MATERIALS | BARRIER FILM
Under certain conditions, a blend of LLDPE and EVOH can be extruded directly to produce “three-layer films” – but without using tie-layers, says Schulman
resistance – and as part of this, developed a new measuring method. Nova tested a range of three- and nine-layer films, containing recyclable and non-recyclable resins – in- cluding traditional high barrier materials like EVOH, as well as its own Surpass LLDPE and HDPE resins. It found that some formulations using only polyolefins had a grease barrier the was compara- ble with EVOH – and that its HDPE had a particularly strong effect.
EVOH protection Guojun Zhang, an R&D engineer at A. Schulman in the US, told delegates at the same event that multi-layer barrier structures could be simplified if EVOH – the oxygen barrier layer – could be protected in a different way. EVOH (ethylene vinyl alcohol) is commonly used in barrier structures because of its high oxygen bar- rier. However, its sensitivity to moisture means that it must usually be protected within a multi-layer structure of its own – which makes manufacturing the film more complex. Rather than doing this – which usually means
making at least a five-layer structure – he proposed protecting the EVOH by incorporating it into a polymer blend. “This is a traditional method for improving the gas barrier of a polymer – by blending it with another higher barrier polymer,” he said. The approach is different to some previous efforts, he said, as it uses a pre-compounding
process to ensure that the multi-layer morphology forms after resins are extruded into thin films. This is because the choice of polymeric compounds – in this case, EVOH and a highly dispersible polyolefin, with proper compatibility and a viscosity match – ensures that they will form into ‘bands’ when extruded, which approximates to the structure of a multi-layer film. One formulation, called B24, gave similar barrier
properties as actual multi-layered films. This film could easily be embedded into other multi-layer films, said Zhang.
Barrier performance
Barrier film is most commonly developed for food packaging, but it can also be used in agricultural film – to retain species such as fumigants and other chemicals. Frank Balemans, senior regional technical sales manager at Kuraray, told last year’s Agricultural Films conference how barrier properties were vital to total impermeable film (TIF) – which helps to reduce the use of fumigants by ensuring that it is not lost to the environment after dosing. Methyl bromide, for instance, is used as a fumigant but is an ozone-depleting substance. For this reason, its use must be limited. Balemans showed a seven-layer TIF structure, incorporating a middle layer of EVOH. “The EVOH layer should be less than 10% of the total structure to have good mechanical proper- ties,” he said. Tests by the University of California Davis, for
instance, showed that TIF helped to keep methyl iodide soil retention levels at 2,000ppm after 180 hours – where it was around 100ppm for standard LDPE film after the same period. In a similar way, high barrier films can be used to
improve the performance of silage film – prevent- ing the degradation of feed such as wet corn. It can also prevent mould, infestation and aroma loss in harvest such as grains.
This AFM image shows that Schulman’s directly extruded B24 blend (left) has a similar structure to a traditional multi-layer film (right)
22 FILM & SHEET EXTRUSION | May 2018
Nylon boost At AMI’s recent Multilayer Packaging Films confer- ence in Germany, Ube said that it had devised a new approach to retortable – or sterilisable – film,
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