Method for producing nonwoven material by hydroentangling and nonwoven material

Abstract

The invention relates to a for producing a nonwoven material by a wet-laid process comprising a hydroentangling step. A fibre dispersion comprising cellulose-based fibres is prepared and a fibrous web is formed by supplying the fibre dispersion on a screen. The fibrous web is hydroentangled in at least one hydroentangling station, and the hydroentangled fibrous web is dried for obtaining the nonwoven material. A strength composition comprising at least one cationic strength component having functional groups capable of forming a covalent bond with a carboxyl group and / or a hydroxyl group is added to the fibre dispersion before formation of the fibrous web. The invention relates also the obtained nonwoven material.

Description

[0001] METHOD FOR PRODUCING NONWOVEN MATERIAL BY HYDROENTANGLING AND NONWOVEN MATERIAL

[0002] The present invention relates to a method for producing a nonwoven material by using hydroentangling according to the preambles of the enclosed independent claims. The invention relates also to a nonwoven material obtained by the method of the invention.

[0003] Nonwoven materials are nowadays used in a great variety of applications, ranging from hygiene articles to agriculture, automotive industry and more. Nonwoven materials are especially used for industrial and household cleaning purposes, personal hygiene care, beauty care and for medical care. As the environmental consciousness has increased, both among the industry as well as among the consumers, there is a growing interest to reduce the environmental impact of nonwoven production and the nonwovens itself. Due to the large production volumes and broad range of application fields, even small improvements in nonwoven manufacture produce significant impacts.

[0004] Nonwoven webs can be manufactured by a wet-laid process where an aqueous fibre dispersion is deposited on a screen, i.e. wire, and the nonwoven web is formed on the screen by draining the water away through the screen. It is desired that the strength of the nonwoven web is similar in all directions of the plane, due to the random orientation of fibres. The web strength can be increased by web bonding. Web bonding can be achieved mechanically by using hydroentanglement, also called spunlacing. In a hydroentanglement step, fine high pressure water jets are used to mechanically consolidate the fibrous web. These high pressure water jets cause the fibres in the web to entangle with each other, whereby the strength of the web is increased. The pressure used in hydroentanglement step is directly proportional to the strength effect which is achieved and for improved result the nonwoven web can go through repeated hydroentanglement steps or cycles before web drying. The high pressure and multiple cycles required make the hydroentanglement a very energy intensive manufacturing step in nonwoven manufacture. Increase in the jet pressure or cycle number in hydroentanglement step for higher strength, increases the energy consumption of the process even more. However, the strength that can be achieved with hydroentanglement is limited: even with repeated hydroentanglement cycles or steps or higher jet pressures, the strength of the nonwoven material cannot be increased over a certain maximum limit.

[0005] The strength of the nonwoven material also depends on its grammage, which means that the manufacture of low grammage nonwovens with good strength properties may be challenging. It is possible to increase the strength of the nonwoven materials by using synthetic, petroleum-based polymer fibres for their manufacture. However, use of petroleum-based polymer fibres can make the disposal of the nonwoven material problematic after the use. As the regulations and costs relating to handling of waste that cannot be recirculated, reused, properly sorted or composted, are increasing, there is a general interest to minimise the amount of petroleum-based polymers in the products. At the same time, there is a certain desire to lower the grammage of nonwoven materials, as it provides savings in raw material usage.

[0006] Chemical bonding has also been used for improving the strength properties of nonwoven materials. In chemical bonding, a liquid bonding agent has been applied to the nonwoven web after its formation by impregnation, coating or spraying.

[0007] There is a need to improve the manufacturing process of wet-laid nonwoven materials to allow the use of lower pressure and / or less cycles in the hydroentanglement process without compromising the strength properties of the final nonwoven material. Furthermore, it would be both economically and environmentally beneficial to be able to minimise the amount of used fibres, especially polymer fibres, as well as chemical bonding agents without compromising the strength properties of the obtained nonwoven material.

[0008] It is an object of the present invention to reduce or even eliminate the above- mentioned problems appearing in the prior art. It is an object of the present invention to provide a method for producing a nonwoven material with improved properties, especially strength properties, by using hydroentangling.

[0009] Another object of the present invention is to provide enhanced strength properties for wet-laid nonwoven material.

[0010] In order to achieve among others the objects presented above, the invention is characterized by what is presented in the characterizing parts of the enclosed independent claims.

[0011] Some preferred embodiments of the invention will be described in the other claims.

[0012] The embodiments and advantages mentioned in this text relate, where applicable, to all aspects of the invention, to the method, product and use, even though it is not always specifically mentioned.

[0013] A typical method according to the present invention, for producing a nonwoven material by a wet-laid process comprising a hydroentangling step, comprises

[0014] - preparing a fibre dispersion comprising cellulose-based fibres,

[0015] - forming a fibrous web by supplying the fibre dispersion on a screen,

[0016] - hydroentangling the fibrous web in at least one hydroentangling station,

[0017] - drying the hydroentangled fibrous web for obtaining the nonwoven material, wherein a strength composition comprising at least one glyoxalated cationic polyacrylamide is added to the fibre dispersion before formation of the fibrous web.

[0018] A typical nonwoven material according to the present invention is obtained by a method according to the invention and has a fibre matrix comprising cellulose- based fibres, wherein a strength composition comprising at least glyoxalated cationic polyacrylamide is uniformly distributed in the fibre matrix. Now it has been surprisingly found that it is possible to significantly improve the strength properties, especially the wet strength, of the produced nonwoven material by an addition of a strength composition, which comprises at least glyoxalated cationic polyacrylamide, to the fibre suspension prior web formation. It is assumed, without wishing to be bound by any theory that the glyoxalated cationic polyacrylamide is possibly absorbed on the surfaces of the anionic fibres, followed by formation of covalent bonds between the carboxyl groups and / or hydroxyl groups of the fibre surfaces and the aldehyde groups of glyoxalated cationic polyacrylamide. This is assumed to result in linkages between the fibres through hemiacetal formation. The wet strength obtained by glyoxalated cationic polyacrylamide is of temporary nature, as the bonding between the fibres and the aldehyde groups of glyoxalated cation polyacrylamide is reversible in the presence of excess water. Temporary wet strength degrades fairly rapidly on soaking the fibre web in water. It was highly unexpected that this temporary wet strength effect could be preserved through hydroentanglement, and be still observed in the produced nonwoven material. The addition of the strength composition to the fibre dispersion before the hydroentangling makes it possible to decrease the jet pressure in the hydroentangling step, while maintaining the strength of the produced nonwoven material. Decreasing the jet pressure provides significant energy savings in the production process of a nonwoven material. The improved strength properties that are obtained by the addition of the strength composition to the fibre dispersion prior web formation also enable the lowering of the grammage of the produced nonwoven material, wherein the method may also provide savings in raw material costs.

[0019] Even if the strength properties of the produced nonwoven material are improved, the flushability of the nonwoven material is maintained or at least remain at an acceptable level, which was also unexpected. Furthermore, there are indications that the addition of the strength composition comprising glyoxalated cationic polyacrylamide may improve the liquid absorptive capacity of the produced nonwoven material. In the present context, the term “nonwoven material” is understood as an engineered fibrous assembly, primarily planar, which has been given a designed level of structural integrity, i.e. a measurable level of tensile strength, by using hydroentanglement step, and excluding materials obtained by weaving, knitting or paper making. The nonwoven material according to the present invention is obtained by wet-laid process, where the cellulose-based fibres, and optional other fibres, are engineered to a level of structural integrity primarily by physical and / or chemical means other than hydrogen bonding between the fibres. The nonwoven material of the present context fulfils the definitions of ISO standard 9092 and CEN EN 29092, in force in August 2024.

[0020] The present invention is suitable for producing a nonwoven material by a wet-laid process comprising a hydroentangling step. In the wet-laid process, an aqueous fibre dispersion is supplied on a screen and the water is drained through the screen, whereby the fibrous nonwoven web is formed on the screen. The manufacture of nonwoven material by using wet-laid process is known for a person skilled art as such. The formed fibrous web is mechanically bonded in a hydroentangling step by using pressurized water jets, in at least one hydroentangling station. Preferably the fibrous web is bonded by using hydroentangling, by passing the web through at least one hydroentanglement station multiple times, i.e. the hydroentangling step comprises a multiple hydroentanglement cycles. The number of required hydroentanglement cycles is dependent on the nonwoven material to be produced and it may be 1 - 7 cycles, such as 1 - 5 cycles. The pressure used in the hydroentanglement step can be different in each cycle. According to one embodiment of the present invention the hydroentanglement cycle(s) or hydroentanglement station(s) may use a fluid pressure in a range of 10 - 250 bar, preferably 10 - 60 bar.

[0021] After the bonding of the fibrous web by hydroentanglement, the hydroentangled fibrous web is dried for obtaining the final nonwoven material.

[0022] The strength composition according to the invention is added to the aqueous fibre dispersion before the fibrous web is formed, i.e. before the fibre dispersion is supplied on the screen and before the hydroentanglement of the web. The addition of the strength composition into the fibre dispersion provides appropriate contact time between the cationic strength component(s) of the strength composition and the cellulose-based fibres of the fibre dispersion.

[0023] According to the present invention, the cationic strength component comprises at least glyoxalated cationic polyacrylamide. The strength composition thus comprises or consists of glyoxalated cationic polyacrylamide. Use of glyoxalated cationic polyacrylamide is advantageous from a sustainability point of view, as polyacrylamide can be produced according to the principles of biomass-balance, in which the majority of fossil based raw materials are replaced by biobased and renewable mass-balanced feedstocks.

[0024] The glyoxalated cationic polyacrylamide may be obtained by reacting a cationic linear polyacrylamide (base polymer) with glyoxal, whereby a cationic polyacrylamide polymer having pendant glyoxalated groups is obtained. The cationic polyacrylamide, which is used as a base polymer, may be a copolymer obtained by polymerisation of acrylamide or a primary amine-containing monomer and at least one cationic monomer. The primary amine-containing monomer may be selected from methacrylamide, ethylacrylamide, N-ethyl methacrylamide, N- butyl methacrylamide, or N-ethyl methacrylamide, or any combination thereof. The cationic monomer may be selected from diallyl dimethyl ammonium chloride (DADMAC), [3-(acrylamide)propyl]trimethyl-ammonium chloride (APTAC), and [3- (methacrylamido)propyl]trimethyl-ammonium chloride (MAPTAC) and any combination thereof. The cationic polyacrylamide used as base polymer, and thus the glyoxalated polyacrylamide, respectively, may comprise only one type of cationic monomer, or it may comprise two or more types of cationic monomers. Preferably, the cationic monomer may be diallyl dimethyl ammonium chloride (DADMAC).

[0025] The cationic polyacrylamide polymer may be obtained by a polymerisation of acrylamide or a primary amine-containing monomer and at least 5 mol-%, preferably at least 7 mol-%, more preferably at least 10 mol-%, of at least one cationic monomer, as defined above. According to an embodiment of the invention cationic polyacrylamide polymer may be obtained by a polymerisation of acrylamide or a primary amine-containing monomer and 5 - 40 mol-%, preferably 7 - 30 mol-%, more preferably 10 - 25 mol-%, sometimes 10 - 20 mol-% of at least one cationic monomer, as defined above. The percentages are calculated based on the total moles of the polymerisable monomers in the polymerisation.

[0026] According to one embodiment of the present invention the strength composition may comprise or consist of glyoxalated cationic polyacrylamide, which has a charge density in a range of 0.5 - 2.5 meq / g or 0.5 - 2 meq / g, preferably 0.75 - 1.9 meq / g, more preferably 1 - 1.8 meq / g, at pH 4.3. The charge densities were measured as described in the experimental part. The defined charge density values enable effective interaction between the strength composition and the negatively charged groups on the surface of the cellulose-based fibres without disturbing the formation of the nonwoven fibrous web.

[0027] The weight average molecular weight of the glyoxalated cationic polyacrylamide may be in a range of 100 000 - 1 000 000 g / mol, preferably 200 000 - 700 000 g / mol, more preferably 250 000 - 600 000 g / mol or 250 000 - 550 000 g / mol. The weight average molecular weight is determined by using size-exclusion chromatography, calibrated by polyethylene oxide (PEO) calibration standards.

[0028] The strength composition may be added to the fibre dispersion in amount that provides a dosage of glyoxalated cationic polyacrylamide in a range of 0.1 - 10 kg / ton dry fibre dispersion, preferably 0.3 - 6 kg / ton dry fibre dispersion, more preferably 0.5 - 5 kg / ton dry fibre dispersion.

[0029] In addition to glyoxalated cationic polyacrylamide, the strength composition may comprise one or several additional cationic strength components. The additional cationic strength component suitable for use in the present method comprise at least functional groups, which are capable of forming covalent bonds with carboxyl groups and / or hydroxyl groups. This means that the additional cationic strength component can form covalent bonds with the cellulose-based fibres of the fibre dispersion. The additional cationic strength component may preferably be a synthetic polymer. According to one embodiment of the invention the additional cationic strength component is a synthetic cationic polymer which has a charge density in a range of 0.5 - 2.7 meq / g, preferably 0.7 - 2.5 meq / g, more preferably 1 .0 - 2.3 meq / g. The operating pH range for the synthetic cationic polymer is 4.5 - 8.5. Within that pH range the synthetic cationic polymer preferably has the defined charge density. In this manner the additional cationic strength component is able to effectively form covalent bonds with the cellulose-based fibres of the fibre dispersion.

[0030] The strength composition may be added in amount that provides a total dosage of the glyoxalated cationic polyacrylamide and optional the additional cationic strength component in a range of 0.1 - 10 kg / ton dry fibre dispersion, preferably 0.3 - 6 kg / ton dry fibre dispersion, more preferably 0.5 - 5 kg / ton dry fibre dispersion.

[0031] In addition to glyoxalated polyacrylamide, the cationic strength component may comprise an additional cationic strength component having functional groups capable of forming a covalent bond with a carboxyl group and / or a hydroxyl group, such as a polyamidoamine epihalohydrin. According to one preferable embodiment the additional cationic strength component is a polyamidoamine epihalohydrin polymer, preferably polyamidoamine epichlorohydrin (PAE) polymer. Polyamidoamine epihalohydrin polymer has a polyamidoamine backbone, which is obtained as a result of a condensation reaction between a polycarboxylic acid, such as adipic acid, and a polyamine, such as diethylenetriamine. A subsequent reaction of the backbone with epihalohydrin, such as epichlorohydrin, results a crosslinked polymer structure, where highly reactive azetidinium groups are created along the polyamidoamine backbone. The amount of azetidinium groups may be controlled, for example, by the epihalohydrin / amine ratio. The polyamidoamine epihalohydrin polymer may have a molar ratio of epihalohydrin to secondary amine group at least 0.8. In some embodiments the molar ratio of epihalohydrin to secondary amine group can be in a range of 0.8 - 1.5, for example 0.9 - 1 .3 or 0.95 - 1 .2. Polyamidoamine epihalohydrin polymer may have a weight average molecular weight in a range of 150 000 - 600 000 g / mol, preferably 200 000 - 500 000 g / mol, more preferably 250 000 - 450 000 g / mol. The weight average molecular weight may be measured by using SEC / GPC determination with PEO (polyethyleneoxide) calibration as described in the following. The weight average molecular weight MW is determined by size-exclusion chromatography (SEC) using Agilent 1100 SE chromatography equipment with integrated pump, autosampler and degasser. Eluent is a buffer solution (0.3125 M CH3COOH + 0.3125 M CH3COONa) with a flow rate of 0.5 ml / min at 35 °C. Typical sample concentration is 2 - 4 mg / ml, with an injection volume of 50 pl. Ethylene glycol (1 mg / ml) is used as a flow marker. The used column set consists of three columns (one TSKgel PWXL guard column and two TSKgel GMPWXL columns). Refractive index detector by Agilent is used for detection (T=35°C). Molecular weight is determined using conventional column calibration with polyethylene oxide) / poly(ethylene glycol) narrow molecular weight distribution standards (Polymer Standards Service).

[0032] The additional cationic strength component of the strength composition may comprise or consists of polyamidoamine epihalohydrin polymer which has a charge density of 1.0 meq / g or higher, preferably 1.2 meq / g or higher, more preferably 1.4meq / g or higher, even more preferably 1.5 meq / g or higher, determined at pH 9.5. The charge density of the polyamidoamine epihalohydrin polymer may be <2.7 meq / g, preferably <2.5 meq / g, more preferably <2.3 meq / g, determined at pH 9.5. The charge density can be determined by titration with potassium salt of polyvinylsulfate. Mutek PCD-03 is used for end point detection.

[0033] The strength composition may be added to the fibre dispersion in amount that provides a dosage of additional cationic strength component, such as polyamidoamine epihalohydrin polymer, in a range of 0.1 - 10 kg / ton dry fibre dispersion, preferably 0.3 - 6 kg / ton dry fibre dispersion, more preferably 0.5 - 5 kg / ton dry fibre dispersion. In addition to the strength composition, it is possible to add one or several sizing agents, such as natural and / or synthetic polymers, for example cationic starch, alkenyl ketene dimer (AKD) and / or alkenyl succinic anhydride (ASA), or any of their combinations, to the fibre dispersion before formation of the fibrous web. According to one preferable embodiment, the sizing agent is cationic starch. The sizing agent(s) can be added before and / or after the addition of the strength composition. Use of sizing agent(s) may further improve the obtained wet strength properties of the produced nonwoven material, and they may also affect positively the water absorption properties of the produced nonwoven material.

[0034] According to one aspect of the present invention a strength composition comprising glyoxalated cationic polyacrylamide (GPAM), and optional polyamide- amine-epichlorohydrin (PAE) polymer, is used in manufacture of a nonwoven material web by hydroentangling for improving strength properties of the nonwoven material web. For example, the present invention is able to improve the strength properties of the nonwoven material web in cross-direction of the web, which means that the produced nonwoven material web can show similar strength properties, e.g. wet and / or dry tensile strength, both in machine direction and cross-direction of the web. Similar or equal strength properties both in machine direction and cross-direction make the nonwoven material suitable for a variety of different uses. According to one embodiment, the nonwoven material may have

[0035] - at least 15 %, preferably at least 20 %, higher dry tensile strength, and / or

[0036] - at least 30 %, preferably 40 % higher wet tensile strength, measured from the material in a machine cross-direction, and / or

[0037] - at least 10 %, preferably at least 15 %, higher wet tensile strength, measured from the material in a machine direction, than an identical nonwoven material manufactured without any chemical addition to the fibre dispersion.

[0038] According to one embodiment, a typical method for producing a nonwoven material by a wet-laid process comprising a hydroentangling step, comprises

[0039] - preparing a fibre dispersion comprising cellulose-based fibres,

[0040] - forming a fibrous web by supplying the fibre dispersion on a screen, - hydroentangling the fibrous web in at least one hydroentangling station,

[0041] - drying the hydroentangled fibrous web for obtaining the nonwoven material, wherein a strength composition comprising at least one cationic strength component having functional groups capable of forming a covalent bond with a carboxyl group and / or a hydroxyl group is added to the fibre dispersion before formation of the fibrous web.

[0042] The fibre dispersion, used in the present method for making the nonwoven material, comprises cellulose-based fibres. The fibre dispersion preferably comprises 90 - 100 weight-%, preferably 95 - 100 weight-%, of cellulose-based fibres. According to one preferable embodiment the fibre dispersion consists of cellulose-based fibres.

[0043] The cellulose-based fibres may comprise or be selected from natural cellulose fibres, man-made cellulosic fibres, i.e. man-made fibres of cellulosic origin, or any mixture thereof.

[0044] Natural cellulose fibres may be selected from wood fibres, such as softwood or hardwood fibres; seed hair fibres, such as cotton, kapok or milkweed fibres; leaf fibres, such as sisal, abaca or pineapple fibres; bast fibres, such as flax, hemp, jute or kenaf fibres; or any combinations thereof. The natural cellulose fibres may also comprise cellulosic fibres of microbiological origin. The natural cellulose fibres may have a fibre length in a range of 0.5 - 3.5 mm, preferably 1 - 3 mm.

[0045] The man-made cellulosic fibres may comprise regenerated cellulosic fibres, such as viscose fibres, lyocell fibres, modal fibres, acetate fibres, triacetate fibres, cupro fibres, loncell™ fibres, Infinna™ fibres or any combinations thereof. The manmade cellulosic fibres are preferably staple fibres. The man-made cellulosic fibres may have a fibre length in a range of 3 - 15 mm, preferably 5 - 13 mm, more preferably 5 - 12 mm.

[0046] The cellulose-based fibres, both natural cellulose fibres and man-made cellulosic fibres may comprise virgin fibres or recycled fibres or a mixture of virgin fibres and recycled fibres. The fibre dispersion may also comprise solely virgin fibres and / or solely recycled fibres. The cellulose-based fibres may alternatively or in addition be fibres originating from agricultural waste or residues, or fibres originating from food processing and / or beverage industry waste, such as citrus industry waste.

[0047] According to one embodiment of the invention, the fibre dispersion may comprise 50 - 100 weight-%, preferably 60 - 99 weight-%, more preferably 60 - 95 weight- %, calculated as dry, of natural cellulose fibres. The fibre dispersion may comprise 0 - 50 weight-%, typically 1 - 40 weight-%, more typically 5 - 40 weight-% of manmade cellulosic fibres. The use of strength composition makes it possible to increase the amount of natural cellulosic fibres in the fibre dispersion, without endangering or reducing the strength properties of the produced nonwoven material. Increased amount of natural cellulose fibres makes the nonwoven material more sustainable and easier to deposit after the use. Furthermore, the natural cellulose fibres absorb moisture / water, and may thus provide the obtained nonwoven material with advantageous absorption characteristics.

[0048] In addition to cellulose-based fibres the fibre dispersion may comprise synthetic fibres, such as synthetic polymer fibres, carbon fibres, and / or glass fibres. The synthetic polymer fibres can be thermoplastic polymer fibres, such as polyolefin fibres, e.g. polyethylene fibres, polypropylene fibres; polyamide fibres, such as polyaramid fibres; polyester fibres; polylactide fibres; or any mixture thereof. The fibre dispersion typically comprises synthetic fibres in an amount <10 weight-%, preferably <5 weight-%, more preferably <1 weight-%. According to one embodiment the fibre dispersion is free of synthetic fibres selected from synthetic polymer fibres, carbon fibres, and / or glass fibres, especially synthetic polymer fibres. The addition of the strength composition according to the present invention may enable production of nonwoven material which provide adequate strength properties without use of synthetic fibres, especially synthetic polymer fibres.

[0049] The nonwoven material of the present invention obtained by a method according to the invention may have a weight in a range of 10 - 260 g / m2, preferably 30 - 150 g / m2, more preferably 40 - 70 g / m2. The nonwoven material may have, for example, the weight in a range of 30 - 100 g / m2, preferably 45 - 90 g / m2, more preferably 50 - 70 g / m2. The addition of the strength composition to the fibre dispersion makes it possible to produce nonwoven materials with a low grammage, without deterioration of its strength properties. This saves resources in form of reduced use of raw materials. The products with low grammage also provide a major impact on CO2 emissions from the product transportation, since more product can be transported with same emission amount.

[0050] The present invention is suitable for manufacturing nonwoven material that is used as flushable wipes and / or biodegradable wipes. In general, the present invention is suitable for manufacturing nonwoven material, for which the strength and disintegration properties are major technical characteristics. The manufactured nonwoven material can be used, for example, as personal or industrial wiping products (personal / industrial wipes), household items (e.g. tabletops), agricultural items or as geotextiles. The manufactured nonwoven material can preferably be disposed after use by flushing, composting, disintegration, recycling or similar end- of-life methods.

[0051] EXPERIMENTAL

[0052] Some embodiments of the invention are described in the following non-limiting examples.

[0053] Determination of Charge Density

[0054] In the following, the charge density of the used glyoxalated polyacrylamides were determined as follows.

[0055] The particle charge measurement was done with Charge Analyzing System CAS touch. The titrator can be used to determine cation ic / an ionic and acid / base demands of aqueous charge systems. The method is based on polyelectrolyte titration, where the charge of the investigated solution is changed. In this case, the adsorption of colloid particles to the piston and cell of the device changes, and at the same time the flow potential changes. The titration is used to determine the point of zero charge (i.e. the isoelectric point), the point where the total charge of the solution passes zero. Based on the consumption of the titration solution, the total charge of the sample can be determined. The charge of the titration solution is opposite to the initial charge of the solution. 100 ml samples of each chemical were prepared with 200 ppm solids content. Based on the starting potential, samples were titrated with Poly-DADMAC or PES-Na. The results are expressed as milliequivalents per gram (meq / g).

[0056] Manufacture of Nonwoven Material

[0057] A wet-laid nonwoven was manufactured by using fibre dispersion comprising wood pulp (GP Cellulose), average fibre length 2.8 mm, and viscose fibres (1.7 dtex, Kelheim Fibers), fibre length 10 mm. The wood pulp was refined by beating to freeness of 15 SR. The fibre dispersion comprised 60 weight-% of wood pulp and 40 weight-% of viscose fibres, calculated as dry.

[0058] A strength composition comprising cationic glyoxalated polyacrylamide was added to the fibre dispersion together with starch (sizing agent) before formation of nonwoven fibrous web. The specifics for the used chemicals are given in Table 1 and the addition amounts are given in Table 2. Wetlaying was done with sheet former (Herman Manufacturing, USA) producing nonwoven material samples. Target areal density of the nonwoven material was 60 g / m2. The fibrous web was subjected to hydroentanglement treatment with a low pressure profile, to demonstrate the synergetic effect of the mechanical and chemical bonding. Hydroentanglement treatment was done with 5 passes at a speed of 5m / min in a hydroentanglement unit (Andritz Perfojet), and the pressure profile of was t30bar|30barf50bar|50barf50bar. The obtained nonwoven material samples were oven dried at 120 °C under tension for 10 minutes. Table 1 Chemicals used.

[0059] Table 2 Test scheme and chemical dosages.

[0060] *dry fibre

[0061] Testing of Nonwoven Material and Results

[0062] Samples of nonwoven material were conditioned in 23 °C, 50 % RH (relative humidity) for 20 minutes before testing. The following properties were tested from the samples of nonwoven material by using identified test methods / Nonwovens Standard Procedures (NWSP): l / l / ef tensile strength’. EDANA - NWSP 1104R0 (20), five parallel measurements Dry tensile strength’. EDANA - NWSP 1104R0 (20), five parallel measurements Liquid absorptive capacity (LAC): EDANA - NWSP 0101 R0 (20), five parallel measurements

[0063] Flushability: evaluated based by the disintegration in a drainline test. The sheet samples were put in to 1 liter water and shaken for 3 hours at 100 rpm. Direction was changed every 45 minutes. The contents were poured through a 12.5 mm sieve and pass level determined as >50% passes. Two parallel measurements were conducted.

[0064] The measured characteristics of manufactured nonwoven materials are given in Table 3.

[0065] Table 3 Characteristics of manufactured nonwoven materials.

[0066] *O-reference samples were only measured from one direction, since it is expected that the MD and CD properties are alike (no hydroentanglement).

[0067] From the results of Table 3 it can be clearly seen that in the cross-direction an improvement of 44 % in the dry tensile strength was obtained when glyoxalated polyacrylamide was added to the fibre dispersion before formation of the nonwoven web and hydroentanglement step, in comparison to hydroentanglement in absence of glyoxalated polyacrylamide. The improvement in wet tensile strength was obtained without deterioration of flushability properties, which remained at acceptable level. A significant wet tensile strength improvement up to 92% in cross-direction and 38% in machine direction was obtained when glyoxalated polyacrylamide was added to the fibre dispersion before formation of the nonwoven web and hydroentanglement step, in comparison to hydroentanglement in absence of glyoxalated polyacrylamide. An improvement in liquid absorptive capacity of the produced nonwoven material could also be seen.

[0068] Although certain embodiments and examples have been described in detail above, those having ordinary skill in the art will clearly understand that many modifications are possible in the embodiments and examples without departing from the teachings thereof. All such modifications are intended to be encompassed within the claims of the invention.

Claims

CLAIMS1. Method for producing a nonwoven material by a wet-laid process comprising a hydroentangling step, the method comprising- preparing a fibre dispersion comprising cellulose-based fibres,- forming a fibrous web by supplying the fibre dispersion on a screen,- hydroentangling the fibrous web in at least one hydroentangling station,- drying the hydroentangled fibrous web for obtaining the nonwoven material, wherein a strength composition comprising at least glyoxalated cationic polyacrylamide is added to the fibre dispersion before formation of the fibrous web.

2. Method according to claim 1 , characterised in that the cationic strength component comprises glyoxalated cationic polyacrylamide, which has a charge density in a range of 0.5 - 2.5 meq / g or 0.5 - 2 meq / g, preferably 0.75 - 1.9 meq / g, more preferably 1 - 1.8 meq / g, measured at pH 4.3.

3. Method according to claim 1 or 2, characterised in that the glyoxalated cationic polyacrylamide has a weight average molecular weight in a range of 100 000 - 1 000 000 g / mol, preferably 200 000 - 700 000 g / mol, more preferably 250 000 - 550 000 g / mol.

4. Method according to claim 1 , 2 or 3, characterised in that the cationic strength component comprises an additional cationic strength component having functional groups capable of forming a covalent bond with a carboxyl group and / or a hydroxyl group, such as a polyamidoamine epihalohydrin.

5. Method according to claim 4, characterised in that the additional cationic strength component comprises a polyamidoamine epihalohydrin polymer which has a charge density >1.0 meq / g, preferably >1.2 meq / g, more preferably >1.4 meq / g, even more preferably >1 .5 meq / g, determined at pH 9.5.

6. Method according to any of preceding claims 1 - 5, characterised in that the strength composition is added in amount that provides a dosage of the glyoxalatedcationic polyacrylamide in a range of 0.1 - 10 kg / ton dry fibre dispersion, preferably 0.3 - 6 kg / ton dry fibre dispersion, more preferably 0.5 - 5 kg / ton dry fibre dispersion.

7. Method according to any of preceding claims 1 - 6, characterised in that the method further comprises addition of at least one sizing agent, such as cationic starch, alkenyl ketene dimer and / or alkenyl succinic anhydride, to the fibre dispersion before the formation of the fibrous web.

8. Method according to any of preceding claims 1 - 7, characterised in that the fibre dispersion comprises 90 - 100 weight-%, preferably 95 - 100 weight-%, of cellulose-based fibres.

9. Method according to any of preceding claims 1 - 8, characterised in that the cellulose-based fibres are selected from natural cellulose fibres, man-made cellulosic fibres or any of their mixtures.

10. Method according to claim 9, characterised in that the fibre dispersion comprises 50 - 100 weight-%, preferably 60 - 99 weight-%, more preferably 60 - 95 weight-%, calculated as dry, of natural cellulose fibres.11 . Method according to any of preceding claims 1 - 10, characterised in that the hydroentanglement station(s) use(s) a fluid pressure in a range of 10 - 250 bar, preferably 10 - 60 bar.

12. Nonwoven material obtained by a method according to any of claims 1 - 11 , having a fibre matrix comprising cellulose-based fibres, wherein a strength composition comprises at least glyoxalated cationic polyacrylamide is uniformly distributed in the fibre matrix.

13. Nonwoven material according to claim 12, characterised in that it has a weight in a range of 10 - 260 g / m2, preferably 30 - 150 g / m2, more preferably 40 - 70 g / m2.

14. Nonwoven material according to claim 12 or 13, characterised in that it has- at least 15 %, preferably at least 20 %, higher dry tensile strength, and / or- at least 30 %, preferably 40 % higher wet tensile strength, measured from the material in a machine cross-direction, and / or- at least 10 %, preferably at least 15 %, higher wet tensile strength, measured from the material in a machine direction, than an identical nonwoven material manufactured without any chemical addition to the fibre dispersion.

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