Plastic Derived Bitumen Modifiers (w-Binder) from Pyrolysis in Sustainable Road Construction

There are current largescale efforts within the paving industry to move towards the use of sustainable binder alternatives to bitumen, which is a nonrenewable and highly impacting resource. The use of waste plastics as binder materials within asphalt concrete is considered as a practical and cost-effective alternative, especially as the growth of new recycling capacities is becoming more crucial. Furthermore, plastic-derived bitumen modifiers from the thermochemical treatment of plastics could be a viable solution to the current limitations associated with plastic bitumen modifiers (PMB), while producing asphalt with enhanced rheological properties and failure resistances. This study provides a novel contribution to outlining the potential of highdensity polyethylene (HDPE) thermal pyrolysis waxes in the modification of bitumen (w-binder) and subsequent hot mix asphalt (HMA) mixtures (w-asphalt), as well as in reclaimed asphalt (RAP) rejuvenation. In the interest of product and process optimisation, it establishes key relationships between pyrolysis process parameters, the chemical and thermal properties/ mechanisms of the wax modifiers and the rheological and mechanical performance of the modified binders/mixtures. Finally, dense graded asphalt concrete modified with an optimal HDPE pyrolysis wax (6 wt% of the binder) and 20% RAP was produced and its resistance to key pavement deterioration modes was determined. The optimal wax was produced at higher pyrolysis temperatures and nitrogen flowrates (having the lowest vapour residence times.) Such process parameters had a crucial role in the resultant wax chemistry and thermal ageing behaviours. Oxidation and polymerization reactions were key mechanisms identified during wax thermal ageing and their effect on the resultant binder and mixture properties were highlighted. The asphalt mixtures produced had enhanced or unaffected resistance to the key failure modes studied, with the RAP + HDPE pyrolysis wax mixture showing superior performance. The HDPE pyrolysis wax acted as a sufficient rejuvenating agent to mitigate the otherwise adverse effects to fatigue resistance of high RAP content in HMA mixtures. This application of plastic pyrolysis wax could help to reduce the amount of nonrenewable materials used for HMA production, increasing the usage of recyclable and secondary materials within flexible pavements in the effort to approach a circular economy

Pyrolysis of polyolefin plastic waste and potential applications in asphalt road construction: A technical review

Pressures on the current plastic waste management infrastructure has made the growth of new sustainable recycling capacities crucial. Pyrolysis is an emerging thermochemical technology that may be utilised at a large scale to aid in reaching the EU 2030 targets for plastic waste. Plastic valorisation via this process could gain increased competitiveness with conventional methods through the use of concepts such as ‘Design for Recycling’, identifying further marketable applications for pyrolysis end co-products. This paper presents a review on the pyrolysis of the most abundant plastic waste polyolefins, low-density polyethylene (LDPE), high-density polyethylene (HDPE) and polypropylene (PP), with a focus on the heavy wax products. A sizeable research gap in its known applications outside of the petrochemical and chemical feedstock industries was identified. Furthermore, its potential use in the hot mix asphalt (HMA) layers of flexible roads as an alternative binder material and aggregate is discussed. A plastic-derived bitumen modifier could be a viable solution to the current limitations associated with plastic bitumen modifiers (PMB), while producing asphalt with enhanced rheological properties and failure resistances. Consequently, future trends in research may include obtaining a full understanding of the capacity for pyrolysis products from waste polyolefins in bitumen modification. The key relationships between pyrolysis process parameters and the subsequent product properties, modification mechanisms and binder performance may also be explored. This application pairing process for pyrolysis products from plastic wastes may also be more extensively adopted in sustainable infrastructure, as well as other industries

Investigation of high-density polyethylene pyrolyzed wax for asphalt binder modification: Mechanism, thermal properties, and ageing performance

The thermal pyrolysis of high-density polyethylene in a fixed bed reactor has been studied in the temperature range of 450–550 °C with two different nitrogen carrier gas flowrates, 2 and 4nullLnullmin−1, to study the effect of these process parameters as well as the resultant vapour residence times on the formation of wax and its chemical and thermal properties. The technology had a high selectivity to waxes, with a yield of up to 91.87% wax from high-density polyethylene at 500 °C using a nitrogen carrier gas flowrate of 4nullLnullmin−1 and subsequent 1.76 second vapour residence time, calculated using the ideal gas law. The waxes were characterised using techniques including gas-chromatography-mass spectroscopy (GC-MS), Fourier transform infrared spectroscopy (FTIR), thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). The process operating temperature especially and its subsequent effect on vapour residence times within the reactor had a considerable impact on both the chemical and thermal properties of the waxes. Higher operating temperatures yielded more olefinic waxes due to the promotion of degradation radical mechanisms such as β-scission. They were observed to have higher melting points and thermal stability. An investigation was conducted to assess the thermal properties and ageing performance of the waxes. Thermal conditioning in an ashing oven at 170 °C for 0–6 hours was conducted with a detailed analysis of GC-MS and FTIR at each stage of thermal exposure to further support thermal characterisation results. The changes in chemical composition were attributed mainly to oxidation and polymerization ageing reactions and were seen to be more prominent in the more unsaturated waxes produced at higher pyrolysis temperatures. The wax produced at 550 °C was determined the optimal wax for binder modification in hot-mix asphalt pavement design due to lower volatile/mass loss. A lower temperature range was suggested for optimal blending conditions to further reduce loss of volatiles with initial blending and storage