Natural calcium carbonate (NCC) is a commonly used abrasive in toothpaste. Typical commercial grades include 325 mesh (D97 ≈ 45 μm) and 400 mesh (D97 ≈ 38 μm). In recent years, toothpaste formulation technology has advanced rapidly. Ultrafine grinding processes have also made significant progress. As a result, finer natural calcium carbonate abrasives are becoming an industry trend.
Studies have confirmed that abrasive particle size significantly affects key performance indicators of fluoride toothpaste. These include pH value, total soluble fluoride (TSF) retention, viscosity, and abrasivity. In this study, natural calcium carbonate abrasives with D97 values ranging from 10–45 μm were selected. Fluoride toothpaste samples were prepared using a unified formulation. The samples were aged at 25°C, 40°C, and 50°C. Their pH value, TSF retention, viscosity, and abrasivity were measured regularly. These results were used to systematically explore the influence of abrasive particle size on toothpaste performance.
Effect on Toothpaste pH Value
The pH value of fluoride toothpaste increases as the D97 particle size decreases.
For the same sample, pH decreases over time during aging. The pH fluctuation at 50°C is significantly larger than at 40°C.
These observations are related to two main reactions during aging:
- Smaller abrasive particles have a larger specific surface area. This accelerates CaCO₃ dissolution and the release of hydroxide ions. The pH rises accordingly.
- Sodium monofluorophosphate (SMFP) gradually hydrolyzes and produces hydrofluoric acid. This acid reacts with calcium ions and phosphate ions to form insoluble fluorapatite. Hydrogen ions are released, causing the pH to fall.
Both reactions accelerate at higher temperatures.
This results in larger pH fluctuations during high-temperature aging.
From the pH curves, toothpaste containing D97 = 25.4 μm abrasives always shows higher pH values than the 21.5 μm sample. Likewise, the 43.9 μm sample remains above the 35.7 μm sample. This occurs because the 25.4 μm and 43.9 μm abrasives contain more free alkali. These alkaline impurities (mainly Ca(OH)₂ or CaO) increase the initial pH of the toothpaste.
Effect on TSF Retention
TSF retention decreases significantly as the D97 particle size becomes smaller.
For the same sample, TSF retention also decreases with higher aging temperatures or longer aging times.
Key reasons include:
- Smaller particle size increases specific surface area. Higher aging temperatures increase calcium ion and hydrofluoric acid concentrations. Both factors accelerate fluorapatite formation.
- Longer aging promotes continuous SMFP hydrolysis and ongoing fluorapatite precipitation. This further lowers TSF retention.
From the retention curves, samples prepared with D97 = 25.4 μm and 21.5 μm abrasives show almost identical behavior.
However, the 43.9 μm sample shows significantly lower retention than the 35.7 μm sample.
This is due to higher free alkali content in the 25.4 μm and 43.9 μm abrasives.
These impurities raise the pH and enhance alkaline reactions.
Free alkali reacts faster with hydrolyzed SMFP than CaCO₃ does, accelerating fluorapatite formation and reducing TSF retention.
Effect on Toothpaste Viscosity
Toothpaste viscosity increases as the D97 particle size decreases.
For the same sample, viscosity also increases at higher temperatures or with longer aging.
The reasons are as follows:
- Smaller particle size and higher temperature increase calcium ion concentration.
Calcium ions strengthen the cross-linking effect on CMC.
Longer aging continues to increase cross-linking, which raises viscosity. - Smaller particles increase the solid–liquid interface.
This increases flow resistance in the paste and contributes to higher viscosity.
Effect on Toothpaste Abrasivity
After 12 weeks of room-temperature aging, abrasivity (Ra) increases as the D97 particle size decreases. This occurs because smaller abrasive particles increase the number of particles in the system. More particles participate in abrasion within the same groove during brushing. Their combined mechanical action deepens the grooves, increasing abrasivity.
In addition, smaller particle sizes result in a narrower particle size distribution. More uniform abrasive particles strengthen the cumulative abrasion effect. This explains why the sample with D97 = 25.4 μm shows relatively high abrasivity.
Conclusion
The toothpaste industry is moving toward finer and more precisely controlled abrasive materials. Achieving stable D97 values in the 10–45 μm range requires accurate, clean, and reliable ultrafine grinding systems.
Epic Powder has more than 20 years of experience in ultrafine powder engineering. The company provides a full range of grinding, classification, and modification equipment.
This includes ball mills, air classifier mills, jet mills, and precision air classifiers. With these advanced powder-processing technology, Epic Powder supports the development of high-performance fluoride toothpaste formulations and enables manufacturers to achieve consistent and reliable product quality.
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— Posted by Emily Chen