METHOD FOR PRODUCING A CELLULOSE PRODUCT HAVING LIPID RESISTANCE TECHNICAL FIELD A method is provided for producing a cellulose-based material having lipid resistance. BACKGROUND Oleophobic barrier function (i.e. lipid resistance) is an important property in many paper, paperboard or containerboard applications, especially for products intended for food contact. Some examples include cellulose- based trays, containers and cups for hot, cold, dry, wet and frozen food and beverages. A known method for obtaining greaseproof food packages is to treat the material with fluorinated chemicals, i.e. poly- and perfluoroalkyl substances (PFAS). Products treated this way are often used e.g. as fast food containers, wrappers, plates and bags. However, PFAS present in such materials risk to migrate into the edible content of the package, ending up being metabolised and resulting in the presence of breakdown products in the human body. Exposure to perfluorinated compounds has been shown to be associated with negative health effects, such as increased risk of cancer. Extensive use of PFAS also lead to environmental pollution, leading to bioaccumulation of these toxic compounds. Alternative ways of providing eco-friendly grease barriers have been suggested, e.g. in WO2018/045248 which describes a method for biobased derivatization of cellulose substrates by means of binding saccharide fatty acid esters to cellulose fibers whereby barrier functions may be achieved. However, there is still a need for improved methods of obtaining oleophobic barrier function in cellulose-based materials, that enables production of non-toxic and preferably biodegradable products for food contact applications. SUMMARY It is an object of the present disclosure to provide an improved method of producing a cellulose-based material for lipid resistance, which eliminates or alleviates at least some of the disadvantages of the prior art methods. According to the present invention, the object is obtained by means of a method for producing a cellulose-based product having lipid resistance, said method comprising the steps of: -providing an aqueous mixture comprising cellulose pulp, said mixture having a solid content between 0.1– 50%; -adding a saccharide fatty acid ester (SFAE) to the mixture in an amount between 0.5 – 10 wt% based on the total mixture fiber content; adding a retention aid chemical to said mixture arranged to improve retention of said SFAE to the cellulose fibers, and -producing a cellulose-based product from said mixture, wherein the electrostatic charge balance of the mixture after adding SFAE, used for producing the cellulose-based end product is adjusted to be on the anionic side. The mixture comprising a blend of at least cellulose pulp, SFAE and retention aid chemical is then used for producing a cellulose-based product with functional barrier against oil and grease, i.e. a material having oleophobic properties. A cellulose-based product made according to the method of the invention is at the same time free from harmful additives such as PFAS; it is based on a renewable natural resource and it is biodegradable. These are advantageous properties in particular for cellulose products intended for food contact which are often disposable, i.e. they are used only once before they are discarded. A single-use product made according to the invention can be recycled for re-use of the cellulose fibers, or in case the product ends up in the nature, it will degrade without leaving any toxic substances that risk to accumulate in the ecosystem.

The invention also relates to a cellulose-based product having lipid resistance obtainable by means of the method described above, wherein said material comprises an oil Cobb30 value below 30 - 35 g/m ² .

According to one aspect of the invention, said product is a compostable/biodegradabe food packaging unit from a molded pulp material, wherein the packaging unit comprises a food receiving or carrying compartment.

By means of adjusting the electrostatic charge balance of the pulp mixture such that it is kept on the anionic side, retention of the SFAE onto the cellulose fibers present therein is significantly improved, leading to an improved functionality with regards to grease barrier property of the material produced from the mixture.

According to another aspect of the invention, the electrostatic charge of a filtrate sample from the pulp mixture is below 20 μeq/l as anionic charge (i.e. between 0 to -20 μeq/l), preferably below 15 μeq/l and more preferably below 10 μeq/l as anionic charge.

According to another aspect of the invention, the electrostatic charge of a filtrate sample from the pulp mixture is within the interval -4 μeq/l to -15 μeq/l (the sign indicating the anionic charge). According to another aspect of the invention, the electrostatic charge of the pulp mixture is adjusted to be on the anionic side by means of controlling the electrostatic charge balance between the SFAE and the retention aid chemical in the mixture. The electrostatic charge of the pulp mixture is measured indirectly by defecting the charge of a filtrate sample taken from the pulp mixture. As an example, electrostatic charge balance of a filtrate sample can be analysed with a Mütek PCD-05 Particle Charge Detector, which is widely used in such applications in the pulp and paper industry.

According to one aspect of the invention, the retention aid chemical is a cationic retention agent.

According to another aspect of the invention, the retention aid chemical is an anionic retention agent.

According to another aspect of the invention, the retention chemical is selected from the group comprising polyethylenimines, polyallyldimethylmethacrylates, polyamines and cationic polyacrylamide and combinations thereof.

According to another aspect of the invention, one or more substance/s is/are used to adjust the electrostatic charge balance, such as AKD, PAAE, cationic starch, and/or anionic substances such as silica, bentonite and/or anionic polyacrylamides.

According to one aspect of the invention, the saccharide fatty acid ester as a homogeneous dispersion is added to said mixture separate from adding of the retention chemical, i.e. adding of SFAE and retention chemical is performed in a stepwise order. This leads to that precipitation of SFAE is avoided before if is added to the mixture and a uniform distribution of SFAE and retention chemical is achieved in the mixture.

According to another aspect of the invention, the saccharide fatty acid ester is mixed with the retention chemical whereupon the resulting blend is added to said mixture.

According to another aspect of the invention, the saccharide is a disaccharide selected from the group comprising sucrose, raffinose, maltodextrose, galactose, a combination comprising glucose, a combination comprising fructose, a combination comprising maltose, lactose, a combination comprising mannose, a combination comprising erythrose, isomaltose, isomaltulose, trehalose, trehalulose, cellobiose, laminaribiose, chitobiose and combinations thereof.

According to another aspect of the invention, the pH in the mixture is 5 - 9, preferably 6 - 8. With too low pH (<5) or too high pH (>9) there is an increasing risk of decomposition of saccharide fatty acid ester due to hydrolysis reaction.

According to another aspect of the invention, the aqueous mixture has a solid content between 0.1- 40wt%. According to the invention, said "mixture" may be in the form of an intermediate formed fiber product, with a high aqueous content and a fiber content of 20 - 50wt%, preferably 30 - 40wt%, meaning that the fibers have been formed and pressed into an intermediate aqueous product whereupon said SFAE is added.

According to another aspect of the invention, the aqueous mixture has a solid content between 0.1- 6wf%, preferably between 0.1 - 2wt%. According to another aspect of the invention, the aqueous mixture has a solid content between 0.2 - 1.5wt%. A mixture having a low solid content may be referred to as a "slurry". The choice of solid content of pulp slurry is depending on various factors. While a low content is beneficial for providing good formation during wet forming, a high content is beneficial for products with high basis weight.

According to another aspect of the invention, the cellulose-based product is a three dimensional molded product, such as a molded food packaging product, made from cellulose fibers. According to this aspect, the method of the invention comprises the additional steps of;

- providing a forming tool having a three-dimensional shape comprising a forming portion, bringing said forming portion into contact with the aqueous mixture so that said forming portion is covered with a wet layer of pulp; and dewatering the layer of pulp on the forming tool at temperatures >100°C to a dry content of at least 70wt%, preferably at least 80%, to achieve the three dimensional molded structure. In one aspect, the layer of pulp present on said forming tool is dewatered by means of press-drying performed at temperatures >100°C, preferably at temperatures between 120-250°C or more preferably between 150- 220°C.

According to a further aspect of the invention, the fiber mixture used to produce the molded product consists of chemithermomechanical pulp (CTMP), thermomechanical pulp (TMP), chemical pulp or semichemical pulp, or a combination thereof.

Additional aspects of the technology are described in the following description text and claims. DETAILED DISCLOSURE

It is to be understood that the foregoing general summary and the following detailed disclosure are exemplary and exemplary only, and are not restrictive of any subject matter claimed. As known to the person skilled in the art, "cellulose" is an organic compound with formula (C ₆ H ₁₀ O ₅ ) _n , which is a polysaccharide consisting of a linear chain of numerous linked glucose units. Cellulose is the most abundant organic polymer on Earth and is obtained from e.g. plants, wood and cotton. As used herein, "cellulose-containing material" or "cellulose-based material" means a composition which consists essentially of cellulose. For example, such material may include, but is not limited to, paper, paper sheets, paperboard, paper pulp, molded fiber products and/or a carton for food storage. As used herein, "saccharide fatty add esters" are a group of surfactants synthesized from esterification of a saccharide and fatty adds. The group of substances comprises a wide range of saccharide fatty acid esters with different degrees of substitution (DS) with fatty acid molecules connected to the hydroxyl groups of a saccharide molecule. The lipophilic character of such a saccharide fatty ester can be described by hydrophilic-lipophilic balances (HLB), wherein the polar saccharide represents a hydrophilic end of the molecule, while the fatty add chain represents a lipophilic end thereof.

As used herein, "lipid resistance" or "oleophobic" means the property of being lipid repellent (sometimes also called "greaseproof") tending to repel and not absorb lipids, grease, fats and the like. In a related aspect, the lipid resistance may be measured by a Cobb Unger oil test (ISO 535) or a TAPPI T441 Kit test.

In the present technology, the term "compostable" or "biodegradable" means the capability of being broken down into innocuous products by the action of living things such as microorganisms.

As used herein, the term "recyclable" means a material that is treatable or that can be processed with used and/or waste items so as to make said material suitable for reuse.

The method and product of the invention will now be described in more detail, for improved understanding but not in a limiting sense.

According to the invention, a method is described for producing a cellulose-based product having lipid resistance. The method comprises at least the steps of providing an aqueous mixture comprising cellulose pulp, said mixture having a solid content between 0.1- 50%; adding a saccharide fatty add ester (SFAE) to the mixture in an amount between 0.5 - 10 wt% based on the total mixture fiber content; and adding a retention aid chemical to said mixture arranged to improve retention of said SFAE to the cellulose fibers. According to the invention, the electrostatic charge balance of the pulp mixture is adjusted to be kept on the anionic side. The mixture comprising at least cellulose pulp, SFAE and retention aid chemical is further used for manufacturing of a cellulose based product having lipid resistance.

In one aspect of the invention, the manufacturing of a cellulose-based product from said mixture includes a step of heat exposure at temperatures >100°C, causing the SFAE to bind to the cellulose-based material. In one aspect of the invention, the cellulose-based product manufactured according to the method of the invention is a molded fiber product. Manufacturing molded fiber products and structure can be done by wet forming, wherein a forming fool is dipped into an aqueous pulp mixture 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. According to the invention, SFAE is added to the aqueous pulp mixture having a fiber content between 0.1-50wt%. The amount of added SFAE is chosen between 0.5 - 10 wt% based on the total mixture fiber content. A retention aid chemical is also added to the mixture to retain the anionic SFAE to the cellulose fibers.

It has been found that by actively controlling the electrostatic charge of the mixture to be anionic, the end product will obtain improved lipid resistance properties (as demonstrated in the hereinafter following Examples). Adjustment of the electrostatic charge of said mixture can be performed in various ways by means of selecting type/s as well as amount of additive/s. For instance, the amount of added retention chemical can be chosen so that the electrostatic charge balance between the SFAE and additives in the mixture is kept on the anionic side, i.e. the mixture is negatively charged. By choosing a cationic retention chemical, retention function can be combined with possibility of adjusting the electrostatic charge balance. It is also conceivable to use dual systems, where combined retention aids and other agents are added to the pulp mixture to optimise retention as well as electrostatic charge balance. Preferably, the electrostatic charge of the pulp mixture is below 20 μeq/l as anionic charge, preferably below 15 μeq/l, and more preferably below 10 μeq/l. Preferably, the electrostatic charge of the pulp mixture is in the interval 4 to 15 μeq/l as anionic charge. The electrostatic charge of the mixture can be measured by means of detecting the cationic demand in the filtrate of the pulp, as known to the person skilled in the art. An example of instrument for detecting pulp charge is Mütek Particle Charge Defector. EXAMPLES

A pulp mixture prepared by disintegrating a market pulp and diluted to 0.3% solid content with tap water. The selected pulp could be chemifhermomechanical pulp (CTMP), thermomechanical pulp (TMP), chemical pulp or semichemical pulp, or a combination thereof. Sucrose fatty acid ester (SFAE) formulation at a solid content of 10-30% is diluted to 2% solid content before adding to the pulp mixture. Other chemicals, retention aids or sizing agent, or wet strength agent, are added at a solid content about 0.2% to the pulp mixture. The chemicals can be added in different combinations, and with different orders to achieve the desired effects.

In one embodiment of the invention, SFAE is added to the pulp mixture followed by the addition of retention aid. In another embodiment of the invention, SFAE is added after the addition of retention aid. In a third embodiment, SFAE and retention aid are premixed before adding to the pulp mixture. After each addition of the chemicals, sufficient mixing is ensured with a laboratory stirrer placed in the pulp mixture.

The samples are formed in a laboratory sheet former at 0.3% solid content. The formed handsheets are dewatered with blotter paper under pressing of a steel plate to reach about 20% solid content. The dewatered and pressed samples are dried between two heated plates at a constant pressure of 25 bar, and a temperature of 120-250 °C. The drying time is adjusted to reach a solid content >90%. The experimental procedure mimics the industrial molded fiber process with the key steps: a) stock preparation with chemical additives; b) forming a 2D or 3D structure with a forming unit; c) dewatering and predrying of the wet formed structure with pressing, with or without heat; d) drying the structure under pressure and heat to its final form as ready product, if required finished with a die-cut step.

The basis weight of the samples is 300-400 gsm, adjusted to give a thickness of 0.5-lmm and a sheet density of 500-600 kg/m3. The barrier properties, OGR and water resistance, of the samples are measured on a flat surface representing the inner surface of a molded fiber product.

The results are seen in the graph of Figure 1, showing Oil Cobb Unger vs. Tappi KIT as a measure of OGR.

A large number of test samples were measured for oil and grease resistance (OGR) with Cobb Unger method (ref) and Tappi KIT, which is commonly known as the KIT test, a method describes a procedure for testing the degree of repellency and/or the antiwicking characteristics of paper or paperboard treated with fluorochemical sizing agents, or PFAS. For comparison, two market salad bow! samples made of molded fiber material but with PFAS as OGR barrier were used as reference. The KIT number of 2.5 is the average number of two measurements of the same sample. Although there seems to be a reasonable correlation between results of the two measurements, the KIT number doesn 't give a reliable measurement of OGR for the products when KIT number is below 3, especially below 2. Based on benchmarking, a Cobb 30 Unger value of 30-35 gsm shows sufficient OGR property, for instance, for applicability as salad bowl or single use food tray. Figure 2 shows the filtrate charge vs. OGR. Herein, it is shown that a good OGR can be achieved with SFAE. The level of OGR is close to what is achievable with a conventional PFAS. To reach a desired level of OGR, retention aid must be used. The retention aid could be a polyethyleneimine (PEI), a cationic polyacrylamide (CRAM), or a polyamine. The common denominator is that they carry high cationic charge, however with different molecular weights, and of different charge densities. With increased amount of retention aid, the OGR effects improves. To reach an adequate OGR level, it is recommended to have sufficient retention aid to give a negative filtrate charge between 0 to -15 μeq/l. As seen in Fig. 2, the optimal effect is obtained at filtrate charge close to -5 μeq/l, such as within the interval -4 μeq/l to -15 μeq/l (i.e. between 4 - 15 μeq/l as anionic charge). In selecting a suitable amount of retention aid, there is also an incentive to minimize the charge of retention aid due to cost reason, and environmental concerns as many of the retention aids have limited acceptance level for food contact applications. At filtrate charge close to zero, the electrostatic system becomes instable and the charge measurement fluctuates around zero charge point by the Mütek PCD instrument thus the filtrate charge is given a reading of zero. The OGR effect deteriorates at filtrate charge close to zero.

Figure 3 shows retention of SFAE vs. filtrate charge. The positive OGR effect of SFAE at low filtrate charge is due to improved retention of the SFAE in the product sample, as shown in Figure 3. It is noted that CRAM has higher retention efficiency as compared to PEI (Figures 3-4). To reach an oil Cobb 30 of 30-35 gsm, 1.5 kg/t CPAM or poiyamine is required. With PEI, >2kg/t is required.

Figure 4 shows OGR effect of SFAE with different retention aids. Similar to SFAE, there is a need to use retention aid also for PFAS. The efficiency of retention aid varies more in the case of PFAS. CRAM is more efficient than PEI (see Figure 5). In Figure 5, SFAE is compared with PFAS.

Table 1 below summarizes all sample data for Figures 2-5. In all test points, a CTMP with a freeness of 400-600ml was used.

Although the present invention has been described in relation to a number of embodiments, these are not to be considered limiting for the invention. The skilled person may provide other embodiments falling within the claims by combining various aspects and embodiments as required.