Hot-pressed boards based on recycled high-density polyethylene tetrapack: Mechanical properties and fracture behavior

In this research, the mechanical properties and fracture behavior of composites based on recycled high-density polyethylene and recycled Tetrapack have been investigated. The matrix and filler were recovered from landfills, ground into flakes of ∼1.6 cm² size, washed and physically mixed before putting the mixture in a cast, introduced in an oven at 250℃, and pressed applying 1 Metric Ton pressure. Mixtures with varying concentration of tetrapack flakes were prepared. Mechanical properties such as Young’s modulus, yield stress, and ultimate tensile stress were obtained from uniaxial tensile deformation tests carried out at room temperature. The results showed that the tetrapack flakes were effective reinforcers, increasing the Young’s modulus and yield stress relative to neat high-density polyethylene. However, it was also found that the filler acts as stress concentrator where mechanical failure initiates. Scanning electron microscopy showed that fracture of the composite occurred mainly by the lack of adhesion between polymer matrix and filler. Moreover, a percolation threshold was reached at about 5% g/g concentration of tetrapack, beyond which mechanical properties are severely compromised. Interestingly, this investigation also showed that the color of the high-density polyethylene flake, that is, the pigment, strongly influences the mechanical properties of the composite. Thus, boards hot-pressed from individual colors were also investigated. The results showed that pigments, like those used in gray color, favored higher degree of crystallinity, as measured by differential scanning calorimetry, and therefore higher Young’s modulus.