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AALTO UNIVERSITY

Literature Review of Cellulose Acetylation

Puu-0.4100 Advanced Biomaterial Chemistry and Technology course report

Wang Lei

1/11/2013

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Abstract

The aim of this paper is to have a review of acetylation of cellulose. In total eleven

papers are reviewed and summarized in this paper. Different acetylation methods are

discussed and compared. Acetylation processes follow the first-order kinetic law. There

are two distinguished reaction mechanisms: when there is a diluent, cellulose undergoes

acetylation without changing its morphology; otherwise cellulose morphology is

disturbed. The crystalline structure of cellulose nanocrystal is preserved after

acetylation. Acetylated cellulose exhibited an increased solubility and dispersion in

various solvents. Acetylated MFC films have good gas barrier properties, but poor

water vapor transfer rate.

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1. Introduction

Natural cellulose fiber is one of the most abundant and low-cost renewable rawmaterials. Meanwhile people are actively searching for alternatives for fossil based

packaging materials. That is why cellulose, especially nano-scaled cellulose, hasattracted a lot of attention due to its appealing intrinsic properties such as nano-scaled

dimensions, high surface area, unique morphology, low density, and mechanicalstrength, as well as the fact that they are biodegradable[6]. In packaging polymer

material, it is important to have strong mechanical properties and to provide good

control of mass transfer between food and the environment. A lot of research focuses on

isolation and modification of nanocellulose and utilize it as reinforcement in polymers,

so that it can be competitive with conventional packaging materials.

Despite all the great potential of nanocelluloses mentioned above, the difficulty of

dispersing highly polar cellulose fiber in non-aqueous medium or polymers is one of the

main challenges. The difficulty of uniformly dispersing nano-sized materials in liquidsis mainly because of their high surface energy. Moreover due to the hydroxyl group

located on surface of cellulose, the surface is hydrophilic. In order to decrease the

hydrophilic characteristics of the fibers and improve the surface adhesion between the

continuous and dispersed phases, chemical modifications of the cellulose are needed [4;

8].

Acetylation is one of the most commonly used modification methods. In acetylationreactions, OH group of cellulose is substituted with acetyl group; therefore the

hydrophilic property is modified to more hydrophobic. Meanwhile moderateacetylation does not change the original crystalline structure of cellulose, so the desired

properties are also preserved. [6; 7]

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2. Acetylation methods

Depending on the purpose of the research, acetylation methods can be different. Table 1

lists the reaction characteristics of 8 papers among those I reviewed [1-8]. As we cansee, most of the research used acetic anhydride as reactant; catalysts such as H2SO4 andHClO4 are used in most cases, since it can increase the reaction speed; temperature is

also a variable for reaction. Both homogeneous and heterogeneous processes are used.

Table 1 [1-8] Acetylation reaction characteristics

Raw material reactant and solvent catalyst Thetergeneous/

homogeneous

130-40% acetylate d

cel lulose

acetic anhydride

/acetic chlorine

withHCLO4/H2SO4/

ZnCl2.../without100/ R.T Hom oge ne ous

2 A nim al cel lulose acetic anhydride H2SO4 60 Both

3 Ce l lulose ace tic anhydride /A MIMCl no 80/100 Hom oge ne ous

4 Ce l luloseacetic anhydride

0,1 vo l% an d 0.4 v ol % H2S O4 30 H ete rge ne ou s

5 Ce l luloseTritylchloride/pyridine/ac

etic anhydridepyridine 90 Hom oge ne ous

6 Cellulose nanocrystal p yridine /ace tic anhydride pyridine 80 Hom oge ne ous

7 Microfibri l Ce llulose ace tic anhydride no 70 He te rgene ous

8 Ke naf f iber pyridine /ace tic anhydride pyridine 100 Hom oge ne ous

After the reaction, successful acetylation can be indicated from FTIR with theappearance of new peak at around 1746 cm-1 which indicates formed ester groups and

the decrease in the intensity of peaks at 3342cm1

which is assigned to OH stretchingof the cellulose after the modification. [3; 4; 6; 7]

The following conclusions can be drawn from the reaction:

Higher temperature results in higher degree of substitution (DS). But when

temperature is too high, cellulose starts to degrade. [1;9]

DS increases when reaction time extends. [1;9]

Uncatalyzed reaction is more selective for primary hydroxyl group. [1]

Catalyst such as H2SO4, HClO4 can boost the reaction and lower the selectivity.ZnCl2 does not affect the reaction so much. [1]

One commercial way to produce cellulose acetate is to use ketene as a reactant. The

manufacture of cellulose acetate by direct addition of ketene is patented as Nightingale.

Samples reacted with ketene with an acetyl content up to 17% preserved their fibrousstructure with only slight degradation. The ketene acetylation was accompanied by an

objectionable polymerization of ketene which produced brownish coloration of thesample. The color could be removed by hot alcohol. [10] The mechanism of ketene

accelerate acetylation reaction can be explained so that ketene can generate aceticanhydride under the presence of water and acetic acid [11].

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Moreover, there is a possibility that ketene can react directly with carbohydrates [11].

Weili Wu et al. also conducted cellulose acetylation with acetic anhydride and iodine ascatalyst, and then compared acetylated cellulose with original cellulose. The

conclusions are similar with other acetylation processes. [9] I think in the future, a study

can be conducted on comparing iodine as a catalyst with conventional acid.

3. Heterogeneous and homogeneous cellulose acetylation

Depending on the purpose of the research, there can be homogeneous and

heterogeneous acetylation processes. Most of the modifications use heterogeneous

reactions, so that the core of the cellulose is preserved; and also because of a lack ofgood cellulose solvents. However, homogenous reaction can create more options to

induce novel functional groups, also open new avenues for the design of products, and

offer opportunity to control the total degree of substitution (DS) value. To date, a

number of solvent systems have been found, such as DMAc/LiCl, DMF/N2O4, NMNO,and DMSO/TBAF and some molten salt hydrates, such as LiClO4*3H2O, and

LiSCN*2H2O. However, limitations remain, such as toxicity, cost, difficulty for solventrecovery, or instability in the above processing [3].

Heterogeneous acetylation is realized by modifying cellulose with acetic anhydride

without swelling cellulose, which means that the reaction starts from the surface of

cellulose. In many cases, only surface modification of cellulose is needed so that themorphological structure and mechanical properties of cellulose will be preserved.

Giovoma et al. conducted a study on heterogeneous acetylation by using only acetic

acid/acetic anhydride reacting with cellulose under mild reaction conditions (30).

They pointed out that the heterogeneous acetylation reactions follow the first-order

kinetic law. The SEM image showed surface damage only occurred at high degrees of

substitution, which is very important for the applications in composites. Structure and

morphological changes should be avoided. [4]

Ionic liquids are a class of emerging novel solvents for cellulose. It is considered a

green solvent due to its low vapor pressure which makes it easy to recycle. Jin Wu et al.

conducted a study on homogeneous acetylation process with ionic liquid1-allyl-3 methylimidazoliumchloride (AMIMCl). They found out that ionic liquids can

boost acetylation reaction without a catalyst. There are more options to control the

degree of substitution [3].

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4. Properties that are affected by acetylation

Crystallinity

The acetylated cellulose presents a lower degree of crystallinity compared

with that of the original cellulose due to the substitution of the hydroxyl

groups by acetyl groups, which weakens the inter- and intra-molecular

hydrogen bonds of cellulose [9].

Dispersibility

Acetylated cellulose nanocrystal (ACN) has improved dispersion ability in 6tested solvents include water, dichloromethane, acetone, toluene,

tetrahydrofuran (THF) and DMF due to the weakened intra- and

intermolecular bonds [6]. Mehdi et al. pointed out in their paper that

acetylated nanofibers are very stable and well dispersed in both acetone andethanol for 2 months [8].

Wettability and polarity

Water has a higher affinity for cellulose nanocrystal (CN) than ACN.

Acetylation changes the surface of fibers from hydrophilic to more

hydrophobic. As a result, water wettability decreases [6, 9].

Thermal properties

The decomposition temperature of ACN was around 15C higher than CN,

which can be explained by the replacement of hydroxyl groups with themore stable acetyl groups. This means that ACN has a better thermal

stability than the original cellulose nanocrystals, which is an advantage forthe improvement of thermal performance of the nanocomposites [6, 9].

Size distribution of nanofibers and acetylated nanofibersThe average size of distribution for acetylated nanofibers was improved. The

overall average size is smaller due to degradation of cellulose [8].

Mechanical properties

The Youngs modulus and tensile strength of the tested acetylated bacterialnanofibril cellulose are lower than that of the original bacterial cellulose due

to the lower degree of crystallinity and the less dense network structure.

However, the difference is insignificant, which means acetylated cellulose is

still suitable for reinforcement in composites [9].

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5. Acetylation relative reaction rate and kinetics

It is of fundamental importance to have knowledge on relative reaction rates between

different hydroxyl groups. However, uneven penetration of reaction agents to

amorphous and crystalline region makes the studying of reaction rate very difficult. Carl

J. et al conducted a study of reaction rate by using cellulose acetate which contains 30-

40% of acetyl because they are soluble in acetic acid. They observed that the primary

hydroxyls acetylated more rapidly than the secondary. When the amount of primary

hydroxyls was plotted against secondary hydroxyls on log-log base, it is a fairly straight

line. Acetylation is a second-order reaction which depends on concentrations of the

hydroxyl and the anhydride. To make it simpler, the concentration of the anhydride is

not taken as a variable by assuming it does not affect the relative rates of different

hydroxyl groups. Then the esterification reaction can be simplified to a concurrent first-order reaction [1]. Let x and y be the number of primary and secondary hydroxyls

respectively. So we can derive the rate equation as follows:

d x = K1 * x * d t and d y = K2 * y * d t

Hence, d x / x= k1/k2 * d y / y

By integration,

log x= k1/ k2 log y + c (1)

Equation (1) is a correlation between the reaction rate of the primary and the secondaryhydroxyl. k1/ k2 is the relative reaction rate.

By conducting experiments under different conditions, with/without catalyst, different

temperature and different reactant, conclusions were drawn that without catalyst,

acetylation is more selective for primary alcohol; but the reaction is very slow. With

catalyst, the primary OH only reacts two times faster than the secondary group. [1]

Much later, Giovanna et al also studied acetylation kinetics. The difference is that he

tried to concentrate on the heterogeneous acetylation processes. During his research,

mild acetylation condition (30) was applied and two different concentrations of acid

catalyst were used. By plotting the acetylation degree (r) along with different reactiontimes, the plotted curve represented also a first-order kinetic law.

(2)

where ris the plateau value (t = ; and the term (r+ A) is the initial rvalue (t = 0),

i.e., the rvalue of the unmodified fibers. Equation (2) very satisfactorily fits the

experimental results [4].

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6. Cellulose acetylation mechanisms

There are two heterogeneous acetylation mechanisms that can be distinguished

depending on whether a non-swelling diluent is used or not. The first method is called

fibrous process, where diluents such as toluene, benzene or amyl acetate are used as

reaction medium. In fibrous process, cellulose triacetate (CTA) generated from the

reaction remains insoluble and there is a direct conversion of cellulose into solid CTA

without change in the gross morphology of the fibers. The other method is named

homogeneous process in Jeans paper where there is no diluent; CTA is solubilized in

reaction medium as it is produced. However, this method is not totally homogeneous in

the descriptive sense of the word; it is essentially a heterogeneous acetylation which

ends up in a homogeneous product (i.e., the CTA solution). In this situation, cellulose

morphology is disturbed before its total acetylation and dissolution. [2]

Sassi et al. conducted a study on the course of acetylation of cellulose microcrystal and

native cellulose fragments at ultra-structural level, under both homogenous and fibrous

processes. They found out that in both the homogeneous and the fibrous acetylation

process, the cellulose crystals appear to be acetylated on their surface, which means

acetylation does not lead to a crystalline swelling. In case of fibrous acetylation, the

cellulose acetate stays where it was produced and surrounds the unreacted core of

cellulose crystals. In case of homogeneous acetylation, it was observed that the surface

of cellulose chains undergo continuous stripping as they become acetylated. This will

lead to a reduction in the diameter of the crystal while the longitudinal dimension stays

more or less constant. Because the sufficiently acetylated cellulose part will dissolve inacetylating medium, acetylated part is lifted from the surface of cellulose [2].

A schematic drawing of homogeneous acetylation process can be found in Figure 1.

Chains that are sufficiently acetylated have become soluble in acetylating medium.Those still in the process of acetylation are lifted from crystal chain surface. The crystal

is indented by a series of grooves that correspond to the missing cellulose chain. [2]

Figure 1 A schematic drawing of homogeneous acetylation process [2]

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Ning Lin et al. also studied morphologies and crystalline properties of acetylated

cellulose nanocrystalline (ACN). And they arrived at similar conclusion with Sassi that

the rod-like shape of CN was preserved during the course of acetylation, but the size is

decreased a bit. In addition, the outline of ACN is blurry. However, the crystalline

characteristic is maintained. [6]

Furthermore in the paper by Galina et al., they stated a different view of acetylation

mechanism for microfibrill cellulose that the acetylation is not only happening on thesurface of cellulose but also involves some bulky parts. It is proven by the fact that

degree of substitution (DS), obtained by titration, and the degree of surface substitution(DSS), detected by XPS, are different. In their study, toluene and acetone are used to

exchanging water out of cellulose solution. [7]Furthermore in Mehdis paper, they also suggested that cellulose acetylation involves

swelling, so the acetylation happens not only on the surface but also the bulk part. But

in their case, pyridine was used as catalyst. [8]

To my understanding, when a diluent such as toluene or cellulose solvent such as ionic

liquid are used, it opens cellulose structure or causes swelling of cellulose. In this case,the acetylation happens not only on the surface.

7. Regio-selective acetylation of cellulose

The solubility of cellulose acetate (CA) depends strongly on the distribution of

acetylation in hydroxyl groups in the anhydroglucose units (AGU). Yoshisuke et al.

pointed out that CA in solution is generally in a quasi-flexible chain state, which means

that the chain stiffness is not a constant; it is between flexible and semi-flexible. The

solubility and clustering in solutions are related to intra- and intermolecular hydrogenbond formation that would be controlled by both chain architecture and the

surroundings. To study the influences of different hydroxyl group on cellulose acetate

chain dynamics and solubility, it is important to control the sequence of different

positions. The control can be achieved by regio-selective substitution of hydroxylgroups in cellulose. Three cellulose derivatives were prepared by the regio-selective

substitution; 6-O- triphenylmethylcellulose (6TC); 2,3-di-O-acetyl-6-O-triphenylmethylcellulose (2,3Ac-6TC) and 2,3-di-O-acetylcellulose (2,3AcC ) regio-

selectively substituted cellulose deacetate. Figure 2 shows the scheme of preparation ofthese cellulose derivatives.

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Figure 2 A schematic figure of preparation of cellulose derivatives.

The 6TC chain was characterized by the architecture that every C-6 position hydroxyl in

the chain was completely substituted by the hydrophobic triphenylmethyl group and that

all the C-2,3 positions hydroxyls remained. On the other hand, 2,3Ac6TC was

characterized by C-2 and -3 positions acetylated. Thus, 6TC and 2,3Ac6TC are

expected not to form intermolecular hydrogen bonds, or not to induce any association,

in polar solvents because of the lack of C-6 position hydroxyls. Intramolecular H-bonds

are normally formed between C-3 position OH and neighboring O-5 ring oxygen or

between C-2 OH and C-6 OH. The latter is the more important for the formation of

intramolecular bonds [5].

Yoshisuke et al. found out that in in 6TC/DMSO system, there is one dynamical cluster

and the size is at most 10 times larger than single chains. It was explained that to protectbulky trityl groups in C-6 position from precipitation in the hydrophilic DMSO

atmosphere, a temporary hydrophilic cover is formed by many hydrophilic hydroxylsthat exist at C-2 and -3 positions to cover the hydrophobic core. This hydrophobic

interaction is a dynamic association that is not thermodynamically stable but a

temporary buildup of 6TC [5].

8. Acetylated MFC and PLA/ACN composite film properties

As mentioned before, cellulose has potential to be applied in packaging materials.

However, due to their hydrophilic surface, cellulose nanofillers cannot be

homogeneously dispersed in other polymeric matrices. In many cases acetylation is

used as modification for cellulose to increase the hydrophobicity and to improve

cellulose surface properties. Following properties related with film applications are

modified by acetylations:

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Gas barrier properties of MFC film

Both pure and partially acetylated MFC films fulfill gas barrier requirement due

to the high crystallinity and densely packed cellulose microfibril. [7]

Water vapor transfer rate (WVTR)

It was expected that acetylation modification will reduce water vapor

permeability because acetylation changes the cellulose surface from hydrophilicto hydrophobic. However both of them exhibit high WVTR compare toconventional packaging material. [7, 8]

Mechanical properties of PLA/ACN nanocomposites

Tensile strength increases until 6 wt% load level then decreases. All PLA/ ACNcomposites exhibited a dramatically increased Youngs modulus. Elongation

decreases all the way because of the presence of rigid nanoctystals. It wassuggested by Ning Lin et al., that with an appropriate amount of nanofabril,

ACN can inhibit self-aggregation and promote dispersion in PLA matrix, thus

acting as reinforcement. But when the amount of nanofiller exceeds a certain

amount, ACN starts to aggregate and may damage the original PLA structure

resulting in a decrease in strength and elongation properties. [6]

Thermal properties of PLA/ ACN nanocomposites.

When the ACN content was less than 2 wt%, the rigid nanocrystals dispersed

homogeneously in the PLA matrix, it restricts the motion of amorphous and free

domains with interactions between nanofillers and the matrix. This interaction

causes a higher energy requirement for thermal transformation which means an

increase in Tg,mid(glass transition point at midpoint) and heat capacity Cp. But

when the amount of nanofillers is increased to 4-6%, the excess ACN affects

the interactions between crystalline and amorphous domains, so it decreases

Tg,midand Cp. However, when a further increase in the amount of ACN causes

self-aggregation it will restrain the motion of the amorphous domains and againincrease Tg,midand Cp. [6]

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9. Conclusion and discussion

Natural fibers have a lot potential in polymer applications. However, due to their

intrinsic drawbacks, modifications are normally required. Among all, acetylation is one

of the most significant reactions for the derivatization and modification of cellulose.Iodine can act as a satisfying catalyst for acetylation. A further study could be

conducted on comparing different catalysts. Acetylated cellulose exhibited increased

solubility and dispersion in various solvents, which is stable for months. It also changes

the surface of cellulose from hydrophilic to hydrophobic due to the substitution of OH

group to acetyls. Acetylated nanocellulose generally preserves the good mechanical

properties of nano-scaled fibers. Acetylation processes follows first-order kinetic law.There are two distinguished reaction mechanisms depending on whether a non-swelling

diluent is used. When there is a diluent, cellulose undergoes acetylation withoutchanging its morphology: in case of no diluent, morphology is disturbed before its total

acetylation and dissolution. Acetylated MFC film has good gas barrier properties and

shows better water vapor transfer rate result. But more research is needed to furtherconquer the high WVTR barrier. The crystalline structure of cellulose nanocrystals is

preserved after acetylation. Acetylated cellulose nanocrystal can disperse

homogeneously in PLA matrix up to 8 wt%. When it is 6 wt% the tensile strength of

ACN/PLA composites was enhanced by 60% and Youngs modulus is 1.5 fold greater

than that of PLA [6].

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References:

1. Carl.J. Malm et al; Relative rates of acetylation of the hydroxyl groups in

cellulose acetate; 7.1952.

2. Jan-francois Sassi et al; Ultrastructural aspects of cellulose acetylation of

cellulose; Cellulose 1995, (2) 111-127.

3. Jin Wu et al; Homogeneous acetylation of cellulose in a new ionic liquid;

Biomacromolecules 2005, 5, 266-268.

4. Giovanna Frisoni et al; Natural cellulose fibers: heterogeneous acetylation

kinetics and biodegradation behavior; Biomacromolecules 2001, 2, 476-482.

5. Yoshisuke Tsunashima et al; Regioselectively substituted 6-o- and 2,3-di-o-

acetyl-6- triphenylmethylcellulose: its chain dynamics and hydrophobic

association in polar solvents; Biomacromolecules 2001, 2, 991-1000.

6. Ning Lin et al; Surface acetylation of cellulose nanocrystal and its reinforcingfunction in poly(lactic acid); Carbohydrate Polymers 83 (2011) 18341842

7. Galina Rodionova et al; Surface chemical modification of microfibrillated

cellulose: improvement of barrier properties for packaging applications;

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10. Carl Hamalainen et al; Partial acetylation of cotton cellulose bv ketene; may1949

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