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VEEGUMā D, A Smectite Clay for Fluoride-Stable Dentifrice Products

Marian Andersson and Peter A. Ciullo, R.T. Vanderbilt Company, Inc.

Water-washed natural smectite clays have served as dentifrice binders for many years. These clays are valued because they ensure a "short" rather than a stringy or pituitous product. The toothpaste ribbon extruded onto the brush breaks cleanly and abruptly as the dispenser is withdrawn. These clays are also used to maintain product rheology and to prevent the dentifrice from separating into liquid and solid phases, even at the temperature extremes that diminish the effectiveness of organic binders.

The use of smectite clays in dentifrices containing sodium fluoride is questioned however, because of their potential to decrease the amount of fluoride ion available to tooth enamel. The extent to which a smectite clay will reduce the amount of available fluoride is determined by two factors. The primary consideration is the amount of Ca²⁺ introduced incidentally with the clay. Less significant is the slightly cationic nature of the edges of smectite clay platelet.

Each macroscopic smectite particle is composed of thousands of submicroscopic platelets stacked in sandwich fashion with a layer of water between each. The faces of these platelets carry a negative charge because of substitutions (e.g., Mg²⁺ for Al³⁺) within the mineral lattice. Edges carry a slight positive charge due to lattice discontinuities. The net negative charge of the platelet is balanced mostly by sodium ions, but other inorganic cations, including Ca²⁺, can be present in minor amounts. These charge balancing ions are considered exchangeable because they can be readily substituted by other cations. Calcium ions may also become available in small amounts from residual calcite (calcium carbonate) or dolomite (calcium magnesium carbonate) that remains with the clay even after beneficiation.

When sodium fluoride is used as the therapeutic ingredient, calcium ions adversely affects fluoride bioavailability because of the precipitation of calcium fluoride (CaF₂). The cationic nature, albeit slight, of the clay particle edges can ionically attract F^- and thereby also reduce bioavailability, although to a much smaller degree. Table 1 compares Magnesium Aluminum Silicate N.F. Type 1A, the most widely used smectite clay in dentifrices during the past several decades, to VEEGUM D, a smectite clay with a lower calcium content. Test samples were made by adding sodium fluoride at a typical dentifrice use level to prehydrated clay dispersions. These were analyzed for F^- using an ion selective electrode technique based on the USP Sodium Fluoride Oral Solution assay. Fluoride was measured when prepared and then every two weeks for three months. The MAS contained 1.5% calcium, mainly in the form of carbonates, while the VEEGUM D contained half this amount, mainly as exchangeable cations. Although the excess of calcium in MAS, compared to VEEGUM D, is mostly or solely in the relatively insoluble form of carbonate minerals, the fluoride recoveries in Table 1 best reflect total relative calcium levels.



Table 1. Fluoride Recovery: VEEGUM D vs. MAS

Although VEEGUM D is obviously more suitable than MAS Type 1A for sodium fluoride dentifrices, it should preferably show no interference with fluoride bioavailability. Interference may be prevented through the use of water-soluble phosphate compounds, such as tetrapotassium pyrophosphate (TKPP) and tetrasodium pyrophosphate (TSPP), which are otherwise commonly employed in dentifrice products as anticalculus agents. When added to smectite dispersions before the introduction of sodium fluoride, these phosphates are believed to sequester Ca²⁺, thus preventing precipitation of CaF₂.

Soluble phosphates, particularly TSPP and TKPP, have been recognized for many years as efficient peptizing agents when added to dispersions of smectite clays. In this capacity the phosphate anion is attracted to the slightly cationic edges of the clay platelets. This prevents the formation of a cohesive colloidal structure, which normally provides characteristic rheological properties through the attraction of the slightly positive-charged platelet edges to the negative-charged platelet faces. Peptized dispersions therefore remain very fluid. The attraction of phosphate anion to clay platelet edges likewise makes them unavailable to subsequently added F^-.

Table 2 illustrates the effect of phosphate addition on the availability of F^- in a VEEGUM D dispersion. When phosphate is added to the VEEGUM D dispersion prior to the sodium fluoride, total fluoride recovery is achieved.



Table 2. Total Fluoride Recovery with Phosphate Added to VEEGUM D

Table 3 provides an example of a toothpaste that maintains full fluoride availability. The binder system is based on the synergistic combination of smectite clay with xanthan gum and cellulose gum. For optimum fluoride stability, the phosphate is added after the clay is dispersed and before any other ingredients are added to the water. In this way, the clay is prevented from tying up any of the subsequently added fluoride. The phosphate also peptizes the clay. This allows the preparation of high solids dispersions, so that an effective amount of VEEGUM D can be used even in "lean" formulations - those with considerably less free water than the example in Table 3.

Formula:	% by Weight
VEEGUM D	1.50
Water	41.11
Tetrasodium Pyrophosphate	1.00
Titanium Dioxide	0.30
VANZAN® NF (Xanthan Gum; R.T. Vanderbilt)	0.80
CMC 7HF (Cellulose Gum; Aqualon)	0.30
Methyl Paraben	0.20
Propyl Paraben	0.05
Sorbitol, 70%	10.00
Glycerin	10.00
Zeodentā 113 (Hydrated Silica; Huber)	30.00
Saccharin	0.20
Water	2.80
Sodium Fluoride	0.24
Rhodaponā LSB (SLS, 29%; Rhodia)	1.50
Fluoride Assay:
When prepared	1080 ppm
3 months at RT	1080 ppm
3 months at 45oC	1080 ppm

Table 3. Fluoride-Stable Toothpaste with VEEGUM D

Conclusion:

The smectite clays most suitable as binders in sodium fluoride dentifrices are those free of residual carbonate minerals. VEEGUM D is the preferred binder because it ensures proper dentifrice rheology and stability, with minimal inhibition of fluoride recovery. The incorporation of soluble pyrophosphate with VEEGUM D maximizes fluoride availability.

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