Cooling biaxially oriented

A newer development is the manufacture of bi-axially oriented bottles, in particular for PETP. The pre-form is injection- moulded and rapidly cooled in the mould it remains amorphous (PETP crystallises very slowly) by heating above its Tg (65°) it passes into the rubbery state, and can then be blown-up and, simultaneously, longitudinally stretched. The biaxial orientation thus obtained, accelerates the crystallisation, and, at the same time, results in a very fine crystalline texture, so that a thin-walled, strong, transparent and heat-resistant bottle is obtained. 

Different methods are used. An online ordinary common blown or cast film line uses a machine direction orienter (MDO) on the front end of biaxially oriented film heated chamber extrusion line. If only the machine direction is to be stretched, a series of precision controlled heated rolls can be used. Film is fed through a series of rolls where it is sequentially heated, drawn around rolls that increase in rotational speed providing the stretching action, annealed around larger diameter roll(s), and cooled on a final roll(s). 

Materials. Biaxially oriented polypropylene (PP) films of 50 um thickness were obtained from 3M and have been described (2). PMDA-ODA (PI) was Kapton H polyimide from Dupont. Copper-plated PTFE films were obtained from Spire Corporation (Bedford, MA). They were prepared using the Ion Beam Enhanced Deposition (IBED) process in which a 100 nm thick Cu film was vapor-deposited onto a PTFE substrate in the presence of a beam of 400 eV Ar+ ions of 25 uA/cm2 (IQ). Shortly before SIMS analysis, the Cu film was removed slowly by peeling at 90° in ambient conditions. Metal-coated PI films were prepared by sputtering 50 nm Cr and 1 um Cu onto a 50 um thick Kapton film on both sides. Thermal annealing was performed in a vacuum chamber at 2xl0 6 torr using a quartz lamp as the heating source. The samples were held for 15 min at the desired temperature and then cooled down to ambient temperature inside the chamber for about 2 hours. Just prior to SIMS analysis, the metal films were peeled slowly at 90° and then immediately introduced into the vacuum chamber of the instrument. 

Tubular film may be biaxially oriented by passing cooled film through a pair of nip rolls, reheating it to a uniform temperature and then passing it between a further pair of rolls. Between the two pairs of rolls a mandrel of air is introduced in order to expand the film. The process is similar to that shown for poly(vinylidene chloride) in Figure 17.4. 

The double-bubble process involves the extrusion and blowing of a tube of molten plastic in a downward direction. The tube is then cooled, most often using a water bath, reheated to just below the melt temperature, and reinflated. The reinflation along with the increase in haul-off speed provides biaxial orientation. Typically the next step is annealing to relieve thermal stresses and stabilize the film. The double-bubble process is most often applied to PP film, but is also used with multilayer PP or PE-based films. One of the major advantages is that this process can deliver a high-clarity film with precise shrink characteristics and very uniform flatness. 

A new generation antifog additive has been introduced recently to control the formation of water droplets in food packaging of biaxially oriented PP (BOPP) and cast PP, which offers control of droplet formation in packaging for pre-cut salads and other fresh produce, as well as in hot food packaged before it cools down, such as fresh bread and meat cooked in a store. 

The term stretching is used to denote separate operations carried out after reheating of the already cooled extrudate or calendered sheet. It is mostly performed at lower temperatures than drawing and therefore requires higher stresses. Both drawing and stretching can be used to obtain monoaxial or biaxial orientations. 

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. 

Storage Keep tightly closed store in cool dry place Uses Impact modifier for engineering thermoplastics adhesion modifier for rubber compds., TPOs, adhesives adhesive for biaxially oriented nylon and PC films textile adhesive food-pkg. adhesives, paper/paperboard Features Bonds at low temps. 

Production of flexible packaging begins with extrusion of plastic resin to yield sheet or film. For film, two main processes are used. In the cast-film process, the resin is extruded through a slit-shaped die onto a water-cooled chrome roUer, where it is solidified. After the edges are trimmed off, the film is wound onto a roll. The film may be oriented by stretching it in the machine direction (uniaxi-ally oriented) or in both the machine and cross directions (biaxially oriented) to improve mechanical and barrier properties. This process is also used to produce plastic sheet for thermoforming or other applications. 

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