Barrier materials

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. 

Of the common commercial resias and films, PVDC has the best water-vapor and oxygen-barrier properties. High crystallinity confers resistance to the permeation of odors and flavors, as weU as to fat and oil. Because of its high chloride content, PVDC tends to corrode processing equipment, which increases manufacturing costs. Unlike other high oxygen-barrier materials, PVDC is almost insensitive to water and water vapor. 

Thermoform able sheet may be mono- or multilayer with the latter produced by lamination or coextmsion. Multilayers are employed to incorporate high oxygen-barrier materials between stmctural or high water-vapor barrier plastics. Both ethylene vinyl alcohol copolymers and poly(vinyhdene chloride) (less often) are used as high oxygen-barrier interior layers with polystyrene or polypropylene as the stmctural layers, and polyolefin on the exterior for sealing. 

Incorporating EVOH as high oxygen barrier with polypropylene is used for packaging tomato catsup, barbecue sauce, mayoimaise, pickle reUsh, and other foods. Bottles fabricated from internal and external layers of polypropylene contain EVOH as the principal high oxygen-barrier material. 

The scalping of flavor and aroma by a package can be minimised by placing a barrier material as near as possible to the food. The ingress of undesirable permeants from the environment can be minimised by placing a barrier polymer between the food and the environment, not necessarily near the food. 

Barrier Layers. Depending on composition, barrier layers can function simply as spatial separators or they can provide specified time delays by swelling at controlled rates or undergoing reactions such as hydrolysis or dissolution. Suitable barrier materials include cellulose esters and water-permeable polymers such as gelatin and poly(vinyl alcohol) (see Barrier polymers). 

Breakthrough time In the context of chemical protective clothing, the rime between initial contact of the chemical on the barrier material surface and the analytical detection of the chemical on the other side of the material. 

Material of Construction (Barrier Material) Heating Water with Steam Heating Water with Heat Transfer Oil CooHng Organic Liqitid with Water Cooling Viscous Organic Liqitid with Water... 

The efficacy of polymers when used to protect metals from corrosive environments is influenced by their efficiency as barrier materials. When applied to metals by some techniques, such as fluidised bed coating, there is always the danger of macro-diffusion through pinholes which are gross imperfections in the surface and which do not have to be visible to be very much greater than the dimension of penetrating molecules. 

Supply is related to cost. If basic materials become scarce, changes will follow. Thus constraints in the petroleum supply may affect the polymer industry, which is based almost entirely on petrochemicals. We can look for expanded use of paper-based materials to provide factors such as bulk, tensile properties, and thermal resistance combined with the low weights of proper barrier materials. Composites will continue to grow in quantity and complexity. 

Technical Advances In Packaging With Flexible Barrier Materials , ASTM Special Technical Publ 548, ASTM, Phila (1972) 23) Anon,... 

Interconnect. Three-dimensional structures require interconnections between the various levels. This is achieved by small, high aspect-ratio holes that provide electrical contact. These holes include the contact fills which connect the semiconductor silicon area of the device to the first-level metal, and the via holes which connect the first level metal to the second and subsequent metal levels (see Fig. 13.1). The interconnect presents a major fabrication challenge since these high-aspect holes, which may be as small as 0.25 im across, must be completely filled with a diffusion barrier material (such as CVD titanium nitride) and a conductor metal such as CVD tungsten. The ability to fill the interconnects is a major factor in selecting a thin-film deposition process. 

Copper is intrinsically a better metal than aluminum for the metallization of IC s. Latest developments in MOCVD show that it can be readily deposited without major changes in existing processing equipment. Diffusion problems are minimized and it appears that present barrier materials, such as titanium nitride or titanium-tungsten alloys, should provide adequate diffusion barriers for the copper-silicon couple, certainly up to the highest temperatures presently used in IC s processing (see Ch. 6). The development of CVD copper for semiconductor metallization is on a considerable scale at this time.Clt ]... 

To prevent or reduce diffusion, a barrier is placed between the two materials. The ideal barrier material must not react with the materials it separates and have suitable electrical and thermal properties. Many designs call for the barrier to be deposited with constant thickness inside very narrow (0.35 im) and deep holes (aspect ratio of 2 to 1 or more). 

Three major barrier materials are now in use and all three are deposited either by CVD or by sputtering. P l... 

Tantalum Nitride as Diffusion Barrier. Tantalum nitride (TaN) produced by MOCVD has excellent potential as a barrier material, comparable to TiN. The resistivity of TaN thin films can be lowered by rapid thermal annealing in nitrogen. 

Glass-ionomer cements become less susceptible to desiccation as they age, because a greater proportion of the water in older cements has become tightly bound . Early contact with moisture is also damaging, and this problem is overcome clinically to some extent by using some sort of protection such as clear nail varnish to seal the cement during its early life (Wilson McLean, 1988). However, this does not give perfect results, and as yet there is no ideal barrier material for this purpose (Earl, Hume Mount, 1985). 

D. Allen. High barrier materials for blister packaging. Pharm. Med. Packaging News, 8, 37 (2000). 

Kanel, S.R., Greneche, J.M. and Choi, H. (2006) ArsenicfV) removal from groundwater using nano scale zero-valent iron as a colloidal reactive barrier material. Environmental Science and Technology, 40, 2045—2050. 

One of the PVA derivatives extensively used in this field is ethylene-vinyl alcohol copolymer (EVOH). Their blends with different polyolefins are also effective as oxigen barrier materials. 

Syntactic foams are composites made of micro-balloons or hollow macrospheres bound into a plastic matrix. The polyimide syntactic foams are used in specific electronic equipment for microwave and RF applications. They act as structural, dielectric and heat-barrier materials. Micro-balloons can be made of polyimides for special applications. 

Ube developed the PA nanocomposite named Ecobesta to replace PVDF or other traditional barrier materials. The all-polyamide structure offers recycling advantages compared to traditional multi-materi designs. It incorporates ... 

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... 

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