Railway sleepers made from recycled plastics?

In rail, greenhouse gas (GHG) emissions mainly come from the energy consumed for transport and the materials used in infrastructure – steel and concrete having large carbon footprints. According to a research team from the University of Tampere (Finland), an alternative in the field of construction and maintenance could come from recycled plastics (usually incinerated).

To demonstrate this, first author Heikki Luomala and his two colleagues Rami Halme and Ilari Jönkkäri chose to focus on railway sleepers. Indeed, these represent a significant quantity of material with a lifespan of up to 60 years. The sleepers therefore have a definite impact on GHG emissions. The idea of ​​the three Tampere scientists is described in detail in the journal Frontiers in Sustainability dated December 12, 2024.

The stages of production of plastic sleepers

The plastics studied in the new study come either from electrical and electronic equipment, or from waste from the recycling of liquid packaging cartons. In the first case, once the metal is removed, the majority plastic is acrylonitrile butadiene styrene (ABS). In the second case, the cardboard repulping process separates the cellulose fibers from the aluminum and plastics. This waste contains polyethylene (PE), propylene (PP), ethylene vinyl alcohol and polyethylene terephthalate (PET) as the main plastics. The production of test sleepers follows six distinct steps: compression, milling, extrusion, granulation, drying and injection molding. First, compression is carried out at 200°C for approximately 4 minutes. Milling then removes the excess material. The two ribbons of recycled plastic leaving the extrusion are then cut to obtain small granules. These are dried at 70°C for almost 2 hours. Finally, the procedure ends with injection molding (at 210°C for 10 seconds for cardboard repulping rejection, and at 230°C for 13 seconds for ABS). At the end of the line, the researchers receive sleepers made of recycled plastics that are 2.6 meters long, 25 centimeters wide and 15 centimeters thick.

The tests undergone by these sleepers were primarily aimed at verifying that they could keep their shape despite the pressure of a load and strong increases in temperature. In this game, the ABS sleepers were the most convincing.

Indeed, while the sleepers resulting from cardboard repulping rejects tended to soften at 55°C, the ABS sleepers did not move and even showed a solidity and hardness almost three times greater. According to the authors of the study, these could therefore be adapted to all types of railways. To get an idea of ​​the savings made on GHG emissions, just imagine the recycling of all the waste from repulping cardboard in the form of sleepers (reinforced with steel bars) in Finland in one year. By replacing concrete, they would reduce emissions by 3,610 tonnes of CO equivalent₂. And if ABS recycling were added to this, the annual reduction in emissions could reach 24,200 tonnes of CO equivalent.₂. Until these plastic sleepers are placed on the tracks, future research should carry out test specimens at scale in order to observe their behavior over the long term.