Liaoning Xinda Talc Group Co., Ltd. - Yang Yongli

1. Definition: Refractive Index is a dimensionless physical quantity, meaning it has no unit. It represents the ratio of the propagation speed of light in a vacuum to that in a medium (such as talc, resin, etc.), with the formula: n=C/V, where c is the speed of light in a vacuum and v is the speed of light in a medium, both in the same unit. Therefore, the ratio n is a pure numerical value.

2. Refractive index of other common materials (comparative reference):

3. Factors affecting the refractive index of talc:

3.1 Chemical Composition:

The chemical formula of talc is 3MgO.4SiO2.H2O. When magnesium (Mg ² ⁺) is replaced by iron (Fe ² ⁺) and aluminum (Al ³ ⁺) ions, it will cause an increase in refractive index. The refractive index of talc with high iron content can reach over 1.59.

3.2 Mineral Structure:

Talc is a layered silicate, and its structural density and lattice arrangement affect the path of light propagation, thereby altering its refractive index.

3.3 Associated minerals and impurities:

Talc often coexists with quartz (refractive index 1.54-1.55), calcite (1.48-1.66), chlorite (1.57-1.62), and other minerals. The presence of impurity minerals can cause the overall refractive index to deviate from the range of pure talc.

3.4 Crystallinity and particle size:

The refractive index of coarse-grained talc is closer to the theoretical value, while fine-grained or cryptocrystalline talc may cause measurement fluctuations due to scattering effects.

4. Principle of Transparent Optics in Coatings:

According to optical principles, when the refractive indices of two materials are close, the scattering of light at the interface is reduced, thereby improving transparency. So when the refractive index of talc is close to that of resin, the particles of talc will be more evenly dispersed in the resin, reducing scattering and making the paint film more transparent. On the contrary, if the difference between the two is large and there is more scattering, the transparency will decrease and the paint film will appear cloudy.  

5. Practical application:

Choosing talc and resin with refractive index matching in coatings, with epoxy resin n ≈ 1.58 and talc n ≈ 1.57, can reduce light scattering and improve transparency. In addition, it is necessary to optimize transparency by combining particle size analysis and dispersion experiments.

The refractive index of talc is one of the important parameters for identifying its quality and origin. In the application of the coating industry, high-purity and low refractive index talc is more favored; High refractive index talc containing impurities may be used in specific ceramic or filler fields.