**1. Introduction**

Recycling concepts for fiber-reinforced plastic (FRP) materials should be developed early on to improve the materials' ecological impact [1]. The reuse of FRP is a challenge due to the difficulty in the separation of the materials, and mechanical properties gradually deteriorate owing to the shortening of the reinforcing fibers with each recycling cycle. This affects both the matrix material used and the fibers. A reuse in the form of continuous fibers in the typical composite technologies is not possible, and alternative routes need to be established for the shortened fibers. This phenomenon has been extensively researched and reviewed in various articles. Examples and case studies were summarized by Pimenta et al. [2] and applications of recycled fiber composites were reviewed by Pickering et al. [3]. Oliveux et al. [4] gave examples of how recycled discontinuous fibers compare to virgin material. Pickering [5] analyzed and compared mechanical recycling and the use of fibers as filler materials to pyrolysis processes. This knowledge can nowadays be considered state-of-the-art, as seen in [6].

Since the fiber length deteriorates during recycling, discontinuous fiber technologies are particularly well-suited for the recycling of these materials, as can be seen in [2,3]. However, the material supply into, e.g., the injection molding process, still proves difficult. The authors present a novel method in this study. Here, end-of-life carbon fibers and production waste are blended with recycled thermoplastic matrix fibers and processed into a bundle of aligned fibers, or a "sliver." This sliver can then be processed via injection molding. Additionally, the sliver can be further processed into a yarn, which allows for established textile processing methods.

In the case of this study, the sliver was processed in a so-called injection molding compounder (IMC) utilizing a direct fiber feed for fiber incorporation. The injection molding compounder was first introduced by KraussMa ffei and enables the direct compounding and injection molding in one process step (single-stage) by combining a twin-screw extruder with an injection molding unit. The IMC and its advantages compared to a two-stage process (separate compounding and injection molding processes) have been proven in various research papers, e.g., by Truckenmüller [7]. Further studies analyzed the use of the IMC, e.g., for the processing of natural fibers [8] or the improvement of resulting fiber length—e.g., see Willems et al. [9] and Wellekötter et al. [10]. It could be shown that the IMC's single-stage process results in longer fibers compared to regular injection molding, thereby improving mechanical properties compared to short-fiber-reinforced thermoplastics [10,11]. The direct fiber feed module draws the fibers into the twin-screw extruder of the IMC near the end of the compounding step, reducing fiber damage and allowing for higher fiber length in the final part.

#### *1.1. Recycling of Carbon Fibers and Fiber-Reinforced Plastics*

Studies on the development of the composites market (e.g., [12–14]) indicate that the demand for FRP will increase significantly in the near future. For this reason, it is vital to address the challenges of recycling at an early stage. FRPs cannot be easily reused after their lifecycles, nor can they be easily separated, sorted, and treated. In addition, structural damage, such as delamination, is not necessarily visible from the surface, increasing the amount of FRP waste through the replacement of parts to comply with safety regulations.

Thermal recovery (combustion) and thus the recovery of stored energy is possible, although it is extremely unsatisfactory from an ecological and economic point of view, as shown by Meng et al. [1]. However, the reuse of parts from thermoplastic matrices as fillers (see below) in less stressed plastic parts is well-established (examples are summarized in [2,5].

Any other recycling process requires a high degree of purity of the fibers and matrices. Solvolysis (e.g., [4,15,16] and pyrolysis (e.g., [2,4,17,18] (combustion in the absence of oxygen) are already being applied industrially to recover carbon fibers. In pyrolysis, the matrix material is completely burned and energetically recovered, while the carbon fibers have a higher degradation temperature, remain largely inert, and thus retain their properties, albeit with shortened fibers due to previous processing. The fibers also lose their sizing during this process [2–4,12,18,19].

Since the fibers are not infinitely long anymore, but rather have a fiber length distribution depending on the parts which they incorporated, most of the conventional textile processing methods are not applicable. Further processing is limited to reusing these fibers in either compression molding, as reinforcements in the injection molding process, or as fibers for wet- or dry-laid non-wovens [3,18,20–23].

A promising approach is to align the fibers according to the load path and use their full potential as a pre-aligned non-woven (e.g., [23]). Several groups blended recycled carbon fibers with thermoplastic matrix fibers to improve processability and enable reuse in thermoplastic applications as hybrid non-wovens [22], yarns or tapes [24], and filaments for 3D-printing [25].

#### *1.2. Short Fiber Reinforced Thermoplastics*

In injection molding, fillers enable cost reduction, reduce shrinkage, and can even improve processing properties [11,26]. In contrast to fillers, plastics are reinforced to improve the material's mechanical behavior. Reinforcement is achieved with a component that has higher mechanical strength and sti ffness compared to the matrix. A su fficiently large length–thickness ratio (l/d) of the reinforcing component is required for e ffective reinforcement. Discontinuous fibers are often used for this purpose [11].

In addition to a high fiber–matrix adhesion, the fiber length defines the mechanical properties, as illustrated by Thomason [27] and shown in Figure 1. Fiber length influences sti ffness, strength, and impact resistance. In general, the longer the fibers in a matrix, the better their mechanical properties [11,27].

**Figure 1.** Influences of fiber length on stiffness, tensile strength, and impact strength.

It is, therefore, evident that the longest possible fiber length is desirable for recycled fibers during injection molding to optimize the properties of the new part. In this study, test specimens were produced from different material combinations in a direct processing method (single-stage) [11]. Compared to a regular two-step process, the one-step process allows for longer fibers and improved fiber qualities [11]. This is due to the elimination of repeated melting, the reduction of the fiber path in the extruder, and the lack of a pelletizing step [11]. Both short fibers and endless roving can be introduced into the already fully-plasticized melt via a direct-fiber-feed module. In addition, inexpensive raw materials can be used instead of cost-intensive fiber-reinforced pellets, in combination with large proportions of recycled material.

#### *1.3. Recycling of Plastics*

The recyclability of plastics is determined by their atomic and structural properties, intermolecular forces, and degree of cross-linking, and by different fillers and reinforcing materials [11,28]. Plastics can be recycled in various ways. Material recycling is the reuse of thermoplastic waste by remelting the material and thus creating new products, retaining the macromolecular structure of the polymer. This process is carried out on thermoplastics that have few or no impurities. It is also possible to filter out mixed and contaminated plastics by separation processes before recycling them [29]. If the processing is too expensive because of high levels of impurities or mixing, or if the quality is insufficient after too many recycling cycles, raw-material recycling can be performed. In this case, the macromolecular structure of the polymer is broken down, and the raw material is used for other products. If both methods fail, thermal recycling is an option, i.e., burning the plastic to generate energy [30,31].

Various factors determine the recyclability of thermoplastics and can influence the quality of the products. In addition to possible contamination by dust, paint residues, or mixing with other polymers, a recycling-related degradation of the polymer chains must be taken into account. Multiple uses and processing can lead to thermal, mechanical, hydrolytic, and oxidative degradation. Consequently, recycled plastics have different mechanical properties compared to virgin material. The change in molecular structure also influences the flow behavior of the polymer melt, possibly causing discoloration, burns, or bubbles in the product [32,33]. Important factors in thermal decomposition are the processing temperature and residence time. The presence of water can cause hydrolytic decomposition, especially in polycondensation products, such as polyamides. Furthermore, an oxidative decomposition process can occur during processing in an oxygen-rich environment [32,33].

In this study, a polyamide was used as a matrix material. A fraction of recycled material was added for all experiments. The recycling of the polyamide has been extensively analyzed, and a short summary of key insights is given below.

In 2003, the mechanical behavior of different fractions of recycled material (15, 30, 50, and 100%) of polyamide 6 (PA6) was investigated by Maspoch et al. [30]. The PA6 used for recycling came from fiber production waste and contained 20% glass particles and 10% glass fibers. The findings show that a small proportion of only 15% recycled material significantly reduced the Young's modulus, the maximum tensile stress, and the elongation at break. Similar behavior was shown for bending properties. Furthermore, reductions in the maximum tensile stress and Young's modulus were observed after three recycling cycles.

In 2013, Crespo et al. [34] investigated the effects of recycling PA6. Virgin plastic was recycled five times and analyzed after each cycle on injection-molded test specimens. The test results showed almost constant values for tensile strength and elongation at break. After the fifth cycle, a decrease of less than 2% in tensile strength and a decrease of 1% in elongation at break was observed. In comparison, the impact strength was significantly reduced even after the first recycling cycle. After the fifth cycle, the impact strength was reduced by more than 30% [34].

In another study, Su et al. [35] also investigated the effects of recycled PA6 on the mechanical properties of injection-molded parts; 16-fold recycling of the plastic was performed. A slight increase in yield stress was observed after each recycling cycle. Similarly to the study by Crespo et al., the elongation at break remained constant until it decreased after the 13th cycle. The flexural strength and modulus increased steadily. The impact strength steadily decreased, similarly to the results from Crespo et al., attributed to the shortening of the molecular chains and the broader distribution of the chain lengths. The originally soft and tough PA6 became hard and brittle with the large number of recycling cycles [35].

Recent studies [36] on the recycling of cast PA6 waste showed that the behavior of the material could be adjusted by compounding. Cast PA6 waste was converted into an extrudable polyamide with suitable additives. Rheological and mechanical analyses revealed that the lubricant leads to higher viscosity and impact strength. Recyclates modified with an oxidizing agen<sup>t</sup> resulted in higher degrees of crystallinity due to the reduced molecular weight and increased stiffness and tensile strength in the final material. Due to the shorter polymer chains, the recyclates had lower viscosities than comparable virgin grades. The material could, therefore, be processed by injection molding.

The authors of this paper further analyzed the recyclability of polyamide by comparing a single-stage and a two-stage process [10]. In the single-stage process, significantly longer fibers and thus better mechanical properties were achieved using direct feed module. The investigations showed that longer fibers could be reached at lower fiber contents. However, the influence of the recycled matrix fraction was negligible.

Pimenta [2] gives a detailed overview and a list of further studies about the incorporation of recycled glass and carbon fibers into the injection molding process (without using recycled matrix material). These studies are, therefore, not be presented here.
