Factors Affecting Accurate Measurement of Fuel Permeation Through Polymers

摘要

Because any polymer contains some space distributed between its molecules, when it is exposed to a fluid (liquid or gas) the physical possibility exists for fluid absorption if the fluid molecules or atoms are small enough to fit into local regions of this distributed space during a kinetic movement. Some polymers contain much more of this internal free space or free volume than others. For any such polymer/fluid system, the chemical and morphological structure of a polymer and the size of fluid molecules (and hence fluid viscosity, if a liquid) - and other, thermodynamic, considerations - influence whether the degree of fluid absorption, and of the related phenomenon permeation, will be significant. Permeation, again a molecular phenomenon, is the most fundamental means by which fluid can pass through a polymer. It is a small but significant - and ever-present -factor in fuel containment by polymeric systems. Permeation is one of a series of interconnected phenomena associated with fluid/polymer interactions, which can be described as follows: Absorption When a polymer is immersed in a fluid, the latter is absorbed ("taken up" by the polymer and distributed throughout it) due to the continuous process of adsorption followed by diffusion, until an equilibrium situation is achieved. For practical reasons, absorption tests are restricted to immersion in liquids. Adsorption The first stage applying when a polymer is placed in contact with a fluid is that the latter adsorbs into the polymer surface regions, reaching equilibrium solubility here essentially instantaneously. Diffusion A fluid present inside a polymer at a high concentration region compared with surrounding regions will diffuse away from the first region, taking a finite time, until an equilibrium situation is achieved. The diffusant molecules move in steps into free volume holes as they form according to kinetic theory. Permeation When a fluid is placed in contact with one side only of a polymer membrane, the former can permeate right through the polymer. The process combines adsorption, diffusion, and eventual evaporation; it could also be viewed as combining "oneway absorption" and evaporation. Permeation tests are generally fast for gases and slow for liquids, and fast for elastomers and slow for thermoplastics and thermosets.