Which substances increase the flexibility and adhesion of organic coating films?

Plasticizers are substances added to organic coatings to enhance their flexibility and adhesion. They work by reducing the glass transition temperature of the polymer in the coating, which allows the material to remain more pliable rather than becoming brittle. This is particularly important in applications where the coated surface may experience movement or deformation, as it helps maintain a strong bond between the coating and the substrate while preventing cracking and peeling.

In contrast, thinners are primarily used to reduce the viscosity of the coating materials for easier application. Driers are additives that speed up the drying process of paints and coatings, which does not directly impact flexibility or adhesion. Fungicides are used to prevent the growth of mold and mildew in coatings, but they do not play a role in enhancing flexibility or adhesion. Therefore, the option that most accurately describes substances that improve both flexibility and adhesion in organic coatings is plasticizers.