Meaning
Mass transfer process describes the time-dependent migration of chemical species from a solid polymer matrix into a surrounding liquid medium. Studying continuous extraction dynamics allows engineers to predict how additives like plasticizers or uv stabilizers will behave when a part is exposed to fluids over its service life. The rate of this movement is influenced by the molecular weight of the migrant and the free volume within the polymer chains.
Diffusion Mechanism
Molecular movement begins when the concentration gradient between the polymer surface and the solvent creates a driving force. In continuous extraction dynamics, the solvent penetrates the polymer structure, causing the chains to relax and increasing the mobility of trapped additives. This process often follows fickian laws where the initial migration is proportional to the square root of time.
As the surface layer is depleted, the rate slows down because the molecules must travel from deeper within the part to reach the interface.
Matrix Resistance
Polymer crystallinity acts as a physical barrier that restricts the movement of migrating species. While amorphous regions allow for relatively fast continuous extraction dynamics, the tightly packed crystalline structures in resins like hdpe or ppa slow the process significantly. Temperature also plays a role, as higher heat increases the kinetic energy of the molecules and expands the space between polymer chains.
Equilibrium Limit
Extraction eventually ceases when the chemical potential of the migrant is equal in both the solid and the liquid phases.
