Meaning
Molecular movement describes the process by which small molecules or chains migrate through the bulk of a plastic material. Polymer diffusion governs how gases, liquids and additives move within the matrix over time. This characteristic is essential for determining the barrier properties of packaging and the long term stability of colored parts.
The rate of movement is influenced by the crystallinity of the resin and the size of the diffusing molecule.
Barrier Performance
Packaging for food or chemicals relies on low rates of gas transmission to protect the contents. High levels of polymer diffusion allow oxygen to enter or flavor compounds to escape, shortening the shelf life of the product. Multi layer structures often combine different resins to create a more effective path for blocking these molecules.
Ethylene vinyl alcohol is a common choice for its ability to slow down the movement of gases. The thickness of the barrier layer is calculated based on the known diffusion rates of the target molecules.
Additive Migration
Functional chemicals like antioxidants or ultraviolet stabilizers must remain distributed throughout the part to be effective. If polymer diffusion is too high, these additives may move to the surface and bloom, creating a white powder or an oily film. This not only ruins the appearance but also leaves the bulk of the material unprotected from degradation.
Controlled migration is sometimes desired in applications like slip agents for films. In those cases, the molecules must move to the surface at a specific rate to provide consistent lubrication.
Environmental Resistance
Exposure to solvents can cause a plastic part to swell or soften as the liquid enters the matrix. Rapid polymer diffusion of a solvent into the polymer structure can lead to environmental stress cracking. Resistance to this process is a major requirement for parts used in automotive fuel systems or medical fluid delivery.
Amorphous resins typically allow for faster movement than semi crystalline types because they have more free volume between the chains. Testing for chemical compatibility often focuses on measuring the mass gain as molecules penetrate the part. Understanding these limits prevents the selection of materials that would fail prematurely in the presence of harsh cleaning agents.
