GB1573883A - Process for the production and the use of whiskers of calcium sulphat - Google Patents

Description

(54) A PROCESS FOR THE PRODUCTION AND THE USE OF WHISKERS OF CALCIUM SULPHATE (71) We, BAYER AKTIENGESELLSCHAFT a body corporate organised under the laws of the Federal Republic of Germany, of 509 Leverkusen, Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a process for the production of whiskers of calcium sulphate or more particularly calcium sulphate hydrates by crystallisation from aqueous acid calcium sulphate solutions free from crystal nuclei, and to the use of these whiskers.

Whiskers of the type in question are also known as filaments or rubber fibres, fibre crystals or needles and are characterised by having a length to mean diameter .atio of at least 6:1.

It is known that calcium sulphate, both in dihydrate and in semihydrate form, occurs in a large number of crystalline forms, including particularly frequently needles. However, these needles generally coalesce to form clusters or starshaped aggregates, so that they can no longer be regarded as whiskers irrespective of their dimensions. According to W. O. Milligan, J. Amer. Chem. Soc. 59, 1456 et seq, 1937, anhydrite and semihydrate in the form of 1 to 2 mm long crystals are formed in 55V,, nitric acid at temperatures of 85"C and 50"C, respectively. The crystals consist of a mixture of fine needles and a predominant proportion of starshaped aggregates of needles or flake-like crystals.

U.S. Patent No. 3,822,340 describes the preparation of such whiskers by heating an aqueous suspension of calcium sulphate dihydrate under pressure to temperatures above about 110 C to 1500C, followed by stabilisation to prevent rehydration.

It is known that whiskers which are characterised by the above-mentioned length-to-diameter ratio show considerable rigidity and retain this rigidity even when embedded in a variety of materials, for example plastics, thereby contributing significantly towards reinforcing soft mouldings. The end products obtained by incorporating whiskers into binders or forming agents combine high strength with relatively low weight.

Unfortunately, conventional methods for producing whiskers on a commercial scale are extremely expensive and uneconomical. Thus, U.S. Patent No. 3,915,927, for example, describes the production of fibres by a reaction carried out in pressure vessels in the presence of saturated steam at temperatures in the range from 140 to 200"C. Accordingly, whiskers have hitherto only been adopted for commercial use in exceptional cases.

Recently the absolute size of whiskers has also been recognised as a quality feature in addition to their geometrical form, in other words the strength of the whiskers per volume increases with their length.

Accordingly, the present invention provides a process for the production of whiskers of calcium sulphate or calcium sulphate hydrates, which have a length-tomean cross-section ratio of at least 6:1, by crystallisation and subsequent separation of the whiskers, wherein a hot, acid aqueous calcium sulphate solution is freed from crystal nuclei, subsequently cooled to a temperature of from 65"C to 400 C, wherein, within the temperature range of 65"C to 400C the cooling is effected slowly, wherein the solution is maintained at a temperature, optionally with addition of seeds, until the saturation concentration has been reached, after which the solution is further cooled and the whiskers which have been crystallised are recovered.

The invention also relates to whiskers produced in this way which are useful as fire-retarding materials.

The invention also relates to agents based on paper and/or pulp and/or ceilulose containing whiskers which have a length to mean cross-section ratio of at least 6:1 and which are produced by crystallisation and separation from hot, aqueous acid calcium-sulphate-containing solutions free from crystal nuclei by a process as just described.

The present invention relates to the use of calcium sulphate whiskers which have a length to mean cross-section ratio of at least 6:1, and which are produced by crystallisation and separation from hot, aqueous acid calcium-sulphate-containing solutions free from crystal nuclei by the process according to the invention, as a reinforcing and/or filling materials for shaped articles, layers, coating and/or films on an inorganic and/or organic basis which contain paper and/or pulp and/or cellulose.

It has surprisingly been found that substantially uniform whiskers free from any troublesome intergrowth can be formed by simple measures and that, at the same time, it is possible to obtain crystals in a size which, hitherto, has only been possible as the result of elaborate preparation techniques. To this end, a solution of calcium sulphate in an acid or acid mixture is initially prepared by stirring a calcium-sulphate-containing material into the acid solution and heating the solution or suspension.

The acid solution or, where a relatively large quantity of calcium sulphate is added, the suspension obtained is heated to temperatures above about 70"C in order to dissolve any crystals formed. The heating temperature selected may be as high as the boiling point of the solution which depends upon the composition and concentration of the solution. However, the heating temperatures normally applied are in the range from about 75"C to 1000C, depending upon the concentration of the solution. The heating time amounts to only a few minutes, although it should be selected so that the calcium sulphate is dissolved up to saturation level.

An acid calcium sulphate solution free from crystal nuclei is then prepared by the process according to the invention. For example, this solution may be obtained by denucleating either an unsaturated or a saturated calcium sulphate solution by hot filtration, i.e. by filtration at the above-mentioned heating temperatures.

However, the clear filtrate of the saturated solution should preferably be reheated.

The reheating temperature should be at least 50C above the filtration temperature and at most at the boiling point of the solution, i.e. at around 100"C. The presence of a solution free from crystal nuclei is of crucial important for the successful accomplishment of the following process steps. In the absence of denucleation of the hot solution for example by filtering the unsaturated solution or by preferably reheating the filtered saturated solution, the above-mentioned polycrystalline aggregates are frequently formed in addition to the acicular whiskers, even when cooling in the range from 65 to 400C is carried out in the manner described hereinafter. In the context of the invention, an acid solution is a solution having a pH-value below 7 and preferably below 4.

After filtration, the clear solution is cooled in any known way to temperatures of from about 70"C to 650C, but to no lower than 65"C. The cooling rate has no bearing upon the shape and size of the required whiskers. After reaching the temperature of approximately 650 C, however, cooling should take place slowly and under control. Particularly large whiskers are obtained when cooling is carried out with careful stirring and when the temperature passes through the range from 650C to at least 400 C over a period of at least about 50 minutes and preferably over 70 minutes. It is also possible to carry out cooling over a somewhat shorter period in order to obtain whiskers which still show adequate dimensions. The temperature selected within this range should be maintained until the saturation concentration at the selected temperature between 65"C and 40"C has been adjusted through the crystallisation of dihydrate. Any oversaturation still present may readily be detected, for example by cooling a sample and subsequently examining it (after about 10 minutes) under a microscope. In the presence of an oversaturation, star shaped aggregates of relatively small needles or of flake-like crystals which have agglomerated into star-shaped aggregates are formed in addition to the large needles having the dimensions of whiskers.

For calcium sulphate dihydrate, the saturation concentration in aqueous 10% hydrochloric acid amounts to 66 p/l for example at a temperature of 100"C, to 46 g/l at 65"C and to 30 g/l at 400C.

After the saturation concentration at the selected temperature has been reached, the solution may be recooled relatively quickly without star-shaped or wafer-shaped aggregates being formed in addition to the already formed acicular crystals. This is because, after the saturation equilibrium has been adjusted, there is surprisingly no further nucleus formation during further cooling, instead the calcium sulphate still present in solution continues to be deposited onto the already formed needles.

If crystallisation should fail to occur even when the solution is cooled to a temperature below 60"C, it must be initiated by the addition of nuclei. The nuclei used may either be already formed whiskers or calcium sulphate hydrates. After inoculation at a temperature in the range from 65 to 400 C, the uniform pattern of the acicular whiskers always appears. The crystals are then obtained from the cooled solution by filtration, are washed until they are free from acid and then dried. By drying at a temperature below about 80"C, the unchanged calcium sulphate dihydrate is obtained as end product.At temperatures above at least 105"C, the so-called anhydrite-a modification distinguished by its high absorption capacity for water-is formed. insoluble anhydrite is formed by drying at temperatures above about 200"C, but we have found that it is of advantage to heat the material to higher temperatures, for example around 400 C to ensure conversion of the desired insoluble anhydrite.

Calcium sulphate shows a solubility steeply increasing with temperature, preferably in aqueous solutions of monobasic acids. Accordingly, the described process may be carried out particularly effectively in aqueous solutions of, for example, hydrochloric acid, nitric acid and perchloric acid which represent the particularly preferred acids according to the invention. It is also possible, however, to carry out the process with other acids, for example polybasic inorganic acids for example, phosphoric acid, sulphuric acid or organic acids for example, sulphosalicyclic acid. Although other acids may also be used, they would not appear to be of any practical significance on account of their shallow solubility gradients.

Any naturally occurring or commercially obtained calcium sulphates free from or containing water may be used as the starting material for the process according to the invention. One particular advantage of the process according to the invention is that waste gypsums may be used as the starting material, even those containing organic substances as impurities, for example from the production of hydrofluoric acid, from the production of phosphoric acid or from the neutralisation of excess sulphuric acid in the sulphonation of organic compounds.

This is because, during the dissolution, crystallisation and filtration process, they either remain on the filter as residue or are dissolved in the acid and remain in the solvent phase after the whiskers have crystallised out.

The dissolution and crystallisation process may be carried out in acids of any concentration, although for practical reasons it is carried out with advantage at the highest solubility gradients. In the case of the above-mentioned acids, this range is from about 5 to 15% by weight.

For practical reasons, the difference between the temperature at which the calcium sulphate is dissolved and the temperature prevailing at the end of the cooling step is advantageously selected to be as large as possible in order fully to utilise the solubility gradient, although the process may also be carried out with small temperature gradients.

Irrespective of the type of acid used, crystals of calcium sulphate dihydrate with a length to mean diameter ratio of at least 6:1, preferably 10:1 and, with particular preference, 20:1 and far greater, for example 100--150:1, are formed by the process according to the invention. The whiskers normally have a thickness of from 0.0005 to 0.03 mm, highly uniform crystallisation at the upper or lower end of the above-mentioned fluctuation range being obtained by varying the cooling conditions. Relatively large crystals are always formed when crystallisation begins in the upper temperature range and also continues in that range until the solubility equilibrium corresponding to this temperature has been adjusted.

Whiskers produced by the process according to the invention may be used for a very wide variety of applications. For example, it is possible to embed them in plastics and to use them as reinforcing materials for inorganic hydraulic binders, for example in the building sector. In addition, they may generally be used as reinforcing materials for organic and/or inorganic materials and/or fillers and/or extenders. These two properties may be combined with particular effect so that the whiskers may be effectively used as a filling reinforcing material, optionally even in combination with other fillers or reinforcing fibres, for example asbestos fibres or glass fibres.

In general, the whiskers produced by the process described above may be used in any shaped articles, coating layers and films on an inorganic and/or organic basis. In the context of the present invention, films also include in particular paper, card, cardboard, etc., in addition to the usual organic films.

One particularly preferred application for the whiskers produced by the process described above is in the production of paper. It is standard practice to fill paper with mineral fillers, such as kaolin (China clay), chalk, barium sulphate, titanium dioxide, etc., in order to improve its properties. Calcium sulphate is also used in small quantities in the form of aniline. However, calcium sulphate in needle form, i.e. in the form of whiskers, has hitherto never been used in the manufacture of paper.

The inventional mineral fillers are added to the paper in small quantities, based on cellulose fibres. Thus, the maximum quantity in which these mineral fillers are added amounts to around 30% by weight, based on cellulose fibres. By contrast, whiskers can be worked into paper in far larger quantities. Thus, the whiskers obtained by the process described above may readily be processed into paper in quantities of from 700 , to 1000 ,; by weight, based on cellulose. With a high filler content, paper can be produced more economically and shows significantly improved properties.For example, the volume and absorbency of the papers thus produced increase and, by virtue of the fibre structure of the whiskers, the strength of the paper does not decrease as drastically as is comparatively the case where conventional mineral fillers are used. Thus, paper to which around 100% of the whiskers produced in accordance with the invention have been added shows substantially the same strength as paper containing 30% of the usual mineral fillers.

Papers filled with such a high percentage of whiskers have another particular advantage, i.e. they are non-inflammable. By virtue of their high absorbency, papers such as these can be effectively impregnated with resins, for example melamine resins, and dried and pressed to form laminates. Laminates produced in this way comply for example with the non-inflammability requirements of the aircraft and shipbuilding industry, although they may also be universally used as fire retarding materials, optionally together with other fibres, for example asbestos fibres.

Non-impregnated papers may also be effectively processed for example with asbestos fibres etc to form special-purpose papers and special-purpose cardboard and may also be used in a variety of different sectors, for example as typing and printing paper, with the usual ash content, but greater strength; as a substantially non-inflammable raw material for wallpaper; as an insulating material for building purposes, optionally impregnated with synthetic resins, according to the application envisaged, and as non-inflammable laminate papers for the production of laminates.

Whiskers used in this way also have an effect similar to that of a pigment, with the result that they may be used with particular effect in cases where filling material or reinforcing material is also required to show pigment-like properties.

The whiskers produced by the process according to the invention may be or should be stabilised, depending upon their desired application. Stabilisation preferably protects the whiskers against moisture and, for this reason, should surround them as completely as possible. U. S. Patent No. 3,822,340 for example describes the stabilisation of whiskers with hydrolysed proteins and anionic polycarboxylic acid polymers. However, the whiskers produced in accordance with the invention may be generally stabilised by at least periodically bringing them into contact with suitable organic and/or inorganic compounds for a sufficiently long period.

Thus, the whiskers can be protected for example by a wet treatment, i.e. for example by mashing the whiskers with stabilisers or by the deposition of stabilisers.

In the deposition method, the whiskers act as seeds for the stabilisers which are precipitated from their solutions by suitable additions. It is possible to apply both organic and also inorganic stabilisers by the deposition method.

For example, oxide aquates of silicon, titanium, zinc and/or aluminium can be deposited as a protective layer onto the whiskers. A water-repelling effect is obtained for example by the deposition of silicones or their intermediate products or, with even greater effect, by other hydrophobic organic substances.

Where substances having a sufficiently high vapour pressure can be used as stabilisers, it is also possible to pass an inert gas stream through these substances or their solutions and then to guide it over the whiskers. The whiskers to be stabilised may also be coated by fluidisation, in which case a suitable, charged inert gas stream may be used as the fluidisation medium. On the other hand, the fluidised or stationary whiskers may also be spray-coated with stabilisers.

Suitable stabilisers may be added to the whiskers after their formation and before their separation in the reaction vessel itself, whereby the whiskers separated off are adequately protected in a simple manner.

The present invention is illustrated by the following Examples.

EXAMPLE 1 69 g of calcium sulphate dihydrate were dissolved while stirring at 950C in 1000 cc of 10% hydrochloric acid. By filtration through a heated suction filter, this solution was freed from the excess calcium sulphate dihydrate and any insoluble impurities which may be present. The filtrate was cooled in air to 650C and then stirred for 2 hours, during which the temperature was kept above 50"C by additional heating.

In the solution, large needle-shaped crystals of calcium sulphate dihydrate began to separate out at 650C. The solubility equilibrium was reached after 2 hours at 500 C. The suspension was then cooled while stirring to 25"C.

After cooling, the crystals consisted entirely of acicular monocrystals with a thickness of from 0.005 mm to 0.025 mm and a length of from 0.1 mm to about 4 mm. The crystals were filtered off and washed with water until free from acid. After they had been dried at 500 C, they formed a loose, silky mass in which monocrystals were clearly discernible with the naked eye. The crystallised material did not contain any star-shaped crystal aggregates, nor any so-called swallowtail twins, such as normally occur as the main constituent in monocrystalline gypsum.

The reaching of the solubility equilibrium during crystallisation of the acicular crystals above 40"C was followed by microscopic examination of the suspension.

On cooling of a sample on the specimen holder, the calcium sulphate dihydrate still present in oversaturated solution crysallised out in the form of star-like crystal aggregates which were clearly visible under 100-fold magnification. The nonappearance of crystals such as these indicated that the solubility equilibrium had been reached.

EXAMPLE 2 (Comparison Example) A solution of 69 g of calcium sulphate dihydrate in 1000 cc of 10% hydrochloric acid was prepared at 950C and filtered in the same way as described in Example 1. In contrast to Example 1, the filtrate was cooled as quickly as possible while stirring to a temperature below 40"C, i.e. 35"C, and stirred until the end of crystallisation. Crystallisation was over after 4 hours, by which time the temperature had fallen to 25"C.

Although the suspension contained some crystals in the form of needles having a diameter of from 0.001 to 0.003 mm, the crystals present in the suspension consisted predominantly of three dimensional star-shaped aggregates composed of flake-like crystals with a diameter of approximately 0.100 mm.

A dull, colourless loose mass of monocrystalline calcium sulphate dihydrate was obtained after filtration, washing and drying.

The yield of crystalline material was identical with that obtained in Example 1, amounting to 30 g of calcium sulphate dihydrate (=75 /n of the theoretical yield).

EXAMPLE 3 In a 1200 litre capacity stirrer-equipped vessel, a 10% nitric acid was heated to 75"C, followed by the addition to saturation level of a waste gypsum from the chemical industry containing organic materials as impurities. The excess gypsum together with undissolved organic and inorganic impurities were separated off by filtration in a filter press. The clear filtrate was heated to 800C in the crystallisation vessel and then rapidly cooled to 65"C, followed by further cooling to 500C over a period of about 120 minutes. Since crystallisation began very hesitantly, the solution was inoculated with a suspension of 0.5 kg of plaster of paris (calcium sulphate semihydrate) in 2 litres of water.Before addition to the nitric acid solution, the suspension required for inoculation was stirred for about 30 minutes to enable the semihydrate to be converted into dihydrate. After the inoculated solution had been stirred for 2 hours at 50"C, examination under a microscope showed that the solubility equilibrium had been reached at that temperature.

The contents of the crystallisation vessel were then cooled while stirring to 25 C.

Although on cooling to 250C crystallisation partially occured at less than 40"C in exactly the same way as in Example 1 due to the oversaturation occurring, the product was completely free from the above-described star-shaped crystal aggregates of flake-like monocrystals.

After the crystals filtered off had been washed and dried, they formed a loose silky colourless mass. The filtrate could be repeatedly used, the quantity of gypsum dissolved on each occasion only corresponding to that which was separated during crystallisation.

Part of the dried calcium sulphate dihydrate was dried at 1050C, resulting in the formation of so-called soluble anhydrite. This soluble anhydrite was converted into insoluble anhydrite by tempering at around 400"C. During this dehydration and conversion, the outer crystal form of the material remained fully intact. Under a polarisation microscope, it could be seen that even the soluble anhydrite represents a polycrystalline pseudomorphosis of the original fibre crystals of calcium sulphate dihydrate. Surprisingly the strength of the whiskers was not reduced during conversion into the polycrystalline form. The insoluble anhydrite thus produced formed loose silky mass which was scarcely any different from the original material.

The starting materials characterised below were used for the following Examples: I. Polymer I.1. The plastic referred to as polyamide-6 in the following Examples was a polymer of -caprolactam having a solution viscosity of 2.8, as measured on a 1% solution in m-cresol at 250 C.

I.2. The plastic referred to as polycarbonate in the following Examples was a polycondensation product, produced in solution, of bisphenol A and phosgene having a relative solution viscosity of 1.31, as measured on a 0.5% solution in methylene chloride at 250C.

I.3. The plastic referred to as polybutylene terephthalate in the following Examples was a polycondensation product of terephthalic acid and 1,4 butane diol having a relative solution viscosity of 1.9, as measured on a 0.5 /" solution in phenoVtetrachloroethane (1:1) at 250C.

II. Reinforcing fibres It.1. The glass fibres used in the following Examples were produced from E glass and can be characterised by the following data: Elasticity modulus approximately 70,000 N/mm2 Tensile strength approximately 2,500 N/mm2 Fibre length approximately 3 mm Fibre diameter approximately 10 jum.

II.2. Calcium sulphate whiskers.

The anhydrous CaSO4-whiskers produced in accordance with the invention were used.

EXAMPLE 4 4500 g of polyamide-6 were mixed on a roll stand with 1500 g of calcium sulphate whiskers and the resulting mixture homogenised in the melt in a twinscrew extruder with screws rotating in the same direction (feed zone 230"C, barrel temperature 2550C, nozzle temperature 2500 C), granulated and finely moulded in an injection moulding machine (melt temperature 2800 C, mould temperature 400 C) to form standard test specimens of which the mechanical properties were determined immediately after injection moulding.

Flexural strength (DIN 53 452)~ISO 178 115 N/mm2 E-modulus (from bending test) (DIN 53 457)~ISO 178 3437 N/mm2 Impact strength (DIN 53 453)=ISO/R 179 32 kJ/m2 By way of comparison, the following mechanical properties were measured on test specimens produced in the same way as in Example 4 from non-reinforced polyamide-6: Flexural strength (DIN 53 452) 72 N/mm2 E-modulus (from bending test) (DIN 53 457) 1390 N/mm2 Impact strength (DIN 53 453) unbroken EXAMPLE 5 Test specimens were produced as in Example 4 from polyamide-6 (4200 g) and calcium sulphate whiskers (1500 g) with E-glass fibres (300 g).These test specimens were found to have the following mechanical properties: Flexural strength (DIN 53 452)128 N/mm2 E-modulus (from bending test) (DIN 43 457) 4560 N/mm2 Impact strength (DIN 53 453) 36 kJ/m2 Example 6 (Comparison Test) Test specimens were produced as described in Example 1 from polyamide-6 (5700 g) and glass fibres (300 g).The test specimens thus produced were found to have the following mechanical properties: Flexural strength (DIN 53 452) 95 N/mm2 E-modulus (from bending test) (DIN 53 457) 1435 N/mm2 Impact strength (DIN 53 453) unbroken EXAMPLE 7 4500 g of polycarbonate were mixed with 1500 g of CaSO4-whiskers and the resulting mixture was homogenised in the melt in a twin-screw extruder with screws rotating in the same direction (feed zone 250"C, barrel temperature 2800 C, nozzle temperature 2800 C), granulated and subsequently moulded in an injection moulding machine (melt temperature 3070C, mould temperature 900 C) to form standard test specimens which were found to have the following mechanical properties:: Flexural strength (DIN 53 452)152 N/mm2 E-modulus (from bending test) (DIN 53 457) 6129 N/mm2 Impact strength (DIN 53 453) 41.3 kJ/m2 By way of comparison, test specimens produced as described in Example 7 from non-reinforced polycarbonate were found to have the following mechanical properties:: Flexural strength (DIN 53 452) 97 N/mm2 E-modulus (from bending test) (DIN 53 457) 2123 N/mm2 Impact strength (DIN 53 453) unbroken EXAMPLE 8 4500 g of polybutylene terephthalate were mixed on a roll stand with 1500 g of CaSO4-whiskers and the resulting mixture was homogenised twice in the melt in a single-screw extruder (feed zone 2400 C, barrel temperature 2500 C, nozzle temperature 2500 C), granulated and finally moulded in an injection moulding machine (melt temperature 2750C, mould temperature 400 C) to form standard test specimens which were found to have the following mechanical properties:: Flexural strength (DIN 53 452)121 N/mm2 E-modulus (from bending test) (DIN 54 457) 3630 N/mm2 Impact strength (DIN 53 453) 27.3 kJ/m2 By way of comparison, test specimens produced as described in Example 8 from non-reinforced polybutylene terephthalate were found to have the following mechanical properties: Flexural strength (DIN 53 452) 95 N/mm2 E-modulus (from bending test) (DIN 53 457) 2289 N/mm2 Impact strength (DIN 53 453) unbroken EXAMPLE 9 Test specimens produced as described in Example 5 from polybutylene terephthalate reinforced with 25% by weight of CaSQ4-whiskers and 5% by weight of E-glass fibres were found to have the following properties:: Flexural strength (DIN 53 452)142 N/mm2 E-modulus (from bending test) (DIN 53 457) 5509 N/mm2 Impact strength (DIN 53 453) 37.5 kJ/m2 EXAMPLE 10 (Comparison Test) Standard test specimens produced as in Example 5 from 5700 g of polybutylene terephthalate and 300 g of E-glass fibres were found to have the following mechanical properties: Flexural strength (DIN 53 452) 118 N/mm2 E-modulus (from bending test) (DIN 53 457) 2796 N/mm2 Impact strength (DIN 53 453) 34 kJ/m2 In all the Examples, sizes and wet strengtheners were added to the fibres and fillers in the usual quantities and the pulps were dispersed in water.

EXAMPLE 11 Writing and printing papers with a weight of approximately 60=100 g/m2 were produced a) 50-60% by weight: and b) 90-100% by weight of gypsum whiskers, based on cellulose, were incorporated into a pulp of which 50 ,/ consisted of bleached sulphate pulp and 50 ,' of bleached sulphite pulp. The papers had very high strengths.

EXAMPLE 12 Raw papers intended for wallpaper having a weight of approximately 80 to 110 g/m2 were produced.

a) 100=120% by weight; b) 200220 / by weight; and c) 290-310% by weight of gypsum whiskers, based on cellulose, were added to 400,, of bleached sulphate pulp and 60% of mechanical wood pulp. The raw wallpapers had very high strengths.

EXAMPLE 13 Raw coated papers having a weight of from about 40 to 100 g/m2 were produced.

a) 50=60% by weight; b) 130=160% by weight: and c) 190=210% by weight of gypsum whiskers, based on cellulose, were incorporated into a mixture of 40=60% of bleached sulphate pulp (softwood), 30= 400,, of bleached sulphate pulp (hard wood) and up to 30% of bleached sulphite pulp. The raw coated papers had excellent strengths.

EXAMPLE 14 Non-inflammable laminate papers having a weight of approximately 80 g/m2 were produced.

30",; of a standard commercial-grade titanium dioxide and a) 590=610% by weight and b) 680-710% by weight of gypsum whiskers, based on cellulose, were incorporated into a mixture of 70=80% of bleached sulphate pulp eucalyptus and 20=30? of bleached sulphate pulp hardwood. These papers also showed very good strengths.

EXAMPLE 15 Non-inflammable papers having a weight of approximately 400 g/m2 were produced.

a) 690=710%, by weight and b) 780810 ,n by weight of gypsum whiskers, based on cellulose, were incorporated into a mixture of 50% of bleached sulphate pulp and 50% of bleached sulphite pulp. The papers showed very good strengths.

WHAT WE CLAIM IS: 1. A process for the production of whiskers of calcium sulphate or calcium sulphate hydrates with a length to mean cross-section ratio of at least 6:1 by crystallisation and subsequent separation of the whiskers, wherein a hot, acid aqueous calcium sulphate solution is freed from crystal nuclei, subsequently cooled to a temperature of from 65 C to 400 C, wherein, within the temperature range of 65 C to 40 C the cooling is effected slowly, and wherein the solution is maintained at a temperature within the range 65 C to 400 C. optionally with addition of seeds, until the saturation concentration has been reached, after which the solution is further cooled and the whiskers which have been crystallised are recovered.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (11)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   EXAMPLE 10 (Comparison Test) Standard test specimens produced as in Example 5 from 5700 g of polybutylene terephthalate and 300 g of E-glass fibres were found to have the following mechanical properties: Flexural strength (DIN 53 452) 118 N/mm2 E-modulus (from bending test) (DIN 53 457) 2796 N/mm2 Impact strength (DIN 53 453) 34 kJ/m2 In all the Examples, sizes and wet strengtheners were added to the fibres and fillers in the usual quantities and the pulps were dispersed in water.

   EXAMPLE 11 Writing and printing papers with a weight of approximately 60=100 g/m2 were produced a) 50-60% by weight: and b) 90-100% by weight of gypsum whiskers, based on cellulose, were incorporated into a pulp of which 50 ,/ consisted of bleached sulphate pulp and 50 ,' of bleached sulphite pulp. The papers had very high strengths.

   EXAMPLE 12 Raw papers intended for wallpaper having a weight of approximately 80 to 110 g/m2 were produced.

   a) 100=120% by weight; b) 200220 / by weight; and

   c) 290-310% by weight of gypsum whiskers, based on cellulose, were added to 400,, of bleached sulphate pulp and 60% of mechanical wood pulp. The raw wallpapers had very high strengths.

   EXAMPLE 13 Raw coated papers having a weight of from about 40 to 100 g/m2 were produced.

   a) 50=60% by weight; b) 130=160% by weight: and

   c) 190=210% by weight of gypsum whiskers, based on cellulose, were incorporated into a mixture of 40=60% of bleached sulphate pulp (softwood), 30= 400,, of bleached sulphate pulp (hard wood) and up to 30% of bleached sulphite pulp. The raw coated papers had excellent strengths.

   EXAMPLE 14 Non-inflammable laminate papers having a weight of approximately 80 g/m2 were produced.

   30",; of a standard commercial-grade titanium dioxide and a) 590=610% by weight and b) 680-710% by weight of gypsum whiskers, based on cellulose, were incorporated into a mixture of 70=80% of bleached sulphate pulp eucalyptus and 20=30? of bleached sulphate pulp hardwood. These papers also showed very good strengths.

   EXAMPLE 15 Non-inflammable papers having a weight of approximately 400 g/m2 were produced.

   a) 690=710%, by weight and b) 780810 ,n by weight of gypsum whiskers, based on cellulose, were incorporated into a mixture of 50% of bleached sulphate pulp and 50% of bleached sulphite pulp. The papers showed very good strengths.

   WHAT WE CLAIM IS: 1. A process for the production of whiskers of calcium sulphate or calcium sulphate hydrates with a length to mean cross-section ratio of at least 6:1 by crystallisation and subsequent separation of the whiskers, wherein a hot, acid aqueous calcium sulphate solution is freed from crystal nuclei, subsequently cooled to a temperature of from 65 C to 400 C, wherein, within the temperature range of 65 C to 40 C the cooling is effected slowly, and wherein the solution is maintained at a temperature within the range 65 C to 400 C. optionally with addition of seeds, until the saturation concentration has been reached, after which the solution is further cooled and the whiskers which have been crystallised are recovered.
2. 2. A process as claimed in Claim 1, wherein the acid aqueous calcium

   sulphate-containing solution is heated to a temperature of from 75"C to 1000C prior to cooling.
3. 3. A process as claimed in Claim 1 or 2, wherein the solution is cooled to a temperature of 65"C and subsequently further cooled with stirring over a period of at least 50 minutes.
4. 4. A process as claimed in any of Claims 1 to 3, wherein the acid aqueous solution has a pH below 4.
5. 5. A process as claimed in any of Claims 1 to 4, wherein the calcium sulphate is used in the form of solution in hydrochloric acid, nitric acid or perchloric acid.
6. 6. A process for preparing calcium sulphate whiskers which have a length to mean cross-section ratio of at least 6:1 substantially as herein described with reference to any of the specific Examples.
7. 7. Calcium sulphate whiskers having a length te mean cross-section ratio of at least 6:1 when prepared by a process as claimed in any of Claims I to 6.
8. 8. A fire-retarding material comprising calcium sulphate whiskers as claimed in Claim 7.
9. 9. A reinforcing and/or filling agent comprising paper and/or pulp and/or cellulose together wlth calcium sulphate whiskers as claimed in Claim 7.
10. 10. A reinforcing and/or filling agent comprising an inorganic or organic shaped article, layer, coating or film together with calcium sulphate whiskers as claimed in Claim 7.
11. 11. A fire-retarding material or a reinforcing and/or filling agent substantially as herein described with reference to any of the specific Examples.