Machine and transverse direction

Anisotropic material In an anisotropic material the properties vary, depending on the direction in which they are measured. There are various degrees of anisotropy, using different terms such as orthotropic or unidirectional, bidirectional, heterogeneous, and so on (Fig. 3-19). For example, cast plastics or metals tend to be reasonably isotropic. However, plastics that are extruded, injection molded, and rolled plastics and metals tend to develop an orientation in the processing flow direction (machined direction). Thus, they have different properties in the machine and transverse directions, particularly in the case of extruded or rolled materials (plastics, steels, etc.). 

In an anisotropic material, the properties depend on the direction in which they are tested. For example, rolled metals, which are anisotropic, tend to develop a crystal orientation in the rolling direction. Thus rolled and sheet-metal products have different mechanical properties in the two major directions. Also, extruded plastic film can have different properties in the machine and transverse directions. These materials are oriented biaxi-ally and are anisotropic. (As reviewed above under EXTRUSION, Orientation). 

A further peculiarity of conventional M/E membranes is that autoclaving produces uneven shrinkage between machine and transverse directions with the result that a disk which is circular before autoclaving becomes somewhat elliptical after auto-... 

Inasmuch as all samples were perfectly round 47 mm disks before "dry" autoclaving, the standard deviation of the measured diameters after autoclaving is a measure of ellipticity,/.e. uneven shrinkage in machine and transverse directions. 

We will use readily available plastic films to demonstrate stress-strain behavior. Students should be able to relate the physical behavior of thin films to the concepts of orientation and crystallinity. They should be able to explain terms such as cold drawing, yielding, and machine and transverse directions. 

In the intermittent contact mode phase images shown in Fig. 3.74 a clear contrast between the constituent phases can be observed both for the compression-molded hmsPP (panel a), as well as for the blown film (in the machine and transverse direction in panels band c, respectively). The soft EPR phase appears with dark phase contrast indicating that a substantial amount of energy is dissipated in this phase compared to the PP phase. The EPR particles measure 1 pm or smaller in diameter dispersed in the polypropylene matrix. The horizontal lines and the occasional vertical streaks in the images are artifacts of microtoming. 

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. 

Thread sealant tape, on the other hand, must perform after it has been applied to the pipe thread. Lower density enhances the drapability (deformation) of the tape around the threads. A balance between the tensile strength in the machine and transverse directions in the range of 15 1 d is required to achieve the desirable deformability. Elongation of 100-200 in the transverse direction and better than 800% in the machine direction are desirable. The amount of fibrillation determines the tensile properties and deformability of the tape. Too little fibrillation will yield a tape with insufficient tensile strength while too much fibrillation will create a hard tape lacking enough deformation. 

Orientation Index (01) - This is a factor that characterizes the orientation of polymers in polytet-rafluoroethylene tubing in machine and transverse directions. Orientation index (01) provides a numerical value to monitor the disparity of orientation ... 

MD and TD referes to machine and transverse directions film properties. 

The most common manufacturing method for BOPP is the fiat tenter stretching process where a thick sheet is first cast, then conditioned for subsequent drawing. Machine and transverse direction stretching are performed on-line and can be adjusted to secure the required orientation balance. ... 

FIGURE 21.30 Comparison of tear strength for hydrated and dry Gore-Select membranes (25 om) and Nafion 112 membranes in machine and transverse directions. (Reproduced from Cleghorn, S. et al., in Handbook of Fuel Cells Fundamentals, Technology and Applications, Vielstich, W., Gasteiger, H.A., and Lamm, A., eds., Vol. 3, John Wiley Sons, Chichester, U.K., 2003. With permission.)... 

In this way states of biaxial orientation are produced the ratio of the orientations in machine and transverse directions can be controlled within certain limits by adjusting the blow-up ratio with respect to the drawdown. Moreover, the overall level of orientation can be increased by decreasing the melt temperature and increasing the cooling rate. 

One approach to characterising the molecular orientation in both uniaxially and biaxially oriented samples of PET makes use of the ratio of the absorption bands near 1250 and 1725 cm, the first of which shows parallel dichroism and the second perpendicular dichroism. An equation is developed that relates this ratio to the molecular orientation with respect to the direction of measurement. Thus, it is possible to determine individually the orientation functions with respect to the machine and transverse directions (131). 

Figure 20.4 shows the SEM of the Nitto Denko separator that resembles more of a nonwoven texture similar to the SEM of a separator sample made through a wet process. The Nitto Denko separator has more shrinkage in both machine and transverse directions. The separator shows the shutdown behavior when heated close to melting temperature of the polyolefin, but the melt integrity is poor. 

The most popular method used for quality control uses a pendulum device (Fig. 6.3) to measure the resistance to tear propagation (ASTM D1922). For this test, specified test samples are die cut and include a slit at one end. The tabs formed on either side of the slit are clamped into the device so that when the pendulum is released a so-called trouser leg tear force is applied along the specimen in the direction of the slit. The test device reports the maximum resistive force carried in the film. Commonly, ten specimens in both the machine and transverse direction of the film are tested. 

In many film applications the product is subjected to puncture or impact loads. These types of loads are applied perpendicular to the plane of the film. Therefore, the stresses act biaxially (in both the machine and transverse directions simultaneously) and are not represented well by a uniaxial test. Impact resistance tests are designed to biaxially load the film to measure its energy absorbing capability. 

The most commonly measured mechanical properties for films are tensile strength, tear strength, and impact resistance. The first two are generally measured in both the machine and transverse directions (MD and TD). The last one is a biaxial test, examining MD and TD simultaneously. 

Compare the molecular orientation and the film tensile and tear strengths in the machine and transverse directions. 

Linear low-density polyethylene, LLDPE, was blended with 2-25 wt% LDPE to give films with large difference of the tensile strength in the machine and transverse direction... 

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