Gas barrier materials

Clay-polymer nanocomposites have proven to be interesting candidates as gas barrier materials preventing permeation of volatile gases by creating a long path for diffusion and as flame-retardant materials. Previous work mainly involves the utihzation of cationic clays, although LDH materials... 

A similiar comparison can be made between permeability data of polyesters and polycarbonates containing "polyester-like" monomers. For example, polycarbonates 26 and 27, which contain predominantly bisphenols which are structurally similiar to the repeat unit (circled) of an alkyl terephthalate polymer (PBT), possess essentially the same low permeability as that polyester (Table V). Furthermore, there has been a recent announcement (3 ) that aliphatic polycarbonates, such as polypropylene and polyethylenecarbonate, are being developed as potential gas barrier materials. In summary these results suggest that the relative low permeabilities of commercial polyesters and high permeabilities of commerical polycarbonates are not a direct consequence of the ester or carbonate links, but are due instead to the structure of the monomers they are prepared from, i.e., the aliphatic diol and aromatic bisphenol, respectively. 

There are other procedures used, and recommended by the manufacturers of specialized proprietary equipment, to recondition cells in a shorter period of time. Periodic maintenance in a qualified service center is necessary for optimum performance of the battery. The efficacy of each should be verified and the added expense and complexity entailed in the use of these proprietary reconditioning devices justified in specific applications. Care must always be exercised in their use, however, to avoid sustained high-rate overcharge, which may damage the gas barrier material. 

One indication of the significant importance of the two factors of (1) temperature compensation of charger voltage and (2) effective maintenance practices is the existence of large-scale field data which document real-time faUure differences of up to 100 1. These life differences exist between weU-maintained batteries in temperature-compensated systems on the one hand and identical battery designs in uncompensated CP systems with frequent and poorly managed maintenance procedures on the other. Recent improvements in gas barrier materials have significantly reduced these faUures. 

Attempts have been made to perform thermal retorting ia a gas barrier flexible pouch or tray. The retort pouch, under development for many years, has a higher surface-to-volume ratio than a can and employs a heat seal rather than a mechanical closure. Similarly, plastic retort trays have higher surface-to-volume ratios and are usually heat seal closed. Plastic cans iatended for microwave reheating are composed of bodies fabricated from multilayer plastic including a high oxygen barrier material, plus double-seam aluminum closures. 

Nylon. Nylon is the designation for a family of thermoplastic polyamide materials which in film form are moderate-oxygen barriers. The gas-barrier properties are equal to odor and flavor barrier properties important in food appHcations. Nylon films are usually tough and thermoform able, but are only fain moisture barriers (see Polyamides). 

Some heavier gauge flexible materials, usually containing nylon, are thermoformed, ie, heated and formed into three-dimensional shapes. Such stmctures are used to provide high gas-barrier, heat-sealable containment for processed meat or cheese. 

Oil Repellent. Fluorochemicals are the only class of material that can provide oil repeUency without altering the porosity of the paper or paperboard. Physical barriers to oil penetration are used primarily for their moisture- or gas-barrier properties, with retarded oil penetration as a secondary benefit. The most common od-repeUent additives are long-chain perfluoroalkyl phosphate salts of ammonia or diethanol amine. Commercial sources include Scotchban (3M), Zonyl (DuPont), and Lodyne (Ciba Specialties). There are also a fluorochemical carboxylate salt, Lodyne (Ciba Specialties), and fluorochemical copolymers, eg, Scotchban (3M). The widest range of oily fluid holdout is provided by the fluorochemical copolymers. 

Certain copolymers of this type have been found to have excellent gas barrier properties, with the dry polymer having an oxygen permeability only about 1/lOth that of polyvinylidene chloride. Unsurprisingly, the copolymer has a high moisture absorption and a high moisture vapour transmission rate. Where the material is swollen by water, gas permeability is also higher. 

Because of the excellent gas barrier properties, EVOH is of interest as a packaging material. However, because of its high water absorption it is usually used as an internal layer in a co-extruded film, sheet, bottle or tube. For example, the system HDPE-EVOH-EVA may be used as a barrier film for packaging cereals, and the system polystyrene-EVOH-polystyrene for packaging coffee and cream, whilst the system polystyrene-EVOH-polyethylene has the additional advantage of heat scalability. 

Water Permeation of Nitrile Copolymers. While the AN content bears a direct relationship to the gas barrier, the water permeability presents quite an anomaly. If the water permeation of the commercial SAN films (25% AN) is measured, the rate is higher than that of polystyrene. Thus it appears that films with greater AN content have even higher water permeation rates. It was discovered, however, (I, 7) that as the AN content increases there is a shift in permeation, and the higher AN/S materials show water barriers of excellent quality. Table VIII... 

It is likely that excellent gas barrier properties exhibited by nanocomposite polymer systems will result in their substantial use as packaging materials in fumre years. 

If waterproofing or dampproofing treatments that are effective gas barriers and that can be sealed at joints and penetrations could be identified, then walls could be made radon resistant. Acceptable dampproofing or waterproofing treatments are specifically listed in building codes in many areas of the United States these lists are periodically amended as new materials come into use. These coatings apply primarily to basement walls. 

Bentonite clay expands when moist to create a waterproof barrier. Bentonite is sold in various forms, including panels and mats. Bentonite is not as resistant to chemicals as the thermoplastic membranes, nor is it puncture resistant. The major flaw of bentonite as a radon barrier, however, is that it is only tightly expanded when wet. This is acceptable for a waterproofing material, but not for a gas barrier. 

Neither poly(ethylene 2,6-naphthoate) (PEN), nor copolymers of PEN and PET, are new materials, but they continue to receive considerable attention, due to their relatively high Tm and T values, and attractive tensile, flexural and gas-barrier... 

Y. H. Yang, L. Bolling, M. A. Priolo, J. C. Grunlan, Super gas barrier and selectivity of graphene oxide-polymer multilayer thin films., Advanced materials (Deerfield Beach, Fla.), vol. 25,... 

TPX is used in mold-releasing films, printed circuit mold-releasing materials, various containers, etc., because of its heat resistance, chemical resistance, mold-releasing properties, transparency and other properties (4). In application areas, which require mechanical strength, gas barrier properties and high temperature mechanical strength, among other properties, composites are needed. 

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