Oriented unbalanced

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. 

In conventional tenter orientation, the sequence of steps is as described above (MD—TD). In some cases it is advantageous to reverse the draw order (TD—MD) or to use multiple draw steps, eg, MD—TD—MD. These other techniques are used to produce "tensilized" films, where the MD tensile properties are enhanced by further stretching. The films are generally unbalanced in properties and in extreme cases may be fibrillated to give fiber-like elements for special textile appHcations. Tensilized poly(ethylene terephthalate) is a common substrate for audio and video magnetic tape and thermal transfer tape. 

The fact that both normal and abnormal machine dynamics tend to generate unbalanced forces in one or more directions increases the analyst s ability to determine the root-cause of deviations in the machine s operating condition. Because of this, measurements should be taken in both radial and axial orientations. 

Another configuration, called an unbalanced design, has piston orientations that are neither in-phase nor 180° out-of-phase. In these configurations, the impact forces generated as each piston changes direction are not balanced by an equal and opposite force. As a result, the impact energy and the vibration amplitude are greatly increased. 

Barker and Grimson (1991) modeled the flow of deformable particles after a free-draining floe whose shape, orientation, and internal structure ranged between the extremes of an extended chain and a folded globule. They interpreted the unhindered motions of free-flowing, deformable droplets to result from an unbalanced force imposed by the flow field, resulting in rotations around the particles center of mass this rotation is superimposed on the steady translational motion. 

In Figure 6, one can see, as with amorphous materials, that the more unbalanced the biaxial orientation is the greater are the reductions in permeability. Interpretations of the transport data for biaxially and uniaxially drawn PET samples can be explained by observing conformational changes in the polymer backbone itself. Apparently, the chain packing efficiency of the amorphous phase improves as the number of trans isomers in the ethylene glycol unit increases. Polarized infrared analysis of uniaxially and biaxially oriented systems indicates that the fraction of... 

SVI should be measured in both machine and transverse directions to determine the extent of isotropy in the tube. If the SVI values are very low (< 1 %) in both directions or equal, the sample is isotropic. This means that the sample has been adequately sintered and stresses have been relieved. If the values are both high, sintering has been poor. If the transverse value is high and the machine direction value is low, then orientation is primarily in the machine direction and it is unbalanced. 

In most molecules with symmetrically disposed orbits and paired spins the effects cancel out, but where there is an unbalanced moment a substance shows what are called paramagnetic properties, and orientates itself in line with a magnetic field. The susceptibility, of a body is defined by the relation y = IjH, where H is the applied field and I is the magnetic moment per unit mass. The magnetic moment is called forth by the field in so far as this orientates the permanent moments. Orientation becomes less easy as thermal motion increases, and consequently the susceptibility decreases with increase of temperature, sometimes in accordance with the law jT. 

Biaxially oriented materials can be balanced (stretched the same amount in both directions), or unbalanced (stretched more in one direction than in the other). Unbalanced oriented films will have different values of tensile and tear strength when measured in the machine direction (MD) (the direction of film travel when it is made), than when measured in the cross-machine direction (CD) (perpendicular to film travel). Unbalanced films are the most commonly produced commercially. Stretching of films is expressed as a percent of the unstretched material dimension. For example, a 400% stretch ratio indicates that the film is four times its initial length. 

Ply orientations in a laminate are taken with reference to a particular loading direction, usually taken to be the direction of the maximum applied load, which, more often than not, coincides with the fibre direction to sustain the maximum load, and this is defined as the 0° direction. It is usual to choose balanced, symmetric laminates in design. A balanced laminate is one in which there are equal numbers of-1-0 and - 0 plies a symmetric laminate is one in which the plies are symmetric in terms of geometry and properties with respect to the laminate mid-plane. Hence, a laminate with a stacking sequence 0/90/-I-45/-45/-45/-I-45/90/0, which is written (0/90/ 45), is both balanced and symmetric. Balanced, symmetric laminates have a simplicity of response. In contrast, an unbalanced, asymmetric laminate will, in general, shear, bend, and twist under a simple axial loading. 

LCP film produced by the counter-rotating die tends to curl because of thermally induced stresses in the upper and lower surfaces of the film during melt cooling and solidification. In Figure 11.10, the top is oriented at -1-0 and the bottom at -0 due to the action of the counter-rotating die on the melt. As the LCP film cools, the orientation persists, and the top layer will tend to shrink most in the direction perpendicular to -1-0, while the bottom will shrink most perpendicular to —0. The top surface will be in compression in the -1-0 direction because of the shrinkage in the bottom surface in that direction. Thus, there are unbalanced residual stresses about the midplane of the film that cause curl. The curl is most pronounced when 0 =45°. 

Molecular orientation is another important factor influencing mechanical properties of coextruded films (11). Biaxial orientation can greatly improve film strength. However, uniaxial or highly unbalanced orientation causes poor transverse properties, which result in easy splitting of coextruded films in the machine direction. This tendency may occur even when a relatively thin layer responds to unidirectional orientation and propagates failure to thicker adjacent layers. 

Many polymeric materials consist of dipoles (chemical bonds which have an unbalanced distribution of charge in a molecule) and traces of ionic impurities. If a polymer containing polar groups is heated so that an immobile dipole becomes mobile, an increase in permittivity is observed as the dipole starts to oscillate in the alternating electric field. This effect is referred to as a dipole transition and has a characteristic relaxation time (t) associated with it (76). When exposed to an electric field, the dipoles tend to orient parallel to the field direction and the ions move toward the electrodes, where they form layers. The dipole relaxation time... 

When a liquid phase becomes very thin, both faces of this film interact. The nature of the interactions may be electrical or of shorter range of interaction [55l (attractive van der Waals forces) or even of very short range of interaction (steric repulsive forces between hydrocarbon chains in lipid bilayers or repulsive hydration forces between oriented water molecules around polar heads of molecules merging in aqueous films between two lipid drop or in soap films). When two faces of such films approach one another, repulsive and attractive forces are unbalanced, giving rise to a constraint, corresponding to a non-equilibrium value of the thickness of the liquid film h. Hydrodynamic instabilities of planar films (dielectric or aqueous) have been widely investigated in the last ten years [54] [55] r59l They are... 

ISO 3167 specifically deals with the production of multipurpose test specimens. It requires the use of a balanced, two-cavity mold with a defined gate design to prepare test specimens used to measure properties of plastics materials. Two identical cavities with large gates and a balanced runner system promotes filling and packing consistently and nniform orientation, minimizes shear effects, and produces uniform specimen. Figure 20-2 illustrates differences between old style, multicavity, unbalanced family mold and ISO test specimens (6). 

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