Replacing cast polypropylene with cast PET for thin packaging films may bring product and production benefits according to SML of Austria.
The company has been experimenting with running PET – for which it makes calendering lines – on a converted cast film line and says that tests by its customers on the resulting films “have revealed an interesting potential for various applications and shown qualities superior to CPP film for certain properties.” In particular PET brings a level of transparency, gloss and stiffness which cannot be matched by CPP.
During the tests SML produced PET films from 12 to 300 microns and put them through various conversion processes. Slitting gave no problems on standard slitter-rewinders for CPP films. Cast PET films in a thickness of 17, 20 and 30 microns were slit and rewound to 76 mm wide rolls.
Twist wrap: These narrow rolls were used for the twist-wrapping of confectionery products. A 30 micron film was processed successfully on a Theegarten Pactec line at 1,600 twists/minute. No modifications were needed to switch the line from CPP to cast PET film and SML says it has customers who have already achieved positive results with 25 micron PET film. The advantage of cast PET for twisting is explained by SML as the film’s very good twisting behaviour, without untwisting, which it says is a distinct advantage over CPP film – especially since the cast PET film is a mono-layer structure only.
Metallisation: Metallisation tests with films of 25 to 70 microns produced gloss and adhesion values that were twice as high as those of a standard CPP film. Another advantage is the (naturally) very high level of surface tension which means that no corona pre-treatment is needed before metallisation.
Printing: Many customers judged the film to be readily printable.
Laminating: Laminates with various substrates such as LDPE, OPP and BOPET films, and also with thin sheet metal, were produced without problems.
Sealing: Sealing tests showed that at a given temperature heat seal strength was higher than that of a CPP film.
Perforating: Cast PET film was perforated off-line at 150 m/min, showing potential for air-permeable packaging for bread and pastries (pictured).
The line SML used to produce its thin cast PET films was a full scale machine with a 150 mm 33 D extruder, which ran at 200 m/min (the line is capable of 1,000 m/min) producing 2,700 mm trim width film. Modifications to the line included pre-drying equipment, special temperature regulation on the cooling rollers, improved web guidance through the entire line and a progressive winding technique. SML says that such a machine could also produce CPP, opening scope for a wider product range. The film was wound to mother rolls on SML’s type 1200 turret winder.
The trials so far have been with mono-layer film, but SML says that co-extrusion will be possible in the future.