PRODUCT AND A FIBER PRODUCT

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) 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.

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

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, 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 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 50%, preferably at least 60%; and

-removing said dried layer from the forming tool to achieve the three dimensional molded product, wherein the 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.

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/amount 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. It has surprisingly been found that the printability of a three dimensional molded, fiber based product is significantly improved by means of a method and a product according to the invention. Presence of a metal salt onto the surface of the three dimensional molded product provides a surface with 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).

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.

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

According to one aspect of the invention, said product is achieved by means of a wet molding procedure. According to this aspect, the cellulose fiber material is an aqueous composition or slurry having a consistency between 0.05-10wt%, preferably 0.2- 1.5wt%. Said forming tool is brought into contact with said slurry e.g. by immersion so that said forming surface of the forming tool is 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 gsm in dry weight. According to another aspect of the invention, press drying of the wet layer is performed at temperatures >100°C, preferably at temperatures between 120-250°C or more preferably between 150-220°C. 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.

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. It is possible to dry the layer of pulp either to a level where the material is still damp, i.e. containing moisture, or to a level where the material is substantially dry. A damp layer of pulp comprises a a dry content of at least 50%, preferably at least 60%. A substantially dried layer of pulp comprises a dry content of at least 70%, preferably at least 80%. 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. 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 wet 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.

According to one aspect of the invention, said product is achieved by means of a dry molding procedure. According to this aspect, the cellulose fiber material is a fiber-based sheet material having a solid content of 30-95wt%.

According to the invention, a metal salt in aqueous form is applied onto one surface of the molded product, preferably sprayed onto a surface thereof. It is to be understood that "spray" means in the form of a plurality of liquid droplets or particles, and that the metal salt in spray form may be delivered by means of a precision device for dispersion of freely flowing dissolved metal salt into said spray form. The droplets or particles may be in micro scale with sizes ranging from 1-900 pm 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 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 of the invention, said metal salt spray comprises functional chemicals such as non-stick chemicals (e.g. lubricants) and colorants.

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

According to another aspect of the invention, the aqueous metal salt spray is applied onto the surface of the three dimensional molded product being in damp condition, i.e. not being fully dried. Spraying metal salt onto a celluluse based product which has a certain moisture content is advantageous since it leads to better adhesion of the metal salt onto the surface, and improved lateral spreading of the metal salt ions across the surface due to surface energy. Being in aqueous form, the metal salt will adhere better to a surface which has a certain moisture compared to a dry one. In the present context, "damp" means that the molded product comprises a dry content less than 70wt%, preferably less than 60wt%.

According to another aspect of the invention, the method further comprises imprinting a pattern onto said three dimensional molded product 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. Preferably, said imprinting is performed by any one of inkjet, flexographic or screen printing. Preferably, the imprinting is performed using any one of pigment based colorant ink, anionic dye based ink or hybrid ink.

Furthermore, the imprinting is performed in-line in a molding line or at a line.

According to another aspect of the present invention, said cellulose fiber material comprises a cellulose nanomaterial such as microfibrillated cellulose (MFC). Thanks to presence of MFC in the fiber material, the resulting molded pulp product achieves an improved printability once it is dried. This is due to that the metal salts applied by means of spraying will bind to the surface of the structure because of the MFC-content, meaning that free metal salt ions are available on the surface of the molded structure, leading to subsequent good printability when using water based ink. Thus, use of cellulose nanomaterial enhances the retention of metal salts in the cellulose material, and it also improves 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 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 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 another aspect, said cellulose fiber material also comprises one or more additional functional chemicals selected from the group comprising cationic polymers, nanopigments, amphoteric polymers and anonionic polymers. The metal 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 cellulose fiber material 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 yet another aspect of the invention, the grammage of the molded product is preferably 5-450 gsm or more preferably 10-200 gsm. According to yet another aspect of the invention, said molded product comprises a density between 350-1500 kg/m3, preferably 400-1200 kg/m3 or most preferably 500-900 kg/m3.

The 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 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. According to a preferred aspect, the three dimensional molded fiber/pulp product is a packaging product and comprises a first, outer side intended for being decorated with an imprinted pattern, and a second inner side intended to provide a barrier against grease, oil, gas, water etc.

Said outer side is thus arranged to comprise metal salt ions for enhancing printability, whereas the inner side may comprise a barrier 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 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. Thus, the products 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 of conventional wet forming procedures and dry 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 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 subjected to application of an aqueous metal salt, e.g. in spray form, onto at least one surface intended for subsequent imprinting. Production of such molded article may be done by wet molding methods or dry 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. 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 50, preferably at least 60%wt%. 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 layers 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. 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 is covered with metal salt ions originating from said applied metal salt spray.

In the following, an example of a dry molding method for manufacturing a three dimensional molded article with improved/enhanced printability will be described in a non-limiting way. In dry molding procedure, said cellulose fiber material is a fiber based sheet material having a solid content of 30-95wt%. Preferably, the fiber based sheet is made from pulp 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, recycled paper and board, broke, nanopulp, dissolving pulp, and regenerated fibers and mixtures thereof.

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.