Pigments play an important role in the coatings industry, and the dispersibility and wettability of pigments directly affect the performance and quality of coatings. Critical surface tension (γ c) is a key parameter used to assess the ease of surface wetting of pigments, thereby providing guidance for the formulation and application of coatings. This article will explore the concept, influencing factors, and determination methods of critical surface tension, and explain its role in pigment wettability.
1. The concept of critical surface tension
Critical surface tension refers to the ability of a liquid to start wetting the surface of a pigment when the interfacial tension of the liquid is equal to or slightly greater than the surface free energy of the pigment. In short, it is an indicator of the wettability of a pigment. The lower the critical surface tension, the easier it is for the liquid to wet and disperse on the surface of the pigment.
2. Influencing factors
The chemical composition and structural properties of pigments are the main factors that affect the critical surface tension. Inorganic pigments usually have high critical surface tension because of their high energy on the surface and easy dispersion. In contrast, organic pigments have lower critical surface tension because of their low energy on the surface and difficult to wet and disperse. For example, the critical surface tensions of phthalocyanine blue and toluidine red are 31.3mN/m and 27.5mN/m, respectively.
3. Determination method
The Zisman method is a common method for determining the critical surface tension of pigments. This method uses several liquids with different surface tensions to measure their contact angles with the pigment under test. Then, plot the cosine of the contact angle with the surface tension of the liquid to obtain a straight line, extrapolate it to a position where the cosine is 1 (the contact angle is 0), and the surface tension at this time is the critical surface tension of the pigment under test.
4. The role of critical surface tension in pigment wettability
Critical surface tension is an important parameter that can affect the dispersibility and wettability of pigments in coatings. Lower critical surface tension makes pigments easier to wet and disperse in liquids, thereby improving paint quality and stability. By controlling the critical surface tension of pigments, paint manufacturers can adjust the formulation of coatings for better paint performance and results.
Summarize
Critical surface tension is an important parameter for evaluating the wettability and dispersibility of pigments, affecting the performance and quality of paints. Its measurement method, the Zisman method, provides a way to quantitatively assess the critical surface tension of pigments. By understanding critical surface tension, paint manufacturers can optimize paint formulations for better pigment wettability and dispersibility, thereby improving paint quality and applicability.
