**1. Introduction**

The early constructions of vehicles like automobiles, locomotives, and aircraft were designed using dense metals with high strength capabilities. However, recent advancements in material science have enabled fiber-reinforced composites to replace traditional metals because of higher strength-to-weight ratios [1–3]. For instance, aluminum has traditionally been one of the most common metals used in the aerospace industry, but its usage dropped from 50% in the Boeing 777 aircraft to only 20% in the Boeing 787 [1]. The advantages that fiber reinforced composite materials have over traditional metal materials include: (1) light weight, (2) high sti ffness and strength, (3) corrosion resistance, and (4) design flexibility. These attributes have been embraced by the automotive industry, which has increased its use of fiber-reinforced composite materials to improve fuel e fficiency and reduce greenhouse gas emissions.

Among all of the di fferent types of reinforcements utilized on a commercial scale, 65% of the revenue generated by the sale of fiber reinforced materials comes from glass fiber [4]. In 2020, the global glass fiber reinforced composite market is expected to grow to about 60 billion dollars [5]. Glass fiber is especially attractive as a reinforcement for composites because of its low cost, superior mechanical and physical properties (e.g., sti ffness and strength, impact resistance, stability, and durability). The tensile modulus and the strength of E-glass fiber are around 72 GPa and 3.5 GPa, respectively. This outperforms aluminum with a tensile modulus of 68.9 GPa and tensile strength of 310 MPa [6,7].

Thermotropic liquid crystalline polymers (TLCPs) are another type of reinforcement that is being extensively studied and used in both academia and industry [8,9]. Tremendous e fforts have been made toward the development of TLCPs that exhibit high modulus and strength coupled with outstanding melt processability [8,10–12]. The drawn TLCP filaments display a modulus of up to 100 GPa and tensile strength of about 1.5 GPa, which is comparable to the properties of E-glass fiber [13].

Both glass fiber and TLCP have excellent mechanical performance, high strength-to-weight ratio, and chemical resistance, but glass fiber is still a more attractive reinforcement choice over TLCP for three major reasons. First, TLCPs are more expensive than glass fiber. Depending on the grade of TLCP, the cost may range from eight to 12 dollars per pound [14]. Second, glass fiber has a higher tensile strength than TLCPs, especially when the TLCP is not generated using the fiber spinning or strand extrusion process. The full reinforcing potential of the TLCP fiber cannot be achieved under other processing techniques. Finally, glass fiber reinforced composites have lower mechanical anisotropy than their TLCP-filled counterparts. This is primarily due to the TLCP fibrils being created in situ under strong unidirectional elongation and shear flow [15].

One of the advantages of using TLCPs in reinforcing thermoplastic materials is the processability. It is well known that the incorporation of glass fibers into thermoplastics results in a substantial increase in viscosity, which gives rise to the di fficulty in processing and high energy consumption [8]. During the processing of a TLCP composite melt, rigid chain TLCP molecules adopt highly oriented states relative to the partially oriented flexible chain molecules displayed by conventional thermoplastics [16]. Therefore, the melt viscosity of TLCP reinforced composites become much lower than that of glass-filled composites, leading to more facile processing. In addition, the surface smoothness of TLCP reinforced composites is greater than that of composites reinforced by glass fiber only [17]. The higher surface smoothness is related to the diameter of TLCP fibrils, which is one order of magnitude less than glass fiber. Additionally, the density of TLCP is around 1.4 g/cm3, which is about half the density of E-glass fiber (2.58 g/cm3) [7,18]. The composite parts that utilize TLCP exhibit a lower weight than those of glass fiber, making the TLCP composite an attractive material, specifically for transportation applications. Finally, the recyclability of the TLCP composite has been found to be superior to glass-filled systems [19]. TLCP is able to regenerate a highly oriented molecular structure during the reprocessing while glass fiber would su ffer severe fiber breakage during the recycling process.

To capitalize on the advantages of using TLCPs and glass fibers, composites consisting of both TLCPs and glass fiber as reinforcements in thermoplastics have been studied [20–25]. The in situ hybrid composite consists of three components: microscopic TLCP fibrils, macroscopic conventional fibers (e.g., glass and carbon fiber), and the matrix polymer [25]. Bafna et al. [23] reported the use of glass fiber and TLCP reinforcements to enhance the mechanical properties and reduce the mechanical anisotropy of in situ TLCP composites with polyetherimide (PEI) as the matrix material. Tensile and flexural moduli increased and the anisotropy reduced with increasing glass fiber content. Furthermore, the creep performance of PEI composites improved when TLCP and glass fiber were blended together. Another study looked to combine the advantages of short fiber composites and the TLCP composite. He et al. [25] investigated the mechanical, rheological, and morphological properties of hybrid in situ carbon fiber or glass fiber/TLCP composite systems. Improvement in tensile and flexural properties, lower melt viscosity, and more oriented fibers in the flow direction have been observed.

Although composite materials have a variety of advantages, one of the major challenges for fiber reinforced composites is their recyclability. The disposal of composite waste in an environmentally friendly manner is a crucial task to our society. Typically, fiber-reinforced composite materials are very di fficult and energy intensive to recycle due to the nature of heterogeneity, technology limitations, high recycling cost, and low quality of recycled products. More restrictive environmental legislation drives the market toward recycling and reusing fiber reinforced composites. There are three major recycling methods to reclaim fiber reinforced composites: (1) thermal process, (2) solvolysis, and (3) mechanical recycling [26–29]. Mechanical recycling has less environmental impact, can recover both fiber and matrix polymer, and requires no use of solvents compared to thermal and solvent methods [30]. Mechanical recycling uses the principles of shredding or crushing the composite part into small particulates and then feeding these into a manufacturing machine to produce recycled parts. The recycled composites have very limited applications. Usually, the recycled composites, acting as

"filler", are blended with virgin materials to make a product with similar performance as new "virgin" parts. The incorporation level of the recycled composites is usually no more than 10 wt % to minimize the negative impact from the recycled materials [30].

Mechanical recycling is considered environmentally friendly and cost-e ffective. However, the application of this method is hindered because the recycling process reduces the performance of subsequent composite parts. Extensive investigations have been carried out on the influence of mechanical recycling on the properties of glass or carbon fiber reinforced composites [31–34]. The mechanical properties of fiber reinforced composites decreased significantly after mechanical recycling, which is mainly due to fiber attrition during the recycling process. The need for developing a recyclable and high-performance composite is becoming extremely urgent.

Previous work has explored the e ffect of mechanical recycling on the properties of TLCP and glass fiber reinforced polypropylene [19]. The results illustrated that the TLCP composite had superior recyclability relative to that of its glass fiber reinforced counterpart. It is of grea<sup>t</sup> interest to determine whether there exists a formulation of TLCP, glass fiber, and matrix polymer that may be mechanically recycled without compromising the mechanical performance of the subsequent composite part. The objective of this work was to utilize glass fiber and TLCP to develop a recyclable and high-performance in situ polypropylene-based hybrid composite. The in situ hybrid composites were mechanically recycled once by injection molding and grinding processes. The processing temperature was optimized through rheological analyses to improve the processability and reduce the thermal degradation of polypropylene. The optimal formulation of glass fiber and TLCP enables the best combination of high recyclability, high mechanical properties, and low mechanical anisotropy of the hybrid composite.

#### **2. Materials and Methods**
