MATERIALS | STRETCH & SHRINK FILM
Above: Efekt Plus used Sabic’s Cohere in stretch film formulations to maintain cling force and cut costs
with similar cling force – despite the lower percent- age – as well as a cost advantage, he said. In a separate project with secondary packaging specialist Maillis, Sabic helped the company create a thin, stiff stretch film using its Supeer 8315 grade. The films were just 10 microns thick, and used Supeer to replace either LLDPE or mLLDPE. The result, said Sabic, was films with higher stretch force, reduced stretching and a constant steep tensile curve. In its third example – a project with ForumPlast and DuoPlast – it used a combination of Supeer and Cohere to replace EVA in stretch hoods, while improving properties at a competitive price. In the original structure, a middle EVA layer, sandwiched between two mLLDPE layers, was replaced by a new structure: two outer layers of Supeer/LDPE, sandwiching a middle layer that combined Cohere, Supeer and LDPE. Some of the advantages of the new design included: higher, more consistent holding force; more stable COF; better protrusion resistance; and higher transparency. Van Hoof added that Sabic is adding m-C6 LLDPE grades to the Supeer portfolio.
Propagation properties Many mechanical properties determine the final performance of pallet stretch film – including maximum elongation, holding force, and the tendency to rupture. Many of them can have a negative effect on one another – which can have a huge influence on cost. “One parameter – tear propagation – if often
forgotten, but improving resistance to this has important benefits,” said Jelle Dendauw, CEO of ESTL in Belgium. For this reason, he said it is important to meas-
ure the mechanical resistance of stretch film after an initial cut or hole has been made in it. This is done in several stages. First, the film is brought to a
22 FILM & SHEET EXTRUSION | July/August 2018
condition of pre-stretch and second stretch, and the line speed is adjusted to zero. The film is then clamped so that it cannot move. A 20mm vertical incision is then made in the film, and the clamp is released. The film is then pulled at 5mm/s until the film breaks. The ‘conclusion’ can be measure in several ways, including: time to breakage (the total time until the film is completely broken); drop time (the time between maximum force and the film breaking); and maximum load factor – which is the film’s maximum force divided by the initial force. Dendauw cited two case studies carried out by
ESTL: one on wrapper performance, and one on pallet stability.
In the first instance, ESTL tested three different films (suppled by SML of Austria), each 23 microns thick. Pre-stretch began at 300% and moved up in 20% increments – in each case repeated the test 10 times, and noting the number of film breaks. The test ended when it became impossible to repeat more than seven times out of 10. Under these conditions, the three films failed at pre-stretch values of 360, 380 and 420%. In the second case, two different 23 micron films
were put through similar stress-strain curves, for wrapping a pallet load of carbonated soft drinks. One film offered much less dynamic deflection under acceleration, which simulated transport conditions. Overall, Dendauw concluded that a higher tear
propagation resistance led to: higher load stability; fewer film breaks for a similar ultimate stretch; and, a higher overall equipment efficiency (OEE) of the wrapper – even in extreme conditions.
Two-way stretch Martin Leinert, of the Fraunhofer Institute for Process Engineering and Packaging (IVV) in Germany, told delegates how his organisation has devised machinery for stretching flat, semi-finished
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