Permeability to gases

HDPE has a better resistance to permeation by gases than LDPE (Table 3.22). 

Polymer Nitrogen Oxygen Water vapour Carbon dioxide Helium 

13 mm) is dependant on the molecular size, shape, wettability, and soundness of the fabricated membrane. 

Attempts have been made to relate permeation rates through thin films to absorption of thicker films, sheets, tubes, pipe, etc. This has been generally unsuccessful. Thicker films and sheets represent an average set of properties obtainable from many thin films 

However, comparative data on identical tests can be used to predict performance in many thin film and coating applications. 

Increased permeability with temperature parallels the decrease in specific gravity with increased temperature in the resin. This corresponds with increased spacing between molecules and increasing molecular activity which allows easier diffusion of the gas through the specimen.tfi 

Test Method ASTM D1434 ASTM D1434 ASTM D1434 

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Polymeric vinylidene chloride generally produced by free radical polymerization of CH2 = CCl2. Homopolymers and copolymers are used. A thermoplastic used in moulding, coatings and fibres. The polymers have high thermal stability and low permeability to gases, and are self extinguishing. 

Permeability to gases Low Low Low Very low Low Very low Low Very low Higt... 

During the 1970s there was considerable interest for a time in copolymers with a high acrylonitrile content for use as barrier resins, i.e. packaging materials with low permeability to gases. Problems associated with free acrylonitrile have, however, led to the virtual disappearance of these materials from the market. 

Aliphatic polycarbonates have few characteristics which make them potentially valuable materials but study of various aromatic polycarbonates is instructive even if not of immediate commercial significance. Although bisphenol A polycarbonates still show the best all-round properties other carbonic ester polymers have been prepared which are outstandingly good in one or two specific properties. For example, some materials have better heat resistance, some have better resistance to hydrolysis, some have greater solvent resistance whilst others are less permeable to gases. 

They are generally more permeable to gases than crystalline polymers, more so above T, than below (see later). 

Good Thermal and Oxidative Stability High Permeability to Gases Excellent Atomic Oxygen (Plasma) Resistance Physiological Inertness (Biocompatibility)... 

The chemical modification of poly (2,6-dimethyl-l,4-phenylene oxide) (PPO) by several polymer analogous reactions is presented. The chemical modification was accomplished by the electrophilic substitution reactions such as bromination, sulfonylation and acylation. The permeability to gases of the PPO and of the resulting modified polymers is discussed. Very good permeation properties to gases, better than for PPO were obtained for the modified structures. The thermal behavior of the substituted polymers resembled more or less the properties of the parent polymer while their solution behavior exhibited considerable differences. 

Table VI. The Permeability to Gases of PPO Modified by Friedel-Crafts Reactions...

It is important to note that some types of dewar necks are made of plastic materials which are permeable to gases and in particular to He. The permeation phenomenon has a strong dependence on temperature and is negligible at 4K (see e.g. ref. [6]). If the dewar remains for a long time at room temperature in an atmosphere containing He gas, the vacuum space is slowly filled with He which must be pumped before the filling with cryogenic liquids. 

To perform optimally, the char, or similar barrier should be continuous, coherent, adherent and oxidation-resistant. It should be a good thermal insulator (which implies closed-cell character) and it should have low permeability to gases, to liquid pyrolysate, and to molten polymer. Moreover, the char must be formed in a timely manner before the polymer is extensively pyrolyzed. 

Poly(organosiloxane)s have a number of outstanding properties such as thermal and oxidative stability, water and chemical resistance, electric insulating capacity, selective permeability to gases, and biocompatibility. 

Vulcanised butyl rubber is very similar to vulcanised natural rubber in various physical characteristics but has better resistance to oxidation and has low permeability to gases. Hence, it is widely used in tubes for cycles, scooters, motor cars, etc. It is also used as rubber in many other applications. 

Because flexible films have excellent strength and good electrical and thermal properties, have low permeability to gases, water, oil, odour and could be sterilised so they are used in capacitors, cables, magnetic recording tapes, typewriter ribbons, food packaging, tracing film, book covers, etc. The film is coated with aluminium by vacuum disposition and used for decoration and in textiles. 

Hybrid polymers are composites containing one organic component (e.g polymer) and another part, distinct in nature to the matrix (e.g. the nanomaterial), and these parts interact through chemical bonding so that the polymer layer bonded to the filler is distinct from the polymer matrix. The properties of these materials are improved by the introduction of the nanomaterial into the polymer matrix. Some of the properties that can be enhanced include the Young s modulus, resistance to flammability, low permeability to gases, and increased heat resistance. 

Polymethylpentene is more permeable to gases than some other polyolefins. 

Cellulosics have high permeabilities to gases, evaluated at ... 

ISO 2285 2001 Rubber, vulcanized or thermoplastic - Determination of tension set under constant elongation, and of tension set, elongation and creep under constant tensile load ISO 2782 1995 Rubber, vulcanized or thermoplastic - Determination of permeability to gases ISO 3384 1999 Rubber, vulcanized or thermoplastic - Determination of stress relaxation in compression at ambient and at elevated temperatures ISO 3865 1997 Rubber, vulcanized or thermoplastic - Methods of test for staining in contact with organic material... 

Copolymers of vinyl chloride and vinylidene chloride are widely used (Equation 6.60). Two extremes are employed. The lesser known has a high vinyl chloride content compared with the vinylidene chloride comonomer. These copolymers are more easily dissolved and have greater flexibility. The most widely known copolymer has about 90% vinylidene chloride and 10% vinyl chloride and is known as Saran. This copolymer has low permeability to gases and vapors and is transparent. Films are sold as Saran Wrap. 

Polysiloxanes also possess a variety of other useful properties such as hydro-phobicity and exceptionally high permeability to gases. Indeed, snails can live submerged beneath the surface of low molecular weight poly(dimethylsiloxane)... 

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