ORIGINAL PAPER

Gas-phase esterification of microfibrillated cellulose (MFC)films

Galina Rodionova • Bard Hoff •

Marianne Lenes • Øyvind Eriksen •

Øyvind Gregersen

Received: 3 July 2012 / Accepted: 13 February 2013

� Springer Science+Business Media Dordrecht 2013

Abstract The barrier properties of microfibrillated

cellulose (MFC) films were improved by heteroge-

neous gas-phase esterification using various combina-

tions of trifluoroacetic acid anhydride, acetic acid and

acetic anhydride. The temperature, reagent ratio and

reaction time were varied in the experimental design.

The effects of two different purification procedures on

the barrier properties of esterified MFC films were

investigated. Washing with water did not affect the

barrier properties compared to those of the films that

were not washed, while the use of diethyl ether led to

improved barrier properties as measured by the contact

angle (CA) of water. The chemical composition of the

modified films was studied by X-ray photoelectron

spectroscopy and Fourier transform infrared spectros-

copy. Alterations in hydrophobicity and oxygen per-

meability were evaluated using dynamic CA and

oxygen transmission rate measurements, respectively.

Keywords Gas-phase esterification �Acetic anhydride � Trifluoroacetic anhydride �Microfibrillated cellulose (MFC) � Hydrophobicity �Oxygen barrier

Introduction

Microfibrillated cellulose (MFC) has numerous unique

properties including low oxygen permeability, high

fibril strength, the capacity to form films, biodegrad-

ability and abundance (Syverud and Stenius 2009;

Minelli et al. 2010). MFC is a hydrophilic material that

is incompatible with hydrophobic matrices. Hence,

modification is needed to obtain improved water

repellence. A controlled solvent-free or gas/vapor-

phase reaction is attractive in this respect. Such reaction

will occur mainly at the accessible regions of the

microfibrils, thus preserving the integrity of the cellu-

lose crystalline regions (Cunha et al. 2007). Easy

adjustment for full-scale processes may also be a

benefit of the method.

An esterification reaction is a simple, convenient

method for modification of fibril properties. Different

derivatization procedures using fluorine-containing

compounds have been used for the hydrophobization

of cellulose substrates (Liebert et al. 1994; Sahin et al.

2002; Navarro et al. 2003; Cunha et al. 2006). Among

others, trifluoroacetic anhydride (TFAA) is an effec-

tive esterification agent in the preparation of partial

cellulose esters (Tsuzuki et al. 1980; Hamalainen et al.

G. Rodionova (&) � B. Hoff � Ø. Gregersen

Department of Chemical Engineering,

Norwegian University of Science and Technology,

7491 Trondheim, Norway

e-mail: galina.rodionova@chemeng.ntnu.no

M. Lenes � Ø. Eriksen

Paper and Fibre Research Institute, 7491 Trondheim,

Norway

123

Cellulose

DOI 10.1007/s10570-013-9887-5

1959). However, liquid-phase treatments might lead to

depolymerization of the cellulose chains (Fengel and

Stoll 1989). Vapor-phase reactions using mixtures of

TFAA/AcOH or Ac2O/TFAA have previously been

applied for the esterification of filter paper and tunicate

cellulose films. The reactions were carried out at room

temperature and led to a significant improvement in

the hydrophobicity of the material (Yuan et al. 2005).

Another efficient gas-phase technique was recently

reported for the surface esterification of freeze-dried

tunicin and bacterial celluloses using palmitoyl chlo-

ride (Berlioz et al. 2009). Vapor-phase labeling with

TFAA is also a widely used technique for character-

ization of cellulose hydroxyl accessibility (Tasker and

Badyal 1994; Buchholz et al. 1997).

A large difference in the esterification reaction rates

and obtained degrees of substitution is observed for

different cellulosic materials, e.g., cotton fibers, tunicin

cellulose, Whatman cellulose powder or kraft pulp

fibers (Cunha et al. 2006; Tsuzuki et al. 1980; Yuan et al.

2005). These differences might be due to differences in

accessibility, molecular weights or specific surface

areas of the substrates. The effects of gas-phase

esterification of MFC, however, have not been reported.

Given this background, the purpose of the present study

was to verify the potential of gas-phase esterification of

MFC films as a method to increase the hydrophobicity

and improve the barrier properties of the films. The

modified films were analyzed using dynamic CA

measurements, oxygen transmission rate, X-ray photo-

electron spectroscopy and Fourier transform infrared

spectroscopy to identify the effects of various esterifi-

cation and work-up procedures.

Experimental

Materials

MFC was produced from kraft pulp made up of

Norway spruce (containing 83.6 % cellulose and

15.6 % hemicelluloses) by pre-treatment in a Clafin

mill combined with homogenization as described by

Eriksen et al. (2008). MFC films of approximately

44 lm thickness were obtained from a suspension of

0.1 % MFC in water by simple filtration through filter

paper supported by a metal mesh and a polyamide

filter cloth. After most of the water was drained, the

films were dried at 100 �C for 2 h.

Gas-phase esterification

The general esterification procedure was as follows:

two MFC films (basis weight 17 g m-2, diameter

6 cm) were dried overnight at 100 �C and placed in a

reaction vessel connected to a vacuum pump through a

cooling trap (Fig. 1). Glass beads were placed into the

reaction vessel to ensure good mixing of the reagents.

The system was evacuated for 30 min prior to

introduction of liquid reagents. The reaction was

carried out using a vapor mixture of TFAA/AcOH or

TFAA/Ac2O. The temperature (22, 30, 40 and 50 �C),

the reagent ratio (1:1, 1:2 and 2:1) and the reaction

time (30 and 40 min) were varied in the experimental

design.

Purification procedures

Method A: Evaporation

The modified films were evacuated for 30 min (0.86 bar

at 40 �C).

Method B: Water washes

The modified films were washed thoroughly with

distilled water for 15 min at 22 �C and dried in an

oven for 2 h at 100 �C.

Method C: Ether washes

The modified films were washed with diethyl ether,

dried for 20 h (0.86 bar at 40 �C) and further evac-

uated at 150 �C for 40 min to remove residual

anhydride.

Characterization methods

Fourier transform infrared spectroscopy

The reaction product was confirmed by Fourier

transform infrared spectroscopy (FT-IR) using a Bio-

Rad Excalibur FTX 3000 spectrophotometer. All

spectra were recorded between 400 and 4000 cm-1.

Cellobiose octaacetate (2.0 mg) was used as a refer-

ence for the acetate peak. Untreated and acetylated

MFC films were dried overnight at 70 �C prior to

analysis.

Cellulose

123

Thickness of MFC films

The thickness of the MFC films was measured

according to the ISO 534 standard for paper and board

materials.

Contact angle

The CA with water was measured on the esterified

MFC films using a Dynamic Absorption Tester DAT

1100 at 22 �C. A minimum of eight readings were

taken on each sample to exclude the possible influence

of surface heterogeneity.

Oxygen transmission rate

The oxygen transmission rate was measured with a

Mocon oxygen transmission rate tester (OX-TRAN

Model 1/50) at 2.2 bar partial oxygen pressure using 2

parallels for each sample at 0 % RH.

X-ray photoelectron spectroscopy

XPS measurements were performed using a Kratos

AXIS 165 spectrometer with an Al-Ka X-ray source

(12.5 kV). Wide-scan spectra were recorded with 80 eV

pass energy, and high-resolution regional spectra were

obtained with 20 eV pass energy. All spectra were

recorded from the sample at an electron take-off angle of

90�. The chemical bonds of carbon atoms were deter-

mined from the chemical shift using high-resolution

energy. Quantification was performed by curve-fitting

the C1s high-resolution spectra region using Gaussian

distributions. The degree of surface substitution (DSS)

of the trifluoroacetyl groups on the MFC film surfaces at

several nanometers in depth was determined. The DSS

was calculated from the surface atomic composition

based on peak intensity. Pure MFC film was used as a

reference (Ostenson et al. 2006).

Results and discussion

Microfibrillated cellulose (MFC) films were esteri-

fied using mixtures of either trifluoroacetic anhy-

dride (TFAA)/acetic acid or TFAA/acetic anhydride.

When TFAA reacts with acetic acid, it gives a

mixed anhydride. The equilibrium in this reaction

strongly favors the product (Scheme 1) (Emmons

et al. 1953).

In cases where TFAA and acetic anhydrides are

used in esterifications, it is known that trace amounts

of acid catalyze the reaction to the mixed anhydride

(Collet et al. 1975). At any time, 3 different anhy-

drides could be present in the mixture, and both

acetyl and trifluoroacetyl groups will be introduced in

reactions with cellulose hydroxyls. The final outcome

depends on the relative amounts of the components

used in the esterification. Trifluoroacetyl groups are

labile, however, and may be solvolyzed by acetic

acid or hydrolyzed by washing with water (Winter

and Scott 1968). The overall esterification of cellu-

lose can be represented by the following reaction

(Scheme 2).

Cooling trap

MFC film

Heater

Fig. 1 Apparatus for gas-phase esterification

Cellulose

123

The presence of water in the acidic reaction medium

may lead to a slower reaction rate and hydrolytic chain

degradation. Anhydrous reaction conditions were

ensured by drying and storing the MFC films carefully

and by performing the reactions under vacuum. First,

the gas-phase acetylation of MFC was studied by

varying the reagent ratio, reaction time and tempera-

ture. To determine how the reaction affected the

hydrophobicity of the MFCs, the modified MFCs were

dried under vacuum without being washed. The results

are summarized in Table 1.

The esterified MFCs (Table 1, entries 2–9) showed

a significant increase in CA with water. No direct

correlation between the variable reaction parameters

and the CA measurements was found. In an attempt to

further improve hydrophobicity, esterification reac-

tions were performed using a 1:1 mixture of TFAA

and Ac2O at different temperatures (Entry 10–12). As

O

F

F

F

O

O

F

F

F

O

OH

O

F

F

F

O

O

O

OH

F

F

F

++

Scheme 1 Formation of a mixed anhydride

OO

OOCellCellO

HO OH

OH

OH

HO OHTFAA/

AcOH orAc

2O

OO

OOCellCellO

HO OH

O

OH

HO OH

O

OO

OOCellCellO

HO OH

O

OH

HO OH

OF3C

OO

OOCellCellO

HO OH

O

OH

O OH

O

CF3

O

Scheme 2 Reaction

scheme for esterification of

cellulose

Table 1 The contact

angles of the MFC films

after esterification with

TFAA/AcOH and TFAA/

Ac2O mixtures at different

reaction conditions

(evacuated after

esterification)

Entry Reagents Reagent ratio Reaction time

(min)

Temperature

(�C)

Contact angle

at 0.2 s

1 None MFC untreated – – 41.2 ± 4.3

2 TFAA/AcOH 1:2 30 22 74.3 ± 2.4

3 TFAA/AcOH 1:2 40 22 73.0 ± 2.7

4 TFAA/AcOH 2:1 30 22 79.2 ± 2.9

5 TFAA/AcOH 2:1 40 22 70.3 ± 5.5

6 TFAA/AcOH 1:2 30 40 89.7 ± 12

7 TFAA/AcOH 1:2 40 40 73.0 ± 1.8

8 TFAA/AcOH 2:1 30 40 69.0 ± 2.1

9 TFAA/AcOH 2:1 40 40 66.6 ± 1.8

10 TFAA/Ac2O 1:1 30 30 96.9 ± 2.5

11 TFAA/Ac2O 1:1 30 40 88.7 ± 2.8

12 TFAA/Ac2O 1:1 30 50 83.3 ± 6.7

Cellulose

123

shown in Table 1, a further increase in CA was

observed. The effect was most pronounced at 30 �C

(Entry 10).

The modified MFC films, as described above,

contain esters of both acetyl and trifluoroacetyl

groups. Moreover, trace amounts of excess reagents

that might also be present could affect the measure-

ment of various properties. Selected MFC films were

post-treated using different washing methods to

remove these traces. First, the samples were washed

with water after the reaction, followed by drying. The

MFC films washed with water did not show any

change in CA, within experimental error. Evacuation

treatment is a mild method for removing unreacted

compounds present on the surface of a film. However,

this method is not effective for eliminating compo-

nents absorbed in the deeper layers. Washings with

diethyl ether were used for this purpose, as well as to

preserve the water resistance of the film. The triflu-

oroacetyl groups are expected to withstand washing

with ether. Table 2 shows the CA of MFC films after

esterification followed by diethyl ether washing.

Table 2 The contact

angles of the MFC films

modified with TFAA/AcOH

and TFAA/Ac2O mixtures

(washed with diethyl ether

after esterification)

Entry Reagents Reagent ratio Reaction time

(min)

Temperature

(�C)

Contact angle

at 0.2 s

13 None Not washed – – 41.2 ± 4.3

14 None Washed – – 65.1 ± 2.6

15 TFAA/AcOH 1:2 30 22 85.9 ± 6.6

16 TFAA/AcOH 2:1 30 22 86.1 ± 3.9

17 TFAA/AcOH 1:2 30 40 80.7 ± 2.6

18 TFAA/AcOH 2:1 30 40 79.9 ± 4.2

19 TFAA/Ac2O 1:1 30 30 78.9 ± 4.8

20 TFAA/Ac2O 1:1 30 40 94.2 ± 2.9

Fig. 2 FT-IR spectra of MFC films esterified with a mixture of TFAA and AcOH

Cellulose

123

Fig. 3 Low-resolution and high-resolution survey spectra of unmodified MFC film (a) and esterified MFC film (b)

Table 3 The degrees of surface substitution and contact angles of the MFC films modified with TFAA/AcOH and TFAA/Ac2O

mixtures (washed with diethyl ether after esterification)

Entry Reagents Reagent

ratio

Reaction time

(min)

Temperature

(�C)

CA at 0.2 s DSS

21 TFAA/AcOH 1:2 30 40 80.7 ± 2.6 0.18

22 TFAA/AcOH 2:1 30 40 79.9 ± 4.2 0.43

23 TFAA/Ac2O 1:1 30 30 78.9 ± 4.8 0.55

Cellulose

123

Washing with ether increased the hydrophobicity of

the non-esterified MFC films (Rodionova et al. 2011),

as seen in the different CA of entries 13 and 14. This

result might be due to a reorientation of the hydroxyl

groups towards the interior of the microfibrils at

elevated temperature (Borgin 1961). Even higher CA

values were obtained by esterifying the MFC films.

The reactions at 22 �C showed positive effects on the

hydrophobicity of the film at both of the reagent ratios

investigated (entries 15–16). The highest CA value,

94.2�, was recorded for the sample modified at 40 �C

with 1:1 TFAA/Ac2O.

Titration methods are widely used to determine the

degree of substitution (DS) of esterified celluloses

(Tsuzuki et al. 1980; Liebert et al. 1994). It was

observed that even thoroughly ground MFC film

particles could not be swollen or mercerized in sodium

hydroxide solution. Therefore, X-ray photoelectron

spectroscopy (XPS) analysis was used to determine the

surface degree of substitution (DSS). Fourier transform

infrared spectroscopy (FT-IR) spectra were taken to

confirm the introduction of ester functionalities.

Figure 2 shows FT-IR spectra of MFCs treated with

TFAA/AcOH. Cellobiose octaacetate was used as a

standard. The analysis showed a weak carbonyl ester

peak at approximately 1750 cm-1 (Shirley et al.

1989). The presence of the fluorine-containing moie-

ties was confirmed by the occurrence of new peaks in

the range of 1000–1500 cm-1 (Bellamy 1975).

The XPS analysis provided insight into the surface

chemical composition of the MFC films before and

after esterification. The XPS survey spectra are shown

in Fig. 3. Carbon and oxygen were the major elements

detected in the unmodified samples, whereas the

additional presence of fluorine on the surface of

the treated samples confirmed the occurrence of the

esterification reaction. High-resolution C1s XPS spec-

tra of the same samples were also obtained. The usual

carbon peaks were detected: C1 from carbon bonded

to other carbon and/or hydrogen only; C2 from carbon

bonded to one oxygen; C3 from carbon bonded to one

oxygen by double bond or two oxygen atoms by single

bonds; C4 from carbon with three bonds to oxygen

atoms, carboxyl or carbonyl groups. The spectra

revealed the increased contribution of C4 carbons,

which can be ascribed to the O–C=O and C–F bonds.

The degree of surface substitution and CA values of

the corresponding samples are summarized in Table 3.

As discussed previously, the esterified MFCs showed a

significant increase in CA values compared to the

unmodified samples. The DSS values of the films

esterified with TFAA/AcOH mixtures were clearly

influenced by the reagent ratios. The lower DSS in

entry 21 might be due to the lack of TFAA required for

the formation of mixed anhydride. The highest DSS

values were obtained for the samples treated with a 1:1

mixture of TFAA and Ac2O.

Some changes in oxygen permeability are to be

expected upon esterification of MFCs. The perme-

ability may either decrease or increase as a result of

enhanced or reduced crystallinity. Introduction of the

relatively bulky trifluoroacetyl groups may cause an

increase in the free volume, facilitating permeation. In

the present study, the differences in the oxygen

transmission rates (OTR) between the MFC films

were small, from 3.68 cm3 m-2 day-1 for unmodified

films (Table 2, entry 13) up to 3.89 cm3 m-2 day-1

for the films reacted with TFAA/AcOH (Table 2,

entry 17).

Conclusions

It was confirmed that gas-phase esterification with

mixtures of TFAA/AcOH or TFAA/Ac2O has poten-

tial utility as an efficient solvent-free method for

hydrophobization of MFC films. It was verified that

the reagent ratio and reaction temperature had signif-

icant effects on the CA of modified films. The highest

degrees of substitution were observed for samples

treated with the TFAA/Ac2O mixture.

Acknowledgments The authors would like to thank Dr.

Leena-Sisko Johansson and Dr. Joseph M. Campbell for

assistance with the XPS measurements, Professor Torbjørn

Helle for the linguistic help, and the project partners in the

Sustain Barrier project at PFI for their financial support.

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