be obtained by utilizing the

freeze-drying technique on cellulose generated by Gluconobacter strains of bacteria, which

bio-synthesize open porous networks of cellulose fibers with relatively large amounts of

nanofibrills dispersed inside.

Olsson et al.[49]

Demonstrated that these networks can be further impregnated with

metalhydroxide/oxide precursors, which can readily be transformed into grafted magnetic

nanoparticles along the cellulose nanofibers. The magnetic cellulose foam may allow for a

number of novel applications of nanocellulose and the first remotely actuated magnetic super

sponges absorbing 1 gram of water within 60 mg cellulose aerogel foam were reported.

Notably, these highly porous foams (>98% air) can be compressed into strong magnetic

nanopapers, which may find use as functional membranes in various applications.

6. Surface modification

The surface modification of nanocellulose is currently receiving a large amount of

attention.[50]

Nanocellulose displays a large amount of hydroxyl groups at the surface which

can be reacted. However, hydrogen bonding strongly affects the reactivity of the surface

hydroxyl groups. In addition, impurities at the surface of nanocellulose such as glucosidic

and lignin fragments need to be removed before surface modification to obtain acceptable

reproducibility between different batches.[51]

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Uses of Nanocellulose

1. Paper and paperboard

The potential of nanocellulose applications in the area of paper and paperboard manufacture

is obvious. Nanocelluloses are expected to enhance the fiber-fiber bond strength and, hence,

have a strong reinforcement effect on paper materials. Nanocellulose may be useful as a

barrier in grease-proof type of papers and as a wet-end additive to enhance retention, dry and

wet strength in commodity type of paper and board products.

2. Composite

As described above the properties of the nanocellulose makes an interesting material for

reinforcing plastics. Nanocellulose has been reported to improve the mechanical properties of

e.g. thermosetting resins, starch-based matrixes, soy protein, rubber latex, poly(lactide). The

composite applications may be for use as coatings and films, paints, foams, packaging.

3. Food

Nanocellulose can be used as a low calorie replacement for today‘s carbohydrate additives

used as thickeners, flavour carriers and suspension stabilizers in a wide variety of food

products and is useful for producing fillings, crushes, chips, wafers, soups, gravies, puddings

etc. The food applications were early recognised as a highly interesting application field for

nanocellulose due to the rheological behaviour of the nanocellulose gel.

4. Hygiene and absorbent products

Different applications in this field include:

Super water absorbent (e.g. for incontinence pads material)

Nanocellulose used together with super absorbent polymers

Use of nanocellulose in tissue, non-woven products or absorbent structures

Use as antimicrobial films

5. Emulsion and dispersion

Apart from the numerous applications in the area of food additives, the general area of

emulsion and dispersion applications in other fields has also got some attention. Oil in water

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applications were early recognized. The area of non-settling suspensions for pumping sand,

coal as well as paints and drilling muds was also explored by the early investigators.

6. Oil recovery

Hydrocarbon fracturing of oil-bearing formations is a potentially interesting and large-scale

application. Nanocellulose has been suggested for use in oil recovery applications as a

fracturing fluid. Drilling muds based on nanocellulose has also been suggested.

7. Medical, cosmetic and pharmaceutical

The use of nanocellulose in cosmetics and pharmaceuticals was also early recognized. A wide

range of high-end applications have been suggested:

Freeze-dried nanocellulose aerogels used in sanitary napkins, tampons, diapers or as

wound dressing

The use of nanocellulose as a composite coating agent in cosmetics e.g. for hair,

eyelashes, eyebrows or nails

A dry solid nanocellulose composition in the form of tablets for treating intestinal

orders

Nanocellulose films for screening of biological compounds and nucleic acids

encoding a biological compound

Filter medium partly based on nanocellulose for leukocyte free blood transfusion

A buccodental formulation, comprising nanocellulose and a polyhydroxylated organic

compound

Powdered nanocellulose has also been suggested as an excipient in pharmaceutical

compositions

nanocellulose in compositions of a photoreactive noxious substance purging agent

8. Other applications

Nanocellulose used to activate the dissolution of cellulose in different solvents

Regenerated cellulose products, such as fibers films, cellulose derivatives

Tobacco filter additive

Organometallic modified nanocellulose in battery separators

Nanocellulose reinforcement of conductive materials

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The use of nanocellulose in loud-speaker membranes

High-flux membranes

Various process of Nanocellulose

A variety of techniques have been used to make nanocellulose.

1. By mechanical fibrillation

The fibrillation of pulp fiber to obtain nano-order-unit web-like network structure, called

microfibrillated cellulose, is obtained through a mechanical treatment of pulp fibers,

consisting of refining and high pressure homogenizing processes. The refining process used

is common in the paper industry, and is accomplished via a piece of equipment called a

refiner. In a disk refiner, the dilute fiber suspension to be treated is forced through a gap

between the rotor and stator disks, which have surfaces fit-ted with bars and grooves, against

which the fibers are subjected to repeated cyclic stresses.

This mechanical treatment brings about irreversible changes in the fibers, increasing their

bonding potential by modification of their morphology and size. In the homogenization

process, dilute slurries of cellulose fibers previously treated by refining are pumped at high

pressure and fed through a spring high pressure loaded valve assembly. As this valve opens

and closes in rapid succession, the fibers are subjected to a large pressure drop with shearing

and impact forces. This combination of forces promotes a high degree of microfibrillation of

the cellulose fibers, resulting in microfibrillated cellulose [52]

.

The refining process is carried out prior to homogenization due to the fact that refining

produces external fibrillation of fibers by gradually peeling off the external cell wall layers (P

and S1 layers) and expos-ing the S2 layer and also causes internal fibrillation that loosens the

fiber wall, preparing the pulp fibers for subsequent homogenization treatment [53]

.

Nakagaito et al. [54]

studied how the degree of fibrillation of pulp fibers affects the mechanical

properties of high strength cellulose composites. It was found that fibrillation solely of the

surface of the fibers is not effective in improving composite strength, though there is a

distinct point in the fibrillation stage at which an abrupt increase in the mechanical properties

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of composites occurs. In the range between 16 and 30 passes through refiner treatments, pulp

fibers underwent a degree of fibrillation that resulted in a stepwise increment of mechanical

properties, most strikingly in bending strength. This increase was attributed to the complete

fibrillation of the bulk of the fibers. For additional high pressure homogenization-treated

pulps, composite strength increased linearly against water retention values, which

characterize the cellulose‘s exposed surface area, and reached maximum value at 14 passes

through the homogenizer.

2. By electro spinning of polymer

Electro spinning derived from electrostatic spinning. Electro spinning has been recognized as

an efficient technique for the fabrication of polymer nanofibers. Various polymers have been

success-fully electrospun into ultrafine fibers e.g. cellulose acetate. There are basically three

components to fulfill the process: a high voltage supplier, a capillary tube with a pipette or

needle of small diameter, and a metal collecting screen. In the electrospinning process a high

voltage is used to create an electrically charged jet of polymer solution or melt out of the

pipette. Before reaching the collecting screen, the solution jet evaporates, and is collected as

an interconnected web of small fibers [55, 56]

.

One electrode is placed into the spinning solution/melt; the other is attached to the collector.

The electric field is subjected to the end of the capillary tube that contains the solution fluid

held by its surface tension. This induces a charge on the surface of the liquid. The potential

difference depended on the properties of the spinning solution, such as polymer molecular

weight and viscosity. When the distance between the spinneret and the collecting device was

short, spun fibers tended to stick to the collecting device as well as to each other, due to

incomplete solvent evaporation. Mutual charge repulsion and the contraction of the surface

charges to the counter electrode cause a force directly opposite to the surface tension [57]

. As

the intensity of the electric field is increased, the hemispherical surface of the fluid at the tip

of the capillary tube elongates to form a conical shape known as the Taylor cone [58]

. By

further increasing in the electric field, a critical value is attained with which the repulsive

electrostatic force overcomes the surface tension and the charged jet of the fluid is ejected

from the tip of the Taylor cone. The discharged polymer solution jet undergoes an instability

and elongation process, which allows the jet to become very long and thin. Meanwhile, the

solvent evaporates, leaving behind a charged polymer fiber. In the case of the melt the

discharged jet solidifies when it travels in the air.

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Most of the polymers were dissolved in some sol-vents before electrospinning. When the

solid polymer or polymer pellet is completely dissolved in a proper amount of solvent that is

held, for example, in a glass container, it becomes a fluid form called polymer solution. The

polymer fluid is then introduced into the capillary tube for electrospinning.

Both the dissolution and the electrospinning are essentially conducted at room temperature

with atmosphere condition.

Polymers, molten in high temperature, can also be made into nanofibers through

electrospinning. Instead of a solution, the polymer melt is introduced into the capillary tube.

However, different from the case of polymer solution, the electrospinning process for a

polymer melt has to be per-formed in a vacuum condition [59]

. Namely, the capillary tube, the

traveling of the charged melt fluid jet and the metal collecting screen must be encapsulated

within a vacuum. A schematic diagram to interpret electrospinning of polymer nanofibers is

shown in Figure 6.

A polymer solution, such as cellulose acetate dissolved in 2:1 acetone: dimethyl acetamide

was introduced into the electric field. The polymer filaments were formed, from the solution,

between two electrodes bearing electrical charges of opposite polarity. One of the electrodes

was placed into the solution and the other onto a collector. Once ejected out of metal

spinnerets with a small hole, the charged solution jets evaporated to become fibers which

were collected on the collector [60]

.

Figure 8. Schematic diagram to show polymer nanofibers by electrospinning.

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3. From sea animals

Colloidal suspensions of cellulose whiskers in water were prepared as following; the shells of

the tunicates cut into small fragments and bleach by three successive treatments with sodium

hypochlo-rite in dilute acetic acid. Heat the mixture to 70–80°C and keep at this temperature

for 1 hour.

After the third cycle, the tunicate mantles isolate via decanting, wash with ice water, and

disintegrate in blender into an aqueous suspension (tunicate content ~3% w/w). The

disintegrated mantles sub-sequently hydrolyze by adding concentrated sulfu-ric acid, heating

the mixture to 80°C, and rigorous stirring at this temperature for 20 minute to yield a

suspension of cellulose whiskers. After washing with water until the pH is neutral, adding

water so that the whisker concentration, suspension of cellu-lose whiskers will be obtained [61]

.

4. From microcrystalline cellulose in organic solvent

The microcrystalline cellulose was swelled and partly separated to whiskers by chemical and

ultra sonification treatments. Dimethyl acetamide with 0.5 wt% LiCl solution was used as

swelling agent. The microcrystalline cellulose in LiCl/dimethyl acetamide was 10 wt% which

was agitated using a magnetic stirrer for 12 hour at 70°C to swell the microcrystalline

cellulose particles. The slightly swelled particles were then sonicated in an ultra-sonic bath

for 3 hours over a period of 5 days with long intervals between each sonication treatment, to

separate cellulose nano whiskers [62]

.

5. By acid hydrolysis

Suspensions of nanocrystalline cellulose were pre-pared. Hydrolysis was carried out with

sulfuric acid with constant stirring. Immediately following the acid hydrolysis, the suspension

dilute 10-fold with deionized water to quench the reaction. The suspension centrifuges at

6000 rpm for 10 min to concentrate the cellulose and to remove excess aqueous acid. The

resultant precipitate should be rinsed, recentrifuged, and dialyzed against water for 5 days

until constant neutral pH [63]

.

6. From bacterial cellulose

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Cellulose can be synthesized by some bacteria. For example, the cellulose was produced by

static cultivation of Acetobacter xylinum, sub species BPR2001, in a fructose/CSL medium at

30°C [64]

. The bacteria were grown in 400 ml Erlenmeyer flasks containing 100 ml of media.

In order to remove the bacteria and to exchange remaining media, the produced cellulose

pellicles were boiled in 1 M NaOH at 80°C for 1 hour fol-lowed by repetitive boiling in

deionised water. To prevent drying and to avoid contamination, the washed cellulose was

stored in diluted ethanol in a refrigerator.

The advantage in using bacterial cellulose as a model for plant cellulose lies in its high purity,

fine fibrils (high surface area) [65],

high tensile strength and water-holding capacity. So,

bacterial cellulose has been used as a reinforcing in nanocomposites [66]

.

Preparation and characterization of

microfibrillated cellulose

Microfibrillated celluloses have impressive mechanical properties, making the material ideal

as a composite reinforcement and, concurrently, to reduce the utilization of petroleum based

components. For example, a 10% phenol-formaldehyde (PF) resin and MFC composite

obtained a 370 MPa bending strength, in comparison to a 2.5% PF resin and pulp composite

that obtained approximately 150 MPa bending strength. Today, MFCs with diameters in

the 10-100 nm range can be produced from various sources such as wood and non-

wood fibers, bacteria, and animal-derived cellulose. MFCs are typically produced by four

mechanical methods: homogenization, microfluidization, micro-grinding, and cryocrushing,

all of which consume energy at different levels.

The production of MFC by fibrillation of cellulose fibers into nano-scale elements requires

intensive mechanical treatment. However, depending upon the raw material and the degree of

processing, chemical treatments may be applied prior to mechanical fibrillation. These

chemical processes are aimed to produce purified cellulose, such as bleached cellulose pulp,

which can then be further processed. There are also examples with reduced energy demand in

which the isolation of cellulose microfibrils involves enzymatic pre-treatment followed by

mechanical treatments. Depending upon the raw materials and fibrillation techniques, the

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cellulose degree of polymerization, morphology and nanofiber aspect ratio may vary.

Examples of cellulose nanofiber preparation methods, including MFC, are shown in Table 2.

A major obstacle that needs to be overcome is the high energy consumption connected to the

mechanical disintegration of the fibers into MFC, which often involves several passes

through the disintegration device. Values around 20,000–30,000 kWh/ton are not uncommon.

Even higher values reaching 70,000 kWh/ton have also been reported. By combining the

mechanical treatment with certain pre-treatments (e.g., chemical or enzyme) it is possible to

decrease the energy onsumption significantly to the level of 1,000 kWh/ tone.

1. Pretreatments Utilized in the Production of Microfibrillated Cellulose

Harsh mechanical pretreatments damage the microfibril structure by reducing molar mass and

degree of crystallinity and can also be energy intensive. For these reasons, chemical

pretreatments such as acid hydrolysis, alkaline, enzymatic hydrolysis, and TEMPO

mediated oxidation have been introduced and each improves processing in different

ways, all reducing processing energy consumption.

Acid hydrolysis

Acid hydrolysis assists in processing by making the cell wall brittle and removing

amorphous regions. This embrittlement is expected to be the result of the molecular

reduction in cellulose chain length, which also results in a 5 to 10 fold decrease in

the degree of polymerization and the production of irregular fragments . A major

disadvantage to utilizing the acid pretreatment, however, is that it can result in a

significantly lower molecular weight cellulose, mainly microcrystalline cellulose (MCC),

which reduces the reinforcing effect of the material if used in composites.

Alkaline pretreatments

Before mechanical processing, a number of researchers have applied alkaline treatment of

fibers in order to distrupt the lignin structure and help to separate the structural linkages

between lignin and carbohy-drates. Purification by mild alkali treatment results in the

solubilization of lignin and remaining pectins and hemicelluloses. Alkali extrac-tion needs to

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be carefully controlled to avoid undesirable cellulose degradation and to ensure that

hydrolysis occurs only at the fiber surface so that intact nanofibers can be extracted.

Alkaline pretreatment improve processing by swelling the pulp fibers, making defibrillation

easier. The swelling promotes water uptake within the fiber, reducing hydrogen bonding,

and resulting in less energy required to defibrillate the macrofibrils into microfibrils. It

is also expected that alkaline pretreatments result in the reorganization of the microfibrils

because of inhomogeneous swelling; this reorganization also promotes defibrillation because

of reduced hydrogen bonding.

TEMPO-mediated oxidation

TEMPO-mediated oxidation is expected to result in improved MFC individualization by

electrostatic repulsion from the introduction of negatively charged surface groups. The

utilization of acid hydrolysis, alkaline, and enzymatic hydrolysis in the wood and

paper industry is commonly used, however, TEMPO is an emerging technology not

commonly used in industrial applications related to the pulp and paper industries. A

major challenge for the application of TEMPO industrially is cost, but a significant

energy reduction in producing these materials could overcome this challenge and

promote the production of these renewable materials for new applications.

Oxidation pre-treatment

Saito et al. introduced an oxidation pre-treatment of cellulose, applying 2,2,6,6 tetramethylpi-

peridine-1-oxyl (TEMPO) radicals before mechanical treatment in a Waring blender.

TEMPO-mediated oxidation is a promising method for surface modifi-cation of native

celluloses, by which carboxylate and aldehyde functional groups can be introduced into solid

native celluloses under aqueous and mild conditions. In the case of such oxi-dations the

nature of the products obtained is highly dependent on the starting materials. When regener-

ated and mercerized celluloses are used, water-soluble b-1,4-linked polyglucuronic acid

sodium salt (cellouronic acid) with a homogeneous chemical structure can be obtained

quantitatively as the oxidized product. On the other hand, when native celluloses are used, the

initial fibrous morphology is mostly maintained, even after the TEMPO-mediated oxidation

under harsh conditions. In this case, the oxidation occurred only at the surface of the

microfibrils, which became negatively charged. This negative charge resulted in repulsion of

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the nanofibers, thus easing fibrillation. As can be seen in Fig. 2. NaBr and NaClO are

generally used as additional catalyst and primary oxidant respectively at pH 9–11. In order to

avoid undesirable side reactions under alkaline conditions, such as significant

depolymerization or discoloration of the oxidized cellulose due to the presence of aldehyde

group residuals, Saito et al. applied a TEMPO/NaClO/NaClO2 system under neutral or

slightly acidic conditions. These authors demonstrated that the new oxidation system allowed

almost complete maintenance of the original DP, uniform nanofiber distribution (& 5 nm in

width) and a material free of aldehyde groups. Films prepared from TEMPO-oxidized

cellulose gels had high transparency, high toughness and low density.

Enzymatic pre-treatment

Enzymatic pre-treatments enable the manufacture of MFC with significantly reduced energy

consumption. In nature, cellulose is not degraded by a single enzyme but a set of cellulases

are involved. These can be classified as A- and B-type celluloses, termed cellobiohydrolases,

which are able to attack highly crystalline cellulose and C- and D-type cellulases or

endoglucanases which generally require some disorder in the structure in order to degrade

cellulose. Cellobiohydrolases and endoglucanases show strong synergistic effects. During

preparation of MFC, isolated cellulases have been applied which modify rather than degrade

the cellulose prepared MFC by treating bleached kraft pulp with OS1, a fungus isolated from

trees infected with Dutch elm disease. A significant shift towards lower fiber diameters

occurred as a result of this enzyme treatment. Fungus OS1 had only mild activity against

cellulose, which is of interest as this minimizes the loss of cellulose during MFC preparation.

Henriksson et al. (2007) and Pa¨a¨kko¨ et al. (2007) found that endoglucanase pre-treatment

facilitates disintegration of cellulosic wood fiber pulp into MFC nanofibers.

Moreover, the MFC produced from enzymatically pretreated cellulosic wood fibers showed a

more favorable structure than nanofibers produced by subjecting pulp fiber to strong acid

hydrolysis. Pretreated fibers subjected to the lowest enzyme concentration (0.02%) were

successfully disintegrated while molecular weight and fiber length were well preserved

(Henriksson et al. 2007). Lo´ pez-Rubio et al. (2007) and Svagan et al. (2007) also combined

mechanical and enzymatic treatments. The cell wall delamination was carried out by treating

the pulp in four separate steps: a refining step using an Escher– Wyss refiner in order to

increase the accessibility of the cell wall to the subsequent enzyme treatment, an enzymatic

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treatment step using monocomponent endoglucanase, a second refining stage, and finally a

step in which the pulp slurry was passed through a high-pressure microfluidizer.

Mechanical treatments

1. Refining and high-pressure homogenization

When processing with a homogenizer, pulp is passed through either one or two

stages, where the fibers are subjected to rapid pressure drops, high shear, and impact forces

against a homogenization valve and an impact ring , Figure 1.3. The pressure drop is

typically 8,000 psi (55 MPa) in Manton-Gaulin 15MR homogenizers, and the fibers

are cycled through the homogenizer approximately 10-20 times. Besides the larger

energy consumption, a main disadvantage of the homogenization of wood fibers is that

long fibers often clog the system, particularly at the in-line valves, which then must

be disassembled and cleaned.

However, the homogenizer can easily be scaled to industrial production and can be operated

continuously.

The manufacture of MFC is now generally per-formed by a mechanical treatment consisting

of refining and high pressure homogenizing process steps. Using a disk refiner, the dilute

fiber suspension is forced through a gap between rotor and stator disks. These disks have

surfaces fitted with bars and grooves against which the fibers are subjected to repeated cyclic

stresses.

This mechanical treatment brings about irreversible changes in the fibers, increasing their

bonding potential by modification of their morphology and size. However, mechanical

refining methods tend either to damage the microfibril structure by reducing molar mass and

degree of crystallinity or fail to sufficiently disintegrate the pulp fiber. The refining process is

carried out prior to homogenization due to the fact that refining results in external fibrillation

of fibers by gradually peeling off the external cell wall layers (P and S1 layers) and exposing

the S2 layer. Internal fibrillation loosens the fiber wall which prepares the pulp fibers for

subsequent homogenization treatment.

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During homogenization dilute slurries of refined cellulose fibers are pumped at high pressure

and fed through a spring-loaded valve assembly. As this valve opens and closes in rapid

succession, the fibers are subjected to a large pressure drop with shearing and impact forces.

This combination of forces promotes a high degree of microfibrillation of the cellulose fibers,

resulting in the production of MFC. Zimmermann et al. (2004) and Lo´ pez-Rubio et al.

(2007) reported the mechanical fibrillation process using a microfluidizer in the

homogenization step. Such mechanical dispersion of pulp fibers leads to fibril structures with

diameters between 20 and 100 nm and estimated lengths of several tens of micrometers.

When a cellulosic pulp fiber suspension is homoge-nized the procedure is often repeated

several times in order to increase the degree of fibrillation. For example, Leitner et al. (2007)

ran a suspension of sugar beet pulp cellulose through a high-pressure laboratory homogeniser

operated at 300 bar for 10–15 cycles. However, with increasing homogenization cycles, the

energy demand increases and can be as high as 30,000 kWh/t (Nakagaito and Yano 2004;

Lindstro¨m 2007). Iwamoto et al. (2005) reported that after 14 cycles, further homogenizing

up to 30 cycles did not improve fibrillation. This observation was supported by Malainine et

al. (2005) who achieved the desired fibrillation by applying 15 passes through a laboratory

homogenizer operated at 500 bar. Dufresne et al. (2000) also used the same operating

conditions to produce MFC from potato pulp.

2. Microfludization:

Processing with a microfluidizer reduces the likelihood of clogs because it has no in-line

moving parts. Pulp is passed through an intensifier pump that increases the outlet pressure to

40,000 psi (276 MPa), followed by an interaction chamber which defibrillates the fibers by

shear forces and impacts against the channel walls and colliding streams. The

microfluidizer operates at a constant shear rate, compared to the homogenizer, which

operates at a constant processing volume. The interaction chamber can be designed with

different geometries to produce different sized materials and plugging can be resolved using

reverse flow through the chamber, Figure 1.4. For example, in the case of a ―Y‖ shaped

orifice, the high pressure inlet is the bottom of the ―Y‖ and is then split into two

streams. These streams collide with the walls at a 45

o angle and then enter a high shear zone which results in the streams interacting with the

wall at 90o and a reduction in pipe diameter, both effective in reducing particle size.

The two streams then meet and collide with each other in the high impact zone, before exiting

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through the low pressure outlet. The utilization of piping angles and colliding streams

reduces the need for moving parts, such as utilized by the homogenizer, reducing processing

and plugging issues.

3. Microgrinding

Modified commercial grinders with specially designed disks have been used by some

researchers in order to fibrillate cellulose fibers. In such equipment the cellulose slurry is

passed between a static grind stone and a rotating grind stone revolving at * 1,500 rpm. The

fibrillation mechanism of the grinder treatment can be explained as follows: the cell wall

structure consisting of nanofibers in a multi-layered structure and hydrogen bonds is broken

down by the shearing forces generated by the grinding stones and then nano-sized fibers are

individualized from the pulp. As an example, Taniguchi and Okamura (1998) obtained

microfibrillated fibers having diameters in the range 20–90 nm by a unique super-grinding

procedure. When homogenized cellulosic pulp was subjected to a grinder treatment by

Iwamoto et al. (2005, 2007), the fibril bundles were further fibrillated and 10 repetitions of

the grinder treatment resulted in uniform nanofibers 50–100 nm wide. During the grinding

process, the shearing force generated by the grinding stones could degrade the pulp fibers,

which might affect the reinforcing potential of MFC and therefore the phys-ical properties of

composites based on the fibrillated pulp fibers.

As a result of the complicated multilayered structure of plant fibers and interfibrillar

hydrogen bonds, a common feature of all disintegration methods is that the material obtained

consists of aggregated nanofibers with a wide distribution in width. However, Abe et al.

(2007) also reported an efficient extraction of wood cellulose nanofibers as they exist in the

cell wall, with a uniform width of 15 nm, by a very simple mechanical treatment. This result

was achieved by keeping the material in the water-swollen state after the removal of lignin

and hemicellulose, thus avoiding the generation of strong hydrogen bonding between the

cellulose bundles, which often takes place during drying processes.

During micro-grinding, wood fibers are forced through a gap between a rotary and a

stator disk, Figure 1.5; these disks have bursts and grooves that contact the fibers to

disintegrate them into the sub-structural components. Contact with the hard surfaces

and repeated cyclic stresses result in the defibrillation of the fibers.

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Typically, the material used for the disks is silicon carbide with a grit class of 46. The disks

can be produced using different grit classes and different groove configurations to alter flow

patterns during processing. Disk maintenance and replacement can be a drawback since

wood pulp fibers can wear down the grooves and grit. However, the main advantage

of processing with the micro-grinder is that the mechanical, fiber shortening

pretreatment utilized with other processing techniques is not required.

4. Cryocrushing

Alemdar and Sain (2008a) extracted MFC from wheat straw and soy hulls via mechanical

treatment involving cryocrushing followed by disintegration and fibrillation. These authors

found that almost 60% of the nanofibers had a diameter within a range of 30–40 nm and

lengths of several thousand nanometers. Cryocrushing is an alternative method for producing

nanofibers in which fibers are frozen using liquid nitrogen and high shear forces are then

applied. When high impact forces are applied to the frozen fibers, ice crystals exert pressure

on the cell walls, causing them to rupture and thereby liberating microfibrils (Wang and Sain

2007a). The cryocrushed fibers may then be dispersed uniformly into water suspension using

a disintegrator before high pressure fibrillation. Bhatnagar and Sain (2005) obtained

nanofibers with an estimated diameter of 5–80 nm by applying cryocrushing of chemically

treated flax, hemp, and rutabaga fibers. Cryocrushing combined with a high-pressure fibrilla-

tion process was used also by Wang and Sain (2007a, b) for the isolation of nanofibers with

diameters in the range 50–100 nm from soybean stock.

The main issue when processing wood pulps to produce MFC with the homogenizer and the

microfluidizer is fiber length, which causes fibril entanglement and clogging of the

equipment. Mechanical, chemical, and enzymatic pretreatments are used to reduce fiber size

and/or to pre-defibrillate the fibers, thus reducing the frequency of equipment clogging.

Such pretreatments also remove the primary cell wall, where the microfibrils are

organized randomly, which efficiently exposes the more organized fibrils located in the

secondary cell wall for further processing. Alternatives for mechanical reduction of fiber

size include disk refiners, PFI mills, manual cutting, and Valley beaters, which can be used

prior to the production of MFC.

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Figure 9. The main steps involved in the preparation of cellulose nanoparticles

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Data Table 3. Examples of cellulose nanofiber preparation procedures

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Technology Selection of NC in Bangladesh

For Bangladesh, I would like to suggest Never-dried bleached (BHW) and unbleached

(UBHW) Kraft hardwood pulps as main raw materials for the production of nanocellulose.

Never-dried bleached (BHW) and unbleached (UBHW) kraft hardwood pulps were

obtained from pulp mills. The bleached hardwood sample contained approximately 1.3%

total lignin and the unbleached sample contained approximately 2.4% total lignin. The

average length of the fibers was approximately 0.96 mm and the average diameter was

approximately 20.5 µm for both samples. As a pretreatment step to reduce fiber length, pulps

were mechanically refined in a laboratory Valley beater for a total refining time of 3 hours at

2% solids content and a 0.5 kg load. The resulting slurries of refined fibers were stored at 4o

C until needed for further processing to produce MFC.

Date Table 4. Parameters used to procedure MFC from bleached and unbleached wood pulp

fibers by homozenization, microfluidization & micro grinding

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Homogenization of the refined fiber slurries was performed with a two-stage 15MR

Manton-Gaulin homogenizer at approximately 0.7% solids content. The operation pressure

was maintained at 55 MPa (8000 psi), and processing was temporarily ceased when the

temperature of the slurry reached approximately 90o C to prevent pump cavitation.

Processing recommenced when the samples had cooled to approximately 45oC. Samples

were collected after 1, 4, 8, 12, 16, and 20 homogenizer passes and stored at 4oC.

Microfluidization of the refined fiber slurries was performed at 0.7% solids content with a

M-110EH and a M-110P processor at operating pressures of 69 MPa (10,000 psi),

138 MPa (20,000 psi), and 207 MPa (30,000 psi). The main difference between the M-

110EH and the M-110P units was that the first was pilot/production scale equipment, whereas

the second was for laboratory use. The chambers utilized were the G10Z (Z configuration,

100 µm orifice diameter) and H210Z (Z configuration 200 µm), and samples were taken

intermittently for up to 20 passes.

Micro-grinding of the refined fiber slurries was performed with a 10-inch Masuko Super

MassColloider at 0.7% and 3% solids and at a rotor speed of 1,500 rpm. Samples were taken

intermittently for analysis during processing for up to 9 passes. Micro-grinding was also

performed on a sample without a refining pretreatment, as the micro-grinder operation

can accommodate untreated fibers at original fiber lengths.

Finally, in cryocrushing, liquid nitrogen is used to freeze the water in the wood pulp and a

mortar and pestle are used to produce a high impact force to liberate the fibrils from the cell

all

Energy calculations for the micro-grinder and the homogenizer

Energy calculations for the micro-grinder and the homogenizer were performed using the

amperage as measured with an ammeter, the flow rate, and the voltage of the equipment. In

the case of the homogenizer, calculations accounted for its sinusoidal amp usage.

The microfluidizer energy calculations were provided from Microfluidics, Inc. Specific

energy consumption was reported in units of kJ per kg of MFC and kWh per metric

ton MFC produced on a dry basis.

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Experiment for determination of properties:

Specific surface area (SSA) of the fibers and microfibrils was determined using a Congo red

dye adsorption method. Aqueous suspensions of fibrous samples were adjusted to pH

6 and treated with varying amounts of Congo red, incubated at 60oC for 24 hours,

and centrifuged at 12,000 rpm (14,000 g) for 15 minutes. UV-Vis absorption at a 500 nm

wavelength was measured to determine Congo red concentration in the supernatant.

This information was used to determine the adsorbed mass and thus the respective

SSA assuming an area per molecule of 1.73 nm 2 by the depletion method, as

discussed in our previous work.

It is possible that Congo red specific surface area (SSA) is affected by the chemical

composition of the samples, and therefore, comparisons of different fiber/fibril types must be

exercised with caution. To account for this issue, specific surface areas of freeze-

dried bleached and unbleached fibers/microfibrils were also measured with a nitrogen

adsorption technique, which was assumed to be independent of the chemical

composition of the MFC samples. Based on the Congo red and BET techniques, it

was determined that the unbleached samples adsorbed about 1.81 times more Congo red per

unit of BET surface area than bleached samples. Thus, all measured Congo red SSA values

for the unbleached samples were divided by 1.81 to provide a comparative SSA

measurement between the samples in the wet state.

Prior to film production, the MFC suspension was de-aerated under vacuum for 10

minutes in an ultrasound bath followed by manual shaking. Films were produced

using a casting-evaporation technique onto petri dishes with a target weight per unit

area (basis weight) of 30g/m2. Dried films were conditioned at 23 oC and 50 %

ambient relative humidity. Typical time required for drying and conditioning was five days.

Film thickness and roughness were determined using standard TAPPI methods (T411

1997; T555 1999) with a Lorentzen and Wettre Micrometer 51 (L&W, Stockholm, Sweden)

and a L&W Parker Print Surface Tester, respectively. Roughness was measured on both the

air and dish side surfaces with a modified clamp pressure of 3.4 kPa (0.5 psi). The weight

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per unit area (or basis weight) was determined using TAPPI standard T410 (T410 1998) and

the apparent film density was calculated using the thickness and the measured basis weight.

The average and standard deviation of five measurements were reported. It is noted that

conventional paper testing methods used here to determine the thickness of the films

are likely to overestimate the thickness, and, therefore, underestimate the film density

(Chinga-Carrasco and Syverud 2010). Optical properties (opacity, color, ISO brightness,

and scattering coefficient) were measured using TAPPI standard methods (T452, T519,

T527) and a Technidyne Color Touch 2 ISO Model (Technidyne Corporation, New

Albany, Indiana, USA) (T452 1998; T519 1996; T527 1994). The average and standard

deviation of five measurements were reported.

Water vapor transmission rate (WVTR) was determined using a wet cup method.

Film samples were cut into 3.9 cm diameter circles and restrained above 50 ml of water in a

closed container. The container was placed on a gravimetric balance (Mettler Toledo

PG 203-S, Mettler Toledo, Columbus, Ohio, USA) with sensitivity of 0.001grams

interfaced with a computer for data acquisition. Ambient conditions, temperature and

relative humidity, were kept constant during all measurements at 23oC and 50% RH,

respectively. Weight data were taken every 3 seconds for at least one hour and the

slope of the generated water loss curve and film thicknesses were used to calculate

the specific WVTR for each sample. Sample data and calculations appear in Appendix

1, section 5. Water adsorption was determined by placing a 3.9 cm diameter circle of the

MFC film in a petri dish containing 30 ml of deionized water. The weight of the

film before and after 10 minutes of immersion in the water was obtained to

determine the amount of water adsorbed. The average and standard deviation of three

measurements were reported.

Tensile strength of the MFC films was determined using an Instron 4411 apparatus

(Instron, Norwood, Massachusetts, USA) with a modified TAPPI standard testing procedure

(T404, 1992). Samples were 15 mm wide and the clamp span was set to 25.4 mm. Crosshead

speed was set to 4 mm/min. Tensile index, the tensile strength divided by the basis weight,

was reported to account for variations in film basis weight that could skew the

tensile strength results. The average and standard deviation of three measurements

were reported. Example Instron data and toughness calculations appear in Appendix 1,

section 2.

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Preparation of Nanocomposite Films

1. From regenerated cellulose

Microcrystalline cellulose powder is produced byacid hydrolysis of amorphous domains,

which results in high crystallinity. Microcrystalline cellu-lose was activated for 6 hours in

distilled H2O at room temperature. Subsequently, the cellulose was dehydrated in ethanol,

acetone, and N,N-dimethyl acetamide for 4 hours each. After decanting N,N-dimethyl

acetamide from the dehydrated cellulose, LiCl/N,N-dimethyl acetamide solution was poured

onto cellulose sample and stirred for 5 minutes.

The solutions were then poured into Petri dish, and left at ambient atmosphere for 12 hour.

After this time a 5–8 mm thick transparent gel had formed which was washed in distilled

water and dehydrated between gently compressed sheets of paper.

The final nanocomposite films were optically trans-parent and had a thickness between 0.2

and 0.5 mm [96].

2. By solution casting

For preparing solid polymer nanocomposite film, combine appropriate amounts of the

nanoreinforcement‘s solution and dissolved polymer matrix. Two processing conditions can

be used to prepare the composites film from this mixture. The mixture cast in a Petri dish and

put in a drying oven under vacuum. The chosen temperature allows the solvent evaporation

and the film formation (i. e. polymer particles coalescence). A so-called evaporated film is

obtained and materials compression in mold under heating and pressure [92, 97].

The second route used to elaborate composite film, the mixture is first freeze-dried to allow

water sublimation, and a compact soft powder is obtained. This powder then press under

heating and pressure [62].

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3. By extrusion

For mixing dry material with suspension solution; the composite materials will be

compounded usinga co-rotating twin-screw extruder with a gravimetric feeding system for

dry materials and a peristaltic pump for the cellulose whiskers suspension. Figure 7, shows a

schematic picture of the compounding process.

4. By electrostatic layer-by-layer

Electrostatic layer-by-layer self-assembled films have been exploited for the fabrication of

sophisticated nanocomposite incorporated the linear polymer cellulose sulfate. In this

method, a charged solid substrate is exposed to a solution of oppositely charged

polyelectrolyte, followed by rinsing.

The polymeric material adhering to the surface has more than the stoichiometric number of

charges required for charge neutralization, thereby reversing the surface charge. This allows

for easy adsorption of the next oppositely charged polyelectrolyte, also resulting in charge

reversal. The amount of adsorbed polymer is self-limiting as a result of rinsing and allows for

stepwise film growth.

Resultant films and coatings show long-life stability as well as selfhealing characteristics.

Structured layer-by-layer films have potential applications as antireflective coatings , wave-

guides, bio/optical sensors, separation technologies and drug delivery systems. Conven-

tionally, layer-by-layer assembly has employed solution-dipping (or dipcoating) in beakers of

various sizes containing dilute aqueous polymer solutions. This inexpensive method works

for most substrates independent of shape but has not always resulted in adequately

homogeneous films. Alter-natively, spin-coating is the most widely used technique for

obtaining uniform films in lithography and other micromachining applications. The spin-

coating process involves the acceleration of a liquid solution on a rotating substrate and is

characterized by a balance of centrifugal forces (spin speed) and viscous forces (solution

viscosity). Films created by this way have been found to be consistent and reproducible in

thickness.

Nanocrystalline cellulose is amenable to sequential film growth by layer-by-layer assembly,

as presented schematically in Figure 8.

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Thin multilayered films incorporating polyelectrolyte layers such as poly (allylamine

hydrochloride) and nanocrystalline cellulose layers were prepared by the electrostatic layer-

by-layer methodology, as well as by a spin coating variant. Both techniques gave rise to

smooth and stable thin films, as confirmed by atomic force microscopy surface morphology

measurements as well as scanning electron microscopy investigations. Films prepared by

spincoating were substantially thicker than solution-dipped films. Thus both techniques are

viable for producing structured nanocomposites, where the large aspect ratio cellulose units

may serve to strengthen the elastic polymer matrix.

Major Equipments used in this Process

1. Homogenization:

When processing with a homogenizer, pulp is passed through either one or two

stages, where the fibers are subjected to rapid pressure drops, high shear, and impact forces

against a homogenization valve and an impact ring (Nakagaito and Yano 2004), Figure

1.3. The pressure drop is typically 8,000 psi (55 MPa) in Manton-Gaulin 15MR

homogenizers, and the fibers are cycled through the homogenizer approximately 10-20

times (Turbak et al. 1983; Andresen et al. 2006; Andresen et al. 2007; Erkisen et al.

2008; Herrick et al. 1983; Stenstad et al. 2008; Syverud and Stenius 2009). Besides the

larger energy consumption, a main disadvantage of the homogenization of wood fibers

is that long fibers often clog the system, particularly at the in-line valves, which then

must be disassembled and cleaned.

However, the homogenizer can easily be scaled to industrial production and can be operated

continuously.

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2. Microfludization:

Processing with a microfluidizer reduces the likelihood of clogs because it has no in-line

moving parts. Pulp is passed through an intensifier pump that increases the outlet pressure to

40,000 psi (276 MPa), followed by an interaction chamber which defibrillates the fibers by

shear forces and impacts against the channel walls and colliding streams

(Microfluidics Incorporated, 2010). The microfluidizer operates at a constant shear

rate, compared to the homogenizer, which operates at a constant processing volume. The

interaction chamber can be designed with different geometries to produce different sized

materials and plugging can be resolved using reverse flow through the chamber, Figure 1.4.

For example, in the case of a ―Y‖ shaped orifice, the high pressure inlet is the bottom

of the ―Y‖ and is then split into two streams. These streams collide with the walls at a 45

o angle and then enter a high shear zone which results in the streams interacting with the

wall at 90o and a reduction in pipe diameter, both effective in reducing particle size.

The two streams then meet and collide with each other in the high impact zone, before exiting

through the low pressure outlet. The utilization of piping angles and colliding streams

reduces the need for moving parts, such as utilized by the homogenizer, reducing processing

and plugging issues.

Figure 10: The homogenzier

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During micro-grinding, wood fibers are forced through a gap between a rotary and a

stator disk, Figure 1.5; these disks have bursts and grooves that contact the fibers to

disintegrate them into the sub-structural components (Nakagaito and Yano 2004).

Contact with the hard surfaces and repeated cyclic stresses result in the defibrillation

of the fibers.

Figure 11. The microfluidizer

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Typically, the material used for the disks is silicon carbide with a grit class of 46. The disks

can be produced using different grit classes and different groove configurations to alter flow

patterns during processing. Disk maintenance and replacement can be a drawback since

wood pulp fibers can wear down the grooves and grit. However, the main advantage

of processing with the micro-grinder is that the mechanical, fiber shortening

pretreatment utilized with other processing techniques is not required.

The main issue when processing wood pulps to produce MFC with the homogenizer and the

microfluidizer is fiber length, which causes fibril entanglement and clogging of the

equipment. Mechanical, chemical, and enzymatic pretreatments are used to reduce fiber size

and/or to pre-defibrillate the fibers, thus reducing the frequency of equipment clogging.

Figure 12. The micro-grinder

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Such pretreatments also remove the primary cell wall, where the microfibrils are

organized randomly, which efficiently exposes the more organized fibrils located in the

secondary cell wall for further processing (Montanari et al. 2005). Alternatives for

mechanical reduction of fiber size include disk refiners, PFI mills, manual cutting, and Valley

beaters, which can be used prior to the production of MFC.

Process Economics

Energy consumption for the processing methods studied varied significantly because of

parameters such as number of passes and flow rate. Homogenization, for example,

had a much slower processing flow rate than the microfluidizer and the micro-grinder,

which significantly increased the amount of energy required for processing. The

processing rates were approximately 2 kg/min, 1 kg/min, and 0.2 kg/min for micro-

grinding, microfluidiziation, and homogenization, respectively. Increasing the pressure

of the microfluidizer also significantly increased energy consumption, as the pump

required more amps to reach higher pressures. In summary, to produce MFC films

with maximum obtainable properties for each processing method, the total energy

required was approximately 9,180 kJ/kg for the microfluidizer with pretreatment, 9,090

kJ/kg for the grinder with pretreatment, and 5,580 kJ/kg for the grinder without

pretreatment and 31,520 kJ/kg for the homogenizer with pretreatment. The energy

consumption values reported herein are within the range of literature reported values and

slightly greater than those reported for MFC materials prepared after chemical pretreatments

(Siro and Plackett 2010).

Assuming an energy cost of $60 per megawatt-hour, homogenization with the refining

pretreatment was the least cost effective method in producing MFC, with an energy cost of

$650/ton, Table 5.5. When comparing the energy and raw material costs of homogenization

to common plastic market prices, homogenization is not economically feasible ($1300

per ton for homogenized MFC vs $1400 per ton market value for high density polyethylene),

as further processing (and therefore cost) would be required to utilize MFCs as

plastics or reinforcements. Microfluidization at 69 MPa (10,000 psi) (with the

pretreatment) was $140/ton and micro-grinding was $95/ton and $210/ton, without and

with the pretreatment, respectively. The ability to eliminate the pretreatment with

micro-grinding significantly reduces the energy costs. Assuming raw material costs of

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$350/ton for unbleached hardwood, $650/ton for bleached hardwood, and $100 for

recycled fiber, it can further be expected that processes in which the unbleached fibers did

not cause processing issues, such as micro-grinding and microfluidization will be favored for

producing MFCs. The most cost efficient production of MFC could be obtained by

utilizing recycled fibers and the micro-grinder without a pretreatment.

Pollution

In nanocellulose manufacturing process there is no adverse effect in Environment.

Safety Aspects

There is no adverse effect in environment of nanocellulose manufacturing, so there is no

safety aspects needed

Data Table 3: Comparison between petroleum-based plastic market values & raw material & energy costs of MFCs produced with homogenization,microfluidization and

micro-grinding

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Conclusion

In the present work, MFCs produced from wood pulps containing extractives,

hemicelluloses, and lignin were used to produce films by a casting-evaporation

technique. Film physical, mechanical, optical, and water interaction properties were

investigated. These properties were hypothesized to depend on the chemical composition of

the MFCs and films; therefore, the elucidation of the effect of pulp type (chemical

composition) on such properties is the focus of my research so that the employment of such

materials, either alone or mixed in hydrophobic matrices (for example in packaging and

composite manufacture) can be realized.

Reducing the processing energy requirement is one of the most important requisites to

produce MFCs on an industrial scale. This research has shown that compared to

homogenization, microfluidization with a refining pretreatment and the micro-grinding

of wood fibers are production methods which require less energy and produce MFC films

with better mechanical properties. It was hypothesized that the microfluidizer was

more successful at producing these properties at lower energy input due to

significantly higher shear rates (10,000,000+ and 650,000 s-1 for the microfluidizer

and homogenizer, respectively) and by the utilization of colliding streams instead of

moving parts to break down the cellulosic fibers into microfibrillated cellulose.

Because microfluidization uses a constant shear rate to defibrillate the material (in

contrast to the homogenizer which is a constant volume process) it was likely that the size

distribution of the materials was smaller and more uniform, resulting in better film formation

and better mechanical properties. It was also possible that the collision of the materials in the

high impact zone results in curling of the microfibrils; the more uniform distribution of

microfibril diameters resulted in microfibrils which could consolidate more during

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drying, and the curling resulted in higher mechanical properties through a distribution of

tensile stress upon stretching of the material.

This consolidation is in agreement with the higher densities of the films produced

from microfluidization. Production of MFCs with a homogenizer, however, resulted

inmicrofibrils with the highest specific surface area and films with the lowest water vapor

transmission rate.

Overall, it is concluded that films produced in this study by the microfluidizer and the micro-

grinder had superior physical, optical, and water interaction properties than homogenization

suggesting that these materials could be produced in a more economically feasible

way for potential packaging applications.

Reference:

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