1.Non-polarity of the molecular structure

　　Among main printing substrates for plastic film packaging, PE is a non-polar macromolecular organic compound with low critical surface tension, obvious inertia and poor compatibility with printing ink. Each unit of PP contains a methyl group, a group with low polarity. PP is also a non-polar macromolecular organic compound, but it features higher critical surface tension than PE, so it is more compatible with printing than PE.

　　2.Low surface energy

　 Plastic films like PE, PP, PET and PVC feature low surface energy. For macromolecular polymers, the lower the surface energy is, the lower the surface tension is and the harder the surface will be wetting. These are problems polyolefin plastic has. To ensure that printing ink can fully wet the printing substrates, the surface tension of the printing substrates is required to be equal to or higher than that of the printing ink. Surface finishing is the only way to improve surface energy of polyolefin plastic.