Polyesters oxygen permeability

Figure 8. Dry oxygen permeability of 38 m oriented polypropylene, 135 jX. ffl low density polyethylene and 13 m polyester, treated with varying thicknesses of the silane primer. These data were recorded with a Modern Controls Inc. Oxtran 1000 permeability analyzer.

Figure 13.34 A plot of oxygen permeability of 10-mil thick amorphous compression molded films vs. clay loading, tor a series of polyester nanocomposites. PETG is glycol-modified poly(ethylene terephthalate), PET is poly(ethylene terephthalate), and PEN is poly(ethylene naphthalate). The effective aspect ratio of these composites is in the range of 150-200.

Attention has been paid to the possible use of multilayer coextrusion of engineering resins for dual ovenable containers, as multilayer packaging could increase shelf stability and thus eliminate the need for freezing or refrigeration. One multilayer combination in commercial use is a sheet of Lexan polycarbonate sandwiched between layers of Ultem polyetherimide. Thermoformed trays of this material are reported to have better break resistance at lower temperatures and higher stiffness at temperatures up to 232°C, as compared to CPET. They are also insert, compatible with polyester lidding materials, and have lower oxygen permeability than PET materials. 

Physical and thermal properties Polyesters are formed via a condensation reaction. PET and poly(butylene terephthalate) (PBT) are not crosslinked so are thermoplastic, but cured polyesters are crosslinked and thermosets. Polyesters may be reinforced with glass or carbon fibres or particles. Addition of 30% or more carbon fibre induces electrical conductivity in polyesters. Despite its high polarity, PET Is a good electrical insulator at ambient conditions because its Tg is considerably higher than room temperature. Polyesters have good abrasion resistance and are tough. PET is impervious to water but has low permeability to oxygen. 

The optical properties, clarity and gloss, are also improved and biaxially oriented polystyrene and polypropylene films are used for envelope windows and overwraps. Biaxially oriented polyvinylidene chloride film is also used for food packaging in view of its good mechanical and optical properties, but for economic reasons it is only used if its low permeability to gases, especially oxygen, is required. The electrical properties are improved by biaxial orientation and the applications of polyester, polycarbonate and polypropylene films in capacitors are expected to show a rapid growth. In these electrical applications crystalline films are always annealed in order to improve the dimensional heat stability. 

Taking poly(ethylene terephthalate) (PET) as an example of an aromatic polyester, this material is less susceptible to oxidation than many other polymers, such as polypropylene (PP), from a chemical reactivity point of view, and from its relative permeability to oxygen (Table 7.1) [2]... 

Gas Parmaability. The oxygen, carbon dioxide, and water vapor permeability of polyesters depend not only on composition but also on degree of crystallinity, orientation, and temperature. Regardless of the variable, PEN always exhibits significantly reduced gas permeability relative to PET. Some examples are illustrated in Table 6. 

The effect of copolymer composition on free volume and gas permeability of PECT copolymers as well as PET and PCT homopolymers was studied by Hill et al. (97). The free volume was studied by positron annihilation lifetime spectroscopy (PALS) in order to determine the relative size and concentration of free volume cavities in the copolymers. The logarithm of the permeability to oxygen and carbon dioxide increased linearly with the %mol content of 1,4-CHDM units in the copolymer, which was in agreement with the free volume cavity size and relative concentration observed by PALS measurements. Light et al. (98) studied the effect of sub-T relaxations on the gas transport properties of PET, PCT and PECT polyesters. They observed that modification of PET with 1,4-CHDM increased the magnitude of the p-relaxation, as well as the diffusion and solubility coefficients for oxygen and CO. ... 

Polyethylene naphthalate (PEN), a relatively new polyester, is part of the PET family and has a great potential as a resin for bottles. PEN resin is more opaque, shows five times lower oxygen and carbon dioxide permeability coefficients, has a higher glass transition temperature, is stronger, and is stiffer than PET. This makes it more suitable for hot fiUing and a excellent material for carbonated beverages. Bottles made of PEN provide the product with additional ultraviolet protection. PEN bottles can be returnable, refiUable, and re-... 

Shellac is an excellent film-forming natural polyester resin secreted by insects. It consists in a complex mixture of aliphatic and aUcyclic acids, which serves as moisture barrier of tablets and pellets due to its low permeability to water and oxygen [26, 27]. Shellac films protect drugs from degradation in the gastric environment. However, its use is limited by stability problans. 

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