SE544591C2 - Method of producing an imprintable cellulose fiber product and a fiber product - Google Patents

Abstract

The invention discloses a method for producing a three dimensional molded structure from cellulose fibers, comprising the steps of:-providing a cellulose fiber material comprising cellulose pulp, said material having a solid content between 0.1-95%;-providing a forming tool having a three dimensional shape including a forming surface, and bringing said forming surface into contact with the cellulose fiber material;-press drying the cellulose fiber material contacted by the forming tool at temperatures >20°C to a dry content of at least 70%, preferably at least 80%; and-removing said dried layer from the forming tool to achieve the three dimensional molded product,wherein said method also comprises the step of applying aqueous metal salt spray such that at least one outer surface of the achieved three dimensional molded product comprises at least one metal salt ion originating from said applied metal salt spray.

Description

METHOD OF PRODUCING AN IMPRINTABLE CELLULOSE FIBERPRODUCT AND A FIBER PRODUCT Technical fieldThe present invention relates to a method of producing athree dimensional cellulose fiber based structure by means of fiber molding.

BackgroundThere is a growing interest for producing cellulose based, three dimensional (3D) products, e.g. for use as packagingapplications for foodstuff, tableware, trays, technical products,electronic equipment and/or consumer goods. Several advantagesare associated with the use of natural fibers (such as cellulosefibers) for manufacturing packages. Being a renewable resource,natural fibers provide a sustainable alternative to other packagingmaterials such as aluminum and plastics, and furthermore naturalfibers are both recyclable and biodegradable. Natural fibers includecellulose fibers of any natural origin, such as derived from woodpulp and/or plants.

There is also a demand in packaging industry of increasingcolor, product differentiation, and novelty in addition topersonalized prints, and to provide eye catching shapes. In additionto such aesthetic considerations an element of physical protection isalso required for the goods in question.

Molding of cellulose fiber materials provides a way ofachieving renewable articles with various three dimensional shapes,which may be used to differentiate products available for sale in a given marketplace.Manufacturing molded fiber products can be done by wetforming, wherein a forming tool is dipped into an aqueous pulpcomposition followed by compression-molding performed underheat, resulting in a dried fiber product having a shapecomplementary to the shape of the mold. Typically, said tool isperforated or porous so that water can be removed from thesuspension or wet pulp during forming during a dewatering/dryingstep. The forming tool is selected in order to control the surfaceroughness. In case of an egg box, for example, the outer surfacecan be made smooth in order to enable printed label to adhere.Preparation of a smoother surface often leads to that the reverseside is coarse. Thus, if making a smooth inner side, the outersurface will be coarse which makes direct printing difficult,especially printing of four colors (CMYK color model). Also, aproblem associated with various forming techniques is roughnessvariations of the molded product which causes problems withuneven print quality.

It is also possible to dry mold products such as trays from acellulose fiber sheet or web. Dry molding can be done in variousways, e.g. by press-forming a wetted sheet combined with appliedheat using a forming tool. However, dry forming has disadvantagesassociated with poor flexibility and elasticity of the cellulose sheetor web material, limiting the 3D-formability and/or leading to therisk of cracks appearing in the material upon forming.

In US2013248130, a compression-molded tray of fibermaterial coated with a removable film is described, andWO200605761O also presents a method and a machine for makingfiber products such as food trays by means of fiber molding from a stock of pulp.In addition to designing the shape of a product, it is desirableto also add colorful decorations and adornments as well asinformative content onto its surface. For example, it is common toadd a label and/or etiquette onto paper based containers andpackages as an information carrier. However, labels/etiquettesrequire additional production step and also consumes extra materialin the form of label components. Direct printing is also done butmostly with one color and mainly for coding or simple color printing,e.g. egg boxes.

Application of ink onto the surface of a molded article oftenleads to problems with dot resolution and that the spreading andabsorption of ink color is hard to regulate, which cause bleedingand wicking. Surface roughness and use of higher fiber content leadto reduced print density and hence greater use of colorants toattain a certain density level. Hence, there is a need forimprovements when it comes to printability of molded pulp products.

Obiects of the invention It is an object of the present invention, to provide a methodfor manufacturing a fiber based, three dimensional molded articlecomprising a surface with enhanced printability. It is also an objectof the present invention to provide a three dimensional moldedarticle which is based on cellulose fibers, and which comprises asurface with enhanced printability properties. By "enhancedprintability" means that printing a pattern onto said surface can bedone substantially without bleeding, wicking and with a highresolution and especially for 4-color (or more) prints. Low ink spreading and adjustable ink absorption is desirable for controllingboth print quality but also print durability and associated problems such as ink smearing, print thru, rub-off or hidden rub-off.

SummaryThe objects of the invention are at least partially obtained by means of a method for producing a three dimensional moldedstructure from cellulose fibers according to claim 1. Said methodcomprises at least the steps of: -providing a cellulose fiber material comprising cellulose pulpand microfibrillated cellulose (MFC), said material having a solidcontent between O.1-95°/0; -providing a forming tool having a three dimensional shapeincluding a forming surface, and bringing said forming surface intocontact with the cellulose fiber material; -press drying the cellulose fiber material contacted by theforming tool at temperatures >20°C to a dry content of at least70%, preferably at least 80%; and -removing said dried layer from the forming tool to achievethe three dimensional molded product, wherein the method also comprises the step of applyingaqueous metal salt spray such that at least one outer surface of theachieved three dimensional molded product comprises at least onemetal salt ion originating from said applied metal salt spray, and -imprinting a pattern onto said three dimensional molded product using a water based ink.

It has surprisingly been found that the printability of a threedimensional molded, fiber based product is significantly improvedby means of a method and a product according to the invention.

Application of a metal salt onto the surface of the three dimensionalmolded product provides a surface with enhanced printability forinks, especially those with pigment colorants. The technology issuitable for inkjet but can also be applied for flexographic or screenprinting. Thanks to the invention, several advantages are achieved.In addition to enhanced printability of the molded product, the useof expensive chemicals is avoided or at least reduced, especiallyfossil based ones. Furthermore, printing of primers or complexsurface treatment processes are not needed. Thus, the solutiondoes not only improve feathering and bleeding but also printdensity and ink adhesion. Thanks to the invention, threedimensional molded pulp products can be decorated with morevariable colored prints using multiple colors without the risk ofinferior print quality, and even 3D effects can be accomplished bymeans of printing thanks to the enhanced printability of thesubstrate (i.e. the molded pulp product).

|II The term "cellulose fiber materia referred to herein is to beinterpreted as a material comprising natural cellulose-based fibers,including aqueous pulp compositions and/or fiber based sheet orweb materials. Any cellulosic fibers known in the art, includingcellulose fibers if any natural origin, such as those derived fromvegetable pulp or agricultural-based pulp, can be used in thecellulose fiber material. Non-limiting examples of cellulosic fiberssuitable for use in this invention are cellulose fibers derived fromsoftwoods such as pines, firs and spruces, as well as fibers derivedfrom eucalyptus, bagasse, bamboo and other ligneous and cellulose SOU FCeS.

The present invention relates to fiber molding of a 3D product. Such fiber molding may be performed using wet forming ordry forming as explained in the Background section of the presentapplication. The present invention relates to products that havebeen obtained by means of wet molding procedure or dry molding procedure.

According to one aspect of the invention, said product isachieved by means of a wet molding procedure. According to thisaspect, the cellulose fiber material is an aqueous composition orslurry having a consistency between 0.O5-1Owt°/0, preferably 0.2-1.5wt°/0. Said forming tool is brought into contact with said slurrye.g. by immersion so that said forming surface of the forming toolis covered with a wet layer of pulp from said aqueous composition,whereafter the layer of pulp present on said forming tool is press-dried and dewatered. Preferably, the wet layer of pulp is 5-150 gsmin dry weight. According to another aspect of the invention, pressdrying of the wet layer is performed at temperatures >100°C,preferably at temperatures between 120-250°C or more preferablybetween 150-220°C. Said press drying can be applied in one orseveral steps depending on the end structure. Also, press dryingcan be done by two complementary forming tools laminating andcompressing the cellulose fiber to be dried.

The forming tool can be brought into contact with the saidaqueous pulp composition by means of immersion into thecomposition, whereupon cellulose fibers are drawn onto the formingportion for instance by means of vacuum suction. Next, the layer ofpulp present on said forming portion is dried and/or dewatered to adry content of at least 70%. Drying can be accomplished with orwithout heating, pressing or any other mechanical support thatimproves dewatering and formation. Combination of elevated temperatures and pressure is a conceivable procedure. Said"elevated temperature" is here to be interpreted as temperatures>100°C. The dried layer is removed from the forming tool toachieve a single layer three dimensional molded structure withenhanced printability. Furthermore, the layer of pulp present onsaid forming portion may be press dried with a pressure between0.2-50 bar, preferably 0.5-15 bar, more preferably 1-10 bar. Incase of drying the wet pulp present on said forming portion bymeans of applying elevated temperatures, such temperatures ispreferably between 100-350°C, preferably 120-250°C, morepreferably between 150-220°C.

According to one aspect of the invention, said product isachieved by means of a dry molding procedure. According to thisaspect, the cellulose fiber material is a fiber based sheet material having a solid content of 3O-95wt°/According to the invention, a metal salt in aqueous form isapplied onto one surface of the molded product by spraying onto asurface thereof. It is to be understood that "spray" means in theform of a plurality of liquid droplets or particles, and that the metalsalt in spray form may be delivered by means of a precision devicefor dispersion of freely flowing dissolved metal salt into said sprayform. The droplets or particles may be in micro scale with sizesranging from 1-900 um in diameter. It is conceivable to have a multi-layer spray or single spray arrangement.

According to another aspect, said metal salt is one of CaCl2,Ca(OAc)2, MgCl2 or AlCl3, or mixtures thereof. According to anotheraspect, the metal salt is added in combination with one or more of the following additives: a cationic polymer, humectants,nanopigments and/or cross-linked polymers. A possible ratiobetween metal salt vs additive is 1:1OO - 100:According to another aspect of the invention, said metal saltspray comprises functional chemicals such as non-stick chemicals (e.g. lubricants) and colorants.

According to another aspect of the present invention, saidspray also comprises one or more additional functional chemicalsselected from the group comprising cationic polymers, nanopigments, amphoteric polymers and anonionic polymers.

The method comprises imprinting a pattern onto said threedimensional molded product using a water based ink. It is alsoconceivable to use an ink that comprises solvent and water. The inkcan also be a varnish or a combination of ink and varnish. Ink cancomprise one colorant or both dye and pigment, said pigmentsoften being anionic. Preferably, said imprinting is performed by anyone of inkjet, flexographic or screen printing. Preferably, theimprinting is performed using any one of pigment based colorantink, anionic dye based ink or hybrid ink. Furthermore, the imprinting is performed in-line in a molding line or at a line.

Furthermore, the cellulose fiber material comprisesmicrofibrillated cellulose (MFC). The use of cellulose nanomaterialenhances the retention of metal salts in the material, and it alsoimproves the strength of the end structure. According to one aspect, said cellulose nanomaterial is anionic MFC, or native MFC.

The term "cellulose nanomaterial" referred to herein is to be interpreted as materials comprising cellulose and encompassesmicrofibrillated cellulose (MFC) as well as cellulose nanocrystals(nanocrystalline cellulose) and mixtures thereof. This means thatone dimension (diameter) of the fibers is within the scale of 1-1000nm (mean average fiber or fibril diameter). Microfibrillated cellulose(MFC) or so called cellulose microfibrils (CMF) shall in the context ofthe present invention mean a micro-scale cellulose particle fiber orfibril with at least one average or mean dimension less than 1000nm. MFC comprises partly or totally fibrillated cellulose orlignocellulose fibers. The cellulose fiber is preferably fibrillated tosuch an extent that the final specific surface area of the formedMFC is from about 1 to about 500 m2/g, such as from 10 to 400m2/g or more preferably 50-300 m2/g when determined for asolvent exchanged and 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 highshear disintegration or liberation of fibrils. One or several pre-treatment steps are usually required in order to make MFCmanufacturing both energy-efficient and sustainable. The cellulosefibers of the pulp to be supplied may thus be pre-treatedenzymatically or chemically, for example to reduce the quantity ofhemicellulose or lignin. The cellulose fibers may be chemicallymodified before fibrillation, wherein the cellulose molecules containfunctional groups other (or more) than found in the originalcellulose. Such groups include, among others, carboxymethyl,aldehyde and/or carboxyl groups (cellulose obtained by N-oxylmediated oxidation, for example "TEMPO"), quaternary ammonium(cationic cellulose). The cellulose may also be methylated orphosphorylated. After being modified or oxidized in one of theabove-described methods, it is easier to disintegrate the fibers intoMFC.

The microfibrillar cellulose may contain some hemicelluloses;the amount is dependent on the plant source. Mechanicaldisintegration of the pre-treated fibers, e.g. hydrolysed, pre-swelled, or oxidized cellulose raw material is carried out withsuitable equipment such as a refiner, grinder, homogenizer,colloider, friction grinder, ultrasound sonicator, single - or twin-screw extruder, fluidizer such as microfluidizer, macrofluidizer orfluidizer-type homogenizer. Depending on the MFC manufacturingmethod, the structure might also contain fines, or nanocrystallinecellulose or e.g. other chemicals present in wood fibers or otherlignocellulosic fibers used in papermaking processes. The structuremight also contain various amounts of micron size fiber particlesthat have not been efficiently fibrillated. The amount of these fiberparticles can be determined e.g. in fiber analysator which is knownfor a skilled person in the art.

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

According to a preferred aspect of the invention, said pulp isselected from the group comprising wood pulps, non-wood pulps,unbleached chemical pulp, defibrated fiber material, bagasse,straws, bamboo, spruce CTMP, eucalyptus CTMP, spruce HT CTMP,kraft pulp, sulphate, sulphite, PGW, GW, DIP, recycled paper andboard, coated and uncoated broke, RMP, TMP, CMP, CSP NSSC nanopulp, dissolving pulp, and regenerated fibers and mixturesthereof. It is understood that other cellulosic material such aschemical or semi-chemical pulp of wood or non-wood material canbe added as part of the pulp stock. Preferably, said pulp is a pulpfiber or fiber mixture with a Schopper Riegler value abovemeasured according to the SR standard.

According to another aspect, said cellulose fiber material alsocomprises one or more additional functional chemicals selectedfrom the group comprising cationic polymers, nanopigments,amphoteric polymers and anonionic polymers. The metal salt incombination with specific cationic polymers enhance ink rubresistance and water fastness. Water fastness refers to thesensitivity of the color adhesion (once imprinted onto the surface of a material) to humidity.

According to another aspect, said cellulose fiber materialfurther comprises one or more co-additives selected from the groupcomprising nanoparticles, cationic mordants, cross-linkers, non-ionic polymers such as PVOH, PEG, cationic fillers, pigments or fillers with high surface area, preferably >10 g/mAccording to yet another aspect of the invention, thegrammage of the molded product is preferably 5-450 gsm or more preferably 10-200 gsm.

According to yet another aspect of the invention, said moldedproduct comprises a density between 350-1500 kg/m3, preferably400-1200 kg/m3 or most preferably 500-900 kg/mThe present invention further also relates to a three-dimensional molded pulp product manufactured by means of a method according to the invention.The present invention further also relates to a three-dimensional molded pulp product comprising more than one layer,whereof at least one layer is a molded structure according to claim1 constituting an imprinting layer made from a mixture aspreviously described, further where said imprinting layer isarranged as an outer layer of said multilayer product. According toa preferred aspect, the three dimensional molded fiber/pulp productis a packaging product and comprises a first, outer side intended forbeing decorated with an imprinted pattern, and a second inner sideintended to provide a barrier against grease, oil, gas, water etc.Said outer side is thus arranged to comprise metal salt ions forenhancing printability, whereas the inner side may comprise abarrier layer, e.g. moisture barrier or grease barrier. This means,according to this aspect only one side of the product is treated with metal salt spray.

Description of Embodiments The present description is directed to production of threedimensional molded pulp articles with enhanced printability.Examples of a three dimensional molded pulp article include in anon-limiting way containers, trays and packages. Thus, theproducts made by means of the method of the present invention may be referred to as packages and/or packaging material.

Although the present description relates to the context ofconventional wet forming procedures and dry forming procedures,the invention is not limited thereto. The skilled person appreciatesthat the invention may contemplate any fiber-based manufacturing method, including 3D printing techniques.According to the invention, presence of metal salt in a surfacelayer of a molded article leads to improved surface printability e.g. when using inkjet printing technology.

It is thus within the ambit of the present invention to providea 3D molded product comprising at least one outer surface or aportion of an outer surface which has been subjected to applicationof an aqueous metal salt, e.g. in spray form, onto at least onesurface intended for subsequent imprinting. Production of suchmolded article may be done by wet molding methods or dry molding methods.

In the following, an example of a wet molding method formanufacturing a three dimensional molded article withimproved/enhanced printability will be described in a non-limiting way.

An aqueous pulp suspension (also referred to as"composition") is provided with the consistency of O.O5-1Owt°/0. Thepulp may be any one of wood pulps, non-wood pulps, unbleachedchemical pulp, defibrated fiber material, bagasse, straws, bamboo,spruce CTMP, eucalyptus CTMP, spruce HT CTMP, kraft pulp,sulphate, sulphite, PGW, GW, DIP, recycled paper and board, broke,RMP, TMP, CMP, CSP NSSC nanopulp, dissolving pulp, andregenerated fibers or mixtures thereof. A cellulose nanomaterial inthe form of microfibrillated cellulose (MFC) is added to the pulpsuspension. Said MFC is preferably anionic MFC, or native MFC, or agrafted version thereof. A 3D shaped forming tool comprising a forming portion is brought into contact with the pulp suspension, forinstance by immersing said tool into the slurry bath. Said formingportion is arranged to represent a 3D mirror image of the article tobe formed. Pulp is drawn onto the forming portion e.g. by means ofvacuum suction until a layer of desired thickness has been formed,whereupon the forming tool is removed from the slurry. At thisstage, the forming portion is covered with a wet layer of pulp, saidwet layer comprising between 5-150 gsm in dry weight. Next, thewet layer of pulp is dewatered to a dry content of at least 7Owt°/0.Dewatering and/or drying can be done in various ways. In a wetcuring procedure, the wet layer is pressed under elevatedtemperatures to be compressed and dried to a certain thickness,thereby yielding a smooth external surface for the end structure. Ina dry curing process, the wet layer is subjected to heated airthereby removing moisture, which results in an end structure with amore textured finish. This way, a single layer molded fiber productis formed.

Manufacturing multilayered molded fiber products can beaccomplished for instance by applying more than one fibrous layerson top of each other in consecutive molding production steps. Forinstance, a layer of metal salt-containing pulp can be molded onto apre-molded pulp layer already present on the forming tool. Thevarious layers of a multilayered product may hereby providedifferent functions, such as rigidity, barrier properties, etc. In amultilayered product, the imprint-enhancing layer is to form theprinting surface, or an outer layer. According to the invention, thehot press temperature range for a wet molded procedure is around 150-220 degrees C, with a press range around 1-10 bar.

Once the 3D shaped molded product has been dried, an aqueous metal salt spray is applied such that at least one outer surface of the achieved three dimensional molded product iscovered with metal salt ions originating from said applied metal salt spray.

In the following, an example of a dry molding method formanufacturing a three dimensional molded article withimproved/enhanced printability will be described in a non-limitingway.

In dry molding procedure, said cellulose fiber material is afiber based sheet material having a solid content of 3O-95wt°/0.Preferably, the fiber based sheet is made from pulp selected fromthe group comprising wood pulps, non-wood pulps, unbleachedchemical pulp, defibrated fiber material, bagasse, straws, bamboo,spruce CTMP, eucalyptus CTMP, spruce HT CTMP, kraft pulp,recycled paper and board, broke, nanopulp, dissolving pulp, and regenerated fibers and mixtures thereof.

The present invention has been described with regard topreferred embodiments. However, it will be obvious to a personskilled in the art that a number of variations and modifications canbe made without departing from the scope of the invention as described herein.

Claims (18)

1. 1. A method for producing a three dimensional molded product fromcellulose fibers, comprising the steps of: -providing a cellulose fiber material comprising cellulose pulp andmicrofibrillated cellulose (MFC), said material having a solid contentbetween 0.1-95°/0; -providing a forming tool having a three dimensional shape includinga forming surface, and bringing said forming surface into contact with thecellulose fiber material; -press drying the cellulose fiber material contacted by the formingtool at temperatures >20°C to a dry content of at least 70%, preferably atleast 80%; and -removing said dried layer from the forming tool to achieve thethree dimensional molded product, cha racterized in that said method also comprises the stepšš of;__ _~;¿-applying aqueous metal salt spray such that at least one outersurface of the achieved three dimensional molded product comprises atleast one metal salt ion originating from said applied metal salt ~ = ' ~ * \ ~ _ 1 i _ z i \ _ i _ ~\W* .-* :* x* ~*\ ~* :\ ~ .~*“ t* t* v* i: fo. v: e w* än ~'\ f* \ f* r* :tæ \ w* -\ \ e* se. f* w* v -\ ~* ~ \ SW“ i š u: šzfx: Éèiššu m n š..\.« :du : :Luš nåns: šul Hzkßmixfu m: k-'kšk-muk.

2. A method according to claim 1, wherein said cellulose fibermaterial is an aqueous composition having a consistency between 0.05-1Owt°/0, preferably 0.2-1.5wt°/0, and wherein said forming tool is broughtinto contact with said aqueous composition so that said forming surface ofsaid forming tool is covered with a wet layer of pulp from said aqueouscomposition, whereafter the layer of pulp present on said forming tool is press-dried and dewatered.

3. A method according to claim 2, wherein the layer of pulp present on said forming tool is dewatered by means of press-drying.

4. A method according to any one of claims 2-3, wherein the wet layer of pulp is 5-150 gsm in dry weight.

5. A method according to any one of claims 2 - 4, wherein pressdrying is performed at temperatures >100°C, preferably at temperaturesbetween 120-250°C or more preferably between 150-220°C.

6. A method according to wherein cellulosefiber material is a fiber based sheet material having a solid content of 30-95Wt°/o. A method according to wherein said imprinting is performed by any one of inkjet, flexographic or screen printing. A method according to wherein the imprinting is performed using any one of pigment based colorant ink, anionic dye based ink or hybrid ink. . A method according to any one of the previous claims, whereinsaid cellulose pulp is selected from the group comprising: wood pulps,non-wood pulps, unbleached chemical pulp, defibrated fiber material,bagasse, straws, bamboo, spruce CTMP, eucalyptus CTM P, spruce HTCTMP, kraft pulp, sulphate, sulphite, PGW, GW, DIP, recycled paper andboard, broke, RMP, TMP, CMP, CSP NSSC nanopulp, dissolving pulp, and regenerated fibers and mixtures thereof. A method according to any one of the previous claims,wherein said metal salt is selected from the group comprising: CaCl2,Ca(OAc)2, MgCl2 or AlCl3, or mixtures thereof. A method according to any one of the previous claims,wherein the dose of metal salt is 1-50 kg/tn or more preferably 5-35kg/tn. A method according to any one of the previous claims,wherein said spray also comprises one or more additional functionalchemicals selected from the group comprising cationic polymers, nanopigments, amphoteric polymers and anonionic polymers. 1; A method according to any one of the previous claims,wherein said cellulose fiber material also comprises one or more co-additives selected from the group comprising nanoparticles, cationicmordants, cross-linkers, non-ionic polymers such as PVOH, PEG, cationicfillers, pigments or fillers with high surface area preferably with a surface area >10 g/m A method according to any one of the previous claims, wherein said MFC is fflaå-æšy-anionic MFC, or native MFC. wherein the grammage of the molded product is gsm or more preferably 10-200 gsm. A method according to any one of the previous claims,wherein said molded product comprises a density between 350-1500kg/m3, preferably 400-1200 kg/m3 or most preferably 500-900 kg/m A three-dimensional molded pulp structure made by means ofa method according to any one of claims 1 - comprising a dose of metal salt between 1-50 kg/tn, or more preferably 5-35 kg/tn. A three-dimensional molded pulp structure comprising morethan one layer, whereof at least one layer is an imprinting layer obtainedby means of a method according to any one of claims 1 - furtherwhere said imprinting layer is arranged as an outer layer of said multilayerstructure and comprises a dose of metal salt between 1-50 kg/tn, or morepreferably 5-35 kg/tn.