Meaning
Mass transport phenomena involve the movement of small molecules or additives through a polymer matrix in response to a mechanical force. Stress induced diffusion occurs when external loads or internal residual stresses create a concentration gradient that drives the migration of plasticisers and dyes. This movement can lead to a change in the local properties of the part and may cause surface defects or premature failure.
Mechanical Gradient
Pressure differences within the solid part act as a driving force for the transport of solutes. In a moulded component with uneven wall thicknesses, stress induced diffusion can cause additives to accumulate in the areas of highest stress. This can lead to a local softening of the material or a change in its chemical resistance, which might not be apparent during the initial quality inspection.
Solute Migration
Movement of the secondary components through the polymer network is a slow but persistent process. Stress induced diffusion is particularly relevant for flexible PVC or other highly plasticised materials where the small molecules are not chemically bonded to the polymer chains. Over time, these molecules can move to the surface, creating a sticky film or causing the part to become brittle as it loses its internal lubricant.
Part Failure
Long term stability of a product is often threatened by these internal changes in composition. Stress induced diffusion can lead to the formation of micro-cracks or voids as the additives leave the bulk of the material. This is a common cause of failure in components that are under constant tension, as the loss of plasticiser at the stress point accelerates the growth of cracks and leads to a catastrophic break.
Because the diffusion process is driven by the internal stress state, the failure often occurs in areas that were otherwise considered safe during the initial design phase.
