CA2437616A1 - Manufacturing of nano-fibrils from natural fibres, agro based fibres and root fibres - Google Patents

Abstract

Natural grown fibers are gaining a renewed interest, especially as a glass fiber substitute, partly due to ecological concerns. Natural fibers like hemp, flax, sisal, and jute;
Agro-based fibres such as Bagasse, wheat straw etc and root crops like Rutabaga fibers hold potential for such innovations due to their availability and low cost. These natural fibers are bundles of individual strands of fibers held together by means of interface of pectin and lignin.
Apart from the long fibers there are fibers, which are on much smaller scale and have 5-50 nm diameters and are thousands of manometer long. The objective is to isolate these nano-sized microfibrils, which are embedded in hydrated, amorphous matrix of hemicelluloses and pectin.
Chemical treatment is done to remove the impurities like hemicelluloses and extractives since higher cellulose content in the fibre leads to higher strength and stiffness of the fibre and then individualization of the cellulose microfibrils done by using mechanical shear force with the help of cryocrushing and high-pressure defibrillization. These namo-sized microfibrils[also referred as namo-fibrils] could contribute towards producing composite materials of very high strength at a low cost in an environmentally friendly manner suitable for biocompatible medical devices, packaging and other high strength structural applications.

Description

Specific Area of Invention This invention pertains to the development of a process for production of cellulose nano fibrils from Root crops like Rutabaga, Natural fibres like I-hemp, Flax Bast fibres, Kenaf; l~gro based fibres like Soy, Wheat, Corn, and Bagasse and Wood fibres as thermo mechanical pulp and Kraft pulp. This invention describes the various steps used in manufacturing process of nano fibrils.
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f 'v aCkgr~urid Natural cellulose fibres have advantages over glass fibre as they are less expensive, Iow bulls density, good moldability, biodegradable, abundantly availahle from renewable resources and have a high specific strength. Like synthetic fibres, natural fibres in the form of woven or nonwoven mats present outstanding opportunity to develop a new class of advanced lightweight composites.
Much work has been done on the elongated fibres from plants such as Hemp, Flax and Jute as in the research work by Batra S.K. Long vegetable fibres, Handbook of fibre science and technology, Vol. 4, Fibre chemistry, New York: Marcel Dekl~er, 1985.p.727-807, Hornsby PIZ, Hinrichsen E, Tarverdi K. "Preparation and properties of polypropylene composites reinforced with wheat and flax straw fibres" . Part II Analysis of composite microstructure and mechanical properties, Journal of Material Science 1997; 32:1009-i5 and Simonsen John, Utilizing straw as a filler in thermoplastic building materials, Construction a:r~d Building Materials, 1996 vol 10, no. 6 ,pp 435-440.
These fibres are few microns in diameter and 3-4 millimetres long as shown by Fengel D, Wegener G, Wood, chemistry, ultra structure, reactions, pp 268-269. However there is tremendous potential of utilizing the natural fibres, agro base; fibres & root crops sources (which are found in abundance) with non-elongated nano-si:aed microfibrils for making thermoplastic nano-composites. These are fibres on much smaller scale and having 5-50 nm diameter and are thousands of nanometer length as illustrated by D.G.Hepworth, &
D.M.Bruce, "The mechanical properties of a composite manufactured from non fibrous vegetable tissue and PVA", Composites Part A: Applied Scic°nce and Manufacturing Issue 31(2000) pp 283-285 . These nano-sized microfibrils demonstrate high strength properties comparable to synthetic fibres.
Nano-sized microfibrils are cellulose chains aggregating to form a fibril, a long thread like bundles of molecules stabilized laterally by hydrogen bonds between hydroxyl groups of adj acent molecules. The molecular arrangement of these fibx~llar bundles is called microfibrils.
These nano-sized microfibrils (also referred as cellulose nano-fibrils) are around 10 nm diameter and 30-100 cellulose molecules in extended chains as mentioned by A.Stamboulis, C.A.Baillie, T.Peijs, "Effects of environmental conditions on. mechanical and physical properties of flax fibers" Composites Part A: Applied Science and Manufacturing Issue 32(2001), pp 1105-1115. These nano-sized microfibrils are embedded in the matrix of hemicelluloses and pectins. Fibre cell wall is consists of primary cell wall and three secondary cell walls. Each cell wall contains a complex matrix of lignin, hemicelluloses and pectins surrounded by these nano-sized microfibrils.
The challenge of this work is to isolate nano-sized microfibrils from the secondary cell wall by chemical and mechanical treatments and then evaluate their strength in terms of aspect ratio of the individualized nano-fibrils.
brief Description of Invention This invention is focussed on the nan-fibrils, which are embedded in the secondary cell wall.
These cellulose nano-fibrils have diameter in the range of 5-ti0 nm and the length in thousands of nanometre. A method has been developed to manufacture these nano-fibrils at a high yield from natural fibres.
A process for obtaining cellulose nano fibrils from natural fil7res involves heating a pulp suspension to 80-90 °C, extracting the cellulosic material with an acid (dilute HCl of 1M cone.) followed by the extraction of pulp with base (cone. less than. 3%, by wt/wt), and then pouring liquid nitrogen into the pulp and keeping the sample in liquid nitrogen for S-10 minutes to freeze the cell wall water and then applying high impact for hreaking the cell walls and hence liberating the microfibrils from the secondary cell wall. This step is then followed by high pressure defibrillation by passing the 2% w/w suspension of the LNZ crushed sample through high pressure defibrillator using PANDA 2K by NIRO SOAVI ~ unit subjecting the treated suspension to a high pressure. The resultant suspension after 30 min contains nano-fibrils in the range of 25-60 nm diameters. The resultant suspension has .above 18% yield of nano-fibrils entangled together. The invention includes a process to disperse the nanofibrils and prevent them from further agglomeration .
The invention demonstrates a much higher aspect ratio of the nano fibrils than the long fibres.
Further the invention relates the use of natural fibres in cornl~ination with the plastic polymers and bioplastics to produce composite with high strength andl stiffness and still having ultra lightweight.
The natural fibres used in this invention to isolate nano fibrils are:

~sHemp Flax Soy ~ Kenaf Wood fibres ~ Wheat straw ~sCorn ~sBagasse ~sRutabaga ~ Turnip Descripti~n of Drawings Figure 1: Mechanism of Acid Hydrolysis in polysaccharides Figure 2: SEM of Flax Bast fibres after chemical treatment with low pressure defibrillation Figure 3: Optical microscope of low pressure defibrillated flax bast fibres e4verage diameter of nano-fibrils:
Figure 4: Flax nano fibrils after 30 min , average diameter 54 nm Figure 5: AFM picture of Flax fibre after 30 min at a pressure above 500 MPa for defibrillation, average diameter. 30-60 nm Figure 6: Flax nano-fibrils after high pressure defibrillation [20 min above 500 bars]
Figure 7: Rutabaga nano-fibrils after high pressure defibrillation [5 min above 500 bars, average diameter 35 nm Figure 8: Wheat straw nano-fibrils after high pressure defibrillation [~0 min above 500 bars] average diameter 30 nm Figure 9: Hemp nano-fibrils after high pressure defibrillation [20 min above 500 bars]
average diameter 60 nm Comparison of extent of defibrilliaation after ~rarious passes Figure 10: Flax bast fibres Figure 11: Rutabaga Figure 12: Wheat straw Figure 13: Hemp fibres Figure 14: Chart for diameter and aspect ratio for Flax Figure 15: Chart for diameter and aspect ratio for Rutabaga Figure 16: Chart for diameter and aspect ratio for Wheat Figure 17: Chart for diameter and aspect ratio for hemp Figure 18: ATR Chart showing the removal of pectins and hemicelluloses after chemical treatments Figure 19: Comparison charts for untreated, acid treated and acid & all~ali treated samples showing the percentages of hemicelluloses, cellulose, lignin and pectin contents Figure 20: Flax nano-fibrils diameter distribution curve Detailed description of the Invention As previously mentioned, this invention is based on developing a technique of the production of cellulose nano fibrils from natural fibres like hemp, flax, from root crops and also from agro based fibres.
,Swellihg of'Fabres:
Swelling agents primarily strong electrolyte solvents have been employed to pre-treat cellulose. Two types of swelling agents have been known: one is intercrystalline and the other intracrystalline. For example water can penetrate and loosen only the amorphous region of cellulose; this is considered as an intercrystalline swelling agent. Gn the other hand swelling agents such as certain salts and alkali solutions affect both amorphous and crystalline regions of cellulose. They are called intracrystalline swelling agents. In other words intracrystalline swelling agents are effective in loosening the crystalline region of cellulose.
The expanded capillary structure of swollen fibres may cause a significant increase in surface area of the cellulose fibre, and the total area of the swollen material may be as much as 100-fold greater than the area that results after drying the material. This enlarges the fine structure so that the substrate is more accessible to cellulose and also it facilitates the diffusion as explained by L.T.Fan, M.M.Gharpuray 8z Y.H.Lee Cellulose; Hydrolysis 1987]

Fibres [except root crop fibres] were soaked in 17.5% w/w sodium hydroxide solution overnight to swell the cell wall to enable large chemical molecules to penetrate through the crystalline region of the cellulose.
Sample preparation:
Fibres were made into pulps with water in l0:lwater to fibre ratio then the sample was vacuum filtered for further chemical treatment.
~'hemical Treatment The structure of lignocelluloses in the cell wall resembles that of a reinforced concrete pillar with cellulose fibres being the metal rods and lignin the natural cement. Nigh order molecular packing of cellulose in its crystalline regions hinders the heterogeneous chemical reactions on the external surface of crystallities. Chemical treatments have been extensively used for the removal of lignin/pectins surrounding cellulose and destroying its crystalline structure.
Although cellulose possesses excellent strength and good stability, yet it can be degraded by resorting to a variety of chemical and physical processes under certain conditions. The most common manifestation of its deterioration is a decrease in DP. This decrease is accompanied by a chemical modification of cellulose molecule such as an increase in its reducing power or development of reactive groups along the chains.
Acid Treatment - Extraction of Pectin Acids serve primarily as catalysts for hydrolysis of cellulose than as reagents for pre-treatment.
When cellulose is hydrolyzed in an acidic medium to glucose, the ? -1-4.
glucosidic bonds of cellulose chains molecules are split by the addition of the water molecules, this addition yields fragments of shorter chain lengths while preserving the basic; structure. ~ne of the new-formed end groups of chain molecules is a potential aldehyde group possessing reducing power. The objective of the acid treatment is to remove the pectin, extractives and hemicellulose since cellulose fibres are embedded in the matrix of hemicellulose and pectin. Acid treatment will hydrolyse these impurities and will liberate the cellulose fibres.

Hydrolysis of cellulose with acid proceeds through the formation of hydrocellulose to soluble polysaccharides and then to simple sugars. This occurs only after the crystalline structure of the cellulose is destroyed by its dissolution or swelling in hot dilute acid.
Native Cellulose -.Stable Cellulose-~--Soluble Polysaccharides --P Calucose The extracted pulp was treated with dilute hydrochloric acid [1 ti~I) at 80 °C+/-5 for two hours.
Acid hydrolysis proceeds in three steps [Figure 1). In the fir<.;t step the proton ofthe catalyzing acid interacts with the glycosidic oxygen linking two sugar units of polysaccharide [I], forming a conjugate acid [II). This step is followed by a slow cleavage of C-O bond forming an intermediate carbo canon [III). The carbo canon finally reacts with a water molecule, resulting in a stable end product and release of proton. [Fengel I7, Wegener G, Wood, chemistry, ultra structure, reactions, pp268-269) After acid treatment, the sample is removed from the hot water bath, cooled and washed with distilled water abundantly until the filtrate becomes neutral, ohen the sample was vacuum filtered for further treatment.
r~lkalf Treatment Alkali treatment of lignocellulosic material causes swelling leading to an increase in surface area, decrease in the DP, decrease in cryastallinity, and separation of structural linl~ages between lignin and carbohydrate and disruption of lignin structure. Therefore to remove the remaining hemicelluloses and pectin from the sample, alkali treatment is done.
At elevated temperatures, polysaccharides are attacked by alkali and dissolution of undegraded polysaccharides takes place. Pectins are naturally soluble in aqueous medium.
This alkali treatment results in solubilization ofpectins and hemicellulose.
Cell-OH + NaOH ~ Cell-ONa -~ + HZO+ surface impurities The samples were weighed and 2% w/w sodium hydroxide solution is added to the sample.
The sample with NaOH solution is placed over a hot water bath maintained at 80 °C+ 5 °C for two hours with constant stirring for better impregnation of alkali into the fibres. After 2 hrs sample was removed from the bath and cooled and then washed with abundant distilled water until becomes neutral, then the sample was vacuum filtered .and residue was frozen.

It is noteworthy that until this stage nano-sized microfibrils are still associated with the call wall. Therefore, mechanical treatment is necessary to shear them apart from the cell wall.
Liquid Nit~ogeh Gushing:
The chemically treated residue was frozen and then frozen pulp was crushed with liquid nitrogen. Liquid nitrogen is a cryogenic gas at 77 K temperature at standard atmospheric pressure. The objective of the cryocrushing is to form ice cystals within the cells. When we apply high impact on the frozen pulp, ice crystals exert pressure on the cell wall and when we apply high mechanical impact, the cell wall ruptures and liberates the microfibrils.
Then the pulp, pulp is washed abundantly with distilled water and a 2% w/w suspension is which Disintegration:
Above liquid nitrogen treatcd sample is made to 2% w/w suspension in distilled water and fibres were dispersed evenly in a disintegrator for 10 minutes at 2000 RPM
speed.
High P~essua~e De~b~illatio~a The suspension was then subjected to high shear defibrillation and cell wall rupture process by exposing the fibre to very cold temperature under water saturation condition and high pressure shearing with and without exposing them to a high pressure defibrillation done with PANDA
2K by NIRO SOAVI S.p.A? . The passage of the suspension through this flow passages under high pressure and controlled flow action subjects the fluid to a condition of high turbulence and shear that creates the efficient mechanism of reduction in size [to submicron level]. The sample was repeated exposed to chemical treatment, freeze drying followed by this defibrillation at 0-150 MPa pressure.
The samples were analysed using transmission electron microscopy, which showed that high-pressure defibrillation led to the individualization of the nanc~-sized rnicro.fibrils.
IiAaterials Flax fast fibres and hemp fibres were used as a source for natural fibres.
Rutabaga and Turnip were used as a source of Root crops, Wheat straw and bagaa.se are used as agro based natural fibres and thermo mechanical pulp and Draft pulp were used as wood fibres source. All these fibres were treated and then defibrillated to produce cellulose nano-fibrils.
The average diameter range obtained from was in the range of 5-60 nm.
Examples Experiments were undertaken to demonstrate the production of cellulose nano fibrils from different raw materials under dLifferent conditions.
Defibrillation vio~thout chemical treatments Natural fibres have length in the range of 5-25 mm and they exist in the form of fibre bundles.
Fibres are tightly attached to each other with pectin, which a.ct as cement to bind them together as a bundle of fibres. Without removing the pectic substancfa it is very difficult to individualise fibres for defibrillation. Moreover the opening of the nozzle in the defibrillator equipment is approximately 1-2 mm and when we pass the suspension of fibres of length 2-25 mm, it chokes the nozzle opening and no circulation is possiible for defibrillation.
Defibrillation vrrith chemical treatments and an~ithout high pressure In one experiment, chemical treatment of the fibres was done but high pressure was not applied for the defibrillation of the fibrils. Then the samples were an;~lyzed in optical microscope and also using Scanning electron rr~icroscopy.
SEM in figure 2 and optical microscope in figure 3 show the fibrils after defibrillation without high pressure and it is clear that still the fibrils are attached to each other and are in the form of bundles. Therefore it is essential to apply a high pressure to iisolate theses fibrils from each other.
Impact of various che~rtical treatments ~Iarious chemical treatments are done to minimise the content of impurities like hemicelluloses, lignin, pectins and minerals in the sample used for isolation of nano-fibrils.
Figure 13 shows the decline in hemicelluloses and lignin conaent and constant increase of ?
cellulose. Higher content of cellulose will lead to a better stiffness and strength of the fibrils.?

Impact of number of passes Flax, Hemp and Wheat straw fibres, when examined under TEM and AFM
demonstrated better extent of defibrillation after 2Q passes but Rutabaga root crop showed a deterioration nn fibrillation with increased number of passes (figure 9). At lower number of passed, TEM
pictures showed that nano fibrils are still entangled to each other and these fibrils diameter is in the range of microns. Increasing the number of passes shows the better isolation of nano fibrils and better yield.
Conclusions ,mss Cellulose nano-fibrils have much higher aspect ratio than the long fibres; hence they have better strength properties.
~s By performing chemical and mechanical treatments we c;an isolate nano-sized microfibrils.
,mss Different stages of chemical treatments lead to increase in cellulose content in the sample and hence increasing the yield of nano fibrils.
.mss Liquid nitrogen crushing before the high-pressure defibrillization plays an important role in rupturing the cell wall and in liberating the microfibrils from cell wall.
,mss Cell rupture technique is better suited for isolation the nano-fabrils than only the shear action.
,e~s T'EM and AFM techniques are used to identify the nano-fibrils and in determining the diameter and length of the nano-fibrils.
Potential Applications The invention process will be ~:csed widely as a ultra light weight composite in automobiles interior and exterior parts.
Due to its lightweight and high strength its potentials application will be in aerospace industry.
Since these nano-composites will be biodegradable with tremendous stiffness and strength , they find application in the medical field such as blood bags, cardiac devices, valves as a reinforcing biomaterial.

Claims (10)

1. Nano-sized microfibrils can be produced from the secondary cell wall by a unique chemi-mechanical unit operation.

2. Overall yield of nano sized microfibrils is above 20%.

3. Nano-fibrils are in the diameter range of 5-60 nm.

4. Aspect ratios of nanofibrils are between 20 to 200.

5. Process for the production of nano-sized cellulose microfibrils from secondary cell wall plant pulp containing cellulose, pectins, hemicelluloses, proteins and mineral materials, comprise of the following steps:
a. hydrolyzing the pulp with acid and base at a temperature between about 80-degree C to extract the pectins and hemicelluloses to form a suspension;
b. vacuum filtration of the suspension after acid and alkali treatments to get a solid residue c. freezing the solid residue from above step and the putting the frozen sample in liquid nitrogen and then applying high impact on the sample to fracture the cell wall.
d. disintegration of the solid residue in 2% w/w suspension for 10 minutes at RPM
e. defibrillating the above suspension at high pressure with a high mechanical shear along with the cell rupture technique.
f. a chemi-mechanical process that stabilizes the nanofibril dispersions g. hydrolysis occurs at a temperature above 20 ° C

6. A process in which alkaline extraction step utilizes caustic soda of concentration above 1 % w/w

7. A process where after each chemical treatment the percentages of hemicelluloses, pectins & lignin decreases and percentage of cellulose increases.

8. Nanofibrils contain more than 98% cellulose molecules

9. Nanofibrils are easy to disperse in suspension and in solid form

10. A process that involves making products for medical, packaging and industrial applications based on bioplastics and plastics by film casting, molding and extrusion process.