Biaxial orientation mechanisms

With the current lack of understanding of uniaxial orientation process for PHB apparent in the previous section, it would be surprising if the biaxial orientation mechanisms of the biopolymer had been fully characterized. Solution-cast foils, particularly if they contain a small... 

One of the aims of the present research at Leeds University, of which the spectroscopic studies form a major part, has been to gain an understanding of mechanical behaviour. Both the uniaxially oriented and the biaxially oriented materials discussed in this review have also been the subject of studies of mechanical anisotropy and deformation. It is therefore of some interest to indicate the key guidelines which are emerging from these related studies. 

The biaxially oriented PET sheets have been extensively studied with regard to their mechanical anisotropy and all nine independent elastic constants have been determined by a variety of experimental techniques 38,39). The complete set of compliances for a one-way drawn sheet of draw ratio 5 1 is shown in Table 7. It is interesting to note that these compliances clearly reflect the two major structural features, the high chain axis orientation and the preferential orientation of the benzene rings... 

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.)... 

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. 

Other PET grades are manufactured for packaging films, as well as for the production of video and audio tapes. These PET types are often standard grades with an IV of 0.64 dL/g. To reduce the sticking tendency of the final product, solid additives such as Si02 or china clay with specific particle sizes and particle-size distributions are incorporated by master-batch processes. The final product, the so-called BOPET, is a biaxial oriented PET film with high mechanical strength and a thickness between 1 and 180 im. 

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. 

The state of the art is a combination of linear low-density polyethylene (PE-LLD) with hmestone for diaper back sheets. By biaxial orientation, the balance of mechanical permeability requirements can be obtained. Because of its PE-like characteristics Ecoflex is a good choice for a biodegradable back sheet. Ecoflex provides a good regulatory basis for these applications because of the positive results of the primary skin irritation test according to OECD 404, as well as the guinea pig test according to OECD 406. 

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. 

Film can be produced either by extrusion tubular blowing or flat process. Each has its advantages and disadvantages. These processes result in film with a molecular orientation predominantly in the machine direction (MD). As reviewed later, orienting the film can be in two orthogonal directions that develop superior optical, mechanical, and physical properties. The process is known as biaxial orientation and it can be applied to both tubular and flat film. 

The stretching mechanism of an equal biaxial stretching machine was modified. Tests with plasticised PVC showed that the machine could produce oriented sheets under different stretching modes - uniaxial (constant width), simultaneous (equal and unequal) biaxial and sequential. If plasticised PVC was biaxially oriented to the same draw ratios by different stretching modes, similar enhancements of mechanical properties occurred. 12 refs. 

Tang, H.I. Hiltner, A. Baer, E. Biaxial orientation of polypropylene by hydrostatic solid state extrusion. Part III. Mechanical properties and deformation mechanisms. Polym. Eng. Sci. 1987, 27 (12), 876. 

The effects of orientation on the mechanical properties of polymers at both small and large deformations depend on the mode of orientation, which determines the preferred average chain alignment. For example, the mechanical properties obtained after uniaxial orientation (which biases the chain end-to-end vectors in one favored direction) differ from those obtained by biaxial orientation (which biases these vectors in two favored direction). Furthermore, the mechanical properties obtained after simultaneous equibiaxial orientation (where orientation in the two favored directions is imposed simultaneously, at equal rates, and to equal extents) often differ from those obtained after sequential orientation in the two favored directions, as well as after orientation by different amounts and/or at different rates in those two directions. See Seitz [35] for a review of the effects of uniaxial and biaxial orientation on the fracture of polystyrene, which fails by brittle fracture or crazing, under uniaxial tension and impact. 

Monoaxially and biaxially oriented films of fluoropolymer are made by melt extrusion of the resin into flat webs or tubes. The main function of orientation is to enhance the mechanical properties of the film such as tensile break strength and tear resistance. The decision to orient is usually made according to the requirements of the end use for mechanical properties. All process surfaces that contact molten fluoropolymers must be corrosion resistant because of the formation of corrosive compounds such as HF and HCl from the high-temperature degradation of these plastics. 

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