Nanofibrillar cellulose: introduction

Transcription

1 Nanofibrillar cellulose: introduction Eero Kontturi Department of Forest Products Technology School of Chemical Technology Aalto University Cellulose Chemistry 22 nd June 2016

2 Outline (1) Nanocellulose types (2) Background on nanofibrillar cellulose (NFC) (3) Mechanical isolation of NFC (4) Chemically isolated NFC (TEMPO-oxidation, periodate oxidation) (5) Bacterial cellulose

3 Nanofibrillar cellulose (NFC) Long threads of individualized cellulose microfibrils

4 Cellulose nanocrystals (CNCs) Rigid rods of crystalline cellulose 1 µm

5 Basic practical difference NFC vs. CNC Nanofibrillar cellulose (NFC) Cellulose nanocrystals (CNC) After preparation: gel After preparation: suspension

6 Types of nanocellulose: terminological issues (1) Nanofibrillar cellulose Synonyms (used in literature) for mechanically isolated nanofibrillar cellulose: - microfibrillar cellulose - cellulose nanofibrils - cellulose microfibrils - cellulose nanofibres (2) Cellulose nanocrystals Synonyms used in literature: - cellulose whiskers - cellulose nanowhiskers - cellulose microfibrils - microcrystalline cellulose - nanocrystalline cellulose Note: microcrystalline cellulose is in its more common use a completely different material (micron-sized cellulose crystals).

7 Note on bacterial cellulose Bacterial cellulose are readily isolate microfibrils synthesized by certain species of bacteria Essentially, bacterial cellulose is nanofibrillar cellulose (or cellulose nanofibres) Often, however, bacterial cellulose is classified as a third nanocellulose species in its own right

8 Why do we want nanocellulose? Alternatives to plastics - reduce the use of oil - ultimate goal: renewable, biodegradable materials with good strength properties Why nanocellulose? - high strength - low density - renewable and abundant - very high aspect ratio (length/width especially in the case of nanofibrils) - cellulose possesses peculiar properties

9 Why do we want nanocellulose? Example Nanocomposite made of poly(lactic acid) as a continuous matrix and nanofibrillar cellulose as the reinforcing phase More esoteric potential applications - viscosity enhancers - security papers (liquid crystal phases)

10 Nanofibrillar cellulose: background

11 Ultrastructure of native cellulose Individual fibre Cellulose microfibril Diameter: 2-20 nm (In wood: 3-4 nm)

12 Ultrastructure: cellulose microfibrils Aggregates: nm (or more) 200 nm Individual microfibrils: ~3.5 nm AFM image of a surface of bleached birch kraft pulp; sample untreated. Imaged by M. Suchy TEM image of longitudinal cross-section of chlorite delignified pine cell wall; freeze-dried and stained. A. Heyn J. Ultrastructure Res. 1969, 26, 52.

13 Remark on terms Cellulose microfibril is a term defined by biologists close to a hundred years ago is a nanomaterial by definition (at least one of the dimensions is <100 nm) Cellulose nanofibril is a term coined after nanotechnology was already an established field (latter half of 2000s) is essentially an isolated microfibril

14 Nanofibrillar cellulose Preparation of nanofibrillar cellulose aims at isolating the individual microfibrils (nanofibrils) from the cell wall structure. Seminal challenges in isolation: - tight, hierarchical structure of the plant cell wall - inherent tendency of cellulose to aggregate

15 Nanofibrillar cellulose: preparation by mechanical disintegration of the fibre cell wall

16 Preparation of nanofibrillar cellulose: mechanical disintegration EARLY EXAMPLES OF INDIVIDUALIZATION OF MICROFIBRILS METHOD: ULTRASONICATION S.K. Asunmaa Tappi 1967, 49, 319. Gardner and Blackwell J. Polym. Sci. C 1971, 36, 327. From aspen holocellulose From valonia alga

17 Preparation of nanofibrillar cellulose: mechanical disintegration First attempt to isolate microfibrils for materials science purposes. Turbak et al. J. Appl. Polym. Sci. Appl. Polym. Symp. 1983, 37, 815.

18 Preparation of nanofibrillar cellulose: mechanical disintegration Enzymatic pretreatment to bleached sulphite pulp. TEM AFM Microfibrils and microfibril aggregates, ca nm in size. Pääkkö et al. Biomacromolecules 2007, 8, 1934.

19 Preparation of nanofibrillar cellulose: mechanical disintegration Z-shaped fluidizer is probably the most common instrument for mechanical disintegration leading to NFC preparation

20 Preparation of nanofibrillar cellulose: mechanical disintegration Wood powder, delignified by chlorite, hemicellulose matrix leached out by alkaline treatment 1 pass through Masuko grinder Highly monodisperse 15 nm wide microfibril aggregates Abe et al. Biomacromolecules 2007, 8, 3276.

21 Preparation of nanofibrillar cellulose: effect of hemicellulose When using mechanical disintegration, increased hemicellulose content facilitates nanofibril separation Iwamoto et al. Biomacromolecules 2008, 9, 1022.

22 Preparation of nanofibrillar cellulose: mechanical disintegration Masuko grinder is another common tool for mechanical disintegration for NFC preparation

23 Preparation of nanofibrillar cellulose: different raw materials Wheat straw pulp Wheat straw NFC Zimmermann et al. Carbohydr. Polym. 2010, 79, 1086

24 Preparation of nanofibrillar cellulose: different raw materials Optical microscopy images Enough passes through homogenizer results in somewhat nanosized fibrils. Hassan et al. Wood Sc. Technol. 2012, 46, 193.

25 Preparation of nanofibrillar cellulose: enzymatic posttreatment Post-treatment after mechanical disintegration with endoglucanases makes the NFC diameter more uniform (NFC prepared from eucalyptus pulp) Wang et al. Cellulose 2016, 23, 1859.

26 TEMPO-mediated oxidation

27 Preparation of nanofibrillar cellulose: chemical isolation TEMPO-mediated oxidation - 2,2,6,6-tetramethyl-1-piperidinyloxy radical (TEMPO) is an oxidation catalyst - TEMPO-NaBr-NaClO system selectively oxidized primary alcohols in polysaccharides, i.e., C6 position in cellulose Pioneered for polysaccharides: de Nooy et al. Carbohydr. Res. 1995, 269, 89. Pioneered for cellulose: Isogai and Kato Cellulose 1998, 5, 153.

28 Preparation of nanofibrillar cellulose: chemical isolation TEMPO-mediated oxidation of native fibres Mechanical stirring Centrifugation supernatant RESULT: highly monodisperse microfibrils (3-4 nm width) Saito et al. Biomacromolecules 2006, 7, Saito et al. Biomacromolecules 2007, 8, 2485.

29 Preparation of nanofibrillar cellulose: chemical isolation Why do we get individual microfibrils from TEMPO-oxidation? Only the surface of the microfibrils is oxidized electrostatic repulsion. Image from: Okita et al. Biomacromolecules 2010, 11, 1696.

30 Effect of fibre drying (hornification) on TEMPO-oxidation When wet fibres are dried once, they shrink and their swelling properties are not restored to the original level Some microfibrils are irreversibly aggregated (hornification) Usually, this hornification phenomenon results in reduced reactivity due to decreased accessibility

31 Effect of fibre drying (hornification) on TEMPO-oxidation Unexpectedly, hornification has no effect on the extent of TEMPO-oxidation Saito et al. Biomacromolecules 2007, 8, 2485.

32 Water retention of TEMPO-oxidized fibres Saito et al. Biomacromolecules 2007, 8, 2485.

33 Preparation of nanofibrillar cellulose: chemical isolation Effect of starting material wood pulp cotton tunicate bacterial cellulose Saito et al. Biomacromolecules 2006, 7, 1687.

34 Preparation of nanofibrillar cellulose: chemical isolation TEMPO-mediated oxidation of cellulose reduces DP, especially with regenerated cellulose grades Extensive survey on DP: Isogai et al. Cellulose 2009, 16, 117. However, at neutral conditions in a TEMPO/NaClO/NaClO 2 system, the DP reduction is minimized Original DP: 680 DP after TEMPO/NaClO/NaClO 2 (oxidation for 3 days, ph 5.8): 490 DP after TEMPO/NaBr/NaClO ( 2 hours, ph 10): 40 Hirota et al. Carbohydr. Polym. 2009, 78, 330.

35 Preparation of nanofibrillar cellulose: chemical isolation Neutral conditions (TEMPO/NaClO/NaClO 2 ) system result in straighter microfibrils (less defects). Saito et al. Biomacromolecules 2009, 10, 1992.

36 Different microfibril sizes used for TEMPO-oxidation Okita et al. Biomacromolecules 2010, 11, 1696.

37 Different microfibril sizes used for TEMPO-oxidation TEMPO-oxidation appears to apply to the entire microfirbil surface Okita et al. Biomacromolecules 2010, 11, 1696.

38 Hydrophobic TEMPO-NFC via ion exchange Okita et al. Biomacromolecules 2011, 12, 518.

39 Hemicellulose effect with TEMPO-oxidation High content of hemicelluloses results in thicker fibrils and lower surface charge after TEMPO-oxidation Tanaka et al. Biomacromolecules 2016 DOI: /acs.biomac.6b00316

40 Hemicellulose effect with TEMPO-oxidation High content of hemicelluloses results in thicker fibrils and lower surface charge after TEMPO-oxidation Tanaka et al. Biomacromolecules 2016 DOI: /acs.biomac.6b00316

41 TEMPO-oxidation to NFC: different raw materials Puangsin et al. Int. J. Biol. Macromol. 2013, 59, 208

42 TEMPO-oxidation to NFC: different raw materials NFC from hemp bast holocellulose via TEMPO-oxidation of differing extents Extensive TEMPO-oxidation can sometimes cut the fibrils Puangsin et al. Int. J. Biol. Macromol. 2013, 59, 208

43 Other chemical means to obtain NFC: periodate oxidation

44 Process with periodate oxidation for NFC

45 Periodate oxidation reaction Because periodate oxidation oxidises two OH groups per monomer, it breaks the cellulose crystal with prolonged reaction times When preparing nanofibrillar cellulose, breaking the crystal would destroy the integrity of NFC Periodate oxidation must be performed delicately in order to separate the fibrils but not to break the crystal Liimatainen et al. Biomacromolecules 2012, 13, 1592.

46 Effect of periodate oxidation on crystallinity Pulp crystallinity Crystallinity goes down but when the reaction is controlled, the cellulose crystallites are not totally broken Periodate reaction is confined mainly on the surface Liimatainen et al. Biomacromolecules 2012, 13, 1592.

47 Appearance of NFC gained from periodate oxidation Liimatainen et al. Biomacromolecules 2012, 13, 1592.

48 Bacterial cellulose

49 Nanofibrillar cellulose: bacterial cellulose A species of bacteria (acetobacteria xylinum) is able to produce pure cellulose microfibrils from sugars Individual microfibrils are formed on spot Macroscopically, bacterial cellulose forms a gel like many other types of nanofibrillar cellulose Bacterial cellulose is the only pure form of nanocellulose: no hemicellulose or lignin is synthesized together with it The microfibrils from bacterial cellulose are larger than in plant cellulose: cross section > nm 7 nm Iguchi et al. J. Mater. Sci. 2000, 35, 261.

50 Bacterial cellulose microfibril Initially 7 13 nm microfibril crosssections merge laterally into 7 (70-145) nm crosssections upon drying

51 Bacterial cellulose growth Bacterial cellulose can be grown under aerobic conditions at the air/water interface

52 Bacterial cellulose growth Nata de coco is a popular dessert in Philippines It is bacterial cellulose It can be purchased from ethnic supermarkets The easiest way to gain bacterial cellulose (sugar can be removed by alkaline extraction)

53 Summary Nanocellulose is either cellulose nanofibers (nanofibrillar cellulose) or cellulose nanocrystals (cellulose nanowhiskers) Main methods to prepare nanofibrillar cellulose: mechanical disintegration of the fibre cell wall, TEMPO-oxidation, or harvest cellulose synthesized by bacteria (bacterial cellulose) TEMPO-oxidation is the only method to produce isolated nanofibrils with homogeneous width (determined by the botanical source) Mechanically disintegrated cellulose can end up with different sizes and properties depending on the raw material and