Engineering response of biomedical grade isotactic polypropylene reinforced with titanium nitride nanoparticles for material extrusion three-dimensional printing

Abstract

Herein, a new generation of printable bio-composite nanofilaments is explored. The aim was to explore the efficiency of Titanium nitride (TiN) as a reinforcing agent for the Fused Filament Fabrication (FFF) 3D printing technique. TiN, a common ceramic in medical devices, in nanopowder form, was used as a polymer enhancer for the also popular medical devices polypropylene thermoplastic. Such filaments with superior performance are needed for demanding app­lications, in which common polymers do not meet the specifications. To fully comprehend the material qualities and behaviour, a variety of tests, including mechanical testing, thermal and rheological investigation, and spectroscopic analysis, were carried out following standards. The effect of the filler percentage was also considered. The morphological properties of the materials were also assessed. The mechanical response of the nanocomposite with a ‑2.0 wt. % filler concentration showed the best improvements in all experiments conducted, including tensile (41.5%), flexural (33.7%), and impact (18.0%) strength. The microhardness was improved 44.8% by the 2.0 wt. % filler concentration nanocomposite. Overall, the efficiency of TiN as a reinforcement agent in FFF 3D printing was proven, making the proposed nanocomposites a sufficient solution when improved performance is required.

Graphical Abstract

Keywords:

• Polypropylene (PP)
• titanium nitride (TiN)
• mechanical characterization
• nanocomposites
• three-dimensional (3D) printing
• material extrusion (MEX)

1. Introduction

In the last decades, there has been a lot of scientific and industrial interest in additive manufacturing (AM), encompassing a variety of sectors, such as advanced fibre-reinforced polymer (FRP) composites (Shanmugam et al., 2021), and scaffolds for tissue engineering (Alam et al., 2020; Imran et al., 2023). With its capabilities for large-scale manufacturing and prototyping, AM is currently at the forefront of processes for industrial production (Mehrpouya et al., 2019). The most prominent AM technology is three-dimensional (3D) printing, which has been extensively studied in recent years and is steadily becoming more established for applications in industry in place of traditional manufacturing processes (Garzon-Hernandez et al., 2020; Rouf et al., 2022). Fused Filament Fabrication (FFF) is distinguished as a very promising 3D printing method because of its adaptability, scalability, and affordability (Kim et al., 2021; Ngo et al., 2018).

FFF is currently constrained by the limitations of the existing thermoplastic materials, which affect the quality of 3D-printed items and the final characteristics of 3D-printed objects (Vidakis et al., 2022). The physical and chemical characteristics of the filaments could provide the 3D-printed parts with multifunctional features, adding value to the prospects of the final products (Ee & Yau Li, 2021). Additionally, polymer mixtures and composites feature rheological properties which can be adjusted to produce high-quality 3D printed products with elevated performance (Chacón et al., 2017). The need for 3D printing-compatible materials with multifunctional qualities—enhanced mechanical properties and other physical traits—is now on the rise (Gnanasekaran et al., 2017). To this end, both inorganic, such as ceramics (Vidakis et al., 2022), and organic fillers, such as biochar (Vidakis et al., 2023) and cellulose (Vidakis et al., 2022; 2023) have been used as additives, with both categories achieving notable results, i.e., nanocomposites suitable for specific types of applications. For example, in the medical field, medical-grade materials and organic fillers are preferable (Vidakis et al., 2022; 2023), while antibacterial performance is also a desired specification (Vidakis et al., 2022).

In the same category as other polyolefin thermoplastic materials including low-density polyethylene (LDPE), high-density polyethylene (HDPE), Polystyrene (PS), and polyethylene (PE), is the polypropylene (PP) thermoplastic, featuring a semi-crystalline structure (Van Belle et al., 2020). PP is the most widely used thermoplastic among them (Koerner et al., 2007; Razavi, 2001). PP is a popular polymer used in various industrial applications, such in the automotive industry (Hariprasad et al., 2020; Jansz, 1999), in medical (Adomavičiūtė et al., 2018; Gopanna et al., 2019; Ma et al., 2021; Van Lierde, 2004), and energy (Li et al., 2021) applications, due to its high energy storage capacity. This is mainly due to its exceptional mechanical qualities, including mechanical strength, as well as its accessibility, production simplicity, and great chemical stability (Jin et al., 2020; Mohan Bhasney et al., 2020). Its durability is such that is used as reinforcement in fibre form in concrete (Liu et al., 2021). For these reasons, along with its ability to be sterilized, its use in medical applications has increased over time (Adomavičiūtė et al., 2018; Ma et al., 2021; Van Lierde, 2004). Thermal shrinkage is a challenge in 3D printing because it results from the crystallization of PP while cooling. The bonding of nearby filament strands, the fusing of layers, and the adherence of the printed layers to the build plate are all impacted by this shrinking. Consequently, to achieve successful and trustworthy 3D printing with PP materials, these aspects must be carefully considered and addressed (Valino et al., 2019). The effect of the 3D printing settings on the morphology of the PP 3D printed parts has been investigated in the literature (Petersmann et al., 2020).

Polypropylene nanocomposites, including both syndiotactic polypropylene (s-PP) and isotactic polypropylene (i-PP), have been the subject of several research studies (Thomann et al., 1996; Tsioptsias et al., 2020; Uehara et al., 1996). Numerous features of polypropylene have improved significantly as a result of these investigations, including its crystallization behaviour (Seo et al., 2000; Truong et al., 2011), electrical conductivity (Foss & Dannhauser, 1963; Wang et al., 2020), and thermomechanical characteristics (Mourad, 2010). Sustainable, eco-friendly additives, such as recycled glass fibres (Sam-Daliri et al., 2022; 2023) and basalt fibres (Ghabezi et al., 2022) have been used as reinforcement for recycled PP polymer in 3D printing as well, highlighting the sustainability of the material and the potential of sustainable eco-friendly composites in 3D printing. Because of its toughness, chemical resistance, bacterial resistance, and transparency, PP finds application in a variety of medical fields, particularly in packaging. Medical-grade PP also demonstrates excellent resistance to steam sterilization. PP is extensively utilized in the production of disposable syringes, offering numerous advantages over traditional glass syringes including crack resistance, lightweight design, easy disposability, leak prevention, environmental friendliness, sterilizability, and clarity. Additionally, PP is employed in the manufacture of non-absorbable surgical sutures (Joseph et al., 2021). These results demonstrate the versatility and potential of polypropylene nanocomposites. Overall, the introduction of nanoparticles in polymeric matrices can induce various types of properties and enhancements in the matrices, from areas such as the improvement of health safety in food applications (Chen et al., 2020) to treatment in medicine (Ziental et al., 2020), up to the improvement of corrosion resistance in coatings (Yuan et al., 2020).

In a recent study, Aumnate et al. focused on adding graphene-polylactic acid (PLA) microcapsules to improve the characteristics of polypropylene (PP) for FFF 3D printing. Surprisingly, the 3D-printed device showed no signs of shrinking or warping even with a low concentration of graphene (0.75 wt. %). Additionally, the construct’s mechanical performance when 3D printed using a graphene nanocomposite with a 30% volume percentage infill outperformed pure PP. These results show the possibility of graphene reinforcement to enhance the mechanical characteristics of PP items produced by 3D printing (Aumnate et al., 2021). The effects on the characteristics of PP polymer when adding TiO₂ as a nano-additive at low weight-to-weight percentages were examined in another investigation conducted by the current research team. According to the results, the characteristics of the nano-compounds enhanced as the filler concentration increased. The material’s processability, however, was not noticeably impacted (Vidakis et al., 2022).

The potential of nitride particles as fillers for the development of polymer nanocomposites has been assessed in numerous research. The thermomechanical behaviour of the generated composite materials has been considerably improved thanks to the efficient reinforcement provided by these nanoparticles when added to polycarbonate and other matrices (Guan et al., 2019; Khan et al., 2022). Nitride nanoparticle incorporation in polymeric matrices has the potential to improve the mechanical and thermal characteristics of the matrices, broadening the scope of their potential applications (Vidakis et al., 2023). The durability of metal nitrides as additives to polymers has been investigated and quantified (Setoura & Ito, 2021).

Due to its outstanding resistance to wear, corrosion, and erosion, Titanium Nitride (TiN), a ceramic substance, has attracted substantial attention. Due to these characteristics, titanium nitride is highly sought-after for applications where toughness and resistance to mechanical deterioration are essential (Santecchia et al., 2015; Wu et al., 1990). Due to its durability, it is used in coating in various demanding applications, such as in fuel cells (Bi et al., 2021). As expected, it is used in coatings for cutting tools, which are among the most demanding fields of application (Bouzakis et al., 1998; 2000). In recent research, mercaptopropylmethyldimethoxysilane (MPMDMS) was used with titanium nitride (TiN) nanoparticles, combined with polyurethane (PU) to create a nanocomposite covering. The outcomes showed that the newly created PU-MPMDMS/TiN nanocomposite has improved hydrophobic characteristics, as shown by an angle of contact with water (WCA) of 157°. According to the study’s findings, adding MPMDMS/TiN nanoparticles to polyurethane resin produced good hydrophobic, and mechanical qualities (Xavier & N, 2022). As expected, the superior properties make TiN a common material in medical applications, mainly in coatings (Gao et al., 2020; Jin et al., 2013; Kazemi et al., 2020). Recently, studies have reported its reinforcing performance as an additive in polymeric matrices, in FFF 3D printing. More specifically, studies have been reported with Polylactic Acid (PLA) (Petousis et al., 2023), Acrylonitrile Butadiene Styrene (ABS) (Vidakis et al., 2023) Polycarbonate (PC) (Vidakis et al., 2022), and medical grade Polyamide 12 (PA12) (Vidakis et al., 2022). The reinforcing effect varies from less than 20% improvement in the tensile strength (Vidakis et al., 2023), to more than 45% (Vidakis et al., 2022), showing a high potential as a reinforcing agent for the improvement of the mechanical performance of polymers in FFF 3D printing. Such differentiation in the results justifies the need for individual studies for each polymeric material.

An innovative strategy is presented herein to assess the potential of titanium nitride (TiN) as a nano-additive in medical-grade PP polymeric matrix, aiming to comprehend, evaluate, and report its effect on the PP mechanical performance. The aim was to propose nanocomposites for the FFF 3D printing technique with improved mechanical performance, expanding the fields of application and the efficiency of the PP polymer. TiN was selected as an additive, due to its use in medical applications, as presented above. The hypothesis was to verify its efficiency as a reinforcement agent in FFF 3D printing and evaluate aspects such as its effect on the thermal stability of the polymer, processability issues, etc. No similar research and nanocomposites in FFF 3D printing have been presented so far to the authors’ best knowledge. The PP thermoplastic is also used in medical applications and herein a medical grade was assessed, toward the development of a new generation of biocompatible nanocomposites, with increased performance. Such PP/TiN nanocompounds were successfully created for the first time to be compatible with material extrusion (MEX) 3D printing, also exploiting the advantages of the method. Through the use of additive manufacturing, this technique allowed the incorporation of TiN particles into the PP matrix, resulting in the creation of new nanocomposites with improved and customized properties. The PP/TiN nanocomposites’ mechanical response has been assessed, and attempts have been made to improve the mechanical characteristics by adjusting the TiN additive’s content in the nanocomposites. The goal of the research was to find the equilibrium between the toughness, strength, and other important attributes of the PP/TiN nanocomposites by methodically examining the impact of TiN nanoparticle concentration. Thermogravimetric Analysis (TGA) was used to evaluate the effect of the TiN additive on the thermal stability of the PP matrix. At the same time, the authors wanted to ensure that the temperatures used during the extrusion process employed for the nanocomposite filament production, do not cause any degradation in the nanocomposites. This investigation shed light on the stability of the nanocompounds at various temperature ranges and the thermal degradation of the prepared nanocomposites. The chemical and elemental composition of the nanocomposites were also investigated and identified using Raman and Energy-dispersive X-ray spectroscopy (EDS). Along with the analyses already described, morphological studies were carried out. Atomic force microscopy (AFM) was used to inspect the filaments’ surface morphology at a microscopic level, yielding details regarding their topography and surface characteristics. The final 3D-printed objects’ morphological properties were examined using scanning electron microscopy (SEM). Combining AFM and SEM allowed for an in-depth comprehension of the morphological features of the filaments and the 3D-printed items, yielding important information about the MEX printing procedure. Among the various weight-to-weight percentages (wt. %) of TiN filler examined, the PP/TiN nanocomposite with 2 weight percent additive content showed the most noticeable increase in all mechanical parameters. In the field of MEX 3D printing, the creation of such materials with improved mechanical performance is widely desired because it increases the potential uses of this additive manufacturing technique.

2. Materials and methods

Figure 1 shows the method developed and followed for the current investigation, which includes the drying process of the raw materials, the methods engaged to manufacture the test samples, the examination of the filament’s quality, and the subsequent investigation of the thermal, morphological, rheological, and mechanical characteristics of the 3D printed specimens.

Figure 1. Workflow for the presented research (A) raw materials, (B) drying process, (C) filament extrusion, (D) filament drying, (E) filament quality control, (F) filament mechanical testing, (G) samples 3D printing, (H) samples quality control, (I) three-point-bending mechanical testing, (J) Charpy impact test, (K) rheology, (L) morphological characterization with SEM.

2.1. Materials

Raw materials were used to create the study’s nanocompounds, employing the material extrusion procedure. For this research, KRITILEN PP, a coarse powder obtained from Plastika Kritis S.A. (Greece), served as the matrix material. Titanium nitride (TiN) was employed to improve the PP polymer’s mechanical performance. It was purchased from Nanographi in Ankara, Turkey, in the form of nanopowder with the following characteristics: Cubic form, size 20.0 nm, true density 5.30 gr/cm (Imran et al., 2023), purity of 99.2%, and a melting point of 2950 °C (data from the manufacturer web site, accessed 10/02/2024, https://nanografi.com/nanoparticles/titanium-nitride-tin-nanopowder-nanoparticles-purity-99-2-size-20-nm-cubic/).

2.2. Nanopowder examination and nanocomposite fabrication

Scanning electron microscopy (SEM) was utilized to evaluate the morphological characteristics and the chemical composition (in Energy-dispersive X-ray spectroscopy – EDS mode) of the TiN powder. The JSM-IT700HR, a field emission SEM made by Jeol Ltd, Tokyo, Japan, was the apparatus used for this study. SEM enabled high-resolution photographs for the examination of the TiN powder, allowing for the microscopic characterization of its shape, size, and elemental analysis through Energy-dispersive X-ray spectroscopy (EDS). Figure 2 shows the results of the EDS analysis. Figure 2C shows the identification and the amount of the titanium (Ti) and nitrogen (N) elements, which displayed the most noticeable peaks in the EDS analysis. Additionally, oxygen was found (about 10.70% of the mass), which may have been caused by moisture or exposure to outside air when the nanoparticles were being handled or stored. Chloride was also discovered in a small amount of the mass (0.42%). Even in tiny amounts, the detection of chlorine may suggest the existence of trace pollutants or impurities in the TiN nanopowder. The elemental composition of the TiN particles agrees with the components mentioned in the powder manufacturer’s specifications (Elemental Analysis according to manufacturer: Ti of 77.8 wt. %, N of 21.90 wt. %, C of 0.0002 wt. %, and Fe of 0.0015 wt. %). The form of the TiN particles was verified by the SEM images, as shown in Figures 2D, and E. Figure 2F displays the EDS mapping for the Titanium (Ti) element of the TiN nanopowder. Ti is evenly distributed in the observation region, as the mapping demonstrates.

Figure 2. The investigation of TiN nanopowder: (A) a picture of the TiN nanopowder that was used in the research, (B) the weighing of the powder to guarantee the precise amount of the TiN additive in the nanocompounds, (C) an examination of the composition of the elements using EDS, (D) SEM pictures taken at 10,000× and (E) 100,000x, and (F) EDS map showing an even distribution of the Ti element in the inspection area.

The raw materials underwent a drying procedure at 60 °C for 24 hours before the creation of the nanocomposites to eliminate any remaining moisture. To guarantee the final nanocomposites’ quality and integrity, this procedure was essential. By varying the weight percentage (wt. %) of TiN, which ranged from 0.0 to 6.0 wt. %, five distinct material blends were created. In each one, the remaining weight was the PP material. No other additives were used. The loading was increased in the nanocomposites and tests were carried out. When the performance of the nanocomposites started to constantly decrease, this was an indication that the filler started to saturate the matrix, so the loading was not further increased. Thus the 6 wt. % maximum loading in the nanocomposites.

During the manufacturing process, a powerful blender working for 30 minutes at typical ambient conditions of temperature (23 °C) at 4000 rpm, was used for the initial dispersion of TiN nanoparticles throughout the PP polymer. After the blending stage, the mixtures went through one more drying cycle. A Noztek extruder (from Noztek, Shoreham-by-Sea, England), was used to turn the produced mixture into filaments (operating at 230 °C and a fixed speed of 53 rpm, since this is adjustable on the extruder, and a die tip with 1.8 mm diameter). This initial step was chosen due to its efficiency in producing nanocomposite filament rapidly, where the focus is on mixing the additive with the matrix, rather than achieving precise filament diameter.

Then, using a 3devo shredder (from 3devo, Utrecht, Netherlands), these filaments were crushed into pellets. The pellets underwent additional processing, and a 3devo Composer extruder (from 3devo, Utrecht, Netherlands), also from Utrecht, Netherlands, was used to create the final filaments. To successfully combine the PP matrix with TiN particles, to make filaments suited for 3D MEX printing, the 3devo Composer extruder, featuring a special screw for raw materials mixing (according to its manufacturer), was used. The first heating zone of the extruder was preheated to 195 °C; the second and third zones were adjusted to 210 °C; and the last heating zone was set to 205 °C. To provide appropriate cooling while preserving the desired filament integrity, the fan speed was adjusted to 55%. The screw’s rotating speed was set to 5 rpm. The production of filaments with a 1.75 mm diameter was accomplished by carefully regulating the heating and cooling conditions. This settings profile was selected after preliminary tests and by consulting the corresponding literature (Vidakis et al., 2022). It was optimized for the unfilled PP and employed in all the nanocompounds to ensure the same manufacturing conditions that allow the comparison of the experimental results in the nanocomposites. The study implemented a two-step extrusion process involving the consecutive use of two distinct filament extruders. This methodology aimed to subject the nanocompounds to additional thermomechanical mixing processes within the extruders, with the goal of achieving optimal dispersion of particles in the matrix.

Pure PP filament was also extruded employing the identical procedure to create a standard against which to compare other materials. The two extrusion techniques previously presented were used to guarantee a uniform distribution of TiN particles within the composites.

2.3. Manufacturing of 3D-printed samples

The two extrusion techniques previously stated were employed to create filaments made of virgin polymer and PP/TiN nanocompounds, which were then individually and successively fed into a MEX 3D printer (Intamsys Funmat-HT, made by Intamsys in China) for the manufacturing of the 3D-printed samples for the experimental procedure. The Intamsuite software platform, also by Intamsys, was used to produce the required G-codes for the samples’ fabrication in the 3D printer. Before starting the samples’ fabrication, parameters and settings were set through a procedure of trial and error to improve the process of 3D printing. The authors consulted the corresponding literature as well (Vidakis et al., 2022). These parameters comprise, among other things, the printing speed, layer height, temperature, and infill density. Same with the extrusion process, the parameters were optimized for the unfilled PP and employed in all nanocompounds, to have comparable results, not influenced by the alteration of the implementation parameters. The relevant G-codes were created once the desired 3D printing settings had been established. The 3D printing procedure was then started utilizing successively the PP/TiN nanocompounds and pure polymer filaments to create the corresponding samples. The specimens used in each experiment were made to fulfil the dimensional standards listed in the respective ASTM specifications:

• ASTM D638-02a: This specification offers instructions for carrying out tensile tests to assess the mechanical qualities of polymeric materials.

• ASTM D790-10: This specification describes how to carry out flexural testing to evaluate the strength and stiffness of polymeric materials.

• ASTM D6110-02: The Charpy notched impact characteristics of polymers are tested per this standard.

• ASTM D384-11e1: The Vickers microhardness measurements were carried out following this standard.

The instructed ASTM requirements were followed, resulting in standardized testing procedures that ensured consistency and comparability between the outcomes of various trials.

Since the samples were 3D-printed at a 100% infill rate, the interior of the object was set to be completely solid (the 3D-printed structure still, is expected to have internal pores, which are normal for such structures (Liao et al., 2019)). The internal space of the object is filled with a grid-like structure using the rectilinear infill pattern. Additionally, for each succeeding layer, the raster orientation was changed from +45 degrees to −45 degrees. The 3D-printing variables used in the research are depicted in Figure 3. It should be noted that the bed temperature (90 °C) was necessary to reduce warping, which is a common issue when 3D printing PP samples (Spoerk et al., 2020). So, this temperature contributed to the adhesion of the samples on the built plate.

Figure 3. The settings for 3D printing to manufacture the samples and the related ASTM geometrical requirements for each of the tests.

2.4. Raman spectroscopy

For the Raman spectroscopy measurements, we used the LabRAM HR Raman Spectrometer by HORIBA Scientific in Kyoto, Japan. The microscope’s resolution was measured to be 1.7 μm lateral and 2 μm axial resolution. The acquisition spectral range was between 50 and 3900 cm⁻¹, which required three consecutive optical windows. The exposure time at each point was 5 seconds with 5 accumulations.

The Raman microscope used has the following specifications. Raman excitation occurred with a 532 nm solid-state laser module with a maximum output power of 90 mW. An Olympus objective lens (LMPlanFL N, Tokyo, Japan) with 0.5 Numerical Aperture, 50× magnification, and 10.6 mm operational working distance. A grating with 600 grooves resulted in a Raman spectral resolution of approximately 2 cm⁻¹. A Neutral Density filter with 5% transmittance limited the Laser power to 2mW on the sample. Each sample following irradiation was inspected visually and there were no observable changes or any discolorations.

2.5. Assessment of rheological properties and thermal characteristics

The response of the nanocomposites to temperature was examined using thermogravimetric analysis (TGA). The evaluations were conducted in a nitrogen (N₂) atmosphere utilizing a PerkinElmer Diamond apparatus made by PerkinElmer, Inc., which is based in Waltham, Massachusetts, in the United States. From the initial ambient temperature, the temperature was raised at a 10 °C/minute rate, reaching a maximum of 550 °C. DSC analysis was employed as well to examine the nanocomposites’ thermal characteristics. The Discovery Series DSC-25 instrument, made by TA-Instruments in New Castle, Delaware, United States, was used to perform the DSC measurements. The temperature was originally raised to 300 °C, then decreased to 25 °C (temperature rotation range of 25–300–25 °C). The samples were heated for the measurements at a typical rate of 15 °C per minute. All the measurements were conducted under an inert atmosphere by implementing nitrogen (N₂) as a purge gas. Within the required temperature range and heating rate, this particular DSC instrument enabled an accurate study of the thermal behaviour and characteristics of the nanocomposites.

To assess the rheometry properties of the nanocompounds, two parallel plates, and a temperature-controlled environmental test chamber were added to a DHR-20 Rheometer (Discovery Hybrid Rotational Rheometer) from TA Instruments (New Castle, Delaware, United States). Data were recorded for a total duration of ten seconds for each data point to prevent overheating and failure. The flow properties of the polymer were evaluated via a hole with a predetermined diameter and length under specific pressure and temperature conditions. Rotational rheometry studies and melt flow rate (MFR) measurements were coupled. The international standard for MFR ASTM D1238-13 was followed for these measurements.

2.6. Evaluation of the manufactured filaments

The surface quality, diameter, and tensile strength of the filaments were thoroughly evaluated before the 3D printing procedure for the samples’ manufacturing. This assessment aimed to guarantee the filaments’ quality and compatibility with the specified 3D printing standards. The diameter of the filaments was continually tracked throughout the production procedure using a closed-loop control mechanism to ensure adherence to requirements. To maintain the desired filament diameter, this device gave real-time feedback on the produced diameter, and adjustments were made accordingly (a built-in feature of the 3devo filament extruder). A digital caliper was also employed to independently confirm and verify the precision of the diameter dimensions. The Imada MX2 device, made by Imada Inc. in Northbrook, Illinois in the United States, was used to measure the tensile strength of the filaments. By holding the filaments securely and applying a steady testing speed of 10 mm/min, this device performed the tensile strength tests. To obtain accurate and representative results for the tensile strength analysis, a total of five specimens from each composite were evaluated. No standard was used for this test. To the authors’ best knowledge, there is no standard available for these types of tests (tensile test of filament).

By utilizing atomic force microscopy (AFM), the morphology of the side surface of each nanocomposite filament was investigated and the surface roughness of the observed region was calculated. Increased surface roughness (rougher surface) negatively affects the tribological characteristics (Gu et al., 2020) of the produced filament, since the filament is in a semi-solid phase during the extrusion process (Rashid & Koç, 2021). This, as a result, is expected to negatively affect the processability of the filament, as it affects the forces between the filament and the nozzle during the extrusion process (Fischer et al., 2023; Kattinger et al., 2022) when 3D printing the parts.

The XE7 AFM equipment from Park Systems, a company with its headquarters in Seoul, South Korea, was used to carry out the AFM measurements. It was possible to accurately capture on a micro-scale the surface characteristics and topography of the composites. The observations were made under typical room conditions. To observe and examine the surface morphology of the samples, high-resolution imaging capabilities afforded by AFM were used. A cantilever developed by Nanosensors, specifically the US-made NCHR type, was used to capture the photos. The cantilever worked at a frequency of 300 kHz and had a diameter of 10 nm. Employing the intermittent contact approach and a 0.5 Hz scanning rate, the images were captured. A continuous working set point was kept throughout the imaging process to guarantee trustworthy, high-quality photographs that were undisturbed by outside influences. The setting point had an amplitude greater than 70% of the cantilever’s natural oscillation, which reduced the impact of outside influences on the captured photographs. Observations were made in an area of 10 × 10 μm on the side surface of the filaments.

2.7. Mechanical properties

Mechanical tests were executed in accordance with the ASTM specifications to mechanically assess the characteristics of the 3D-printed objects under various loading conditions. The tests were intended to comprehend the mechanical performance of the specimens. The specimens were tested in controlled conditions (55% humidity and 23° C temperature), and the results were measured and recorded meticulously to ensure their accuracy and reliability. The samples’ capacity to endure applied forces, their overall strength, and their stiffness were assessed following ASTM standards. In all tests, the sample made of pure PP was the control sample. The mechanical test results were also correlated with the remaining tests performed in the study, to further assess and interpret the effect of the TiN nanoparticles addition in the PP thermoplastic.

A total of five samples for each composite per test were created and evaluated for the analysis of the PP/TiN compounds. Utilizing Imada Inc.'s (Northbrook, Illinois, USA) Imada-MX2 apparatus, the assessment of the tensile responses of the PP/TiN compounds was achieved. The samples were put through tensile tests using standard grips at a constant strain rate of 10 mm/min. The samples were tested for flexural strength using the same Imada MX2 equipment. The samples were mounted on supports with a 52 mm span length for three-point bending flexural testing. The samples endured a 10 mm/min strain rate during the test while a progressive bending force was applied to evaluate their flexural characteristics. Flexural tests were terminated when a 5% strain was reached, in accordance with the standard.

Charpy notched specimens were exploited to evaluate the material’s impact properties. The Terco MT-220 apparatus from Kungens-Kurva, Sweden, was used to conduct the impact testing. This apparatus had a 367 mm height for discharging the hammer, which made the impact circumstances uniform for all samples. A Vickers microhardness measurement was also performed. During the test, a stress of 200 gF was applied for 10 seconds. The InnovaTest 300 device, made in Maastricht, the Netherlands, was used to conduct this microhardness test. All of the measurements performed contributed to assessing the material’s overall mechanical qualities and its viability for various applications.

2.8. Morphology of 3D-printed specimens

SEM was utilized to analyze the shape and structure of the 3D-printed specimens’ fractured and lateral surfaces. In particular, the Tokyo, Japan-based Jeol Ltd.'s field emission JSM-IT700HR model was used for the examination of the samples. A high vacuum with a power of 20.0 kV was used for operating the SEM. At various magnifications, pictures of the specimens were taken after they had been sputtered with a thin layer of gold. This thorough method helped to better comprehend the fracture mechanism and the quality of the 3D-printed specimens by offering insightful information about their morphology and structure at various scales.

3. Results

3.1. Raman spectroscopy outcomes

The Raman spectra of the pure PP and the PP/TiN nanocomposites are visible in Figure 4A. The following Table 1 presents the relevant Raman peaks from the pure PP sample, which are supported by literature. As presented in Figure 4B, there are clear differences from PP Pure. Particularly, there are some gradual decreases of Raman intensity at 398, 528, 808, 840, 973, 1168, 1329, 1458, and 2967 cm⁻¹ present in all samples, indicating a concentration decrease of the related bonds as described in the following Table 2. Moreover, there are some Raman bands showing a gradual increase of Raman intensity at 2832, 2858, 2873, 2904, 2925, and 2942 cm⁻¹ indicating a concentration increase of the bonds described in Table 2.

Figure 4. Raman spectra from (A) PP Pure and PP/TiN 1 wt. %, 2 wt. %, 4 wt. %, and 6 wt. %, and (B) PP Pure, PP/TiN 1 wt. %, 2 wt. %, 4 wt. %, and 6 wt. %, when PP Pure is removed.

Table 1. Significant Raman peaks of PP pure identified and their associated assignments.

Wavenumber (cm^−1)	Intensity	Raman peak assignment
398	Medium	O-C-O bend (Resta et al., 2015)
809	Major
839	Major	γ(C–OH)ring (Bichara et al., 2016; Synytsya et al., 2003)
940	Small	C-H bending (Resta et al., 2015; Stuart, 1996)
973	Medium
998	Small	C-H in-plane bending (Stuart, 1996)
1036	Small	C-CH3 stretching (Lin et al., 2021)
1151	Medium	Skeletal deformation (Makarem et al., 2019)
1168	Medium
1329	Major	C- O- C stretch (Resta et al., 2015)
1360	Medium	C–C-H, C-O–H, and O-C-H (Makarem et al., 2019)
1435	Medium	C-H^−3 deformation (Stuart, 1996) C-H^−2 deformation (Stuart, 1996; Zimmerer et al., 2019)
1457	Major	C-H3 symmetric bending (Lin et al., 2021; Resta et al., 2015; Stuart, 1996);
2721	Small	C = O stretching (Liu et al., 2006)
2841	Major	C-H2 symmetric stretching (Makarem et al., 2019)
2869	Major	C-H2 symmetric stretching (Makarem et al., 2019); C-H symmetric stretching (Liu et al., 2006)
2885	Major	CH2 symmetric stretching (Liu et al., 2006; Makarem et al., 2019)
2907	Major	CH vibration (Makarem et al., 2019)
2926	Major	CH2 asymmetric stretching (Makarem et al., 2019)
2954	Major	CH2 asymmetric stretching (Makarem et al., 2019)
2964	Major	Asymmetric vibration of νas(CH3) (Montoro et al., 2014)

Table 2. Significant Raman peak intensity differences and their related assignments, between PP pure and PP/TiN samples.

Wavenumber (cm^−1)	Raman spectrum changes
398	Gradual decrease O-C-O bend (Resta et al., 2015)
528	Gradual decrease Anatase crystal phase of TiO2 (Perevedentseva et al., 2021)
808	Gradual decrease Si–O–Si bending (Gatin et al., 2022; Luiz et al., 2007)
840	Gradual decrease γ(C–OH)ring (Bichara et al., 2016; Synytsya et al., 2003)
973	Gradual decrease
1168	Gradual decrease in Skeletal deformation (Makarem et al., 2019)
1329	Gradual decrease C-O-C stretch (Resta et al., 2015)
1458	Gradual decrease C-H3 symmetric bending (Lin et al., 2021; Resta et al., 2015; Stuart, 1996)
2832	Increase CH vibration (Chalmers et al., 2002)
2858	Increase CH2 symmetric stretching (Makarem et al., 2019)
2873	Increase C-H2 symmetric stretching (Makarem et al., 2019); C-H symmetric stretching (Liu et al., 2006)
2904	Increase CH stretching (Stuart, 1996)
2925	Increase CH2 asymmetric stretching (Makarem et al., 2019)
2942	Increase CH stretching and bending (Zou et al., 2009)
2967	Gradual decrease Asymmetric vibration of νas(CH3) (Montoro et al., 2014)

From Table 2, we observe in the PP/TiN samples a decrease in the Raman peaks (1458, 2967 cm⁻¹), which refer to CH₃ concentration (methylated parts of the molecule). This fact, in combination with the increase of Raman C-H peaks at (2832, 2873, 2904, and 2942 cm⁻¹) could be explained as the bond released from CH₃ is replaced with a C-H bond. Taking into account the fact that the dissociation energy of C-H is higher than that of CH₃, it could be an explanation of the sample’s mechanical properties increase (May et al., 2000).

3.2. Thermogravimetric Analysis and differential scanning calorimetry

Figure 5A displays the weight loss of the investigated composites and pure PP as a function of the temperature as it was derived through the TGA charts. These charts demonstrate how weight varies with temperature and provide important information regarding the material’s thermal stability and patterns of degradation. Figure 5A’s results make it clear that the TiN particle addition had limited to no influence on how resistant the PP material was to heat degradation. At a temperature of roughly 400 °C, which is similar to the point at which PP starts to lose weight, all the composites start to lose weight. The addition of TiN nanoparticles causes a negligible shift of the compounds’ degradation to higher temperatures. The attached bar graphs in Figure 5A show the weight that was still present after the TGA was finished. The filler percentage of each nanocompound is consistent with this weight, as can be shown. In Figure 5B the findings of the DSC investigation are presented. The introduction of the TiN nanoparticles did not cause any significant altering regarding the melting temperature, the temperature at which the PP thermoplastic changes phase, of the investigated composites. Also, the introduction of the TiN nanoparticles altered the melting enthalpy by decreasing its value. This behaviour helps the printability of the material especially for high-speed printing applications, because for the same amount of heat energy, more volume of polymer can be melted.

Figure 5. Study of the thermal performance of pure PP and PP/TiN compounds using (A) TGA curves and (B) heat-flow curves (DSC) at various temperatures.

3.3. Rheometric evaluation

Figure 6A displays the results of the rheological measurements. Particularly, the samples’ viscosity and stress are displayed on logarithmic axes as functions of shear rate. All samples showed an overall drop in viscosity as the shear rate increased, pointing to a non-Newtonian, pseudoplastic, or shear-thinning tendency (Petousis et al., 2023; Saadat et al., 2010) as seen in the rheological graphs. Increased shear rate results in a decrease in viscosity (Figure 6A). When TiN concentration increases, the movability and the tangled polymer chains can be impacted, which may decrease the composite’s viscosity at greater shear rates.

Figure 6. Pure PP polymer and PP/TiN composites’ rheological analyses are presented in (A) viscosity and stress vs. shear rate and (B) melt flow vs. filler percentage.

Figure 6B displays the MFR (in g/10 min) as a result of the TiN additive weight percent for the melt flow rate measurements. When compared to PP/TiN composites, pure PP has the highest MFR. Consequently, a reduced viscosity is indicated by a greater MFR. This is consistent with the behaviour seen in the rheological graphs, where the presence of TiN nanoparticles reduced viscosity, which in consequence reduced MFR.

3.4. Analysis of filaments’ performance

It has already been mentioned that the filaments under investigation were made using a 3devo composer extrusion device outfitted with a closed-loop system that controls the filament diameter. The incorporated system can generate a consistent-sized filament with a precise diameter by measuring the filament diameter in real time and adjusting the extrusion procedure within tolerable limits. Figures 7A and B display two arbitrarily chosen parts of the generated filaments. As shown the produced filament is within a 1.65 mm – 1.85 mm diameter range, which is accepted for FFF 3D printing. The optical stereoscope used to take these pictures was the OZR5 (from KERN & SOHN GmbH, located in Albstadt, Germany). The figures also display graphs from the measured diameters of raw PP and PP/TiN 4.0 percent nanocomposites. The optical stereoscopy pictures revealed filaments with a nearly flawless surface and a few minor flaws. The findings of the filament tensile investigations, which are depicted in Figure 7C, demonstrate an improvement in tensile strength for all TiN nanocompounds. The compound with 4.0 wt. % TiN had up to 41.5% greater than the unfilled PP filament tensile strength, the greatest increase recorded in the tests. Additionally, Figure 7D shows how the presence of TiN nanoparticles affected the stiffness (tensile modulus of elasticity) of the filaments that were created. When compared to the PP matrix, the stiffness of the PP/TiN 4.0 wt. % compound was increased by 34.0%.

Figure 7. Extruded filament segments are being monitored in real-time and here two different filament types are presented: (A) pure PP and (B) PP/TiN 4 wt. %. (C) Results of the filaments’ tensile tests and (D) the results for the tensile modulus of elasticity.

All of the created filaments’ lateral surfaces were inspected with AFM. All of the filaments had similar or higher surface roughness than the pure PP filament, according to the findings, which are demonstrated in Figure 8. The filament with the greatest percentage of TiN particles (6.0 wt. %) revealed the most notable rise in all three surface roughness factors, with all three (3) surface roughness factors, Rq, Ra, and Rz, revealing an increase in their values. The association between the surface roughness metrics (Rq for Root Mean Square roughness, Ra for Arithmetic Average roughness, and Rz for Maximum Height roughness) and the TiN concentration in the compounds is depicted in Figures 8G-I. All the surface roughness metrics increase constantly with the increase of the TiN loading in the nanocompounds.

Figure 8. (A) The study’s usage of an atomic force microscope. Images acquired from the lateral surfaces of the investigated filaments employing AFM are displayed in (B) for pure PP, (C) PP/TiN 1 wt. %, (D) PP/TiN 2 wt. %, (E) PP/TiN 4 wt. %, and (F) PP/TiN 6 wt. %. Graphs showing the relationship between surface roughness parameters and TiN content in the compounds: (G) Rq, (H) Ra, and (I) Rz.

3.5. Mechanical performance

Tensile testing in accordance with ASTM D638-02a was performed on the 3D-printed parts to evaluate their respective mechanical properties. A graph showing the correlation between measured strain (mm/mm) and tensile stress (MPa) for a random specimen of each composite material and the raw PP is shown in Figure 9A. A bar chart of the mean tensile strength (in MPa) for each material in conjunction with the filler percentage is shown in Figure 9B. The experiment’s results reveal that all nanocomposites exhibited greater tensile strength than the unfilled PP polymer, with the highest reported increase of 41.5% achieved by the 2.0 weight percent additive concentration nanocompound. The tensile modulus of elasticity (in MPa) results are shown in Figure 9C. The findings followed a similar trend with the sample with 2.0 wt. % of TiN providing the best results, with stiffness improvement of 26.8%, while the sample with 6.0 wt. % of TiN exhibited a minor decrease in its performance, more possibly owed to the saturation of the filler in the matrix.

Figure 9. Results from measuring the tensile strength of samples made via 3D printing. (A) Randomly chosen 3D-printed specimens from each nanocomposite material are shown in illustrative graphs of tensile stress vs strain, (B) outcomes of the tensile strength (average value and deviation from the five samples tested), and (C) outcomes of tensile modulus of elasticity (average value and deviation from the five samples tested).

Figure 10 lists the flexural properties of specimens produced by pure PP and PP/TiN nanocomposites. A stress-strain curve is shown in Figure 10A for one random specimen of each nanocomposite. Corresponding to the requirements of ASTM D790-10, the standard values of flexural strength were determined at an ultimate strain of 5%. The maximum flexural strength (Figure 10B) of 53.2 MPa was attained by the nanocomposite with a loading of 2.0 wt. %, demonstrating an increase of 33.7% compared to pure PP. It should be noted that the flexural strength of the compounds with the highest TiN content (6.0 wt. %) was slightly lower than that of the raw PP material, again possibly attributed to the saturation of the filler in the matrix. With a 9.1% increase over the raw PP material, the nanocomposite loaded with 2.0 wt.% TiN also showed the highest flexural modulus of elasticity (Figure 10C). In contrast, the flexural modulus of elasticity was found to be reduced in the nanocomposites containing 1.0, 4.0, and 6.0 weight percent TiN.

Figure 10. The outcomes of the flexural tests (A) Stress-strain curves for samples tested for flexural strength; a random specimen from each nanocomposite’s five 3D printed specimens. In accordance with the requirements of ASTM D790, the experiment was stopped at 5% strain. (B) the results for the flexural strength and its standard deviation, and (C) the findings for the flexural modulus of elasticity and its standard deviation.

The toughness values (MJ/m³) for the manufactured specimen are shown in Figure 11. The energy that the materials absorbed during testing were calculated using the stress-strain graphs. The tensile and flexural toughness measurements for the materials were calculated using the integral of the pertinent stress vs strain curves. Figures 11A and B, which depict the results of specimen testing, indicate that all composites had tensile and flexural toughness values that are equivalent to or greater than those of pure PP polymer, with the exception of the specimens that include 6.0 weight percent of TiN nanoparticles, which showed a minor decline. Particularly, the PP/TiN 2.0 wt.% composite exhibits improvements in tensile and flexural toughness of 8.7% and 17.2%, respectively.

Figure 11. (A) Tensile toughness, and (B) Flexural toughness for all created specimens.

Figure 12A displays the outcomes of the impact experiments, whereas Figure 12B displays the results of the Vickers micro-hardness testing. The mean Charpy impact strength (measured in kJ/m²) and Vickers micro-hardness (measured in HV) for all the materials inspected were assessed to take into consideration the various filler concentrations. These measurements provide data on the impact durability and toughness characteristics of the materials at different filler concentrations. As shown in Figure 12A, the impact strength rises with filler concentration up to 2.0 wt. %, but there is no further enhancement beyond this point. On the other hand, Figure 12B demonstrates a linear and gradual increase in the micro-hardness values as the filler content rises. At the greatest percentage of 6.0 wt. %, the micro-hardness reaches a value of 24.3 HV, which is 44.8% higher than that of virgin PP. These findings show that the materials’ hardness properties improve as filler concentration increases. Such a performance in this test was rather expected, due to the high-hardness characteristics of the TiN additive, which were expected to be induced in the nanocomposites.

Figure 12. (A) the Charpy impact strength and (B) Vickers microhardness for all nanocomposites.

3.6. Morphological analysis

SEM was employed to examine the cracked and lateral surfaces of the 3D-printed objects (tensile test specimens were inspected, after they failed in the test). Figures 13A, D, and G depict SEM photos of the side surfaces of the 3D-printed samples. The samples shown in these images are PP/TiN composites with weight percentages of 0.0, 2.0, and 4.0 wt. % respectively. Imperfections in the layer thickness and uneven layer morphologies are seen in the case of the side surfaces of the nanocomposites, while the pure PP shows a 3D printing structure without defects. The fracture surface topography at 30× magnification of each of these samples is shown in Figures 13B, E, and H. Following, the fracture surfaces were magnified 300 times to enable a complete analysis of their morphological characteristics (Figures 13C, F, & I). Microvoids were found in the PP/WC 4 wt.% in Figure 13H, mostly close to the specimen’s margins. Even at a 100% infill ratio, these micro voids in the 3D-printed structure are expected due to the 3D printing technique utilized, which creates the object layer by layer. These voids are expected to be inflated after the mechanical testing of the samples. The fracture surface shows minimum deformations for the pure PP and the 2 wt. % nanocomposite, while the topography significantly differs in the case of the 4 wt. %, with a more ductile fracture mechanism and visible deformation in the filament strands. As the filler loading increases, the shape of the layers slightly worsens, and it becomes not so uniform. This can be attributed to the significant reduction in the MFR by the addition of the TiN nanoparticles in the PP matrix. This instructs that the 3D printing settings need adjustment for the TiN-loaded composites. In the study, the 3D printing settings were optimized for the pure PP and the same parameters were used in the composites as well, to be able to have comparable results. Regarding the fracture surface, the 2 wt. % sample showed a rather similar morphology to the pure PP sample. The main difference is the increased size of the pores in the pure PP sample. The sample with 4 wt. % loading presented a different morphology, with more pores, which are larger in size. Such an internal structure is expected to have contributed to the reduced mechanical properties of the specific samples compared to the 2 wt. % ones.

Figure 13. The SEM images for (A) a lateral view of a pure PP sample at 150× magnification, (B) a fractured view of a pure PP sample at 30× magnification, (C) a fractured view of a pure PP sample at 300× magnification, (D) a lateral view of a PP/2.0% TiN sample at 150× magnification, (E) a fractured view of a PP/2.0% TiN sample at 30× magnification, (F) a fractured view of a PP/2.0% TiN sample at 300× magnification, G) a lateral view of a PP/4.0% TiN sample at 150× magnification, (H) a fractured view of a PP/4.0% TiN sample at 30× magnification, (I) a fractured view of a PP/4.0% TiN sample at 300× magnification.

Investigating the TiN additive’s distribution inside the nanocomposites and identifying potential agglomerations are the goals of Figure 14. The PP/TiN 1.0 wt. % composite is shown in Figure 14A in an SEM image magnified 1000 times, demonstrating no presence of TiN agglomerations. The PP/TiN 6.0 wt. % (Figure 14B) subjected to the same investigation revealed the presence of agglomerated TiN particles. Figure 14C gives a larger magnification (30,000×) of the indicated area in Figure 14B to provide further insight. Figure 14D shows an EDS graph that is concentrated on the area seen in Figure 14B, emphasizing the spread of the Ti element (which is depicted in the EDS map presented in Figure 14E), the main constituent of the TiN additive. Furthermore, a significant amount of bulk Carbon (depicted in the EDS map presented in Figure 14F) was identified, which is expected in polymeric materials. Because of their organic molecules, carbon is a crucial component in compounds such as the ones studied here.

Figure 14. (A) PP/TiN 1.0 wt. % at 1000× magnification (B) PP/TiN 6.0 wt. % at 1000× magnification, (C) PP/TiN 6.0 wt. % at 30,000× magnification, (D) EDS analysis of PP/TiN 6.0% obtained from a region containing an agglomeration of TiN nanoparticles, and (E), (F) EDS mapping of Ti and C elements distribution in the nanocompound, correspondingly.

4. Discussion

Figure 15 displays a synopsis of the experimental results in the mechanical tests carried out herein on both the nanocomposites and the pure PP material. The inclusion of TiN particles resulted in most cases in improved material characteristics. This can be attributed to the reinforcing mechanism occurring by the addition of the nanoparticles, which is well explained and documented in the literature. More specifically, the mechanical properties may have been enhanced by the chemical processes and the interactions occurring at the point of contact between the PP matrix and the TiN particles (Chang et al., 2020; Crosby & Lee, 2007; Navarro Oliva et al., 2023; Nguyen et al., 2016; Zhang et al., 2022). All the compounds that were created and tested performed mechanically in a better way than the unfilled PP polymer. The composite with a TiN load of 2.0 wt. % had overall the best mechanical performance. This finding suggests that, among the examined compounds, the 2.0 weight percent loading is the optimum option among the ones tested. Further increasing the TiN filler percentage, results in the mechanical properties degrading. This implies that higher TiN concentrations in the PP matrix eventually reach a saturation level. It is important to keep in mind that the current study did not investigate the percolation threshold of the prepared nanocomposites because it was not within the scope of the analysis. Moreover, the microhardness exhibits a steady and upward trend up to a 6-weight percent additive loading, improving by 44.8% over raw PP. In applications, in which the surface stiffness of the parts is required, such a loading can confront the specifications. It should be noted that filaments with 4 wt. % loading had the highest tensile strength, while 3D printed samples with 2 wt. % showed the highest tensile strength. This can be attributed to the effect of the 3D printing process on the performance of the materials. The filament is solid material, while the 3D printed samples are not solid, even at 100 infill percentage, due to the 3D printing structure, which affects their performance.

Figure 15. Spider graph illustrating the results of the mechanical tests. The mechanical performance of the unfilled PP thermoplastic is indicated by the blue-coloured zone. The nanocomposites with the highest performance in each one of the mechanical properties assessed are indicated in the table on the right side of the figure.

SEM images demonstrate that there is no substantial increase in the micro-voids on the surfaces of the 3D-printed items as the additive percentage of the PP/TiN compound is increased. Additionally, the SEM analysis provides visible evidence equally distributed throughout the matrix material. No particle clustering was located in all filler concentrations, except the highest one of 6 wt. %, in which a low number of particles clustering can be observed in the fracture surfaces. Still, the increase in the filler loading worsens the 3D printing structure, with layers having a non-uniform shape and fusion with the other layers. This can be attributed to the change in the rheological properties of the nanocomposites. The MFR was significantly decreased, making the flow of the material more difficult and thus affecting the quality of the 3D printing structure. This outcome instructs for adjustments in the 3D printing settings, to be optimum for each nanocompound. This was not done within the context of the research, to have comparable results, as already mentioned above. Such an optimization in the 3D printing settings per nanocompound is expected to further increase the mechanical performance of the nanocompounds prepared herein. Regarding the fracture surfaces, pure PP specimens show micro-voids in comparison to specimens with larger filler concentrations. As was previously mentioned, it is normal to find such flaws or structural variations in parts produced utilizing the MEX 3D printing technology. The increase of the TiN loading to 4 wt. % changes the topography of the fracture surface, as more voids are visible, and an overall less solid internal 3D printing structure is presented. Through the SEM investigation, the distribution of the nanoparticles in the matrix was also evaluated. No agglomerations were located in the lower-loading nanocompounds. The two-step extrusion process followed contributed to this outcome. At higher loadings, agglomerations were located in the SEM images. This is expected to be due to the filler saturation in the matrix, which also had a negative effect on the mechanical performance of the nanocompounds with loadings higher than 2 wt. %. The EDS mapping for the Ti element in the fracture areas of the tensile samples, also validates the uniform distribution of the nanoparticles, at least up to the highest loading of 6 wt. %, in which some agglomerations were found, as mentioned above. Still, the sufficient particle distribution is also validated by the acceptable deviation in the mechanical tests, which indicates a similar composition in the nanocomposites in all the samples tested.

The TGA investigation has demonstrated that the materials employed in the MEX technique are not negatively impacted by the temperatures employed during processing. This is significant because it shows that the 3D printing process and the mechanical properties of the specimens created using the produced composites will not be negatively impacted by any potential material deterioration brought on by high temperatures. Additionally, no significant change in the thermal properties of the unfilled PP occurred with the addition of the TiN nanoparticles.

According to the available data, no prior reports have been documented on the mechanical characteristics of similar composites in the literature. This emphasizes the study’s novelty because it addresses an existing gap in knowledge and presents novel viewpoints on the mechanical properties of the composites under investigation. Still, as mentioned above, TiN nanoparticles have been introduced to polymeric matrices in MEX 3DP. More specifically, TiN nanoparticles have been introduced to Polylactic Acid (PLA) (Petousis et al., 2023), Acrylonitrile Butadiene Styrene (ABS) (Vidakis et al., 2023) Polycarbonate (PC) (Vidakis et al., 2022), and medical grade Polyamide 12 (PA12) (Vidakis et al., 2022). As stated above, the reinforcing effect varies from less than 20% improvement in the tensile strength (Vidakis et al., 2023), to more than 45% for the medical-grade PA12 (Vidakis et al., 2022). A 41.5% increase in the tensile strength of the PP polymer is reported herein, which is remarkably close to the highest increase of 45% reported so far for the medical-grade PA12 (Vidakis et al., 2022), showing the potential of the PP polymer for use in applications requiring increased mechanical behaviour from the polymeric materials.

5. Conclusions

Herein, the performance of Titanium Nitride (TiN) ceramic nanoparticles as a strengthening factor for the PP polymeric material in MEX 3DP was investigated. The addition of TiN ceramic nanopowder to the PP matrix validated the idea and notably increased the PP polymer’s mechanical responsiveness. Nanocomposites were produced using a thermomechanical extrusion method, which led to the fabrication of filaments for MEX 3DP and by extension to 3D printed samples, tested for their mechanical performance. The main findings are summarized as follows:

• The tensile and flexural strength increased considerably as compared to pure PP, with an increase of 41.5% and 33.7%, respectively (achieved by the 2 wt. % TiN nanocomposite).

• The inclusion of TiN led to an increase in the tensile and flexural toughness of 8.7% and 17.2%, respectively.

• There was an increase in the tensile and flexural elastic moduli of 26.8% and 9.1%.

• The introduction of 2 wt. % TiN nanoparticles in the PP matrix, which achieved these results, is considered the optimum loading for the nanocomposites prepared herein with a thermomechanical extrusion process for MEX 3DP.

• The TiN ceramic greatly increased microhardness at the 6 wt. % nanocomposite, verifying its high wear resistance performance, with a maximum improvement of 44.8%.

• The thermal stability of the PP polymer was negligibly affected by the introduction of the TiN nanoparticles.

• The rheological properties (MFR) were drastically reduced. The decrease in the 3D printing structure quality observed in the SEM images in the high-loaded nanocomposites can be attributed to this change in the MFR, which instructs for adjustments in the applied 3D printing settings for the nanocomposites. This was decided not to be implemented herein to have comparable results since all the nanocomposites and the unfilled PP were prepared and tested under the exact same process and settings.

Future research can focus on industrializing the method of production and determining the precise percolation threshold of the TiN filler in the polymer matrix. By adjusting the 3D-printing settings, it is feasible to maximize the benefits of the composite material by enhancing the reinforcement effect of the TiN filler in the PP material. By fine-tuning these issues and promoting the usage of TiN-reinforced PP composites in various industries, the study’s findings would be more useful in real-world applications. The applications of TiN-based composites could be expanded by further studies in these sectors.

Ethical approval

Not applicable.

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations.

Disclosure statement

No potential competing interest was reported by the authors.

Additional information

Funding

This research received no external funding.