Surface modification of Microfibrillated Cellulose films by Gas-Phase Esterification: Improvement of Barrier Properties.

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1 Surface modification of Microfibrillated Cellulose films by Gas-Phase Esterification: Improvement of Barrier Properties. G. Rodionova*, M. Lenes**, Ø. Eriksen**, B. H. Hoff*, Ø. W. Gregersen* * Norwegian University of Science and Technology (NTNU), **Paper and Fibre Research Institute (PFI) ABSTRACT The purpose of the present study was to verify the potential of gas phase acetylation on MFC films to increase the hydrophobicity and improve the barrier properties of the films. This was done by esterification with a mix of trifluoroacetic acid anhydride (TFAA) and acetic acid (AcOH). Due to the high reactivity of the TFAA at 40 o C, esterifications were obtained when TFAA was in excess in the reaction mixture. The reaction was confirmed by FT-IR and the hydrophobicity improvement was evaluated using contact angle measurements. INTRODUCTION Microfibrillated cellulose (MFC) is one possible biodegradable material showing promising properties as a barrier film [1]. MFC is a high strength and large aspect ratio film forming material. Films made from MFC may be used for the preparation of multifunctional barrier materials. By introducing specific functional groups on the fibril surface, significant improvement of hydrophobicity, compatibility with plastics and oxygen barrier may be obtained. Solvent-free and gas/vapor-phase reactions appear as promising ways for altering both physical and chemical characteristics of cellulosic materials [2]. Moreover these methods seem to be suitable for application in full-scale processes with a minimum impact on the environment. EXPERIMENTAL The films were prepared from a 0.1 % MFC water suspension by natural dewatering and drying. The pure MFC suspension was obtained by high-pressure homogenization of softwood kraft pulp [3]. The esterification was carried out using a vapour mixture of AcOH and TFAA. The temperature (22 o C and 40 o C), the reagent ratio (1:2 and 2:1) and the reaction time (10+20, 20+20) were varied in the experimental setup. Reaction time was divided into two parts first the time from addition of the reagents until vacuum was introduced and second, the time from full vacuum until the end of reaction. The experimental setup used is shown in Figure 1. Heater MFC film Cooling trap Figure 1: Experimental setup for gas-phase esterification The reaction product was verified by Fourier transform infrared spectroscopy (FT-IR) using a Bio-Rad Excalibur FTX 3000 spectrophotometer. A spectral grade KBr pellet containing 2.0 mg of cellobiose octaacetate was used as a standard. Untreated and esterificated MFC films were properly dried before the analysis. Contact angle measurements with water were done using a Dynamic Absorption Tester DAT 1100 at room temperature. A minimum of ten readings were taken on every sample to exclude possible influence of the heterogeneity of the surface. RESULTS AND DISCUSSION The results show a significant increase in contact angle with water after esterification. Figure 2 shows examples of contact angle measurements on unmodified (41.2 o ) and modified films (79.2 o ) respectively, where the increased hydrophobicity is clearly illustrated.

2 a b Figure 2: a) liquid drop on pure MFC film surface, b) liquid drop on esterified MFC film surface Table 1 presents the contact angle for the check time 0.2 sec at different reaction conditions. All the films modified at 40 o C showed higher contact angle results than the ones esterified at room temperature. The best results were registered for the films esterified in the presence of lower amount of TFAA and probably lower degree of esterification. Unmodified MFC films have a contact angle of 41.2 o. Table 1: Contact angle of modified MFC films at different reaction conditions TFAA/AcOH ratio Reaction time, min Temperature, o C Contact angle at 0,2 sec 1: ±2.9 2: ±1.8 Figure 3 shows the FT-IR spectra of the standard choosen as a fully acetylated cellobiose unit, and MFC films with low and high degree of esterification. The spectra clearly show carbonyl peaks of acetyl groups at around 1750 cm -1 [4] Figure 3: FT-IR spectra of esterified MFC films: 1-cellulose octaacetate (standard), 2-MFC film with CA 66.6 o, 3- MFC film with 79.2 o CA CONCLUSIONS It was confirmed that gas-phase esterification with the mixture of TFAA and AcOH has a potential for being used as a method for hydrophobization of the MFC films. So far it was verified that the reaction had positive effect on contact angle of the films compared to unmodified samples. ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support form the Research Council of Norway, Södra Cell, Dynea, Korsnäs, Elopak and Peterson. References: 1. Syverud, K., and Stenius, P., Strength and barrier properties of MFC films, Cellulose 16, (2009). 2. Yuan, H., Nishiyama, Y., and Kuga, S., Surface esterification of cellulose by vapor-phase treatment with trifluoroacetic anhydride, Cellulose 12, (2005). 3. Eriksen, Ø., Syverud, K., and Gregersen, Ø.W., The use of microfibrillated cellulose produced from kraft pulp as strength enhancer in TMP pulp, Nordic Pulp & Paper Research Journal, 23(3), (2008). 4. Shirley, K., Yu, T., and Green, J. B., Determination of total hydroxyls and carboxyls derivatization by infrared spectroscopy, Analytical Chemistry 61, (1989).

3 Surface modification of Microfibrillated Cellulose films by Gas-Phase Esterification: Improvement of Barrier Properties Espoo, September 29 G. Rodionova, B. H. Hoff, Ø. W. Gregersen (NTNU) M. Lenes, Ø. Eriksen(PFI)

4 Overview Microfibrillated cellulose (MFC) Why do we modify MFC chemically? Production of MFC films Gas-phase esterification Characterisation methods Esterification - Experiments and results - Degree of substitution by titration (DS) - Surface hydrophobicity - Fourier transform infrared spectroscopy (FTIR) Conclusions

Microfibrillated cellulose (MFC) Material properties: Microfibrillated cellulose gel abundant renewable biodegradable film forming low density hydrophilic microfibrillated cellulose (MFC)

5 Microfibrillated cellulose (MFC) Material properties: Microfibrillated cellulose gel abundant renewable biodegradable film forming low density hydrophilic microfibrillated cellulose (MFC) nanofibrillar cellulose (NFC) cellulosic fibrillar fines nanoscale fibrillated cellulose Various terms for MFC (D 10-40nm L 1000 nm) MFC behaves as a gel even at low concentrations (0,1%)

6 Why do we modify MFC? Main challenges: hydrophilic nature lack of compatibility with hydrophobic matrices Modification of MFC - Solvent-free or gas/vapor-phase reactions are promising ways for altering the physical and chemical characteristics of MFC - Also for application for full-scale processes

7 Production of MFC films Film preparation: 1.Natural dewatering on a metal stand 2.On Petri dishes 3.Using dynamic sheet former MFC forms films: High strength Dense paper like structure Low permeability High translucency Factors affecting the MFC film strength: The aspect ratio The surface chemistry The DP of cellulose molecules on the fibrils Pure MFC film made from a water dispersion

8 Gas-phase esterification of MFC Reaction scheme An esterification reaction can be used for modification of OH-groups and change properties of fibrils to the more hydrophobic and more resistant to water Reaction scheme for acetylation with a mixture anhydride (TFAA trifluoroacetic anhydride; AcOH acetic acid; TFA - trifluoroacetic acid)

9 Characterisation methods

Determination of DS Titration Titration is a laboratory method used to determine the unknown concentration of a known reactant (analyte). Titrant is added to analyte until reaction is complete.

10 Determination of DS Titration Titration is a laboratory method used to determine the unknown concentration of a known reactant (analyte). Titrant is added to analyte until reaction is complete. The final quantity of titrant shows the amount of analyte present. MFC films were desintegrated into powder before analyses. The amount of ester groups in modified MFC films was determined by titration with hydrochloric acid.

11 Fourier transform infrared spectroscopy (FTIR) Measurement technique for collecting infrared spectra Infrared spectra from the MFC films were used to evaluate success of the reaction by determining: O-H bond C=O bond - The presence of a specific bond in the cellulose substrate - Comparing height of the different peaks and indicating the diffrence in the degree of substitution Infrared spectrum of acetylated MFC

12 Contact angle measurements The contact angle is the angle at which a liquid/vapour interface meets the solid surface Measurements were made on modified and non-modified MFC films at different check times Wetting CA 90 Repellency CA 90 Dynamic Absorption Tester (Paper and Fiber Research Institute, Trondheim, Norway)

13 Gas-phase esterification Experiments and results

14 Materials and methods MFC was produced from kraft pulp made of Norway spruce by mechanical treatment in a Claflin conical refiner combined with homogenization. MFC films were prepared from suspension of 0.1 % MFC in water by simple filtration through a paper filter, supported by a metal mesh and a polyamide filter cloth. After draining most of the water, samples were dried. The esterification was carried out using a vapour mixture of AcOH and TFAA. Variables: The temperature The reagent ratio The reaction time The reaction time was divided into two parts first the time from addition of the reagents until vacuum was introduced and second, the time from full vacuum until the end of reaction.

15 Materials and methods To remove unreacted compounds after esterification MFC films were subjected to: 1. evacuation for 30 min 2. washing with distilled water Experimental series : 1. MFC films evacuated after esterification: - Films modified at different reaction times and reagent ratios at room temperature - Films modified at different reaction times and reagent ratios at 40 o C 2. MFC films washed after esterification: - Films modified at different reaction times and reagent ratios at 40 o C

16 Experimental setup MFC film Cooling trap Heater

17 Gas-phase esterification AcOH +TFAA Gas-phase surface esterification was applied to obtain hydrophobicity and improve barrier properties of MFC films Reaction conditions: 1. Vapour mixture: Acetic acid (AcOH) + trifluoroacetic anhydride (TFAA) ratio 1:2 and 2:1 2. Temperature: 22 o C and 40 o C 3. Reaction time: 30 min and 40 min TFAA reacts with AcOH to give a mixed anhydride. Practical observations during synthesis indicate that the equilibrium is strongly displaced towards the side of the mixed anhydride. O O O O O O F F F O F F F + OH F F F O + F F F OH

18 Characterization methods Titration (quantification of ester degree of substitution) - MFC films were grinded into powder before analysis - DS was calculated from the acetyl content derived based on HCl and NaOH consumptions FTIR (qualitative evaluation of reaction success) - Analysis was performed on a Bio-Rad Excalibur FTX 3000 spectrophotometer - A spectral grade KBr pellet containing 2.0 mg of cellobiose octaacetate was used as a standard - Untreated and esterificated MFC films were properly dried before the analysis Contact angle (water repellency) - Measurements with water were done using a Dynamic Absorption Tester DAT 1100 at room temperature - A minimum of ten readings were taken on every sample

19 Titration Determination of ester DS Reagents ratio (TFAA : AcOH) Reaction time, min Temperature, o C DS 1 : : :

20 Contact angle Hydrophobicity measurements MFC films treated with vapour mixtures of AcOH and TFAA were evacuated for 30 minutes after reaction Reagents ratio (TFAA : AcOH) Reaction time, min Temperature, o C Contact angle, 0.2 sec 1 : ±2.4 1 : ±2.7 2 : ±2.9 2 : ±5.5 1 : ±12 1 : ±1.8 2 : ±2.1 2 : ±1.8

21 Contact angle Hydrophobicity measurements MFC films treated with vapour mixtures of AcOH and TFAA were carefully washed with distilled water after reaction Reagents ratio (TFAA : AcOH) Reaction time, min Temperature, o C Contact angle, 0.2 sec 1 : ±1.0 1 : ±2.7 2 : ±1.9

22 Contact angle A liquid drop on pure MFC film surface A liquid drop on esterified MFC film surface

23 FTIR results Cellobiose octaacetate C=O peak MFC film with high CA MFC film with low CA

24 Conclusions Microfibrillated cellulose films were successfully modified through gas-phase esterification with TFAA and AcOH. It was confirmed by FTIR spectra, titration and contact angle measurements. It was verified that the structural modification had increased the contact angle of the films compared to unmodified samples. Reagents ratio had the most significant influence on obtained results. An effective technique to make hydrophobic films was established.

25 Acknowlegement The authors gratefully acknowledge the financial support of:

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