% One entry per reference. The key is what you pass to \cite.
@article{peterson2018,
  author  = {Emma Peterson and Karl Wenzel},
  title   = {Thermal stability of virgin polyethylene grades under nitrogen},
  journal = {Polymer Degradation and Stability},
  volume  = {155},
  pages   = {88-97},
  year    = {2018}
}

@article{ferreira2020,
  author  = {Joao Ferreira and Ines Duarte and Miguel Castro},
  title   = {Residual contamination in post consumer polyolefin streams},
  journal = {Waste Management},
  volume  = {112},
  pages   = {34-43},
  year    = {2020}
}

@article{tanaka2019,
  author  = {Hiroshi Tanaka and Yuki Sasaki},
  title   = {Heating rate effects in the thermogravimetry of polyolefin blends},
  journal = {Thermochimica Acta},
  volume  = {678},
  pages   = {178-186},
  year    = {2019}
}