Exergy-Based Assessment of Polymers Production and Recycling: An Application to the Automotive Sector

: In the last century, the economic growth has been accompanied by a worldwide diffusion of polymers for multiple applications. However, there is a growing attention to the environmental pollution and energy consumption linked to the unconditional use of plastic. In the present work, exergy is used as a measure of the resource consumption during the life cycle of polymers. Nine commercially diffused polymers are chosen, and their production chains are identiﬁed according to the “grave to cradle” approach. The global Embodied Exergy (EE) is calculated as the sum of the contribution of each step of the chain, including the production process and the Exergy Replacement Cost (ERC) of the fossil fuel. Then, recycling routes and the associated exergy consumption are analysed. Thermodynamic recycling indexes are developed depending on the ﬁnal product, namely the crude polymeric material and the oil derivatives or structural molecules. The main results show that some commonly used polymers have a considerable impact in terms of EE (e.g., PET). Recycling indexes encourage the recycling processes, which are always energetically convenient (from 10% to 60% of exergy savings) compared with the production from virgin raw material. Results from EE calculation are used for the thermodynamic assessment of the plastic content of vehicle components, to obtain useful information for recycling practices development.


Introduction
Nowadays, polymeric materials are widely diffused in the everyday life of people all around the world. In the last decades, they have become a milestone of the industry and the economy, with a production of 57.9 million of tons in Europe in 2019. This constitutes only 16% of the global production, being Asia the major producer (51%) [1]. The spread of the worldwide use of plastic is strictly linked to the growth of the petrochemical industry. Currently, the major feedstock for polymers production is still coming from by-products of oil and gas refineries: heavy hydrocarbons (e.g., kerosene and naphtha) or saturated hydrocarbons (e.g., ethane and propane) [2]. According to an estimation of Hamman [3], between 1.3% and 2.1% of primary hydrocarbon resources consumed each year are diverted to hydrocarbon feedstocks for the production of plastics world-wide. It corresponds to an average energy consumption (e.g., energy in the feedstock) of 45 MJ/kg of plastic. Moreover, the additional energy for processing the polymers ranges from 36 to 54 MJ/kg of plastic. Considering the European 2018 production, it means that between 0.531 and 0.797 Gtoe of primary energy have been invested for polymer manufacturing.
Despite polymeric materials are usually referred to as 'plastics', they are composed by a great variety of materials designed to cover the different needs of the end products. More than 350 different types of polymers are currently commercially available [4]. All polymers can be classified in one of the following two categories, depending on the polymerization process: thermoplastics, a family of polymers that can be melted when withdrawn from the nature according to eight categories of resources. The CEENE method is then applied to a number of different products and materials. A CEENE assessment of post-industrial plastic waste is presented in [32].
The inclusion of exergy-based indicators into the Industrial Ecology (IE) have been implemented by Stanek et al. [33], who applied the Thermo-Ecological Cost (TEC) theory to the analysis of different energy and technological systems (e.g., hard coal production). Exergy analysis offers also advanced tools for the evaluation of non-renewable mineral resources, as demonstrated by Valero et al. [34][35][36]. They presented a new thermodynamic approach, based on quantifying the exergy costs required to replace the extracted minerals. In this way, exergy is a measure of the 'distinction' from the surroundings and the thermodynamic rarity of minerals is defined as the total amount of exergy resources needed to obtain a mineral from a completely degraded state (called 'Thanatia'), using the best prevailing technologies [37]. The total exergy cost is the sum of the energy associated with conventional mining, beneficiation, smelting and refining processes, plus the exergy theoretically invested in concentrating the resources from Thanatia to mines ore grade, i.e., their Exergy Replacement Cost (ERC). An interesting application is found in [38,39], where thermodynamic rarity is used to rank the critical metals used in passenger car and as a weighting factor for assessing their downcycling. A comprehensive metal assessment of two passenger cars (conventional and battery electric models) in terms of mass and thermodynamic rarity is also presented in [40].
The general method used to account for the total exergy of the resources invested in a product life cycle is the calculation of the Cumulative Exergy Consumption (CExC) [41]. In this work, the concept of CExC is combined with the methodology and the definition of thermodynamic rarity.
Currently, no examples are present of use of exergy for comparing the resources invested in producing polymers from virgin (primary) material with those from secondary materials through recycling. Besides, no applications are present for the thermodynamic assessment of vehicle plastic components.
The aim of the present work is to define and assess the exergy life cycle of polymeric materials and to develop exergy-based indicators comparing polymers production from primary and secondary raw materials. Besides, a thermodynamic assessment of the vehicle plastic components is performed with the aim of obtaining useful information for developing recycling practices.

Materials and Methods
The general scheme of the adopted methodology which will be detailed in the next paragraphs is presented in Figure 1. systems and plant management [29,30]. Dewulf et al. [31] have developed the Cumulative Exergy Extraction from the Natural Environment (CEENE) indicator, which quantifies the exergy withdrawn from the nature according to eight categories of resources. The CEENE method is then applied to a number of different products and materials. A CEENE assessment of post-industrial plastic waste is presented in [32]. The inclusion of exergy-based indicators into the Industrial Ecology (IE) have been implemented by Stanek et al. [33], who applied the Thermo-Ecological Cost (TEC) theory to the analysis of different energy and technological systems (e.g., hard coal production). Exergy analysis offers also advanced tools for the evaluation of non-renewable mineral resources, as demonstrated by Valero et al. [34][35][36]. They presented a new thermodynamic approach, based on quantifying the exergy costs required to replace the extracted minerals. In this way, exergy is a measure of the 'distinction' from the surroundings and the thermodynamic rarity of minerals is defined as the total amount of exergy resources needed to obtain a mineral from a completely degraded state (called 'Thanatia'), using the best prevailing technologies [37]. The total exergy cost is the sum of the energy associated with conventional mining, beneficiation, smelting and refining processes, plus the exergy theoretically invested in concentrating the resources from Thanatia to mines ore grade, i.e., their Exergy Replacement Cost (ERC). An interesting application is found in [38,39], where thermodynamic rarity is used to rank the critical metals used in passenger car and as a weighting factor for assessing their downcycling. A comprehensive metal assessment of two passenger cars (conventional and battery electric models) in terms of mass and thermodynamic rarity is also presented in [40].
The general method used to account for the total exergy of the resources invested in a product life cycle is the calculation of the Cumulative Exergy Consumption (CExC) [41]. In this work, the concept of CExC is combined with the methodology and the definition of thermodynamic rarity.
Currently, no examples are present of use of exergy for comparing the resources invested in producing polymers from virgin (primary) material with those from secondary materials through recycling. Besides, no applications are present for the thermodynamic assessment of vehicle plastic components.
The aim of the present work is to define and assess the exergy life cycle of polymeric materials and to develop exergy-based indicators comparing polymers production from primary and secondary raw materials. Besides, a thermodynamic assessment of the vehicle plastic components is performed with the aim of obtaining useful information for developing recycling practices.

Materials and Methods
The general scheme of the adopted methodology which will be detailed in the next paragraphs is presented in Figure 1.

Polymers Production Routes
Nowadays the main feedstock for plastics production is still found in by-products of oil and gas refineries. Most of these hydrocarbons (e.g., naphtha, ethane, propane, gas oil) have little commercial value and must be separated and processed in order to obtain lightweight unsaturated olefins [2]. To this end, the most common process is the steam cracking [42]; the process energy demand is consistent and depends on the feedstock characteristics.
Ethylene, propylene, butadiene, benzene, toluene and xylene are the main building blocks for creating the macromolecules of polymers and are mainly obtained by steam cracking of naphtha, gas oil or light hydrocarbons [14]. The creation of polymers from monomers is accomplished through the polymerization process. Temperature, pressure, catalysts and energy requirement vary in order to create the conditions for the building blocks to combine and bond. Catalysts can be used to start or speed up the reaction [43]. The most common mechanisms of polymerization are by addition or condensation. In addition polymerization (e.g., PE, PP and PVC) the growth of polymer occurs by reaction between a monomer and a reactive site; no by-products are generated. In condensation polymerization (e.g., PET, PA and PC), the reaction between the repeating unit and the growing chain produces by-products. Table 1 reports a brief description of the production routes identified for the 9 polymers analysed in this work. The selected polymers are among the most commercially diffused; they are also the ones with the highest weight percentage in the vehicle plastic composition presented in the next Sections.  [14,42,[44][45][46][47]).

Polymer Abbreviation Production Process
Polyethylene/Polypropylene PE/PP Addition polymerization of ethylene (C 2 H 4 ) for PE and propylene (C 3 H 6 ) for PP, obtained from steam cracking of naphtha.

Polyvinyl Chloride PVC
Chlorine (Cl 2 ) is extracted from salt (NaCl) by electrolysis and it reacts with ethylene for producing Ethylene Dichloride (EDC); cracking of EDC produces Vinyl Chloride Monomer (VCM) and HCl, which is used to produce additional EDC by oxychlorination. Polymerization of VCM occurs by addition in aqueous medium.

Acrylonitrile Butadiene Styrene ABS
Emulsion polymerization of acrylonitrile, polybutadiene and styrene. Acrylonitrile (C 3 H 3 N) is obtained by the reaction between propylene and ammonia (derived from natural gas); polybutadiene comes from polymerization of butadiene (C 4 H 6 ) from naphtha cracking; styrene is produced from ethylbenzene dehydrogenation Polyurethane PU Condensation polymerization between a diisocyanate (e.g., MDI) and a polyol. MDI production starts with a condensation reaction between aniline (C 6 H 7 N) and formaldehyde (CH 2 O) for producing MDA, which reacts with phosgene (COCl 2 ) to produce MDI. A polyol is the result of an alkoxylation (ethylene oxide EO + OH group), with glycerine as initiator.

Polyethylene Terephthalate PET
Polymerization of terephthalic acid (PTA) (or dimethyl terephthalate DMT) and ethylene glycol (EG). PTA is obtained by oxidation of p-xylene (C 8 H 10 ) with acetic acid as solvent, while EG (C 2 H 6 O 2 ) comes from hydrolysis of EO.
Styrene Butadiene Rubber SBR Polymerization of styrene and butadiene, followed by vulcanization with sulphur (S).
Ethylene Propylene Diene Rubber EPDM Solution polymerization of ethylene, propylene and diene (e.g., hexadiene C 6 H 10 ), followed by vulcanization with S or peroxide.

Polymers Exergy Life Cycle
In order to calculate the exergy invested along all the polymer production chain, some assumptions are made on its structure. According to the "grave to cradle" path [48], polymers production phases are considered as follows: (i) polymerization; (ii) production of monomers or 'building blocks' from oil and gas heavy by-products (referred as 'naphtha'); (iii) production of naphtha from fossil fuel (referred as 'coal'); (iv) fossil fuels from organic matter (referred as 'wood'). The first two phases have been described in Section 2.1; details on phase (iii) and (iv) are reported below.
• Naphtha from coal: naphtha is produced from the processing of fossil fuels [49].
Although the most common commercial route is the one from petroleum refinery, there are historical examples of naphtha production from coal through direct liquefaction or Fisher Trops (FT) reaction as well as from destructive distillation of biomass [50]. In this work fossil fuel is modelled as coal since it appears inclusive of all the characteristics of the generic fossil fuel. Therefore, direct liquefaction from coal is assumed, resulting the most efficient process in terms of yield of naphtha (i.e., 10%, considering that the black sub-bituminous coal and the lignite are more suitable for this process). The invested fossil energy (excluding the feedstock energy) is 38 MJ/kg of naphtha [49]. • Coal from wood: coal is chosen in the model also for its convenience at the time of calculating the Exergy Replacement Cost (ERC), presenting a more stable composition than oil. In its general definition, the ERC corresponds to the natural bonus of having resources concentrated in a deposit. The ERC of fossil fuels has not been previously considered by Valero and Valero 'due to the impossibility of reproducing the photosynthetic process that once created the resource' [37]. This makes sense if oil, coal and natural gas are considered strictly as fuels, which are destined to be finally burned. Anyway, if fossil fuels are used as raw materials, as in the case of polymer production, it becomes theoretically possible to come back to the 'grave' with recycling. According to Whiting and Carmona [51], the ERC of fossil fuels can be evaluated considering the cumulative exergy cost of equivalent fossil fuels (first generation bio-fuels) production (e.g., bioethanol for gasoline, biogas for natural gas, biochar for coal); furthermore, they extend the boundary of the analysis including the 'solar radiation to crop' factor to ERC calculation. In this work, only the 'crop to fuel' part of the described ERC is taken, which represents the exergy invested in processing and concentrating the natural primary resources into viable deposits.
In line with [51], charcoal produced from timber is considered as the alternative biofuel for ERC evaluation; the invested exergy is composed by the feedstock exergy of the biomass, 54.5 MJ/kg of coal, and an external contribution for the process amounting to 28.1 MJ/kg of coal. All the previously reported values of invested energy refer to the main product unit (i.e., by-products are not included in the calculation).
The sum of all the contributions in terms of materials and exergy invested in the different steps of polymer production chain gives the Embodied Exergy (EE) of the materials, as reported in Table 2. The exergy data are expressed in MJ of exergy/kg of final product (i.e., polymeric material). The energy consumption is divided into direct fossil fuel use, electricity and heat. The chemical exergy of fossil fuel b f uel ch is calculated by means of the Szargut correction factor ϕ of Lower Heating Value (LHV) (Equation (1)) [41]. The method is applied also for calculating the feedstock exergy of polymers, namely the primary exergy of the initial fossil fuel embodied in the final product. The value of ϕ is 1.06 for oil fuel and 1.04 for natural gas, while for each polymer it is evaluated by means of Equation (2) for fuels with O/C < 0.667, according to the ultimate analysis (carbon C, hydrogen H, oxygen O and nitrogen N). 'Heat' refers to steam consumption and its exergy is evaluated as the sum of two contributions: physical exergy (b steam ph = (h − h 0 ) − T 0 (s − s 0 )), where h, s, h 0 and s 0 are the specific enthalpy (kJ/kg) and entropy (kJ/kg·K) of the considered and reference state (T 0 = 288 K, p 0 = 1 atm) respectively; chemical exergy of liquid water (b water ch = 50 kJ/kg). If the conditions are not specified in literature, steam is considered saturated at 16 atm.

Polymers Recycling Routes
Recycling methods are usually referred to as primary, secondary, tertiary and quaternary recycling [59]. Primary and secondary recycling techniques are based on mechanical treatment of discarded polymers in order to obtain the starting material. The primary recycling is usually performed by the manufacturer itself for post-industrial waste (closed-loop recycling) [7]. The secondary recycling is the most common and involves a series of steps after collection, namely cleaning, drying, shredding, contaminant separation, addition of additives, agglomeration, pelletization and extrusion. The mechanical characteristics of recycled polymers can be degraded, so that they are commonly used in manufacturing less value products [6]. Only thermoplastic polymers can undergo mechanical recycling because they can be re-melted and reprocessed into end products [60]. Tertiary recycling consists in the recovering of monomers through depolymerisation processes, such as solvolysis, thermolysis and pyrolysis (thermal recycling) or glycolysis and methanolysis (chemical recycling). Many thermosets plastics can be chemically recycled in order to recover their constituent molecules [59]. The expression quaternary recycling is used to indicate the energy recovery from plastics through incineration [60].
Due to the variety of existing recycling processes, an extent literature review has been performed in order to identify the most suitable considering the specific application in vehicles. A brief description of the recycling processes and the associated exergy consumption (expressed in MJ of exergy per kg of recycled material) are reported in Table 3.  PET Secondary PET reprocessing process consists in a first section to remove impurities (pre-washing, magnetic separation, x-ray separation of PVC) and in a second to recover PET and by-products (HDPE and fines) by flotation. The material is then dried, screened and stored.

SBR/EPDM Tertiary
Devulcanization is the most delicate phase, because a selective rupture of sulphur bonds (S-S or C-S) must be achieved without breaking the hydrocarbon bonds. The most common method is a thermal process carried out in steam-heated autoclave at a certain temperature (225 • C) and pressure (28-30 bar)  Thermodynamic Recycling Indexes In order to evaluate the recycling process, exergy-based recycling indexes are developed, depending on the final product, namely the new crude polymeric material (primary product) or the oil derivatives (secondary products). Examples of developing of exergybased indicators for products life cycle are present in [66]. Figure 2 can be useful for understanding the exergy flows. A new polymer can be obtained by mechanical recycling (as in the case of PE, PP, PVC, ABS and PET) or via chemical recycling through decomposition into the constituent macromolecules and consequent re-polymerization (as for PU, PA6.6, PET, SBR, EPDM). According to this, different recycling indexes are adopted: • , the mechanical recycling index (Equation (3)) is defined as a comparison between the embodied exergy of the mechanical recycling ( ) and the exergy of the production from virgin material, starting from naphtha, ( , the tertiary recycling index (Equation (4)) is defined as the ratio between the exergy necessary for re-obtaining the polymer via depolymerisation ( + ) and the one for producing it from naphtha.

•
, the global recycling index (Equation (5)) is calculated as the ratio between the embodied exergy of the recycling (secondary or tertiary) and the one of the entire production chain starting from the biomass, in order to give a broader order of magnitude.
• , the chemical recycling index (Equation (6)) compares the embodied exergy of the production of oil derivatives from polymers ( ) with the one from fossil fuel ( → ). This indicator is introduced since, for some polymers (PE, PP, PVC), the chemical recycling consists in a decomposition into secondary products (hydrocarbon molecules).
It has to be considered that these indexes are strictly relative to the processes of materials manufacturing and recycling; they do not take into account the exergy invested in dismantling the end-of-life products or collecting and transporting the waste materials. The values of the indexes are given in percentage; low values mean that the recycling process is advantageous in terms of invested exergy compared to production from virgin materials.

Polymers in Vehicles
Data on polymeric composition of vehicles are provided by SEAT S.A. They refer to a 2017 SEAT Leon model of approximately 1270 kg, of which 16.6% are non-metallic A new polymer can be obtained by mechanical recycling (as in the case of PE, PP, PVC, ABS and PET) or via chemical recycling through decomposition into the constituent macromolecules and consequent re-polymerization (as for PU, PA6.6, PET, SBR, EPDM). According to this, different recycling indexes are adopted: • REC mec , the mechanical recycling index (Equation (3)) is defined as a comparison between the embodied exergy of the mechanical recycling (Ex rec ) and the exergy of the production from virgin material, starting from naphtha, (Ex oil_prod + Ex pol ). • REC ter , the tertiary recycling index (Equation (4)) is defined as the ratio between the exergy necessary for re-obtaining the polymer via depolymerisation (Ex depol + Ex pol ) and the one for producing it from naphtha. • REC gl , the global recycling index (Equation (5)) is calculated as the ratio between the embodied exergy of the recycling (secondary or tertiary) and the one of the entire production chain starting from the biomass, in order to give a broader order of magnitude. • REC ch , the chemical recycling index (Equation (6)) compares the embodied exergy of the production of oil derivatives from polymers (Ex depol ) with the one from fossil fuel (Ex coal→oil prod ). This indicator is introduced since, for some polymers (PE, PP, PVC), the chemical recycling consists in a decomposition into secondary products (hydrocarbon molecules).
It has to be considered that these indexes are strictly relative to the processes of materials manufacturing and recycling; they do not take into account the exergy invested in dismantling the end-of-life products or collecting and transporting the waste materials. The values of the indexes are given in percentage; low values mean that the recycling process is advantageous in terms of invested exergy compared to production from virgin materials.

Polymers in Vehicles
Data on polymeric composition of vehicles are provided by SEAT S.A. They refer to a 2017 SEAT Leon model of approximately 1270 kg, of which 16.6% are non-metallic materials (i.e., glass, polymers and ceramics). As reported in Table 4, 21 polymers are identified, composed by 14 thermoplastics and 9 thermosets. Adhesives and resins are not included (even if they can be polymer-based materials). The vehicle plastic composition is compared with data found in the literature, showing good accordance for typology and quantity of polymers. Only the polymers with a weight percentage higher than 2% were chosen for the analysis, namely PE, PP, PVC, ABS, PU, PA66, PET, SBR and EPDM. They also occur to be the most commercially diffused and with existent recycling practices.

Vehicle Components
The developed thermodynamic concepts and values are used for the analysis of the plastic content of a vehicle. In addition to the data on the total polymeric material contained in a SEAT Leon, the composition of some vehicle components has been provided by SEAT S.A., as reported in Table 5. The analysed car parts are chosen between the ones with significant plastic content as well as for their facility at the time of being eventually removed for recycling. Many vehicle polymers incorporate additives for enhancing mechanical characteristics, strength, fire resistance or for coloring [67]. The composition of some of these chemical substances is not declared by producers, which only report the weight content. The most common declared additive are the ones reported in Table 6; their feedstock exergy has to be included in the calculation of the EE of the corresponding polymer. The global EE of each car part is calculated, in order to account for the distribution among the various polymers. In case of no recycling and total shredding, the EE is totally dispersed. Therefore, the evaluation is useful also to give information at the time of planning recycling practices, together with the developed recycling indexes.

Comparison of Polymer Production EE
First, the global EE of the polymers production chain (from 'Biomass' to 'Polymer' in Figure 2) is calculated (Figure 3a), evaluating the contribution of each step (Figure 3b).

TiO2, F2
Carbon black C 34.2 [41] The global EE of each car part is calculated, in order to account for the distribution among the various polymers. In case of no recycling and total shredding, the EE is totally dispersed. Therefore, the evaluation is useful also to give information at the time of planning recycling practices, together with the developed recycling indexes.

Comparison of Polymer Production EE
First, the global EE of the polymers production chain (from 'Biomass' to 'Polymer' in Figure 2) is calculated (Figure 3a), evaluating the contribution of each step (Figure 3b).  Figure 3b. The repartition is similar for all polymers, with approximately 60% of exergy embodied in the biomass for coal production, 32% in the external contribution to the 'biomass-to-coal' process, 4% in naphtha production from coal and the remaining 4% in polymerization process and feedstock. The major differences between polymers are linked to feedstock and process exergy, which strongly influence the global balance. The polyolefin (PE, PP, PVC) and the PU have the lowest values of EE, since the production processes are quite plain and the Results show a wide range of values of EE for the analysed polymers, ranging from 0.036 toe/kg of PVC to 0.479 toe/kg of SBR. The average values of percentage contribution of each step to the global EE are reported in Figure 3b. The repartition is similar for all polymers, with approximately 60% of exergy embodied in the biomass for coal production, 32% in the external contribution to the 'biomass-to-coal' process, 4% in naphtha production from coal and the remaining 4% in polymerization process and feedstock. The major differences between polymers are linked to feedstock and process exergy, which strongly influence the global balance. The polyolefin (PE, PP, PVC) and the PU have the lowest values of EE, since the production processes are quite plain and the major constituent hydrocarbons (ethylene and propylene) have high yield from naphtha. An increase in the complexity of the molecules lead to a growth in the process exergy as well as in the quantity of required primary fossil fuel. This is the case of ABS, SBR and PET and, to a lesser extent, of PA6.6 and EPDM. The use of butadiene represents the major burden in the production of ABS (20 wt% of butadiene) and SBR (75 wt%), since it has a particularly low yield from naphtha (1:27); butadiene is present also in EPDM, even in lower quantities (10 wt%). The second more influencing factor is the presence of benzene (yield from naphtha 1:12) for styrene production. Despite its large commercial use, PET is the second most important in terms of global EE; in fact, the production of PTA requires para-xylene, which is extracted from heavy reformate of naphtha with very low yield (4 wt%).

Comparison of Recycling Indexes
According to the data reported in Section 2.3, the thermodynamic recycling indexes are calculated for each polymer. Results are graphically reported in Figure 4a-d.
the second most important in terms of global EE; in fact, the production of PTA requires para-xylene, which is extracted from heavy reformate of naphtha with very low yield (4 wt%).

Comparison of Recycling Indexes
According to the data reported in Section 2.3, the thermodynamic recycling indexes are calculated for each polymer. Results are graphically reported in Figure 4a-d. Among the polyolefin, PE is the one with the highest (75%), followed by PP, PVC and ABS. Mechanical recycling is the most convenient option for PET, with an exergy saving of about 60% with respect to production from virgin materials; recycling through depolymerisation appears not so convenient, since the value of is about 89%. This picture is confirmed by the real practice since PET is one of the most mechanically recycled polymers (other than one of the most diffused). In terms of tertiary recycling, PA6.6 has the lowest value of , less than 50%. This should encourage the recycling of polyamide, better if in closed loop, which is not so diffused so far. Even the values of of rubbers (50% for SBR and 58% for EPDM) appear not as high as for justifying the almost total absence of recycling practices in the world. Looking at the broader vision, Among the polyolefin, PE is the one with the highest REC mec (75%), followed by PP, PVC and ABS. Mechanical recycling is the most convenient option for PET, with an exergy saving of about 60% with respect to production from virgin materials; recycling through depolymerisation appears not so convenient, since the value of REC ter is about 89%. This picture is confirmed by the real practice since PET is one of the most mechanically recycled polymers (other than one of the most diffused). In terms of tertiary recycling, PA6.6 has the lowest value of REC ter , less than 50%. This should encourage the recycling of polyamide, better if in closed loop, which is not so diffused so far. Even the values of REC ter of rubbers (50% for SBR and 58% for EPDM) appear not as high as for justifying the almost total absence of recycling practices in the world. Looking at the broader vision, the values of REC gl are quite low as expected. In fact, the exergy invested in the recycling process is less than 2% of the total exergy necessary for obtaining the polymer from virgin material, starting from the primary exergy of the biomass. Finally, it is interesting to notice the values of REC ch of the polyolefin (24.6% for PP, 42.4% for PE and 37% for PVC). Considering this quite low exergy cost of the petrochemicals production, the depolymerisation could be a promising solution for obtaining secondary products to sell in the market if the mechanical recycling is not possible. It has to be considered that all these values refer to the processes only, excluding the collection and transport exergy cost of waste polymers as well as raw materials.

Vehicle Components
A first thermodynamic assessment of the vehicle is conducted by calculating the global EE of the vehicle polymeric content. Results are reported in Table 7, where a comparison with the rarity of the metals is presented [38]. It is evident that the exergy embodied in the polymeric materials is several orders of magnitude greater than the metals rarity. However, the analysis of the contribution of the single steps highlights that the exergy associated to the processing from raw materials is pretty similar. The real difference is represented by the ERC of fossil fuel. The 'grave to cradle' path for theoretically reintroducing the fossil fuel derivatives into their 'dead state' (so in the condition where they are organic material) is much more complicated and exergy intensive than the re-concentration of minerals from the Thanatia's grade.

Vehicle Components
According to the material composition reported in Table 5, the total EE of the four vehicle components plastic content is calculated (the EE of additives is included). A comparison with the specific values (referred to the total amount of polymers) is reported in Figure 5. The highest value (0.8 toe) associated to the floor covering is due to the presence of a large quantity of PET (83.5% of the total EE), which is a polymer with the highest values of EE together with SBR (also present in this component). Floor covering is the third component in terms of weight between the analysed, so its specific EE value (0.29 toe/kg) is higher than the one of dashboard (0.07 toe/kg) and rear bumper (0.16 toe/kg). The highest value (0.8 toe) associated to the floor covering is due to the presence of a large quantity of PET (83.5% of the total EE), which is a polymer with the highest values of EE together with SBR (also present in this component). Floor covering is the third component in terms of weight between the analysed, so its specific EE value (0.29 toe/kg) is higher than the one of dashboard (0.07 toe/kg) and rear bumper (0.16 toe/kg). On the other hand, the instrumental cluster has the lowest value of global EE (0.09 toe), since its weight is considerably lower than the others; this also implies that its EE specific value is high (0.26 toe/kg) since the resources are more concentrated. The dashboard has the highest weight and the smallest EE specific value (0.07 toe/kg), but its total EE (0.6 toe) is lower than the one of the floor covering and the rear bumper, since it is mainly composed by PP. Figure   Figure 5. Comparison between total and specific EE values of vehicle components polymeric content.
The highest value (0.8 toe) associated to the floor covering is due to the presence of a large quantity of PET (83.5% of the total EE), which is a polymer with the highest values of EE together with SBR (also present in this component). Floor covering is the third component in terms of weight between the analysed, so its specific EE value (0.29 toe/kg) is higher than the one of dashboard (0.07 toe/kg) and rear bumper (0.16 toe/kg). The highest value (0.8 toe) associated to the floor covering is due to the presence of a large quantity of PET (83.5% of the total EE), which is a polymer with the highest values of EE together with SBR (also present in this component). Floor covering is the third component in terms of weight between the analysed, so its specific EE value (0.29 toe/kg) is higher than the one of dashboard (0.07 toe/kg) and rear bumper (0.16 toe/kg). On the other hand, the instrumental cluster has the lowest value of global EE (0.09 toe), since its weight is considerably lower than the others; this also implies that its EE specific value is high (0.26 toe/kg) since the resources are more concentrated. The dashboard has the highest weight and the smallest EE specific value (0.07 toe/kg), but its total EE (0.6 toe) is lower than the one of the floor covering and the rear bumper, since it is mainly composed by PP. Figure 6 reports the detailed distribution of the EE between the constituting polymers of the components.

Recycling Considerations
The polymeric composition of the vehicle components is a fundamental factor in defining recycling practices. All the exergy still embodied in the vehicle (in this case, in form of plastics) would be totally lost in case of no recycling and the material dispersed in case of landfill disposal or incineration. This means that the same amount of resources (EE of polymers) are needed in order to re-obtain the components; the aim of recycling is to recover the value and to spend only a percentage of the total resources for having the final products.
In theory, all these polymers are recyclable, in the sense that at least one recycling industrial process exists. In real practice the most recycled are PP, PE and PET. Moreover, many factors influence the practical implementation of recycling: • the compatibility with the other polymers and the difficulty in separating them; • the presence of additives that can affect the recycling process; • the form on which the polymer is present (e.g., PET in the floor covering is in form of fibre, which is not commonly recycled differently from the 'bottle' PET material); • the recycling volumes that can be achieved.
All these qualitative elements (together with the developed recycling indexes) have to be considered in order to assess the recyclability of the vehicle components and they will be further discussed in future works.

Conclusions and Discussion
An exergy-based assessment of polymeric materials has been performed in order to compare the resources invested in producing polymers from virgin material with those from secondary materials through recycling. Besides, the calculated data have been used to analyse the plastic composition of vehicle components with the aim of obtaining useful information for recycling.

Recycling Considerations
The polymeric composition of the vehicle components is a fundamental factor in defining recycling practices. All the exergy still embodied in the vehicle (in this case, in form of plastics) would be totally lost in case of no recycling and the material dispersed in case of landfill disposal or incineration. This means that the same amount of resources (EE of polymers) are needed in order to re-obtain the components; the aim of recycling is to recover the value and to spend only a percentage of the total resources for having the final products.
In theory, all these polymers are recyclable, in the sense that at least one recycling industrial process exists. In real practice the most recycled are PP, PE and PET. Moreover, many factors influence the practical implementation of recycling: • the compatibility with the other polymers and the difficulty in separating them; • the presence of additives that can affect the recycling process; • the form on which the polymer is present (e.g., PET in the floor covering is in form of fibre, which is not commonly recycled differently from the 'bottle' PET material); • the recycling volumes that can be achieved.
All these qualitative elements (together with the developed recycling indexes) have to be considered in order to assess the recyclability of the vehicle components and they will be further discussed in future works.

Conclusions and Discussion
An exergy-based assessment of polymeric materials has been performed in order to compare the resources invested in producing polymers from virgin material with those from secondary materials through recycling. Besides, the calculated data have been used to analyse the plastic composition of vehicle components with the aim of obtaining useful information for recycling.
First, the global Embodied Exergy of 9 different polymers has been calculated tracking back the exergy invested in the production process, considering polymerization, naphtha production from fossil fuel and Exergy Replacement Cost of coal. The set of analysed polymers have been chosen between the ones with a weight percentage higher than 2% basing on the plastic composition of a 2017 SEAT Leon vehicle.
The analysis of the entire chain enlarges the vision, showing that the major exergy investment occurs in the first steps where the primary natural resources (e.g., biomass) are concentrated in form of fossil fuel to be further utilized. Then, in the strictly production phase, the complexity of the processes for obtaining the constituent molecules is what determines the EE of the polymers.
Data on the best available recycling processes have been collected and exergy-based recycling indicators have been defined and calculated for each polymer, according to the type of recycling (secondary or tertiary) and the nature of the final products (new polymer or secondary materials). The resulted scenario confirms as quite convenient some practises that are already in use, such as the mechanical recycling of PET or of some polyolefin (PP, PVC) over the chemical one. These results are in line with the one of [20], where mechanical and feedstock recycling of plastic waste are compared with a LCA methodology. Also the results of [70] confirmed the environmental benefit of PET mechanical recycling under different scenarios.
From our analysis it also emerges that some scarcely diffused recycling processes are not so prohibitive from an exergy perspective, at least considering the comparison with the production process from virgin materials. Even if the transport and collection of waste polymers is not accounted for in the calculation of the EE, the fact that all the recycling indexes are lower than 100% (some of them even significantly) leaves a positive margin for further exergy consumption. This difference is even more marked in the comparison of the recycling process with the entire production chain (starting from the biomass up to the polymer), being the values of REC gl in the order of 2%. All these factors are an encouragement not only to pursue and improve recycling technologies, but also to optimize the connection between the producers of intermediate materials so that all stakeholders can benefit from the savings derived from recycling. This is a crucial point for the developing of plastic waste circular economy and it is also confirmed by the conclusion of [71].
Finally, a thermodynamic assessment of the plastic content of some vehicle components is presented. In the first place, the calculation of the total EE of the components gives an idea of the order of magnitude of the MJ of exergy that are definitively dispersed in case that the materials are not reused or recycled. The methodology applied to the single component can be useful to reveal which polymer can be critical with respect to the others at the time of recycling. Since many heterogenous factors are involved in defining the 'recyclability' concept, this will be the object of further investigation as well as the evaluation of the polymer substitution.