Ethylene vinyl alcohol, packaging

Sugared cereals are often packaged in aluminum foil or barrier plastic, eg, ethylene vinyl alcohol, laminations to retard water vapor and flavor transmission (see Wheat and other cereal grains). 

With the exception of ethylene vinyl acetate added in the 1980 s, the list of materials and polymers approved as packaging for food irradiated products has remained static for decades. This article supplies details of the approved list, which includes such polymers as polyethylene terephthalate and polyvinyl chloride. The article provides an update on the latest proactive move to expand the list of packaging materials and polymers approved for the irradiation of foods. The expanded list would include ethylene vinyl alcohol, PVC film, ionomers, nylon 66, 6/12 and copolyesters among others. 

Multilayered materials can be readily formed using thermoforming including food packaging that may involve inclusion of layers of ethylene-vinyl alcohol copolymers, PS, polyolefins, and/or copolymers of vinylene dichloride and vinyl chloride. Microwavable food trays from (crystallized) PET are manufactured using thermoforming. 

In addition, further layers, e.g., polymeric oxygen barrier layers, such as ethylene vinyl alcohol or poly(amide) layers, can be incorporated into the packaging material. A most simple structure for the build up of the layers and an example for a packaging article are shown in Figure 2.4. 

When an EVA copolymer is saponified, ethylene vinyl alcohol (EVOH) units are introduced (17,18). EVOH copolymers are excellent in melt moldability, gas barrier properties, oil resistance, antistatic property and mechanical strength, and are used as various types of packages in the form of a film, a sheet, a container, etc. 

Perhaps the most exciting developments are yet to feature in fruit juice packaging. PET can be laminated with other plastics, such as nylon and ethylene vinyl alcohol (EVOH), to give extremely good barrier properties, and polyethylene naphthalate (PEN) may enable production of a plastic bottle that can be pasteurised at high temperatures. 

Extrusion. Sheet, film, and profiled articles are made by extrusion (20). The resin is melted and forced through a die plate or head. Variations include multilayer and blown film applications. In multilayer coextrusions, different combinations of plastics are separately but concurrently extruded to form layered sheet or film. In the packaging industry, specialty resins such as high barrier ethylene—vinyl alcohol copolymers are combined with heat- and impact-resistant thermoplastics for food packages. The properties of each resin layer are additive, as opposed to the "averaging of property" in blends. Multilayers are also used for blow-molded containers, films, and sheet products (see also Film AND sheeting materials). 

Barrier Plastics. When plastics replace metals and glass in packaging, their permeability is often a limiting property. Barrier performance generally increases with density and crystallinity. The most promising barrier plastics include ethylene/vinyl alcohol, polyvinylidene chloride, polyacrylonitrile, and polyethylene naphthoate. These are used most efficiently by laminating them to commodity plastics such as polyethylene and polyethylene terephthalate. 

Different barrier plastics meet different requirements. A very popular barrier plastic is EVOH (ethylene-vinyl alcohol copolymer) that can be tailored to the needs of packages and other products.80 Generally the thickness ranges from 0.5 to 3.0% of the wall thickness it can be thicker if higher barrier is required. Generally EVOH thickness greater than 8% of the container sidewall can lead to internal structural failures that can fail on drop tests. Also a very thick layer tends to be difficult to process consistently. The EVOH s crack resistance improves as its ethylene content increases. 

BMs commonly include from two to seven layers, although more are also used. The construction usually includes one or more barrier layers. These are plastics with a particular resistance to the transmission of water vapor or gases such as oxygen or carbon dioxide. Examples are ethylene vinyl alcohol (EVOH), nylons, and polyvinylidene chloride (PVDC). Their presence greatly enhances the performance of the BM as a package for foodstuffs, beverages, and other critical products. The barrier materials are all deficient in some respect such as price, mechanical strength, and moisture resistance. Thus not used as a material of sole construction for BM. Their use is in thin layers shielded by other more robust and economical body plastics. 

There are two polymers used in a maj ority ofhigh barrier food packages copolymers containing polyvinylidene chloride (Saran ) and ethylene-vinyl alcohol copolymer (Eval, Selar-OH ). Packages in which either polyvinylidene chloride or ethylene-vinyl alcohol copolymer (EvOH) provide the barrier are invariably multilayer since neither of these materials offers the properties required to make a good monolayer structure. There are advantages and disadvantages associated with each of these materials, and the... 

Ethylene vinyl alcohol barrier resins are never used as single layer films in packaging but rather are utilized in coextruded structures combined with less expensive structural polymers. 

Ethylene vinyl alcohol (EVOH) is used to produce barrier resins impervious to most gases. This property makes the material especially useful for food packaging. EVOH resins however are sensitive to moisture. As humidity increases the permeability of vapor and odor also increases. Therefore, EVOH is coextruded with other polymers such as nylon or polypropylene to minimize this problem. 

Ethylene vinyl alcohol is also used in production of plastic bottles, thermoform film and sheet, and flexible squeeze tubes. It is being experimented with for microwave food packaging applications, but this use is not yet commercial. It could, however, provide a large new market for EVOH if the application can be successfully demonstrated. 

American Can s Omni retortable plastic can marks a major development in food packaging. The five layer can, used by Hormel, Del Monte, and Campbell Soup Company, consists of polyolefin structural layers, adhesive layers incorporating desiccants, and ethylene vinyl alcohol (EVOH) polymer as the barrier layer. 

Extruded ethylene vinyl alcohol copolymer (EVAL) is used as an oxygen-barrier packaging material. Dry EVAL has extremely low oxygen permeability, but the vinyl alcohol part of the copolymer is hydrophilic, and in the swollen wet state its permeability becomes higher than that of PVDC (Fig. 11.5). 

In this chapter we discuss PVT and surface properties of three sets of random copolymers. Monomer units are ethylene, vinyl alcohol, and vinyl acetate, as well as styrene and acrylonitrile. Random copolymers comprising these monomers are used widely. As an example, ethylene-vinyl alcohol random copolymers (EVOHs) have excellent gas barrier properties. They are used for food-packaging films or in fuel tank liners [Takahashi et al., 1999 Alvarez et al., 2003 Ito et al., 2003 Lopez-Rubio et al., 2003 Muramatsu et al. 2003]. 

A widely popular container or carton used to store liquid or solid food is coated/laminated with various barrier and sealant materials, for example, a high-density polyethylene (HOPE), a heat seal-able polymer material such as low-density polyethylene (LDPE), or a heat-sealable barrier material such as an ethylene vinyl alcohol copolymer (EVOH), which can emit unwanted odors created during extrusion coating or package converting/sealing. 

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