Properties biaxially oriented

General Description Polypropylene is produced commercially in different forms, depending upon the desired properties. Polypropylene films can be stretched, or oriented, to modify the properties. Biaxially oriented polypropylene (BOPP), and oriented polypropylene (OPP) films are polypropylene films that have been oriented biaxially and uniaxially, re-spectively.f ]... 

Orientation of polymers enhances many of their properties, particularly the mechanical, impact, barrier and optical properties. Biaxial orientation has the added advantage of allowing this enhancement in two directions, avoiding any weakness in the transverse direction. Biaxial orientation is particularly important in films, where it allows the production of thinner films having superior mechanical, optical and barrier properties and, if required, the ability to shrink when reheated. For example, shrink and barrier properties are particularly important in meat packaging, whereas good mechanical and optical properties and no shrinkage are desired in box overwrap applications of tape cassettes and CDs. 

Property Biaxially oriented film Uniaxially oriented film... 

Biaxial Orientation. Many polymer films require orientation to achieve commercially acceptable performance (10). Orientation may be uniaxial (generally in the machine direction [MD]) or biaxial where the web is stretched or oriented in the two perpendicular planar axes. The biaxial orientation may be balanced or unbalanced depending on use, but most preferably is balanced. Further, this balance of properties may relate particularly to tensile properties, tear properties, optical birefringence, thermal shrinkage, or a combination of properties. A balanced film should be anisotropic, although this is difficult to achieve across the web of a flat oriented film. 

Oriented polypropylene film (OPP) may be classified as heat-set and non-heat-set, blown and tentered, coextmded and coated. Orientation improves the cold-temperature resistance and other physical properties. Heat-set biaxially oriented polypropylene film (BOPP) is the most widely used protective packagiag film ia the United States. It is used to wrap bakery products, as lamination pHes for potato and com chips, and for pastas and numerous other flexible pouch and wrapping appHcations. Nonheat-set OPP is used as a sparkling, transparent shrink-film overwrap for cartons of candy. 

Although PS is normally considered a rather brittle material, biaxial orientation imparts some extremely desirable properties, particularly in regard to an increase in elongation. Thus, the 1.5—2% of elongation normally associated with unoriented PS can become as high as 10%, depending on the exact conditions of preparation (267,268). 

Polymers can exist in a number of states. They may be amorphous resins, rubbers or fluids or they can be crystalline structures. TTie molecular and the crystal structures can be monoaxially or biaxially oriented. Heterogeneous blends of polymers in different states of aggregation enable materials to be produced with combinations of properties not shown by single polymers. 

Structurally the difference between PEN and PET is in the double (naphthenic) ring of the former compared to the single (benzene) ring of the latter. This leads to a stiffer chain so that both and are higher for PEN than for PET (Tg is 124°C for PEN, 75°C for PET is 270-273°C for PEN and 256-265°C for PET). Although PEN crystallises at a slower rate than PET, crystallization is (as with PET) enhanced by biaxial orientation and the barrier properties are much superior to PET with up to a fivefold enhancement in some cases. (As with many crystalline polymers the maximum rate of crystallisation occurs at temperatures about midway between Tg and in the case of both PEN and PET). At the present time PEN is significantly more expensive than PET partly due to the economies of scale and partly due to the fact that the transesterification route used with PEN is inherently more expensive than the direct acid routes now used with PET. This has led to the availability of copolymers and of blends which have intermediate properties. 

Stretching a polymer in two perpendicular directions, either successively or by blowing a bubble of molten material, leads to its biaxial orientation, which strongly improves mechanical properties in the stretching directions and/or gas permeability (e.g., biaxial orientation of polypropylene leads to BOPP (for biaxially oriented polypropylene) or biaxial orientation of poly(ethylene terephthalate) gives CC>2-impermeable bottles for carbonated beverages.) (Characterisation methods for determining molecular orientation are considered in Chapter 8.)... 

This stretching, similar to the drawing of fibres, which promotes orientation and crystallite formation, is called biaxial orientation. It gives the film added strength and gas-barrier properties. In some processes, monoaxial (uniaxial) drawing is employed, e.g., polypropylene, which is then slit into thin strips and fabricated into heavy duty sacks, carpet backing, etc. The stenter process is used to make biaxial oriented poly(vinylidene dichloride) ("ding" film), polyester, polyamide and polypropylene films. 

A second type of anisotropic system is the biaxially oriented or planar random anisotropic system. This type of material is illustrated schematically in Figure 2A. Four of the five independent elastic moduli are illustrated in Figure 2B in addition there are two Poisson s ratios. Typical biaxially oriented materials are films that have been stretched in two directions by either blowing or tentering operations, rolled materials, and fiber-filled composites in which the fibers are randomly oriented in a plane. The mechanical properties of anisotropic materials arc discussed in detail in following chapters on composite materials and in sections on molecularly oriented polymers. 

Biaxially Oriented Poly(Ethylene 2,6-Naphthalene) Films Manufacture, Properties and Commercial Applications... 

PEN has excellent gas-barrier properties. The gas-permeation coefficients of unoriented cast film and biaxially oriented films for PEN and PET are given in Table 10.4. 

A biaxially oriented polyester film made of a copolymer consisting of 80 % ethylene 2,6-naphthalenedicarboxylate has excellent adhesion, surface hardness and wear resistance, while also providing significant transparency and antireflection properties. This material, when laminated to glass, is excellent for the prevention of shattering [82],... 

ISO 1874-2 1995 Plastics - Polyamide (PA) moulding and extrusion materials - Part 2 Preparation of test specimens and determination of properties ISO 15987 2003 Plastics - Film and sheeting - Biaxially oriented polyamide (nylon) films. 

Mechanical properties of polymers are improved by orientation processes [22, 24, 29]. Because of the biaxial orientation, films from blown film lines exhibit, e.g., a high puncture and tear propagation resistance. Blown film lines can be designed to produce flexible PE-LD or PE-HD films of 5-200 pm. 

Table V. Properties of Crosslinked, Biaxially Oriented Copolymer Film...

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). 

Biaxially oriented PPS film is transparent and nearly colorless. It has low permeability to water vapor, carbon dioxide, and oxygen. PPS film has a low coefficient of hygroscopic expansion and a low dissipation factor, making it a candidate material for information storage devices and for thin-film capacitors. Chemical and thermal stability of PPS film derives from inherent resin properties. PPS films exposed to toluene or chloroform for 8 weeks retain 75% of their original strength. The UL temperature index rating of PPS film is 160°C for mechanical applications and 180°C for electrical applications. Table 9 summarizes the properties of PPS film. 

A process that greatly improves blow molded product properties is that of injection stretch blow molding, which introduces biaxial orientation in crystallizable polymers. There are two variants of this process involving the molding of a preform as the first step. The thermomechanical paths of the two process variants are shown in Fig. 14.19. In the... 

Trogamid T can be processed without difficulty by the normal methods used for thermoplastics (5). In this respect it offers fewer problems than polyacetal or polycarbonate, for example. Some Trogamid T properties can be improved by biaxial orientation. The effects, however, are lower than with crystalline polymers. 

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