Note: Descriptions are shown in the official language in which they were submitted.
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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
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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
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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,
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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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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.-
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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
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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
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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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yleld stress
N/mm change in %
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PP 35 0
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.
