GB1597668A - Process for the production of inorganic fibres based on calcium sulphate - Google Patents

Process for the production of inorganic fibres based on calcium sulphate Download PDF

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Publication number
GB1597668A
GB1597668A GB19980/78A GB1998078A GB1597668A GB 1597668 A GB1597668 A GB 1597668A GB 19980/78 A GB19980/78 A GB 19980/78A GB 1998078 A GB1998078 A GB 1998078A GB 1597668 A GB1597668 A GB 1597668A
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Prior art keywords
fibres
calcium sulphate
length
dihydrate
weight
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GB19980/78A
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Evonik Operations GmbH
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SKW Trostberg AG
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    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08K—Use of inorganic or non-macromolecular organic substances as compounding ingredients
    • C08K7/00—Use of ingredients characterised by shape
    • C08K7/02—Fibres or whiskers
    • C08K7/04—Fibres or whiskers inorganic
    • C08K7/08—Oxygen-containing compounds
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
    • C01F11/00—Compounds of calcium, strontium, or barium
    • C01F11/46—Sulfates
    • C01F11/466—Conversion of one form of calcium sulfate to another
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B14/00—Use of inorganic materials as fillers, e.g. pigments, for mortars, concrete or artificial stone; Treatment of inorganic materials specially adapted to enhance their filling properties in mortars, concrete or artificial stone
    • C04B14/38—Fibrous materials; Whiskers
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2004/00—Particle morphology
    • C01P2004/01—Particle morphology depicted by an image
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2004/00—Particle morphology
    • C01P2004/10—Particle morphology extending in one dimension, e.g. needle-like
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2004/00—Particle morphology
    • C01P2004/60—Particles characterised by their size
    • C01P2004/61—Micrometer sized, i.e. from 1-100 micrometer

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  • Chemical & Material Sciences (AREA)
  • Organic Chemistry (AREA)
  • Engineering & Computer Science (AREA)
  • Ceramic Engineering (AREA)
  • Materials Engineering (AREA)
  • Inorganic Chemistry (AREA)
  • Civil Engineering (AREA)
  • Geology (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Structural Engineering (AREA)
  • Health & Medical Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Medicinal Chemistry (AREA)
  • Polymers & Plastics (AREA)
  • Compounds Of Alkaline-Earth Elements, Aluminum Or Rare-Earth Metals (AREA)
  • Inorganic Fibers (AREA)
  • Paper (AREA)

Description

(54) PROCESS FOR THE PRODUCTION OF INORGANIC FIBRES BASED ON CALCIUM SULPHATE (71) We, SKW TROSTBERG AKTIEN GESELLSCHAFT (formerly known as Siid- deutsche Kalkstickstoff-Werke Aktiengesellschaft). of D-8223 Trostberg, Federal Republic of Germany. a Joint-Stock Company organised under the laws ofthe 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 a process for the production of inorganic fibres based on calcium sulphate.
It is known that, in aqueous solution, the temperature at which calcium sulphate dihydrate or hemihydrate can be converted into anhydrite can be considerably lowered.
When heated in the air, calcium sulphate dihydrate and hemihydrate only lose their water of crystallisation at a temperature of from about 120 C. or 190 C., respectively, the products obtained being non-fibrous aggregates.
Van't Hoff et al. (Z. phys. Ch., 45, 257 et seq./1903) found that gypsum, in the presence of various materials, such as sodium chloride, magnesium chloride, magnesium sulphate or nitric acid, can, when heated in aqueous solution. be converted into hemihydrate or anhydride crystals, depending upon the temperature employed.
According to A. E. Hill, boiling for three days in 15 to 20% sulphuric acid leads to the formation of anhydrite crystals with a length of 20 to 30 y (J.A.C.S., 56, 1071--1078/1934), Japanese Patent Specification No. Sho 51-87 494 describes the hydrothermal production of gypsum whiskers by treating calcium sulphate in organic solvents diluted with water or in the presence of mineral acids, their ammonium or metal salts, as well as urea, in organic solvents diluted with water.
According to U.S. Patent Specification No.
3,977,890, the formation of calcium sulphate hemihydrate fibres takes place hydrothermally under pressure at temperatures of 125 to 140 C. in the presence of boric acid.
Finally, in the periodical Ryusan to Kogyo, 29(6), 113-125/1976, M. Tanaka describes a process for the production of fibres based on a-calcium sulphate hemihydrate from waste gas desulphurisation gypsum which is obtained by oxidation of the initially obtained calcium sulphite under pressure at about 130 C. The fibres obtained have a length of from 100 to 200y and a diameter of from 1 to 2,u. By heating to temperatures above 350 C., these hemihydrates fibres can be converted into anhydrite fibres.
All the above-mentioned known processes suffer from disadvantages: either non-fibrous products are obtained or the process requires the use of organic solvents and thus involves a considerable expense for the recovery of these solvents or these known processes require a considerable expense for apparatus since the fibre formation must be carried out under pressure. Furthermore, the fibres obtained have a maximum length of only 0.2 mm.
Therefore, it is an object of the present invention to overcome the disadvantages of the known processes and to produce fibres of calcium sulphate dihydrate, hemihydrate or anhydrite directly from calcium sulphate compounds. This process is to be economic and technically simple to carry out, without the use of organic solvents, is to operate at atmospheric pressure with high space-time yields and is to provide high fibre quality, i.e.
a large ratio of length to diameter of the fibres, as well as fibres with a length of over 0.2 mm.
Thus, according to the present invention, there is provided a process for the production of inorganic fibres based on calcium sulphate with a ratio of length to diameter of more than 50: 1 and a length of at least 0.2 mm., wherein calcium sulphate is converted isothermally into calcium sulphate dihydrate, hemihydrate or anhydrite fibres in an aqueous solution of a proton donor, in a salt solution or in a mixture thereof at atmospheric pressure and at a temperature of from 40 to 120on., in the presence of a solid phase, said proton donor being hydrochloric acid and/or nitric acid and said salt being a halide and/or a sulphate of a mono-, di- or trivalent metal, with the proviso that the halide is not a fluoride and with the further proviso that when the proton donor is nitric acid, it is used in admixture with the salt.
According to the process of the present invention, the calcium sulphate employed as starting material can be natural gypsum building gypsum or synthetic or natural anhydrite, which usually contain about 90% by weight of calcium sulphate. Gypsums which are obtained in the course of technical reactions can also be employed so long as they do not contain any impurities which disturb fibre formation. Calcium sulphate dihydrate is preferably used as starting product. As solid phase, still undissolved calcium sulphate is initially present which dissolves in the course of the conversion, the solid fibres formed forming new phases.
Surprisingly, it is possible, according to the process of the present invention, to produce calcium sulphate fibres at a temperature of from 40 to 120 C. with high space-time yields, the fibre formation admittedly taking place via the dissolved phase but calcium sulphate only going into solution to the extent of the fibre formation so that, in all, dissolving of all of the introduced calcium sulphate is not necessary. The fibres produced in this way have substantially equal lengths and diameters and are free from nonfibrous components. This can be readily appreciated from the accompanying drawings, which show two microphotographs of fibres obtained by the process according to the present invention.
The proton donor used can be a technical grade aqueous hydrochloric acid and/or nitric acid. The use of dilute nitric acid and especially of 0.05 to 0.2 molar nitric acid has proved to be particularly advantageous.
Apart from the hydrochloric and/or nitric acid, which acts as a proton donor, halides (excluding fluorides) of mono-. di- and trivalent metals can also be employed, for example sodium chloride, potassium chloride, calcium chloride. zinc chloride aluminium chloride, potassium bromide and potassium iodide. However, it is especially preferred to use lithium chloride, magnesium chloride and zinc chloride since the fibres obtained in the presence of these compounds are especially beautifully shaped and have an especially favourable ratio of length to diameter.
The salt solution is preferably at least 1 molar.
Besides the use of aqueous solutions of pure salts, mixtures of salts can also be employed. Mixtures of salts with the protondonating acids have also proved to be useful.
Thus, for example, when using an aqueous solution of magnesium chloride/nitric acid, calcium sulphate fibres are obtained with especially pronounced longitudinal growth and, in comparison with the fibre length, only a small thickness. Furthermore, the low content of water of crystallisation when using such mixtures is also noticeable, which points towards mixtures of calcium sulphate hemihydrate with calcium sulphate anhydrite fibres as the theoretical content of water of crystallisation of pure calcium sulphate hemihydrate of 6.9% by weight shows. The assumption of the concurrent presence of hemihydrate and anhydrite fibres was confirmed by X-ray diffractometric investigations.
Besides the salts or proton donors employed, the reaction temperature and the concentration of the calcium sulphate, which is preferably 5 to 50% by weight, in the reaction solution play a part in the fibre formation and in the nature of the fibres obtained. If the proportion of calcium sulphate in the reaction mixture is 10% by weight and the reaction temperature is about 40on., then more than 10 hours are necessary in order to recognise a fibre formation. When the concentration of solid material in the reaction mixture is about 30% by weight, fibres can be obtained after only about 10 to 15 minutes. In order to save time, it is preferable to work at a temperature of at least 60çC.
It is especially interesting that the formation of fibres by the process according to the present invention proceeds isothermally, i.e.
the conversion of the introduced calcium sulphate, the fibre formation and the precipitation of the fibres formed takes place at one and the same temperature. Thus, no energyand time-consuming heating and cooling procedures are needed in order to crystallise out the fibres from the reaction solution. The fibre suspensions can, after formation thereof, be separated off in known manner, for example by centrifuging, from the aqueous salt or acid solution and the mother liquor can be recycled for use again for the next batch. Due to recycling of the mother liquor, there is obtained a practically quantitative fibre yield, calculated upon the calcium sulphate employed.
Furthermore, it is especially surprising that, in the process according to the present invention, calcium sulphate-anhydrite fibres are obtained from calcium sulphate dihydrate or from gypsums consisting essentially of calcium sulphate dihydrate, without a laborious subsequent dewatering of the initially formed calcium sulphate dihydrate fibres having to be carried out. Thus, the process is characterised by an especially high economy since it permits the production not only of dihydrate but also of anhydrite fibres without the use of pressure, merely in dependence upon the reaction temperature and possibly of the reaction medium in one and the same plant in a relatively short reaction time.If the reaction is carried out in such a manner that calcium sulphate dihydrate fibres are formed, these can subsequently be dewatered at temperatures above 100 C. and at a water vapour pressure greater than 2 bar.
The fibres produced by the process according to the present invention can, because of their high strength values, be used as additives for the improvement of the mechanical properties of matrix materials of the most varied kinds. They are preferably used, for example, for strengthening synthetic resins, for improving the tear strength of paper and cardboard, for reinforcing inorganic binding agents and for producing fabrics, fleeces, insulation mats and fire-protection mats.
The calcium sulphate anhydrite fibres produced by the process according to the present invention have the further advantage of being temperature stable up to about 1100"C. This property means that the fibres are generally useful for strengthening materials which melt below 1100 C., for example glass, aluminium and alloys thereof. Furthermore, the tempèrature stability of the fibres can be utilised by using the fibres as flameprotection agents in paper, cardboard and certain kinds of textiles.
The following Examples are given for the purpose of illustrating the present inven tion: Example 1.
40 g. Waste gypsum obtained from citric acid production (75% by weight calcium sulphate dihydrate) were treated in a solution of 22 g. lithium chloride in 100 ml. water for two hours at a temperature of 105 C. Monocrystalline fibres were obtained with a length of 0.5 to 2 mm. The average ratio of length to diameter was about 100:1, the content of water of crystallisation was 7.5% by weight and the yield was 24%, referred to the gypsum used.
Example 2.
0.05 Mol calcium sulphate dihydrate was treated in 100 ml. of a 4 molar potassium chloride solution for one hour at 105 C.
Monocrystalline fibres were obtained with a length of 0.5 to 0.8 mm. The average ratio of length to diameter was about 150:1, the content of water of crystallisation was 2.4% by weight and the yield was over 95%, referred to the dihydrate used.
Example 3.
40 g. Natural gypsum (about 91% by weight calcium sulphate dihydrate) were treated in a solution of 54.4 g. zinc chloride in 100 ml. water for two hours at 105 C.
Monocrystalline fibres were obtained with a length of 0.5 to 1.6 mm. The average ratio of length to diameter was about 100:1, the content of water of crystallisation was 9.5% by weight and the yield was 29 g.
Example 4.
5 g. Commercially-available building gypsum (about 90% by weight calcium sulphate hemihydrate) were treated in a solution of 40.6 g. magnesium chloride in 100 ml. water for two hours at 105 C. Monocrystalline fibres were obtained with a length of 0.6 to 1.1 mm. The average ratio of length to diameter was about 120:1, the content of water of crystallisation was 5.4% by weight and the yield was 4 g.
Example 5.
Synthetic anhydrite was converted into calcium sulphate dihydrate by one of the usual processes. 0.03 mol of this calcium sulphate dihydrate was treated with 100 ml.
of a 2 molar potassium bromide solution for two hours at 100 C. Monocrystalline fibres were obtained with an average length of 0.4 mm. and with an average ratio of length to diameter of about 100:1. The content of water of crystallisation was 6.4% by weight and the yield, referred to the dihydrate used, was over 95%.
Example 6.
0.03 Mol calcium sulphate dihydrate was treated in 100 ml. of a 2 molar potassium iodide solution for one hour at 105 C.
Monocrystalline fibres were obtained with an average length of 0.25 mm. and with an average ratio of length to diameter of about 60: 1. The content of water of crystallisation was 5.9% by weight and the yield was over 95%, referred to the dihydrate used.
Example 7.
0.25 Mol calcium sulphate dihydrate was treated at 105 C. in a solvent mixture of 0.09 molar nitric acid/2 molar magnesium chloride for one hour. Monocrystalline fibres were obtained with a length of 0.5 to 1.4 mm.
The average ratio of length to diameter was 150:1, the content of water of crystallisation was 2.8% by weight and the yield was 70%, referred to the dihydrate used.
WHAT WE CLAIM IS: 1. A process for the production of inorganic fibres based on calcium sulphate with a ratio of length to diameter of more than 50: 1
**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

  1. **WARNING** start of CLMS field may overlap end of DESC **.
    of calcium sulphate dihydrate, without a laborious subsequent dewatering of the initially formed calcium sulphate dihydrate fibres having to be carried out. Thus, the process is characterised by an especially high economy since it permits the production not only of dihydrate but also of anhydrite fibres without the use of pressure, merely in dependence upon the reaction temperature and possibly of the reaction medium in one and the same plant in a relatively short reaction time. If the reaction is carried out in such a manner that calcium sulphate dihydrate fibres are formed, these can subsequently be dewatered at temperatures above 100 C. and at a water vapour pressure greater than 2 bar.
    The fibres produced by the process according to the present invention can, because of their high strength values, be used as additives for the improvement of the mechanical properties of matrix materials of the most varied kinds. They are preferably used, for example, for strengthening synthetic resins, for improving the tear strength of paper and cardboard, for reinforcing inorganic binding agents and for producing fabrics, fleeces, insulation mats and fire-protection mats.
    The calcium sulphate anhydrite fibres produced by the process according to the present invention have the further advantage of being temperature stable up to about 1100"C. This property means that the fibres are generally useful for strengthening materials which melt below 1100 C., for example glass, aluminium and alloys thereof. Furthermore, the tempèrature stability of the fibres can be utilised by using the fibres as flameprotection agents in paper, cardboard and certain kinds of textiles.
    The following Examples are given for the purpose of illustrating the present inven tion: Example 1.
    40 g. Waste gypsum obtained from citric acid production (75% by weight calcium sulphate dihydrate) were treated in a solution of 22 g. lithium chloride in 100 ml. water for two hours at a temperature of 105 C. Monocrystalline fibres were obtained with a length of 0.5 to 2 mm. The average ratio of length to diameter was about 100:1, the content of water of crystallisation was 7.5% by weight and the yield was 24%, referred to the gypsum used.
    Example 2.
    0.05 Mol calcium sulphate dihydrate was treated in 100 ml. of a 4 molar potassium chloride solution for one hour at 105 C.
    Monocrystalline fibres were obtained with a length of 0.5 to 0.8 mm. The average ratio of length to diameter was about 150:1, the content of water of crystallisation was 2.4% by weight and the yield was over 95%, referred to the dihydrate used.
    Example 3.
    40 g. Natural gypsum (about 91% by weight calcium sulphate dihydrate) were treated in a solution of 54.4 g. zinc chloride in 100 ml. water for two hours at 105 C.
    Monocrystalline fibres were obtained with a length of 0.5 to 1.6 mm. The average ratio of length to diameter was about 100:1, the content of water of crystallisation was 9.5% by weight and the yield was 29 g.
    Example 4.
    5 g. Commercially-available building gypsum (about 90% by weight calcium sulphate hemihydrate) were treated in a solution of 40.6 g. magnesium chloride in 100 ml. water for two hours at 105 C. Monocrystalline fibres were obtained with a length of 0.6 to 1.1 mm. The average ratio of length to diameter was about 120:1, the content of water of crystallisation was 5.4% by weight and the yield was 4 g.
    Example 5.
    Synthetic anhydrite was converted into calcium sulphate dihydrate by one of the usual processes. 0.03 mol of this calcium sulphate dihydrate was treated with 100 ml.
    of a 2 molar potassium bromide solution for two hours at 100 C. Monocrystalline fibres were obtained with an average length of 0.4 mm. and with an average ratio of length to diameter of about 100:1. The content of water of crystallisation was 6.4% by weight and the yield, referred to the dihydrate used, was over 95%.
    Example 6.
    0.03 Mol calcium sulphate dihydrate was treated in 100 ml. of a 2 molar potassium iodide solution for one hour at 105 C.
    Monocrystalline fibres were obtained with an average length of 0.25 mm. and with an average ratio of length to diameter of about 60: 1. The content of water of crystallisation was 5.9% by weight and the yield was over 95%, referred to the dihydrate used.
    Example 7.
    0.25 Mol calcium sulphate dihydrate was treated at 105 C. in a solvent mixture of 0.09 molar nitric acid/2 molar magnesium chloride for one hour. Monocrystalline fibres were obtained with a length of 0.5 to 1.4 mm.
    The average ratio of length to diameter was 150:1, the content of water of crystallisation was 2.8% by weight and the yield was 70%, referred to the dihydrate used.
    WHAT WE CLAIM IS: 1. A process for the production of inorganic fibres based on calcium sulphate with a ratio of length to diameter of more than 50: 1
    and a length of at least 0.2 mm.. wherein calcium sulphate is converted isothermally into calcium sulphate dihydrate. hemihydrate or anhydrite fibres in an aqueous solution of a proton donor. in a salt solution or in a mixture thereof at atmospheric pressure and at a temperature of from 40 to 120 C. in the presence of a solid phase. said proton donor being hydrochloric acid and/or nitric acid and said salt being a halide of a mono-. di- or trivalent metal, with the proviso that the halide is not a fluoride and with the further proviso that when the proton donor is nitric acid. it is used in admixture with the salt.
  2. 2. Process according to claim 1 wherein the starting material used is natural gypsum.
    building gypsum. synthetic or natural anhydrite or gypsum obtained as a by-product.
  3. 3. Process according to claim 2. wherein the starting material used is calcium sulphate dihydrate or hemihydrate.
  4. 4. Process according to any of the preceding claims, wherein the halide used is a chloride of lithium, magnesium. zinc or aluminium or a mixture thereof.
  5. 5. Process according to any of the preceding claims, wherein a halide of a mono-.
    di- or trivalent metal, excluding the fluorides, is used in admixture with a proton donor.
  6. 6. Process according to any of the preceding claims, wherein at least I molar salt solution is used.
  7. 7. Process according to any of the preceding claims, wherein 5 to 506Sc by weight of calcium sulphate is present in the reaction mixture.
  8. 8. Process according to claim I for producing calcium sulphate fibres, substantially as hereinbefore described and exemplified.
  9. 9. Calcium sulphate fibres, whenever produced by the process according to any of claims I to 8.
GB19980/78A 1977-07-01 1978-05-16 Process for the production of inorganic fibres based on calcium sulphate Expired GB1597668A (en)

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
DE2729842A DE2729842B2 (en) 1977-07-01 1977-07-01 Process for the production of inorganic fibers based on calcium sulfate

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GB1597668A true GB1597668A (en) 1981-09-09

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JP (1) JPS5418927A (en)
AT (1) AT361103B (en)
CA (1) CA1121973A (en)
DE (1) DE2729842B2 (en)
FR (1) FR2395965A1 (en)
GB (1) GB1597668A (en)
IT (1) IT1109145B (en)
SE (1) SE431234B (en)

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2053874B (en) * 1977-12-29 1982-07-28 Idemitsu Kosan Co Production of calcium sulphate
US4801355A (en) * 1987-01-16 1989-01-31 United States Gypsum Company Tabular acicular gypsum and method of filling paper
FR2629069B1 (en) * 1988-03-24 1990-11-23 Lafarge Sa Platres PROCESS FOR THE PREPARATION OF CALCIUM SULPHATE WITH A LONGLINE CRYSTALLINE STRUCTURE HAVING A LENGTH AND A FACTOR OF MASTERED SHAPE
FR2640961B1 (en) * 1988-12-22 1993-04-02 Lafarge Platres
US7273579B2 (en) * 2004-01-28 2007-09-25 United States Gypsum Company Process for production of gypsum/fiber board

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR932226A (en) * 1946-08-13 1948-03-16 Cie Des Phosphates Et Du Chemi Process for promoting needle formation in gypsum precipitates
GB1038025A (en) * 1963-08-16 1966-08-03 Bpb Industries Ltd Lightweight material and production thereof
US4029512A (en) * 1974-08-05 1977-06-14 Johns-Manville Corporation Method for the preparation of fibrous insoluble calcium sulfate anhydrite
SE7601262L (en) * 1975-02-07 1976-08-09 Ici Ltd FIBER-ARMED MATERIAL AND KIT FOR ITS MANUFACTURE
JPS51109295A (en) * 1975-03-22 1976-09-28 Idemitsu Kosan Co KAISHITSUSARETAKETSUSHOSEISETSUKOSENINOSEIZOHOHO
DE2621611C3 (en) * 1976-05-14 1981-06-25 Skw Trostberg Ag, 8223 Trostberg Inorganic fibers, processes for their production and their use

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IT7868375A0 (en) 1978-06-13
JPS5418927A (en) 1979-02-13
IT1109145B (en) 1985-12-16
SE7807379L (en) 1979-01-02
DE2729842B2 (en) 1979-05-23
SE431234B (en) 1984-01-23
CA1121973A (en) 1982-04-20
DE2729842A1 (en) 1979-01-25
ATA477378A (en) 1980-07-15
AT361103B (en) 1981-02-25
FR2395965A1 (en) 1979-01-26

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