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Adhesion in Plastics          Coating Thickness          Permeability

The Measurement of Permeability

Barrier Coatings: Measurement of Permeability

An apparatus is typically used to determine the permeation of gases through films. Specifically, the apparatus is modelled to (i) establish initially a vacuum on one side, and (ii) maintain an atmosphere of gas pressure on the other side of a film. The rate of gas permeation or permeability is calculated through the rate of increase of pressure on the vacuum side. The units for permeability are typically cm3 of gas diffused times mils of film thickness per day per 100 in2 per atmosphere of pressure. Typically, the measurement of the diffusion of specific gas molecules is made possible through sophisticated instruments where gas mixtures are of interest. For example, a film with sensitive gas permeability to the presence of moisture necessitates varying the relative humidity to quantify the effect, and for this, we need to measure mixtures. For practical purposes, to measure permeability for high barrier films, these instruments can also detect gas concentrations at very low pressures.

Barrier Coatings: Permeation of Molecules through Polymers

A molecule that is soluble in the film and can diffuse through the film can permeate a plastic film. As such, in an amorphous film, small-sized molecules find sites to occupy in loosely entangled bundles of molecules or occupy in the spaces between the molecules. A successful diffusion is marked with free movement of the diffusing molecules from site to site. In case of the disruptive polymer chain or bundle of chains, until the natural vibration of the chain opens up a hole, the molecule must wait. Besides, the rest of the chain and holes open rather easily for a relatively flexible polymer molecule, providing ready accommodation for the movement of one part of the chain. In contrast, stiff and bulky polymer molecules demonstrate relatively slow and infrequent movement. As such, amorphous (without a clearly defined shape or form) polymers with a high diffusion rate for small molecules are poor barriers. And this barrier increases substantially for stiff and bulky polymers, like, polyarylates containing large aromatic units.

To produce a high barrier to small molecules, crystallinity serves as an effective way because the diffusion rate of small molecules is reduced by any mechanism that reduces the mobility of the polymer system. The density and order of polymer crystallites limit the diffusion and dissolution of small molecules through them. This suggests the perfect barrier disposition of a polymer with a completely crystalline structure. This paper will later discuss the impracticability of achieving these structures and their limitations in yielding commercially useful coatings. All real crystalline polymers comprise amorphous or disordered regions which lead to the structure’s permeability. Thus, the level of crystallinity determines the level of barrier.

Barrier Coatings: Crystalline Polymers

The specific chemical structure of the polymer molecule, besides crystallinity, is another major factor that influences the gas barrier. It affects the efficiency of packing, intramolecular bonding (existing or taking place within a molecule), and rigidity. The amorphous or more disordered regions impact the rate of permeability as well as the degree of crystallinity. This leads to the mention of hydrogen bonding; this key force greatly increases intermolecular attraction in nylon. Truly high barriers result in only a few crystalline polymers from a favourable combination of crystallinity and chemical structure. The factors causing high melting points and high barrier are the same in the case of the first three barrier polymers; with thermal degradation occurring well before melting. Hence, during polymerization other monomers should be introduced into these polymer chains to lower the crystallinity.