Mechanical properties biaxial

One of the main methods for improving the mechanical properties of linear polymers is their drawing that can be uniaxial (fibres), biaxial (films), planar symmetrical (films-membranes) etc. As a result of polymer deformation, the system changes into the oriented state fixed by crystallization. 

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.)... 

Uniaxial deformations give prolate (needle-shaped) ellipsoids, and biaxial deformations give oblate (disc-shaped) ellipsoids [220,221], Prolate particles can be thought of as a conceptual bridge between the roughly spherical particles used to reinforce elastomers and the long fibers frequently used for this purpose in thermoplastics and thermosets. Similarly, oblate particles can be considered as analogues of the much-studied clay platelets used to reinforce a variety of materials [70-73], but with dimensions that are controllable. In the case of non-spherical particles, their orientations are also of considerable importance. One interest here is the anisotropic reinforcements such particles provide, and there have been simulations to better understand the mechanical properties of such composites [86,222],... 

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. 

Polyolefin foams are easier to model than polyurethane (PU) foams, since the polymer mechanical properties does not change with foam density. An increase in water content decreases the density of PU foams, but increases the hard block content of the PU, hence increasing its Young s modulus. However, the microstructure of semi-crystalline PE and PP in foams is not spherulitic, as in bulk mouldings. Rodriguez-Perez and co-workers (20) showed that the cell faces in PE foams contain oriented crystals. Consequently, their properties are anisotropic. Mechanical data for PE or PP injection mouldings should not be used for modelling foam properties. Ideally the mechanical properties of the PE/PP in the cell faces should be measured. However, as such data is not available, it is possible to use data for blown PE film, since this is also biaxially stretched, and the texture of the crystalline orientation is known to be similar to that in foam faces. 

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. 

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). 

Billiar, K.L., and Sacks, M.S. (2000) Biaxial mechanical properties of native and glutaraldehyde-treated aortic valve cusp Part II - A structural constitutive model. Journal of Biomechanical Engineering 122, 327-335... 

Fig. 13.39 Mechanical properties of biaxially (helicoidally) oriented PS. [Reprinted by permission from K. J. Cleereman, Injection Molding of Shapes of Rotational Symmetry with Multiaxial Orientation, Soc. Plast. Eng. J., 23, 43 (1967).]...

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. 

Kim, J. C., Cakmak, M. and Zhou, X., Effect of composition on orientation, optical and mechanical properties of biaxially drawn PEN and PEN/PEI blend films. Polymer, 39, 4225-4234 (1998). 

External factors that will influence polymer mechanical properties are temperature or thermal treatment, temperature history, large differences in pressure, and environmental factors such as humidity, solar radiation, or other types of radiation. The mechanical properties of polymer are also sensitive to the methods and variables used for testing, such as strain deformation as well as the rate at which the strain is performed. Finally, the mechanical behavior of polymeric materials and the values of their mechanical properties will be sensitive to the kind of strain that is imposed by the applied force, namely, tension, compression, biaxial, or shear. 

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