Some thermoplastic polymers crystallize on cooling where as others don’t. crystallization takes place during cooling in the mould, with the formation of microscopic-ordered regions called spherulites. Clarifiers improve the optical properties of plastics by eliminating the very large spherulites that scatter light. They do this by initiating many more spherulites than normal and under these conditions they tend to have diameters smaller than the wavelength of light.
Clarifiers are therefore also nucleating agents, but not all nucleating agents can be called clarifying agents. Because the change in crystallization behavior with some nucleating agents may not produce clear plastic products, although it will has other benefits.
Clarifiers usually have some compatibility with the polymer. Some have sufficiently low melting points to melt in the polymer and form a network of nucleation sites, whereas others remain solid, providing a more limited number of sites.
Nucleating agents also alter the crystallization process by starting nucleation at several sites. The benefits are better mechanical properties and shorter cycle times resulting from more rapid solidification. Nucleating agents can complement the role of impact modifiers, increasing impact strength by eliminating very large spherulites.