AND A FIBER STRUCTURE

Technical field

The present invention relates to a method of producing a three dimensional cellulose fiber based structure by means of fiber molding.

Backg round

There is a growing interest for producing cellulose based, three dimensional (3D) products, e.g. for use as packaging applications for foodstuff, tableware, trays, technical products, electronic equipment and/or consumer goods. Several advantages are associated with the use of natural fibers (such as cellulose fibers) for manufacturing packages. Being a renewable resource, natural fibers provide a sustainable alternative to other packaging materials such as aluminum and plastics, and furthermore natural fibers are both recyclable and biodegradable. Natural fibers include cellulose fibers of any natural origin, such as derived from wood pulp and/or plants.

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

Molding of cellulose fiber materials provides a way of achieving 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 (also referred to as formed fiber products) and structures can be done by wet forming, wherein a forming tool is dipped into an aqueous pulp composition followed by compression-molding performed under heat, resulting in a dried fiber product having a shape complementary to the shape of the mold. Typically, said tool is perforated or porous so that water can be removed from the suspension or wet pulp during forming during a dewatering/drying step. The forming tool is selected in order to control the surface roughness. In case of an egg box, for example, the outer surface can be made smooth in order to enable printed label to adhere. Preparation of a smoother surface often leads to that the reverse side is coarse. Thus, if making a smooth inner side, the outer surface will be coarse which makes direct printing difficult, especially printing of four colors (CMYK color model). Also, a problem associated with various forming techniques is roughness variations of the molded product which causes problems with uneven print quality.

In US2013248130, a compression-molded tray of fiber material coated with a removable film is described, and W02006057610 also presents a method and a machine for making fiber products such as food trays by means of fiber molding from a stock of pulp.

In addition to designing the shape of a product or structure, it is desirable to also add colorful decorations and adornments as well as informative content onto its surface. For example, it is common to add a label and/or etiquette onto paper based containers and packages as an information carrier. However, labels/etiquettes require additional production step and also consumes extra material in the form of label components. Direct printing is also done but mostly 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 often leads to problems with dot resolution and that the spreading and absorption of ink color is hard to regulate, which cause bleeding and wicking. Surface roughness and use of higher fiber content lead to reduced print density and hence greater use of colorants to attain a certain density level. Hence, there is a need for improvements when it comes to printability of molded pulp products.

Objects of the invention

It is an object of the present invention, to provide a method for manufacturing a fiber based, three dimensional molded article comprising a surface with enhanced printability. It is also an object of the present invention to provide a three dimensional molded article which is based on cellulose fibers, and which comprises a surface with enhanced printability properties. By "enhanced printability" means that printing a pattern onto said surface can be done substantially without bleeding, wicking and with a high resolution and especially for 4-color (or more) prints. Low ink spreading and adjustable ink absorption is desirable for controlling both print quality but also print durability and associated problems such as ink smearing, print thru, rub-off or hidden rub-off.

Summary

The objects of the invention are at least partially obtained by means of a method for producing a three dimensional molded structure from cellulose fibers according to claim 1. Said method comprises at least the steps of: providing an aqueous composition comprising cellulose pulp and at least one metal salt in a substantially homogeneous mixture, wherein said mixture has a solid content between 0.05-10wt% ;

-providing a forming tool having a three dimensional shape comprising a forming portion, and bringing said forming portion into contact with the aqueous composition so that said forming portion is covered with a wet layer of pulp at an amount of between 5-150 gsm in dry weight;

-dewatering the layer of pulp contacted by the forming tool at temperatures >100°C to a dry content of at least 70wt% to achieve the three dimensional molded structure. Preferably, the layer of pulp is dewatered to dry content of at least 80wt%, preferably at least 85wt%.

It has surprisingly been found that the printability of a three dimensional molded, fiber based structure is significantly improved by means of a method and a structure according to the invention. Addition of a metal salt into the aqueous solution (i.e. slurry) provides enhanced printability for inks, especially those with pigment colorants. By means of providing a layer of formed fiber comprising high amounts of metal salt forming free metal ions and metal complexes near the surface, printability properties are improved. The technology is suitable for inkjet but can also be applied for flexographic or screen printing. In addition to enhanced printability of the molded product, the use of expensive chemicals is avoided or at least reduced, especially fossil based ones. Furthermore, printing of primers or complex surface treatment processes are not needed. Thus, the solution does not only improve feathering and bleeding but also print density and ink adhesion. Thanks to the invention, three dimensional molded pulp products can be decorated with more variable colored prints using multiple colors without the risk of inferior print quality, and even 3D effects can be accomplished by means of printing thanks to the enhanced printability of the substrate (i.e. the molded pulp product).

According to one aspect of the present invention, said mixture also comprises a cellulose nanomaterial such as e.g. microfibrillated cellulose (MFC). The use of cellulose nanomaterial enhances the retention of metal salts in the material, and it also improves the strength of the end structure. Enhancing the retention of the metal ions and/or metal complexes onto the surface by means of adding MFC leads to the advantage that the metal salts do not come off the formed fiber layer once it is dried, which can happen in case of retention additives are absent. In one aspect of the invention, MFC is pre-mixed with salt before it is added to the material composition. This enables charge reversal of MFC and it also gives a more homogenous suspension. Flerein, the term "aqueous composition" can also be referred to as "aqueous suspension".

The term "cellulose nanomaterial" referred to herein is to be interpreted as materials comprising cellulose and encompasses microfibrillated cellulose (MFC) as well as cellulose nanocrystals (nanocrystalline cellulose) and mixtures thereof. This means that one dimension (diameter) of the fibers is within the scale of 1-1000 nm (mean average fiber or fibril diameter). Microfibrillated cellulose (MFC) or so called cellulose microfibrils (CMF) shall in the context of the present invention mean a micro-scale cellulose particle fiber or fibril with at least one average or mean dimension less than 1000 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 500 m2/g, such as from 10 to 400 m2/g or more preferably 50-300 m2/g when determined for a solvent 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 high shear disintegration or liberation of fibrils. One or several pre treatment steps are usually required in order to make MFC manufacturing both energy-efficient and sustainable. The cellulose fibers of the pulp to be supplied may thus be pre-treated enzymatically or chemically, for example to reduce the quantity of hemicellulose or lignin. The cellulose fibers may be chemically modified before fibrillation, wherein the cellulose molecules contain functional groups other (or more) than found in the original cellulose. Such groups include, among others, carboxymethyl, aldehyde and/or carboxyl groups (cellulose obtained by N-oxyl mediated oxidation, for example "TEMPO"), quaternary ammonium (cationic cellulose). The cellulose may also be methylated or phosphorylated. After being modified or oxidized in one of the above-described methods, it is easier to disintegrate the fibers into MFC.

The microfibrillar cellulose may contain some hemicelluloses; the amount is dependent on the plant source. Mechanical disintegration of the pre-treated fibers, e.g. hydrolysed, pre swelled, or oxidized cellulose raw material is carried out with suitable equipment such as a refiner, grinder, homogenizer, colloider, friction grinder, ultrasound sonicator, single - or twin- screw extruder, fluidizer such as microfluidizer, macrofluidizer or fluidizer-type homogenizer. Depending on the MFC manufacturing method, the structure might also contain fines, or nanocrystalline cellulose or e.g. other chemicals present in wood fibers or other lignocellulosic fibers used in papermaking processes. The structure might also contain various amounts of micron size fiber particles that have not been efficiently fibrillated. The amount of these fiber particles can be determined e.g. in fiber analysator which is known for a skilled person in the art.

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.

According to another embodiment, the aqueous suspension may comprise a mixture of different types of fibers, such as microfibrillated cellulose, and an amount of other types of fiber, such as kraft fibers, fines, reinforcement fibers, synthetic fibers, dissolving pulp, rejects, recycled pulp or paper packaging materials, coated or uncoated broke, TMP, CTMP or PGW. The suspension may also comprise other process or functional additives, such as fillers, pigments, wet strength chemicals, retention chemicals, cross-linkers, softeners or plasticizers, adhesion primers, wetting agents, biocides, optical dyes, fluorescent whitening agents, de-foaming chemicals, hydrophobizing chemicals such as AKD, ASA, waxes, resins etc. Other additives that may be added are various inkjet print enhancing chemicals such as cationic mordants, nanofillers etc. These will enhance the print quality effect but also provide e.g. other effects. The term "cellulose fiber material" referred to herein is to be interpreted as a material comprising natural cellulose-based fibers, including aqueous pulp compositions and/or fiber based sheet or web materials. Any cel lulosic fibers known in the art, including cellulose fibers if any natural origin, such as those derived from vegetable pulp or agricultural-based pulp, can be used in the cellulose fiber material. Non-limiting examples of cel lulosic fibers suitable for use in this invention are cellulose fibers derived from softwoods such as pines, firs and spruces, as well as fibers derived from eucalyptus, bagasse, bamboo and other ligneous and cellulose sources.

Said press drying can be applied in one or several steps depending on the end structure. Also, press drying can be done by two complementary forming tools laminating and compressing the cellulose fiber to be dried.

According to one aspect of the invention, aqueous composition has a solid content between 0.05-10wt%.

The forming tool can be brought into contact with the said aqueous pulp composition by means of immersion into the composition, whereupon cellulose fibers are drawn onto the forming portion for instance by means of vacuum suction. Next, the layer of pulp present on said forming portion is dried and/or dewatered to a dry content of at least 70%. Drying can be accomplished with or without heating, pressing or any other mechanical support that improves 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 to achieve a single layer three dimensional molded structure with enhanced printability.

As explained above, the aqueous composition has a solid content (i.e. a consistency) of preferably between 0.05 - 10 wt%, more preferably 0.2-1.5wt%.

Furthermore, the layer of pulp present on said forming portion may be press dried with a pressure between 0.2-50 bar, preferably 0.5-15 bar, more preferably 1-10 bar.

In case of drying the pulp present on said forming portion by means of applying elevated temperatures, such temperatures is preferably between 100-350°C, preferably 120-250°C, more preferably between 150-220°C.

It is also within the ambit of the present invention to produce a multilayer three dimensional molded structure with enhanced printability. A "layer" can also be referred to as a "ply". According to one example, a multilayer three dimensional molded structure may be produced in that the surface of said forming portion of the forming tool is pre-coated with a first layer of cellulose fibers. Said pre-coated first layer may have a different function such as providing a certain rigidity or barrier property. A formed fiber layer made from pulp containing metal salt is thereafter applied on top of said first layer to provide an outer surface with enhanced printability. More than two layers are also conceivable in a multiply structure, preferably 2 - 5 layers. In one embodiment, at least one of the layers in a multilayer three dimensional molded structure is made of MFC. A combination of metal salts and microfibrillated cellulose enhances the effect and provide a more even ink receptive surface. Also, addition of MFC will not only enhance metal salt retention but also increase surface smoothness and surface strength.

According to one aspect of the invention, the metal salt- containing wet layer of pulp present on the forming portion during production of said three dimensional molded structure is 5-150 gsm in dry weight.

According to a preferred aspect of the invention, said pulp is selected 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 and board, coated and uncoated broke, RMP, TMP, CMP, CSP NSSC nanopulp, dissolving pulp, and regenerated fibers and mixtures thereof. It is understood that other cellulosic material such as chemical or semi-chemical pulp of wood or non-wood material can be added as part of the pulp stock. Preferably, said pulp is a pulp fiber or fiber mixture with a Schopper Riegler value above 50 measured according to the SR standard.

According to yet another aspect of the invention, the method further comprises imprinting a pattern onto said three dimensional molded structure using a water based ink or a solvent based ink. It is also conceivable to use an ink that comprises both solvent and water. The ink can also be a varnish or a combination of ink and varnish. Ink can comprises one colorant or both dye and pigment, said pigments often being anionic. According to another aspect, said metal salt is preferably added to the pulp, or in case of cellulose nanomaterial such as MFC being present in the material, to the MFC before it is mixed with the pulp, or to the mixture of pulp and MFC. Pre-mixing MFC and metal salt before adding to the pulp stock can be advantageous because it leads to charge reversal of MFC, and also prevents the salts from precipitating other bonding agents that may be present in the pulp.

According to another aspect, said metal salt is one of CaCI2, Ca(OAc)2, MgCI2 or AICI3, or mixtures thereof.

According to another aspect, 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 ratio between metal salt vs additive is 1:100 - 100:1.

According to another aspect, the dose of metal salt is 1-50 kg/tn or more preferably 5-35 kg/tn measured for the outer ply in case of a multilayered ply. It is understood that the dose of metal salt is measured on the dried/dewatered product. Moreover, the skilled person understands that the salt amount can be determined by means of ToF-SIMS or other spectroscopic of chemical analysis methods.

According to another aspect, said aqueous composition also comprises one or more additional functional chemicals selected from the group comprising cationic polymers, nanopigments, amphoteric polymers and anonionic polymers. The salt in combination with specific cationic polymers enhance ink rub resistance and water fastness. Water fastness refers to the sensitivity of the color adhesion (once imprinted onto the surface of a material) to humidity.

According to another aspect, said aqueous composition further comprises one or more co-additives selected from the group comprising nanoparticles, cationic mordants, cross-linkers, non ionic polymers such as PVOH, PEG, cationic fillers, pigments or fillers with high surface area, preferably >10 g/m2.

According to another aspect, said cellulose nanomaterial is anionic MFC, or native MFC.

According to another aspect, the grammage of the molded structure is preferably 5-450 gsm or more preferably 10-200 gsm.

According to another aspect, molded structure comprises a density between 350-1500 kg/m3, preferably 400-1200 kg/m3 or most preferably 500-900 kg/m3.

The present invention also relates to a moldable aqueous suspension of cellulose pulp, at least one metal salt and optionally microfibrillated cellulose in a substantially homogeneous mixture. The present invention also relates to the use of a moldable aqueous suspension comprising cellulose pulp and at least one metal salt in a substantially homogeneous mixture, for improving the printability of a three dimensional molded structure made from said aqueous suspension. Said aqueous suspension may optionally comprise also cellulose nanomaterials such as MFC, and at least one metal salt in a substantially homogeneous mix. 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 claim 1 constituting an imprinting layer made from a mixture as previously described, further where said imprinting layer is arranged as an outer layer of said multilayer product.

Description of Embodiments

The present description is directed to production of three dimensional molded pulp articles with enhanced printability. Examples of a three dimensional molded pulp article include in a non-limiting way containers, trays and packages.

Although the present description relates to the context of conventional wet forming procedures, the invention is not limited thereto. The skilled person appreciates that the invention may contemplate any fiber-based manufacturing method, including 3D printing techniques.

According to the invention, presence of metal salt in a cellulose based molded pulp article, or at least presence of metal salt in a surface layer 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 provide a 3D molded product comprising at least one outer surface or a portion of an outer surface which has been made from a pulp slurry comprising metal salt additive/s. Production of such molded article may be done by wet molding methods. In the following, an example of a wet molding method for manufacturing a three dimensional molded article with improved/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 0.05-10wt%. The pulp may be any one of 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 and board, broke, RMP, TMP, CMP, CSP NSSC nanopulp, dissolving pulp, and regenerated fibers or mixtures thereof.

A cellulose nanomaterial such as e.g. microfibrillated cellulose (MFC) may be added to the pulp suspension. At least one metal salt is also added to the pulp and mixed to achieve a substantially homogeneous suspension. The cellulose nanomaterial may be pre mixed with the metal salt before they are added to the pulp. Furthermore, said metal salt/s may be added during fibrillation of MFC or during disintegration of the pulp. Said MFC is preferably anionic MFC, or native MFC, or a grafted version thereof.

A 3D shaped forming tool comprising a forming portion is brought into contact with the pulp suspension, for instance by immersing said tool into the slurry bath. Said forming portion is arranged to represent a 3D mirror image of the article to be formed. Pulp is drawn onto the forming portion e.g. by means of vacuum suction until a layer of desired thickness has been formed, whereupon the forming tool is removed from the slurry. At this stage, the forming portion is covered with a wet layer of pulp, said wet layer comprising between 5-150 gsm in dry weight. Next, the wet layer of pulp is dewatered to a dry content of at least 70wt%. Dewatering and/or drying can be done in various ways. In a wet curing procedure, the wet layer is pressed under elevated temperatures to be compressed and dried to a certain thickness, thereby yielding a smooth external surface for the end structure. In a dry curing process, the wet layer is subjected to heated air thereby removing moisture, which results in an end structure with a more textured finish. This way, a single layer molded fiber product is formed.

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

Imprinting a pattern onto the surface of the three dimensional molded product will now be described. The three dimensional molded article (produced by means of wet- or dry molding as described above) is subjected to imprinting a pattern e.g. by inkjet printing onto the metal salt-containing surface (i.e. the printing surface). Said inkjet printing is done using one or several printheads, hereby using one or several colors such as four color model CMYK. Furthermore, printing can be done in-line, at-line or off-line e.g. at the moulded pulp line. Printing can also be done off- line e.g. after or before packing or filling line. The printing can also be done using other suitable printing technologies such as screen printing. One advantage with digital printing is, however, the possibility for more flexible printing, variable and personalized printing and non-contact printing mode.

The preferred inks is a water based ink comprising preferably pigment based colorants. The inks typically contains multiple additives including rheology modifiers, dispersants, biocides, humectants etc. It should also be understood that inks can be so- called hybrid inks comprising both dye and pigment based colorants. The inks can also contain one or several co-solvents in addition to water.

It is also preferred that the surface intended for printing of said substrate is hydrophilic or such that the contact angle for water is < 80 degrees or pref. < 60 degrees after 10 seconds when measured with distilled water. The surface should preferably be hydrophilic for rapid liquid ink carrier medium uptake. By "carrier medium" means a solvent, which could be water or water/solvent mixture. Solvents could be e.g. various alcohol or e.g. glycols. A fast absorption of the carrier medium is advantageous since it reduce the risk of intercolor bleeding. At the same time, fast absorption may also lead to too much penetration of the colorant which should be avoided. One way to evaluate the hydrophilicity and ability of the solvent to spread is by e.g. contact angle measurements.

Preferably, said ink is water based due to cost and safety. Solvent or co-solvent inks may also be possible but then the fixation mechanism might be different. It is believed that metal salts electrostatically interacts with the anionic colorants in the ink. There are a number of water based inks on the market. Inks which are especially suitable for this tye of technology are e.g. branded under the name ColorLok. HP Desktop Officejet 8000 is example of a printer having ink which is reactive with the metal salts.

The solution according to the invention is also suitable for use of anionic dye based inks or hybrid inks meaning dye combined with a colorant. Pigment colorants have some advantages over dye based colorants such as light stability. Some properties, on the other hand, are better for the dye based colorants and therefore a mix of various colorants could offer a broader technical effect compared to a one-colorant based ink. Typically, the ink or colorants are anionicically charged, e.g. pigment colorants are anionic charged through electrostatic and/electrosteric stabilization.

The printed products can also be hybrid printed, and/or varnished or laminated afterwards. In this context, the hybrid printing means that two or more printing techniques are combined, e.g. screen printing and inkjet printing. A problem with screen printing is limited speed, and that personalized or variable print jobs cannot be done. On the other hand, a combinaton of screen and inkjet printing would offer different solutions to the same print job such as variable print with inkjet and non-variable print made with screen. It is also possible to use one or both for making varnishing including spot vanishing.

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