How does glass transition temperature affect the way a coating film forms?

In the chemistry of surface coatings, the glass transition temperature ($T_g$) is a critical factor in determining how polymer particles coalesce to form a continuous film.

• Rubbery State ($T>T_g$): When polymer particles are above their $T_g$, they exist in a rubbery state. This flexibility allows the particles to flow together and coalesce effectively under surface-mediated forces once applied to a substrate, resulting in a clear, continuous film.1
• Glassy State ($T<T_g$): When a polymer is below its $T_g$, it is in a rigid, glassy state that prevents natural coalescence. To facilitate film formation in these systems, a coalescent—a solvent that acts as a plasticizer—must be added to temporarily lower the effective $T_g$. Once the film has formed, the coalescent evaporates, leaving behind a solid polymer film.1

Understanding $T_g$ is essential for controlling the mechanical and thermal properties of the final coating, as it dictates the physical state of the polymer during the application and curing processes.12