The migration and diffusion of plasticizers in PVC products can cause yellowing, hardening, and brittleness of the products, which will affect the performance of the products. What's more serious is that the largest amount of phthalate esters (PAEs) plasticizers have potential carcinogenic risks, and they will endanger human health and pollute the environment after they are moved out of PVC products. Therefore, studying the migration mechanism and inhibition methods of plasticizers in PVC products is of great significance for inhibiting the occurrence of migration and developing non-migrating PVC products.
1. Migration mechanism of plasticizer
The force between the plasticizer and PVC molecules is weak van der Waals force and hydrogen bond, and the plasticizer is easy to migrate and diffuse to the outside during processing and use.
There are four main ways of migration and diffusion of plasticizers:
①Volatilize from the surface of PVC products to the atmosphere;
②The liquid phase contacted by the PVC product is extracted;
③ Migration and diffusion occurs in the solid or semi-solid material contacted by the PVC product;
④Exuded from PVC products under pressure.
The migration and diffusion of the plasticizer mainly goes through three stages: the plasticizer diffuses to the inner surface, forms a "recumbent" state on the inner surface, and diffuses away from the surface of the product.
The migration and diffusion of the plasticizer can be regarded as the transfer of small molecular substances in the polymer, which is the migration in the gaps between the polymer segments. Polymer molecular chain segments usually include linear, branched and cross-linked types, and there will be arrangement and stacking between the chain segments. Therefore, the internal spatial network structure of the polymer is extremely complex, and there are crystalline and amorphous regions at the same time. Amorphous regions are mostly amorphous, with more gaps between segments than crystalline regions. Therefore, the migration and diffusion speed of small molecules in the crystalline region is lower than that of the amorphous region. The morphology of crystal regions, amorphous regions, and molecular segments will change with changes in external conditions such as temperature.
2. Factors affecting plasticizer migration
The main factors affecting the migration and diffusion of plasticizers in PVC products are temperature, PVC product characteristics, plasticizer molecular characteristics, plasticizer content, medium, etc.
① Temperature: Temperature can significantly affect the occurrence of diffusion. The higher the temperature, the greater the thermal activation energy of atoms, and the easier migration will occur.
②PVC product characteristics: mainly reflected in the free volume of the chain segment and the movement characteristics of the chain segment. When the minimum free volume required for the movement of the plasticizer molecules is less than the free volume of the polymer segment, the plasticizer molecules can diffuse through the polymer segment. The greater the free volume in PVC, the easier it is for plasticizer molecules to migrate. The higher the degree of unsaturation of the PVC molecular chain, the stronger the mobility and the easier the plasticizer to diffuse.
③The molecular characteristics of plasticizer: The relative molecular weight of plasticizer directly affects its migration ability. The larger the relative molecular weight, the more difficult it is to diffuse in PVC products. When the plasticizer molecular volume is the same, the plasticizer with flat or long molecules is easier to diffuse than the plasticizer with spherical molecules.
④Plasticizer content: when other conditions are the same, the greater the plasticizer content, the easier migration will occur.
⑤Medium: Generally speaking, the plasticizer is easier to be drawn out by the oil phase solvent, but more difficult to be drawn out by the water phase.