Tenter process

Orientation of styrene-based copolymers is usually carried out at temperatures just above T. BiaxiaUy oriented films and sheet are of particular interest. Such orientation increases tensile properties, flexibility, toughness, and shrinkability. PS produces particularly clear and sparkling film after being oriented biaxiaHy for envelope windows, decoration tapes, etc. Oriented films and sheet of styrene-based polymers are made by the bubble process and by the flat-sheet or tentering process. Eibers and films can be produced by uniaxial orientation (237) (see Eilmand SHEETING materials). 

Two methods are widely used for producing biaxially oriented PP (BOPP) film. One is the tenter process, and the other is the tubular or bubble process. In both, homopolymer of about 3 g/10 min MFR is most widely used, although random copolymer is used for better heat scalability. 

In the tenter process, extruded sheet is drawn sequentially, first in the machine direction and then in the transverse or cross direction. Draw ratios of 4 x 7 to 6 X 10 are common. Film thickness ranges from 0.5 to 2.5 mils. 

A special kind of BOPP film, known as opaque film, is used for packaging products such as candy, chocolate bars, and soaps and for labels, such as those now used on most soft drink bottles. Opaque film is produced in the tenter process from PP to which a fine filler (e.g., calcium carbonate) or an incompatible polymer has been added. 

Another commercial application for flat-die coextrusion is biaxially oriented multilayer films (11) made with the tentering process to improve mechanical properties. Tentered film is biaxially oriented by stretching in the longitudinal and transverse direction, either sequentially or simultaneously, at uniform optimum temperature. In sequential stretching, the multilayer extrudate is cooled to a suitable orientation temperature on a first set of rolls and then stretched in the machine direction between a second set of rolls which is driven faster than the first set. The uniaxially stretched film then enters a tentering frame, which has traveling clips that clamp the edge of the film. The clips are mounted on two... 

Stretching the film only in the transverse direction may also be accomplished by using a tenter (23). The tenter process uses clips on a chain (Fig. 8). The clips grab each side of the cast web and transport the web through an oven. Once the film is heated to the orientation temperature, the rails that the chains ride on diverge. This method alone is seldom employed in practice. 

Biaxially oriented PP (BOPP) film is a highly transparent, stiff film. The film can be produced using a blown film or tenter process. Stretching in both the machine direction and transverse direction is carried out at a temperature below the melting point of the polymer and results in the partial orientation of molecules in the direction of stretch. The film produced by the blown process is nearly isotropic in both the directions while the film produced by the tenter process tends to be more highly oriented in the direction of machine. The cost of the tenter process equipment is much higher in comparison to blown film equipment. 

Simultaneous tentering is possible, involving complex movements of the film edge grips so that the film is stretched in the machine and transverse directions at the same time. However, the process is mechanically complicated and it is difficult to adjust the balance between the stretch directions, so the two-step tenter process (Fig. 8.12) is the one usually adopted for most films. 

Figure 8.12 Tenter process for biaxially oriented fiim.

This type of film can be produced using a blow film or a tenter process. Stretching in both the machine direction and transverse direction is carried out at a temperature below the melting point of the polymer and results in the partial orientation of molecules in the direction of stretch. The... 

---