1. Introduction
Marine debris presents a significant global environmental challenge, with an estimated 6.4 million metric tons entering the oceans annually [
1]. Among the various sources of this pollution, abandoned, lost, or discarded fishing nets are particularly concerning due to their substantial contribution to plastic waste in marine ecosystems. Historically, fishing nets were crafted from natural fibers such as cotton, jute, and hemp. However, since the 1950s, synthetic polymers have largely replaced these materials owing to their superior durability and resistance to water degradation [
2]. As a result, modern nets persist in the marine environment for decades, compounding pollution levels.
Derelict fishing gear, including nets and ropes, constitutes a major component of marine plastic pollution. Studies indicate that such gear accounts for approximately 46% of the 79,000 metric tons of plastic waste in the Great Pacific Garbage Patch [
3]. Additionally, a meta-analysis estimated that 5.7% of all fishing nets are lost to the marine environment annually [
4]. Even after being discarded, these nets continue to harm marine organisms through a process known as "ghost fishing," wherein entanglement leads to injury and mortality [
5].
Fishing nets are predominantly manufactured from nylon (polyamides). Nylon, a semi-crystalline thermoplastic, is highly valued for its thermal stability, mechanical strength, abrasion resistance, and durability. These properties make it a preferred material not only for marine applications but also for advanced uses in the automotive industry [
6].
In automotive manufacturing, the adoption of lightweight polymer materials has been driven by the need to reduce fuel consumption and minimize the carbon footprint of vehicles [
7]. This trend is further supported by global regulatory frameworks. In the European Union, Regulation EU 2019/631 mandates that all new passenger cars achieve emissions of less than 49.5 g CO₂/km by 2030 [
8]. Similarly, the United States Environmental Protection Agency (EPA) has set a target for new passenger cars to emit less than 73 g/mile of CO₂ by Model Year 2032 [
9].
The automotive industry utilizes approximately 35% of all polyamide materials [
10], with polyamide 6 (PA6) and polyamide 66 (PA66) being the most widely used. PA6 is commonly found in components like door handles, mirrors, fuel caps, wheel covers, gears, and bearings, while PA66 is often employed in under-the-hood applications including gears and leaf springs [
6]. Both PA6 and PA66 are often reinforced, most notably with glass fibers or carbon fibers, to increase rigidity, enhance mechanical properties, and reduce overall weight [
11,
12,
13,
14]. Güler et al. [
15] used a PA66 glass-fiber reinforced composite to manufacture an automobile hinge component and found that it satisfies the optimal dimensions of the component and reduces the weight of the component. Ishikawa et al. [
16] used carbon-fiber reinforced PA6 composite to manufacture a chassis of a vehicle originally made of aluminum alloy.
Despite the many advantages of glass fibers in composite applications, their production is highly energy-intensive and relies heavily on fossil fuels [
17]. Life cycle assessments reveal that manufacturing glass fibers can demand five to ten times more non-renewable energy than producing natural fibers [
18,
19]. Natural fibers are also less dense and often more cost-effective, providing additional benefits such as weight reduction [
20]. However, incorporating these fibers into polyamides poses a challenge because the high melting temperatures of polymers like PA6 and PA66 can degrade the fibers, ultimately diminishing the composite’s mechanical properties [
21].
Various natural fibers have been investigated in the literature as reinforcements for PA6 composites. For instance, Abdullah et al.[
22] fabricated kenaf fiber–reinforced PA6 composites and reported increases in both storage modulus and tensile modulus at a 10% fiber weight loading, primarily attributed to the enhanced stiffness provided by the fibers. Erbas Kiziltas et al. [
23]examined the impact of hemp, flax, and kenaf fibers, as well as their blend, on the thermal properties of PA6. Their results showed an increase in storage modulus for all fiber types, with the highest improvement (68% at room temperature) observed at a 20% fiber content.
Among the natural fibers of interest is switchgrass (Panicum virgatum L.), a warm season grass native to North America and Mexico, known for thriving in poor soils and unfavorable conditions and reaching heights of up to 366 cm [
24]. Switchgrass fibers stand out for their low cost, good quality, and sustainability, making them a promising reinforcement in polymer matrices [
25]. As demonstrated by Van den Oever et al. [
26], reinforcing polypropylene with 30 wt% switchgrass fibers yielded a 2.5-fold increase in flexural modulus. However, to our knowledge, no published studies have focused on using switchgrass fibers to reinforce PA6.
In parallel, the automotive sector has increasingly turned to recycled materials for more sustainable manufacturing. For instance, in 2021, Ford announced that its Bronco Sport features components made from 100% recycled ocean plastic [
27], and in 2022, BMW announced the use of discarded fishing nets in both visible and non-visible interior and exterior trim pieces for its Neue Klasse electric car series, aiming for the recycled material to constitute 30% of the total material content by 2025 [
28]. Yet, no published investigations have explored composites composed solely of natural fibers and a 100% recycled fishing net matrix for automotive applications.
To address these gaps, this study investigates the mechanical and thermal properties of a switchgrass fiber–reinforced composite fabricated from 100% recycled nylon fishing nets, aiming to offer a more sustainable alternative to commonly used PA6 in automotive components.
4. Conclusions
This study demonstrates the successful development of sustainable composites using recycled fishing net-derived PA6 reinforced with switchgrass fibers for automotive applications. Incorporating switchgrass fibers up to 30 wt% improves mechanical properties significantly. Compared to unreinforced PA6, the FN30SG composite shows a 23% rise in tensile strength and a 126% increase in Young’s modulus, making it competitive with commercial PA6 grades, especially for applications where stiffness is more critical than ductility.
Melt flow rate (MFR) measurements reveal a notable decrease in flow with higher fiber content, dropping from 19.35 g/10 min for neat PA6 to 8.63 g/10 min for FN30SG. This reduction in flow indicates increased melt viscosity, which can complicate processing due to reduced mold filling efficiency. However, beyond 20 wt% fiber content, MFR values level off, suggesting that fiber aggregation may limit further viscosity increases.
Thermal analysis shows that switchgrass fibers affect the crystallization of PA6, as DSC results indicate lower crystallization temperatures and overall crystallinity at higher fiber loadings. DMA confirms that storage modulus is enhanced below the matrix glass transition temperature. SEM images demonstrate strong fiber-matrix adhesion up to 20 wt% fiber, with some interfacial deterioration at 30 wt% loading. Alongside the plateau in mechanical properties and processing difficulties, these results suggest an optimal performance range of 20–30 wt% fiber.
Overall, recycled fishing net-derived PA6/switchgrass fiber composites offer a promising, more sustainable alternative to virgin PA6, particularly for automotive components needing high stiffness and moderate strength. Future work should focus on refining processing conditions and exploring surface treatments to further improve fiber-matrix adhesion at higher fiber content.