DE68909810T2 - Mainly inorganic fibers and processes for their production. - Google Patents

Abstract

A substantially inorganic fibrous sheet material and a method and a filler for the production thereof are disclosed. The sheet material comprises (a) cationic inorganic fibres, (b) a cationic inorganic filler including a mixture of talc and soapstone, (c) an anionic binder including an anionic polyelectrolyte and, optionally, cellulose fibres, and (d) strength-improving additives including thermoplastic polymer particles. To produce the sheet material, a stock is formed of the above-mentioned components, the stock charge being adjusted such that a substantially charge-neutral system is obtained, and the stock being formed into a sheet. The filler consists of a mixture of talc and soapstone.

Description

The present invention relates to a mainly inorganic fibrous sheet material, to a method for producing such a sheet material by paper technology and to a filler for producing such a sheet material.

By "papermaking" is meant the technology used to make paper, i.e. preparing an aqueous suspension or stock from the various components and dewatering the suspension to form a sheet which is then subjected to drying and, if necessary, subsequent treatment such as calendering.

In the conventional production of fibrous sheet materials, cellulose fibers are the main component. Cellulose fibers are disadvantageous in that they give the sheet material less dimensional stability and a tendency to become moldy. The sheet material is therefore unsuitable for use in damp environments, for example.

It is known to partially replace the cellulose fibers of such sheet materials with inorganic fibers and fillers. The components are bonded together in the conventional way by whole-material gluing or by applying a binding agent in a size press or by a combination of these.

It is already known from European patent application 78850011.4 (publication no. 1539) to produce a carrier material in the form of a fiber felt starting from mineral fibers and cellulose fibers in an aqueous suspension that also contains an acrylate binder. After After the suspension has been drained on a sieve, the sheet thus formed is pressed and dried, after which a binder in the form of a styrene-butadiene latex is added. A disadvantage of this known material is that the styrene-butadiene latex releases formalin and that, in addition, the high cellulose fiber content of the material makes it unstable to moisture and mold.

It is already known from the DK patent application 5436/76 (corresponding to GB-A-1,597,369) to produce a composite material that contains cellulose fibers, film-forming, particulate or fibrous polymer material, polyelectrolyte and possibly an inorganic particulate or fibrous material. This material also has a high cellulose fiber content (15-40% by weight), which is why the material does not meet the requirements for dimensional stability and resistance to moisture and mold.

GB-A-2,031,475 relates to a paper product with a high filler content. The fibrous mass of the paper product is preferably wood pulp, but may be based on rock wool, ceramic fibres, glass fibres etc. The paper product contains at least one finely divided mineral or organic filler and as examples of mineral fillers chalk, kaolin, talc, magnesia, dolomite, mica, clays, asbestos, aluminium hydrate etc. are mentioned. The specific filler according to the present invention, which comprises a mixture of talc and steatite, is not mentioned. In the manufacture of the paper product, the filler is added to the fibrous mass, followed by addition of a first ionogenic substance, then a second ionogenic substance of opposite polarity, etc., the alternating addition of ionogenic substances of opposite polarities being terminated when groups of filler particles are obtained which can give a stable suspension which can be dewatered on the wire of a paper machine.

In the above-mentioned prior art method, the cellulose fibers of the sheet material have only been partially replaced by inorganic fibers. One reason why a relatively high cellulose fiber content in the sheet material has been maintained despite the disadvantages of lower dimensional stability and resistance to moisture and mold is that the material must have a given wet strength to make its manufacture by papermaking possible. In conventional papermaking, the wet strength is provided by the cellulose fibers, which have a binding effect through hydrogen bonds during the formation of the paper web. Consequently, rapid development of wet strength requires rapid formation and dewatering, which is also advantageous in that it offers an opportunity to increase production capacity.

However, this desire for rapid dewatering has caused difficulties in the manufacture of sheet materials containing inorganic fibers such as glass fibers, since inorganic fibrous materials do not form hydrogen bonds and therefore do not contribute bond strength themselves. Rapid dewatering thus results in lower wet strength and to compensate for this, it has therefore been necessary in the prior art method to maintain a relatively high cellulose fiber content in the material and also to add wet strength agents in the form of, for example, melamine resins.

One application where the requirements placed on the material are particularly high is wallpaper for bathrooms or washrooms, so-called wet room wallpaper, e.g. vinyl plastic layers on a base material.

This type of wallpaper material is difficult because it is bordered on the long sides and can be glued together using a Joint sealing fluid is welded. After this, the wallpaper must not move under the effect of moisture and/or heat, otherwise the wallpaper will bulge and peel away from the wall, opening the joint in an unattractive manner, allowing moisture to penetrate behind the wallpaper. Previous base materials were paper, but this has now been replaced by non-woven fiberglass materials that are resistant to moisture and mold. Non-woven fiberglass, on the other hand, suffers from the disadvantage that the vinyl plastic must be applied in at least two steps - pre-treatment (sealing) and finishing - otherwise the vinyl plastic will flow through the non-woven fiberglass material. The two-step application means a significant increase in price. In addition, many people shy away from working with non-woven fiberglass materials because they can cause skin irritation.

The present invention aims to avoid the disadvantages of the prior art materials and to provide a material which is mainly inorganic, i.e. consists of at least 50% by weight of inorganic material, and which is dimensionally stable and resistant to temperature changes, moisture and mold.

This is achieved according to the invention by a unique combination of components and steps for manufacturing a sheet material therefrom.

In particular, the present invention provides a mainly inorganic fibrous sheet material which is characterized by comprising

(a) 1-80 wt% cationic inorganic fibres;

(b) 1-80% by weight of cationic inorganic filler consisting of a mixture of talc and steatite in a weight ratio of about 30:70 to 70:30;

(c) 3-20 wt.% anionic polyelectrolyte as anionic binder; and

(d) 2-20 wt.% of starch-improving (strength-increasing) additives containing thermoplastic polymer particles.

The invention also provides a process for producing a mainly inorganic, fibrous sheet material by paper technology, characterized by the formation of a stock from

(a) 1-80 wt.% cationic inorganic fibres;

(b) 1-80% by weight of cationic inorganic filler consisting of a mixture of talc and steatite in a weight ratio of about 30:70 to about 70:30;

(c) 3-20 wt.% anionic polyelectrolyte as anionic binder; and

(d) 2-20 wt.% of starch-improving additives containing thermoplastic polymer particles,

wherein the charge of the stock is controlled so that a predominantly charge-neutral system is achieved;

and the formation of a sheet from the whole fabric.

The invention also provides a filler for producing a primarily inorganic fibrous sheet material, characterized in that the filler consists of a mixture of talc and steatite in a weight ratio of about 30:70 to about 70:30.

Further characteristic features of the invention will become apparent from the following description and the appended claims.

Among the components of the sheet material according to the invention, the inorganic filler is a particularly characteristic and important component. The inorganic The filler of the invention is cationic and includes a mixture of talc and steatite. In addition to its role as a filler, the mixture of talc and steatite acts as a drainage regulator, in particular to provide a controlled slow drainage. In addition, the filler contributes to creating a product which is "dead", ie a product which is dimensionally stable and does not change under the effect of humidity and temperature changes.

The function of the filler as a drainage regulator is due to the fact that, unlike conventional sheet material production by paper technology, the present invention does not require maximum drainage, but instead requires slower and controlled drainage. The reason for this is that, since inorganic fibrous materials do not provide wet strength by themselves, the present invention uses the water of the sheet material suspension to contribute to the wet strength initially. For this purpose, the water of the sheet material suspension must be retained until the sheet material has developed sufficient dry strength through the added binders. The mixture of talc and steatite used by the present invention has been found to have unique and extremely advantageous properties for this purpose. The reason for this has not been fully elucidated.

The collective name "talc" usually includes (a) the mineral talc, (b) steatite, which is a compact variant of talc, and (c) the rock-like steatite (soapstone, soapstone).

The mineral talc is a hydrated magnesium silicate with the ideal composition Mg₃Si₄O₁₀(OH)₂. The talc content of commercial talc is high and is usually around 97% by weight. To produce commercial talc Talc mineral is crushed and finely ground and then purified by flotation to produce a talc product with a high talc content and whiteness.

Steatite is a natural product which mineralogically consists of a mixture of talc in with a high chlorite content and smaller amounts of carbonate and amphibole. For example, steatite from Handöl in Sweden has the following mineral composition

Talc about 67 wt.%

Chlorite about 18 wt.%

Carbonate about 8 wt.%

Amphibole about 3 wt.%

Ore material about 4 wt.%

To achieve maximum results in terms of drainage and wet strength, the talc and steatite in the filler mixture of the invention should have different average particle sizes. Thus, it is preferred that the talc be coarsely grained and have an average particle size of about 10-20 µm, whereas the steatite be finely grained or micronized and have an average particle size of about 5-10 µm. The relative proportions of talc and steatite in the filler mixture of the invention can vary within wide limits from a weight ratio of about 30:70 to about 70:30, more preferably from about 40:60 to about 60:40, and most preferably about 50:50. The filler content in the sheet material of the invention can also vary within wide limits from 1-80 wt.%, more preferably 45-70 wt.%.

In addition to the particularly characteristic cationic inorganic filler described above, the sheet material of the invention also contains cationic inorganic fibers. The content of inorganic fibers in the sheet material can vary within wide limits of 1-80 wt.%, more preferably about 20-40 wt.%. In principle, the inorganic fibres may be freely selected from existing inorganic fibres provided that they are cationic, ie have a positive charge. If the inorganic fibre is not itself cationic, it can be made cationic by subjecting the fibre to a surface treatment in a manner known per se so that the fibre acquires a positive surface charge. It is preferred in the present invention that the inorganic fibres are mineral fibres, and most preferred are glass fibres. The fibre length and diameter are not critical, but it is preferred that the average fibre diameter is in the range 1-20 µm and that the average fibre length is in the range 2-20 mm, preferably 5-15 mm.

The sheet material according to the invention can comprise, as a further cationic component, a cationic polyelectrolyte in an amount of up to about 2% by weight.

A polyelectrolyte is a polymer with the character of an electrolyte, which means that it dissociates into ions in aqueous solution like an electrolyte and is electrically conductive. Depending on whether the polymer main chain is positively or negatively charged during dissociation, the polyelectrolyte is called cationic or anionic. The starting material of cationic polyelectrolytes are derivatives of esters and amides. A characteristic feature of these polyelectrolytes is an ammonium group, which can be in the form of a salt of a tertiary amine or as a quaternary ammonium group. The starting material for anionic polyelectrolytes is usually acrylic acid and methacrylic acid.

The cationic polyelectrolyte, together with the other cationic components, i.e. the inorganic filler and the inorganic fibers, serves to balance the negative charges of the anionic components of the sheet material. In principle, such balancing of the positive and negative charges to obtain charge neutrality can be achieved by appropriate adjustment of the amounts of inorganic filler and inorganic fibers, and the cationic polyelectrolyte is therefore not an indispensable component of the sheet material of the invention. However, it has been found that, particularly in the continuous manufacture of the sheet material of the invention, it is both simpler and more practical to effect this balancing and the achievement of charge neutrality by adding a cationic polyelectrolyte, and the invention therefore prefers to do so. As will be discussed in more detail below, the cationic polyelectrolyte is preferably added in repeated doses.

The cationic polyelectrolyte can be selected from all available polyelectrolytes of this type, and since these are well known in themselves, a detailed list does not seem necessary here.

However, the cationic polyelectrolytes commercially available from Stockhausen under the trade names Prestol 436k and Prestol 521k and from Röhm GmbH, Federal Republic of Germany, under the trade name ROHAFLOC KL 925 may be mentioned as examples.

In addition to the above-mentioned cationic components, the sheet material also contains anionic components in such an amount that they form a charge-neutral system together with the cationic components. As previously mentioned, the anionic components include a binder comprising an anionic polyelectrolyte and optionally also cellulose fibers. The use of anionic polyelectrolytes as dispersing and flocculants is already known, but the inventor is not aware that they have already been used as binders or as in the present invention to form a aqueous suspension or a stock to develop dry strength in the sheet material formed. In general, the anionic polyelectrolytes are in the form of dilute aqueous solutions with a concentration of about 10-35% by weight. Since anionic polyelectrolytes are well known per se, a detailed list of them does not seem necessary. However, as examples, mention may be made of the flocculating polymers commercially available under the trade name Prestol, e.g. Prestol 2935/74 manufactured by Stockhausen, and the anionic polyelectrolyte Prodefloc N2M from Prodeco in Italy and PLEX 4911 from Röhm GmbH. In the sheet material according to the invention, the anionic polyelectrolyte content is 3-20% by weight, preferably about 5-10% by weight.

As mentioned above, the sheet material of the invention may optionally contain cellulosic fibers to improve the dry strength of the material. However, a high cellulose content has a negative effect on the dimensional stability of the sheet material and its resistance to moisture and mold, which is why the cellulose fiber content of the invention is kept as low as possible, such as at most about 8% by weight, preferably at most about 5% by weight. The cellulose fibers are preferably added in an amount of 2-5% by weight, more preferably about 4-5% by weight. For making sheets on a take-off felt paper machine, the cellulose fibers can be omitted entirely. This differs from the prior art method which generally uses cellulose fibers in amounts of 30% by weight and more. Furthermore, in contrast to the prior art method, the cellulose fibers of the present invention are not long, but have preferably been refined to a drainage resistance of about 75-100º Schopper-Riegler (ºSR).

In addition to the above-mentioned cationic and anionic components, the sheet material of the invention also contains a strength-improving additive comprising thermoplastic polymer particles. By "particles" is meant here not only grains but also flakes and fibers. The strength-improving additive is intended to impart dry strength to the sheet material by heating the thermoplastic polymer particles during sheet manufacture, e.g. in the drying section or in a hot calender, to cause them to sinter or fuse together at their points of contact or intersection while retaining mainly their original configurations, so that the particles form a coherent three-dimensional network structure or reinforcement which imparts dry strength to the sheet material.

The thermoplastic polymer particles are contained in the sheet material according to the invention in an amount of about 2-20 wt.%.

In principle, the charge of the thermoplastic polymer particles is not particularly critical and they may be, for example, anionic or non-ionic, but it is particularly preferred that they are non-ionic. The thermoplastic polymer particles may be freely selected from available thermoplastic polymer particles or mixtures thereof. However, since the temperature in the drying section of the paper machine used to produce the sheet material and also in the hot calender is about 70-120°C, the thermoplastic polymer particles should be allowed to soften within this temperature range, preferably about 70-80°C. A particularly preferred thermoplastic polymer in connection with the invention is polyvinyl alcohol. Among other useful thermoplastic polymers may be mentioned polyethylene, polypropylene, polyvinyl chloride, polyester and polycarbonate. As already mentioned, the thermoplastic polymers may Polymer particles are in the form of fibers. The fiber dimensions are not critical, but it is preferred that they have an average length of about 0.6-12 mm, preferably about 1-6 mm and a linear weight of about 0.5-10 dtex, preferably about 1-6 dtex.

The sheet material of the invention normally has a thickness of about 0.1-1.5 mm, preferably about 0.1-0.5 mm, and most preferably about 0.15-0.25 mm, but greater thicknesses can be obtained without difficulty by multi-layer techniques (e.g. using multiple headboxes).

The manufacture of the mainly inorganic fibrous sheet material according to the invention will now be described. As mentioned in the introduction, the sheet material is manufactured by paper technology, and since paper technology is a well-known and well-established method, it does not seem necessary to give a detailed description of this method and the equipment used. Instead, the description will be directed hereinafter to the characteristic features of the invention, i.e. the method by which the components are combined to form the sheet material and the order in which this is done.

In the process according to the invention, the inorganic fibers, preferably surface-treated glass fibers according to the above, are slurried in water. The glass fiber concentration is preferably about 1% by weight, since the fibers tend to form clumps in higher concentrations, e.g. above about 3% by weight.

Then the inorganic filler, which includes a mixture of talc and steatite according to the above, is added to the glass fiber suspension and preferably cellulose fibers are also added to contribute to the strength of the material.

The anionic polyelectrolyte is then added to the resulting slurry or stock.

After the anionic polyelectrolyte has been added, a charge neutral system must be brought about and, if necessary, the desired charge neutrality is adjusted by adding cationic material. As previously mentioned, charge neutrality can in principle be achieved by the relative adjustment of, on the one hand, the amounts of the anionic polyelectrolyte and any cellulose fibers present and, on the other hand, the cationic inorganic filler and the cationic inorganic fibers. Such adjustment can conveniently be carried out during the batch-wise production of the sheet material according to the invention. In continuous sheet production using conventional paper technology, e.g. on a Fourdrinier machine or a self-feed machine, such a controlled adjustment is not easily achieved and it is therefore preferred to add a cationic polyelectrolyte, whereby a charge neutral system is quickly brought about.

All the required cationic polyelectrolytes can be added at once, but it has surprisingly been found to be advantageous if the cationic polyelectrolyte is added in several, preferably two, doses. Thus, it has been found that the addition of the cationic polyelectrolyte at once results in about 60-70% precipitation of the remaining anionically charged components of the sheet material, whereas a 95-100% precipitation is obtained if the cationic polyelectrolyte is added in two doses.

The higher precipitation can be easily observed visually as the initially slightly milky suspension becomes clear during precipitation. When the cationic polyelectrolyte is added in batches is added, the first charge is preferably added to the stock after the anionic polyelectrolyte has been added, followed by the second charge immediately before the headbox. The majority of the cationic polyelectrolyte, eg 60-95 wt.%, preferably about 85-95 wt.%, is added with the first charge. When nonionic thermoplastic polymer particles are used, such as the nonionic polyvinyl alcohol particles preferred according to the invention, the system must be charge neutral if a uniform distribution of the nonionic thermoplastic polymer particles is to be achieved.

In addition to the above-mentioned main components of the sheet material according to the invention, one or more additives known in the papermaking industry, such as hydrophobic agents, antifoaming agents, colorants, optical brighteners, retention agents and lubricants, can also be added.

Care should be taken to ensure that the aqueous suspension or stock has a suitable ionic strength, which can be controlled by means of alkali or alkaline earth salts such as sodium, magnesium or calcium salts, of which calcium salts such as calcium chloride give the best result. The addition of salt reduces the repulsion of the electrical double layer between the less charged surfaces of the upper and lower sides of the filler particles. Normally, the calcium ion content is supplied by the water hardness, which should be about 18-26º dH (German hardness), but if the water hardness is unsuitable, it is adjusted by adding a calcium salt so that the calcium content is about 7-18 mol/m³ H₂O, which corresponds to about 0.4-1.0 kg CaCl₂/m³ H₂O.

When all components have been added to the stock, the stock is fed through the headbox onto the screen and passed through the press and drying sections of the paper machine in a conventional manner. According to the invention it is preferred that the first part of the drying section is heated to a maximum temperature (about 90-120°C) in order to rapidly achieve dry strength of the sheet material. In order to avoid fibre raising, the second part of the drying section should have a lower temperature (about 60-70°C). Furthermore, it is preferred to calender the sheet material after the drying section at a temperature of about 70-120°C and a pressure of about 18-25 N/mm. As already mentioned, the strength-improving thermoplastic polymer of the sheet material should be selected so that it begins to soften on the surface at about 70°C, whereby the thermoplastic particles are bonded together at the crossing points during treatment in the drying section and the hot calender. However, the thermoplastic particles should not completely melt and flow into each other at the temperatures used in the drying section or the hot calender.

The sheet material according to the invention finds a variety of applications, e.g. in wall, floor and roof covering materials (e.g. in roofing felt) and in foamed products, such as polyurethane, polyvinyl chloride and phenolic plastics. The sheet material according to the invention can be included as a carrier or base material, but can also be included, e.g. as an intermediate layer or used separately.

The sheet material according to the invention contains, as mentioned above, at least 50% by weight of inorganic material. Preferably, the sheet material contains 70-80% by weight or more of inorganic material and, as far as the inventor is aware, it has not previously been possible to produce sheet materials with these high contents of inorganic fibers and fillers using paper technology.

To further illustrate the invention, the following non-limiting examples are given.

Examples 1 - 3

Three pieces of sheet material were prepared using the components and contents (wt%) given in Table 1.

The steatite used in the examples was type H340 from Handöl in Sweden. This is a finely crushed steatite with an average diameter of 5-10 µm. The talc content is about 67 wt% and the combustion loss is about 8 wt%. The oil absorption value is 55 g oil/100 g steatite and the melting point is 1,500ºC.

The talc used in the examples was of the type Finntalk P40 from Outokumpu Oy, Finland, which has an average diameter of about 10-20 µm, a talc content of about 97 wt%, a combustion loss of 7 wt% and an oil absorption value of 32 g oil/100 g talc. The melting point is 1375ºC.

The thermoplastic particles used in the examples were polyvinyl alcohol flakes of the Moviol type from Hoechst and polypropylene fibers of the Pulpex P type from Herkules. Table 1 Examples Glass fibers (cationic, length 10 mm, diameter 3 µm) Steatite Talc Cellulose (long-fiber pine sulfate cellulose, which has been ground and bleached, 95º SR) Cationic polyelectrolyte (ROHAFLOC KL 925 from Röhm) Anionic polyelectrolyte (PLEX 4911 from Röhm) Thermoplastic particles (PVA)

First, about 1.0 kg CaCl₂/m³ H₂O is added to the stock water to give a water hardness of 23º dH. Then, the cationic glass fiber is added and slurried to a slurry at a concentration of about 1.0 wt.%. To the slurry are added the cationic steatite and the talc. The steatite is grayish green and the talc is practically white. In cases where the composition contains cellulose fibers, these are also added to the slurry.

Then the anionic polyelectrolyte is added and the system now begins to transform into a charge neutral system. To ensure that the system is indeed a charge neutral system, the cationic polyelectrolyte is added in several (two) doses. Finally, the thermoplastic particles are added to the charge neutral system.

The resulting slurry or stock is fed to the paper machine and the water is sucked off immediately before the drying section. The first part of the drying section is heated to a maximum temperature (about 90-120ºC) to rapidly increase the dry strength of the sheet. After the drying section, the sheet is hot calendered at a pressure of 23 N/mm.

The properties of the as-prepared sheet materials were then investigated and the results obtained are given in Table 2. Table 2 Examples Basis weight Thickness Density Air resistance Tensile index, machine direction Tensile index, cross direction Elongation, machine direction Elongation, cross direction Bending force Dimensional stability, machine direction % * Dimensional stability, cross direction % * Z-strength Coal ash (ash content) * Dimensional stability values are the difference in percent of the dimensions of the material in the machine or cross direction before and after soaking in water for 10 minutes.

Claims (15)

1. Mainly inorganic, fibrous sheet material, characterized in that it comprises the following (a) 1-80 wt.% cationic inorganic fibers; (b) 1-80% by weight of cationic inorganic filler consisting of a mixture of talc and steatite in a weight ratio of about 30:70 to about 70:30; (c) 3-20 wt.% anionic polyelectrolyte as anionic binder; and (d) 2-20 wt.% starch improving additives containing thermoplastic polymer particles. 2. The sheet material of claim 1, wherein the talc has an average particle diameter of about 10-20 µm and the steatite has an average particle diameter of about 5-10 µm. 3. The sheet material of claim 1 or 2, further comprising up to about 2% by weight of cationic polyelectrolyte. 4. Sheet material according to any one of the preceding claims, further comprising up to about 8% by weight cellulosic fibers. 5. Sheet material according to claim 4, wherein the cellulose fibers have a freeness of about 75-100º SR. 6. Sheet material according to one of the preceding claims, wherein the thermoplastic polymer particles are thermoplastic polymer flakes. 7. The sheet material of claim 6, wherein the thermoplastic polymer particles are nonionic thermoplastic polymer flakes. 8. Sheet material according to claim 6 or 7, wherein the thermoplastic polymer particles consist of polyvinyl alcohol. 9. A process for producing a mainly inorganic, fibrous sheet material by paper technology, characterized by the formation of a stock, based on the weight of the sheet material, from (a) 1-80 wt.% cationic inorganic fibres; (b) 1-80% by weight of cationic inorganic filler consisting of a mixture of talc and steatite in a weight ratio of about 30:70 to about 70:30; (c) 3-20 wt.% anionic polyelectrolyte as anionic binder; and (d) 2-20% by weight of starch improving additives containing thermoplastic polymer particles; wherein the charge of the stock is controlled so as to achieve a predominantly charge-neutral system; and the formation of a sheet from the whole fabric. 10. The method of claim 9, wherein the stock is formed from an inorganic filler consisting of a mixture of talc having an average particle diameter of about 10-20 µm and steatite having an average particle diameter of about 5-10 µm. 11. The method of claim 9 or 10, wherein the whole material loading is controlled by adding up to about 2 wt% cationic polyelectrolyte, preferably in multiple doses. 12. Process according to one of claims 9-11, wherein thermoplastic polymer particles, preferably made of polyvinyl alcohol, are added as starch-improving additives. 13. A process according to any one of claims 9-12, wherein up to about 8% by weight of cellulose fibers, preferably with a degree of grinding of about 75-100º SR, is also added as an anionic binder. 14. A filler for making a primarily inorganic, fibrous sheet material, the filler consisting of a mixture of talc and steatite in a weight ratio of about 30:70 to about 70:30. 15. The filler of claim 14, wherein the talc has an average particle diameter of about 10-20 µm and the steatite has an average particle diameter of about 5-10 µm.