Technical field

The present document relates to a method for producing a high density web or film comprising nanofibrillated polysaccharide, wherein a filler material is introduced comprising nanobubble carbon dioxide as a precursor.

Background

Films manufactured from nanofibrillated or microfibrillated polysaccharides, such as microfibrillated cellulose (MFC), are known to provide papers or paper boards, polymer coatings or different types of laminated products, having very good oxygen barriers or very low air permeability. MFC films can also provide good grease or fat resistance. Modified MFC or MFC films can also provide some solvent or water vapor resistance. MFC films can also provide aroma barrier in different types of paper or film products. These papers or paperboards are useful in many different applications, for instance in the pharmaceutical or food packaging industry. However, when forming these films or webs in a paper making machine, one drawback or difficulty, is the rate at which the dewatering may take place.

Currently, films from MFC are either made on plastic films, where the plastic film work as a carrier substrate or the MFC is coated directly on plastic or a paper or paperboard, i.e. a base board, which then act as a base substrate. In the latter case, dewatering can occur in two directions, i.e.

absorption of liquid phase into base board or through the base board if thin substrate and evaporation of the liquid into the air. In case of the coating on a plastic film carrier substrate, the absorption is basically non-existing and therefore all water removal must take place via evaporation or through a one- directional water removal via pressing. The use of dewatering chemicals and fillers are known to affect dewatering behavior in a paper machine, but their use and efficiency on MFC web are less efficient.

There are some highly refined papers being made today, in which MFC technology, and the production of MFC webs, is used. These refined papers are produced in a conventional paper machine, but various changes have been made to the process in order to enable production of such webs. One of the biggest challenges is the dewatering, where the main solution for this problem today is simply to slow down the speed at which the paper machine is operating, which of course impairs the efficiency of the paper making process. Another solution that is more commonly adopted, is to make a more coarse MFC, which then enable higher production speeds. The disadvantage of this solution is that the oxygen barrier properties are not very good, which is even more pronounced at low grammage.

There is thus a need for a production method of a MFC web or film which is more efficient in terms of dewatering and that provide high barrier properties even at low grammages or coat weights. Further there is a need to be able to feed and maintain retention of nanoparticles without using high amount of retention chemicals.

Summary

It is an object of the present disclosure, to provide an improved manufacturing method for dense films or web of microfibri Hated

polysaccharide, which eliminates or alleviates at least some of the

disadvantages of the prior art methods for manufacturing these types of webs or films.

The invention is defined by the appended independent claims.

Embodiments are set forth in the appended dependent claims and in the following description.

According to a first aspect there is provided a method for producing a film or web in a paper making process, said method comprising the steps of: providing an aqueous solution comprising a microfibrillated polysaccharide, wherein the microfibrillated polysaccharide is any one of a microfibrillated cellulose, nanofibrillated cellulose, fibrillated cellulose fibers and cellulose fibrils or a mixture thereof, having a dry content in the range of from 0.1 to 5.0 weight-%, introducing a filler material into said aqueous solution, wherein the method further comprises the step of forming said filler material directly in the aqueous solution by introducing precursor materials into said solution, whereby said precursor materials react to form the filler material onto or into the fibrils of the microfibrillated polysaccharide; wherein said filler material is formed from gas bubbles from carbon dioxide having a diameter in the range of from 10 to 800 nm.

By "aqueous solution" is meant any solution comprising the

microfibrillated polysaccharide in the wet end of a paper making machine, e.g. the stock solution.

By using carbon dioxide bubbles having a nano size, i.e. nanobubbles, to form the filler material, a nanosized filler, or "nanofiller" material may be obtained. The nanobubble carbon dioxide may also provide for higher reaction efficiency, improved mixing behavior, and a reduced need for gas or other precursors.

It has surprisingly been found that the formation or introduction of such a nanofiller into or onto the fibrils of the microfibrillated polysaccharide provides a film or web having a high content of microfibrillated polysaccharide, i.e. a dense film or web, which has improved dewatering characteristics. By the filler, i.e. the nanofiller, forming onto or into the fibrils an inorganic-organic fibril/fiber composite may thus be formed, for which the dewatering process is much faster than compared to a conventionally formed web or film of a microfibrillated polysaccharide.

The filler material may be any one of a precipitated calcium carbonate and a precipitated magnesium carbonate, or other metal salt carbonate or a mixture thereof.

The filler material may be a precipitated calcium carbonate (PCC).

By using nanobubbles of carbon dioxide to form the filler material it is possible to form a nano-PCC which provides for a web or film having improved dewatering characteristics. According to one embodiment the method may further comprise the step of forming said filler material directly in the aqueous solution by.

The carbon dioxide bubbles may then be a precursor material.

The filler material may according to one embodiment be introduced into the aqueous solution in an in-line process.

By using nanobubbles of carbon dioxide, which stability can be adjusted, to form the filler material it is possible to provide for a very quick introduction and mixing of the filler material, into the aqueous solution through an in-line process. This means that the filler material may be formed directly prior to the introduction into the aqueous solution, and introduced in a continuous manner into the solution, or formed directly into the stream of the paper or board making machine, i.e. directly in the presence of e.g. MFC.

Further the in-line process allows for a very quick formation of the filler material if it is formed by introducing the precursor materials directly into the aqueous solution. The stability of the nanobubble makes it possible to more easily control the mixing and reaction efficiency of the filler and/or filler precursors. An example of the in-line PCC process is described in

WO2014072912.

According to one embodiment of the first aspect the filler material may be prepared or introduced into the aqueous solution in an off-line process or an at-line process. The aqueous solution may thus contain the MFC as defined above.

By the filler material being introduced in an off-line process is meant that the filler material is formed at a location separate from the paper making process, and then transported to the paper making machine for introduction into the process. By at-line is meant that the filler material is formed directly in connection to the paper making machine and then introduced into the process. This means that precursor materials are mixed, and thus allowed to react, to form the nanofiller prior to it being introduced in the paper making process.

According to one embodiment the precursor materials may comprise any one of calcium carbonate (CaCOs), calcium oxide (CaO), calcium hydroxide (Ca(OH)2), calcium bicarbonate (Ca(HC03)2), sodium bicarbonate (NaHCOs), magnesium hydroxide (Mg(OH)2), and magnesium oxide (MgO), or a mixture thereof.

According to one embodiment the grammage of the film or web may be less than 100 g/m ² , or less than 70 g/m ² , or less than 50 g/m ² , or less than 30 g/m ² ' or less than 20 g/m ² .

The amount of filler material in said film or web may, according to one embodiment be less than 20% of the dry content of the microfibrillated polysaccharide, or less than 15% of the dry content of the microfibrillated polysaccharide, or less than 10% of the dry content of the microfibrillated polysaccharide.

By keeping the amount of filler material relatively low there is a minimal negative effect on barrier and strength properties.

According to a second aspect there is provided a film or a web material comprising microfibrillated polysaccharide or microfibrillated cellulose obtained by the method according to the first aspect.

According to a third aspect there is provided a paper or paper board material obtained from a film or web material according the second aspect.

This film or web, may be an intermediate product, and can be further treated or laminated according to technologies known for a person skilled in the art.

Description of Embodiments

In the production of highly refined paper and paperboard materials for instance for use in the food packaging industry, or different types of coating or laminates, fiber webs or fiber films comprising microfibrillated polysaccharides are often used. The end product often has properties such as low air permeability, or good oxygen barrier capabilities.

The microfibrillated polysaccharide may be any types of highly fibrillated fibers, such as a microfibrillated cellulose, nanofibrillated cellulose, , and cellulose fibrils or a mixture thereof. Microfibrillated polysaccharide is defined here as including bacterial cellulose or nanocellulose (BNC) spun with either traditional spinning techniques or then with electrostatic spinning. In these cases, the material is preferably a polysaccharide but not limited to solely a polysaccharide. Also cellulose whiskers, or regenerated cellulose is included in this definition. The microfibrillated polysaccharide may also be synthetic biofibers that have been fibrillated or spun into nanosize fibers.

The microfibrillated polysaccharide can further be mixed together with normal lignocellulose or reinforcing pulps both in low or high amounts. It is further possible to use a purified microfibrillated polysaccharide or a

chemically, or physically modified grade thereof.

According to one embodiment the microfibrillated polysaccharide is a microfibrillated cellulose. The microfibrillated cellulose (MFC) is also known as nanocellulose. It is a material typically made 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, esparto (alfa) grass, bagasse (cereal straw, rice straw, bamboo, hemp), kemp fibers, flax and other cellulosic fiber sources. MFC can also be derived from parenchymal cell walls or other non-wood fiber sources.

In microfibrillated cellulose the individual microfibrils or elementary fibrils have been partly or totally detached from each other. A microfibrillated cellulose fibril is very thin (~2-20 nm) and the length is normally between 100 nm to 10 pm. However, the microfibrils may also be longer, for example between 10-200 pm, but lengths even 2000 pm can be found due to wide length distribution. Fibers that has been fibrillated to a short length and which have microfibrils on the surface and microfibrils that are separated and located in a water phase of a slurry are included in the definition MFC.

Furthermore, whiskers are also included in the definition MFC.

In order to produce the paper or paperboard qualities a high density fiber web or film is usually used. By "high density" or "high concentration" web or film is meant that the dry content of the microfibrillated polysaccharide is in the range of from 0.5 to 5 weight-% of an aqueous solution in which the microfibrillated polysaccharide is dissolved. According to one embodiment the dry content is in the range of from 2 to 5 weight-%. According to another embodiment the dry content is in the range of from 3 to 5 weight-%. The aqueous solution may be a stock solution in the paper making process. The stock solution may also comprise other fibers, additives and paper chemicals.

These types of high density webs or films are often difficult to dewater in an efficient manner.

Fillers are materials which are used in the paper making process to confer different types of qualities to the paper or paper board. They may also be used, for instance, simply to provide for a cheaper product or pigmentation of the product. Many different types of fillers may be used, depending on the desired end product, and one of the most common filler materials used in the paper making process is precipitated metal carbonates, such as precipitated calcium carbonate or precipitated magnesium carbonate, or other types of carbonates based on primarily alkaline earth metals. The filler material may also be a mixture of different metal carbonates.

One conventional way of forming precipitated calcium carbonate (PCC) is to heat calcium carbonate which then forms lime and carbon dioxide gas. CaCOs + Heat→ CaO + CO ₂ †

The lime is then added to water to form calcium hydroxide (hydrated lime or slake). CaO + H ₂ O→ Ca(OH) ₂

The slaked lime is then combined with carbon dioxide gas and calcium carbonate reforms, and since it is insoluble in water, it precipitates out.

Ca(OH) ₂ + CO ₂ → CaCOs j + H ₂ O

The above is a simplified reaction scheme, in practice this process is more complex, but also well known to the person skilled in the art. For instance CaCO3 may also be formed from salts, such as CaCI2 and sodium bicarbonate.

Further to this gypsum (for example CaSO ₂ x 2 H ₂ O) may be used as a source of Ca ions. Gypsum is often used as filler, however it has a high dissolution in water (about 2- 2,5 g/l at 25C), most of the dissolved Ca ions could be regained via CO ₂ addition.

In the paper making industry this can either be done off-line, i.e. the precursor materials, e.g. the slaked lime and carbon dioxide can be combined to produce the PCC at a facility which is separate from the paper making process. This can also be done "at-line" which means that the filler material is produced in direct connection to the paper making process, i.e. no

transportation from a separate facility. The production may also be an "in-line production or process" which means that the precipitated calcium carbonate is produced directly into the flow of the paper making stock, i.e. the captured carbon dioxide is combined with slaked lime milk inline, instead of being produced separately from the paper making process.

Separate production of PCC further requires the use of retention chemicals to have the PCC adsorbed or fixed onto the fibers. An in-line PCC process is generally recognized as providing a clean paper machine system, and there is a reduced need of retention chemicals. An in-line PCC process is for instance disclosed in WO201 1/1 10744.

Alternatively the precursor materials may comprise other metal hydroxides, such as magnesium hydroxide in order to form a precipitated magnesium carbonate, or any other suitable metal hydroxide which can be precipitated in the presence of carbon dioxide to form a carbonate. According to one embodiment fly ash comprising high amount of CaO or MgO can be used.

Carbon dioxide is thus one of the most common precursors of filler materials, such as PCC. According to the present invention the carbon dioxide is introduced in the form of microsized or nanoszied gas bubbles or so called nanobubbles. These bubbles may also be called microbubbles.

According to one embodiment the nanobubbles may have a diameter in the range of from 10 to 800 nanometers (nm). According to one embodiment the nanobubbles may have a diameter in the range of from 10 to 600 nm. According to another embodiment the nanobubbles may have a diameter in the range of from 10 to 400 nm. According to yet an alternative embodiment the nanobubbles may have a diameter in the range of from 10 to 100 nm. According to one alternative the gas bubbles may have different size distributions, i.e. there is a mixture of bubbles having different sizes or diameters. This means that a main part of the bubbles may have a diameter in the range of 10 to 400 nm, but there may be bubbles having a larger diameter present in the mixture. The micro or nanobubbles are quite stable, which may provide for an improved or increased mixing and reaction efficiency when forming the filler material by providing the carbon dioxide precursor in the form of

nanobubbles.

The filler material formed by introducing these nanobubbles will also be of micro or nanosized particles, that is a nanofiller material is formed, e.g. a nano-PCC. According to one embodiment the diameter of these nanofiller particles may be in the range of from 100 to 1000 nm.

This means that the formed nanofiller, due to its small size, also can be more easily mixed into the high density or high concentration solution comprising the nanofibrillated polysaccharide.

The filler material may be produced at sub or super critical conditions, for instance at high pressure or high temperature, or combination of both.

The filler material may be further surface modified, for instance by adding chemicals such as dispersants, coupling agents, flame retardants, binders, hydrophobic chemicals, etc.

The introduction of the nanofiller, or alternatively the precursors forming the nanofiller, into the high density solution of nanofibrillated polysaccharide has shown to provide for a paper making process in which the web or film may be more easily dewatered, i.e. has a higher dewatering rate.

This means that a highly refined paper or paperboard may be produced from the web or film in a more efficient manner.

According to one embodiment the amount of filler material, i.e. the nanofiller material, is less than 20% of the dry content of the microfibrillated polysaccharide, or less than 15% of the dry content of the microfibrillated polysaccharide, or less than 10% of the dry content of the microfibrillated polysaccharide.

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.