FI20235764A1 - A method for forming a filament comprising cross-linked fibers and a filament - Google Patents

Abstract

A method for forming a filament comprising cross-linked fibers, the method comprising providing an aqueous suspension comprising at least natural non-modified microfibrillated cellulose (MFC) fibers and a dichlorotriazine-based crosslinker as a cross-linking agent, and drying the aqueous suspension to form at least one filament, wherein the filament comprises at least 50 % wt. preferably at least 70 % wt, MFC.

Description

A METHOD FOR FORMING A FILAMENT COMPRISING CROSS-LINKED

FIBERS AND A FILAMENT

TECHNICAL FIELD OF THE INVENTION

The invention relates to fiber suspensions and filaments in general. More specifically, the invention relates to a method for forming a filament comprising cross-linked fibers and a filament.

BACKGROUND OF THE INVENTION

In processes relating to manufacture of e.g. filaments to be utilized in for instance woven fabric production, where the filaments can be made from a fiber suspension, the properties of the formed filaments are important to ensure that the end product has desired properties and/or that the processing of the filaments is manageable or according to appropriate specifications.

It is especially advantageous to obtain filaments that have a high tenacity (wet and/or dry) and/or high elongation. These are characteristics that provide — strength to the filaments and may make processing them easier and/or may give end products such as fabrics that are resilient to wear and tear.

Alkali resistance is another quality that is preferred to be improved, as chemicals utilized in washing products are usually alkalic.

In the prior art, fiber suspensions and methods for producing filaments from such exist, where cross-linking chemicals are used to improve some property of the suspension or formed filaments. In these methods, fibers in the e cellulose suspensions are cross-linked to each other. Cross-linked cellulose

S fibers are typically made by reacting cellulose with multifunctional agents that © are capable of reacting with the hydroxyl groups of the anhydroglucose

O 25 repeating units of the cellulose either in the same chain, or in neighboring ? chains simultaneously. Formaldehyde and urea-formaldehyde products were = among the first agents used to cross-link cellulosic fibers. Formaldehyde, 3 however, has many adverse effects on health and is a known human

O carcinogen. &

Monomers having multifunctional groups, such as carboxylic acid groups and aldehyde groups, have also been used as cross-linking agents for cellulosic fibers. For example, alkanepolycarboxylic acids are capable of cross-linking cellulose fibers by forming an ester bond with the fiber's hydroxyl groups. One problem associated with the use of alkanepolycarboxylic acid is that the cellulosic fibers cross-linked thereby tend to lose their cross-linking upon storage, and revert to un-cross-linked fibers.

Some methods are also known where cross-linking of fibers is carried out not in the fiber suspension but as a post-processing or finishing step on a formed filament. These methods may especially be related to properties of the filament affecting the appearance of e.g. fabric, such as reduction of pilling, by applying chemicals to a formed filament to reduce the pilling or aid in giving an appearance of a cleaner surface by removing split fibrils e.g. via enzymatic treatments.

It would be advantageous to discover a cross-linking agent to be used in a fiber suspension, where a formed filament may be provided with selected characteristics, where the cross-linking agent would advantageously provide several different desired characteristics. The use of cross-linking agents that are not harmful for the environment or humans is desirable, while preferably also enabling a method for forming filaments that is easy, effective, and may be carried out in an environmentally friendly manner.

SUMMARY OF THE INVENTION

The object of the invention is to alleviate at least some of the problems in the prior art. In accordance with one aspect of the present invention, a method is provided for forming a filament comprising cross-linked fibers, the method comprising & - providing an aqueous suspension comprising at least natural non-

N 25 modified microfibrillated cellulose (MFC) fibers and a dichlorotriazine- <Q based crosslinker as a cross-linking agent, and > - drying the agueous suspension to form at least one filament, wherein

E the filament comprises at least 50 % wt. preferably at least 70 % wt, 3 MFC. 2 30 Through embodiments of the invention, filaments may be formed where the

S fibers are cross-linked to provide one or more selected gualities. The selected gualities may be related to wet and/or dry tenacity and/or elongation (or a combined property, such as workability, which will be considered further below), alkali resistance, and/or dye affinity, for instance.

Through the present invention, filaments may have a higher wet tenacity as compared to prior art filaments, while maintaining a dry tenacity that is similar to that of the prior art or does not differ from it at least more than a threshold amount. It is known that many cross-linking agents that increase wet tenacity of a filament will simultaneously decrease the dry tenacity, which may be undesirable.

Improved alkali resistance may be provided, which is beneficial as during textile processing, filaments may be subjected to NaOH or alkalic conditions.

With the present invention, a formed filament may not dissolve in alkalic conditions or at least may dissolve less than prior art filaments.

A high dimensional stability of fabric that is knitted utilizing filaments that may be formed through the present invention may also be provided, due to advantageous wet strength properties and/or less shrinkage of the formed filaments.

In one embodiment, the cross-linking agent may be 2-sodium hydroxy-4,6- dichloro-1,3,5-triazine.

The cross-linking agent 2-sodium hydroxy-4,6-dichloro-1,3,5-triazine has been known to be used in Lyocell filaments in a finishing step to reduce fibrillation. The cross-linking agent will only be present on the surface of the filament and additionally, as it is supplied to the formed filament, other gualities of the filament, such as tenacity and/or elongation, may not be improved as desired.

With the present invention, an agueous fiber suspension may be provided & where a cross-linking agent is supplied in the aqueous suspension, such that

N 25 the cross-linking agent is not applied in a post-processing step only after a <Q filament if formed from the suspension. This eliminates a step as compared > to prior art methods where a cross-linking agent is supplied in a finishing step.

E Elimination of process steps may lead to reduction of manufacturing + eguipment, reduction of produced waste, such as waste water.

O

3 30 The cross-linking agent may penetrate the amorphous region of the cellulose < material. Cross-linking may occur between different regions in separate cellulose fibrils and/or cross-linking may occur between different separate fibrils of the suspension or filament. This type of cross-linking behavior may lead to many of the benefits associated with the present invention. The cross-

linking is in the invention beneficially carried out by providing the cross-linking agent before filament formation, when preparing the aqueous suspension.

Furthermore, as the cross-linking agent is provided already in the suspension, before filament formation, filaments with improved or selected characteristics, such as improved or selected tenacity and/or elongation, were seen to be formed. Additionally, fibrillation may be prevented or reduced. The reduced fibrillation may be seen in the filaments as well as in fabric produced from the filaments. A produced fabric may be wrinkle-free or exhibit less wrinkling than other fabrics or without the cross-linking agent present.

In embodiments of the invention, the cross-linking agent may reduce swelling of fibers or filaments, reducing deterioration of fibrils in the filaments. The reduced swelling property may also reduce wrinkling of a produced fabric.

A crosslinked textile made from filaments according to the invention may also exhibit higher abrasion resistance. It has been shown that a fabric made with filaments that do not have cross-linked fibers has abrasion resistance of about 3000 rubs, while a fabric made with filaments according to the invention has abrasion resistance of about 4000-5000 rubs, and also exhibits less weight change.

In the traditional Lyocell cross-linking process, the cross-linking agent cannot be added at the suspension-phase because while dissolving pulp as the raw material of lyocell, the pulp needs to be dissolved in an NMMO solution for further extrusion in high pressure and temperature. Conversely, the present invention is based on filaments produced with nano/microfibrillated cellulose, e which does not have to be chemically treated, i.e. the cellulose is in cellulose

S 25 | form and non-regenerated.

S The present invention may provide control of the process in terms of > reproducibility and distribution of the cross-linking agent, which may

E contribute to providing desirable wet processing characteristics of the formed 3 filaments. 2 30 It was discovered that with the present invention, also dye affinity of filaments

S may be improved. Penetration and retention of the dye may be improved as compared to filaments without the cross-linking agent. When the cross-linking agent is provided in the suspension as in the present invention, the cross- linking agent may penetrate an entire formed filament. Thus, the improved dye affinity is not only at the surface of the filament but also throughout the filament. A shade of a dyed filament or fabric may be more even and/or deeper than compared to those in the prior art. The dyeability may be increased, whereby less amount of dye may be needed. 5 The used cross-linking agent may, in addition to providing a cheaper way for cross-linking fibers, allow circumvention of using other chemicals such as formaldehyde, which may be harmful.

An amount of cross-linking agent may be 0.1-10 % wt, such as 0.5-7 % wt, 3- 4 % wt, or or 3-5 % wt of the dried filament. The amount of cross-linking agent that is required may thus be low, keeping an amount of chemicals that is required for the process low.

A “dried” filament may refer to a filament that has a dry material content of at least 70% wt. The dry material content may be between 70% wt and 100 % wt.

The amount of cross-linking agent that is utilized may be selected based on the MFC-type used in the aqueous fiber suspension. More specifically, the amount of cross-linking agent may be selected based on a hemicellulose content of the MFC.

For instance, in some embodiments, if the hemicellulose content of the MFC is under 5% wt of the dry weight of the MFC, the amount of crosslinking agent may be between 2-8% wt, such as 3-5 % wt of the dried filament. If the hemicellulose content of the MFC is over 5% wt, such as over 10% wt of the dry weight of the MFC, the amount of crosslinking agent may then be under & 2% wt, such as about 1% wt of the dried filament.

N

O 25 In an embodiment of the invention, the method may comprise adjusting an

S initial pH of the suspension comprising MFC before adding the cross-linking

I agent. A base may be added to the suspension untilthe pH of the suspension - is /-12, such as 8-11. At least after final preparation, i.e. after all constituents

S have been added to the suspension, the pH of the suspension may most 2 30 preferably be 8-9 or 8.5-9. The adjusted pH may result in a preferred tenacity

S and/or elongation of the filament. If the pH is too high, the tenacity and/or elongation may decrease beyond a preferable amount.

The method may comprise providing the aqueous suspension with selected components in a selected adding order.

Selected components in an aqueous suspension may comprise at least MFC, at least one base, such as NaOH, the at least one triazine-based cross-linking agent, at least one strength resin, optionally polyamideamine-epichlorohydrin (PAE), anionic polyacrylamide (APAM), and/or glyoxalated polyacrylamide (GPAM), optionally at least one hydrophobic adhesive, such as alkyl ketene dimer (AKD), and optionally at least one dispersion agent, such as carboxymethylcellulose (CMC).

At least some of the selected components may provide a synergistic effect.

For instance, at least one wet strength resin, such as PAE may boost some of the effects that are provided through use of the cross-linking agent. PAE may be advantageous in use, as it may be less harmful than other possible wet strength resins.

An adding order of the selected components may affect the properties of the suspension, filament, and/or an end product such as fabric. Specifically, a selected adding order may result in selected properties of the filament, such as selected or preferred elongation and/or tenacity.

In one embodiment, a selected adding order may be MFC, the at least one base, the at least one cross-linking agent, the at least one wet-strength resin, optionally hydrophobic adhesive, optionally dispersion agent, and optionally a further wet-strength resin. Yet, also other adding orders are possible.

Selected components and selected adding order may in one embodiment & comprise: MFC, NaOH, the at least one triazine-based cross-linking agent,

N 25 PAE, CMC, and APAM. Further components may also be provided, their

I adding order not being specified.

O

2 The filament may be formed by extruding the aqueous suspension through a > nozzle to form a filament comprising cross-linked fibers. Such processes for

S forming filaments will be known to the skilled person. In particular, the 2 30 aqueous suspension may be extruded through at least one nozzle onto a

S drying surface, where the extruded agueous suspension is dried to form a dry filament.

In some embodiments, a method for forming a filament may additionally comprise a curing step, wherein the filament is subjected to a selected temperature for a selected time period to heat the filament. The curing step may occur after the drying of the filament.

A selected temperature for the curing step may be 70-200 °C, preferably 80- 150 °C.

A selected time period for the curing step may be 1-60 minutes, preferably 5- 30 minutes.

A method for forming a filament may additionally comprise a humidifying step, wherein the extruded suspension comprising said cross-linking agent is subjected to a selected relative humidity for a selected time period. The humidifying step may occur after the drying of the filament.

A selected relative humidity for the humidifying step may be 10-80%, preferably 50-70%, more preferably 60-65%. If a humidity is too high, hydrogen bonds of the filament may be broken down, reducing wet strength/tenacity of the filament and/or too high humidity may hinder the cross-linking reaction. The filament may exhibit improved qualities when a curing step is carried out in addition to a humidifying step. Only a humidifying step may lead to filaments with less desirable qualities.

A selected time period for the humidifying step may be at least 10 days, such as at least 1-15 days, or at least 3-6 days, such as at least 4-5 days. The time period for the humidifying step may depend on the temperature and/or relative humidity used in the humidifying step. For example, if the temperature is & under about 23 °C and the relative humidity is about 65%, the time period

N 25 — may be about at least10 days.

O

S In embodiments of the invention where a curing step and humidifying step are

I performed separately, the curing step preferably occurs before the > humidifying step. +

S One embodiment of the invention may comprise a combined humidifying and

N 30 curing step, wherein the suspension comprising said cross-linking agent may

N be subjected to a selected temperature and selected relative humidity for a selected time period. A combined humidifying and curing step may comprise subjecting the suspension comprising said cross-linking agent to steam,

comprising temperature of 80-150 °C, preferably 100-120 °C, relative humidity of 50-70%, preferably 60-65%, and for a time period of 5 minutes to 6 days, preferably 4-5 days.

With embodiments of the invention where a curing and humidifying step are provided, the properties of the filament may be adjusted or optimized to exhibit e.g. selected or desired tenacity, elongation, and/or alkali resistance.

In one other aspect of the invention, a filament is provided according to independent claim 18.

A formed filament may exhibit a structure where the cross-linking agent has penetrated into the entire filament (as opposed to methods where cross-linker is applied as a finishing step). The fibers may thus be cross-linked throughout the filament and not only on the surface.

BRIEF DESCRIPTION OF THE DRAWINGS

Next the invention will be described in greater detail with reference to exemplary embodiments in accordance with the accompanying drawings, in which:

Figure 1 shows structures of possible cross-linking agents and

Figure 2 shows an example of a crosslinking reaction.

DETAILED DESCRIPTION

Percentage values presented herein refer to an amount of percentage by © weight (wt.%) of dried filament, unless otherwise indicated. The word

S “comprising” may be used as an open term, but it also comprises the closed

O term “consisting of”. & In this text, the term “filament” refers to cellulose fibrils that are linked to each

E 25 other after an extrusion process. The formed filament may be a monofilament, < with the term “monofilament” referring here to the extruded and formed

O

5 filament, where the filament is a continuous length of individual fibrils grouped & and extending generally along the longitudinal dimension of the cellulose

N monofilament.

In the present disclosure, the expressions “non-regenerated cellulose” or

“natural cellulose” refer to cellulose or cellulose fibrils or fibers that have not undergone chemical or physical modification of its macromolecular structure.

Non-regenerated MFC as discussed here is substantially non-regenerated and consists mainly of crystalline structure of cellulose I. Cellulose | may have structures la and IB. Man-made cellulosic fibers commonly used e.g. in the pulp and paper industry are regenerated and the crystalline structure is mainly other than cellulose |. Conversion of cellulose | to cellulose II (or other forms, like cellulose III or cellulose IV) is irreversible. Thus, these forms are stable and cannot be converted back to cellulose |.

Cellulose material (pulp) is built up by a cellulose fiber matrix. The fibers that form such a matrix are fibril bundles which in turn consist of microfibrils.

Through a fibrillation process the cellulose fibers are separated into a three- dimensional network of microfibrils with a large surface area. These entangled fibrils are called microfibrillated cellulose (MFC). The width of entangled fibrils in MFC may be between 50 nm to 2 um and length or longitudinal dimension may be between 100 nanometers to 500 micrometers, such as 100 nanometers to 200 micrometers. The MFC utilized in connection with the present invention may non-regenerated MFC.

The method for manufacturing MFC is not limited. MFC may be isolated from cellulose fibers using methods known within the art through high pressure, high temperature and high velocity impact homogenization, for instance. The homogenization process is used to delaminate or disintegrate the cell walls of the fibers and to liberate their sub-structural fibrils and microfibrils.

Enzymatic and/or mechanical pre-treatments of wood fibers may also be used.

N

N Cellulose used in this invention may originate from any plant-based material.

O Plant based raw material may be wood or non-wood material. The wood

S material can be based on softwood tree, such as spruce, pine, fir, larch,

I Douglas-fir or hemlock, or hardwood tree, such as birch, aspen, poplar, alder, > 30 eucalyptus or acacia, or any mixture of above. The non-wood material may

S be as cotton, hemp, flax, sisal, jute, kenaf, bamboo, peat, or coconut. Non- 2 wood based natural cellulose fibers may also be derived from agricultural

S residues, grasses, or other plant substances such as straw, leaves, bark, seeds, hulls, flowers, vegetables, or fruits. Woody plants have a good availability, small environmental burden and the quality of fiber is good. The above applies both to non-regenerated cellulose and also regenerated and processed forms of cellulose.

The present invention provides a method for providing filaments comprising cross-linked fibers, where an aqueous suspension is provided, comprising at least natural non-modified microfibrillated cellulose (MFC) fibers and at least one cross-linking agent, wherein the suspension is dried to form at least one filament comprising 50 % wt. preferably at least 70% wt. MFC. A composition of the aqueous suspension may be selected according to desired properties of the suspension and/or produced fiber or filament.

In one embodiment, the aqueous suspension comprises water, cellulosic fibers (MFC), the cross-linking agent, and at least one dispersion agent, typically a cellulose derivative.

During manufacturing of filament, the aqueous suspension may be directed through a small nozzle where fibers align (orient) well with the flow. The nozzle may feed the aqueous suspension to a surface, after which the filament is dried in order to obtain fibrous monofilament. The fibrous monofilament may be produced via a single step process. A thus manufactured fibrous monofilament is continuous but it may be post processed into shorter lengths. Thickness of the fibrous monofilament may be affected at least in part by adapting manufacturing speed, aqueous suspension concentration and nozzle geometry.

In the drying process, the water content of the filament is essentially eliminated or at least significantly reduced.

Of the dry weight of a formed filament, 80-98 % wt. may be MFC, while 2-20 & % wt. may be the cross-linking agent and other additives, such as one or more

N 25 dispersion agent(s), strength additive(s) hydrophobic adhesive(s), ? pigment(s), and/or other modifier(s). The additives can be used for tailoring > properties of a produced filament or fabric as appreciated by the skilled

E person.

S The dispersion agent may be a cellulose derivative such as 2 30 carboxymethylcellulose (CMC), hydroxyethyl cellulose (HEC), ethyl < hydroxyethyl cellulose (EHEC), methyl cellulose (MC), hydroxypropyl methyl cellulose (HPMC), hydroxyethyl methyl cellulose (HEMC), methyl ethyl hydroxyethyl cellulose (MEHEC); hydroxypropyl cellulose (HPC); ethyl cellulose (EC) and starch or any mixture thereof. In one embodiment the dispersion agent is carboxymethyl cellulose (CMC), optionally with an additional dispersion agent.

The dispersion agent has an effect on shear strength of the fibrous monofilament. The dispersion agent may be used in an amount of 0.5 to 20 wt.-% of the total weight of the dry fibrous monofilament. In one embodiment

CMC is used 5 to 20 wt.-%, or 2 to 16 wt.-% or about 10 wt.-% of the total weight of the material fibrous monofilament.

For example, CMC may be used in an amount of 0.5 to 20 wt.-% of the total weight of the dry fibrous monofilament. In one embodiment CMC is used 5 to 20 wt.-% or about 10 wt.-% of the total weight of the material fibrous monofilament. In one embodiment CMC is used 4 to 5 wt.-% of the total weight of the material fibrous monofilament. In one embodiment CMC is used 14 to 16 wt.-% of the total weight of the material fibrous monofilament.

Strength additive(s) or agent(s) may be dry strength agents such as polyacrylamide resin (amphoteric / anionic / cationic), starch, vegetable gums, carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA) and latex or a wet strength agent such as cationic glyoxylated resins, polyamidoamine- epichlorohydrin — resins (PAE), polyamine-epichlorohydrin — resins, ureaformaldehyde (UFH), epoxide resins, amphoteric polyacrylamides (APAM), glyoxalated polyacrylamides (G-PAM), or polyethylene oxide (PEO).

The strength agent may be G-Pam. The amount of G-Pam may be 0.5 to 3 wt.-% of the total weight of the material fibrous monofilament, such as 2 wt.- % of the total weight of the dry fibrous monofilament. Use of G-Pam allows modifying the wet strength level from temporary towards permanent.

The strength agent may be APAM. The amount of APAM may be 0.5 to 5 wt.- ? % of the total weight of the dry fibrous monofilament, such as 2 to 4 wt.-% dry > weight of the fibrous monofilament. The higher is the amount of APAM the

E better is the elasticity of the nonwoven fabric. APAM is a super-flocculant also + usable as an additional dispersion agent. It improves alignment of fibers is

S 30 the suspension while they are extruded through a small nozzle on to a solid

S surface. &

The strength agent may be PEO in an amount of 0.5 to 5 wt.-% of the total weight of the dry fibrous monofilament, such as 1 to 3 wt.-% of the fibrous monofilament. PEO increases elasticity of the monofilament and the fabric here described.

A hydrophobic adhesive may be alkyl ketene dimer (AKD), alkenylsuccinic anhydride (ASA), rosin, natural waxes or modified sun flower based adhesive (MSOHO) or any mixture thereof.

Said hydrophobic adhesive may be AKD. The amount may be 0.5 to 10 wt.- % of the total weight of the dry fibrous monofilament, such as 2 to 5 wt.-% of the total weight of the dry fibrous monofilament. As a hydrophobic adhesive,

AKD may reduce the adsorption properties of the monofilament or or a produced fabric. AKD may also increase strength of the monofilament or fabric.

In one embodiment, an aqueous suspension may comprise at least MFC, at least one base, such as NaOH, the at least one cross-linking agent, at least one strength resin, optionally polyamideamine-epichlorohydrin (PAE), anionic polyacrylamide (APAM), and/or glyoxalated polyacrylamide (GPAM), optionally at least one hydrophobic adhesive, such as alkyl ketene dimer (AKD), and optionally at least one dispersion agent, such as carboxymethylcellulose (CMC).

Figure 1 shows example structures of molecules that may be utilized as cross- linking agents. Fig. 1A exhibits a general structure of dichlorotriazine-based cross-linking agents that may be used as cross-linking agents in the present invention. X may be OR, NHR, or SR, where R is a hydrogen atom, a sodium or alkyl chain, or a chromophore group.

Fig. 1B shows the structure of 2-sodium hydroxy-4,6-dichloro-1,3,5-triazine, & which has been used as a cross-linking agent in the example test cases given 3 25 — further below.

S Figure 1C shows a general example structure of one other group of

I dichlorotriazine-based cross-linking agents, where R is an alkyl group, while > Fig. 1D shows another general example structure of one further group of

S dichlorotriazine-based cross-linking agents, where D is a chromophore. The 2 30 example of Fig. 1D relates to a group of molecules known as reactive dyes. oo

N Figure 2 shows an example of a cross-linking reaction that may occur in some embodiments of methods of providing a cross-linked filament according to the invention. Here, the cross-linking agent is 2-sodium hydroxy-4,6-dichloro-

1,3,5-triazine. The reaction is a two-step reaction, where the first step (1) may occur at a lower temperature than the second step (2). The first step of the reaction may occur when conditions of a temperature of at least between 5 °C and 25 °C are provided, and thus may take place at room temperature.

This step of the reaction may begin taking place as soon as the aqueous suspension is prepared. The first step is related to the cross-linking agent molecule being linked to a cellulose molecule by the cellulose molecule replacing one of the CI atoms of the cross-linking agent.

The second step of the reaction may take place at a temperature that is higher than room temperature, such as above 80 °C or above 100 °C. In this step of the reaction, a cross-linking agent that has undergone the first step of the reaction, and thus has one of the Cl atoms replaced by a cellulose molecule, has the other remaining CI atom replaced by a further cellulose molecule.

After the second step of the cross-linking reaction, two cellulose molecules are thus cross-linked together. The second step of the reaction may be enabled or enhanced by carrying out a curing step as part of the method for providing a filament, which will be discussed further below.

The alkali resistance of the filament comprising cross-linking agent (referred to herein also as cross-linked filament) produced through the present invention was tested by preparing such a filament and comparing to a reference filament. The aqueous solutions used to prepare the cross-linked filament and the reference filament where otherwise the same, except for the cross-linked filament comprising the cross-linking agent (3% wt.) and NaOH.

An amount of 5 grams of the reference filament and the filament according to

Q 25 — the invention were both subjected to a solution of 0.5 mol NaOH for a time

N period of 10 minutes. After dipping the filaments in the NaOH solution for 10

O minutes and subseguent drying in room temperature of 24 hours, the

S filaments were weighed, and it was observed that the cross-linked filament

I had retained a weight of 5g, while the reference filament had lost weight of > 30 0.07 grams, i.e. weighing 14% less (0.43 grams) after the NaOH treatment. It

S may thus be concluded that the cross-linked filament does not dissolve in the 2 alkalic conditions, thereby providing alkalic resistance as compared to the

S reference fiber.

Table 1 shows the effect of the amount of cross-linking agent used on the properties of a formed filament prepared from aqueous suspensions with two different types of MFC. The workability refers to property of the formed filament, where workability (cN/tex%) = elongation * tenacity * 10. Tenacity is the customary measure of strength of a fiber or yarn. It is usually defined as the ultimate (breaking) force of the fiber (in gram-force units) divided by the linear density. Mostly tenacity is expressed as cN/tex.

In the examples, the used cross-linking agent is 2-sodium hydroxy-4,6- dichloro-1,3,5-triazine. The percentages of components refer to % wt. of dry filaments. The aqueous suspensions were otherwise similar in terms of other constituents and their amounts, except for the type of MFC used and the amount of cross-linking agent. Test numbers 1-6 comprised MFC having a hemicellulose content of under 5% and test numbers 7-11 comprised a hemicellulose content of over 5%. Other components of the aqueous suspension are NaOH, PAE, AKD, CMC, and A-PAM.

Table 1: amount of crosslinker crosslinker (CL); tenacity (T); elongation (E); workability (W)

CL T E W T E W

(dry) dry) (dry) (wet) (wet) (wet)

Je Jo [Wf

J fre oe fw ow fw fm

N

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O sf mf [ww rr

Efe fm fw [ow fw Jw +

El fe fr fr =

O pepe to

PR srt i"

Pe Tw Te fe fe pe +

Table 1 shows that when the hemicellulose content of the MFC is under 5%, the wet workability is increased as the amount of cross-linking agent is increased from 0% to 4%, but at crosslinker percentages of 8% and 10%, the wet workability is somewhat decreased from that which is shown at 4% crosslinker.

With hemicellulose content of MFC of over 5%, the wet workability is increased as the amount of cross-linker is increased from 0% to 3%, while at cross-linker percentages of 5% and 10%, the wet workability is decreased from that which is seen at 3% cross-linking agent.

Therefore, an optimal amount of cross-linking agent may depend on the type of MFC that is used in the aqueous solution. The preferable amount of cross- linking agent may depend on the hemicellulose content of the MFC.

Table 2 exhibits the effect of an adding order of selected components to the aqueous suspension. In all of the test cases, the components of the suspensions are the same except for the amount of cross-linking agent, which was 2-sodium hydroxy-4,6-dichloro-1,3,5-triazine. The other components are

NaOH, CMC, A-PAM, AKD, PAE , and MFC (remaining portion), where percentages are weight percentages of the filament after drying. The dry conditions workability is given, which is determined after drying the filament = but before curing and humidifying. The initial wet, cured workability refers to

N wet workability after the drying and a curing step. Wet workability is given also 3 as after a humidifying step, which has occurred after determination of the 2 initial wet cured workability. During the humidifying step, the filament was

I 25 stored in atmospheric conditions, 23 °C and RH 65% for 10 days. The curing 3 and humidifying process steps will be further considered in connection with

S table 4.

O Table 2: amount of crosslinker (CL); Tenacity (T); elongation (E), workability (W)

ja oy [ese [Met oa des [ Emr Ew few

Gc CO am CO

Coc Ca a fan

EE Cc joja e jee ep Cc joja je join ee je Ps jej joja je jois

Table 2 shows that the highest wet workability (both cured initial and cured and humidified) is achieved by test number 3, which relates to an agueous suspension for which the adding order is MFC, NaOH, cross-linking agent,

PAE, AKD, CMC, and A-PAM. The amount of cross-linking agent is 5 % wt (2-sodium hydroxy-4,6-dichloro-1,3,5-triazine).

Table 2 also shows that wet workability (both cured initial and cured and humidified) with 3% cross-linking agent is improved over the reference case with no cross-linking agent, while 5% cross-linking agent yields even higher wet workability. n With the reference case, a decrease of wet workability is exhibited when

S comparing the cured and humidified value to the initial cured value. However,

O when the cross-linking agent is present, the wet workability is even further = increased after the humidifying to provide substantially higher workability than 9 15 for the reference case. a - For the cases with 3% cross-linking agent, the wet workability after

S humidifying compared to the initial, before humidifying is increased by 53%

Q and 46% in the performed tests. Cases with 5% cross-linking agent show an & even higher enhancement of wet workability after 10 days as compared to the initial, with increases of 119% and 95%.

It can be further observed that the dry workability of the cases where cross-

linking agent is used is comparable to the reference case, or is at least not dramatically reduced. The dry workability is decreased compared to the reference case only by about 10% maximum, with reductions in all cases being between about 5%-10%. Also notable is that the wet workability (especially the case where both curing and humidifying is carried out) provided in cases where cross-linking agent is employed may be at a similar level as the dry workability. The wet workability in the cured and humidified condition with the cases with cross-linking agent shows only about 3%-23% lower values than the dry workability. With test number 3 where the amount of cross-linking agent is 5 % wt and the adding order is MFC, NaOH, Cross- linking agent, PAE, AKD, CMC, and A-PAM, the difference is only about 3%.

It is also seen, based on tests 4-5, that with the adding order MFC, NaOH,

PAE, AKD, CMC, Cross-linking agent, and A-PAM, an amount of 5 % wt. cross-linking agent gives higher (better) workability (initial and cured) than 3 % wt. of cross-linking agent.

Therefore, in a preferred embodiment, selected components and adding order may be: MFC, NaOH, cross-linking agent, PAE, AKD, CMC, and A-

PAM. Also other adding orders are, of course, possible. Furthermore, selected components may also comprise other components. Selected components may comprise further components and/or some components, such as AKD, may be left out. AKD may be used to improve e.g. softness feeling of filament, but its use is optional. A selected adding order may be:

MFC, at least one base, cross-linking agent (2-sodium hydroxy-4,6-dichloro- 1,3,5-triazine was used in the example case), and at least one wet-strength resin. NaOH may be replaced by another base, PAE and/or A-PAM may be & replaced by other wet-strength resins, and/or CMC may be replaced by

N another dispersion agent.

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S Table 3 shows the effect of pH of the aqueous suspension (a final pH, after

I all constituents have been added) on the filament properties. In the test cases - 30 2,and3, an amount of NaOH is varied, while once more 2-sodium hydroxy-

S 4,6-dichloro-1,3,5-triazine is used as the cross-linking agent. For the test case 2 rows where an amount of crosslinker is 0%, the components of the

S suspensions are the same except for the amount of NaOH. These components are MFC, CMC, A-PAM, and PAE. Additionally, test case 2 shows rows for an amount of NAOH of 1g, giving 0.5 M of NaOH.

Table 3: amount of crosslinker (CL); amount of 0.5 M NaOH (B); tenacity (T); elongation (E); workability (W). The wet strength properties of cured samples: initial or humidified ones.

FF en oe [om [or [ie % |(g) |pH cN/ctex | % cN/dtex | % wi. fer fas fre fr js ol er

Eo Cc för fa js as = ff je fom fr iso [= [s fre fa | frn an en

Table 3 thus shows that the workability is higher when the pH of the suspension is about 8.5 as compared to when the pH is at a level of 12-14.

Additionally, when no NaOH is used, the wet workability exhibits a drop with time (from wet, cured, initial to wet, cured, humidified) that is higher than a corresponding drop in cases where 1 g of NaOH was used to obtain a pH of 8.5. It may be concluded that a pH of about 8-9 or 8.5-9 (after final preparation of the suspension) may be advantageous. @ Altering of the pH of the suspension was in the above example carried out by & using a base. Also some other form of catalyst may be used to facilitate the 8 cross-linking reaction.

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© 15 The aqueous suspension may also be provided with one or more dyes, such z as direct dyes and/or reactive dyes. In tests comparing e.g. fabric made from 3 filaments formed by a method according to the invention where the agueous 5 suspension is provided with a dye to fabric made of filaments including dyes

N where an agueous suspension does not contain the cross-linking agent of the

N 20 invention, it may be observed upon visual inspection that the color of the fabric made from filaments formed by a method according to the invention is deeper than the color of the comparison reference fabric.

In methods for forming the filament, the method may comprise a curing step and a humidifying step or a combined curing and humidifying step. The filament may be subjected to a selected temperature for a selected time period and to a selected relative humidity for a selected time period. Here, a filament may have been formed such that the suspension comprising the cross-linking agent has been extruded through a nozzle and preferably also already dried. In other words, the curing and humidifying is performed on a formed filament. The curing preferably occurs before the humidifying (or they are simultaneous).

The used relative humidity and temperature affect the time periods that are preferably employed in the curing and/or humidifying step or a combined curing and humidifying step.

Examples giving favorable results in terms of wet tenacity and elongation may be relative humidity of 60-65 % for at least 4-5 days for a humidifying step and 100- 120 degrees Celsius for 10-20 minutes for the curing step. The conditions for curing and/or humidifying step may depend on the scale or the amount of filament that is to be treated, for instance.

It is notable that the curing and humidifying may enable aiding of penetration of the cross-linking agent into the amorphic regions of the fibrils, enhancing the cross-linking and improving filament properties.

Table 4 shows the effect of curing and humidifying on properties of a filament comprising cross-linked fibers. The storing procedure refers to a storage time during which the filaments have been stored in room temperature and RH 30 e conditions. Determining of filament properties after storing may give an

S 25 indication of how the properties develop. As is indeed shown by table 4, © properties of a formed filament determined directly after e.g. humidifying may

O differ from those seen after storing.

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E Table 4: Wet strength properties after curing, humidifying and storing a + sample filament in different conditions.

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S Procedure Conditions cN/dtex | % om jee [rr rma [20 re zn jo jon ja

Storing 0.63 6.5 41

RH30,64hrs

Curing + e. RH60 1.09 7.05 77

Humidifying 24hrs rir joe fra fre fe

Storing 1.21 7.33 88

RH30, 48hrs

From table 4, it may be observed that after a curing step with conditions 120 °C, 20 min and a humidifying step of conditions 23 °C, RH60, 24hrs, the tenacity determined after storing in room temperature at RH30 for 64 hours is decreased to a value of 0.63 from a value of 0.99 which was determined directly after the first humidifying step. This may indicate that the humidifying of 24 hours is not long enough to ensure that properties of the filaments are retained. n The same sample was then subjected to a curing and humidifying step with

S 10 curing conditions 120 °C, 20 min and humidifying conditions 23 °C, RH60, 48

O hrs. After this the sample was once more subjected to a further humidifying = step with conditions 23 °C, RH60, 48hrs. After this and a storing at room © temperature and RH30 for 48 hours, it was observed that the properties of

E the filament (in particular tenacity) did not decrease after the storing. It may 3 15 thus be determined that after the longer humidifying periods (i.e. not leaving 5 the humidifying at only 24 hours), the cross-linking properties have been

N maximized and the filament properties no longer show detrimental decrease

N in value after storing.

The invention has been explained above with reference to the aforementioned embodiments, and several advantages of the invention have been demonstrated. It is clear that the invention is not only restricted to these embodiments, but comprises all possible embodiments within the spirit and scope of inventive thought and the following patent claims.

The features recited in dependent claims are mutually freely combinable unless otherwise explicitly stated.

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Claims (1)

1. A method for forming a filament comprising cross-linked fibers, the method comprising - providing an aqueous suspension comprising at least natural non- modified microfibrillated cellulose (MFC) fibers and a dichlorotriazine- based crosslinker as a cross-linking agent, and - drying the aqueous suspension to form at least one filament, wherein the filament comprises at least 50 % wt. preferably at least 70 % wt,

MFC.

2. The method of any previous claim, comprising adjusting an initial pH of the suspension comprising MFC, before adding said cross-linking agent, using a base until the pH of the suspension is 7-12, such as 8-

11.

3. The method of any previous claim, wherein the amount of cross-linking agent is 0.1-10 % wt, such as 0.5-7 % wt, or 3-5 % wt of the dried filament.

4. The method of claim 3, wherein the amount of cross-linking agent is selected based on the MFC-type and associated hemicellulose content of the MFC, such that the amount of crosslinking agent is decreased with increasing amount of hemicellulose content.

5. The method of claim 4, wherein if the hemicellulose content of the MFC is under 5% wt of the dry weight of the MFC, the amount of crosslinking > agent is between 2-8% wt, such as 3-5 % wt of the dried filament, and < if the hemicellulose content of the MFC is over 5% wt, such as over O 25 10% wt of the dry weight of the MFC, the amount of crosslinking agent S is under 2% wt, such as about 1% wt of the suspension.

E 6. Themethodofany previous claim, the method comprising providing the 3 aqueous suspension with selected components in a selected adding s order. N 30 7. The method of claim 6, wherein the selected components comprise at least MFC, at least one base, such as NaOH, the at least one cross- linking agent, at least one strength resin, optionally polyamideamine-

epichlorohydrin (PAE), anionic polyacrylamide (APAM), and/or glyoxalated polyacrylamide (GPAM), optionally at least one hydrophobic adhesive, such as alkyl ketene dimer (AKD), and optionally at least one dispersion agent, such as carboxymethylcellulose (CMC).

8. The method of claim 7, wherein the selected adding order is: MFC, the at least one base, the at least one cross-linking agent, the at least one wet-strength resin, optionally hydrophobic adhesive, optionally dispersion agent, and optionally a further wet-strength resin.

9. The method of claim 7 or claim 8, wherein the selected components and adding order comprises: MFC, NaOH, cross-linking agent, PAE, CMC, and APAM.

10. The method of any previous claim, wherein the cross-linking agent is 2- sodium hydroxy-4,6-dichloro-1,3,5-triazine.

11. The method of any previous claim, wherein the filament is formed by extruding the aqueous suspension through a nozzle to form a filament comprising cross-linked fibers.

12. The method of any previous claim, additionally comprising a curing step, wherein the filament is subjected to a selected temperature for a selected time period to heat the filament after drying.

13. The method of claim 12, wherein said selected temperature is 70-200 °C, preferably 80-150 °C. e 14. The method of claim 12 or claim 13, wherein said selected time period S is 1-60 minutes, preferably 5-30 minutes. O <Q 15. The method of any previous claim, additionally comprising a > 25 humidifying step, wherein the filament is subjected to a selected relative E humidity for a selected time period after drying. > IS 16. The method of claim 15, wherein said selected relative humidity is 10- & 80%, preferably 50-70%, more preferably 60-65%. N 30

17. The method of claim 15 or claim 16, wherein said selected time period is at least 10 days, such as at least 1-15 days, or at least 3-6 days, preferably at least 4-5 days.

18. The method of any previous claim 1-11, additionally comprising a combined humidifying and curing step, wherein the filament is subjected to a selected temperature and selected relative humidity for a selected time period, optionally wherein the combined humidifying and curing step comprises subjecting the filament to steam, comprising temperature of 80-150 °C, preferably 100-120 °C, relative humidity of 50-70%, preferably 60-65%, and for a time period of 5 minutes to 6 days, preferably 4-5 days.

19. A filament produced by the method of any previous claim. O N O N O 3 O O + O N LO O N oo Al