Water vapor barrier

Hard baked goods such as cookies and crackers have a relatively low water and high fat content. Water can be absorbed, and the product loses its desirable texture and becomes subject to Hpid rancidity. Packagiag for cookies and crackers includes polyolefin-coextmsion film pouches within paperboard carton sheUs, and polystyrene trays overwrapped with polyethylene or oriented polypropylene film. Soft cookies are packaged in high water-vapor-barrier laminations containing aluminum foil. 

OPP producers have expanded the core, creating a foam stmcture with lower density, greater opacity, and a stiffer, more paper-Hke feel. Vacuum metallisation increases opacity and water-vapor barrier properties. 

Oriented polypropylene film has exceUent water-vapor barrier but poor gas barrier properties exceUent clarity, or opacity in newer forms and good heat-seal properties in packagiag appHcations. 

Polyester. Poly(ethylene terephthalate) [25038-59-9] (PET) polyester film has intermediate gas- and water- vapor barrier properties, very high tensile and impact strengths, and high temperature resistance (see Polyesters, thermoplastic). AppHcations include use as an outer web in laminations to protect aluminum foil. It is coated with PVDC to function as the flat or sealing web for vacuum/gas flush packaged processed meat, cheese, or fresh pasta. 

Ethylene vinyl acetate copolymer (EVA) forms a soft, tacky film with good water-vapor barrier but very poor gas-barrier properties. It is widely used as a low temperature initiation and broad-range, heat-sealing medium. The film also serves for lamination to other substrates for heat-sealing purposes. 

Polyacrylonitrile (PAN) films have outstanding oxygen and CO2 barrier properties, but only modest water-vapor barrier properties. They are for processed-meat and fresh pasta packaging laminations where an oxygen barrier is required for vacuum or gas flush packaging. 

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. 

In order to improve the water vapor barrier property of Ecoflex or Ecovio , a wax-based masterbatch can be used. Depending on the concentration, this masterbatch reduces the water vapor transmission rates of Ecoflex films by up to... 

An edible film should have good water vapor barrier properties (low or no water permeation and diffusion through film), which should not increase or increase very little with increasing relative vapor pressure (Lawton, 1996). Films should withstand mechanical stress and strain to such an extent that they do not break easily under a decent mechanical force (Talja et al, 2008). Thus, composition of starch-based films is an important factor influencing its barrier and mechanical properties. Also, starch-based edible films may have an impact on the sensory and textural characteristics of the food. 

However, the drawback of the use of HDPE has been the curling of the material caused by it. The reason for this is the post-crystallization taking place in the HDPE layer after its extrusion. This problem can be circumvented by replacing the HDPE by COC. COCs exhibit good water vapor barrier properties, and in addition, they are amorphous. Thus, neither post-crystallization nor curling caused by this occur in the extruded COC layer (64). 

Biaxially oriented polypropylene (BOPP) films have higher stiffness than cast films and consequently can be used in much thinner gauges. Homopolvmers are used almost exclusively to provide maximum stiffness and water-vapor barrier. Oriented films are produced by the tenter frame and bubble processes. 

Polyethylene is water-resistant and is a good water vapor barrier. Less than 0.1% water is absorbed in a 2-inch (5-centimeter), 1/8-inch (3-millimeter) thick disk of polyethylene in 24 hours. Transmission of other gases is high when compared with that of most other plastics. Polyethylene is not satisfactory for retention of vacuum. 

In the same publication, a dressing that limits pain is most desirable which usually means an occlusive or water vapor barrier. However, a semipermeable dressing would be preferable that would control the amount of water vapor loss. A dressing that conforms to any contour, and a dressing that does not firmly attach to the tissue or interfere with the natural healing process are most desirable for superficial and full thickness skin wounds as well as deep tissue wounds. Further, a dressing that limits body water loss to less than 35 g of water vapor transmitted per m2 per hour is considered low enough to maintain a moist environment for most wounds. 

Microscopic pinholes primarily affect water-vapor barrier properties. The oxygen barrier is mainly affected by macroscopic defects. 

PP possesses good water vapor barrier and fat resistance properties. Normal PP films have limited food packaging applications (e.g. packaging of bread) because of their low cold temperature resistance. Copolymer mixtures with ethylene are used to improve cold resistance and heat sealability as well as material strength and, above all, seal strength. 

Type of Polymer Specific Gravity (ASTM D 792) Water Vapor Barrier Gas Barrier Resistance to Grease and Oils... 

Polybutandiol sebacate, polyhexandiol sebacate, polynonandiol sebacate, and polydecandiol sebacate esters were previously prepared by Bastioli [2] and used as biodegradable water vapor barriers. 

Koelsch, C. (1994). Edible water vapor barriers properties and promises. Trends Food Sci. Technol. 5, 76-81. 

Park, H.J., and Chinnan, M.S. (1995). Gas and water vapor barrier properties of edible films from protein and cellulosic materials. J. Food Eng. 25,497-507. 

TABLE 5. Water Vapor Barrier Property of the Films. 

A number of copolymers of vinylidene chloride are used in practice. Copolymers with acrylonitrile are used in low flammability fibers (modacrylic fibers). These fibers begin to lose weight when heated at 285-308° C due to dehydrohalogenation [41] but do not ignite easily. A tercopolymer butadiene-styrene-vinylidene chloride is used in fabrics industry and in paper industry. Other copolymers include PVC/PVDC, used for fibers and for films with low permeability to gases and water vapors (barrier films), etc. 

Sobral, P.J.A., Menegalli, F.C., Hubinguer, M.D., and Roques, M.A. Mechanical, water vapor barrier and thermal properties of gelatin based edible films. Food Hydrocolloids, 15, 423, 2001. 

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