Polymer laminates barrier properties

In the late 1960s a new class of AN copolymers and multipolymers was introduced that contain >60% acrylonitrile. These are commonly known as barrier resins and have found thek greatest acceptance where excellent barrier properties toward gases (5), chemicals, and solvents are needed. They may be processed into bottles, sheets, films, and various laminates, and have found wide usage in the packaging industry (see Barrier polymers). 

Poly(vinyhdene chloride) (PVDC) film has exceUent barrier properties, among the best of the common films (see Barrier polymers). It is formulated and processed into a flexible film with cling and tacky properties that make it a useful wrap for leftovers and other household uses. As a component in coatings or laminates it provides barrier properties to other film stmctures. The vinyUdene chloride is copolymerized with vinyl chloride, alkyl acrylates, and acrylonitrile to get the optimum processibUity and end use properties (see Vinylidene chloride monomer and polymers). 

Layered Structures. Whenever a barrier polymer lacks the necessary mechanical properties for an appHcation or the barrier would be adequate with only a small amount of the more expensive barrier polymer, a multilayer stmcture via coextmsion or lamination is appropriate. Whenever the barrier polymer is difficult to melt process or a particular traditional substrate such as paper or cellophane [9005-81-6] is necessary, a coating either from latex or a solvent is appropriate. A layered stmcture uses the barrier polymer most efficiently since permeation must occur through the barrier polymer and not around the barrier polymer. No short cuts are allowed for a permeant. The barrier properties of these stmctures are described by the permeance which is described in equation 16 where and L are the permeabiUties and thicknesses of the layers. 

One the attempt to give improved mechanical properties to TPX is to laminate the polymer and a resin containing polar groups, such as ethylene vinyl acetate and PA. These polymers exhibit excellent gas barrier properties, and the lamination of TPX makes an improvement in the gas barrier properties. Furthermore, PA, in particular biaxially-oriented PA, shows an excellent rigidity, toughness, impact resistance (4). 

In packaging, thin PVDC films are used as barrier layers in laminates. PVDC dispersion coatings provide very good barrier properties on paper, regenerated cellulose, OPP and other plastic films. The coatings can also be manufactured so that they are heat sealable. Because of their heat sealability, fat and oil resistance and good flexibility, PVDC polymers are exceptional packaging materials. 

Barrier properties of organic polymers cannot equal metals and glass. Plastics offer so many other advantages that we often try to compromise or laminate to optimize overall balance of properties. It is known that molecular rigidity and crystallinity improve barrier... 

Besides the coextruded laminate structure in Fig. 4b, cases c-f are also viable structures for some applications. Chapter 11(47) discusses the addition of inorganic fillers to EVOH copolymer to achieve large increases in barrier properties in some applications. The effects of different loadings of mica flake in several polymers other than EVOH was also recently reported to be effective (80). 

Most aseptic foods are packaged in a variety of polymeric materials. The plastic polymers used in aseptic packages are either in a pure or a coextruded form to optimize barrier properties. These plastics are not as inert as metal and glass containers resulting in a shorter shelf life for laminated packages when compared to metal and glass containers. 

Poly(ethylene-CO-vinyl alcohol) is a thermoplastic used extensively in laminates for food containers due to its excellent film forming and oxygen barrier properties. Limited evidence for the disappearance of the polymer was provided [93]. Whether or not the copolymer is biodegradable is apparently related to the size and distribution of the ethylene blocks. 

Wood, WE (2001). Improved aroma barrier properties in food packaging with cyclodextrins. In Polymers, Laminations and Coatings Conf, TAPPI, "ifn yil. 

Multilayer co-extrusion is another technique used in the preparation of starch/ synthetic sheets or films [164, 263-266], in which TPS is laminated with appropriate biodegradable polymers to improve the mechanical, water-resistance and gas-barrier properties of final products. These products have shown potential for applications such as food packaging and disposable product manufacture. Three-layer co-extrusion is most often practiced, in which a co-extrusion line consists of two single-screw extruders (one for the inner starch layer and the other for the outer polymer layers) a feedblock a coat-hanger-type sheet die and a three-roll calendering system [164]. Biodegradable polyesters such as PCL [164, 264], PLA [164, 263], and polyesteramide, PBSA and poly(hydroxybutyrate-co-valerate) [164] are often used for the outer layers. These new blends and composites are extending the utilization of starch-based materials into new value-added products. 

Coextrusion is the simultaneous extrusion of two or more layers of different polymers to make a film. Coextruded films have three main advantages over other types of film they have very high barrier properties, similar to laminates but produced at a lower cost they are thinner than laminates and are therefore easier to use on filling equipment and the layers do not separate. There are three main groups of polymers that are coextruded ... 

Rosin and its derivatives are frequently used as polymer additives in different capacities. Their role as plasticizers is illustratedby recent work related to the paper coating for foodpackaging with a 3-hydroxybutirate/3-hydroxy valerate copolymer (PHBW) [112, 113]. The addition of tall oil rosin to the copolymer was found to improve its water vapour barrier properties as well as to reduce the pinhole density of the corresponding laminated papers. 

Polylactides (PLA) may have many potential applications for an important set of products but some of their properties should be improved to obtain similar performance to petroleum-based commodities. One of the most important current applications of PLA is food packaging, in particular for short-shelf-life products with common applications such as rigid containers, drinking cups, over-wrap and lamination films. PLA production and consumption are expected to increase therefore research into the variation of PLA mechanical and barrier properties is currently very active. The control of barrier properties in PLA films is possible by modification of the polymer network through the formation of intramolecular and inter-molecular covalent crosslinking, for example by applying thermal treatment, or by modifying the chemical composition. Another method is to incorporate fillers, in particular layered nanoclays, and this will be the subject of this chapter. 

The other main application field of aPP is related to its tackiness. Papers are laminated by roll-coating with aPP-based adhesives. The aPP layer between the papers imparts not only good adhesion but also barrier properties. aPP and related materials can be used to laminate paper to other substrates such as polymer films. aPP may serve also as the basic ingredient of hot-melt adhesives (HMA), even in HMA with some pressure sensitivity. 

Metallization is used to apply a thin coating of metal, typically a form of aluminum, to a polymer film. This provides improved oxygen and water barrier properties as well as barrier to light. The best known use of metallized film is probably potato chip bags. Metallized films may also be used for nuts and salty snacks. Metallized films may be coated to provide sealability or may be laminated to... 

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