WO2007071387A2

WO2007071387A2 - Plant fibre, fibre-based moulded body, and method for producing plant fibres provided with novolak

Info

Publication number: WO2007071387A2
Application number: PCT/EP2006/012302
Authority: WO
Inventors: Wolfgang Heep; Manfred Dunky; Wolfgang Kantner; Beate Stephani; Steffen Tobisch; Detlef Krug
Original assignee: Dynea Erkner Gmbh
Priority date: 2005-12-20
Filing date: 2006-12-20
Publication date: 2007-06-28
Also published as: WO2007071387A3; DE102005061222A1

Abstract

The invention relates to a plant fibre provided with hardener-free novolak. The plant starting materials are broken down into thermomechanically decomposing aggregates or chemo-thermomechanically decomposing aggregates, in the presence of hardener-free novolak, and the hardener-free novolak is supplied before or during the decomposition. The invention also relates to moulded bodies produced using inventive fibres, and to a method for producing the inventive plant fibre.

Description

Vegetable fiber, fiber-based shaped articles and methods of making novolak-treated vegetable fibers

The invention relates to a fiber-based shaped article which is produced using novolak-treated vegetable fibers and is characterized in particular by a reduced formaldehyde emission behavior and better hydrophobic properties. The invention further relates to a process for the preparation of a vegetable fiber provided with novolak.

For the production of moldings, such as mats, plates or moldings, vegetable pulps, especially wood fibers are used. The vegetable fibers, in particular wood fibers, are produced by mixing vegetable raw materials, for example pieces of wood, thermomechanically or chemothermothermally, such as e.g. in Druckzerfaserungsaggregaten, preferably in refiners, be digested. It should be noted that the sharper the digestion conditions in terms of temperature, pressure and time in these processes, the more formaldehyde from the vegetable raw materials, especially from wood, is emitted.

In refiners, the fibers are made from the vegetable starting materials by defibering between grinding discs. In this case, for example, in the case of wood as starting material, one usually works with pressures in a range of 7 to 11 bar and temperatures of up to 185 ° C. and a residence time of three to six minutes. Above the above conditions, there may be significant emissions of formaldehyde.

The fiber production / fiber pulping process by means of said methods can be controlled so that fibers of a given Geometry and structure can be obtained.

Paraffins may also be added during the fiber manufacturing / pulping process. The paraffins improve the sliding properties of the fibers, thereby reducing the energy expenditure in the fiber production / pulping process.

Likewise, the paraffins are used to improve the hydrophobization of the fibers produced and the moldings produced from these fibers.

The disadvantage, however, is that the paraffins can be added only in an amount of up to about 3.0% by weight, based on the absolutely dry, vegetable fiber-containing starting material, for example absolutely dry wood (atro).

As the amount of paraffin increases further during the fiber manufacturing / fiber pulping process, the bonding properties of the fibers thus produced deteriorate.

Due to the reduced adhesiveness of the fibers produced with the addition of increased quantities of paraffin, the property profile of the shaped bodies produced from these fibers likewise deteriorates.

Furthermore, vegetable fibers, especially wood fibers, depending on the type of wood, different amounts of naturally occurring and in the vegetable raw material, especially wood, chemically bound formaldehyde. The chemically bound formaldehyde can be released at least partially during fiber production / fiber pulping and / or production of the shaped body as well as during use and application of the shaped body. The object of the present invention is to provide vegetable fibers, in particular wood fibers, in which the formaldehyde release in the fiber production / fiber pulping process is significantly reduced and improves the hydrophobic properties of the fiber become.

WO 99/14022 A1 discloses a process for producing particleboard and fiberboard in which a binder is added to the starting materials during defibration. Disadvantageously, special conditions must be met, so that it does not lead to premature curing of the binder. Thus, the defibration must be carried out in particular at low temperatures, which leads to a partial drastic deterioration of the fiber properties and thus to a minimization of use of the materials produced therewith. In any case, in order to obtain the intended adhesive effect of the binder, the added binder must cure more or less completely.

DE 100 00 689 A1 discloses a process for the production of non-woven fabric parts, in which a thermosetting resin or resin-hardener mixture is superficially applied and heated on a nonwoven fabric, which also disadvantageously must be taken to ensure that the temperature below the light-off temperature of Resin is, so it does not come to an early curing. The aim of the addition of the resin or the resin-hardener mixture is in any case sufficient curing, so as to be able to accomplish the intended effect as an adhesive (binder) between the individual fibers.

DE 28 17 699 discloses a multi-stage process for the production of moldings, wherein in a first step fiber mats on which a phenol-formaldehyde resin is deposited by acid addition, are provided and sprayed in a second step hardener and subsequently the shaping is carried out under hot pressing , The disadvantage is that it is a complex multi-step process in which the released during the defibration of the starting materials formaldehyde is released into the environment. As a further considerable disadvantage, the release of formaldehyde is forced even after the production process, since the subsequent superficial spraying of the hardener does not achieve complete mixing of the binder and the hardener and thus does not crosslinked binder components are present.

A further object of the present invention is to provide a method in which the emission of formaldehyde is significantly reduced during the fiber production / fiber pulping process and which enables the production of low formaldehyde emission potential fibers.

Another object of the invention is to produce a fiber-based molded article in which the formaldehyde emission potential is significantly reduced.

Another object is to provide a molded article having improved hydrophobic properties.

The object underlying the invention is achieved by providing a vegetable fiber, which is provided with hardener-free novolak, wherein the vegetable starting materials are digested in the presence of hardener-free novolak in thermo-mechanically disintegrating aggregates or chemo-thermo-mechanically disintegrating aggregates and the hardener-free novolak before or during the Digestion is added.

The essential task of the added hardener-free novolak is to chemically bond the formaldehyde coming from the wood (fiber) substance and, where appropriate, in the fiber production process, and thus prevent a corresponding release of formaldehyde. The amount of novolak added is preferably such that sufficient reactive novolac is available as compared to the formaldehyde to be estimated for the formaldehyde-absorbing reaction. Any addition of hardener to the novolak added according to the invention is in any case beyond the purpose of the invention described here and does not represent an improvement of the effect intended according to the invention, but rather a complication of the system due to the imminent pre-hardening during of the fiber manufacturing process. In any case, the amount of hardener added may not be so great that it is no longer possible to provide a sufficient amount of reactive novolak for the reaction with the formaldehyde present in the wood or arising during the fiber production process.

Preferred developments are specified in the subclaims 2 to 6.

The object underlying the invention is further solved by a molded body based on plants, wherein the shaped body contains a vegetable fiber according to any one of claims 1 to 6 b i s solved.

The object of the invention is further achieved by providing a process for producing a hardener-free novolak fiber according to any one of claims 1 to 6, the process comprising the step of: digesting vegetable starting materials in the presence of hardener-free novolak in thermo-mechanically disintegrating aggregates or Chemo-thermo-mechanically releasing aggregates, wherein prior to or during the digestion process harder Novolak is added.

The inventors have surprisingly found that it is possible to significantly lower the formaldehyde emission potential of manufactured vegetable fiber materials, especially wood fibers, and fiber-containing moldings or fiber moldings made therefrom, when the vegetable raw materials are loaded with hardener-free novolak during the fiber manufacturing / fiber pulping process.

An essential aspect of the method according to the invention is that reactive, ie not cured, novolak is still available as formaldehyde scavenger even after the fibers have been processed.

Surprisingly, it has also been found that the hydrophobic properties of the fiber materials thus produced and shaped articles produced therefrom are further improved. According to the invention, the hardener-free novolak is added before or during the digestion of the starting materials, ie the defibration process. As a result, several advantages are achieved.

On the one hand, hardener-free novolak has thermoplastic properties and acts as a lubricant during the pulping or defibering of the starting materials. As a result, the energy required for the defibration of the starting materials is lowered, whereby a significant proportion of the originally given energy costs can be saved. The service life of the units is increased.

On the other hand, the hardener-free novolak and the fibers are brought into intensive contact with each other, so that the novolak is not only superficially applied to the fibers, but at least partially diffuses into the fibers. Thus, the hardener-free novolak is preferably present both on and in the fiber. The hardener-free novolak applied on the surface or penetrated into the fiber can therefore reliably bind in particular also the formaldehyde which is already formed during the production of the fibers.

The fiber according to the invention is preferably characterized in that hardener-free novolak is also contained in the fiber. This allows binding of the formaldehyde directly at or after formation. After incorporation of the fiber according to the invention into a shaped body, even when using formaldehyde-containing binder to provide a solid fiber-resin composite, even harder-free novolak is present in and on the fiber, so that also bound in the finished molded body released formaldehyde can be. The release of further formaldehyde from the molding can be done, for example, during the curing process or subsequently by released from the curing agent of the binder formaldehyde.

It is therefore an essential feature of the method according to the invention that the added novolak does not react with hardener added and hardens, but rather preferably in uncured or, after reaction with the generated in the wood or in the fiber production formaldehyde or optionally when curing an added conventional binder, only partially cured, but in any case is still reactive form.

The novolac used in the present invention is not a binder. The novolak resins partially used in the prior art contain hardeners. These novolak hardener mixtures are binders. A novolak-hardener mixture can not be used in the present invention, as it would during curing of the fiber to a curing of the novolak, whereby the digestion device, preferably a refiner, would be destroyed.

The formaldehyde released during the fiber production / fiber pulping process is largely chemically bound by the hardener-free novolak, whereby partial curing of the novolak occurs or may occur.

Complete, in particular premature, complete curing does not take place according to the invention. In any case, complete curing would prevent a further reaction with formaldehyde present in the wood or arising during fiber production or processing and thus not be able to develop the inventively desired effect of the novolak.

The chemical bonding of formaldehyde significantly reduces the formaldehyde emission potential of such fibers. Due to the binding capacity of the hardener-free novolak, the formaldehyde emission is markedly reduced even in the further processing of these inventive vegetable fibers and in the production of moldings and the use and application of the moldings.

Fibrous tablets are traditionally made using vegetable Fibers, especially wood fibers, and usually of formaldehyde-containing binders, which comprise about 90% of the binders used worldwide for the production of such moldings, such as mats, sheets or moldings produced.

Moldings produced by using the vegetable fibers according to the invention are likewise distinguished by significantly reduced formaldehyde emissions, since the formaldehyde emitted from the formaldehyde-containing binders also contributes to the partial curing of the novolak during the production of moldings under pressure and heat, and thus the formaldehyde is largely removed from the system and can not be emitted.

In view of the fact that formaldehyde has a great hazard potential for humans and the environment, the present invention makes it possible to significantly reduce the human health burden during the fiber production process, the molding production and its use and application.

The vegetable fiber can be made from any suitable vegetable

Starting material, which is commonly used for the production of fiber moldings, be prepared. The plant fibers can come from annual or perennial plants.

As vegetable fiber, e.g. Flax, hemp, bast, kenaf, jute or mixtures thereof.

According to a preferred embodiment, the vegetable fiber is a wood fiber.

The wood fiber can be made of different wood materials or different types of wood, for example, pine, spruce, beech, poplar, etc.

The vegetable fibers of the invention preferably have a length from 0.05 mm to 30 mm, preferably from 0.1 to 20 mm.

Also very suitable fiber lengths of 0.1 to 10 mm, more preferably from 0.1 to 8 mm, have proven. According to a further preferred embodiment, the fiber length is 0, 1 b is 5 mm.

It is further preferred that the novolak is uncured or only partially crosslinked in and / or on the fiber according to the invention and thus also at a later time, such as e.g. Formaldehyde releasably releases any released formaldehyde during the production of moldings using formaldehyde-containing binders under pressure and heat. This liberated formaldehyde contributes to the partial cure of the novolak. Furthermore, it is favorable that the proportion of novolak is 0.05 to 15% by weight, preferably 0.1 to 10% by weight, in each case based on the weight of absolutely dry fiber (atro).

It has surprisingly been found that even small amounts of hardener-free novolak in the fiber and / or on the fiber surface are sufficient to bind the formaldehyde released during the fiber production / fiber pulping process and / or the molding production as far as possible.

In the production of a shaped body according to the invention also using formaldehyde-containing binders does not complete curing of added before or during the fiber production novolac, as given by the commonly used synthetic binder amounts of formaldehyde are insufficient to completely through-harden the hardener-free novolak.

This allows the above-mentioned inventive embodiment of the method, in which even after the Faserherstellungs- and

Processing process is still available reactive novolac.

Thus, about 50 mg of formaldehyde / kg of UF resin, with a molar ratio of formaldehyde (F) to urea (U) of 1, 05, while the Fiber board production delivered.

The amounts of formaldehyde released during the pulping process are between 0.08 and 30 mg / l, depending on the starting material used and the digestion conditions chosen.

Although these amounts of formaldehyde released are low with respect to the curing of novolak and thus bring about only a slight to only partial curing, these amounts are significant in terms of health risks to humans and the environment. An essential advantage of the present invention is thus that on and in the fiber or in the - also using formaldehyde-containing binders - produced moldings sufficiently uncured novolak is present, which leads to a significant reduction in the amount of formaldehyde released into the environment , which significantly reduces the health risk.

Due to the pronounced in the novolaks, water-repellent methylene bridges, a hydrophobing of the plant fiber of the invention and the molding is achieved in addition. The hydrophobization of the vegetable fibers, preferably wood fibers, increases the resistance of the vegetable fibers or of the shaped bodies produced from these vegetable fibers to moisture and moisture, since the water is largely repelled by the hydrophobic surface.

It has also surprisingly been found that the hardener-free novolak used according to the invention improves the sliding properties of the vegetable starting materials during the fiber production / fiber pulping process, so that the paraffins traditionally used for hydrophobing and simultaneously reducing the energy expenditure during the fiber pulping reduced or even omitted in their amounts can be. The hardener-free novolak has thermoplastic properties until cured, which, with good distribution of the novolak, lead to the fibers produced according to the invention for improving the sliding properties and improved hydrophobization of the fibers or of the molded articles produced therefrom.

The hardener-free novolak used according to the invention preferably contains no curing agent, such as, for example, hexamethylenetetramine. For the purposes of the invention, a "hardener-free" novolak is also understood to mean a novolak which may contain minor amounts of hardener, but these amounts are so low that the novolak can only crosslink incompletely The proportion of hardener in the., During the fiber pulping or during the mold body production has released formaldehyde

"Hardener-free" novolak is preferably less than 1.0% by weight, more preferably less than 0.8% by weight, even more preferably less than 0.5% by weight, based in each case on the total weight of curing agent and novolak ,

The novolaks to be used according to the invention are defined as polycondensates based on phenolic components, in particular phenol, and aldehyde components, in particular formaldehyde, prepared in the acidic pH range, in a molar ratio of the phenolic components, in particular phenol, to the aldehyde components, in particular formaldehyde 1: <1, preferably 1: 0.5 to 1: 0.9 (molar ratio of phenolic component: aldehyde component).

The hardener-free novolak can be present in the form of powders, pellets, particles, pieces, melt, emulsion and / or dispersion and can be added in this form in the process according to the invention.

Since the production or digestion of the vegetable fibers based on vegetable raw materials mechanical or chemithermomechanical digestion, the hardener-free novolak can be added in solid or liquid form. The addition of the hardener-free novolak can, for example. When using refiners for fiber pulping in the chip bunker, cooker, the screw plug and / or the grinding chamber done.

In view of the fact that the moldings produced using vegetable starting materials are widely used in the interior finishing of houses or in the manufacture of furniture, the invention represents an important contribution to improving the living conditions of humans and animals.

It is further preferred that the shaped body contains synthetic and / or natural binders and optionally further additives.

Synthetic binders are preferably selected from the group of formaldehyde-based condensation resins, e.g. Urea-formaldehyde (UF), melamine-formaldehyde (MF), melamine-urea-formaldehyde (MUF), melamine-urea-phenol-formaldehyde (MUFF), phenol-formaldehyde (PF), phenol-melamine-formaldehyde (PMF), Phenol-urea-formaldehyde (PUF),

Resorcinol-phenol-formaldehyde (RPF) resorcinol-phenol-urea-formaldehyde (RPUF) and mixtures thereof, and / or from the group of thermoplastic binders, e.g. based on polypropylene (PP), polyethylene (PE), polyvinyl chloride (PVC), polyvinyl alcohol (PVA), polyvinyl acetates (PVAc) and mixtures thereof and / or from the group of polyurethanes and isocyanates, e.g. polymeric diphenylmethane diisocyanate (PMDI) and mixtures thereof.

According to a further preferred embodiment, the molding contains the synthetic binder preferably in the amount of 0.5 to 50 wt .-%, more preferably from 1 to 40 wt .-%, each calculated as a so-called solid resin and based on the total weight of the molding. The remainder may be vegetable fibers as well as optional additives. Natural binders are preferably selected from the group consisting of carbohydrates, tannins, lignins, starches, proteins and mixtures thereof.

According to a further preferred embodiment, the molding contains the natural binders preferably in the amount of 1 to 50 wt .-%, more preferably from 5 to 40 wt .-%, even more preferably from 10 to 30 wt .-%, each calculated as Solid and based on the total weight of the molding. The remainder may be vegetable fibers as well as optional additives.

According to another preferred embodiment, the additives are selected from the group consisting of water repellents, flame retardants, antifungal agents, bactericides, dyes, pigments, conductivity enhancers and mixtures thereof.

The additives may be present in the molding according to the invention in an amount of from 0.1 to 20% by weight, preferably from 0.5 to 15% by weight, more preferably from 1 to 10% by weight, in each case calculated as the solids on the total weight of the molding, be present.

The proportions of all components, calculated as solid resin or solid, add up to 100% by weight in the molding according to the invention.

The shaped body according to the invention is preferably a plate, mat or a molded part.

Due to the significantly reduced formaldehyde emissions, the moldings of the invention are particularly suitable in the living and working area. For example, the moldings of the invention can be used as a base substrate for the production of furniture or floor coverings, such as laminate floors, or in the automotive interior. The fibers produced according to the invention can also be used for the production and use of moldings for outdoor use when suitable synthetic binders can be selected from the groups of the binders described above.

The process according to the invention is surprisingly simple. It is only essential that the fiber production process or the fiber preparation is carried out in the manner described in the presence of hardener-free novolac.

In the method according to the invention surprisingly and advantageously also much more intense conditions during the application of "hardener-free" novolac can be applied, without causing a significant, preferably no, formaldehyde emission.

The process according to the invention can be carried out both under the conditions usually used and well above the conditions usually used, e.g. with regard to the pressure of 7 to 11 bar, in particular above 11 bar. As suitable pressures in the range of 12 to 100 bar, for example. From 12 to 20 bar have been found.

The process according to the invention can be carried out, for example, at temperatures of 150 ^° C. or more, for example of 165 ^° C. or more. Furthermore, temperatures of up to 185 ° C and temperatures above 185 ° C, for example. From 190 ° C to 270 ⁰ C or 195 ⁰ C to 250 ⁰ C are applied. Furthermore, it has been found that both the usual residence times of three to six minutes and longer residence times under the commonly used and / or elevated pressure and / or temperature conditions are possible, such as. From 7 to 50 minutes or from 8 to 30 minutes , Likewise, a shortening of the residence time usually used at elevated pressure and temperature conditions is possible and useful, such as. A residence time of 1, 5 min at 12 bar / 190 ° C. The vegetable fibers obtained using the specified pressure and / or temperature and / or time conditions and using "hardener-free" novolac, in particular wood fibers, and the moldings produced therefrom are distinguished by improved properties, in particular with regard to formaldehyde emission and swelling.

The present invention is illustrated by, but not limited to, the following example.

Example:

As starting material, pine wood chips were used in excess in a refiner in the presence of water and (i) 1% by weight of paraffin (Sasol 138, Sasol) or (ii) 1% by weight of paraffin (Sasol 138) and 1% by weight. -% hardener-free novolak or (iii) 1 wt .-% hardener-free novolak at a temperature of 180 ⁰ C and a pressure of 10 bar thermomechanically digested until no fiber bundles were present and the fiber length was about up to 20 mm. The data in% by weight refers to atro wood (absolutely dry wood).

Subsequently, the digested fibers were blended in a conventional manner in each case with 17 wt .-% UF resin, calculated as a solid resin based on atro fiber, in a Blow-Iine. Fibrous plates having a plate thickness of 10 mm and a bulk density of 900 kg / m ^{3 were produced} by hot pressing from the resulting fiber-resin mixture.

The formaldehyde content was determined as a perforator value in mg / 100g fiberboard in accordance with the test methodology described in DIN EN 120.

The transverse tensile strength was determined according to EN 319, the thickness swelling after 24 hours of water storage according to EN 317.

The results are shown in Table 1. Table 1

Result:

The fibers obtained according to the different digestion variants were processed into 10 mm thick fiberboards having a density of 900 kg / m ³ after addition of 17% by weight UF resin, calculated as solid resin based on atro fiber, as described above.

From the comparison, it can be seen that a complete replacement of conventionally used paraffin with hardener-free novolac can be made without causing deterioration in transverse tensile strength and swelling. In the emission of formaldehyde, there is even a significant improvement in those fiberboards whose fibers were digested in the presence of hardener-free novolac.

Although the simultaneous use of paraffin and hardener-free novolac during fiber pulping results in an improvement in the swelling, but a significant deterioration in the transverse tensile strength and no improvement in the formaldehyde emission. The very good result shows that it is surprisingly possible to replace paraffin normally used as lubricant and for hydrophobization or to improve the swelling by hardener-free novolak, wherein there is no impairment of strength or swelling, but at the same time the formaldehyde absorption capacity is significantly increased. From a health point of view, this is a significant advance over the state of the art.

Claims

claims

1. vegetable fiber, characterized in that the vegetable fiber is provided with hardener-free novolak, wherein the vegetable starting materials are digested in the presence of hardener-free novolak in thermo-mechanically disintegrating aggregates or chemo-thermo-mechanically disintegrating aggregates and added the hardener-free novolak before or during the digestion becomes.

2. Vegetable fiber according to claim 1, characterized in that the vegetable fiber is made of annual and / or multi-year plants and in particular is a wood fiber.

3. Vegetable fiber according to claim 1 or 2, characterized in that the vegetable fiber has a length of 0.05 mm to 30 mm, preferably 0.1 mm to 20 mm.

4. Vegetable fiber according to one of claims 1 to 3, characterized in that the novolak is present in and / or on the vegetable fiber unvemetzt or only in imperfectly crosslinked form.

5. Vegetable fiber according to claims 1 to 4, characterized in that the proportion of novolak 0.05 to 15 wt .-%, calculated as a solid and based on the weight of absolutely dry fiber.

6. Vegetable fiber according to one of the preceding claims, characterized the novolak is prepared in the molar ratio of the phenolic component to aldehyde, in particular phenol to formaldehyde, of 1 to less than 1 in the acidic pH range.

7. A shaped body of fiber base, characterized in that the shaped body contains vegetable fibers according to one of claims 1 to 6.

8. Shaped body according to claim 7, characterized in that the shaped body contains natural and / or synthetic binders and optionally further additives.

9. Shaped body according to claim 8, characterized in that the synthetic binder from the group of formaldehyde-based condensation resins, such as urea-formaldehyde (UF), melamine-formaldehyde (MF), melamine-urea-formaldehyde (MUF), melamine-urea Phenol-formaldehyde (MUPF), phenol-formaldehyde (PF), phenol-melamine-formaldehyde (PMF), phenol-urea-formaldehyde (PUF), resorcinol-phenol-formaldehyde (RPF) resorcinol-phenol-urea-formaldehyde (RPUF) and mixtures thereof, from the group of thermoplastic binders, such as based on polypropylene (PP), polyethylene (PE), polyvinyl chloride (PVC) ₁ polyvinyl alcohol (PVA), polyvinyl acetates (PVAc) and mixtures thereof, and / or from the group polyurethanes and isocyanates such as polymeric diphenylmethane diisocyanate (PMDI) and / or mixtures thereof.

10. Shaped body according to claim 8, characterized in that the natural binder selected from the group consisting of carbohydrates, tannins, lignins, starches, proteins and mixtures thereof or other materials based on natural raw materials becomes.

11. Shaped body according to claim 8, characterized in that the additives are preferably selected from the group consisting of water repellents, flame retardants, fungicides, bactericides, dyes, pigments, conductivity-increasing substances and mixtures thereof.

12. Shaped body according to one of claims 7 to 11, characterized in that the shaped body is a plate, mat or molded part.

13. A process for the preparation of a hardener-free novolac fiber according to any one of claims 1 to 6, which process comprises the following step: digesting vegetable starting materials in the presence of novolac in thermo-mechanically disintegrating aggregates or chemithermomechanical disintegrating aggregates, wherein before or Hardener novolac is added during the digestion process.

14. The method according to claim 13, characterized in that the vegetable starting materials are digested to a fiber length of 0.05 to 30 mm, preferably 0.1 to 20 mm.

15. The method according to claim 13 or 14, characterized in that the novolak in an amount of 0.05 to 15 wt .-%, preferably 0.1 to 10 wt .-%, calculated as a solid and based on the weight of the absolute dry fiber, is added.