CA1065202A

CA1065202A - Calcium sulphate fibres having inorganic coatings

Info

Publication number: CA1065202A
Application number: CA304,774A
Authority: CA
Inventors: Alois Aignesberger; Ekkehard Weinberg; Walter Lukas
Original assignee: SKW Trostberg AG
Current assignee: Evonik Operations GmbH
Priority date: 1977-08-25
Filing date: 1978-06-05
Publication date: 1979-10-30
Also published as: GB1597669A; FR2401243B1; IT7868591A0; SE431566B; DE2738415A1; DE2738415B2; FR2401243A1; AT369441B; ATA617778A; JPS5446996A; SE7807380L

Abstract

Abstract The invention provides calcium sulphate fibres provided with inorganic coatings, the coatings consist-ing of sparingly soluble, firmly adhering inorganic compounds, the proportion by weight thereof on the coated fibres being from 0.5 to 10% by weight. The invention also provides processes for the production of these coated fibres, which are useful for strength-ening cellulose products, synthetic resins and inorganic binding agents.

Description

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The present invention is concerned with calcium Aulphate fibres having inorganic coatings and with the production and use thereof.
Calcium sulphate dihydrate occurs in nature in the form of lumps or powder and is generally referred to as gypsum. Its uRe 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 sul-phate in aqueous solution at comparatively high temper-atures and with the use of pressure, which ~atisfies technical requirements. However, because of it~ water 301ubility, the calcium sulphate fibres so produced suffer from the disadvantage that they cannot be uni-versally used with succe~s. Thus, it has already been qugge~ted in German Patent Specification No.2,314,645 ; to stabilise calcium sulphate, hemihydrate and anhydrite fibre~ agalnst rehydration. For the stabili~ation according to this German Patent Specification No.2,314,645, calcium sulphate fibres and e~pecially tho~e ln the hemi-hydrate or anhydrite form, are pro~ided with organic coatings, for example with waxes, protein hydrolysate~

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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 100C. and, in any case, above 300C. Since, however, modern constructional work materials, such as polycarbonates, polyamides, aromatic polyesters, aromatic polyimines and the like, are worked up at temperatures of up to or even above 300C., fibers coated with the coatings known from this German Patent Specification ~o. 2,314,645 cannot, in many cases, be incorporated ' -: 10 as strengthening materials.
Therefore, it is an object of the present invention to overcome the disadvantages of the previously known products and to provide calcium sulphate fibers which have a low solubility ~
and are provided with protective coatings which are stable at ~.
temperatures of more than 300C. and/or the surfaces of which are very full of fissures or have knub-like projections~
Thus, according to the present invention, there are provided calcium sulphate fibers provided with inorganic coatings, wherein the coatings are of firmly adhering inorganic compounds that are sparingly soluble in aqueous systems, the proportion by weight thereof on the coated fibers being from 0.5 to lC% by weight.
In another aspect of the invention there is provided a process for the production of the coated fibers of the inventionwhich process comprises contacting in an aqueous medium, the calcium sulphate fibers with a water soluble salt that forms with calcium sulphate a compound that is sparingly soluble in aqueous systems.

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The coatings may suitably comprise an inorganic material selected from calcium carbonate, calcium fluoride, calcium silicate, calcium phosphate and barium sulphate.
It is a characteristic of the coated fibers accord-ing to the present invention that, due to the presence 106520~ -., thereon of the sparingly soluble inorganic coating, the origi~ally smooth fi~re surface can, due to knub-like growths, have a surface which is very full of fissures.
According to the present invention, the calcium sulphate fibres are preferably hemihydrate or anhydrite fibre~. 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 150C., for example a~ 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 lea~t one calcium salt with a dilute aqueous sol-ution 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 achieve-ment of the desired fibre length, leaving to stand, po~sibly at an elevated temperature; or syntheti~ 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 ~olution and/or concentrating the ~olvent, or it is pos~ible to produce calcium sulphate - : ,, - . : ~ . -- , : ~ :. -, 106SZ(~Z

fibres isothermally at a temperature of from 40 to 120C.
in an aqueous solution of a proton donor, in a salt solution oX in a mixture thereof in the presence of a solid phase and to convert into calcium sulphate di-hydrate, hemihydrate or anhydrite fibres. 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 sol-uble salt which form~ sparingly soluble salts on the fibre surface of the calcium sulphate fibres. According - to another method, calcium sulphate fibres are added to a solution of a salt which forms sparingly soluble com-pounds with calcium sulphate. According tc a third process, the coating formation takes place simultane--ously with the fibre production by adding to the fibre production medium salts which form sparingly soluble compounds with calcium sulphate.
Examples of compounds which form sparingly sol-uble, firmly adhering inorganic coatings include 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, ~()65ZOZ

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 sub-strate. When using silicates as salts forming sparingly soluble, firmly adhering inorganic coatings, it is preferable either to work under strongly alkaline con-ditions or, after application of the coating, to carry out a strongly acidic after-treatment, for example with hydrochloric acid. Under these conditions, the solub-ility of the calcium sulphate fibres reduced by the silicate coating can again be drastically reduced.
Surprisingly, with the use of the process accord-ing to the present invention, there are formed thin, dense, firmly adhering inorganic coatings on the calcium sulphate fibres, with maintenance of the fibre structure, which coatings protect the fibres from dissolving, the proportion 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 concentrat-ions of the salt solutions, dense and smooth coatings are obtained, whereas in the case of comparatively ~hort .. , , . . - . . .

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reaction times and/or comparatively low concentrations of the salt solutions, ]cnub-like coatings are obtained.
The fibres according to the present invention have a considerably reduced solubility in water which is 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 inorganlc coatings do not change or do not substantially change their lowered solubility even in the case of comparatlvely 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 (thenmoplasts and duroplasts), 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 iB about a hundred times lower but, nevertheless, bring about a considerable increase in the strength of the paper.
This substantially lower solubility thereby pro-vides the following advantages: the retention of the .

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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 neg-ligibly low.
The calcium sulphate fibres according to the invention with knub-like inorganic coatings, the pro-duction of which is described in Examples 2 and 8, are, due to their excellent adhesion, especially effective for strengthening duroplasts and thermoplasts, 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 calciumcarbonate coatinqs.
Exam~le 1.
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 sol-ution passed through the filter after 10 minutes. Coated calcium sulphate dihydrate fibres were obtained with a solubility of ~ 10 mg. CaS04.2H20/1. (the solubil~ty of the dihydrate fibres in water is normally about 2500 mg./l.). Even in the case of filtering, the fibre structure remains substantlally unchanged.

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10$52(~2 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 am~ient temp-erature, the fibres are filtered off. The fibre quality, in comparison with the starting substance, was practic-ally unchanged, and the solubility of the partly coated fibres was 800 mg./l. Here, too, similarly to the fibres according to Example 8, the surface was strongly knubbed.

2. Calcium sul~hate fibres with caIcium fluoride coatinqs.
2.1. Calcium sulPhate d 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 ~V 150:1) was 82 mg./l.-i~.......................................................................... .
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 sub~e~uently stirred for 30 minutes and .

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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 sol-ubility of 2.89 g./l.).

3. Calcium sulphate dihYdrate fibres with barium sulphate coatinqs.
Example 5.
5 g. Calcium sulphate dihydrate fibres were sus-pended in 200 ml. water and the pH value of the sol-ution adjusted to about 2 with hydrochloric acid. -After adding 250 mg. barium chloride, stirring was continued for 20 minutes, followed by filtering aff the fibres. The solubility of the coated fibres was about 20 mg./l., the fibre quality, in comparison with the starting material, being unchanged.

4. Calcium sulphate fibres with calcium silicate coatinqs.
4.1. Calcium sul~hate 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 : ~ , - , , ~ ~ ' ' ' ~ ' : , '' ' . ': : : . :~
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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. ~nhYdrite fibres.
Example 7.
16 ml. Aqueous sodium silicate ~olution (100 g.
sodium silicate in 200 ml. water) were mixed with 50 ml.
of an aqueous sodium hydroxide olution (pH 14) and sub-sequently 3 g. anhydrite fibres were added thereto.
After stirring for two hours, the fibres were filtered off, no impairment of the fibre quality being ascertain-able. 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. Their surface was strongly"knubbed", as could be seen from microphotographs thereof.

5. Calcium phosphate coatinas.~
Example 9.

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2 g. Calcium sulphate dihydrate or hemihydrate fibre~ 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./
1. in the case of coated calcium sulphate anhydrite fibres.

6. Exam~les 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 - .......... _ . ........ .

Exam~le 11.
Improvement of the mechanical properties of poly-propylene (PP) by the addition of anhydrite fibres with knub-like surface.

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PP + 20 wt.% short 32 8 5 glass fibres _ .
PP + 20 wt.yo anhydrite 31 -11.4 _ 45 +28.5 ~+ = with knubs.
Exam~le 12.
Improvement of the mechanical properties of a -duroplast by the addition of anhydrite fibres.

¦ ~ yield stre~s ~/mm change in %
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epoxide resin 40 0 epoxide re~in ~ 2 wt.% 47 +17 5 anhydrite fibres epoxide resin ~ 2 wt.%
anhydrite fibres N+ _ _ _ +27.5 N+ = with knubs.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. Calcium sulphate fibers provided with inorganic coatings, wherein the coatings consist of firmly adhering inorganic compounds that are sparingly soluble in aqueous systems, the proportion by weight thereof on the coated fibers being from 0.5 to 10% by weight, the inorganic com-pound being selected from the group consisting of calcium carbonate, calcium fluoride, calcium silicate, calcium phosphate and barium sulphate.

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

3. Calcium sulphate fibers according to claim 1 or 2, wherein the coatings consist of calcium compounds that are sparingly soluble in water.

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

5. A coated fiber comprising a calcium sulphate fiber having an average length to diameter of more than 100:1, and a coating of an inorganic material firmly adhered to the fiber, said coating being stable at temperatures of more than 300°C and sparingly soluble in an aqueous system, such that the coated fiber has a solubility in water of up to 100 mg/
liter, and said coating comprising 0.5 to 10%, by weight, of the coated fiber; said inorganic material being selected from the group consisting of calcium carbonate, calcium fluoride, calcium silicate, calcium phosphate and barium sulphate.

6. A coated fiber according to claim 5, wherein said fiber is a calcium sulphate hemihydrate fiber.

7. A coated fiber according to claim 5, wherein said fiber is a calcium sulphate anhydrite fiber.

8. A coated fiber according to claim 5, 6 or 7, wherein said inorganic material is a calcium compound selected from the group consisting of calcium carbonate, calcium fluoride, calcium silicate and calcium phosphate.

9. A coated fiber according to claim 5, 6 or 7, wherein said inorganic material is barium sulphate.

10. A coated fiber comprising a calcium sulphate fiber having an average length to diameter of more than 100:1, and a coating of an inorganic material firmly adhered to the fiber, said coating being stable at temperatures of more than 300°C and sparingly soluble in an aqueous system; said coating having a knub-like structure imparting a fissured surface to the fiber; and comprising 0.5 to 10%, by weight, of the coated fiber said inorganic material being selected from the group consisting of calcium carbonate, calcium fluoride, calcium silicate, calcium phosphate and barium sulphate.

11. A coated fiber according to claim 10, wherein said fiber is a calcium sulphate hemihydrate fiber.

12. A coated fiber according to claim 10, wherein said fiber is a calcium sulphate anhydrite fiber.

13. A coated fiber according to claim 10, 11 or 12, wherein said inorganic material is barium sulphate.

14. A coated fiber according to claim 10, 11 or 12, wherein said inorganic material is a calcium compound selected from the group consisting of calcium carbonate, calcium fluoride, calcium silicate and calcium phosphate.

15. A process for the production of coated fibers comprising calcium sulphate fibers having a coating of a firmly adhering inorganic compound that is sparingly soluble in aqueous systems; said coating being present in an amount of 0.5 to 10%, by weight, of the coated fibers; which process comprises contacting in an aqueous medium the calcium sulphate fibers with a water soluble salt that forms with calcium sulphate a compound that is sparingly soluble in aqueous systems; said inorganic compound being selected from the group consisting of calcium carbonate, calcium fluoride, calcium silicate, calcium phosphate and barium sulphate.

16. A process according to claim 15, wherein to a suspension of calcium sulphate fibers is added a soluble salt which forms a compound that is sparingly soluble in an aqueous system.

17. A process according to claim 15, wherein calcium sulphate fibers are added to a salt solution that forms with calcium sulphate a compound that is sparingly soluble in an aqueous system.

18. A process according to claim 16 or 17, wherein the salt is a silicate and the process is carried out under strongly alkaline conditions.

19. A process according to claim 16 or 17, wherein the salt is a silicate and including a strongly acidic after-treatment.

20. A process according to claim 15, wherein the coating formation is carried out simultaneously with the fiber pro-duction by adding to a fiber production medium a salt that forms with calcium sulphate fibers a compound that is sparingly soluble in an aqueous system.

21. Cellulose products, synthetic resins and inorganic binding agents, strengthened with coated fibers as defined in claim 1 or 2.

22. Cellulose products, synthetic resins and inorganic binding agents, strengthened with a coated fiber as defined in claim 5 or 10.