**1. Introduction**

Problems related to petroleum resource depletion and the potential dangers of microplastics are continuously raised, and the need for eco-friendly materials continues to rise. The consumption of reusable or biodegradable natural polymers is essential, especially when taking raw materials into account. Since ancient times, natural fibers, such as wool, silk, and cotton, have been used in clothing, and cellulose, which is a wood-based polymer, has been actively employed in the paper industry. In addition, since the inception of FRPs, studies using natural fibers as reinforcing agents have been repeatedly reported.

Natural fibers are not only abundant and inexpensive, but are also biodegradable due to their nature-derived properties. Therefore, researchers have examined their application in FRP reinforcement [1]. However, the hydrophilic properties of natural fiber reduce compatibility with synthetic polymer matrices. Chemical modification of natural fiber surfaces has been attempted to improve the compatibility between natural fibers andmatrix polymers [2]. Although, this surface modification process mostly inhibits the economical and environmentally-friendly properties of natural fibers.

Nanocellulose is a nanomaterial obtained from wood and agricultural byproducts [3–6], as well as from bacteria [7,8], and has a diameter numbering tens of nanometers [9]. This nanomaterial has low density, high strength, and excellent transparency [10]. As a result, research is being performed to utilize nanocellulose in various applications ranging from food packaging materials, composite-material nanofillers, electronic devices, and biomaterials [11]. Due to the nanomorphology of nanocellulose and the resulting strong mechanical strength, composite materials with various general-purpose polymers, such as polyolefin and polyester [9], are also being developed [12]. In addition, as biodegradable polyesters such as polylactic acid (PLA), polycaprolactone (PCL), and polybutylene

**Citation:** Bang, J.; Lee, H.; Yang, Y.; Oh, J.-K.; Kwak, H.W. Nano/Micro Hybrid Bamboo Fibrous Preforms for Robust Biodegradable Fiber Reinforced Plastics. *Polymers* **2021**, *13*, 636. https://doi.org/10.3390/polym1304 0636

Academic Editor: José Miguel Ferri

Received: 4 February 2021 Accepted: 18 February 2021 Published: 20 February 2021

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succinate (PBS) were developed, studies to apply nanocellulose to compensate for the insufficient mechanical strength were also actively conducted [7]. However, nanocellulose also has hydrophilic properties similar to natural fibers and wood-based biomass, which are their raw materials. Thus, insufficient compatibility with a polymer matrix is considered a limitation [13]. Biomimetic methods have been tried to overcome this drawback. When bacteria biosynthesized nanocellulose, a water-soluble polymer, such as hydroxyethyl cellulose or polyvinyl alcohol, was introduced to prepare a nanocellulose/polymer composite [7,14]. These biomimetic nanocellulose/polymer composites showed higher tensile properties than composites prepared by simple mixing [12]. However, this method has the disadvantage of interfering with the cellulose biosynthesis of bacteria, and mass production is impossible [15,16].

Recently, attempts have been made to use nanocellulose as a "green" binder in the manufacture of preforms for hierarchical composites. Lee et al. used bacterial cellulose as a binder to prepare sisal fiber preforms for the reinforcement of polyacrylated epoxidized soybean oil (AESO) [16]. As a result, it is evident that the mechanical properties of the preform improved significantly. The mechanical properties of poly AESO/sisal preforms manufactured by thermal curing following vacuum-assisted resin injection are also extensively improved. It is known that the excellent binding and adhesion ability of these nanocelluloses can be clearly expressed in natural fibers, including sisal and silk fibers [17,18]. However, most of the research results indicate that bacterial cellulose was used as a binder [19], and there has been no effort to manufacture a hierarchical fibrous-composite preform using microfibers and nanocellulose produced from the same tree species.

Bamboo is a plant resource that is cultivated worldwide [20], especially in Asia and South America. Since ancient times, bamboo has been steadily used in furniture and construction materials due to its rigid mechanical properties [20]. In addition, as bamboo fiber extraction technology advances, attempts to use it as a sustainable raw material for high-performance composite materials and advanced carbon materials are steadily progressing. [21–23]. Furthermore, various types of nanocellulose manufacturing technologies utilize bamboo fibers as raw materials [24–26]. Therefore, in the development of eco-friendly composite materials, it is very attractive to develop a facile preforming process using the hierarchical nano/micro morphology of bamboo fiber [27,28].

This study's primary purpose is to fabricate a fibrous composite preform with improved mechanical properties through a multiscale, hybridizing process using bambooderived microfibers and cellulose nanofibers. First, the optimal preparation conditions (temperature and time) were established to minimize the energy consumption required to manufacture the fibrous composite preform. The physicochemical properties of the prepared micro/nano bamboo-fibrous composite preform were analyzed according to the nanocellulose content, and the changes in mechanical properties were confirmed accordingly. Finally, FRP was prepared using PBS, which is an actual biodegradable polyester, and changes in mechanical properties of FRP due to the reinforcing effect of nano/microfiber preforms were observed.
