Toward a circular economy: zero-waste manufacturing of carbon fiber-reinforced thermoplastic composites

Fiber-reinforced composites are crucial for lightweighting in aerospace, wind energy, and automotive industries due to their high strength-to-weight ratio and excellent fatigue properties. However, the automotive industry faces unique challenges concerning end-of-life costs and recyclability mandates (e.g., the European Union's End-of-Life Vehicle Directive requiring at least 95% reuse or recovery). Traditional thermosetting matrix composites are not easily recyclable, posing a significant hurdle. Thermoplastic matrix composites, such as those using PPS, offer an alternative due to their recyclability. Furthermore, the increasing demand for carbon fiber necessitates the use of recycled carbon fiber, which offers significant cost and energy savings compared to virgin fiber production. While thermoplastic composite recycling has been studied, focusing on discontinuous and continuous fiber composites and injection molding, challenges remain in retaining fiber length and achieving homogenous fiber dispersion, crucial for maximizing the mechanical properties of the remanufactured composites. This research explores these challenges, focusing on the effects of fiber length and dispersion on the properties of recycled carbon fiber/PPS composites.

Existing literature highlights the challenges and approaches in thermoplastic composite recycling. Some studies have investigated the recycling of injection-molded composites, observing significant reductions in tensile strength and modulus after recycling, often attributed to fiber length reduction. Other research has explored integrating continuous fiber-reinforced thermoplastic grinds into injection molding compounds, showing improvements in strength and modulus but still limited by short fiber lengths. The critical fiber length, the minimum length required for a fiber to break before being pulled out of the matrix, is a key parameter. Previous work also investigated compression molding of thermoplastic composite scrap to retain fiber length, achieving mixed results in terms of strength retention. This study builds on these findings, focusing on controlling fiber length through sieving and evaluating its impact on mechanical properties and microstructural homogeneity.

Recycled carbon fiber/PPS scrap from organosheet composite manufacturing was size-reduced using a hammer mill and sieved through various mesh sizes (6.35 mm, 4.75 mm, 2.36 mm, 2.0 mm, 0.85 mm, and 0.425 mm) to control fiber length. Composites were then manufactured using both compression and injection molding. Compression molding involved using organosheets, hand-cut platelets, sieved recyclate, and wet-laid (WL) mats (with both recycled and virgin fibers). Injection molding utilized sieved material compounded with neat PPS, with a commercial 50 wt% carbon fiber/PPS compound serving as a benchmark. Thermal properties were evaluated using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Tensile properties were determined using ASTM D3039 and D638 standards, with strain measured using extensometry and digital image correlation (DIC), respectively. Microstructural analysis employed optical microscopy to determine void content and fiber volume fraction, and scanning electron microscopy (SEM) to examine failure surfaces and fiber dispersion. A dispersion index (DI) was calculated to quantify fiber dispersion. Statistical analysis, including two-sample t-tests, was used to compare material properties.

Thermal analysis showed increased PPS crystallinity with added carbon fibers, influenced by molding pressure and cooling rate. Compression-molded zero-waste composites exhibited lower tensile properties than organosheet composites, with decreased strength and modulus observed with shorter fiber lengths. The failure strain decreased significantly, attributed to localized failure in matrix-rich regions due to microstructural inhomogeneity. Statistical analysis showed that strain-to-failure for many sample types was not significantly different, but strength and modulus were more strongly affected by particle size. A simple Rule of Mixtures reasonably predicted the modulus of the mixed recyclate composite but not the strength. Injection-molded specimens achieved higher tensile strength than compression-molded ones due to fiber alignment and reduced defects. The commercial compound showed higher modulus but lower tensile strength due to short fiber lengths and agglomerations. Statistical analysis revealed that strength and modulus decreased with decreasing fiber length, while the strain-to-failure was similar for recycled fiber composites. The dispersion index (DI) indicated that poor fiber dispersion led to reduced strength, although longer fiber lengths could offset this effect. SEM imaging confirmed the increased presence of matrix-rich regions in the zero-waste composites, and fiber pull-out increased with decreasing fiber length in the injection-molded specimens. Void content was generally low in the sieved composites but higher in the organosheet and wet-laid composites. The fiber volume fraction decreased with decreasing sieve size, likely due to the presence of neat PPS particulates. The economic analysis suggested that mechanical recycling, with size exclusion using sieving, could be a more viable alternative recycling strategy than pyrolysis, offering lower energy usage and reduced greenhouse gas emissions.

The findings demonstrate that both fiber length and microstructural homogeneity significantly influence the mechanical properties of recycled fiber-reinforced composites. Sieving effectively reduces variability in properties, but it does not address fiber length retention over multiple recycling cycles. The observed reduction in strength, despite long fiber lengths in some cases, highlights the critical importance of homogenous fiber dispersion in achieving optimal mechanical performance. The lower energy usage of mechanical recycling with sieving compared to pyrolysis suggests that this strategy may offer a more sustainable path toward a circular economy for carbon fiber-reinforced composites. The economic analysis points to the need for further optimization of recycling logistics to enhance economic feasibility.

This study demonstrates the successful recycling and remanufacturing of carbon fiber/PPS composites using size-exclusive sieving to control fiber length and reduce variability. While sieving improves the properties of recycled composites, maintaining fiber length and ensuring homogeneous dispersion remain critical challenges. Future research should focus on developing technologies that retain fiber length and improve fiber alignment and dispersion during remanufacturing to enable the transition to a truly circular economy for fiber-reinforced composites.

The study's limitations include the use of a specific type of recycled carbon fiber and PPS resin. The results may not be directly generalizable to other types of fibers or thermoplastic matrices. The economic analysis focused on energy consumption and labor costs, but did not include all factors like capital investment and transportation costs comprehensively. Future work could explore a broader range of materials and factors to enhance the generalizability of the results.