# Thermoforming Process Effect on Performances of Thermoplastic/Recycled Carbon Fiber Composites
## 1 Introduction
Subsequently, these mats can be thermoformed with traditional manufacturing techniques, such as compression molding, to produce composites with relatively high fiber volume fractions and good mechanical properties. [PERSON] et al. [22] produced nonwoven mats based on rCFs with a paper-making process and then compression-molded them with an epoxy resin layer to manufacture rCFRCs. The samples' nominal volume fraction was around 30%, resulting in the same stiffness and strength as the aerospace grade 2024-T4 aluminum alloy. [PERSON] et al. [22] produced rCF/PPS composites from nonwoven mats consisting of commingled rCFs and polyphenylene-sulfide (PPS) fibers through compression molding. The experimental results demonstrated that interlaminar fracture energies and mechanical properties significantly increased. However, it also negatively affected the interlaminar fracture resistance. Therefore, studies of these mats' processing, post-processing, and mechanical behavior are particularly interesting. Coupling Nonwoven mats made by rCF with polyamide (PA6) enables high abrasion resistance, heat resistance and excellent mechanical properties [23]. On the other hand, the use of MAPP, polypropylene modified with maleic anhydride (MA), allows the polymer to retain all the advantages of PP, including excellent chemical and mechanical properties, thermal stability, and low density [24]. Additionally, the presence of maleic anhydride enhances compatibility between the matrix phase and recycled carbon fibers (rCFs) [25]. The effect of MA in improving interfacial adhesion is primarily attributed to chemical interactions, such as the formation of covalent and hydrogen bonds, between the functional groups on the CF surface and the MA groups in the MAPP [25]. Fiber recovery is achievedthrough a modified pyrolysis process, where a furnace burns the resin. According to ELG Carbon Fibre, pyrolysis provides a recycled material that maintains 90% of the carbon fiber's tensile strength. To convert the recycled carbon fiber, ELG Carbon Fibre use a carding line capable of providing up to 250 tonnes per year of carbon fiber or carbon fiber/thermoplastic nonwoven mats [26]. The recycled carbon fibers are initially mixed with thermoplastic fibers, polyamide PA6 and maleic anhydride-modified polypropylene (MAPP) to ensure homogeneous distribution. Subsequently, rCFs mixed with thermoplastic fibers are fed into the carding machine, which is equipped with toothed cylinders that align and distribute the fibers into a thin and uniform web. This fibrous web is then layered to achieve the desired basis weight. The resulting mats can be subjected to thermoforming processes [27]. This paper aims to investigate the optimal process condition of compression-molding for repurposing recycled carbon fibers nonwoven mats. Two types of pre-forms were considered: Carbiso TM-PA6/60 and Carbiso TM-MAPP/60, namely rCF/PA6-60 and rCF/MAPP, respectively. Two different approaches were explored: direct compression-molding in a one-shot fabrication (Method 1--M1) and lamination with stepwise consolidation (Method 2--M2). Differential scanning calorimetry (DSC) was used to evaluate how processing affected crystallinity. The influence of porosity on the elastic modulus, induced by the compression molding process, was assessed by dynamic mechanical analysis (DMA). The results show that with up to two layers, M2 yields better performance in terms of porosity and storage modulus; however, with more layers, performance differences between the two methods diminish. The viscoelastic properties also highlight differences in interphase behavior: PA6-based composites exhibit a reinforcement modulus of 60 GPa, while MAPP-based ones reach 90 GPa. The higher reinforcing efficiency in MAPP composites is attributed to a stronger interphase enabled by maleic anhydride, as evidenced by lower tan\(\delta\) values, indicating reduced energy dissipation.