Silica capable of being used in toothpaste compositions

Details Silica capable of being used in toothpaste compositions Silica capable of being used in toothpaste compositions

1999-02-19

2001-02-13

Rose, Shep K. (Department: 1614)

Drug, bio-affecting and body treating compositions

Dentifrices

C423S335000, C423S339000, C051S308000

Reexamination Certificate

active

06187292

ABSTRACT:

The present invention relates to an abrasive precipitation silica with a median diameter of at least 10 &mgr;m and with good grain cohesion, which can be used in toothpaste compositions, in particular in anti-tartar compositions, and to a process for preparing the said silica; the invention also relates to toothpaste compositions containing the said silica.
The abrasive agents (in particular silica) used in toothpaste compositions generally have a median particle diameter of less than or equal to 10 &mgr;m, less than 1.5% of the weight of the said particles having a diameter of greater than 44 &mgr;m.
Certain formulations, in particular those with an anti-tartar effect, can require the use of larger-sized abrasive agents.
A so-called “crunchy” effect, i.e. a granular sensation in the mouth, and throughout brushing, may be desired by the consumer. An abrasive can achieve this effect only if it has a sufficient grain cohesion during brushing.
This problem can be solved by silica gels (continuous three-dimensional rigid structure) with a BET specific surface generally of greater than 300 m
2
/g. However, the process for preparing these gels requires long and expensive washing and filtration steps.
Precipitation silicas (aggregates consisting of discrete particulate entities bound together by weak bonds) are obtained by processes which are simpler to carry out, but do not have a sufficient level of grain cohesion for the desired application.
The Applicant has found an abrasive silica with a median diameter of at least 10 &mgr;m and with sufficient grain cohesion, which can be obtained by a simple process, such as a process for the preparation of silica by precipitation.
A first subject of the invention consists of an abrasive precipitation silica which can be used in toothpaste compositions, this silica having
a BET specific surface of about 15 to 300 m
2
/g, preferably of about 20 to 250 m
2
/g
a DOP oil uptake of about 40 to 160 ml/g, preferably of about 50 to 140 ml/g
a median particle diameter of at least 10 &mgr;m, generally of about 12 to 30 &mgr;m
a particle cohesion factor of at least 85% approximately for a median particle diameter of about 12 &mgr;m to 20 &mgr;m.
The said silica also has a CTAB specific surface of about 10 to 120 m
2
/g, preferably of about 15 to 100 m
2
/g.
The BET specific surface is measured according to the Brunauer—Emmet—Teller method described in “The Journal of the American Chemical Society”, Vol. 60, page 309, February 1938 and corresponding to ISO standard 5794/1 (Annex D).
The DOP oil uptake is determined according to ISO standard 787/5 using dioctyl phthalate.
The silica particle cohesion is quantified using a specific test of cohesion by ultrasound; this test allows the change in the median diameter d
50
of a silica suspension to be evaluated by measuring the particle size before and after the ultrasound treatment.
According to this test, the particle size measurement (by laser scattering using a Laser Sympatec granulometer) is carried out on a silica suspension treated with ultrasound using a Vibracell Bioblock sonicator (600 W power rating), equipped with a probe 19 mm in diameter, a timer and a converter, according to the following operation:
Preparation of the Silica Suspention
A homogeneous suspension of 15 g of silica in 135 g of water is prepared using a Rayneri impeller-disperser; 70 g of suspension are then transferred into a 50 ml glass flask.
Ultrasound Treatment
The probe, which has been preadjusted, is introduced into the flask to a depth of 4 cm, without contact with the glass walls; the timer is programmed with pulsation over a period of 2000 seconds, so as to obtain an active ultrasound cycle of 600 seconds.
The ultrasound cycle is started after the flask is sealed.
Checking the Particle Size
After manual homogenization of the sealed flask, about 2 ml of homogeneous suspension are removed with a pipette, and this suspension is then poured into the granulometer cuvette, after adjusting the level of water, if necessary, in order to obtain an optical concentration of 20%±3%.
After ultrasound dispersion of the suspension in the cuvette for 30 s, the median diameter d
50
is measured using a 100 mm focal device.
The cohesion factor of the test silica is then calculated; the higher the cohesion of the silica, the higher this factor.
Cohesion Factor
The initial median particle diameter d
50i
is measured on the homogeneous silica suspension before the ultrasound treatment.
The final median diameter d
50f
is measured on the homogeneous silica suspension after the ultrasound treatment.
The cohesion factor CF, as a (%) is calculated according to the following equation:
CF
=(
d
50f
/d
50i
)×100
A CF value of 100 corresponds to the maximum cohesion value.
The CTAB specific surface is the external surface area determined according to the standard NFT 45007 (November 1987).
A second subject of the invention consists of a process for preparing abrasive silica with good grain cohesion, which can be used in toothpaste compositions, by reacting an alkali metal M silicate, with an SiO
2
/M
2
O ratio of about 2 to 4, preferably of about 3 to 3.8, with an acidifying agent, optional maturation of the silica mash formed, separation and optional drying of the silica suspension recovered and optional grinding, the said process being characterized in that the operation for forming the silica mash is carried out according to the following steps:
a first step consisting in using an initial stock solution consisting of water, an electrolytic salt from the group of alkali metals or alkaline-earth metals and optionally an acidobasic agent, at a temperature of about 70 to 98° C., preferably of about 80 to 95° C., the amount of electrolyte present being about 0.1 to 1 mol of alkali-metal electrolytic salt or of about 10 to 100 mmol of alkaline-earth-metal electrolytic salt per liter of stock solution;
a second step consisting in introducing into the said stock solution the alkali metal silicate as an aqueous solution and the acidifying agent, under conditions such that the pH of the reaction medium remains more or less constant and at a value of about 4 to 7, preferably of about 5.5 to 7, and particularly of about 5.8 to 6.7, the said reagents being introduced until the desired silica concentration is obtained in the said medium, the reaction medium being maintained at a temperature of about 70 to 98° C., preferably of about 80 to 95° C.;
and, after optional maturation, a third optional step consisting in acidifying the reaction medium until a silica mash with a pH of less than 6, preferably less than 5, most particularly of about 4, is obtained.
The choice of the silicate and of the acidifying agent to carry out the process of the invention is made in a manner which is well known per se.
The alkali metal silicate is advantageously a sodium or potassium silicate. sodium silicates may be mentioned most particularly.
The said silicate is used in the form of an aqueous solution with a concentration, expressed as SiO
2
, of about 150 to 400 g/l, preferably of about 200 to 400 g/l.
The acidifying agent generally used is a strong inorganic acid such as sulphuric acid, nitric acid or hydrochloric acid, or an organic acid such as acetic acid, formic acid or carbonic acid. Preferably, it is sulphuric acid. This acid can be used in dilute or concentrated form, preferably in the form of an aqueous solution with a concentration of about 60 to 400 g/l.
Among the electrolytes mention may be made in particular of the metal salt of the starting silicate and of the acidifying agent, i.e. preferably sodium sulphate.
An acidobasic agent can be used in the initial stock solution to ensure that the pH of the said stock solution is close to that chosen for the second step of the process, it being possible for this pH to be adjusted using a strong acid or a strong base. Acidobasic agents which may be mentioned in particular are alkali metal hydrogen phosphates.
The said stock solution can also contain silicates; however, the amount of optional silicate ions,

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