HEISKANEN ISTO (FI)
BACKFOLK KAJ (FI)
KAZEMAHVAZI SOHRAB (SE)
| WO2018078558A1 | 2018-05-03 | |||
| WO2017046755A1 | 2017-03-23 | |||
| WO2016097964A1 | 2016-06-23 |
| CN111519476A | 2020-08-11 | |||
| US20200277738A1 | 2020-09-03 |
| CLAIMS 1. A method for producing a three-dimensional molded article from cellulose fibers, comprising the steps of: -providing a pulp molding composition; -molding a three-dimensional precursor structure from said composition wherein said structure comprises a dry content between 15-80wt%; -providing an aqueous suspension comprising microfibrillated cellulose and wet strength agent; -applying the aqueous suspension onto at least one face of said precursor structure to create a surface barrier layer on said structure; and -dewatering and drying said molded precursor structure under elevated temperature >150°C to a dry content of >88wt%, preferably >94wt%, more preferably >96% to achieve the three-dimensional molded article. 2. A method according to claim 1, wherein said aqueous suspension comprises 1-100 kg/tn wet strength agent, more preferably 5-75 kg/tn, even more preferably 10-50 kg/tn based on the dry content of the surface barrier layer. 3. A method according to any one of claims 1-2, wherein said wet strength agent is selected from the group comprising: polyamide epichlorohydrin, polyethylene imine, dialdehyde starch, polyacryl amides, glyoxal or melamine formaldehyde, and urea formaldehyde melamine or a combination thereof. A method according to any one of claims 1-3, wherein said microfibrillated cellulose is fine fibril derived from enzymatically treated and fluidized chemical pulp or homogenized pulp or disinegrated by aqueous counter collision (ACC) or high pressure drop methods or treated in a high shear rotor stator mixer. A method according to any one of claims 1-3, wherein said microfibrillated cellulose is coarse fibril, derived from fibrillated chemical pulp, said fibril coarse having a Shopper Riegler value between 50-95, more preferably between 60-93. A method according to any one of the previous claims, wherein said microfibrillated cellulose and wet strength agent have been co-fibrillated or co-mixed before applied onto said precursor structure as an aqueous suspension. A method according to the invention, wherein the aqueous suspension further comprises a sizing agent, preferably selected from the group comprising alkyl ketene dimer (AKD), rosin sizes such as soap or rosin emulsions, alkyl succinic anhydride (ASA), polyurethane and styrene maleic anhydrides. A method according to any one of the previous claims, wherein the pulp molding composition is an aqueous composition having a dry content between 0.05 - 10wt%, preferably between 0.2-1.5wt%. A method according to any one of the previous claims, wherein the pulp molding composition comprises cellulose fibers or a mix of cellulose fibers 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, sulphate, sulphite, PGW, GW, DIP, recycled paper and board, broke, R.MP, TMP, CMP, NSSC, dissolving pulp, and regenerated fibers and mixtures thereof. A method according to any one of the previous claims, wherein the grammage of the three-dimensional molded article is 50 - 500 gsm in dry weight. A method according to any one of the previous claims, wherein the dewatering and drying of said molded precursor structure is performed as a one-sided dewatering. A method according to any one of the previous claims, wherein the surface barrier layer of the three-dimensional molded article has a basis weight between 5-50gsm, preferably 10- 30gsm, preferably between 7-20gsm. A method according to any one of the previous claims, wherein applying the aqueous suspension is non-contact application, preferably a series of separate non-contact application steps of aqueous suspension onto the precursor structure, preferable at least two consecutive application steps. A method according to claim 13, wherein said non-contact application is performed by spraying liquid droplets onto the surface of the precursor structure. 15. A method according to any one of the previous claims, wherein said three-dimensional molded article has a repulpability of a reject rate (as determined according to the PTS R.H 021/97 test method) below 10%, preferably below 5%. 16. A method according to any one of the previous claims, wherein the three-dimensional molded article has a Cobbeo value (as determined according to standard ISO 535:2014 after 60 seconds) below 50 g/m2, preferably below 30 g/m2. 17. A method according to any one of the previous claims, wherein the three-dimensional molded article has an oil Cobbso value (as determined according to standard SCAN-P 37:77 after 30 seconds) below 9 g/m2, preferably below 5 g/m2. 18. A method according to any one of the previous claims, wherein the three-dimensional molded article has a KIT barrier >5, preferably >10 (TAPPI method 559, 3M KIT test). 19. A method according to any one of the previous claims, wherein the three-dimensional molded article has an air permeance, L&W Code 168 air permeance tester (pm/Pa s at 20 kPa), less than 1000 and more preferably less than 500 and most preferably less than 210 (SCAN P26 or ISO 5636-1). 20. A method according to any one of the previous claims, wherein the three-dimensional molded article has a Gurley Hill value (L&W Code 166) >20 000, more preferably >30 000 and most preferably >40 000 s/100 ml (determined according to the standard ISO 5636/6). 27 A method according to any one of the previous claims, wherein said three-dimensional molded article has an average density between 350-1500 kg/m3, preferably 400-1200 kg/m3 or more preferably 500-900 kg/m3. A method according to any one of the previous claims, wherein said three-dimensional molded article comprises a top side with the surface barrier layer, and a back side opposite to said top side, wherein the density of said top side is >800 kg/m3, more preferably > 850 kg/m3, and the density of said back side is 300 - 800 kg/m3. A method according to any one of the previous claims, wherein said three-dimensional molded article comprises a wet strength >1%, preferably between 2-10%. A three-dimensional molded pulp article made by means of a method according to any one of claims 1 - 23, comprising a wet tensile strength >1%, preferably between 2-10%, and a repulpability characterized by a reject rate (as determined according to the PTS R.H 021/97 test method) below 10%, preferably below 5%. A three-dimensional molded pulp article according to claim 24, wherein the surface barrier layer comprises 1-100 kg/tn wet strength agent, more preferably 5-75 kg/tn, even more preferably 10-50 kg/tn based on the dry content of the surface barrier layer, and wherein the substrata layer is void of any wet strength agent. A three-dimensional molded pulp article comprising more than one layer, whereof at least one layer is made by means of a method according to any one of claims 1 - 23, further where said layer is arranged as an outer layer of said multilayer article. 29 |
Technical field
The present invention relates to a method of producing a three- dimensional cellulose-based structure by means of fiber molding.
Background
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 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 allowing for composting. Natural fibers include cellulose fibers of any natural origin, such as derived from wood pulp and/or plants.
Manufacturing molded fiber products and structures can be done by wet forming, wherein an aqueous pulp composition is applied to a forming tool to form a fiber matt followed by compression-molding performed under elevated temperatures, 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, such as in a dewatering/drying step. i It is common to add so called wet strength agents (WSA) to the furnish upon molding procedures. Wet strength chemicals improve the strength properties of the paper both in wet and dry state by crosslinking the cellulose fibres with covalent bonds that do not break upon wetting. Wet strength agents also improve mechanical properties under humid conditions. A disadvantage associated with addition of WSA is that the repulpability of the material is decreased, i.e. the fraction of reusable fibers upon recycling is reduced, or harsher process condition than normally used at the mill is required. Another disadvantage with WSA is that the chemical retention in the sheet or substrate is important in order to achieve the effects. One way is to combine WSA with a specific retention chemical in order to increase the fixation. The harsh dewatering conditions in fiber molding present challenging conditions for obtaining good and efficient retention of the chemicals. To obtain sufficient wet strength, the preferred technical solution is to inrease dosage of WSA and with higher dosage of retention chemicals. As a consequence, the use of higher concentration of WSA deteriorates disintregration of the molded product and hence reduces repulpability.
Thus, there is a need for improvements when it comes to producing three dimensional products, and articles made from molded cellulose fibers, at least when it comes to improving repulpability.
Objects of the invention
It is an object of the present invention to provide an improved method for manufacturing a cellulose-based, three dimensional molded fiber article, resulting in a product having good wet strength, grease resistance and water- and vapor resistance and having an improved recyclability compared to known molded fiber products.
Summary
The objects of the invention are obtained by means of a method for producing a three-dimensional molded article from cellulose fibers according to claim 1.
Said method comprises at least the steps of: -providing an aqueous pulp molding composition; -wet molding a three-dimensional precursor structure from said composition, wherein said structure comprises a dry content between 15-80wt%;
-providing an aqueous suspension comprising microfibrillated cellulose and wet strength agent;
-applying the aqueous suspension onto at least one face of said precursor structure to create a surface barrier layer on said structure;
-dewatering and drying said molded precursor structure under elevated temperature >150°C to a dry content of >88wt%, preferably >94wt%, more preferably >96% to achieve the three- dimensional molded article.
Said achieved three-dimensional molded article thus comprises a base substrate originating from the aqueous pulp molding composition, and a surface barrier layer (herein sometimes also referred to as "top layer" or "top barrier layer") originating from the aqueous suspension. It is to be understood that the procedure of molding a three- dimensional precursor structure" in this context may include the steps of providing a forming tool having a three dimensional shape and 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 from the pulp molding composition.
It is also to be understood that said "precursor structure" refers to an intermediate, not-yet ready structure, that in the present case has been formed into a three-dimensional shape, and which is still in a wet or semi-wet state (dry content between 15-80wt%).
Moreover, the term "recycling" used herein refers to the process of converting waste cellulose-based materials into new cellulose-based materials and products. The recyclability of a material depends on its ability to reacquire the properties it had in its virgin state.
During recycling of cellulose based products, the fibers are converted back into pulp in a so called "repulping" procedure. In other words, recyclability and repulpability means that preconsumer waste can be collected, disintegrated and reused for example when making new formed fiber products. Post-consumer waste can be collected and treated in various ways before being reused in e.g. molded pulp. The treatment before reuse depends on the packed product and level of contaminants. Repulpability can be determined according to the PTS standard (PTS R.H 021/97).
According to a preferred aspect of the invention, said aqueous pulp molding composition used for molding the precursor structure is free from wet strength agent. According to the present invention, a top barrier layer is formed on a surface of the molded article. It has surprisingly been found that a top layer comprising a mixture of MFC and WSA, which have been applied onto a wet precursor substrate as a pre-mixture leads to several advantages. One advantage is that the method allows for the use of significantly reduced dosages of WSA compared to known products, while still retaining enough wet strength of the endproduct. Lower amounts of WSA in its turn leads to increased repulpability of the material, i.e. to a lower reject rate of the material once it is being recycled. Unlike prior art formed fiber products where WSA is present evenly throughout the material, the present invention leads to that a majority, i.e. >90%, of the WSA is concentrated to one top layer of the article. Furthermore, applying the aqueous suspension onto a wet precursor substrate ensures better economy, better infiltration of MFC and chemicals into the base substrate, and significantly better chemical retention.
Also, the method according to the invention leads to advantageous properties of the molded end-product. The top barrier layer provides surprisingly good oil and grease resistance (OGR.) as well as water repellence measured as KIT and Cobb values.
In one embodiment, the MFC content of the top barrier layer may be in the range of 70 to 99 weight%, in the range of 80 to 99 weight%, or in the range of from 90 to 99 weight% of the solids of the top barrier layer.
Microfibrillated cellulose (MFC) shall in the context of the patent application mean a nano scale cellulose particle fiber or fibril with at least one dimension less than 1000 nm. MFC comprises partly or totally fibrillated cellulose or lignocellulose fibers.
The liberated fibrils have a diameter less than 1000 nm, whereas the actual fibril diameter or particle size distribution and/or aspect ratio (length/width) depends on the source and the manufacturing methods.
The smallest fibril is called elementary fibril and has a diameter of approximately 2-4 nm (see e.g. Chinga-Carrasco, G., Cellulose fibres, nano fibrils and microfibrils, : The morphological sequence of MFC components from a plant physiology and fibre technology point of view, Nanoscale research letters 2011, 6:417 , while it is common that the aggregated form of the elementary fibrils, also defined as microfibril Fengel, D., Ultrastructural behavior of cell wall polysaccharides, Tappi J., March 1970, Vol 53, No. 3.), is the main product that is obtained when making MFC e.g. by using an extended refining process or pressure-drop disintegration process. Depending on the source and the manufacturing process, the length of the fibrils can vary from around 1 to more than 10 micrometers. A coarse MFC grade might contain a substantial fraction of fibrillated fibers, i.e. protruding fibrils from the tracheid (cellulose fiber), and with a certain amount of fibrils liberated from the tracheid (cellulose fiber).
There are different acronyms for MFC such as cellulose microfibrils, fibrillated cellulose, nanocellulose, fibril aggregates, cellulose microfibers, cellulose fibrils, microfibrillar cellulose, microfibril aggregrates and cellulose microfibril aggregates. MFC can also be characterized by various physical or physical-chemical properties such as large surface area or its ability to form a gel-like material at low solids (1-5 wt%) when dispersed in water. 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 300 m 2 /g, such as from 1 to 200 m 2 /g or more preferably 50-200 m 2 /g when determined for a freeze-dried material with the BET method.
According to the invention, the MFC should preferably be a mechanically fibrillated fiber from either non-mechanically pretreated or mechanically pre-treated pulp. The pulp can also be enzymatically pre-treated before fibrillation step. Further, the MFC should preferably have the following characteristics: FS5 fines should preferably lower than 100%, and preferably lower than 90% (as determined the Valmet FS5 fiber image analyzer). "FS5 fines" here refers to the projection area of particles with a size under 0.2mm of the total surface area of all measured objects x 100%. FS5 fibrillation level (P14) should preferably be higher than 1.5% and more preferably higher than 1.8% and more preferably higher than 2%.
According to one aspect of the invention, the MFC is fine fibrils derived from enzymatically treated and fluidized chemical pulp or homogenized pulp or disinegrated by Aqueous counter collision (ACC) or high pressure drop methods or treated in a high shear rotor stator mixer. Before subjecting the enzymatically pre-treated pulp to high shear disintegration, the pulp can be mechanically activated or fibrillated in one or several steps. The enzymatically treated pulp can be provided as never-dried, dewatered or dried before further subjecting to the activation and fibrillation step. According to another aspect of the present invention, said MFC is coarse fibril, derived from fibrillated chemical pulp, said fibril coarse having a Shopper Riegler value between 50-95, more preferably between 60-93. The Schopper-Riegler value can be determined through the standard method defined in EN ISO 5267-1.
It is within the scope of the present invention to mix fine and coarse MFC and apply together with WSA as said aqueous solution to form the top barrier layer.
According to one aspect of the present invention, the aqueous suspension comprises wet strength agent of an amount between 1- lOOkg/tn, more preferably 5-75kg/tn and most preferably 10- 50kg/tn (measured in dry weight of top layer). As previously explained, the aqueous suspension is applied onto a surface of the molded precursor substrate thus forming a top layer.
According to another aspect of the present invention, said wet strength agent is selected from the group comprising : polyamide epichlorohydrin, polyethylene imine, dialdehyde starch, polyacryl amides, glyoxal or melamine formaldehyde, polyamidoamineepichloro hydrin and urea formaldehyde melamine or a combination thereof.
According to another aspect of the present invention, said microfibrillated cellulose and WSA have been co-fibrillated and/or co-mixed before applied onto said precursor structure as an aqueous suspension. Co-fibrillation and/or co-mixing leads to facilitated preparation of the aqueous solution, e.g. due to that MFC alone is sometimes difficult to mix with water. The mixing is even more difficult if the WSA is added with a retention agent or if the WSA has cationic or cellulose fibril reactive sites since this can causes substantial increase in gel viscosity or formations or flocs. Thus, in one aspect of the invention, WSA is added together with additional functional chemicals such as retention agents to fiber before subjecting the resulting mixture to fibrillation treatment.
According to another aspect of the invention, said aqueous suspension further comprises a sizing agent, preferably selected from the group comprising: alkyl ketene dimer (AKD), rosin sizes such as soap or rosin emulsions, alkyl succinic anhydride (ASA), polyurethane or styrene maleic anhydrides. In a preferred aspect, the sizing agent is AKD. Preferably, the amount of AKD is between 0.05 -100 kg/tn, more preferably 0.1-50 kg/tn, and even more preferably 0.5-40 kg/tn and more preferably 2-20 kg/tn based on the dry content of the top layer.
According to another aspect of the invention, said aqueous suspension further comprises dye and/or pigment to achieve a shaded/colored product providing possibilities of controlling the appearance of the final article.
According to another aspect of the invention, the viscosity of the aqueous suspension is 5-2000 mPas, such as 10-500 mPas as determined with Brookfield, i.e. Brookfield viscosity determined at 100 rpm according to standard SCAN-P 50:84.
According to yet another aspect of the present invention, said aqueous suspension is applied onto the precursor structure by means of a non-contact application method, e.g. by spraying liquid droplets onto the surface of the precursor structure, or by means of curtain coating. Non-contact application onto the precursor structure may be repeated several times in order to reach the desired grammage of the top layer, preferable in at least two consecutive steps, or at least ten steps. Spraying of aqueous suspension can be carried out using methods known in the art. The spraying can for example be electrostatically assisted or ultrasound assisted. It can also be performed by a co-axial spray nozzle, as steam, high air pressure or by use of a rotary disk atomizer.
It is also within the scope of the present invention to apply said aqueous suspension onto the precursor structure by means of contact applications, such as immersing the precursor structure into a bath of aqueous suspension comprising MFC and WSA or by means of pad printing technology (i.e. using a resilient pad for transferring the suspension onto the surface structure)
According to another aspect of the present invention, the pulp molding composition for wet molding the precursor structure is an aqueous composition having a solid content between 0.05 - 10wt%, preferably between 0.2-1.5wt%.
According to another aspect of the present invention, the pulp molding composition comprises cellulose fibers or a mix or cellulose fibers 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, sulphate, sulphite, PGW, GW, DIP, recycled paper and board, broke, R.MP, TMP, CMP, NSSC, dissolving pulp, and regenerated fibers and mixtures thereof. In a preferred aspect, the pulp molding composition is based on CTMP. CTMP comprise bulky and stiff fibers and provide more rigidity to the final structure. CTMP pulp can also be deficient of fines e.g. by fractionation or washing. Less fines in the bulk layer leads to faster dewatering during production of the molded structure.
According to yet another aspect of the present invention, the grammage of the three-dimensional molded article is 50 - 500 gsm in dry weight.
Preferably, dewatering and drying the precursor structure comprising the top barrier layer is performed by pressing the wet fiber layer under applied heat, optionally with vacuum suction. According to another aspect of the present invention, dewatering and drying said molded precursor structure is performed as a onesided dewatering. In case of dewatering and drying by means of vacuum suction, one-sided dewatering is performed by applying the vacuum on the non-coated side of the structure. This leads to that the top barrier layer is to some extent drawn into the base substrate, leading to improved retention and integration of the MFC and WSA.
According to yet another aspect of the invention, the top barrier layer has a grammage between 5-50gsm, preferably 10-30gsm, preferably between 7-20gsm.
According to yet another aspect of the invention, the top barrier layer comprises <10 pinholes/m 2 , preferably <5 pinholes/m 2 , more preferably pinhole free according to standard EN13676:2001. ii According to yet another aspect of the invention, the surface barrier side of the three-dimensional molded article has a water Cobbeo value (as determined according to standard ISO 535:2014 after 60 seconds) below 50 g/m 2 , preferably below 30 g/m 2 . According to yet another aspect, the substrate comprises internal sizing such as AKD, and the three-dimensional molded article has a Cobbeo value below 30 g/m 2 .
According to yet another aspect of the invention, the three- dimensional molded article has an oil Cobbso value (as determined according to standard SCAN-P 37:77 after 30 seconds) below 9 g/m 2 , preferably below 5 g/m 2 .
According to yet another aspect of the invention, the three- dimensional molded article has a KIT barrier >5, preferably >10 such as 12 (TAPPI method 559, 3M KIT test).
According to yet another aspect of the invention, the three- dimensional molded article has an air permeance, L&W Code 168 air permeance tester (pm/Pa s at 20 kPa), less than 1000 and more preferably less than 500 and most preferably less than 210 (SCAN P26 or ISO 5636-1).
According to yet another aspect of the invention, the three- dimensional molded article has a Gurley Hill value (L&W Code 166) >20 000, more preferably >30 000 and most preferably >40 000 s/100 ml (determined according to the standard ISO 5636/6).
According to yet another aspect of the present invention, said three-dimensional molded article has an average density between 350-1500 kg/m3, preferably 400-1200 kg/m3 or more preferably 500-900 kg/m3.
According to yet another aspect of the present invention, said three-dimensional molded article comprises a top side comprising said top barrier layer, and a back side opposite to said top side. According to this aspect, the density at said top side is >800 kg/m 3 , more preferably > 850 kg/m 3 , and the density at said back side is 300 - 800 kg/m 3 .
According to yet another aspect of the present invention, said dried three-dimensional molded article has a repulpability characterized by a reject rate (as determined according to the PTS R.H 021/97 test method) below 10%, preferably below 5%. Hereby, the molded article according to the invention is suitable for recycling, since a vast majority of its fiber content can be re-used.
According to yet another aspect of the present invention, said three-dimensional molded article comprises a wet strength >1%, preferably between 2-10% (measured as percent wet strenght over dry strength).
According to yet another aspect of the present invention, said three-dimensional molded article has a wet rub turbidity between 1- 25 Nephelometric Turbidity Units (NTU) measured with a nephelometer (ISO 7027) after 20s.
The present invention further comprises a three-dimensional molded pulp article comprising more than one layer, whereof at least one layer is made by means of a method according to the invention, further where said top barrier layer is arranged as an outer layer of said multilayer article. Detailed description
The present description is directed to production of three- dimensional molded pulp articles with barrier properties and improved repulpability. Examples of three-dimensional molded pulp articles include in a non-limiting way bowls, cups, capsule, pots, containers, trays and packages.
The present description relates to the context of wet molding procedures.
According to the invention, concentrating wet strength agent only to a surface of a 3D molded fiber article, wherein said wet strength agent has been applied in an aqueous suspension together with MFC to create a surface barrier layer, leads to a product with improved recyclability, yet having a good wet strength as well as barrier properties.
In one embodiment, the molded article is intended to contain foodstuff. In such a variant, the surface comprising the top barrier layer is arranged at the food-contact side of the product.
It is thus within the ambit of the present invention to provide a 3D molded fiber article comprising at least one outer surface, or a portion of an outer surface, which has been subjected to application (e.g. spraying, coating or immersion) of an aqueous suspension comprising a mixture of WSA and MFC. Said aqueous suspension is to be applied onto a substrate in wet state, whereafter the article is subjected to drying and dewatering under heat treatment, and preferably also applied pressure. In the following, an example of a wet molding method for manufacturing a three-dimensional molded fiber article with barrier properties and improved/enhanced recyclability will be described in a non-limiting way.
An aqueous pulp composition is provided with consistency between 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, sulphate, sulphite, PGW, GW, DIP, recycled paper and board, broke, R.MP, TMP, CMP, NSSC, dissolving pulp, and regenerated fibers or mixtures thereof. Preferably, the pulp is CTMP.
A 3D shaped forming tool comprising a forming portion is brought into contact with the pulp composition, 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. Next, the wet layer of pulp is dewatered to a dry content between 15-80wt%. Hereby, an intermediate precursor structure has been obtained, which is still supported by the shape of the forming tool.
In a next step, an aqueous suspension is provided, comprising microfibrillated cellulose and wet strength agent mixed with each other to a homogenous mixture. The aqueous suspension is applied onto at least one face of said precursor structure e.g. by means of a non-contact application method to create a layer on said structure. An example of a non-contact application method is spraying liquid droplets onto the precursor structure. Application of the aqueous suspension can be performed in many consecutive sessions, e.g. spraying sessions, e.g. such as spraying at least two times on the same surface, or at least five times or at least twenty times, until a desired thickness of the sprayed layer is achieved. The top layer originating from the aqueous suspension will provide barrier properties, and the wet strength agent present in the suspension will contribute to make the article stable and rigid.
The precursor structure having been covered with a top barrier layer of mixed WSA and MFC is then to be further dewatered and dried. Preferably, dewatering of said molded precursor structure is performed as a one-sided dewatering by means of applying suction at the non-coated side of the structure. Sucking from the untreated side leads to that the top layer is partially drawn into the substrate and gets somehow integrated therewith, and binding of the top surface components is improved. Preferably, the side of the structure comprising the barrier layer is pressed against a support arrangement, e.g. a mesh. As a result, a dense and stable top barrier layer is achieved.
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. According to the invention, the hot press temperature range for a wet molded procedure is between 150-220 °C, with a press range between 1-10 bar. This way, a single layer 3D molded fiber article is formed, having a dry content of >88wt%, preferably >94wt%, more preferably >96%.
It is conceivable to produce multilayered molded fiber products. 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. The various layers of a multilayered product may hereby provide different functions, such as rigidity, barrier properties, etc. In a multilayered product according to the present invention, the foodcontact layer of the end-product is arranged to comprise the top barrier layer described herein.
The present invention has been described with regards 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.
Examples
Materials used
Pulp - CTMP pulp was used with and without additives, i.e. internal sizing agent and/or wet strength agent (WSA). Fine fibril - This was a chemical pulp that was enzymatically treated and then fluidized. Among the characteristic properties of this material is high water retention value properties, but due to its fineness, it is difficult to retain on a wire. If blended with a furnish/pulp, it will pass through the wire or holes in the dewatering unit. The material is gel like at low solid contents 2-4 wt%.
Coarse fibril - This is a fibrillated fiber prepared from a chemical pulp. The characteristic properties of this is that it is easier to retain mechanically in a fiber matrix or on a dewatering wire. On the other hand, it increases also drainage resistance. The preferred Shopper Riegler level is 50-95 and more pref. 60-93. The SR for fibril coarse used here was about 93.
The internal sizing agent used was alkyl ketene dimer (AKD).
The wet strength agent (WSA) used here was polyamide epichlorohydrin.
Measurement and evaluation methods
The following methods and standards apply both to the definitions of the appended claims and to the measurements performed in the examples below.
Twelve cellulose-based test sheets (referred to below) were prepared and analyzed with regards to the following properties:
-Oil Cobb30 (gsm)
-Water Cobb60 (gsm) (ISO 535:2014) -KIT (TAPPI method 559, 3M KIT test) -WVTR. (g/m2 24h) (ASTM Fl 249) -OTR. (g/m2 24h) (ASTM D-3985) -Wet rub (30s, turbidity NTU) -Wet rub (20s, turbidity NTU)
-Wet strength (expressed as the ratio of wet to dry tensile force at break)
-PTS repulpability
With regards to evaluation of wet rub, measurements were performed using a conventional rotational wet abrasion tester to determine wet rub resistance of the coatings. A volume of 50 ml distilled water was poured over the respective sample, and after 30s wetting time, the rotational wet abrasion of the sample is started. After 40s and 20s respectively, the rotation was stopped and water was poured off from the sample. Turbidity of the water was then measured using a nephelometer (ISO 7027).
The results from the performed tests for the respective sheet are summarized in Table 1, appended herewith.
Results
The sheets numbered 1 - 12 are described in the following.
1. Base sheet (comparative example)
A 300 gsm sheet was prepared on a 2D molding wire using vacuum suction. The wet sheets were dried using a hot plate (under pressure) at 180 C. No barrier properties were obtained. 2. Base sheet with WSA and internal sizing (comparative example)
Same as #1 but with added AKD and WSA. Clear improvement especially in water cobb (water uptake) but the sheet has still poor wet resistance. Due to the presence of WSA, this grade will have limited repulpability.
3. Base sheet with integrated fine fibril top layer and WSA After wet forming the base, a 10 gsm (dry weight) wet layer of fine fibril pre-mixed with WSA was applied by spraying, and then the product was further dewatered and dried. This recipe shows a significant reduction in oil uptake (Oil Cobb). The amount of WSA corresponds to 0.86 kg/tn of the whole product, i.e. a reduction with more than 50% compared to #2.
4. Base sheet with integrated fine fibril top layer
This is a corresponding test point to #3 but without the WSA. The oil absorbency confirms that the fine fibril gives a significant reduction on oil uptake and also in water uptake. The oil and grease resistance show also excellent KIT value (max value) confirming that the coating is uniform and forms a dense layer with good adhesion to the base substrate. The gas and water vapor barriers are also detectable showing that the film is dense and not destroyed during coating, which is probably due to the fact that steam and water is removed in one side direction, i.e. a one-sided dewatering was performed.
5. Base sheet with integrated top layer comprising fine fibril and sizing agent This sample shows the benefit of adding sizing agent to the integrated top layer instead of adding the chemical to the bulk furnish. Compared to the reference #1, an increased wet rub resistance is obtained.
6. Base sheet with integrated top layer comprising fine fibril, AKD and WSA
In this trial, high barrier level is obtained and especially for KIT. Surprisingly high WVTR is obtained which indicates that the specific manufacturing technique is actually densifying the top layer structure without causing blistering and pinholes. Very low wet rub resistance value, i.e. very low amount of material is releases confirming high surface strength.
7. Base sheet with top layer comprising fine fibril, AKD and WSA
Same as the #6 but the top layer grammage has been increased to about 20 gsm. In this case, very low OTR and WVTR. was obtained which is surprising.
8. Base sheet with top layer comprising fine fibril, AKD and WSA
Same as the #6 but the top layer grammage has been increased to about 30 gsm. In this case, very low OTR and WVTR was obtained which is surprising.
9. Base sheet with top layer comprising fine fibril, AKD and WSA and dispersing agent
Same as the #8 but the top layer recipe has been modified with a dispersing agent for enhancing runnability and flow behavior. WVTR and KIT are almost on same level whereas OTR. is increased (less gas barrier).
10. Base sheet with top layer comprising coarse fibril, AKD and WSA
This corresponds to sample #6 with the exception that the fibril grade is coarser. At 10 gsm amounts, no barrier was obtained.
11. Base sheet with top layer comprising coarse fibril, AKD and WSA
This corresponds to #10 with the exception that the coating weight is about 30 gsm.
12. Base sheet with top layer comprising coarse fibril, AKD and WSA and dispersing agent
This corresponds to #11 with the exception that the top layer contains a dispersing agent (low molecular weight CMC). The oil barrier and oil uptake values are very good.
Comments
The results show that applying a mixture of WSA together with MFC in a coating layer not only improves the liquid and grease barrier but also enhances wet strength, measured as both wet tensile strength and, especially, wet rubbing strength, which otherwise has shown to be solved with WSA addition in the pulp furnish. The efficient use of WSA also results in significantly reduced dosage of WSA, and the benefit is larger the higher basis weight the product has. Combination of WSA and AKD in the MFC provides additional advantages showing synergistical effects that is not obtained with AKD and WSA separately.