SE1550647A1 - Production of nanosized precipitated calcium carbonate and use in improving dewatering of fiber webs - Google Patents

Abstract

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 having a dry content in the range of from 0.1 to 5.0 weight-%; introducing a filler material into said aqueous solution, whereby said filler material is formed into or onto fibrils of said microfibrillated polysaccharide; wherein said filler material is formed from gas bubbles of carbon dioxide having a diameter in the range of from 10 to 800 nm.

Description

PRODUCTION OF NANOSIZED PRECIPITATED CALCIUM CARBONATEAND USE IN IMPROVING DEWATERING OF FIBER WEBS Technical field The present document relates to a method for producing a high density webor fiber comprising nanofibrillated polysaccharide, wherein a filler material isintroduced comprising nanobubble carbon dioxide as a precursor.

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

Currently, films from MFC are either made on plastic films, where the plasticfilm work as a carrier substrate or then MFC is coated directly on plastic or apaper or paperboard, i.e. a base board, which then act as a base substrate. lnthe latter case, dewatering can occur in two directions, i.e. absorption of liquidphase into base board or through the base board if thin substrate andevaporation of the liquid into the air. ln case of the coating on a plastic filmcarrier substrate, the absorption is basically non-existing and therefore allwater 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 dewateringbehavior in a paper machine, but their use and efficiency on MFC web areless efficient.

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

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

Summarylt is an object of the present disclosure, to provide an improved manufacturing method for dense films or web of microfibrillatedpolysaccharide, which eliminates or alleviates at least some of thedisadvantages of the prior art methods for manufacturing these types of websor films.

The invention is defined by the appended independent claims.Embodiments are set forth in the appended dependent claims and in thefollowing description.

According to a first aspect there is provided a method for producinga film or web in a paper making process, said method comprising the stepsof: providing an aqueous solution comprising a microfibrillated polysaccharidehaving a dry content in the range of from 0.1 to 5.0 weight-%, introducing a filler material into said aqueous solution, whereby said filler material is formedinto or onto fibrils of said microfibrillated polysaccharide; wherein said fillermaterial is formed from gas bubbles from carbon dioxide having a diameter inthe 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 micro or even 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 providefor higher reaction efficiency, improved mixing behavior, and a reduced needfor gas or other precursors. lt has surprisingly been found that the formation or introduction of sucha nanofiller into or onto the fibrils of the microfibrillated polysaccharideprovides a film or web having a high content of microfibrillatedpolysaccharide, i.e. a dense film or web, which has improved dewateringcharacteristics. By the filler, i.e. the nanofiller, forming onto or into the fibrilsan inorganic-organic fibril/fiber composite may thus be formed, for which thedewatering process is much faster than compared to a conventionally formedweb or film of a microfibrillated polysaccharide.

The microfibrillated polysaccharide may be any one of amicrofibrillated cellulose, nanofibrillated cellulose, nanocrystalline cellulose,fibrillated cellulose fibers and cellulose fibrils or a mixture thereof and the fillermaterial may be any one of a precipitated calcium carbonate and aprecipitated magnesium carbonate, or other metal salt carbonate or a mixturethereof.

The filler material may according to one embodiment be a precipitatedcalcium carbonate (PCC).

By using nanobubbles of carbon dioxide to form the filler material it ispossible to form a nano-PCC which provides for a web or film havingimproved dewatering characteristics.

According to one embodiment the method may further comprise thestep of forming said filler material directly in the aqueous solution by introducing precursor materials into said solution, whereby said precursormaterials react to form the filler material onto or into the fibrils of themicrofibrillated polysaccharide.

The carbon dioxide bubbles may then be a precursor material.

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

By using nanobubbles of carbon dioxide, which stability can beadjusted, to form the filler material it is possible to provide for a very quickintroduction and mixing of the filler material, into the aqueous solution throughan in-line process. This means that the filler material may be formed directlyprior to the introduction into the aqueous solution, and introduced in acontinuously into the solution, or formed directly into the stream of the paperor 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 fillermaterial if it is formed by introducing the precursor materials directly into theaqueous solution. The stability of the nanobubble makes it possible to moreeasily control the mixing and reaction efficiency of the filler and/or fillerprecursors. An example of the in-line PCC process is described inWO2014072912.

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

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

According to one embodiment the precursor materials may comprisecomprises any one of calcium carbonate (CaCOß), calcium oxide (CaO), calcium hydroxide (Ca(OH)2), calcium bicarbonate (Ca(HCOs)2), sodiumbicarbonate (NaHCOß), magnesium hydroxide (Mg(OH)2), and magnesiumoxide (MgO), or a mixture thereof.

According to one embodiment the grammage of the film or web may beless than 100 g/m2, or less than 70 g/m2, or less than 50 g/m2, or less than 30g/m2- or less than 20 g/m2.

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

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

According to a second aspect there is provided a film or a web materialcomprising microfibrillated polysaccharide or microfibrillated celluloseobtained by the method according to the first aspect.

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

This film or web, may be an intermediate product, and can be furthertreated or laminated according to technologies known for a person skilled inthe art.

Description of Embodiments ln the production of highly refined paper and paperboard materials forinstance for use in the food packaging industry, or different types of coating orlaminates, fiber webs or fiber films comprising microfibrillated polysaccharidesare often used. The end product often has properties such as low airpermeability, or good oxygen barrier capabilities.

The microfibrillated polysaccharide may be any types of higly fibrillatedfibers, such as a microfibrillated cellulose, nanofibrillated cellulose,nanocrystalline cellulose, and cellulose fibrils or a mixture thereof.Microfibrillated polysaccharide is defined here as including bacterial celluloseor nanocellulose (BNC) spun with either traditional spinning techniques or then with electrostatic spinning. ln these cases, the material is preferably apolysaccharide but not limited to solely a polysaccharide. Also cellulosewhiskers, microcrystalline cellulose or regenerated cellulose andnanocellulose crystals is included in this definition. The microfibrillatedpolysaccharide may also be synthetic biofibers that have been fibrillated orspun into nanosize fibers.The microfibrillated polysaccharide can further be mixed together with normallignocellulose or reinforcing pulps both in low or high amounts. lt is furtherpossible to use a purified microfibrillated polysaccharide or a chemically, orphysically modified grade thereof.According to one embodiment the microfibrillated polysaccharide is amicrofibrillated cellulose. The microfibrillated cellulose (MFC) is also knownas nanocellulose. lt is a material typically made from wood cellulose fibers,both from hardwood or softwood fibers. lt can also be made from microbialsources, agricultural fibers such as wheat straw pulp, esparto (alfa) grass,bagasse (cereal straw, rice straw, bamboo, hemp), kemp fibers, flax andother cellulosic fiber sources. MFC can also be derived from parenchymal cellwalls or other non-wood fiber sources.ln microfibrillated cellulose the individual microfibrils or elementary fibrils havebeen partly or totally detached from each other. A microfibrillated cellulosefibril is very thin (~2-20 nm) and the length is normally between 100 nm to 10um. However, the microfibrils may also be longer, for example between 10-200 um, but lengths even 2000 um can be found due to wide lengthdistribution. Fibers that has been fibrillated to a short length and which havemicrofibrils on the surface and microfibrils that are separated and located in awater phase of a slurry are included in the definition MFC. Furthermore,whiskers are also included in the definition MFC. ln order to produce the paper or paperboard qualities a high densityfiber web or film is usually used. By “high density” or “high concentration” webor film is meant that the dry content of the microfibrillated polysaccharide is inthe range of from 0.5 to 5 weight-% of an aqueous solution in which themicrofibrillated polysaccharide is dissolved. According to one embodiment the dry content is in the range of from 2 to 5 weight-%. According to anotherembodiment the dry content is in the range of from 3 to 5 weight-%.

The aqueous solution may be a stock solution in the paper makingprocess. The stock solution may also comprise other fibers, additives andpaper chemicals.

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

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

One conventional way of forming precipitated calcium carbonate (PCC) is toheat calcium carbonate which then forms lime and carbon dioxide gas.CaCOs + Heat -> CaO + CO2T The lime is then added to water to form calcium hydroxide (hydrated lime orslake). CaO + H20 -> Ca(OH)2 The slaked lime is then combined with carbon dioxide gas and calciumcarbonate reforms, and since it is insoluble in water, it precipitates out.Ca(OH)2 + C02 ~> CaCOs i + H20 The above is a simplified reaction scheme, in practice this process is morecomplex, but also well known to the person skilled in the art. For instanceCaCOs may also be formed from salts, such as CaCl2 and sodiumbicarbonate.

Further to this gypsum(for example CaSOz x 2 H20) may be used as asource Ca ions. Gypsum is often used as filler, however it has a highdissolution in water (about 2- 2,5 g/l at 25C), most of the dissolved Ca ionscould be regained via C02 addition. ln the paper making industry this can either be done off-line, i.e. the precursormaterials, e.g. the slaked lime and carbon dioxide can be combined toproduce the PCC at a facility which is separate from the paper makingprocess. This can also be done “at-line” which means that the filler material isproduced in direct connection to the paper making process, i.e. notransportation from a separate facility. The production may also be an “in-lineproduction or process” which means that the precipitated calcium carbonateis produced directly into the flow of the paper making stock, i.e. the capturedcarbon dioxide is combined with slaked lime milk inline, instead of beingproduced separately from the paper making process.

Separate production of PCC further requires the use of retentionchemicals to have the PCC adsorbed or fixed onto the fibers. An in-line PCCprocess is generally recognized as providing a clean paper machine system,and there is a reduced need of retention chemicals. An in-line PCC process isfor instance disclosed in WO2011/110744.

Alternatively the precursor materials may comprise other metalhydroxides, such as magnesium hydroxide in order to form a precipitatedmagnesium carbonate, or any other suitable metal hydroxide which can beprecipitated in the presence of carbon dioxide to form a carbonate. Accordingto one embodiment fly ash comprising high amount of CaO or MgO can beused.

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

According to one embodiment the nanobubbles may have a diameter in therange of from 10 to 800 nanometers (nm). According to one embodiment thenanobubbles may have a diameter in the range of from 10 to 600 nm.According to another embodiment the nanobubbles may have a diameter inthe range of from 10 to 400 nm. According to yet an alternative embodimentthe nanobubbles may have a diameter in the range of from 10 to 100 nm.According to one alternative the gas bubbles may have different sizedistributions, 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 diameterin the range of 10 to 400 nm, but there may be bubbles having a largerdiameter present in the mixture.

The micro or nanobubbles are quite stable, which may provide for animproved or increased mixing and reaction efficiency when forming the fillermaterial by providing the carbon dioxide precursor in the form ofnanobubbles.

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

This means that the formed nanofiller, due to its small size, also can bemore easily mixed into the high density or high concentration solutioncomprising 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 byadding chemicals such as dispersants, coupling agents, flame retardants,binders, hydrophobic chemicals, etc.

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

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

According to one embodiment the amount of filler material, i.e. thenanofiller material, is less than 20% of the dry content of the microfibrillatedpolysaccharide, or less than 15% of the dry content of the microfibrillatedpolysaccharide, or less than 10% of the dry content of the microfibrillated polysaccharide.ln view of the above detailed description of the present invention, othermodifications and variations will become apparent to those skilled in the art.

However, it should be apparent that such other modifications and variationsmay be effected without departing from the spirit and scope of the invention.

Claims (12)

1. 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 microfibrillatedpolysaccharide having a dry content in the range of from 0.1 to 5.0weight-% ;introducing a filler material into said aqueous solution, whereby saidfiller material is formed into or onto fibrils of said microfibrillatedpolysaccharide;characterized inthatsaid filler material is formed from gas bubbles of carbon dioxide having a diameter in the range of from 10 to 800 nm.

2. The method as claimed in claim 1, wherein the microfibrillatedpolysaccharide is any one of a microfibrillated cellulose, nanofibrillatedcellulose, nanocrystalline cellulose, and cellulose fibrils or a mixture thereof.

3. The method as claimed in any one of claims 1 or 2,wherein said filler material is any one of a precipitated calcium carbonate anda precipitated magnesium carbonate, or a mixture thereof.

4. The method as claimed in any one of claims 1 or 2, wherein thefiller material is a precipitated calcium carbonate (PCC).

5. The method as any one of the preceding claims,wherein the method further comprises the step of forming said filler materialdirectly in the aqueous solution by introducing precursor materials into saidsolution, whereby said precursor materials react to form the filler material ontoor into the fibrils of the microfibrillated polysaccharide. 12

6. The method as claimed in any one of the preceding claims, whereinsaid filler material is introduced into the aqueous solution in an in-line process.

7. The method as claimed in any one of claims 1 to 4 wherein saidfiller material is introduced into the aqueous solution in an off-line process or an at-line process.

8. The method as claimed in claim 5, wherein said precursor materialscomprises any one of carbon dioxide, calcium carbonate (CaCOs), calciumoxide (CaO), calcium hydroxide (Ca(OH)2), calcium bicarbonate (Ca(HCOs)2),sodium bicarbonate (NaHCOß), magnesium hydroxide (Mg(OH)2), andmagnesium oxide (MgO), or a mixture thereof.

9. The method as claimed in any one of the preceding claims,wherein the grammage of the film or web is less than 100 g/m2, or less than70 g/m2, or less than 50 g/m2, or less than 30 g/m2- or less than 20 g/m2.

10.The method as claimed in any one of the preceding claims,wherein the amount of filler material in said film or web is less than 20% of thedry content of the microfibrillated polysaccharide, or less than 15% of the drycontent of the microfibrillated polysaccharide, or less than 10% of the drycontent of the microfibrillated polysaccharide.

11.A film or a web material comprising microfibrillatedpolysaccharide or microfibrillated cellulose obtained by the method asclaimed in any one of claims 1 to 10.

12.A paper or paper board material obtained from a film or webmaterial according to claim 11.