Comparative life cycle assessment of carbon fiber reinforced compositecomponents for automotive industry

Archimede Forcellese. Università Politecnica delle Marche, Via Brecce Bianche 12, 60131 Ancona, Italy. Tommaso Mancia. Università Politecnica delle Marche, Via Brecce Bianche 12, 60131 Ancona, Italy. Michela Simoncini. Università eCampus, Via Isimbardi 10, 22060 Novedrate (CO), Italy. Serena Gentili. Università Politecnica delle Marche, Via Brecce Bianche 12, 60131 Ancona, Italy. Marco Marconi. Università degli Studi della Tuscia, Largo dell’Università, 01100 Viterbo, Italy. Alessio Vita. Università Politecnica delle Marche, Via Brecce Bianche 12, 60131 Ancona, Italy. Corresponding author: alessio.vita@univpm.it Alessia Nardinocchi. HP Composites s.p.a., Zona Ind. Campolungo, 63100 Ascoli Piceno (AP), Italy. Vincenzo Castorani. HP Composites s.p.a., Zona Ind. Campolungo, 63100 Ascoli Piceno (AP), Itally.


Intr Introduction oduction
Moving towards sustainable processes is a mandatory task to realize greener components for the automotive industries [1]. This is especially true as these components are used in eco-friendly products such as electric vehicles (EVs) [2].
However, as well known, the efficiency of EVs is limited mainly due to the high weight of batteries. A method to improve the kilometric range of EVs is to lighten structural and non-structural components [3]. In this context, CFRP (Carbon Fiber Reinforced Polymer) composites, due to their impressive resistance-to-weight and stiffness-to-weight ratios, are increasingly attracting the attention of car manufacturers as valid replacements for metals [4,5]. Moreover, different studies demonstrated that the use of CFRP in substitution of steel for the production of car structures, in a life cycle perspective, can lead to a reduction of environmental impacts, especially if long lifetime is assured [6,7]. This can be attributed to the lowest fuel consumption of the vehicle realized in composite materials even though their ESAFORM 2021. MS06 (Chains & Sustainability)), 10.25518/esaform21.2542 2542/1 manufacturing and the EoL (End of Life) phases result in higher impacts respect to the ones of steel [8]. Indeed, CFRP composites (especially those based on thermoset matrix) cannot be recycled and their production processes require significant quantities of energy.
According to literature, the production of raw carbon fibers can account for more than 50% of the total environmental loads of the manufacturing of a CFRP product [9]. However, an important share of these impacts is related to the manufacturing processes exploited to realize composite components. Indeed, to consolidate thermoset matrix and to obtain the higher mechanical performances, composites are cured at high temperature and pressure, thus requiring an intensive use of energy carriers.
One of the most used manufacturing techniques for producing high performance CFRP products is vacuum bag molding with autoclave curing. It needs long lay-up and curing times, as well as high manufacturing costs. In addition, this process is related to high environmental loads as compared to other manufacturing techniques [6,10].
OOA (Out-Of-Autoclave) methods allow to overcome the above-mentioned limitations of autoclave curing. As an example, dry fibers technologies such as Resin Transfer Molding (RTM) have been recognized as valid alternative methods to produce high performance composite components. To shorten processing time, injection pressure can be increased, thus performing High-Pressure RTM (HP-RTM) instead of Low-Pressure (LP-RTM) and Compression (C-RTM) RTM [11][12][13]. However, to do that, heavier and more expensive equipment (such as molds, pumps, presses, etc.) are required. From the environmental point of view, this results in an increase of the environmental loads [14].
Another OOA method which is gaining the attention of industries is PBM (Pressure Bag Molding) [15]. Using this method, it is possible to reduce the cycle time of at least 30% respect to the autoclave process. However, as reported by Vita et al. [16], PBM could results in higher environmental impacts respect to vacuum bag molding with autoclave curing as CFRP molds are used. RTM is a closed-mold process as reinforcing dry fibers are placed inside a closed mold before the injection of a liquid thermoset resin. The mold is heated to the adequate temperature that allows the matrix curing. LP-RTM is a modification of the conventional RTM process, the less expensive one in terms of equipment used. It employs lower resin injection pressure and final hydrostatic pressure during the curing cycle. Vacuum is used to clamp molds and helps the resin flow across the fiber pack. The standard cycle time has a duration of 30-60 minutes, considering the typical injection pressure of 10-20 bars [12]. HP-RTM is able to reduce cycle times to less than 10 minutes using injection pressures up to 150bars. These pressures cause high tooling costs and movements of dry fibers inside the molds (a binder is used to hold the fibers in position) [17]. C-RTM further reduces cycle times (injection and impregnation) respect to the HP-RTM. During the injection phase, a gap is present between the dry preform and the countermold, Comparative life cycle assessment of carbon fiber reinforced compositecomponents for au...

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which gives less resistance to the resin flow. The injection pressure is typically equal to 5-10 bars [13].
The autoclave processing is performed laminating a preimpregnated material, prepreg, on a CFRP mold. Then, a vacuum bag is realized and the curing occurs in an autoclave at high temperature and pressure for long time, typically more than 100°C, 2 bar and 2 hours, respectively. The CFRP mold is realized stacking prepreg sheets over a plastic master, typically made of a polyurethane foam (Ureol®) and performing a complete vacuum bag process with autoclave curing.
The useful life of a plastic master is 10 uses and then disposed in landfills, while the mold can be used, before surface degrades, for 150 times [18].
In the PBM process, a silicone counter-mold, manufactured by a curing reaction in an oven, is used to consolidate the CFRP prepreg sheets, laminated over a mold, at a pressure of 6-8bar. The mold is typically made of aluminum and heavy mass is required to withstand the high press pressure generated by a press and by the countermold. The demolding phase requires an energy intensive cooling system [15].

Life cycle assessment
The Life Cycle Assessment methodology has been used to assess and compare the five different process alternatives.
Analyses have been conducted by following the methodology and guidelines foreseen in the ISO 14040 -14044 standards, which includes four steps: (i) Goal and scope definition, to define objective, functional unit and system boundaries; (ii) Life Cycle Inventory (LCI), to "decompose" the system under analysis in different unit processes and perform an input-output analysis; (iii) Life Cycle Impact Assessment (LCIA), to calculate the impact in terms of different impact and damage categories; (iv) Results Interpretation, to analyze the obtained results, identify criticalities and define possible improvement strategies.
The Simapro 8.0.5.13 software tool, which includes the Ecoinvent 3.1 as database for secondary data, has been used to model the analyses and calculate the results.

Goal and scope Goal and scope
The objective of the study consists in comparing the environmental impacts of five process alternatives for the manufacturing of CFRP components for the automotive sector: (i) Autoclave; (ii) Pressure bag molding; (iii) Compression resin transfer molding; (iv) High pressure resin transfer molding; (v) Low pressure resin transfer molding. The study can be useful for manufacturing companies involved in the CFRP sector in order to choose the best process alternative to pursue environmental sustainability of their manufacturing activities.
The part considered in the study is a fender of a high-performance sport car with a surface of about 1,3 m2 and a weight of the finished part of about 1 kg. Despite the five different process alternatives, generally lead to parts with different mechanical properties (parts manufactured through autoclave processes have the best performances followed by PBM and then RTM), in this case the CFRP fenders can be considered comparable from the LCA point of view since no advanced mechanical performances are required, and its adoption in sports car are only related to vehicle lightweighting strategies (i.e. substitution of metal parts). Therefore, the common functional unit can be defined as "the production of a CFRP fender with the abovementioned dimensions, through different manufacturing processes".
The study can be classified as a cradle to gate analysis, since all the unit processes from material production to • ReCiPe midpoint [22] to calculate the impact in terms of 18 different categories in order to have a comprehensive overview about the environmental performance of the system under analysis; • ReCiPe endpoint [22] to normalize and weigh the 18 midpoint indicators, obtaining results in terms of three damage categories (i.e. Ecosystems, Resources, Human health) and finally an aggregated single score.
A graphic explanation of the presented manufacturing processes and the relative system boundary is reported in Fig. 1.

Lif Life cy e cycle in cle inv vent entory ory
The LCI is based on measured and estimated data, as well as data derived from relevant literature studies. The following

R Results assessment and int esults assessment and interpr erpretation etation
The first Life Cycle Impact Assessment (LCIA) has been performed by jointly considering all the midpoint indicators included in the ReCiPe methodology. The following Table 2  In this way the worst alternative has a 100% value and the savings can be easily visualized.
T  Going into more details in one of the most relevant categories for the manufacturing sector, and in particular for CFRP [10,[19][20][21], the following Fig. 3 reports the split of contributions for the Global Warming Potential indicator (i.e. ReCiPe Climate Change). In this case the best alternative is the C-RTM, followed by the LP-RTM, the HP-RTM, the Autoclave, and finally the PBM as the most impactful process. Split of contributions highlight that for almost all the process the most critical flow is the production of carbon fibers, included within the Cutting contribution for Autoclave in case of Autoclave/PBM, which requires consumption of energy for the prepregging phase [9].
Considering Autoclave, the second largest contribution is the Tooling (about 36% of the total impact), that is mainly due to the manufacturing of the Ureol master (more than 90% of the Tooling contribution). For PBM, the contribution of Tooling is relevant but minor than in case of Autoclave (29,61 vs 49,86 kgCO2eq). The most penalizing contribution is certainly the Curing, and specifically the critical flow is the energy consumption for heating the mold. In addition, in this case also the De-molding phase generates impacts due to the energy consumption needed for cooling the mold before the part extraction. Such two contributions make the PBM the worst process in terms of GWP.
Considering RTM, instead, the different dimensions of aluminum molds for the three variants (380 kg for C-RTM, 1610 kg for HPRTM and 610 kg for LP-RTM) cause relevant differences in terms of GWP impacts (46,02 kgCO2eq, 17,61 kgCO2eq and11,27 kgCO2eq, respectively). In addition, the mold dimensions are directly correlated to the electric energy needed for mold heating during the resin curing, another aspect that enlarge the differences among C-RTM, LP-RTM and HP-RTM (in order of total impact). Finally, a ReCiPe endpoint analysis has been performed, as reported in the following Fig. 4 with the results previously obtained with the ReCiPe midpoint analyses. The obtained results show that: • The manufacturing of carbon fiber and epoxy resin is the most relevant environmental load for the analyzed processes; • RTM techniques are associated to lower environmental impacts with respect to both PBM and autoclave; • C-RTM results the greener manufacturing process, mainly due to the lightness of the mold; • Autoclave and PBM are the worst processes. This can be attributed to the prepregging phase and to the high impacts Comparative life cycle assessment of carbon fiber reinforced compositecomponents for au...