1. Introduction
With benefits such as toolless material processing, high geometric freedom, fast prototyping, and cost-efficient small-scale production, additive manufacturing (AM) has the potential to revolutionize the manufacturing industry [
1]. Depending on the form of material and the type of extruder, extrusion-based AM can be divided into filament fused fabrication (FFF) and fused granular fabrication (FGF), among others [
2]. FFF uses high-quality, not too brittle or too flexible filament with a specific and constant diameter [
3]. So, only certain materials with the appropriate mechanical properties can be processed by FFF. In comparison, the FGF method is not so limited by the variety of materials [
3], while all industrial polymers can be found as pellets [
4]. Using polymeric pellets as a feedstock material can improve production times by up to 200 times [
5] and reduce costs by a factor of 10. This is due to the fact that an additional filament-extruding step is not required during the pellet-based AM process [
6]. In addition, the one-step preparation of feedstock for FGF excludes a second thermal processing of the polymers, which always reduces their molar mass [
3].
Biodegradable polylactic acid (PLA) filaments are one of the most widely used extrusion-based 3D printing feedstocks [
3]. PLA is an eco-friendly polymer material [
7] based on plant materials [
8]. It is a three-carbon-membered thermoplastic with one hydroxyl and one carbonyl at the end. It has a prolonged biodegradation rate and is brittle despite its low degree of crystallinity [
9]. However, the role of biodegradable plastics in solid waste management is somewhat controversial because of their slow degradation rate and their possible interference with plastic recycling efforts [
10]. Therefore, increasing the production of PLA might cause some problems, mainly related to managing the waste generated after its use [
11]. One way to utilize PLA waste is composting [
12]. However, this method is used to degrade industrial waste, where a large amount of waste is collected daily [
12]. Hence, considering that PLA and the construction of composting facilities are expensive [
13,
14], reprocessing scrap materials could be an interesting way to save costs [
15,
16].
Some studies revealed that coupling open-source 3D printers with polymer processing could offer the basis for a new paradigm of the distributed recycling process [
1,
8,
17,
18]. The main conclusion from these studies is that 3D printing with recycled PLA is a viable option. A common disadvantage of filament-fed printers reported in these works is nozzle clogging during the reprinting of recycled materials. On the other hand, these studies showed the decreasing tendency of mechanical properties with the addition of recycled content [
1,
8,
17,
18]. This limitation can be solved by adding nanofillers, which could also add functionalities to the produced nanocomposites [
19]. The physical, chemical, and mechanical improvements are significantly higher than the more traditional polymer composites with micron-sized fillers [
20].
Among the many inorganic materials available today, nano-TiO
2 has received most of the attention [
9] because it is nearly non-toxic, inert, optically transparent, biocompatible, environmentally friendly, and inexpensive. Therefore, nano-TiO
2 has been widely introduced into polymers to improve heat resistance, radiation resistance, mechanical and electrical properties [
13], and bacteriostatic and photocatalytic activity [
21]. So, the nanoalloy of Ti has a great potential to act as a reinforcing material in PLA composites compared with natural fillers [
12]. According to the literature, plentiful dangling bonds exist on the surface of nano-TiO
2, which could interact with polymer molecules, thus improving the properties of nanocomposites [
22]. The literature also revealed that 0.5% to 8% nanofiller reinforcement is sufficient to strengthen the polymer mechanically and thermally. Apart from that, PLA is less susceptible to photodegradation than TiO
2 nanoparticles; hence, TiO
2 nanoparticles can improve the photodegradability of PLA [
12].
Several investigators have fabricated nanocomposites by reinforcing titanium dioxide in the PLA matrix. The results of Zhuang et al. [
10] show that the thermal and mechanical properties are markedly improved when the content of TiO
2 is 3 wt% in the PLA/TiO
2 nanocomposites prepared by in situ polymerization. Buzarovska et al. [
23] produced nanocomposites with 0.5, 1, 2, 5, and 10 wt% TiO
2 by solution casting. Zhang et al. [
22] employed a vane extruder to compound PLA/TiO
2 nanocomposites with 0, 0.5, 1.0, 2.0, 5.0, 10.0, and 15.0 wt% TiO
2 and prepared the samples by injection molding. The prepared nanocomposites showed improved thermal stability for all samples and improved tensile strength in the samples by up to 2%. Nakayama et al. [
24] proved that the tensile behavior of PLA films with 10% nano-TiO
2 was similar to pure PLA. All these studies showed a rising trend in tensile strength when a uniform dispersion of nanoparticles in the matrix of PLA is achieved up to a certain amount of nano-TiO
2. This upper limit depends on the manufacturing technique, and to the best of our knowledge, nobody has reported the maximum amount of nano-TiO
2 that produces the best enhancement of the tensile strength on FGF PLA-printed parts. Thus, it can be concluded that the effects of different processing flow fields on the degree of dispersion and the mechanical behavior have not been investigated in detail yet [
22]. Also, in 3D printing, mechanical performance depends on the product’s layer adhesion [
25], as the bonding strength between two consecutive layers is a weak point of layer-by-layer construction [
26].
The aim of this research is to study the possibility of enhancing PLA recyclability by hybridizing reprocessed PLA (rPLA) with virgin PLA and nano-TiO2 and using the resulting material as a feedstock for FGF to produce high-quality parts. To achieve this goal, two types of nanocomposite pellets incorporating neat PLA with nano-TiO2 and a blend of neat and rPLA with nano-TiO2 were prepared. Secondly, samples were FGF-printed from the prepared nanocomposite pellets. This printing technology was preferred because it reduces nozzle clogging during printing and makes filament production unnecessary, saving PLA from additional thermal degradation. Then, the morphology, thermal, and mechanical properties of the produced samples were investigated. Subsequently, the influence of the addition of nano-TiO2 and rPLA to neat PLA on the interlayer adhesion of 3D printed samples was analyzed. Finally, the mass fraction of TiO2 that improves the mechanical properties of PLA nanocomposites produced by FGF was determined. Although not the focus of this study, the addition of nano-TiO2 may also provide further functionalities to the nanocomposites, such as UV resistance and antibacterial activity. Once the feasibility of using the proposed nanocomposites for FGF and their mechanical reinforcement capabilities are proven, this study will serve as a basis for investigating these additional properties of PLA.
The novelty of this work lies in using nanocomposite pellets of PLA and rPLA with the addition of nanoscale titanium dioxide as a feedstock for FGF technology. To the best of our knowledge, there has been no previous report on the preparation and properties studies of 3D printed nanocomposites from PLA with nano-TiO2 (PLA/TiO2) and from a mixture of PLA and rPLA with nano-TiO2 (PLA/rPLA/TiO2).