EP3512993B1 - Method for producing a wet-laid nonwoven fabric - Google Patents

Description

The invention relates to a method for producing a wet-laid nonwoven fabric.

Known processes for producing nonwovens from natural fibers such as cellulose fibers generally include the formation of a fibrous web and subsequent dewatering, such as drying. Different methods of nonwoven formation are known from the prior art. The formation of the fibrous web is usually carried out by a wet laying process on an inclined wire former with a very low consistency of the fibrous suspension, in particular with a solids content of 0.01 to 0.1% by weight based on 100% by weight of the fleece obtained. EP2634297 discloses a method for producing a nonwoven fabric with fibers and binding fibers.

As a rule, natural fibers form hydrogen bonds with each other as soon as they are placed in water. This makes it possible for nonwoven webs to be produced from natural fibers without the use of binders in the fiber suspension. Such bonds do not occur with artificially produced fibers, such as fibers made from synthetically produced polymers and especially with industrially produced, inorganic fibers. Until now, appropriate chemical binding agents or thermal binding fibers had to be used in order to bind such fibers to one another and thus obtain a stable fleece using the wet-laying process. On the one hand, such chemical binders can be added to the fiber suspension as chemical reagents. On the other hand, wet-laid nonwoven webs were subsequently soaked in a binder section with such a binder. Both processes have the disadvantage that a certain amount of chemical additives are required, the storage, handling and disposal of which can be problematic. When the nonwoven web is impregnated using the binder section, contamination occurs due to encrusting binder, which, if not removed quickly, results in a lot of cleaning effort. As an alternative to chemical solidification processes, thermal solidification processes are used Binding fibers in the form of thermoplastics are added to conventional fibers of the pulp suspension. These are melted in a later process step, then surround the fibers and stick them together after they have cooled.

However, the previous chemical and thermal solidification processes have the disadvantage that the binders or binding fibers used are not suitable for high temperature applications above, for example, 300 ° C.

The present invention relates to the aforementioned generic objects.

The present invention is based on the object of specifying a method of the type mentioned at the outset, with which the aforementioned problems can be eliminated in the simplest and most reliable way possible. In particular, a method is to be specified in which the addition of a chemical binder or thermoplastic binding fibers in the fiber suspension can be largely dispensed with. This happens as soon as artificially produced fibers are used for wet-laid nonwovens - especially in high-temperature applications.

The task is solved according to the independent claims. Particularly preferred and advantageous embodiments of the invention are set out in the subclaims.

For the purposes of the invention, a fibrous web is to be understood as meaning a fabric or tangle of fibers of limited length made from a fibrous suspension, e.g. continuous fibers (filaments) or cut yarns. The fibrous web initially has such a low strength that it itself is not load-bearing.

A nonwoven fabric or nonwoven web in the sense of the invention is a structure made of fibers that in some way form a nonwoven (ie a fiber layer or a Fiber pile) have been put together and, for example, connected to one another in some way. For the purposes of the present invention, it is a wet-laid nonwoven, i.e. a hydraulically (also: hydrodynamically) formed nonwoven. In other words, a nonwoven is a consolidated, in particular finally consolidated, fibrous web. In other words, the fibrous web is an intermediate product of the finally produced, fully solidified nonwoven web. Such a nonwoven material is considered to have been finally solidified if, due to the solidification, it essentially has such a high strength that it is suitable for its intended use, for example for its further processing into corresponding products such as hygiene articles. Hydraulic pre-consolidation is understood to mean a consolidation that does not yet convert the fibrous web into a nonwoven fabric because it does not achieve the necessary degree of consolidation. For the purposes of the present invention, (final) solidification can also be a combination of (also multi-stage) water jet solidification - i.e. a hydraulic solidification process - and additional impregnation using a binder - i.e. a chemical solidification process. Following the impregnation of the nonwoven web using the binder that was applied to it in a binder section, the nonwoven web can be dried. Optionally, subsequent mechanical consolidation, for example using a needle machine, can further increase the strength of the nonwoven web.

Nonwovens within the meaning of the invention do not include fiber structures produced by crossing or intertwining yarns, as occurs in weaving, knitting, knitting, lace production, braiding and the production of tufted products. Films and papers are also not considered nonwovens.

The term hydroentanglement or waterjet needling is a hydraulic consolidation process for producing a strong bond between the fibers of a nonwoven. This causes the fibers to become entangled so that the fleece is compacted and solidified by swirling, e.g. B. focused high-pressure water jets act on the fibrous web.

For the purposes of the invention, fiber suspension is to be understood as meaning a mixture of a liquid - such as water - and fibers.

Forming wire and/or carrying wire are usually designed as endless, self-contained loops, for example running on rollers. They can be set up in such a way that the fibrous web can be water jet needled on it. This means that the corresponding forming fabric and/or supporting fabric is permeable to water so that the water jets can pass through it.

A former, such as an inclined sieve former, in the sense of the invention is assigned a forming sieve which, at least in some sections - for example along a first section - runs at an angle to the horizontal. At least one headbox is then arranged in this section in such a way that it applies the fibrous suspension to the top of the forming fabric. Topside means that the fiber suspension is applied to the top of the forming fabric. This is the side that, on the one hand, faces away from the rollers on which it rotates and, on the other hand, faces the outlet of the headbox. On the underside, i.e. in the area of the underside of the forming fabric, at least one drainage element can be arranged for draining the fibrous suspension that has just been applied. The headbox can in turn be assigned to the inclined wire former. As a rule, the inclined screen former is arranged in such a way that the first section rises in the direction of the deposited fibrous web at an angle to a horizontal plane.

According to the present invention, when it is said that the fiber suspension is essentially free of binders, what is meant is that it contains less than 10% by volume, preferably less than 5% by volume and particularly preferably no binders. Binders are substances that cause bonding of the fibers reach each other, so that, for example, there is a firm bond between the fibers. The term binder includes chemical binders which, for example, are applied to the fibrous web in liquid form or mixed into the fibrous suspension. They connect the fibers together through adhesion.

The decomposition temperature is the temperature at which the material of the fibers decomposes chemically or thermally. The decomposition temperature is characteristic, for example, of materials that do not melt, such as thermosets. The melting temperature is the temperature at which the material, for example the fiber, changes from the solid state to the melt.

The term modulus of elasticity refers to a material parameter from materials engineering that describes the relationship between stress and strain during the deformation of a solid body with linear-elastic behavior. The term bending stiffness means the product of the modulus of elasticity with the corresponding moment of inertia. For example, with the same moment of inertia, one material or a fiber made from it is more rigid than the other if it has a higher modulus of elasticity in comparison. For the purposes of the invention, a fiber is flexible if it or its material has a modulus of elasticity that is below 10 GPa and rigid if the modulus of elasticity is at least 10 GPa.

Fire-retardant is a material that maintains its function over a certain period of time in the event of a fire - i.e. does not melt or decompose - and is, for example, flame-retardant.

The term binding fibers means fibers that have a lower modulus of elasticity than the fibers of the fibrous suspension according to the invention and are therefore more flexible - with the same moment of inertia as the fibers according to the invention. The admixture of the binding fibers to the rigid fibers according to the invention enables the latter to form as a result of the entanglement rigid fibers, which thus form a matrix, solidify with one another. The binding fibers thus indirectly enable better solidification of the rigid fibers with one another. The binding fibers can - but do not necessarily have to - like conventional fibers known from thermal solidification processes, also be meltable, i.e. made from thermoplastics. In this case, these binding fibers can be fused together by thermal activation, such as thermofusion or thermocalendering. For this purpose, the material of these binding fibers can have a melting temperature below 300° C.

Particularly when it comes to high temperature applications of at least 300° C, the comparatively rigid fibers as well as the binding fibers should be made from a material that has a decomposition or melting temperature of at least 300° C. The statement at least 300°C means above 300°C and therefore includes higher temperatures, e.g. above 350°C or above 500°C. In such areas of application, the advantages according to the invention are particularly satisfactory. This is where the previous chemical and thermal solidification processes fail. Above these temperature(s), the chemical binders as well as the thermal binding fibers break down, causing the rigid fibers to lose their bond and the fleece to dissolve. When we talk about high-temperature applications, this means that the nonwovens produced with the invention or the end products in which the nonwovens are processed in an ambient temperature of at least 300 ° C, preferably above 350 ° C and particularly preferably above of 500° C can be operated or used. This contrasts with low temperature applications below 300°C.

Such nonwovens can preferably be made from glass, metal, mineral, ceramic or carbon fibers. One then speaks of technical nonwovens. Such fibers can also be plastic fibers such as aramid fibers, but also mineral fibers such as basalt fibers. For metallic fibers, for example, steel, stainless steel or titanium fibers come into consideration. The materials mentioned have a modulus of elasticity of at least 10 GPa. They are then comparative hard, brittle and rigid and cannot easily intertwine and tangle with one another. It is therefore particularly advantageous if, in addition to the fibers, binding fibers that are less rigid are used.

Regardless of the embodiments shown, both the comparatively flexible and the comparatively rigid fibers are preferably designed to be fire-retardant.

For example, if the nonwoven web is hydraulically solidified on the forming fabric - and preferably finally there - then the overall length of the device for producing a nonwoven web in the running direction of the nonwoven web to be produced can be significantly reduced. However, it would be conceivable to design the hydraulic consolidation in several stages. Pre-solidification by water jet consolidation could initially take place on the forming fabric and the final solidification could take place in a further process step outside the forming fabric.

In principle, it would be conceivable to produce a nonwoven web that is completely free of chemical binding agents and (thermal) binding fibers. However, applications are conceivable in which, following the hydraulic bonding, the nonwoven web is also thermally or chemically bonded, for example by impregnating it with a binder. This further increases the degree of strength of such a material. Nonwovens produced in this way can be usefully used in low-temperature applications below 300° C.

It would also be conceivable to additionally structure the nonwoven web, which has been solidified - preferably using water jets. This can also be done using appropriate structuring devices, for example water jets. Then not only can the strength of such a nonwoven web be improved, but it can also be given a predetermined structure.

In order to dry the solidified nonwoven web quickly and effectively, it can be dewatered mechanically, for example using a press, using a vacuum suction or thermally using a dryer (e.g. using through-flow drying technology, then called a through-flow dryer).

The present invention further relates to the product produced directly using the method according to the invention, i.e. the nonwoven material itself.

A method for converting a device for producing a wet-laid nonwoven web is also described. This means that existing devices that specialize in thermal or chemical binders can be converted to hydraulic consolidation quickly, easily and cost-effectively. The overall length of such a converted device can even be reduced if the previous binder section is removed.

Fig. 1

eine stark schematisierte Darstellung einer Vorrichtung zur Herstellung eines erfindungsgemäßen Vliesstoffes in einer Seitenansicht gemäß einer ersten Ausführungsform;

Fig. 2

eine stark schematisierte Darstellung einer Vorrichtung zur Herstellung eines erfindungsgemäßen Vliesstoffes in einer Seitenansicht gemäß einer weiteren Ausführungsform.

The invention is explained in more detail below with reference to the drawings without restricting the generality. In the drawings shows:

Fig. 1

a highly schematic representation of a device for producing a nonwoven fabric according to the invention in a side view according to a first embodiment;

Fig. 2

a highly schematic representation of a device for producing a nonwoven fabric according to the invention in a side view according to a further embodiment.

In the Fig. 2 a device for wet-laying a nonwoven web according to the invention is shown schematically in a side view and therefore not to scale. The device comprises a former, here designed as an inclined wire former 1. This is assigned an endless forming screen 2, which here rotates on rollers. The latter rotates relative to the fixed inclined wire former 1. Above the forming wire 2, a headbox 1.1 is arranged. The latter is dem Inclined screen former 1 assigned. A fibrous suspension can be fed to the headbox 1.1 and can be applied via an outlet of the headbox 1.1 to the forming wire 2, more precisely to its upper side. The fiber suspension usually has a water-fiber mixture. The forming screen 2 is designed so that it allows the water to pass through. Below the forming fabric 2, on the side facing the headbox 1.1, a drainage box 1.2 is arranged for removing the water from the pulp suspension. The drainage box 1.2 is assigned to the inclined screen former 1.

When the device is operating as intended, the pulp suspension passes through the outlet of the headbox 1.1 onto the forming fabric 2, which moves past the headbox 1.1 or the drainage box 1.2 via the rollers. The water flows through the forming fabric 2 into the drainage box 1.2. The fibers from the pulp suspension remain attached to the forming wire 2 and are transported further with it. In this way, a corresponding fibrous web F is continuously deposited or formed on the forming fabric 2.

The forming fabric 2 is - viewed in its running direction or in the running direction of the fibrous web F - inclined upwards against the horizontal in a first section. The inclined screen former 1 is arranged in this first section, i.e. the fibrous web F is formed on this section. The first section is delimited by the upper rollers, which follow one another in the running direction of the supporting sieve 2. At least two such upper rollers are provided for this purpose. In the illustration shown, the forming fabric 2, which here rotates in a clockwise direction, rises from the bottom left to the top right in the said first section.

After its formation, the fibrous web F is passed under the solidification device 4 on the forming wire 2 for its hydraulic consolidation. The latter is assigned a plurality of water jet nozzles 4.1, which are here above the forming fabric 2 and an outlet 4.2 for water, which is below the forming fabric 2. As shown, the forming fabric 2 runs in the area in which the water steel nozzles 4.1 and the drain 4.2 are arranged horizontally or at least in sections essentially parallel to the horizontal plane. According to this embodiment, the fibrous web F is finally solidified on the forming fabric 2 to form the nonwoven web V.

The former thus forms the forming section of the device. In the running direction of the nonwoven web V to be produced, in the present case a binder section of the device immediately follows the forming section. This comprises an application device 7, which is arranged above a transport sieve 5 which runs horizontally or at least in sections essentially parallel to the horizontal plane. By means of the application device 7, the finally hydraulically consolidated nonwoven web V can be impregnated with a chemical binder. In the running direction of the nonwoven web V to be produced (in the view of Figure 2 from left to right) a thermal drying device, for example, can be connected directly to the binder section in order to dry the nonwoven web V provided with a binder (not shown).

In the Figure 1 is a further development of the embodiment from Figure 2 shown. Essentially the same components are shown there and labeled accordingly as in Figure 2 . In addition, however, a further solidification device 4 is arranged. This directly adjoins the forming fabric 2 as seen in the running direction of the nonwoven web V to be produced. The further solidification device 4 also includes a plurality of water jet nozzles 4.1, which here are arranged above a supporting sieve 3 and an outlet 4.2 for water, which is arranged below a supporting sieve 3. The further solidification device 4 is therefore located in the running direction of the nonwoven web to be produced after the forming section and the optional binder section (each immediately upstream).

As shown in the two figures, the (first) hydraulic consolidation device 4 is assigned a pre-consolidation device 6 in the running direction of the nonwoven web to be produced. In principle, this can be set up analogously to the hydraulic consolidation device 4, but with a lower Pressure can be operated as the solidification device 4, which is, for example, only 5 to 25 bar. The respective solidification device 4, however, can be operated with a pressure of 15 to 400 bar.

As shown in the figures, the fibrous web F is solidified from one side, here the top, i.e. the side facing away from the forming wire 2 or the supporting wire 3. In principle, it would also be conceivable to additionally solidify the fibrous web F from its underside. This could be done according to the Figure 1 The further solidification device 4 may be immediately followed by a further solidification device, not shown, in the running direction of the nonwoven web V to be produced. This could be upstream of the binder section shown in the figures in the direction mentioned. In the embodiment of the Figure 2 This would be arranged immediately downstream of the forming fabric 2 or the forming section in the same running direction. Such a solidification device could comprise a cylinder around which the fibrous web F at least partially wraps for its (final) solidification. A large number of water jet nozzles are then directed onto the cylinder in order to apply water jets to the fibrous web, which is partially guided around the cylinder, from its underside.

Regardless of the embodiment shown, the rigid fibers according to the invention are mixed into the fiber suspension. The fiber suspension can be essentially free from the addition of a (chemical) binder. Binding fibers can also preferably be added to these fibers - either already in the fiber suspension - or shortly before the (first) hydraulic solidification. As stated at the beginning, the binding fibers can be designed to be comparatively pliable in relation to the fibers according to the invention in order to provide a higher overall strength for the nonwoven web during hydraulic solidification by entangling them.

In principle, it would be conceivable to dispense with the binder section, regardless of the embodiment shown. A conventional device for producing such a nonwoven web generally includes the following: Figures 1 and 2 shown Components, but no (pre-)consolidation devices 4 and 6. In order to convert such a conventional device according to the invention, the (pre-)consolidation devices 4 and 6 mentioned are now mounted at the locations mentioned and the binder section is then preferably removed. Instead of this, for example, a conventional transport sieve can also be provided. This means that conventional devices that thermally or chemically bond the fibers to produce a nonwoven web without hydraulic consolidation can be equipped with one. Such a conventional device can be converted quickly, easily and inexpensively to a device according to the invention.

Reference symbol list

1

Inclined screen former

1.1

Headbox

1.2

drainage box

2

forming screen

3

carrying sieve

4

Solidification device

4.1

Water jet nozzles

4.2

Sequence

5

Transport sieve

6

Pre-consolidation device

7

Order setup

F

fibrous web

v

Nonwoven web

Claims (9)

1. Method for producing a wet-laid non-woven web (V), comprising the following steps:

   a) feeding a fibrous slurry to a forming fabric (2) to deposit a fibrous web (F) thereon, wherein the fibrous slurry comprises industrially created, inorganic fibres or fibres of synthetically created polymers and the fibrous slurry is substantially free of - preferably chemical - binders, i.e. that the fibrous slurry contains less than 10% by volume of binders, preferably less than 5% by volume of binders and particularly preferably no binders, wherein applicable binders are agents that bond the fibres together;

   b) entangling the fibrous web (F) to create the non-woven web (V) by water-jet entanglement of the fibrous web (F),

   wherein, in addition to the fibres, the fibrous slurry also comprises binding fibres, or the latter are fed to the fibrous web (F) before or during the entangling operation,

   characterized in that the fibres are selected such that their decomposition or melting temperature is at least 300°C, and the material of the fibres has a modulus of elasticity of at least 10 GPa, whereas the binding fibres have a modulus of elasticity which is less than 10 GPa.
2. Method according to one of Claims 1, characterized in that the material of the fibres comprises glass, metal, mineral, ceramic or carbon.
3. Method according to either of Claims 1 and 2, characterized in that the fibres have an average length of 2 mm to 40 mm.
4. Method according to one of Claims 1 to 3, characterized in that the binding fibres are plastics fibres, such as thermoplastic or refractory fibres, and/or in that the binding fibres are also selected such that their decomposition or melting temperature is at least 300°C.
5. Method according to one of Claims 1 to 4, characterized in that the hydraulic entanglement of the fibrous web (F) to produce the non-woven web (V) is preferably effected on the forming fabric (2) and is preferably a final entanglement operation.
6. Method according to one of Claims 1 to 5, characterized in that hydraulic pre-entanglement by water-jet needling is effected on the forming fabric (2) and the final entangling operation is effected outside the forming fabric (2) in a further process step.
7. Method according to one of Claims 1 to 6, characterized in that, after the hydraulic entangling operation by means of water jetting, the non-woven web (V) is also chemically entangled, e.g. is impregnated with a binder.
8. Method according to one of Claims 1 to 7, characterized in that the non-woven web (V) is dewatered mechanically, e.g. by means of a press, or by means of vacuum dewatering or thermally by means of a dryer.
9. Non-woven web produced from a fibrous slurry substantially free of - preferably chemical - binders, wherein the thus wet-laid fibrous web (F) comprises industrially created, inorganic fibres or fibres of synthetically created polymers and also binding fibres and is entangled by water jetting to produce the non-woven web (V) and is produced by a method according to one of Claims 1 to 8, wherein the fibres have a decomposition or melting temperature of at least 300°C, and the material of the fibres has a modulus of elasticity of at least 10 GPa, whereas the binding fibres have a modulus of elasticity which is less than 10 GPa.

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