The natural calcium carbonate abrasive used in toothpaste usually has 325 mesh (D97 ~ 45 mesh) μ m) And 400 mesh (D97 about 38 μ m) In recent years, with the development of toothpaste technology and the progress of grinding technology, it has become a trend to use natural calcium carbonate abrasive with finer particle size. Research shows that the particle size of calcium carbonate abrasive has an important influence on the pH value, total soluble fluoride retention, viscosity, and friction value of fluoride toothpaste.
The particle size of D97 is 10-45 μ The natural calcium carbonate abrasives were made into fluoride toothpaste according to the same formula. The toothpaste was aged at 25 ℃ (or room temperature), 40 ℃, and 50 ℃. The pH value, total soluble fluoride retention rate, viscosity, and friction value of the toothpaste were tested regularly to investigate the effect of the particle size of natural calcium carbonate abrasives on the performance of fluoride toothpaste.
1. Effect of particle size of calcium carbonate abrasive on pH value of toothpaste
The pH value of fluoride toothpaste basically increased with the decrease of particle size of natural calcium carbonate D97, while the pH value of the same toothpaste mainly decreased with the extension of aging time, and the pH fluctuation range of the toothpaste aged at 50 ℃ was larger than that aged at 40 ℃; The decrease of abrasive particle size (an increase of specific surface area) accelerated the dissolution of calcium carbonate, increasing pH value of toothpaste, while smfp gradually hydrolyzed hydrofluoric acid, and then reacted with calcium ion and phosphate to form insoluble fluorapatite, while releasing hydrogen ion, resulting in the decrease of pH value of toothpaste, Both reactions were accelerated with the increase of aging temperature, resulting in a larger fluctuation range of pH value of toothpaste.
In the pH curve, the D97 particle size of 25.4 μm was above the D97 particle size of 21.5 μm, and the D97 particle size of 43.9 μm was above the D97 particle size of 35.7 μm; The reason is that the natural calcium carbonate abrasive D97 with a particle size of 25.4 μm and 43.9 μ m contains a little more free alkali (mixed with calcium hydroxide or calcium oxide), which makes the pH value of toothpaste higher.
2. Effect of particle size of calcium carbonate abrasive on TSF retention of toothpaste
In general, the TSF retention rate of fluoride toothpaste decreased with the decrease of D97 particle size, while the TSF retention rate of the same toothpaste decreased with the increase of aging temperature or aging time; the reason is that the decrease of abrasive particle size (the increase of specific surface area) and the increase of aging temperature make the increase of calcium ion or hydrofluoric acid in the liquid phase, and then accelerate the reaction of forming insoluble fluorapatite. The longer aging time makes the hydrolysis and precipitation of stop continue.
The TSF curve of D97 particle size 25.4 μm coincides with that of 21.5 μm, and that of D97 particle size 43.9 μm is lower than that of D97 particle size 35.7 μm; The reason is that the natural calcium carbonate abrasive D97 with the particle size of 25.4 μm and 43.9 μm contains a little more free alkali, the pH value of the corresponding toothpaste is also higher, and the alkalinity of free alkali in the liquid phase is higher than that of calcium carbonate, so the reaction with hydrolyzed smfp to form insoluble fluorapatite is faster, which leads to the decrease of TSF retention rate.
3. Effect of particle size of calcium carbonate abrasive on the viscosity of toothpaste
The viscosity of fluoride toothpaste increased with the decrease of particle size of natural calcium carbonate D97, while the viscosity of the same toothpaste increased with the increase of aging temperature or aging time. There are two reasons
(1) The decrease of abrasive particle size and the increase of aging temperature lead to the increase of calcium ion in the liquid phase and the increase of the cross-linking effect of calcium ion on CMC, while the extension of aging time leads to the increase of cross-linking effect and the increase of paste viscosity;
(2) The decrease of abrasive particle size also increased the solid-liquid interface in toothpaste, increased the flow resistance of paste, and contributed to the increase of paste viscosity.
4. Effect of particle size of calcium carbonate abrasive on friction value of toothpaste
The friction value Ra of fluoride toothpaste aged at room temperature for 12 weeks increased with the decrease of particle size of natural calcium carbonate D97; The decrease of particle size leads to the increase of abrasive particles in toothpaste, and there are more natural calcium carbonate abrasive particles in the same groove during brushing and grinding, which makes the groove easy to deepen due to the superposition of their effects, thus leading to the increase of toothpaste friction value.
In addition, the width coefficient of particle size distribution narrows with the decrease of particle size, which also strengthens the superposition effect of natural calcium carbonate abrasive particles. That is to say, there are more natural calcium carbonate abrasive particles with similar size in the same groove, which also has a certain effect on increasing the friction value of toothpaste. This may be the reason for the high friction value of D97 toothpaste with a particle size of 25.4 μm.