Machine direction stretching

Poly(ethylene terephthalate) film is produced by quenching extruded film to the amorphous state and then reheating and stretching the sheet approximately three-fold in each direction at 80-100°C. In a two-stage process machine direction stretching induces 10-14% crystallinity and this is raised to 20-25% by... 

The lactone blends also expanded the processing window for film stretching. The highest machine-direction stretch ratio before break for the two lactone-grade films tested was 54 1 and 5 7 1, compared to 5.1 1 for the non-lactone film And in the transverse-direction stretching, the temperature window expanded by 16°C-17°C, compared with 14°C for the nonlactone film. 

This equation is sometimes used instead of the take-up ratio to provide an indication of the machine direction stretching. It represents the degree of thickness reduction from the die gap to the final film, both easily obtainable values. Of course, thickness reduction during processing occurs in the transverse direction as well as the machine direction. 

Increasing the take-off speed increases film length and uniaxial orientation but reduces film thickness. The machine-direction stretching of the film is gauged by the drawn-down ratio (DDR)... 

Subsequently, a second or even more (15) machine direction stretching stations may be used. The web may have to be further heated before it is stretched a second time. This process can produce higher molecular orientation in films than... 

This machine direction stretching process is also used as the first orientation step in sequential biaxial film manufacture. 

SGqUGntial Biaxial OriGntation. The standard process for sequential biaxial orientation is machine direction orientation followed by transverse orientation. This is a continuous process. The machine direction stretch occurs similar to the process described in the monoaxial section (Fig. 5). However, a second draw station is typically not present in this process. In the pure monoaxial orientation case, the process often tries to maximize the molecular orientation in the... 

Heaters will soften the tube and the tube is inflated with air. The initial filling of the tube with air requires good timing by the operator. As air is pumped into the expanding tube, the operator pulls the tube away faster than the top nip supplies cast tube. Once the inflated bubble reaches the bottom of the stretch tower, a second nip closes. This second nip seals the air in the tube. The second nip runs at a speed greater than the first and provides the machine direction orientation. The amount of air pumped into the tube before the second nip closes is one of the primary factors in the transverse direction stretch ratio. Other process variables that contribute to the transverse stretch ratio are the web temperature and machine direction stretch ratio. Pressure in the tube may be increased by narrowing the frame used to collapse the bubble. 

The main difference between the flat film methods is how the clips are accelerated through the tenter. In one commercial process, the clips ride in a screw (30). As the pitch of the screw increases, the clips accelerate in the machine direction (Fig. 11). The clips may be passed from one screw to another. This type of process has large constraints on the machine direction orientation. Once the screw is machined, the machine direction stretch ratio is fixed. Changing from one screw to another can demand excessive time. A machine direction orienter prior to the inlet of the tenter oven may be used with this type of tenter (31). This permutation of this process will allow for changing the overall machine direction stretch ratio without changing the screws. 

The main feature of this type of process is the very high level of flexibility attainable in the machine direction stretch. The strain profile may be linear, exponential, logarithmic, or stepped. The amoimt of stretch can easily be programmed to occur in a given temperature zone in the oven. Changing the machine direction stretch ratio can easily be programmed. It takes just... 

Example No. Width/Thickness of Extruded Film, cm/mm Latent Solvent /wt% Film Temperature at Machine Direction Stretch, °C Fast Roll Speed /Slow Roll Speed /MD Stretch Ratio, cm/min Preheating Air Temperature at Transverse Stretch, °C Width/ Thickness of the Oriented Film, cm/mm Temperature/ Duel Time iu DryiugZone, C/sec... 

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. 

The films and sheets are often anisotropic. Properties are different in the processing and transverse directions. Stretching enhances the performances in the machine direction. 

Heat-shrinkable sheets (thickness 0.040 to 0.120 inch, or 1 to 3 mm) and films (thickness 0.001 to 0.020 inch or 0. 025 to 0.5 mm) are fabricated from many of the same materials as shrinkable tubing.91 They are produced by extrusion as a tube, sheet or blown film. Irradiation is done by the equipment shown in Figure 8.10. Orientation (stretching) after irradiation can be done by several methods, namely by differentially heated and driven rolls (in the machine direction) or by a tenter frame (see Figure 8.11) in the transverse direction. If desired, biaxial stretching can be done. 

Extruded films are sometimes bi-axially stretched, in particular PE and PP films, which gain in strength and transparency by biaxial orientation. The stretching takes place at elevated temperatures, but below the melting point. The edges of the film are gripped by a series of clips, which diverge in the transverse direction and simultaneously accelerate in the machine direction. 

A Qualitative Tests. Hold an unmarked TD strip at each end. Quickly pull the strip apart and observe the behavior Take another strip, and this time start to pull the strip slowly, then release. What do you observe Repeat the pulling and releasing, but each time pull a little harder. Finally, slowly pull the strip, keeping force on it without releasing it. Watch various parts of the strip as you stretch it. It might be instructive to hold each strip up to the light and look through it as you stretch them. Repeat these operations with strips cut in the machine direction. Record your observations and note whether the MD and TD strips exhibited the same behavior. 

If your bag had no visible lines, can you determine the machine direction based on the stretching behavior of films labeled A and B ... 

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