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Title:
METHOD FOR TREATING A NANOCELLULOSE FILM AND A FILM TREATED ACCORDING TO THE METHOD
Document Type and Number:
WIPO Patent Application WO/2020/044209
Kind Code:
A1
Abstract:
The present invention relates to a method for treating a film comprising nanocellulose with an aqueous solution comprising tetraalkylammonium hydroxide, such as tetraethylammonium hydroxide and/or tetralkylammonium chloride, such as tetraethylammonium chloride or tetrabuthylammonium chloride to partly dissolve the film and to improve the barrier properties of the film. The invention also relates to a film produced according to the method and to a package comprising said film.

Inventors:
LAKOVAARA MATIAS (FI)
HANSSON SUSANNE (SE)
SVENSSON ADRIANNA (SE)
SIRVIÖ JUHO (FI)
LIIMATAINEN HENRIKKI (FI)
Application Number:
PCT/IB2019/057174
Publication Date:
March 05, 2020
Filing Date:
August 27, 2019
Export Citation:
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Assignee:
STORA ENSO OYJ (FI)
International Classes:
D21H11/18; C08L1/02; C08B15/02; C08B15/08; C08J5/18
Domestic Patent References:
WO2017163167A12017-09-28
WO2017072124A12017-05-04
WO2017221137A12017-12-28
Other References:
SOYKEABKAEW, NATTAKAN ET AL.: "All-cellulose nanocomposites by surface selective dissolution of bacterial cellulose", CELLULOSE, vol. 16, no. 3, 2009, pages 435 - 444, XP019672338
YOUSEFI, HOSSEIN ET AL.: "Direct fabrication of all-cellulose nanocomposite from cellulose microfibers using ionic liquid- based nanowelding", BIOMACROMOLECULES, vol. 12, no. 11, 22 September 2011 (2011-09-22), pages 4080 - 4085, XP055697216
SIRVIÖ, J. A. ET AL.: "Rapid preparation of all-cellulose composites by solvent welding based on the use of aqueous solvent", EUROPEAN POLYMER JOURNAL, vol. 97, December 2017 (2017-12-01), pages 292 - 298, XP055697218
ABE, MITSURU ET AL.: "Maintenance-free cellulose solvents based on onium hydroxides", ACS SUSTAINABLE CHEMISTRY & ENGINEERING, vol. 3, no. 8, 14 July 2015 (2015-07-14), pages 1771 - 1776, XP055697220
WEI, WEI ET AL.: "Room temperature dissolution of cellulose in tetra-butylammonium hydroxide aqueous solvent through adjustment of solvent amphiphilicity", CELLULOSE, vol. 24, no. 1, 12 November 2017 (2017-11-12), pages 49 - 59, XP036128527, DOI: 10.1007/s10570-016-1113-9
ZHONG, CHAO ET AL.: "Wheat straw cellulose dissolution and isolation by tetra-n-butylammonium hydroxide", CARBOHYDRATE POLYMERS, vol. 94, no. 1, 15 April 2013 (2013-04-15), pages 38 - 45, XP055697225
Attorney, Agent or Firm:
LINNÉ, Nina (SE)
Download PDF:
Claims:
Claims

1. A method for treating a film comprising nanocellulose with an aqueous solution comprising tetraalkylammonium hydroxide, such as tetraethylammonium hydroxide and/or tetralkylammonium chloride, such as tetraethylammonium chloride or tetrabuthylammonium chloride to partly dissolve the film and to improve the barrier properties of the film.

2. The method according to any of the preceding claims wherein the film is treated with the aqueous solution for a period of 1 second to 60 minutes.

3. The method according to any of the preceding claims wherein the treatment with the aqueous solution is done at a temperature of 20-50°C, preferably at a temperature of 20-30°C.

4. The method according to any of the preceding claims wherein a solvent is applied to the treated film to remove the aqueous solution and terminate the dissolution reaction.

5. The method according to claim 4 wherein the solvent is a water miscible solvent preferably an alcohol and/or water.

6. The method according to any of the preceding claims wherein the treated film is subjected to a pressure of 40-900 kPa for a period of less than 10 minutes.

7. The method according to claim 6 wherein the treated film is subjected to an

increased temperature of 50-200°C either before or during the treatment at increased pressure.

8. The method according to any of the preceding claims wherein the nanocellulose is microfibrillated cellulose (MFC)

9. A film treated according to the method in any of the claims 1-8.

10. The film according to claim 8 wherein the film has an OTR value below 100 at 38°C and 90% RH measured according to ASTM F 1927-98.

11. A liquid or food-packaging material, being a laminate of one or more layers of the film according to any one of claims 9-10, with one or more base layers of paper or paperboard and/or one or more layers of a polymer.

Description:
METHOD FOR TREATING A NANOCELLULOSE FILM AND A FILM TREATED ACCORDING TO THE METHOD

TECHNICAL FIELD

The present invention relates to methods for treating nanocellulose films, as well as the modified nanocellulose films themselves so as to improve the barrier properties of said films.

BACKGROUND

Microfibrillated cellulose (MFC), which is a kind of nanocellulose, comprises partly or totally fibrillated cellulose or lignocellulose fibers. The liberated fibrils have a diameter less than 100 nm, whereas the actual fibril diameter or particle size distribution and/or aspect ratio

(length/width) depends on the source and the manufacturing methods. The smallest fibril is called elementary fibril and has a diameter of approximately 2-4 nm (see e.g. Chinga- Carrasco, G., Nanoscale research letters 2011, 6 :417), while it is common that the aggregated form of the elementary fibrils, also defined as microfibril, is the main product that is obtained when making MFC e.g. by using an extended refining process or pressure-drop disintegration process (see Fengel, D., Tappi J ., March 1970, Vol 53, No. 3.) . Depending on the source and the manufacturing process, the length of the fibrils can vary from around 1 to more than 10 micrometers. A coarse MFC grade might contain a substantial fraction of fibrillated fibers, i.e. protruding fibrils from the tracheid (cellulose fiber), and with a certain amount of fibrils liberated from the tracheid (cellulose fiber) .

Films or coatings made from fine cellulosic fibers such as nanofibers or microfibrillated cellulose can for instance be used in packaging applications. One important requirement for these films or coatings is that the barrier properties, such as gas permeability, is low and not altered by environmental conditions, form of storage or post-converting .

In some applications, the films are provided with additional physical barriers, such as polymer or plastic laminates in order to provide the film or coating with additional features or improved barrier properties required for more demanding applications. The use of plastics and other functional chemical is however sometime undesirable from an environmental and safety point of view.

Films or coatings made from microfibrillated cellulose (MFC) or nanofibrillated cellulose (NFC) are known to provide good oxygen or gas barrier properties including aroma barrier. It is known, however, that the films or coatings comprising high amount of MFC or NFC are very sensitive to moisture. When exposing such films in environment containing higher moisture such as in tropical conditions, the gas barrier properties are significantly reduced . Prior art teaches the use of various co-additives in the film comprising MFC or NFC or to laminate or co-extrude films e.g . with thermoplastic polymer that provides good water vapor transmission resistance. Unfortunately, many of the proposed solutions in the prior art are not industrially scalable or do not solve problems with bidirectional moisture sensitivity.

There is thus a need for a method for improving the barrier properties of a film comprising fine cellulosic fibers such as nanocellulose e.g . microfibrillated cellulose, especially at high humid conditions.

SUMMARY

In a first aspect, a method for increasing the barrier properties, especially the oxygen barrier properties, of a nanocellulose film is provided. The method comprises treating a film comprising nanocellulose with an aqueous solution comprising tetraalkylammonium hydroxide, such as tetraethylammonium hydroxide and/or tetralkylammonium chloride, such as tetraethylammonium chloride or tetrabuthylammonium chloride to partly dissolve the film and to improve the barrier properties of the film.

The film may be treated with the aqueous solution for a period of 1 second to 60 minutes, preferably for a period of 10 seconds to 10 minutes and the treatment with the aqueous solution is preferably done at a temperature of 20-50°C, even more preferred at a temperature of 20-30°C.

A solvent is preferably applied to the treated film to remove the aqueous solution and terminate the dissolution reaction. The solvent is preferably a water miscible solvent preferably an alcohol and/or water.

The film may be subjected to a pressure of 40-900 kPa for a period of less than 10 minutes after treatment with the aqueous solution. It may also be preferred that the film is subjected to an increased temperature of 50-200°C either directly before or during the treatment at increased pressure. The film can either be in a semi-wet or dry state prior to the treatment with increased pressure and/or heat, and it can also be a part of the drying process of the treated film.

The nanocellulose is preferably microfibrillated cellulose (MFC). In a second aspect, a nanocellulose film is provided that has been treated according to the method above. The film preferably has an Oxygen Transmission Rate (OTR) value below 100 at 38°C and 90% RH, preferably below 50 at 38°C and 90% RH and even more preferred below 20 at 38°C and 90% RH. The OTR value is measured according to ASTM F 1927-98.

In a third aspect, a liquid or food-packaging material is provided, which is being a laminate of one or more layers of the film according to the invention, with one or more base layers of paper or paperboard and/or one or more layers of polymer.

Further details of the technology are presented in the following description and the dependent claims.

DETAILED DISCLOSURE

The present invention relates to treatment of a nanocellulose film to improve the barrier properties, especially the oxygen barrier properties at high relative humidity (RH) . Barrier properties are very important when producing films that will be exposed to high humidity for example in packages. Another important feature is also the mechanical properties of the films, and the present invention is also believed to improve both the flexibility and the dry and wet strength of the film.

The nanocellulose film according to the present invention is subjected to a treatment by using a solution that can interact with the hydroxyl groups on cellulose and partly dissolve the film. It has surprisingly been found that it is possible to only partly dissolve a film comprising high amounts of nanocellulose and be able to form a film (referred to herein as "all-cellulose composite film") that has very good barrier properties, especially at high humid conditions. It is well known to produce all cellulose composites from cellulose fibers but it was very surprising that a similar technology worked so well on a film comprising a nanocellulose, such as microfibrillated cellulose which is a material comprising fibrils and not fibers.

The treatment to party dissolve the nanocellulose film is done by subjecting the film to an aqueous solution of a tetraalkylammonium hydroxide, such as tetraethylammonium hydroxide and/or a tetraalkylammonium chloride, such as tetraethylammonium chloride or

tetrabuthylammonium chloride. The concentration of the aqueous solution used is preferably between 10-50%. The dry content of the film being subjected to the aqueous solutions is high, preferably above 80 % by weight, even more preferably above 90% by weight or even more preferred above 94% by weight. It is important that the film is strong enough to withstand dissolution during the treatment with the aqueous solution. Lower dry contents decreases the strength of the film and increases the risk of dissolution of the film. The film is preferably submerged into the aqueous solution. The film may also be exposed to the aqueous solution by spraying, coating, spreading, vapour deposition and/or printing the solution onto the surface/s of the film.

The treatment with the aqueous solution may occur for a period of 1 second to 60 minutes, preferably for a period of 10 seconds to 10 minutes and even more preferably for a period of 10 seconds to 5 minutes and at a temperature between 20-50°C, preferably at a temperature between 20-30°C.

After the treatment with the aqueous solution, the nanocellulose film may be treated with a solvent to remove the aqueous solution to terminate the dissolution reaction and remove the aqueous solution and to precipitate the dissolved parts. The solvent used to terminate the dissolution of the film is preferably a water miscible solvent, preferably an alcohol, such as ethanol, methanol or isopropanol and/or water. It is also possible to use sequential washing with different solvent/s in each washing step. It is important to terminate the dissolution reaction in time so that non-dissolved fibrils remain in the film to give the film good strength properties.

The treated film may thereafter be subjected to an increased pressure to further improve the barrier and strength properties of the film.The pressure used is preferably between 40-900 kPa and the pressing may last for a period of less than 10 minutes, preferably between 1 second to 10 minutes. It is preferred that the pressing is done at elevated temperatures. Temperatures used during pressing may be between 50-200°C, preferably between 100- 150°C. The pressing may be done in any conventional equipment such as presses or calenders. By combining the use of pressing, preferably hot pressing of the treated film the properties, such as the barrier and mechanical properties, of the film can be improved. The film can either be in a semi-wet or dry state prior to the treatment with increased pressure and/or heat, and it can also be a part of the drying process of the treated film. It may be preferred that the film has a dry content of 60-99% by weight during treatment at increased pressure. It has been found that the properties of the film, such as barrier and mechanical properties, increases even more when applying pressure and heat to the treated

nanocellulose film, i.e. the all cellulose composite film.

The treated film is preferably dried after being subjected to the aqueous solution and the solvent/s. The film may be dried using any drying equipment known to a person skilled in the art. Nanocellulose

By nanocellulose is meant any one of microfibrillated cellulose (MFC), nanofibrillated cellulose (NFC), bacterial cellulose and/or nanocrystalline cellulose.

Nanocellulose Films

With nanocellulose film is meant a thin substrate with good gas, aroma or grease or oil barrier properties, preferably oxygen barrier properties. The film preferably has a basis weight of less than 100 g/m 2 and a density in the range from 700-1400 kg/m 3 .

The film may be produced by applying a suspension comprising nanocellulose, for example microfibrillated cellulose onto a wire. The wire may be porous felt wire or made from polymer of metal. It may also be possible to apply the suspension comprising nanocellulose by casting the suspension onto a substrate. The substrate may be a polymer or metal substrate. The casted fibrous web can then be dried and optionally peeled off from the substrate. The applied nanocellulose suspension is thereafter dried to form a nanocellulose film.

The film preferably comprises 70-100% by weight of nanocellulose, for example

microfibrillated cellulose based on total dry weight of the film, preferably between 80-95% by weight of nanocellulose.

The film may further comprise other additives such as any one of a starch, carboxymethyl cellulose, a filler, retention chemicals, flocculation additives, deflocculating additives, wet strength additives, dry strength additives, plasticizers and polymers, such as polyvinyl acetate (PVAc), polyvinyl alcohol (PVOH), polyvinyl alcohol-acetate (PVOH/Ac), ethylene vinyl alcohol (EVOH), softeners or mixtures thereof.

MFC and MFC films

Microfibrillated cellulose (MFC) or so called cellulose microfibrils (CMF) shall in the context of the present invention mean a micro-scale cellulose particle fiber or fibril with at least one dimension less than 100 nm. MFC comprises partly or totally fibrillated cellulose or lignocellulose fibers. The cellulose fiber is preferably fibrillated to such an extent that the final specific surface area of the formed MFC is from about 1 to about 300 m 2 /g, such as from 1 to 200 m 2 /g or more preferably 50-200 m 2 /g when determined for a freeze-dried material with the BET method.

Various methods exist to make MFC, such as single or multiple pass refining, pre-treatment followed by refining, or high shear disintegration or liberation of fibrils. One or several pre- treatment steps are usually required in order to make MFC manufacturing both energy- efficient and sustainable. The cellulose fibers of the pulp to be supplied may thus be pre treated enzymatically or chemically, for example to reduce the quantity of hemicellulose or lignin. The cellulose fibers may be chemically modified before fibrillation, wherein the cellulose molecules contain functional groups other (or more) than found in the original cellulose. Such groups include, among others, carboxymethyl, aldehyde and/or carboxyl groups (cellulose obtained by N-oxyl mediated oxidation, for example "TEMPO"), or quaternary ammonium (cationic cellulose). After being modified or oxidized in one of the above-described methods, it is easier to disintegrate the fibers into MFC.

The microfibrillar cellulose may contain some hemicelluloses; the amount is dependent on the plant source. Mechanical disintegration of the pre-treated fibers, e.g. hydrolysed, pre-swelled, or oxidized cellulose raw material is carried out with suitable equipment such as a refiner, grinder, homogenizer, colloider, friction grinder, ultrasound sonicator, single - or twin-screw extruder, fluidizer such as microfluidizer, macrofluidizer or fluidizer-type homogenizer.

Depending on the MFC manufacturing method, the product might also contain fines, or nanocrystalline cellulose or e.g. other chemicals present in wood fibers or other lignocellulosic fibers used in papermaking processes. The product might also contain various amounts of micron size fiber particles that have not been efficiently fibrillated.

MFC can be produced from wood cellulose fibers, both from hardwood or softwood fibers. It can also be made from microbial sources, agricultural fibers such as wheat straw pulp, bamboo, bagasse, or other non-wood fiber sources. It is preferably made from pulp including pulp from virgin fiber, e.g. mechanical, chemical and/or thermomechanical pulps. It can also be made from broke or recycled paper.

The above described definition of MFC includes, but is not limited to, the proposed TAPPI standard W13021 on cellulose nano- or microfibril (CNF or CMF, respectively) defining a cellulose nanofiber material containing multiple elementary fibrils with both crystalline and amorphous regions, having a high aspect ratio with width of 5-30 nm and aspect ratio usually greater than 50.

Also provided is a nanocellulose film obtained or obtainable via the methods described herein.

The nanocellulose films have uses as liquid barriers in e.g. liquid- or food- packaging material. In one aspect, a liquid or food packaging material is provided which is a laminate of one or more layers of the nanocellulose film as described herein, with one or more base layers of paper or paperboard and/or one or more layers of a polymer. The polymer may preferably be polyethylene (PE), polypropylene (PP) and/or polyethylene terephthalate (PET). The film may be coated or laminated with one or more layers of polymer/s on one or both sides of the film. The polymer laminate can for example be used as a pouch in food packaging.

All details of the methods described above are also relevant for the nanocellulose film, as described herein.

EXAMPLES

Example 1 :

Material

A 3% MFC suspension in water, comprising 100% by weight of MFC based on total fiber amount was used. The MFC was made of bleached softwood kraft pulp.

MFC Film preparation

The MFC film was produced by rod coating the MFC suspension on a metal plate, which was pre-heated to 105 °C. The estimated temperature during drying was 70 °C. The obtained film had a grammage of 50.1 g/m 2 and was 49.9 pm thick.

Film treatment

The formed film was immersed in a 35 wt.% tetraethylammonium hydroxide (TEAOH) solution for 30 s. The film was thereafter washed with ethanol 4 times for approximately 5 min at a time, followed by washing in water and drying. The film was vacuum dried in a vacuum drier (Karl Schroder KG, Germany) for 10 min at temperature of 93°C, at negative pressure of 930 mbar.

As a reference a film without treatment with TEAOH was used.

Results

OTR measurements were carried out on the dried films, and the OTR results are presented in Table 1. It can be seen from Table 1 that the OTR values at high humidity has strongly improved for the film treated with the solution according to the invention. The OTR were measure according to ASTM F 1927-98. Table 1. OTR values for the films

Example 2:

Material A mixture of two types of 3% MFC grades (in water) was used for the film preparation. It contained 75% of a coarse grade and 25% of a fine grade. Clay was added and the final mixture contained 4.76% clay.

MFC Film preparation

MFC films were obtained by using Doctor Blade technique. The MFC sludge was coated on a warm metal plate. The obtained film was 34 pm thick.

Film treatment

The film was immersed in 35 wt.% tetraethylammonium hydroxide (TEAOH) solution for 30 s (film A) or 60 s (films B and C). Then they were washed with ethanol 4 times for

approximately 5 min at a time, followed by washing in water. Wet films were stretched between two metal wires and gently pressed with an iron. Then they were pressed in a pellet press for 60 s at temperature of 105°C (film A and B) or 150°C (film C), at lObar.

Results

All the films were pre-conditioned at 23°C and 80% humidity for at least 48h before the measurements. OTR measurements were carried out on the dried films, and the OTR results are presented in Table 2. It can be seen from Table 2 that the OTR values at high humidity has strongly improved for the films treated with the solution according to the invention. The OTR were measure according to ASTM F 1927-98. Table 2. OTR values of the films

While the invention has been described with reference to a number of embodiments and examples, the skilled person can freely amend and combine these embodiments within the scope of the claims. The scope of the invention is defined by the appended claims.