Transmission Efficiency of plastic Films Part 1

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TRANSMISSION EFFICIENCY OF PLASTIC FILMS The lower tendency towards delaminating is possibly due to the fact that the interfacial surfaces of coextruded films do not oxidize because they never come into contact with the air. Normally such oxidation does occur on the surface of any polymeric film as it emerges hot from the extruder. Tiny crystals are then formed on the surface and these interfere with the subsequent bonding during lamination. The position is not always so simple however during early work on producing coaxially extruded laminates of such dissimilar films as polyethylene and nylon, difficulties in bonding were encountered. The problems were solved as mentioned earlier by introducing a gaseous oxidative medium in the annular space between the coaxially extruded plies. The paradox may perhaps be explained by the fact that in the case of the coextruded film the bonding occurred between the two oxidized surfaces while the polymers were still molten so that no actual crystals were formed. Another way of overcoming difficulties in bonding two particularly incompatible films is to use ‘bridge’ materials which are compatible with both of the former materials. In some cases this may involve a graduated ‘bridge’ leading to finished laminates of five or seven materials. This sort of approach is only possible economically because of the very thin layers which are possible using the co extrusion process. The advantage of flexibility in obtaining a wide range of properties depends on the possibilities (already mentioned) of producing very thin component plies. Apart from the obvious factor of cost there is often a limit to the thickness of a laminate which is set by handle- ability (stiffness etc.) or even clarity. In such cases the number of different properties which can be built-in by using different single plies is also limited. There are also disadvantages to the use of co extrusion. One of these is the difficulty of utilization of the scrap produced during the extrusion process. When extruding a single ply film, a fair proportion of scrap film can be fed into the extruder and used again since with most plastics there is no danger of chemical degradation With co extrusion, however the problem is that a mixture of polymers is produced with no intervening stage and separation of the plies even when possible would not be economic. This disadvantage will of course be aggravated in the case of short runs. Co extrusion will therefore be best for long runs with steady running conditions. On the other hand one can visualize certain composite films where the trim could be used to produce a blended core with more specialized polymers as the outer plies. There is of course a limit to this type of scrap utilization. Another disadvantage is the fact that laminates with sandwich print cannot be produced. This is a particular disadvantage in food packaging when a scuff-proof decoration may well be required but where printing cannot be on the inside of the ply in contact with the food. 10
2.3 BLOWN FILM EXTRUSION: TRANSMISSION EFFICIENCY OF PLASTIC FILMS A typical set-up for blown film extrusion is shown below, in this instance the molten polymer from the extruder enters the die from the side but entry can also be affected from the bottom of the die. Once in the die the molten polymer is made to flow round a mandrel and emerges through a ring shaped die opening in the form of a tube. The tube is expanded into a bubble of the required diameter by an air pressure maintained through the centre of the mandrel. The expansion of the bubble is accompanied by a corresponding reduction in thickness. The pressure of the air is kept constant in order to ensure uniform thickness and width of film. Other factors that affect film thickness are extruder output, haul-off speed and temperatures of the die and along the barrel. These must also be strictly controlled. As with any extrusion process film blowing becomes more as speeds are increased. The limiting factor here is the rate at which the tubular extrudate can be cooled. Cooling is usually achieved by blowing air against the outside surface of the bubble. Under constant air flow conditions an increase in extrusion speed results in a higher ‘frost’ line (the line where solidification of the extrudate commences) and this leads to bubble instability. Increasing the air flow gives more rapid cooling and lowers the ‘frost’ line but this is limited in its application because too high a velocity of the air stream will distort the bubble. Various designs of air cooling rings have been worked 11
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TRANSMISSION EFFICIENCY OF PLASTIC
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4. ANTI-STATIC ADDITIVES TRANSMISSI
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Butyl, GRS, polyurethane, neoprene
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Transmission Efficiency of plastic Films Part 1

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