CA1088040A

CA1088040A - Alumina

Info

Publication number: CA1088040A
Application number: CA244,448A
Authority: CA
Inventors: Anthony R. Emery; John W. Case
Original assignee: British Aluminum Co Ltd
Current assignee: British Aluminum Co Ltd
Priority date: 1975-01-24
Filing date: 1976-01-26
Publication date: 1980-10-21
Also published as: DE2602402C2; FR2298510B1; IT1054458B; FR2298510A1; DE2602402A1; GB1537823A

Abstract

ABSTRACT OF THE DISCLOSURE
The specification describes a method of producing ground alpha alumina trihydrate such that it is suitable for incorporation in a toothpaste. The method includes the steps of grinding the substance to a mean particle diameter of less than 25 microns in the presence of a sufficient quantity of a potable organic acid or acid reacting salt to convert all free caustic soda on the surface of the particle to a sodium salt. Such a method reduces the possibility of the combined particles and toothpaste corroding the aluminum tubes used to package the product.

Description

~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.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of producting ground alpha alumina tri-hydrate for use in the preparation of a toothpaste comprising grinding the alumina to a mean particle diameter of less than 25 microns in the presence of a sufficient quantity of potable organic acid or acid reacting salt to convert all free caustic soda on the surface of the particles to the sodium salt.

2. A method according to Claim 1 in which the potable organic acid is selected from the group of acids consisting of carboxylic acids and hydroxycarboxylic acids.

3. A method according to Claim 1 in which the acid reacting salt is selected from the group consisting of sodium-bisulphate, aluminum sulphate and sodium dihydrogen phosphate.

4. A method according to Claim 1 in which the potable organic acid is benzoic acid.

5. A method according to Claim 1 in which the potable organic acid is citric acid.

6. A method according to Claim 4 in which said acid is present in the quantity of up to 1%.

7. A method according to Claim 4 in which said acid is present in the quantity in the range of .5% to .7%.

8. A method according to Claim 1 in which the potable acid is present in the range of from .4% to .6%.

9. A method according to Claim 1 in which said acid reacting salt is aluminum sulphate present in the range from .3% to .5%, by volume.

10. A method according to Claim 1 in which said acid reacting salt is sodium bisulphate present in the range from .3 to .5% by volume.

11. A method according to Claim 1 in which said acid reacting salt is sodium dihydrogen phosphate present in the range of from .5% to 1% by volume.

12. A method according to Claims 1, 2 or 3, in which the additive in the form of a powder is mixed with coarse alpha alumina trihydrate in a blender, to give the required proportion and wherein the mixture is ground in a mill.

13. A method according to Claims 1, 2 or 3, in which the alpha alumina trihydrate and the additive in the required pro-portion are fed together into a mill and ground.

14. A method according to Claims 1, 2 or 3, in which the alpha alumina trihydrate premixed with the additive to a higher ratio of additive to alpha alumina trihydrate than the required proportion in the final product is fed into a mill together with coarse alumina trihydrate to give the required proportion and the mixture subsequently ground.

15. A method according to Claims 1, 2 or 3, in which the alpha alumina trihydrate premixed with the additive to a higher ratio of additive to alpha alumina trihydrate than the required proportion in the final product is fed into a mill together with coarse alumina trihydrate to give the required proportion and the mixture subsequently ground in a fluid energy mill.