Properties barrier

Many polymers are used in barrier applications, either to keep contents in or contaminants out. Food packaging is an excellent example of such usage. Plastic films and containers of many types are used to package food. Blow molded bottles often contain numerous layers, each of which provides specific benefits. Polyethylene layers are excellent water barriers, polyvinyl alcohol is a good oxygen barrier, and polyethylene terephthalate impedes the diffusion of carbon dioxide from carbonated drinks. Other barrier applications include toothpaste tubes, diaper backsheets, tarpaulins, and geomembranes, which are used to line containment ponds and landfill pits. 

The moisture barrier properties and humidity resistance make COCs suitable for container and film applications. The low moisture absorption prevents swelling. 

Sample Clay content (wt%) IV ) Flexural strength Flexural modulus HDT (18.6kgfcm ) UV ) trans. (%) CO, Barrier Clarity 

As mentioned above, the plate-like-shaped day is indeed effident in maximizing the path length of a diffusing molecule. According to Nielsen [29], the tortuousity factor Tis defined as the ratio of the actual distance d, that a diffusient must travel, to the shortest distance d, that it would travel in the absence of obstacles. T can be expressed in terms of the length L, width W, and volume fraction p of the sheet-Uke particle as follows  

The incorporation of clay minerals to polymers is an attractive approach to enhance gas barrier properties, as has been demonstrated with some polar thermoplastic polymers and thermosets [1]. Polyethylene, and especially HOPE, is commonly used in food packaging, where sometimes is needed to achieve an atmosphere with low concentration of oxygen and high of carbon dioxide. Consequently, a high permeability to these molecules is required in HOPE. The understandable idea is that molecules diffusing in the polymer will be slowed by increasing tortuosity as they meet essentially impermeable clay mineral layers in their path through the polymer and have to find a way around them [ 1 ]. 

Carrera et al. [51] proposed that the increase in barrier properties which were found at low contents of clay were due to defects at the interface which had less resistance to permeation. At higher content of OMt, and since the crystallinity was maintained as that of the pure polymer, the barrier properties were increased as a result of the tortuous path created by the exfoliated structure of the OMt. Monsivais-Barro et al. [52], proposed that the tortuosity effect caused by the dispersed nanolayers in HDPE improve the gas barrier properties in some cases while in other cases the interfacial regions or the free 

From a morphological point of view, there are four fundamental parameters that can be adjusted to maximize the barrier properties of clay nanocomposites. 

Dispersion The degree of dispersion of the nanoplatelets is determined by the degree of delamination of the clay. The fully delaminated (exfoliated) nanocomposite presents much higher values for the tortuosity factor and the aspect ratio in comparison with the partially delaminated (intercalated) nanocomposite. This means that the clay particles that grow as aggregates or books of sheets must be broken up or exfoliated into individual sheets that have a thickness of the order of 1 nm, with lengths and widths of the order of 500 nm. 

Compatibility The particles must be compatible with the polymeric substrate. Since natural clays are alumino-silicates, they must be prepared or functionalized so that they will be compatible with a polymer. For natural clay products, this process can be quite complex, given the wide chemical variability of different samples, even from the same mine. 

Orientation Once the particles are dispersed in the polymer, they must be oriented so that the flat surface of the clay is parallel to the surface of the packaging material to maximize the barrier effect. Several models have been developed in order to describe the mass transfer within the nanocomposites. Most models assume that the platelets have a regular and uniform shape (rectangular, sanidic, or circular) and form a regular array in space. They are either parallel to each other or have a distribution of orientations, with the 

Reaggregation During processing, these particles must be kept from reaggregating or clumping up, thus destroying the benefits of the approach. 

Thermoplastics are widely used as packaging materials due to their low cost, excellent chemical resistance, good barrier properties and the potential for recycling. In fact, over 35% of all thermoplastic is used in packaging. They are also used in a variety of other applications where barrier properties are required. These include water and gas pipes, as well as car petrol tanks. 

Permeability of a material to small molecule penetrants, such as oxygen and water, increases with the solubility of the small molecule in the matrix [52] and with the diffusion coefficient in that material [53]. Polymers are very sensitive to plasticisation by small molecules. Thus, the presence of small molecules may greatly increase the diffusion coefficient. Based on these observations, one can envision ways to decrease permeability by reducing the solubility and/or the diffusion coefficient. 

Molecules can neither dissolve in, nor diffuse through, mineral fillers to any appreciable extent. Therefore the presence of filler reduces the solubility of the diffusant in the composite material, and thereby the permeability, in proportion to the volume fraction of filler. 

In addition, the presence of impermeable filler in a polymer forces the diffusant molecule to travel further around the filler particles. This physical blocking effect is known as tortuosity, because the filler forces the diffusant to take a more indirect, or tortuous, path through the material. The degree of tortuosity imposed is dependent upon the anisotropy and orientation of the filler particles with respect to the direction of diffusion. For example, platy particles oriented perpendicularly to the diffusion vector will be particularly effective in retarding diffusion. The permeability of a composite can be calculated using an equation that allows for the reduction in permeant solubility and for the tortuosity (Equation 8.3). Where P and Pp are the permeability of the composite and the unfilled polymer, respectively. The terms w and t refer to the width and thickness of the filler and (pp and (pf represent the volume fraction of polymer and filler. 

As mentioned previously, the addition of filler may also change the amount of crystallinity in the polymer. As polymer crystals are impermeable even to low molecular weight species, an increase in crystallinity also results in improved barrier properties, through increased tortuosity [54], This effect is expected to be especially prevalent for fillers that induce a high degree of transcrystallinity. 

Some authors have reported significant porosity in nanocellulose films [67-69], which seems to be in contradiction with high oxygen barrier properties, whereas Christin et al. measured a nanocellulose film density close to the density of crystalline cellulose (cellulose 16 crystal structure, 1.63 g/cm ), indicating a very dense film with a porosity close to zero. 

Henriksson et al. [69] further used them to prepare porous cellulose nanopaper of remarkably high toughness. Nanopapers based on wood 

Changing the surface functionality of the cellulose nanoparticle can also affect the permeability of nanocellulose films. Films constituted of negatively charged cellulose nanowhiskers could effectively reduce permeation of negatively charged ions, while leaving neutral ions virtually unaffected. Positively charged ions were found to accumulate in the membrane [70]. 

From a fundamental point of view, different underlying morphologies are sensed differently for various properties. Thus mechanical measurements might be more responsive to, say, phase continuity, while thermal measurements might be more sensitive to the extent of actual molecular mutual solution. It is therefore interesting to bring to bear barrier properties as another class of molecular probes by which an IPN can be studied. The behavior of permeating small molecules can yield much information 

From an applications point of view, IPNs are potentially useful as coating materials, packaging, etc. Frisch et recently studied the 

More recently, Frisch and Frisch reported on the toluene vapor transmission through an SIN film based on poly(l,4-oxybutylene) glycol, MW 1000, and 4,4 -methylene bis(cyclohexyl isocyanate) (H12MDI), and epoxy resin. The coefficient of diffusion decreased, as did the permeability, as the epoxy content was increased. 

Chitosan plasticised glycerol Whey plasticised glycerol 

Because of their hydrophilicity, IWSPs are excellent barriers to non-polar substances, such as lipids and some aroma compounds [83]. However, this characteristic makes their gas barrier properties very much dependent on the humidity conditions of the measurements, and maybe the reason of variation of data presented in the literature. 

The oxygen permeability of both amylose and amylopectin rich starch films is reported to be as good as that from commercial EVOH at ambient humidity (relative humidity 15%), but above 20% such efficacy is lost [85]. Investigation of the gas permeability of potato starch films embedded with LDPE showed similar oxygen barrier characteristics to hydrophilic barriers, but with increasing water content this property was gradually lost [86]. 

The hydrophilic nature of IWSPs makes them unsuitable as moisture barriers for packaging applications unless certain modifications are applied. 

Most natural-based polymers show a large water take up and high water permeability. They also change their mechanical and barrier properties in high moisture conditions, which is also a major disadvantage. This water sensitivity can develop a more spontaneous, non-controllable degradation under the influence of bacteria. 

Eshelby s method, to predict the coefficients of thermal expansion of composites containing either aligned axisymmetric or nonaxisymmetric ellipsoidal particles. 

For composites consisting of axisymmetric particles (e.g., spheres, fibers, and disks) and epoxy resin, they predicted that (i) the coefficients of thermal expansion for spherical inclusions are the same in all directions and decrease modestly as the volume fraction of particles increases and (ii) with the increase in aspect ratio, the thermal expansion becomes anisotropic. For composites consisting of three-dimensional nonaxisymmetric ellipsoidal particles (i.e., day platelets) and nylon 6, they predicted that (i) the coefficient of thermal expansion in the longitudinal direction decreases as the primary and secondary aspect ratios increase and (ii) the value in the transverse direction decreases as the primary aspect ratio increases but decrease with the secondary aspect ratio. 

Xu et al. [35] used the relative permeability theory in combination with the tortuous pathway model to examine the barrier properties of polymer nanocomposites containing impermeable and oriented clay layers. It was found that 

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It possesses outstanding barrier properties to gases, especially water vapor. It is surpassed only by the fully fiuorinated polymers in chemical resistance. A few solvents dissolve it at temperatures... 

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). 

The properties of SAN resins depend on their acrylonittile content. Both melt viscosity and hardness increase with increasing acrylonittile level. Unnotched impact and flexural strengths depict dramatic maxima at ca 87.5 mol % (78 wt %) acrylonitrile (8). With increasing acrylonitrile content, copolymers show continuous improvements in barrier properties and chemical and uv resistance, but thermal stabiUty deteriorates (9). The glass-transition... 

Acrylonitrile copolymeri2es readily with many electron-donor monomers other than styrene. Hundreds of acrylonitrile copolymers have been reported, and a comprehensive listing of reactivity ratios for acrylonitrile copolymeri2ations is readily available (34,102). Copolymeri2ation mitigates the undesirable properties of acrylonitrile homopolymer, such as poor thermal stabiUty and poor processabiUty. At the same time, desirable attributes such as rigidity, chemical resistance, and excellent barrier properties are iacorporated iato melt-processable resias. 

Acrylonitrile—methyl acrylate—iadene terpolymers, by themselves, or ia blends with acrylonitrile—methyl acrylate copolymers, exhibit even lower oxygen and water permeation rates than the iadene-free copolymers (110,111). Terpolymers of acrylonitrile with iadene and isobutjlenealso exhibit excellent barrier properties (112), and permeation of gas and water vapor through acrylonitrile—styrene—isobutjleneterpolymers is also low (113,114). 

Some cast (unoriented) polypropylene film is produced. Its clarity and heat sealabiUty make it ideal for textile packaging and overwrap. The use of copolymers with ethylene improves low temperature impact, which is the primary problem with unoriented PP film. Orientation improves the clarity and stiffness of polypropylene film, and dramatically increases low temperature impact strength. BOPP film, however, is not readily heat-sealed and so is coextmded or coated with resins with lower melting points than the polypropylene shrinkage temperature. These layers may also provide improved barrier properties. 

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). 

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. 

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). 

Nylon films are used in lamination or coated form to ensure heat sealabiHty and enhance barrier properties. The largest uses are as thermoforming webs for twin-web processed meat and cheese packagiag under vacuum or in an inert atmosphere. Other uses include bags for red meat, boil-ia-bags, bag-in-box for wine, and as the outer protective layer for aluminum foil in cookie and vacuum coffee packages. 

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. 

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. 

To enhance water-vapor- or gas-barrier properties, layers of different plastics may be injected together or sequentially. Multilayer injection-molded pieces may be prepared as packaging or for blowing into bottle or jar shapes. 

Figure 4c also describes the spontaneous polymerisation ofpara- s.yX en.e diradicals on the surface of soHd particles dispersed in a gas phase that contains this reactive monomer (16) (see XylylenePOLYMERS). The poly -xylylene) polymer produced forms a continuous capsule sheU that is highly impermeable to transport of many penetrants including water. This is an expensive encapsulation process, but it has produced capsules with impressive barrier properties. This process is a Type B encapsulation process, but is included here for the sake of completeness. 

The web materials used to form dmms and cans can be customized to each packaging apphcation. The materials are selected based on the package properties of moisture and humidity resistance, nonstick resistance, or barrier properties required. The adhesives used to bond the phes together include sihcates, poly(vinyl alcohol), and poly(vinyl acetate) (10). Sihcate adhesives are most commonly used in the manufacture of dmm packages. 

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