# Using Plastic Waste in a Circular Economy Approach to Improve the Properties of Bituminous Binders
## 1 Introduction  
This manuscript refers to a part of a broader research project that was carried out to validate an initial hypothesis: would it be feasible and beneficial to use a waste-derived by-product, resulting from a sustainable production of middle and heavy hydrocarbons (chemical recycling), for the improvement of some properties of different binders used in bituminous mixtures for flexible pavements? Here is described a part of the laboratory work to confirm some aspects of this hypothesis. It started with the production and characterization of waxes resulting from pyrolysis, a thermochemical process that can be used, for example, to produce liquid fuel as an alternative to refine it from fossil sources (in a specific reaction system, developed at the IST--University of Lisbon--Portugal). The waxes were produced by thermal cracking (pyrolysis) of high-density polyethylene (HDPE) recovered from plastic waste. However, it is generally assumed that they can also be produced via co-processing with other plastic residues such as cable insulation (originating in waste from electric and electronic equipment (WEEE)), and rubber from end-of-life tires. The produced waxes were added to four standardized types of binders (all paving bitumen), and the effect on their properties was assessed. In the first phase (described in this manuscript), the considered experimental program included, among other tests, the following: thermogravimetric and spectroscopic analysis, determination of the needlepenetration and softening temperature, testing of dynamic viscosity, and assessment of the rheologic properties of several materials and blends. Among other goals, the one mentioned in ninth place in the U.N. 2030 Agenda for Sustainable Development ([[https://sdsg.un.org/2030](https://sdsg.un.org/2030) agenda]([https://sdsg.un.org/2030](https://sdsg.un.org/2030) agenda), accessed on 23 February 2022) defines short/medium-term objectives centred on building more resilient infrastructure, fostering innovation, and promoting sustainable industrialization. In this sense, as explained by [PERSON] et al. [1] when referring to the road infrastructure pavement sector, an appropriate contribution to the fulfilment of this goal can be given by the incorporation of different materials (including waste) that improve the performance of bituminous mixtures. These materials comprise not only a variety of polymers [2] (such as styrene-butadiene-styrene (SBS), poly(ethylene terephthalate) (PET), high-density polyethylene (HDPE), low-density polyethylene (LDPE), polystyrene (PS), polyurethane (PU), polypropylene (PP), cross-linked polyethylene (XLPE or PEX), acrylonitrile butadiene styrene (ABS), ethylene-vinyl acetate (EVA), styrene-isoprene-styrene (SIS), and others), Sulphur, mineral acids, and sustainable polymers from recycled waste plastics [3] but also different types of waves (natural or synthetic). Bituminous binders may already contain waves, present in the oils that gave rise to them, but their characteristics are different from PE waxes, and their effects are also diversified [4]. Usually, PE waxes offer good chemical and water resistance, excellent distribution and fluidity, high softening points, and better chemical stability than those resulting from oil refining [5]. As recently highlighted by [PERSON] et al. [6], who studied the use of chars (from post-waste plastic processing) in civil engineering, pyrolysis is a sustainable answer to convert some wastes into fuels (liquids and gases). They pointed out that pyrolysis \"_does not produce toxic soot_\" (which includes polycyclic aromatic hydrocarbons (PAH)), can offer an excellent calorific potential (around 40 MJ-kg-1, comparable with conventional fuels), and the waxes can be used as \"_viscosity-reducing materials_\", especially in polymer-modified bitumen (PMB) (reducing heating costs in asphalt plants). In this regard, other authors found similar calorific values in waxes obtained in pyrolysis of HDPE (up to 45.4 MJ-kg-1) [7] and waste polypropylene (about 43 MJ-kg-1) [8].