WO2020058803A1

WO2020058803A1 - A method for improving adhesion during production of film

Info

Publication number: WO2020058803A1
Application number: PCT/IB2019/057603
Authority: WO
Inventors: Cecilia LAND HENSDAL; Isabel KNÖÖS
Original assignee: Stora Enso Oyj
Priority date: 2018-09-19
Filing date: 2019-09-10
Publication date: 2020-03-26
Also published as: EP3853264A4; SE542944C2; EP3853264A1; SE1851113A1

Abstract

The present invention relates to a method for manufacturing at least one layer of a film characterized by that the method comprises the steps of: providing a first suspension comprising microfibrillated cellulose, providing a second suspension comprising microfibrillated dialdehyde cellulose, mixing the first suspension with the second suspension to form a mixture, in which mixture less than 5% of the anhydroglucose units (AGU) of the total cellulose present therein comprise a dialdehyde, applying said mixture to a substrate to form a fibrous web, and drying said web to form at least one layer of said film. The present invention further relates to a film comprising said at least one layer of film.

Description

A METHOD FOR IMPROVING ADHESION DURING PRODUCTION OF FILM

Technical field

The present invention relates to a film having improved adhesion properties during forming. More particularly, the present invention relates to a method of manufacturing such a film and a film produced.

Background

During casting of films comprising native microfibrillated cellulose, it may happen that the film detaches from the substrate before it is dewatered. The film then runs the risk of becoming non-flat or wrinkled, which ruins the quality of the film. To overcome this problem, a plasticizer such as sorbitol is usually used as an additive, even up to 30% addition. However, sorbitol is a moisture sensitive chemical which deteriorates the oxygen barrier properties of the film, especially at high relative humidity in the air.

Hence, there is a need to find a solution of producing films with good adhesion properties during forming of film, while avoiding the use of moisture sensitive additives.

Summary

It is an object of the present invention, to provide an improved method for manufacturing a film comprising native microfibrillated cellulose, having improved adhesion properties while maintaining barrier properties of the film.

The object of the present invention is at least partially achieved by means of a method for manufacturing a least one layer of a film wherein the method comprises the steps of:

providing a first suspension comprising microfibrillated cellulose, providing a second suspension comprising microfibrillated dialdehyde cellulose,

mixing the first suspension with the second suspension to form a mixture, in which mixture less than 5% of the anhydroglucose units (AGU) of the total cellulose comprise a dialdehyde,

applying said mixture to a substrate to form a fibrous web and drying said web to form at least one layer of said film.

According to one aspect of the invention, in said mixture, below 5%, preferably between 0.1 - 3%, more preferably between 0.1 - 2% of the anhydroglucose units (AGU) of the total cellulose in the mixture comprise a dialdehyde.

The skilled person understands that the DA-MFC added in the second suspension can have different degrees of oxidation, defined as the portion of AGUs that have a dialdehyde. The amount of DA-MFC needed to be added depends on the degree of oxidation in the corresponding suspension. A mixture of DA-MFC and native MFC naturally has an average degree of oxidation that is lower than that of the DA-MFC part of the mixture. In that sense, said mixture containing both MFC and DA-MFC could theoretically be interpreted as a DA-MFC with low degree of oxidation.

It has surprisingly been found that use of DA-MFC as an additive leads to improved adhesion properties upon film making, i.e. a balanced adherence between the casted film suspension and the underlying substrate. By “balanced adherence” means in this context that the casted film suspension adheres well enough to the substrate during drying to not detach therefrom (where detaching of the film would lead to wrinkles and bubbles in the resulting film surface), and at the same time does not stick too hard to the surface. Too strong attachment of the film onto the substrate during making could lead to problems with detaching the dried film once it is ready. The dry content of the mixture applied to the substrate is preferably between 1 -10% by weight. Depending on the substrate onto which the mixture is applied, the dry content of the mixture may vary.

According to another aspect of the invention, the m icrof ibri I lated dialdehyde cellulose of said second suspension has an average degree of oxidation between 10-50%, preferably between 30-40%.

According to another aspect of the invention, the second suspension comprises also native MFC in addition to microfibrillated dialdehyde cellulose.

According to another aspect of the invention, drying of the web to form at least one layer of said film is performed under heat treatment of said web. Such heat treatment may be achieved by means of heating the substrate onto which the mixture is cast. In one example, said substrate is a polymer, plexi glass or metal substrate, and heat treatment is accomplished by means of heating said polymer, plexi glass or metal substrate so that it has reached a temperature between 45 - 120°C, preferably between 50 - 80°C, when the mixture is applied. The skilled person understands that other conventional heat treatment methods are also conceivable such as IR-drying. Drying the film under heat treatment leads to the advantage of improved crosslinking between the microfibrillated fibers in the film.

The mixture may further comprise additives, preferably any one of a starch, carboxymethyl cellulose, a filler, clay, talcum, polyvinyl alcohol (PVOH), retention chemicals, flocculation additives, deflocculating additives, dry strength additives, softeners, or mixtures thereof. It may be possible to add additives that will improve different properties of the mixture and/or the produced film such as latex and/or polyvinyl alcohol for enhancing the ductility of the film. It may be possible to add the additive to the first suspension, the second suspension and/or to the mixture.

The present invention further relates to a film comprising a mixture of microfibrillated cellulose and microfibrillated dialdehyde cellulose, wherein the film is manufactured from a mixture in which less than 5% of the

anhydroglucose units (AGU) of the total cellulose present in the mixture comprise a dialdehyde. The film preferably has a grammage of less than 50 g/m², preferably between 10-50 g/m²

According to one aspect, said film is a multilayer film comprising more than one layer. For instance, a layer of polyethylene (PE) on at least one side of the film comprising microfibrillated dialdehyde cellulose also improves strainability of the film and provides heat-sealing properties.

According to one aspect of the invention, at least one layer of the film is a water vapor barrier film comprising any one of polyethylene (PE), polypropylene (PP), polyamide, polyethylene terephthalate (PET) or ethylene vinyl alcohol (EVOH). The water vapor barrier film has preferably a

grammage between 10 - 60 g/m², preferably 30 - 50 g/m².

In one aspect, said film is a multilayer film wherein at least one layer of the film is a metallized barrier layer. By“metallized barrier layer” means a thin layer of metal providing barrier properties reducing permeability to e.g.

oxygen, water, water vapour and light. According to one aspect of the invention, said metallized barrier layer is a physical vapour deposited metal or metal oxide layer, or a chemical vapour deposited metal or metal oxide layer, wherein said metal or metal oxide is selected from the group consisting of aluminium, aluminium oxides, magnesium oxides, silicium oxides, copper, magnesium and silicon. Preferably, the metallized barrier layer has a weight between 50 - 250 mg/m², preferably between 75 - 150 mg/m².The present invention further relates to a packaging material comprising a base material laminated with at least one layer of the film previously described comprising a mixture of a microfibrillated cellulose and a microfibrillated dialdehyde cellulose. Said base material is preferably paper or paperboard. According to one aspect of the invention, the paper or paperboard of said packaging material has a grammage between 20 - 500 g/m², for instance between 80 - 400 g/m². The packaging material may comprise a multilayer film which in addition to the film comprising a mixture of a microfibrillated cellulose and a microfibrillated dialdehyde cellulose also comprises one or more of a water vapor barrier layer and/or a metallized barrier layer previously described According to another aspect, the invention also relates to the use of DA-MFC as an additive for improving adhesion properties in manufacturing of film, where said film comprises microfibrillated cellulose.

Description of Embodiments

The method according to the present invention relates to providing a first suspension comprising microfibrillated cellulose and mixing said first suspension with a second suspension which in its turn comprises

microfibrillated dialdehyde cellulose (DA-MFC), to form a mixture. According to the invention, in said mixture less than 5% of the anhydroglucose units (AGU) of the total cellulose present therein comprise a dialdehyde.

The mixture is then applied onto a substrate to form a fibrous web and said web is thereafter dried to form at least one layer of the film. It has surprisingly been found that by forming a mixture comprising microfibrillated cellulose and microfibrillated cellulose from dialdehyde cellulose (DA-MFC) the adhesion between the mixture and the substrate is improved to such an extent that DA-MFC may replace moisture-sensitive additives such as sorbitol.

The at least one layer of the film is produced by applying said mixture to a substrate to form a fibrous web and drying said web to form at least one layer of said film. The drying of said web may be done in any conventional way, however preferably under heat treatment. The dry content of the at least one layer of the film after drying is preferably above 95% by weight.

The substrate may be a polymer, plexi glass or metal substrate onto which the mixture is casted. The cast coated fibrous web is dried in any conventional manner and thereafter optionally peeled off from the substrate. It may be possible to cast or coat more than one layer onto the substrate forming a multilayer film. It is possible to produce a film comprising more than one layer wherein at least one of the layers comprises the mixture according to the invention. It may also be possible that more than one layer of the film comprises the mixture according to the invention. It may also be possible that one or more layers of the film only comprises microfibrillated cellulose of the first suspension, i.e. not microfibrillated dialdehyde cellulose (DA-MFC). It may also be possible that one or more layers of the film is a water vapor barrier film comprising any one of polyethylene (PE), polypropylene (PP), polyamide, polyethylene terephthalate (PET) or ethylene vinyl alcohol

(EVOH). It may also be possible that one or more layers of the film is a metallized barrier layer, comprising any one of aluminium, aluminium oxides, magnesium oxides, silicium oxides, copper, magnesium and silicon. The film may comprise two, three, four, five, six, seven, eight, nine, ten or more layers.

A packaging material including a layer of the mixture according to the invention may comprise further barrier layers for preventing migration of air, water and flavors through the base material. According to one aspect a packaging material may include at least one water vapor barrier layer comprising any one of polyethylene (PE), polypropylene (PP), polyamide, polyethylene terephthalate (PET) or ethylene vinyl alcohol (EVOH). The packaging material may also include one or more metallized barrier layer(s) comprising any one of aluminium, aluminium oxide, magnesium oxide or silicium oxide.

Microfibrillated cellulose (MFC) or so called cellulose microfibrils (CMF) shall in the context of the present application mean a nano-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 m²/g, such as from 1 to 200 m²/g or more preferably 50-200 m²/g when determined for a freeze-dried material with the BET method. The term“native MFC” refers to MFC that is made from conventional chemical, chemimechanical and/or mechanical pulp without further chemical treatment, e.g. said native MFC is lacking special functional groups. Various methods exist to make MFC, such as single or multiple pass refining, pre-hydrolysis 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, carboxym ethyl, 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 or NFC.

The nanofibrillar 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 in papermaking process. 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 (CMF) 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.

Dialdehyde cellulose (DA-MFC) is typically obtained by reacting cellulose with an oxidising agent such as sodium periodate. During the periodate oxidation, selective cleavage of the C2-C3 bond of the

anhydroglucose unit (AGU) of cellulose takes place, with concurrent oxidation of the C2- and C3-OFI moieties to aldehyde moieties. In this manner, crosslinkable functional groups (aldehyde groups) are introduced to the cellulose. The microfibrillated dialdehyde cellulose in the second suspension should in this context mean a dialdehyde cellulose treated in such way that it is microfibrillated. The production of the microfibrillated dialdehyde cellulose is done by treating dialdehyde cellulose for example by a homogenizer or in any other way such that fibrillation occurs to produce microfibrillated dialdehyde cellulose. The microfibrillated dialdehyde cellulose in the second suspension preferably has an oxidation degree between 10-50%, preferably between 30- 40%. The degree of oxidation was determined according to the following description: after the dialdehyde cellulose reaction, the amount of C2-C3 bonds in the cellulose that are converted to dialdehydes is measured. The degree of oxidation is the amount of C2-C3 bonds that are converted compared to all C2-C3 bonds. This is measured with a method by H. Zhao and N.D. Heindel,“Determination of Degree of Substitution of Formyl Groups in Polyaldehyde Dexran by the Flydroxylamine Hydrochloride Method”, Pharmaceutical Research, vol. 8, pp. 400-402, 1991 , where the available aldehyde groups reacts with hydroxylamine hydrochloride. This forms oxime groups and releases hydrochloric acid. The hydrochloric acid is titrated with sodium hydroxide until pH 4 is reached, and the degree of oxidation is thereafter calculated from according to the formula below. The received aldehyde content is divided by two to get the value of the degree of oxidation, since an oxidized anhydroglucose unit has two aldehyde groups.

VNaO_H = the amount of sodium hydroxide needed to reach pH 4 (I)

CNaOH = 0,1 mol/l

rrisampie = dry weight of the analysed DAC sample (g)

M_w = 160 g/mol, which is the molecular weight of the dialdehyde cellulose unit

The mixture may further comprise additives, preferably any one of a starch, carboxymethyl cellulose, a filler, retention chemicals, clay, talcum, polyvinyl alcohol, flocculation additives, deflocculating additives, dry strength additives, softeners, or mixtures thereof. It may be possible to add additives that will improve different properties of the mixture and/or the produced film. It may be possible to add the additive to the first suspension, the second suspension and/or to the mixture.

Brief description of the figure

Figure 1 shows a photo of a film with undulated surface due to poor adhesion during film making. Example

Example I: DA-MFC as an adhesion enhancer

A first suspension of native microfibrillated cellulose (MFC) was diluted to a solids content of 3 wt%. 10 wt% of polyvinyl alcohol was added and also 5 wt% of clay or talcum, according to Table 1. (Added relative to 100% MFC) A second suspension with a solids content of 3 wt% was also prepared by mixing 60% microfibrillated dialdehyde cellulose (DA-MFC) with a degree of oxidation of 40%, with 40% MFC. The total degree of oxidation in the second suspension was thus 24%.

A first mixture with no added DA-MFC, i.e. corresponding to said first suspension only, is referred to as Test 1 , whereas Tests 2, 3 and 4 were prepared by mixing the first suspension with increased added amounts of the second suspension, ending up with mixtures No. 2, 3 and 4 having 1wt%, 5wt% and 7wt% respectively of said second suspension. For Tests 2,3 and 4 the portion of AGUs in the mixture containing a dialdehyde was thus 0.24%,

1.2% and 1.68% respectively.

The mixtures were used for casting of films on a heated Plexiglas or on a plasma treated polypropylene (PP) substrate, where the degree of plasma treatment was 100%. For Test 1 , the film was cast and dried on a heated Plexiglas substrate, described in the following. A metal plate was preheated in an oven at 70°C for over 2h. Coating was performed in room temperature of 23°C, by placing a frame with dimension of approximately an A4 sheet on the Plexiglas, applying the mixture inside the frame and using a rod like tool to fill out said frame and create the fibrous web. The frame was removed from the Plexiglas, and next said Plexiglas with the web was placed on top of the preheated metal plate consequently heating also said Plexiglas to a temperature between 45 - 55°C, whereupon the web was allowed to dry to form a film.

Tests 2, 3 and 4 were cast on a plasma treated polypropylene substrate in room temperature.

Films were successfully casted with all the tested mixtures when adding the DA-MFC mixture (Table 1 ), with varying weigh% content of DA- MFC as explained above. The films with clay needed a higher amount of DA- MFC than the film with talcum in order to achieve an acceptable adhesion.

The adhesion to the substrate of the respective films was evaluated by visual inspection of the film surface after drying, and by evaluating film behaviour opon detaching the film from the substrate after drying. In order to meet the requirement for a“good ahesion”, the casted film should firstly present an essentially planar/flat surface on the substrate with no undulations and/or detached areas or bubbles appearing across the film surface.

Secondly, the film should also detach from the substrate after that the film is ready, i.e. it should be possible to peel off the film from the substrate after completion of the film making without rupturing said film. If the film adhesion to the substrate is too strong, the film will rupture when trying to detach it. In other words, a“good adhesion” represents a sophisticated balance between too weak adhesion leading to bubbles and undulations in the film, and too strong adhesion leading to ruptures upon removing the dried film from the substrate.

In Table 1 , a“good adhesion” fulfilling the above defined requirements of presenting a planar surface upon completion of film drying, and upon completion being detachable without rupturing, is marked as“OK”.“Almost OK” is to be interpreted as a few observed wrinkles, i.e. less than three wrinkles for one film sheet of a approximately A4 size.“Too low” is to be interpreted as a comparably large number of wrinkles, i.e. more than three wrinkles for one film sheet of approximately A4 size.

Figure 1 shows a photo of a film made from a mixture of MFC, PVOFI, talcum and 0.24wt% DA-MFC, where the adhesion was too low leading to undulations spreading across the film during the drying phase of film making process.

Table 1 : Adhesion test result with varying amounts of added DA-MFC with D.O. of 24% The results from Example 1 show that microfibrillated dialdehyde cellulose may serve as an additive for the purpose of enhancing adhesion during casting of barrier films comprising microfibrillated cellulose.

The positive results implies that addition of small amounts of DA-MFC to a mixture intended for film making could replace conventional moisture sensitive additive in the form of sorbitol.

In view of the above detailed description of the present invention, other modifications and variations will become apparent to those skilled in the art. However, it should be apparent that such other modifications and variations may be effected without departing from the spirit and scope of the invention.

Claims

1. A method for manufacturing at least one layer of a film characterized by that the method comprises the steps of:

mixing the first suspension with the second suspension to form a mixture, in which mixture less than 5% of the anhydroglucose units (AGU) of the total cellulose present therein comprise a dialdehyde,

applying said mixture to a substrate to form a fibrous web, and drying said web to form at least one layer of said film.

2. A method according to claim 1 , wherein in said mixture between 0.1 - 3%, more preferably between 0.1 - 2% of the anhydroglucose units (AGU) of the total cellulose in the mixture comprise a dialdehyde.

3. The method according to any of the preceding claims wherein the dry content of the mixture applied to the substrate is between 1-10% by weight.

4. The method according to any of the preceding claims, wherein the microfibrillated dialdehyde cellulose of said second suspension has an average degree of oxidation between 10-50%, preferably between 30-40%.

5. The method according to any of the preceding claims wherein the substrate is a polymer, plexi glass or metal substrate.

6. The method according to any one of claims 1 - 5, wherein said drying of the film comprises heating the temperature of said substrate to between 45-120°C.

7. The method according to any one of the preceding claims, wherein said mixture further comprises any one of a starch, carboxymethyl cellulose, a filler, clay, talcum, retention chemicals, flocculation additives, deflocculating additives, dry strength additives, softeners, polyvinyl alcohol or mixtures thereof.

8. A film obtainable according to the method of any one of the previous claims having a basis weight of less than 50 g/m², preferably between 10-50 g/m²

9. The film according to claim 8, wherein said film is a multilayer film comprising more than one layer.

10. The film as claimed in any one of the claims 8 - 9, wherein said film is a multilayer film and wherein at least one layer of the film is a water vapor barrier film comprising any one of polyethylene (PE), polypropylene (PP), polyamide, polyethylene terephthalate (PET) or ethylene vinyl alcohol (EVOH).

11. The film according to claim 10, wherein the water vapor barrier film has a grammage between 10 - 60 g/m², preferably 30 - 50 g/m².

12. The film according to any one of claims 8 - 11 , wherein said film is a multilayer film and wherein at least one layer of the film is a metallized barrier layer.

13. The film according to claim 12, wherein said metallized barrier layer is a physical vapour deposited metal or metal oxide layer, or a chemical vapour deposited metal or metal oxide layer.

14. The film according to claim 13, wherein said metal or metal oxide is selected from the group consisting of aluminium, aluminium oxides, magnesium oxides, silicium oxides, copper, magnesium and silicon.

15. The film according to any one of claims 12 - 14, wherein said metallized barrier layer has a weight between 50 - 250 mg/m², preferably 75 - 150 mg/m².

16. A packaging material comprising a base material and at least one layer of the film as claimed in any one of claims 8-15.

17. The packaging material according to claim 16, wherein the base material is paper or paperboard.

18. The packaging material according to claim 17, wherein the paper or paperboard has a grammage between 20 - 500 g/m²

19. The packaging material according to claim 17, wherein the paper or paperboard has a grammage between 80 - 400 g/m².

20. Use of microfibrillated dialdehyde cellulose as an additive in the manufacturing of a film according to any one of claims 8 - 15, for improving adhesion properties during film making.