GB1597669A

GB1597669A - Calcium sulphate fibres

Info

Publication number: GB1597669A
Application number: GB22437/78A
Authority: GB
Original assignee: SKW Trostberg AG
Current assignee: Evonik Operations GmbH
Priority date: 1977-08-25
Filing date: 1978-05-25
Publication date: 1981-09-09
Also published as: JPS5446996A; DE2738415B2; SE431566B; CA1065202A; ATA617778A; IT7868591A0; FR2401243A1; DE2738415A1; SE7807380L; FR2401243B1; AT369441B

Description

(54) CALCIUM SULPHATE FIBRES (71) We, SKW TROSTBERG AKTIENGESELLSCHAFT (formerly known as Süddeutsche Kalkstickstoff-Werke Aktiengesellschaft), of D-8223 Trostberg, Federal Republic of Germany, a Joint-Stock Company orgamsed under the laws of the Federal' Republic of 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: The present invention is concerned with calcium sulphate fibres having inorganic coatings and with the product and use thereof, The term "calcium sulphate fibres" used herein is to be understood to mean needle-like crystals of calcium sulphate in which the ratio of average length to diameter is at least 10:1.

Calcium sulphate dihydrate occurs in nature in the form of lumps or powder and is generally referred to as gypsum. Its use as constructional material depends upon the ability of the hemihydrate produced from the dihydrate again to form the dihydrate with water and thereby to solidify.

Besides this modification, naturally occurring fibrous calcium sulphate dihydrate is also known. Furthermore, German Patent Specification No. 2,314,645 describes a process of producing fibrous calcium sulphate in aqueous solution at comparatively high temperatures and with the use of pressure, which satisfies technical requirements. However, because of its water solubility, the calcium sulphate fibres so produced suffer from the disadvantage that they cannot be universally used with success. Thus, it has already been suggested in German Patent Specification No. 2,314,645 to stabilise calcium sulphate, hemihydrate and anhydrite fibres against rehydration. For the stabilisation according to this German Patent Specification No. 2,314,645, calcium sulphate fibres and especially those in the hemihydrate or anhydrite form, are provided with organic coatings, for example with waxes, protein hydrolysates or, preferably, with anionic polycarboxylic acid polymers. However, such organic coatings are expensive. Furthermore, they suffer from the disadvantage of decomposing at temperatures above about 100 C. and, in any case, above 300 C. Since, however, modern construction work materials, such as polycarbonates, polyamides, aromatic polyesters, aromatic polyimines and the like, are worked up at temperatures of up to or even above 300 C., fibres coated with the coatings known from this German Patent Specification No. 2,314,645 cannot, in many cases, be incorporated as strengthening materials.

Therefore, it is an object of the present invention to overcome the disadvantage of the previously known products and to provide calcium sulphate fibres (as hereinbefore defined) which have a solubility in water of not more than 800 mg./litre and are provided with protective coatings which are stable at temperatures of more than 300 C. and/or the surfaces of which are very full of fissures or have burl-like projections.

Thus, according to the present invention, there are provided calcium sulphate fibres (as hereinbefore defined) provided with inorganic coatings, wherein the coatings consist of firmly adhering inorganic compounds, the proportion by weight thereof on the coated fibres being from 0.5 to 10% by weight, the coated calcium sulphate fibres having a solubility in water of not more than 800 mg./litre.

It is a characteristic of the coated fibres according to the present invention that, due to the presence thereon of the inorganic coating, the originally smooth fibre surface can, due to burl-like growths, have a surface which is very full of fissures.

According to the present invention, the calcium sulphate fibres are preferably hemihydrate or anhydnte fibres. The production of such fibres is known. Thus, it can take place by heating an aqueous slurry of calcium sulphate dihydrate at a temperature of from 105 to 1500C., for example as described in German Patent Specification No. 2,314,645; or it is possible to obtain especially thin fibres with a ratio of average length to diameter of more than 100:1 by reacting a dilute aqueous solution of at least one calcium salt with a dilute aqueous solution of a stoichiometric amount of a water-soluble sulphate at a pH value of from 8 to 13 or by reacting a dilute aqueous suspension of calcium oxide and/or calcium carbonate with excess dilute sulphuric acid at an elevated temperature and thereafter, for the achievement of the desired fibre length, leaving to stand, possibly at an elevated temperature; or synthetic or natural gypsum can be suspended in water, a proton donor is added thereto, the gypsum is brought into solution at an elevated temperature and fibre formation is brought about by mixing with an aqueous sulphate solution and/or cooling of reaction solution and/or concentrating the solvent; or it is possible to produce calcium sulphate fibres isothermally at a temperature of from 40 to 1200C. in an aqueous solution of a proton donor, in a salt solution or in a mixture thereof in the presence of a solid phase and to convert into calcium sulphate dihydrate, hemihydrate or anhydrite fibres (see our co-pending British Patent Application No. 19980/78). However, the fibres which can be used according to the present invention are not limited to those obtained by the above-described processes.

The coated calcium sulphate fibres according to the present invention can be produced by various methods. According to one process, production takes place by adding to a suspension of calcium sulphate fibres a soluble salt which forms firmly adhering compounds on the fibre surface to give coated calcium sulphate fibres having a solubility in water of not more than 800 mg./litre. According to another method, calcium sulphate fibres are added to a solution of a salt which forms a coating of firmly adhering compounds with calcium sulphate, the coated calcium sulphate fibres having a solubility in water of not more than 800 mg./litre. According to a third process, the coating formation takes place simultaneously with the fibre production by adding to the fibre production medium salts which form firmly adhering compounds with calcium sulphate to give coated calcium sulphate fibres having a solubility in water of not more than 800 mg./litre.

Examples of compounds which form the desired firmly adhering inorganic coatings includes those water-soluble salts, the ions of which form salts with the calcium ion or sulphate ion which are more sparingly soluble than calcium sulphate. Such compounds include, for example, sodium carbonate, sodium fluoride, sodium silicate, trisodium phosphate and barium chloride. On the surface of the calcium sulphate crystals there form, in aqueous solution, thin and dense layers of the corresponding calcium salts or of barium sulphate, which layers protect the calcium sulphate crystal substrate. When using silicates as salts for forming the firmly adhering inorganic coatings, it is preferable either to work under strongly alkaline conditions or, after application of the coating, to carry out a strongly acidic after-treatment, for example with hydrochloric acid. Under these conditions, the solubility of the calcium sulphate fibres reduced by the silicate coating can again be drastically reduced.

Surprisingly, with the use of the process according to the present invention, there are formed thin, dense, firmly adhering inorganic coatings of the calcium sulphate fibres, with maintenance of the fibre structure, which coatings protect the fibres from dissolving, theproportion by weight of the coating on the coated fibres being 0.5 to 10% by weight.

In general, in the case of comparatively long reaction times and/or of comparatively high concentrations of the salt solutions, dense and smooth coatings are obtained, whereas in the case of comparatively short reaction times and/or -comparatively low concentrations of the salt solutions, burl-like coatings are obtained.

The fibres according to the present invention have a considerably reduced solubility in water which is usually less than 10 mg./litre in comparison with 2500 to 2900 mg./litre in the case of uncoated fibres.

The calcium sulphate fibres according to the present invention with inorganic coatings do not change or do not substantially change their lowered solubility even in the case of comparatively long storage in water or in the case of repeated slurrying in fresh water.

Therefore, they can be used for strengthening matrix materials, for example synthetic resins (thermoplastic and thermosetting), inorganic binding agents (gypsum, cement and lime), cellulose products (cardboard and paper) and the like.

The calcium sulphate fibres according to the present invention with smooth surfaces are preferably employed in paper and cardboard production since, in comparison with uncoated fibres, they have a solubility which is about a hundred times lower but, nevertheless, bring about a considerable increase in the strength of the paper.

This substantially lower solubility thereby provides the following advantages: the retention of the fibres in the case of sheet formation is considerably increased and the additional water loading when using the fibres according to the present invention is negligibly low.

The calcium sulphate fibres according to the invention with burl-like inorganic coatings, the production of which is described in Examples 2 and 8, are, due to their excellent adhesion, especially effective for strenghtening thermosetting and thermoplastic synthetic resins, the strengthening action being especially evident in the case of polyolefins (cf.

Example 11).

The following Examples are given for the purpose of illustrating the present invention:1. Calcium sulphate dihydrate fibres with calcium carbonate coatings Example I In a suction filter funnel, 200 ml. of a 0.5 molar sodium carbonate solution were poured over 4 g. calcium sulphate dihydrate fibres in such a manner that the solution passed through the filter after 10 minutes. Coated calcium sulphate dihydrate fibres were obtained with a solubility of < 10 mg. CaSO4.2H2O/l. (the solubility of the dihydrate fibres in water is normally about 2500 mg./l.). Even in the case of filtering, the fibre structure remains substantially unchanged.

Example 2 10 g. Sodium carbonate were suspended in 200 ml. water and 4 g. calcium sulphate dihydrate fibres added thereto. After stirring for 5 minutes at ambient temperature, the fibres are filtered off. The fibre quality, in comparison with the starting substance, was practically unchanged, and the solubility of the partly coated fibres was 800 mg./l. Here, too, similarly to the fibres according to Example 8, the fibres had a markedly burl-like surface.

2. Calcium sulphate fibres with calcium fluoride coatings.

2.1 Calcium sulphate dihydrate fibres.

Example 3 50 g. Gypsum were suspended in 80 ml. 6% nitric acid and the mixture boiled for about 5 minutes. After filtering off insolubles, 10 g. sodium fluoride were added, while stirring, and the clear reaction solution then left to cool. The precipitated fibres were filtered off and washed neutral with an ammoniacal sodium fluoride solution (10 g./l.). The solubility of the fibres (length 0.5 - 1.5 mm., L:D K 150:1) was 82 mg./l.

2.2 Anhydrite fibres.

Example 4 20 g. Sodium fluoride were dissolved in 500 mg. water and 10 g. anhydrite fibres added thereto. The suspension was subsequently stirred for 30 minutes and thereafter the fibres were filtered off. The average length of the coated fibres was then about 80% of that of the starting substance. The solubility was less than 10 mg./l. (untreated anhydrite fibres have a solubility of 2.89 g./l.); 3. Calcium sulphate dihydrate fibres with barium sulphate coatings.

Example 5.

5 g. Calcium sulphate dihydrate fibres were suspended in 200 ml. water and the pH value of the solution adjusted to about 2 with hydrochloric acid. After adding 250 mg. barium chloride, stirring was continued for 20 minutes, followed by filtering off the fibres. The solubility of the coated fibres was about 20 mg./l., the fibre quality, in comparison with the starting material, bein unchanged.

4. Calcium sulphate fibres with calcium silicate coatings.

4.1. Calcium sulphate dihydrate fibres.

Example 6 10 g. Calcium sulphate dihydrate fibres were stirred for about 2 hours at ambient temperature in a saturated aqueous sodium silicate solution, the fibre quality thereby remaining substantially unchanged. After filtering off the fibres, their solubility was about 300 mg./l. Subsequent washing of the fibres with dilute hydrochloric acid brought about a reduction of solubility to about 25 mg./l.; the fibre quality was maintained.

4.2. Anhydrite fibres Example 7.

16 ml. Aqueous sodium silicate solution (100 g. sodium silicate in 200 ml. water) were mixed with 50 ml. of an aqueous sodium hydroxide solution (pH 14) and subsequently 3 g.

anhydrite fibres were added thereto. After stirring for two hours, the fibres were filtered off, no impairment of the fibre quality being ascertainable. The solubility of the fibres was less than 10 mg./l.

Example 8 16 ml. of a solution of 100 g. sodium silicate in 200 ml. water were mixed with 200 ml. of an aqueous sodium hydroxide solution (pH 14) and subsequently 3 g. anhydrite fibres were added thereto. After stirring for 1.5 hours, the fibres were filtered off, no impairment of the fibre quality being ascertainable. The solubility of the fibres was about 600 mg./l. The fibres had a markedly burl-like surface, as could be seen from microphotographs thereof.

5. Calcium phosphate coatings.

Example 9 2 g. Calcium sulphate dihydrate or hemihydrate fibres were briefly boiled with 20 ml. of a solution of 30 g. trisodium phosphate in 1 litre water and the fibres were filtered off. The fibre quality corresponded to that of the starting material; the solubility was 100 mg./l. in the case of coated calcium sulphate anhydrite fibres.

6. Examples of use.

Example 10 Anhydrite fibres produced according to Example 7 (solubility < 10 mg./l.) were used for strengthening paper (raw material: beech cellulose): breaking increase in length strength (m) referred to zero value 0 value 2.290 +2% fibres 2.770 +16.6% +10% fibres 2.500 + 9.1% for comparison: anhydrite fibres +2% fibres 2.470 + 7.9% without coatings Example 11 Improvement of the mechanical properties of polypropylene (PP) by the addition of anhydrite fibres with a burl-like surface.

yield stress N/mm2 change in % PP 35 0 PP + 20 wt.% short glass fibres 32 -8.5 PP + 20 wt.% anhydrite fibres 31 -11.4 PP + 20 wt.% anhydrite fibres N+ 45 +28.5 N+ = with burl-like surface Example 12 Improvement of the mechanical properties of a thermosetting resin by the addition of anhydrite fibres.

yield stress N/mm2 change in % epoxide resin 40 0 epoxide resin + 2 wt.% anhydrite fibres 47 +17.5 epoxide resin + 2 wt.% anhydrite fibres N+ 51 +27.5 N+ = with burl-like surface.

WHAT WE CLAIM IS: 1. Calcium sulphate fibres (as hereinbefore defined) provided with inorganic coatings, wherein the coatings consist of firmly adhering inorganic compounds, the proportion by weight thereof on the coated fibres being from 0.5 to 10% by weight, the coated calcium sulphate fibres having a solubility in water of not more than 800 mg./litre.

2. Calcium sulphate fibres according to claim 1, wherein the coatings have a burl-like structure which imparts a very fissured surface to the fibres.

3. Calcium sulphate fibres according to claim 1 or 2, wherein the coatings consist of calcium compounds.

4. Calcium sulphate fibres according to claim 1 or 2, wherein the coatings consist of barium sulphate.

5. Coated calcium sulphate fibres according to claim 1, substantially as hereinbefore described and exemplified.

6. Process for the production of coated fibres according to claim 1, wherein to a suspension of calcium sulphate fibres is added a soluble salt which forms firmly adhering inorganic compounds on the fibre surface to give coated calcium sulphate fibres having a solubility in water of not more than 800 mg./litre.

7. Process for the production of coated fibres according to claim 1, wherein calcium sulphate fibres are added to a salt solution which forms a coating of firmly adhering compounds with calcium sulphate, the coated calcium sulphate fibres having a solubility in water of not more than 800 mg./litre.

8. Process according to claim 6 or 7, wherein, when the salt used is a silicate, the process is carried out under strongly alkaline conditions or a strongly acidic after-treatment is carried out.

9. Process for the production of fibres according to claim 1, wherein the coating formation is carried out simultaneously with the fibre production by adding to the fibre production medium safts which form firmly adhering compounds with calcium sulphate fibres to give coated calcium sulphate fibres having a solubility in water of not more than 800 mg./litre.

10. Process for the production of coated fibres according to claim 1, substantially as hereinbefore described and exemplified.

11. Coated fibres, whenever produced by the process according to any of claims 6 to 10.

12. Cellulose products, synthetic resins and inorganic binding agents, whenever strengthened with coated fibres according to any of claims 1 to 5 and 11.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. yield stress N/mm2 change in % epoxide resin 40 0 epoxide resin + 2 wt.% anhydrite fibres 47 +17.5 epoxide resin + 2 wt.% anhydrite fibres N+ 51 +27.5 N+ = with burl-like surface. WHAT WE CLAIM IS:

1. Calcium sulphate fibres (as hereinbefore defined) provided with inorganic coatings, wherein the coatings consist of firmly adhering inorganic compounds, the proportion by weight thereof on the coated fibres being from 0.5 to 10% by weight, the coated calcium sulphate fibres having a solubility in water of not more than 800 mg./litre.