Plastic pad printing ink is a solvent-based type of ink formulated as a one- or two-component ink for printing on plastics. In general, there are two types of plastics: solvent-sensitive (simple plastics) and non-solvent-sensitive (complex plastics). For solvent-sensitive plastics, pre-treatment is not required. These plastics have a surface energy level that is the same as or above the pad printing ink. Complex plastics, on the other hand, have low surface energy. As a result, pre-treatment is required for good ink adhesion. An example of an ink adhesion promoter for complex plastics is Natron polypropylene primer.