Processing and characterizing repulping reject of liquid package board

Liquid package board (LPB) packages typically have the smallest global warming potential compared to other package solutions, but recycling of the integrated plastic and aluminum fractions of the packages is still limited. In the fiber recycling process the plastic and aluminum fractions with some residual fibers are concentrated to the repulping reject (Polyal). This study investigates composition and properties of repulping rejects from recycling of separately collected LPB packages. The rejects were modified in different ways, hot pressed, grinded and finally compounded with a twin screw extruder. Compositions of the compounds were characterized with Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA) and their properties were studied with tensile testing and melt flow rate (MFR) measurements. All studied compounds had relatively good processability and mechanical properties. Properties of compounds were improved by extraction of residual fibers and melt filtering of polyethylene terephthalate (PET) flakes. Especially melt filtering offered a significant improvement in strain at break from 11% to 33%. It was also found that high density polyethylene (HDPE) should be extracted from the reject and recycled separately since the extraction is relatively easy, does not cause drastic changes in the compound properties and recycled HDPE has more potential applications than the Polyal compounds.