Best practice: Development of a recyclable, paper-like film Combination of blown film and foam extrusion
© IKVFlexible plastic packaging is often made of polyolefins. These materials are increasingly being filled with high proportions of calcium carbonate to produce so-called “paper-like films”. These films offer ecological and functional advantages, as they save fossil raw materials, have a natural look and improved printability and are similar in feel to paper. They are mainly used in the packaging of fresh produce, butter and margarine as well as for labels and carrier bags.
A key problem with these highly filled films is their increased density of over 1 g/cm³, which means that they cannot be recognized as polyolefins in the sink-float process and therefore cannot be returned to the plastics cycle. As this sorting process is based on density separation, highly filled films sink and are not identified as recyclable plastics.
A process combination of blown film and foam extrusion could be a solution to reduce the density of the film without compromising its desired mechanical properties too much.
Schaumbildung zur Dichteverringerung
To improve recyclability, a combination of blown film and foam extrusion processes is used. A chemical blowing agent is added to the melt in order to reduce the density of the middle layer and thus lower the overall film density to below 1 g/cm³. The challenge is to maintain the mechanical properties of the film, in particular tensile strength and dead-fold behavior.
Optimierung der Schaumstruktur
Three key parameters influence the film structure and properties:
Propellant content
An optimum quantity must be found to ensure a homogeneous cell structure. Too little blowing agent leads to insufficient foam formation, while too much creates large, unstable bubbles and greatly reduces the viscosity of the material.
Molecular weight and side chain length
These influence the melt strength and the elongation behavior during extrusion. Long side chains increase stability and lead to finer foam formation.
Mass flow ratio of the layers
The foam profile can be influenced by varying the throughput of the individual extruder layers. A high addition of mass in the middle layer improves the homogeneity of the cell structure, but can impair the mechanical properties.
Herstellung der optimierten Folien
In the extrusion pilot plant, a series of tests is being carried out involving the extrusion of three-layer films with a foamed middle layer made of PE-HD and PE-LD.
Various propellant quantities and material combinations are tested. The aim is to produce a stable film structure that remains mechanically resilient and has a density of less than 1 g/cm³.
Analyse und Ergebnisse
The tests show that the optimized combination of parameters can successfully reduce the density without significantly impairing the functionality of the film:
Reduced density:
The highly filled film achieves a density of less than 1 g/cm³ through foaming and can therefore be sorted using the float-sink method.
Mechanical properties:
The tensile strength initially decreases due to foaming, but improves with a finer cell structure.
Dead-fold behavior:
By specifically modifying the material composition, the dead-fold angle was optimized to 86.5° so that the film retains its paper-like properties.
Conclusion
The combination of blown film and foam extrusion offers an innovative solution to the problem of recycling paper-like films. Through targeted process adjustments, sustainable packaging materials can be created that meet both the needs of the industry and the requirements for recyclability.
Dr.-Ing. Lisa Leuchtenberger-Engel
Head of the Extrusion and Rubber Technology Department
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