FI20215083A1

FI20215083A1 - Kartongprodukt med hög beständighet och förfarande för framställning därav

Info

Publication number: FI20215083A1
Application number: FI20215083A
Authority: FI
Inventors: Duncan Mayes
Original assignee: Fiberdom Oy
Priority date: 2021-01-25
Filing date: 2021-01-25
Publication date: 2022-07-26
Also published as: AU2022210496A1; WO2022157425A1; EP4281616A1; CA3206132A1; FI130540B

Abstract

The object of the invention is high durability cardboard product and a method for the manufacturing thereof. The method comprises the steps of applying a cross-linking formulation comprising at least one cross-linking acid and at least one polyol to a pulp sheet material, pre-drying the pulp sheet material treated with the cross-linking formulation and pre-heating a single sheet or combined multiple sheets of the pre-dried pulp sheet material treated with the cross-linking formulation. At least one cardboard product is shaped and cut out of the preheated pulp sheet material, and finally subjected to a final curing step.

Description

HIGH DURABILITY CARDBOARD PRODUCT AND METHOD FOR THE MANU-

FACTURING THEREOF Object of the Invention The present invention relates to a high durability cardboard product and a method for the manufacturing thereof. Background of the Invention The invention has been developed to provide a technically and economically viable replacement to the broad range of single use plastics. The new technology will ad- dress the increasing demand for packaging and single use products which are de- rived from fully renewable raw materials and which can be fully recycled and re-pro- cessed in a circular economy. Currently produced single use plastics used in a range of applications from packaging to eating utensils and other single use items are be- ing banned in Europe and a number of other countries around the World. The current renewable material solution is cardboard, which is derived from wood or other ligno-cellulosic raw materials and is commonly used for a broad range of packaging, storage and logistics, and single use utensils such as plates and cups. The actual formulation of cardboard varies significantly depending on the intended end use with the most basic form being brown kraft liner box board through to more complex white coloured liquid, grease and oxygen barrier containing board which may also have other printed media applied to the surfaces. Many countries are now developing effective recycling systems to collect and re-process the card- board materials for re-use into new cardboard products. N 25 A challenge with current cardboard products is the continued durability of the card- = board when used in moist or varying humidity environments or in applications 7 where the cardboard may come into contact with high humidity, moisture, liguids, - or grease. When exposed to such conditions, cardboard will lose its strength, hold E moisture and increase the risk of microbial growth and will often no longer provide & 30 the needed functionality it was intended for and limit its use as a replacement to = conventional thermoplastic materials such as polypropylene, polyethylene and poly- vinyl chloride (PVC). To address these limitations barriers are often applied to the surfaces of the cardboard which prevent or limit the negative impacts of moisture and grease.

Traditionally fossil based polymer films such as linear low-density poly- ethylene (LLDPE) has been used but in more recent times a number of advance- ments in bio derived films and barriers have been in development and are beginning to enter the market.

However, a number of challenges remain which still creates limitations on the use of cardboard as a replacement to plastics.

The application of a film coating on the surfaces helps to protect the direct surfaces but does not limit the risk of moisture and grease incursion and degradation from the edges of the boards and at joints and if the barrier is broken the functionality is lost.

In addition, the use of a plastic barrier increases the complexity of the recycling and repro- cessing of the board as the very different materials need to be carefully separated before re-processing is possible.

The present invention provides novel cardboard products produced from 100% re- newable materials with significantly enhanced strength properties and moisture and liguid resistance without the use of secondary layer films.

Additionally, the products show improved anti-microbial properties and are fully recyclable with existing card- board materials and products.

Prior Art The present invention discloses a novel cross-linking reaction of cellulosic pulp ma- terials in the form of sheets or rolls for the preparation of high-durability cardboard products achieving their final dimensional stability and strength after curing.

A sim- ultaneous or subseguent hydrophobation reaction may be employed to further en- hance the hydrophobic properties of the product.

N 25 The use of the combination of citric acid or similar carboxylic acids and sorbitol or = similar polyols as a base formulation to facilitate an esterification reaction with the 7 hydroxyl groups within the acid and also within wood fibres is well known and at it - earliest incorporated to a patent US3661955 (A) with the title "Polyesters of citric E acid and sorbitol” having a priority date of 3.11.1969 and a later and even more rel- & 30 evant patent CA2443901 C with the title "Cross-linked pulp and method of making = the same” having a priority date of 11.4.2001. The Canadian patent primarily refers N to the use of carboxylic acid or maleic acid as the cross-linking additive which is to be applied via various means to pulp sheets with intended outcome of an esterification reaction with the cellulosic pulp fibres to improve wet strength in the use of hygiene products. The above-mentioned prior art focuses on improving the absorbency and wet resili- ency properties of cellulosic pulp sheet materials and do as such solve a problem opposite the one of the present invention, which aims for a compact high-durability product, mainly as an alternative to conventional single use plastic products. Description of the Invention The novel invention relates to the use of a cellulosic material for the preparation of bio-based high durability cardboard products. The sheet pulp material, herein refer- ring to all kinds of pulp sheets, which can be of a determined size or continuous sheets. Preferably the reaction is carried out on sheets, bales, or rolls of industrially availa- — ble market pulp without any other pre-processing steps. Cellulose fibres or pulp of different origin are suitable for the process. The esterification process is carried out by use of a base chemical solution contain- ing a reactive cross-linking acid, preferably a tricarboxylic acid, and a water-soluble polyol containing multiple hydroxyl groups. The ratio of the tricarboxylic acid to pol- — yol and the solid's ratio to the base solvent are varied depending on the end appli- cation and the desired properties of the cardboard material. The enhanced strength properties and moisture and liquid resistance of the end product in accordance to the present invention is achieved by a final curing step. — The hydrophobic properties of the material can be further enhanced by the addition

N < 25 of a novel functional hydrophobation emulsion. ? The emulsion comprises organic and commercially available substances with hydro-

LO N phobic functionality mainly derived from essential methylene groups forming a non-

I & polar moiety of the molecule. The emulsion is formed in a base solvent such as wa- ter or an organic solvent with similar functionality, such as alcohols. A non-ionic sur-

O 0 30 factant may be added as an emulsifying agent.

N

O N The cross-linking formulation and the optional hydrophobation emulsion are synthe- sised separately at temperatures that enables formation of a liguid formulation,

often a temperature of around 60° and higher is necessary.

The obtained reaction formulations are applied to the pulp material to be treated either as a blend or as separate formulations using methods known in the art, such as by submersion, spraying or impregnation.

The uptake of the reaction formulation can be enhanced by use of microwave treatment.

Any excess formulation is extracted and may be re- used in the process.

The hydrophobation emulsion can be applied during the treat- ment step with the cross-linking reagent, prior to the final curing step, or as a com- bination of any of these.

In a preferred embodiment, the chemically treated pulp material is in the form of — sheets or rolls, whereby the sheets may be applied on top of one another, optionally in a cross-layer formation, to increase the structural properties and dimensional sta- bility of the final cardboard product.

The chemically treated cardboard material is preferably formed and cut into a de- sired shape before final curing of the end product.

Preferable end products are dif- ferent packing materials, such as boxes and inserts shaped to hold a product in place.

Due to the non-toxic characteristics of the reagents and raw material, the material is well suited for, but not limited to, end-use in the food industry.

Especially preferable end products are eating utensils, such as cutlery.

Besides single use forks, knives and spoons, also other tableware, such as plates, cups and bowls are further examples of cardboard products of the invention.

The cardboard material can also be pre-cut into foldable boxes or wrappings that can be used for take-away meals, or transport boxes or the like.

Summary of the Invention N 25 The object of the invention is a method for manufacturing of a bio-based high-dura- = bility cardboard product according to the characterising part of claim 1, wherein a 7 cross-linking formulation comprising at least one cross-linking acid and at least one - polyol is applied to a pulp sheet material, the pulp sheet material treated with the E cross-linking formulation is pre-dried, a single sheet or combined multiple sheets of & 30 the pre-dried pulp sheet material treated with the cross-linking formulation is pre- = heated, at least one cardboard product is shaped and cut out of the pre-heated pulp N sheet material, and said at least one cardboard product obtained is subjected to a final curing step.

Preferably the cross-linking acid and polyol is selected from a range of agents approved in the food and pharmaceutical industries, preferably a tricarboxylic acid selected from 1-hydroxypropane-1,2,3-tricarboxylic acid, propane- 1,2,3-tricarboxylic acid, 2-hydroxynonadecane-1,2,3-tricarboxylic acid, 2-hydroxy- propane-1,2,3-tricarboxylic acid, benzene-1,3,5-tricarboxylic acid and prop-1-ene- 5 1,2,3-tricarboxylic acid and a polyol selected from but not limited to xylitol, sorbitol and erythritol.

In another preferred embodiment, an optional functional hydrophobation emulsion is applied to the pulp sheet material as a mix with the cross-linking formulation, subsequent to treatment with the cross-linking formulation and/or on the formed product of the pulp sheet material treated with the cross-linking formulation prior to curing of the cardboard product. The cross-linking formulation, the hydrophobation emulsion or the mix thereof may be applied to the pulp sheet material in a pressur- ised environment through impregnation as a mist in semi-gaseous or atomised form. Alternatively, the cross-linking formulation, the hydrophobation emulsion, or the combination thereof is applied to the pulp sheet material in liguid form, such as by submersion, spray or curtain coating methods, and any excess solution removed in the process is optionally re-used. In a further preferred embodiment the cross- linking formulation, the hydrophobation emulsion, or the combination thereof is ap- plied to the pulp sheet material under microwave treatment, whereby the pulp sheet material and formulation are combined within a microwave device. Preferably the cross-linking formulation, the hydrophobation emulsion, or the combination thereof is applied to the pulp sheet material at a temperature from 80°C to 100°C.

_ 25 In a further preferred embodiment, at least two pulp sheets are arranged on top of O each other prior to shaping, cutting and curing, optionally in a cross-layer orienta- 5 tion, preferably in a 90° angle with respect to the predominant fibre direction of the O adjacent layer. The shaping and/or cutting of the product may take place at temper- I atures ranging from 120-170°C, preferably from 140-170°C. The final curing of the > 30 product may take place at a temperature ranging from 150°C to 200°C.

N A further object of the invention in that the cardboard product is a bio-based high N durability cardboard product according to the characterising part of claim 11. The product is formed from at least one layer of a cellulosic pulp sheet material, wherein the pulp sheet material comprises moieties of at least one polyol and at least one organic cross-linking acid being at least partially cross-linked to the cellulose struc- ture of the pulp sheet through ester bonds. In a further embodiment of the inven- tion, the cardboard product additionally comprises at least one hydrophobic agent.

Said at least one cross-linking agent, optionally together with a hydrophobic agent, is preferably present in the cardboard product in a total amount of at least 5 wt-% based on the total weight of the product. The cardboard product may be formed out of a cross-linked pulp sheet or multiple combined sheets consolidated in to one piece, whereby the main surfaces of the cardboard product are flat or curved sur- faces of essentially even thickness, optionally with a three-dimensional structure for improved product strength or other functionality. In a further preferred embodiment of the invention the product is suitable for containing or handling food, such as eat- ing utensils, boxes and wrappings used during transportation of meals, preferably the product is single use cutlery and tableware.

Drawings The invention is hereinafter described in detail with reference to the following draw- ings, wherein: Figure 1 is a flowchart presenting the general steps of the process of the inven- tion. Figure 2 presents the weight percentage gain WPG (%) of different cardboard products and reference materials described in Examples 2 and 3. — 25 O Figure 3 presents spectra obtained by Fourier transform infrared (FTIR) spec- 5 troscopy for the paper surface of a commercial paper cup used as ref- O erence (uncoated surface) and a pulp sheet material of the invention, I the specific parameters of which is presented in Example 4. * 30 & Definitions

LO N Pulp sheet material is within this application referring to a an essentially flat, sheet- N like pulp material. The pulp sheet material may be in the form of sheets of a deter- mined size. It may also refer to long sheets of paper pulp, possibly continuous sheets that may be rolled or folded for easier handling. The pulp sheet material may be manufactured from any kind of pulp known in the art, and any combination of pulp of different origin.

—Bio-based is herein to be understood as a material or a compound that is obtainable from a natural source or any combination of such materials or compounds. Herein the term bio-based also includes synthetically produced equivalents to such com- pounds and mixes consisting essentially of such compounds. The term bio-based also refers to any unprocessed or processed renewable material, especially plant- based materials. The term recyclable herein refers to a product being recyclable together with con- ventional products produced from the same base raw material, namely pulp. There is no need to separate binding or functional agents prior to recycling as these are chosen from a range of agents that can be fully blended into the recycled material without significant negative effects, such as increased toxicity, formation of harmful components, formation of lumps, such as from plastic films, etc. The term reaction formulation herein refers to the cross-linking formulation, ie. the base formulation, the hydrophobation emulsion, i.e. the functional emulsion, or a combination of these. The reaction formulation is water-based or prepared in an or- ganic solvent with similar functionality. The term cross-linking agent and hydrophobic agent herein refers to the active in- _ 25 —gredient of the reaction formulation. The total solids content of cross-linking agent O and or hydrophobic agent in the final product comprises both reacted moieties of 5 the agents and unreacted agents in solid state.

O I Detailed Description of the Invention > 30 The pulp sheet material used as raw material in the present invention is preferably & an industrially available market pulp in the form of a sheet either cut to determined = size or in the form of a continuous sheet that may be roll or folded for easier han- N dling of the material. No other pre-processing steps, such as hammer milling, sepa- rating, de-fibrillating or refining or air lay mat forming, are required. A broad variety of pulp types are suitable for the process including but not limited to thermo-me- chanical, chemi-thermomechanical pulp (CTMP), softwood and hardwood kraft pulps, dissolving pulp, recycled pulp, pulp derived from alternative agricultural ligno- cellulosic fibres such as hemp, flax, bagasse, palm, rice stems and the likes.

The pulp sheet material, preferably in the form of bales or rolls of pulp sheet are fed into the treatment and manufacturing line. (1) For the esterification process, a base chemical solution is used. This base formulation contains at least one reactive or- ganic cross-linking acid where one or more of the hydrogen atoms have been re- placed by a carboxyl group and preferably containing at least three carboxyl groups.

Preferable is use of an acid well known and approved in the food and pharmaceuti- cal industries such as, but not limited to, 1-hydroxypropane-1,2,3-tricarboxylic acid, propane-1,2,3-tricarboxylic acid, 2-hydroxynonadecane-1,2,3 tricarboxylic acid, 2- hydroxypropane-1,2,3-tricarboxylic acid, benzene-1,3,5-tricarboxylic acid and prop- 1-ene-1,2,3-tricarboxylic acid. Additionally, the chemical solution contains a water- — soluble polyol which contains multiple hydroxyl groups. The polyol is preferably se- lected from a range of polyols obtainable from natural sources, preferably widely used and approved in the food industry such as, but not limited to, xylitol, sorbitol and erythritol. These primary components are synthesised in a base of water or similar functional organic solvent in a variety of formulated ratios between 1:1 to — 5:1 cross-linking acid to polyol, in one preferred embodiment the cross-linking acid to polyol ratio is 3:1. The solid's ratio to the base formulation is preferably between 5 and 50% by weight depending on the end application and desired properties of stiffness, bending strength and moisture resistance. The formulation is prepared at a temperature where the cross-linking acid and polyol are dissolved under stirring in = 25 — the solvent used. A temperature range of 60—119°C is preferred. The base formula- N tion thus obtained is herein referred to as the cross-linking formulation.

7 The enhanced strength properties and moisture and liguid resistance of the end - product of the present invention is achieved by a curing process that may be per- E formed using a variety of technigues. The strength and hydrophobicity properties of & 30 the final product can be increased by performing a densifying step prior to or in = combination with the final curing step (6).

N In order to increase the hydrophobic properties of the final product, a hydrophoba- tion emulsion may be added (2, 6). The hydrophobation emulsion comprises organic and commercially available substances with hydrophobic properties as hydrophobic agents, the primary constituents of which include at least one substance selected from fatty acid esters, fatty alcohols, other hydrophobic organic acids and hydrocar- bons or additional or alternative functional substances selected from a range of pen- — tacyclic triterpenoids such as, but not limited to oleanolic acid, betulin and betulinic acid.

Such hydrophobic agents may be derived from natural oils and waxes, in one preferred embodiment the hydrophobic agent is carnauba wax.

The hydrophobation emulsion is formed in a base solvent, possibly in combination with a non-ionic sur- factant commonly used in the art for oil in water emulsions.

The base solvent can be water or an organic solvent with similar functionality, such as ethanol.

The cross- linking formulation and functional emulsion are synthesised separately at tempera- tures enabling the formation of the formulation and the emulsion.

The cross-linking formulation may be prepared at a lower temperature, such as from 10°C up to a pre-reaction temperature of the esterification process, often below 120°C.

The aim is to apply the solution to the cellulosic pulp material in a form where the esterifica- tion process of the cross-linking solution is not yet initiated, thus enabling formation of cross-linking between the cellulosic structure, whereby the amount of available hydroxyl groups is reduced within the substrate.

The hydrophobation emulsion usu- ally requires a temperature where the hydrophobic agent is in liquid form, for most waxes the temperature should be above 60°C.

Preferable temperatures for the preparation of the reaction formulations ranges from about 60°C to 119°C, even more preferably from 80°C to 100°C.

The duration of this preparation step is often around 1 hour or more.

As noted above, special care should be taken not to rise the temperature to a temperature initiating a rapid esterification reaction.

The cross- _ 25 linking formulation and the functional emulsion may be blended upon completion of O the independent synthesis steps within the same or a similar temperature range.

Al- 5 ternatively, the functional emulsion may be applied separately to the pulp sheet ma- O terial treated with the cross-linking formulation.

The ratio of the solids content of I the functional emulsion to the solids content of the cross-linking formulation is pref- > 30 erably from about 0.1% to 15% in the mixed formulation.

Preferably, the surfactant & ratio of the functional emulsion ranges between 0.1% and 50% by weight of the 5 solids content of the emulsion.

N Upon final synthesising of the reaction formulations, i.e. the base formulation, the functional emulsion or the mixed formulation, the pulp sheet material is exposed to the combined or separate reaction formulation via a range of alternative methods known in the art, including submersion, spraying or impregnation in a pressurised environment or any combination of these.

The reaction formulation is preferably ap- plied to the pulp sheet material at a temperature ranging from 10°C up to a temper- ature still not initiating an esterification reaction within the formulation, preferably the temperature is above 60°C, more preferably from 80°C to 119°C, and even more preferably from 80°C to 100°C.

Most preferred is a temperature from about 90°C to 95°C.

For the mixed formulation and the hydrophobation solution a temper- ature above 60°C is often necessary to achieve good results.

Herein, the term reac- — tion formulation refers to the cross-linking formulation, the functional emulsion or the mix of these two prepared as described above.

In one embodiment, bales or rolls of pulp sheet are impregnated (2a) with the mix of the cross-linking formulation and the functional emulsion, only the cross-linking formulation, or the functional emulsion subsequent to a treatment of the pulp sheet — material with the cross-linking formulation.

Preferably the cross-linking formulation or the mix of the cross-linking formulation and the hydrophobation emulsion is applied throughout the material to be treated.

This enables the cross-linking reaction to take place through the whole cross-section of the pulp-sheet material, thus giving maximum stability and ensuring good interfa- cial properties when combining multiple sheet layers.

In applications where higher flexibility of the cardboard is needed, the cross-linking agent may be applied only on the surface of the pulp sheet material or by using a cross-linking formulation having a lower total cross-linking agent concentration. = When the hydrophobation emulsion is applied separately, this may be added only < 25 onto the surface, such as by spray coating after formation and cutting of the prod- 5 uct shape.

The surface may be treated partially or entirely, and it is also possible to O apply the functional emulsion only on one surface of the pulp sheet.

The hydropho- Ek bation emulsion may be added during the initial treatment step and/or prior to cur- N ing of the end product.

An increase in contact angle of the cardboard material is 3 30 achieved through this additional step, which is beneficial in applications where the N cardboard product is exposed to moisture for a prolonged time or when water repel- N lent properties are needed.

In one embodiment where the pulp sheet material is impregnated (2a) with at least one of the reaction formulations, the process may be carried out at an internal pres- sure between 2-10 bar and a spray release of the liquid reaction formulation at a temperature above 80°C to a pressure chamber to create a mist-based impregna- tion with gradual release of pressure to atmospheric pressure. In a further embodi- ment utilising the impregnation approach, the reaction formulation may be formed into a very fine mist by use of an atomisation technique, whereby the separate or combined formulation is introduced into the vacuum chamber at high velocity through suitably fine nozzles which cause the fine atomisation to a mist as it enters the chamber which enhances the absorption of the formulation into the pulp sheets, bales or rolls. In another embodiment, at least one reaction formulation is added in liquid form. (2b) Individual sheets may for example be submersed in the reaction solution. The individual sheets or a continuous sheet material can be conveyed through a bath containing the reaction formulation, preferably at a temperature above 60°C and up to the pre-reaction temperature of the esterification reaction, even more preferably above 80°C and below 120°C, even more preferably below 100°C. The residence time depends on the thickness of the material, often 10-30 s is sufficient. The up- take of the solution may be enhanced by a longer residence time, such as 1 minute or more. Excess solution is then removed from the pulp sheet material, for example by conveying it into a mangle, which may be mildly heated having a temperature around 60°C, for dispersion and homogenisation. Excess liquid may alternatively be removed by use of vacuum or other techniques known in the art. At least one reac- tion formulation may also be applied to the pulp sheet material in liquid form as a N 25 spray or by a curtain coating process.

O = Use of microwave treatment at the point of combining the solution with the pulp 7 material has been found to significantly enhance the solution uptake both in the ini- - tial treatment stage and also in the retention of the solids post drying. Upon reach- E ing the desired weight percentage gain (WPG), the pulp sheets or pulp rolls are re- & 30 moved and, if necessary, excess solution is extracted by use of, for example, a = mangle or via vacuum. This excess solution may be recycled for re-use. The tar- N geted solids content to be retained in the pulp sheets or rolls will be determined based on the final application and controlled with residence time, temperature and possibly through regulated pressure and varying solids ratio within the solution.

For most applications, a WPG of 100-160% in wet form is targeted after application of the combined or separate reaction formulations.

By optimising the parameters, a WPG of 200-300% may be achieved in wet form, thus resulting in a solids content increase of around 75% in the final product.

The application of the cross-linking and optional hydrophobic agents in a water solu- tion, or similar functionality solvent, has been found beneficial with respect to the interaction between the substrate and the cross-linking agent.

The cardboard prod- uct obtained shows a very good dimensional stability and reduced water uptake af- ter curing even at relatively low solid content of the cross-linking agent and optional hydrophobic agent in the final product, such as around 20-30 wt-% presented in the examples.

The pulp-sheet material should be pre-dried (3) after application of the at least one reaction formulation.

This may be carried out by removal of any excess liquid, for example by use of vacuum or a mangle, as mentioned above, and/or by evaporation of the solvent.

When the solvent is evaporated by the use of heat, the temperature range is preferably 50-104°C.

In one preferred embodiment the temperature is raised gradually, thus removing moisture slowly and simultaneously pre-heating the treated pulp sheet material prior to the curing process.

Preferably, the moisture content is reduced to below 10% during the pre-drying step, more preferably lower than 5%. A higher moisture content will be likely to cause deformation of the sur- face during the forming and curing step or partial separation of combined layers.

The pre-drying may for example be carried out by transferring chemically treated — pulp sheet material to a pre-heated oven, preferably at a temperature ranging from O 25 50-104%, even more preferred is a temperature of 80—100°C, where the pulp 5 sheet material is dried to a moisture content which is close to ambient with the in- 0 door climate of the production plant.

The equilibrium moisture content (EMC) in a Ek relative humidity (RH) of 55-60% is estimated to be 7-9%. The targeted dry solids N content of the cross-linking agents, optionally together with a hydrophobic agent, 3 30 remaining in the pulp sheets or rolls after drying is determined based on the end N application, preferably being at least 5% wt-% based on the total weight of the N product.

Often a total solids content of between 10 wt-% and 50 wt-% is targeted.

In some embodiments, a solids content above 50 wt-% is preferred.

The pulp sheet material, often in the form of bales of sheets or as rolls can be stored in the mid process to be transferred to the forming line or transported to an- other production unit. When no storage is needed, for example in production lines using a continuous process and a single sheet or roll material, the temperature may be raised further in the pre-drying step in order to pre-heat and eliminate absorbed humidity from the material before the forming step.

The individual pulp sheets or the continuous unrolled pulp sheet are conveyed to the consolidation and forming of the product. For high-strength end products at least two sheets are laid on top of each other, preferably 2-5 sheets. In these appli- cations, simultaneous treatment of more than one bale or roll of pulp sheet material may be beneficial. The individual layers of pulp sheet material may optionally be ori- ented in a cross-layer formation whereby the predominantly single direction ori- ented fibres of one sheet are turned in a different direction in the next layer, prefer- ably in a 90° angle, to build a cross layer structure before entering the consolidation and forming process. In an especially preferred embodiment, three layers of treated pulp sheets are stacked with the middle layer oriented at a 90° angle to the outer layers fibre direction prior to further processing. The primary objective of the cross layering of the fibre directions is to further increase the structural properties and di- mensional stability of the final cardboard products.

The treated pulp-sheet material is then pre-heated (4) to further decrease the mois- ture content of the material and evacuate any absorbed humidity from the mid stor- age as well as to raise the temperature to a pre-reaction level ready for consolida- tion and forming (5) of the final products. This pre-heating process may be applied to the pulp sheet or sheet layups by means of infra-red radiation, high-frequency, N 25 — microwave or conventional hot air heating technologies. It may be carried out rap- = idly to a temperature of about 90-110°C after which the sheets are subjected to an 7 optional consolidation process followed by forming and cutting (5) of the product - and a final curing processes (6). The step of pre-heating and the step of forming E and/or cutting of the final product may be carried out simultaneously. Preferable is & 30 a continuous process, such as the one described in the patent "Method for manufac- = turing products made from fibre material, and disposable products made by this N method” with the application number PCT/FI2020/050511.

In a further preferred embodiment, and an advancement of the aforementioned pa- tent, the pulp sheet material will be consolidated under pressure.

The pressure used may range from, for example, 300 kN to 1500 kN.

This further enhances the strength and hydrophobicity properties of the product.

Preferably a partial cross-

linking chemical reaction between the reaction formulation and the hydroxyl groups of the cellulosic pulp fibre is initiated by temperatures ranging between 120°C to 170°C.

The heating may be carried out separately or simultaneously with the above optional consolidation step.

When the optional hydrophobation solution is added, a pressing temperature below the melting temperature of the hydrophobic agent is preferred to avoid this to leach out of the material prior to curing.

Simultaneous heating and consolidation of the pulp sheet material treated with the at least one reaction formulation may be carried out by transferring a single sheet or a multiple sheet layup, preferably as an unrolled continuous sheet, to a heated roller or plate press to densify and/or consolidate said sheet or sheet layup by means of heated compression and densification plates or rollers, preferably at a temperature of about 120°C to 140°C.

This step can possibly include some basic pre-shaping and pre-activation of the chemical reactions.

After this, the densified single or multiple consolidated sheets are transferred to a forming station, such as a heated roll die press or plate press.

The temperature is at this stage preferably — raised to around 140-170°C for pre-esterification.

The final form shaping (8) may be carried out using a pressure of at least 300-1500 kN.

The duration of the pres- sure and heat treatment is often 5-10 s, whereby only partial esterification is initi- ated and the cardboard material may be easily formed into desired shape.

The final product may be formed using any known technology suitable for the material, such = 25 — as hydraulic mould press, roll forming, and cutting with such technologies as stamp- N ing, rotary embossing and cutting or laser cutting. 7 The product is formed in one piece out of a cross-linked pulp sheet of essentially - even thickness, whereby the main surfaces of the cardboard product are flat or E curved surfaces of even, optionally with a three-dimensional structure for improved & 30 product strength or other functionality.

For example, in the production of single use = cutlery, the gripping surface and edges that are not used for cutting or holding food are often folded or rounded for increased comfort and strength of the product.

Simi- larly, for many packing materials it is preferable to form the material into a shape that protects an item from breaking and holding it in place. Connecting pieces and shapes may also be formed allowing at least two cardboard products to be attached to each other or to be placed in connection to each other. The cellulosic pulp sheet material is finally cured through an esterification process taking place between the at least two carboxyl groups of the cross-linking acid and the hydroxyl groups of the cellulose as well as the polyol. This reduces the amount of available hydroxyl groups of the cellulose within the substrate and forms a cross- linked structure providing improved dimensional stability within the material and en- hanced durability and hydrophobicity properties. This final reactive curing step (6) is applied once the articles have been formed and/or cut out to the final shapes as de- scribed (5).

The final curing step is preferably carried out at temperatures ranging between 150°C and 200°C, more preferably between 170°C and 190°C, for a required time to complete the chemical cross-linking reaction and fixate any additional functional additives. The duration depends on the size and thickness of the product and also the heating medium used. A variety of heating mediums known in the art, such as infrared radiation (IR), high frequency or heated fan ovens, may be used for the curing step. For a single layer cardboard product or a product consisting of less than four layers, a curing time of between 10 and 30 min is often sufficient. In a pre- ferred embodiment, a curing time of 1 minutes is applied for a single or double layer product at a temperature between 170°C and 190°C. Any unreacted offcuts from the cutting process can be diverted before the final curing step and further pro- cessed and formed into alternative products and finally reacted under the same final curing step.

S 25 The final durable cardboard products thus obtained can then be carried forward to a 5 packing station for bagging and labelling. N The cardboard product of the present invention is formed from at least one layer of E a cellulosic pulp sheet material, wherein the pulp sheet material comprises moieties 2 of at least one polyol and at least one organic cross-linking acid being at least par- D 30 tially cross-linked to the cellulose structure of the pulp sheet through ester bonds. O Additionally, the cardboard product may comprise at least one optional hydrophobic agent. Said at least one cross-linking agent, optionally together with a hydrophobic agent, may be present in the cardboard product in a total amount of at least 5 wt- % based on the total weight of the product. The product may be a single sheet cardboard material or combined multiple sheet cardboard material and any product produced thereof. The pulp sheet material treated may be chosen from a variety of thicknesses. Consequently, the cardboard product may be a very rigid, thick cardboard, possibly of combined pulp sheets, as well as a paper like, very thin single sheet cardboard and anything between these. The cardboard product may be formed in one piece out of a cross-linked pulp sheet or multiple combined sheets consolidated in to one piece, whereby the main sur- faces of the cardboard product are flat or curved surfaces of essentially even thick- ness, optionally with a three-dimensional structure for improved product strength or other functionality. In one preferred embodiment, the cardboard product of the present invention is suitable for containing or handling food. Examples of such prod- ucts are eating utensils, boxes and wrappings used during transportation of meals, preferably the product is single use cutlery and tableware.The resulting products obtained by the process as described above will have significantly enhanced strength properties, improved moisture and liquid resistance, and additionally im- proved anti-microbial properties when compared to conventional cardboard prod- — ucts. Furthermore, the cardboard product thus obtained is fully recyclable and re- pulpable with existing cardboard based materials and products. Examples In the following examples different cardboard pieces were prepared and tested. The N 25 — parameters used in the tests, such as reaction times and pressures, may also be ap- = plied to pulp sheet material treated with other reaction formulations presented in 7 the description of the invention. a 3 Example 1: Strength properties for single layer cardboard

O 5 30 Asingle ply pulp sheet was treated with a water based cross-linking formulation N comprising citric acid and sorbitol in a ratio of 3:1 and having a total solids content of 18%. After a pre-drying process, the sheet was pressed at 700kN for 10 seconds at a press temperature of 170°C after which sheet was cured at 180°C for 15 minutes. Several strength tests were performed on the obtained treated pulp sheet having the dimension of 25 mm x 135 mm. A single ply pulp sheet of the same kind was used as reference. No cross-linking formulation was applied to the reference sheet. A similar pressure treatment at 700 kN for 10 seconds and at a temperature of 170°C was performed for the reference. The results are presented in table 1, show- ing a WPG of 27% corresponding to a solid content of around 21 wt-%, a signifi- cantly improved bending resistance at different degrees, as well as improved bend- ing stiffness and Taber stiffness compared to the reference material. Table 1: Strength properties for the cardboard product of Example 1 Final durable card- Measurement board sheet ww ff Bending resistance 5° 1195 Bending resistance 7.5° 1801 Bending resistance 15° 1276 3554 N Bending resistance 30° 1722 5350

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Example 2: WPG after water soaking test Two different test sets of single ply pulp sheet were prepared using slightly different concentration of the cross-linking formulation. Two test pieces of cardboard material were prepared from each solution and the effect of pressure treatment on the water uptake was studied. Four reference pieces were prepared using the same heat and pressure treatment without the addition of the cross-linking formulation. All card- board pieces had a dimension of 20 mm x 100 mm and were soaked in water for 5 minutes at 23°C. Test Set 1: Single ply pulp sheet treated with a solution of citric acid and sorbitol in a ratio of 3:1 and having a solids content of 15%. No pressure was applied to test piece 1a prior to curing at 180°C for 15 min. Test piece 1b was pressed at 700kN and a temperature of 170°C prior to curing at 180°C for 15 min. Test Set 2: Single ply pulp sheet treated with a solution of citric acid and sorbitol in a ratio of 3:1 and having a solids content of 17.5%. No pressure was applied to test — piece 2a prior to curing at 180°C for 15 min. Test piece 2b was pressed at 700kN and a temperature of 170°C prior to curing at 180°C for 15 min. The reference materials were prepared as follows. Ref 1a was not pressed and not cured. Ref 2a was not pressed but cured at 180°C for 15 min. Ref 1b was pressed at 700kN at 170°C and not cured. Ref 2b was pressed at 700kN at 170°C and cured at 180°C for 15 min. The results from the soaking test are presented in figure 2 and indicates that the water uptake of the material is significantly decreased by the treatment with the — cross-linking formulation followed by the curing step. The water uptake was further O decreased by pressure treatment prior to curing. 5 25

N - Example 3:

T N A hydrophobation emulsion was prepared by emulsifying carnauba wax in an 3 amount of 6 wt-% in water at a temperature ranging from about 95°C to 100°C. N Cremophor& RH 40 from BASF was used as surfactant. A cross-linking formulation N 30 was prepared at a similar temperature by dissolving citric acid and sorbitol in a 3:1 ratio in water to a total solids content of 20%. The functional emulsion and the cross-linking formulation were mixed at a similar temperature to form a uniform for- mulation and a pulp sheet material of 20 mm x 100 mm was soaked in the mixed formulation at a temperature of from about 95°C to 100°C for 1 min. The pulp sheet material was dried at 100°C for 1 hour leading to an average WPG of 75%. The final curing was performed at 180°C for 15 min. The hydrophobation treatment significantly increased the wetting contact angle of the material and the moisture resistance. After a 5 min water soaking test the WPG of the hydrophobic cardboard piece was 4.4%, calculated as an average value for five test pieces. The test result is presented in the diagram of figure 2 as Test set 3.

Example 4: FTIR analysis of single layer cardboard A single ply pulp sheet was treated with a cross-linking formulation prepared ac- cording to the description of the invention containing citric acid and sorbitol in a 3:1 ratio and having a total solids content of 22%. After pre-drying of the pulp material, a pressure of 700kN was applied for 10 seconds at a press temperature of 170°C. The cardboard thus obtained was cured at 180°C for 15 minutes. A FTIR-analysis was performed on the material obtained (SET4) and the spectrum obtained was compared to a reference spectrum (Cup board). A commercial paper cup was used as reference material and the analysis was performed on the un- coated side of the paper cup. The results are presented in Figure 3. The FTIR-analysis confirms the presence of carbonyl groups within the structure of the cardboard product of the present invention, thus indicating the presence of es- = ter bonds within the material and a cross-linked structure.

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Claims

Claims

1. Method for manufacturing of a bio-based high-durability cardboard product, characterised in that: -a cross-linking formulation comprising at least one cross-linking acid and at least one polyol is applied to a pulp sheet material, - the pulp sheet material treated with the cross-linking formulation is pre-dried, - a single sheet or combined multiple sheets of the pre-dried pulp sheet material treated with the cross-linking formulation is pre-heated, - at least one cardboard product is shaped and cut out of the pre-heated pulp sheet material, and - at least one cardboard product obtained is subjected to a final curing step.

2. Method according to claim 1, characterised in that the cross-linking acid and polyol is selected from a range of agents approved in the food and pharmaceutical industries, preferably a tricarboxylic acid selected from 1-hydroxypropane-1,2,3-tri- carboxylic acid, propane-1,2,3-tricarboxylic acid, 2-hydroxynonadecane-1,2,3-tricar- boxylic acid, 2-hydroxypropane-1,2,3-tricarboxylic acid, benzene-1,3,5-tricarboxylic acid and prop-1-ene-1,2,3-tricarboxylic acid and a polyol preferably selected from but not limited to xylitol, sorbitol and erythritol.

3. Method according to claim 1 or 2, characterised in that an optional functional hydrophobation emulsion is applied to the pulp sheet material as a mix with the cross-linking formulation, subsequent to treatment with the cross-linking formula- _ 25 — tion and/or on the formed product of the pulp sheet material treated with the cross- O linking formulation prior to curing of the cardboard product. 5 O 4. Method according to any of the preceding claims, characterised in that the I cross-linking formulation, the hydrophobation emulsion or the mix thereof is applied > 30 to the pulp sheet material in a pressurised environment through impregnation as a & mist in semi-gaseous or atomised form. N 5. Method according to any one of claims 1-3, characterised in that the cross- linking formulation, the hydrophobation emulsion, or the combination thereof is applied to the pulp sheet material in liquid form, such as by submersion, spray or curtain coating methods, and any excess solution removed in the process is option- ally re-used.

6. Method according to any of the previous claims, characterised in that the cross-linking formulation, the hydrophobation emulsion, or the combination thereof is applied to the pulp sheet material under microwave treatment.

7. Method according to any of the previous claims, characterised in that the cross-linking formulation, the hydrophobation emulsion, or the combination thereof is applied to the pulp sheet material at a temperature from 80°C to 100°C.

8. Method according to any of the previous claims, characterised in that at least two pulp sheets are arranged on top of each other prior to shaping, cutting and cur- ing, optionally in a in a cross-layer orientation, preferably in a 90° angle with re- spect to the predominant fibre direction of the adjacent layer.

9. Method according to any of the previous claims, characterised in that the shap- ing and/or cutting of the product takes place at a temperature ranging from 120— 170°C, preferably from 140-170°C.

10. Method according to any of the previous claims, characterised that the final curing of the product takes place at a temperature ranging from 150°C to 200°C. _ 25 11. Bio-based high durability cardboard product, characterised in that the card- O board product is formed from at least one layer of a cellulosic pulp sheet material, 5 wherein the pulp sheet material comprises moieties of at least one polyol and at 0 least one organic cross-linking acid being at least partially cross-linked to the cellu- I lose structure of the pulp sheet through ester bonds. = 0) 2 12. Bio-based high durability cardboard product according to claim 11, character-

LO N ised in that the cardboard product additionally comprises at least one hydrophobic

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13. Bio-based high durability cardboard product according to claim 11 or 12, char- acterised in that said at least one cross-linking agent, optionally together with a hydrophobic agent, is present in the cardboard product in a total amount of at least wt-% based on the total weight of the product. 5

14. Bio-based high durability cardboard product according to any one of claims 11— 13, characterised in that the cardboard product is formed in one piece out of a cross-linked pulp sheet or multiple combined sheets consolidated in to one piece, whereby the main surfaces of the cardboard product are flat or curved surfaces of essentially even thickness, optionally with a three-dimensional structure for im- proved product strength or other functionality.

15. Bio-based high durability cardboard product according to any one of claims 11— 14, characterised in that the product is suitable for containing or handling food, — such as eating utensils, boxes and wrappings used during transportation of meals, preferably the product is single use cutlery and tableware.

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