Finely powdery magnesium hydroxide and a process for preparing thereof

A finely powdery magnesium hydroxide especially suitable as a flame-retardant filler for plastic compounds, in which the particles are provided optionally with a thin coating of a surfactant. The grain size of the magnesium hydroxide, measured by laser diffraction, is below 10 .mu.m. The median value of the grain size is greater than 0.8 .mu.m, and the maximum of the median value of the grain size is 3 .mu.m. The contents of water-soluble ionic impurities, viz. Ca.sup.++, Na.sup.+, K.sup.+, SO.sub.4 --, Cl.sup.-, of the magnesium hydroxide are below the following limits (in parts by weight): Ca.sup.++ <1000 ppm, Na.sup.+ <20 ppm, K.sup.+ <20 ppm, SO.sub.4 --<1500 ppm, Cl.sup.- <1000 ppm. The contents of Mn, Cu and Ni are below the following limits (in parts by weight): MnO<100 ppm, NiO<100 ppm, CuO<10 ppm. The process for preparing the magnesium hyroxide of this invention starts with a magnesium oxide obtained by spray roasting a magnesium chloride solution from which previously foreign matter has been removed, adding water thereto for hydration, whereupon the resulting magnesium hydroxide is removed by filtration and the filter cake material is subjected to a post-wash with water.

The present invention relates to a finely powdery magnesium hydroxide 
suitable especially as flame-retardant filler for plastic compounds, the 
particles thereof being provided optionally with a thin coating of a 
surfactant. 
Frequently, finely powdery magnesium hydroxide is used as flame-retardant 
filler for plastic compounds, especially those on basis of thermoplastic 
materials. In such a use relatively high amounts of magnesium hydroxide 
are added to the plastics, the weight of the magnesium hydroxide 
frequently amounting to the half up to the double weight of the plastic. 
Kinds of finely powdery magnesium hydroxide often have properties which 
exert a negative influence on the mechanical properties of plastic 
compounds to which such a magnesium hydroxide has been added as filler. 
Often e.g. a tendency of such plastic compounds and articles produced from 
such plastic compounds respectively to take up relatively high amounts of 
water, to a decrease of the tensile strength and to an increased ageing 
may be observed. 
Frequently, the flowability and formability of such plastic compounds and 
the appearance of the surface of articles formed from such plastic 
compounds is adversely influenced by the properties of certain kinds of 
finely powdery magnesium hydroxide, e.g. by the grain structure and the 
contents of water thereof. 
DE-C3-2 624 065 describes a magnesium hydroxide having the special 
structure of particles which should eliminate in use of said magnesium 
hydroxide in plastic compounds disadvantages as mentioned above. The 
particles should have a deformation in the &lt;101&gt;-direction of not more 
than 3.times.10.sup.-3, a crystallite size in the &lt;101&gt;-direction of more 
than 800 .ANG. and a specific surface, determined according to BET, of 
less than 20 m.sup.2 /g. According to DE-C3-2 659 933 in addition to that 
structure of the particles of the magnesium hydroxide said particles are 
covered with anionic surfactants so as to eliminate the above mentioned 
disadvantages. 
Now it has been found that the above mentioned structural parameters are 
not suitable for the characterization of a magnesium hydroxide to be used 
for the above mentioned purpose, by which the above mentioned 
disadvantages can be eliminated reliably. Despite of such a structure in 
the practice, the mentioned disadvantages may occur. 
It is an object of the present invention to provide a finely powdery 
magnesium hydroxide as defined above, which in use as flame-retardant 
filler in plastic compounds eliminates reliably the mentioned 
disadvantages and which, also after a longer period of time, does not 
induce both any impairment of the chemical-physical properties, especially 
of the electrical insulating property and of the chemical resistance, and 
any impairment of the mechanical properties and of the dimensional 
stability and does not favor disadvantageous changes of the plastic 
compounds under the influence of moisture and other environmental 
influences. The finely powdery magnesium hydroxide to be prepared should 
allow also a simple processing of plastic compounds, in which it serves as 
flame-retardant filler, and should allow to obtain good strength 
properties of articles prepared from such plastic compounds as well as a 
uniformly closed surface of said articles, which does not show any trouble 
by surface faults. 
The finely powdery magnesium hydroxide of the present invention is 
characterized by a grain size, measured by laser diffraction, of below 10 
.mu.m, the median value of the grain size being greater than 0.8 .mu.m and 
3 .mu.m at the utmost and further characterized in, that the magnesium 
hydroxide contains water-soluble ionic impurities, viz. Ca.sup.++, 
Na.sup.+, K.sup.+, SO.sub.4.sup.--, Cl.sup.-, in amounts below the 
following limits (in parts by weight): 
EQU Ca.sup.++ &lt;1000 ppm, Na.sup.+ &lt;20 ppm, K.sup.+ &lt;20 ppm, 
EQU SO.sub.4.sup.-- &lt;1500 ppm, Cl.sup.- &lt;1000 ppm, 
and that the magnesium hydroxide contains Mn, Cu and Ni in amounts below 
the following limits (in parts by weight): 
EQU MnO&lt;100 ppm, NiO&lt;100 ppm, CuO&lt;10 ppm. 
Magnesium hydroxide having such features allows to achieve well the above 
mentioned objects. 
In experiments carried out during the development of subject matter of this 
invention is was found that by keeping the values of water-soluble ionic 
impurities of the magnesium hydroxide in the mentioned ranges the 
insulating resistance and the electrical breakdown resistance of bodies or 
articles consisting of a plastic compound filled with such a magnesium 
hydroxide are definitely higher than in case of using other usual kinds of 
magnesium hydroxide, which fact is particularly observed, if such bodies 
or articles are exposed to the influence of moisture. Furthermore by 
keeping the amounts of water-soluble ionic impurities of the magnesium 
hydroxide in the mentioned ranges the swelling of plastic compounds 
occurring under the influence of moisture, which swelling was observed 
often with use of usual kinds of magnesium hydroxide, is prevented 
effectively, and by using of the magnesium hydroxide of the invention very 
good mechanical properties (e.g. strength, elongation at break and 
dimensional stability) of the articles produced from the plastic compounds 
as well as a good resistance thereof against phenomena of ageing are 
achieved. Keeping the amounts of contaminations by heavy metals within the 
mentioned ranges contributes to these features, which fact probably can be 
explained that thereby a chemically disadvantageous influence on the 
plastic material (e.g. oxidative degradation) is substantially eliminated. 
Observing the above mentioned upper limit of the grain size contributes to 
the good mechanical properties and the large avoidance of disadvantageous 
effects of extraneous influences, especially of moisture, by allowing to 
achieve a high surface quality of the bodies or articles produced from the 
plastic compounds, that means a dense and closed surface counteracting to 
the penetration of foreign matter. Equally, the mentioned grain size is 
advantageous for obtaining a good tensile strength. Also the observation 
of the above mentioned range of the median value (designated "d.sub.50 ") 
of the grain size of the magnesium hydroxide is advantageous for the 
achievment of favorable mechanical properties of the plastic compounds and 
also with respect to the flame-retardant effect. It should be mentioned 
that the stated values of grain sizes are based on a measurement of the 
grain size with laser diffraction. A measurement of the grain size by 
other measuring methods may give other values. The measurement with laser 
diffraction does not detect an amount of up to 1% (by weight) of oversized 
particles. 
Therefore, it appears to be particularly advantageous that bodies or 
articles produced from plastic compounds containing the magnesium 
hydroxide of the invention as a filler show several favorable mechanical 
properties in combination. Thus, e.g. very good values of tensile strength 
und simultaneously good values of elongation at break may be obtained in 
elastomeric plastic compounds. 
A preferred embodiment of the magnesium hydroxide of the invention for 
obtaining a particularly good resistance to influences of moisture 
provides that the amount of Ca.sup.++, Na.sup.+, SO.sub.4.sup.--, Cl.sup.- 
is below the following limits: 
EQU Ca.sup.++ &lt;500 ppm, Na.sup.+ &lt;10 ppm K.sup.+ &gt;10 ppm, 
EQU SO.sub.4.sup.-- &lt;800 ppm, Cl.sup.- &lt;500 ppm. 
As to the contents of heavy metals in the magnesium hydroxide a preferred 
embodiment provides that the amount of Mn, Cu and Ni is below the 
following limits: MnO&lt;50 ppm, NiO&lt;50 ppm, CuO&lt;5 ppm. Thereby the catalytic 
promotion of phenomena of degradation of the plastic material can be 
eliminated substantially completely. 
As to the flame-retardant effect of the finely powdery magnesium hydroxide 
it is also advantageous if the loss at red heat of the magnesium hydroxide 
is &gt;30.0%. 
The electrical conductivity of the magnesium hydroxide, determined 
according to DIN 53208 on an aqueous suspension, is preferably &lt;500 
.mu.S/cm, preferably &lt;300 .mu.S/cm. 
For obtaining favorable mechanical properties of the plastic compounds and 
the articles produced therefrom and also for achieving the flame-retardant 
effect of the magnesium hydroxide a maximum value of the grain size of 7 
.mu.m and a median value of the grain size of 1.+-.0.2 .mu.m proved to be 
especially advantageous. 
As to the processing the magnesium hydroxide with the plastic material to 
give a plastic compound and as to the dispersion of the magnesium 
hydroxide in the plastic material and also as to an influence on the 
E-modul of the plastic compounds filled with the magnesium hydroxide it is 
advantageous if the ratio of the diameter of the primary particles of the 
magnesium hydroxide to the height of said primary particles, designated 
"aspect ratio", is between 2 and 6. A ratio of the diameter of said 
primary particles to the height thereof of between 3 and 4 is particularly 
favorable for the dispersibility. 
The optional provision of a thin coating of a surfactant on the particles 
of the magnesium hydroxide is mainly advantagous for a further improvement 
of the dispersibility and for a further improvement of the mechanical 
properties of the plastic compounds. Relatively small amounts of up to 2%, 
related to the weight of the magnesium hydroxide, may be sufficient. 
The present invention also relates to a process for preparing the finely 
powdery magnesium hydroxide, said process comprising adding water to 
magnesium oxide obtained by spray roasting from a magnesium chloride 
solution from which previously foreign matter has been removed, which 
magnesium oxide contains Ca.sup.++, Na.sup.+, K.sup.+, SO.sup.--, 
Cl.sup.-, in amounts below the following limits (in parts by weight): 
EQU Ca.sup.++ &lt;10 000 ppm, Na.sup.+ &lt;1000 ppm, K.sup.+ &lt;1000 ppm, 
EQU SO.sub.4.sup.-- &lt;3000 ppm, Cl.sup.- &lt;100 000 ppm, 
and Mn, Cu and Ni in amounts below the following limits (in parts by 
weight): MnO&lt;150 ppm, NiO&lt;150 ppm, CuO&lt;15 ppm, allowing to react the 
suspension with stirring, removing then the magnesium hydroxide formed in 
suspension by filtration and subjecting the filter cake material to one or 
several post-washes with completely desalted water, dewatering the filter 
cake material again and finally drying thereof. Preferably completely 
desalted water is used for the hydration of the magnesium oxide. 
In the process of the invention the suspension preferably is allowed to 
react with stirring at a temperature of between 55.degree. to 100.degree. 
C. For a rapid and complete progress of the hydration and for obtaining 
the features essential for the magnesium oxide of the invention it is 
advantageous to stir the suspension stirring at a temperature of between 
80.degree. and 90.degree. C. 
In the scope of the process of the invention it is possible in a 
particularly simple way to achieve the special properties and features 
respectively of the finely powdery magnesium hydroxide if the magnesium 
chloride solution in turn is prepared by digestion of a magnesium silicate 
material or magnesium hydrosilicate material, such as olivine, serpentine, 
garnierite and the like with hydrochloric acid and subsequent purification 
of the digestion pulp. 
The production of the magnesium oxide in the process of the invention is 
effected by spray roasting of a magnesium chloride solution. By this 
technique a magnesium chloride solution is sprayed within a reactor, in 
which an atmosphere of hot gas generated by burners is present. This 
results in the substantially complete pyrohydrolysis of the magnesium 
chloride, whereas other components of the magnesium chloride solution, 
e.g. watersoluble potassium, sodium or calcium salts, are not modified. 
It should be mentioned that also processes for preparing magnesium 
hydroxide and magnesium oxide different from the above mentioned spray 
roasting technique may be used. Thus, for the production of magnesium 
hydroxide and magnesium oxide from sea water lime milk or dolomite milk is 
added to the sea water, resulting in the precipitation of the magnesium 
hydroxide, the latter being separated by sedimentation and washed 
subsequently, whereupon the formed magnesium hydroxide is converted into 
the magnesium oxide by thermical treatment. Such a preparation of 
magnesium hydroxide and magnesium oxide is exposed to several 
disadvantageous influences by extraneous substances present in the sea 
water and in the lime milk or dolomite milk, which may result in the 
disadvantageous presence thereof in the end product. 
Furthermore, the invention relates to the use of the finely powdery 
magnesium hydroxide of the invention as flame-retardant filler in plastic 
compounds. A preferred embodiment of the invention relates to the use of 
the finely powdery magnesium hydroxide of the invention as flame-retardant 
filler in plastic compounds, the plastic component of which being a 
thermoplastic substance. 
Finally the invention relates to a plastic compound containing a plastic 
material and as flame-retardant filler a finely powdery magnesium 
hydroxide of the invention. A preferred plastic compound is characterized 
in that the compound contains as plastic component a thermoplastic 
material and as flame-retardant filler a finely powdery magnesium 
hydroxide of the invention. 
In the following examples the invention is illustrated in more detail.

EXAMPLE 1 
10 l of completely desalted water are added into a reaction vessel and 
heated to a temperature of 70.degree. C. 850 g of a magnesium oxide having 
the chemical analysis and grain size analysis shown in table I, column 1, 
prepared by pyrohydrolysis of a magnesium chloride solution, are 
introduced into the above solution and stirred sufficiently by means of a 
stirrer for 3 hours. Subsequent to the hydrothermical treatment the 
product is filtered and washed with water. After drying a product is 
obtained, the chemical analysis and grain size analysis of which is given 
in table II, column 1. The electrical conductivity of the magnesium 
hydroxide thus obtained was determined according to DIN 53208 on an 
aqueous suspension with 265 .mu.S/cm. The primary particles have a ratio 
diameter to height of 3 to 4. 
EXAMPLE 2 
10 l of completely desalted water are added into a reaction vessel and 
heated to a temperature of 85.degree. C. 2 kg of a magnesium oxide having 
the chemical analysis and grain size analysis shown in table I, column 2, 
prepared by pyrohydrolysis of a magnesium chloride solution, are stirred 
into the solution and subjected to a hydrothermical treatment for 5 hours. 
Then the product is filtered at 85.degree. C. and washed with water. After 
drying a product is obtained, the chemical analysis and grain size 
analysis of which is given in table II, column 2. The electrical 
conductivity, determined according to DIN 53208 on an aqueous suspension, 
was 382 .mu.S/cm. The primary particles showed a ratio of diameter to 
height of 5 to 6. 
EXAMPLE 3 
1500 g of a magnesium hydroxide prepared according to Example 1 were mixed 
intensively with 15 g of an alkoxysilane in a rapid mixer for 15 minutes 
and thereby modified in the surface. 
EXAMPLE 4 
For preparing a plastic compound 100 parts by weight of an elastomeric 
ethylene-propylene-diene-polymer (EPDM) in form of a powder were mixed 
intimately with 200 parts by weight of a magnesium hydroxide prepared 
according to Example 1. Then specimens were prepared from this plastic 
compound by injection molding and said specimens were examined according 
to DIN 53670. The results of the examination are mentioned in column A of 
table III. 
EXAMPLE 5 
Example 4 is repeated using the same polymer with the exception that a 
magnesium hydroxide obtained by Example 3 is employed. The results of the 
examination of the specimens prepared from the plastic compound are 
mentioned in column B of table III. 
COMATIVE EXAMPLE 1: 
Example 4 is repeated using the same polymer with the exception that a 
commercial magnesium hydroxide prepared from sea water is employed. The 
results of the examination of the specimens prepared from the plastic 
compound are mentioned in column C of table III. 
COMATIVE EXAMPLE 2 
Example 4 is repeated using the same polymer with the exception that a 
magnesium hydroxide modified on the surface, as described in DE-C3-2 659 
933, is employed. The results of the examination of the specimens prepared 
from the plastic compound are mentioned in column C of table III. 
From table III it is immediately evident that the specimens prepared from 
plastic compounds containing magnesium hydroxide of the invention as a 
filler (columns A and B) show a high tensile strength and simultaneously a 
good elongation at break as well as a low swelling at storage in water. 
Speciments prepared from plastic compounds containing a commercial 
magnesium hydroxide prepared from sea water (table III, column C) show a 
distinctly lower tensile strength and an increased swelling at storage in 
water compared with the values of columns A and B of table III. The use of 
another known magnesium hydroxide being modified on its surface 
(comparative example 2, table III, column D) results in a lower swelling, 
but decreases highly the tensile strength. 
EXAMPLE 6 
For the production of a plastic compound 100 parts by weight of a 
polypropylene of Type PP 8400 (Huls-Chemie) were mixed intimately with 150 
parts by weight of a magnesium hydroxide obtained by Example 1. Then 
specimens were prepared from said plastic compound by injection molding 
and examined then with respect to tensile strength and elongation at break 
according to DIN 53455, to impact resistance according to DIN 53453, to 
combustion properties according to ASTM D 2863-77 and to flame resistance 
according to UL 94/V (3 mm). Furthermore the flow length of the plastic 
compound at injection molding at 240.degree. C. was determined according 
to an internal comparative method as measure of processability. The 
obtained values are mentioned in table IV, column A. With respect to the 
combustion properties the LOI (limiting oxygen index) value was 
determined, which corresponds to the minimum amount of oxygen (% O.sub.2) 
of the environmental atmosphere necessary to maintain the burning down. UL 
94/V (3 mm) are guidelines of Underwriters Laboratory for conducting 
examination of flame-resistance of specimens having a thickness of 3 mm in 
vertical position; V-0 . . . means the best result, V-1 . . . a moderate 
result H.B. . . . high burning. 
EXAMPLE 7 
Example 6 is repeated using the same polyproplene such as in Example 6 with 
the exception that a magnesium hydroxide obtained by Example 3 is 
employed. The results of the examination of the specimens prepared from 
the plastic compound by injection molding are mentioned in column B of 
table IV. 
COMATIVE EXAMPLE 3 
Example 6 is repeated using the same polyproplene such as in Example 6 with 
the exception that a commercial magnesium hydroxide prepared from sea 
water and used in comparative example 1 is employed. The results of the 
examination of the specimens prepared from the plastic compound by 
injection molding are mentioned in column C of table IV. 
COMATIVE EXAMPLE 4 
Example 6 is repeated using the same polyproplene such as in Example 6 with 
the exception that a magnesium hydroxide modified on its surface and used 
in comparative example 2 is employed. The results of the examination of 
the specimens prepared from the plastic compound by injection molding are 
mentioned n column D of table IV. 
COMATIVE EXAMPLE 5 
Specimens were prepared from the polypropylene used in examples 6 and 7 and 
the comparative examples 3 and 4 by injection molding without addition of 
magnesium hydroxide and subjected to the examinations mentioned in these 
examples and comparative examples. The results given in column E of table 
IV were obtained. 
TABLE I 
______________________________________ 
Magnesium oxide 
1 2 
______________________________________ 
Chemical analysis 
MgO (from the difference) 
% by weight 98.2 94.1 
SiO.sub.2 " 0.005 0.02 
CaO " 0.50 0.52 
Al.sub.2 O.sub.3 
" 0.010 0.002 
Fe.sub.2 O.sub.3 
" 0.007 0.004 
MnO " 0.0005 0.003 
NiO " 0.003 0.002 
Na.sub.2 O " 0.02 0.018 
K.sub.2 O " 0.02 0.012 
SO.sub.4.sup.-.sup.- 
" 0.04 0.065 
Cl.sup.- " 1.2 5.28 
specific surface BET 
m.sup.2 /g 5.0 4.7 
Grain analysis: 
median value d.sub.50 
.mu.m 2.43 2.68 
upper grain limit 
.mu.m 24.6 24.6 
______________________________________ 
TABLE II 
______________________________________ 
Magnesium hydroxide 
1 2 
______________________________________ 
Chemical analysis 
loss at red heat at 1000.degree. C./ 
2 hours % by weight 
30.5 30.52 
SiO.sub.2 " 0.012 30.52 
Fe.sub.2 O.sub.3 " 0.005 0.002 
Al.sub.2 O.sub.3 " 0.003 0.002 
CaO " 0.006 0.001 
Mg(OH).sub.2 (from the difference) 
" 99.9 99.9 
Na.sub.2 O " &lt;0.001 &lt;0.001 
K.sub.2 O " &lt;0.001 &lt;0.001 
SO.sub.4.sup.-.sup.- 
" 0.028 0.017 
Cl.sup.- " 0.014 0.082 
CuO ppm &lt;5 &lt;5 
MnO " 4 20 
NiO " 20 14 
specific surface BET 
m.sup.2 /g 11 14.5 
Grain analysis: 
median value d.sub.50 
.mu.m 1.19 1.41 
upper grain limit 
.mu.m 6.0 5.0 
______________________________________ 
TABLE III 
______________________________________ 
Determinations of specimens 
Kind of Plastic compound 
examination A B C D 
______________________________________ 
Shore A-hardness 85 86 81 79.5 
DIN 53505 
Tensile strength/Nmm.sup.-2 
7.0 10.0 4.4 2.8 
DIN 53504 10.2 12.8 6.3 2.8 
original 6.6 7.8 4.0 3.1 
7 days at 135.degree. C..sup.1) 
28 days storage in water 50.degree. C..sup.2) 
Elongation at break/% 
224 185 212 534 
DIN 53504 179 145 178 479 
original 247 230 422 464 
7 days at 135.degree. C..sup.1) 
28 days storage in water 50.degree. C..sup.2) 
Swelling at storage 
in water/%.sup.2) 
Day 1 0.9 0.4 1.5 0.4 
Day 3 1.5 0.8 3.8 0.7 
Day 7 2.0 1.3 8.2 0.9 
Day 14 2.5 1.7 9.8 1.4 
Day 21 2.7 1.8 10.5 1.8 
Day 28 2.8 2.0 11.8 1.9 
vulcanization 
minimum of torque M.sub.L 
10.0 9.8 5.4 2.5 
maximum of torque M.sub.H 
64.1 63.2 57.8 37.3 
______________________________________ 
.sup.1) Ageing by hot air was carried out in accordance with DIN 53508 by 
storage at 135.degree. C. for 7 days. 
.sup.2) The storage in water was carried out in accordance with DIN 53521 
by subjecting the specimens to the contacting medium water for a period o 
28 days at 50.degree. C. 
TABLE IV 
______________________________________ 
Kind of Plastic compound 
examination A B C D E 
______________________________________ 
Tensile strength (N/mm.sup.2) 
25.0 20.8 18.2 18.0 23.0 
Elongation at break (m/m) 
0.035 0.34 0.026 0.22 &gt;1 
impact resistance (KJ/m.sup.2) 
10.0 w.f. 3.0 w.f. w.f. 
LOI (% O.sub.2) 
27.0 n.d. n.d. 23.8 17.1 
UL 94/V (3 mm) V-0 V-0 V-1 H.B. H.B. 
flowing length at injection 
13.5 14.0 6.0 15.0 15.0 
molding 240.degree. C. (cm) 
______________________________________ 
impact resistance w.f. = without fracture 
LOI n.d. = not determined