Note: Descriptions are shown in the official language in which they were submitted.
~088040
This invention relates to a method of producing
ground alpha alumina trihydrate hereafter referred to as
hydrate of alumina and is particularly although not
exclusively concerned with such alumina having a mean
particle size suitable for incorporation into a toothpaste
and being non-corrosive to aluminium.
Hydrate of alumina has become, in the last 10 years, a
common constituent of toothpastes but as commercially
produced by the Bayer process it normally contains a small
quantity of caustic soda both fixed in the crystals and free
on the surface of the crystals. When the crystals are -
ground to a particle size acceptable for toothpaste
manufacture the proportion of caustic soda on the surface
of the ground particles increases so that toothpaste
incorporating such particles would be sufficiently strongly
alkaline as to corrode an a~uminium tube after long storage
therein. It is therefore normal practice to pack sueh a
product in lined tubes to prevent corrosion.
It is known to incorporate a potable organic acid with
the ground particles in a toothpaste formulation but this
causes a reaction to occur involving the formation of
sodium salts within the toothpaste. This method of
addition is, in itself, undesirable as the amount oE the
organic acid to be added is likely to vary with each batch
of ground particles.
Accordingly, it is an object of the present invention
to provide an improved method of producing ground hydrate
of alumina.
The present invention therefore provides a method of
:
~088(~0
producing ground alpha alumina trihydrate comprising grinding
the hydrate of alumina to a mean particle diameter suitable for
incorporation into a toothpaste e.g. less than 25 microns
in the presence of a sufficient quantity of a potable
organic acid or acid reacting salt to convert all the free
caustic soda on the surface of the particles to a sodium
salt.
The potable organic acid or acid reacting salt may
include carboxylic acids such as benzoic acid, or
hydroxycarboxylic acids such as citric acid, acid reaction
salts such as sodium bisulphate, aluminium sulphate and
sodium dihydrogen phosphate.
The quantity may be up to 1.0% and is preferably 0.5%
to 0.7% for benzoic acid; 0.4% to 0.6% for citric acid;
0.3% to 0.5% for aluminium sulphate; 0.3% to 0.5% for
sodium bisulphate and 0.5% to 1.0% for sodium dihydrogen
phosphate. It will be understood that the optimum quantity
will depend upon the source of hydrate of alumina.
The additive, in the form of a powder, is mixed with
the coarse alpha alumina trihydrate in a blender to give
the required proportion and the mixture ground in a mill.
The alpha alumina trihydrate and the additive in the
required proportions may be fed together into the mill and
ground or the alpha alumina trihydrate premixed with the
additive to a higher ratio of additive to alpha alumina
trihydrate than the required proportionsin the final
product may be fed into the mill together with coarse alpha
alumina trihydrate to give the required proportion and
ground. A mill suitable for giving a dry-ground product
iO88(~40
of the correct particle size grading is used, the milling
conditions being adjusted to give the required product
which may have a typical mean particle diameter of 6.5,
8.0, 11.5 and 17.0 microns or any other in the range 1-25
microns and produce a pH value to give the right pH range.
Preferably a fluid energy mill is used.
EXAMPLE
Hydrate of alumina having a low soluble soda content
as produced in the Bayer process for the preparation of
toothpaste grades had a soda content fixed in the crystal
of approximately 0.23% Na2O together with soluble soda on
the surface of less than 0.05% giving a total of approx-
imately 0.26% Na2O. On grinding this coarse hydrate of
alumina part of the soda fixed in the crystals becomes
soluble, the amount that becomes available depending on
the degree of grinding. For a product ground to a mean
particle size of 8 microns, the limit of soluble soda is
0.055% and an average figure is 0.03% Na2O. It is
essentially this soluble soda which has to be neutralised
by the acidic additives.
100 kg of coarse hydrate of alumina were ground in the
presence of 0.3% of benzoic acid to give a product of mean
particle size 8 microns. A similar batch of 100 kg was
ground without benzoic acid. The pH of the two grades of
ground hydrate of alumina produced was compared in a 20%
aqueous slurry.
pH
Treated hydrate of alumina 5.7
Untreated hydrate of alumina 9.4
The two grades of ground hydrate of alumina were made
up into toothpastes according to the formula below:
Ground hydrate of alumina 52.0
Glycerine 20.0
Carboxymethylcellulose 1.1
Sodium lauryl sulphate 1.5
Flavour 0.8
Sodium saccharate 0.2
Water 24_4
100 . O
Toothpastes prepared from both the treated and
untreated ground hydrate of alumina were tested for pH
with the results shown below:
Toothpaste from treated hydrate of alumina pH 7.4
Toothpaste from untreated hydrate of alumina pH 9.5
It will be seen that the pH of the toothpaste prepared
from the treatecl material is higher than that measured for
the water slurry. This is due to the alkaline nature of
the other constituents of the toothpaste formulation. On
the toothpaste made from untreated material which was much
more alkaline, this increase in pH was smaller. ;~
The two toothpastes were then subjected to a simple
corrosion test in which pieces of aluminium metal were
partially immersed in samples of the paste, the containing
glass tube sealed with a stopper and the tubes and contents
held in a thermostat at 43C (110F) for a suitable period
of time. The results were as follows:
10~8040
2 weeks 110F
Treated hydrate of alumina satisfactory
Untreated hydrate of alumina gassing and corrosion
The amount of organic acid used will not exceed the
stoichiometric amount necessary to neutralise the whole of
the caustic soda present in the coarse unground hydrate of
alumina and in practice quantities less than this are
adequate. This is because some of the soda is still fixed
in the crystals even after grinding and does not react
with the additives.
Although the example given above utilises benzoic
acid it will be understood that similar results may be
obtained using citric acid, sodium bisulphate, aluminium
sulphate or sodium dihydrogen phosphate.
Although the pH value of the toothpaste incorporating
the ground hydrate of alumina changes in the manner shown
above, it is not claimed that corrosion is dependent solely
upon pH, however, the reduction of pH is a means of reducing
corrosion.
Some furher examples are given below.
~)88~40
CORROSION TESTS WITH VARIOUS ADDITIVES
Additive ph of Observations on Corrosion
Paste Sample
1% sodium dihydrogen 9.9 Stained surface; slight
phosphate (1) corrosion
None 9.5 Dull, corroded surface
0.3% benzoic acid7.4 Bright, non-corroded surface
0.6% " " 6.2 ll ll ll ll
0.4% citric acid 6.5 Dull surface; very slight
corrosion
0.3~ aluminium (2) 6.2 Bright, non-corroded surface
sulphate
0.4% sodium (3) 5.3 Bright, non-corroded surface
bisulphate
(1) This sample was milled in a vibratory mill.
The result shows promise but is not as satis-
factory as if fluid energy milling had been used.
(2) Commercial grade pure aluminium sulphate used
containing approximately 17 wt% A12O3.
(3) Pure sodium bisulphate monohydrate (NaHSO4.H2)) r
used. In the paste formulation
hydroxyethylcellulose is used instead of
carboxymethylcellulose as otherwise a lumpy
paste is produced.
`` io88()~
THE EFFECT OF METHOD OF GRINDING IN OF ADDITIVES ON THE
CORROSION BEHAVIO~ROF THE RESULTING PASTE
,
Additive and Method pH of Observatio~ on Corrosion
of milling paste Sample
.
0.65% benzoic acid 5.2 Very bright non-corroded
in fluid energy mill surface :
0.65% benzoic acid Slightl~ dull surface
in vibratory mill
for 4-5 hours 7.6
0.65% benzoic acid in
roller mill 20 hours 9.0 Dull surface
0.3% aluminium Bright non-corroded
sulphate in fluid 6.2 surface
energy mill
0.3% aluminium Slightly dull surface
sulphate in vibrat-
ory mill 4-5 hours 8.2 .
The Table shows that the pH of the paste is lower
when using the product from the fluid energy mill and
that the corrosion behaviour is superior.
If a fluorided toothpaste is required, the non-corrosive
hydrate of alu~ina can be used, as shown in the Table below.
Tests show that an appreciable proportion of the fluoride remains
soluble even on long storage. The fluoride was added as
. sodium monofluorophosphate (MFP) to be equivalent to 0.:1%
- total fluorine content, by weight on the paste.
.
~(~88~0
CORROSION TESTS ON TOOTHPASTES WITH FLUORIDE PRESENT
AS SODIUM MONOFLUOROPHOSPHATE
._
Additive pH of Observations on Corrosion
paste Sample
none 9.9 Stained brown surface
0.6% benzoic acid 7.6 Bright non-corroded surface
0.3% aluminium
sulphate 7.3 Bright non-corroded surface
0.4% sodium
bisulphate 7.3 Bright non-corroded surface
0.4% citric acid 8.1 Bright non-corroded surface
It will be noted that in the presence of MFP the pH
of the paste is higher than when MFP is not added although
no corrosion is produced in the samples with milled-in
additives.
In some cases it is desired to use a mixed fluoride
formulation e.g. 0.1% by weight fluorine as MFP plus 0.05%
fluorine by weight as sodium fluoride. With this
formulation it is possible to use the hydrate of alumina
with milled-in benzoic acid or sodium bisulphate but not
aluminium sulphate as shown in the Table below.
~(~&040
CORROSION TESTS ON TOOTHPASTES WITH FLUORIDE PRESENT AS
MFP PLUS SODIUM FLUORIDE
Additive pH of Observations on Corrosion
paste Sample
none 10.2 Very severe corrosion
0.65% benzoic acid 8.0 Bright surface but pale
brown colour
0.3% aluminium
sulphate 10.3 Patchy etched brown surface
0.4% sodium .
bisulphate 9.7 Fairly bright surface
Only very slight corrosion is observed with the
hydrate of alumina containing benzoic acid or sodium
bisulphate but severe corrosion when no additive is milled
in or when aluminium sulphate is used.
Tests again show that an appreciable amount of the
fluoride remains soluble even on long storage.
