An internal plasticizer is actually part of the polymer. Generally the internal plasticizer is the second monomer introduced during the polymerization of the polymer. Since the second monomer is copolymerized in the molecular structure of the polymer, the regularity of the polymer molecular chain is reduced, that is, the crystallinity of the polymer molecular chain is reduced. For example, vinyl chloride-ester acid copolymers are more flexible than vinyl chloride homopolymers. The temperature range of the internal plasticizer is relatively narrow, and it must be added during the polymerization process, so the internal plasticizer is used less.
External plasticizer is a low molecular weight compound or polymer. Adding it to the polymer that needs to be plasticized can increase the plasticity of the polymer. The external plasticizer is generally a high-boiling, less volatile liquid or a low melting point solid, and most of them are ester organic compounds. Usually they do not react chemically with the polymer, and the interaction with the polymer is mainly the swelling effect at elevated temperature, forming a solid solution with the polymer. External plasticizers have comprehensive properties, are easy to produce and use, and are widely used. Nowadays people generally refer to plasticizers as external plasticizers. Dioctyl phthalate (DOP) and dibutyl phthalate (DBP) are both external plasticizers.